Valorization of Sodium Lignosulfonate for Enhanced Humification and Copper Immobilization During Municipal Sludge Composting

Valorization of Sodium Lignosulfonate for Enhanced Humification and Copper Immobilization During Municipal Sludge Composting | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Valorization of Sodium Lignosulfonate for Enhanced Humification and Copper Immobilization During Municipal Sludge Composting Minghui Li, Shangchun Chen, Genjia Xu, Meihua Zhao, Changya Chen, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7877970/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 10 Mar, 2026 Read the published version in Water, Air, & Soil Pollution → Version 1 posted You are reading this latest preprint version Abstract Valorization of industrial by-products like sodium lignosulfonate (SL) offers a sustainable strategy for environmental remediation. This study provides a systematic evaluation of SL's dual-purpose application to enhance humification and immobilize copper (Cu) during municipal sludge composting. The effects of SL addition at 0% (CK), 3% (T3), and 9% (T9) dry weight over a 60-day composting period were investigated. The 9% SL treatment (T9) significantly accelerated compost maturity, achieving a germination index (GI) of 96.4% compared to 85.9% in the control. EEM-PARAFAC analysis revealed that SL addition promoted the transformation of protein-like and quinone-like intermediates into stable humic acid (HA)-like substances. Consequently, the final HA content and degree of polymerization (DP) in T9 were substantially higher than in the control. Critically, this enhanced humification directly facilitated Cu immobilization. The DTPA-extractable (bioavailable) Cu in T9 was reduced to 96.40 mg/kg, a 66% reduction from its initial value and significantly lower than the 150.73 mg/kg in the final control compost. Pearson analysis confirmed a strong negative correlation between HA content and bioavailable Cu. The findings demonstrate that SL is a highly effective amendment for producing safer, higher-quality compost, offering a novel strategy for the synergistic valorization of industrial and municipal waste streams. Composting Sodium lignosulfonate Humification Heavy metal immobilization Waste valorization Copper Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Highlights Sodium lignosulfonate enhances sludge composting and improves compost quality. The addition of 9% sodium lignosulfonate significantly increases humic acid content. Bioavailable copper was reduced by 66% with 9% sodium lignosulfonate. Enhanced humification is the key mechanism for copper immobilization. 1. Introduction Municipal sludge, a voluminous by-product of wastewater treatment, represents both a significant disposal challenge and a potential resource for agricultural valorization (Izydorczyk et al., 2021 ; Nkinahamira et al., 2019 ). Composting is widely recognized as a key strategy to stabilize sludge and convert it into a valuable soil conditioner (Aro and Fatehi, 2017 ; Y. Wu et al., 2025 ). However, the efficacy of sludge composting is often constrained by two interconnected factors, inefficient humification and the presence of heavy metals (Chen et al., 2025 ). Inefficient humification results in a compost product with low stability and limited agronomic value, as the formation of stable humic substances (HS) is a pivotal process governing compost quality (Guo et al., 2019 ; Zhao et al., 2022 ). Simultaneously, municipal sludge is a sink for heavy metals like copper (Cu), which, due to its high bioavailability and toxicity, poses ecological risks and restricts the land application of composted sludge (Guo et al., 2023 ; Liu et al., 2025 ; Wei et al., 2022 ). Interestingly, these two challenges are mechanistically linked. The process of composting can effectively passivate heavy metals by transforming them into more stable, less bioavailable forms (Cyubahiro et al., 2023 ; Wu et al., 2023 ; X. Zheng et al., 2022 ). This immobilization is primarily driven by complexation and chelation with newly formed HS, particularly humic acids (HA) (Ma et al., 2024 ; Tang et al., 2019 ). The abundant oxygen-containing functional groups (e.g., carboxyl, phenolic hydroxyl) in HA create strong binding sites for metal ions (X. Wang et al., 2021 ). Therefore, enhancing the rate and quality of humification is a direct and sustainable strategy for mitigating the environmental risks associated with heavy metals in sludge compost. To accelerate humification, various additives rich in aromatic precursors, such as lignin, have been explored (Cui et al., 2021 ; Liu et al., 2025 ; Long et al., 2024 ). Sodium lignosulfonate (SL), an abundant and low-cost water-soluble by-product of the sulfite pulping process, is an ideal candidate. Its rich aromatic structure and high density of phenolic hydroxyl and sulfonic acid groups make it both an excellent precursor for HS synthesis and a potent chelating agent for metal ions (Liu et al., 2025 ). Recent studies have acknowledged the potential of SL in composting. For instance, research has shown that SL can promote the formation of humic substances in paper mills and municipal sludge composting (Chen et al., 2025 ; W. Wang et al., 2021 ). Other work has focused on SL's role in the passivation of heavy metals through the formation of insoluble sulfates and phosphates (Liu et al., 2025 ). However, a systematic investigation that quantitatively links the dose-dependent enhancement of humification quality with the degree of heavy metal immobilization is still needed. Specifically, the pathway by which SL-derived precursors are transformed into distinct humic fractions (e.g., fulvic vs. humic acids) and how this transformation directly governs the reduction in Cu bioavailability remains to be fully elucidated. This study aims to fill this gap by systematically investigating the dual-purpose valorization of SL in a Cu-contaminated municipal sludge composting system. This work evaluates the capacity of SL to act as a humic precursor, accelerating the transformation of fulvic acids (FA) to stable HA and increasing the overall degree of polymerization in a dose-dependent manner. It further determines how this enhanced humification leads to a quantifiable reduction in the bioavailable (DTPA-extractable) fraction of Cu through increased complexation. To confirm this mechanistic pathway, the statistical correlation between key humification indices (e.g., HA content, HA/FA ratio) and Cu bioavailability is established. By elucidating these relationships, this research provides new mechanistic insights into the synergistic recycling of industrial and municipal wastes, offering a practical strategy to produce safer, high-value compost. 2. Materials and Methods 2.1. Raw Materials and Additives The primary raw materials for composting were dewatered municipal sludge and pine ( Pinus massoniana ) sawdust. The anaerobically digested dewatered sludge was obtained from the Wastewater Treatment Plant in Guangzhou, Guangdong Province, China. The sawdust (particle size ≤ 2 cm) was sourced from a local timber market. SL, derived from a sulfite pulping process, was obtained from a secondary fiber paper mill and used as the additive. All materials were stored at 4°C before use. The detailed physicochemical properties of the raw materials were analyzed and are presented in Table 1 . Table 1 Physicochemical characteristics of the raw materials and additives. Parameter Sludge Sawdust SL Moisture (%) 37.69 6.86 6.74 pH 7.66 ± 0.11 7.43 ± 0.12 10.08 ± 0.25 EC 1.58 0.25 5.89 TOC (g/kg, dry wt.) 210.48 573.76 326.41 TN (g/kg, dry wt.) 18.57 4.64 C/N Ratio 11.34 123.72 - Ash Content (%) 36.74 1.18 11.25 Total Cu (mg/kg, dry wt.) 2720.65 2.2. Experimental Design and Composting Operation The composting experiment was conducted in cylindrical plastic reactors (30 cm inner diameter, 50 cm height), each with a working volume of approximately 25 L. To minimize heat loss, each reactor was insulated with a 10-cm-thick layer of polyurethane foam. A perforated plate was placed at the bottom of each reactor to facilitate passive aeration. The experiment consisted of three treatments, each performed in triplicate. The treatments include Control (CK) with no SL addition, T3 with the addition of 3% SL (by dry weight of sludge and sawdust), and T9 with the addition of 9% SL (by dry weight of sludge and sawdust). For each reactor, sludge and sawdust were mixed at a 6:1 ratio (dry weight basis) to achieve an initial C/N ratio of approximately 28, which is considered optimal for microbial activity in sludge composting (Pan et al., 2012 ). The corresponding dose of SL was then added and thoroughly homogenized. The moisture content of each mixture was adjusted to 60% (w/w) using deionized water. Finally, 0.5% (w/w) of mature compost from a previous batch was added as an inoculum to accelerate the process. The total initial weight of the mixture in each reactor was approximately 15 kg. The composting process was carried out for 60 days. To ensure aerobic conditions, the compost piles were manually turned every three days for the first 20 days (thermophilic phase) and weekly thereafter. Moisture content was monitored weekly by weight and replenished with deionized water to maintain a range of 55–60%. 2.3. Sampling and Physicochemical Analysis Samples were collected on days 0, 5, 10, 15, 20, 30, 40, 50, and 60. At each sampling point, composite samples were obtained by mixing materials collected from the upper, middle, and lower layers of each reactor. Each composite sample was divided into two subsamples. One was immediately stored at 4°C for germination index (GI) analysis. The other was air-dried, ground to pass through a 0.15 mm sieve, and stored for subsequent chemical analysis. Temperature was recorded twice daily (9:00 and 16:00) at the center of each compost pile using a digital thermometer, with the average value reported as the daily temperature. Moisture content was determined by drying samples at 105°C to a constant weight. Organic matter (OM) content was determined by mass loss on ignition in a muffle furnace at 550°C for 8 h (Y. Chen et al., 2021 ). Total organic carbon (TOC) content was calculated as 58% of OM (Zmora-Nahum et al., 2005 ), while Alkaline potassium persulfate ultraviolet spectrophotometry was used for the determination of total nitrogen (TN). The germination index (GI) was determined using Chinese cabbage seeds ( Brassica rapa subsp. pekinensis ) as described by Liu et al. ( 2020 ) to assess phytotoxicity (Y. Liu et al., 2020 ). 2.4. Humification and Copper Speciation Analysis The extraction of HS, including HA and FA, was performed using an alkaline solution (0.1 M NaOH + 0.1 M Na 4 P 2 O 7 ) following the procedure described by Zhou et al. ( 2014 ) (Zhou et al., 2014 ). The degree of polymerization (DP) was calculated as the HA/FA ratio. Fluorescence excitation-emission matrix (EEM) spectroscopy was conducted on the HS extracts using a Hitachi F-7000 fluorescence spectrophotometer. To minimize inner-filter effects, all extracts were diluted with ultrapure water to achieve a UV absorbance below 0.1 at 254 nm. EEM spectra were recorded with excitation wavelengths from 200 to 400 nm and emission wavelengths from 280 to 550 nm. Raman and Rayleigh scattering were removed from the spectra before analysis. Parallel Factor (PARAFAC) analysis was performed on the EEM dataset using the drEEM toolbox in MATLAB 2016a to identify the underlying fluorescent components. For heavy metal analysis, the total Cu content was determined by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, Agilent 5110) after microwave digestion of the samples with a mixture of concentrated nitric acid (HNO₃) and hydrogen peroxide (H₂O₂) in a 3:1 (v/v) ratio. The bioavailable fraction of Cu was determined by extraction with a diethylenetriaminepentaacetic acid (DTPA) solution, a method widely used to estimate plant-available metals, as described by Lindsay and Norvell ( 1978 ) (Lindsay and Norvell, 1978 ). 2.5. Data Analysis The experimental data were arranged using Microsoft Excel 2016 software. Parallel Factor Analysis was conducted using MATLAB 2016a on the 3D fluorescence scanning data after removing Raman scattering and Rayleigh scattering. SPSS 22.0 was used for data correlation analysis. 3. Results and discussion 3.1. Effect of SL on Compost Maturity and Physicochemical Evolution The addition of SL significantly influenced the physicochemical evolution and final maturity of the sludge compost, as evidenced by changes in temperature, OM dynamics, and phytotoxicity (Fig. 1 ). As shown in Fig. 1 a, all treatments entered the thermophilic phase (> 50°C) after a turning operation on day 4, reaching peak temperatures of 56.3°C (CK), 58.5°C (T3), and 59.5°C (T9). The higher peak temperature and prolonged thermophilic period in the T9 treatment suggest that SL addition promoted microbial metabolism (Rastogi et al., 2020 ). This can be attributed to the readily available carbon in the water-soluble SL, which may have fueled initial microbial activity, as well as the higher TOC load providing more substrate for sustained thermophilic decomposition. Notably, only the T9 treatment met the requirement for adequate sanitation by maintaining a temperature above 50°C for more than five days, whereas the other two groups did not. The degradation of OM is a primary indicator of composting efficiency. The final OM degradation ratios were 33.3% (CK), 31.1% (T3), and 28.8% (T9) (Fig. 1 b). Interestingly, the T9 treatment showed the lowest OM degradation. This is likely because SL, with its complex aromatic structure, is more recalcitrant to microbial decomposition than other organic materials (Dorogaya et al., 2023 ; Khatami et al., 2019 ). This presents a notable trade-off; while a lower degradation rate may imply reduced mass stabilization, it also indicates greater carbon sequestration in the final compost product, a significant environmental benefit for soil amendment. Consequently, the C/N ratio in all treatments decreased over time, stabilizing at final values of 12.98 (CK), 13.31 (T3), and 14.36 (T9) (Fig. 1 c). While traditional guidelines often cite a C/N ratio below 20 as an indicator of maturity values below 15 are also widely accepted, particularly for nitrogen-rich feedstocks like sewage sludge where the initial nitrogen content is high (Ma et al., 2022 ; H. Wu et al., 2025 ). Therefore, the final C/N ratios achieved in all treatments are indicative of a mature compost product. The higher final C/N in the T9 treatment is a direct result of the addition of carbon-rich, nitrogen-free SL. The GI provides a direct measure of compost phytotoxicity and maturity (Tang et al., 2023 ). The final GI values were 85.9% (CK), 92.3% (T3), and 96.4% (T9) (Fig. 1 d). All final composts can be considered non-phytotoxic, with GI values well above the 80% threshold (Zhang et al., 2019 ). The superior GI in the T9 treatment is particularly significant. It reflects the combined effect of efficient degradation of organic phytotoxins (e.g., low-molecular-weight organic acids) during the robust thermophilic phase, and the successful immobilization of inorganic phytotoxins, namely bioavailable copper (Z. Chen et al., 2021 ; Wang et al., 2022 ), which is discussed in detail in Section 3.3 . This demonstrates that the addition of SL is beneficial in promoting the overall maturity and safety of the final compost product. 3.2. SL-Driven Enhancement of the Humification Process The addition of SL fundamentally altered the humification pathway by accelerating the conversion of labile precursors into stable, high-molecular-weight humic substances. This was elucidated by integrating chemical analyses of humic fractions with advanced spectroscopic characterization using EEM-PARAFAC. PARAFAC analysis successfully identified three key fluorescent components within the dissolved organic matter (Peter et al., 2024 ; Wang et al., 2014 ), including a humic acid (HA)-like substance (C1), a quinone-like intermediate (C2), and a protein-like substance (C3) (Fig. 2 ). The evolution of these components reveals the underlying mechanism of SL's action. The protein-like substances (C3), derived from microbial biomass and sludge proteins, served as a primary nitrogen-containing precursor (X. Liu et al., 2020 ; Wu et al., 2011 ) and were gradually consumed during composting in all treatments (Fig. 2 a). The quinone-like substances (C2) are known to be highly reactive intermediates in the formation of humic substances (Cory and McKnight, 2005 ; W. Wang et al., 2021 ). In all treatments, the relative content of C2 initially increased and then decreased as it was consumed in polymerization reactions (Fig. 2 b). Critically, the final relative content of C2 was highest in the control (CK), suggesting a bottleneck in the humification pathway. In contrast, the lower final C2 levels in the T3 and T9 treatments indicate a more efficient consumption of these reactive intermediates, facilitated by the presence of SL. This accelerated consumption of precursors in the SL-amended treatments led directly to a more significant accumulation of stable humic products. The relative content of the HA-like component (C1) increased throughout the process in all treatments, with the most pronounced increase observed in T9 (Fig. 2 c). This spectroscopic evidence is directly corroborated by the chemical measurements of humic fractions (Fig. 3 ). The final HA content in T9 reached 15.18 mg/g, a substantially greater accumulation compared to the 8.17 mg/g observed in the control (Fig. 3 b). This increase in HA was accompanied by a corresponding decrease in FA content (Fig. 3 c), confirming the conversion of smaller, less stable FA molecules into larger, more complex HA polymers. The degree of polymerization (DP), calculated as the HA/FA ratio, serves as a key indicator of humification quality and compost maturity (Y. Chen et al., 2021 ). The DP value increased in all treatments during the maturation phase, reaching final values of 0.77 (CK), 0.81 (T3), and 0.99 (T9) (Fig. 3 d). The significantly higher DP in the T9 treatment provides quantitative proof that SL addition promotes a more advanced state of humification. The overall transformation is visually confirmed by the EEM fluorescence spectra (Fig. 4 ). At the start of composting, all samples showed prominent fluorescence in both the FA-like (Region III) and HA-like (Region V) regions. By day 60, a clear shift was observed with the fluorescence intensity in the FA-like region diminished while the peak in the HA-like region intensified, particularly in the T9 treatment (Fig. 4 f). This visual evidence reinforces the conclusion that SL facilitates the conversion of FA-like substances into more stable HA-like structures. Collectively, the integrated chemical and spectroscopic data provide compelling evidence that SL acts as a powerful humification catalyst. It promotes the efficient transformation of protein- and quinone-like precursors into structurally stable, high-molecular-weight humic acids, thereby enhancing the overall quality of the final compost. 3.3. Humification-Linked Immobilization of Copper The enhanced humification observed in the SL-amended treatments directly translated into a significant reduction in the bioavailability of copper, a key environmental outcome of the composting process. As composting progressed, the total Cu content in all treatments increased slightly due to a concentration effect caused by the degradation of OM (Fig. 5 a). However, from an environmental risk perspective, the bioavailable fraction is of greater concern. The DTPA-extractable Cu content (DCC) and its extractability rate (DCE) showed a marked decrease in all treatments over the 60 days (Fig. 5 b). The most substantial reduction was observed in the T9 treatment, where DCC decreased from an initial 285.47 mg/kg to a final value of 96.40 mg/kg. This represents a 66% reduction and is significantly lower than the final DCC of 150.73 mg/kg in the control (CK). Correspondingly, the final Cu extractability rate in T9 was only 2.9%, compared to 4.9% in CK, demonstrating a profound stabilization effect. This significant immobilization of Cu is attributed to two primary mechanisms. First and foremost, the enhanced formation of stable humic acids in the SL-amended treatments provided abundant binding sites for Cu ions. The deprotonation of carboxyl and phenolic hydroxyl functional groups on HA molecules facilitates strong complexation and chelation with Cu(II), effectively sequestering it from the bioavailable pool (de Melo et al., 2016 ). The clear inverse relationship between the rising HA content (Fig. 3 b) and the falling DCC (Fig. 5 b) strongly supports this pathway. Second, the SL additive itself introduced a high density of oxygen-containing functional groups, such as sulfonic acid and phenolic hydroxyl groups, which have a strong affinity for Cu ions and can directly form stable chelates (Ma et al., 2019 ; Šćiban et al., 2011 ). It is essential to acknowledge that other factors, such as pH, may influence metal bioavailability (Adamczyk-Szabela and Wolf, 2022 ; Nkinahamira et al., 2022 ; Zhou et al., 2022 ). However, a dramatic difference in Cu immobilization between the T9 and CK treatments strongly suggests that the enhanced humification is the dominant differentiating factor. Therefore, the results confirm that the addition of SL effectively reduces the environmental risk of Cu in sludge compost, primarily by fostering a humic-rich matrix with a high capacity for metal complexation. 3.4. Synthesis and Correlation Analysis To statistically validate the mechanistic links between SL addition, humification, and copper immobilization, a Pearson correlation analysis was performed on key physicochemical parameters across all treatments and time points (Fig. 6 ). This analysis provides a quantitative synthesis of the entire composting process. The study confirms that compost maturity, represented by a high GI, is strongly and positively correlated with the degree of humification. Specifically, GI shows a highly significant positive correlation with HA content and the degree of polymerization (DP) (p < 0.01), while being negatively correlated with indicators of immaturity such as OM content (p < 0.01) and FA content (p < 0.05). This statistically reinforces the conclusion that a more advanced state of humification leads to a safer, less phytotoxic final product. The humification pathway itself is clarified by the strong negative correlation between the HA-like component (C1) and the protein-like precursor (C3) (p < 0.01). This provides statistical evidence that the consumption of labile proteinaceous materials is directly linked to the formation of stable humic acids, confirming the transformation pathway observed in the EEM-PARAFAC analysis. Crucially, the analysis provides direct statistical proof for the central hypothesis of this study. The bioavailable Cu fraction (DCC) exhibits a highly significant negative correlation with HA content (p < 0.01) and the overall GI (p < 0.01). Conversely, DCC is positively correlated with indicators of less-humified material, such as FA content (p < 0.05) and the protein-like precursor C3 (p < 0.01). This demonstrates that as the compost becomes more humified with higher HA and lower FA and protein-like content, the bioavailability of copper decreases significantly (W. Zheng et al., 2022 ). Taken together, this correlation analysis provides a robust statistical synthesis of the entire process. SL addition enhances the conversion of labile precursors (C3) into stable humic acids (HA), which not only improves overall compost maturity (GI) but also serves as the key mechanism responsible for the significant immobilization of bioavailable copper. 4. Conclusions This study demonstrates that sodium lignosulfonate (SL) is a highly effective dual-purpose additive for the valorization of municipal sludge through composting. The addition of 9% SL acted as a potent humification catalyst, accelerating the conversion of labile precursors into stable, high-molecular-weight humic acids, as evidenced by a significantly higher final humic acid content and degree of polymerization. Critically, this enhanced humification was identified as the primary mechanism for copper immobilization. The resulting humic-rich matrix provided abundant binding sites for complexation, leading to a profound reduction in the bioavailable fraction of copper and significantly mitigating the compost's environmental risk. The mechanistic link between these processes was confirmed by a robust negative correlation between humic acid content and bioavailable copper, providing quantitative evidence for this pathway. Ultimately, this work establishes a practical and scalable strategy for the synergistic recycling of industrial and municipal waste streams, advancing circular economy principles by using a low-cost by-product to produce safer, high-value compost from contaminated sludge. Declarations Acknowledgments The study was financially supported by the National Natural Science Foundation of China (52170128, 42477225, and 42207298), and the Natural Science Foundation of Guangdong Province (2024A1515010980). Ethics Approval and Consent to Participate Not applicable. Consent for Publication All authors have read and approved the final manuscript. Data Availability The data generated during the study will be made available on request. Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Funding The study was financially supported by the National Natural Science Foundation of China (52170128, 42477225, and 42207298), and the Natural Science Foundation of Guangdong Province (2024A1515010980). Author contributions Minghui Li : Formal analysis, Data curation, Investigation, Writing-original draft. Shangchun Chen : Formal analysis, Data curation. Genjia Xu : Software，Validation. Meihua Zhao : Methodology, Funding acquisition, Resources, Supervision, Project administration. Changya Chen : Formal analysis, Validation, Software. François Nkinahamira : Conceptualization, Resources, Supervision. References Adamczyk-Szabela, D., Wolf, W.M., 2022. The Impact of Soil pH on Heavy Metals Uptake and Photosynthesis Efficiency in Melissa officinalis, Taraxacum officinalis, Ocimum basilicum. Molecules 27. https://doi.org/10.3390/molecules27154671 Aro, T., Fatehi, P., 2017. Production and Application of Lignosulfonates and Sulfonated Lignin. ChemSusChem 10, 1861–1877. https://doi.org/10.1002/cssc.201700082 Chen, Q., Chen, Y., Hu, Z., Lin, H., Huang, W., Wei, W., 2025. Enhancement of humic substances formation in aerobic composting of municipal sludge by adding sodium lignosulfonate at different phases. Biomass Convers. Biorefinery 15, 12621–12632. https://doi.org/10.1007/s13399-024-05981-9 Chen, Y., Chen, Yaoning, Li, Y., Liu, Y., Li, H., Jiang, H., Luo, X., Tang, P., Chen, L., Yan, H., 2021. Evolution of humic substances and the forms of heavy metals during co-composting of rice straw and sediment with the aid of Fenton-like process. Bioresour. Technol . 333, 125170. https://doi.org/10.1016/j.biortech.2021.125170 Chen, Z., Fu, Q., Cao, Y., Wen, Q., Wu, Y., 2021. Effects of lime amendment on the organic substances changes, antibiotics removal, and heavy metals speciation transformation during swine manure composting. Chemosphere 262, 128342. https://doi.org/10.1016/j.chemosphere.2020.128342 Cory, R.M., McKnight, D.M., 2005. Fluorescence Spectroscopy Reveals Ubiquitous Presence of Oxidized and Reduced Quinones in Dissolved Organic Matter. Environ. Sci. Technol. 39, 8142–8149. https://doi.org/10.1021/es0506962 Cui, H., Ou, Y., Wang, L., Yan, B., Li, Y., Bao, M., 2021. Critical passivation mechanisms on heavy metals during aerobic composting with different grain-size zeolite. J. Hazard. Mater. 406, 124313. https://doi.org/10.1016/j.jhazmat.2020.124313 Cyubahiro, E., Luo, Z., Wang, H., Luo, Y., Nkinahamira, F., Habumugisha, T., Wang, Z., 2023. Effect of montmorillonite biochar composite amendment on thallium bioavailability in contaminated agricultural soils and its mitigated health risk. Environ. Sci. Pollut. Res. 30, 47882–47891. https://doi.org/10.1007/s11356-023-25668-4 de Melo, B.A.G., Motta, F.L., Santana, M.H.A., 2016. Humic acids: Structural properties and multiple functionalities for novel technological developments. Mater. Sci. Eng . C 62, 967–974. https://doi.org/10.1016/j.msec.2015.12.001 Dorogaya, E.S., Kuzina, E.V., Suleimanov, R.R., Yurkevich, M.G., Bakhmet, O.N., 2023. On the possibility of utilizing sodium lignosulfonate as a nano-organic foundation for creating soil-like bodies in the purposes of technogenic-degraded land rehabilitation. Nanotechnologies Constr. A Sci. Internet-Journal 15, 359–372. https://doi.org/10.15828/2075-8545-2023-15-4-359-372 Guo, S., Nkinahamira, F., Adyari, B., Zhang, Y., Hu, A., Sun, Q., 2023. Fate and Spatial–Temporal Variation of 23 Elements at 7 Wastewater Treatment Plants in Southeast City of China. Water 15, 1226. https://doi.org/10.3390/w15061226 Guo, X., Liu, H., Wu, S., 2019. Humic substances developed during organic waste composting: Formation mechanisms, structural properties, and agronomic functions. Sci. Total Environ . 662, 501–510. https://doi.org/10.1016/j.scitotenv.2019.01.137 Izydorczyk, G., Mikula, K., Skrzypczak, D., Trzaska, K., Moustakas, K., Witek-Krowiak, A., Chojnacka, K., 2021. Agricultural and non-agricultural directions of bio-based sewage sludge valorization by chemical conditioning. Environ. Sci. Pollut. Res . 28, 47725–47740. https://doi.org/10.1007/s11356-021-15293-4 Khatami, S., Deng, Y., Tien, M., Hatcher, P.G., 2019. Formation of water-soluble organic matter through fungal degradation of lignin. Org. Geochem . 135, 64–70. https://doi.org/10.1016/j.orggeochem.2019.06.004 Lindsay, W.L., Norvell, W.A., 1978. Development of a DTPA Soil Test for Zinc, Iron, Manganese, and Copper. Soil Sci. Soc. Am. J. 42, 421–428. https://doi.org/10.2136/sssaj1978.03615995004200030009x Liu, G., Hu, Z., Huang, W., Zheng, Y., Yang, F., Su, Y., 2025. Sodium lignosulfonate enhances the insoluble sulfates and phosphates formation of heavy metal passivation in municipal sludge aerobic composting. Int. J. Environ. Sci. Technol. 22, 8619–8630. https://doi.org/10.1007/s13762-024-05872-6 Liu, X., Hou, Y., Li, Z., Yu, Z., Tang, J., Wang, Y., Zhou, S., 2020. Hyperthermophilic composting of sewage sludge accelerates humic acid formation: Elemental and spectroscopic evidence. Waste Manag. 103, 342–351. https://doi.org/10.1016/j.wasman.2019.12.053 Liu, Y., Ma, R., Li, D., Qi, C., Han, L., Chen, M., Fu, F., Yuan, J., Li, G., 2020. Effects of calcium magnesium phosphate fertilizer, biochar and spent mushroom substrate on compost maturity and gaseous emissions during pig manure composting. J. Environ. Manage . 267, 110649. https://doi.org/10.1016/j.jenvman.2020.110649 Long, Y., Zhu, N., Zhu, Y., Shan, C., Jin, H., Cao, Y., 2024. Hydrochar drives reduction in bioavailability of heavy metals during composting via promoting humification and microbial community evolution. Bioresour. Technol . 395, 130335. https://doi.org/10.1016/j.biortech.2024.130335 Ma, F., Zhu, T., Wang, Y., Li, X., Chang, M., Zhao, C., Wang, Z., Quan, H., 2024. Humic acid enhances adsorption effect: Application foundation of high-temperature composting products for remediation of heavy metals pollution. Biochem. Eng. J. 209, 109415. https://doi.org/10.1016/j.bej.2024.109415 Ma, J., Khan, M.A., Xia, M., Fu, C., Zhu, S., Chu, Y., Lei, W., Wang, F., 2019. Effective adsorption of heavy metal ions by sodium lignosulfonate reformed montmorillonite. Int. J. Biol. Macromol. 138, 188–197. https://doi.org/10.1016/j.ijbiomac.2019.07.075 Ma, R., Liu, Y., Wang, J., Li, D., Qi, C., Li, G., Yuan, J., 2022. Effects of oxygen levels on maturity, humification, and odor emissions during chicken manure composting. J. Clean. Prod . 369, 133326. https://doi.org/10.1016/j.jclepro.2022.133326 Nkinahamira, F., Guo, S., Cao, M., Zhang, Y., Asefi, B., Sun, S., Feng, M., Sun, Q., Yu, C.-P., 2022. A closed-loop system to recycle rare earth elements from industrial sludge using green leaching agents and porous β-cyclodextrin polymer composite. Resour. Conserv. Recycl . 180, 106152. https://doi.org/10.1016/j.resconrec.2021.106152 Nkinahamira, F., Suanon, F., Chi, Q., Li, Y., Feng, M., Huang, X., Yu, C.-P., Sun, Q., 2019. Occurrence, geochemical fractionation, and environmental risk assessment of major and trace elements in sewage sludge. J. Environ. Manage. 249, 109427. https://doi.org/10.1016/j.jenvman.2019.109427 Pan, I., Dam, B., Sen, S.K., 2012. Composting of common organic wastes using microbial inoculants. Biotech 2, 127–134. https://doi.org/10.1007/s13205-011-0033-5 Peter, P.O., Ifon, B.E., Nkinahamira, F., Lasisi, K.H., Li, J., Hu, A., Yu, C.-P., 2024. Harnessing the composition of dissolved organic matter in lagoon sediment in association with rare earth elements using fluorescence and UV–visible absorption spectroscopy. Sci. Total Environ . 908, 168139. https://doi.org/10.1016/j.scitotenv.2023.168139 Rastogi, M., Nandal, M., Khosla, B., 2020. Microbes as vital additives for solid waste composting. Heliyon 6, e03343. https://doi.org/10.1016/j.heliyon.2020.e03343 Šćiban, M.B., Klašnja, M.T., Antov, M.G., 2011. Study of the biosorption of different heavy metal ions onto Kraft lignin. Ecol. Eng. 37, 2092–2095. https://doi.org/10.1016/j.ecoleng.2011.08.006 Tang, J., Zhuang, L., Yu, Z., Liu, X., Wang, Y., Wen, P., Zhou, S., 2019. Insight into complexation of Cu(II) to hyperthermophilic compost-derived humic acids by EEM-PARAFAC combined with heterospectral two dimensional correlation analyses. Sci. Total Environ . 656, 29–38. https://doi.org/10.1016/j.scitotenv.2018.11.357 Tang, R., Liu, Y., Ma, R., Zhang, L., Li, Y., Li, G., Dingmei Wang, Lin, J., Li, Q., Yuan, J., 2023. Effect of moisture content, aeration rate, and C/N on maturity and gaseous emissions during kitchen waste rapid composting. J. Environ. Manage . 326, 116662. https://doi.org/10.1016/j.jenvman.2022.116662 Wang, G., Yang, Y., Kong, Y., Ma, R., Yuan, J., Li, G., 2022. Key factors affecting seed germination in phytotoxicity tests during sheep manure composting with carbon additives. J. Hazard. Mater. 421, 126809. https://doi.org/10.1016/j.jhazmat.2021.126809 Wang, W., Hou, Y., Huang, W., Liu, X., Wen, P., Wang, Y., Yu, Z., Zhou, S., 2021. Alkali lignin and sodium lignosulfonate additives promote the formation of humic substances during paper mill sludge composting. Bioresour. Technol. 320, 124361. https://doi.org/10.1016/j.biortech.2020.124361 Wang, X., Lyu, T., Dong, R., Liu, H., Wu, S., 2021. Dynamic evolution of humic acids during anaerobic digestion: Exploring an effective auxiliary agent for heavy metal remediation. Bioresour. Technol. 320, 124331. https://doi.org/10.1016/j.biortech.2020.124331 Wang, Zichao, Gao, M., Wang, S., Xin, Y., Ma, D., She, Z., Wang, Zhe, Chang, Q., Ren, Y., 2014. Effect of hexavalent chromium on extracellular polymeric substances of granular sludge from an aerobic granular sequencing batch reactor. Chem. Eng . J. 251, 165–174. https://doi.org/10.1016/j.cej.2014.04.078 Wei, S., Li, Zichen, Sun, Y., Zhang, J., Ge, Y., Li, Zhili, 2022. A comprehensive review on biomass humification: Recent advances in pathways, challenges, new applications, and perspectives. Renew. Sustain. Energy Rev . 170, 112984. https://doi.org/10.1016/j.rser.2022.112984 Wu, H., Wang, C., Zhou, J., Cong, H., Gao, Y., Cai, W., Feng, S., Zhang, C., 2025. Feedstock optimization with low carbon to nitrogen ratio during algal sludge aerobic composting: Quality and gaseous emissions. Bioresour. Technol . 416, 131811. https://doi.org/10.1016/j.biortech.2024.131811 Wu, J., Zhang, H., He, P.-J., Shao, L.-M., 2011. Insight into the heavy metal binding potential of dissolved organic matter in MSW leachate using EEM quenching combined with PARAFAC analysis. Water Res . 45, 1711–1719. https://doi.org/10.1016/j.watres.2010.11.022 Wu, S., Tursenjan, D., Sun, Y., 2023. Impact of compost methods on humification and heavy metal passivation during chicken manure composting. J. Environ. Manage . 325, 116573. https://doi.org/10.1016/j.jenvman.2022.116573 Wu, Y., Yu, Z., Zhao, M., Chen, C., Nkinahamira, F., 2025. Effects of an applied electric field on the composting process of organic wastes: A review. J. Environ. Chem . Eng. 13, 119387. https://doi.org/10.1016/j.jece.2025.119387 Zhang, J., Ying, Y., Yao, X., 2019. Effects of turning frequency on the nutrients of Camellia oleifera shell co-compost with goat dung and evaluation of co-compost maturity. PLoS One 14, e0222841. https://doi.org/10.1371/journal.pone.0222841 Zhao, B., Wang, Y., Sun, H., Xu, Z., 2022. Analysis of humus formation and factors for driving the humification process during composting of different agricultural wastes. Front. Environ . Sci. 10, 1–12. https://doi.org/10.3389/fenvs.2022.954158 Zheng, W., Yang, Z., Huang, L., Chen, Y., 2022. Roles of organic matter transformation in the bioavailability of Cu and Zn during sepiolite-amended pig manure composting. J. Environ. Manage. 314, 115046. https://doi.org/10.1016/j.jenvman.2022.115046 Zheng, X., Zou, D., Wu, Q., Wang, H., Li, S., Liu, F., Xiao, Z., 2022. Review on fate and bioavailability of heavy metals during anaerobic digestion and composting of animal manure. Waste Manag . 150, 75–89. https://doi.org/10.1016/j.wasman.2022.06.033 Zhou, S., Kong, F., Lu, L., Wang, P., Jiang, Z., 2022. Biochar — An effective additive for improving quality and reducing ecological risk of compost: A global meta-analysis. Sci. Total Environ . 806, 151439. https://doi.org/10.1016/j.scitotenv.2021.151439 Zhou, Y., Selvam, A., Wong, J.W.C., 2014. Evaluation of humic substances during co-composting of food waste, sawdust and Chinese medicinal herbal residues. Bioresour. Technol . 168, 229–234. https://doi.org/10.1016/j.biortech.2014.05.070 Zmora-Nahum, S., Markovitch, O., Tarchitzky, J., Chen, Y., 2005. Dissolved organic carbon (DOC) as a parameter of compost maturity. Soil Biol. Biochem. 37, 2109–2116. https://doi.org/10.1016/j.soilbio.2005.03.013 Additional Declarations No competing interests reported. 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1","display":"","copyAsset":false,"role":"figure","size":154442,"visible":true,"origin":"","legend":"\u003cp\u003eChanges in (a) Temperature, (b) OM, (c) C/N, and (d) GI during the composting process. 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(HA: humic acid; FA: fulvic acid; HS: humic substance; DP: degree of polymerization.)\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-7877970/v1/4534388940a1b217e2b2b1b0.png"},{"id":96160868,"identity":"93c95f4c-87cd-4623-b82d-f1bbf8752a77","added_by":"auto","created_at":"2025-11-18 08:50:02","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":156598,"visible":true,"origin":"","legend":"\u003cp\u003eExcitation-emission matrix spectra of initial and final compost samples of different treatments: (a) CK-0d, (b) T3-0d, (c) T9-0d, (d) CK-60d, (e) T3-60d, and (f) T9-60d.\u003c/p\u003e","description":"","filename":"image4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7877970/v1/6e17b839dae2248cf6454abb.jpeg"},{"id":96160867,"identity":"9236878d-7a13-406f-873b-6356907f983d","added_by":"auto","created_at":"2025-11-18 08:50:02","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":60966,"visible":true,"origin":"","legend":"\u003cp\u003eChanges in (a) total Cu content, and (b) DCC and DCE during the composting process. (DCC: DTPA-extractable Cu content; DCE: DTPA-extractable Cu extractability rate.)\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-7877970/v1/a4f103c9e56c249f0c7fa2a3.png"},{"id":96160871,"identity":"653548e0-0ef1-4e3b-ae32-78c3c400c02f","added_by":"auto","created_at":"2025-11-18 08:50:02","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":140045,"visible":true,"origin":"","legend":"\u003cp\u003eThe Pearson correlation heatmap analysis of physicochemical properties, humification parameter, and heavy metals in samples from different composting treatments. N = 21. Significance levels: *p \u0026lt; 0.05, **p \u0026lt; 0.01, T: Temperature; C/N: carbon nitrogen ratio; OM: organic matter; GI: germination index. Cu: total Cu content; HA: \u0026nbsp;humic acid; FA: fulvic acid; HS: humic substance; DP: HA/FA ratio; DCC: DTPA-extractable Cu content; DCE: DTPA-extractable Cu extractability rate; C1~C3: Component 1~3.\u003c/p\u003e","description":"","filename":"image6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7877970/v1/2d10dab9be819558f4220f62.jpeg"},{"id":104739637,"identity":"3a54af89-19a3-4844-8741-0c5b4441a7cc","added_by":"auto","created_at":"2026-03-16 16:11:17","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1748974,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7877970/v1/87ca6bdf-9c37-4b0e-8f44-f0c79f28b66c.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Valorization of Sodium Lignosulfonate for Enhanced Humification and Copper Immobilization During Municipal Sludge Composting","fulltext":[{"header":"Highlights","content":"\u003cul type=\"disc\"\u003e\n \u003cli\u003eSodium lignosulfonate enhances sludge composting and improves compost quality.\u003c/li\u003e\n \u003cli\u003eThe addition of 9% sodium lignosulfonate significantly increases humic acid content.\u003c/li\u003e\n \u003cli\u003eBioavailable copper was reduced by 66% with 9% sodium lignosulfonate.\u003c/li\u003e\n \u003cli\u003eEnhanced humification is the key mechanism for copper immobilization.\u003c/li\u003e\n\u003c/ul\u003e"},{"header":"1. Introduction","content":"\u003cp\u003eMunicipal sludge, a voluminous by-product of wastewater treatment, represents both a significant disposal challenge and a potential resource for agricultural valorization (Izydorczyk et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Nkinahamira et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Composting is widely recognized as a key strategy to stabilize sludge and convert it into a valuable soil conditioner (Aro and Fatehi, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Y. Wu et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). However, the efficacy of sludge composting is often constrained by two interconnected factors, inefficient humification and the presence of heavy metals (Chen et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Inefficient humification results in a compost product with low stability and limited agronomic value, as the formation of stable humic substances (HS) is a pivotal process governing compost quality (Guo et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Zhao et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Simultaneously, municipal sludge is a sink for heavy metals like copper (Cu), which, due to its high bioavailability and toxicity, poses ecological risks and restricts the land application of composted sludge (Guo et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Liu et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Wei et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eInterestingly, these two challenges are mechanistically linked. The process of composting can effectively passivate heavy metals by transforming them into more stable, less bioavailable forms (Cyubahiro et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Wu et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; X. Zheng et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). This immobilization is primarily driven by complexation and chelation with newly formed HS, particularly humic acids (HA) (Ma et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Tang et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). The abundant oxygen-containing functional groups (e.g., carboxyl, phenolic hydroxyl) in HA create strong binding sites for metal ions (X. Wang et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Therefore, enhancing the rate and quality of humification is a direct and sustainable strategy for mitigating the environmental risks associated with heavy metals in sludge compost.\u003c/p\u003e\u003cp\u003eTo accelerate humification, various additives rich in aromatic precursors, such as lignin, have been explored (Cui et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Liu et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Long et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Sodium lignosulfonate (SL), an abundant and low-cost water-soluble by-product of the sulfite pulping process, is an ideal candidate. Its rich aromatic structure and high density of phenolic hydroxyl and sulfonic acid groups make it both an excellent precursor for HS synthesis and a potent chelating agent for metal ions (Liu et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Recent studies have acknowledged the potential of SL in composting. For instance, research has shown that SL can promote the formation of humic substances in paper mills and municipal sludge composting (Chen et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; W. Wang et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Other work has focused on SL's role in the passivation of heavy metals through the formation of insoluble sulfates and phosphates (Liu et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). However, a systematic investigation that quantitatively links the dose-dependent enhancement of humification quality with the degree of heavy metal immobilization is still needed. Specifically, the pathway by which SL-derived precursors are transformed into distinct humic fractions (e.g., fulvic vs. humic acids) and how this transformation directly governs the reduction in Cu bioavailability remains to be fully elucidated.\u003c/p\u003e\u003cp\u003eThis study aims to fill this gap by systematically investigating the dual-purpose valorization of SL in a Cu-contaminated municipal sludge composting system. This work evaluates the capacity of SL to act as a humic precursor, accelerating the transformation of fulvic acids (FA) to stable HA and increasing the overall degree of polymerization in a dose-dependent manner. It further determines how this enhanced humification leads to a quantifiable reduction in the bioavailable (DTPA-extractable) fraction of Cu through increased complexation. To confirm this mechanistic pathway, the statistical correlation between key humification indices (e.g., HA content, HA/FA ratio) and Cu bioavailability is established. By elucidating these relationships, this research provides new mechanistic insights into the synergistic recycling of industrial and municipal wastes, offering a practical strategy to produce safer, high-value compost.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1. Raw Materials and Additives\u003c/h2\u003e\u003cp\u003eThe primary raw materials for composting were dewatered municipal sludge and pine (\u003cem\u003ePinus massoniana\u003c/em\u003e) sawdust. The anaerobically digested dewatered sludge was obtained from the Wastewater Treatment Plant in Guangzhou, Guangdong Province, China. The sawdust (particle size\u0026thinsp;\u0026le;\u0026thinsp;2 cm) was sourced from a local timber market. SL, derived from a sulfite pulping process, was obtained from a secondary fiber paper mill and used as the additive. All materials were stored at 4\u0026deg;C before use. The detailed physicochemical properties of the raw materials were analyzed and are presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003ePhysicochemical characteristics of the raw materials and additives.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eParameter\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSludge\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSawdust\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eSL\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMoisture (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e37.69\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e6.86\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e6.74\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003epH\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e7.66\u0026thinsp;\u0026plusmn;\u0026thinsp;0.11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e7.43\u0026thinsp;\u0026plusmn;\u0026thinsp;0.12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e10.08\u0026thinsp;\u0026plusmn;\u0026thinsp;0.25\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eEC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1.58\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.25\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e5.89\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTOC (g/kg, dry wt.)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e210.48\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e573.76\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e326.41\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTN (g/kg, dry wt.)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e18.57\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e4.64\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC/N Ratio\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e11.34\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e123.72\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAsh Content (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e36.74\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e1.18\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e11.25\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTotal Cu (mg/kg, dry wt.)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e2720.65\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2. Experimental Design and Composting Operation\u003c/h2\u003e\u003cp\u003eThe composting experiment was conducted in cylindrical plastic reactors (30 cm inner diameter, 50 cm height), each with a working volume of approximately 25 L. To minimize heat loss, each reactor was insulated with a 10-cm-thick layer of polyurethane foam. A perforated plate was placed at the bottom of each reactor to facilitate passive aeration. The experiment consisted of three treatments, each performed in triplicate. The treatments include Control (CK) with no SL addition, T3 with the addition of 3% SL (by dry weight of sludge and sawdust), and T9 with the addition of 9% SL (by dry weight of sludge and sawdust). For each reactor, sludge and sawdust were mixed at a 6:1 ratio (dry weight basis) to achieve an initial C/N ratio of approximately 28, which is considered optimal for microbial activity in sludge composting (Pan et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). The corresponding dose of SL was then added and thoroughly homogenized. The moisture content of each mixture was adjusted to 60% (w/w) using deionized water. Finally, 0.5% (w/w) of mature compost from a previous batch was added as an inoculum to accelerate the process. The total initial weight of the mixture in each reactor was approximately 15 kg. The composting process was carried out for 60 days. To ensure aerobic conditions, the compost piles were manually turned every three days for the first 20 days (thermophilic phase) and weekly thereafter. Moisture content was monitored weekly by weight and replenished with deionized water to maintain a range of 55\u0026ndash;60%.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3. Sampling and Physicochemical Analysis\u003c/h2\u003e\u003cp\u003eSamples were collected on days 0, 5, 10, 15, 20, 30, 40, 50, and 60. At each sampling point, composite samples were obtained by mixing materials collected from the upper, middle, and lower layers of each reactor. Each composite sample was divided into two subsamples. One was immediately stored at 4\u0026deg;C for germination index (GI) analysis. The other was air-dried, ground to pass through a 0.15 mm sieve, and stored for subsequent chemical analysis. Temperature was recorded twice daily (9:00 and 16:00) at the center of each compost pile using a digital thermometer, with the average value reported as the daily temperature. Moisture content was determined by drying samples at 105\u0026deg;C to a constant weight. Organic matter (OM) content was determined by mass loss on ignition in a muffle furnace at 550\u0026deg;C for 8 h (Y. Chen et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Total organic carbon (TOC) content was calculated as 58% of OM (Zmora-Nahum et al., \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2005\u003c/span\u003e), while Alkaline potassium persulfate ultraviolet spectrophotometry was used for the determination of total nitrogen (TN). The germination index (GI) was determined using Chinese cabbage seeds (\u003cem\u003eBrassica rapa\u003c/em\u003e subsp. \u003cem\u003epekinensis\u003c/em\u003e) as described by Liu et al. (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) to assess phytotoxicity (Y. Liu et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4. Humification and Copper Speciation Analysis\u003c/h2\u003e\u003cp\u003eThe extraction of HS, including HA and FA, was performed using an alkaline solution (0.1 M NaOH\u0026thinsp;+\u0026thinsp;0.1 M Na\u003csub\u003e4\u003c/sub\u003eP\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e) following the procedure described by Zhou et al. (\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) (Zhou et al., \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The degree of polymerization (DP) was calculated as the HA/FA ratio. Fluorescence excitation-emission matrix (EEM) spectroscopy was conducted on the HS extracts using a Hitachi F-7000 fluorescence spectrophotometer. To minimize inner-filter effects, all extracts were diluted with ultrapure water to achieve a UV absorbance below 0.1 at 254 nm. EEM spectra were recorded with excitation wavelengths from 200 to 400 nm and emission wavelengths from 280 to 550 nm. Raman and Rayleigh scattering were removed from the spectra before analysis. Parallel Factor (PARAFAC) analysis was performed on the EEM dataset using the drEEM toolbox in MATLAB 2016a to identify the underlying fluorescent components.\u003c/p\u003e\u003cp\u003eFor heavy metal analysis, the total Cu content was determined by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, Agilent 5110) after microwave digestion of the samples with a mixture of concentrated nitric acid (HNO₃) and hydrogen peroxide (H₂O₂) in a 3:1 (v/v) ratio. The bioavailable fraction of Cu was determined by extraction with a diethylenetriaminepentaacetic acid (DTPA) solution, a method widely used to estimate plant-available metals, as described by Lindsay and Norvell (\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1978\u003c/span\u003e) (Lindsay and Norvell, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1978\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e2.5. Data Analysis\u003c/h2\u003e\u003cp\u003eThe experimental data were arranged using Microsoft Excel 2016 software. Parallel Factor Analysis was conducted using MATLAB 2016a on the 3D fluorescence scanning data after removing Raman scattering and Rayleigh scattering. SPSS 22.0 was used for data correlation analysis.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.1. Effect of SL on Compost Maturity and Physicochemical Evolution\u003c/h2\u003e\u003cp\u003eThe addition of SL significantly influenced the physicochemical evolution and final maturity of the sludge compost, as evidenced by changes in temperature, OM dynamics, and phytotoxicity (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, all treatments entered the thermophilic phase (\u0026gt;\u0026thinsp;50\u0026deg;C) after a turning operation on day 4, reaching peak temperatures of 56.3\u0026deg;C (CK), 58.5\u0026deg;C (T3), and 59.5\u0026deg;C (T9). The higher peak temperature and prolonged thermophilic period in the T9 treatment suggest that SL addition promoted microbial metabolism (Rastogi et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). This can be attributed to the readily available carbon in the water-soluble SL, which may have fueled initial microbial activity, as well as the higher TOC load providing more substrate for sustained thermophilic decomposition. Notably, only the T9 treatment met the requirement for adequate sanitation by maintaining a temperature above 50\u0026deg;C for more than five days, whereas the other two groups did not.\u003c/p\u003e\u003cp\u003eThe degradation of OM is a primary indicator of composting efficiency. The final OM degradation ratios were 33.3% (CK), 31.1% (T3), and 28.8% (T9) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). Interestingly, the T9 treatment showed the lowest OM degradation. This is likely because SL, with its complex aromatic structure, is more recalcitrant to microbial decomposition than other organic materials (Dorogaya et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Khatami et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). This presents a notable trade-off; while a lower degradation rate may imply reduced mass stabilization, it also indicates greater carbon sequestration in the final compost product, a significant environmental benefit for soil amendment. Consequently, the C/N ratio in all treatments decreased over time, stabilizing at final values of 12.98 (CK), 13.31 (T3), and 14.36 (T9) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). While traditional guidelines often cite a C/N ratio below 20 as an indicator of maturity values below 15 are also widely accepted, particularly for nitrogen-rich feedstocks like sewage sludge where the initial nitrogen content is high (Ma et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; H. Wu et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Therefore, the final C/N ratios achieved in all treatments are indicative of a mature compost product. The higher final C/N in the T9 treatment is a direct result of the addition of carbon-rich, nitrogen-free SL.\u003c/p\u003e\u003cp\u003eThe GI provides a direct measure of compost phytotoxicity and maturity (Tang et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). The final GI values were 85.9% (CK), 92.3% (T3), and 96.4% (T9) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed). All final composts can be considered non-phytotoxic, with GI values well above the 80% threshold (Zhang et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). The superior GI in the T9 treatment is particularly significant. It reflects the combined effect of efficient degradation of organic phytotoxins (e.g., low-molecular-weight organic acids) during the robust thermophilic phase, and the successful immobilization of inorganic phytotoxins, namely bioavailable copper (Z. Chen et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Wang et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), which is discussed in detail in Section \u003cspan refid=\"Sec11\" class=\"InternalRef\"\u003e3.3\u003c/span\u003e. This demonstrates that the addition of SL is beneficial in promoting the overall maturity and safety of the final compost product.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.2. SL-Driven Enhancement of the Humification Process\u003c/h2\u003e\u003cp\u003eThe addition of SL fundamentally altered the humification pathway by accelerating the conversion of labile precursors into stable, high-molecular-weight humic substances. This was elucidated by integrating chemical analyses of humic fractions with advanced spectroscopic characterization using EEM-PARAFAC. PARAFAC analysis successfully identified three key fluorescent components within the dissolved organic matter (Peter et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Wang et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), including a humic acid (HA)-like substance (C1), a quinone-like intermediate (C2), and a protein-like substance (C3) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The evolution of these components reveals the underlying mechanism of SL's action. The protein-like substances (C3), derived from microbial biomass and sludge proteins, served as a primary nitrogen-containing precursor (X. Liu et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Wu et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2011\u003c/span\u003e) and were gradually consumed during composting in all treatments (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). The quinone-like substances (C2) are known to be highly reactive intermediates in the formation of humic substances (Cory and McKnight, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; W. Wang et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). In all treatments, the relative content of C2 initially increased and then decreased as it was consumed in polymerization reactions (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). Critically, the final relative content of C2 was highest in the control (CK), suggesting a bottleneck in the humification pathway. In contrast, the lower final C2 levels in the T3 and T9 treatments indicate a more efficient consumption of these reactive intermediates, facilitated by the presence of SL. This accelerated consumption of precursors in the SL-amended treatments led directly to a more significant accumulation of stable humic products. The relative content of the HA-like component (C1) increased throughout the process in all treatments, with the most pronounced increase observed in T9 (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThis spectroscopic evidence is directly corroborated by the chemical measurements of humic fractions (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The final HA content in T9 reached 15.18 mg/g, a substantially greater accumulation compared to the 8.17 mg/g observed in the control (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). This increase in HA was accompanied by a corresponding decrease in FA content (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec), confirming the conversion of smaller, less stable FA molecules into larger, more complex HA polymers. The degree of polymerization (DP), calculated as the HA/FA ratio, serves as a key indicator of humification quality and compost maturity (Y. Chen et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). The DP value increased in all treatments during the maturation phase, reaching final values of 0.77 (CK), 0.81 (T3), and 0.99 (T9) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). The significantly higher DP in the T9 treatment provides quantitative proof that SL addition promotes a more advanced state of humification.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe overall transformation is visually confirmed by the EEM fluorescence spectra (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). At the start of composting, all samples showed prominent fluorescence in both the FA-like (Region III) and HA-like (Region V) regions. By day 60, a clear shift was observed with the fluorescence intensity in the FA-like region diminished while the peak in the HA-like region intensified, particularly in the T9 treatment (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef). This visual evidence reinforces the conclusion that SL facilitates the conversion of FA-like substances into more stable HA-like structures. Collectively, the integrated chemical and spectroscopic data provide compelling evidence that SL acts as a powerful humification catalyst. It promotes the efficient transformation of protein- and quinone-like precursors into structurally stable, high-molecular-weight humic acids, thereby enhancing the overall quality of the final compost.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e3.3. Humification-Linked Immobilization of Copper\u003c/h2\u003e\u003cp\u003eThe enhanced humification observed in the SL-amended treatments directly translated into a significant reduction in the bioavailability of copper, a key environmental outcome of the composting process. As composting progressed, the total Cu content in all treatments increased slightly due to a concentration effect caused by the degradation of OM (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). However, from an environmental risk perspective, the bioavailable fraction is of greater concern. The DTPA-extractable Cu content (DCC) and its extractability rate (DCE) showed a marked decrease in all treatments over the 60 days (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb).\u003c/p\u003e\u003cp\u003eThe most substantial reduction was observed in the T9 treatment, where DCC decreased from an initial 285.47 mg/kg to a final value of 96.40 mg/kg. This represents a 66% reduction and is significantly lower than the final DCC of 150.73 mg/kg in the control (CK). Correspondingly, the final Cu extractability rate in T9 was only 2.9%, compared to 4.9% in CK, demonstrating a profound stabilization effect. This significant immobilization of Cu is attributed to two primary mechanisms. First and foremost, the enhanced formation of stable humic acids in the SL-amended treatments provided abundant binding sites for Cu ions. The deprotonation of carboxyl and phenolic hydroxyl functional groups on HA molecules facilitates strong complexation and chelation with Cu(II), effectively sequestering it from the bioavailable pool (de Melo et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). The clear inverse relationship between the rising HA content (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb) and the falling DCC (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb) strongly supports this pathway. Second, the SL additive itself introduced a high density of oxygen-containing functional groups, such as sulfonic acid and phenolic hydroxyl groups, which have a strong affinity for Cu ions and can directly form stable chelates (Ma et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Šćiban et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). It is essential to acknowledge that other factors, such as pH, may influence metal bioavailability (Adamczyk-Szabela and Wolf, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Nkinahamira et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Zhou et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). However, a dramatic difference in Cu immobilization between the T9 and CK treatments strongly suggests that the enhanced humification is the dominant differentiating factor. Therefore, the results confirm that the addition of SL effectively reduces the environmental risk of Cu in sludge compost, primarily by fostering a humic-rich matrix with a high capacity for metal complexation.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e3.4. Synthesis and Correlation Analysis\u003c/h2\u003e\u003cp\u003eTo statistically validate the mechanistic links between SL addition, humification, and copper immobilization, a Pearson correlation analysis was performed on key physicochemical parameters across all treatments and time points (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). This analysis provides a quantitative synthesis of the entire composting process. The study confirms that compost maturity, represented by a high GI, is strongly and positively correlated with the degree of humification. Specifically, GI shows a highly significant positive correlation with HA content and the degree of polymerization (DP) (p\u0026thinsp;\u0026lt;\u0026thinsp;0.01), while being negatively correlated with indicators of immaturity such as OM content (p\u0026thinsp;\u0026lt;\u0026thinsp;0.01) and FA content (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05). This statistically reinforces the conclusion that a more advanced state of humification leads to a safer, less phytotoxic final product.\u003c/p\u003e\u003cp\u003eThe humification pathway itself is clarified by the strong negative correlation between the HA-like component (C1) and the protein-like precursor (C3) (p\u0026thinsp;\u0026lt;\u0026thinsp;0.01). This provides statistical evidence that the consumption of labile proteinaceous materials is directly linked to the formation of stable humic acids, confirming the transformation pathway observed in the EEM-PARAFAC analysis. Crucially, the analysis provides direct statistical proof for the central hypothesis of this study. The bioavailable Cu fraction (DCC) exhibits a highly significant negative correlation with HA content (p\u0026thinsp;\u0026lt;\u0026thinsp;0.01) and the overall GI (p\u0026thinsp;\u0026lt;\u0026thinsp;0.01). Conversely, DCC is positively correlated with indicators of less-humified material, such as FA content (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05) and the protein-like precursor C3 (p\u0026thinsp;\u0026lt;\u0026thinsp;0.01). This demonstrates that as the compost becomes more humified with higher HA and lower FA and protein-like content, the bioavailability of copper decreases significantly (W. Zheng et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Taken together, this correlation analysis provides a robust statistical synthesis of the entire process. SL addition enhances the conversion of labile precursors (C3) into stable humic acids (HA), which not only improves overall compost maturity (GI) but also serves as the key mechanism responsible for the significant immobilization of bioavailable copper.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eThis study demonstrates that sodium lignosulfonate (SL) is a highly effective dual-purpose additive for the valorization of municipal sludge through composting. The addition of 9% SL acted as a potent humification catalyst, accelerating the conversion of labile precursors into stable, high-molecular-weight humic acids, as evidenced by a significantly higher final humic acid content and degree of polymerization. Critically, this enhanced humification was identified as the primary mechanism for copper immobilization. The resulting humic-rich matrix provided abundant binding sites for complexation, leading to a profound reduction in the bioavailable fraction of copper and significantly mitigating the compost's environmental risk. The mechanistic link between these processes was confirmed by a robust negative correlation between humic acid content and bioavailable copper, providing quantitative evidence for this pathway. Ultimately, this work establishes a practical and scalable strategy for the synergistic recycling of industrial and municipal waste streams, advancing circular economy principles by using a low-cost by-product to produce safer, high-value compost from contaminated sludge.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe study was financially supported by the National Natural Science Foundation of China (52170128, 42477225, and 42207298), and the Natural Science Foundation of Guangdong Province (2024A1515010980).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics Approval and Consent to Participate\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for Publication\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll authors have read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data generated during the study will be made available on request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe study was financially supported by the National Natural Science Foundation of China (52170128, 42477225, and 42207298), and the Natural Science Foundation of Guangdong Province (2024A1515010980).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMinghui Li\u003c/strong\u003e : Formal analysis, Data curation, Investigation, Writing-original draft.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eShangchun Chen\u003c/strong\u003e: Formal analysis, Data curation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eGenjia Xu\u003c/strong\u003e: Software，Validation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMeihua Zhao\u003c/strong\u003e: Methodology, Funding acquisition, Resources, Supervision, Project administration.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eChangya Chen\u003c/strong\u003e: Formal analysis, Validation, Software.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFrançois Nkinahamira\u003c/strong\u003e: Conceptualization, Resources, Supervision.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAdamczyk-Szabela, D., Wolf, W.M., 2022. The Impact of Soil pH on Heavy Metals Uptake and Photosynthesis Efficiency in Melissa officinalis, Taraxacum officinalis, Ocimum basilicum. \u003cem\u003eMolecules\u003c/em\u003e 27. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/molecules27154671\u003c/span\u003e\u003cspan address=\"10.3390/molecules27154671\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eAro, T., Fatehi, P., 2017. Production and Application of Lignosulfonates and Sulfonated Lignin. \u003cem\u003eChemSusChem\u003c/em\u003e 10, 1861\u0026ndash;1877. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1002/cssc.201700082\u003c/span\u003e\u003cspan address=\"10.1002/cssc.201700082\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eChen, Q., Chen, Y., Hu, Z., Lin, H., Huang, W., Wei, W., 2025. Enhancement of humic substances formation in aerobic composting of municipal sludge by adding sodium lignosulfonate at different phases. \u003cem\u003eBiomass Convers. Biorefinery\u003c/em\u003e 15, 12621\u0026ndash;12632. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s13399-024-05981-9\u003c/span\u003e\u003cspan address=\"10.1007/s13399-024-05981-9\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eChen, Y., Chen, Yaoning, Li, Y., Liu, Y., Li, H., Jiang, H., Luo, X., Tang, P., Chen, L., Yan, H., 2021. Evolution of humic substances and the forms of heavy metals during co-composting of rice straw and sediment with the aid of Fenton-like process. \u003cem\u003eBioresour. Technol\u003c/em\u003e. 333, 125170. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.biortech.2021.125170\u003c/span\u003e\u003cspan address=\"10.1016/j.biortech.2021.125170\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eChen, Z., Fu, Q., Cao, Y., Wen, Q., Wu, Y., 2021. Effects of lime amendment on the organic substances changes, antibiotics removal, and heavy metals speciation transformation during swine manure composting. \u003cem\u003eChemosphere\u003c/em\u003e 262, 128342. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.chemosphere.2020.128342\u003c/span\u003e\u003cspan address=\"10.1016/j.chemosphere.2020.128342\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eCory, R.M., McKnight, D.M., 2005. Fluorescence Spectroscopy Reveals Ubiquitous Presence of Oxidized and Reduced Quinones in Dissolved Organic Matter. \u003cem\u003eEnviron. Sci. Technol.\u003c/em\u003e 39, 8142\u0026ndash;8149. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1021/es0506962\u003c/span\u003e\u003cspan address=\"10.1021/es0506962\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eCui, H., Ou, Y., Wang, L., Yan, B., Li, Y., Bao, M., 2021. Critical passivation mechanisms on heavy metals during aerobic composting with different grain-size zeolite. \u003cem\u003eJ. Hazard. Mater.\u003c/em\u003e 406, 124313. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jhazmat.2020.124313\u003c/span\u003e\u003cspan address=\"10.1016/j.jhazmat.2020.124313\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eCyubahiro, E., Luo, Z., Wang, H., Luo, Y., Nkinahamira, F., Habumugisha, T., Wang, Z., 2023. Effect of montmorillonite biochar composite amendment on thallium bioavailability in contaminated agricultural soils and its mitigated health risk. Environ. \u003cem\u003eSci. Pollut. Res.\u003c/em\u003e 30, 47882\u0026ndash;47891. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s11356-023-25668-4\u003c/span\u003e\u003cspan address=\"10.1007/s11356-023-25668-4\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003ede Melo, B.A.G., Motta, F.L., Santana, M.H.A., 2016. Humic acids: Structural properties and multiple functionalities for novel technological developments. \u003cem\u003eMater. Sci. Eng\u003c/em\u003e. C 62, 967\u0026ndash;974. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.msec.2015.12.001\u003c/span\u003e\u003cspan address=\"10.1016/j.msec.2015.12.001\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eDorogaya, E.S., Kuzina, E.V., Suleimanov, R.R., Yurkevich, M.G., Bakhmet, O.N., 2023. On the possibility of utilizing sodium lignosulfonate as a nano-organic foundation for creating soil-like bodies in the purposes of technogenic-degraded land rehabilitation. Nanotechnologies Constr. \u003cem\u003eA Sci. Internet-Journal\u003c/em\u003e 15, 359\u0026ndash;372. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.15828/2075-8545-2023-15-4-359-372\u003c/span\u003e\u003cspan address=\"10.15828/2075-8545-2023-15-4-359-372\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGuo, S., Nkinahamira, F., Adyari, B., Zhang, Y., Hu, A., Sun, Q., 2023. Fate and Spatial\u0026ndash;Temporal Variation of 23 Elements at 7 Wastewater Treatment Plants in Southeast City of China. \u003cem\u003eWater\u003c/em\u003e 15, 1226. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/w15061226\u003c/span\u003e\u003cspan address=\"10.3390/w15061226\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGuo, X., Liu, H., Wu, S., 2019. Humic substances developed during organic waste composting: Formation mechanisms, structural properties, and agronomic functions. \u003cem\u003eSci. Total Environ\u003c/em\u003e. 662, 501\u0026ndash;510. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.scitotenv.2019.01.137\u003c/span\u003e\u003cspan address=\"10.1016/j.scitotenv.2019.01.137\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eIzydorczyk, G., Mikula, K., Skrzypczak, D., Trzaska, K., Moustakas, K., Witek-Krowiak, A., Chojnacka, K., 2021. Agricultural and non-agricultural directions of bio-based sewage sludge valorization by chemical conditioning. \u003cem\u003eEnviron. Sci. Pollut. Res\u003c/em\u003e. 28, 47725\u0026ndash;47740. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s11356-021-15293-4\u003c/span\u003e\u003cspan address=\"10.1007/s11356-021-15293-4\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKhatami, S., Deng, Y., Tien, M., Hatcher, P.G., 2019. Formation of water-soluble organic matter through fungal degradation of lignin. \u003cem\u003eOrg. Geochem\u003c/em\u003e. 135, 64\u0026ndash;70. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.orggeochem.2019.06.004\u003c/span\u003e\u003cspan address=\"10.1016/j.orggeochem.2019.06.004\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLindsay, W.L., Norvell, W.A., 1978. Development of a DTPA Soil Test for Zinc, Iron, Manganese, and Copper. \u003cem\u003eSoil Sci. Soc. Am. J.\u003c/em\u003e 42, 421\u0026ndash;428. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.2136/sssaj1978.03615995004200030009x\u003c/span\u003e\u003cspan address=\"10.2136/sssaj1978.03615995004200030009x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLiu, G., Hu, Z., Huang, W., Zheng, Y., Yang, F., Su, Y., 2025. Sodium lignosulfonate enhances the insoluble sulfates and phosphates formation of heavy metal passivation in municipal sludge aerobic composting. \u003cem\u003eInt. J. Environ. Sci. Technol.\u003c/em\u003e 22, 8619\u0026ndash;8630. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s13762-024-05872-6\u003c/span\u003e\u003cspan address=\"10.1007/s13762-024-05872-6\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLiu, X., Hou, Y., Li, Z., Yu, Z., Tang, J., Wang, Y., Zhou, S., 2020. Hyperthermophilic composting of sewage sludge accelerates humic acid formation: Elemental and spectroscopic evidence. \u003cem\u003eWaste Manag.\u003c/em\u003e 103, 342\u0026ndash;351. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.wasman.2019.12.053\u003c/span\u003e\u003cspan address=\"10.1016/j.wasman.2019.12.053\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLiu, Y., Ma, R., Li, D., Qi, C., Han, L., Chen, M., Fu, F., Yuan, J., Li, G., 2020. Effects of calcium magnesium phosphate fertilizer, biochar and spent mushroom substrate on compost maturity and gaseous emissions during pig manure composting. \u003cem\u003eJ. Environ. Manage\u003c/em\u003e. 267, 110649. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jenvman.2020.110649\u003c/span\u003e\u003cspan address=\"10.1016/j.jenvman.2020.110649\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLong, Y., Zhu, N., Zhu, Y., Shan, C., Jin, H., Cao, Y., 2024. Hydrochar drives reduction in bioavailability of heavy metals during composting via promoting humification and microbial community evolution. \u003cem\u003eBioresour. Technol\u003c/em\u003e. 395, 130335. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.biortech.2024.130335\u003c/span\u003e\u003cspan address=\"10.1016/j.biortech.2024.130335\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMa, F., Zhu, T., Wang, Y., Li, X., Chang, M., Zhao, C., Wang, Z., Quan, H., 2024. Humic acid enhances adsorption effect: Application foundation of high-temperature composting products for remediation of heavy metals pollution. \u003cem\u003eBiochem. Eng. J.\u003c/em\u003e 209, 109415. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.bej.2024.109415\u003c/span\u003e\u003cspan address=\"10.1016/j.bej.2024.109415\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMa, J., Khan, M.A., Xia, M., Fu, C., Zhu, S., Chu, Y., Lei, W., Wang, F., 2019. Effective adsorption of heavy metal ions by sodium lignosulfonate reformed montmorillonite. \u003cem\u003eInt. J. Biol. Macromol.\u003c/em\u003e 138, 188\u0026ndash;197. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.ijbiomac.2019.07.075\u003c/span\u003e\u003cspan address=\"10.1016/j.ijbiomac.2019.07.075\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMa, R., Liu, Y., Wang, J., Li, D., Qi, C., Li, G., Yuan, J., 2022. Effects of oxygen levels on maturity, humification, and odor emissions during chicken manure composting. \u003cem\u003eJ. Clean. Prod\u003c/em\u003e. 369, 133326. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jclepro.2022.133326\u003c/span\u003e\u003cspan address=\"10.1016/j.jclepro.2022.133326\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eNkinahamira, F., Guo, S., Cao, M., Zhang, Y., Asefi, B., Sun, S., Feng, M., Sun, Q., Yu, C.-P., 2022. A closed-loop system to recycle rare earth elements from industrial sludge using green leaching agents and porous β-cyclodextrin polymer composite. \u003cem\u003eResour. Conserv. Recycl\u003c/em\u003e. 180, 106152. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.resconrec.2021.106152\u003c/span\u003e\u003cspan address=\"10.1016/j.resconrec.2021.106152\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eNkinahamira, F., Suanon, F., Chi, Q., Li, Y., Feng, M., Huang, X., Yu, C.-P., Sun, Q., 2019. Occurrence, geochemical fractionation, and environmental risk assessment of major and trace elements in sewage sludge. \u003cem\u003eJ. Environ. Manage.\u003c/em\u003e 249, 109427. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jenvman.2019.109427\u003c/span\u003e\u003cspan address=\"10.1016/j.jenvman.2019.109427\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003ePan, I., Dam, B., Sen, S.K., 2012. Composting of common organic wastes using microbial inoculants. \u003cem\u003eBiotech\u003c/em\u003e 2, 127\u0026ndash;134. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s13205-011-0033-5\u003c/span\u003e\u003cspan address=\"10.1007/s13205-011-0033-5\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003ePeter, P.O., Ifon, B.E., Nkinahamira, F., Lasisi, K.H., Li, J., Hu, A., Yu, C.-P., 2024. Harnessing the composition of dissolved organic matter in lagoon sediment in association with rare earth elements using fluorescence and UV\u0026ndash;visible absorption spectroscopy. Sci. \u003cem\u003eTotal Environ\u003c/em\u003e. 908, 168139. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.scitotenv.2023.168139\u003c/span\u003e\u003cspan address=\"10.1016/j.scitotenv.2023.168139\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eRastogi, M., Nandal, M., Khosla, B., 2020. Microbes as vital additives for solid waste composting. \u003cem\u003eHeliyon\u003c/em\u003e 6, e03343. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.heliyon.2020.e03343\u003c/span\u003e\u003cspan address=\"10.1016/j.heliyon.2020.e03343\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eŠćiban, M.B., Klašnja, M.T., Antov, M.G., 2011. Study of the biosorption of different heavy metal ions onto Kraft lignin. \u003cem\u003eEcol. Eng.\u003c/em\u003e 37, 2092\u0026ndash;2095. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.ecoleng.2011.08.006\u003c/span\u003e\u003cspan address=\"10.1016/j.ecoleng.2011.08.006\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eTang, J., Zhuang, L., Yu, Z., Liu, X., Wang, Y., Wen, P., Zhou, S., 2019. Insight into complexation of Cu(II) to hyperthermophilic compost-derived humic acids by EEM-PARAFAC combined with heterospectral two dimensional correlation analyses. \u003cem\u003eSci. Total Environ\u003c/em\u003e. 656, 29\u0026ndash;38. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.scitotenv.2018.11.357\u003c/span\u003e\u003cspan address=\"10.1016/j.scitotenv.2018.11.357\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eTang, R., Liu, Y., Ma, R., Zhang, L., Li, Y., Li, G., Dingmei Wang, Lin, J., Li, Q., Yuan, J., 2023. Effect of moisture content, aeration rate, and C/N on maturity and gaseous emissions during kitchen waste rapid composting. \u003cem\u003eJ. Environ. Manage\u003c/em\u003e. 326, 116662. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jenvman.2022.116662\u003c/span\u003e\u003cspan address=\"10.1016/j.jenvman.2022.116662\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang, G., Yang, Y., Kong, Y., Ma, R., Yuan, J., Li, G., 2022. Key factors affecting seed germination in phytotoxicity tests during sheep manure composting with carbon additives. \u003cem\u003eJ. Hazard. Mater.\u003c/em\u003e 421, 126809. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jhazmat.2021.126809\u003c/span\u003e\u003cspan address=\"10.1016/j.jhazmat.2021.126809\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang, W., Hou, Y., Huang, W., Liu, X., Wen, P., Wang, Y., Yu, Z., Zhou, S., 2021. Alkali lignin and sodium lignosulfonate additives promote the formation of humic substances during paper mill sludge composting. \u003cem\u003eBioresour. Technol.\u003c/em\u003e 320, 124361. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.biortech.2020.124361\u003c/span\u003e\u003cspan address=\"10.1016/j.biortech.2020.124361\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang, X., Lyu, T., Dong, R., Liu, H., Wu, S., 2021. Dynamic evolution of humic acids during anaerobic digestion: Exploring an effective auxiliary agent for heavy metal remediation. \u003cem\u003eBioresour. Technol.\u003c/em\u003e 320, 124331. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.biortech.2020.124331\u003c/span\u003e\u003cspan address=\"10.1016/j.biortech.2020.124331\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang, Zichao, Gao, M., Wang, S., Xin, Y., Ma, D., She, Z., Wang, Zhe, Chang, Q., Ren, Y., 2014. Effect of hexavalent chromium on extracellular polymeric substances of granular sludge from an aerobic granular sequencing batch reactor. \u003cem\u003eChem. Eng\u003c/em\u003e. J. 251, 165\u0026ndash;174. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.cej.2014.04.078\u003c/span\u003e\u003cspan address=\"10.1016/j.cej.2014.04.078\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWei, S., Li, Zichen, Sun, Y., Zhang, J., Ge, Y., Li, Zhili, 2022. A comprehensive review on biomass humification: Recent advances in pathways, challenges, new applications, and perspectives. \u003cem\u003eRenew. Sustain. Energy Rev\u003c/em\u003e. 170, 112984. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.rser.2022.112984\u003c/span\u003e\u003cspan address=\"10.1016/j.rser.2022.112984\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWu, H., Wang, C., Zhou, J., Cong, H., Gao, Y., Cai, W., Feng, S., Zhang, C., 2025. Feedstock optimization with low carbon to nitrogen ratio during algal sludge aerobic composting: Quality and gaseous emissions. \u003cem\u003eBioresour. Technol\u003c/em\u003e. 416, 131811. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.biortech.2024.131811\u003c/span\u003e\u003cspan address=\"10.1016/j.biortech.2024.131811\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWu, J., Zhang, H., He, P.-J., Shao, L.-M., 2011. Insight into the heavy metal binding potential of dissolved organic matter in MSW leachate using EEM quenching combined with PARAFAC analysis. \u003cem\u003eWater Res\u003c/em\u003e. 45, 1711\u0026ndash;1719. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.watres.2010.11.022\u003c/span\u003e\u003cspan address=\"10.1016/j.watres.2010.11.022\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWu, S., Tursenjan, D., Sun, Y., 2023. Impact of compost methods on humification and heavy metal passivation during chicken manure composting. \u003cem\u003eJ. Environ. Manage\u003c/em\u003e. 325, 116573. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jenvman.2022.116573\u003c/span\u003e\u003cspan address=\"10.1016/j.jenvman.2022.116573\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWu, Y., Yu, Z., Zhao, M., Chen, C., Nkinahamira, F., 2025. Effects of an applied electric field on the composting process of organic wastes: A review. \u003cem\u003eJ. Environ. Chem\u003c/em\u003e. Eng. 13, 119387. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jece.2025.119387\u003c/span\u003e\u003cspan address=\"10.1016/j.jece.2025.119387\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhang, J., Ying, Y., Yao, X., 2019. Effects of turning frequency on the nutrients of Camellia oleifera shell co-compost with goat dung and evaluation of co-compost maturity. \u003cem\u003ePLoS One\u003c/em\u003e 14, e0222841. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1371/journal.pone.0222841\u003c/span\u003e\u003cspan address=\"10.1371/journal.pone.0222841\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhao, B., Wang, Y., Sun, H., Xu, Z., 2022. Analysis of humus formation and factors for driving the humification process during composting of different agricultural wastes. \u003cem\u003eFront. Environ\u003c/em\u003e. Sci. 10, 1\u0026ndash;12. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3389/fenvs.2022.954158\u003c/span\u003e\u003cspan address=\"10.3389/fenvs.2022.954158\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZheng, W., Yang, Z., Huang, L., Chen, Y., 2022. Roles of organic matter transformation in the bioavailability of Cu and Zn during sepiolite-amended pig manure composting. \u003cem\u003eJ. Environ. Manage.\u003c/em\u003e 314, 115046. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jenvman.2022.115046\u003c/span\u003e\u003cspan address=\"10.1016/j.jenvman.2022.115046\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZheng, X., Zou, D., Wu, Q., Wang, H., Li, S., Liu, F., Xiao, Z., 2022. Review on fate and bioavailability of heavy metals during anaerobic digestion and composting of animal manure. \u003cem\u003eWaste Manag\u003c/em\u003e. 150, 75\u0026ndash;89. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.wasman.2022.06.033\u003c/span\u003e\u003cspan address=\"10.1016/j.wasman.2022.06.033\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhou, S., Kong, F., Lu, L., Wang, P., Jiang, Z., 2022. Biochar \u0026mdash; An effective additive for improving quality and reducing ecological risk of compost: A global meta-analysis. \u003cem\u003eSci. Total Environ\u003c/em\u003e. 806, 151439. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.scitotenv.2021.151439\u003c/span\u003e\u003cspan address=\"10.1016/j.scitotenv.2021.151439\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhou, Y., Selvam, A., Wong, J.W.C., 2014. Evaluation of humic substances during co-composting of food waste, sawdust and Chinese medicinal herbal residues. \u003cem\u003eBioresour. Technol\u003c/em\u003e. 168, 229\u0026ndash;234. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.biortech.2014.05.070\u003c/span\u003e\u003cspan address=\"10.1016/j.biortech.2014.05.070\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZmora-Nahum, S., Markovitch, O., Tarchitzky, J., Chen, Y., 2005. Dissolved organic carbon (DOC) as a parameter of compost maturity. \u003cem\u003eSoil Biol. Biochem.\u003c/em\u003e 37, 2109\u0026ndash;2116. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.soilbio.2005.03.013\u003c/span\u003e\u003cspan address=\"10.1016/j.soilbio.2005.03.013\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Composting, Sodium lignosulfonate, Humification, Heavy metal immobilization, Waste valorization, Copper","lastPublishedDoi":"10.21203/rs.3.rs-7877970/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7877970/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eValorization of industrial by-products like sodium lignosulfonate (SL) offers a sustainable strategy for environmental remediation. This study provides a systematic evaluation of SL's dual-purpose application to enhance humification and immobilize copper (Cu) during municipal sludge composting. The effects of SL addition at 0% (CK), 3% (T3), and 9% (T9) dry weight over a 60-day composting period were investigated. The 9% SL treatment (T9) significantly accelerated compost maturity, achieving a germination index (GI) of 96.4% compared to 85.9% in the control. EEM-PARAFAC analysis revealed that SL addition promoted the transformation of protein-like and quinone-like intermediates into stable humic acid (HA)-like substances. Consequently, the final HA content and degree of polymerization (DP) in T9 were substantially higher than in the control. Critically, this enhanced humification directly facilitated Cu immobilization. The DTPA-extractable (bioavailable) Cu in T9 was reduced to 96.40 mg/kg, a 66% reduction from its initial value and significantly lower than the 150.73 mg/kg in the final control compost. Pearson analysis confirmed a strong negative correlation between HA content and bioavailable Cu. The findings demonstrate that SL is a highly effective amendment for producing safer, higher-quality compost, offering a novel strategy for the synergistic valorization of industrial and municipal waste streams.\u003c/p\u003e","manuscriptTitle":"Valorization of Sodium Lignosulfonate for Enhanced Humification and Copper Immobilization During Municipal Sludge Composting","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-18 08:49:57","doi":"10.21203/rs.3.rs-7877970/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"3b6d14d3-2d57-4723-99eb-0ffeffdb865b","owner":[],"postedDate":"November 18th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-03-16T16:06:11+00:00","versionOfRecord":{"articleIdentity":"rs-7877970","link":"https://doi.org/10.1007/s11270-026-09352-0","journal":{"identity":"water-air-and-soil-pollution","isVorOnly":false,"title":"Water, Air, \u0026 Soil Pollution"},"publishedOn":"2026-03-10 16:00:10","publishedOnDateReadable":"March 10th, 2026"},"versionCreatedAt":"2025-11-18 08:49:57","video":"","vorDoi":"10.1007/s11270-026-09352-0","vorDoiUrl":"https://doi.org/10.1007/s11270-026-09352-0","workflowStages":[]},"version":"v1","identity":"rs-7877970","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7877970","identity":"rs-7877970","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}