Tailoring Hydrophobicity and Selectivity in Organo-Modified Steel Slag Adsorbents for Aromatic Hydrocarbon Remediation from Petroleum Wastewater

Tailoring Hydrophobicity and Selectivity in Organo-Modified Steel Slag Adsorbents for Aromatic Hydrocarbon Remediation from Petroleum Wastewater | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 4 July 2025 V1 Latest version Share on Tailoring Hydrophobicity and Selectivity in Organo-Modified Steel Slag Adsorbents for Aromatic Hydrocarbon Remediation from Petroleum Wastewater Authors : Shuang Lin 0009-0004-6881-0760 , Changsheng Qu , and Dongyao Xu [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.175162525.57126181/v1 Published Colloids and Surfaces A: Physicochemical and Engineering Aspects Version of record Peer review timeline 232 views 140 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Industrial steel slag (SS) was converted into efficient adsorbents for petroleum wastewater treatment via acid activation coupled with organic modification. Acid activation exposed a hydroxylated silica-rich matrix (SRS), enabling quaternary ammonium surfactant grafting to create organo-SRSs with enhanced hydrophobicity. Among the prepared materials, BC-SRS exhibited superior performance, achieving a maximum naphthalene (NA) adsorption capacity of 505.9 mg/g due to synergistic π–π stacking, hydrophobic interactions, and weak electrostatic forces. Adsorption followed pseudo-second-order kinetics and Freundlich isotherms, with thermodynamic analysis indicating spontaneous and endothermic behavior for BC-SRS. The material demonstrated excellent reusability and selectivity under complex water chemistries. This work presents a sustainable strategy for converting industrial waste into high-efficiency adsorbents, advancing green remediation and circular resource utilization. 1. Introduction The rapid development of petroleum refining and petrochemical industries has led to the widespread discharge of hydrocarbon-rich wastewater. Polycyclic aromatic hydrocarbons (PAHs) represent a major class of persistent and toxic pollutants in the petroleum wastewater. Naphthalene (NA), the most abundant low-molecular-weight PAH, is frequently detected in refinery effluents, oil spills and industrial discharges, which is classified as a priority pollutant due to its carcinogenicity, mutagenicity, and bioaccumulation potential [1-3]. The relatively high water-solubility of NA enables easy migration through aquatic systems, posing severe threats to aquatic organisms and human health [4,5]. Conventional treatment methods such as biodegradation or oxidation often fall short in effectively removing naphthalene, particularly at low concentrations [6]. Therefore, developing efficient and selective removal strategies for NA from petroleum-contaminated wastewater is of critical environmental and public health importance [7,8]. Among the available technologies, adsorption has been extensively explored for the removal of aromatic organic pollutants due to its operational simplicity, low cost, and high efficiency [3,9]. While conventional adsorbents such as activated carbon and mesoporous silica exhibit excellent adsorption performance, their high production costs and limited regenerability hinder large-scale application. In the global steel market, China produced 1.03 billion tons of crude steel in 2021. However, nearly 15% to 20% of solid waste was obtained during the processing of steel smelting industry [10]. Steel slag (SS) as a byproduct of steel manufacturing composed primarily of CaO, SiO₂, Fe₂O₃, and Al₂O₃, offers significant advantages as a low-cost, abundant, and chemically active precursor material [11]. Its intrinsic porosity, alkalinity, and magnetic separation potential further support its application in wastewater treatment [10]. The effective reutilization of SS not only contributes to environmental protection but also embodies the principles of sustainable waste valorization and green resource utilization. Despite the abundance and alkaline nature, pristine SS exhibits limited adsorption capacity for organic pollutants due to its heterogeneous mineral composition, low specific surface area, and poorly developed pore structure [12]. To overcome these limitations, modification strategies including high-temperature calcination to improve porosity, compositing with biochar to enhance surface functionality, and acid treatment to selectively remove inactive alkaline phases [13, 14] are explored. Acid leaching has proven particularly effective, as it dissolves labile metal oxides (e.g., CaO, MgO) while enriching the residual matrix in amorphous silica (SiO₂), thereby increasing both the structural homogeneity and the chemical reactivity of SS [14]. The resulting silica-rich slag (SRS) not only provides a more stable framework but also offers abundant hydroxyl and silanol groups, which serve as anchoring sites for further chemical functionalization. This structural transformation significantly enhances the potential of steel slag as a platform material for adsorbent development. In particular, surface modification using amphiphilic or cationic surfactants enables tuning of interlayer spacing, regulation of surface polarity, and introduction of hydrophobic or π-conjugated functional domains that favor interactions with nonpolar organic molecules such as naphthalene [15, 16]. This dual-step approach-acid activation followed by surfactant-assisted functionalization-opens a promising pathway for engineering low-cost, high-performance hybrid adsorbents from industrial solid waste. Such materials offer strong potential for selective removal of aromatic contaminants from complex wastewater matrices, aligning with the goals of sustainable resource utilization and environmental remediation [10]. In the NA adsorption process, the mechanism is predominantly governed by non-covalent interactions, including π–π stacking, hydrophobic association, and van der Waals forces. Depending on the functional groups on the adsorbent surface, additional contributions from hydrogen bonding and electrostatic attraction may also enhance the interaction strength [17]. For example, Wang et al. demonstrated that activated biochar with oxygenated surface functionalities significantly enhanced the π–π and hydrogen bonding interactions with naphthalene, leading to increased adsorption capacity (290.5 mg/g) [18]. Similarly, theoretical studies using density functional theory (DFT) have shown that materials with electron-donating groups or transition metal sites can facilitate strong charge-transfer interactions with PAHs, reinforcing adsorption affinity and selectivity [17]. In terms of the acid-treated steel slag, enriched in amorphous silica, provides a structurally uniform and chemically tunable substrate that can be further modified for targeted adsorption applications. Surface functionalization with amphiphilic surfactants not only increases hydrophobicity but also regulates interfacial polarity and interlayer spacing, thus creating an optimized microenvironment for the selective adsorption of nonpolar aromatic compounds like naphthalene [6]. Such structural tailoring has been reported to markedly improve the adsorption capacity and regeneration potential of silica-based materials for aromatic hydrocarbons [7,8]. Therefore, the integration of silica enrichment via acid leaching and subsequent surfactant-assisted modification provides a promising platform for designing efficient, cost-effective adsorbents tailored for PAH removal. This approach not only advances the valorization of industrial solid wastes such as steel slag but also contributes to the development of sustainable remediation technologies for oil-derived organic contaminants. In this study, we present a novel approach for synthesizing a naphthalene-selective adsorbent via acid purification of SS followed by surfactant modification. The structural property and surface chemistry of the resultant organo silica-rich slag (organo-SRSs) are characterized by XRD, TG-DTG, N 2 adsorption, XPS, water contact angle and Zeta potentials. Moreover, the adsorption processes of organo-SRSs towards NA are fully discussed in terms of the influence factors, the adsorption kinetics, isotherms and thermodynamics, as well as the adsorption selectivity, applicability, regenerability and the adsorption mechanisms. This work not only contributes to the sustainable utilization of industrial byproducts but also provides insights into the design of tailored adsorbents for the removal of organic pollutants targeted at the petroleum waste from aqueous environments. 2. Materials and Experiments 2.1 Materials The source material steel slag (SS) is derived from the impurities oxidation in metallic charge, the slag-forming material addition, and the desulfurization products from Xu Gang Steel Group Co., Ltd. The naphthalene (NA) and benzalkonium chloride (BC), dodecyl dimethyl benzyl ammonium bromide (DDBAB), cetyltrimethylammonium bromide (CTAB), and hexadecyl dimethyl benzyl ammonium chloride (HDBAC) are bought from Shanghai Aladdin Biochemical Technology Co., Ltd. (China). The surfactant structures are shown in Figure S1. 2.2 Synthesis of organo-SRSs Organo-SRSs are synthesized in the sequence of acid activation and organic modification of SS. The acid activation is conducted by mixing 1 g of SS into 100 mL of HNO 3 solution (the concentration of HNO 3 is 200 mg/L). After reacting under the conditions of 60 o C and 8h in a water bath oscillator, the black powder (SRS) is obtained by centrifugating, washing and drying at 60 o C. Organo-SRSs is synthesized by the one-pot hydrothermal method. The detailed procedures are: A certain amount of surfactants (0.5 g) is dissolved in 30 mL of deionized water in 100 mL of autoclave reactor. After adding 0.1 g of SS into the surfactant solution, the mixed solution is reacted under 80 o C for 12 h. The resultant powders are obtained by cooling, centrifugating and drying, which are denoted as BC-SRS, DDBAB-SRS, CTAB-SRS and HDBAC-SRS, respectively. Details of characterization, adsorption tests, the adsorption kinetics, isotherms, thermodynamics, as well as the selectivity tests are listed in the Supporting Information. 3. Results and Discussion 3.1 Structural Characterization To reveal the structural evolution of SS through acid activation and organic modification processes, the XRD patterns of SS, SRS and organo-SRSs are shown in Figure 1a. The sharp and well-defined peaks in SS at 2 θ = 18°, 26°, 29°,32° and 34° correspond to Ca(OH)₂, SiO₂, CaCO₃, Fe₂O₃, and FeO, respectively, reflecting the complex mineralogical composition and heterogeneous crystallinity of SS (consistent with the XRF results in Table S1) [19]. The necessity of acid activation is revealed by the prominent diffraction peaks of Ca(OH)₂ and CaCO₃ in SS, which demonstrate that the surface of SS is rich in basic species with limited accessibility for modification or contaminant adsorption. After acid activation, SRS exhibits weakened Ca- and Fe-associated peaks and emerges an additional peak centered at around 2 θ = 21°characteristic of the mesostructured SiO₂[20]. After the acid activation process, decalcification and partial de-mentalization lead to the successful exposure of SiO₂ matrix with high surface hydroxyl density, contributing to enhanced potential for organic functionalization processes. The XRD patterns of organo-SRSs reveal further insights into the interfacial interactions and structural alterations. Compared with SRS, all the organo-SRS samples exhibit noticeable changes in the intensity of the silica-related peak at around 2 θ = 21°, suggesting that surfactant molecules interact intimately with the silica framework through intercalation into pore channels or adsorption onto the surface. Compared with DDBAB-SRS, CTAB-SRS and HDBAC-SRS, BC-SRS exhibits the most significant structural perturbation with the gentlest character peak at 21°, which is attributed to strong π-π interactions between the benzyl ring and the siloxane surface, combined with dense packing of the cationic head groups within the disordered silicate domains. The TG curves of SRS and organo-SRSs are shown in Figure 1b. By evaluating thermal behavior across the temperature ranges of 30-600 °C, different weight losses can correspond to physiosorbed water loss, decomposition of organics, and residual stability, respectively. SRS displays a continuous weight loss of 3.1% below 100 °C, which is primarily attributed to the evaporation of physically adsorbed water [21], indicative of the inherent hydrophilicity of SRS due to the abundant surface silanol groups (-Si-OH). The weight loss between 150 °C and 450 °C corresponds to the thermal decomposition of the grafted or adsorbed surfactants. BC-SRS shows the smallest overall mass loss, retaining 95.3 wt% at 600 °C. Given the aromatic structure of BC and its relatively short alkyl chain, it is reasonable to attribute this behavior to stronger interfacial interactions (π–π stacking and cation–π interactions between the benzyl group and the silica-rich surface) resulting in a more compact configuration. CTAB-SRS shows the most substantial mass loss (5.9 wt% with a residual mass of 94.1 wt%), indicating the highest degree of surfactant incorporation. This is consistent with long alkyl chain in CTAB facilitates cooperative hydrophobic packing and possibly bilayer formation on the SRS surface. However, the earlier onset of decomposition (200 °C) and steeper slope suggest that CTAB is weakly bound on the SRS surface. In contrast, DDBAB-SRS displays a residual weight of 95.3 wt% similar with BC-SRS, whereas the decomposition of HDBAC-SRS closely resembles that of CTAB-SRS. This suggests that while both DDBAB and HDBAC introduce comparable levels of organic content, the benzyl group in DDBAB may promote more effective π–π or hydrophobic interactions, thereby enhancing thermal stability slightly relative to HDBAC. The N 2 adsorption-desorption results (Table 1, Figures 1c and 1d) provide a comprehensive depiction of the textural properties and porosity evolution of SS, SRS and organo-SRSs. SS exhibits a low S BET of 7.8 m²/g, with pore volume and diameter undetectable, indicating a dense, non-porous structure. This observation consists of the BET and BJH profiles (Figures 1c and 1d). The limited surface area of SS is attributable to the presence of crystalline phases such as Ca(OH)₂ and Fe-based oxides, whose compact morphologies afford limited internal porosity. SRS undergoes a dramatic increase in surface area to 112.2 m²/g, with a pore volume of 0.24 cm³/g, a narrow pore width of 0.57 nm and an average pore diameter of 10.2 nm. The sharp uptake observed in the low relative pressure region ( P / P ₀ < 0.1) of the BET isotherm confirms the prevalence of microporosity, while the H3-type hysteresis loop at higher P / P ₀ indicates slit-like mesopores[22]. The BJH curve exhibits a dominant peak centered around 0.55-0.60 nm, consistent with the formation of well-defined micropores due to the dissolution of calcium and iron phases, and the exposure of amorphous SiO₂ networks. Upon organic functionalization, a significant decrease in S BET is observed, ranging from 24.3 to 29.2 m²/g, with pore volumes converging around 0.13-0.14 cm³/g in the organo-SRSs. This reduction in accessible surface area is attributed to the partial blockage of pore entrances and surface coverage by the surfactant molecules. Specifically, BC-SRS retains the highest surface area (29.2 m²/g) that preserve mesopore access while modifying interfacial properties. DDBAB-SRS with a similar pore structure (pore width = 2.77 nm, average diameter = 19.8 nm) displays a slightly lower S BET (27.5 m²/g) and similar pore volume (0.14 cm³/g), suggesting a more extensive surfactant incorporation due to its longer alkyl chain. The BJH curve reveals textural disorder and suppressed mesopore volume, indicative of denser surface coverage and possible formation of bilayer-type assemblies within pore domains. CTAB-SRS and HDBAC-SRS exhibit further reductions in S BET (24.3 m²/g and 27.3 m²/g, respectively). Both materials possess slightly larger pore widths (3.17 nm), reflecting the spatial rearrangement of long-chains within mesopores. Notably, the average pore diameter of CTAB-SRS reaches 25.7 nm, the largest among all samples, suggesting that the linear alkyl chains may induce pore widening through surface restructuring or interfacial tension effects. Conversely, HDBAC-SRS, while maintaining a similar pore width, has a smaller average diameter (18.9 nm), potentially due to stronger π–π interactions between the benzyl headgroup and the silanol-rich surface. Figure 1. The XRD patterns of SS, SRS and organo-SRSs (a), TG (b), BET (c) and BJH curves (d) of SRS and organo-SRSs Table 1 N 2 adsorption-desorption results of SS, SRS and organo-SRSs SS 7.8 NA NA NA SRS 112.2 0.24 0.57 10.2 BC-SRS 29.2 0.14 2.77 17.1 DDBAB-SRS 27.5 0.14 2.77 19.8 CTAB-SRS 24.3 0.13 3.17 25.7 HDBAC-SRS 27.3 0.14 3.17 18.9 The morphology and water contact angle (WCA) of SS, SRS and organo-SRSs are shown in Figure 2. SS displays angular, compact particles with smooth surfaces, indicative of a crystalline, hydrophilic matrix as confirmed by a low WCA of 12.6° (Figure 2a). After acid activation (Figure 2b), SRS exhibits increased surface roughness and partial porosity due to decalcification and silica enrichment, leading to a moderate WCA increase to 30.4°. BC-SRS (Figure 2c) presents a lamellar, partially collapsed structure, and a WCA of 50.2°, indicating enhanced hydrophobicity from π-π interactions between the benzyl headgroup and surface silanols. Compared with BC-SRS, DDBAB-SRS (Figure 2d) shows a granular morphology and a moderate WCA of 38.2°. CTAB-SRS with longer alkyl chain (Figure 2e) exhibits a fibrous, sponge-like structure with a WCA of 51.5°, arising from the coverage of hydrophobic chain. HDBAC-SRS (Figure 2f) with a rod-like anisotropic morphology, achieves the highest WCA of 68.2°. Collectively, these results demonstrate that surface hydrophobicity and morphology are strongly governed by surfactant molecular architecture, with dual-functional (aromatic-alkyl) structures yielding superior water repellency and interfacial modification. Figure 2. The SEM and WCA of SS (a), SRS (b), BC-SRS (c), DDBAB-SRS (d), CTAB-SRS (e) and HDBAC-SRS (f) Figure 3. The XPS survey of SRS and organo-SRSs (a and d), high-resolution spectra of Si element (b and e) and O element (c and f) Based on the full survey and high-resolution XPS spectra shown in Figures 3a-f, the surface chemical environments of SRS and organo-SRSs were analyzed to investigate the interactions. In the full spectra (Figures 4a and 4d), all samples exhibit prominent peaks corresponding to Si 2p (~104 eV) and O 1s (~532 eV), with additional peaks at C 1s (~285 eV) and N 1s (~400 eV) in the organo-SRSs[23], indicating the successful grafting of surfactants. The pristine SRS shows negligible N 1s intensity, whereas BC-SRS and DDBAB-SRS display stronger N 1s peaks, further evidence more effective amine or quaternary ammonium group loading. The intensity of the C 1s peak also increases substantially upon surface modification, supporting the incorporation of long alkyl chains. High-resolution Si 2p spectra (Figures 4b and 4e) reveal slight shifts in the binding energy upon surface functionalization. The Si 2p peak of pristine SRS is centered at 103.85 eV, corresponding to Si⁴⁺ in SiO₂[24]. After modification, the Si 2p peaks shift slightly to lower values: 103.53 eV (BC-SRS), 103.46 eV (DDBAB-SRS), 103.27 eV (CTAB-SRS), and 103.44 eV (HDBAC-SRS). This downward shift in binding energy indicates an electron-donating effect from the organic moieties, suggesting successful chemical interaction between the silanol groups and surfactant headgroups. Among all, BC-SRS shows the most significant shift (Δ E = –0.32 eV), implying the strongest interaction between BC and the silicate surface. Similarly, the O 1s spectra (Figures 4c and 4f) show deconvoluted peaks, which correspond to lattice oxygen (Si–O–Si) and surface hydroxyl or C=O oxygen, respectively. The main O 1s peak of pristine SRS appears at 533.07 eV, which shifts to 532.70 eV (BC-SRS), 532.63 eV (DDBAB-SRS), 532.40 eV (CTAB-SRS), and 532.56 eV (HDBAC-SRS) after modification. BC-SRS shows the largest shift in O 1s binding energy, further supporting a stronger chemical bonding or denser surface coverage. The decreased binding energy is consistent with electron redistribution due to the replacement of polar silanol groups with less electronegative organic functionalities. XPS results confirm that all four surfactants were successfully anchored onto the SRS surface, leading to characteristic chemical shifts in both Si 2p and O 1s spectra. Among them, BC-SRS exhibits the strongest interaction with the substrate due to the favorable affinity and steric configuration of BC molecules. These results are consistent with the TGA and adsorption performance data, supporting the conclusion that BC-SRS possesses superior surface modification and interaction strength. 3.2 Adsorption Tests 3.2.1 The effects of adsorbent dosage The influence of adsorbent dosage on the removal efficiency (R%) and adsorption capacity (q e ) of SRS and organo-SRSs for naphthalene (NA, initial concentration of 300 mg/L) is illustrated in Figure 4a. As the dosage increases from 1 g/L to 9 g/L, SRS and organo-SRSs exhibit a consistent increase in removal efficiency due to the enhanced availability of active surface sites. However, the adsorption capacity simultaneously decreases with the adsorbent dosage increasing. At the low dosage of 1 g/L, SRS only reaches a removal efficiency of 37.8% and of 113.4 mg/g. HDBAC-SRS achieves the highest removal efficiency of 96.2% and a maximum adsorption capacity of 192.4 mg/g, indicating superior affinity toward NA driven by combined π–π and hydrophobic interactions. This performance surpasses that of BC-SRS (92.8%, 185.6 mg/g), CTAB-SRS (89.3%, 178.2 mg/g) and DDBAB-SRS (84.6%, 167.4 mg/g). The decline trends of R% and q e becomes more pronounced at the adsorbent dosage of 9 g/L, reflecting the saturation of the adsorption sites and dilution of adsorptive driving force per unit mass. Nonetheless, the removal efficiencies of HDBAC-SRS, BC-SRS, and CTAB-SRS are higher than 98%. Mechanistically, the superior adsorption performance of HDBAC-SRS and BC-SRS can be attributed to the synergistic effects of hydrophobic alkyl chains and aromatic benzyl groups, which enable strong van der Waals and π-π interactions with NA. The relatively high adsorption capacity of CTAB-SRS is due to the contribution of low surface energy and efficient hydrophobic adsorption microenvironment of long alkyl chain. Considering both the adsorption capacity, removal efficiency and adsorbent input, 1 g/L is chosen as the optimal adsorbent dosage in the subsequent tests. Figure 4. The effects of adsorbent dosage (a), time (b), NA concentration and temperature on the adsorption capacity of BC-SRS (c), DDBAB-SRS (d), CTAB-SRS (e) and HDBAC-SRS (e) 3.2.2 The effects of time and adsorption kinetics The adsorption kinetics demonstrate a rapid initial uptake followed by a gradual approach to equilibrium (Figure 4b). Within the first 30 min, organo-SRSs exhibit significant increases in adsorption capacity, with CTAB-SRS, HDBAC-SRS, and BC-SRS rapidly reaching 172.54, 145.09 and 175.09 mg/g, while DDBAB-SRS shows a comparatively slower uptake, attaining only 105.18 mg/g at 30 min. The rapid initial increase in adsorption capacity is due to abundant active site availability and high concentration gradients, facilitating strong π–π stacking and hydrophobic interactions. Over the ensuing 240 minutes, adsorption plateaus are observed, with BC-SRS achieving the highest equilibrium capacity (203.7 mg/g), followed by CTAB-SRS (192.6 mg/g), HDBAC-SRS (189.5 mg/g), and DDBAB-SRS (153.2 mg/g). The superior performance of BC-SRS may be attributed to its more accessible surface architecture and efficient π–π interactions via benzyl groups, while the relatively lower capacity of DDBAB-SRS suggests less efficient packing of the surfactant within the mesoporous network and low hydrophobicity. These trends confirm that the adsorption of NA on organo-SRSs is strongly influenced by surfactant molecular structure, interfacial hydrophobicity and surface accessibility. Figure 5. The pseudo-second order (a), intra-particle diffusion (b) and isotherms for the adsorption of NA on BC-SRS (a), DDBAB-SRS (b), CTAB-SRS (c) and HDBAC-SRS (d) The kinetic behavior of NA adsorption onto organo-SRSs was evaluated using pseudo-first-order, pseudo-second-order and intra-particle diffusion models in Table 2[25-27]. Pseudo-second-order model demonstrated the best fit as evidenced by the highest R 2 values (with all the R 2 values higher than 0.99) and the lowest AIC values[28], indicating that the adsorption process is governed by chemisorption involving electron sharing or exchange between NA and the surface of organo-SRSs. The higher rate constants (k 2 ) for BC-SRS (4.65 × 10⁻³ g mg⁻¹ min⁻¹) and HDBAC-SRS (4.29 × 10⁻³ g mg⁻¹ min⁻¹) are higher than CTAB-SRS and DDBAB-SRS, demonstrating the faster kinetics and consistent with the higher adsorption capacities in the kinetic curves. Intra-particle diffusion model produced relatively high R 2 values, with the intercepts of all the curves are non-zero, implying that intra-particle diffusion is involved in the adsorption process. Notably, the values of C (indicative of the thickness of boundary layer) of BC-SRS and HDBAC-SRS are higher, indicating the existence of π–π stacking interactions between the adsorbed NA molecules and those dissolved in the solution. The kinetic results highlight the importance of surfactant structure in modulating both adsorption rate and equilibrium uptake: (i) aromatic moieties (e.g., especially benzyl groups in BC and HDBAC) enhance π–π interactions and promote rapid NA capture, while long alkyl chains improve hydrophobic affinity but may introduce diffusion resistance. (ii) the adsorption of NA onto organo-SRSs is primarily controlled by chemisorption mechanism, with additional contributions from film diffusion and intra-particle transport processes. Table 2 Kinetic parameters for the adsorption of NA on organo-SRSs BC-SRS DDBAC-SRS CTAB-SRS HDBAC-SRS q e,exp 204.68 153.33 193.84 193.34 Pseudo-first-order q e,cal 108.85 57.97 123.96 123.96 k 1 (10 -2 ) 3.12 2.04 2.14 2.14 R 2 0.9250 0.7994 0.7754 0.7754 Pseudo-second-order AIC 126.73 143.55 151.91 152.68 q e,cal 208.23 202.43 189.75 187.26 k 2 (10 -5 ) 3.99 2.44 2.78 2.85 R 2 0.9919 0.9984 0.9987 0.9952 AIC 103.84 98.72 96.15 94.61 Intra-particle diffusion k id 4.57 6.15 2.28 4.29 C (10 2 ) 1.48 0.71 1.59 1.11 R 2 0.9675 0.9961 0.9674 0.9972 AIC 110.21 108.36 101.48 102.37 3.2.3 The effect of NA concentration and adsorption isotherms With the NA concentration increasing from 200 mg/L to 600 mg/L, the adsorption capacity of organo-SRSs increases markedly, indicating enhanced driving force for mass transfer between the bulk solution and the adsorbent surface (as shown in Figures 4 c-f). Higher solute concentrations elevate the probability of solute-adsorbent collisions and promote occupation of available active sites. Notably, BC-SRS exhibits the most significant increase in adsorption capacity with increasing, reaching a maximum adsorption capacity of 505.9 mg/g. This sharp increase reflects the high affinity of BC-SRS for NA due to synergistic π-π interactions from the benzyl group and favorable hydrophobic partitioning effects. In contrast, DDBAB-SRS achieves a lower of 228.2 mg/g, which may be attributed to reduced surface accessibility or steric hindrance from its long alkyl chain leading to partial pore blockage. CTAB-SRS and HDBAC-SRS exhibit intermediate performance, with q m values reaching 264.9 mg/g and 344.5 mg/g, respectively. The linear increase trend in the adsorption capacity suggests multilayer adsorption or progressive site activation at higher concentrations. The strong hydrophobicity introduced by the long alkyl chains in CTAB and HDBAC facilitates the NA adsorption, while the additional π–π interactions from the benzyl group of HDBAC further enhance site affinity, contributing to its superior performance over CTAB-SRS. The variation in adsorption response across the four organo-SRSs underscores the critical role of surfactant molecular structure in governing site availability, diffusion accessibility, and adsorption affinity under high-concentration conditions. The adsorption isotherms in Table 3 offer a detailed comparison of the adsorption behaviors of NA on organo-SRSs[29-31]. The Freundlich model provides the best overall fit as indicated by consistently high correlation coefficients, low SSE and minimal AIC values, suggesting that NA adsorption occurs heterogeneously over multilayer sites with varying affinities, which aligns well with the diverse surface microenvironments on the surface of organo-SRSs. The Khan model incorporates non-linearity through a third parameter, providing suitable fitting performance. CTAB-SRS shows favorable agreement with the Khan model at 298 K ( R 2 = 0.9987, AIC = 64.6), while the SSE value remains higher than that of the Freundlich or Redlich-Peterson models, suggesting limited improvement despite added complexity. Notably, the Redlich-Peterson model demonstrates strong fitting performance across the temperature range. The parameter g (typically 0 < g < 1) remains close to 1 for most systems (e.g., 0.98-0.99 for CTAB-SRS and HDBAC-SRS), indicating a predominantly Langmuir-like mechanism with slight heterogeneity. The low SSE and competitive AIC values (e.g., AIC = 43.7 for HDBAC-SRS at 328 K) support its suitability as a flexible descriptor for organo-SRS surfaces with quasi-uniform but adaptable binding domains. In summary, the adsorption of NA on organo-SRSs is best described by the Freundlich model, highlighting the presence of heterogeneous, multilayer adsorption processes dominated by π–π stacking and hydrophobic interactions. The Redlich-Peterson model also offers strong descriptive accuracy, confirming that the surface microenvironments are energetically diverse yet structurally consistent. Table 3 Adsorption isotherm constants for the adsorption of NA on organo-SRSs BC-SRS DDBAB-SRS CTAB-SRS HDBAC-SRS 298 K 313 K 328 K 298 K 313 K 328 K 298 K 313 K 328 K 298 K 313 K 328 K Langmuir q max 9.94E05 8.26E05 1.07E06 201.36 431.94 201.36 3.61E02 3.38E02 3.34E02 5.29E02 7.15E02 6.51E02 K L 2.39E-06 3.70E-06 3.53E-06 4.99E-02 1.69E-03 4.99E-02 9.43E-03 8.16E-03 6.05E-02 5.79E-03 0.30E-02 2.51E-03 R 2 0.6060 0.8671 0.5487 0.9621 0.9863 0.9668 0.9464 0.9646 0.9909 0.9030 0.9472 0.9782 SSE 35.97 11.76 42.92 2.85 0.47 1.84 3.18 2.16 0.49 7.38 4.48 1.56 AIC 85.2 97.4 108.2 56.4 59.9 62.5 72.1 75.8 79.3 63.7 61.0 58.4 Freundlich K f 24.81 0.32 0.005 13.84 2.67 4.59 29.90 23.57 15.43 14.39 7.52 5.90 n 4.28 0.67 0.39 2.08 1.43 1.77 2.60 2.46 2.17 1.78 1.49 1.48 R 2 0.9923 0.9917 0.9707 0.9498 0.9927 0.9368 0.8972 0.9084 0.9663 0.9515 0.9732 0.9911 SSE 0.12 0.89 3.34 1.77 0.25 1.30 6.11 5.57 1.80 3.69 2.72 0.63 AIC 49.6 52.3 55.9 42.7 44.2 45.6 53.4 54.9 56.1 45.1 43.5 42.0 Khan q max 1.40E03 1.47E03 287.51 1.58E03 4.81E03 6.29E03 4.38E03 4.54E04 1.34E03 2.69 2.83 3.85 a 31.72 4.96 9.69 2.99 0.30 16.04 6.61 70.03 2.65 0.44 0.33 0.32 b 4.05E-04 1.19E-03 2.45E-03 1.03E-03 0.44 1.03E04 5.73E-04 4.48E-05 1.26E-03 20.08 4.30 1.89 R 2 0.9591 0.9932 0.9645 0.9820 0.9909 0.9648 0.9699 0.9967 0.9957 0.9515 0.9731 0.9911 SSE 3.74 0.59 3.37 0.64 0.32 0.72 2.34 0.25 0.29 4.61 2.85 0.79 AIC 68.1 71.7 75.9 60.8 62.3 63.8 60.2 62.5 64.4 61.4 59.3 57.2 Redlich-Peterson a 5.03E04 5.85E03 3.40E04 1.51 4.99E02 0.55 2.20 1.75 1.59 4.04E04 1.29E04 5.09E03 b 1.14E07 1.85E04 6.41E06 4.77E-04 1.88E02 1.09E-05 1.79E-04 6.74E-05 7.12E-04 2.81E03 1.71E03 8.62E02 g 1.33 0.50 1.52 1.35 0.30 1.82 1.58 1.71 1.30 0.44 0.33 0.32 R 2 0.9367 0.9875 0.9560 0.9820 0.9909 0.9697 0.9719 0.9988 0.9949 0.9515 0.9732 0.9914 SSE 5.78 1.11 4.18 0.64 0.31 0.62 2.09 0.09 0.34 4.61 2.84 0.79 AIC 51.4 54.8 59.0 45.5 47.1 48.9 47.6 49.3 51.1 46.3 45.0 43.7 3.2.4 The effect of temperature and adsorption thermodynamics The effect of temperature on the adsorption performance of organo-SRSs provides critical insights into the thermodynamic nature and interfacial behavior (Figures 4c-f). Temperature variations (25 °C, 40 °C, and 55 °C) lead to a distinct impact on both the equilibrium adsorption capacity and removal efficiency, reflecting differences in enthalpic favorability, adsorbate mobility, and surfactant-layer stability. Consistent enhancement in both the adsorption capacity and removal efficiency of BC-SRS is observed with increasing temperature. At an initial concentration of 600 mg/L, the increases from 462.8 mg/g at 25 °C to 505.9 mg/g at 55 °C, which suggests an endothermic process facilitated by increased molecular kinetic energy that overcomes activation barriers for diffusion and interaction with benzyl-functionalized sites. The consistent increase across all concentration levels indicates a thermodynamically favorable sorption mechanism dominated by π–π stacking and hydrophobic interactions. In contrast, DDBAB-SRS exhibits a pronounced decrease in adsorption performance with rising temperature. At 600 mg/L, the drops from 228.8 mg/g at 25 °C to 142.4 mg/g at 55 °C. The inverse temperature dependence implies an exothermic adsorption mechanism governed by physisorption or weak van der Waals interactions. Moreover, the long alkyl chain and benzyl headgroup in DDBAB may form a densely packed, temperature-sensitive surface layer whose structural integrity or binding configuration is disrupted at elevated temperatures, thereby reducing the effective adsorption interface. CTAB-SRS and HDBAC-SRS display intermediate behaviors, suggesting that while higher temperature enhances molecular diffusion and surface contact, the probability of site saturation or competitive desorption also increases, balancing out the thermodynamic gain. The thermodynamic parameters are summarized in Table 4[32-34]. The Gibbs free energy change (Δ G ) is negative at each temperature (298 K, 313 K, and 328 K), confirming that the NA adsorption is spontaneous. Notably, BC-SRS exhibits the most negative Δ G values, ranging from -2.07 kJ/mol to -2.54 kJ/mol, indicating a thermodynamically more favorable adsorption process compared to other organo-SRSs. In contrast, CTAB-SRS presents less negative values (e.g., -1.36 kJ/mol at 298 K), suggesting a relatively weaker driving force for adsorption. The enthalpy change (Δ H ) distinguishes the thermodynamic nature. BC-SRS shows a substantially negative enthalpy value of -50.6 kJ/mol, indicative of an exothermic adsorption process dominated by strong interactions such as π–π stacking and hydrophobic affinity between NA and the surface of BC-SRS. Similarly, the higher Δ H of DDBAB-SRS (-49.2 kJ/mol) implies energetically favorable but possibly less stable surface-solute interactions at elevated temperatures. In contrast, the lower Δ H of CTAB-SRS (-13.9 kJ/mol) suggests that the adsorption is primarily governed by weaker van der Waals forces or partitioning into the hydrophobic layer, rather than specific chemical interactions. The entropy change (Δ S ) values are negative across all the organo-SRSs, indicating decreased randomness at the solid-liquid interface during the adsorption process. The strongest decrease of Δ S is observed for DDBAB-SRS (-43.6 J/mol·K), reflecting a more ordered structure formed upon adsorption, potentially due to tight packing or restricted mobility of the long-chain surfactant layer. Table 4 Thermodynamic parameters for the adsorption of NA on organo-SRSs T = 298 K T = 313 K T = 328 K BC-SRS -2.07 -2.54 -2.51 -27.3 -50.6 DDBAB-SRS -1.42 -1.61 -2.17 -43.6 -2.9 CTAB-SRS -1.36 -1.51 -1.65 -32.2 -13.9 HDBAC-SRS -1.76 -2.11 -2.32 -20.1 -22.8 3.2.5 The effect of solution pH and surface character of organo-SRSs Under varying solution pH values, the adsorption of NA on organo-SRS displays a pronounced bell-shaped dependence, reaching the maximum adsorption capacity in the mildly acidic range (the maximum adsorption capacities of BC-SRS, DDBAB-SRS, CTAB-SRS and HDBAC-SRS are 202.0, 151.9, 182.6 and 183.5 mg/g at pH 5, respectively) and declining at both lower and higher solution pH values (Figure 6a). Figure 6 The effect of pH (a), regeneration tests (b), the selectivity of BC-SRS (c), the effect of anions (d) and the adsorption of analogous polycyclic and monocyclic aromatic compounds (f) Zeta-potential measurements elucidate the role of electrostatic forces[35]. The surfaces of organo-SRSs transition from positive potentials at pH 1 (+13 mV to +15 mV) to strongly negative values at pH 9 (–15 mV to –27 mV). At low pH, the positively charged interface is highly hydrated and disfavors the adsorption of neutral hydrophobic solutes. At high pH, excessive surface charge leads to extensive electrical double layers that retain water molecules and disrupt surfactant packing, weakening hydrophobic and π–π interactions. Under the condition of pH 5, the zeta potentials cross through mild negativity (–8 mV to –10 mV), striking an optimal balance: electrostatic repulsion of water is reduced, surfactant alkyl chains are maximally exposed, and aromatic headgroups engage most effectively in π–π stacking with NA molecules. These results underscore that, although NA is neutral, the interfacial charge modulates the hydration environment and surfactant conformation, thereby indirectly controlling hydrophobic partitioning and π–π driven adsorption. 3.2.6 Regeneration and selectivity tests In order to assess the reusability and stability of organo-SRSs, five consecutive adsorption–desorption cycles were conducted (initial NA concentration = 300 mg/L, adsorbent dosage = 1.0 g/L). The regeneration was performed via ethanol-assisted desorption followed by drying at 60 °C. BC-SRS displayed superior recyclability, with the adsorption capacity decreasing from 198.5 to 189.7, 180.2, 172.4 and 164.6 mg/g, respectively. This can be attributed to the stronger π–π stacking interactions between the benzyl moieties and NA. In contrast, DDBAB-SRS, CTAB-SRS and HDBAC-SRS exhibited a slightly faster capacity decline, with retention ratios of 70.2%, 62.3% and 61.9%, respectively. The greater loss observed in HDBAC-SRS may be due to partial desorption inefficiency or loss of surfactant functionality during regeneration[36]. To evaluate the adsorption selectivity of BC-SRSs toward NA in complex petroleum-derived wastewater matrices, competitive adsorption experiments were conducted in the presence of four representative co-contaminants: phenol (Ph), o-cresol (OC), benzothiophene (BT), and n-decane (ND). These compounds span polar hydroxylated aromatics, sulfur-containing heterocycles, and nonpolar aliphatic hydrocarbons. Under identical conditions ( C 0 = 300 mg/L, adsorbent dosage = 1.0 g/L), BC-SRS demonstrated a pronounced selectivity for NA, achieving an adsorption capacity of 201.4 mg/g, while Ph, OC, BT, and ND were adsorbed to a significantly lesser extent, at 84.7, 72.9, 96.3, and 45.2 mg/g, respectively. The superior selectivity of BC-SRS toward NA is attributed to the synergistic contribution of π–π stacking and hydrophobic partitioning. NA, being a planar, electron-rich aromatic hydrocarbon, exhibits strong π–π interactions with the benzyl headgroups of BC. In contrast, Ph and OC possess hydroxyl substituents that disrupt π–π stacking and introduce hydrogen-bonding competition with the aqueous phase, thereby diminishing their affinity onto the surface of BC-SRS. Although BT retains aromaticity, the electron distribution across its sulfur-containing heterocycle leads to weaker π–π overlap and reduced adsorption strength relative to NA. ND, lacking π electrons, is adsorbed only via weak van der Waals and hydrophobic forces, resulting in the lowest uptake amount. To further assess robustness of BC-SRS under complex water chemistries, the influence of inorganic anions Cl⁻, NO₃⁻, SO₄²⁻, and HCO₃⁻-on was investigated (Figure 6d). These anions were introduced at 10 mM concentrations into NA solutions ( C 0 = 100 mg/L, pH = 5.0, dosage = 1.0 g/L) to simulate realistic ionic backgrounds. The relatively minor reduction in adsorption capacity in the presence of Cl⁻ (-1.6%) and NO₃⁻ (-4.4%) suggests that monovalent anions exert negligible competition or interference with the NA adsorption process. This behavior is consistent with a mechanism dominated by non-electrostatic interactions, namely π–π stacking between the naphthalene ring and the benzyl group of BC, as well as hydrophobic partitioning into the alkyl-rich interfacial layer. These interactions are largely insensitive to the ionic strength or the identity of monovalent co-ions due to the non-polar character of naphthalene and the organic surface layer of BC-SRS. The presence of SO₄²⁻ and HCO₃⁻ led to pronounced declines in adsorption, particularly for HCO₃⁻ (–17.7%), which may be attributed to modifications of surface hydration layers, pH buffering effects, or competitive hydrogen bonding with the surfactant headgroups. SO₄²⁻ may hinder access to adsorption sites by altering the electrical double layer or by forming transient hydration shells around the sorbent surface. HCO₃⁻ can shift local pH and possibly alter the conformation of the surfactant layer or introduce weak competition via interfacial interactions. However, even in the most disruptive scenario (HCO₃⁻), BC-SRS retained over 82% of its original adsorption capacity, indicating strong site-specific recognition and high selectivity for aromatic hydrocarbons. These results reinforce that the dominant mechanism for NA adsorption on BC-SRS is π–π stacking and hydrophobic affinity, rather than ion-exchange or electrostatic attraction. The limited sensitivity to anionic interference highlights the stability and functional resilience of BC-SRS to serve as a suitable candidate for real-world applications involving saline, alkaline, or multicomponent wastewater matrices where competitive adsorption or ionic screening is unavoidable. 3.2.7 Adsorption mechanism To elucidate the adsorption mechanism of BC-SRS toward NA, structurally analogous polycyclic and monocyclic aromatic compounds were selected as probe molecules, including 1-methylnaphthalene (1-MNA), 2-naphthol (2-NA), anthracene (AT), and benzene (BEN). These compounds vary systematically in molecular planarity, π-electron density, hydrophobicity, and functional group polarity, allowing for a mechanistic deconvolution of interaction types governing naphthalene adsorption. All experiments were conducted under identical conditions (C₀ = 300 mg/L, pH = 5.0, dosage = 1.0 g/L). The high adsorption capacity for NA confirms its strong interaction with BC-SRS. The slightly lower uptake of 1-MNA is attributed to steric hindrance from the methyl group, which disrupts optimal face-to-face π–π interaction with the benzyl headgroups of BC. Although the structure of 2-NA is planar and aromatic, it contains a polar hydroxyl group that competes with water molecules for hydrogen bonding and diminishes hydrophobic interactions, leading to substantially lower adsorption. The three-ring system of AT allows for enhanced π–π interaction relative to benzene, yet its larger molecular size may result in steric constraints within the modified interfacial microdomain, thus limiting its adsorption compared to naphthalene. The significantly lower adsorption of BEN underscores the necessity of extended π-electron conjugation and hydrophobic surface area for high-affinity adsorption. These comparative results suggest that π–π stacking and hydrophobic interactions are the dominant mechanisms, with electronic delocalization, planarity, and molecular volume all contributing to adsorption efficiency. The diminished affinity for polar or sterically hindered analogs further confirms that the interfacial microenvironment created by the benzyl-substituted surfactant preferentially stabilizes nonpolar, electron-rich planar PAHs through selective molecular recognition. BC-SRS acts as a tailored sorbent for naphthalene via cooperative interfacial π-electron interactions and hydrophobic encapsulation. Figure 7. The XPS survey of spent BC-SRS (a), high-resolution spectra of O (b), Si (c) and C element (d) The XPS analysis of BC-SRS after NA adsorption reveals significant shifts in binding energies, indicating changes in the surface chemical environment and providing mechanistic insight into the adsorption process. The C 1s spectrum exhibits two distinct peaks centered at 284.80 eV and 286.41 eV are retained at 284.80 eV and 286.42 eV, while a new peak emerges at 288.55 eV, which can be attributed to π–π interactions between the aromatic rings and the adsorbent surface, or potentially to weak donor–acceptor interactions involving the π-system of NA and polar surface sites[37,38]. Similarly, in the O 1s spectrum, the binding energies of the Si–O–Si lattice and surface hydroxyl groups (532.70 eV and 530.15 eV, respectively) shift slightly to 532.75 eV and 529.90 eV[39], suggesting a local change in the electron cloud density around oxygen atoms due to dipole-dipole interactions or weak hydrogen bonding between the hydroxylated silica framework and adsorbed NA molecules. Such shifts indicate a perturbation of the silica surface environment and support the involvement of non-covalent interactions in the adsorption process. The Si 2p peak also exhibits a slight shift from 103.53 eV to 103.56 eV[40], consistent with minimal electron density redistribution around silicon atoms, potentially induced by the altered polarization state of adjacent siloxane networks in the presence of aromatic adsorbates. Taken together, the XPS data provide comprehensive evidence that NA adsorption on BC-SRS is governed by a combination of π–π stacking, hydrophobic interactions, and weak electrostatic or hydrogen bonding contributions. The emergence of a new C1s peak and the systematic binding energy shifts in O1s and Si2p regions clearly point to multimodal physisorption behavior, highlighting the effectiveness of the BC-modified surface in selectively enriching aromatic hydrocarbons via synergistic non-covalent interactions. These findings validate the tailored surface chemistry of BC-SRS as particularly well-suited for the efficient removal of hydrophobic and π-conjugated pollutants such as naphthalene from aqueous environments. 4. Conclusion To realize the valorization of industrial waste steel slag (SS) for environmental remediation, a series of organo-functionalized silica-rich materials (organo-SRSs) were synthesized via acid activation followed by surface modification with quaternary ammonium surfactants. Acid treatment facilitated selective decalcification and partial de-metalization of SS, exposing a high-density hydroxylated SiO₂ matrix conducive to subsequent organic grafting. The resulting organo-SRSs exhibited enhanced hydrophobicity, increased organic moiety loading, and reduced surface area and porosity. Among them, BC-SRS demonstrated the strongest interaction with the substrate, attributed to the steric compatibility and molecular affinity of the BC modifier. Compared with unmodified SRS (62.9 mg/g), the adsorption capacities of organo-SRSs towards naphthalene (NA) significantly increased, reaching 191.7 mg/g for BC-SRS, 180.4 mg/g for DDBAB-SRS, 181.9 mg/g for CTAB-SRS, and 181.3 mg/g for HDBAC-SRS at an adsorbent dosage of 1 g/L. Notably, BC-SRS achieved a maximum uptake of 505.9 mg/g, the highest among all samples, driven by synergistic π–π stacking, π–π interactions, and hydrophobic forces. Kinetic and isotherm studies confirmed that the pseudo-second-order, intra-particle diffusion, and Freundlich models best described the adsorption behavior. Thermodynamic analysis revealed that NA adsorption on BC-SRS was spontaneous and endothermic, whereas exothermic processes dominated for the other organo-SRSs. The superior performance of BC-SRS was further supported by its excellent reusability over five adsorption–desorption cycles, high selectivity toward NA in the presence of coexisting contaminants in petroleum-derived wastewater, and structural robustness under diverse water chemistries. Mechanistic investigations confirmed that π–π stacking, hydrophobic interaction, and weak electrostatic or hydrogen bonding played dominant roles in NA uptake, influenced by factors such as aromaticity, molecular planarity, and hydrophobic surface affinity. This study not only highlights BC-SRS as a promising high-performance adsorbent for aromatic hydrocarbon removal but also exemplifies a sustainable approach to transforming industrial solid waste into functional environmental materials, aligning with the principles of waste valorization, green chemistry, and circular economy for long-term environmental sustainability. 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Keywords adsorption naphthalene organic modification steel slag Authors Affiliations Shuang Lin 0009-0004-6881-0760 China University of Mining and Technology - Beijing View all articles by this author Changsheng Qu Chinese Research Academy of Environmental Sciences View all articles by this author Dongyao Xu [email protected] China University of Mining and Technology - Beijing View all articles by this author Metrics & Citations Metrics Article Usage 232 views 140 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Shuang Lin, Changsheng Qu, Dongyao Xu. Tailoring Hydrophobicity and Selectivity in Organo-Modified Steel Slag Adsorbents for Aromatic Hydrocarbon Remediation from Petroleum Wastewater. Authorea . 04 July 2025. DOI: https://doi.org/10.22541/au.175162525.57126181/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. 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