Transport and retention of n-hexadecane in cadmium-/naphthalene-contaminated calcareous soil sampled in a karst area

Transport and retention of n-hexadecane in cadmium-/naphthalene-contaminated calcareous soil sampled in a karst area | 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 Transport and retention of n-hexadecane in cadmium-/naphthalene-contaminated calcareous soil sampled in a karst area Yiting Huang, Yankui Tang, Yi Liang, Zhenze Xie, Jipeng Wu, Jiajie Huang, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-2006218/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 26 Jun, 2023 Read the published version in Environmental Geochemistry and Health → Version 1 posted 7 You are reading this latest preprint version Abstract Petroleum hydrocarbon pollutants in karst areas have aroused widespread concern due to their toxicity. It is crucial to gain knowledge on transport and retention of petroleum hydrocarbons in karst areas. Calcareous soils in karst areas were contaminated by cadmium/naphthalene due to the industrial and agricultural activities, however, the fates of petroleum hydrocarbons in these contaminated calcareous soils have been rarely studied. In this study, n-hexadecane was selected as a model petroleum hydrocarbon. Batch experiments were conducted to explore the adsorption behavior of n-hexadecane on cadmium-/naphthalene-contaminated calcareous soils at various pH, and column experiments were performed to investigate the transport and retention of n-hexadecane under various flow velocity. The results showed that Freundlich model can well describe the adsorption behavior of n-hexadecane on all samples (R 2 >0.9). According to the adsorption coefficient ( K f ), the presence of cadmium/naphthalene dramatically increased the adsorption efficiency of n-hexadecane on calcareous soils, and the increasing pH value reduced the adsorption capacity in all groups. The transport of n-hexadecane in all samples were well described by two kinetic sites model of Hydrus-1D with R 2 >0.9. The higher effluent concentration of n-hexadecane indicates that n-hexadecane can more effectively break through the cadmium-/naphthalene-contaminated calcareous soil with high flow velocity, which means the lower n-hexadecane retention in this situation. The observation can be explained by the electrostatic repulsion between n-hexadecane and each soil sample. These findings have important implications for the government of groundwater in calcareous soils from karst areas. N-hexadecane Freundlich pH Column experiments Flow velocity Hydus-1D Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction Petroleum hydrocarbons are inevitably released into soils by the spill and leakage from tanks at petrol stations and industrial sites, then they transport downward along the soil layer and enter groundwater. The extensive and long-term petroleum hydrocarbons pollutants pose a threat such as carcinogenic, teratogenesis, and gene mutations to public health through the food chain (Wu et al., 2017 ). Some works have found that the fate of petroleum hydrocarbons may definitely influence the remediation efficiency (Ossai et al., 2020 ). N-hexadecane, a hydrophobicity fraction represents the ignitability of diesel fuel, has been usually chosen as the model petroleum hydrocarbon by many researchers to study on the environmental risks and the remediation (Brils et al., 2002 ; Samaei et al., 2020 ; Samaei et al., 2022 ). Thus, to better select the treatment option for remediation, it is necessary to investigate the transport and retention of n-hexadecane in soil environments. To date, the transport and retention of petroleum hydrocarbons in soils have been studied. Transport of petroleum hydrocarbons in quartz sand is promoted by increasing pH due to the increasing electrostatic repulsion between contaminations and particle surfaces (Wang et al., 2020b ). Ca 2+ provides the adsorption sites of petroleum, thus elevated concentration of Ca 2+ can increase the retention of petroleum in soil (Luo et al., 2022 ; Zhang et al., 2012 ). The high-speed and short-term infiltration of petroleum hydrocarbons into deeper soil layers are also affected by the minimum content of clay and the lower ionic strength (Rosales et al., 2014 ; Wang et al., 2020b ). Furthermore, the stronger leaching as torrential rain and flood changes the release of petroleum hydrocarbons in soil (Dror et al., 2002 ). In fact, one serious challenge impacting the soil remediation of n-hexadecane is the presence of other contaminations. Contaminated natural soils with n-hexadecane at petrol stations and industrial sites have been usually found heavy metals or Polycyclic Aromatic Hydrocarbons (PAHs), and the mixture may bring more risk to public health (Khudur et al., 2018 ; Li et al., 2021 ). Heavy metals cause long-term pollution in soils due to the lower bioavailability (Jampasri et al., 2016 ). PAHs, the toxic compounds in diesel oil and crude oil with uniquely stable structures, still retain in soils for a long time after n-hexadecane have been degraded (Zhou et al., 2019 ). Moreover, heavy metals change the organic matter, water holding capacity and pH value of soils (Zhao et al., 2017 ; Zheng et al., 2016 ). Besides, hydrophobic PAHs change the properties of soils by covering with a thin insulating film (Steliga and Kluk, 2020 ), lowering porosity, increasing resistance to penetration, aggregating particles in plaques, etc (He et al., 2013 ; Li et al., 2012 ; Wu et al., 2013 ). However, the influence of heavy metals/PAHs in soils on n-hexadecane transport has been overlooked by previous studies. The presence of dissolution fissures, crevices, and channels in karst areas cause stronger interaction between surface water and groundwater (Yang et al., 2019 ). As a result, the groundwater is more easily to be contaminated in karst areas than in non-karst areas (Lü et al., 2020 ). Thus, it is necessary to pay more attention to the process of how petroleum hydrocarbons in soil enter groundwater, since calcareous soils in karst areas have been seriously contaminated by petroleum hydrocarbons (Guo et al., 2020 ; Peng et al., 2022 ). Moreover, as an stability derivative of PAHs (Chen et al., 2009 ; Xu et al., 2022 ), naphthalene has effectively retained in karst areas by abundant soil organic matter (SOM) (Sun et al., 2019 ). Higher abnormal enrichment of cadmium (Cd) in karst than in non-karst due to the natural weathering of carbonate rock, the higher pH value, and the increasing soil organic carbon (SOC) (Albert et al., 2021 ; Wen et al., 2020 ). Therefore, three types of calcareous soil samples (origin calcareous soil, Cd-contaminated calcareous soil and naphthalene-contaminated calcareous soil) were chosen for studying the transport and retention of n-hexadecane due to their different properties. The objectives of this study are to (1) reveal the effect of Cd/naphthalene and pH on n-hexadecane adsorption, (2) investigate the effect of Cd/naphthalene and flow velocity on the transport and retention of n-hexadecane, and (3) explore the mechanisms governing the transport of n-hexadecane in vertical direction of Cd-/naphthalene-contaminated calcareous soil. The results can contribute to the development of palliative measures and management strategies to reduce the risks of environmental health. Methods And Materials Reagents N-hexadecane and naphthalene were obtained from Shanghai Macklin Biochemical Co., Ltd with a purity > 98%. Cadmium chloride was purchase from Tianjin Kermel Chemical Reagent Co., Ltd. KCl was purchased from Sinopharm Chemical Reagent Co., Ltd, Shanghai, China. Methanol, acetone, and n-hexane were supplied by T. JKE MAO Chemical Reagents CO., LTD. In this study, all reagents were at least analytical reagent, and all solutions were prepared using MIilli-Q water (resistance of 18.2 MΩ.cm at 25℃). Preparation and characterization of calcareous soil samples Calcareous soil samples were collected at the 0–20 cm below the ground surface from Fusui, Guangxi Zhuang Autonomous Region, China (22°56'43.6″N, 107°15'46.7″E). Calcareous soil samples were air dried at room temperature (25 ± 1℃) for one week, and they were sieved through a 2-mm mesh before experiments. The physical and chemical properties of the soil samples were provided in supplementary materials (Table S1). Table S1 presented that the background concentration of Cd in collected calcareous soils was below the detection limit of standard from China. N-hexadecane and naphthalene are not detected. According to Soil Environmental Quality Risk Control Standard for Soil Contamination of Development Land in China (risk screening values for soil contamination), the concentration of Cd and naphthalene was configured to be 20 mg/kg and 70 mg/kg in tested conditions, respectively. After soil samples and contamination solution were evenly mixed, the mixtures were placed in a fume hood for 90 days. The origin calcareous soil, Cd-contaminated calcareous soil and naphthalene-contaminated calcareous soil were marked as OS, CS and NS, respectively. Fourier transform infrared spectroscopy (FTIR, IRTracer-100, Shimadzu, Japan) was used to examine functional group in freeze-dried soil samples at wave numbers from 400 to 4000 cm − 1 . X-ray diffraction (D8 Dicover, Bruker Axs Gmbh, Germany) was used to analyze the crystalline constituents of freeze-dried soil samples. The concentration of dissolved organic matter (DOM) was detected by TOC-VCPH (Shimadzu, Japan). The zeta potential was measured by Nanobrook Ommi, Brookhaven (America). The fluorescence spectrometer (F-7000, Hitachi, Japan) was used to measure the three-dimensional excitation-emission matrix spectroscopy (3D-EEM) under the flowing conditions: excitation wavelengths of 200–450 nm (every 5 nm), emission wavelengths of 250–600 nm (every 1 nm), scan speed of 12000 nm/min, the excitation slit of 5 nm and emission slit of 5 nm. Batch experiments Before batch experiment, 0.5, 1.0 and 2.0 g soil samples were respectively mixed with 50 mL of 100 mg/L n-hexadecane solution into 100 mL conical flasks, and the mixtures were shaken for 12 h at 140 rpm and 25 ± 1 ℃. The ratio of soil samples to solution was selected 1.0 g for the stability of n-hexadecane quantification in batch experiments. In adsorption kinetic experiments, all conical flasks with parafilm (PM996, America) were shaken at various time scales (0.25, 0.5, 1, 2.5, 4, 8 and 12 h) in dark under the conditions of 140 rpm and 25 ± 1 ℃. In desorption kinetic experiments, the residues of preliminary adsorption kinetic experiments were mixed with 50 mL solution without n-hexadecane, and the mixtures was shaken at the same conditions as adsorption kinetic experiments. In adsorption isotherm experiments, 1.0 g soil sample was transferred to 50 mL various concentrations of n-hexadecane solution (50, 60, 80, 90 and 100 mg/L), and then the mixture was shaken at 140 rpm (25 ± 1 ℃) in dark for the equilibrium time, respectively. Moreover, the pH was adjusted with value of 5, 7, 9 using 0.1 mol/L HCl or NaOH for investigating the effect of pH on n-hexadecane adsorption behavior by isotherm experiments. All supernatants were collected after the mixtures were centrifuged at speed of 8000 rpm for 10 min. All the experiments were determined in triplicate using the standard experiment, and the concentration of n-hexadecane in liquid phase was detected by UV spectrophotometer (Shimadzu, Japan) at 257 nm. Column experiments The soil sample was wet-packed into a cylindrical column (3 cm inner diameter × 12 cm length). The schematic of experimental setup for column experiment is shown in Fig. S1. A peristaltic pump was used to pump influents from the bottom into column. The packed column was firstly saturated with background solution (1 mM KCl solution, which was used to minimize any volatilization loss (Rosales et al., 2014 )) with the set velocity (1, 2, and 4 mL/min) for 3 h pre-equilibration and air-removing. After that, 2.0 pore volumes (PVs) tracer (5 mM KCl solution) was injected into the column, and then several PVs of background solution was not stopped injecting until tracer was monitored to be zero. The tracer was measured by Conductivity Meter (DDSJ-308A, Shanghai INESA Scientific Instruments Co., Ltd, China). Breakthrough curves (BTCs) of tracer were used to determine longitudinal dispersion coefficient (cm 2 /min) in a column. Fig. S2 showed the BTCs of tracer can be well fitted with R 2 > 0.99 by STANMOD software, and a high degree of symmetry in column. The BTCs peak of tracer nearly reached the inlet concentration (the ratio was 93.6%), suggesting that KCl was a non-reactive tracer in this work. The following step was, 2.0 PVs of n-hexadecane solution (100 mg/L) with background solution was pumped into column, and then the column was flushed by several PVs of background solution. The effluents of n-hexadecane were determined by UV spectrophotometer (Shimadzu, Japan) at 257 nm. The BTCs of n-hexadecane were expressed as relative concentration ( C / C 0 ) versus PV, where C and C 0 were the concertation of n-hexadecane in the effluents and influents, respectively. The BTCs of n-hexadecane were fitted by Hydrus-1D, and the details of this model were described by (Simunek et al., 1999 ). Moreover, two groups were set to assure the consistency the reproducibility of column experiment. To better know the residue of n-hexadecane in a column, the packed column was excavated in 12 layers (from top to bottom) after column experiment was finished, and each layer was determined by Ultrasonic Extraction and Gas Chromatograph (GC, Shimadzu GC-2010, Japan). The parameter of GC system was referred by Determination of Extractable Petroleum Hydrocarbons (C10-C40) in China (HJ 894–2017). Results And Discussion Properties of soil sample The FTIR spectrum of OS, CS and NS is given in Fig. 1. The transmittance peak at 3443 cm -1 , 1636 cm -1 and 1034 cm -1 represent the stretching of -OH, C=C / C=O and Si-O-Si, respectively. The XRD pattern of OS, CS and NS (Fig. 2) showed that all soil samples were composed of saponite, kaolinite, rodolicoite and vanadium oxide. The FTIR spectrum and XRD pattern of OS, CS and NS indicated that the coating of Cd/naphthalene did not change the surface functional group and the mineralogical composition of calcareous soils. Table 1 is the zeta potential of OS, CS and NS. The OS, CS and NS are all negatively charged in the condition of this work, and the presence of n-hexadecane increased the negative surface potentials of OS, CS and NS since surface coating by hydrophobic compounds can change the zeta potential of porous media (Song et al., 2011; Yang et al., 2017b). Table 1 The zeta potential of all soil samples concentration of n-hexadecane Zeta potentials (mV) CS OS NS 0 mg/L -15.84 -20.76 -23.03 100 mg/L -29.11 -23.47 -24.63 The DOM concentration of CS (313.10 mg/kg) and NS (331.30 mg/kg) was higher than that of OS (88.35 mg/kg) (Table S1), suggesting that microbial communities in calcareous soil may respond to Cd/naphthalene (Schwarz et al., 2011; Wang et al., 2020a). DOM may control the fate of n-hexadecane in calcareous soil due to its amphipathic molecules. Fluorescence spectra of OS, CS and NS (Fig. 3) showed the intensity of tyrosine-like fluorescent peak in CS/NS were more significant than in OS, suggesting that CS/NS have more aromatic ring amino acids (Yan et al., 2019). The intensity of tryptophan-like fluorescent peak in CS/OS was stronger than in NS. Fulvic acid relates to the carbonyl and carboxyl groups, and it can increase the solubility of hydrocarbon (Lu et al., 2013). Clearly, the intensity of fulvic acid peak in CS was greater than in OS/NS. Fluorescence components of DOM in OS, CS and NS were shown in Fig. 4. It can be seen that the humic acid content of NS was smaller than that of OS/CS. It may result in the greater adsorption capacity of NS since the humic acid can generally inhibit the adsorption of hydrocarbon on porous media (Wang et al., 2017). Adsorption and desorption kinetic of hexadecane on calcareous soil samples N-hexadecane may be firstly trapped in calcareous soil through molecular adsorption, and later desorption may constitute the rate determining step. All samples were fitted well by the pseudo-first-order dynamics model with R 2 > 0.9 (Fig. S3 and Table S2). Adsorption equilibrium time for n-hexadecane on OS, CS and NS were all 4 h, suggesting that the presence of Cd/naphthalene cannot change the adsorption equilibrium time. It may be causing by the homologous properties of calcareous soil samples (Yang et al., 2013). The adsorption rate constant ( k 1 ) of OS was higher than that of CS/NS, which means Cd/naphthalene can increase the adsorption efficiency of n-hexadecane on calcareous soils. Desorption equilibrium time for n-hexadecane on OS, CS and NS were all 1 h, indicating that Cd/naphthalene cannot change the desorption equilibrium time. The k 1 of n-hexadecane adsorption on OS was lower than CS/NS (Table S2), which means Cd/naphthalene can reduce the desorption efficiency of n-hexadecane on calcareous soils. In all cases, adsorption efficiency was higher than desorption efficiency, suggesting the desorption behavior of n-hexadecane cannot greatly influence the adsorption behavior in this work. On the whole, Cd/naphthalene can improve the adsorption capacity and decrease the desorption capacity of n-hexadecane on calcareous soils, but they cannot change the equilibrium time. Effect of pH on the adsorption isotherm of n-hexadecane on the calcareous soil samples In most cases, Freundlich (Fig. 5 and Table 2) is better than Langmuir (Fig. S4 and Table S3), which means the adsorption behavior of n-hexadecane on soil samples may not dependent on monolayer adsorption. In Table 2, K f value (at pH=7) of n-hexadecane adsorption on soil samples was lower than that of on loess soil (Jiang et al., 2016) and modified diatomite (Xu et al., 2020). It can be summarized that Cd/naphthalene cannot obviously improve the adsorption efficiency of n-hexadecane on calcareous soils in comparison with other soils. At pH=7, K f value of OS was lower than that of CS/NS, suggesting that Cd/naphthalene can improve the higher n-hexadecane adsorption capacity on calcareous soil. Previous studies have also found that adding metals can increase the adsorption of hydrocarbon on porous medias (Saeedi et al., 2018). Cd-coating porous medias can enhance the adsorption of anionic and neutral hydrocarbon compounds since the polar functional group in porous media was complexation with Cd (Wang et al., 2017). However, in this study, the surface functional group in calcareous soils did not change after Cd/naphthalene coating, therefore the improved adsorption efficiency may not absolutely depend on function group’s change. Furthermore, the aggregation and hydrophobicity of porous media causing by Cd-coating can increase the adsorption efficiency of hydrocarbons (Nguyen et al., 2013). The negative charge of porous media surface can be shielded after coating naphthalene (Yang et al., 2017a), therefore the adsorption of n-hexadecane was inhibited. Moreover, the n value of all cases was lower than 1, indicating that adsorption sites in the surface of all soil samples (OS, CS and NS) were limitation although Cd/naphthalene can open more adsorption sites for n-hexadecane. Table 2 The parameters of Freundlich Soil sample pH K f a 1/ n b R 2 OS 5 0.10±0.15 2.30±0.26 0.95 7 0.00±0.01 2.10±0.36 0.98 9 0.02±0.03 2.76±0.32 0.98 CS 5 0.26±0.38 1.89±0.33 0.95 7 0.05±0.08 2.27±0.42 0.95 9 0.01±0.02 2.48±0.25 0.98 NS 5 1.27±1.40 1.56±0.25 0.95 7 0.82±0.82 1.65±0.22 0.96 9 0.06±0.09 2.17±0.30 0.95 a K f is the coefficient of Freundlich which positively associated with adsorption capacity. b n is the sorption intensity. In recent years, rainwater with higher pH value (pH=8.0) was found in a typical karst area (Zeng et al., 2020). Furthermore, previous studies have found that the adsorption of diesel oil on loess soil was weakened by increasing pH (Jiang et al., 2016) since pH improve the dissolution and dispersion of diesel oil (Delle Site, 2001; Pradubmook et al., 2003). However, the adsorption of naphthalene on biochar colloid had almost no effected by pH (Yang et al., 2017b) due to the little dissociation degree of nonpolar hydrocarbons in polar solution (Grządka, 2011). Therefore, the potential influence of pH on the n-hexadecane adsorption was investigated in this study. As shown in Fig. 5 and Table 2, it can be observed that the elevated pH (from 5 to 9) caused decreasing adsorption efficiency of n-hexadecane on OS, CS and NS. In this study, pH value was set from 5 to 9, thus the adsorption of n-hexadecane on each soil sample at pH=5 was the maximum. The adsorption of n-hexadecane on OS was weaker than CS/NS at pH=5 and pH=7. The K f value of OS was higher than CS in alkaline environment (at pH=9). The n-hexadecane adsorption on NS remained stable from 5 to 7, but it was changed from pH=7 to pH=9. Some studies reported that the release of Cd/naphthalene in soils depended on pH since the bonds between contaminations and soils were broken by pH (Kicińska et al., 2022; Yang et al., 2001). Furthermore, the release of Cd/naphthalene in soils may open more adsorption sites for n-hexadecane. Moreover, the presence of impurities in n-hexadecane, such as long-chain carboxylic acids (Fang et al., 2015), and the unbalanced hydrophobic/hydrophilic properties at oil/water boundary in solution (Li and Bhushan, 2015) may influence the adsorption of hydroxyl ions (OH - ), thus n-hexadecane carries negative surface charge. The electronic mobility of the n-hexadecane and the adsorption sites on soil samples may change along with the increasing pH at aqueous phase (Kim et al., 2012; Li and Bhushan, 2015). Therefore, pH can influence the adsorption behavior of n-hexadecane on soil samples. Effect of flow velocity on transport of n-hexadecane in the calcareous soil samples The BTCs of n-hexadecane in the calcareous soil samples with the addition of various flow velocity are presented in Fig. 6. As shown in Fig. 6, all BTCs were symmetric in shape, implying that there was the physical equilibrium transport in column. The maximum value of C / C 0 were 28 %, 35 % and 48 % in OS, CS and NS with the set flow velocity of 1 mL/min, respectively, suggesting n-hexadecane may more effectively breakthrough the column of CS/NS than that of OS. Similarly, the maximum value of C / C 0 in CS/NS was higher than in OS at the set flow velocity of 2 and 4 mL/min. The two kinetic sites model of Hydrus-1D well described the BTCs of n-hexadecane in each soil sample with different Darcy velocity according to the higher correlation coefficient (R 2 ) between observed and simulated data (Table 3). The characteristic of BTCs for n-hexadecane in all soil sample column can be reflected by S max and K att of Hydrus-1D (Table 3). In all case, the S max value of OS was higher than that of CS/NS, which means greater irreversible retention of n-hexadecane in OS. The greater value of K att for CS/NS at all Darcy velocity reflects a rapid release process of n-hexadecane. The mass recovery rate of n-hexadecane in effluent are showed in Table 3. There is a higher mass recovery rate for n-hexadecane in the effluent of CS/NS packed column than that of OS packed column. On the whole, the transport of n-hexadecane in soil samples followed the order of CS/NS > OS at the same Darcy velocity, which means Cd/naphthalene can improve the transport efficiency of n-hexadecane. In Table 2, negative charge density of CS/NS was more than that of OS. The more negative charge of soil samples, the stronger electrostatic repulsions between the soil samples and the negatively charged n-hexadecane were, which is likely responsible for promoting the transport of n-hexadecane in CS/NS packed column (Wu et al., 2020). Furthermore, although naphthalene-coating cannot dramatically change the surface negative charge of calcareous soils, nonpolar naphthalene can lead to charge-shielding (Yang et al., 2017b). The previous studies have proved that the transport of petroleum hydrocarbons with negative charge were inhibited by the decreasing pH due to the reduction in electrostatic repulsion (Cai et al., 2017; Wang et al., 2020b). Our previous batch experiments also showed that the electronic mobility of the n-hexadecane increased along with the increasing pH at aqueous system (Fig. 5 and Table 2). Moreover, the unique hydrological and geological structures in karst areas lead to variable flow velocity, and pollutants are more easily to rapidly transport in the pipeline systems of karst (Jiang et al., 2018). Therefore, this section only investigated the potential influence of flow velocity on the n-hexadecane transport in Cd-/naphthalene-contaminated calcareous soils. In Fig. 6, high value of C / C 0 for n-hexadecane in OS, CS and NS were observed along with high flow velocity, which means the amount of n-hexadecane transport increased with the increasing flow velocity. Similar results had observed by some studies (Alazaiza et al., 2021; Alazaiza et al., 2020). In Table 3, it should be pointed out that the value of S max decreased with high Darcy flow (0.69 cm/min) while the value of K att was to be the minimum at Darcy flow of 0.12 cm/min. It suggested that smaller Darcy velocity may improve the irreversible retention but inhibit the reversible retention of n-hexadecane in calcareous soil samples. The mass recovery rate of n-hexadecane transport increased when the set flow velocity reached 4 mL/min (Table 3), indicating the transport of n-hexadecane can be improved. Overall, n-hexadecane was more likely to break through CS/NS column with a high flow velocity. Table 3 the parameters of fitted model Column No. Soil sample Set flow (mL/min) Darcy velocity (cm/min) The mass recovery rate (%) S max1 (mg/g) a K att (min -1 ) b R 2 1 OS 1 0.12 30.96 0.3384 0.0224 0.9711 2 OS 1 0.12 27.81 0.3896 0.0279 0.9401 3 OS 2 0.31 32.87 0.2855 0.0825 0.9827 4 OS 2 0.31 35.14 0.2639 0.0861 0.9429 5 OS 4 0.69 42.08 0.2368 0.0886 0.9818 6 OS 4 0.69 40.03 0.2100 0.0827 0.9598 7 CS 1 0.12 30.80 0.2512 0.0172 0.9719 8 CS 1 0.12 28.99 0.2368 0.0216 0.9768 9 CS 2 0.31 35.36 0.1639 0.0377 0.9771 10 CS 2 0.31 37.51 0.1468 0.0361 0.9524 11 CS 4 0.69 57.68 0.1270 0.0652 0.9429 12 CS 4 0.69 54.21 0.1011 0.0681 0.9577 13 NS 1 0.12 43.98 0.2377 0.0125 0.9681 14 NS 1 0.12 40.11 0.2033 0.0153 0.9767 15 NS 2 0.31 56.98 0.1309 0.0394 0.9888 16 NS 2 0.31 57.19 0.1214 0.0365 0.9459 17 NS 4 0.69 58.66 0.1160 0.0490 0.9695 18 NS 4 0.69 56.98 0.1270 0.0424 0.9872 a the maximum solid-phase retention capacity of n-hexadecane in attachment site. b the attachment rate. Effect of flow velocity on retention of n-hexadecane in the calcareous soil samples As shown in Fig. 7, the maximum concentrations of n-hexadecane were retained in -12 cm, suggesting that n-hexadecane may be migrated downward by gravity force after leaching. The mass recovery rate of n-hexadecane effluent in CS/NS packed column was higher than in OS packed column (Table 3). Clearly, Cd/naphthalene reduced the retention content of n-hexadecane at various set flow velocity. Previous studies found that soil components such as SOM play an important role in retaining petroleum hydrocarbons (Adam et al., 2002; Cai et al., 2019). The DOM concentration of CS (313.10 mg/kg) was higher than that of OS (88.35 mg/kg) (Table S1), and the intensity of fulvic acid peak in CS was stronger than in OS (Fig. 3). It may be explained that the transport of hydrophobic substance was enhanced by increasing the concentration of fulvic acid (Dong et al., 2021; Sojitra et al., 1996; Yu et al., 2011). Moreover, batch experiments showed a higher n-hexadecane adsorption efficiency of NS than that of OS, since soil samples can expose additional attachment sites to n-hexadecane in shaking condition. However, adsorption sites are not available in column experiment, and the n-hexadecane transport may more affect by hydrodynamics (Wang et al., 2020b). In Fig. 7, when the flow velocity was 1 mL/min, the retention content of n-hexadecane in each soil column reached maximum. The effect of flow velocity on n-hexadecane retention is more pronounced at 4 mL/min. In Table 3, the mass recovery rate of n-hexadecane in effluent improved when flow velocity raised from 1 mL/min to 4 mL/min. Some studies also reported an increasing velocity result in an easier transport of contaminations in soil (Jiang et al., 2019; Yang et al., 2020). Macroscopically, the increasing flow velocity caused the decreasing residence of n-hexadecane in OS, CS and NS column, respectively. Conclusion This study provides valuable insights into the transport and retention of n-hexadecane in calcareous soil in karst areas. The results showed that the surface functional group and the mineralogical composition in calcareous soil did not change after Cd/naphthalene coating. Cd/naphthalene dramatically increased the adsorption efficiency of n-hexadecane on calcareous soil, but the function was limitation. The increasing pH value reduced the adsorption capacity in all groups due to the changed surface charges of soil samples and n-hexadecane. The higher transport efficiency and the lower retention efficiency of n-hexadecane in CS/NS depended on the more negative charge coating. Higher flow velocity is a key factor determining the fate of n-hexadecane in calcareous soils in karst areas. As a result, n-hexadecane is more likely to move far in cadmium-/naphthalene-contaminated calcareous soil with high flow velocity in karst areas and thereby impose serious risk to groundwater. Declarations Acknowledgments This study was financially supported by the National Natural Science Foundation of China (No. 51668006) and the Innovation Project of Guangxi Graduate Education (No. YCSW2021015 and No. YCBZ2021016). Statements & Declarations Funding This work was supported by the National Natural Science Foundation of China (No. 51668006) and the Innovation Project of Guangxi Graduate Education (No. YCSW2021015 and No. YCBZ2021016). Competing interests The authors have no relevant financial or non-financial interests to disclose. Author contributions All authors contributed to the study conception and design. All authors read and approved the final manuscript. Yiting Huang : Methodology; Software; Investigation; Writing-Original Draft; Visualization; Project administration. Yankui Tang : Conceptualization; Resources; Writing-Review & Editing; Supervision; Funding acquisition. Yi Liang : Methodology; Investigation; Software; Editing. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-2006218","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":133472236,"identity":"a23f4557-3f64-46fa-aa32-33f6d27d047d","order_by":0,"name":"Yiting Huang","email":"","orcid":"","institution":"School of Civil Engineering and Architecture, Guangxi University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Yiting","middleName":"","lastName":"Huang","suffix":""},{"id":133472237,"identity":"8aa9999c-f5a5-423e-9d28-02c731d17164","order_by":1,"name":"Yankui 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NS\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-2006218/v1/835cfac0e89c6e56ea61b3cb.png"},{"id":26118239,"identity":"a5fc1e09-3fbe-4799-980a-c4f1b0dbbe38","added_by":"auto","created_at":"2022-09-06 14:39:57","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":79958,"visible":true,"origin":"","legend":"\u003cp\u003eXRD pattern of OS, CS and NS\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-2006218/v1/5ae47fe37901cd4e38867b98.png"},{"id":26118991,"identity":"b5fbe4f4-bed2-45cc-8d26-0d7f54624d5a","added_by":"auto","created_at":"2022-09-06 14:49:56","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":194764,"visible":true,"origin":"","legend":"\u003cp\u003eFluorescence spectra of (a) OS, (b) CS and (c) NS\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig03.png","url":"https://assets-eu.researchsquare.com/files/rs-2006218/v1/73e01c7e231a2b8107c50509.png"},{"id":26118718,"identity":"c5ebe6fb-c687-47a4-b8ee-0d28f56f507f","added_by":"auto","created_at":"2022-09-06 14:44:56","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":118022,"visible":true,"origin":"","legend":"\u003cp\u003eFluorescence components of DOM in OS, CS and NS. Ⅰ. Aromatic protein, Ⅱ. Aromatic protein, Ⅲ. Fulvic acid-like, Ⅳ. Soluble microbial by-product-like, Ⅴ. Humic acid-like\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-2006218/v1/aea69d274867d25befbf7a63.png"},{"id":26118236,"identity":"9b4c3f6a-7e7d-4b63-946a-227b3dd0115d","added_by":"auto","created_at":"2022-09-06 14:39:56","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":64052,"visible":true,"origin":"","legend":"\u003cp\u003eAdsorption isotherm of n-hexadecane on (a) OS, (b) CS and (c) NS with various pH (5, 7, 9) fitted by Freundlich model. The ratio of soil sample and n-hexadecane solution was 1:50. The various concentration of n-hexadecane were set 50, 60, 80, 90 and 100 mg/L. Adsorption time was set 4 h (140 rpm, 25±1 ℃). Q\u003csub\u003ee \u003c/sub\u003eis the amount of n-hexadecane adsorbed by soil samples. C\u003csub\u003ee\u003c/sub\u003e is the concentration of n-hexadecane\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig05.png","url":"https://assets-eu.researchsquare.com/files/rs-2006218/v1/3618cf44b1e61672b15c66b1.png"},{"id":26117878,"identity":"67459345-797d-403d-8fde-61416756fdf7","added_by":"auto","created_at":"2022-09-06 14:34:56","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":72471,"visible":true,"origin":"","legend":"\u003cp\u003eThe breakthrough curves of n-hexadecane in (a) OS, (b) CS and (c) NS with the set flow velocity of 1, 2, and 4 mL/min. The influent concentration of n-hexadecane was set 100 mg/L. \u003cem\u003eC\u003c/em\u003e/\u003cem\u003eC\u003c/em\u003e\u003csub\u003e0 \u003c/sub\u003eis normalized effluent concentration, where \u003cem\u003eC\u003c/em\u003e and\u003cem\u003e C\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e were the concertation of n-hexadecane in the effluents and influents, respectively\u003c/p\u003e","description":"","filename":"Fig06.png","url":"https://assets-eu.researchsquare.com/files/rs-2006218/v1/8fcd7bf1c2909315f3b101ca.png"},{"id":26117879,"identity":"f722d824-5017-4596-93a1-9066eaf605e3","added_by":"auto","created_at":"2022-09-06 14:34:57","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":49942,"visible":true,"origin":"","legend":"\u003cp\u003eRetention profiles of n-hexadecane in (a) OS, (b) CS and (c) NS column under different flow velocity1, 2, and 4 mL/min. The influent concentration of n-hexadecane was set 100 mg/L\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig07.png","url":"https://assets-eu.researchsquare.com/files/rs-2006218/v1/455092a56687c6c4e0eeac2b.png"},{"id":44733631,"identity":"e26aedfb-abb0-46f2-8929-b8d54d0a0cdc","added_by":"auto","created_at":"2023-10-16 22:09:07","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1044979,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-2006218/v1/1e6f5f67-fd7b-48db-b0cf-e9eb0b449ef8.pdf"},{"id":26118237,"identity":"b6ef2cfa-8f79-4f44-a888-ef75c9ed5e92","added_by":"auto","created_at":"2022-09-06 14:39:56","extension":"doc","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":378368,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementartmaterials.doc","url":"https://assets-eu.researchsquare.com/files/rs-2006218/v1/98d784f337207c876ee0b743.doc"},{"id":26118719,"identity":"0dfc8b4a-e57d-4138-a7ac-b86ed02ca93f","added_by":"auto","created_at":"2022-09-06 14:44:57","extension":"doc","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":4568576,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementartmaterialsamendment.doc","url":"https://assets-eu.researchsquare.com/files/rs-2006218/v1/b7bada3630ce0c3889af10c0.doc"}],"financialInterests":"No competing interests reported.","formattedTitle":"Transport and retention of n-hexadecane in cadmium-/naphthalene-contaminated calcareous soil sampled in a karst area","fulltext":[{"header":"Introduction","content":"\u003cp\u003ePetroleum hydrocarbons are inevitably released into soils by the spill and leakage from tanks at petrol stations and industrial sites, then they transport downward along the soil layer and enter groundwater. The extensive and long-term petroleum hydrocarbons pollutants pose a threat such as carcinogenic, teratogenesis, and gene mutations to public health through the food chain (Wu et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Some works have found that the fate of petroleum hydrocarbons may definitely influence the remediation efficiency (Ossai et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). N-hexadecane, a hydrophobicity fraction represents the ignitability of diesel fuel, has been usually chosen as the model petroleum hydrocarbon by many researchers to study on the environmental risks and the remediation (Brils et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Samaei et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Samaei et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Thus, to better select the treatment option for remediation, it is necessary to investigate the transport and retention of n-hexadecane in soil environments.\u003c/p\u003e \u003cp\u003eTo date, the transport and retention of petroleum hydrocarbons in soils have been studied. Transport of petroleum hydrocarbons in quartz sand is promoted by increasing pH due to the increasing electrostatic repulsion between contaminations and particle surfaces (Wang et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2020b\u003c/span\u003e). Ca\u003csup\u003e2+\u003c/sup\u003e provides the adsorption sites of petroleum, thus elevated concentration of Ca\u003csup\u003e2+\u003c/sup\u003e can increase the retention of petroleum in soil (Luo et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Zhang et al., \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). The high-speed and short-term infiltration of petroleum hydrocarbons into deeper soil layers are also affected by the minimum content of clay and the lower ionic strength (Rosales et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Wang et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2020b\u003c/span\u003e). Furthermore, the stronger leaching as torrential rain and flood changes the release of petroleum hydrocarbons in soil (Dror et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). In fact, one serious challenge impacting the soil remediation of n-hexadecane is the presence of other contaminations. Contaminated natural soils with n-hexadecane at petrol stations and industrial sites have been usually found heavy metals or Polycyclic Aromatic Hydrocarbons (PAHs), and the mixture may bring more risk to public health (Khudur et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Li et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Heavy metals cause long-term pollution in soils due to the lower bioavailability (Jampasri et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). PAHs, the toxic compounds in diesel oil and crude oil with uniquely stable structures, still retain in soils for a long time after n-hexadecane have been degraded (Zhou et al., \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Moreover, heavy metals change the organic matter, water holding capacity and pH value of soils (Zhao et al., \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Zheng et al., \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Besides, hydrophobic PAHs change the properties of soils by covering with a thin insulating film (Steliga and Kluk, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), lowering porosity, increasing resistance to penetration, aggregating particles in plaques, etc (He et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Li et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Wu et al., \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). However, the influence of heavy metals/PAHs in soils on n-hexadecane transport has been overlooked by previous studies.\u003c/p\u003e \u003cp\u003eThe presence of dissolution fissures, crevices, and channels in karst areas cause stronger interaction between surface water and groundwater (Yang et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). As a result, the groundwater is more easily to be contaminated in karst areas than in non-karst areas (L\u0026uuml; et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Thus, it is necessary to pay more attention to the process of how petroleum hydrocarbons in soil enter groundwater, since calcareous soils in karst areas have been seriously contaminated by petroleum hydrocarbons (Guo et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Peng et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Moreover, as an stability derivative of PAHs (Chen et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Xu et al., \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), naphthalene has effectively retained in karst areas by abundant soil organic matter (SOM) (Sun et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Higher abnormal enrichment of cadmium (Cd) in karst than in non-karst due to the natural weathering of carbonate rock, the higher pH value, and the increasing soil organic carbon (SOC) (Albert et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Wen et al., \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Therefore, three types of calcareous soil samples (origin calcareous soil, Cd-contaminated calcareous soil and naphthalene-contaminated calcareous soil) were chosen for studying the transport and retention of n-hexadecane due to their different properties. The objectives of this study are to (1) reveal the effect of Cd/naphthalene and pH on n-hexadecane adsorption, (2) investigate the effect of Cd/naphthalene and flow velocity on the transport and retention of n-hexadecane, and (3) explore the mechanisms governing the transport of n-hexadecane in vertical direction of Cd-/naphthalene-contaminated calcareous soil. The results can contribute to the development of palliative measures and management strategies to reduce the risks of environmental health.\u003c/p\u003e"},{"header":"Methods And Materials","content":"\u003cp\u003eReagents\u003c/p\u003e \u003cp\u003eN-hexadecane and naphthalene were obtained from Shanghai Macklin Biochemical Co., Ltd with a purity\u0026thinsp;\u0026gt;\u0026thinsp;98%. Cadmium chloride was purchase from Tianjin Kermel Chemical Reagent Co., Ltd. KCl was purchased from Sinopharm Chemical Reagent Co., Ltd, Shanghai, China. Methanol, acetone, and n-hexane were supplied by T. JKE MAO Chemical Reagents CO., LTD. In this study, all reagents were at least analytical reagent, and all solutions were prepared using MIilli-Q water (resistance of 18.2 MΩ.cm at 25℃).\u003c/p\u003e \u003cp\u003ePreparation and characterization of calcareous soil samples\u003c/p\u003e \u003cp\u003eCalcareous soil samples were collected at the 0\u0026ndash;20 cm below the ground surface from Fusui, Guangxi Zhuang Autonomous Region, China (22\u0026deg;56'43.6\u0026Prime;N, 107\u0026deg;15'46.7\u0026Prime;E). Calcareous soil samples were air dried at room temperature (25\u0026thinsp;\u0026plusmn;\u0026thinsp;1℃) for one week, and they were sieved through a 2-mm mesh before experiments. The physical and chemical properties of the soil samples were provided in supplementary materials (Table S1). Table S1 presented that the background concentration of Cd in collected calcareous soils was below the detection limit of standard from China. N-hexadecane and naphthalene are not detected. According to Soil Environmental Quality Risk Control Standard for Soil Contamination of Development Land in China (risk screening values for soil contamination), the concentration of Cd and naphthalene was configured to be 20 mg/kg and 70 mg/kg in tested conditions, respectively. After soil samples and contamination solution were evenly mixed, the mixtures were placed in a fume hood for 90 days. The origin calcareous soil, Cd-contaminated calcareous soil and naphthalene-contaminated calcareous soil were marked as OS, CS and NS, respectively.\u003c/p\u003e \u003cp\u003eFourier transform infrared spectroscopy (FTIR, IRTracer-100, Shimadzu, Japan) was used to examine functional group in freeze-dried soil samples at wave numbers from 400 to 4000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. X-ray diffraction (D8 Dicover, Bruker Axs Gmbh, Germany) was used to analyze the crystalline constituents of freeze-dried soil samples. The concentration of dissolved organic matter (DOM) was detected by TOC-VCPH (Shimadzu, Japan). The zeta potential was measured by Nanobrook Ommi, Brookhaven (America). The fluorescence spectrometer (F-7000, Hitachi, Japan) was used to measure the three-dimensional excitation-emission matrix spectroscopy (3D-EEM) under the flowing conditions: excitation wavelengths of 200\u0026ndash;450 nm (every 5 nm), emission wavelengths of 250\u0026ndash;600 nm (every 1 nm), scan speed of 12000 nm/min, the excitation slit of 5 nm and emission slit of 5 nm.\u003c/p\u003e \u003cp\u003eBatch experiments\u003c/p\u003e \u003cp\u003eBefore batch experiment, 0.5, 1.0 and 2.0 g soil samples were respectively mixed with 50 mL of 100 mg/L n-hexadecane solution into 100 mL conical flasks, and the mixtures were shaken for 12 h at 140 rpm and 25\u0026thinsp;\u0026plusmn;\u0026thinsp;1 ℃. The ratio of soil samples to solution was selected 1.0 g for the stability of n-hexadecane quantification in batch experiments. In adsorption kinetic experiments, all conical flasks with parafilm (PM996, America) were shaken at various time scales (0.25, 0.5, 1, 2.5, 4, 8 and 12 h) in dark under the conditions of 140 rpm and 25\u0026thinsp;\u0026plusmn;\u0026thinsp;1 ℃. In desorption kinetic experiments, the residues of preliminary adsorption kinetic experiments were mixed with 50 mL solution without n-hexadecane, and the mixtures was shaken at the same conditions as adsorption kinetic experiments. In adsorption isotherm experiments, 1.0 g soil sample was transferred to 50 mL various concentrations of n-hexadecane solution (50, 60, 80, 90 and 100 mg/L), and then the mixture was shaken at 140 rpm (25\u0026thinsp;\u0026plusmn;\u0026thinsp;1 ℃) in dark for the equilibrium time, respectively. Moreover, the pH was adjusted with value of 5, 7, 9 using 0.1 mol/L HCl or NaOH for investigating the effect of pH on n-hexadecane adsorption behavior by isotherm experiments.\u003c/p\u003e \u003cp\u003eAll supernatants were collected after the mixtures were centrifuged at speed of 8000 rpm for 10 min. All the experiments were determined in triplicate using the standard experiment, and the concentration of n-hexadecane in liquid phase was detected by UV spectrophotometer (Shimadzu, Japan) at 257 nm.\u003c/p\u003e \u003cp\u003eColumn experiments\u003c/p\u003e \u003cp\u003eThe soil sample was wet-packed into a cylindrical column (3 cm inner diameter \u0026times; 12 cm length). The schematic of experimental setup for column experiment is shown in Fig. S1. A peristaltic pump was used to pump influents from the bottom into column. The packed column was firstly saturated with background solution (1 mM KCl solution, which was used to minimize any volatilization loss (Rosales et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2014\u003c/span\u003e)) with the set velocity (1, 2, and 4 mL/min) for 3 h pre-equilibration and air-removing. After that, 2.0 pore volumes (PVs) tracer (5 mM KCl solution) was injected into the column, and then several PVs of background solution was not stopped injecting until tracer was monitored to be zero. The tracer was measured by Conductivity Meter (DDSJ-308A, Shanghai INESA Scientific Instruments Co., Ltd, China). Breakthrough curves (BTCs) of tracer were used to determine longitudinal dispersion coefficient (cm\u003csup\u003e2\u003c/sup\u003e/min) in a column. Fig. S2 showed the BTCs of tracer can be well fitted with R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.99 by STANMOD software, and a high degree of symmetry in column. The BTCs peak of tracer nearly reached the inlet concentration (the ratio was 93.6%), suggesting that KCl was a non-reactive tracer in this work. The following step was, 2.0 PVs of n-hexadecane solution (100 mg/L) with background solution was pumped into column, and then the column was flushed by several PVs of background solution. The effluents of n-hexadecane were determined by UV spectrophotometer (Shimadzu, Japan) at 257 nm. The BTCs of n-hexadecane were expressed as relative concentration (\u003cem\u003eC\u003c/em\u003e/\u003cem\u003eC\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e) versus PV, where \u003cem\u003eC\u003c/em\u003e and \u003cem\u003eC\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e were the concertation of n-hexadecane in the effluents and influents, respectively. The BTCs of n-hexadecane were fitted by Hydrus-1D, and the details of this model were described by (Simunek et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). Moreover, two groups were set to assure the consistency the reproducibility of column experiment.\u003c/p\u003e \u003cp\u003eTo better know the residue of n-hexadecane in a column, the packed column was excavated in 12 layers (from top to bottom) after column experiment was finished, and each layer was determined by Ultrasonic Extraction and Gas Chromatograph (GC, Shimadzu GC-2010, Japan). The parameter of GC system was referred by Determination of Extractable Petroleum Hydrocarbons (C10-C40) in China (HJ 894\u0026ndash;2017).\u003c/p\u003e"},{"header":"Results And Discussion","content":"\u003ch2\u003eProperties of soil sample\u003c/h2\u003e\n\u003cp\u003eThe FTIR spectrum of OS, CS and NS is given in Fig. 1. The transmittance peak at 3443 cm\u003csup\u003e-1\u003c/sup\u003e, 1636 cm\u003csup\u003e-1\u003c/sup\u003e and 1034 cm\u003csup\u003e-1\u003c/sup\u003e represent the stretching of -OH, C=C / C=O and Si-O-Si, respectively. The XRD pattern of OS, CS and NS (Fig. 2) showed that all soil samples were composed of saponite, kaolinite, rodolicoite and vanadium oxide. The FTIR spectrum and XRD pattern of OS, CS and NS indicated that the coating of Cd/naphthalene did not change the surface functional group and the mineralogical composition of calcareous soils. Table 1 is the zeta potential of OS, CS and NS. The OS, CS and NS are all negatively charged in the condition of this work, and the presence of n-hexadecane increased the negative surface potentials of OS, CS and NS since surface coating by hydrophobic compounds can change the zeta potential of porous media (Song et al., 2011; Yang et al., 2017b).\u003c/p\u003e\n\u003cp\u003e \u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 1\u003c/strong\u003e The zeta potential of all soil samples\u003c/p\u003e\n\u003ctable width=\"0\"\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd rowspan=\"2\" width=\"154\"\u003e\n\u003cp\u003econcentration of n-hexadecane\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd colspan=\"3\" width=\"406\"\u003e\n\u003cp\u003eZeta potentials (mV)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"139\"\u003e\n\u003cp\u003eCS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"139\"\u003e\n\u003cp\u003eOS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"127\"\u003e\n\u003cp\u003eNS\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"154\"\u003e\n\u003cp\u003e0 mg/L\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"139\"\u003e\n\u003cp\u003e-15.84\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"139\"\u003e\n\u003cp\u003e-20.76\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"127\"\u003e\n\u003cp\u003e-23.03\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"154\"\u003e\n\u003cp\u003e100 mg/L\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"139\"\u003e\n\u003cp\u003e-29.11\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"139\"\u003e\n\u003cp\u003e-23.47\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"127\"\u003e\n\u003cp\u003e-24.63\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eThe DOM concentration of CS (313.10 mg/kg) and NS (331.30 mg/kg) was higher than that of OS (88.35 mg/kg) (Table S1), suggesting that microbial communities in calcareous soil may respond to Cd/naphthalene (Schwarz et al., 2011; Wang et al., 2020a). DOM may control the fate of n-hexadecane in calcareous soil due to its amphipathic molecules. Fluorescence spectra of OS, CS and NS (Fig. 3) showed the intensity of tyrosine-like fluorescent peak in CS/NS were more significant than in OS, suggesting that CS/NS have more aromatic ring amino acids (Yan et al., 2019). The intensity of tryptophan-like fluorescent peak in CS/OS was stronger than in NS. Fulvic acid relates to the carbonyl and carboxyl groups, and it can increase the solubility of hydrocarbon (Lu et al., 2013). Clearly, the intensity of fulvic acid peak in CS was greater than in OS/NS. Fluorescence components of DOM in OS, CS and NS were shown in Fig. 4. It can be seen that the humic acid content of NS was smaller than that of OS/CS. It may result in the greater adsorption capacity of NS since the humic acid can generally inhibit the adsorption of hydrocarbon on porous media (Wang et al., 2017).\u003c/p\u003e\n\u003cp\u003e \u003c/p\u003e\n\u003ch2\u003eAdsorption and desorption kinetic of hexadecane on calcareous soil samples\u003c/h2\u003e\n\u003cp\u003eN-hexadecane may be firstly trapped in calcareous soil through molecular adsorption, and later desorption may constitute the rate determining step. All samples were fitted well by the pseudo-first-order dynamics model with R\u003csup\u003e2\u003c/sup\u003e\u0026gt; 0.9 (Fig. S3 and Table S2). Adsorption equilibrium time for n-hexadecane on OS, CS and NS were all 4 h, suggesting that the presence of Cd/naphthalene cannot change the adsorption equilibrium time. It may be causing by the homologous properties of calcareous soil samples (Yang et al., 2013). The adsorption rate constant (\u003cem\u003ek\u003csub\u003e1\u003c/sub\u003e\u003c/em\u003e) of OS was higher than that of CS/NS, which means Cd/naphthalene can increase the adsorption efficiency of n-hexadecane on calcareous soils. Desorption equilibrium time for n-hexadecane on OS, CS and NS were all 1 h, indicating that Cd/naphthalene cannot change the desorption equilibrium time. The \u003cem\u003ek\u003csub\u003e1 \u003c/sub\u003e\u003c/em\u003eof n-hexadecane adsorption on OS was lower than CS/NS (Table S2), which means Cd/naphthalene can reduce the desorption efficiency of n-hexadecane on calcareous soils. In all cases, adsorption efficiency was higher than desorption efficiency, suggesting the desorption behavior of n-hexadecane cannot greatly influence the adsorption behavior in this work. On the whole, Cd/naphthalene can improve the adsorption capacity and decrease the desorption capacity of n-hexadecane on calcareous soils, but they cannot change the equilibrium time.\u003c/p\u003e\n\u003ch2\u003eEffect of pH on the adsorption isotherm of n-hexadecane on the calcareous soil samples\u003c/h2\u003e\n\u003cp\u003eIn most cases, Freundlich (Fig. 5 and Table 2) is better than Langmuir (Fig. S4 and Table S3), which means the adsorption behavior of n-hexadecane on soil samples may not dependent on monolayer adsorption. In Table 2,\u003cem\u003e K\u003csub\u003ef\u003c/sub\u003e\u003c/em\u003e value (at pH=7) of n-hexadecane adsorption on soil samples was lower than that of on loess soil (Jiang et al., 2016) and modified diatomite (Xu et al., 2020). It can be summarized that Cd/naphthalene cannot obviously improve the adsorption efficiency of n-hexadecane on calcareous soils in comparison with other soils. At pH=7, \u003cem\u003eK\u003csub\u003ef\u003c/sub\u003e\u003c/em\u003e value of OS was lower than that of CS/NS, suggesting that Cd/naphthalene can improve the higher n-hexadecane adsorption capacity on calcareous soil. Previous studies have also found that adding metals can increase the adsorption of hydrocarbon on porous medias (Saeedi et al., 2018). Cd-coating porous medias can enhance the adsorption of anionic and neutral hydrocarbon compounds since the polar functional group in porous media was complexation with Cd (Wang et al., 2017). However, in this study, the surface functional group in calcareous soils did not change after Cd/naphthalene coating, therefore the improved adsorption efficiency may not absolutely depend on function group\u0026rsquo;s change. Furthermore, the aggregation and hydrophobicity of porous media causing by Cd-coating can increase the adsorption efficiency of hydrocarbons (Nguyen et al., 2013). The negative charge of porous media surface can be shielded after coating naphthalene (Yang et al., 2017a), therefore the adsorption of n-hexadecane was inhibited. Moreover, the \u003cem\u003en\u003c/em\u003e value of all cases was lower than 1, indicating that adsorption sites in the surface of all soil samples (OS, CS and NS) were limitation although Cd/naphthalene can open more adsorption sites for n-hexadecane.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 2\u003c/strong\u003e The parameters of Freundlich\u003c/p\u003e\n\u003ctable width=\"0\"\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd width=\"92\"\u003e\n\u003cp\u003eSoil sample\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"52\"\u003e\n\u003cp\u003epH\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"164\"\u003e\n\u003cp\u003e\u003cem\u003eK\u003csub\u003ef \u003c/sub\u003e\u003c/em\u003e\u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"130\"\u003e\n\u003cp\u003e1/\u003cem\u003en \u003c/em\u003e\u003csup\u003eb\u003c/sup\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"109\"\u003e\n\u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd rowspan=\"3\" width=\"92\"\u003e\n\u003cp\u003eOS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"52\"\u003e\n\u003cp\u003e5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"164\"\u003e\n\u003cp\u003e0.10\u0026plusmn;0.15\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"130\"\u003e\n\u003cp\u003e2.30\u0026plusmn;0.26\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"109\"\u003e\n\u003cp\u003e0.95\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"52\"\u003e\n\u003cp\u003e7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"164\"\u003e\n\u003cp\u003e0.00\u0026plusmn;0.01\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"130\"\u003e\n\u003cp\u003e2.10\u0026plusmn;0.36\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"109\"\u003e\n\u003cp\u003e0.98\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"52\"\u003e\n\u003cp\u003e9\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"164\"\u003e\n\u003cp\u003e0.02\u0026plusmn;0.03\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"130\"\u003e\n\u003cp\u003e2.76\u0026plusmn;0.32\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"109\"\u003e\n\u003cp\u003e0.98\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd rowspan=\"3\" width=\"92\"\u003e\n\u003cp\u003eCS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"52\"\u003e\n\u003cp\u003e5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"164\"\u003e\n\u003cp\u003e0.26\u0026plusmn;0.38\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"130\"\u003e\n\u003cp\u003e1.89\u0026plusmn;0.33\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"109\"\u003e\n\u003cp\u003e0.95\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"52\"\u003e\n\u003cp\u003e7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"164\"\u003e\n\u003cp\u003e0.05\u0026plusmn;0.08\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"130\"\u003e\n\u003cp\u003e2.27\u0026plusmn;0.42\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"109\"\u003e\n\u003cp\u003e0.95\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"52\"\u003e\n\u003cp\u003e9\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"164\"\u003e\n\u003cp\u003e0.01\u0026plusmn;0.02\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"130\"\u003e\n\u003cp\u003e2.48\u0026plusmn;0.25\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"109\"\u003e\n\u003cp\u003e0.98\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd rowspan=\"3\" width=\"92\"\u003e\n\u003cp\u003eNS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"52\"\u003e\n\u003cp\u003e5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"164\"\u003e\n\u003cp\u003e1.27\u0026plusmn;1.40\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"130\"\u003e\n\u003cp\u003e1.56\u0026plusmn;0.25\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"109\"\u003e\n\u003cp\u003e0.95\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"52\"\u003e\n\u003cp\u003e7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"164\"\u003e\n\u003cp\u003e0.82\u0026plusmn;0.82\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"130\"\u003e\n\u003cp\u003e1.65\u0026plusmn;0.22\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"109\"\u003e\n\u003cp\u003e0.96\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"52\"\u003e\n\u003cp\u003e9\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"164\"\u003e\n\u003cp\u003e0.06\u0026plusmn;0.09\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"130\"\u003e\n\u003cp\u003e2.17\u0026plusmn;0.30\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"109\"\u003e\n\u003cp\u003e0.95\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u003csup\u003ea\u003c/sup\u003e \u003cem\u003eK\u003csub\u003ef\u003c/sub\u003e \u003c/em\u003eis the coefficient of Freundlich which positively associated with adsorption capacity.\u003c/p\u003e\n\u003cp\u003e\u003csup\u003eb\u003c/sup\u003e \u003cem\u003en\u003c/em\u003e is the sorption intensity.\u003c/p\u003e\n\u003cp\u003eIn recent years, rainwater with higher pH value (pH=8.0) was found in a typical karst area (Zeng et al., 2020). Furthermore, previous studies have found that the adsorption of diesel oil on loess soil was weakened by increasing pH (Jiang et al., 2016) since pH improve the dissolution and dispersion of diesel oil (Delle Site, 2001; Pradubmook et al., 2003). However, the adsorption of naphthalene on biochar colloid had almost no effected by pH (Yang et al., 2017b) due to the little dissociation degree of nonpolar hydrocarbons in polar solution (Grządka, 2011). Therefore, the potential influence of pH on the n-hexadecane adsorption was investigated in this study. As shown in Fig. 5 and Table 2, it can be observed that the elevated pH (from 5 to 9) caused decreasing adsorption efficiency of n-hexadecane on OS, CS and NS. In this study, pH value was set from 5 to 9, thus the adsorption of n-hexadecane on each soil sample at pH=5 was the maximum. The adsorption of n-hexadecane on OS was weaker than CS/NS at pH=5 and pH=7. The\u003cem\u003e K\u003csub\u003ef\u003c/sub\u003e\u003c/em\u003e value of OS was higher than CS in alkaline environment (at pH=9). The n-hexadecane adsorption on NS remained stable from 5 to 7, but it was changed from pH=7 to pH=9. Some studies reported that the release of Cd/naphthalene in soils depended on pH since the bonds between contaminations and soils were broken by pH (Kicińska et al., 2022; Yang et al., 2001). Furthermore, the release of Cd/naphthalene in soils may open more adsorption sites for n-hexadecane. Moreover, the presence of impurities in n-hexadecane, such as long-chain carboxylic acids (Fang et al., 2015), and the unbalanced hydrophobic/hydrophilic properties at oil/water boundary in solution (Li and Bhushan, 2015) may influence the adsorption of hydroxyl ions (OH\u003csup\u003e-\u003c/sup\u003e), thus n-hexadecane carries negative surface charge. The electronic mobility of the n-hexadecane and the adsorption sites on soil samples may change along with the increasing pH at aqueous phase (Kim et al., 2012; Li and Bhushan, 2015). Therefore, pH can influence the adsorption behavior of n-hexadecane on soil samples.\u003c/p\u003e\n\u003ch2\u003eEffect of flow velocity on transport of n-hexadecane in the calcareous soil samples\u003c/h2\u003e\n\u003cp\u003eThe BTCs of n-hexadecane in the calcareous soil samples with the addition of various flow velocity are presented in Fig. 6. As shown in Fig. 6, all BTCs were symmetric in shape, implying that there was the physical equilibrium transport in column. The maximum value of \u003cem\u003eC\u003c/em\u003e/\u003cem\u003eC\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e were 28 %, 35 % and 48 % in OS, CS and NS with the set flow velocity of 1 mL/min, respectively, suggesting n-hexadecane may more effectively breakthrough the column of CS/NS than that of OS. Similarly, the maximum value of \u003cem\u003eC\u003c/em\u003e/\u003cem\u003eC\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e in CS/NS was higher than in OS at the set flow velocity of 2 and 4 mL/min. The two kinetic sites model of Hydrus-1D well described the BTCs of n-hexadecane in each soil sample with different Darcy velocity according to the higher correlation coefficient (R\u003csup\u003e2\u003c/sup\u003e) between observed and simulated data (Table 3). The characteristic of BTCs for n-hexadecane in all soil sample column can be reflected by \u003cem\u003eS\u003csub\u003emax\u003c/sub\u003e\u003c/em\u003e and \u003cem\u003eK\u003csub\u003eatt\u003c/sub\u003e\u003c/em\u003e of Hydrus-1D (Table 3). In all case, the \u003cem\u003eS\u003csub\u003emax\u003c/sub\u003e\u003c/em\u003e value of OS was higher than that of CS/NS, which means greater irreversible retention of n-hexadecane in OS. The greater value of \u003cem\u003eK\u003csub\u003eatt\u003c/sub\u003e\u003c/em\u003e for CS/NS at all Darcy velocity reflects a rapid release process of n-hexadecane. The mass recovery rate of n-hexadecane in effluent are showed in Table 3. There is a higher mass recovery rate for n-hexadecane in the effluent of CS/NS packed column than that of OS packed column. On the whole, the transport of n-hexadecane in soil samples followed the order of CS/NS \u0026gt; OS at the same Darcy velocity, which means Cd/naphthalene can improve the transport efficiency of n-hexadecane. In Table 2, negative charge density of CS/NS was more than that of OS. The more negative charge of soil samples, the stronger electrostatic repulsions between the soil samples and the negatively charged n-hexadecane were, which is likely responsible for promoting the transport of n-hexadecane in CS/NS packed column (Wu et al., 2020). Furthermore, although naphthalene-coating cannot dramatically change the surface negative charge of calcareous soils, nonpolar naphthalene can lead to charge-shielding (Yang et al., 2017b).\u003c/p\u003e\n\u003cp\u003eThe previous studies have proved that the transport of petroleum hydrocarbons with negative charge were inhibited by the decreasing pH due to the reduction in electrostatic repulsion (Cai et al., 2017; Wang et al., 2020b). Our previous batch experiments also showed that the electronic mobility of the n-hexadecane increased along with the increasing pH at aqueous system (Fig. 5 and Table 2). Moreover, the unique hydrological and geological structures in karst areas lead to variable flow velocity, and pollutants are more easily to rapidly transport in the pipeline systems of karst (Jiang et al., 2018). Therefore, this section only investigated the potential influence of flow velocity on the n-hexadecane transport in Cd-/naphthalene-contaminated calcareous soils. In Fig. 6, high value of \u003cem\u003eC\u003c/em\u003e/\u003cem\u003eC\u003c/em\u003e\u003csub\u003e0 \u003c/sub\u003efor n-hexadecane in OS, CS and NS were observed along with high flow velocity, which means the amount of n-hexadecane transport increased with the increasing flow velocity. Similar results had observed by some studies (Alazaiza et al., 2021; Alazaiza et al., 2020). In Table 3, it should be pointed out that the value of \u003cem\u003eS\u003csub\u003emax\u003c/sub\u003e\u003c/em\u003e decreased with high Darcy flow (0.69 cm/min) while the value of \u003cem\u003eK\u003csub\u003eatt\u003c/sub\u003e\u003c/em\u003e was to be the minimum at Darcy flow of 0.12 cm/min. It suggested that smaller Darcy velocity may improve the irreversible retention but inhibit the reversible retention of n-hexadecane in calcareous soil samples. The mass recovery rate of n-hexadecane transport increased when the set flow velocity reached 4 mL/min (Table 3), indicating the transport of n-hexadecane can be improved. Overall, n-hexadecane was more likely to break through CS/NS column with a high flow velocity.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 3\u003c/strong\u003e the parameters of fitted model\u003c/p\u003e\n\u003ctable width=\"0\"\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003eColumn No.\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eSoil sample\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003eSet flow (mL/min)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003eDarcy velocity (cm/min)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003eThe mass recovery rate (%)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e\u003cem\u003eS\u003csub\u003emax1\u003c/sub\u003e\u003c/em\u003e (mg/g) \u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e\u003cem\u003eK\u003csub\u003eatt \u003c/sub\u003e\u003c/em\u003e(min\u003csup\u003e-1\u003c/sup\u003e) \u003csup\u003eb\u003c/sup\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003e1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eOS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003e1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003e0.12\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003e30.96\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e0.3384\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.0224\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.9711\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003e2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eOS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003e1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003e0.12\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003e27.81\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e0.3896\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.0279\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.9401\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003e3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eOS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003e2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003e0.31\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003e32.87\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e0.2855\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.0825\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.9827\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003e4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eOS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003e2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003e0.31\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003e35.14\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e0.2639\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.0861\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.9429\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003e5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eOS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003e4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003e0.69\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003e42.08\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e0.2368\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.0886\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.9818\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003e6\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eOS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003e4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003e0.69\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003e40.03\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e0.2100\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.0827\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.9598\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003e7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eCS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003e1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003e0.12\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003e30.80\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e0.2512\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.0172\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.9719\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003e8\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eCS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003e1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003e0.12\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003e28.99\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e0.2368\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.0216\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.9768\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003e9\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eCS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003e2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003e0.31\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003e35.36\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e0.1639\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.0377\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.9771\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003e10\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eCS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003e2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003e0.31\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003e37.51\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e0.1468\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.0361\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.9524\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003e11\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eCS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003e4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003e0.69\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003e57.68\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e0.1270\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.0652\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.9429\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003e12\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eCS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003e4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003e0.69\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003e54.21\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e0.1011\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.0681\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.9577\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003e13\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eNS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003e1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003e0.12\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003e43.98\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e0.2377\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.0125\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.9681\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003e14\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eNS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003e1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003e0.12\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003e40.11\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e0.2033\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.0153\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.9767\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003e15\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eNS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003e2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003e0.31\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003e56.98\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e0.1309\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.0394\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.9888\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003e16\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eNS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003e2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003e0.31\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003e57.19\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e0.1214\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.0365\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.9459\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003e17\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eNS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003e4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003e0.69\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003e58.66\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e0.1160\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.0490\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.9695\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"83\"\u003e\n\u003cp\u003e18\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"104\"\u003e\n\u003cp\u003eNS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"113\"\u003e\n\u003cp\u003e4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"142\"\u003e\n\u003cp\u003e0.69\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"186\"\u003e\n\u003cp\u003e56.98\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"120\"\u003e\n\u003cp\u003e0.1270\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.0424\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"118\"\u003e\n\u003cp\u003e0.9872\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u003csup\u003ea\u003c/sup\u003e the maximum solid-phase retention capacity of n-hexadecane in attachment site.\u003c/p\u003e\n\u003cp\u003e\u003csup\u003eb\u003c/sup\u003e the attachment rate.\u003c/p\u003e\n\u003ch2\u003eEffect of flow velocity on retention of n-hexadecane in the calcareous soil samples\u003c/h2\u003e\n\u003cp\u003eAs shown in Fig. 7, the maximum concentrations of n-hexadecane were retained in -12 cm, suggesting that n-hexadecane may be migrated downward by gravity force after leaching. The mass recovery rate of n-hexadecane effluent in CS/NS packed column was higher than in OS packed column (Table 3). Clearly, Cd/naphthalene reduced the retention content of n-hexadecane at various set flow velocity. Previous studies found that soil components such as SOM play an important role in retaining petroleum hydrocarbons (Adam et al., 2002; Cai et al., 2019). The DOM concentration of CS (313.10 mg/kg) was higher than that of OS (88.35 mg/kg) (Table S1), and the intensity of fulvic acid peak in CS was stronger than in OS (Fig. 3). It may be explained that the transport of hydrophobic substance was enhanced by increasing the concentration of fulvic acid (Dong et al., 2021; Sojitra et al., 1996; Yu et al., 2011). Moreover, batch experiments showed a higher n-hexadecane adsorption efficiency of NS than that of OS, since soil samples can expose additional attachment sites to n-hexadecane in shaking condition. However, adsorption sites are not available in column experiment, and the n-hexadecane transport may more affect by hydrodynamics (Wang et al., 2020b). In Fig. 7, when the flow velocity was 1 mL/min, the retention content of n-hexadecane in each soil column reached maximum. The effect of flow velocity on n-hexadecane retention is more pronounced at 4 mL/min. In Table 3, the mass recovery rate of n-hexadecane in effluent improved when flow velocity raised from 1 mL/min to 4 mL/min. Some studies also reported an increasing velocity result in an easier transport of contaminations in soil (Jiang et al., 2019; Yang et al., 2020). Macroscopically, the increasing flow velocity caused the decreasing residence of n-hexadecane in OS, CS and NS column, respectively.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThis study provides valuable insights into the transport and retention of n-hexadecane in calcareous soil in karst areas. The results showed that the surface functional group and the mineralogical composition in calcareous soil did not change after Cd/naphthalene coating. Cd/naphthalene dramatically increased the adsorption efficiency of n-hexadecane on calcareous soil, but the function was limitation. The increasing pH value reduced the adsorption capacity in all groups due to the changed surface charges of soil samples and n-hexadecane. The higher transport efficiency and the lower retention efficiency of n-hexadecane in CS/NS depended on the more negative charge coating. Higher flow velocity is a key factor determining the fate of n-hexadecane in calcareous soils in karst areas. As a result, n-hexadecane is more likely to move far in cadmium-/naphthalene-contaminated calcareous soil with high flow velocity in karst areas and thereby impose serious risk to groundwater.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eAcknowledgments\u003c/p\u003e\n\u003cp\u003eThis study was financially supported by the National Natural Science Foundation of China (No. 51668006) and the Innovation Project of Guangxi Graduate Education (No. YCSW2021015 and No. YCBZ2021016).\u003c/p\u003e\n\u003cp\u003eStatements \u0026amp; Declarations\u003c/p\u003e\n\u003cp\u003eFunding\u003c/p\u003e\n\u003cp\u003eThis work was supported by\u0026nbsp;the National Natural Science Foundation of China (No. 51668006) and the Innovation Project of Guangxi Graduate Education (No. YCSW2021015 and No. YCBZ2021016).\u003c/p\u003e\n\u003cp\u003eCompeting interests\u003c/p\u003e\n\u003cp\u003eThe authors have no relevant financial or non-financial interests to disclose.\u003c/p\u003e\n\u003cp\u003eAuthor contributions\u003c/p\u003e\n\u003cp\u003eAll authors contributed to the study conception and design. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eYiting Huang\u003c/strong\u003e: Methodology; Software; Investigation; Writing-Original Draft; Visualization; Project administration.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eYankui Tang\u003c/strong\u003e: Conceptualization; Resources; Writing-Review \u0026amp; Editing; Supervision; Funding acquisition.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eYi Liang\u003c/strong\u003e: Methodology; Investigation; Software; Editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eZhenze Xie\u003c/strong\u003e: Methodology; Material preparation; Investigation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eJipeng Wu\u003c/strong\u003e: Data collection; Analysis.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eJiajie Huang\u003c/strong\u003e: Data collection; Experiment.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eShanxiong Wei\u003c/strong\u003e: Experiment; Editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eShaojiang Nie\u003c/strong\u003e: Experiment; Software.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTao Jiang\u003c/strong\u003e: Experiment.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAdam, G., Gamoh, K., Morris, D.G., Duncan, H., 2002. 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Environmental Science and Pollution Research, 26(32): 33192\u0026ndash;33201.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"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":"environmental-geochemistry-and-health","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"egah","sideBox":"Learn more about [Environmental Geochemistry and Health](https://www.springer.com/journal/10653)","snPcode":"10653","submissionUrl":"https://submission.nature.com/new-submission/10653/3","title":"Environmental Geochemistry and Health","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"N-hexadecane, Freundlich, pH, Column experiments, Flow velocity, Hydus-1D","lastPublishedDoi":"10.21203/rs.3.rs-2006218/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-2006218/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003ePetroleum hydrocarbon pollutants in karst areas have aroused widespread concern due to their toxicity. It is crucial to gain knowledge on transport and retention of petroleum hydrocarbons in karst areas. Calcareous soils in karst areas were contaminated by cadmium/naphthalene due to the industrial and agricultural activities, however, the fates of petroleum hydrocarbons in these contaminated calcareous soils have been rarely studied. In this study, n-hexadecane was selected as a model petroleum hydrocarbon. Batch experiments were conducted to explore the adsorption behavior of n-hexadecane on cadmium-/naphthalene-contaminated calcareous soils at various pH, and column experiments were performed to investigate the transport and retention of n-hexadecane under various flow velocity. The results showed that Freundlich model can well describe the adsorption behavior of n-hexadecane on all samples (R\u003csup\u003e2\u003c/sup\u003e\u0026gt;0.9). According to the adsorption coefficient (\u003cem\u003eK\u003c/em\u003e\u003csub\u003e\u003cem\u003ef\u003c/em\u003e\u003c/sub\u003e), the presence of cadmium/naphthalene dramatically increased the adsorption efficiency of n-hexadecane on calcareous soils, and the increasing pH value reduced the adsorption capacity in all groups. The transport of n-hexadecane in all samples were well described by two kinetic sites model of Hydrus-1D with R\u003csup\u003e2\u003c/sup\u003e\u0026gt;0.9. The higher effluent concentration of n-hexadecane indicates that n-hexadecane can more effectively break through the cadmium-/naphthalene-contaminated calcareous soil with high flow velocity, which means the lower n-hexadecane retention in this situation. The observation can be explained by the electrostatic repulsion between n-hexadecane and each soil sample. These findings have important implications for the government of groundwater in calcareous soils from karst areas.\u003c/p\u003e","manuscriptTitle":"Transport and retention of n-hexadecane in cadmium-/naphthalene-contaminated calcareous soil sampled in a karst area","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2022-09-06 14:34:54","doi":"10.21203/rs.3.rs-2006218/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major revision","date":"2023-05-15T11:17:18+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2023-05-08T07:51:33+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"3c3db035-356c-4b13-80c5-6c03e8b9d7b9","date":"2023-05-08T07:36:47+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2022-09-12T16:07:01+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2022-09-04T05:17:29+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2022-09-01T16:49:27+00:00","index":"","fulltext":""},{"type":"submitted","content":"Environmental Geochemistry and Health","date":"2022-08-28T08:29:19+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"environmental-geochemistry-and-health","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"egah","sideBox":"Learn more about [Environmental Geochemistry and Health](https://www.springer.com/journal/10653)","snPcode":"10653","submissionUrl":"https://submission.nature.com/new-submission/10653/3","title":"Environmental Geochemistry and Health","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"4e02e126-51ca-4e8b-9e46-fc79bae2cabf","owner":[],"postedDate":"September 6th, 2022","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2023-10-16T21:50:20+00:00","versionOfRecord":{"articleIdentity":"rs-2006218","link":"https://doi.org/10.1007/s10653-023-01664-y","journal":{"identity":"environmental-geochemistry-and-health","isVorOnly":false,"title":"Environmental Geochemistry and Health"},"publishedOn":"2023-06-26 21:24:42","publishedOnDateReadable":"June 26th, 2023"},"versionCreatedAt":"2022-09-06 14:34:54","video":"","vorDoi":"10.1007/s10653-023-01664-y","vorDoiUrl":"https://doi.org/10.1007/s10653-023-01664-y","workflowStages":[]},"version":"v1","identity":"rs-2006218","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-2006218","identity":"rs-2006218","version":["v1"]},"buildId":"FbvkV6FR0MCFSLy54lSbu","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}