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

Studying the transport of petroleum hydrocarbons in cadmium-/naphthalene-contaminated calcareous soils is crucial to comprehensive assessment of environmental risks and developing appropriate strategies to remediate petroleum hydrocarbons pollution in karst areas. 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 better described the adsorption behavior of n-hexadecane in all cases (R2 > 0.9). Under the condition of pH = 5, it was advantageous for soil samples to adsorb more n-hexadecane, and the maximum adsorption content followed the order of: cadmium/naphthalene-contaminated > uncontaminated soils. The transport of n-hexadecane in cadmium/naphthalene-contaminated soils at various flow velocity was well described by two kinetic sites model of Hydrus-1D with R2 > 0.9. Due to the increased electrostatic repulsion between n-hexadecane and soil particles, n-hexadecane was more easily able to breakthrough cadmium/naphthalene-contaminated soils. Compared to low flow velocity (1 mL/min), a higher concentration of n-hexadecane was determined at high flow velocity, with 67, 63, and 45% n-hexadecane in effluent from cadmium-contaminated soils, naphthalene-contaminated soils, and uncontaminated soils, respectively. These findings have important implications for the government of groundwater in calcareous soils from karst areas.


Introduction
Petroleum hydrocarbons are inevitably released into soils by the spill and leakage from tanks at petrol stations and industrial sites.The widespread and longterm presence of petroleum hydrocarbon pollutants poses significant threats to public health, including carcinogenicity, teratogenicity, and gene mutations, which can be transmitted through the food chain (Wu et al., 2017).Petroleum hydrocarbons can be Vol:.( 1234567890) transported to deeper soil layers through the force of rain-runoff.Subsequently, it is difficult to determine the scope of contamination when designing strategies of soil remediation (Ossai et al., 2020).
To date, many studies have reported on the transport behavior of petroleum hydrocarbons in the soils to support remediation technologies.For example, the clay content, pH, ionic strength, and concentration of Ca 2+ in soils have been found to result in the migration of petroleum hydrocarbons (Luo et al., 2022;Rosales et al., 2014;Wang et al., 2020b;Zhang et al., 2012).Additionally, the characteristics of soil samples also play an important role in the transport of petroleum hydrocarbons.Wang et al., 2020b have reported that the petroleum colloids were more easily retained in sandy soil compared to quartz.In fact, one of the factors impacting the transport of petroleum hydrocarbons is the co-occurrence of other contaminations.The soils contaminated with petroleum hydrocarbons at petrol stations and industrial sites have been found to contain heavy metals or Polycyclic Aromatic Hydrocarbons (PAHs), which has been found to pose additional risks to public health (Khudur et al., 2018;Li et al., 2021).Heavy metals, due to their lower bioavailability, contribute to long-term soil pollution (Jampasri et al., 2016).PAHs, the toxic compounds with structurally stable characteristics present in diesel oil and crude oil, have persisted in soils even after the degradation of other hydrocarbons (Zhou et al., 2019).
Heavy metals alter the organic matter, water holding capacity and pH value of soils (Zhao et al., 2017;Zheng et al., 2016).Hydrophobic PAHs modify soil properties by forming a thin insulating film, reducing porosity, increasing the resistance of penetration, and promoting the formation of aggregated particles (He et al., 2013;Li et al., 2012;Steliga & Kluk, 2020;Wu et al., 2013).As a consequence, the transport of petroleum hydrocarbons in these contaminated soils may become more and more complex.Some studies have showed that the presence of heavy metals increased the mobility of petroleum hydrocarbons (Saeedi et al., 2018a(Saeedi et al., , 2018b)), while in some cases, there was only a slight relationship between the retention and distribution of petroleum hydrocarbons and heavy metals (Eeshwarasinghe et al., 2019;Huang et al., 2019).Furthermore, different components of petroleum hydrocarbons exhibit various migration behaviors in soil environments (Li et al., 2022;Zhang et al., 2022).
However, the interaction of coexistence compounds has been overlooked.To comprehensively support the soil remediation, it is crucial to gain more knowledge on petroleum hydrocarbons migration in contaminated soils.
The presence of dissolution fissures, crevices, and channels in karst areas enhances the interaction between surface water and groundwater (Yang et al., 2019).Consequently, groundwater is more susceptible to contamination in karst areas compared to nonkarst areas (Lü et al., 2020).Therefore, it is crucial to investigate the processes by which petroleum hydrocarbons enter calcareous soil layers in karst areas where severe contamination has been reported (Guo et al., 2020;Peng et al., 2022).Additionally, naphthalene, as a stability derivative of PAHs, has been found to be effectively retained in karst areas (Chen et al., 2009;Sun et al., 2019;Xu et al., 2022).The higher abnormal enrichment of cadmium (Cd) in karst areas compared to non-karst attributes to natural weathering of carbonate rock, higher pH values, and increasing soil organic carbon (SOC) content (Albert et al., 2021;Wen et al., 2020).
In this work, n-hexadecane was selected as a model petroleum hydrocarbon.Three types of calcareous soil samples (original 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 on n-hexadecane adsorption under various pH, (2) investigate the effect of Cd/naphthalene on the transport and retention of n-hexadecane at various flow velocity, 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.

Reagents and instrumentation
N-hexadecane and naphthalene were obtained from Shanghai Macklin Biochemical Co., Ltd with a purity greater than 98%.Cadmium chloride was purchase Vol.: (0123456789) 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 grade and all solutions were prepared using Milli-Q water (with a resistance of 18.2 MΩ cm at 25 °C).
Fourier transform infrared spectroscopy (FTIR, IRTracer-100, Shimadzu, Japan) was used to examine the functional group in the freeze-dried soil samples at wave numbers from 400 to 4000 cm −1 .X-ray diffraction (D8 Discover, Bruker Axs Gmbh, Germany) was used to analyze the crystalline constituents of the 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 Omni, 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 (Ex) wavelengths of 200-450 nm (every 5 nm), emission (Em) wavelengths of 250-600 nm (every 1 nm), scan speed of 12,000 nm/min, the excitation slit of 5 nm and emission slit of 5 nm.

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).The calcareous soil samples were air dried at room temperature (25 ± 1 °C) for one week and 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 the collected calcareous soils was below the detection limit of the Chinese standard.Both n-hexadecane and naphthalene in soils were 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 and 70 mg/kg in tested conditions, respectively.After absolutely mixing the soil samples with the contamination solution, the mixtures were placed in a fume hood for 90 days.The original calcareous soil, Cd-contaminated calcareous soil and naphthalene-contaminated calcareous soil were marked as OS, CS and NS, respectively.

Batch experiments
Before batch experiments, 0.5, 1.0 and 2.0 g soil samples were, respectively, mixed with 50 mL of the 100 mg/L n-hexadecane solution into 100 mL conical flasks.The mixtures were shaken for 12 h at 140 rpm and 25 ± 1 °C.The ratio of soil samples to solution was selected 1.0 g to ensure the stability of n-hexadecane quantification in the 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 °C.In the desorption kinetic experiments, the residues from preliminary adsorption kinetic experiments were mixed with 50 mL of the solution without n-hexadecane.The mixtures were shaken at the same conditions as adsorption kinetic experiments.In adsorption isotherm experiments, 1.0 g soil sample was transferred to 50 mL of various concentrations of n-hexadecane solution (50, 60, 80, 90, and 100 mg/L).The mixtures were shaken at 140 rpm (25 ± 1 °C) in dark for an equilibrium time.Moreover, the pH value in the solutions was adjusted to 5, 7, 9 using 0.1 mol/L HCl or NaOH for investigating the effect of pH on the adsorption behavior of n-hexadecane in the isotherm experiments.
All supernatants were collected after the mixtures were centrifuged at a speed of 8000 rpm for 10 min.The concentration of n-hexadecane in supernatants was detected by UV spectrophotometer (Shimadzu, Japan) at 257 nm.The experiments were conducted in triplicate using the standard procedure.

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 experiments 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 Vol:.( 1234567890) loss (Rosales et al., 2014)) with the set velocity (1, 2, and 4 mL/min) for 3 h for pre-equilibration and air-removing.After that, 2.0 pore volumes (PVs) tracer (5 mM KCl solution) were injected into the column, and then, several PVs of background solution were not stopped injecting until the concentration of the tracer was monitored to be zero.The tracer concentration 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 were well fitted with R 2 > 0.99 by STANMOD software, indicating a high degree of symmetry in column.The BTCs peak of tracer nearly reached the inlet concentration (with 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) mixed with background solution were pumped into column, and then, the column was flushed by several PVs of background solution.The effluents of n-hexadecane were determined by UV a 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 in Text S1 of Supplementary Materials (Simunek et al., 1999).Moreover, two groups were set up to ensure the consistency and the reproducibility of the 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 completed, and each layer was determined by Ultrasonic Extraction and Gas Chromatograph (GC, Shimadzu GC-2010, Japan).The parameters of GC system were referred by Determination of Extractable Petroleum Hydrocarbons (C 10 -C 40 ) in China (HJ 894-2017).

Results and discussion
The characterization of the soil samples The FTIR spectrum of OS, CS and NS is shown in Fig. 1.The transmittance peak at 3443, 1636, and 1034 cm −1 represented the stretching of -OH, C=C/ C=O, and Si-O-Si, respectively.The XRD patterns 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 indicated that the coating of Cd/ naphthalene did not change the functional group and the mineralogical composition of the calcareous soils.Table 1 presented the zeta potential of OS, CS and NS.The OS, CS and NS all exhibited negatively charged under the condition of this work, and the presence of n-hexadecane increased the negative surface potentials of OS, CS and NS.This was because the surface coating by hydrophobic compounds can modify the zeta potential of porous media (Song et al., 2011;Yang et al., 2017b).
The concentration of DOM in CS (313.10 mg/kg) and NS (331.30mg/kg) was higher than that in OS (88.35 mg/kg) (Table S1), suggesting that the microbial communities in calcareous soil responded to the presence of Cd/naphthalene (Schwarz et al., 2011;Wang et al., 2020a).The fluorescence spectra of OS, CS and NS (Fig. 3) showed a higher intensity of the tyrosine-like fluorescent peak (Ex: 200-250 nm, Em: 280-330 nm) in CS/NS compared to OS, suggesting that a higher content of aromatic ring amino acids in CS/NS (Yan et al., 2019).The intensity of tryptophan-like fluorescent peak (Ex: 200-250 nm, Em: 330-380 nm) in CS/OS was stronger than in NS.Fulvic acid, which is associated with the carbonyl and carboxyl groups, can increase the solubility of hydrocarbons (Lu et al., 2013).Clearly, the intensity of fulvic acid peak (Ex: 200-250 nm, Em: 380-500 nm) in CS was greater than in OS/NS.Fluorescence components of DOM in OS, CS and NS are shown in Fig. 4.
It can be seen that the humic acid content in NS was smaller than that in OS/CS, which may result in the higher adsorption capacity of NS since the humic acid generally inhibits the adsorption of hydrocarbons onto porous media (Wang et al., 2017).
The adsorption and desorption kinetic of n-hexadecane on calcareous soil samples All samples were fitted well by the pseudo-first-order dynamics model with R 2 > 0.9 (Fig. S3; Table S2).
The adsorption equilibrium time for n-hexadecane on OS, CS and NS was all 4 h, suggesting that the presence of Cd/naphthalene did not alter the adsorption equilibrium time.This was caused by the homologous properties of soil samples (Yang et al., 2013).The adsorption rate constant (k 1 ) of OS was higher than that of CS/NS, indicating that Cd/naphthalene enhanced the adsorption efficiency of n-hexadecane on calcareous soils.The desorption equilibrium time for n-hexadecane on OS, CS and NS was all 1 h, suggesting that Cd/naphthalene did not change the desorption equilibrium time.The k 1 value of n-hexadecane adsorption on OS was lower than that of CS/ NS (Table S2), which means Cd/naphthalene reduced the desorption efficiency of n-hexadecane on calcareous soils.In all cases, the adsorption efficiency was higher than the desorption efficiency, suggesting the desorption behavior of n-hexadecane did not greatly influence the adsorption behavior in this word.On the whole, Cd/naphthalene improved the adsorption capacity and decreased the desorption capacity of n-hexadecane on calcareous soils, but it did not alter the equilibrium time.

Effect of pH on the adsorption isotherm of n-hexadecane on the calcareous soil samples
In most cases, the Freundlich model (Fig. 5; Table 2) provided a better fit than the Langmuir model (Fig. S4; Table S3), indicating that the adsorption behavior of n-hexadecane on soil samples may not dependent on monolayer adsorption.In Table 2, K f value of n-hexadecane adsorption on soil samples (at pH = 7) was lower than that on loess soil (Jiang et al., 2016) and modified diatomite (Xu et al., 2020).It can be summarized that Cd/naphthalene did not significantly At pH = 7, K f value of OS was lower than that of CS/ NS, suggesting that Cd/naphthalene improved the adsorption capacity of n-hexadecane.Previous study has also found that an elevated metals increased the adsorption of hydrocarbons on porous media (Saeedi et al., 2018b).Cd-coating porous media enhanced 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 did not absolutely depend on function group's change.The study which was reported by (Nguyen et al., 2013) indicated that the aggregation and hydrophobicity of porous media caused by Cd-coating increased the adsorption efficiency of hydrocarbons.This may be one of the reasons why the presence of Cd enhanced the adsorption of hexadecane on soil samples in this study.Additionally, the negative charge of porous media surface was shielded after coating naphthalene (Yang et al., 2017a); therefore, the adsorption of n-hexadecane was slightly inhibited.Moreover, the n value in all cases was lower than 1, indicating that adsorption sites in the surface were limitation although Cd/naphthalene has opened more adsorption sites for n-hexadecane.
In recent years, rainwater with a higher pH value (pH = 8.0) has been found in a typical karst area (Zeng et al., 2020).Previous studies have reported that the adsorption of diesel oil on loess soil was weakened by increasing pH since pH improved the dissolution and dispersion of diesel oil (Delle Site, 2001;Jiang et al., 2016;Pradubmook et al., 2003).However, the adsorption of naphthalene on biochar colloid did not depend on pH (Grządka, 2011;Yang et al., 2017b).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) decreased adsorption efficiency of n-hexadecane on OS, CS and NS.It was also found that the maximum adsorption of n-hexadecane on each soil sample occurred at pH = 5.At pH = 5 and pH = 7, the adsorption of n-hexadecane on OS was weaker compared to CS/ NS.In alkaline environment (at pH = 9), the K f value of OS was higher than that of CS.The adsorption of n-hexadecane on NS remained stable from pH = 5 to 7, but it exhibited the changes from pH = 7 to 9. Several studies have reported that the release of Cd/ naphthalene in soils depended on pH, as the bonds between contaminations and soils can be broken by pH (Kicińska et al., 2022;Yang et al., 2001).Additionally, the unavoidable 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 result in the negative surface charges on n-hexadecane (Li & Bhushan, 2015).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 & Bhushan, 2015).Therefore, pH influenced the adsorption behavior of n-hexadecane on soil samples.

Effect of flow velocity on transport of n-hexadecane in the calcareous soil samples
The unique hydrological and geological structures in karst areas result in variable flow velocities.Therefore, this section only investigated the potential influence of flow velocity on the n-hexadecane transport in Cd-/naphthalene-contaminated calcareous soils.
The BTCs of n-hexadecane in the calcareous soil samples with the various flow velocities are presented in Fig. 6.As shown in Fig. 6, all BTCs exhibited a symmetric shape, implying that the physical equilibrium transport in the column.The maximum values of C/C 0 were 28, 35, and 48% in OS, CS, and NS, respectively, when the set flow velocity was 1 mL/min.This suggests that n-hexadecane was more effectively able to breakthrough CS/NS compared to 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 accurately described the BTCs of n-hexadecane in each soil sample with different Darcy velocity, as indicated by the higher correlation coefficient (R 2 ) between observed and simulated data (Table 3).The BTCs for n-hexadecane can be characterized by the parameters 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 a greater irreversible retention of n-hexadecane in OS.Conversely, the higher K att value for CS/NS at various Darcy velocity indicated a rapid release process of n-hexadecane.The mass recovery rate of n-hexadecane in effluent is shown in Table 3.There was a higher mass recovery rate for n-hexadecane in the effluent of the CS/NS packed column than that of the 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 improved the transport efficiency of n-hexadecane.
In Table 2, negative charge density of CS/NS was higher than that of OS.The higher negative charge of soil samples, the stronger electrostatic repulsions between the soil samples and the negatively charged n-hexadecane were, which was responsible for promoting the transport of n-hexadecane in CS/ NS packed column (Wu et al., 2020).Furthermore, although naphthalene-coating did not significantly change the surface's negative charge of calcareous soils, nonpolar naphthalene can lead to chargeshielding (Yang et al., 2017b).
The previous studies have proved that the transport of petroleum hydrocarbons with a negative charge was inhibited by the lower 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 with the increasing pH in aqueous system (Fig. 5; Table 2).In Fig. 6, a high value of C/C 0 for n-hexadecane in OS, CS and NS was observed with 6 The 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.C/C 0 is normalized effluent concentration, where C and C 0 were the concertation of n-hexadecane in the effluents and influents, respectively Vol:. ( 1234567890) high flow velocity, indicating an increased transport of n-hexadecane.Similar results have been reported in previous studies (Alazaiza et al., 2020(Alazaiza et al., , 2021;;Dror et al., 2002).In Table 3, it should be noted that S max decreased with higher Darcy flow (0.69 cm/min), while K att was lower at a Darcy flow of 0.12 cm/min.It suggested that smaller Darcy velocity improved 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 an improved transport of n-hexadecane.Overall, n-hexadecane was more likely to breakthrough CS/NS column at a higher flow velocity.

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 at a depth of − 12 cm, suggesting that n-hexadecane has migrated downward due to the force of gravity after leaching.The mass recovery rate of n-hexadecane in effluent from CS/NS packed column was higher than that in OS packed column (Table 3).Clearly, Cd/naphthalene reduced the retention content of n-hexadecane at various set flow velocity.Previous studies have found that soil components played an important role in retaining petroleum hydrocarbons (Adam et al., 2002;Cai et al., 2019).The concentration of DOM in CS (313.10 mg/kg) was higher than that in OS (88.35 mg/kg) (Table S1), and the intensity of the fulvic acid peak in CS was stronger compared to OS (Fig. 3).This can be attributed to the enhanced transport of hydrophobic substances through an increased concentration of fulvic acid (Dong et al., 2021;Sojitra et al., 1996;Yu et al., 2011).Furthermore, batch experiments in this study showed NS exhibited a higher n-hexadecane adsorption efficiency compared to OS, as soil samples can expose additional attachment sites for n-hexadecane under shaking condition.However, in column experiments, adsorption sites were not available, and the n-hexadecane transport was more influenced by hydrodynamics (Wang et al., 2020b).In Fig. 7, it can be observed that the retention content of n-hexadecane reached its maximum the when the flow velocity was 1 mL/min.The effect of flow velocity on n-hexadecane retention was more pronounced at 4 mL/min.Table 3 showed that the mass recovery rate of n-hexadecane in effluent improved as the flow velocity increased from 1 to 4 mL/min.Some studies have also reported an increasing velocity led to 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 contaminated calcareous soils in karst areas.The results showed that although n-hexadecane was easily adsorbed by soil samples (including OS, CS, and NS), the high pH value in calcareous soils in karst areas may contribute to n-hexadecane release.At pH = 5, the presence of Cd/naphthalene in the soils significantly enhanced the adsorption capacity of n-hexadecane, thereby facilitating the formation of co-contaminations in soils.However, the presence of Cd and naphthalene has limited ability to increase the adsorption of n-hexadecane on calcareous soils compared to previous studies.
The mineralogical compositions and functional groups in Cd/naphthalene-contaminated soils were similar to uncontaminated soil.However, slight differences were observed among soil samples in DOM composition, DOM content, and zeta potential.Notably, n-hexadecane was more easily able to breakthrough CS and NS, which carried more negative charges, due to the increase in electrostatic repulsion.Similar results were observed at different flow velocity.Furthermore, compared to low flow velocity (1 mL/min), the transport of n-hexadecane in CS, NS, and OS, respectively, increased at high flow velocity.Correspondingly, n-hexadecane exhibited a greater content of retention in soil samples at low flow velocity.As a result, n-hexadecane has a higher possibility of being transported to deeper layers at a higher velocity in cadmium-/naphthalene-contaminated calcareous soils in karst areas.Consequently, the coexisting contaminations must be considered when conducting soil remediation.

Fig. 3
Fig. 3 Fluorescence spectra of a OS, b CS and c NS

Fig. 4
Fig. 4 Fluorescence 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

Fig. 5
Fig. 5 Adsorption 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,

Fig. 7
Fig. 7 Retention 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

Table 1
The zeta potential of all soil samples Vol.: (0123456789)

Table 2
The parameters of Freundlich a K f is the coefficient of Freundlich which positively associated with adsorption capacity b n is the sorption intensity

Table 3
the parameters of fitted model a The maximum solid-phase retention capacity of n-hexadecane in attachment site