Lead and Chromium Immobilization Process Subjected to Different Freeze-Thaw Treatments in Soils of the Northeastern Qinghai-Tibet Plateau

,e freeze-thaw cycle is one of the important processes that affected heavy metal behaviors in soil. However, information regarding the adsorption and desorption behavior of heavy metals in soils under different freeze-thaw conditions is relatively less. ,erefore, different freeze-thaw conditions including unfrozen, 15 freeze-thaw cycles at 60% water content, and 15 freeze-thaw cycles at 100% water content were investigated. ,en the adsorption and desorption behaviors of Pb and Cr in freeze-thaw soils were studied. Results showed the Pb and Cr adsorption amount mostly decreased with increasing water-soil ratio, and the soil performance of Pb and Cr adsorption at same water-soil ratios showed variation under different freeze-thaw conditions. ,e Pb isothermal adsorption was higher for most freeze-thaw treatments compared to the control.,e soil performance of Cr isothermal adsorption showed variation under different freeze-thaw conditions. Most electrostatic binding of Pb and Cr were stronger under unfrozen and freeze-thaw conditions than unfrozen conditions. Most Pb and Cr adsorption kinetics patterns of freeze-thaw treated soils were rapid than unfrozen conditions. ,ese results implied that freeze-thaw cycles could change the soil adsorption and desorption patterns of Pb and Cr. ,erefore, further studies are urgently needed to investigate Pb and Cr immobilization mechanisms in soils during freeze-thaw cycles. Hence, these findings provided useful information on Pb and Cr immobilization process in soils that underwent freeze-thaw cycles to offer an additional insight into predicting Pb and Cr behaviors in cold and freezing environments.


Introduction
e soil freeze-thaw cycle (FTC) is a common natural phenomenon in the high-and middle-latitude regions of the northern hemisphere; the Qinghai-Tibet Plateau is located in the temperate zone with an alpine climate [1]. Significant effects of freeze-thaw cycles on soil characteristics have been widely reported [2][3][4]. For example, freeze-thaw cycles (FTCs) can significantly alter soil aggregate stability and density [5,6]. Soil community structures, microbial activities, and extractable N concentration are also affected by FTCs [7][8][9][10]. Organic mineralization can also be affected by FTCs [11]. Multiple freeze-thaw treatments (FT-treatments) also caused a significant increase in the release of dissolved organic carbon [12]. However, different experiment results have been found. e methodological differences in FTC experiments in different soils cause the different research results. Large FTC amplitudes and rapid FTC rates are reported to produce large effects on soil properties, whereas moderate FTC amplitudes and slow FTC rates have minimal effects [13,14]. Additionally, the freezethaw time also differs significantly among different researches [15].
Compared with emerging contaminants, heavy metals might pose higher risks [16][17][18]. Heavy metal pollution and emerging contaminants have attracted attention widely [19][20][21][22]. FTC is one of the critical processes that influence heavy metal behaviors through changing soilspecific surface area, organic matter content, physical structure, and other soil properties [23][24][25]. Many studies have demonstrated that the geochemistry behavior of some heavy metals can be altered by FTCs [26,27]. It is generally regarded that adsorption and desorption of heavy metals in unfrozen soils were higher than the soil with FT-treatment [3,27], but this study has different results. e soil adsorption amount of heavy metals increased with the decreased freezing temperature and FTC frequency [3,28]. FTCs can also transport the particulate metals from soil environment to aquatic environment [25,29]. Moreover, FTCs are important functions for the transform of heavy metals from organic or Fe-Mn oxides fractions to exchangeable fractions [30,31]. However, most studies adopted extreme freeze-thaw conditions; for example, the researchers usually adopted −25°C as freezing temperature and 20°C as thawing temperature [25,32]. e minimum temperature in a natural environment below −20°C rarely exists in soil when the FTCs occur. erefore, the researchers should adopt a suitable temperature for the freeze-thaw conditions. FTC is a typical climate characteristic in northwest China. Little information has been available on the adsorption and desorption of heavy metals in soils of Qinghai-Tibet Plateau experiencing FTCs. e immobilization processes of heavy metals in soil would be impacted by the property change caused by FTCs, which can cause variations in the soil environment. In this study, we took the soils of the northeastern Qinghai-Tibet Plateau in China as the research objects and probed into the behaviors of the adsorption and desorption of Cr and Pb in soils treated with different freezethaw conditions. e main targets of this were to (1) investigate isothermal adsorption and desorption behavior of Pb and Cr in the freeze-thaw treated (FT-treated) soils and (2) study kinetics of adsorption and desorption behavior of Pb and Cr in the FT-treated soils. e ground of the Qinghai-Tibet plateau is mainly classified into two major categories on the map of permafrost on the Qinghai-Tibetan Plateau (QTP 1996): permafrost and seasonally frozen [1]. e freezing-thawing phenomenon is distributed in this region widely. e freezing and thawing temperatures of the lab-simulated experiments were set as −15°C and +5°C, respectively.

2.2.
Freeze-aw Procedure. About 200 g of soil samples were put into plastic bags. Soil moistures were adjusted to 60% and 100% water holding capacity. e freezing temperature (−15°C) and thawing temperature (+5°C) were selected to simulate the common winter temperature in northwest China. In the simulated experiments, the soil samples with different water contents were frozen under −15°C for 24 h and then thawed under 5°C for the next 24 h; the above processes were set as one FTC. e freeze-thaw processes of samples were conducted by a refrigerator (BCD-649WE, Qingdao Haier Co. Ltd., Qingdao, China). After 15 FTCs, samples of FT-treatment were air-dried and labeled as F15 + W60% and F15 + W100%, respectively. e unfrozen soil was kept under 5°C and labeled as F0 + W0%.

Pb and Cr Adsorption and Desorption Experiments.
For each FT-treatment, about 1.0 g of treated soil sample was put into a 50 mL centrifuge tube. All the experiments were conducted at a fixed ionic strength (NaNO 3 , analysis pure, Sinopharm Chemical Reagent Co. Ltd.). In order to illustrate the effect of water-soil ratio on adsorption of FT-treated soils, the solutions of 0.01 mol/L NaNO 3 containing 20 mg/L Pb 2+ (Pb(NO 3 ) 2 , analysis pure, Sinopharm Chemical Reagent Co. Ltd.) and 10 mg/L Cr 6+ (K 2 Cr 2 O 7 , analysis pure, DaMao Chemical Reagent Factory) were added to each tube. e Pb 2+ and Cr 6+ solution of volumes consisted of 10, 30, 50, and 80 mL, respectively. e Pb 2+ contents of solutions were 0.2, 0.6, 1.0, and 1.6 mg, while the Cr 6+ contents of solutions were 0.1, 0.3, 0.5, and 0.8 mg, respectively. en the tubes were shaken at 180 times/min continuously for 2 h; the samples were centrifuged at 4000 rpm for 10 min and filtered through a 0.45 μm cellulose acetate syringe filter.
e Pb and Cr concentration of the supernatants was measured by plasma emission spectrometer ( ermo ICAP 6500 DUO, ermo Fisher, USA). e adsorption capacity of Pb and Cr were calculated as follows: where S a is the adsorption capacity (mg/kg), C 0 is the initial Pb or Cr concentration in mg/L, C 1 is the Pb or Cr concentration of equilibrium (mg/L), V is the volume of added solution (L), and m is the weight of soil sample (kg).
For each sample, about 1.0 g of air-dried soil sample was placed in a 50 mL centrifuge tube. A 20 mL solution of 0.01 mol/L NaNO 3 containing varying concentrations of Pb and Cr was added to each tube. Solutions containing various concentrations (5,10,20,50, and 100 mg/L) of 20 mL Pb 2+ (Pb(NO 3 ) 2 solution) and various concentrations (5,10,20,30, and 50 mg/L) of 20 mL Cr 6+ (K 2 Cr 2 O 7 solution) were prepared. ese experiments aimed to assess isothermal adsorption of FT-treated soils. en the tubes were shaken at 180 times/min continuously for 2 h and then centrifuged at 4000 rpm for 10 min. e supernatants were filtered through a 0.45 μm cellulose acetate syringe filter.
Following the adsorption experiments of Pb or Cr, the centrifuged residues were weighed to calculate the content of residual solutions. en 20 mL of 0.01 mol/L NaNO 3 solutions were added to the centrifuged residues. e tubes were shaken 2 Journal of Chemistry for 2 h. e concentrations of Pb or Cr in the supernatant were measured as described above. e amount of desorbed Pb or Cr was calculated according to the following equation: where S d is the desorption capacity in mg/kg, C 2 is the Pb or Cr concentration of equilibrium (mg/L), and V 1 is the volume remaining in the residue. e isothermal adsorption behavior of Pb and Cr in the soil is calculated by the Freundlich and Langmuir models.
e Freundlich model explains site adsorption intensity and heterogeneity between adsorbate and adsorbent [33]. e Freundlich equation has the following form: where Q e is Pb or Cr adsorbed by soil (mg/kg), C e is Pb or Cr concentration in adsorption equilibrium (mg/L), n is constant, and k is affinity coefficient (L/mg). Langmuir model assumes that maximum adsorption corresponds to a saturated monolayer of solute molecules on the surface of the adsorbent [33], and the energy of adsorbance is constant. e Langmuir model is calculated according to the following equation: where Q e is Pb or Cr adsorbed by soil (mg/kg), C e is Pb or Cr concentration in adsorption equilibrium (mg/L), Q m is the adsorption maximum of Pb or Cr adsorbed by soil, and k is affinity coefficient (L/mg). In order to illustrate the soil kinetics of adsorption, about 1.0 g of treated soil samples were put into a 50 mL solution centrifuge tube. e solutions of 0.01 mol/L NaNO 3 containing 20 mg/L Pb 2+ (Pb(NO 3 ) 2 solution) and 10 mg/L Cr 6+ (K 2 Cr 2 O 7 solution) were added to each tube. Supernatants were collected from the tubes at 5, 10, 30, 60, 240, and 480 minutes. e following experiment processes were the same as described above, and the concentrations of Pb 2+ or Cr 6+ in the supernatant were measured as described above.
e pseudo-first-order model is a conventional model examined to analyse the kinetics of adsorption processes. e pseudo-first-order kinetics model supposes that the diffusion step controls the adsorption and its adsorption is proportional to the difference in adsorption capacity at equilibrium and at any time [34]. e Weber-Morris model described intraparticle diffusion in the solid phase [34]. e pseudo-first-order (5) and Weber-Morris (6) models are described as follows: where t is reaction time (h), Q e and Q t are adsorption capacity at equilibrium and at any time t (mg/kg), and k 1 and k 2 denote pseudo-first-order rate constant and intraparticle diffusion equation constant, respectively. C is a constant.

Effect of Water-Soil Ratio on Adsorption of FT-Treated
Soils. e soil type in the sampling sites was sandy clay loam, and soil pH was an alkaline. e Pb and Cr concentrations of the selected soils would not affect the adsorption and desorption experiment because exchangeable Pb and Cr contents were under very low level. e soil performance of Pb and Cr adsorption at various water-soil ratios under different FT-treated are shown in Figure 1. e different soil adsorption trends of Pb and Cr responded more or less constantly with change in the water-soil ratios. e Pb and Cr adsorption amount mostly decreased with increasing the water-soil ratio. However, the soil performance of Pb and Cr adsorption at same water-soil ratios showed variation under different freeze-thaw conditions. e FTCs with different freezing conditions exerted different effects on soils of the northeastern Qinghai-Tibet Plateau. At water-soil ratios of 10:1, 30:1, and 80:1, the Pb adsorption amount of soils in S-1 under different FT-treated (F15 + W60% and F15 + W100%) decreased compared to the unfrozen soil. At water-soil ratio of 50:1, the Pb adsorption amount in S-1 increased at 15 FTCs and 60% water content and decreased at 15 FTCs and 100% water content compared to the unfrozen soil. At water-soil ratio of 10:1, the Pb adsorption amount of FT-treated soils at 15 FTCs and 100% water content in S-2 was maximum value compared to the unfrozen and FT-treated soils (F15 + W60%), and the Pb adsorption amount of FT-treated soils at 15 FTCs and 60% water content in S-2 was maximum value (125.07 mg/kg and 9.9 mg/kg) at water-soil ratios of 30:1 and 80:1.
e Pb adsorption amount of soils in S-2 under different FT-treated (F15 + W60% and F15 + W100%) decreased compared to the unfrozen soil at a water-soil ratio of 50:1. At a water-soil ratio of 10:1, the Pb adsorption amount of unfrozen soil in S-6 was maximum value (173.9 mg/kg), the Pb adsorption amount of soils under different FT-treated (F15 + W60% and F15 + W100%) decreased compared to the frozen soil. In the soils of S-3, the Pb adsorption amount of soils under different FT-treatments (F15 + W60% and F15 + W100%) decreased compared to the unfrozen soil at all the water-soil ratios; conversely, the soils in S-5 increased under different FT-treatments (F15 + W60% and F15 + W100%) at all the water-soil ratios. At different water-soil ratios, the Pb adsorption amount of FT-treated soils at 15 FTCs and 60% water content in S-4 was the maximum value compared to the unfrozen and FT-treated soils (F15 + W100%). At a water-soil ratio of 50:1 of soil in S-6, the changing trend was the same as a water-soil ratio of 10:1. At water-soil ratios of 30:1 and 80:1, the Pb adsorption amount in S-6 approached maximum values (159.36 mg/kg and 12.5 mg/kg) at 15 FTCs and 60% water content compared to the unfrozen soil; conversely, the Pb adsorption amount of FT-treated soil (F15 + W100%) in S-6 decreased compared to the unfrozen soil. Base on the experimental results, the soil performance of Pb adsorption at the same water-soil ratios showed variation under different freeze-thaw conditions. e Cr adsorption patterns at the same water-soil ratios were different under different FT-treatments. At water-soil ratios of 10:1 and 50:1 of soil in S-1, the Cr adsorption Journal of Chemistry amount of FT-treated soil in S-1 decreased compared to the unfrozen soil; conversely, the Cr adsorption amount of FTtreated soil increased at a water-soil ratio of 30:1. At a watersoil ratio of 80:1 in S-1, the Cr adsorption amount slightly increased at 15 FTCs and 60% water content and decreased at 15 FTCs and 100% water content compared to the unfrozen soil. At a water-soil ratio of 10:1 in S-2, the Cr adsorption amount of FT-treated soil increased compared to the unfrozen soil and decreased at water-soil ratios of 30:1, 50:1, and 80:1. At water-soil ratios of 10:1 and 30:1 in S-3, the Cr adsorption amount of FT-treated soil increased compared to the unfrozen soil and decreased at water-soil ratios of 50:1 and 80:1. At water-soil ratios of 10:1, 30:1, and 50:1 of soil in S-4, the Cr adsorption amount of FT-treated soil increased compared to the unfrozen soil. At water-soil ratio of 80:1 in S-4, the Cr adsorption amount slightly increased at 15 FTCs and 60% water content and slightly decreased at 15 FTCs and 100% water content compared to the unfrozen soil. At water-soil ratios of 10:1 and 30:1, the Cr adsorption amount of FT-treated soil decreased in S-5 compared to the unfrozen soil and increased at water-soil ratios of 50:1 and 80:1. At water-soil ratios of 10:1 and 30:1 of soil in S-6, the Cr adsorption amount of FT-treated soils in S-6 decreased compared to the unfrozen soil. At water-soil ratios of 50:1 and 80:1 in S-6, the Cr adsorption amount slightly increased at 15 FTCs and 60% water content and decreased at 15 FTCs and 100% water content compared to the unfrozen soil. e results indicated that Cr adsorption amount at same watersoil ratios was different under different FT-treatments.

Isothermal Adsorption and Desorption of FT-Treated Soils.
Pb needs strict control since it can enrich in different matrices [17]. e adsorption and desorption of Pb in soils with different FT-treatments were shown in Figures 2(a), 2(b), 2(e), and 2(f )). A greater increase in adsorption was seen with increasing Pb concentrations from 0 to 20 mg/kg for the FT-treatments in the soils of S-1 and S-2, compared to adsorption with Pb concentrations ranging from 20 to 100 mg/kg. A greater increase in adsorption was seen with increasing Pb concentrations from 0 to 100 mg/kg for the most FT-treatments in the soils of S-3, S-5, and S-6. A greater increase in adsorption was seen with increasing Pb concentrations from 0 to 100 mg/kg for the treatment of 15 FTCs and 60% water content in S-4. A slight increase in adsorption was seen with increasing Pb concentrations from 0 to 50 mg/kg, and a greater increase ranging from 50 to 100 mg/kg for the treatment of 15 FTCs and 60% water content in S-4. e soil adsorption amount increased by the Pb concentration ranging from 0 to 50 mg/kg and decreased by the Pb concentration ranging from 50 to 100 mg/kg for the treatment of 15 FTCs and 100% water content in S-4. A high slope at Pb concentration illustrated a high proportion of available adsorption sites distribute on the surface of soil particles. In general, the adsorption of Pb was higher for most FT-treatments compared to the control. e desorption levels of Pb in soils by 0.01 mol/L NaNO 3 with different FT-treatments were shown in Figures 2(e) and 2(f )). About 0.08-10.52% (average value: 3.18%) of adsorbed Pb was desorbed from the soil without FTCs, while 0.02-10.67%  Figure 1: Relationship between water-soil ratio and Pb or Cr adsorption amount in soils under different freeze-thaw conditions. F0 and F15 refer to the number of freeze-thaw cycles (0 and 15 cycles), and W0%, W60%, and W100% refer to water ratio of field capacity (0%, 60%, and 100% water ratio).
(average value: 2.91%) of adsorbed Pb was desorbed from FT-treated soils at the same initial concentration of Pb.
ese data indicated that electrostatic binding of Pb in most soil samples was stronger under FTCs.
e Pb Langmuir and Freundlich parameters and data values are shown in Table 1. e 1/n in the Freundlich model is a measure of the heterogeneity of adsorption sites on the adsorbent surface. Surface site heterogeneity increases with 1/n approaches 0. Conversely, surface site homogeneity increases with 1/n approaches unity [35]. For the control with unfrozen soil, the 1/n value was higher at 0.87 ± 0.01 in soils of S-1, while the 1/n values for the FT-treatments (F15 + W60% and F15 + W100%) were lower at 0.78 ± 0.24 and 0.78 ± 0.21, respectively. is indicated that surface site homogeneity decreases with FT-treatment. e soils in S-4 and S-6 were similar to the soils of S-1 under the same FT-  e results suggested that the binding energy between soil colloid and Pb increased under FT-treatments. e adsorption and desorption of Cr in soils with different FT-treatments are shown in Figures 2(c), 2(d), 2(g), and 2(h)). A slight change in adsorption was shown with increasing Cr concentration from 0 to 50 mg/kg for the most FT-treatments in the soils of S-1, S-4, and S-5. A greater increase in adsorption was seen with increasing Cr concentrations from 0 to 20 mg/kg for the most FT-treatments in the soils of S-2 and S-3, compared to adsorption with Cr concentrations ranging from 20 to 50 mg/kg. A greater increase in adsorption was seen with increasing Cr concentrations from 0 to 50 mg/kg for the most FT-treatments in the soils of S-6. In general, the adsorption of Cr had no regular pattern among these soil samples under different FTtreatments and unfrozen conditions. Desorption amount of Cr in soils by 0.01 mol/L NaNO 3 with different FT-treatments are shown in Figures 2(g) and 2(h)). About 14.11-39.95% (average value: 27.03%) of adsorbed Pb was desorbed from the soil without FT-treatment, while 3.13-45.47% (average value: 25.34%) of adsorbed Cr was desorbed from FT-treated soils at the same initial concentration of Cr. ese results indicated that the electrostatic binding of Cr in most soil samples was stronger under freeze-thaw conditions. e Cr Langmuir and Freundlich parameters in different soils under different freeze-thaw conditions are shown in Table 2. Compared to the control with no FTtreatment, the 1/n value was higher at 1.75 ± 0.81 under 15 FTCs and 100% water content in soil of S-1, while the 1/n values for the FT-treatment (F15 + W60%) were lower at 0.13 ± 0.82. For the control with unfrozen soil, the 1/n value was higher at 0.71 ± 0.51 in soils of S-3, while the 1/n values for the FT-treatments (F15 + W60% and F15 + W100%) were lower at 0.62 ± 0.47 and 0.50 ± 0.17, respectively.
ese results indicated that surface site homogeneity decreased with FT-treatment. Conversely, the 1/n values in soils of S-2, S-4, and S-6 under FTtreatments increased compared to the control with no Note. "-" means that these data could not be fitted using the Freundlich and Langmuir models. F0 and F15 refer to the number of freeze-thaw cycles (0 and 15 cycles), and W0%, W60%, and W100% refer to water ratio of field capacity (0%, 60%, and 100% water ratio). 6 Journal of Chemistry FT-treatment.
ese results indicated that surface site homogeneity increased with FT-treatments in soils of S-2, S-4, and S-6. Compared to the control with no FTtreatment, the 1/n value was higher at 1.02 ± 0.46 under 15 FTCs and 60% water content in soil of S-5, while the 1/ n value for the FT-treatment (F15 + W100%) was lower at 0.54 ± 0.42. According to Table 2, most data of Cr adsorption could not be fitted using the Langmuir model. e results indicated that different adsorption patterns were observed in different FT-treatments and different sampling areas.

Adsorption Kinetics of FT-Treated Soils.
In order to optimize the time required for access to equilibrium condition, a series of adsorption experiments were performed.
is investigation also allowed verifying the kinetics of the process. e adsorption of Pb and Cr from aqueous solution to about 1 g of soil was studied by contacting the solid and liquid phases in the range of 5-480 minutes (Figure 3).
Base on pseudo-first-order model analysis, the equilibrium of adsorption of Pb ions was not achieved after 480 minutes for soil under unfrozen in S-1, 458 minutes for soil under 15 FTCs and 60% water content, and 376 minutes for soil under 15 FTCs and 100% water content. Equilibrium of adsorption of Pb ions was achieved after 3.33 h for soil under unfrozen in S-3, 0.72 h for soil under 15 FTCs and 60% water content, and 0.90 h for soil under 15 FTCs and 100% water content. Equilibrium of adsorption of Pb ions was not achieved after 480 min for soil under unfrozen in S-5, 0.70 h for soil under 15 FTCs and 60% water content, and 0.74 h for soil under 15 FTCs and 100% water content. ese results demonstrated that the kinetics of Pb adsorption under FTtreatments were more rapid than the unfrozen soil in the S-1, S-3, and S-5. Equilibrium of adsorption of Pb ions was achieved after 7.84 h for soil under unfrozen and FTtreatment (F15 + W100%) in S-2; it was seen that the adsorption of Pb ions onto soil under 15 FTCs and 60% water content was more rapid than unfrozen and other FTtreatment. e adsorption of Pb ions onto soil in S-6 under 15 FTCs and 60% water content was more rapid than unfrozen and other FT-treatment. e results indicated that most Pb adsorption patterns on soils under FT-conditions were rapid than unfrozen conditions. Table 3 lists the kinetic parameters for the models on the adsorption of Pb onto soils in six sites. R 2 for the Pb pseudofirst-order model ranged from 0.46 to 0.94. Most of the R 2 data were larger than 0.6, except soil in S-2 under unfrozen condition, soil in S-3 under FT-treatment (F15 + W100%), and soil in S-6 under unfrozen condition. R 2 for the Pb Weber-Morris model ranged from 0.02 to 0.92. Soils in S-4 and S-3 did not fit well in some conditions. Compared to the control with no FT-treatment, the Q e value was higher at 25.41 ± 2.76 under 15 FTCs and 100% water content in soil of S-1, while the Q e value for the FT-treatment (F15 + W60%) were lower at 20.46 ± 2.96. For the control with unfrozen soil, the Q e value was higher at 41.05 ± 7.4 in soils of S-2, while the Q e values for the FT-treatments (F15 + W60% and F15 + W100%) were lower at 28.   Table 4, parts of data could not be fitted using the pseudo-first-order model and Weber-Morris model. R 2 for the Pb pseudo-first-order model ranged from 0.001 to 0.91. R 2 for the Cr Weber-Morris model ranged from 0.02 to 0.95. Most of the R 2 data of the pseudo-firstorder model and Weber-Morris model were lower than 0.8, the correlations were weak in most soil samplings under different conditions. For the control with no FT-treatment, the Q e value was lower at 37.63 ± 1.84 in soils of S-3, while the Q e values for the FT-treatments (F15 + W60% and F15 + W100%) were higher at 50.45 ± 3.93 and 55.03 ± 2.98, respectively. e soils in S-4, S-5, and S-6 were similar to the soils of S-3 under the same FT-treatments. e results indicated that most Cr adsorption amounts on soils under freeze-thaw conditions were higher than unfrozen conditions. , and W0%, W60%, and W100% refer to water ratio of field capacity (0%, 60%, and 100% water ratio).

Conclusions
Results showed that different FT-treatments and soil water content affected the lead and chromium immobilization process in the northeastern Qinghai-Tibet Plateau. e different soil adsorption trends of Pb and Cr responded more or less constantly with change in the water-soil ratios.
e Pb and Cr adsorption amount mostly decreased with increasing the water-soil ratio, and the soil performance of Pb and Cr adsorption at the same water-soil ratios showed variation under different freeze-thaw conditions. e adsorption of Pb was higher for most FT-treatments compared to the control. e soil performance of Cr isothermal adsorption showed variation under different freeze-thaw conditions. Most electrostatic binding of Pb and Cr was stronger under unfrozen and freeze-thaw conditions than unfrozen conditions. Most Pb adsorption kinetics patterns of FT-treated soils were rapid than unfrozen conditions; the Note. F0 and F15 refer to the number of freeze-thaw cycles (0 and 15 cycles), and W0%, W60%, and W100% refer to water ratio of field capacity (0%, 60%, and 100% water ratio). Note. "-" means that these data could not be fitted using the pseudo-first-order and Weber-Morris models. F0 and F15 refer to the number of freeze-thaw cycles (0 and 15 cycles), and W0%, W60%, and W100% refer to water ratio of field capacity (0%, 60%, and 100% water ratio).
Q e values in soils of S-3, S-4, and S-6 under FT-treatments increased compared to the control with no FT-treatment.
Most Cr adsorption kinetics patterns on soils under freezethaw conditions were rapid than unfrozen conditions, and most Cr adsorption amounts on soils under freeze-thaw conditions were higher than unfrozen conditions. ese results implied that FT-treatment could change the soil adsorption and desorption patterns of Pb and Cr. erefore, further studies are urgently needed to investigate the lead and chromium immobilization mechanism of heavy metals in soils during FTCs.
Data Availability e data are available on request.

Conflicts of Interest
No potential conflicts of interest are reported by the authors.