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Cd(II) and Pb(II) adsorption in karst soils amended with litter extract from Ficus virens
⁎Corresponding authors. lwb062@163.com (Wenbin Li), dhongyan119@163.com (Hongyan Deng)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
In this study, different proportions of Le were used to amend purple, yellow, and yellow–brown soils of karst areas to investigate the effect of litter extract (Le) from Ficus virens on Cd(II) and Pb(II) adsorption in karst soils. The basic physicochemical properties of Le-amended karst soils were determined, and the microscopic morphology of Le-amended karst soils was detected. The batch method was used to study the isothermal adsorption characteristics of Cd(II) and Pb(II) adsorption on different Le-amended karst soils. Physicochemical properties and microscopic morphology results showed that Le was amended on the karst soil surface and changed the surface properties of the karst soil. The maximum adsorption capacity range of Cd(II) and Pb(II) by Le-amended karst soils was 92.10–241.61 and 108.91–262.12 mmol/kg, respectively, and the peak value was reached when the karst soils were amended with 20 % of Le. Increasing the pH and temperature were beneficial to Cd(II) and Pb(II) adsorption in each Le-amended karst soil. The adsorption amount of Cd(II) and Pb(II) initially increased and then decreased with the increase in ionic strength. Cd(II) and Pb(II) adsorption was a spontaneous, endothermic, and entropy-increasing process, and electrostatic attraction, precipitation, complexation, and ion exchange were the main adsorption mechanisms. The adsorption amount of Cd(II) and Pb(II) by Le-amended yellow–brown soil maintained approximately 85 % of original sample after three rounds of regeneration.
Keywords
Litter extract
Ficus virens
Karst soil
Cd(II) and Pb(II)
Adsorption amount
1 Introduction
With the rapid development of industrialization and urbanization, heavy metal pollution is increasing, resulting in an environmental crisis that cannot be ignored by human beings and nature (He et al., 2013). Heavy metal ions are widely available and toxic, and they enter the environment through various pathways to jeopardize human health and life (Zhan et al., 2018). Pb(II) and Cd(II), which are mainly introduced into the soil as fertilizers, pesticides, municipal sewage irrigation, and industrial wastes (Cerqueira et al., 2011) are the major heavy metal pollutants in China (Hu et al., 2014). High levels of Pb(II) and Cd(II) may induce serious environmental pollution, affecting soil, water, atmosphere, and human health (Kahvecioğlu et al., 2023). Studies found that Cd(II) is the most significant pollutant, with an exceedance rate of 29.6 % for Cd(II) pollutants and 1.4 % for Pb(II) pollutants of China (Wan et al., 2018); moreover, 30,000 ha of arable land in China is highly polluted (Song et al., 2017). Therefore, exploring methods that can remediate Pb(II) and Cd(II) contamination in soil is important for the protection of the ecological environment and the sustainable development of agriculture.
Current methods used for the remediation of heavy metal-contaminated soils include physical, chemical, and bioremediation methods (Khalid et al., 2017; Sarker et al., 2023). Among them, physical and chemical methods have limitations due to their shortcomings, such as being destructive and costly, not environmentally friendly, and prone to releasing additional pollutants into the environment, causing secondary pollution (Yan et al., 2022). Biomaterial remediation has the advantages of being ecologically friendly and cost effective; therefore, bioremediation is one of the most promising technologies currently used for the remediation of heavy metal-contaminated soils (Gavrilescu, 2022). The removal efficiency of heavy metal pollutants can be improved with the help of biomaterials, and natural biomass has been used during biosorption (Kushwaha et al., 2022). Choudhary et al. (2020) converted pine leaf litter to biochar at a pyrolysis temperature of 550 °C for the adsorption of Pb(II) in water. The results showed that the maximum adsorption of Pb(II) increased significantly as the pH increased from 2 to 5 and above, and the adsorption increased when the temperature increased, showing a positive effect of temperature increase. Khan et al. (2020) found that the synthesized nanoparticles showed a significant increase in Cr(VI) adsorption at a high acidic pH, and abutilon indicum is a valuable resource for enhancing the potential of nanoparticles in Cr(VI) adsorption. Verma et al. (2019) made biochar from teak leaves and perilla leaves for the removal of As(Ⅲ) and As(Ⅴ) from aqueous solution and found that both biochar materials successfully adsorbed As(Ⅲ) and As(Ⅴ) from aqueous solution and the adsorption of As(Ⅲ) by both materials was 666.7 and 454.54 μg/g, respectively. Activated carbon from almond shells was treated by KOH and sulfur doping, and the adsorption capacity value for Cd(II) adsorption was 282.70 mg/g (Saka et al., 2023). The above research shows that natural biomass materials can be a promising option for heavy metal ion adsorption due to their low economic value and abundant possibilities.
Plant litter as natural organic matter has an important role in improving soil fertility and altering soil enzyme activity after decomposition (Lecerf et al., 2021). Studies found that plants have strong applicability to heavy metal stress in the soil environment, and the biological extract from plants have strong heavy metal complexation ability (Thomas, 2021). Soil in karst areas has high calcium content, poor soil and water conservation ability, and poor pollutant enrichment ability (Yan et al., 2022). If soils are amended by biological methods, the pollution repair cost may be reduced; pollution repair efficiency may be enhanced; and the soil structure, organic matter, and microecological environment are improved. However, relevant studies are few. In this study, litter extract (Le) made from the Ficus virens was used as a biological modifier to amend different karst soils, and the basic physicochemical properties and micromorphology of the different Le-amended karst soils were analyzed. Furthermore, the isothermal adsorption properties of Cd(II) and Pb(II) on different samples were explored, and the influences of the environmental factors (temperature, pH, and ionic strength) on the adsorption process were analyzed. The results of the study provide theoretical references to the utilization of bioresources for the remediation of heavy metal contaminated soil.
2 Materials and methods
2.1 Experimental materials
2.1.1 Collection of soil samples
From March to April (rainfall is less than 20 mm per month) in 2023, three sampling points were selected as purple (P), yellow (Y), and yellow–brown (B) soils of the karst area (Luzhou City, Sichuan Province, China). The surface soil sample (0–25 cm, concentrated area of Le) was collected by the S-spot method in a typical area, mixed evenly, dried, ground, and passed through a 0.15 mm nylon sieve for storage. The basic situations around each sampling point are shown in Table 1.
| Soil samples | Longitude and latitude | Altitude (m) | Soil humidity | Soil porosity | Soil structure | Root number |
|---|---|---|---|---|---|---|
| P | E105°50′1.13″ N27°47′8.51″ |
736.70 | Moist | Little | Granular | Little |
| Y | E105°56′45.24″ N27°51′57.89″ |
794.42 | Slightly moist | Much | Block | Little |
| B | E105°56′22.38″ N27°58′37.7″ |
760.08 | Slightly moist | Medium | Granular | Much |
Note: Soil humidity in the table is divided into four grades: wet (water content ≥ 40 %), moist (40 % > water content ≥ 10 %), slightly moist (10 % > water content ≥ 5 %), and dry (water content < 5 %). The number of soil pores is divided into three grades: much (porosity ≥ 60 %), medium (60 % > porosity ≥ 30 %), and little (porosity < 30 %). Soil structure is divided into block, granular, flake, nucleus, and so on. The number of soil roots is divided into three grades: much (root denesty ≥ 20 %), medium (20 % > root denesty ≥ 5 %), and little (root porosity < 5 %).
2.1.2 Preparation of Le
The litter of Ficus virens was obtained from the experimental field of Luzhou City, Sichuan Province, China. The litter was dried and crushed separately, and then the litter powder was mixed with distilled water (dH2O) at a mass ratio of 1:20 (biomass:water). The mixture was shaken for 2 h and centrifuged at 4,800 r/min for 20 min. The extracts were designated as Le and placed in a refrigerator at 4 °C for later use. The dissolved organic carbon of Le was 447.52 mg/L, SUVA254 (aromaticity strength of dissolved organic matter) was 1.55, and SUVA260 (hydrophobic component of dissolved organic matter) was 1.72.
2.1.3 Preparation of pollutant solutions
Solutions of heavy metal pollutants, i.e., Cd(II) and Pb(II), were prepared using Cd(NO3)2 (analytic reagent, Fuchen Chemical Reagent Factory) and Pb(NO3)2 (analytic reagent, Chengdu Cologne Chemical Reagent Factory), respectively.
2.2 Experimental design
2.2.1 Preparation of Le-amended karst soils
The Le was mixed evenly into P, Y, and B soils in accordance with mass ratios of 0 %, 10 %, 20 %, 50 %, and 100 % at natural pressure; reacted at 40 °C for 24 h; dried at 60 °C; and ground. Le-amended P soil (Le-P), Le-amended Y soil (Le-Y), and Le-amended B soil (Le-B) were obtained by passing through a 60-mesh sieve. Different treatments were expressed as follows: P, 10 %Le-P, 20 %Le-P, 50 %Le-P, 100 %Le-P, Y, 10 %Le-Y, 20 %Le-Y, 50 %Le-Y, 100 %Le-Y, B, 10 %Le-B, 20 %Le-B, 50 %Le-B, and 100 %Le-B. Samples were analyzed for pH, cation exchange capacity (CEC), total organic carbon (TOC), and Brunauer–Emmett–Teller specific surface area (SBET) and characterized by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and particle size analyzer.
2.2.2 Isothermal adsorption experiment
The initial concentration gradient of Cd(II) and Pb(II) was set to 0, 20, 50, 100, 150, 200, 300, 400 mg/L and 500 mg/L. Each treatment was repeated thrice under 20 °C, pH = 4, and ionic strength of 0.1 mol/L.
2.2.3 Influence factor experiment
The pH of the initial solution was set to 2, 4, and 6 (experimental temperature: 20 °C; ionic strength: 0.1 mol/L). The experimental temperatures were set to 10 °C, 20 °C, and 30 °C (pH value of the solution: 4; ionic strength: 0.1 mol/L). The ionic strength of the initial solution was set to 0.01, 0.1, and 0.2 mol/L (pH value of the solution: 4; experimental temperature: 20 °C).
2.3 Experimental methods
pH was determined using the HQ411D table pH meter (Hash Company, Colorado, USA; refer to the test method of soil sample, solid–liquid ratio was 1:5). CEC was determined by sodium acetate–ammonium acetate method. TOC was determined using the automatic Aurora 1030C TOC analyzer (Xylem, NewYork, USA). SBET was analyzed using a multipoint BET method through the Gold APP V-Sorb2800P analyzer (Ultrametrics, Beijing, China). SEM was performed using the Japanese Hitachi S-4800 scanning electron microscope. FTIR analysis was performed on the Nicolet iS50 type Fourier transform infrared spectrometer (ThermoFisher, Massachusetts, USA). The chemical states were measured using X-ray photoelectron spectroscope (XPS, ESCA Ulvac-PHI PHI 1600).
Different samples (0.5000 g) were accurately weighed into nine 50 mL polyacrylamide centrifuge tubes and added with 20 mL of Cd(II) and Pb(II) solutions successively. Samples were oscillated at 150 r/min at a constant temperature of 20 °C for 24 h (Li et al., 2016). After centrifugation at 4800 r/min for 15 min, the supernatant was separated through a 0.45 μm filter membrane, and the Cd(II) and Pb(II) adsorption capacities of each tested sample were calculated through the subtraction method. The above experiments were repeated under different influencing factors to compare the differences in Cd(II) and Pb(II) adsorption. Cd(II) and Pb(II) concentrations were determined by flame atomic absorption spectrophotometry (AA900T, PerkinElmer, Massachusetts, USA), and the analytical quality was controlled by inserting standard solution.
2.4 Data analysis
2.4.1 Calculation of equilibrium adsorption capacity
Equilibrium adsorption capacity was calculated according to Eq. (1) (Wang et al., 2022).
2.4.2 Fitting of adsorption isotherms
Based on the adsorption isotherm trend, Langmuir and Freundlich isotherm models were selected to fit the Cd(II) and Pb(II) adsorption isotherm (Zhao et al., 2021), which are respectively expressed in Eqs. (2) and (3):
2.4.3 Calculation of thermodynamic parameters
The parameter b in Langmuir model, and kn in Freundlich models are the apparent adsorption constant that equivalent to the equilibrium constant, then K = b or K = kn. The thermodynamic parameters calculated by K are called the apparent thermodynamic parameters, and the calculation formulas are shown in Eqs. (4)–(6).
Curvexpert 1.4 fitting software was used to fit the adsorption isotherms by the stepwise approximation method. SPSS 16.0 statistical analysis software was used to process the experimental data for variance and correlation analysis. Oringin 2021 software was adopted to improve data plotting. The data were expressed as the means with standard deviation, and different letters indicate significant differences among various amendments. Analysis of variance was performed to determine the effects of amendments, followed by Tukey’s honestly significant difference test. Differences of P < 0.05 were considered significant.
3 Results and discussion
3.1 Basic physicochemical properties of Le-amended karst soils
The basic physicochemical properties of different Le-amended karst soils are shown in Fig. 1(a)–1(c). Le-Ps had higher pH, CEC, and SBET than Le-Ys and Le-Bs. With the increase in Le amendment, the pH and SBET of Le-P decreased, the TOC of Le-P increased, and the CEC of Le-P increased first and then decreased. The pH, CEC, and TOC of Le-Y and Le-B increased, and the SBET of Le-Y and Le-B decreased with increasing Le amendment. The above results were obtained because Le amendment increased the organic matter content on the surface of Le-amended karst soils, resulting in the increased TOC content of Le-amended karst soils (Diao et al., 2022). In addition, the SBET of the Le-amended karst soils decreased due to the mulching effect of Le amendment.
Basic physicochemical properties of different Le-Ps (a), Le-Ys (b), and Le-Bs (c).
3.2 Particle size distribution of Le-amended karst soils
Soil particle size distribution is one of the important physical characteristics of soil (Qi et al., 2018). Fig. 2(a), 2(b), and 2(c) show the particle size distribution of different Le-Ps, Le-Ys, and Le-Bs, respectively. The particle size distribution curves of the tested soil samples were mainly unimodal and have a wave peak near 20 µm. With the increasing Le amendment proportion, the particle size of Le-Ys changed slightly, but the number of large particles of Le-Ps and Le-Bs increased. The above results showed that the particle size of Le-Ps and Le-Bs increased after Le amendment, and the particle size distribution tends to be dispersed. Le amendment changed the pore structure compared with the original soil sample, thereby changing the adsorption capacity of heavy metal ions (Acosta et al., 2009).
Particle size distribution of different Le-Ps (a), Le-Ys (b), and Le-Bs (c).
3.3 FTIR characteristics of Le-amended karst soils
Fig. 3(a), (b), and (c) show the FTIR spectra of different Le-Ps, Le-Ys, and Le-Bs, respectively. Among them, the vibration wave number of 1300–1050 cm−1 is a C–O single bond. The absorption peak with wave number of 3700 cm−1 is O–H or N–H stretching vibration absorption peaks. The absorption peak with a wave number of approximately 1700 cm−1 is mainly the tensile vibration characteristic peak of C=C, indicating that the addition of the extract can introduce more hydroxyl groups into the material, enhance the hydrophilicity of the soil, and contribute to the adsorption of heavy metal ions. The peak positions of the main absorption peaks of the soil samples amended with different proportions of Le are similar, but the relative intensity of the peaks at the same position are different (Bo et al., 2010). The peaks of Le-Ps and Le-Ys are similar, but the peak intensity of Le-Ys at 1090, 1700, and 3700 cm−1 is stronger than that of Le-Ps. The peak position of Le-B is different from that of Le-Ps and Le-Ys at a wave number of approximately 1400 cm−1, and the main effect is the C–H bond. This finding might be related to the difference in the active functional group content of LE, resulting in a different degree of soil acceptance of LE and affecting the amendment degree (Zhao et al., 2023).
FTIR map of different Le-Ps (a), Le-Ys (b), and Le-Bs (c).
3.4 SEM analysis of Le-amended karst soils
Fig. 4(a)–(c) and Fig. 4(d)–(f) are the SEM images of the karst soils before and after 100%Le amendment, respectively. The unamended karst soils mainly showed block-like and foliar-like characteristics with a relatively obvious pore structure. When the karst soils were amended by Le, the original pores were filled, and the structure similar to wall skin and rock was formed. This result was obtained probably because a large amount of dissolved organic matter is attached to the surface of the soil sample, causing the surface tissue to condense. Compared with 100%Le-P and 100%Le-Y, the surface of the 100%Le-Y was more fully filled.
SEM images of P (a), Y (b), B (c), 100%Le-P (d), 100%Le-Y (e), and 100%Le-B (f).
3.5 Isothermal adsorption characteristics of Cd(II) and Pb(II)
The adsorption isotherms of Cd(II) and Pb(II) for each Le-amended karst soils under a temperature of 20 °C, pH of 4, and ionic strength of 0.1 mol/L, are shown in Fig. 5(a)–5(f). Langmuir and Freundlich models were used to fit the adsorption isotherms of Cd(II) and Pb(II) (Table 2). The fitting correlation reached a significant level, and the fitting effect of the Langmuir model was superior to that of the Freundlich model. The adsorption capacity of Cd(II) and Pb(II) in different Le-amended karst soils increased with the increase in equilibrium concentration, gradually tended to saturation, and showed an “L” type trend.
Adsorption isotherms of Cd(II) (a–c), Pb(II) (d–f) by different Le-amended karst soils.
| Sample | Langmuir model | Freundlich model | |||||
|---|---|---|---|---|---|---|---|
| qm (mmol/kg) | b | Correlation coefficient/r | k | n | Correlation coefficient/r | ||
| Cd(II) | P | 92.10 | 2.91 | 0.9204** | 63.63 | 2.52 | 0.9799** |
| 10 %Le-P | 121.06 | 1.51 | 0.9617** | 67.95 | 1.96 | 0.9870** | |
| 20 %Le-P | 181.18 | 0.74 | 0.9829** | 73.81 | 1.62 | 0.9945** | |
| 50 %Le-P | 119.86 | 1.72 | 0.9898** | 70.51 | 2.12 | 0.9928** | |
| 100 %Le-P | 117.35 | 1.33 | 0.9597** | 62.53 | 1.98 | 0.9800** | |
| Y | 93.58 | 3.59 | 0.9788** | 66.47 | 2.62 | 0.9402** | |
| 10 %Le-Y | 125.31 | 1.52 | 0.9760** | 71.00 | 2.13 | 0.9930** | |
| 20 %Le-Y | 134.94 | 1.56 | 0.9781** | 77.66 | 2.08 | 0.9887** | |
| 50 %Le-Y | 110.83 | 2.76 | 0.9874** | 75.19 | 2.44 | 0.9605** | |
| 100 %Le-Y | 108.20 | 2.69 | 0.9899** | 72.55 | 2.44 | 0.9599** | |
| B | 94.01 | 4.56 | 0.9716** | 71.36 | 2.75 | 0.9695** | |
| 10 %Le-B | 130.53 | 1.39 | 0.9783** | 71.03 | 1.95 | 0.9872** | |
| 20 %Le-B | 241.61 | 0.45 | 0.9779** | 73.50 | 1.43 | 0.9891** | |
| 50 %Le-B | 109.94 | 2.86 | 0.9855** | 74.87 | 2.32 | 0.9494** | |
| 100 %Le-B | 105.18 | 2.79 | 0.9764** | 70.64 | 2.32 | 0.9389** | |
| Pb(II) | P | 109.06 | 15.69 | 0.9811** | 169.11 | 2.16 | 0.9058** |
| 10 %Le-P | 179.15 | 8.80 | 0.9922** | 404.98 | 1.46 | 0.9853** | |
| 20 %Le-P | 226.79 | 11.11 | 0.9639** | 737.13 | 1.35 | 0.9538** | |
| 50 %Le-P | 166.94 | 7.43 | 0.9909** | 327.84 | 1.47 | 0.9894** | |
| 100 %Le-P | 162.55 | 7.78 | 0.9923** | 306.45 | 1.53 | 0.9786** | |
| Y | 108.91 | 6.36 | 0.9736** | 136.75 | 1.85 | 0.9867** | |
| 10 %Le-Y | 187.58 | 5.89 | 0.9930** | 339.46 | 1.42 | 0.9809** | |
| 20 %Le-Y | 209.46 | 6.11 | 0.9812** | 423.00 | 1.36 | 0.9703** | |
| 50 %Le-Y | 133.40 | 9.58 | 0.9767** | 229.14 | 1.75 | 0.9825** | |
| 100 %Le-Y | 131.41 | 6.52 | 0.9896** | 188.05 | 1.69 | 0.9888** | |
| B | 143.20 | 11.47 | 0.9935** | 285.37 | 1.70 | 0.9913** | |
| 10 %Le-B | 193.65 | 11.63 | 0.9962** | 561.01 | 1.43 | 0.9911** | |
| 20 %Le-B | 262.12 | 23.76 | 0.9636** | 1619.75 | 1.32 | 0.9601** | |
| 50 %Le-B | 159.64 | 10.71 | 0.9876** | 372.20 | 1.51 | 0.9652** | |
| 100 %Le-B | 151.13 | 11.01 | 0.9844** | 304.86 | 1.68 | 0.9914** | |
Note: ** indicates significant correlation at the P = 0.01 level.
The qm range of Pb(II) by Le-amended karst soils was 109.06–226.79 (Le-Ps), 108.91–209.46 (Le-Ys), and 143.20–262.12 mmol/kg (Le-Bs). The qm range of Cd(II) was 92.10–241.61 mmol/kg, which was lower than that of Pb(II) under the same conditions. The qm values of Le-Ps, Le-Bs, and Le-Ys for Cd(II) and Pb(II) showed an increasing and then decreasing trend with increasing Le amendment ratios and peaked at 20 %. The low Le amendment can increase the adsorption capacity of soil primarily because the high proportion of Le amendment would cover and fill the void of the soil sample, resulting in the reduction of exchangeable points and SBET of the soil surface (Wang et al., 2023), which was also confirmed in Fig. 1. At the same equilibrium concentration, the adsorption amount of the Le-amended soil samples was greater than that of the unamended soil samples. Compared with P soil, the qm values of Cd(II) and Pb(II) of Le-Ps were increased by 49.05 %–107.95 % and 27.42 %–96.72 %, respectively. The qm of Le-Ys and Le-Bs increased by 15.62 %–92.32 % and 5.54 %–157.00 % compared with raw Y and B soil. Overall, Le-amended karst soils had the best adsorption effect on Cd(II) and Pb(II), showing the following trend: 20 %Le amended > 10 %Le amended > 50 %Le amended > 100 %Le amended > unamended. The values of adsorption constant b were all lower than those of unamended soil and negatively correlated with the percentage of Le amendment. The adsorption strengths n were all greater than 1, which indicated a strong adsorption affinity. These results are mainly due to the ability of plant Le to interact with the pollutants, thereby affecting the adsorption processes of the pollutants on the soil solid phase and providing relief to the contaminated soil (Liu et al., 2022).
3.6 Effect of pH on the adsorption of Cd(II) and Pb(II)
When the pH value was in the range of 2–6, the adsorption amount of Cd(II) and Pb(II) by Le-amended karst soils increased with pH, showing a positive correlation (Fig. 6). The adsorption amount of Cd(II) by different Le-amended karst soils increased by 22.64 %–46.88 % (Le-Ps), 13.99 %–81.09 % (Le-Ys), and 42.47 %–58.84 % (Le-Bs). The increase in Pb(II) adsorption by Le-Ps, Le-Ys, and Le-Bs ranged from 4.71 % to 25.21 %, from 10.96 % to 80.84 %, and from 3.86 % to 10.84 %, respectively. The reason is that when the pH is low, the solution contains a large amount of H+, and the Le-amended karst soils are more prone to protonation, which is not conducive to their chelation with Cd(II) and Pb(II), and hinders the ion-exchange adsorption between the test soil samples and Cd(II) and Pb(II), thereby reducing the adsorption amount (Wang et al., 2022). By contrast, the negative electrification of the test soil samples increased with the increase in pH (Huang et al., 2014), The electrostatic gravitational force between the soil samples and Cd(II) and Pb(II) was increased, which promoted the adsorption effect of the test soil samples on heavy metal ions.
Effect of pH on Cd(II) (a, c, and e) and Pb(II) (b, d, and f) adsorption.
3.7 Effect of ionic strength on Cd(II) and Pb(II) adsorption
The adsorption amount of Cd(II) and Pb(II) by each Le-amended karst soil gradually increased with the increase in ionic strength in the range of 0.01–0.2 mol/L (Fig. 7). Compared with the original soil samples, the increase in Cd(II) and Pb(II) adsorption by Le-Ps ranged from 18.52 % to 28.92 % and from 6.66 % to 21.10 %, respectively. The increase brought about by Le-Ys and Le-Bs ranged from 5.67 % to 56.48 % and from 4.19 % to 75.39 %, respectively. When the ionic strength increased from 0.1 mol/L to 0.2 mol/L, the adsorption amount of Cd(II) and Pb(II) for the Le-amended karst soils decreased. The strongest adsorption capacity of Cd(II) and Pb(II) was observed at an ionic strength of 0.1 mol/L. The above results were obtained because with the increase in ionic strength, Na+ and Cl− in the solution increased, and the bilayer structure of each test soil was compressed (Deng et al., 2024). The charge density became larger, and the electronegativity became stronger, which promoted the electrostatic interactions between the surface charge of the soil samples and the heavy metal ions. In addition, the density of adsorption sites on the soil surface increased, which contributed to the increase in the amount of the adsorption of Cd(II) and Pb(II). When the ionic strength was further increased, Na+ in the solution competes with Cd(II) and Pb(II) for adsorption, which affected the electrostatic gravitational force between the soil samples and Cd(II) and Pb(II), and then led to a decrease in the adsorption amount.
Effect of ionic strength on (a, c, and e) and Pb(II) (b, d, and f) adsorption.
3.8 Effect of temperature on the adsorption of Cd(II) and Pb(II)
The effect of temperature on Cd(II) and Pb(II) adsorption by Le-amended karst soils are shown in Fig. 8(a)–8(f). The adsorption amount of Cd(II) and Pb(II) increased with the increase in temperature within 10 °C–30 °C, which showed a positive temperature effect. The increase in Cd(II) and Pb(II) adsorption by Le-Ps, Le-Ys, and Le-Bs were 3.19 %–53.16 %, 4.98 %–30.83 %, and 6.27 %–36.74 %, respectively. The above results were mainly related to the chemical (endothermic) adsorption of Cd(II) and Pb(II) by the Le-amended karst soils, and the increase in temperature made the irregular thermal movement between molecules became more intense, which was favorable to enhancing the adsorption effect. The results proved that after amendment by the Le from Ficus virens, the ion exchange, precipitation, and complexation between Le-amended karst soils and Cd(II) and Pb(II) were enhanced (Zhao et al., 2021).
Effect of temperature on Cd(II) (a, c, and e) and Pb(II) (b, d, and f) adsorption.
The results of thermodynamic parameters for Cd(II) and Pb(II) adsorption by different Le-amended karst soils are shown in Table 3. The apparent free energy (ΔG) of Cd(II) and Pb(II) adsorption at 10 °C and 30 °C were less than 0, indicating that the adsorption process was spontaneous and that the spontaneity was stronger at 30 °C than 10 °C under the same treatment. The apparent enthalpy (ΔH) values of Cd(II) and Pb(II) adsorption were all positive, indicating that the adsorption process was a heat-absorbing reaction. The increase in temperature favored the adsorption of Cd(II) and Pb(II) by Le-amended karst soils, which is consistent with the positive temperature effect shown in Fig. 8 and is also in line with the results of previous studies (Liu et al., 2017). The entropy values (ΔS) of Cd(II) and Pb(II) adsorption were greater than 0, indicating that the adsorption process was an entropy-increasing reaction. This finding was attributed to the different adsorption mechanisms of Cd(II) and Pb(II) by the soil itself and the change in Le amendment percentage, leading to the increase in system chaos.
| Samples | △G (kJ/mol) | △H (kJ/mol) | △S (J/mol/K) | ||
|---|---|---|---|---|---|
| 10 °C | 30 °C | ||||
| Cd(II) | P | −18.06 | −20.10 | 3.96 | 77.74 |
| 10 %Le-P | −16.70 | −18.45 | 3.66 | 71.93 | |
| 20 %Le-P | −15.05 | −16.65 | 3.72 | 66.30 | |
| 50 %Le-P | −17.28 | −18.78 | 3.06 | 71.84 | |
| 100 %Le-P | −16.03 | −18.13 | 4.53 | 72.61 | |
| Y | −18.69 | −20.63 | 3.64 | 78.86 | |
| 10 %Le-Y | −16.93 | −18.47 | 3.21 | 71.11 | |
| 20 %Le-Y | −16.90 | −18.53 | 3.40 | 71.69 | |
| 50 %Le-Y | −18.37 | −19.97 | 3.08 | 75.75 | |
| 100 %Le-Y | −18.25 | −19.90 | 3.20 | 75.75 | |
| B | −19.65 | −21.23 | 2.86 | 79.49 | |
| 10 %Le-B | −16.75 | −18.24 | 3.15 | 70.26 | |
| 20 %Le-B | −13.84 | −15.40 | 3.94 | 62.79 | |
| 50 %Le-B | −18.36 | −20.06 | 3.27 | 76.37 | |
| 100 %Le-B | −18.13 | −20.00 | 3.61 | 76.78 | |
| Pb(II) | P | −22.40 | −24.35 | 3.07 | 89.97 |
| 10 %Le-P | −21.35 | −22.89 | 2.56 | 84.47 | |
| 20 %Le-P | −21.78 | −23.48 | 2.78 | 86.71 | |
| 50 %Le-P | −20.87 | −22.46 | 2.72 | 83.30 | |
| 100 %Le-P | −20.99 | −22.58 | 2.69 | 83.64 | |
| Y | −20.41 | −22.07 | 2.90 | 82.29 | |
| 10 %Le-Y | −20.21 | −21.88 | 2.93 | 81.71 | |
| 20 %Le-Y | −20.41 | −21.97 | 2.73 | 81.69 | |
| 50 %Le-Y | −21.50 | −23.11 | 2.65 | 85.30 | |
| 100 %Le-Y | −20.62 | −22.14 | 2.62 | 82.06 | |
| B | −21.94 | −23.56 | 2.63 | 86.77 | |
| 10 %Le-B | −21.99 | −23.59 | 2.60 | 86.84 | |
| 20 %Le-B | −23.54 | −25.39 | 2.80 | 93.01 | |
| 50 %Le-B | −21.77 | −23.39 | 2.64 | 86.20 | |
| 100 %Le-B | −21.84 | −23.46 | 2.64 | 86.43 | |
3.9 Adsorption mechanism of Cd(II) and Pb(II)
3.9.1 Correlational analyses
The physicochemical properties of soil have an important effect on the adsorption of heavy metal ions. The Pearson correlation analysis between physicochemical properties of Le-amended karst soil and adsorption parameters of Cd(II) and Pb(II) was conducted, and the results were shown in Fig. 9(a)–9(d). When the significance of the normality test result was greater than 0.05, the data obeyed the normal distribution. The adsorption parameters of Cd(II) and Pb(II) by Le-Ps, Le-Ys, and Le-Bs were not significantly correlated with physicochemical properties, indicating that the pH, CEC, TOC, and SBET of Le-amended karst soils had minimal influence on its adsorption capacity of Le-amended karst soils. This result is similar to the previous results of the effect of Le proportion on Cd(II) and Pb(II) adsorption amount, that is, a high Le amendment ratio does not necessarily increase the adsorption effect.
Correlation between physicochemical properties and Cd(II) (a and b) and Pb(II) (c and d) adsorption parameters. Note: * indicates significance at the P = 0.05 level.
3.9.2 XPS analysis
According to the XPS full spectrum analysis in Fig. 10(a), 100 %Le-B mainly contains C1s, O1s, Al3p, Na1s, and Si2p. The high-resolution C1s, O1s, Cd3d, and Pb4f spectra of 100 %Le-B before and after Cd(II) and Pb(II) adsorption are shown in Fig. 10(b)–10(e). The high resolution XPS C1s spectrum was decomposed into three peaks: C–C (284.80 eV), C–O–C (286.55 eV), and O–C = O (288.84 eV). After contamination, the peak intensity of C–C, C–O–C, and O–C = O decreased, indicating that the organic structure of the sample changed after contamination, and these groups may participate in the adsorption of Cd(II) and Pb(II) through surface complexation (Wang et al., 2021). The O1s spectrum shows that the O1s peaks at 531.82 and 532.64 eV are O–H and C–O, respectively. The reduction of C–O and O–H contents after pollution indicates that O-containing groups can promote the adsorption of metal ions (Cheng et al., 2023). Thus, the complexation between the organic phase of 100 %Le-B and Cd(II) and Pb(II) was proven. The adsorption of Cd(II) and Pb(II) by 100 %Le-B was verified by XPS analysis of Cd3d and Pb4f. The binding energies of Cd3d5/2 and Cd3d3/2 after adsorption were 405.82 and 412.85 eV respectively, which proves that Cd(II) was in the oxidation state. According to the analysis of Pb4f, the binding energies of Pb4f7/2 and Pb4f5/2 after adsorption were 138.64 and 143.55 eV, respectively, and Pb(II) was in the oxidation state. In addition, the changes in the intensity of Al2p and Si2p peaks after adsorption also indicated that the adsorption process was accompanied by ion exchange and electrostatic interaction.
XPS survey (a), C1s (b), O1s (c), Cd3d (d), and Pb4f (e) spectra of 100 %Le-B before and after adsorption.
3.9.3 Mechanism analysis
The possible adsorption mechanism found by the above study is shown in Fig. 11. Cd(II) and Pb(II) adsorption by Le-amended karst soils has four forms: electrostatic adsorption, chemical precipitation, complexation, and ion exchange. The negatively charged surface of Le-amended karst soils attracts Cd(II) and Pb(II) by electrostatic attraction to maintain electroneutrality, thereby making the concentration of these ions near the surface of the colloid greater than that of the native solution. Cd(II) and Pb(II) are adsorbed onto the negatively charged adsorption sites of Le-amended karst soils through electrostatic action, and the more charge the soil surface carries, the stronger the adsorption capacity for Cd(II) and Pb(II). The Le-amended karst soils contains various functional groups that have strong complexation and ion-exchange capabilities for Cd(II) and Pb(II), such as complexation between Cd(II) and Pb(II) and carboxyl groups (Wang et al., 2023). Therefore, the above results confirm that karst soils amended by Le can remove Cd(II) and Pb(II) from the solution efficiently, which is an economical and environmentally friendly technical means. In summary, Cd(II) and Pb(II) adsorption by the dissolved organic matter formed by Le is the result of the joint action of different adsorption mechanisms, and it shows excellent adsorption capacity for Cd(II) and Pb(II).
Possible mechanism of Cd(II) and Pb(II) adsorption by different Le-amended karst soils.
3.10 Reuse performance of Le-amended karst soils
The mass loss of materials and the adsorption amount of Cu(II) and Pb(II) after three rounds of regeneration are shown in Fig. 12(a) and 12(b), respectively. The mass loss of 100 %Le-P, 100 %Le-Y, and 100 %Le-B after the first regeneration was approximately 13 %, 10 %, and 6 %, respectively. With the increase in regeneration time, the mass loss of 100 %Le-P, 100 %Le-Y, and 100 %Le-B gradually decreased, and the loss rate of the third regeneration was approximately 5.0 %. After one regeneration, the adsorption amounts of Cd(II) and Pb(II) on 100 %Le-P, 100 %Le-Y, and 100 %Le-B remained over 88 % of that on the non-regenerated soils. With the extension of regeneration time, the adsorption amounts of Cd(II) and Pb(II) decreased, and the decrease rate of 100 %Le-P and 100 %Le-Y was greater than that of 100 %Le-B. Therefore, 100 %Le-B is a good cyclic adsorption soil sample; after three rounds of regeneration, the adsorption effect can still reach approximately 85 % of the raw soil sample. Table 4 compares the adsorption amount of Le-amend B with other similar adsorbents. The comparison reveals that the Le-amended karst soil used in this study exhibited a good adsorption effect on Cd(II) and Pb(II). Compared with other similar adsorbents, Le-B had a better adsorption effect on Cd(II) and Pb(II).
Mass loss (a) and Cu(II) and Pb(II) adsorption amount (b) of 100 %Le-amended karst soils after regeneration. Different lowercase letters indicate significant differences among three regenerations at the P = 0.05 level.
| Pollutant | Adsorbent | qm (mmol/kg) | Reference |
|---|---|---|---|
| Cd(II) | Humic acid amended loess soil | 31.39 | Jiang et al. (2006) |
| Amphoteric-anionic modified yellow brown soil | 77.85 | Liu et al. (2017) | |
| Acidic paddy soil | 27.13 | He and Xu (2022) | |
| Pb(II) | Napa soil | 117.71 | Mawardi and Zainul (2015) |
| Paddy soil | 10.88 | Li et al. (2017) | |
| Organomodified clay soil | 200.26 | Ottard et al. (2023) | |
| Cd(II) and Pb(II) | Le-amended B | 241.61 for Cd(II); 262.12 for Pb(II) | This study |
4 Conclusion
We have successfully prepared Le-amended karst soils for Cd(II) and Pb(II) adsorption, and they had qm values of 92.10–241.61 and 108.91–262.12 mmol/kg, respectively. The optimum adsorption conditions were 30 °C, pH = 6, and an ionic strength of 0.1 mol/L. The adsorption process was a spontaneous, endothermic, and entropy-increasing reaction. The adsorption amount of Cd(II) and Pb(II) by Le-B maintained approximately 85 % of the original sample after three rounds of regeneration. In conclusion, amendment with Le enhanced the functional groups and activities, making Le a promising material for energy production and environmental remediation applications. The Le-amended karst soils proposed in this study have economic, ecological, and high application value for Cd(II) and Pb(II) remediation.
CRediT authorship contribution statement
Mengting Guo: Writing – review & editing, Visualization. Tianjiang Jin: Writing – review & editing. Lina Wen: Supervision, Writing – review & editing. Bixia Wang: Supervision, Writing – review & editing. Jing Lin: Supervision, Writing – review & editing. Wenbin Li: Conceptualization, Methodology, Formal analysis, Validation, Investigation, Writing – review & editing, Visualization. Hongyan Deng: Methodology, Formal analysis, Supervision, Writing – review & editing.
Acknowledgements
The authors wish to acknowledge and thank the Sichuan Transportation Technology Project (2021-ZL-8), the Fundamental Research Funds of China West Normal University (20A022), and the Tianfu Scholar Program of Sichuan Province (2020-17).
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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