Remediation of Cd and Cu Contaminated Agricultural Soils near Oilfields by Biochar Combined with Sodium Humate-Wood Vinegar

Abstract

Soil contaminations by heavy metals near oilfields have been widely reported and are causing great concern. Thus, it is highly desirable to develop cost-effective materials and methods to avoid heavy metal residues contaminating soil and food. An effective, environmentally friendly, and inexpensive remediation material for heavy metal-polluted soil was designed and prepared using biochar (BC) combined with humic acid (HA) resulting from sodium humate (NaHA) simply reacting with wood vinegar (BHW). After adding BHW, the chemical fractions of copper and cadmium in the soil undergo larger changes. Meanwhile, the availability of heavy metals decreases. The maximum adsorption capacity of copper and cadmium in the soil using the BHW is larger than that only using biochar. The adsorption kinetics ensures that the adsorption process of Cd²⁺ and Cu²⁺ ions on BHW is chemical adsorption, which is best fitted using the pseudo-second-order rate equation. The thermodynamics guarantees that the metal ions adsorb on the heterogeneous surface of BHW in multilayer, which is credited to the enhancement of oxygen-containing groups in the biochar combined with the humic acid. The remediation material BHW holds promise for the immobilization of heavy metal in the soils and could be recommended based on its economic feasibility, high efficacy, and environmental safety.

1. Introduction

Heavy metal pollution is a serious global problem, which can cause issues of environmental safety and human health problems. Globally, more than 10 million sites are considered soil contaminated, of which more than 50 percent are polluted with heavy metals [1]. As a cause of rapid economic and societal development, heavy metal soil pollution has increasingly become a serious environmental problem, especially in the field of the petroleum industry and agriculture [2]. The total exceeding standard in China’s (China’s second-level soil environmental quality standard) soil quality rating is 16.1 percent [3]. The soil of agricultural and industrial areas in China is partially contaminated by heavy metals, and the arable land resources are declining [4]. The blowout accident or leakage in a given oilfield will pollute the surrounding soil and make the heavy metals and other harmful substances seriously invade and damage the soil quality. While applying the waste to the soil, excess heavy metals can cause changes in soil fertility and reduce the size and quality of crop plants [5]. Heavy metals often coexist in contaminated soil and can accumulate in the human body through the food chain of soil crops [6]. Subsequently, heavy metals are absorbed by plants into the food chain and accumulated in animals and humans, where they can be toxic [7]. In recent years, extensive attention has been paid towards research on environmentally friendly, low-cost materials for soils contaminated by metal [8].

In situ immobilization of heavy metals via chemical stabilization becomes a cost-effective and particularly convenient method to reduce the availability and uptake of heavy metals by plants [9,10]. Biochar (BC) has proved particularly effective in modifying techniques for adsorbing heavy metals and reducing their potential availability [11,12]. The in situ immobilizing of heavy metals in contaminated soil is an effective and low-cost remediation technique to decrease the bioavailability and mobility of toxic elements [6,13]. As a consequence, compared with other remediation technologies such as mechanical separation, chemical washing, and electrodynamic remediation, biochar remediation of contaminated soil has shown special effects on immobilizing heavy metals in soil and reducing the uptake of heavy metals by plants [10,14]. The foremost mechanisms of biochar immobilizing heavy metals in soil include precipitation, increased physical adsorption, enhanced ion exchange capacity, and alkalization [15]. The degree of stable immobilization of heavy metals by biochar is related to the biogeochemical processes that biochar undergoes in soils [16,17]. In particular, the surface properties of biochar are affected by the chemical, physical, and biological processes that are caused by the aging of biochar [18]. Under aerobic conditions, compost produced by spontaneous microbial oxidation of agricultural straw, organic waste and livestock manure has proved to be a very effective soil conditioner for heavy metal contamination [12].

Recently, wood vinegar (WV) has been used to compost solid wastes and charcoal to adsorb and immobilize metal contaminants, such as nickel, zinc, and copper. Wood vinegar can change the structure of organic matter during composting in metal-contaminated soils. The composted organic matter, such as humic acid substances which are important organic matter in soil, can generate insoluble metal-organic compounds, thus lowering the amount of bioavailable metal in the soil [19].

A few previous studies show humic acid or other organic acids can enhance biochar to immobilize and adsorb heavy metals [20,21], such as lead and nickel. At relatively low doses, biochar, humic substances or wood ash can recover pristine soil from multi-contaminated soils and reduce Pb Cu, and Cd mobility in soils [21]. The formation of complexes between humic acid and Chrome (III) or Chrome (VI) affected the flowability of metal ions and reduced the concentration of free metal species to reduce the heavy metal toxicity of chromium to organisms [22]. Nevertheless, there is very little research on the effects of BC combined with humic acids or other organic acids on the availability and mobility of heavy metals in agricultural soils. The mobility and availability of heavy metals in soils depend on the chemical fractions of them in the soils [12,23]. Therefore, it is necessary to investigate the effect of humic acids or organic acids on biochar interacting with heavy metals, and the subsequent mobility and availability of heavy metals in agricultural soils.

Copper [10] and cadmium [18] are major pollutants in polluting mines, they especially affect farmland near copper mines and oilfields. the mobility and availability of these pollutants are of great concern to China [6,24]. Consequently, this study is focused on agricultural soils contaminated with copper and cadmium [14]. We sought to find out an immobilization material BHW made by BC combined with humic acid resulting from sodium humate simply reacting with wood vinegar. Moreover, wood vinegar is rich in organic acids and generated from the carbonization of wood. The BHW is environmental, high-efficient, economically feasible, and food safe; it therefore holds promise for the recommendation for the remediation of contaminated soils by metals.

2. Materials and Methods

2.1. Soil Samples and Chemicals

The diagram for the experiment route has been presented in Figure S1. A sample of 0–0.20 m surficial soil of agricultural land near the oilfield was taken. The soil was air dried. The soil was purified through a 120-mesh sieve. Then the soil was artificially polluted using a copper nitrate (or cadmium nitrate) solution to prepare a contaminated soil. The chemical and physical properties of the experimental soil were displayed in Table S1. Based on the 2nd level of heavy metal risk control standard (400 mg∙kg⁻¹ for copper and 100 mg∙kg⁻¹ for cadmium) of GB15618-2018 of China, the copper nitrate, (or cadmium nitrate) solution including 400 mg Cu²⁺ (or 100 mg Cd²⁺) was prepared then evenly mixed into 1.0 kg of soil. The contaminated soil samples were cultured for 1 month. The mean value of 3 replicate samples was used for discussion. The information on chemicals in the research was shown in Table S2.

2.2. Preparation Procedure of BHW Remediation Materials

The material composition of every group was expressed in Table S3. We prepared the BHW remediation materials using BC powder, NaHA, and WV solutions having a mass ratio of 1.5:1:1 [25]. BC was mixed with NaHA, crushed thoroughly, dried, and added by WV. The above mixtures were dried in the air to become fine granules. Then the fine granules were sieved by a sifter to get the remediation material. As a control group, under the same experimental conditions, the BC group was composed of only BC used as the control group. In the meantime, under the same processing conditions, the group of BC-NaHA was prepared and consisted of BC and NaHA.

2.3. Remediation Experiment

The six treatment groups for the remediation experiment were respectively named BHW-1, BHW-2, BHW-3, BHW-4, BHW-5, and BHW-6. They were contaminated soils which contain 0.04 g~0.5 g∙kg⁻¹ of BHW materials. The same relative quantities of BC-NaHA treatment groups and BC treatment groups were also prepared, respectively. The treatment groups involving BC, BC-NaHA and BHW were shown in

Table 1. The treatment groups include BC, BC-NaHA and BHW.

	Treatment Dosage (g∙kg−1)	BC	BC-NaHA	BHW
Control group	0	-	-	-
1#	0.04	BC-1	BC-NaHA-1	BHW-1
2#	0.1	BC-2	BC-NaHA-2	BHW-2
3#	0.2	BC-3	BC-NaHA-3	BHW-3
4#	0.3	BC-4	BC-NaHA-4	BHW-4
5#	0.4	BC-5	BC-NaHA-5	BHW-5
6#	0.5	BC-6	BC-NaHA-6	BHW-6

. Each remediation material and the contaminated soil samples were thoroughly blended. Then they were cultured for 30 days under the temperature of 25 ± 1.0 °C and turned several times. After aging, the contaminated soil was employed as a control group without any remediation [25,26]. We prepared 3 replicated samples for every treatment group to guarantee the accuracy of the experimental data.

2.4. The Measurement of Metal Content of Different Fractions

The experimental soil samples were cultured in the same environment for 30 days. The metal contents of theirs were detected by the five-point sampling method. Using 0.01 mol·L⁻¹ CaCl₂ as an extraction agent at the soil-solution ratio of 1:10, the available metal contents in the experimental soil samples were extracted [27,28]. The reduction rate of available copper and cadmium indicates that the incorporation of the remediation material reduces the percentage content of available copper and cadmium compared to the control sample [21,27]. It was necessary to quantify the available, exchangeable and reducible fractions after the remediation treatment for assessing metal mobility [29]. According to Table S4, the chemical compositions of Cu and Cd in control soil and BHW-restored soil were performed using the BCR (European Community Bureau of Reference) sequential extraction method [30].

The contents of Cu²⁺ and Cd²⁺ ions in different fractions were determined by a graphite furnace atomic absorption spectrometer (AAS) [26]. The sample solution was prepared to 100 mL with deionized water and then filtered with a 0.22 µm filter. After that, the filtrate solution was used to measure the content of soluble Cu²⁺ and Cd²⁺ ions by the AAS. The detailed parameters of measurement were followed. The photomultiplier tube voltage is 540 V, at the same time, the lamp current is 7.5 mA. Besides, the slit was 1.3 nm, and the wavelength was 283.3 nm. The atomization temperature was 1900 °C and the ashing temperature was 650 °C [26]. Three replicates were carried out for every sample and the mean value of 3 replicates was used for the research.

2.5. Adsorption Properties of Remediation Materials

2.5.1. Adsorption Kinetics

Using 0.01 mol·L⁻¹ of NaNO₃ as background, Cu²⁺ solution and Cd²⁺ solution were prepared in two 1000 mL beakers, their initial concentrations were 400 mg·L⁻¹ and 100 mg·L⁻¹, respectively. Under the temperature of 25 ± 0.5 °C, stirring with a thermostatic magnetic stirrer, 0.5 g of BHW was added into the solution with Cu²⁺ or the Cd²⁺ solution. At the adsorption time of 5, 10, 30, 60, 120, 180, 240, 360, 480, 600, 720, and 1440 min, the solution samples were quickly drawn with a pipette to pass through a 0.22 µm filter membrane. The content of copper and cadmium in the filtrate was measured to measure their adsorption capacity by an AAS, respectively.

2.5.2. Adsorption Isotherms

0.2 g BHW was put in a 50 mL polyethylene centrifuge tube, after that, with 0.01 mol·L⁻¹ NaNO₃ as the background, a 20 mL Cu²⁺ solution was added to the 50 mL polyethylene centrifuge tube. Then, the original concentrations of the cadmium solutions were set as 5, 10, 20, 50, 80, 100, 200, and 300 mg·L⁻¹. Meantime, the original concentrations of the copper solutions were set as 10, 20, 50, 80, 100, 200, 300, 400, 500, and 600 mg·L⁻¹. The volume of all solutions was set as 50 mL. Then the centrifuge tube was oscillated at 200 r·min⁻¹ for 24 h in a shaking incubator with a constant temperature of 15 °C, 25 °C and 35 °C, respectively. After oscillation, the sample was centrifuged using a centrifuge at 8 × 10³ r·min⁻¹ for 10 min. In succession, the supernatant solution was filtered by a 0.22 µm filter membrane. Then the concentration of copper and cadmium in the filtrate was measured by the AAS, and the adsorption capacity of BHW was calculated.

2.5.3. Competitive Adsorption

The partition coefficient was an important parameter describing the adsorption characteristics of heavy metals in the soil. This was particularly crucial to determine the selection order of remediation materials under the co-competitive action of multiple metal ions through the distribution coefficient. A 0.5 g of soil without BHW was set as Soil-A. A 0.5 g of soil containing 0.2% BHW was set as Soil-B. They were put into 100 mL polyethylene centrifuge tubes, respectively. Two groups of 50 mL mixed solutions contained three initial concentrations of 100, 200 and 300 mg·L⁻¹ of Cd²⁺ and Cu²⁺, respectively. The pH value of the solutions was 5.0 The two groups of blended solutions were poured into Soil-A and Soil-B, respectively. The blended solutions were centrifuged at 8 × 10³ r·min⁻¹ for 10 min, and after oscillated at a constant temperature (25 ± 0.5 °C) for 24 h. The supernatant solutions were passed through a 0.22 µm filter. The concentrations of heavy metals in the filtrate were measured and the adsorption capacity was calculated.

$$K_d=\frac{Q_e}{C_e}$$

(1)

where, K_d is the distribution coefficient of heavy metals at the soil-water interface; C_e means the equilibrium concentration of metal ions in the blended solution, mg·g⁻¹; Q_e refers to the capacity of metals adsorbed by the soil, mg·g⁻¹.

2.6. Data Statistics

2.6.1. Adsorption Capacity

The amount of adsorption of heavy metals was calculated according to the following formula:

$$Q_e=\frac{C_0-C_e}{\mathrm{m}}\times \frac{\mathrm{V}}{1000}$$

(2)

where, Q_e refers to the amount of adsorption of metals, mg·g⁻¹; C₀ means the initial concentration of metals, mg·L⁻¹; V represents the volume of the solution, mL; m is the mass of the soil sample, g; C_e means the equilibrium concentration of metals, mg·L⁻¹.

2.6.2. Adsorption Kinetic

(1)

Pseudo first-order rate equation

$$In\left(Q_e-Q_t\right)=In{Q}_e-K_{1}t$$

(3)

where, Q_e refers to the adsorption amount at equilibrium, mg·g⁻¹; Q_t means the adsorption amount at the time t, mg·g⁻¹; K₁ is the pseudo-first-order rate constant, min⁻¹; t represents the reaction time, min.

(2)

Pseudo-second-order rate equation

$$\frac{t}{Q_t}=\frac{t}{Q_e}+\frac{1}{K_2{Q}_{e^2}}$$

(4)

where, Q_e refers to the adsorption amount at equilibrium, mg·g⁻¹; Q_t means the adsorption amount at the time of t, mg·g⁻¹; K₂ is the pseudo-second-order rate constant, min⁻¹; t represents the reaction time, min.

2.6.3. Adsorption Isotherm

(1)

Langmuir adsorption isotherm

$$\frac{C_e}{Q_e}=\frac{C_e}{Q_{max}}+\frac{1}{K_L{Q}_{max}}$$

(5)

where, C_e refers to the concentration at adsorption equilibrium, mg·L⁻¹; Q_e represents the adsorption capacity at equilibrium, mg·g⁻¹; K_L means a constant related to the size of the adsorption energy, L·mg⁻¹; Q_max refers to the adsorption capacity at maximum, mg·g⁻¹.

(2)

Freundlich adsorption isotherm

$$In{Q}_e=\frac{1}{n}In{C}_e+In{K}_F$$

(6)

where, C_e refers to the concentration at adsorption equilibrium, mg·L⁻¹; K_F represents a constant related to the adsorption strength, mg·g⁻¹; Q_e means the adsorption amount at equilibrium, mg·g⁻¹; n refers to the heterogeneity of the adsorbent. The factor, when n > 1, indicates that there is a strong force between the adsorbate and the adsorbent.

All data of this work are analyzed by the single-factor analysis of variance, the data is the mean ± standard deviation, and lowercase letters indicate significant differences between different groups under the same percentage (p < 0.05).

2.7. Fourier Transform Infrared Spectroscopy and X-ray Photoelectron Spectrometer Measurements

All BHW, BC-NaHA, BC and their immobilization samples were rinsed three times with ultrapure water before and after immobilizing metal ions to remove all the physisorbed metal ions. Then under vacuum, the samples were dried overnight for all measurements [17,25]. Using the KBr disc technique, Fourier Transform Infrared Spectroscopy (FT-IR) spectra of the samples were carried out by a VECTOR-22 Fourier transform infrared spectrometer (Bruker, Germany) [23].

X-ray Photoelectron Spectrometer (XPS) test was run by an XPS spectrometer (AXIS Supra type, Kratos, UK). The spectra of all BHW, BC-NaHA, BC and their immobilization samples were obtained over the range of 0 to 1200 eV. The slit width of the analyzer was 1.9 mm and in energy the analyzer was 300 eV. The binding energy was calibrated using the C1s peak as 284.60 eV [25,31,32].

3. Results and Discussion

3.1. Effect of BHW on the Availability of Cu and Cd

Figure 1 displays the effects of different dosages and types of restoration materials on the available copper and cadmium content in the soil. The addition of BHW material significantly reduced the effective Cu²⁺ and Cd²⁺ content extracted by CaCl₂, and the effective Cu²⁺ and Cd²⁺ content of the three materials was the lowest. Compared with the control group (Figure 1a), at the addition level of 0.8 g·kg⁻¹, BC, BC-NaHA, and BHW reduced the effective content of copper by 28.88%, 38.76%, and 47.50%, respectively, which is higher than the report of Rong et al. [33] that humic acid reduces the effective Cu²⁺ content by 18%. Compared with the control group (Figure 1b), when the added amount was 1.6 g·kg⁻¹, BC, BC-NaHA, and BHW reduced the effective content of copper by 16.67%, 30.14%, and 38.47%, respectively. This is higher than the result of Rong et al. [33] that humic acid reduces the effective Cd²⁺ content by 37%. This implies that the addition of wood vinegar can enhance the ability of BC-NaHA to adsorb copper and cadmium ions. In other words, BHW has a better solidification effect on Cu²⁺ and Cd²⁺ ions in the soil at a lower dose and has a significant effect on reducing the available content of copper and cadmium.

3.2. Effect of BHW on the Chemical Fractions of Cu and Cd

Figure 2 shows the changes in the chemical forms of copper and cadmium in the soil before and after BHW material treatment. After treatment with 0.8 g of BHW material (Figure 2a), the exchangeable and reducible components of Cu²⁺ decreased by 9.30% and 11.79%, respectively. At the same time, the Cu²⁺ of the oxidizable component and the residue component rise by 5.47% and 15.62%, respectively, and the results of the copper form transformation law are consistent with the literature [34,35]. After treatment with 1.6 g of BHW material (Figure 2b), the exchangeable and reducible components of Cd²⁺ decreased by 12.08% and 4.80%, respectively. At the same time, the Cd²⁺ of the oxidizable component and the residual component rise by 8.28% and 15.58%, respectively. The results of the cadmium form transformation law are consistent with the literature [34,35]. Therefore, BHW can effectively reduce the Cu²⁺ and Cd²⁺ contents of the exchangeable and reduced components in the soil, thereby reducing the toxicity and bioavailability of copper and cadmium [19].

3.3. Effect of BHW on Competitive Adsorption

Under the condition of the coexistence of binary heavy metals, the competitive adsorption of binary heavy metals on soil (Soil-A) is shown in Figure 3a, and the competitive adsorption of binary heavy metals on BHW-containing soil (Soil-B). As shown in Figure 3b. At the initial concentration which is set in the experiment, the adsorption amount Q_e of binary heavy metals on Soil-A and Soil-B is in the order of Cu²⁺ > Cd²⁺.

Table 2. The partition coefficient and joint partition coefficient at different initial concentrations of metal ions.

Partition Coefficient	Soil-A		KdΣsp1	Soil-B		KdΣsp2
	Cu	Cd		Cu	Cd
Kd100	0.073	0.028	0.101	0.149	0.039	0.188
Kd200	0.052	0.021	0.073	0.063	0.024	0.087
Kd300	0.066	0.028	0.094	0.087	0.034	0.121

shows the partition coefficient and joint partition coefficient K_dΣsp1 of each metal in soil without BHW (Soil-A), and the partition coefficient and joint partition coefficient K_dΣsp2 of each metal in soil with BHW (Soil-B). In soil and BHW-containing soil systems, the order of the partition coefficient of the Cu-Cd competition system is Cu > Cd, and the priority order of adsorption is Cu > Cd, which is in line with the results reported in the literature [9]. The joint partition coefficient K_dΣsp2 of soil heavy metals with BHW is higher than the joint partition coefficient K_dΣsp1 of soil without BHW, which indicates that BHW increases the rate of soil rapid adsorption of Cu²⁺ and Cd²⁺ ions.

3.4. Adsorption Kinetic

Figure 4a presents the change in the capacity of Cu²⁺ and Cd²⁺ ions adsorbed by BHW over time. From Figure 4a, the adsorption amount of BHW for Cu²⁺ ions rose rapidly in the first 60 min, slowly increased in 60–120 min, and gradually reached equilibrium at 180 min, the equilibrium adsorption capacity was 43.60 mg·g⁻¹. The adsorption capacity of Cd²⁺ ions by BHW increased rapidly in the first 120 min, the slow ascending stage was 120–180 min, and gradually reached equilibrium at 240 min, and the equilibrium adsorption capacity was 26.66 mg·g⁻¹. The reason is that there are abundant adsorption sites on the BHW surface at the initial stage of adsorption. After Cu²⁺ and Cd²⁺ ions occupied the sites rapidly, the surface-active vacancy sites of BHW were greatly reduced. Cu²⁺ and Cd²⁺ ions began to penetrate BHW, resulting in a decreased adsorption rate. As the adsorption time increases, the amount of Cu²⁺ and Cd²⁺ ions absorbed by BHW also increases, because the concentration difference driving force of the solution at high concentration promotes the infiltration of Cd²⁺ and Cu²⁺ ions into BHW material, and the utilization rate of adsorption sites expands.

Figure 4b shows the linear fitting curve diagram of the pseudo-second-order rate equation. The adsorption rate of Cu²⁺ ions by BHW is much faster than that of Cd²⁺ ions.

Table 3. Fitting parameters of adsorption kinetics of BHW adsorption of Cd²⁺ and Cu²⁺.

Linear Model Type	Parameters	Cu2+	Cd2+
Quasi-first-order	Qe (mg·g−1)	42.53	22.57
	K1 (min−1)	0.0316	0.0175
	R2	0.9766	0.9609
Quasi-second-order	Qe (mg·g−1)	48.54	28.49
	K2 (g·mg−1·min−1)	0.000757	0.001293
	R2	0.9985	0.9989

shows the corresponding fitting parameters and calculation parameters of the pseudo-first-order and pseudo-second-order rate equations for the adsorption of Cd²⁺ and Cu²⁺ ions by BHW. In comparison, the calculated values of the pseudo-second-order rate equation (Q_e,cal = 48.54 mg·g⁻¹ and 28.49 mg·g⁻¹) are the closest to the experimental values of equilibrium adsorption capacity (Q_e,exp = 43.60 mg·g⁻¹ and 26.66 mg·g⁻¹). This indicates that this kinetic equation is more suitable to describe the adsorption process of Cu²⁺ and Cd²⁺ ions on BHW. The fitting parameters (R²) of the pseudo-second-order rate equation (R² = 0.9989) are higher, indicating that chemical sorption is the main adsorption of Cu²⁺ and Cd²⁺ ions by BHW material [6].

3.5. Adsorption Isotherms

Figure 5a indicates the relationship between the adsorption capacity of Cd²⁺ and Cu²⁺ ions on BHW and the initial concentration of BHW, respectively. At the same initial concentration, the adsorption capacity of Cd²⁺ and Cu²⁺ ions on BHW gradually increases as the temperature increases. It indicates that the interaction between the adsorbent and solvent surface is reduced with increasing temperature, and more adsorption sites are released, which promoted the adsorption of metal ions.

Figure 5b,c are respectively the linear fitting diagrams of the Freundlich model and Langmuir model for Cd²⁺ and Cu²⁺ adsorption isotherms in the presence of BHW. The related parameters are expressed in

Table 4. Fitting parameters of adsorption thermodynamics for BHW adsorption of Cd²⁺ and Cu²⁺.

Fitting Type	Parameters	Cu(Ⅱ)			Cd(Ⅱ)
		288.15 K	289.15 K	308.15 K	288.15 K	289.15 K	308.15 K
	Qmax (mg·g−1)	43.48	45.87	47.62	25.91	27.86	30.67
Langmuir	KL (L·mg−1)	0.0266	0.0356	0.0514	0.0954	0.1065	0.1457
	R2	0.8526	0.8751	0.9071	0.9566	0.8569	0.8457
	KF (mg·g−1)	2.418	3.276	4.128	2.376	2.875	3.721
Freundlich	n	1.88	2.006	2.052	1.706	1.752	1.701
	R2	0.9844	0.9945	0.9949	0.9926	0.9937	0.9946

. The Freundlich model has a better fit to the experimental results of Cd²⁺ and Cu²⁺ ions, it predicts that Cd²⁺ and Cu²⁺ ions form multilayer adsorption on the surface of the adsorbent BHW while being accompanied by chemical reactions. The maximum adsorption amount of BHW on Cd²⁺ and Cu²⁺ ions is higher than that of the humic acid-based carbon material in the literature [21]. The values of n are greater than 1 and between 1–10, which indicates that Cu²⁺ and Cd²⁺ ions are easy to adsorb on BHW [36]. When the temperature is 288.15 K, 298.15 K, and 308.15 K, the K_F value of the Freundlich model rises with the increase in temperature. The temperature rise is beneficial to reduce the interaction between the adsorbent and the solvent surface because the larger the value of K_F, the more the adsorption amount. Consequently, more adsorption sites are exposed, which promotes the adsorption of metal ions on the BHW material. The fitting parameter K_F of Cu²⁺ adsorption on the BHW material is greater than that of Cd²⁺, which proves that the adsorption of Cu²⁺ ions by BHW is faster than the adsorption of Cd²⁺ ions at the same temperature. It is consistent with the result of the dynamics section.

3.6. Immobilization Mechanism

FT-IR spectra of BC, BC-NaHA, and BHW are shown in Figure 6a. The intensities of the -OH, C=C, C=O, and C-O peaks of the BHW material are much greater than those of the corresponding peaks on the BC, and BC-NaHA materials [37,38,39]. The order of the number of functional groups is BHW > BC-NaHA > BC. The results show that the introduction of WV can enhance the quantity of oxygen-containing groups, increase the adsorption sites on the surface of the material, and can adsorb more metal ions to form metal complexes. Figure 6b shows the FT-IR spectra of Cu²⁺ ions immobilized by BC, BC-NaHA, and BHW. The intensity of hydroxyl peaks on the three materials decreases significantly after Cu²⁺ ion adsorption, among which the absorption peaks of carboxyl and hydroxyl groups weaken significantly after Cu²⁺ ion adsorption by BHW [40,41]. It indicates that compared with BC-NaHA and BC, both hydroxyl and carboxyl groups on BHW materials are involved in the reaction with Cu²⁺ ion. This is consistent with the research results of the literature [42]. Figure 6c shows the FT-IR spectra of Cd²⁺ ions before and after adsorption by BC, BC-NaHA and BHW. After Cd²⁺ ions were immobilized by BC, BC-NaHA, and BHW, the stretching vibration peaks of hydroxyl (-OH), aromatic C=C, C=O, and C-O were reduced, respectively, and the hydroxyl and carboxyl groups on BHW had the most significant weakening effect. It indicates that a large number of hydroxyl and carboxyl groups on BHW react with Cd²⁺ ions to form more metal complexes, which is in line with the research results of Bai et al. [41].

XPS full spectrum of BC, BC-NaHA, and BHW before and after Cd²⁺ and Cu²⁺ ion adsorption is shown in Figure 7. It can be seen from Figure 7, that there are peaks of Na1S and Ca2P on BHW and BC-NaHA materials, but the peaks of Na1S and Ca2P disappear after Cd²⁺ and Cu²⁺ ions are absorbed. The main reason is that Cd²⁺ and Cu²⁺ ions are complexed with BC-NaHA and BHW oxygen-containing functional groups by ion- exchange, instead of Na(ⅰ) ions and Ca²⁺ ions [12].

Figure 8 indicates the C1s high-resolution spectra before and after the adsorption of Cu²⁺ (or Cd(II)) ions by BC, BC-NaHA and BHW. As shown in Figure 8a, C=C/C-C, O-C=O and C-OH in the C1s spectrum are located at 284.60 eV, 288.65 eV and 286.10 eV, respectively [22,32]. Before immobilizing heavy metal ions, the contents of C=C/C-C, O-C=O and C-OH in the spectrum of BC are 66.4%, 13.2% and 20.4%, respectively. As for BC-NaHA, the content of C=C/C-C, O-C=O and C-OH in the spectrum are 59.90%, 12.7% and 27.4%, respectively. However, the contents in the BHW spectrum are 51.1%, 11.1% and 37.8%, respectively. The result shows that the BHW material has the highest oxygen content (48.9%) among the three materials, followed by BC-NaHA (40.1%) and BC (33.6%) due to the introduction of more oxygen-containing functional groups by wood vinegar.

After immobilizing Cu²⁺ (or Cd(II)) ions in Figure 8b,c, the contents of C=C/C-C, O-C=O and C-OH in the BC spectrum are 74.2% (75.3%), 10.74% (10.8%), and 15.10% (14.0%), respectively. The contents are 71.8% (74.6%), 11.8% (10.6%), and 16.4% (14.7%) in the BC-NaHA spectrum, respectively. However, they are 68.8% (72.4%), 13.0% (8.9%), and 18.2% (18.7%), respectively. The result shows that the oxygen contents decrease by 7.8% (8.9%), 17.9% (21.5%), and 11.9% (14.7%), respectively. Compared with the oxygen contents before immobilization, all the oxygen contents significantly reduce, and the oxygen contents of BHW dramatically decrease. Therefore, the hydroxyl and carboxyl groups of BHW and metal ions formed more metal complexes. This result is consistent with that of FT-IR analysis [43].

4. Conclusions

Biochar combined with sodium humate-wood vinegar enhances the ability of soil to absorb and solidify heavy metal ions and reduces the bioavailability of heavy metals; the priority order of adsorption for multiple metals is Cu > Cd. Applying 0.01–0.16% of BHW material, the exchangeable and reducible content is significantly reduced, and the content of oxidizable components and residues is significantly increased; the reduction rate of effective content reaches 38.47–61.40%. The adsorption curve of BHW adsorption of Cd²⁺ and Cu²⁺ shows the Freundlich model for multilayer adsorption. It has better adsorption performance when the pH value is 5–6. BHW materials have obvious chemical adsorption characteristics for pollutants. The heterogeneous structure increases the pore structure, increases the adsorption surface area, and has more active sites. The adsorption mechanisms include chemical adsorption, surface physical adsorption, ion exchange, electrostatic interaction, and coordination. This work provides scientific guidance for the development of low-cost, widely applicable soil remediation materials and application techniques, which can be used on soil contaminated by heavy metals.