2.1 Study area
The Panyi Mine Reclamation Area is located approximately 1.0 km east of the Panyi Mine, in the Panji District, Huainan City (China). A coal gangue hill is located on the southern side of the reclamation area. This reclamation area was formed by the filling and rolling of coal gangue into the coal mining subsidence area, the coal gangue is covered by clay with a design thickness of 100 cm. After that treatment, several tree species including poplar, privet, metasequoia, cedar, and Sichuan juniper were planted in the subsidence area. This area lies in a warm temperate semi-humid continental monsoon climate zone. The average annual temperature is 15.3°C, with the highest and lowest temperatures respectively occurring in July and January, while the average annual precipitation is 926 mm.
2.2 Sample collection and pretreatment
Three sampling lines were established in the Panyi Mine Reclamation Area. Along each line, a sampling point was established every 5 m, for a total of 60 sampling points. At each sampling point, a soil heavy metal sampler was used to collect 0–20 cm of reclaimed soil. Each sample had a mass of at least 1 kg. After removing any impurities, the soil sample was crushed and spread flat on an air-dried soil dish to dry. Each sample was sieved using the quarter method, ground with an agate mortar, passed through a 100-mesh nylon screen, packed in a Ziploc bag, and then labeled for use.
2.3 Experimental method
As mentioned above, HA was selected as the representative DOC molecule. HA was used to prepare DOC solutions with different concentrations.
2.3.1 Adsorption of Cd in reclaimed soil under the influence of DOC
A given sample of reclaimed soil (1.000 g, passed through an 18-mesh screen) was weighed and placed in a 50-mL centrifuge tube. Then, 20 mL of a Cd(NO3)2 solution having a Cd concentration of 0, 0.2, 0.4, 0.6, 0.8, 1, 1.5, 2, 2.5, 3, 4, 5 , 8, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, or 80 mg/L was added to the centrifuge tube, followed by the addition of 1 mL of HA solution whose concentration was 10 mg/L. The centrifuge tube was immediately capped, shaken at 200 r/min at a constant temperature of 25°C for 24 h, and then centrifuged at 4000 r/min for 20 min. Finally, the supernatant was passed through a 0.45-μm filter membrane, and one drop of 1% nitric acid was added to the filtrate followed by shaking for the Cd determination. The Cd adsorption capacity (Qa, in mg/kg) was determined as the difference between the concentration of Cd before and after its adsorption:
where C0 is the concentration of Cd in the initial solution (mg/L), C is the concentration of Cd in the equilibrium solution at an adsorption equilibrium (mg/L), V is the initial volume of liquid added (mL), and m is the mass of the sample (g). A control experiment was simultaneously carried out with an HA solution (60 mg/L).
2.3.2 Desorption of Cd from reclaimed soil under the influence of DOC
The desorption experiment was performed upon completing the adsorption experiment. The reclaimed soil after Cd adsorption was washed once with absolute ethanol to remove any free Cd and placed at a constant temperature (25°C) for 1 h. Next, 20 mL of NaNO3 solution (0.01 mol/L) was added to the soil, followed by its covering and shaking at 200 r/min and 25°C for 24 h. The solution was then centrifuged at 4000 r/min for 20 min. Finally, the supernatant was filtered through a 0.45-μm membrane, and one drop of 1% nitric acid was added to the filtrate followed by shaking for the Cd determination. The Cd desorption capacity (Qd) was determined this way:
The Cd desorption rate (W) was calculated as
2.3.3 Effects of pH on Cd adsorption and desorption in soil
For this, the HA and Cd solutions were respectively set to a fixed concentration of 60 mg/L and 20 mg/L, while the pH of soil samples was adjusted to 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, or 10.5 at 25°C. The specific steps consisted of putting a 1.000-g soil sample (passed through an 18 mesh screen) into a 50-ml centrifuge tube, to which was added 20 ml of Cd(NO3)2 solution (20 mg/L concentration of Cd) followed by the addition of 1 ml of the HA solution (60 mg/L Cd concentration). The pH value of the sample was adjusted accordingly along the gradient (pH 4.5 to 10.5), with each sample removed at 25°C for 24 h, and then centrifuged (4000 r/min) for 20 min. The ensuing supernatant in the centrifuge tube was filtered through a 0.45-μm membrane. After this filtration, one drop of 1% nitric acid was added to the filtered solution and shaken, the concentration of Cd was then determined. According to the difference in the Cd concentration before and after adsorption, the adsorption capacity was calculated using formula (1).
After completing the adsorption experiment, the corresponding desorption experiment was likewise carried out. To do this, soil samples that had adsorbed a certain amount of Cd were washed with anhydrous ethanol (washed once) and maintained at constant temperature (25°C) for 1 h. Next, 20 ml of 0.01 mol/L NaNO3 solution was added and the pH value of a sample adjusted accordingly (pH gradient of 4.5 to 10.5, as used above), then each tube was shaken at 25°C for 24 h and centrifuged for 20 min. The supernatant in the samples’ centrifuge tube was filtered by a 0.45-μm membrane, and this filtrate was then shaken after adding to it one drop of 1% nitric acid. The concentration of Cd was determined. The desorption amount of Cd was calculated using formula (2).
2.3.4 Effect of temperature on Cd adsorption and desorption in soil
To each soil sample, the 60 mg/L HA solution and 20 mg/L Cd solution were added. Under the condition of pH 7.5, the temperature of the samples was adjusted to 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, or 40°C.
Specifically, a 1.000-g soil sample (first passed through an 18 mesh screen) was put into a 50-ml centrifuge tube, to this was added 20 ml of Cd(NO3)2 solution (20 mg/L Cd ), and then 1 ml of the HA solution (60 mg/L Cd) was added to each centrifuge tube. The samples were removed at the eight different temperatures (5°C to 40°C gradient) under constant temperature oscillation (200 r/min) for 24 h and centrifuged (4000 r/min) for 20 min. The supernatant in the centrifuge tube was passed through a 0.45-μm membrane. After this filtration, one drop of 1% nitric acid was added to the filtered solution and shaken, after which the concentration of Cd was determined. According to the difference in the Cd concentration before and after adsorption, the adsorption capacity was calculated using formula (1).
The corresponding desorption experiment was carried out after the completion of the just-described adsorption experiment. The soil that had adsorbed a certain amount of Cd was washed with anhydrous ethanol to remove the free Cd (washed once). The soil was placed at 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, or 40°C for 1 h. Then 20 ml of 0.01 mol/L NaNO3 solution was added to each sample, and these removed in a constant temperature oscillator (200 r/min) for 24 h and centrifuged (4000 r/min) for 20 min. The supernatant in the centrifuge tube was filtered through a 0.45-μm membrane, and the filtrate was shaken after adding to it one drop of 1% nitric acid. The concentration of Cd was determined. The desorption amount of Cd was calculated using formula (2).
2.4 Data processing
SPSS 24.0 software was used for the statistical analysis of the data, Origin8.0 software was used for drawing the figures, and MATLAB mathematical software was used to compile the corresponding calculation program for the gray correlation[40] analysis. According to the statistics for Cd adsorption capacity, desorption capacity, and various factors, 74 samples were analyzed, for which the order number was m = 4 (HA concentration, Cd concentration, temperature, and pH value) P= 0.5. Using the MATLAB calculations, the gray correlation degree of HA concentration, Cd concentration, temperature, and pH value are listed in Table 1. To establish the best adsorption and desorption model as function of HA concentration, Cd concentration, pH value, and temperature (at a 95% confidence interval), these four predictors were entered into a multiple linear regression (but with no interaction terms). In this model, the response variable Y is the adsorption capacity of Cd in the solution, XHA is the concentration of HA, XCd is the concentration of Cd in the solution, XT is the solution temperature, and XPH is the pH of the solution.