Abstract
The content of heavy metals in the soil in Guizhou Province, which is a high-risk area for heavy metal exposure, is significantly higher than that in other areas in China. Therefore, the objective of this study was to evaluate the ability of CaCO3 and clay to accumulate heavy metals in topsoil sample collected from Lixisol using the method of indoor simulation. The results showed that the contents of Cu, Zn, Cd, Cr, Pb, Hg and As in the soil sample were 10.8 mg/kg, 125 mg/kg, 0.489 mg/kg, 23.5 mg/kg, 22.7 mg/kg, 58.3 mg/kg and 45.4 mg/kg, respectively. The soil pH values increased with the CaCO3 concentration in the soil, and the fluctuation of the soil pH values was weak after the CaCO3 concentrations reached 100 g/kg. The adsorption capacity of lime soil increased by approximately 10 mg/kg on average, and the desorption capacity decreased by approximately 300 mg/kg on average. The desorption of all heavy metals in this study did not change with increasing clay content. Pseudo-second-order kinetics were more suitable for describing the adsorption kinetics of heavy metals on the soil material, as evidenced by the higher R2 value. The Freundlich model can better describe the adsorption process of As on lime soil. The process of As, Cr, Cd and Hg adsorption on the soil sample was spontaneous and entropy-driven. Additionally, the process of Cu and Pb adsorption on the soil materials was spontaneous and enthalpy-driven. Generally, the adsorption and desorption of heavy metals in polluted soil increased and decreased, respectively, with increasing CaCO3 content. The effect of calcium carbonate on the accumulation of heavy metals in soil was greater than that of clay. In summary, CaCO3 and pH values in soil can be appropriately added in several areas polluted by heavy metals to enhance the crop yield and reduce the adsorption of heavy metals in soils.
Highlights
There are two main soil types, i.e., Lixisols and Acrisols, in Guizhou Province, of which the former are characterized by high content of calcium and magnesium carbonate (CaCO3, MgCO3), and carbonate plays an important role in the biogeochemical process of heavy metals [1,2]
Ning et al [6] measured the concentrations of heavy metals (Cu, Zn, Mn, Cd, Ni, Pb and Co) in limestone forests in Huaxi district, Guiyang, and the heavy metal pollution was assessed by comparing our results with the background values of heavy metals in limestone soils in Guizhou Province, China
The pH of topsoil in Guizhou Province was 7.64 in this study, which corresponds to weak alkaline soil (Figure 2a)
Summary
There are two main soil types, i.e., Lixisols and Acrisols, in Guizhou Province, of which the former are characterized by high content of calcium and magnesium carbonate (CaCO3, MgCO3), and carbonate plays an important role in the biogeochemical process of heavy metals [1,2]. Ning et al [6] measured the concentrations of heavy metals (Cu, Zn, Mn, Cd, Ni, Pb and Co) in limestone forests in Huaxi district, Guiyang, and the heavy metal pollution was assessed by comparing our results with the background values of heavy metals in limestone soils in Guizhou Province, China. The results showed that the measured contents of Cu, Mn, Cd, Pb, Ni and Co were higher than or similar to these background values. It is necessary to understand the adsorption of heavy metals in topsoil samples collected from Lixisol
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