Effects of citric acid on Cd speciation in biochar-amended contaminated soils

Understanding the roles of low-molecular-weight organic acids (LMWOAs) in the transformation of chemical speciation of heavy metals in contaminated soil amended by biochar is of great significance for assessing the effectiveness of this carbonaceous amendment. The impacts of citric acid (a typical LMWOA) on the redistribution of Cd in contaminated soil amended by different amounts of biochar (1-5%) generated at various pyrolysis temperatures (300 ~ 700 °C) were investigated in this study. Biochar treatments reduced the exchangeable fraction (F1) of Cd after one year of incubation. Meanwhile, biochars produced at lower temperatures with more surface functional groups showed better immobilization performances (e.g., the F1 values decreased from 54.5% (CK) to 31.7% (adding 5% biochar generated at 300 °C) and 37.3% (adding 5% biochar generated at 700 °C), respectively). In comparison, the iron and manganese-bound fraction (F3) and organic bound fraction (F4) of Cd exhibited increased trends because of the increase in Cd linked to organic compounds and mineral oxides of soil particles. The residual fraction (F5) also showed an increased trend within a long remediation action. That is, the application of biochars favored the Cd redistribution into more stable speciations and led to a decrease in mobility. However, the addition of citric acid was beneficial in transforming immobilized fractions of Cd into mobilized fractions due to the H⁺-driven dissolution and release of Cd precipitation (e.g., for the soil treated with 3% biochar generated at 300 °C, the F1 values increased from 35.9% (before adding citric acid) to 43.5% (after adding citric acid)). Moreover, the high ratio of biochar addition could buffer soil pH reduction caused by adding citric acid and inhibit the dissolution of Cd. The findings have implications for assessing the effectiveness of biochar used as an amendment for reducing the risk of Cd in the rhizosphere of contaminated soils.