Cadmium, lead, and zinc immobilization in soil using rice husk biochar in the presence of citric acid

The interaction between biochar and heavy metals in the presence of low molecular weight organic acids is largely unknown, but it is important to understand biochar's influence on heavy metals immobilizing in the rhizosphere. The impacts of different amounts of biochar (5%) and citric acid, coupled with combinations of biochar and citric acid on the redistribution of cadmium, lead, and zinc among their different geochemical forms, including their bioavailability in metal contaminated soil, were investigated in an incubation experiment. Adding biochar, low concentration of citric acid (2 mmol kg(-1) soil), and biochar with a low level of citric acid markedly reduced the acid-soluble and available (CaCl2-extractable) cadmium, lead, and zinc compared to control in which biochar plus low concentration of citric acid proved to be the most effective treatment. These trends were reversed in the case of high concentrations of citric acid (> 5-20 mmol kg(-1) soil) and the combined biochar plus high levels of citric acid treatments. With incubation time, biochar in combination with a low concentration of citric acid transformed the largest amounts of acid-soluble and reducible cadmium, lead, and zinc into oxidizable and residual fractions. These results suggested that a low level of citric acid enhanced metals' immobilization, while higher concentration of citric acid increased their mobilization. The study concluded that biochar with this low level of citric acid did in fact emerge as the best treatment, and can serve as an amendment for metals' immobilization/redistribution in contaminated soils. Graphic abstract

Automated summary

The study found that under low concentrations of citric acid, biochar effectively reduced the solubility and extractability of cadmium, lead, and zinc in soil, but this trend reversed under high concentrations of citric acid. Therefore, the combination of biochar with low levels of citric acid may be the most effective treatment for immobilization and redistribution of metals in contaminated soils.