Using CaCl2 and EDTA extraction methods to predict the bioavailability of heavy metals after biochar application in three soils differing in initial pH

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Abstract Biochar amendment is a promising approach to mitigate soil contamination from agricultural and industrial sources via the immobilization of heavy metals. However, mechanisms of interaction between biochar, soils, pH and contaminants remain poorly understood. This study aimed at: (i) investigate short-term (21 days) changes in soil pH following the application of biochar at different rates, and (ii) evaluate the effectiveness of two single extraction methods (CaCl₂ for exchangeable metals and EDTA for labile metals) in predicting the subsequent bioavailability of Cd, Pb and Zn in three soils representing strongly acidic, acidic and alkaline conditions. Soils were treated with different sawdust biochar rates (5%, 10% and 20%) for 7, 14 and 21 days. The extractions conducted showed that biochar application caused an increase in the immobilization of exchangeable metals in the following order strongly acidic > alkaline > acidic and that the highest increase in pH was observed during the first (0.2035 units day −1 , 10% biochar), second (0.4035 units day −1 , 20% biochar) and third (0.1835 units day −1 , 10% biochar). Biochar was less efficient in immobilizing labile metals, except in the alkaline soil, where their concentrations were significantly lower in 5% and 10% biochar ( P < 0.05 ). Special focus was placed on the strongly acidic agricultural soil due to the rapid increase of its pH overtime without significant efficacy in the removal of metals readily released into the soil environment. This study highlights the importance of soil pH in heavy metal remediation and shows that the rate of pH change after biochar application is a key factor which influences immobilization efficiency. The understanding of this kinetic relationship is essential for predicting and optimizing the short-term effectiveness of biochar amendments.

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