The Use of biochar in the Remediation of Pb, Cd, and Cu-Contaminated Soils. The Impact of biochar Feedstock and Preparation Conditions on Its Remediation Capacity

Soil constitutes an important part of terrestrial ecosystems, prone to be adversely impacted by human activities. During the last decades, several methods have been developed aiming at its remediation, including the use of biochar as a soil amendment. In the present work, we have assessed the reduction of Pb, Cd, and Cu soil concentrations as a function of the mixing ratio of biochar added to soil, as well as the source of biochar employed. Furthermore, we have investigated the effects of biochar addition relating to the chemical forms of heavy metals (HMs) related to their bioavailability and mobility. The concentrations of HMs were determined by the BCR (European Community Bureau of Reference) sequential extraction procedure before and after biochar addition to the soils. Five types of biochar were used, obtained as by-products of sugarcane bagasse (Β), bamboo (ΒΒ), rice straw (RSB), garden waste (GB), and paulownia (PB) treatment, respectively. Biochar derived from sugarcane (B) reduced the availability of metals, as it decreased their concentration in the acid extractable fraction, by 40.5, 66.6, and 50% for Pb, Cd, and Cu, respectively. In addition, (B) application increased the residual fraction of Cu and Pb by 9% and 24.8%, respectively. Biochar derived from garden residues (GB) and paulownia plant (PB) dramatically increased the residual fraction of Cd over 97%, minimizing its availability. Sugarcane-derived biochar appeared to significantly increase Cu and Pb residual fraction concentrations and decrease available Cd concentration. Similar changes are caused by the types of biochar in the following order: biochar from sugarcane > paulownia > garden wastes > bamboo > rice straw. The redistribution of HM concentrations causes a significant improvement of environmental quality in polluted soils, as it limits the mobility and availability of toxic metals to the soil ecosystem. The use of biochar is a low-cost and eco-friendly method for the remediation of contaminated with HMs soils in the framework of a circular economy.