The transformation of typical heavy metals during the process for magnetic harvesting and subsequent liquefaction in ethanol of microalgae

This study reports the transformation of three typical heavy metals (As(III), Hg(II), and Pb(II)) during the process for magnetic harvesting and subsequent liquefaction in ethanol of microalgae. The magnetic harvesting process of Chlorella Vulgaris (CV) was simulated using a co-precipitation method, achieving the highest magnetic harvesting ratio of 98.95 % at a pH of 8 and an iron-algae ratio of 0.4 g/g. Then the magnetically harvested CV was subjected to liquefaction in ethanol to explore the speciation and migration of As(III), Hg(II), and Pb(II). The environmental risk assessment of above three heavy metals in the bio-oil and biochar was assessed according to the Risk Assessment Code (RAC). Hg(II) and Pb(II) were found to be effectively stabilized and immobilized in the biochar, while As(III) exhibited a propensity to migrate into the bio-oil and existed mostly in the dangerous speciation such as fraction associated with Fe and Mn oxides and fraction bound to organic matter. It reflects a high environmental risk and necessitates a pre-removal treatment.