Is vermicompost the possible in situ sorbent? Immobilization of Pb, Cd and Cr in sediment with sludge derived vermicompost, a column study.

The goal of this study was to investigate the immobilization effect of vermicomposted sewage sludge for Pb, Cd and Cr in the sediment under simulated in situ conditions using column test. Positioning 10% dw of vermicompost at the bottom layer of the column resulted in an average decrease of Pb, Cd and Cr in the leachate of 93, 97 and 75.5%, with the accumulated adsorbed amount of 11.80, 4.81 and 5.62 mg g-1, respectively. Fluorescence Excitation‒Emission Matrix (EEM) combined with parallel factor analysis (PARAFAC) was adopted to identify the components in DOM (dissolved organic matter) that were efficient for the immobilization of heavy metals. The 4‒component PARAFAC model established showed that DOM was dominated by a protein‒like material (component C1), and three humic‒like materials (component C2, C3 and C4). The humic substances formed the organo‒metal complexes with Pb, Cr and Cd, hence, the metal ions were sequestered by the sorbent. Also, as calculated by the bivariate coefficients, the C2/C1 ratios can be liable parameters for assessing the retaining capability of vermicompost for heavy metals. Generally, vermicompost can be used as a promising in situ sorbent for the remediation of heavy metal polluted sediments.