Changes in Heavy Metal Speciation and Release Behavior Before and After Sludge Composting Under a Phosphate-rich Atmosphere

Abstract

Sewage sludge is rich in organic matter, N, and P and could be used as a soil amendment to improve the status of soil organic matter, soil structural characteristics, and soil water retention capacity after aerobic composting. However, heavy metals in sewage sludge have become the main bottleneck limiting its land application. In addition, with the large-scale exploitation of phosphate rock resources in our region of interest, a large amount of phosphate tailings needs to be disposed and a large area of abandoned mining lands needs to be reclaimed. Phosphate tailings could be auxiliary materials for sewage sludge composting to immobilize heavy metals, and the compost could be applied for revegetation of the abandoned mining lands. The contents of As, Cr, Cu, Ni, Pb, Cd, and Zn were measured, and a successive extraction procedure was used to investigate the change in speciation of heavy metals in the sludge before and after the phosphate-rich composting. pH-dependent leaching tests were carried out to further evaluate the immobilization effects of composting on heavy metals and the release potential under different pH conditions. The results showed that the contents of heavy metals in the compost satisfied the corresponding threshold for land reclamation. Adding phosphate tailings greatly improved the stability of heavy metals during the composting process. The portion of stable residues of Pb, Cd, As, and Zn in the phosphate-rich compost was 84.00%, 58.00%, 68.50%, and 30.93%, respectively, compared with 68.10%, 30.50%, 40.32%, and 16.48% for the control, compost without adding the phosphate tailings. Meanwhile, the maximum leaching potential of As, Ni, Pb, Zn, and Cu in the phosphate-rich compost decreased from 3.692 mg·kg-1, 0.903 mg·kg-1, 0.217 mg·kg-1, 7.225 mg·kg-1, and 8.725 mg·kg-1 to 0.684 mg·kg-1, 0.586 mg·kg-1, 0.071 mg·kg-1, 2.603 mg·kg-1, and 6.935 mg·kg-1in the control, respectively, for pH 6-8.It could be concluded that the addition of phosphate tailings in the sludge composting lowered the risk of heavy metals in sewage sludge compost to make it favorable for beneficial use in abandoned mining land reclamation.