The Synthesis of Secondary Iron Minerals Induced by Quartz Sand during the Bioleaching Process Improves the Dewaterability of Municipal Sewage Sludge

Abstract

This study elucidated the mechanisms during the bioleaching process when optimizing the dewaterability of municipal sludge using quartz sand. The experiment was conducted with a shaking table and a series of controlled trials designed to investigate the influence of quartz sand on sludge dewaterability. Scanning electron microscopy and X-ray diffraction were applied to explore the quartz sand’s action mechanism. Results indicated that quartz sand could improve the sludge bioleaching efficiency. The optimal reaction time was between 24–48 h and 48–72 h with and without 10 g·L−1 of quartz sand, and a minimum sludge specific resistance to filtration was 1.2 × 1012 and 2.4 × 1012 m·kg−1, respectively. Quartz sand could provide nucleating sites for secondary iron minerals and overcome the unfavorable influence of a low Fe3+ supply rate in the initial bioleaching stage (0–24 h). Because it was conducive to accelerating the initial mineral precipitation, quartz sand could improve bioleaching efficiency. The X-ray diffraction spectrum showed that quartz sand induced changes in the synthesis pathway of secondary iron minerals when the concentration of Fe2+ ≥ 4 g·L−1. This then promoted the transformation of schwertmannite into jarosite during the mineralization process, which immobilizes nutrients such as K+ and NH4+ in the form of jarosite. Accordingly, bioleached sludge dewaterability and its utilization value can be improved. These results provide theoretical reference for improving bioleaching techniques in the treatment of municipal sludge.