Mechanisms of heavy metal migration and removal in sewage sludge by Fe2+/Ca(ClO)2/tannic acid conditioning and risk assessment

Sewage sludge treatment remains a critical environmental challenge due to its high moisture content and heavy metal contamination. The Fe 2+ /Ca(ClO) 2 /tannic acid (TA) combined conditioning has shown great performances on sludge dewatering, but its influence on heavy metal removal and risk assessment was unclear. In this study the speciation transformation of heavy metals and related mechanisms were comprehensively investigated. The results demonstrated that the sludge conditioning simultaneously enhanced sludge dewatering and promoted heavy metal removal under optimized conditions. Notably, Zn, Cd, and Ni were effectively removed from the sludge solid phase, with removal efficiencies of 39.12 %, 52.49 %, and 30.86 %, respectively. Moreover, the conditioning significantly reduced the ecological risk posed by heavy metals, as indicated by a marked decrease in comprehensive risk index from high to medium levels. The mechanisms involved in the combined effects of oxidation, acidification, and ion binding. Oxidation disrupted extracellular polymeric substance structure, while acidification increased surface positivity, enhancing heavy metal desorption. The Fe 2+ /Ca 2+ competed with heavy metals for binding sites. Additionally, pH reduction and TA chelation further facilitated the transfer of heavy metals into the liquid phase. As a result, the potential mobility index for heavy metals Zn, Cd, Ni, Hg, and As decreased by 33.93 %, 35.98 %, 21.40 %, 26.76 %, and 50.57 %, respectively. This study provides a promising strategy for the safe and resource-efficient utilization of sewage sludge.