Enhancing dewaterability and reducing heavy metal leaching in industrial landfill sludge: The role of extracellular polymeric substances

In this study, we explored various conditioning treatments (Fe(II)-NaClO, Fe(III), polyacrylamide (PAM), CaO, MgO, Fe(III) + CaO, and Fe(III) + MgO) aimed at enhancing the dewatering efficiency of industrial textile-dyeing landfill sludge (LS) and reducing heavy metal leaching in the dewatered filtrate. The analysis revealed significant improvements in sludge dewaterability, especially with the combined Fe(III) + CaO and Fe(III) + MgO treatments, with the latter demonstrating a marginally superior performance. The adsorption of hydrophilic loosely bound and tightly bound extracellular polymeric substances (LB-EPS and TB-EPS) onto hydroxide complexes facilitated the formation of a more stable and porous structure, which enhanced the release of bound water. However, certain treatments such as Fe(II)-NaClO and PAM presented challenges to dewatering due to excessive oxidation and the formation of a highly viscous gel structure, respectively. Furthermore, the exploration underscored the critical role of protein concentrations within soluble extracellular polymeric substances (S-EPS) in mediating heavy metal translocation, particularly Zn, Cu, and Ni, from the solid to the liquid phase of LS. An increased risk of leaching was observed with the Fe(II)-NaClO treatment, associated with over-oxidation-induced solubilization of S-EPS proteins. Notably, the findings elucidated effective mechanisms for immobilizing heavy metals. Combined treatments (Fe(III) + CaO and Fe(III) + MgO) were particularly effective in containing heavy metals, with varying mechanisms contributing to their removal. Specifically, the adsorption and precipitation of ferric hydroxide complexes with calcium or magnesium were crucial in the removal of Cr, Pb, and Hg, while the removal of Zn, Cu, and Ni was predominantly facilitated by the formation of heavy metal-protein-hydroxide co-precipitates.