Mercury (Hg) removal from drinking water by enhanced coagulation with minimal dose sulfide: From synergistic...

Mercury (Hg) contamination in drinking water poses a serious risk to public health, but conventional coagulation processes are intrinsically ineffective for removing ultra-trace Hg. Although sulfide exhibits a strong affinity toward mercury, its application in drinking water treatment has long been rarely reported. This study challenges conventional approaches by introducing a novel method for mercury (Hg) removal from drinking water using Enhanced Coagulation with minimal dose Sulfide (EnCS). In this process, a small amount of sulfide was introduced and subsequently immobilized within a Fe-based coagulation system. The addition of minimal sulfide dramatically enhanced Hg removal, increasing efficiency from <55% under conventional coagulation to >90%, with effluent Hg concentrations consistently below 1.0 μg/L. Mechanistic investigations revealed that sulfide rapidly converts dissolved Hg²⁺ into nanoscale HgS colloids, and simultaneously generates high-affinity Fe-S⁻ surface sites during coagulation hydrolysis. These species were efficiently removed through sweep flocculation and surface complexation, while excess Fe hydrolysates effectively sequestered residual sulfide, resulting in a compliant effluent without sulfide and odor. The EnCS process exhibited robust performance across a broad pH range and maintained low residual Fe and turbidity levels, fully complying with drinking water standards. Its engineering feasibility was further validated through successful full-scale application in a municipal drinking water treatment plant (30,000 m³/d), where stable Hg removal was achieved over two years of operation despite fluctuating raw water Hg concentrations. This study provided the first demonstration that sulfide, when precisely dosed and synergistically coupled with Fe-based coagulation, can be safely and effectively applied for trace mercury removal in drinking water systems.