Energy-free cadmium removal from wastewater using a zinc-stainless steel galvanic cell: Performance and mechanism

Recycling cadmium from wastewater not only mitigates environmental risks but also offers economic benefits. However, conventional zinc powder cementation is hindered by excessive zinc consumption and the generation of low-purity cadmium sludge. In this study, we proposed a galvanic cell system composed of a zinc plate coupled with a stainless steel plate, replacing zinc powder as the reductant. Under optimized conditions, the system achieved 99.8 % cadmium removal, with a recovered cadmium purity exceeding 90 %. Mechanistic insights derived from kinetic modeling and electrochemical analysis revealed that cadmium deposition was governed by a mixed control mechanism involving both mass transfer and interfacial reaction. The stainless steel plate provided additional cathodic reduction sites and established a natural potential difference together with the zinc anode. This galvanic interaction promoted active zinc dissolution and facilitated preferential cadmium deposition. Meanwhile, microstructural analysis confirmed the formation of a loose, dendritic cadmium layer, which prevented surface encapsulation and promoted favorable mass transfer conditions for sustained cementation. Compared to traditional zinc powder cementation, this approach eliminates the need for external energy input, reduces zinc consumption, and avoids the generation of hazardous sludge. These findings demonstrate a clean, efficient, and scalable strategy for the removal and recovery of cadmium from wastewater.