A novel Fe recycling method from pickling wastewater producing a KFeS2 whisker for electroplating wastewater treatment

Abstract

Pickling is a common pre- or intermediate treatment in electroplating, enamelling, rolling and other processes. Here, pickling sludge was converted to a novel product of KFeS₂ nanorods via a facile hydrothermal method that effectively removes heavy metals from electroplating wastewater. The pickling sludge was a mixture of weakly crystallised Fe in the form of irregular aggregates. The Fe content of the raw sludge was 35.9 wt% and the main impurities included aluminium, carbon and salt. After hydrothermal treatment, 74.6% of the Fe-containing compounds were converted to KFeS₂ nanorod clusters with a diameter of 300 nm and lengths in the range of 2–5 μm. In the hydrothermal system, Fe³⁺ was dissolved from the surface of the pickling sludge to form Fe(OH)₄⁻, which accelerated the formation of KFeS₂ nanorod clusters. The prepared KFeS₂ nanorods were effective in treating raw electroplating wastewater at pH 12.1, with removal rates of over 99% for Zn and Cu and 63.3% for Ni. These values were higher than those of poly ferric sulphate, poly aluminium chloride, Na₂S, raw sludge and activated carbon powder. The KFeS₂ nanorods were metastable and hydrolysed spontaneously in alkaline electroplating wastewater. The generated Fe–SH groups on the surface formed a complex with metals, namely, Cu, Zn and Ni, and thus were removed with flocculation. Therefore, the KFeS₂ nanorods generated from pickling wastewater recycling could be used as a prospective coagulant for heavy-metal removal in the treatment of alkaline electroplating wastewater.