Efficient Removal of Ferrocyanide by Magnetic Layered Double Oxides and Thermal Decomposition Behavior of Adsorption Products

Abstract

Cyanide, as a highly toxic pollutant, poses a great challenge to the eco-friendly management of cyanide-containing wastewater from gold smelters. In this work, Zn-based magnetic layered double oxides (ZnAl-LDO/CoFe2O4) was prepared and the adsorption performance on ferrocyanide was investigated. The results indicated that ZnAl-LDO/CoFe2O4 exhibited a remarkable maximum adsorption capacity of 620 mg/g for total cyanide. Selective removal of ferrocyanide from highly concentrated thiocyanate and sulfate solutions was achieved, facilitated by solid-liquid separation through magnetic adsorption. Surface complexation and the “memory effect” were identified as the predominant mechanisms underlying ferricyanide adsorption. Formation of Zn2Fe(CN)6·3H2O on the surface of ZnAl-LDO/CoFe2O4, along with the incorporation of ferrocyanide into the interlayer of LDO, facilitated the restoration of the layered structure of layered double hydroxide (LDH). Subsequently, three types of cyanide-containing wastewater from a gold smelter underwent treatment, achieving cyanide concentrations below the discharge standard (<0.5 mg/L). Furthermore, investigation into the thermal decomposition behavior of adsorption products in air and argon atmospheres revealed minimal cyanide leakage (<0.1%) in all cases, indicating complete cyanide removal. In an air atmosphere, the primary phases of the roasting product comprised ZnO and ZnFe2O4, accompanied by a significant morphological change and reduced adsorption capacity (48.8 mg/g). Conversely, in an argon atmosphere, thermal decomposition of adsorption products enabled the recovery of Zn in gaseous form, while the unreacted ZnAl-LDO retained its original morphology and demonstrated superior adsorption performance. Under optimal conditions, the adsorption capacity of the pyrolysis product reached 81.5% of the freshly prepared adsorbent.

Keywords: cyanide-containing wastewater, ferrocyanide, adsorption, magnetic separation, layered double oxides, thermal decomposition