Co-production of high quality NH4SCN and sulfur slow release agent from industrial effluent using calcined MgAl–hydrotalcite

Thiosulfate (S2O32−) is the main impurity in ammonium thiocyanate (NH4SCN) extracted from the effluent from industries such as electroplating or mining. It significantly lowers the quality of NH4SCN. Here we report a method for selective removal of S2O32− from aqueous solutions containing thiocyanate (SCN−) using the material produced by calcination of MgAl–CO3 hydrotalcites also known as layered double hydroxides (LDHs). The calcined materials, which we denote CLDHs, are well known scavengers for anionic contaminants. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) showed that CLDH exhibits selectivity in adsorption between S2O32− and SCN− anions, with preferential adsorption of S2O32− anions resulting in S2O32−-intercalated MgAl–LDHs. Selective removal of S2O32− from an aqueous mixed solution lowered the ratio of S2O32−:SCN− from 1:1 to approximately 1:7, which meets the requirements for production of high quality NH4SCN after recrystallization, or for direct reuse in the industrial process which produced the original effluent. At the same time, the by-product, S2O32−-intercalated MgAl–LDH showed satisfactory S2O32−-release behavior in a simulated solution, showing that is a potential slow-release sulfur fertilizer. Furthermore, its alkaline nature allows it to act as an effective acid soil conditioner. In this way, waste effluent posing an environmental hazard can be converted into two valuable products by a simple and economical process.