Co-production of high quality NH 4SCN and sulfur slow release agent from industrial effluent using calcined MgAl–hydrotalcite

Abstract

Thiosulfate (S 2O 3 2−) is the main impurity in ammonium thiocyanate (NH 4SCN) extracted from the effluent from industries such as electroplating or mining. It significantly lowers the quality of NH 4SCN. Here we report a method for selective removal of S 2O 3 2− from aqueous solutions containing thiocyanate (SCN −) using the material produced by calcination of MgAl–CO 3 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 S 2O 3 2− and SCN − anions, with preferential adsorption of S 2O 3 2− anions resulting in S 2O 3 2−-intercalated MgAl–LDHs. Selective removal of S 2O 3 2− from an aqueous mixed solution lowered the ratio of S 2O 3 2−:SCN − from 1:1 to approximately 1:7, which meets the requirements for production of high quality NH 4SCN after recrystallization, or for direct reuse in the industrial process which produced the original effluent. At the same time, the by-product, S 2O 3 2−-intercalated MgAl–LDH showed satisfactory S 2O 3 2−-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.