Physicochemical properties of wastewater produced from the microwave upgrading process of Indonesian lignite

This study is aimed to examine the utilization of wastewater produced from the microwave irradiation upgrading process of Indonesian lignite. Physicochemical properties including total organic carbon (TOC), chemical oxygen demand (COD), organic functional groups, inorganic anions, inorganic cations, surface tension, pH, and electrical conductivity (EC) of the removed water were analysed. Results showed that TOC and COD increased with increased microwave time. Organic matters in removed water mainly consists of aromatic, aliphatic, and oxygen functional groups. During the microwave process, inorganic anions (SO42−, F− and Cl−) and cations (Na+, K+, Ca2+, etc.) gradually leached out from lignite. Concentrations of the dominant SO42− and Na+ ions initially increased and then decreased due to the competitive effect of leaching rate of ions and removal rate of water. Thus, EC initially increased and then decreased with total ion concentration, whereas surface tension and pH monotonously decreased. Moreover, pH must be improved and COD must be reduced before discharge or reuse of the removed water.