Transformation characteristics of nitrogen, sulfur and chlorine during microwave-assisted hydrothermal treatment of excavated waste

The removal effect of nitrogen, sulfur and chlorine from the excavated waste utilizing the microwave-assisted hydrothermal method was investigated in this work. Speciation changes of nitrogen, sulfur and chlorine in hydrochars with different treatment times were determined by X-ray photoelectron spectroscopy analysis. Results showed that 90% chlorine, 65% nitrogen and 63% sulfur were removed from the excavated waste after treatment at 230 °C for 100 min, while 82% energy and 85% carbon element were recovered at the same time. The conversion of organic nitrogen, sulfur and chlorine into the corresponding water-soluble inorganic forms was enhanced by the combined microwave irradiation and hydrothermal reaction, which promoted the migration of Cl, S, N into the liquid phase. However, some original and generated species of nitrogen, sulfur and chlorine remained in the hydrochars due to the polymerization of organic forms and the adsorption of inorganic forms. These findings suggested that the efficient purification of nitrogen, sulfur and chlorine in excavated waste could be obtained using the microwave-assisted hydrothermal treatment without additives, which provided a reference for the recycling and cleaning utilization of excavated waste.