Removal of pollutants from aqueous solutions by coals and residual cokes obtained from simulated underground coal gasification experiments

Abstract

Groundwater pollution caused by underground coal gasification (UCG) as well as its remediation has attracted significant public attention. In this paper, model gasification experiments on three types of coals with different coalification degrees were carried out using a UCG model test system. The residual coal and coke samples were characterized via SEM, low temperature N2 adsorption, and FT-IR. Thereafter, their adsorption of phenol and Cr(VI) from simulated wastewater as well as the purification of gas washing water were investigated. Compared with the raw coals, the UCG residual cokes presented a higher specific surface area (SBET) of 44.99–381.51 m2/g and total pore volume (Vt) of 0.040–0.228 cm3/g; Cokes obtained from brown coal gasification had the highest SBET and Vt values. Parts of O-containing groups were preserved after the gasification. Phenol and Cr(VI) were removed by all types of coals and residual cokes. The removal rate of phenol increased with SBET and Vt, and cokes presented higher phenol adsorption capacities than those of raw coals. The O-containing groups significantly influenced and favored the removal of Cr(VI). Brown coal and its UCG coke had the highest adsorption capacities, with phenol removal rates of 17.71% and 72.44%, and presented the highest adsorption capacities, with Cr(VI) removal rates of 29.02% and 21.77%, respectively. When UCG residual cokes were used, the removal rates of total organic carbon (TOC) in both types of gas washing water exceeded 82%. Almost all the Cr and Se and some of the Ni were removed from gas washing water.