Environmental impacts of wastewater from low-rank coal handling processes in terms of mutagenicity and acute toxicity

Abstract

Two low-rank coals, a sub-bituminous coal (BA) and a lignite (LY), were treated by hot water extraction (HWE) and hydrothermal treatment (HTT), and the environmental impacts of water-soluble matters eluted from coal by the HWE and HTT processes were evaluated in terms of TOC, mutagenicity, and the acute toxicity against freshwater organisms. When HWE was performed at 80°C, the degree of TOC for LY was much higher than that for BA. However, for HTT, the two coals gave comparable TOC values in L/S ratio of both 100 and 3. The HWE and HTT eluents of two coals were assessed by the Ames mutagenicity assay with Salmonella typhimurium TA100 and TA98 strains, and no notable mutagenicity was observed in the presence or absence of metabolic activation. When the mutagenicity of the 300oC-HTT eluent of LY was analyzed, no notable mutagenicity was also observed. For the HWE and HTT eluents of LY, the acute toxicity test was carried out by use of Daphnia magna and Oryzias latipes. When the 80°C-HWE eluent was tested, almost no toxicity was observed for D. magna and O. latipes. However, when the HTT eluents were examined, the toxicity increased as the elevation of the HTT temperature, and the toxicity of organic matters dissolved in the 350°C-HTT eluent was comparable to that of reference phenolic compounds. From the FTIR analysis of organic matters eluted in the HTT eluents, it is found that the toxicity is caused by the presence of aromatic compounds with hydrophilic substituents, such as carboxyl and hydroxyl groups.