Treatment of mine wastewater using a coal fly ash-based coagulant: Coagulation adsorption isotherm study

Abstract

The mining industry produces large volumes of wastewater that may cause detrimental environmental pollution due to its high concentrations of harmful metal ions. This study used a synthesized coal fly ash-based composite coagulant (containing Al3+, Fe3+, Mg2+, Si4+ and Ca2+) to remove impurities such as Fe, Mg, Mn, Ni, Al, Si, Ca, Zn and total dissolved solids from mine wastewater. Coagulation process optimization was studied by varying the coagulant dosage, pH, temperature and coagulation time. Models that describe coagulation adsorption mechanisms have not been established for adsorption on hydroxide precipitates derived from a coal fly ash-based composite coagulant. The coagulation data for the removal of the mentioned pollutants from mining effluents were analyzed in light of adsorption isotherm, including Langmuir, Freundlich, Temkin and BET isotherm models. Results showed that the coal fly ash-based composite coagulant is effective in removing the studied pollutants in mine wastewater. Floc formation analyses revealed that destabilization by charge neutralization and sweep flocculation happened during coagulation. Analyses of the produced sludge composition showed that adsorption happened during coagulation. In general, the coagulation adsorptions are exothermic processes as indicated by Temkin and BET isotherm models. The assessment of the regression coefficients of determination from the four adsorption models for the fit of the coagulation data showed that the \({R}^{2}\) values for Temkin and BET were in the suitable range, and they could be considered as the coagulation adsorption models for this investigation in general. The application of such a coagulant can be easily adopted in mine effluent treatment plants, thereby reducing mine wastewater pollution issues. This new application of coal fly ash can be extended to the treatment of other wastewaters.