Sorptive removal of Hg-II by red mud (bauxite residue) in contaminated landfill leachate

摘要

The ability of redmud (RM) (bauxite residue) to remove Hg-II from landfill leachate (LL) was assessed. The studied aspects comprised the effects of time, pH, Hg-II concentration and the sorption isotherm, besides the influence of chloride and representative organic ligands. Hg-II removal by RM exhibited a complex kinetics where initial rapid sorption was followed by desorption at longer times. The sorption of Hg-II on RM was strongly pH-dependent. Outstanding maximum sorption was observed at pH similar to 4-5 (>= 99.6%), while it abruptly dropped at higher pH values down to a minimum similar to 28% at pH similar to 10.5. Chloride decreased Hg-II sorption at acid pH and shifted the pH(max) towards higher pH similar to 9.4, which opposes to sorption in LL and suggests Cl- did not primarily control the process in LL. Amongst the organic ligands, acetate and salicylate slightly affected Hg-II sorption. Conversely, glycine affected sorption in a pH-dependent manner resembling that in LL, which suggests the relevant role of the organic nitrogenated compounds of LL. EDTA suppressed Hg-II sorption at any pH. Hg-II speciation modelling and dissolved organic matter (DOM) sorption support complexation of Hg-II by DOM as the primary factor governing the removal of Hg-II in LL. The sorption isotherm was better described by the Freundlich equation, which agrees with the heterogeneous composition of RM. The results indicate that Hg-II sorption on RM is favorable, but reveal differences in sorption and reduced efficiency, in LL media. Notwithstanding, RM possesses a notable capacity to remove Hg-II, even under the unhelpful complexing and competing conditions of LL.