Sorptive removal of HgII by red mud (bauxite residue) in contaminated landfill leachate.

The ability of red mud (RM) (bauxite residue) to remove HgII from landfill leachate (LL) was assessed. The studied aspects comprised the effects of time, pH, HgII concentration and the sorption isotherm, besides the influence of chloride and representative organic ligands. HgII removal by RM exhibited a complex kinetics where initial rapid sorption was followed by desorption at longer times. The sorption of HgII on RM was strongly pH-dependent. Outstanding maximum sorption was observed at pH∼4-5 (≥99.6%), while it abruptly dropped at higher pH values down to a minimum ∼28% at pH∼10.5. Chloride decreased HgII sorption at acid pH and shifted the pHmax towards higher pH∼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 HgII 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 HgII sorption at any pH. HgII speciation modelling and dissolved organic matter (DOM) sorption support complexation of HgII by DOM as the primary factor governing the removal of HgII in LL. The sorption isotherm was better described by the Freundlich equation, which agrees with the heterogeneous composition of RM. The results indicate that HgII sorption on RM is favorable, but reveal differences in sorption and reduced efficiency, in LL media. Notwithstanding, RM possesses a notable capacity to remove HgII, even under the unhelpful complexing and competing conditions of LL.