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Sorptive removal of HgII by red mud (bauxite residue) in contaminated landfill leachate Part A Toxic/hazardous substances & environmental engineering

Rubinos, David A., Barral, María T.
Journal of environmental science and health 2017 v.52 no.1 pp. 84-98
EDTA (chelating agent), acetates, bauxite, chlorides, desorption, dissolved organic matter, landfill leachates, ligands, mercury, models, pH, process control, sorption isotherms
The ability of red mud (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∼4–5 (≥99.6%), while it abruptly dropped at higher pH values down to a minimum ∼28% at pH∼10.5. Chloride decreased Hg ᴵᴵ sorption at acid pH and shifted the pH ₘₐₓ 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 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.