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

ABSTRACT

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.

KEYWORDS:

• bauxite waste
• leachate
• reuse
• sorption
• treatment
• toxic metal

ORCID

David A. Rubinos http://orcid.org/0000-0002-0161-7745