Assessment of red mud as sorptive landfill liner for the retention of arsenic (V).

The sorption of AsV on red mud (bauxite residue), produced in the ALCOA-San Cibrao factory (Spain), was assessed in view of its potential use as sorptive liner of landfills for the attenuation of As-rich leachates. The operating parameters evaluated, using batch-type procedures, comprised the effects of time, solution pH, AsV concentration (sorption isotherm) and presence of phosphate on the AsV sorption. The results showed that the red mud efficiently sorbed AsV. The sorption was fast, with a major fraction of initial AsV being removed in a few minutes or hours of contact, depending on AsV concentration. The kinetic process was well described by the pseudo-second order equation, which points to chemisorption is involved, whereas surface (film) diffusion chiefly governs the rate of AsV sorption for the red mud system. Sorption of AsV was strongly pH-dependent. Maximum removal (>98%) was observed at slightly acidic pH (pHmax = 5.5-6), while AsV sorption considerably decreased at both highly acidic and alkaline pH. The percentages of sorbed AsV decreased with the increasing solution AsV concentration, and the AsV sorption capacity (up to 43.5 mmol/kg) of the red mud was higher (∼4 -fold) at pH ∼6 than at pH ∼9.2 (natural pH of the red mud). The presence of P at equimolar or 1:10 As/P molar ratios reduced AsV sorption by ∼20% and 30%, respectively. Simulations of AsV migration taking into account the effects of dispersion and diffusion through an hypothetical red mud liner, using the sorption parameters and the geotechnical-hydraulic conductivity characteristics of the RM, predicted a deeper migration of AsV in the liner at pH∼9.2 than at pH∼6 and a minimum thickness of ∼90 cm and ∼20 cm, respectively, for a RM liner to decrease the solution AsV concentration from highly toxic 1 mM to a safe <0.133 μM (<10 μg/L) level, after a 35-years period.