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

The sorption of As-v 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, As-v concentration (sorption isotherm) and presence of phosphate on the As-v sorption. The results showed that the red mud efficiently sorbed As-v. The sorption was fast, with a major fraction of initial As-v being removed in a few minutes or hours of contact, depending on As-v 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 As-v sorption for the red mud system. Sorption of As-v was strongly pH-dependent. Maximum removal ( > 98%) was observed at slightly acidic pH (pH(max) = 5.5-6), while As-v sorption considerably decreased at both highly acidic and alkaline pH. The percentages of sorbed As-v decreased with the increasing solution As-v concentration, and the As-v sorption capacity (up to 43.5 mmol/kg) of the red mud was higher (similar to 4-fold) at pH similar to 6 than at pH similar to 9.2 (natural pH of the red mud). The presence of P at equimolar or 1:10 As/P molar ratios reduced As-v sorption by similar to 20% and 30%, respectively. Simulations of As-v 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 As-v in the liner at pH similar to 9.2 than at pH similar to 6 and a minimum thickness of similar to 90 cm and similar to 20 cm, respectively, for a RM liner to decrease the solution As-v concentration from highly toxic 1 mM to a safe < 0.133 mu M ( < 10 mu g/L) level, after a 35-years period.