A Bauxsol™-based Permeable Reactive Barrier for the Treatment of Acid Rock Drainage

Abstract.

Permeable reactive barriers (PRBs) have generally been used to treat low flow and/or low contaminant loads of acid rock drainage (ARD). Bauxsol™, a product made from seawater-neutralized red mud (a by-product of alumina refining) buffers pH at 8.8 and has been shown to remove >99% of heavy metals loadings at >1000 meq/kg, which would make it an ideal medium for PRBs. Unfortunately, Bauxsol™ is very fine-grained (>90% of the material <10 µm) and therefore, possesses a very low hydraulic conductivity. Consequently, sand/Bauxsol™ mixtures were trialed to determine hydraulic conductivity and ARD treatment capacities. Column tests indicated that the sand/Bauxsol™ mixtures maintain a higher hydraulic conductivity when used to treat ARD than when used with tap water. A 75:25 sand: Bauxsol™ filter with a 130 mm thick layer of reactive media had a hydraulic conductivity of 3.06 x 10^-5 m/s and except for Al, treated a highly contaminated ARD to ANZECC (2000) drinking water guidelines. A field trial of a 1 m³ PRB (70:30 sand:Bauxsol™ mixture) with a hydraulic conductivity of 9.8 x 10^-5 m/s treated about 45,000 L of highly contaminated ARD to the ANZECC (2000) drinking water standards, and about 27,000 L of this water had heavy metal loadings reduced to the ANZECC (2000) Protection of Aquatic Ecosystem guideline of >90% species protection.

The metal binding preference of Cu>Pb>Zn>Mn displayed by the PRB is consistent with the pKa of hydration values and it appears that MOH⁺ formation dominates the removal of these metals. However, other removal processes appear to apply for Fe, Al, and Cd. Overall metal removal efficiencies ranged from 91.22% for Mn to >99.99% for Cu. Analysis of dried spent Bauxsol™ indicates that bound metals are not readily leachable, allowing the spent media to be disposed safely in a landfill.