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.
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Munro, L.D., Clark, M.W. & McConchie, D. A Bauxsol™-based Permeable Reactive Barrier for the Treatment of Acid Rock Drainage. Mine Water Environ 23, 183–194 (2004). https://doi.org/10.1007/s10230-004-0061-z
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DOI: https://doi.org/10.1007/s10230-004-0061-z