Removal of Al, Ga, As, V and Mo from alkaline wastewater using pilot-scale constructed wetlands

The study was initiated to evaluate constructed wetland technology as a method for treating alkaline (pH 8.0-8.6) drainage high in Al, Mo, V, As and Ga originating from bauxite residue storage areas. Pilot-scale horizontal flow constructed wetlands were operated over a 40-week period using three filter materials (granitic gravel, bauxite and alum water treatment sludge), and half of the wetlands were planted with Phragmites australis and the other half left unplanted. Gravel was the least effective medium for removing the target elements, while of the two active media, water treatment sludge was more effective than bauxite. Plants removed only small amounts of elements into their above- and below-ground dry matter (0.4-4.9% of that added). Nonetheless, the presence of plants greatly increased the effectiveness of all three media since their presence decreased effluent pH values by 0.5-1.3 pH units and that of the filter media by 0.4 pH units. Removal of elements followed the order Al > Ga > V > As > Mo. For planted wetlands, total elemental removal ranged from 18 to 98% for gravel, 80 to 99% for bauxite, and 93 to 99% for water treatment sludge. The lowest removal was for Mo (ranging from 18% for gravel to 93% for water treatment sludge) and the highest for Al (ranging from 98% in gravel to 99% in water treatment sludge). A sequential fractionation scheme for As, V and Mo on filter material at the end of the experiment showed that for bauxite and water treatment sludge, V and As were concentrated in the NaOH extractable fraction while Mo was concentrated in the less strongly adsorbed NaHCO₃ extractable fraction. It was concluded that a constructed wetland with water treatment sludge as an active filter material is an effective technology for removal of the target elements from the alkali drainage.