AMENDMENT EFFECTS ON pH AND SALT CONTENT OF BAUXITE RESIDUE

Abstract

Bauxite residue, a waste product from the refining of bauxite to alumina, contains excessive Na and an elevated pH. We investigated the use of four chemical amendments to reduce bauxite residue pH and Na content and to improve soil characteristics for reclamation purposes. A control (unamended) and treatments of gypsum, acidic gypsum, sulfuric acid, or elemental sulfur were thoroughly mixed with bauxite residue at rates to reduce the exchangeable sodium percentage (ESP) to 5%, with or without an equivalent of 90 Mg ha−1 wood chips for aeration. Materials were placed in separate 32-L PVC containers, leached with water, and 8 separate pore volumes collected. All treatments were replicated three times. Pore volumes 1 through 5 and 8 were analyzed for pH, Na, Ca, Mg, and sodium adsorption ratio (SAR). The acidic gypsum and acidic gypsum + wood chips treatments significantly lowered the pH (<8.5), leached the greatest amount of Na, Ca, and Mg for all pore volumes, and lowered the SAR as compared with other treatments. On completion of the leaching experiment, we analyzed the 0- to 15-cm and 15- to 30-cm depths in the columns for Na, Ca, Mg, SAR, ESP, pH, and electrical conductivity and compared values with unamended treatment. For all treatments, the Na content and ESP were reduced by an order of magnitude, Ca and Mg content increased, pH was reduced, and SAR was reduced by several orders of magnitude as compared with pre-experiment concentrations. Overall, the acidic gypsum and the acidic gypsum + wood waste treatments showed the most promise for reducing bauxite residue pH and Na content.