Evaluating the potential of inorganic and organic ligands to extract aluminium from an acidic soil using batch and leaching cells

Batch and leaching cell approaches were used to study the effect of a range of inorganic and organic ligands on the distribution of aluminium (Al) between the exchangeable and solution phase and to assess the ability of the ligands to mobilise aluminium in an acidic red earth (Calcic Rhodoxeralf). Anions were chosen to represent a range of potential abilities to form complexes with Al. They included chloride, fluoride, citrate, salicylate, 2,3-dihydroxybenzoic acid, catechol, and 2 different coal-derived fulvates. Batch experiments, using a 1 : 5 soil : solution ratio, showed that citrate, 2,3-dihydroxybenzoic acid, and fluoride had a similar effect on Al sorption and speciation. These anions were effective in decreasing the amount of exchangeable and sorbed Al and increased the amount of complexed Al in solution. In the soil solution, very little Al was in the uncomplexed toxic form (Al3+). Results from the batch experiments may not necessarily give a good indication of the behaviour of Al in the soil at realistic moisture contents. Therefore, those anions that showed the best capacity to remove exchangeable Al and decrease the concentration of toxic Al3+ in the solution were used in leaching cell experiments. These results showed that citrate, 2,3-dihydroxybenzoic acid, and the coal-derived fulvates can significantly decrease exchangeable Al, citrate being the most effective. Citrate and 2,3-dihydroxybenzoic acid resulted in significant amounts of Al being leached from the soil, whereas fluoride and the fulvates resulted in only slightly more Al leaching than chloride. Although the fulvates can remove some of the exchangeable Al, it was not leached from the soil. Both the batch and leaching cell techniques do allow the screening of compounds to decrease the concentration of toxic Al3+on the cation exchange complex of the soil.