EVALUATING THE POTENTIAL OF INORGANIC AND ORGANIC-LIGANDS TO EXTRACT ALUMINUM 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 aluminiu m (Al) between the exchangeable and solution phase and to assess the a bility of the ligands to mobilise aluminium in an acidic red earth (Ca lcic Rhodoxeralf). Anions were chosen to represent a range of potentia l abilities to form complexes with Al. They included chloride, fluorid e, citrate, salicylate, 2,3-dihydroxybenzoic acid, catechol, and 2 dif ferent coal-derived fulvates. Batch experiments, using a 1:5 soil:solu tion ratio, showed that citrate, 2,3-dihydroxybenzoic acid, and fluori de had a similar effect on Al sorption and speciation. These anions we re effective in decreasing the amount of exchangeable and sorbed Al an d increased the amount of complexed Al in solution. In the soil soluti on, very little Al was in the uncomplexed toxic form (Al3+). Results f rom the batch experiments may not necessarily give a good indication o f the behaviour of Al in the soil at realistic moisture contents. Ther efore, those anions that showed the best capacity to remove exchangeab le Al and decrease the concentration of toxic Al3+ in the solution wer e used in leaching cell experiments. These results showed that citrate , 2,3-dihydroxybenzoic acid, and the coal-derived fulvates can signifi cantly decrease exchangeable Al, citrate being the most effective. Cit rate and 2,3-dihydroxybenzoic acid resulted in significant amounts of Al being leached from the soil, whereas fluoride and the fulvates resu lted in only slightly more Al leaching than chloride. Although the ful vates can remove some of the exchangeable Al, it was not leached from the soil. Both the batch and leaching cell techniques do allow the scr eening of compounds to decrease the concentration of toxic Al3+ on the cation exchange complex of the soil.