Competitive Sorption and Displacement of Arsenate by Phosphate On An Aluminum Mining by-Product.

Antagonistic effects between competing ions are important when evaluating a by-product as a potential adsorbent and, or soil amendment. This work aimed to assess: i)  mono- and multielement adsorption and desorption of P and As on an aluminum mining by-product; and, ii) the displacement of previously adsorbed arsenate by phosphate added at increasing doses in solution. For adsorption, 0.3 g of the adsorbent were suspended in 20 mL of 30 mmol L^-1 NaCl, which had its pH adjusted to 5.5 with the addition of predetermined amounts of 1 mol L^-1 HCl and 1 mol L^-1 NaOH. After adjusting the pH, phosphate and arsenate were added as H₃PO₄ and Na₂HAsO₄.7H₂O in single and mix solutions with initial concentrations of 0.67 mmol L^-1 (adsorbent:solution rate of 1:100) After 72h of reaction, samples were centrifuged and the supernatant collected for As and P analyses. The remaining residue was reacted with 30 mL of 30 mmol L^-1 NaCl to promote desorption of previously adsorbed elements. To evaluate the displacement of arsenate by phosphate, we add 30 mL of a 30 mmol L^-1 NaCl containing phosphate in the following concentrations: 0, 0.007, 0.014, 0.1, 1.0, and 2.0 mmol L^-1. The competition between phosphate and arsenate reduced the adsorbed concentration by 3 and 23%, respectively when comparing single versus mix systems. On the other hand, this competition increased phosphate desorption up to 4 fold and arsenate up to 2 fold. The ratio mmol kg^-1 of adsorbed phosphate/mmol kg^-1 of desorbed arsenate decreased upon increasing the concentration of P in the displacement solution (2.3±1.1 and 0.98±0.03 for solutions containing 0.007 and 0.014 mmol L^-1 of phosphate, respectively). Sponsored by CAPES, CNPq, and FAPEMIG.