Factors Affecting the Desorption of Arsenate by Phosphate From Soil Components and Soils.
A. Violante, G Krishnamurti, M Pucci, and M. Pigna. UniversitÓ di Napoli- ITALY, UniversitÓ 100, Portici (Napoli), Italy
Arsenic is a toxic element for humans, animals, and plants and can be present in all natural environments. Sorption of arsenate by soil constituents (mainly metal oxides, and other variable charge minerals) controls the mobility and bioavailability of arsenate in soil-water-plant systems. In contrast to adsorption studies, little information are available on the desorption of arsenate from soil components and soils. Phosphate has been reported to displace adsorbed arsenate from minerals and soils. Application of phosphate fertilizers is a common crop management practice that can have a direct effect on the sorption/desorption behaviour of arsenate and its phytoavailability and ground-water contamination. The aim of this work was to study the desorption of arsenate previously adsorbed on metal oxides (gibbsite, non crystalline Al-hydroxide, goethite and ferrihydrite) and soils (Andisols) by phosphate as a function of: i) pH, ii) residence time (from 3 h to 2 months), and iii) surface coverage (50 or 100%). The adsorption of arsenate onto the metal oxides was affected by the nature of the sorbents and the amounts of arsenate added. The complete adsorption of arsenate onto the samples at 50% of surface coverage was particularly fast and occurred within 2-3 hours for all the sorbents. By adding phosphate (P/As molar ratio of 3) immediately after the complete adsorption of arsenate onto the surfaces of the oxides a high proportion of the metalloid was desorbed after 24 hours from gibbsite (52%) and Al(OH)x (43%), whereas low amounts were desorbed from goethite (18%) and ferrihydrite (23%), indicating that arsenate is held more strongly on to the Fe oxides in comparison to Al oxides. The amount of arsenate desorbed by phosphate after 24 hours decreased with increasing residence time, due to the reorganization of arsenate from desorbable to not desorbable forms on sites of different bonding strength and reactivity. The kinetics of reorganization of arsenate on the metal oxides as a function of residence time could be explained best by the Elovich kinetic model. The kinetics of desorption (from 3 hours to 2 months) of arsenate by phosphate from Andisols as a function of pH (5.0-7.0), by keeping the surface coverage and residence time constant, showed that arsenate desorption increased by increasing the pH of the systems.