Influence of Major Anions on As(V) Adsorption by Synthetic 2-line Ferrihydrite. Kinetic Investigation and XPS Study of the Competitive Effect of Bicarbonate

The potential competitive effect of background electrolytes (Na₂HPO₄ ·2H₂O, NaHCO₃, Na₂SO₄ and NaCl solutions) on arsenate adsorption onto synthetic 2-line ferrihydrite has been studied by means of kinetic batch experiments conducted at pH values from 4.0 to 10.0 and at anionic concentrations of 0.01 and 0.1 M. The results indicate that the adsorptive capacity of ferrihydrite for arsenate decreases strongly in the presence of phosphate species at pH in the range of 4-10 and in the presence of bicarbonate at pH 8.3 as a consequence of their competitive effect. Analogously to phosphate, a surface interaction of inner-sphere type between ferrihydrite and bicarbonate is suggested. Chloride has negligible effects on arsenate adsorption processes, confirming it as an outer-sphere ion that does not compete with the inner-sphere binding peculiar to arsenate onto ferrihydrite. Sulphate exhibits an intermediate behaviour; at 0.01 M concentration, the competitive effect of sulphate is similar to that of chloride, whereas at 0.1 M concentration sulphate shows a moderate influence on arsenate adsorption. The results of the kinetic studies can be summarised by the following order of competitive capacity: phosphate > carbonate > sulphate > chloride. The process of arsenate adsorption follows pseudo-second order kinetics and the reaction half-time notably increases in the presence of strong competitor anions such as phosphate and carbonate with respect to an ineffective competitor anion such as chloride. Modelling of arsenate adsorption with PHREEQC, according to the Generalized Two-Layer Model, confirms that the pH effect is notably less important than the competitive effect of carbonate species in determining the amount of arsenate adsorbed onto ferrihydrite at pH 8.3 in 0.1 M NaHCO₃ solution, whereas the model greatly underestimates the competitive effect of carbonate species at pH 8.3 in 0.01 M NaHCO₃ solution. The results of the batch experiments in 0.1 M NaHCO₃ solution are substantiated by XPS analyses of ferrihydrite after immersion in the same solution, both with and without dissolved arsenate. XPS confirms the interaction between ferrihydrite surface and arsenate; the binding energy of As3d shifts towards higher binding energies after adsorption with respect to the pure compound Na₂HAsO₄·7H₂O taken as reference standard. In presence of carbonate species, the As3d binding energy is found at intermediate values. XPS quantitative analysis shows a depletion of arsenate on ferrihydrite surface, providing further evidence of the competition of the two species (i.e. arsenate and bicarbonate) for the ferrihydrite adsorption sites. Important environmental implications concerning arsenic mobility, as well as possible application in various fields (e.g. irrigation agriculture, soil decontamination, water treatment), might derive from these findings.