Effect of Humic Acid Coating on Arsenite Adsorption on Ferrihydrite-Kaolinite Complexes.
Maria Martin1, Luisella Celi1, Elisabetta Barberis1, Antonio Violante2, Leonard M. Kozak3, and P. M. Huang3. (1) DIVAPRA, Univ of Turin, Grugliasco (Torino), via L. da Vinci 44, Grugliasco, 10095, Italy, (2) Univ of Naples FedericoII, Portici (Napoli), Via UniversitÓ 100, Naples, 80055, Italy, (3) Dept of Soil Science, Univ of Saskatchewan, 51 campus drive, Saskatoon, SK S7N 5A8, Canada
The toxicity and potential bioavailability of arsenic in soils depends, among the other factors, on its state of oxidation: arsenite is more toxic than arsenate and it is generally reported to be less retained by soil colloids. Although the adsorption of arsenite on pure, synthetic iron oxides is well documented, limited information is available on its adsorption on more complex systems that are more likely to exist in soils, where the co-existence of phyllosilicates, iron oxides and soil humic matter commonly occurs. The present work is aimed to evaluate the kinetics of arsenite adsorption on mixed systems, kaolinite/Ferrihydrite (KGa2-Fh) and kaolinite/Ferrihydrite/humic acid (KGa2-Fh-HA), in order to verify the effect of the presence of the humic acids (HA) on the iron oxide coated phyllosilicates on arsenite adsorption. Preliminary isotherms of arsenite and arsenate adsorption on the two complexes (pH 6.5, temperature 298 K, 24 h of interaction) showed that the presence of a 50% HA coverage of the surface significantly lowered the adsorption capacity toward both the As forms. Arsenite was retained in higher amount than arsenate by both the complexes, although with lower binding strength, and the ratio between adsorbed arsenite and arsenate was hardly affected by the HA coverage. Although the kinetics of the adsorption reaction could not be investigated for times shorter than 5 min with conventional kinetic methods, during the first 5 min of interaction, 73% of the arsenite adsorbed at equilibrium was adsorbed from the solution by the KGa2-Fh complex and 60% by the KGa2-Fh-HA complex. Six kinetic models, i.e., the zero-order, first-order, second-order, Elovich, parabolic diffusion and power law equations were used to fit the experimental data. The Elovich model provided the best fitting for the experimental data, with a fairly linear plot covering all the 24 hours of the complete kinetic study. In order to provide the rate constant of arsenite adsorption on the two substrates at 288, 298, 308 and 318 K, the first-order rate equation was selected based on the r2, P and standard error values. Two reaction steps were observed: a fast step between 5 and 20 min and a slow step between 30 and 240 min; after this time, semi-equilibrium was reached. The extremely low temperature dependence of the adsorption kinetics for both the complexes did not allow the determination of the energy of activation and the collision frequency by Arrhenius equation. The rate constant of the fast reaction was higher for the KGa2-Fh complex than for the KGa2-Fh-HA, while they become very similar for the slow reaction. The lowering of the maximum amount of adsorbed arsenite and the lowering of the fast reaction rate constant in the case of the HA covered substrate can be explained in terms of pre-occupation of the positively charged reaction sites by HA, and also in terms of steric hindrance of the large humic molecules hampering the arsenite to reach the surface. The net negative charge displayed by the surface after the adsorption of HA can also cause the electrostatic repulsion of anionic species to some extent, although arsenite is mainly undissociated at pH 6.5. The data indicate that humic substance-sesquioxide-clay mineral interactions deserve close attention in understanding As transformation and mobility in the environment.