Kinetic and Mechanistic Evaluation of Inorganic Arsenic Species Adsorption onto Humic Acid Grafted Magnetite Nanoparticles

Humic acid coated magnetic iron oxide nanoparticles (HA-MNPs) were synthesized, characterized, and studied for the removal of toxic inorganic arsenic species from aqueous media. The adsorption of As(III) and As(V) followed pseudo second-order kinetics, and the observed data were accurately modeled employing the Freundlich adsorption isotherm. Application of the Weber and Morris intraparticle diffusion model to the observed kinetic data suggests that the adsorption occurs in three distinct stages, fast, intermediate, and slow steps. We propose the initial stage is governed by surface association, followed by intraparticle diffusion of arsenic through the HA matrix and, finally, chemical reaction or bonding between the arsenic species and HA functionality. The HA-MNP nanoadsorbent absorbs >95% of the inorganic arsenic species with an absorption capacity of 12.2-12.6 mg/g from aqueous media and is effective under a variety of conditions. Post arsenic adsorption characterization of the nanoparticles suggests that As(III) binds with the carboxylate group of HA through a proposed ester type linkage, while electrophilic reactions can occur between the nucleophilic functional groups present in HA and the electrophilic arsenic atom in As(V). The results obtained demonstrated that HA-MNPs are robust and have promise for effective As(III) and As(V) remediation.