A novel method for fabrication of a binary oxide biochar composite for oxidative adsorption of arsenite: Characterization, adsorption mechanism and mass transfer modeling

Abstract

Arsenite [As(III)] is more mobile and toxic than arsenate [As(V)], making it more difficult and more critical to remove from water. The oxidative adsorption of As(III) is preferred due to its cost and time effectiveness. The chemical mixing process is a common method for preparation of biochar composites, but it is not time-efficient for homogeneous metal impregnation and is challenging to achieve consistent and reproducible physicochemical properties of the composite. The mixing method is even more challenging if two metal oxides are utilized in preparing engineered biochars. The current study proposes a novel microwave- and electrochemical-assisted technique to overcome the shortcomings of the mixing method and create a binary-oxide biochar composite with oxidation and adsorption capabilities. In addition, a new procedure was used to develop a diffusional mass transfer model combined with a genetic algorithm to characterize the adsorption process. Characterization of the fabricated biochar indicated that the deposited Fe and Mn oxides on the biochar surface included a mixture of Mn(II,III,IV) oxides and poorly crystalline α-FeOOH, respectively. The adsorption results suggested a hybrid adsorption mechanism with a high probability of a homogeneous monolayer adsorption process. Based on XANES analyses, no reduction to As(III) was observed for the As(V) adsorbed onto adsorbent, while 79% of the adsorbed As(III) was oxidized to As(V). The intraparticle diffusion and external mass transfer coefficients were estimated using a diffusional mass transfer model. Based on the modeling results, the adsorption of As onto the fabricated adsorbent was mainly controlled by external mass transfer. Overall the As(III) adsorption capacity of untreated biomass increased from 62 to 21,426 μg/g through the modification, indicating the high potential of the proposed procedure for preparation of adsorbents with improved oxidation and adsorption performance.