Fabrication of Fe/Mn oxide composite adsorbents for adsorptive removal of zinc and phosphate

Abstract

The main purpose of our work lies in a new methodology of fabricating sorptive materials possessing bi-functionality to adsorb both cationic and anionic contaminants. As noted in the manuscript, many contaminated soils tend to contain multiple contaminants, and therefore, single-phase adsorbents cannot adequately address those mixed contaminants in a water body. In this respect, bi- or multi-functional adsorbents capable of removing multiple contaminants have great merits and practical advantages. Four types of magnetic Fe/Mn oxide composites were prepared using different precipitation methods with or without post-treatment with H2O2: (1) co-precipitation of Fe2+/Mn2+ in alkaline condition (OS-N), (2) co-precipitation with H2O2 (OS-H), (3) two-step precipitation of Fe2+ and Mn2+ in sequential order (TS-N), and (4) two-step precipitation with H2O2 treatment (TS-H). The composites were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), particle size analyzer, and zeta potential analyzer. Cation-exchange capacity (CEC) and oxidation capacity of the composites were also measured using standard tests. The adsorption capacity of the composites toward Zn and PO4 was evaluated by carrying out adsorption kinetics, adsorption isotherm, and pH effect experiments. The individual composites consisted mainly of Fe/Mn oxide phases significantly differing in particle size, morphology, and mineralogical phases, as well as adsorption capability. Zeta potential and XRD analyses revealed that Zn2+ adsorption (Langmuir adsorption capacity 64.52 mg g−1) of TS-H composite was relatively high among the four composites due to the strong negative charges of birnessite present in the composite. On the other hand, TS-N composite with the greatest point of zero charge (PZC) showed the highest PO4 3− adsorption. The kinetic data of Zn2+ and PO4 3− adsorption were fit to the pseudo-second-order reaction rate model reasonably well, which suggests the adsorption process occurred via chemisorption or chemical bonding between composite’s active sites and adsorbates. Four types of Fe/Mn composites were synthesized by different precipitation methods. Zn2+ and PO4 3− adsorption capacities of the composites varied with the method by which they were synthesized. Post-treatment with H2O2 also yielded composites with enhanced oxidation capacity. The appropriate synthesis method for generating Fe/Mn-based adsorbents depends on the type of contaminants. Lastly, using the multi-functional adsorbents could facilitate removal of both anion and cation, potentially providing cost benefits over conventional adsorption techniques.