| Iron-manganese binary oxide (FeMnOx) is considered to be highly effective for arsenic adsorption from aqueous solutions. However, the agglomeration effect and difficult separation hindered its application in practical engineering. In this paper, reduced graphene oxide (RGO) was used as a supporting matrix to disperse FeMnOx due to its huge specific surface area, so the synthesized novel carbon-based nanocomposite adsorbent (FeMnOx/RGO) as well as starch stabilized adsorbent (FeMnOx/RGO (starch)) highly weakened the agglomeration effect of FeMnOx particles and greatly improved the adsorption efficiency.Different characterization techniques were employed to analyze surface morphology and structure, chemical composition of FeMnOx/RGO composite adsorbent, including transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), Brunauer-Emmett-Teller (BET) method, X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). It was found that FeMnOx/RGO had higher specific surface area (411 m2 g-1) than bare FeMnOx, and the amorphous form of Fe-Mn binary oxide particles were well dispersed on graphene sheets, providing good conditions for arsenic adsorption.The loading ratio of FeMnOx had an influential effect on arsenic removal efficiency. Comparative experiments were conducted and the results demonstrated that as the mass ratio of FeMnOx to FeMnOx/RGO nanocomposites was 45%, active substance FeMnOx possessed better adsorption behavior, and showed 47.05 mg As g-1 FeMnOx and 49.01 mg As g-1 FeMnOx adsorption capacities for As(III) and As(V), respectively, with 7.0 mg L-1 initial concentration.Different stabilizers such as cetyltrimethyl ammonium bromide (CTAB), starch and carboxymethyl cellulose (CMC) were used to stabilize FeMnOx/RGO in water for arsenic adsorption. Experimental results indicated that FeMnOx/RGO (starch) was more efficient for arsenic adsorption than the unstabilized FeMnOx/RGO. As the initial arsenic concentration was 7.0 mg L-1, the adsorption capacities for As(III) and As(V) by FeMnOx/RGO (starch) reached to 72.46 mg As g-1 FeMnOx and 54.71 mg As g-1 FeMnOx, respectively.As(Ⅲ) adsorption was better described by the Freundlich model on both FeMnOx/RGO and FeMnOx/RGO (starch).In contrast, the Langmuir model matched well with As(V) adsorption data. The removal process perfectly obeyed pseudo second-order kinetic model for both As(III) and As(V) adsorbed by the two materials. Effects of various anions on arsenic removal by FeMnOx/RGO and FeMnOx/RGO (starch) were investigated. Coexisted SO42- at different concentrations had little influenc on arsenic removal efficiency, while PO43- showed significant inhibiting effect on arsenic removal. Moreover, the recycling experiment showed that both materials had good recyclability and regeneration performance, which would provide a favorable foundation for engineering application.Electrostatic attraction was confirmed to be involved in the adsorption process via studying the effect of initial pH.As(V) removal by FeMnOx/RGO was highly dependent on pH values, and the removal efficiency of As(Ⅲ) also decreased continuously under alkali conditions. However, no obvious influence of pH on As(III) adsorption by stabilized FeMnOx/RGO (starch) was observed. The study on effect of the Fe:Mn molar ratio and XPS results showed that redox reaction and surface complexation mechanism were also involved in the adsorption. MnO2 could convert a portion of As(III) into As(V), iron(III) oxide was found crucial to As(V) removal, while RGO only acted as a favorable carrier to disperse iron and manganese oxides and had little influence on arsenic removal. Compared to the arsenic removal performance of single metal oxide assembled on RGO (FeOx/RGO, MnOx/RGO), Fe-Mn binary oxide on RGO possessed synergistic effects for As(III) and As(V) removal. Moreover, steric stabilization was operative for starch to stabilize FeMnOx nano particles, while electrosteric stabilization was the predominant mechanism for CTAB and CMC to disperse FeMnOx particles. |
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