A review on multi-synergistic transition metal oxide systems towards arsenic treatment: Near molecular analysis of surface-complexation (synchrotron studies/modeling tools)

Boosting arsenic removal with metastable Fe2+/Mn3+ redox process in MnFe2O4/rGO composites for high capacity and stability

Fe-Mn oxides exhibit significant potential in the application of chemical and electrochemical remediation of groundwater arsenic contamination. However, the mechanism controlling the equilibrium between chemisorption inhibition and capacitive adsorption enhancement at ferromanganese oxide electrodes is unclear, posing significant challenges to achieving both electrochemical arsenic removal efficiency and cycle stability. Here, we introduce for the first time a defect engineering strategy to synthesize defect-rich, reduced graphene oxide-anchored MnFe2O4 composites (MnFe2O4/rGO). The electrochemically efficient arsenic removal capacity (102.6 mg·g-1) and sustained cycling stability (30 cycles with >95 % efficiency) are achieved through the synergistic pseudocapacitive effect of metastable Fe-Mn bimetallic. 80 % of the arsenic removal is due to pseudocapacitive effects driven by reversible redox reactions of metastable Fe2+/Mn3+ in MnFe2O4 tetrahedral coordination revealed by X-ray photoelectron spectrum (XPS). The electronic microenvironment of iron site is modulated by Mn atom reducing the arsenic adsorption energy on MnFe2O4/rGO electrode based on electronic impedance spectrum (EIS) and density function theory (DFT). Continuous flow experiments reveal that this electrochemical system deeply purifies 5 L arsenic-laden groundwater (1 mg·L-1) below World Health Organization's (WHO) drinking water guidelines with lower energy consumption and high selectivity. This study provides valuable insights for tailoring effective, stable electrodes in electrochemical arsenic removal.

Removal of arsenic from water by silver nanoparticles and Fe-Ce mixed oxide supported on polymeric anion exchanger

By encapsulating nanoscale particles of goethite (α-FeO(OH)), hydrous ceric oxide (CeO2·H2O, HCO) and silver nanoparticles (AgNPs) in the pores of polystyrene anion exchanger D201, a novel nanocomposite FeO(OH)-HCO-Ag-D201 was prepared for the effective removal of arsenic from water. The isotherm study shows that FeO(OH)-HCO-Ag-D201 has excellent adsorption performance for As(III) and As(V), with an increased adsorption capacity of As(III) to 40.12 mg/g compared to that of 22.03 mg/g by the composite adsorbent without AgNPs (FeO(OH)-HCO-D201). The adsorption kinetics data showed that the sorption rate of FeO(OH)-HCO-Ag-D201 for As(III) is less than that for As(V), and the adsorption of As(III) and As(V) were consistent with the pseudo-second-order model and the pseudo-first-order model, respectively. Neutral or basic conditions are favored for the adsorption of As(III/V) by FeO(OH)-HCO-Ag-D201. Compared with nitrate/chloride/bicarbonate, sulfate/silicate/phosphate showed more remarkable inhibition of arsenic removal by FeO(OH)-HCO-Ag-D201, whereas natural organic matter showed no interference to the arsenic removal. The As(V) adsorption involved different interactions such as electrostatic attraction and surface complexation, while the adsorption of As(III) involved the part oxidization of As(III) to As(V) and the simultaneous adsorption of As(III) and As(V). In addition to the Ce(IV) in CeO2·H2O acted as an oxidant, the synergistic effect of α-FeO(OH) and AgNPs also contributed to the oxidization of As(III) to As(V). Moreover, the reusable property suggested that this FeO(OH)-HCO-Ag-D201 nanocomposite has great potential for arsenic-contaminated water purification.

Surface Hydroxyl Groups Functionalized Porous CeO2 for Enhanced Selective Adsorption of As(III)

The adsorption technique has been considered as one of the promising methods to remove arsenic ions in aqueous systems. However, the adsorbents are usually poorly selective and have low capacity. Herein, a kind of porous CeO2 containing surface hydroxyl group which synthesized facilely possesses the great performance of selective adsorption of As(III) with 98.57% removal against 14 other coexisting metal ions. The results showed that the processes of As(III) and As(V) uptake were heterogeneous monolayer adsorptions and included external and intraparticle diffusions. The adsorption capacities at pH 2.5 reached 111.24 and 56.89 mg/g for As(III) and As(V), respectively. In addition, it also showed that As(III) could be oxidized to As(V) in the adsorption process. Density functional theory calculations revealed that the OH group in H3AsO30 and the As atom in H3AsO40 have affinity with lattice oxygen (O2-) in CeO2, while the O atom in H2AsO4- preferred the Ce atom in CeO2. This study provides a novel porous CeO2 containing hydroxyl groups for selective and efficient removal of arsenic and elucidates the mechanism of As(III) and As(V) adsorptions.

Modeling of arsenic migration and emission characteristics in coal-fired power plants

After coal combustion, the minerals present in fly ash can adsorb arsenic (As) during flue gas cooling and reduce As emissions. However, a quantitative description of this adsorption behavior is lacking. Herein, the As adsorption characteristics of minerals (Al/Ca/Fe/K/Mg/Na/Si) were investigated, and a model was developed to predict As content in fly ash. Lab-scale experiments and density functional theory calculations were performed to obtain mineral As adsorption potential. Then, the model was established using lab-scale experimental data for 11 individual coals. The model was validated using lab-scale data from ten blended coals and demonstrated acceptable performance, with prediction errors of 2.83-11.45 %. The model was applied to a 350 MW coal-fired power plant (CFPP) with five blended coals, and As concentration in the flue gas was predicted from a mass balance perspective. The experimental and predicted As contents in fly ash were 11.92-16.15 and 9.61-12.55 μg/g, respectively, with a prediction error of 17.39-22.29 %, and those in flue gas were 11.52-16.58 and 5.37-34.04 μg/Nm3. Finally, As distribution in the CFPP was explored: 0.74-1.51 % in bottom ash, 74.05-82.70 % in electrostatic precipitator ash, 0.53-1.19 % in wet flue gas desulfurization liquid, and 0.13-0.73 % in flue gas at the stack inlet.