Preparation Of Fe₃O₄-MnO₂magnetic Nanoplates And Research On Its Adsorption Capacity For Arsenite Removal

Arsenic, a relatively scarce but ubiquitous element, is of serious concern due toits toxicity and carcinogenicity. In natural water, arsenic is primarily present ininorganic forms and exists in two predominant species, arsenite and arsenate. Moreimportantly, As（Ⅲ） is25⁶0times more toxic than As（Ⅴ） and therefore deservesmore attention. As there is no simple treatment for the efficient removal of As（Ⅲ）,an oxidation step is always necessary to achieve higher removal. However, this leadsto a complicated operation and is not cost-effective. To overcome these disadvantage,we proposed a one-pot, low-cost and environmentally-friendly approach forfabrication of tunable Fe₃O₄-MnO₂nanoplates at ambient temperature and pressure,which combined the oxidation property of manganese dioxide and the highadsorption features to As（Ⅴ） of iron oxides.XPS, XRD, TEM, VSM and BET techniques were applied to characterize theprepared adsorbent. It was found that Fe3O4nanoplate cores were coated byamorphous MnO2shells. In addition, the morphology of the as-synthesizednanoparticles could be controlled through manipulating the molar ratio of FeSO4andKMnO₄. The increased KMnO₄dosage led to the decrease in thickness of thenanoplate cores from12nm to <5nm, the increase in edge length from40nm to70nm, and the increase in thickness of the shell from <1nm to5nm. In addition, forthe conditions of common pressure and ambient temperature, the Fe₃O₄-MnO₂nanoplates could be synthesized with an arbitrary molar ratio of FeSO4with KMnO₄in one pot. Batch experimental results showed that the magnetic nanoplates withinitial Fe/Mn molar ratios of3:1could be utilized as promising novel adsorbents forremoval of trace As（Ⅲ） from drinking water.Laboratory experiments were carried out to investigate the influence of initialAs（Ⅲ） concentration, reaction time and solution pH values on arsenite removal.Batch experimental results showed that with the increase of the initial arseniteconcentration, the adsorption capacity increases. The minimum time required toreach equilibrium was4h. As（Ⅲ） could be effectively removed by Fe₃O₄-MnO₂magnetic nanoplates at initial pH range from2to8. The effects of anions such asSO42, PO43, SiO32and CO32, which possibly exist in natural water, on As（Ⅲ）removal were also investigated. The results indicated that phosphate was thegreatest competitor with arsenite for adsorptive sites on the adsorbent. The presenceof sulfate had no significant effect on arsenite removal.Laboratory experiments were carried out to investigate adsorption capacity of Fe₃O₄-MnO₂magnetic core-shell nanoplates with initial Fe/Mn molar ratios of3:1and adsorption kinetics on arsenite removal. Batch experimental results showed thatFreundlich model fitted better the As（Ⅲ） experimental data than Langmuir modeland the maximal adsorption capacitiy of As（Ⅲ） by Fe₃O₄-MnO₂magneticnanoplates with initial Fe/Mn molar ratios of3:1was80.40mg/g. The removal ofAs（Ⅲ） by Fe₃O₄-MnO₂magnetic nanoplates could be described well by thepseudo-second-order kinetic model. At pH7.0,200μg/L of As（Ⅲ） could be easilydecreased to below10μg/L by Fe₃O₄-MnO₂magnetic nanoplates （200mg/L）. Theco-occurring redox reactions between Mn oxide and As（Ⅲ） was confirmed by XPSanalysis. The results showed that As（Ⅲ） was completely oxidized to less toxicAs（Ⅴ）. Moreover, the synthesized magnetic nanoplates have been demonstrated tobe capable of enhancing the adsorption of arsenite through simultaneous oxidationof arsenite to arsenate by MnO2.