Adsorption Performance And Mechanism Of Iron-based Nano-materials Towards Low Concentration Arsenic In Water

Inorganic arsenic?As?compounds have high toxicity and trend to cause a variety of cancerous change of body organs and tissue lesions.Nature As pollution in drinking water has been reported in more than 70 countries.It is worth mentioning that,the contamination of As in groundwater is generally at a low concentration,ranging from 0.01 to 5 mg?L^–1 in most countries.In this work,based on the difficulty of remediation in As contamination,such as its low concentration and high toxicity,we designed and synthesized four kinds of iron-based nano materials for the removal of As in aqueous solution,and also analyzed the adsorption performance and the mechanisms of these iron-based materials to As.Based on the different As?V?adsorption capacities of nano zero valent iron?nZVI?and zero valent iron?ZVI?normalized by specific surface area,the comprehensive mechanism of arsenic adsorption by nZVI at varied pH was deeply analyzed.Batch experiment shows that,the change of pH has smaller influence on the As?V?adsorption by nZVI compared with that by ZVI.X-ray diffraction?XRD?and X-ray absorption near-edge structure?XANES?analysis indicated that,with the coexistence of As?V?species at low concentration,the increasing pH value would not change the phase of ZVI obviously,however,it would decrease the content of lepidocrocite??-FeOOH?but increase the magnetite/maghemite?Fe₃O₄/?-Fe₂O₃?content in the corrosion products of nZVI,whereas the latter has stronger adsorption ability to As?V?.With the increasing of solution pH,the change of corrosion products makes nZVI have a stronger adsorption affinity to As?V?,which partly complements the negative impact from electrostatic repulsive-force.By using Mg?OH?₂ nanoplatelets as supporting material for nZVI,nZVI@Mg?OH?₂composite was prepared and found to have ultra strong adsorption ability to As?V?at low concentrations.The size of nZVI particles range from 30 to 70 nm.Mg?OH?₂ has a self-supported spherical structure,whose diameter is of several micrometers,interweaved by the nanoplatelets the thickness of which is 8-20 nm.In composite material,the content of nZVI is33.66%.Batch adsorption experiment indicated the Langmuir model was more appropriate to depict the adsorption process of As?V?on nZVI@Mg?OH?₂ composite,and the maximum theory adsorption capacity of As?V?is 7.17 mg?g^–1,which is higher than that of pure nZVI and pure Mg?OH?₂.The mechanism study revealed that the variation of corrosion products of nZVI in the presence of Mg?OH?₂ and Mg²⁺was an important factor for increase in adsorption ability to As?V?.X-ray diffraction?XRD?analysis indicated that the weakly basic condition induced by Mg?OH?₂ dissolution decreased the lepidocrocite??-FeOOH?content but increased the magnetite/maghemite?Fe₃O₄/?-Fe₂O₃?content in the corrosion products of nZVI,and the latter has better adsorption affinity to As?V?.Moreover,extended X-ray absorption fine structure spectroscopy?EXAFS?indicated that the coordination between arsenic and iron minerals is influenced by dissolved Mg²⁺,leading to probable formation of magnesium ferrite?MgFe₂O₄?which has considerable adsorption affinity to As?V?.Magnesium ferrite?MgFe₂O₄?nano-plates of rich hydroxyl group and defects has been successfully synthesized and dedicated to remove the low concentration As?V?in aqueous solution.Results from scanning electron microscopy?SEM?showed that the as-synthesized MgFe₂O₄ nano-platelets were about 10 nm in thickness and consisted of many individual nanospheres.Rietveld refinement of X-ray diffraction?XRD?data indicated that the Mg²⁺ions substituted the Fe³⁺ions at both the octahedral and the tetrahedral sites of the crystal structure.Batch adsorption experiment showed that the equilibrium concentration of As?V?could be reduced to 4.9?g?L^-1 when the initial concentration of As?V?is 1 mg?L^-1,which complied with the drinking water standard of WHO(10?g?L^-1).The synthesized MgFe₂O₄ was found to outperform commonly used iron oxide adsorbents?Fe₃O₄,?-Fe₂O₃ and?-Fe₂O₃?in the adsorption of As?V?.Further mechanistic studies proved that the superior adsorption capacity could be attributed to:?1?increased amount of surface hydroxyl groups that resulted from the surface defects.?2?formation of tridentate hexanuclear surface complexes instead of bidentate binuclear complexes,and?3?formation of Mg-OH surface hydroxyl groups and As-Mg monodentate mononuclear surface complexes.This work demonstrated the correlation between the rich surface hydroxyl groups and the superior As?V?adsorption ability of defective MgFe₂O₄.The synthesized material may have potential application in environmental remediation,such as treating As-contaminated water.Zinc ferrite spinel nano particle with different Zn content(Zn_x Fe_3-xO₄)have been synthesized by hydrothermal method.The influence of Zn content and the distribution of Zn atom in spinel crystal structure on the As?V?adsorption by zinc ferrite have been studied.The result shows that,with increasing of x in Zn_xFe_3-x O₄,the crystal size is increased.When x<0.6,Zn atoms mainly occupy the octahedral site in spinel.With x increasing,the octahedral site is saturated,and the Zn atom turns to occupy the tetrahedral site in the range of x>0.6.Batch adsorption experiment shows that when x=0.6,Zn_xFe_3-xO₄ has strongest adsorption ability to As?V?.Langmuir model is more suitable to describe the adsorption process of As?V?by Zn_0.6Fe_2.4O₄,and the theoretical maximum adsorption capacity of As?V?is 7.86 mg?g^–1,which is higher than the performance of Fe₃O₄,?-Fe₂O₃ and?-Fe₂O₃.The adsorption enhancement mechanism of Zn_0.6Fe_2.4O₄ to As?V?can be attributed to the increase of surface hydroxyl groups.