| Arsenic is a highly toxic element which is characterized by the heavy toxicity, great harm, carcinogenicity, mutagenicity and teratogenicity, existing commonly in natural environment. Water pollution caused by arsenic is one of the pressing environmental problems around the world. Iron oxides are known to have the high surface charge, large specific surface area and stable physical-chemical properties for the arsenic removal optimization of adsorbent materials. Humic Acid, as an important component of natural organic matter, is exist with arsenic to co-occur a complicated interaction which exerting an influence on the migration, transformation and bioavailability of arsenic in waters. In this study, akaganeite, a typical type of iron oxides, was selected as the adsorbent of arsenic removal for its advantages of crystalline structure, low-cost and easily obtaining. The impact study of HA on two inorganic forms of arsenic adsorption (arsenate and arsenite) by akaganeite was investigated by using methods of static adsorption experiment and instrumental analysis. The behavior and mechanism of HA effect on arsenic adsorption by akaganeite was emphatically studied.The akaganeite nanocrystalline was prepared by using in-situ hydrolysis of ferric iron ions with mild reaction condition and simple method in this study. The physicochemical properties of the obtained adsorbent were characterized by using instrumental analysis method. With the presence of HA, through static adsorption tested the effect of pH on arsenic adsorption by akaganeite, adsorption and desorption kinetics, thermodynamic process, the effect of foreign ions and the shift of adsorption edges were investigated detailedly.The results of pH effect experiment indicatied that the removal rate of arsenic was almost constant at pH<11 but sharply decreased at pH>11 without HA condition. However, the arsenic adsorption was slightly restrained in the presence of HA and the inhibition ratio was less than 3%. The thermodynamic experimental results indicated that the adsorbing capacity of arsenic was increased with the rise of temperature in the absence of HA but the thermodynamic adsorption behavior of arsenic was inhibited by different concentrations of HA. The Langmuir isotherm model was used to describe the arsenate adsorption very well while the Freundlich model fitted well on arsenite adsorption. Adsorption kinetics pointed out that the Elovich equation described arsenic adsorption with or without HA very well. The study of adsorption edge showed us that HA had relatively smaller effect on the shift of arsenic adsorption edge but arsenic had greater influence on the shift of HA adsorption edge. It is indicated the different competitive capacities of arsenic and HA both on the surface of akaganeite. The co-exsiting ions brought effects at varying degree on arsenic adsorption by akaganeite. Among them, phosphate, silicate and carbonate had bigger influence on arsenate adsorption while sulfate, phosphate and silicate had larger effect on arsenite adsorption. The desorption kinetics indicated that phosphate had a larger effect on the stability of adsorbed arsenate but not arsenite. HA and silicate had no effect on the stability of adsorbed arsenic by akaganeite.The main influence components of HA and the ligand exchange mechanism on arsenic adsorption process were determined and explained by using 3DEEMs, LC-OCD, FTIR and XPS analysis. This study has important significance for in-depth understanding of the fate of arsenic on the surface of iron-containing mineral with the presence of natural organic matter in natural environment. |
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