| The environmental risk of arsenic and fluoride in groundwater has caused worldwide concern over the recent years. New researches have found that concentrations of arsenic and fluoride above WHO drinking water standard have been detected in aquifers of various areas, such as China, Mexico, Argentina and Pakistan. Moreover, there is a good correlation between arsenic and fluoride- contents (R2 = 0.84). Thereby there is an urgent need to develop technologies for the simultaneously removal of the coexisting arsenic and fluoride from groundwater. Because of the ease of handling, sludge-free operation, and possibility of regeneration, the adsorption process appears to be one of the most promising methods for arsenic or fluoride removal from groundwater, especially for small communities. Many new adsorbents have developed for either arsenic removal or fluoride removal. However, very few studies have been carried to synthesize novel adsorbents for simultaneous removal of arsenic and fluoride. Considering the high affinity of alumina for fluoride and that of iron oxide for arsenic, Fe-Al binary metal oxides were synthesized for simultaneous removal of arsenic and fluoride.The Fe-Al bimetal oxides with three different Fe/Al molar ratios were prepared and characterized. The performance of the three adsorbents for fluoride removal, arsenate removal and simultaneous removal of arsenate and fluoride were investigated, and compared to the performance of the synthesized Fe metal oxides under the same conditions in a wide range of. pH. XRD patterns obtained from the synthesized Fe-Al oxides have several peaks that indicate the slightly crystalline structure. With increasing of aluminum content, specific surface area of Fe-Al bimetal oxides was fall-after-rise, and the positively charged in adsorbents surfaces increased steadily.The adsorption experimental results showed that four adsorbents were greatly influenced by pH. The Fe-Al bimetal oxides were more effective in arsenic and fluoride removal compared with Fe metal oxides in a wide pH range. 4Al1Fe was the most effective adsorbent for arsenic and fluoride removal at pH values near neutrality. The Langmuir monolayer adsorption capacity for arsenate and fluoride is 71.43 mg/g and 45.87 mg/g, respectively. The second effective adsorbents were 1Al1Fe and 1Al4Fe. The poorest effective adsorbent was pure Fe metal oxides, and the adsorption capacity for arsenate and fluoride was only 27.32 mg/g and 4.96 mg/g, respectively. The reason for the different adsorption efficiency was that the positively charged and the hydroxyl groups in adsorbents surface was different, which played a major role in the absorption of arsenate and fluoride.The competitive adsorption was obtained in simultaneously removal of arsenate and fluoride. The existence of fluoride had a great influence on the adsorption of arsenate, while arsenate had little impact on the adsorption of fluoride. The results of XPS showed that the content of Al on the surface of Fe-Al bimetal oxides decreased drastically after binding adsorption of As (V) and fluoride, indicating that Al atoms were overlaid by the adsorbed arsenate and fluoride.The above results also illustrated that arsenate and fluoride combined the same binding sites of adsorbent, which caused in the competitive adsorption of arsenate and fluoride. The affinity between fluoride and the binding sites seemed stronger than arsenate, resulting in stronger competition for fluoride absorption.Summarily, the adsorption efficiency of Fe-Al bimetal oxides for simultaneously removal of arsenate and fluoride was much higher than pure Fe metal oxides. Because of the wide existence in nature, low cost and easy access, the iron and aluminum hydroxide was an excellent choice for preparing co-adsorbent to treating the environmental risk of the co-existence of arsenate and fluoride. |
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