| Arsenic contamination in groundwater is one of the most serious environmental problems in the world, and groundwater arsenic removal has become a hot topic. Due to the strong affinity between arsenic and iron minerals, iron oxides/hydroxides were selected as the arsenic removal materials in this study. Several column experiments were conducted to optimize the Fe-coating time and influx concentrations of FeSO4and NaClO. And then, in-situ groundwater arsenic removal experiments were performed under different conditions to estimate arsenic removal capacity of Fe-coating. The XRD, SEM, FT-IR and electron probe analysis was conducted to characterize the feature of Fe-coating and to study the interaction mechanism between As and Fe-coating. The singnifanct finds and results are as follows:1. Fe-coating experiment was achieved through a cyclical four-step procedure alternatively injecting5mmol/L FeSO4solution,2.5mmol/L NaClO solution, and deoxygenated deionized water. After96h, Fe-coating homogeneously formatted without clogging in the sand column. The breakthrough time of Na2HAsO4solution was33h longer than that of fluorescein sodium in the Fe-coated sand column. This indicated that the Fe-coating can effectively remove aqueous As with dynamic adsorption capacity of0.11molAs/molFe when vL=0.45cm/min. And the capacity will significantly increase with decraseing of VL. Since groundwater flowing velocity is generally less than vL=0.45cm/min, thus the Fe-coating absorption effective will remarkably increase in in-situ conditions. The feature of Fe-coating was characterized by SEM, electron probe, EDS and FT-IR before and after arsenic removal experiments. The results showed that original Fe-coating presented as scalelike crystalline iron oxides/hydroxides (such as goethite etc.), and the aqueous As was removed by Fe-coating through formation of Fe-As monodentate mononuclear complexes.2. The in-situ arsenic removal experiments were performed using the same cyclical procedure by changing FeSO4solution into mix solution of FeSO4and As. The results showed that all co-influx arsenic (≤233μg/L) can be removed by the newly formed Fe material during Fe-coating, and the total absorbed arsenic is much lower than the predicted adsorption capacity of the material, implying the potential efficient in in-situ field. The features of Fe-coating preformed by SEM, EDS and FT-IR suggested that the simultaneous influx of arsenic and Fe solution could influence the crystallization of coated Fe oxides and form some poorly crystallized Fe minerals, such as ferrihydrite or amorphous iron oxyhydroxide. Notably, due to the larger surface area, this poorly crystallized Fe oxides/hydroxide is more effective to remove arsenic from groundwater though the formation of Fe-As bidentate binuclear complexes.3. In order to evaluate the stability and efficiency of the material, reductant HS-was added to the As-saturated and Fe-coated sand column. The reductive dissolution of Fe oxides/hydroxides was observed. A abrupt arsenic increasing occurred at the beginning of reductant influx, then effluent arsenic concentration kept stable at a low level. The dissolved iron could react with HS-and form pyrite (FeS2), which would absorb arsenic in the column. It is possible that before the formation of pyrite, arsenic from reductive dissolution was released into water from materials, then when the occurrence of pyrite, released arsenic was re-aborbed into pyrite, removing arsenic from effluent water. However, in summary, in-situ reduction condition should have no evident impacts on the arsenic removal.Coated Fe oxides/hydroxides on sand can be conducted through the cyclical four-step procedure without clogging, which can effectively remove aqueous arsenic. Even under strong reduction condition, the newly formed pyrite can also re-absorb the aqueous arsenic and keep effluent arsenic concentration at a low level. Therefore, this technology should be worked in in-situ groundwater arsenic removal. |
PDF Full Text Request