[Show abstract][Hide abstract] ABSTRACT: Mobilization of arsenic under anaerobic conditions is of great concern in arsenic contaminated soils and sediments. Bacterial reduction of As(V) and Fe(II) influences the cycling and partitioning of arsenic between solid and aqueous phase. We investigated the impact of bacterially mediated reductions of Fe(III)/Al hydroxides-bound arsenic(V) and iron(III) oxides on arsenic release. Our results suggested that As(V) reduction occurred prior to Fe(III) reduction, and Fe(III) reduction did not enhance the release of arsenic. Instead, Fe(III) hydroxides retained their dissolved concentrations during the experimental process, even though the new iron mineral-magnetite formed. In contrast, the release of reduced As(III) was promoted greatly when aluminum hydroxides was incorporated. Thus, the substitution of aluminum hydroxides may be responsible for the release of arsenic in the contaminated soils and sediments, since aluminum substitution of Fe(III) hydroxides universally occurs under natural conditions.
[Show abstract][Hide abstract] ABSTRACT: Lime neutralization and coprecipitation of arsenate with iron is widely practiced for the removal and immobilization of arsenic from mineral processing effluents. However, the stability of the generated iron-arsenate coprecipitate is still of concern. In this work, we developed a two-step coprecipitation process involving the use of iron and aluminum and tested the stability of the resultant coprecipitates. The two-step Fe-As-Fe or Fe-As-Al coprecipitation process involved an initial Fe/As = 2 coprecipitation at pH4 to remove arsenic from water down to 0.25 mg/L, followed by introduction of iron or aluminum (Fe/As = 2, Al/As = 1.5 or 2). The two-step coprecipitates showed higher stability than traditional Fe/As = 4 coprecipitate under both oxic and anoxic conditions. Leaching stability was enhanced when aluminum was applied in the second step. The use of aluminum in the second step also inhibited microbial mediated arsenate reduction and arsenic remobilization. The results suggest that the two-step coprecipitation process is superior to conventional coprecipitation methods with respect to the stability of the generated arsenic-bearing solid waste. The use of Al in the second step is better than Fe to enhance the stability. This work may have important implications to the development of new technologies for efficient arsenic removal from hydrometallurgical solutions and safe disposal in both oxic and anoxic environment.
Water Research 11/2011; 46(2):500-8. DOI:10.1016/j.watres.2011.11.045 · 5.32 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Sediment bound arsenic usually undergoes phase transformation processes when it is transported and buried in deeper settings. This work investigated anaerobic microbial mediated speciation change of the arsenic in offshore sediment and monitored the transformation process of oxyhydroxide associated arsenate to sulfide associated forms. The fate of arsenic and possible pathways of transformation were discussed based on quantitative analysis of aqueous and solid arsenic and iron, and qualitative characterization using X-ray absorption near edge spectroscopy (XANES). Arsenic was released and reduced upon development of anoxic conditions but was resequestered by authigenic minerals later. Most of the arsenic in the sediment was converted to orpiment-like material. Sulfide may have played double roles in arsenic redistribution process, i.e. promoting arsenic release from host oxyhydroxides in early stage and removal of arsenite from solution in the form of arsenic sulfide in later stage. The findings have implications about the pathways of arsenic transformation when arsenate is transported and buried below redox boundaries in offshore sediment.
Water Research 10/2011; 45(20):6781-8. DOI:10.1016/j.watres.2011.10.041 · 5.32 Impact Factor