Study Of Arsenic Adsorption On Fe (Hydr)Oxides And Fe-Mn Binary Oxide And Its Application

With the rapid development of biogas project in china, biogas slurry also increases significantly. Since methane fermentation feedstock such as livestock and poultry contain a lot of heavy metals, which resulting from heavy metal pollution is getting worse and seriously threatening food safety and human health. Arsenic is one of the main heavy metal pollutants in biogas slurry and a highly toxic carcinogen, therefore the treatment of arsenic in biogas slurry become imperative. Iron and manganese oxides, distributing widely in sediment and soil, have a huge surface area and high redox activity and make them play an important role in controlling arsenic contamination in water. This paper regarded iron (hydr)oxides and Fe-Mn binary oxide (FMBO) as research objects. The adsorption mechanism of arsenate (As(Ⅴ)) by ferrihydrite, hematite and hematite at different pH conditions were studied. And the adsorption and oxidation efficiency of arsenate and arsenite (As(Ⅲ)) by FMBO and the influences of co-existing matter which were generally present in biogas slurry were also examined. The results were as follows:(1) As(Ⅴ) was adsorbed on goethite and hematite surfaces only by forming bidentate corner-sharing complex (2C) on the surface at acidic or alkaline solutions (e.g. pH=3.0,9.0). The removal of arsenate by ferrihydrite was via a surface complexation and a surface precipitation reaction at acidic solution (e.g. pH=3.0), with the surface precipitation dominating at longer time intervals, whereas ferrihydrite adsorbed As(V) only via surface complexation.(2) Whether formation of surface precipitate mainly depended on the presence of dissolved Fe3+and the release of Fe3+were determined by the pH of bulk phase and nature of adsorbent. Furthermore, adsorption time and initial concentration of As(Ⅴ) also affect the formation of surface precipitate.(3) Fe-Mn binary oxide had strong adsorption ability to both As(Ⅲ) and As(V) and the maximum adsorption capacity was111.10、71.40mg·g-1respectively. As(Ⅲ) and As(V) adsorbed on FMBO surfaces through forming inner-sphere surface complexes by ligand exchange with hydroxyl groups, and As(Ⅲ) removal by FMBO was through a oxidation and adsorption combined process. During the adsorption process, released Mn2+and increased new adsorption sites resulting from destruction of surface structure were the main reasons why adsorption capacity of As(Ⅲ) was higher than As(V).(4) Zinc ion could promote As(Ⅲ) and As(Ⅴ) adsorption on FMBO and the adsorption capacity increased with the increasing of zinc ion concentration. Phosphate had significant effect on As(Ⅲ) and As(Ⅴ) removal, and the effect of the addition order of phosphate on arsenic adsorption capacity was also different. However, organics such as humic acid, animal protein and carbamide had no significant effect on As(Ⅲ) and As(Ⅴ) removal.