Cr(VI)Removal In Aqueous Solution By Iron Ore And Bacteria Synergistic Reaction

Iron particles and nanoparticles could be used to remediate Cr(Ⅵ)-polluted soils and underground waters with PRB (Permeable Reactive Barrier) and would be oxidized to Fe2O3 gradually while many kinds of minerals which containing iron oxides exist in soils.This article researched on Cr(Ⅵ) removal by pyrite, limonite, magnetite, hematite, pyrrhotite and bacteria and synergistic removal of Cr(Ⅵ) by iron ores and bacteria was studied. Besides, Carboxymethyl Cellulose (CMC), a nontoxic and biodegradable stabilizer, was used to stabilize the nanoscale zero-valent iron and maintain Cr(Ⅵ) removal rate in Cr(Ⅵ) reduction. It could also be used as a carbon source in synergistic removal of Cr(Ⅵ) by iron ores and bacteria.The results are as follows:1. Cr(Ⅵ) could be removed from water by pyrite and the removal efficiency was affected by pyrite size and dose, initial pH and temperature. Cl- and SO42- did not affect the reaction obviously but H2PO4- could inhibit the reaction. At room temperature,10 mg l-1 Cr(Ⅵ) could be removed completely after 30 min by 20 g l-1 pyrite (100～200 mesh) with pH of 3.0, while Cr(Ⅵ) removal rate could reach 91.82% after 120 min at pH 5.5 with other conditions the same as before, and only 52.73% of Cr(Ⅵ) could be removed at pH 9.0. At pH 5.5 and room temperature, reaction capability of pyrite (100～200 mesh) to reduce Cr(Ⅵ) was 1.53 mg Cr(Ⅵ) g-1. FeS2 from the surface of pyrite reacted with Cr(Ⅵ) in the solution and Cr(Ⅵ) was reduced to form Cr3S4, composed of Cr2S3 and CrS, while some CrO2 was produced. They appeared on the surface of pyrite and covered it, which made pyrite lose the ability to reduce Cr(Ⅵ) gradually. Meanwhile, Fe(Ⅱ) in FeS2 was oxidized to Fe(Ⅲ) and could not reduce Cr(Ⅵ) any more.2. Cr(Ⅵ) could also be removed from water by limonite and the removal efficiency was affected by limonite size and dose, initial pH. Temperature did not play an important role in reaction, just like the addition of Cl- and NO3-. But MgO could inhibit the reaction at the beginning of the reaction. FeS2 from the surface of limonite reacted with Cr(Ⅵ) in the solution and Cr(Ⅵ) was reduced to form Cr3S4, composed of Cr2S3 and CrS, while some Cr5Si3 was produced. They appeared on the surface of limonite and covered it, which made limonite lose the ability to reduce Cr(Ⅵ) gradually. At pH 5.5 and room temperature, reaction capability of limonite (100～200 mesh) to reduce Cr(Ⅵ) was 10.03 mg Cr(Ⅵ) g-1.3. In neutral condition, magnetite could hardly remove Cr(Ⅵ) while hematite had the ability. In acid condition, the Cr(Ⅵ) removal abilities of magnetite and hematite were all improved, especially magnetite. In comparison, pyrrhotite had good abilities to remove Cr(Ⅵ) in aquous solution in neutral and acid conditions and FeCr2O4 produced in the reaction would exist on the surface of pyrrhotite. In basic condition, all three iron ores had bad results in removal.4. Indigenous bacteria from Sewage Treatment Plant could be acclimated and screened, and then used in hexavalent chromium reduction in aqueous solution. The microbial reduction of Cr(Ⅵ) was an enzyme catalysis reaction. NaAc was the suitable carbon resource. pH played an important roles in reactions and best results appeared in neutral condition. After short starvation the bacteria still had the ability to removal Cr(Ⅵ).5. A synergistic mechanism was found in Cr(Ⅵ) removal by iron ore and bacteria and better results could be obtained by them together than by them seperately. After 78 h,30 mg l-1 Cr(Ⅵ) could only be reduced to 29.6 mg l-1 by 0.2 g limonite and could be reduced to 5.07 mg l-1 by 2.0 g bacteria. At the same condition,30 mg l-1 Cr(Ⅵ) could be reduced to 3.68 mg l-1 by limonite and bacteria. The synergistic reaction had three reaction pathways. First, Cr(Ⅵ) was adsorbed and reduced by iron ores. Second, bacteria reduced Cr(Ⅵ). Third, Fe(Ⅲ) which exists on the surface of iron ores and in the solution was reduced to Fe(Ⅱ) by bacteria and then Fe(Ⅱ) reacted with Cr(Ⅵ).6. Iron particles and nanoparticles could be used to remove Cr(Ⅵ) and would be oxidized to Fe2O3 gradually. Carboxymethyl Cellulose could disperse iron nanoparticles and iron oxides which were producted from iron nanoparticles to maintain the reaction activity. It could also be used as a carbon source in synergistic removal of Cr(Ⅵ) by iron ores and bacteria and the results could be regarded as the basic researches results in Cr(Ⅵ) removal synergistic reaction of iron nanoparticles and bacteria.