The Removal Of Hexavalent Chromium In Groundwater By The Combinantion Of Zero Valent Iron And Microorganisms

Chromium and its compounds are important chemical material.However,because of improper storage,use and disposal,chromium pollution has become one of the serious environment problems,which poses severe risks to public health and the environment.Zero-valent iron Permeable Reactive Barrier(Fe0-PRB)has been widely employed in the remediation of Cr(Ⅵ)-contaminated groundwater due to its strong reductive capacity,elevating reaction rate and inexpensiveness.However,the passivation of Fe0 during the remediation,which could cause significant decreases of reactivity,permeability and life-span of a Fe0-PRB system,or even failure of the remedy scheme,has become the main problem for long-term performance of Fe0-PRB systems.Considering the above problems,batch experiments were conducted to investigate Cr(Ⅵ)removal by both abiotic ZVI and biotic ZVI systems.Due to the interactive effect between Fe0 and microorganisms,more effective iron corrosion and more reactive minerals were found in the bio-iron system than in the abiotic one,leading to more effective Cr(Ⅵ)removal.The recycle experiments showed that the Fe0-H2 O system lost its efficacy in 30 days,while the bio-iron system maintained high activity even after 62 days.Experiments also showed that the Cr(Ⅵ)removal capacity of Fe0 in the bio-iron system was about 6.16 mg Cr(Ⅵ)/(g Fe0),compared to a value of 0.46 mg Cr(Ⅵ)/(g Fe0)in the Fe0-H2 O system after 4 cycles.Apparently,longer stability and life-span of the bio-iron system could be maintained in term of microbial activity.Higher iron dosages and low pH promoted the Cr(Ⅵ)removal.Organic pollutants,nitrate and sulfate are common in the chromium pollution environment,which will have impact on the Cr(Ⅵ)removal.Thus,batch experiments were conducted to investigate the effect of organic substrate,oxygen,nitrate and sulfate on the removal of Cr(Ⅵ)by biotic ZVI system.The results shown that Additional organic substrates(e.g.,sodium acetate,ethanol,glucose and humid acid)can improve the removal of Cr(Ⅵ)in this combined system,and the Cr(Ⅵ)removal efficiency correspondingly increased 33%,19%,16% and 11% after 3 d reaction time,respectively.The air had little effect on the Cr(Ⅵ)removal in the bio-iron system.However,nitrate exerted serious inhibition on the Cr(Ⅵ)removal,and the inhibitory effect was more serious with increasing nitrate concentration.Due to the interactive effect between microorganisms and Fe0,iron sulfide was generated on the iron surface under sulfate-reducing condition,which could significantly promote the Cr(Ⅵ)removal because of its strong adsorptive and reductive capacity.The Cr(Ⅵ)removal efficiency was thereafter increased 39% after 2 d reaction time when the initial sulfate concentration was 10 mmol/L.The research result have important reference value to the remediation of Cr(Ⅵ)in groundwater by Fe0-PRB.The introduction of microorganisms can enhance the Cr(Ⅵ)removal and the life span of Fe0-PRB.In addition,the effects of iron dosages,pH values,organic substrates and electron acceptors in waters should be taken into account when the bio-iron system is applied for the remediation of Cr(Ⅵ)-contaminated waters.