Removal Characteristics And Mechanism Of Aqueous Micropollutants By Metallic Iron

Micropollutants,with high oxidation state and high toxicityin drinking water andother water sources,are hardly removed by the conventional and advanced treatment processes and are potentially reduced by zero-valent iron. In order to provide theoretical concepts and practical solutions for a better control of these micropollutants,theirremoval characteristics and mechanism by metallic iron were investigated. Carbon tetrachloride(CCl4), haloacetic acids(HAA9) and bromate(Br O3-)were selected as model compounds in this study.Under an empty bed contact time(EBCT) of 2.5-10 min and a reaction temperature of 5-35 oC in packed column experiments, the removal efficiency of monochloroacetic acid(MCAA) by metallic iron was 16.4-81.3%, and that of CCl4, HAA8 and Br O3-wereabout100%. The halogen mass imbalance of CCl4, MCAA and dichloroacetic acid(DCAA)removal might be attributed to theinfluence of dissolved oxygen(DO) on the removal mechanisms.Under a temperature at 25 oC and a reaction time of 2 h in shake flask experiments, the removal of Br O3-by metallic irongradually decreased and that of CCl4 and chloroacetic acids increased first and then decreased withincreasing DO concentration from 0 to 12 mg/L. Theoptimum DO concentrationsfor CCl4, MCAA, DCAA andtrichloroactic acid(TCAA)removal were 1.27, 1.52, 1.52 and 0.75 mg/L, respectively. The removal efficiencies of CCl4, MCAA, DCAA, TCAA and Br O3-were 12.1-45.8%, 1.25-27.5%, 22.4-68.1%, 26.0-80.5% and 52.5-100%, respectively,and their pseudo-first-order reaction constants were0.08-0.29, 0.0002-0.16, 0.12-0.53, 0.18-0.73 and 0.41-3.91 h-1, respectively.The pollutant removal by metallic iron consisted ofmigration, diffusion, adsorption and reduction processes. From outside to inside, the corrosion products on the surface of metallic iron were in an order of lepidocrocite, green rust, magnetite and ferrous oxide. The reduction of pollutants occurred in the conductive layer after penetrating through the outside nonconductive layerof lepidocrocite.The rate-limitingstep of CCl4 and MCAAremoval was the reduction process.DCAA and TCAA belonged tothe reductionand diffusion limitedspecies at the DO value below and above2.59 mg/L. The removal of Br O3- was controlled by the mass transfer process. If the oxidation capacity of pollutant was greater than that of oxygen, the removal of pollutantwill be inhibited inthe presence of DO.Otherwise, itsremoval will be enhanced in a lower DO range of 0-2 mg/L.Based on the pollutant removal mechanism by metallic iron,the removal technology for various micropollutants was optimized.The removal rate of Br O3- in metallic iron packed column at an EBCT of 10 min was close to 100%. The removal rate of CCl4 by the combined air stripping and metallic iron process was 98%, while the ratio between gas and water was 2, and the aeration time and EBCT were 10 and 2.5 min, respectively. The removal rate of HAA9 by sequential metallic iron and biologically active carbon processwas 99%, when operated at the EBCTof2.5 and 10 minin sequence.