Study Cn Effect Of Bicarbonate On Reduction Of Nitrobenzebe By Zero-Valent Iron In Groundwater

Bicarbonate is the major anion in groundwater. In the process of pollutants treatedby zero valent iron, along with the increase of pH, which lead to emerge more CO32-,combined with iron to produce carbonate precipitation, The sediments includingaragonite, calcite, oxides, hydroxyl oxides, hydroxides, carbonates, siderite andchukanovite. Threrfore, the impact of bicarbonate on the type of surface sediment ofFe0can not be ignored. These substances attached to the surface of zero-valent ironhas some impact on the reduction of pollutants. The study of weather intermediatecorrosion products of Fe0having reducing capacity is important to maintain long-termstable operation of ZVI-based PRB.In this paper, we set the nitrobenzene as the target pollutant. Batch experimentswere designed to investigate the influence of different concentrations of bicarbonateon nitrobenzene degradation by zero valent iron in subsurface environment and thetype of sediments on surface of Fe0. The concentrations of bicarbonate were set as0,100,300,500and800mg·L-1, and ferrous hydroxide, chukanovite and sideriteselected as the representative of zero-valent iron corrosion intermediate products,researchs on synthetic conditions of three substances were carried out, as well as thereduction capacity of nitrobenzene, the results showed that:（1） bicarbonate can enhance the zero-valent iron reactivity at concentrations(100~500mg/L), and the promoting effect was inhanced with increasing the bicarbonateconcentration, The system of bicarbonate concentration of800mg/L was notconducive to the reduction of nitrobenzene. The precipitates of the system wereanalyzed by X-ray diffraction. The results indicate that: The sediments were mainlycomposed of Fe3O4and FeOOH in the system of500mg/L. The main components ofsediments in reaction system800mg/L were Fe3O4, Fe, Fe2O3and FeOOH. The typesand quantities of sediments significantly more than reaction system of500mg/L, whichshowed that the kind and quantity of surface sediment of zero-valent iron will beaffected by the concentration of bicarbonate, more precipitates would be generatedwhen the concentration of bicarbonate was high, which led to the hindered of electron transfer between zero valent iron and nitrobenzene, the reduction effect ofnitrobenzene become deterioration. However, the iron carbonate precipitations werenot detected, the iron oxide hydroxide and carbonate may be decomposed in theprocess of solid analysis and sample preparation.（3）Fe2(OH)2CO3could be generated when R=1.1, R′=0.7and R=1.1, R′=0.9, andthere was no other kind of iron compounds. According to the equilibrium conditions,the standard Gibbs free energy of formation of Fe2(OH)2CO3was estimated at-1151.1KJ/mol. On the base of the standard Gibbs free energy of formation ofFe2(OH)2CO3, Pourbaix diagrams of Fe-C-H2O were drawn showing that Fe(OH)2was stable when pH＞11, Eh＜-500mV. Fe2(OH)2CO3could be stable when7.86＜pH＜10.22,-730mV＜Eh＜390mV. the stability domain of FeCO3was6＜pH＜11and Eh＞-750mV. The stability domain of Fe2(OH)2CO3is integrally included insidethe stability domain of FeCO3, which indicates that Fe2(OH)2CO3is metastable withrespect to FeCO3, Fe2(OH)2CO3should more readily converted to FeCO3withdecrease of pH.（4）Fe(OH)2, Fe2(OH)2CO3and FeCO3could reduce nitrobenzene with differentdegrees. The removal rates of nitrobenzene was71.49%,57.31%,48.94%when theexperiments were performed to400hours, respectively. The reduction efficiency ofnitrobenzene followed the sequence of Fe(OH)2, Fe2(OH)2CO3, FeCO3from high tolow. The reaction rate constants were0.003mg/(L h),0.002mg/(L h),0.001mg/(L h),respectively. The precipates of reaction system were analyzed by X-ray diffraction,the results showed that the Fe3O4and FeOOH were the main component in the systemof Fe(OH)2. In the system of Fe2(OH)2CO3, FeOOH was the only sediment. Theprecipates were mainly FeOOH and FeCO3in the system of FeCO3.