Study On Formation Condition Of Iron Carbonate Precipitation And Its Effect On Nitrobenzene

Permeable reactive barriers（PRBs） with zero-valent iron as the reactive material have received extensive attentions in recent years as treatment for contaminated groundwater. Many contaminants can be removed by Fe-PRB, such as heavy metals, inorganic ions, chlorinated organic compounds, nitrobenzene, petroleum hydrocarbons and so on. Corrosion products formed on the Fe⁰ surface through chemical reactions（between inorganic ions and Fe⁰） have major effects on the reactivity, porosity, permeability, and consequent longevity of Fe⁰-PRB technology. Carbonate/bicarbonate is the most common inorganic anions, the iron carbonate precipitate can form such as Fe CO₃, Fe₂（OH）₂CO₃, Fe₆（OH）₁₂CO₃ due to the affect of Carbonate/bicarbonate. The precipitate can cover the surface active sites of Fe⁰ and decrease the activity of Fe⁰, or may serve as a conductor for electrons between the iron core and the contaminants, or as a chain initiator causing more precipitation, the precipitate play an important role on the long-term operation of Fe⁰-PRB.Experiments were conducted in FeCl₂, NaOH, and Na₂CO₃ solutions to investigate the effect of different [OH^-]=0.02, 0.06, 0.1mol/Land R=[Fe²⁺]/[OH^-], R’=[CO₃^2-]/[OH^-] on the formation of iron carbonate precipitate under anaerobic conditions, nitrobenzene is the contaminant, study on the effect of preicipitate(Fe CO₃, Fe₂（OH）₂CO₃, Fe₆（OH）₁₂CO₃) treat nitrobenzene, also, the effect of p H and nitrobenzene concentration on the removal of nitrobenzene by precipitate was studied, in addition, the kinetics of precipitate reduce nitrobenzene was studied. The result indicated:（1） The results showed that FeCO₃, Fe₂（OH）₂CO₃, Fe6（OH）₁₂CO₃ were detected under the different experimental conditions, Fe₆（OH）₁₂CO₃ can formation in the low concentration [OH^-]=0.02mol/L and not formation in the high OH^- concentration solutions. The low OH^- concentration and low R, high OH^- concentration and large R have positive effect on the formation of Fe₂（OH）₂CO₃. The alone Fe CO₃ formation need higher R, R’ and lower OH^- concentration, that means the amounts of Fe²⁺, CO₃^2-are much more than OH^-.（2） FeCO₃, Fe₂（OH）₂CO₃, Fe₆（OH）₁₂CO₃ can reduce nitrobenzene, the reduction ability was Fe₆（OH）₁₂CO₃> Fe₂（OH）₂CO₃> Fe CO₃. The removal of nitrobenzene depended on precipitate in Fe₆（OH）₁₂CO₃ and Fe₂（OH）₂CO₃ systems, and dissolved Fe²⁺ in Fe CO₃ system.（3） The more nitrobenzene, the faster reduction rate, more anline formation. The most nitrobenzene（371mg/L） had a lower removal rate than medium nitrobenzene concentration（184mg/L） in Fe CO₃ and Fe₂（OH）₂CO₃ systems as the lack of precipitate; p H value had different effects in three precipitate systems, as the pH value increase from 7 to 9, the removal of nitrobenzene scarcely influenced in Fe₆（OH）₁₂CO₃ system, but the removal rate increase with the p H rise. This due to Fe₆（OH）₁₂CO₃ can exist stable when the p H is 7-9, slight alkaline had an advantage for the Fe₂（OH）₂CO₃, Fe CO₃ had much Fe²⁺ in high p H solutions.（4） The reaction of three precipitation reduction nitrobenzene could be described by first order reaction kinetics. Reaction rate constant was 0.0068, 0.0054, 0.0051 mg/（L·h） in Fe6（OH）₁₂CO₃, Fe₂（OH）₂CO₃, FeCO₃ systems, respectively. The reaction rate constant increase and then decrease with the increase of nitrobenzene when the nitrobenzene concentration ranged from 48 mg/L to 371mg/L. The reaction rate constant nearly constant in Fe₆（OH）₁₂CO₃ system, and increase from 0.0027 mg/（L·h） to 0.0039 mg/（L·h） in Fe₂（OH）₂CO₃ system, and increase from 0.0022 mg/（L·h） to 0.0048 mg/（L·h） in Fe CO₃ system.