The Remediation Of Para-chloronitrobenzene Contaminated Groundwater By Zero-valent Iron

Chloronitrobenzenes are extensively used in producing pesticides, dyes, pharmaceuticalsand other industrial chemicals. Compared with other aromatics, chloronitrobenzenes areconsidered to be more recalcitrant due to the electron-withdrawing character of chloro andnitro groups on the aromatic ring. The para-chloronitrobenzene (p-CNB) is an importantmember of chloronitrobenzenes, which wide application and improper disposal have resultedin the concerned groundwater environment pollution. The reduction of p-CNB by zero-valentiron (ZVI) was investigated in this paper, and the main results are as blows.In the first section, the effects of initial pH, dissolved oxygen, mixing rate, iron diameter,iron dosage and reaction temperature on the p-CNB conversion by ZVI were studied. Theresults showed that p-CNB could be effectively removed when initial pH value was7andbelow. The reduction process of p-CNB was delayed due to the presence of oxygen inaqueous solution. The reduction rate increased with mixing rate and iron dosage. Fine ironpowders may possess higher reaction efficiency for p-CNB removal due to its higher surfacearea. The reaction could be described using pseudo first-order kinetics. The reaction wasendothermic and the activation energy was29.48kJ/mol.In the second section, batch tests were conducted to evaluate the influences of severalcommon dissolved anions in groundwater on the reduction of p-CNB by ZVI. The resultsshowed that p-CNB reduction was enhanced by both Cl-and SO42-and NO3-. HCO3-couldeither improve or inhibit p-CNB reduction, depending on whether the mixing speed wasintense enough to rapidly eliminate Fe-carbonate complex deposited on ZVI surface. Thereduction rate by ClO4-decreased because its competence with p-CNB for electrons. Thep-CNB reduction was inhibited by PO43-, SiO32-and humic acid in the order of humic acid <PO43-<SiO32-since these ions could form inner-sphere complexes on iron surface. Theresults indicated that common dissolved anions in groundwater should be taken into accountwhen ZVI is applied for contaminated groundwater remediation.In the third section, batch tests were conducted to investigate individual and cooperativeeffects of Ca2+, Mg2+and HCO3-in groundwater on the reduction of p-CNB by ZVI. Theresults showed that Ca2+and Mg2+did not improve p-CNB reduction. HCO3-significantlyenhanced p-CNB conversion rate when its concentration was0-100mg/L. But p-CNBremoval efficiency decreased owing to the formation of ferric carbonates when HCO3-concentration exceeded100mg/L. Due to the formation of CaCO3(or MgCO3) blocking theactive sites where electron transfer took place, the coexistence of Ca2+(or Mg2+) and HCO3- had an inhibitory effect on p-CNB removal. It indicated that the effects of hardness andbicarbonate in groundwater should be taken into account when zero-valent iron is applied forthe remediation of chloronitrobenzenes contaminated groundwater.In the fourth section, ZVI was used to pretreat p-CNB and the major product waspara-chloroaniline (p-CAN). By adding H2O2directly, further p-CAN degradation can beattributed to Fenton oxidation because ferrous ions (Fe2+) released during the ZVI corrosioncould be used as an activator for H2O2decomposition. The control experiments showed thatthe sequential treatment was more effective than Fenton oxidation alone in treating p-CNBwastewater since the removal rate of TOC was improved by about34%. It suggested that theamino function group is more susceptible to oxidative radical attack than the nitro functiongroup.In conclusion, as a kind of environment-friendly material, ZVI could removechloronitrobenzenes from groundwater effectively. The effects of groundwater characteristicparameters should be taken into consideration when ZVI is used for the remediation ofchloronitrobenzenes.