Study On Remediation Of Karst Groundwater Contaminated With Carbon Tetrachloride Using Zero-valent Iron

Chlorinated hydrocarbons are the most prevalent groundwater pollutants that pose arisk to public health. Significant attention has been devoted over the past decade toresearch and field-applications of zero-valent iron(ZVI) for groundwater remediation.The uncertainty of ZVI effectiveness under complex subsurface environment will stillaffected the application of ZVI remedial techniques. According to the characteristics ofCCl4contaminated site in Qiligou water source of Xuzhou city, the researchinvestigated the enhanced degradation of CCl4by iron-based bimetals and the ZVIcoupled with electrokinetics using a series of batch and column bench scale tests toprovide technical parameters for remedial design. CCl4degradation process by spongeiron (a new type of zero-valent iron, ZVI), iron-based bimetal particles and factorsaffecting degradation efficiency, including dosage, catalytic metal type, catalytic metalcontent, initial pH and geochemical constituents of groundwater were investigatedthrough batch experiments. The surface structure and composition of ZVI and bimetalbefore and after reaction with CCl4were determined using SEM-EDS, X-ray diffraction,X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy to revealthe mechanism of reductive dechlorination of CCl4. The electro-kinetic enhanceddegradation of CCl4in the columns packed with ZVI and the factors affecting CCl4degradation efficiency including electrode position and voltage gradient were evaluated.The mechanism of the enhanced CCl4degradation by ZVI coupled with electrokineticswas also discussed by analyzing the concentration variation of target pollutant,intermediate products and the changes in microscopic surface structural of the ZVI. Themain conclusions are as followed:(1) CCl4was effectively degraded by sponge iron and about75percent of CCl4was transformed into chloroform (CF) by hydrogenolysis process. The rate of CFtransformation was slower than that of CCl4, resulting in the CF accumulation. Surfaceacid activation showed low influence on CCl4degradation with ZVI. The CCl4degradation reactions followed pseudo-first-order kinetics, and the apparent first-orderrate constant (kobs) increased linearly with increasing ZVI dosage and the suitabledosage of20g/L was indicated in terms of surface area-normalized rate constants (kSA).The kobsdecreased with the increasing of pH value and the process indicated that thedegradation of CCl4had a better performance under weak acidic condition.(2) Geochemical constituents of groundwater had a significant influence on CCl4degradation with ZVI. The most common anions, including Cl-, SO42-and HCO3-, can promote the degradation of CCl4with the order of HCO3->SO42->Cl-. HCO3-enhancedthe effect of ZVI on CCl4degradation as a buffer and an oxidant providing cathodicreaction; SO42-dissolved the hydroxide on the ZVI surface to promote the degradation;and Cl-accelerated the degradation rate by pitting corrosion on the ZVI surface. Afterreaction, the iron oxides on ZVI surface were FeOOH and Fe2O3in Cl-, SO42-systemand the FeOOH was the only iron oxide in HCO3-system. The existence of Fe2+can alsopromote the degradation of ZVI for CCl4, and the higher of the concentration was, thefaster rate of CCl4degradation shows. Fe2+can be adsorbed on the ZVI surface o formFe (II) complexes, which caused the dissolution of the passive film, to accelerate thedegradation of CCl4. The existence of humic acid could reduce the degradation of CCl4and its inhibiting effect will be increasing with the dosage of humic acid concentration.The FT-IR results showed that surface adsorption of humic acid on ZVI will promotethe passive film formation of FeOOH and Fe2O3.(3) Catalytic metals, including Ni, Cu and Ag, were loaded on the iron substratenon-uniformly to change the surface property and reactivity of ZVI and significantlyenhanced the degradation of CCl4.①The CCl4degradation pathway in the bimetallicsystem includes direct reduction on ZVI surface and catalytic hydrogenation reductionon catalytic metal surface which is the primary way. Fe-Ag had the highest CCl4degradation efficiency among the bimetals tested, followed by Cu-Fe and Ni-Fe. Thedegradation rate order on Ni-Fe, Cu-Fe and Ag-Fe for reducing CCl4agrees with theorder of electro-potential produced between the iron substrate and noble metal deposited.②The variation of CCl4degradation rate is rather sensitive with the loading ratio ofcatalytic metal. The suitable loading ratios of Cu and Ag in Cu-Fe and Ag-Fe bimetalare0.8wt%and0.4wt%, respectively. Cu-Fe and Ag-Fe on CCl4degradation weredivided into slow degradation and accelerated degradation stages, which both followedwith the pseudo-first-order kinetics. The CCl4degradation rates with Cu-Fe (Cu0.8wt%)and Ag-Fe (Ag0.4wt%) separately were41.6times and72.4times as with ZVI inaccelerated degradation stage.③Ag-Fe (Ag0.4wt%) had a better performance onCCl4degradation in faintly acid conditions(pH~6), lower pH (<5) or alkaline conditionswere not conducive to the reductive dechlorination of CCl4. The loss of catalytic metalfrom iron surface caused by strong iron corrosion, to a certain extent, inhibited thedegradation of chlorinated organics in lower pH condition. The existence of humic acidalso inhibited the Ag-Fe degradation on CCl4, and this kind of inhibiting effectincreased with the concentration of humic acid increasing. (4) CCl4was effectively degraded in column packed with sponge iron and thedegradation follows pseudo-first-order kinetics. The rate of degradation was slowlydecreased with time, and the apparent first-order rate constants decreased from0.1178min-1(1d) to0.0669min-1(90d). But CF could not be effectively transformed inZVI-packed column. The SEM/EDS and XRD results showed that secondary mineralsincluding aragonite(CaCO3), Fe3O4/γ-Fe2O3and β-FeOOH deposited on ZVI surfaceafter90d of operation and consequently inhibited ZVI reactivity; The ZVI surfacemicrostructure analysis also confirmed that deposition of calcium carbonates could befound near the entrance face of a reaction column and iron oxides was depositedthroughout the entire column, which was consistent with the results of field operation.(5) ZVI coupled with electrokinetics can promote the degradation of CCl4and CF.The main factors were the electrode position and the voltage gradient. The enhancementof electro-kinetic on CCl4and CF degradation by ZVI was achieved by increasing theoxidation-corrosion of ZVI instead of direct or indirect reductive dechlorination ofchlorinated organics on the surface of cathode when using graphite electrodes. Theimprovement of CF removal efficiency mainly occurred near and behind the anode andthe configuration of anode installed up gradient was more conducive to CF removal.The application of electrokinetics could slow down or accelerate the passivation of ZVI.The ZVI behind anode could be depassivated and ZVI behind cathode could bepassivated with the setting of anode installed up gradient and cathode installed downgradient. Therefore, the cathode should be placed at the end of PRB in field operation tominimize the negative effect of the cathode.