Research On Trichloroethylene Removal Efficiency By Modified Microscale Zero Valent Iron

At present, our country and even many parts of the world are suffering from different extent of groundwater pollution by chlorinated hydrocarbons pollutants, which thread the safety of residents’ drinking water. Using zero-valent iron material as the reducing agent in the construction of in-situ reaction zone is a promising approach for groundwater pollution remediation technologies. Microscale zero valent iron (mZVI) became the ideal medium according to the low price, lower health risks and good reactivity. At present, the main bottleneck of this application is gravity sedimentation and activity reduction of mZVI during the process of migration underground. This paper prepared stabilized mZVI by means of surface modification to realize mZVI’s good suspension stability and reactivity, focused on enhanced the trichloroethylene (TCE) removal from contaminated groundwater and had important guiding significance on adjust measures to local conditions in the in-situ remediation of TCE contaminated groundwater.Firstly, xanthan gum (XG), guargum(GG) and carboxy methyl cellulose(CMC) were employed to coat mZVI for surface modification, two different dosages of each biopolymer (3g/L and 6g/L) were adopted respectively, the optimal stabilizer was selected by investigating the suspension stability of the modified mZVI and its TCE removal efficiency. Secondly, in order to determine the impact mechanism of XG modification on the activity of mZVI, two main TCE removal process (adsorption and reduction) were respectively investigated, and the key chemical indicators of the reaction system (pH, Eh, Fe2+) and iron surface passivation layer were analyzed. At last, considering different water chemical factors in the actual groundwater environment may impact the reactivity of mZVI, the effects of geochemical constituents (humic acid (HA), Ca2+-HCO3-,Na+-Cl-) on TCE removal using XG modified mZVI were investigated using batch experiments.(1)Stabilizer selection:the sedimentation experiment indicated that the relative spectrophotometric values at 10 min from small to large was CMC3 (0.24)< CMC6 (0.45)< GG3(0.75)< GG6(0.98)< XG3 (0.91)< XG6 (0.94). (CMC3 refers to the mZVI suspension solution modified by 3g/L XG and similarly there are CMC6, GG3, GG6, XG3 and XG6). The large decrease of relative spectrophotometric values means the significantly gravity settling occurred; conversely means the suspension stability. The TCE removal batch experiments showed that under the same concentration of stabilizer, XG3 increased the TCE removal efficiency by 31.85% over a 480 h experiment, whereas GG3 and CMC3 increased 15.61% and 9.69% respectively. XG worked best on buffering solution pH and inhibiting surface passivation during the reaction. Overall, considering the raising stability and reactivity, XG was considered the most effective modifier for mZVI.(2)The influence mechanism of XG:the result indicated that TCE removal was attributed to both sorption and reduction processes,the TCE sorption capacity of mZVI was lower in the presence of XG, whereas the TCE reduction capacity was significantly increased. The FTIR spectra confirmed that XG, which is rich in hydrophilic functional groups, was adsorbed onto the iron surface through intermolecular hydrogen bonds, which competitively repelled the sorption and mass transfer of TCE towards reactive sites. The variations in the pH, Eh, and Fe2+ concentration as functions of the reaction time were recorded, indicating that XG worked to buffer the solution pH, inhibit surface passivation, and promote TCE reduction by mZVI.(3)The effects of water chemical factors:In the co-presence of geochemical constituents, TCE adsorption was not significantly impacted, while its reduction rate was increased by the presence of HA (5-20 mg L-1), inhibited with increasing Ca(HCO3)2 concentration (0.4-2 mM) and unaffected by NaCl in the range of 10-100 mM.The promotional effect of HA on TCE reduction was primarily due to the Fe-humate complexes in solution that delayed the formation of passivated precipitates. However, the presence of Ca(HCO3)2 exerted an opposite effect on TCE reduction by facilitating the formation of passivated precipitates, such as green iron rust. NaCl (10 -100 mM) showed no significant effects.