Synthesis Of Stabilized NZVI Composites And Their Reduction Efficacy Toward Metronidazole And Organic Dyes In Waters

The past20years have seen the wide application of NZVI material in rapidly transforming a wide range of recalcitrant contaminants such as polychlorinated biphenyls (PCBs), trichloroethylene (TCE) and arsenic (As) in various water bodies. However, a potential drawback of NZVI material for environmental remediation is its magentism and high surface free energy, which will result in particle agglomeration, thereby reducing its reactivity toward pollutants. To tackle this engineering difficulty, surface modified or supported methods have usually been employed to synthesize less-aggregated NZVI composites with high reactivity and long-term durability. Among all supporters, porous poly(vinylidene fluoride)(PVDF) microfiltration membranes are often selected as stabilizing candidates to immobilize NZVI particles due to its high mechanical strength and thermal stability. Yet, to the best of our knowledge, no report is available on evaluating the reduction efficacy of NZVI immobilized within PVDF membranes toward metronidazole (MNZ) in aqueous environment, although some fragmentary studies had reported the use of NZVI particles for the removal of MNZ, amoxicillin and ampicillin from waters. On the other hand, the excellent discoloration performance of NZVI (whether in its bare or supported forms) against a wide range of organic dyes has been demonstrated in previous studies. But thus far, study on probing the effects of polymer coatings on discoloration efficacy of NZVI toward organic dyes is still limited, though enhanced stability and mobility in aquifers for NZVI modified by various surfactants, polymers and other organic substances has also been reported.MNZ, as a nitroimidazole antibiotic, has been considered as an emerging contaminant in ecosystems. Recently, its potential risk to ecosystems and human beings has become an increasing concern due to its low degradability, potential mutagenicity, and carcinogenicity. On the other side, the water pollution caused by various organic dyes has also sparked a great deal of attention due to their potential toxicity, carcinogenicity and mutagenicity to ecosystems. Therefore, it is of great importance to eliminate or degrade MNZ and organic dyes from aquatic environment. Given the flaws of previous studies, a serious of experiments had been carried out in this dissertation based on two aspects, i.e., probing the reduction efficacy of PVDF supported NZVI toward MNZ and elucidating the effects of polymer coatings (i.e., poly(methyl methacrylate)(PMMA)/anisole hybrids) on performance of NZVI against the discoloration of organic dyes.Firstly, poly(acrylic acid)(PAA)/PVDF-NZVI (PPN) hybrids were fabricated via the processes of in situ functionalization of PVDF, ion exchange and in situ reduction ferrous or ferric ions by KBH4solution in order. Subsequently, the resultant hybrids were characterized by attenuated total reflectance-infrared (ATR-IR) spectra, field emission scanning electronic electron microscope (FE-SEM), X-ray energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Afterwards, batch experiments regarding the removal of MNZ from stimulation wastewater over the well-characterized PPN hybrids were conducted to probe their reactivity and efficacy under differing operating conditions and varying water environment chemistry. Finally, the main contents and results of this part have been outlined as follows:(1) Analysis on characterization indicated that in comparison with bare NZVI, NZVI supported in PVDF membranes exhibited less aggregation and well dispersion. The removal of MNZ from waters by PPN indicated that PPN showed unparalleled advantages over bare NZVI in respect to the reactivity, stability and durability over the reaction course. Further investigations suggested that lowering initial concentration of MNZ and elevating reaction temperature enhanced the removal of MNZ by PPN, and that the acidic and neutral conditions generally demonstrated more favorable effect on MNZ removal than the alkalinity ones. In addition, the presence of Cl-ions slightly improved the reactivity of PPN with MNZ, while the SO42-ions inhibited its reactivity. Kinetics of the MNZ removal by PPN was found to follow a two-parameter pseudo-first-order decay model well, and the activation energy for MNZ degradation by PPN was calculated30.49kJ/mol. Analysis on intermediates and final products implied that hydrogenation reduction was responsible for MNZ degradation by PPN, which involved three chemical reactions like nitro reduction, N-denitration and hydroxyethyl cleavage, and that the possible products were1-(2-hydroxyethyl)-2-methyl-5-aminoimidazole,2-methyl-5-nitroimidazole, and1-(2-hydroxyethyl)-2-methyl-5-imidazole.(2) To further elucidate the influences of hardness (Ca2+) and alkalinity (HCO3-) on performance of PPN hybrids against the reductive removal of MNZ, batch experiments under varying levels of hardness and alkalinity coupling with the presence of other common inorganic ions and dissolved organic matters had also been carried out in this proposal. Results suggested that single presence of Ca2+or HCO3-ions in solutions (whether in their low or high levels) exerted different inhibition to the reduction efficacy of PPN toward MNZ (using the removal of MNZ as a reference), and that the individual effects of Ca2+or HCO3-differed from their collective ones. Further additions of other common inorganic ions into target solutions containing high concentration of Ca2+or HCO3-ions decreased MNZ removal in the order:NH4+>SO42->Mg2+>PO43->NO3-, with removal efficiencies ranging from88.91to75.44%, corresponding to reaction constant rates ranging from0.0952to0.0527min-1. Humic acid (HA), whether in its individual or collective forms with high levels of Ca2+and/or HCO3-ions, introduced relatively larger inhibition to the reactivity of PPN with MNZ. However, the lowest MNZ removal obtained in the case of high Ca2+and HCO3-levels along with20mg/L HA was still above73.37%.On the other hand, PMMA/anisole co-modified NZVI (PNZVI) was also prepared (by pre-synthesis method) to reveal the effects of PMMA/anisole coatings on discoloration ability of NZVI. Results indicated that the hybrid coatings on NZVI surface enhanced its oxidation resistance and reduced its agglomeration. Nevertheless, such coatings introduced an inhibition to discoloration reactivity of NZVI toward the selected organic dyes, i.e., acid fuchsine, sunset yellow, and methyl orange. Further investigations using acid fuchsine as a model suggested that increasing its initial concentration enhanced the inhibition of hybrid coatings to discoloration efficacy of NZVI toward acid fuchsine, while increasing particle dosage, reaction temperature, and solution pH decreased their inhibitory effects. In addition, the presence of chloride ions in solutions enhanced discoloration reactivity of PNZVI toward acid fuchsine whereas inhibited the reactivity for bare NZVI. Studies on discoloration kinetics showed that the discoloration process of organic dyes by PNZVI and bare NZVI could both be well-described by empirical rate equation in spite of the presence of the inhibitory effects of PMMA/anisole coatings. For the PNZVI-dye reaction system, the inhibited mass transfer, blocked reactive sites, restrained electron transfer and reactive species might be responsible for such inhibition caused by hybrid coatings.In general, the reduction efficacy of PPN hybrids toward MNZ in aquatic environment and the effects of PMMA/anisole coatings on discoloration efficacy of NZVI toward organic dyes have been reported for the first time in this dissertation. Results herein implied that PPN composite possessing high reactivity, stability and long-term durability might be a promising material to pretreat antibiotic wastewaters, and that various effects of polymer modifiers used for NZVI modification should be carefully evaluated when fabricating surface stabilized NZVI composite for environmental remediation.