| Abstrakt: |
Metallic iron (Fe0) or zero-valent iron (ZVI) has been extensively used for water remediation during the past three decades. It has been proven to be effective in treating waters polluted with chlorinated aliphatics, dyes, heavy metals, pathogens, radionuclides and more. Fe0 is an abundant, low-cost, and non-toxic reactive metal. Its environmental reactivity is justified by the negative electrode potential of the redox couple FeII/Fe0 (E0 = -0.44 V). These characteristics of Fe0 have been exploited in a wide range of remediation technologies, including both permeable reactive barriers (PRBs) for in-situ groundwater remediation, and filtration systems (Fe0 filters) for decentralized water treatment. Fe0 PRBs and Fe0 water filters employ Fe0 and other solid aggregates (e.g. MnO2, pyrite, sand) to passively remediate polluted waters in a reactive zone. A polluted water flowing through a reactive zone packed with Fe0 is ideally satisfactorily decontaminated, meaning that the contaminant concentrations in the outflow are below regulatory levels (called maximum contamination level - MCL). The voluminous literature on "water remediation using Fe0" is characterized by the huge number of parameters which have been shown to affect the efficiency of Fe0/H2O remediation systems under environmental conditions. Relevant parameters include: (i) Fe0 type, (ii) nature and concentration of pollutants, (iii) pH value, (iv) O2 concentration, (v) water salinity, (vi) water flow velocity (contact time), and (vii) ambient temperature. These factors are interrelated and have been frequently demonstrated as important from isolated sets of experiments. However, because of the lack of (i) a reference Fe0 material, and (ii) a unified experimental procedure, it is practically impossible to compare data obtained with different contaminants, different natural waters and using different Fe0 samples. A major shortcoming of published results from batch experiments is that they have been achieved within very short equilibration times (some few hours or days). More so, they have been mostly achieved under mixing and stirring conditions that do not represent the conditions in filtration systems while additionally considering Fe0 as a reducing agent. The net result is that it is doubtful whether the reported limitations of the Fe0 technology have been accurately identified. To address these gaps, the present thesis was designed to: (i) theoretically discuss the importance of hybrid Fe0/aggregate systems for the sustainability of Fe0 filters (Objective 1), and (ii) experimentally investigate the influence of five common anions (Cl-, F-, HCO3-, HPO4 2-, and SO4 2-) on the efficiency of Fe0/H2O remediation systems (Objective 2). Objective 1 was achieved by a critical literature review, and published as such (review article). Objective 2 was achieved through a series of quiescent batch experiments lasting for up to 45 days using the methylene blue method (MB method). Each system was characterized by the extent of dye discoloration, the final iron concentration and the final pH value. Experiments were conducted in assay tubescontaining 20 or 22 mL of MB, 0.1 of Fe0, 0.5 g of sand, and various concentrations of anions. The MB method is an innovative procedure which enables monitoring the availability of solid iron corrosion products (FeCPs) in a Fe0/sand/H2O system. The results are summarized in two research articles. Results from Objective 1 indicate that hybrid systems which are likely to be sustainable (e.g. Fe0:sand < 10% (v/v)) are yet to be investigated. Experimental results (Objective 2) showed that all five tested anions basically inhibit the remediation process as Fe salts are formed within individual systems, thereby delaying the availability of FeCPs which are the effective contaminant scavengers. For the chloride and sulfate, the present work has established that Cl- and SO42- basically delay the process of contaminant removal in Fe0/H2O systems because the stability of Fe-salts (e.g. FeCl2, FeCl3, FeSO4) delay the availability of solid iron corrosion products. The results also present a better explanation of the ambivalent role of HCO3- (e.g. inhibitory at high concentrations, and enhancing at lower ones). In fact, the impact of HCO3- is not limited to a concentration-dependent buffering effect but reveals the kinetics of the formation of a FeCO3 scale on Fe0. As expected, phosphate inhibition of Fe0 reactivity was observed, since phosphate could form inner-sphere complexes with dissolved Fe and co-precipitates on the Fe0 surface, which inhibits electron transfer from Fe0 to protons (H+ from water dissociation). Lastly, fluoride is demonstrated as a strong inhibitor of Fe0 reactivity because of the formation of very stable FeF63- complexes, delaying the precipitation of FeCPs. Based on the results achieved in this thesis, one of the most productive areas for future work would involve investigating the suitability of lower than 10% (vol/vol). Related small-scale experiments should last for several months or years to interpret the unexpected long-term corrosion kinetics. In addition, better experimental protocols to characterize the influence of co-solutes on the Fe0 remediation systems is possible. More systematic research is needed to design efficient and sustainable real world Fe0-based systems characterized by multi-anion-compositions. Replicating the experiments conducted herein with relevant mixtures of anions (e.g. HCO3 --Cl--SO4 2- or HCO3 --F--SO4 2-), representative for natural waters seems to be to logical next step. [ABSTRACT FROM AUTHOR] |
