Hybridized reactive iron-containing nano-materials for water purification

Abstract

Groundwater is an important source for drinking water in all corners of the globe, and in places like Denmark, it is the primary source for drinking water. Climate change and population growth will only lead to further dependence on groundwater as the supply for drinking water. However, the expanding population and industrialization of human civilization also leads to environmental consequences affecting groundwater sources. Storm-water and agricultural runoff, industrial spillage and dumping, acid mine drainage, and leakage from landfills are a few prime examples of routes of contamination for pollutants to enter groundwater systems. In order to make these contaminated water sources viable for human consumption, the use of reactive iron (i.e. Fe0 or zero-valent iron), and in particular nanoscale zero-valent iron (nZVI), is being employed to reductively degrade and/or adsorb many of these pollutants. However, the use of nZVI, as it currently stands, has its limitations. These limitations are primarily rapid oxidation and aggregation, resulting in loss of reactivity and applicability. Therefore, development of new materials incorporating nZVI and improving synthesis strategies to increase the applicability of nZVI is paramount to its future success as a remediation technique. This PhD project has investigated various materials aimed at solving the reactivity loss of reactive iron to create a robust treatment system capable of treating polluted waters. This PhD project also investigated and developed a procedure to appropriately measure the reactivity of reactive iron for a universal testing method. Coating of nZVI is a common solution to combatting the limitations of the material, in that the coating can surround the nZVI particle and prevent it from interacting with other particles while still allowing for interaction with the aqueous pollutant. This study employed a synthetic organo-functionalized magnesium-based aminoclay (MgAC) for this exact purpose. By varying the ratio of MgAC to nZVI and monitoring the change in physical characteristics and reactivity, a composite material was formed that improved the overall functionality of nZVI. It was determined that the reactive iron (vs. oxidized iron) content, colloidal stability, particle size, and nitrate degradation could all be best enhanced at a weight ratio of 7.5:1 of MgAC:Fe. Another solution, although less common, to combatting the limitations, is to entrap or impregnate a porous material with nZVI. This way acts in a similar manner, except that the nZVI is bound within a complex matrix rather than coated with a protective barrier. A variety of porous polymeric networks, termed covalent organic polymers (COPs), were impregnated with nZVI and evaluated similarly as with the MgAC. All COPs exhibited high uptake of nZVI, approximately 10% by mass. Reactivity quantification proved to be difficult when degrading an azo dye, due to the very high propensity of the COPs to adsorb both the dye and its degradation products. However, these COPs acted as extremely efficient carriers of nZVI for maintaining colloidal stability. In one case, the COP used (COP-19) increased the colloidal stability of nZVI by two orders of magnitude. Building on the application of these composite materials, investigating how best to handle the synthesized materials can prolong their lifetime. To do this, three washing and storage strategies of the MgAC coated nZVI were investigated. They were: washing the particles immediately after synthesis with a NaHCO3 buffer, washing the particles after storing with a NaHCO3 buffer, and washing the particles immediately after synthesis with a MgAC solution. For all the particle reactivity tests done, it was apparent that washing the particles after storing was detrimental to the material. The colloidal stability, reactive iron, and reactivity towards nitrate dropped rapidly through one week of storage. The other strategies, where washing was done immediately was able to preserve the three aforementioned properties much more efficiently though one week of storage, with MgAC washed particles faring better of the two. This pre-washing technique removes residual reactants in the synthesis mixture that can corrode the iron, and furthermore, pre-washing with MgAC adds more of the stabilizer to the material that protects the nZVI even more. Moreover, by looking deeper into the characteristics of uncoated nZVI, depending on the washing method, allowed for more insight to the nature of the mechanisms taking place during storage. It was observed that washing nZVI with MilliQ water after synthesis created an environment where the particles were slightly more oxidized from the start, which led to an increased formation of an iron-hydroxide shell during storage. Not washing nZVI or washing with the reductant NaBH4 prohibited initial oxidation, leading to subsequent iron-oxide formation during storage. This is important, because the hydroxide shell promotes more electron transfer, whereas the oxide shell acts as a depassivation layer. The increased electron transfer then allowed for higher reactivity during storage, up to one week. To make comparison and quantification for researchers, a simple and effective method to assess the reactivity of nZVI is extremely important. And, as it is now, most of the reactivity characterization methods are often analytically intensive, requiring expensive equipment, and often don’t respond uniformly to different nZVI-based materials. This study sought to solve this problem, by developing a simple colorimetric assay that is capable to taking a degradation product produced by nZVI reacting with a compound, and creating a color reaction detectable with a simple spectrophotometer. This was done by utilizing the indophenol reaction, which uses phenol and selected other reagents to produce a blue color. Phenol can be produced from the dehalogenation of 4-chlorophenol by nZVI, and to a greater extent by bimetallic nickel-nZVI. That simple method was then optimized to reduce reagent volumes, nickel concentration, and to broaden the range of detectable compounds. These compounds capable of being used in the color assay with the same set of reagents were ultimately aniline, ammonium, and phenol; all of which can be produced by the degradation reaction from nZVI. Finally, to compare the applicability of the colorimetric assay to common halogenated groundwater contaminants; it was compared to the dehalogenation of TCE, TCA, and atrazine. The colorimetric assay performed similarly to the degradation of those chlorinated compounds; meaning the assay can be a simple tool to assess the reactivity of any nZVI when ultimately targeting more difficult to analyze compounds in real-world sources. Ultimately, the primary goal of this PhD study was to develop a robust nanocomposite material containing nZVI for water treatment systems. Taking the lessons learned from initial composite work using MgAC and COPs, the final material combined granular activated carbon with COP and nZVI. After a lengthy process in developing a method to chemically graft COP material to the surface of activated carbon, it was possible to impregnate that composite material with nZVI. Because of the activated carbon backbone, the final material proved to be an extremely robust material with the structural integrity to be used in a packed-bed column that is common when treating high volumes of water. Although, continued optimization of the material is necessary, preliminary results when adsorbing and degrading contaminants were very promising, outperforming activated carbon alone and just the carbon impregnated with nZVI. Also, a bonus effect was achieved in the process. In that the entire composite material, in particular the COP attached to the surface of the carbon, acted as a protective barrier from the effects of oxidation. The carbon-COP-nZVI composites exhibited nearly 100% reactive iron content upon synthesis, compared to much lower amounts in other reported nZVI composites or the carbon-nZVI produced in this study having only 80% reactive iron content. The results of this PhD concluded in various advances in the application and assessment of nZVI and nZVI composite materials. Various composite materials provided increased colloidal stability and reactivity for nZVI. Various washing and storage strategies elucidated better methods for delivering nZVI to a water contaminant and the underlying mechanisms taking place in the nZVI corrosion process. Finally, novel materials combining three different technologies were developed to eventually lead to a robust water treatment system capable of degrading typically hard to remediate water pollutants.