Iron Oxide/Carbon Nanocomposites for Redox Conversion of Water Pollutants

DoctorGenerally, a variety of harmful chemical reagents were used for wastewater treatment more than a century ago. The above fact led to the renaissance of the advanced oxidation process (AOP) and advanced reduction process (ARP) and they have been widely investigated for complete removal/mineralization of recalcitrant organic contaminants, microorganisms, heavy metals, persistent organic pollutants (POPs), and the non-biodegradable contaminants, etc. It is responsible for the low cost, highly efficient, and available advantages of AOP and ARP technologies, whereas the conventional strategies require external energy, excess amount of chemical reagents, high-cost processes that are not suitable for the next generation water treatment technology. Thus, brand new approaches of AOP- and ARP-based water treatment technologies are the most promising and practical technologies for environmental remediation. At first, the most common ways of treating organic contaminants in the aqueous solution by AOP are as follows: 1) Fenton reaction, 2) electrochemical processes, 3) O3/H2O2/UV, 4) photocatalysis, and 5) sonolysis, etc. The above strategies are toward the same goal that is how to generate more reactive oxygen species (ROS) and remove/mineralize organic contaminants as fast and efficiently as possible. Usually, AOP-based water treatment inevitably contains metal ions and chemical oxidants (e.g., O3, H2O2, persulfate, etc.). For maximizing treatment efficiency, electricity, and light irradiation would be applied as the option. This option could be a double-edged sword. Is it worth producing more CO2 for the environment for removing trace organic contaminants in the water? Therefore, energy and chemical-less processes are required as the ultimate environmental remediation technology. For the next, ARP is the most efficient way to remove the highly oxidized compounds, heavy metals, per- and poly-fluoroalkyl substances (PFAS), and anthropogenic chemicals, etc. It commonly utilizes hydrated electrons (eaq-), H atoms (H●), and UV/sulfite (i.e., sulfite radical anions (SO3●-), sulfur dioxide radical anions (SO2●-)). The above processes are also in the same goal that how to generate more reactive reductants and remove reducible contaminants as fast as possible. The conventional ARP had to utilize electron beam irradiation to the polluted groundwater and the usage of activator with harmful chemical reductants concurrently. Thus, either AOP or ARP had to use more energy and chemicals to remove/mineralize aqueous contaminants. The skeptical opinions from some researchers were raised from the early development of the technique that still needs to improve the removal efficiency and practical application in real environmental remediation. The main purpose of this study is the development of iron oxide/carbon nanocomposite as the novel environmental nanomaterials for initiating redox conversions of aqueous contaminants into the harmless product even under ambient (i.e., energy and/or chemical reagents) conditions. In the first study, the Fe2O3 nanorods loaded on a carbon nanofiber sheet (Fe2O3/CNF), was found to be active in degrading aromatic contaminants spontaneously under the dark and ambient conditions even without requiring any external energy and chemical oxidants (e.g., H2O2 or PMS). The removal of aromatic contaminants was not caused by adsorption but by oxidative degradation since the concurrent generation of intermediates. The degradation was only possible in the presence of dissolved O2 in the aqueous solution which was subsequently reduced to ROS (i.e., O2●- and ●OH). As a result, this Fe2O3/CNF system was proposed that the direct electron transfer from aromatic ring to O2 passing through Fe2O3/CNF with initiating oxidative degradation. For the second study, Fe2O3/CNF could initiate a generalized reductive transformation of aquatic pollutants which usually need chemical reductants (e.g., NaBH4 or hydrazine) and external energy inputs (e.g., electricity). However, Fe2O3/CNF enables the spontaneous reductive transformation of recalcitrant inorganic contaminants under dark and ambient conditions. This abnormal reductive activity was only exhibited on Fe2O3/CNF, whereas either bare CNF or commercial Fe2O3 nanoparticles or their physical mixture have negligible activity. Detailed characterizations of Fe2O3/CNF before and after the reductive reactions suggested that the spontaneous electron transfer was initiated on introducing reducible inorganic substrates and the electrons were transferred from the oxygen-containing functional groups (i.e., C-OH) on the CNF surface to substrates pass through Fe2O3 NRs. In particular, its reductive processes occurring on Fe2O3/CNF were little hindered by the presence of dissolved O2, which makes the application practically viable as it does not require an energy consuming deaeration process before the treatment. Because of the co existence of organic and inorganic contaminants in the real wastewater nearby the manufacturing region. To generalize and simulate the practical wastewater treatment system, we focus the environmental redox reactions on Fe2O3/CNF simultaneously. This study is the first instance of the simultaneous removal of various couples of organic and inorganic contaminants with the synergic effect under dark ambient aerated conditions without needing any energy and harmful chemical reagents. The cation-π interaction promoted spontaneous electron transfer to generate ROS to degrade various organic contaminants. The intrinsic electron transfer from surface oxygen-containing functional groups (i.e., C-OH) on CNF to inorganic contaminant was possibly reducing inorganic contaminant concurrently. In particular, the phenolic intermediates, and in-situ generated O2●- promoted removal of reducible inorganic contaminants which served as external reductants. Besides, the low valent chromium enabled to produce ●OH by the synergic reaction with in situ produced H2O2 during the redox reactions. To utilize Fe2O3/CNF as an ultimate environmental nanomaterial, it could successfully remove a variety of aquatic contaminants under the flowing condition which potentially function as a novel environmental nanomaterial. As a result, an environmental nanomaterial Fe2O3/CNF enabled to initiate a simultaneous and synergic environmental redox conversion under dark ambient aerated conditions spontaneously. Finally, for maximizing organic contaminants degradation activity on iron oxide/carbon nanocomposite, by using a photochemical system to effectively separate photo-excited electrons and holes by introducing carbon paper (CP) in Fe2O3/CP. We focused on a thin Fe2O3 layer coated carbon paper (Fe2O3/CP) as a novel environmental nanomaterial. This system exhibited unusual photoactivity for the complete degradation/mineralization of organic contaminants without needing any external harmful chemical oxidants. The synergic combination of Fe2O3 and CP facilitates the efficient charge separation across the Fe2O3-CP interface. The surface oxygen-containing functional groups CP seem to serve as a critical role as an electron acceptor while the holes on Fe2O3 directly oxidize organic contaminants without generating hydroxyl radicals. Fe2O3/CP exhibited versatile photocatalytic activities for a variety of organic compounds and it showed much higher photocatalytic activity than any other Fe2O3/carbon materials. Additionally, in the case of photo induced activation of peroxymonosulfate (PMS) by photocatalyst (e.g., TiO2) is not studied well. Thus, we reported the activation of PMS on visible light irradiated TiO2 via a charge transfer complex path as introduced in the Appendix