Enhanced Degradation Of Organic Contaminants By Hydrated Electrons On Montmorillonite

As the most active reducing species, the hydrated electron is localized and stabilized within a cavity formed by surrounding water molecules, which could be generated by ultraviolet light irradiation of aromatic compounds such as indole in aqueous solution. When aromatics are excited to their first singlet state by light irradiation, aromatic radical cations and hydrated electrons are produced after charge separation. Previous studies showed that hydrated electrons could reduce many organic and inorganic contaminants. However, due to the strong reductive ability, the lifetime of hydrated electrons in solution is short, as it is quickly consumed by surrounding protons and oxygen or quenched through charge recombination. As a result, there are few documented instances for the degradation of contaminants by hydrated electrons under environmentally relevant conditions. Smectites, including montmorillonite, are 2:1 layered aluminosilicate clay minerals that are widely distributed in soils, subsoils, sediments, and prehistoric clay deposits. Related studies have shown that the smectite clay could effectively stabilize the photoinduced charge separation state thereby prolonging its lifetime from a few milliseconds to several hours. Here, we find that the presence of natural montmorillonite enhance the production of hydrated electrons during the light irradiation of indole molecules. Based on our experimental and spectroscopic results, the mechanism can be described as follows:Indole molecules are initially adsorbed in the galley regions or interlayers of montmorillonite. Upon light irradiation, photolysis of indole molecules associated with montmorillonite produces hydrated electrons and the indole radical cations. Due to the planar layered aluminosilicate structure with embedded negative charges, montmorillonite clay could stabilize the newly formed radical cations, consequently preventing the recombination of hydrated electrons and indole radical cations, which would increase the concentration of hydrated electrons. Here, we hypothesize that the presence of natural montmorillonite could enhances the production of hydrated electrons and the formed hydrated electrons could effectively reduce coexistent contaminants.Nitro-aromatic compounds (NACs) are widely used as pesticides, explosives, solvents and chemical intermediates. Prior studies reported that NACs are ubiquitously distributed in the natural environment and have adversely affected human and ecosystem health. Reduction of NACs is an important process for complete degradation of NACs in the environment. Bacterial and fungi are known to effectively reduce NACs under the proper environmental conditions. In addition, NACs also undergo abiotic reduction through direct reaction with reduced sulfur, iron species and extracellular polymeric substances. The reduction reactions were significantly facilitated in the presence of iron porphyrins and hydroquinone moieties present in dissolved organic matter, which could serve as electron transfer mediators. Photodegradation is also an important dissipation pathway for NACs. In most cases, hydroxyl radicals were involved in the degradation reaction, which is generally considered as a photooxidation process. In this research,1,3-dinitrobenzene (m-DNB) and indole were used as a model NACs and as a representative aromatic compound capable of generating hydrated electrons, respectively. Our experimental results showed that m-DNB can be reduced to 3-nitroaniline (3-NA) by hydrated electrons derived from indole when irradiated by either simulated or natural sunlight. The reaction is significantly facilitated by natural montmorillonite. Indole and m-DNB molecules are initially adsorbed in the galley regions or interlayers of montmorillonite. During the photoreduction process, the role of montmorillonite clay mineral is not only to promote the separation of electron and indole radical cation, but also it acts as a nano-reactor in which both m-DNB and hydrated electrons accumulate in a constrained environment, hence promoting their direct contact. The new photoreduction pathway described herein improves our understanding for the transformation of NACs in the natural environment.PFCs (perfluorinated compouds) have been produced in large quantities, used in many household applications, and disposed of with little or no regulation. Perfluorinated chemicals are highly persistent in the environment, bioaccumulative, and toxic. There is widespread and well documented exposure of humans and wildlife to PFCs, most notably perfluooctanoic acid (PFOA) and perfluorooctanesulfonate (PFOS) which are the common byproducts of the primary commercial products. The hydrated electron is the most reactive reducing species known and hence is highly efficient in the reduction of PFCs. We demonstrated that hydrated electrons can be generated when indole or its derivatives are irradiated with light, especially in the presence of montmorillonite. Hence, we want to use the produced hydrated electrons to degrade PFCs. To improve the yield of hydrated electrons, we tried many reaction conditions. Our experimental results demonstrate the complete defluorination of both PFOA and PFOS by hydrated electrons derived from 3-Indoleacetic acid (IAA) under aerobic condition in the presence of HDTMA-montmorillonite. Furthermore, there was no significant different on degradation (or defluorination) efficiency in the pH range from 4.0 to 10.0. The effect of humic acid on PFOA degradation is also relatively weak. FTIR and EPR results provide evidence that the observed degradation of PFOA was caused by the hydrated electrons derived from IAA exposed to light irradiation. HDTMA-montmorillonite can be conceptualized as a unique nano-reactor that localizes PFOA and IAA in a constrained space, stabilizes IAA radical cations hence promoting formation of hydrated electrons, and provides a matrix that shields the hydrated electrons from quenching by protons and oxygen.