Electrical Discharge Plasma for Wastewater Treatment and Functional Nano-material Synthesis

L, Chandana and Ch, Subrahmanyam (2018) Electrical Discharge Plasma for Wastewater Treatment and Functional Nano-material Synthesis. PhD thesis, Indian Institute of Technology Hyderabad.

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Abstract

Understanding plasma-liquid interactions is a growing interdisciplinary area of research, due to the increasing applications of nothermal discharge plasma in environmental, biological, medical and material synthesis. With the beginning of industrialization, mankind has witnessed environmental concerns. In a way, industrialization has not only brought economic development but also troubled the environment. One of the concerns is visible in the form of environmental and water pollution. Unfortunately, a lot of water bodies are contaminated with chemical and pathogenic substances. Advanced Oxidation Processes (AOPs) are universally recognized tertiary treatments and have a strong potential towards the bio-recalcitrant (non-biodegradable) pollutants. AOPs are a class of oxidation processes, which involve the in-situ generation of hydroxyl radical (•OH), which can oxidize many harmful pollutants, including organic pollutants, heavy metals, and micro-organisms. Among the AOPs, utilization of plasma discharges is of particular interest to the scientific community, as it can generate a wide spectrum of multiple reactive species, intense electric fields, UV radiation and shock waves. In addition, plasma contact with liquid (water) can also generate significant amounts of secondary oxidants like H2O2 and O3 and other reactive oxygen species like •O, HO2•, and O2 •–. Plasma discharges also lead to the formation of powerful reducing agents such as hydrated electrons (eaq –), and hydrogen radical (•H). However, the optimization of discharge plasma reactors for wastewater treatment is a complicated task, which still needs much more exploration. In a similar context, a large number of chemical reduction methods have been reported for the synthesis of nanomaterials, these chemical reductions introduce more impurities/defects in materials and are also harmful to the environment. The present study reports the potentials of an atmospheric pressure plasma jet (APPJ) for handling the above said problems. Typical observation is that the concentration of the secondary oxidants (like H2O2, O3 and HNO3) can be tuned by varying the plasma gas. It was observed that the concentration of active species increased gradually with increasing the plasma treatment time. More acidification and increase in the conductivity were observed while operating in air plasma due to the formation of inorganic acids, whereas, the highest amount of H2O2 formation was observed with oxygen gas. The hydroxyl radicals and hydrogen peroxide are the main contributors to the organic compounds (model dyes) degradation and phenol mineralization. The addition of Fe2+ salt showed a positive effect on the formation of •OH. The addition of a radical scavenger (methylene blue) promoted the reduction of Cr(VI) to Cr(III), whereas addition of iron (II) salt decreased Cr(VI) reduction and promoted the vii competing methylene blue oxidation via Fenton reaction. Interesting observation is that plasma treated water also showed a significant reduction of Cr(VI), probably due to the availability of long lived species, including H2O2. Degradation and mineralization of phenol was carried out by using an APPJ operating under argon/air plasma. The combination of CeO2, Ce0.90Ni0.10O2- and 10 wt % NiO/CeO2 catalysts were studied to improve the phenol degradation efficiency. It was observed that catalyst addition to NTP reactor improves the efficiency of the process. The stable intermediates formed during the discharge process were identified by HR-MS. For the first time, APPJ has been demonstrated as an environmentally benign approach for the redox transformations of As(III) and Cr(VI). The simultaneous treatment of Cr(VI) and As (III) appears to be more beneficial than the removal of individual pollutant, which was assigned due to the synergistic effect between Cr(VI) reduction and As(III) oxidation. The solution pH also showed a significant effect on the redox transformations. Addition of Fe2+ salts not only favored the As(III) oxidation to As(V) but also promoted the immobilization of As(V) by Fe(III) ions. The present APPJ reactor has been tested successfully for the bacterial inactivation in an aqueous medium. Typical results indicated that gaseous (Ar + air) mixture showed the highest bacterial inactivation, possibly due to the right combination of solution pH and active species concentration. The •OH present in the solution contributes significantly to the bacterial inactivation, whereas, the presence of either HNO3 and/or H2O2 is not effective. APPJ has been used for the green synthesis of reduced graphene oxide (rGO). Active species formed during plasma discharge are the main contributors to the reduction as well as oxidation of the GO/RGO. Removal of the functional groups and re-oxidation of unsaturated double bonds present in graphene were successfully characterized by various physico-chemical studies. Yet another application of the APPJ is the green synthesis of highly dispersed colloidal silver nanoparticles (AgNPs) under ambient conditions. The size of the colloidal AgNPs was tuned by adjusting the AgNO3 initial concentration and the applied power. Interestingly the plasma reduced AgNPs are stable even after 30 days, whereas the chemically reduced ones agglomerated within 12 h.

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