During two last decades, numerous technologies and approaches are presented for treating contaminants in liquids and solutions. Among them, thermal plasma has shown rapid kinetic and high destruction efficiencies due to very high temperature and highly active radicals. Hence, the use of thermal plasma for treatment of the contaminants in liquids and solutions has received a lot of attention in view of its low environmental impacts. This review focuses on thermal plasma, and it describes the current status of liquid and solution treatment using this technology. A comprehensive analysis of the available scientific and technical literature on liquid and solution plasma treatment is presented, including the treatment of a variety of contaminants in liquids and solutions via different kinds of thermal plasma. The principles of thermal plasma generation and the available plasma technologies with potential applications to generate valuable products from liquid waste are presented. In addition, the results of the thermal plasma processes for the treatment of specific contaminants are investigated. In light of the investigated literature, thermal plasma is found to have a significant potential to treat the liquid wastes.
A novel direct current (DC) plasma torch, operating with a gas mixture consisting of carbon dioxide and hydrocarbon (methane), has been adapted and used for the first time for treatment of a solution containing dissolved carboxylic acid. Compared with conventional DC thermal plasma torches, this novel torch has high plasma enthalpy, high thermal conductivity, low cost and often readily available operating gas (CO 2 ) and long electrode life-time. Performance of this CO 2 /CH 4 DC thermal plasma torch submerged in aqueous solution was investigated for decomposition of dissolved organic contaminants. Sebacic acid was selected as a representative of organic pollutants in wastewater of chemical process industries. Effect of different operational conditions including treatment time, initial solution pH, and the reactor pressure as well as the role of oxidizing agents such as (H 2 O 2 ) were investigated on decomposition rate of sebacic acid and its intermediate products. It was shown that the submerged CO 2 /CH 4 DC thermal plasma could decompose carboxylic acid. The highest decomposition rate was obtained at neutral solution pH. Adding H 2 O 2 could significantly enhance the decomposition rate due to synergetic effect of ultraviolet radiation (UV) absorption and hydrogen peroxide oxidation. Photo-oxidation was also proposed as the decomposition mechanism with the novel CO 2 / CH 4 plasma torch based on its high level of UV. This work shows the ability of this novel plasma torch for treatment of organic acids in liquids and solutions and therefore offers a new application for the CO 2 /CH 4 thermal plasma in wastewater treatment.
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