Degradation of synthetic organic compounds by sulfate- and hydroxyl radical-based advanced oxidation processes

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Degradation of synthetic organic compounds by sulfate- and hydroxyl radical-based advanced oxidation processes

 

Author: Wang, Yuru
Title: Degradation of synthetic organic compounds by sulfate- and hydroxyl radical-based advanced oxidation processes
Degree: Ph.D.
Year: 2012
Subject: Water -- Purification.
Water chemistry.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Civil and Environmental Engineering
Pages: xxi, 167, 1 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2530220
URI: http://theses.lib.polyu.edu.hk/handle/200/6758
Abstract: Currently, the contamination of water sources by synthetic organic compounds (SOCs) is one of the concerning environmental issues faced throughout the world, leading to a great interest in developing alternative treatment technologies for the removal of SOCs in aqueous medium. Among the existing SOCs, dyes and herbicides contribute to a large portion of the confirmed toxic organics and thus have to be removed prior to the discharge of the treated water. Against the background, the primary objective of this research work is to explore sulfate- and hydroxyl radical based advanced oxidation processes (AOPs) for the elimination of dyes and herbicides in water and wastewaters. Firstly, an oxidation process using sulfate radical (SO₄.⁻) activated by Fe(II)-mediated Oxone® process (FO) were evaluated by monitoring the degradation of a Xanthene dye Rhodamine B (RhB) in an aqueous solution. The effects of reactant dosing sequence, Fe(II)/Oxone® molar ratio and concentration, solution pH, and inorganic salts on the process performance were investigated. Total RhB removal was obtained within 90 min under an optimal Fe(II)/Oxone® molar ratio of 1:1. The RhB degradation was found to be a two-stage kinetics, consisting of a rapid initial decay and a subsequent retarded stage. Additionally, TOC study indicates that stepwise addition of Fe(II) and Oxone® can notably improve the process performance by about 20%, and the retention time required can be greatly reduced compared with the conventional one-off dosing method. On the other hand, it was found that the rapid depletion as well as the slow regeneration of Fe(II) in the above FO process usually terminates the production of SO₄.⁻ and limits the decay rate. To tackle with the problem, a novel electrochemically enhanced FO process (i.e. EFO) was proposed. In this oxidation process, once an electric current is applied between the anode (an iron sheet) and the cathode (a graphite bar), a predetermined amount of Oxone® is added to the reactor. Ferrous ions generated from the sacrificed Fe anode mediate the generation of SO₄.⁻ through the decomposition of Oxone®. The EFO process was evaluated in terms of a selected herbicide 2,4,5-Trichlorophenoxyacetic acid (2,4,5-T) degradation in aqueous solution. Experimental results demonstrated that low solution pH facilitated the system performance due to the dual effects of weak Fenton's reagent generation and persulfate ion generation, whereas the process was inhibited at basic pH levels through non-radical self-dissociation of Oxone® and the formation of Fe(OH)₃. The active radicals involved in the EFO process were identified. The EFO process demonstrates a very high 2,4,5-T degradation efficiency (over 90% decay within 10 min), which justifies the novel EFO a promising process for herbicide removal in water.
Furthermore, the degradation of 2,4,5-T in aqueous solution by photo-assisted Fe(II)/ Oxone® process (FOU) was explored and compared with the FO process. Regarding the involvements of UV, Oxone® and/or transition metal, four different processes (i.e., UV alone, Oxone®/UV, FO, and FOU) were evaluated in terms of 2,4,5-T decay. For the tests involving the UV irradiation, the effect of various wavelengths of UV light was also investigated. The experimental results indicated that direct photolysis of 2,4,5-T was insignificant (< 19%). The process was improved in the presence of Oxone® due to the formation of sulfate and hydroxyl radicals via the photolysis of Oxone® with the UV 254 nm irradiation, which exhibits the best performance (> 80%) in comparing with the others (300, 350, 419 nm). In particular, the FO process was dramatically promoted upon the introduction of UV irradiation (i.e. FOU). Subsequently, the role of UV irradiation was elucidated in-depth by comparing the real-time of [Fe(II)] in the solution during the reaction between the FO process and the FOU process. The effect of initial pH was investigated and optimized to be 3.68. The 2,4,5-T decay by the FOU process under various [Oxone®] and [2,4,5-T] was also examined. Besides, the influences of various anions on the decay of 2,4,5-T by the FOU process were examined in detail. The decay pathways for the transformation of 2,4,5-T by UV alone, Oxone®/UV, and FOU processes were proposed using LC-ESI/MS analysis. Additionally, the efficiencies of UV alone, Oxone®/UV and FOU were further examined in terms of mineralization and Cl- ion accumulation. It was found that the proposed FOU process demonstrates the best removal (100% in 90 min) of the parent and daughter compounds examined with an outstanding mineralization performance (> 83% in 8 h) in comparison with the other two processes investigated in this study. Finally, an improved process by incorporating the merits of both of EFO and FOU processes was developed, in which the solution treated under EFO system is simultaneously irradiated with UV light (i.e., EFOU). This EFOU process was examined through the degradation of 0.2 mM 2,4,5-TCP under various operating conditions. It was found that the EFOU process yields over 92% of probe removal at a very low applied current (1 mA) in natural pH (4.35) condition. It was demonstrated that an acidic condition is favorable to the process. Furthermore, around 80% of 2,4,5-TCP was decayed at pH of 7.70 in 20 min suggesting that the EFOU process is efficient work even at neutral pH conditions. The mode of current-applying and the strategy of tandem addition of Oxone® on the effect of process performance were also investigated. Furthermore, a possible decay pathway of 2,4,5-TCP by EFOU process was proposed based on the identified aromatic intermediates by LC-ESI/MS analysis.

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