Experimental And Theoretical Studies On The Oxidation Mechanism Of Bromophenols By Ozone And Fe(?)

Organic pollutants widely distributed in the aquatic environment.Halogenated organic pollutants are the mainly contributing to the environmental hazards.Comprehensive studies on their degradation including their reaction process,mechanisms,and toxicity assessment can be an effective way to hinder their widespread manufacturing,consumption and disposal process.Advance oxidation technique plays conclusive role in their effective degradation,especially ozonation and Fe(?)are the most easy and reliable techniques for organic pollutant degradation.In series of two projects,pentabromophenol(PBP),bromophenols(BPs)including(monobromphenols(2-BP,3-BP,4-BP),dibromophenols(2,4-DBP,2,6-DBP,3,5-DBP),tribromphenol(2,4,6-TBP)and PBP)were comprehensively studied.In addition,their reaction mechanism,reaction kinetics,toxicity assessment and computational analysis including density functional theory(DFT)proven to be the novel findings of this dissertation.Detail studies of these two projects are given as:(I)Oxidative degradation of PBP in aqueous basic medium was systematically investigated by using two different ozone generators(sources:air and water).A comprehensive series of experiments including reaction kinetics,mechanism,and toxicity assessment were performed.Degradation influencing factors such as p H(7.0,8.0,9.0),humic acid(HA)and anions(Cl^-,SO₄^2-,NO₃^-,and HCO₃^-).PBP was efficiently degraded within five mins(O₃ source:water)and 45 mins(O₃ source:air)at p H 8.0 maintained by phosphate buffer.Reaction kinetics revealed 17 by-products with five possible pathways,including dimers with their isomers and lower bromophenols.Furthermore,the frontier molecular orbital theory was employed to confirm the proposed ozonation pathways,including the breakage of the C-O bond at C₅ and C₄ positions,and the cleavage of the C-C bond at C₃ and C₆ position.Product P5,P14(hydroxyl-nonabromophenyl ether)and P15(dihydroxyl-octabromophenyl ether)were identified with isomers.Ecological structure activity telationships toxicity assessment resulted into the conversion of highly toxic PBP(acute toxicity:LC₅₀=0.11 mg/L for fish,LC₅₀=0.124mg/L for daphnia,and EC₅₀=0.118 mg/L for green algae)to less harmful products aside from dimers.P14(acute toxicity:LC₅₀=1.04×10⁵)found to be more toxic as compare to PBP.From these findings,we concluded that ozonation is an effective and ideal process for PBP degradation.(II)The oxidative degradation of eight bromophenols including monobromophenols(2-BP,3-BP,and 4-BP),dibromophenols(2,4-DBP,2,6-DBP,and 3,5-DBP),a tribromophenol(2,4,6-TBP)and a PBP,by a Fe(?)reaction process at a p H of 8.0 was systematically studied.It was concluded that their degradation rates increased with increasing Fe(?)concentrations in solution.The removal of 2,4,6-TBP,2-BP,and 2,6-DBP was faster than that of the other five BPs,which could be attributed to the position of the substituting Br atom.Moreover,the direct oxidation and coupling reactions greatly influenced the reactivity of the bromophenols with Fe(?).The electron paramagnetic resonance(EPR)analysis confirmed the presence of hydroxyl radicals in present system.The oxidation reaction products of PBP and 2-BP were recognized by an electrospray time-of-flight mass spectrometer;hydroxylation,hydroxyl substitution,the cleavage of the C-C bond,direct oxidation and polymerization via an end linking mechanism were noticeably found in the reaction process,resulting in the formation of polymerization products and causing hydroxylation to occur.Theoretical calculations further determined the possible oxidation sites of 2-BP and PBP.These studies provide the complete oxidative degradation of PBP and BPs by ozonation and Fe(?),the problem of toxic product can be overcome by Fe(?)oxidation,which proved to more effective and efficient to degrade not just only PBP but other BPs as well.These studies may prove that computational analysis including DFT at different level can be authentic approach to strengthen the experimental findings.