The main objective of this research programme was to determine the factors influencing the decolourisation of dyes at low pH by different peroxide species, both in the presence and absence of metal ion catalysts and, therefore, to find a set of optimal conditions for application to wastewater treatment processes. An additional study looked at whether peroxoborates were capable of acting as nucleophiles. The specific aims of the study were: to investigate the in-situ formation of peracetic acid from the equilibrium formed between hydrogen peroxide and acetic acid, and whether this can be achieved without the addition of an acid catalyst such as sulphuric acid; to study the comparative reactivity of in-situ generated peracetic acid and hydrogen peroxide towards a range of dyes used in industry; to investigate the catalytic potential of a range of metal ions towards the reaction between peroxides and dyes; to investigate the structural features of dyes that might influence reactivity (decolourisation); and to investigate the reactivities of other peracid-like peroxide species that can be generated from hydrogen peroxide (peroxoborates and peroxocarbonates).The novel aspects arising from this study were: (a) The development of a new method for the in-situ generation of peracetic acid that gives the same equilibrium yield as established methods yet does not require the addition of an acid catalyst;(the reaction was slow, but there was minimal decomposition and so it is ideal for circumstances that allow the preparation of peracetic acid well in advance of use). (b) The first comprehensive study of the bleaching potential of peracetic acid and hydrogen peroxide towards a wide structural range of dyes both in the presence and absence of metal ions (iron, manganese, silver and copper). (c) The inference that for iron-catalysed bleaching of azo dyes by peracetic acid the catalytic mechanism involves pre-complexation of iron and dye, followed by reaction of the 'activated' complex with peracetic acid rather than a free radical mechanism that might have been expected for such systems. (d) The evidence that, in contradiction to literature studies, peroxoborate species do not act as nucleophiles. As an introduction to this work the reactions of peroxyacids are described in general terms. The experimental work is divided in three parts. In Chapter two, the homogeneous preparation of peracetic acid (PAA) from acetic acid (AA) and hydrogen peroxide (H2O2) was investigated with and without the catalysis of sulphuric acid (H2SO4). The formation of PAA and total peroxide content was determined by iodimetric titration. The reaction was slow in the absence of a strong acid catalyst, and was faster with a sulphuric acid catalyst. There was no loss of total peroxide over the timescales of both reactions, whether a catalyst was used or not. The equilibrium constant for peracetic acid formation at temperature of 20 was found to be 2.04 with a catalyst, and 2.10 without catalyst. The rate constant for the hydrolysis of peracetic acid for both forward and reverse reactions increased when the sulphuric acid concentration was increased from 0.02 M to 0.32 M. Linear relationships were found between the observed rate constants and H+ concentrations at 25oC. Moreover, it was found that the preparation of peracetic acid showed a first-order dependence with respect to peroxide concentration. In Chapter three, the application of this preparation of peroxyacids to the degradation of different types of dyesstuffs was investigated. As we know, physical or other chemical methods for dye degradation are expensive and can generate secondary pollution. In this part of the study the reactions of dyes with hydrogen peroxide and peracetic acid in the absence and presence different metal ions (Fe3+, Cu2+, Mn2+ and Ag+) were investigated. The iron/peroxyacid system was found to be the most effective. Consequently, Chapter 4 evaluates the decolourization of five azo dyes under conditions of bleaching by peracetic acid in the presence of Fe3+ as a catalyst. The experiment was carried out in aqueous acidic media. Dye oxidation systems are complex because: they involve several different tautomers; there is the possibility of dye aggregation at lower dye concentrations; and the oxidant species involved can be either the undissociated peroxide acting as an electrophile, or the dissociated peroxide acting as a nucleophile. The results obtained for the reaction of azo dyes with peracetic acid without added iron, when converted to the second order rate constant for the electrophilic reaction, k2E gave a value of 4.5x10-6 dm3 mol-1 s-1 for orange II, which is very high. This may be due to trace metal ions still being present and catalysing the reaction, possibly from impurities in the dye itself. No metal ion chelators were used in the present study because the bulk of the study was designed to elucidate the effect of metal ions concentration on reaction rate. For the catalysed reactions a significantly increased rate of absorbance decrease with increasing iron concentration was observed. Saturation of iron was also demonstrated at high iron concentrations, suggesting the formation of an iron (III)-dye complex which then reacted with peracetic acid. The maximum rate of reaction was observed at an iron concentration of 0.012 M, and the results showed a reactivity order of Ponceau 4R > Amaranth > (Orange II & Carmosine) > Black PN; Orange 1 was unreactive under these conditions. Also one of the key objectives of this chapter was to determine the optimum conditions for dye degradation in terms of pH and oxidant and catalyst concentrations. The optimum conditions for maximum degradation occurred at the highest pH of 3.0 and at about 1x10-3 M iron. Evidence of the possible involvement of radicals in our studies comes from the observation of a lag phase followed by a more rapid bleaching phase in the oxidation of azo dyes by peracetic acid at the lowest iron concentrations (another possibility is that at these iron concentrations the reactive iron complex forms at a much slower rate). However, this process is slow by comparison with the rate of oxidation at higher iron concentrations that do not exhibit this lag phase; consequently, if free radical mechanisms are suggested then they are not significant compared to the proposed formation of a reactive iron-dye complex.ixThe work contained in final experimental Chapter aimed to clarify whether or not any of the peroxyborate species displayed nucleophilic characteristics and thus accelerated the rate of the reaction of hydrogen peroxide with p-nitrophenyl acetate. The pH range of 6.0 to 8.0 is critical in terms of the distribution of peroxide species for a hydrogen peroxide / boric acid system. The triganol peroxoboric acid, B(OH)2OOH, is the only significant peroxoborate species below pH 6.5. However, above this pH, increased concentrations of the monoperoxoborate anion, B(OH)3OOH, the peroxodiborate anion, (HO)3BOOB(OH)32-, and the diperoxodiborate anion (HO)2B(OO)2B(OH)22-, are formed, with diperoxoborate, B(OH)2(OOH)2- forming at higher hydrogen peroxide concentrations. Therefore this is the ideal pH range in which to elucidate any effects of borate on the reaction of hydrogen peroxide and PNPA. The observed second order rate constants (k2obs) for the reaction between p-nitrophenyl acetate and hydrogen peroxide, and the corresponding second order rate constants, k2, for the reaction of the perhydroxyl anion with p-nitrophenyl acetate was determined by equation:In borate buffer the k2 values were significantly reduced compared to other buffers; this reduction was consistent with the hydrogen peroxide complexing with borate to form a range of non-reactive (towards carbonyl groups) peroxoborate species, thus also reducing the equilibrium concentration of the perhydroxyl anion. There was no evidence for peroxoborate species that could act as nucleophiles, in contradiction of literature claims. Values of k2 in the case of phosphate buffer compared reasonably well with values in the literature of 3140 and 3520 dm3 mol-1 s-1 obtained at pH 6.8 in ionic strengths of 0.02 dm3 mol-1 and 0.1dm3 mol-1 respectively. In carbonate buffer the literature value is 3785 dm3 mol-1 s-1 at pH 10, ionic strength 0.1 M, in borate buffer.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:537456 |
| Date | January 2010 |
| Creators | Unis, Melod |
| Contributors | Deary, Michael |
| Publisher | Northumbria University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://nrl.northumbria.ac.uk/2284/ |
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