2001_A.B.C.alvares_Partial Oxidation of Hydrolysed and Unhydrolysed Textile Azo Dyes by Ozone and the Effect on Biodegradability

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PARTIAL OXIDATION OF HYDROLYSED AND

UNHYDROLYSED TEXTILE AZO DYES BY OZONE

AND THE EFFECT ON BIODEGRADABILITYA. B. C. ALVARES, C. DIAPER and S. A. PARSONS

School of Water Sciences, Cran®eld University, Cran®eld, UK

This work involved a series of batch tests where Acid Yellow 17, and unhydrolysedand hydrolysed Reactive Black 5 textile dyes were ozonated to assess the ef®cacy of partial oxidation and associated decolourization, and parent compound degradation.

Absorbance readings and a decrease in the chemical oxygen demand/total organic carbon(COD/TOC) con®rmed that partial oxidation was the predominant degradation mechanism. Anapplied dose of 0.6 g l 1 caused 100% colour removal through dye chromophore cleavage

whilst higher doses resulted in further degradation. Biodegradability improvement in ozonatedhydrolysed Reactive Black 5 was tracked through biological oxygen demand (BOD5) increaseand BOD/COD and BOD/TOC ratios. Partial oxidation improved the biodegradability of hydrolysed Reactive Black 5 and the optimal applied dose requirement for maximumbiodegradability improvement was 1.8 g l 1, which increased the BOD/TOC and BOD/CODratios from 0 to 0.58 and 0.27, respectively. This suggested that for maximum biodegradabilityimprovement, partial oxidation must proceed beyond decolourization.

Keywords: partial oxidation; ozone pretreatment; textile dye wastewater; biodegradability.

INTRODUCTION

Whilst aerobic biological treatment processes are conven-tionally employed to treat both domestic and industrialwastewaters, it has been shown that they cannot successfullytreat all xenobiotic organic compounds. These compounds,due to their complex nature, emerge from the treatmentprocess unchanged before discharge into the environmentvia ef¯uents and sewage sludges1.

Textile dyes are designed to resist fading and are thusrecalcitrant, resisting biodegradation and imparting colourto receiving waters2. There is currently no de®nitivetreatment for textile dyeing and ®nishing wastewater, and

increasingly stringent consents including those that limitcolour mean that water companies are passing treatmentcosts back to the dyers3,4 . Acid and reactive dyes resistaerobic biodegradation and are particularly problematicbecause of their lower exhaustion rates and high solubility.They are thus found in wastewater at higher concentra-tions than other dye classes and as they do not adsorb easilyto biomass cell walls5, they will not be biodegraded orremoved from the wastestream adsorbed to settled sludge(bioelimination).

The use of ozonation prior to aerobic biological treatmentis a potential solution to reduce both colour and `hard’ TOC

or COD entering receiving waters. Rather than completemineralization to form inorganic salts, carbon dioxideand water, the purpose of ozone pretreatment is partialoxidation. During this step-wise process, complex non-biodegradable parent compounds are converted to simplerby-products that contain in their chemical structure an

increasing percentage of oxygen in the form of hydroxyl,carboxyl or aldehyde function groups. Partial oxidationproducts include aldehydes, ketones and organic acids,which are biodegradable6.

Whilst the cost of ozonation for complete mineraliza-tion of recalcitrant organics may be prohibitive exceptfor low pollutant concentrations, ozone pretreatment in acombined chemical-biological treatment is more economi-cally desirable due to a combination of the following:

· less ozone is required than that needed for completemineralization

· biological processes, with operating costs up to a tenth of

chemical ones, complete the majority of mineralization

7

.A review of the literature8 has shown that biodegradation

of wastewaters by downstream processes can be maximizedby pretreatment doses in the range 0.23 1.04 mg O3 /mgCOD. Combined treatment of a strong textile wastewater(3000mgl 1 COD) with pre-ozonation increased CODremoval through the activated sludge process from 70%to 96%9. A pilot-scale biological trickling ®lter was ableto remove 80% COD from ozonated textile wastewatercompared to 45% without ozone pretreatment10.

This work focused on CI Reactive Black 5 (hydrolysedand unhydrolysed) and CI Acid Yellow 17 textile azo

dyes (Figure 1), both of which are untreatable by activatedsludge11,12. The ®rst stage examined the ef®cacy of partialoxidation and associated colour removal and parent com-pound degradation. Further investigation then followedto assess the effect of partial oxidation on HRB5biodegradability.

103

0957±5820/01/$10.00+0.00q Institution of Chemical Engineers

Trans IChemE, Vol 79, Part B, March 2001



MATERIALS AND METHODS

Commercial grade CI Acid Yellow (100% SandolenYellow E-2GL) and CI Reactive Black 5 (100% DrimareneBlack K3B) were obtained from Clariant. A stock solutionof 50g l 1 CI Reactive Black 5 was hydrolysed by adjustingto pH 11 using sodium hydroxide, followed by boiling atabove 908C for 5 hours to maximize hydrolysis. Workingsolutions of 500mg l 1 in deionized water were preparedfrom hydrolysed CI Reactive Black 5 (HRB5), unhydro-lysed CI Reactive Black 5 (RB5) and CI Acid Yellow 17(AY17).

Ozonation was carried out at uncontrolled pH using aglass stirred batch reactor and a Prominent Ozon Lab0.5/50A ozone generator using dried air as feed gas, a¯owrate of 0.7 l min 1, and an ozone concentration of 15m gl 1, determined iodometrically13. One litre of samplewas ozonated for 180 minutes, during which subsampleswere removed via a sampling port and analysed to track partial oxidation in terms of chemical oxygen demand

(COD), total organic carbon (TOC) and absorbance. Afterthis initial exploration, HRB5 was investigated for bio-degradability improvement by batch ozonating one litresamples for 60±240 minutes under the same conditions.In addition to the tests above, the carbonaceous biologicaloxygen demand (BOD5) of each sample of ozonated andunozonated HRB5 was determined using three differentdilutions.

COD was measured using Hach vials, absorbance scansof unozonated and ozonated samples were obtained usinga Jenway 6505 UV/VIS spectrophotometer, and TOC wasmeasured using a Shimadzu TOC-500A Total Carbon

Analyser.The Blue Book Method14 was used to determine BOD5.The alkaline pH of unozonated HRB5 and acidic pH of ozonated batches were adjusted to pH 6.5±7.5 usingsulphuric acid and sodium hydroxide, respectively. Tertiaryef¯uent from Cran®eld University STW was used as seed.

RESULTS AND DISCUSSION

Partial Oxidation

Parent compound degradation was traced in terms of absorbance reduction at the maximum UV-VIS wavelengths(lmax ). Initial absorbance scans for HRB5, RB5 and AY17showed a major peak both in the visible and ultra-violetregions of the electromagnetic spectrum. Ozonation pro-gressively reduced absorbance in all cases and no newpeaks were formed. A reduction in absorbance at the VIS-lmax indicated decolourization through the cleavage of dyechromophores which in azo dyes are characterized by anN N bond (Figure 1). Degradation of the parent compoundaromatic rings was suggested by reduced absorbance atthe UV-lmax. This lagged behind chromophore destruction,suggesting that colour removal was the ®rst step of partialoxidation. The reduction in absorbance with increasingozonation is shown for HRB5 in Figures 2 (a) and (b).

A comparison of colour, COD and TOC removal for all

three dyes showed similar removal ef®ciencies at the sameapplied dose. For all three dyes, 99±100% transformationof the parent dye compounds through chromophore des-truction was achieved at an applied dose of 0.6 g l 1

(Figure 3). The continued detection of COD and TOCafter the point of 100% colour removal showed that parentcompound degradation was predominantly due to partialoxidation rather than complete mineralization. At themaximum ozone dose, 55± 57% of the initial COD wasremoved and TOC was reduced by 26± 27% (Figure 4). Thisindicated that although there was some mineralization,much of the organic material was degraded by conversion

to other organic molecules10,15,16,17

. Were total minerali-zation the predominant oxidation mechanism, an increasein the COD/TOC ratio would have resulted due to similardegradation rates for both COD and TOC. A decrease inthe COD/TOC ratio was, however, observed with increas-ing ozone dose (Figure 5), indicating the progressive

104 ALVARES et al.


Figure 1. Chemical structure of textile dyes used, (a) CI Acid Yellow 17, and (b) CI Reactive Black 5.



incorporation of oxygen into the ozonation by-productsthrough partial oxidation. Such a decline in COD/TOC

ratio due to ozone pretreatment has also been noted byother authors18,19,20.

KineticsPrevious work on the ozonation of azo dyes suggests

that the degradation of the parent compound and CODfollow pseudo ®rst-order kinetics21. In the current work,semi-log plots for AY17, RB5 and HRB5 show a near lineardependence between the decrease in both colour andCOD and the reaction time (Figures 6 (a) and (b)). The®rst order rate constants for all three dyes were calculated

from the relative change in VIS-lmax vs. reaction time andCOD vs. reaction time. The rate of colour removal was9± 13 times faster than that of COD removal during the sametime period (Table 1), demonstrating that colour removalwas not due to mineralization of the parent molecule butdue to its conversion to other organic molecules by partialoxidation.

Biodegradability Improvement

HRB5 was investigated further for biodegradabilityimprovement due to the commercial importance of reac-

tive dyes, their comparatively low exhaustion rates andbecause the majority of the un®xed dye present in dyebathef¯uent is present in a hydrolysed form.

The results discussed earlier con®rmed that partialoxidation had occurred, with some incidental minerali-zation of organic carbon. An optimal ozonation periodfor partial oxidation for biodegradability improvementwould be one that did not excessively remove COD orTOC, and resulted in an increased BOD5. A higher BODafter ozonation is indicative of a greater availability of organic substrate for aerobic microbial degradation, asnon-biodegradable carbonaceous material is converted tosimpler, more bioamenable compounds14,15,16. A review of

the literature8 has shown that for maximum biodegrad-ability improvement >90% parent compound elimination

105PARTIAL OXIDATION OF HYDROLYSED AND UNHYDROLYSED TEXTILE AZO DYES


Figure 2. Effect of applied ozone dose on degradation of HRB5,(a) absorbance scans showing reduction in absorbance at selected ozonedoses. Peak at 380±392nm is an artefact of the spectrophotometer,(b) corresponding reduction in absorbance at UV and VIS-maximumwavelengths.

Figure 3. Degradation of parent dye compound (measured as VIS-lmax ) by ozonation.

Figure 4. Removal of COD and TOC from dye solutions by ozonation.



is required, possibly due to:

· initial ozonation by-product recalcitrance that requiresfurther oxidation, by which time most of the parentcompound has been eliminated19

· inhibition of biomass activity in the case of toxic parentcompounds so that any remaining parent compoundprevents the biodegradation of oxidation by-products19.

Thus, based on the preliminary ozonation results, theozone doses for investigating HRB5 biodegradabilityimprovement were selected in anticipation of 100% VIS-lmax and >90% UV-lmax absorbance reduction.

Ozonation of HRB5 increased BOD5 (Figure 7), con-®rming that ozonated samples contained by-products that

the domestic WWTW inoculum was able to utilize. Theuse of such an unacclimatized seed provided a robustindication of the general ease of sample biodegradabilitybecause biodegradation occurred without the developmentof specialized enzyme pathways through previous acclima-tization. This implies that the HRB5 ozonation by-productsgenerated in this work could be treatable at a downstream

WWTW without the need for acclimatization.An applied dose of 1.81 g l 1 gave maximum biodegrad-

ability improvement, raising the BOD from 0 to 35 mg l 1

(Figure 7). In many cases, biodegradability parameterswhen examined as a function of ozone dosage havesimilarly revealed an optimum dose for maximum biode-gradability improvement19,22,23. Whilst lower doses maynot promote adequate partial oxidation for maximumbiodegradability improvement, higher doses above theoptimum can diminish biodegradability, possible due tothe complete mineralization of biodegradable ozonationby-products24 or their conversion into recalcitrant ones.

Furthermore, ozonation at doses above the optimum can beinef®cient due to minimal additional partial oxidation orparent compound degradation24.

The BOD5 /COD and BOD5 /TOC ratios are commonly-used indicators of biodegradability improvement25,26,27,28,where a value of 0 indicates nonbiodegradability and anincrease in the ratio re¯ects biodegradability improvement.The ozonation conditions of this experiment increasedthe BOD5 /COD ratio and BOD5 /TOC (Figure 8), whichindicated an augmentation of the biodegradable proportionof the COD and TOC content, represented by BOD5. TheBOD5 /TOC ratio matched the BOD5trend discussed earlier,with a maximum increase occurring on the application

of 1.81gl 1 ozone dose, equivalent to 22 mg O3 /mg initialTOC. This optimum ozone dose for biodegradabilityimprovement was double that required for complete chro-mophore destruction. Further partial oxidation after initialdecolourization was therefore required for maximum bio-degradable by-product formation. This was re¯ected bythe <90% UV absorbance reduction that coincided with themaximum BOD5 /TOC ratio. As in the preliminary explora-tion, the COD/TOC ratio declined, showing the continued

106 ALVARES et al.


Figure 5. Decrease on COD/TOC ratio by ozonation.

Figure 6. Pseudo-®rst order of decay of HRB5, RB5 and AY17, (a) colourremoval, and (b) COD removal.

Table 1. Pseudo-®rst order rate constants for the ozonation of HRB5,RB5 and AY17.

k COLOUR R2 k COD R2

HRB5 0.064 0.99 0.005 0.97RB5 0.073 0.99 0.006 0.99AY17 0.053 0.99 0.006 0.99

Figure 7. Effect of ozone dose on HRB5 BOD5.



progress of partial oxidation. Although the increase in BOD/ COD ratio continued beyond the 1.81 g l 1 ozone dose, ineconomic terms the minimal additional reduction in theCOD/TOC ratio would not merit the use of a higher dose.

A reduction in toxicity after preozonation of varioustextile wastewaters has been reported10,29,30. To address theconcern that ozonation by-products themselves may be

toxic, the BOD5 test was undertaken for each sample usingthree different dilutions. A `sliding’ effect whereby theBOD5 increases with the dilution factor is indicative of sample toxicity13. Thus toxic samples with a high dilutionfactor will give a high BOD5 value because of dilutedtoxicity effects, whilst less dilute samples will give alower BOD5 due to more intensi®ed toxicity effects.This misleading sliding effect related to dilution factorwas unobserved in unozonated and ozonated HRB5samples, demonstrating the absence of such toxicity effects.

Once determined, the optimum ozone dose for biode-gradability improvement can be employed in a laboratory-

scale chemical-biological system for assessment of actualprocess feasibility, performance and optimisation.

Potential treatment cost savingsThe Mogden formula is the UK charging scheme for

ef¯uent discharging to sewer, and is used to determinetrade ef¯uent charges (TEC). Since the Severn-Trent watercompany region is one of the areas in the UK where thetextile industry is concentrated, treatment costs for HRB5were calculated as 42p per cubic metre by incorporatingSevern-Trent treatment charges into the Mogden formula.Assuming the pretreatment of dyebath wastewater beforeit joins the main ef¯uent stream, a comparison was made

between the effects of ozone dose on TOC/BOD ratioand on TEC. The calculations, which relate to the dyeingprocess alone, included the following assumptions:

· unozonated HRB5 COD of 273mg l 1

· ef¯uent ¯owrate of 1000 m3 day 1

· suspended solids concentration of 370 mg l 1

In the context of the HRB5 COD reduction reported inthis work, an annual saving of £18,250 in overall cost of discharge would be possible at the optimum ozone dosethat achieves maximum biodegradability (Figure 9).

CONCLUSIONS

This work has shown that ozone pretreatment decolour-ized AY17, RB5 and HRB5 by partial oxidation andfurther partial oxidation improved the biodegradabilityof HRB5, which would otherwise be untreatable usingconventional aerobic biological processes. In addition thiswork has shown that:

· When using absorbance to assess parent compoundremoval, absorbance at both UV- and VIS-lmax providesinformation regarding process ef®cacy.

· 100% chromophore destruction and hence decolouri-

zation was predominantly achieved through partialoxidation.

· Further partial oxidation following colour removalincreased the BOD5 /COD and BOD5 /TOC ratio of dyesamples indicating improved biodegradability.

· The lag of biodegradability improvement behinddecolourization suggests that partial oxidation needs toproceed beyond colour removal for maximum biodegrad-ability improvement.

· The optimum ozone dose improvement determinedthrough feasibility trials such as this can be used insubsequent laboratory-scale work to assess and enhanceactual process performance using a combined chemical-biological system.

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107PARTIAL OXIDATION OF HYDROLYSED AND UNHYDROLYSED TEXTILE AZO DYES


Figure 8. Effect of ozone dose on HRB5 absorbance, BOD/TOC, BOD/ COD and COD/TOC ratios.

Figure 9. Effect of ozone on potential treatment charges and HRB5biodegradability.



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ACKNOWLEDGEMENTS

Miss A. Alvares was funded by a grant from EPSRC with additionalsupport from Air Products. The authors would also like to thank Prominentfor the supply of the ozonator used in this work.

ADDRESS

Correspondence concerning this paper should be addressed to Dr S.A.Parsons, School of Water Sciences, Cran®eld University, Cran®eld,Bedfordshire MK43 0AL, UK. E-mail: S.A.Parsons@cran®eld.ac.uk

This paper was presented at the IChemE’s Research 2000 Conference,

held at the University of Bath, UK, 6±7 January 2000. It was received 30

September 1999 and accepted for publication after revision 8 January

2001.

108 ALVARES et al.


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2001_A.B.C.alvares_Partial Oxidation of Hydrolysed and Unhydrolysed Textile Azo Dyes by Ozone and the Effect on Biodegradability