Colour in textile effluents - sources, measurement, discharge consents and simulation: a review

Abstract

This paper aims to review the problem of colour in textile ef¯uents, the different classes of dyes available and their contribution to the problem. Through new regulations, pressure is being placed on water companies all over the world to reduce the amount of colour in sewage ef¯uent. Dyes exhibit low toxicity to mammals and aquatic organisms and therefore colour consents are normally applied for aesthetic and industrial reasons rather than for prevention of toxicity. The absorbance, ADMI values and concentrations of dyes in ef¯uent are examined here with particular reference to reactive azo dyes used in cotton processing. Colour consents, the problem of colour in textile wastewaters and the importance for research in this area are also discussed. Dye concentrations of 0.01 g dm À3 up to 0.25 g dm À3 have been cited as being present in dyehouse ef¯uent, depending on the dyes and processes used. ADMI values ranged from 50 to 3890 units for the dyeing of cotton. It was concluded that 1500 ADMI units was a reasonable value to aim for when simulating coloured ef¯uents. Simulated textile ef¯uents may be used for research purposes. These should resemble real wastes as closely as possible, but it is often dif®cult to replicate the ADMI values, absorbance and spectra of real ef¯uents. The concentrations of dye used in simulated ef¯uents examined in literature varied from 0.01 g dm À3 to 7 g dm À3.As absorbance and ADMI values change with the types of dye used, it is dif®cult to relate these values to dye concentrations. A concentration of 0.18 g dm À3 of a Red or Yellow dye or 0.43 g dm À3 of a blue dye would provide an ADMI of approximately 1500 units and ®ts within the range of dye concentrations presented in literature. A dye mixture simulating colour in a real textile ef¯uent is suggested and some limitations of simulating actual wastewaters discussed.

Photocatalytic degradation of textile dyes by hydrogel supported titanium dioxide nanoparticles

“…The effect of ions like chloride, sulphate nitrate and phosphate on the decolouration efficiency was studied and is tabulated in Table 1. The values in the table shows that the presence of Cl -, SO 4 2-and NO 3-anions leads to an increase of the effectiveness of the photo catalytic degradation and the presence of HPO4 2-anion decreases the photocatalytic efficiency. This is because in the case of chloride ions the formation of Cl • radical by the reaction of photo produced OH • with Cl -oxidizes pollutants results in increase in efficiency of degradation.…”

“…This is because in the case of chloride ions the formation of Cl • radical by the reaction of photo produced OH • with Cl -oxidizes pollutants results in increase in efficiency of degradation. [24] Similarly the SO 4 2-ions adsorbed in the surface of TiO 2 react with photo-induced holes (h + ) to form sulphate radical anion ( • SO − 4 ) which is a strong oxidant [25]. The NO 3 -ions, weakly adsorbed on the surface of TiO 2 may cause acidification of the solution with the nitric acid and therefore increase the photocatalytic degradation by decreasing pH.…”

“…The NO 3 -ions, weakly adsorbed on the surface of TiO 2 may cause acidification of the solution with the nitric acid and therefore increase the photocatalytic degradation by decreasing pH. The same interpretation is proposed for HPO 4 2-ions which increase the pH of the solution and consequentlydecrease the photo catalytic degradation.…”

“…Many dyes are visible in water at concentrations as low as 1 mgL -1 . Textile processing wastewaters, typically with dye content in the range 10-200mgL -1 [4] are therefore usually highly coloured and discharge in open waters presents an aesthetic problem. The treatment of textile effluents is of interest due to their toxic and aesthetical impacts on receiving waters.…”

Treatment of dye wastewater with permanganate oxidation and in situ formed manganese dioxides adsorption: Cation blue as model pollutant

“…Dye wastewater, which receives great concern due to its difficulty to treat, contains high intensity of color, high chemical oxygen demand (COD); and perhaps more serious, they are toxic and carcinogenic to aquatic living organisms [11,12]. Biological process is unlikely to achieve good performance if the biodegradability is not satisfactorily increased by chemical oxidation.…”