engleski

Organic synthetic dyes ; persistent water pollutants

Synthetic dyes belong to a large group of organic compounds and they are widely used in our everyday life. Nowadays, there are over 100 000 commercially available dyes. Coloured wastewater is a consequence of batch processes both in the dye manufacturing and in the dye consuming industries. 2 % of dyes that are produced are discharged directly in aqueous effluent, and 10 % are subsequently lost during the textile colouration process. An indication of the scale of this problem is that the annual market for dyes is more than 700 000 tonnes per year. Coloured wastewater is particularly associated with reactive dyes that are used for dyeing natural fibres. These dyes make up approximately 30 % of the total dye market. Reactive dye production is characterized by the great losses (10 - 20 %) caused by the high solubility of the dyes, which creates an economical and ecological problem. A specific problem connected to the reactive dyes production is that they readily hydrolyze thus additionally pollutes wastewater. Reactive dyes are therefore present in wastewater in relatively high concentrations (20 mg - 1 g/l), but even at low concentrations dyes are visible and therefore undesirable in water. Furthermore, reactive dye wastewater contains intermediates with complex structure and there is also a problem of presence of organic halides. This type of wastewater could be toxic to the aquatic life and dangerous to the human health as well. Generally, synthetic dyes are specifically designed to resist fading upon exposure to light, water and oxidizing agents and with very few exceptions they are xenobiotic. So there is a clear need to treat dye wastewater prior to discharge into the effluent. Wastewater can be treated using physical, chemical and biological methods. The commonly used method for the treatment of dye wastewater is the combination of biological oxidation and physical-chemical treatment. However these processes are quite ineffective in decolourization of wastewater since dyes are of recalcitrant nature and hardly removable by coagulation-flocculation or activate carbon adsorption due to their high water solubility. Moreover, these chemical treatment options only provide a separation of the dyes and produce large quantities of sludges. Among oxidation processes there is a group of processes that are recently considered as promising alternative for treatment of dye wastewater. Advanced oxidation processes (AOPs) produce and utilize hydroxyl radical to oxidize organic mater. AOPs as a wastewater treatment processes involve an input of energy either chemical, electrical or radiative, into the water matrix to generate hydroxyl radical which is very powerful and non-selective oxidant. It has been reported that coloured aromatic compounds can be degraded effectively by a variety of homogenous and heterogeneous AOPs. When colour removal method is considered, it is important to be aware that there are inevitable secondary considerations that go beyond the actual removal of colour. These considerations include: sludge production and handling cost requirements, the level of operational skill required, the operational and maintenance attention needed, the long-term system reliability, and the total system costs. Compared with other oxidation processes, processes using Fenton's reagent are relatively economic and easily operated and maintained. Today, Fenton's reagent is used to treat a variety of industrial wastes containing a range of toxic organic compounds like: phenols, formaldehyde, BTEX - benzene, toluene, ethylene and xylene, and complex wastes derived from dyestuffs, pesticides, wood preservatives, plastics additives, and rubber chemicals. Fenton process may be applied to wastewaters, sludges or contaminated soils, with the effects of odour and colour removal, organic pollutant destruction, toxicity reduction, biodegradability improvement, and BOD/COD removal. This research is addressed to Fenton (Fe(II)/H2O2) and Fenton "like" (Fe(III)/H2O2, Fe0/H2O2) processes, as well as Ozone (O3) and Peroxone (O3/H2O2) processes for the treatment of synthetic dye wastewater containing C.I. Reactive Blue 49 (RB49). Also, influence of solid particles, zeolite ZSM5 and Y type, on overall process efficiency was studied. Wastewater decolourization was monitored by UV/VIS spectrophotometry, while mineralization process was monitored by the means of the TOC parameter. Because of sensitivity of Fenton's reagent to different wastewater it is recommended that the Fenton and Fenton "like" processes always should be optimized through laboratory treatability tests. Therefore, a set of experiments with different molar ratios of Fe(II)/H2O2, Fe(III)/H2O2 and Fe0/H2O2 were conducted. O3 and O3/H2O2 processes are strongly pH dependent. Ozone can oxidize organic matter in water either directly or through the hydroxyl radicals produced during the decomposition of ozone. The direct oxidation with molecular ozone is of primary importance under acidic conditions, however it is relatively slow compared to the hydroxyl free radical oxidation in neutral and basic solutions. The increase of pH and the addition of H2O2 to the aqueous O3 solution enhance the O3 decomposition and thus result into the higher rates of OH radicals production. Experimental results expressed as percentage of dye and TOC removal were discussed for each process and compared. The complete decolourization was achieved in all cases. TOC removal ranged between 30-60% depending on applied treatment process. Positive effect of solid particles on the dye mineralization was observed in Fenton processes with maximum 20 % increasement of TOC removal. On the basis of TOC values versus time the order and reaction rate constant have been established. It has been found that AOPs applied for RB49 mineralization follows pseudo-first order kinetic rate.

AOPs; reactive dyes; Fenton reagent; Fenton

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