tio 2 -mediated heterogeneous photocatalysis

Solar photocatalysis (TiO₂/sunlight) in the presence of H₂O₂ was investigated for the treatment of real textile effluent by Garcia et al. (2009). Treatment performance was evaluated in terms of the parameters COD, inorganic ions (ammonium, sulphate, nitrate) and analysis of characteristic UV-visible light absorption bands in the 200–800 nm range. Degussa P-25 grade titania (TiO₂) were used as the photocatalyst (BET = 50 m²/g; 30 nm particle size; 75–80 % anatase and 20–25 % rutile crystal phase). Five effluent samples were collected from a clothing factory located in Brazil (average values: pH at dyeing machine outlet = 13; pH in the balancing tank = 7.5; NH₄–N = 5.1 mg N/L; N_org = 13.4 mg/L; COD = 368.4 mg/L).

0.25 g/L suspensions of titania and 10 mM H₂O₂ was used in the experiments. The pH was adjusted 3.50 by 0.1 M HCl prior to photocatalysis to achieve the best adsorption level. The reactor consisted of an open cylindrical borosilicate glass

with suspensions containing 500 mL textile effluent. The average temperature and solar radiation flux were 30°C and 2.85 MJ/m², respectively. UV-Vis absorption spectra results implied that in the colour region absorbance values completely disappeared after 150 min, whereas for the UV region longer treatment periods (6 h) were necessary and still absorption bands were observed at 228, 254 and 280 nm. H₂O₂ consumption was in the range of 88–100 % for all textile effluent samples. COD removals in the range of 62–100 % could also be achieved after 6 h solar photocatalytic treatment. Not the TOC parameter, but inorganic ions were measured to follow the extent of mineralisation. In all textile effluent samples, variable final concentrations of ammonia, nitrate and sulphate was detected after 6 hours of photocatalytic treatment. The formation of gaseous nitrogen cannot be ruled out. Both nitrogen and sulphate species are expected as oxidation end products of textile dyes present in the dyehouse effluent. Negatively charged ions such as nitrate and sulphate are expected to be adsorbed on the positively charged titania surface at acidic pH’s.

2.5.5 ozonation

Ozone is known to effectively decolourise textile dyes in aqueous solution and in real/simulated dyehouse effluent at mass-transfer limited rates. In a paper authored by Chu et al. (2008) micro bubble technology was employed to increase the mass transfer rate of ozone and enhance the ozone oxidation of real textile industry wastewater (COD = 530–600 mg/L; BOD₅ = 80–116 mg/L; pH = 6.9–8.7;

TSS = 60–80 mg/L). Enhancement of the ozone utilisation would decrease the amount of ozone supply and lower the off-gas (unreacted) ozone concentration.

Experiments were performed using a micro bubble generator (consisting of a recycling pump, an accelerator and an injector) and an ordinary bubble contactor featuring a conventional air diffuser, which is commonly used in ozonation systems, for comparative purposes (wastewater volume = 20 L). Ozone was produced from pure oxygen by an ozone generator. The micro bubble generator produced a milky and high intensity micro bubble solution, which could reach a higher oxygen transfer rate at a lower input gas flow rate (investigated range = 0.02–1.50 L/min). A volumetric oxygen transfer rate of 0.086–0.413 kg/(m³´h) and a total mass transfer coefficient of 0.1072–0.4859 min⁻¹ were obtained at the studied airflow rate range. The decolourisation capacity was determined by scanning the optical spectra of a sample from 200 to 800 nm and integrating the area below the absorbance curve. During the ozonation of real textile wastewater by using the micro bubble oxidation system, the input ozone could be almost completely utilised, and the rate of decolourisation and organic reduction were much faster than those of the bubble contactor. The time required for 80 % colour

removal from real textile wastewater was about 140 and 280 min by ozone micro bubble and conventional bubbles, respectively. The COD removal efficiency in the micro bubble system was higher by 20 %; it reached 70 % (at 200 min) in the micro bubble system and was only 50 % at 360 min for the bubble contactor. pH decreased gradually during ozonation to from the original neutral to acidic values as a consequence of acidic intermediate production. The results revealed that in addition to the enhancement of the mass transfer of ozone, micro bubbles could improve the oxidation of actual textile wastewater.

In another study conducted by Somensi et al. (2010), ozonation of real, untreated textile wastewater (COD = 1505 mg/L; BOD = 91.2 mg/L; surfactants = 1.18 mg/L;

colour as A₄₅₅ = 0.754; pH = 9.1; phenolic compounds = 0.090 mg/L; total iron = 0.77 mg/L; nitrate = 2 mg N/L; phosphate = 12 mg P/L; sulphate = 345.3 mg/L) was conducted in a pilot-scale plant (an air dryer, compressor, ozone generator and a 18 L cylindrical PVC/PE made semi-batch contactor) and the efficiency of this treatment was evaluated based on the parameters colour and soluble organic matter removals the latter being measured as COD at two different pH values 3.0 and 9.1). Ozone gas was produced at a concentration of 20 g/m³ and sparged counter-current in relation to the textile wastewater circulation for a treatment period of 4 h. Identification of intermediate and final degradation products of ozonation were also carried out via gas chromatography-mass spectrometry (GC- MS). Besides, the final ecotoxicity (Lumistox^® test conducted with marine photobacteria to obtain EC₅₀ values) of the pretreated wastewater was measured.

After 4 h of ozone treatment with wastewater recirculation at a flow rate of 0.45 m³/h, the average efficiencies for colour removal were 68 % (at pH 9.1) and 41 % (at pH 3.0), while COD removal was 26 % (at pH 9.1) and 19 % (at pH 3.0) for an ozone feed rate of 20 g/h. Ozonation had a positive impact on the BOD₅/COD (a rough estimate of ultimate biodegradability) ratios obtained for the textile wastewater by a factor of 7. A GC-MS analysis of the ozonated textile wastewater indicated that some oxidation products were present at the end of the pretreatment time.

Among the intermediates, phenols, quinones, hydroquinones, phthalic, muconic, fumaric, maleic were identified, but they disappeared completely after 60 min of ozone treatment. The presence of some aliphatic alcohols originated from the partial hydrolysis of nonylphenol ethoxylated (NPE) compounds, which are widely used in textile mills as dispersants, humectants and emulsifiers are observed in the raw textile wastewater. It should also be noted here that GC-MS methods are not appropriate for the detection of polar or large molecular weight compounds. Thus, it is very probable that some polar degradation products were generated which could not be detected with the analytical method used. In spite of this fact, the bacterial luminescence inhibition test showed a significant toxicity reduction (decrease in the EC₅₀ values from 3.4 % to 28.6 % based on volumetric

dilution ratios) upon comparing the untreated and ozonated textile wastewater’s toxicity. In conclusion, ozonation pretreatment of textile industry wastewater is an important step in terms of improving wastewater biodegradability as well as reducing acute toxicity, which should be removed completely via sequential biological treatment.

2.5.6 Homogenous and heterogeneous Fenton’s processes