The continuous release of organic pollutants from industrial units is deteriorating the quality of aquatic ecosystems. The widespread use of chlorinated organics in the industrial and manufacturing processes has resulted in introduction of toxicity in the aquatic environment. The bleaching unit in textile industry and pulp & paper industry generates effluent loaded with chlorinated aromatic organics (Ahlborg et al. 1980). According to a report, production of one tonne of paper requires 100kg of color-imparting materials and around 2-5kg of organochlorides to the bleaching section resulting in the introduction of CPs in the wastewater generated (Nagarathnamma et al. 1999). CPs possess broad spectrum bactericidal properties, thus, find an application as preservatives in wood processing industry, paints, leather, vegetable fibres and as disinfectant. The TCP is one of the most commonly formed chlorinated compound during dechlorination of water too. These compounds are widely applied in the agricultural fields as herbicides, pesticides, and insecticides. Health organisations and environmental pollution agencies have recommended 0.1 µg/l and 200 µg/l as maximum permissible concentration of chlorophenols in drinking water and wastewater, respectively (ASTDR, 1999). Owing to their widespread use of CPs, residues of such compounds are continuously detected in soils, sediments, surface water, ground water and in the trophic levels of a food chain. There has been an increasing concern towards the adverse effects of chlorophenols towards environmental damage and human health (Mukherjee et al. 2022). CPs are categorised in the priority toxic pollutants listed by USEPA (ASTDR, 2007), and have the ability to bioaccumulate in organisms (de Souza et al. 2021). The toxicity of chlorophenols increases with increase in the number of chlorine atoms present at the ortho- and para- position and are more stable than chlorine present at meta- position. These compounds absorb light with wavelength less than 300nm (lesser fraction of in sunlight) resulting in environmental stability (Bandara et al. 2001). At the same time, conventional treatment methods fail to degrade the pollutant as biological treatment is inefficient towards decomposition due to complex structure of the pollutant. On the other hand, AOPs have emerged as a promising approach towards treatment of such recalcitrant organic pollutants (Yadav et al. 2022). The application of AOPs is reported successful in treatment of wastewater generated from pharmaceuticals (Verma and Haritash 2019), textile (Sharma et al. 2016; Pipil et al. 2021), pesticide, pulp and paper sector (Cameselle and Gouveia 2016) etc. The processes are governed on the principle of in-situ generation of highly reactive oxidising species in the form of hydroxyl radical (Verma and Haritash 2020). Because of the high reduction potential of OH• (2.8 eV), they have the ability to oxidize toxicants like chlorophenols to non-toxic forms as CO2 and H2O. Among AOPs, heterogeneous catalysis has gained popularity because of its inherent destructive potential towards complete mineralization of organic pollutants. TiO2 is the most common semiconductor used in the photocatalysis process (Aljuboury et al. 2016; Balazani et al. 2019). The high photocatalytic activity, high photochemical reactivity, high stability and lesser toxicity towards environment make it compatible for its use (Thirunavukkarasu et al. 2020). When irradiated with light source, it absorbs photons of energy equivalent or more than its bandgap width of 3.2 eV. Electrons from valence band are excited to conduction band leaving behind holes. Thus, formation of electron-hole pair takes place (Eq. 1). Electrons directly react with organic pollutant to form reduction products, whereas holes either react directly with organic compound or react with water to form hydroxyl radicals which carry out oxidation of harmful organics (Eq. 2–7) (Ahmad et al. 2016). The following reactions take place during the TiO2 based photocatalysis:
TiO2 e− + h+ (1)
e− + O → O2−• (2)
h+ + H2O → OH• + H+ (3)
h+ + OH− → OH• (4)
h+ + organic compound → end product (5)
OH• + organic compound → end product (6)
e− + organic compound → end product (7)
There have been several studies on removal of polychlorophenols using photocatalysis and Fenton’s treatment in-silo (Kusvuran et al. 2005; Pera-Titus et al. 2004), but the studies on degradation/mineralisation of TCP are very limited. The studies on MCPs and DCPs reported removal of the contaminants, but the studies on degradation of TCPs and the removal of toxicity is not targeted in most of the studies done so far. Since the toxicity of CPs is higher as Cl is attached at ortho- and para- position, the degradation of TCP in textile and pulp & paper industry effluent is pertinent.
Although the removal efficiency of photocatalytic oxidation is high, the chemical and energy input is also high. In order to minimise the cost of treatment, optimization of regulating/ input parameters should be performed. The present study dealt up with optimization of nano-TiO2, option of removing use of H2O2, and possibility of visible light excitation. In the present study, effort has been made to study the mineralisation (degradation) of TCP based on analysis of intermediates of degradation, if any, supported by the analysis of total organic carbon before and after the photocatalytic oxidation. Finally, the operational cost of treatment was evaluated for all the options tested for the treatment of TCP-contaminated wastewater.