Reuse of a treated textile effluent from cobalt oxide and sulphate radical-based advanced oxidation process

Abstract

Reactive dye waste effluents are the most difficult to treat, as they are highly polluted due to the structure of the dyestuffs and chemicals used during the dyeing process. Due to the water shortage and environmental pollution, textile industries are encouraged to treat the waste effluent produced during dyeing processes so as to facilitate its reuse, as this will contribute to mitigating environmental pollution and minimise water consumption. However, relatively few of the treatment technologies employed for the treatment of textile wastewater are applicable for water that is intended for reuse. Many treatment technologies exist for the treatment of textile waste effluents, but are either limited in efficiency or high in operating and energy cost. Chemical treatment methods such as the cobalt oxide mediated sulphate radical-based advanced oxidation process (CO-SR-AOP) shows promise but have not yet been evaluated for the reuse of textile wastewater in the dyeing process. The purpose of this work is to study the reusability of a treated reactive dye effluent obtained from dyeing cotton fabrics using peroxymonosulfate (PMS) activated by a cobalt oxide (Co3O4) catalyst and using a laboratory-scale continuous wastewater treatment reactor. In order to achieve this, a cobalt oxide catalyst was hydrothermally synthesised, cotton fabrics were bleached as pre-treatment prior to being dyed using blue reactive dye and tap water to produce the necessary textile waste effluent. The produced waste effluent was treated with Oxone (PMS) and a cobalt oxide catalyst; then reused in the next dyeing process, using an identical dyeing recipe. The pH of the treated effluent was corrected to neutral before its reuse. The waste effluent from the first cycle of dyeing was treated before its next reuse. This process was carried out for a maximum of three cycles. The dyed fabrics obtained using the treated effluent were compared with the ones dyed with tap water in terms of colour fastness. The optimisation of the reusability of a treated effluent from cobalt oxide and sulphate radical-based advanced oxidation process was carried out using Design-Expert software version 11.1.2.0 using a Box-Behnken design taken from response surface methodology. The effects of three factors were studied: Oxone level, dye concentration and reuse cycles at low, high and medium levels in fifteen experimental runs. Colour fastness of the dyed fabrics was studied as the response of the trials. Based on the preliminary results, the treated effluent can be reused in two successive reuse cycles without altering the fabric’s quality. To obtain more or less 80% colour removal, waste effluent with 3% dye concentration must be obtained and treated with a high dosage of Oxone (3.5 g/l). Salt can be recovered by using this process, but with a darker shade of dyed fabric as a result, when compared with the reference. Varying dilution factors and standing times of the treated effluent were investigated but did not have significant influence on the colour quality of the dyed fabrics. A useful model was found to predict the colour fastness of dyed fabrics with an effluent treated with the continuous wastewater treatment reactor system using PMS activated by Co 3 O 4 . The study of the interaction effects of all three parameters led to the finding that to obtain good colour fastness grading of the dyed fabrics, the treated effluent can be reused a maximum of two iterations, with a dye concentration of 5% and an Oxone concentration of 1 g/L. The predicted optimum process conditions for this process were 1.3 g/l of Oxone used to treat a waste effluent with 4.4% dye concentration and reuse in a maximum of three reuse cycles.