Decolourisation of metal complex azo dyes and treatment of a dyehouse waste by modified photo-Fenton (UV-vis/ferrioxalate/H₂O₂) process

Abstract

In a laboratory study, decolourisation of three metal complex azo dyes (Acidol Yellow, Acidol Grey and Acidol Scarlet) in mixture by the modified photo-Fenton (UV-vis/ferrioxalate/H₂O₂) process, and treatment of a dyehouse waste containing the three dyes were examined. From decolourisation of the dye mixture, process conditions for decolourisation were selected – <i style="">p</i>H 4, Fe(III) dose 12.5 mg/L, oxalic acid dose 50 mg/L, hydrogen peroxide dose 60 mg/L and irradiation time 90 min. Ninety-two percent decolourisation of the dye mixture (colour 120 SU) occurred in 90 min under irradiation with sunlight intensity in the range 0.5-0.8 kW/m². The process was found effective in decolourisation of highly coloured solutions. Modified photo-Fenton decolourisation of the synthetic dyehouse waste (<i style="">p</i>H 7.5, colour 120 SU, COD 260 mg O₂/L, BOD₅ 125 mg/L, turbidity 1.5 NTU, conductivity 1265 mmho/cm and total solids 875 mg/L) under selected process conditions and irradiation with sunlight or incandescent lamp, was also studied. Ninety-two percent decolourisation (residual colour ca 10 SU) was achieved under 90 min irradiation with sunlight (0.5-0.8 kW/m²) or incandescent lamp (6.5 kW/m²). A depth of 30 cm was found to be the maximum depth for decolourisation. The study has demonstrated that modified photo-Fenton is an effective process for decolourisation of dyehouse waste containing metal complex azo dyes. A scheme for oxidative treatment of the dyehouse waste was formulated and the treatment efficiency was evaluated – 90-92% or 90-92%, 69-70% or 63-73%, and 52-65% or 50-60% removal of colour, COD and BOD₅ were achieved under irradiation with sunlight or incandescent lamp.