Continuous treatment of an azo dye based on a combined ZVI/photo-Fenton setup. Process modelling by response surface methodology

Abstract

The performance of a continuous system that sequentially couples a column filled with zero-valent iron with a photo-Fenton reactor was evaluated under mild acidic conditions. Studies were conducted to evaluate the better coupling strategy between each treatment module and to analyze the effects of several operational parameters on the system behaviour. The azo dye Acid Black 1 was chosen as model pollutant. Decolourization degrees and decreases of TOC values were used for the evaluation of the treatment performance under different conditions; whereas steady state concentrations of Fe(II), Fe(III) and H2O2 together with spectral features in the UV–vis region were analysed to elucidate mechanistic issues. The response surface methodology approach based on a central composite design was used to evaluate and model the effects of key factors on the decolourization degrees. Results show that the spatial separation between the ZVI and H2O2 may lead to substantial improvements of treatment efficiencies provided that configurations involving recirculation loops are avoided. Besides, a strict and continuous pH control during the oxidative stage is critical for obtaining a stable operation and acceptable decolourization efficiencies through the coupled system. The simple parameters developed from UV–vis spectra allowed monitoring the extent of reductive and oxidative transformation pathways. In addition, the statistical analysis shows that reactor pH and hydraulic residence time are the key factors influencing the system response, whereas initial pH and added oxidant concentration are much less significant. The conclusions drawn are relevant from a technological viewpoint since allow a rational design of cost-effective treatments.