A review on the treatment of textile industry effluents through Fenton processes

Effluents stemming from the textile industry are concerning from the environmental viewpoint, given that large volumes are generated and they present a high concentration of organic matter and toxicity. Dyes stand out among the substances present in such effluents because part of them does not fix on the fiber during the dyeing process, leaving the effluent with a strongly colored aspect and difficult to be treated. Due to the limitations and low efficiency of conventional processes, alternative forms for the treatment of textile effluents have been evaluated. Among them, the Fenton oxidation processes stand out, which are based on the use of iron as a catalyst in the conversion of H2O2 into highly reactive free radicals. The catalyst may be solubilized or used in the insoluble form (heterogeneous catalysis). Fenton reaction can be enhanced by combining UV, visible (photo-Fenton, solar photo-Fenton), or ultrasound radiation (sono-Fenton), and electrical current (Electro-Fenton). In this review, a brief mention of textile effluent characteristics is presented, and the fundamentals, advantages, limitations, and recent progress of the Fenton processes are addressed as alternatives to the treatment of textile effluents. A survey on the efficiency of such processes regarding the reduction of important characterization parameters (organic matter concentration, color, turbidity) for real textile effluents also is presented. From the real effluent data, it was noted that these processes are efficient in the reduction of organic pollutants and color. Among the Fenton processes, photo-Fenton showed better efficiencies and less variability for the different effluents treated (reducing on average 82% of COD and 95% of color). It has also been observed that such processes can be also efficient when combined with other processes (e.g. coagulation-flocculation or biological processes). Treated effluents present less toxicity and have been reused in the dyeing stage, although such an approach is still poorly evaluated. The costs for the treatment of real effluents calculated by different authors ranged from 1.1 to 28.9 USD per Kg of COD removed, where the processes that presented the lowest costs were classical Fenton and technologies that use solar energy, such Photo-Fenton and Photo-electro-Fenton.