On the Fenton’s process for the treatment of effluents from the dyeing of agates containing Rhodamine B and ethanol

In the processing of agates, the gemstone pieces are submitted to dyeing processes. One of the dyeing processes applied is with Rhodamine B, which provides a reddish-pink colouration. The practice generates a wastewater containing residual amounts of the dye and ethylic alcohol with high polluting potential. Thus, the objective of this paper is to investigate the chemical parameters of the Fenton´s oxidation process to treat this effluent. The main parameters evaluated were [H2O2]:[Fe2+] molar ratio and the concentration of the reagents H2O2 and FeSO4·7H2O. The wastewater contained a concentration of 772 mg L−1 of Rhodamine B and 3% of ethanol in its composition, giving the effluent a high colour, organic load, and toxicity. In the Fenton reaction treatment, 11.1 g L−1 of ferrous sulphate and 20 mL L−1 of oxygen peroxide (in a molar ratio [H2O2]:[Fe2+] of 7.5:1) were defined as the best dosage, which allowed complete alcohol removal, an average absorbance reduction of 99.9% at 554 nm, a TOC reduction of 93.3%, an increase in surface tension from 58.9 to 64.4 mN m−1 and a toxicity factor (TF) decrease from 526 to 16 with respect to the organism Daphnia similis. The Fenton process combines oxidation, coagulation and air stripping mechanisms that substantially reduces pollutants present in this effluent, at a rougher stage. The results obtained are useful, both for the gemstone sector and others that make use of Rhodamine B dye.