Removal of azo dyes with alginate: relationship between dye structure and removal efficiency

The azo dyes represent between 60-75% of the total of dyes produced nowadays. They are widely used in cosmetic industry, paper, optoelectronic, textile, etc. Unfortunately due to their high resistance to the conventional treatments applied to wastewaters, they pollute the water bodies where are discharged. In this report, the sodium alginate (ALG) is evaluated for the removal of the azo dyes, Direct red 80 (DR-80), Congo red (Rcongo) and Methyl red (Rmetilo). UV-visible, Fourier transform infrared spectroscopy (Ftir) and Raman spectroscopic studies helped to elucidate the formation of tautomers, species responsible for their removal. The removal percentages were: DR-80 (99.9% at ph = 12 and ionic strength = 0.1 M), Rcongo (99.7% at ph = 12 and ionic strength = 0.1 M), and Rmetilo (14.9% at ph = 7 and ionic strength = 0.1 M). The Zimm-Bragg model described adequately the experimental isotherms, suggesting the importance of dye aggregation in their removal efficiency. Additionally, the removal efficiency of seven azo dyes was compared and it was found that the highest removal efficiencies are achieved when dye molecules exhibit high molecular weight, planarity and are positively charged. This allows to predict in a qualitative way, from the azo dye structures, which of them will be removed more efficiently with ALG. (Author)