Optimization of the heterogeneous electro-Fenton process assisted by scrap zero-valent iron for treating textile wastewater: Assessment of toxicity and biodegradability

Abstract

In this work, the heterogeneous electro-Fenton process assisted by scrap zero-valent iron (SZVI) and with graphite electrodes (as anode and cathode) is studied as an alternative way to treat wastewater from a textile plant located in Medellin, Colombia. The effects of the initial pH, SZVI load, and current density in the SZVI-assisted electro-oxidation process (EO/SZVI) are analyzed and optimized using multivariable regression models generated using the Box-Behnken experimental design and convex nonlinear optimization. The EO/SZVI process leads to maximum reductions of approximately 100%, 67%, and 59% in color, chemical oxygen demand (COD), total organic carbon (TOC), respectively, under the optimal operating conditions of pH of 3.5. Further, it leads to an SZVI concentration of 0.6 g/L, and a current density of 20 mA/cm(2) during 30 min of electrolysis. The post-process pollutants are characterized further using molecular weight distribution measurements, ultraviolet-visible spectroscopy, high-performance liquid chromatography, biodegradability, and toxicity. The results show that the treated effluent is polluted mostly by carboxylic acids of low molecular weight. A remarkable enhancement of the biodegradability of the effluent is evidenced by an increase in the BOD (5)/COD ratio (biodegradability index) from 0.15 to 0.54. Although the EO/SZVI process does not improve the toxicity (as measured by the mortality of Artemia salina), the latter is reduced subsequently from 100% to 20% through adsorption using activated carbon (AC). The findings of this study indicate that the EO/SZVI process is an effective and promising alternative for treating textile wastewater.