Optimization of electro-Fenton process for the removal of non-biodegradable organic compounds in instant coffee production wastewater using composite Fe3O4–Mn3O4 nanoparticle catalyst

Abstract

Wastewater from instant coffee production has high organic content. The non-biodegradable compounds of the organic portion, such as caffeine, trigonelline, and diterpene, are difficult to be removed by conventional processes. Electro-Fenton (EF) with particulate catalyst has been found to be a promising method to treat such compounds. Comparing with homogenous Fenton process, EF uses fewer chemicals and generated less sludge. The catalysts can be easily recovered by magnets if they have magnetic properties. In this study, 30–60 nm composite Fe3O4–Mn3O4 nanoparticles were prepared and used as EF catalyst for the treatment of non-biodegradable organic matters in biologically pre-treated wastewater from instant coffee production. These particles were found to contain Fe3O4, γ-Fe2O3, and Mn3O4, which were effective metal sources for Fenton reactions. The correlations between three important operating parameters (i.e., pH, catalyst concentration, and current density) and the treatment efficiency of the non-biodegradable compounds were elucidated by response surface method (Box–Behnken design) via Modde software. An optimal condition was found at a pH of 3.8, a Fe3O4–Mn3O4 concentration of 0.5 g/L, and a current density of 19.6 mA/cm2. After 60 min of treatment at this condition, the highest removal efficiencies of chemical oxygen demand (COD), color, and total organic carbon in the wastewater were 88%, 98%, and 93%, respectively. With this degree of treatment, the COD and color in treated wastewater met the highest treatment level of Vietnam National Technical Regulation on Industrial Wastewater (QCVN 40:2011, column A, 75 mgO2/L for COD, and 50 Pt–Co for color).