Combined process for removal of tetracycline antibiotic – Coupling pre-treatment with a nickel-modified graphite felt electrode and a biological treatment

• Combination of an electrochemical pretreatment and a biological process for tetracycline removal.
• A graphite felt electrode modified by nickel electro-deposition was used.
• BOD5/COD increased from 0 before treatment to 0.46 after tetracycline electrolysis at 0.7 V/SCE.
• Non-pretreated tetracycline was not biodegraded by activated sludge but only biosorbed.
• 69% mineralization was obtained during the combined process (54% during activated sludge cultures).

Biodegradability improvement of tetracycline-containing solutions after an electrochemical pre-treatment was examined. Cyclic voltammetry with a nickel electrode revealed a significant electrochemical activity of tetracycline, in both oxidation and reduction. Electrochemical treatment was therefore performed in a home-made flow cell using a nickel-modified graphite felt electrode as the working electrode. Optimal conditions, namely 100 mg l−1 initial tetracycline, above 0.45 V potential, and between 1 and 6 ml min−1 flow rate, led to a more than 99% conversion yield of tetracycline in oxidation in alkaline conditions, after only a single pass through the percolation cell. However, total organic carbon (TOC) analyses revealed a low mineralization level, i.e., always below 31%, underscoring the importance of a combined electrochemical and biological treatment. This was confirmed by the favorable trends of the COD/TOC ratio, decreasing from 2.7 to 1.9, and the average oxidation state, increasing from 0.044 to 1.15, before and after oxidation pretreatment at 0.7 V and 3 ml min−1 flow rate. Electrolyzed solutions appeared biodegradable, since BOD5/COD increased from 0 to 0.46 for untreated and pretreated TC at 0.7 V/SCE. Biological treatment showed only biosorption for non-pretreated tetracycline, while after 11.5 days of culture, the mineralization of solutions electrolyzed in oxidation was 54%, leading to a 69% overall TOC decrease during the combined process.