Removal of Hydrolyzable and Condensed Tannins from Aqueous Solutions by Electrocoagulation Process

Vegetable tanning effluents have very high and difficult-to-treat chemical oxygen demand (COD). The main environmental damage is due to the low biodegradability of tannins, that can create environmental problems. Tannins are recalcitrant molecules and resist microbial attack; they are toxic to a variety of microorganisms. In this work, electrocoagulation with aluminum electrodes in a batch reactor was applied to remove polyphenolic compounds of vegetable tannins from aqueous solutions. Two types of commercial tannin extracts were used: chestnut, as a representative of hydrolyzable tannins, and mimosa, as a typical condensed tannin. The effects of operating parameters on the efficiency of electrocoagulation—current density, pH, ionic strength, interelectrode distance (IEA), electrolysis time, concentration of tannin extracts, and the like—have been investigated. It has been shown that the removal efficiency of tannin polyphenols increased with the increasing applied current density and increasing ion strength. The optimum current density ($J$) was $47.6\mathrm{A}/{\mathrm{m}}^2$ for chestnut and $71.4\mathrm{A}/{\mathrm{m}}^2$ for mimosa at $\mathrm{1,000}\mathrm{mg}/\mathrm{L}$. Within these values, 97.4 and 98.8% of tannin polyphenols can be removed, respectively. On the other hand, the most effective removal capacity can be achieved at normal pH (without correction) of each tannin solution in the optimum range. However, the removal efficiency of vegetable tannins from water decreases at more acidic and alkaline pH. The addition of NaCl to the solution helps increase removal efficiency and could save power consumption significantly. The optimal electrode distance is determined to be 2 cm for the electrolysis cell employed. The optimal time for achieving maximum removal of polyphenols increases with increasing tannin concentration. Nevertheless, it can be reduced by increasing the applied current.