Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 80:1188–1197 (2005) DOI: 10.1002/jctb.1314 Removal of tannins and polyhydroxy phenols by electro-chemical techniques M Murugananthan, G Bhaskar Raju ∗ and S Prabhakar National Metallurgical Laboratory (Madras Centre), CSIR Madras Complex, Taramani, Chennai-600113, India Abstract: The removal of tannins and their basic molecules such as catechol, pyrogallol, phloroglucinol and resorcinol was studied by electro-flotation and electro-oxidation techniques. Iron rods and triple oxide (TaO 2 /RuO 2 /IrO 2 ) coated titanium rods were used as electrodes for electro-flotation and electro- oxidation respectively. Interaction of tannins and polyhydroxy phenols with iron oxy-hydroxide formed by anodic dissolution of iron was studied by UV-Visible, FT-IR and zeta-potential measurements. GC–MS was used to identify the intermediate compounds formed during electro-oxidation. Catechol, pyrogallol, phloroglucinol and resorcinol were found to be mineralized in the presence of triple oxide (TaO 2 /RuO 2 /IrO 2 ) coated titanium electrodes. Catechol and pyrogallol were found to form insoluble organo-metal complexes while resorcinol and phloroglucinol were found to adsorb on iron oxy-hydroxide. The sludge thus formed was simultaneously separated from the aqueous phase by hydrogen bubbles. It was observed that the power consumption is relatively high for electro-oxidation compared with electro- flotation. The effects of pH and reaction time on their removal were also studied and the results are discussed.  2005 Society of Chemical Industry Keywords: electro-oxidation; electro-flotation; tannins; catechol; pyrogallol; phloroglucinol; resorcinol INTRODUCTION Tannins are extensively used in the leather industry to convert hides and skins into leather. Skins are tanned to achieve the desired flexibility in the dried condition and to protect them from microbial attack or hydrolysis when moist. Tannins are defined as polyphenols with molecular weights ranging from 500 to 20 000 which are capable of precipitating gelatin, certain proteins and alkaloids. Depending on the structure and action towards hydrolytic agents, tannins are usually classified into two broad categories, namely hydrolysable and condensed tannins. Hydrolysable tannins are characterized by a sharp distribution of molecular weight around 2000. Condensed tannins are characterized by a wide molecular weight distribution, a flavonoid structure with mainly phenolic reactive groups, and polymerization occurring under the action of acids to yield amorphous tannin red. Tannins are used as antioxidants and stabilizers and also as depressants in mineral-processing operations. Purification of tannery wastewaters containing tannins, sulfonates, ethoxylates, fatty acids, dyes, proteins and soluble carbohydrates has long been a problem owing to their toxicity. Extensive exposure of skin to hydroxy phenols may cause discoloration, local irritation, eczema or even death due to absorption. Hence it is important to remove these tannin compounds from wastewater. In recent years, electro-chemical techniques have been applied both in the purification of drinking water and in the treatment of various industrial effluents that are not easily biodegradable. Electro-coagulation 1 was suggested for potable water production wherein the Mo content was decreased from 9.95 to 0.006 ppm and Fe from 130 to 0.015 ppm. Nearly 99% of suspended solids were removed from surface water by adopting a three-stage process comprising electro- coagulation, flotation and micro-filtration. 2 Electro- coagulation and flotation techniques were reported for the separation of oil from emulsions, 3 the purification of urban and restaurant wastewater, 4 and textile wastewater, 5 the removal of latex particles, 6 metal ions 7 and sulfur species 8 from aqueous suspensions and for the treatment of industrial liquid waste. 9 Szpyrkowicz et al have suggested the use of Ti/Pt/Ir as anode for the removal of nitrogen from industrial wastewater. 10 Mraz and Krysa 11 have attempted to improve the efficiency of the process and the life of the electrodes by incorporating surface modifications to the electrodes. Electro-chemical oxidation was extensively studied for the treatment of wastewaters containing phenolic compounds. 12 – 15 The present study was aimed at understanding the mechanism of tannins removal from tannery effluents by electro- flotation and electro-oxidation. Accordingly, pure compounds of resorcinol, pyrogallol, catechol and ∗ Correspondence to: G Bhaskar Raju, National Metallurgical Laboratory (Madras Centre), CSIR Madras Complex, Taramani, Chennai- 600113, India E-mail: nmlmc@vsnl.com Contract/grant sponsor: CSIR; contract/grant number: SMM-0010 (Received 11 September 2004; revised version received 20 January 2005; accepted 22 February 2005) Published online 10 May 2005  2005 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2005/$30.00 1188