TREATMENT OF HIGH STRENGTH HEXAMINE- CONTAINING WASTEWATER BY ELECTRO-FENTON METHOD SHANSHAN CHOU 12 *, YAO-HUI HUANG 1 , SHEN-NAN LEE 1* M , GAW-HAO HUANG 1 and CHIHPIN HUANG 2* M 1 Union Chemical Laboratories, Industrial Technology Research Institute, 321, Kuang Fu Road, Section 2, Hsinchu, Taiwan, R.O.C. and 2 Institute of Environmental Engineering, National Chiao Tung University, Hsinchu, Taiwan, R.O.C. (First received January 1998; accepted in revised form June 1998) AbstractÐA novel Electro-Fenton (EF±Fere) method, applied H 2 O 2 and electrogenerated ferrous ion, was investigated for treating the hexamine-containing wastewater. The performance of Fe 2+ generation in the electrolytic system was ®rst evaluated, including the factors of the cathode material, initial pH, initial ferric concentration (Fe i ), and current density. When initial pH exceeded 2.5, the current e- ciency dramatically decreased, which was due to the formation of Fe(OH) 3 . Between 3000 and 10,000 mg/l of Fe i , the initial current eciency of Fe 2+ generation was almost constant (85±87%), which dropped sharply to 39% at 1000 mg/l. In EF±Fere experiments, the COD removal eciency attained above 94% after 5 h of reaction. The relationship between the temperature, dissolved oxygen, and COD was discussed. The changes in hexamine and its oxidation intermediates (methanol, formalda- hyde, formate, ammonium and nitrate) during the reaction were also investigated. Three additional ex- periments using H 2 O 2 /Fe 2+ ,H 2 O 2 /Fe 3+ , and direct electrolysis were also conducted to treat the hexamine-containing wastewater for comparison. The results showed that the EF±Fere method was the most ecient. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐhydrogen peroxide, electrolysis, oxidation, hexamine, wastewater INTRODUCTION A hexamine-containing waste stream has been identi®ed as the most refractory in a petrochemical factory (factory A) (Huang et al., 1997). The COD concentration of this stream ranges from 15,000 to 40,000 mg/l. Hexamine, (CH 2 ) 6 N 4 , is a nonaromatic heterocyclic compound with a symmetrical three- dimensional molecular structure, as shown in Fig. 1. In acidic condition, it can be hydrolyzed to formal- dehyde and ammonia (Pitter and Chudoba, 1990). Smith and Colquhoun (1987) have con®rmed that hexamine is resistant to biodegradation, even when sludge has been acclimated for a long period. Advanced oxidation processes (AOPs) are con- sidered highly promising for treating refractory compounds. Common oxidants involved in AOPs are Fenton's reagent (Walling, 1975), ozone, ozone/ H 2 O 2 , and UV/H 2 O 2 , of which the primary inter- mediate is the hydroxyl radical (OH : ) (Huang et al., 1993). Fenton's reagent, a mixture of hydrogen peroxide and a ferrous salt, is an eective and simple oxidant of organic contaminants. It has been proven eective in treating various organic contami- nants such as dyes, ethers, nitrophenols, chlorinated phenols, aromatic amines, polycyclic aromatics, and photographic wastewater (Korenaga et al., 1989; Ewa, 1991; Sedlak and Andren, 1991; Haag and Yao, 1992; Kuo, 1992; Casero et al., 1997). However, the oxidation of nonaromatic heterocyclic compounds in this way was scarcely investigated (Korenaga et al., 1989). In addition, the application of Fenton's reagent in the destruction of organics has been limited by the slurry system, because ferric hydroxide sludge requires additional separation pro- cess and disposal. Several authors have recently reported the appli- cation of the electrochemical method in Fenton's reaction (electro-Fenton method). These studies can be generally divided into three groups. The ®rst group (EF±H 2 O 2 method) uses Fe 2+ and electro- generated H 2 O 2 , that can be produced from the two-electron reduction of sparged oxygen on graph- ite, reticulated vitreous carbon, or carbon-PTFE cathodes (Sudoh et al., 1986; Tzedakis et al., 1989; Hsiao and Nobe, 1993; Brillas et al., 1996). Its dis- advantage is the low current eciency in acidic con- dition (Do and Chen, 1993). The second group (EF±Feox method) utilizes H 2 O 2 and electrogener- ated Fe 2+ which is produced via the oxidation of iron, the sacri®cial anode (Pratap and Lemley, Wat. Res. Vol. 33, No. 3, pp. 751±759, 1999 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98/$ - see front matter PII: S0043-1354(98)00276-0 *Author to whom all correspondence should be addressed. [Tel.: +886-3-5732638; Fax: +886-3-5732349; E-mail: 790512@ucl.itri.org.tw]. 751