J. Indian Inst. Sci., Sept.–Oct. 2006, 86, 507–514 © Indian Institute of Science. *Author for correspondence. Degradation of azo dye with horse radish peroxidase (HRP) VASANTHA LAXMI MADDHINNI 1, *, HIMA BINDU VURIMINDI 1 AND ANJANEYULU YERRAMILLI 2 1 Center for Environment, Institute of Science and Technology, Jawaharlal Nehru Technological University, Kukatpally, Hyderabad 500 072, India. 2 Trent Lott Geospatial Visualization Centre, and Environmental Sciences, Jackson State University, Mississippi, MS 39217, USA. email: vasu_madineni@yahoo.com; Phone and Fax: +91-40-2315 6133. Received on June 30, 2006; Revised on October 17, 2006. Abstract Azo dyes are recalcitrant carcinogenic compounds and have dermal and immunological effect on human beings. Conventional methods are not effective in the treatment of azo dyes. The oxidation capability of horse radish per- oxidase (HRP) and hydrogen peroxide (H 2 O 2 ) on direct yellow 12 dye was investigated and was found to be very effective treatment methodology. HRP was extracted from horse radish roots, and its performance was evaluated in soluble and immobilized form by conducting batch experiments in the presence of H 2 O 2 . The oxidation of direct yel- low-12 dye was tested as a function of HRP at fixed concentration of H 2 O 2 , and at constant HRP activity (1.8 units/ml). Parameters such as aqueous-phase pH, H 2 O 2 concentration, dye and enzyme concentrations were studied. Keywords: Direct yellow-12 dye, horse radish peroxidase, hydrogen peroxide, immobilization, oxidation. 1. Introduction Among the chemical classes of dyes, azo dyes are considered to be recalcitrant, non- biodegradable and persistent. Treatment of dye-based effluents is considered to be one of the challenging tasks in environmental fraternity. Dyes are complex aromatic compounds, which are normally used for coloration of various substrates like leather, textiles, papers, etc. They are sometimes fused with heavy metals on the structural interface and are consid- ered to have relatively bad consequence on the surrounding environment due to their toxic and inhibitory nature [1–3]. Dye-based effluents are normally not amenable for conven- tional biological wastewater treatment due to their recalcitrant and inhibitory nature [4]. Azo dyes are largely resistant to biodegradation and persist in conventional wastewater treatment processes. Chemical contamination of soil and groundwater is widespread and frequent [5]. The electron-withdrawing nature of the azo linkages obstructs the susceptibil- ity of azo dye molecules to oxidative reactions. Only specialized azo dye-reducing enzymes were found to degrade the dyes [6]. 1.1 Bioremediation is a viable tool for restoration of contaminated subsurface environments A combination of enzymatic catalysis and electrochemical generation of hydrogen peroxide (H 2 O 2 ), which can be a potential alternative to traditional processes, was developed. The