Received: 30 November, 2010. Accepted: 31 March, 2011. Original Research Paper Dynamic Biochemistry, Process Biotechnology and Molecular Biology ©2011 Global Science Books Packed Bed Reactor Model for De-mercurization of Simulated Mercury-Laden Wastewater Sanjukta Ghoshal • Pinaki Bhattacharya • Ranjana Chowdhury * Chemical Engineering Department, Jadavpur University, Kolkata-700 032, India Corresponding author: * ranjana.juchem@gmail.com ABSTRACT Microbial reduction of soluble bivalent mercury to its less toxic elemental metallic form was performed using mercury-resistant Bacillus cereus (JUBT1) isolated from the sludge of chloralkali industries. A lab-scale 1 m long and 0.05 m diameter packed bed biofilm reactor was designed to remove mercuric ions (Hg 2+ ) using isolated bacterial consortium. The bioreactor was continuously fed with sterile simulated wastewater containing HgCl 2 to reduce bivalent mercury to its elemental form by growing bacterial biofilm on porous packing material of the reactor. The performance efficiency of the reactor was studied varying different chemical and hydrodynamic parameters such as inlet concentration of mercury and inlet flow rate of simulated mercury-laden water, among others. The reactor was followed by an activated carbon-based adsorber to remove residual mercury in the reactor effluent. A maximum of 97-98% removal efficiency was obtained with respect to the concentration of Hg 2+ in the inlet water. A deterministic mathematical model was developed to explain the performance of the packed bed reactor. _____________________________________________________________________________________________________________ Keywords: activated carbon filter, biofilm reactor, hydrodynamic parameters, mathematical model, mercury-resistant Bacillus cereus (JUBT1) Abbreviations: See Appendix. INTRODUCTION Mercury (Hg), a highly hazardous heavy metal, is found to be present in appreciable quantity in the solid waste of caustic chlorine industries. Hg is also present in small or appreciable quantity in the industrial effluent of battery industries, goldsmith industries and industries for medical instruments. Due to its extensive applications, wide spread distribution, high toxicity and bio-magnification (Canstein et al. 1999; Okoronkwo et al. 2006), Hg poses a great threat towards environmental pollution. Hg is capable of binding with sulfhydryl, thioester and immidazole groups in its bivalent cationic form and thus inactivates the enzyme function (Horn et al. 1993; Okino 2000) causing absolute system disorder. Use of microorganisms for Hg removal is a very effective technique for the treatment of industrial wastewater (Essa et al. 2005; Ruiz 2005; Sorkhoh et al. 2010). The present techniques to abate Hg pollution are highly energy intensive and thus the process, when used for large scale Hg separation, becomes highly expensive. Ap- plication of bioremediation of contaminated soil is presently gaining a widespread application because of its low energy requirement, low cost (Oehmen et al. 2009; Mathivanan et al. 2010) as well as appreciably high purification efficiency. Bacteria containing Hg–resistance (mer) determinants are capable of reducing mercuric ions. Resistance to bivalent Hg ions in bacteria is conferred by the NADPH (Reduced Nicotinamide Adenine Dinucleotide Phosphate) dependent mercuric reductase, a cytoplasmic flavoenzyme (Horn et al. 1993; Deckwer et al. 1999; Brown et al. 2002; Schneider and Deckwer 2005; Kiyono and Pan-Hou 2006). Studies on biotransformation of Hg contaminated industrial effluent are now being carried out by different research groups with the objectives of transforming soluble bivalent Hg to its less toxic elemental metallic form which can be conveniently separated using conventional downstream processing. Kan- nan and Krishnamoorthy (2006) isolated Hg-resistant Bacil- lus cereus in one of their studies and found that the bacteria efficiently reduce Hg to its elemental form. It is now well established that Hg-resistant bacteria are able to perform this biotransformation with significantly high rate under controlled environmental conditions. Jayasankar et al. (2008) observed that several marine bacteria were highly resistant to Hg and were capable of detoxifying Hg along with some other heavy metals (De and Ramaiah 2007). Although much is known on the isolation, identification and culture medium formulation for Hg-resistant bacteria, only little attention has been paid on the understanding of growth kinetics of Hg-resistant cell as well as bioprocess engineer- ing behaviour of the cells in different contacting devices in order to achieve favourable product distribution. In the present investigation growth kinetics of the iso- lated strain (identified by 16S r DNA technology) of Hg- resistant bacteria namely, B. cereus (JUBT1) has been in- vestigated. A biofilm reactor using immobilized form of the isolated strain has been operated to remove bivalent mercu- ric ions from simulated wastewater. A deterministic mathe- matical model has been developed for the biofilm reactor to predict its performance under the influence of different parameters like inlet concentration of mercuric ions and volumetric flow rate of the feed solution etc. MATERIALS AND METHODS Bacterial strain Hg-resistant bacteria have been isolated from the sludge of chlor- alkali industry initially as a mixed culture. A pure monoculture was purified from the mixed culture using conventional microbio- logical techniques. The morphology and genes of the isolated bac- teria have been established using microbiological, biochemical and 16S-rDNA (ribosomal deoxyribonucleic acid) techniques (Genei, Bangalore). ®