Removal of Mercury from Chemical Wastewater by Microoganisms in Technical Scale IRENE WAGNER-DO ¨ BLER,* ,† HARALD VON CANSTEIN, † YING LI, ‡ KENNETH N. TIMMIS, † AND WOLF-DIETER DECKWER ‡ GBF (National Research Centre for Biotechnology), Division of Microbiology and Division of Biochem ical Engineering, Mascheroder Weg 1, D-38124 Braunschweig, Germ any The enzymatic reduction of Hg(II) to water insoluble Hg(0) by mercury resistant bacteria has been used for removal of mercury from wastewater in technical scale.Pure cultures of seven mercury resistant strains of Pseudomonas were immobilized on carrier material inside a 700 L packed bed bioreactor. Neutralized chloralkali electrolysis wastewater with a mercury concentration of 3-10 mg/L was continuously fed into the bioreactor (0.7 m 3 /h up to 1.2 m 3 /h). A mercury retention efficiency of 97% was obtained within 10 h of inoculation of the bioreactor. At optimum performance, bioreactor outflow concentrations were below 50 μg Hg/L, which fulfill the discharge limit for industrial wastewater. In combination with an activated carbon filter, outflow concentrations below 10 μg Hg/L were always obtained. The retention efficiency of the bioreactor was not affected by fluctuations in inflow conductivity (between 20 and 105 mS/cm), pH (between 6.5 and 7.5), or mercury concentration (between 3 and 10 mg/L) and was between 95% and 99%. Temperature increases up to 47 °C did not impair bioreactor performance. Standby periods up to 6 h could be tolerated without loss in activity. A simple, effective, and robust biotechnology for remediation of mercury polluted wastewater is thus demonstrated. Introduction Mercury pollution of the environment by mining activities and industrial wastewaters has resulted in worldwide con- tamination of large areas of soils and sediments (1-4) and led to elevated atmospheric mercury levels (5). Because of a lack of suitable cleanup technologies, efforts to deal with polluted sites are directed toward the mechanical removal of contaminated material and its deposition elsewhere (6, 7).Such processes are costlyand often result in remobilization of toxic mercury compounds during the dredging process (8). Mercury is one of the most toxic elements. It binds to the sulfhydryl groups of enzymes and proteins, thereby inacti- vating vital cell functions (9). After discharge into the environment,mercuryentersthe sedimentswhere it persists for many decades. It is taken up by aquatic organisms in the form of highly toxic methylmercury and is subsequently biomagnified through the food chain. The health of top predators, e.g. birds, fish, seals, and man, is thereby threatened (10, 11). Mercury poisoning results in severe chronic disease or death (12). Therefore, the discharge of mercury into the environment needs to be prevented by efficient and cost-effective end-of-pipe treatment technolo- gies for mercury emitting industries. One of the core products of the chemical industry is chlorine gas, with a production of 9.2 million tons in Europe in 1998. Today, 60% of the European chlorine production result from chloralkali electrolysis by the socalled amalgam process (13), which is based on electrolytic dissociation of concentrated sodium or potassium chloride solutions with mercury as the cathode of the electrolysis cell. Wastewaters from this process are characterized byhigh mercuryand salt concentrations.Current treatment procedures result in large volumes ofmercury contaminated sludge, e.g. precipitation by hydrogen sulfide, or are expensive, e.g. ion exchange columns. Mercuryresistant bacteria are widespread in nature.Such bacteria reduce soluble Hg(II) to insoluble metallic Hg(0) by means of the cytoplasmic enzyme mercuric reductase, encoded bythe m erA gene (14, 15).Habitats containinghigh levels ofmercuryexist since ancient times (16),and therefore this microbial detoxification mechanism is thought to be a veryold one (17).Here,we exploit it to remove mercuryfrom chloralkali electrolysis wastewater and retain it within a bioreactor. Mercury reduced by the bacteria accumulates in the form of small droplets of metallic mercury within the microbialbiofilm (18),from which it can ultimatelybe eluted and recycled back into the process. We have previouslyshown that laboratorymodelreactors are capable ofremoving90-97%ofmercuryfrom a chloralkali factory wastewater and that the mercury and salt concen- tations commonly encountered in such wastewaters are not inhibitoryto microbialmercuryremoval (19).However,good performance in the laboratory does not guarantee similar efficiency in large scale under process conditions, since important parameters are carefully controlled in the labora- tory and fluctuations are usually avoided. To be applicable to an industrialcleanup problem,the microbiologicalprocess must be reliably scaled up without loss in efficiency. Moreover, it needs to be robust with regard to fluctuations in wastewater parameters typically encountered on-site a factory. Here, we describe the inoculation and operation of a pilot plant developed based on laboratory columns described previously (19). The pilot plant was capable of treating about 50% of the maximum amount of wastewater produced at the chloralkali factory which served as the first testing site. We show here the effect of large fluctuations in mercuryconcentration,salt concentration,temperature,and pH on microbial mercury retention and demonstrate the utility of the new microbiological treatment technology for the cleanup of mercury polluted wastewater at a technical scale. Experimental Section Design and Operation of Pilot Plant. The pilot plant for continuous microbial mercury removal from chloralkali electrolysis wastewater is schematically shown in Figure 1. The pH of the incoming wastewater was neutralized to pH 7.0 ( 0.5 by addition of NaOH (20% w/ w). This was done in two steps and regulated by an adaptive controller. For 1 m 3 ofwastewater,appr.1LofNaOH wasneeded.Subsequently, medium was added to a final concentration of 50 mg/L sucrose and 10 mg/L yeast extract (300 mL of a medium *Corresponding author phone: +49 531 6181 408; fax: +49 531 6181 411; e-mail: iwd@gbf.de. † Division of Microbiology. ‡ Division of Biochemical Engineering. Environ. Sci. Technol. 2000, 34, 4628-4634 4628 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 21, 2000 10.1021/es0000652 CCC: $19.00  2000 American Chemical Society Published on Web 10/06/2000