Reaction Engineering Aspects of Microbial Mercury Removal

Abstract

Mercury‐resistant microorganisms are widespread in natural environments and can effectively be used to demercurize Hg(II)‐contaminated wastewaters as was already demonstrated on an industrial scale. The aim of this paper is to find the performance limits with regard to Hg(II) loadings D cHg,in (dilution rate × Hg(II) inlet concentration) and residual Hg(II) at the reactor outlet and to provide a reasonable basis for an optimal and safe process design. To this end, comprehensive studies were carried out with different single microbes (natural isolates and a genetically engineered strain) as well as microbial consortia in batch and continuous stirred reactors and fixed beds with microorganisms immobilized as films. The rate of the biotransformation (reduction of inorganically and organically bound Hg(II) to elemental Hg(0)) was found to follow a uniform mechanism with inhibition kinetics (Haldane type). Both reactor types are able to cope with high Hg(II) loadings and yield conversions up to 98 %. The stirred vessel is particularly suited for high cHg,in but restricted to low D (D < μmax), while the fixed bed can be operated at high D, say 10 h–1, but can only deal with cHg,in < 10 mg/L due to the limited Hg(II) tolerance of microorganisms. The loading limitations can be removed by appropriate recycle flows for both reactor types. However, irrespective of reactor type used, the residual Hg at the outlet cannot be reduced below the legal discharge limit (50 μg/L) mainly owing to the adsorption of Hg(II) on biomass. Therefore, a separation step following the reactor is required (sand bed, activated carbon filter). Comparing the reactor types exhibits the superiority of the fixed bed system due to its simpler construction, easier operation and higher cost effectiveness. Furthermore, the fixed bed shows better flexibility and robustness to extreme loadings. This justifies a posteriori the choice of a fixed bed reactor applied in the technical process.

Functioning of the mercury resistance operon at extremely high Hg(II) loads in a chemostat: A proteome analysis

“…These adsorption results are in agreement with those found with another P. putida strain (Chang and Hong, 1994). Analysis of effluent streams from a 1 m³ fixed bed bioreactor also showed that dissolved elemental mercury concentrations in the effluent were very low (< 5%), while 60-80% of the mercury was bound to biomass (Leonhäuser et al, 2006).…”

Biosorption of Cr(VI) by three different bacterial species supported on granular activated carbon—A comparative study

“…The binding of metal ions with supported biofilms is relayed by several authors: Quintelas and Tavares [12,13] used a biofilm of Arthrobacter viscosus to remove Cr(VI), Cd(II), Pb(II) and Fe(II), Kang et al [14] applied a biofilm of Pseudomonas aeruginosa to the removal of Cr(III), Ni(II) and Co(II), Leonhauser et al [15] studied the behaviour of a biofilm of A. hydrophila and P. putida in mercury removal. The bacterial extracellular polymeric substances (EPS) synthesis is an aspect of special relevance in the development of biofilms.…”

Model‐based optimization of the feeding policy of a fluidized bed bioreactor for mercury uptake by immobilized Pseudomonas putida cells

“…[20] However, other significant sources of pollution can be also mentioned, such as the use of mercury as cathode in the chloralkali industrial electrolysis at large scale leading to significant mercury emissions (ca. 1 g Hg/t chlorine [21] ;), and highly polluted wastewater (up to 7.6 mg L À1 , [22] ). Mercury is also used in numerous industrial and medical applications (fungicides, disinfectants, dental products, catalysts, igniters, dyes production, etc.).…”