Removal of Cationic Heavy Metals from Drinking Water Supplies through the Ion Exchange Membrane Bioreactor

1. Removal of Cationic Heavy Metals from Drinking Water Supplies through the Ion Exchange Membrane Bioreactor Adrian Oehmen Universidade Nova de Lisboa Portugal

2. Heavy Metal Pollution in Waterways • Mercury is the heavy metal with the highest known toxicity. • Mercury is a bioaccumulative toxin that attacks the central nervous and endocrine systems. • Can cause brain damage • Mercury pollution enters water systems mainly through rainfall and effluents from industrial processes. • In water supplies, mercury exists primarily in the cationic form (Hg2+). • Maximum contaminant level for mercury in drinking water • 1-2 ppb (WHO, US EPA)

3. Ion Exchange Membrane Bioreactor (IEMB) • Combines the transport of an ionic pollutant (e.g. Hg2+) with its simultaneous bioconversion. • Hg2+ is transported through a cation exchange membrane at the expense of a harmless counterion (Na+). • Hg2+ is then converted to Hg0 via biological reduction, stripped from the liquid phase and recovered in the gas phase. • The IEMB concept has successfully been applied for the removal of anionic pollutants such as nitrate, perchlorate and arsenate from drinking water.

4. Cl- Cl- Na+ Na+ Na+ Na+ Hg2+ Hg2+ Hg2+ Transport and Bioreduction through the IEMB Stripping to Gas phase Hg0 Oxygen, carbon source, nutrients Biocompartment Water compartment Cation Exchange Membrane Biofilm

5. Advantages of the IEMB System • Promotes selective and efficient pollutant removal • The associated brine solution from membrane transport is treated • Provides a physical barrier between the polluted water and microbial cells, carbon sources. The initial water matrix is otherwise maintained largely intact. • Thus, prevents secondary contamination of drinking water • The treated water production rate does not depend on the hydraulic retention time of the biocompartment

6. Objectives of this Study • Selection of a suitable cation exchange membrane • Evaluate Hg2+ transport through 11 different commercially available cation exchange membranes by Donnan dialysis. • Biological Hg2+ removal using mixed microbial cultures • Investigate the effect of carbon source on the process performance and Hg2+ reduction kinetics. • Integrate these 2 processes in the ion exchange membrane bioreactor to achieve Hg2+ removal from drinking water

7. Cation Exchange Membrane Selection: Hg2+ Flux • Fumatech FKE membrane exhibited high flux, good mechanical stability and reasonable price

8. Microbial Hg Resistance Mechanism (Wagner-Döbler et al., 2003) • Hg2+ is converted to Hg0, via the MerA enzyme • Hg0 is then stripped from the liquid to the gas phase, and recovered through e.g. adsorption onto various materials

9. Dissolved Oxygen Meter Atomic Absorption Spectrometer pH Meter Filters Gold Trap Air Pump Heater SnCl2 solution (Catalyst) Gas/liquid separator Magnetic Stirrer HCl solution (Carrier) Gold Trap Filters waste N2 gas Mercury Measurement: Methodology Carbon source, nutrients, biomass, Hg2+ Gas-phase measurement N2 gas Gas Flow (with Hg0) Liquid-phase measurement

10. Biological mercury reduction Glucose culture Acetate culture • Two mixed cultures were enriched with Hg2+ reducing organisms using different carbon sources • Glucose • Acetate • Most of the Hg2+ was reduced to Hg0 and stripped to the gas phase

11. Biomass Growth of the Mixed Cultures Glucose culture Acetate culture • Hg2+ bioreduction and biomass growth were not simultaneous • Biomass growth commenced only after Hg was completely removed • Delay in growth of acetate culture • Glucose was partially converted to acetate and tended to accumulate

12. Carbon Source Swap Glucose culture Acetate culture • Glucose culture • Hg2+ reduction rate much slower with acetate as carbon source • Acetate culture • Half-saturation coefficient (KHg) is substantially higher with acetate • Mercuric acetate complexes may be more difficult to biodegrade

13. Comparison of Culture/Carbon Source • Glucose was a more effective substrate for: • Culture selection • Bioreactor operation • Good Hg mass balance recovery was achieved (>85%)

14. IEMB Setup Membrane Water Effluent Air Pump Bio-Medium pH Meter Bio-Effluent Dissolved Oxygen Meter Water Feed Magnetic Stirrer Gold Trap Filters Bio Compartment Water Compartment Gas Flow (with Hg0)

15. IEMB operation • IEMB operated using the glucose mixed culture • Hg2+ removal >98% at an F/A ratio of 1.5 l/(m2h) • Hg also removed in the biocompartment (<10 ppb) F/A = 15 l/(m2h) F/A = 1.5 l/(m2h)

16. IEMB: Effect of Membrane Pre-Treatment • Membrane pre-treatment in HgCl2 increased Hg flux through the membrane

17. Conclusions • A suitable cation exchange membrane (Fumatech FKE) was selected for IEMB operation • Glucose was found to be a more favourable carbon source for the operation of mixed microbial cultures, in terms of: • Enriching an effective microbial community for Hg2+ bioreduction • Maximising the rate of Hg2+ bioreduction, minimsing the mercury half-saturation coefficient (KHg) • The integrated IEMB system was shown to be very effective in removing a high level of Hg (>98%) • Experimental study is currently ongoing to evaluate its potential at low Hg concentrations • Process applicable for the removal of other heavy metals with optimisation of the biocompartment (e.g. biosorption)

18. Acknowledgements • Co-authors:D. Vergel, J. Fradinho, J. L. Capelo, S. Velizarov, J. G. Crespo, M. A. M. Reis • The financial support by Fundação para a Ciência e a Tecnologia (FCT), Portugal through Project No. PPCDT/AMB/57356/2004 and postdoctoral research grant SFRH/BPD/20862/2004.