Electrodialysis/Reverse Osmosis to Recover Dissolved Organics from Seawater Peter H. Pfromm, Tarl Vetter Department of Chemical Engineering, Kansas State University Manhattan, Kansas E. Michael Perdue, Ellery Ingall, Jean-François Koprivnjak School of Earth and Atmospheric Sciences Georgia Institute of Technology Atlanta, Georgia

Overview Introduction and Motivation Electrodialysis Reverse Osmosis Combined Process Process Characterization Experiments/Results Conclusions and Outlook

DOC ~ 1ppm Units: Gigatons C, GtC/yr (1 GtC= 109 tons of carbon) Source: NASA http://earthobservatory.nasa.gov/Library/CarbonCycle

Earth scientists would like to know: Composition of carbon reservoirs Origin/fate of carbon reservoirs The problem with DOC in the oceans: Only 1 gram of carbon in 1000 liters of seawater...... Salt The approach: Engineers and scientists collaborate Develop a new separation approach

What is marine dissolved organic carbon (DOC)? ~30wt% of DOC is “high molecular weight” (HMW) >1000 Da ~70wt% of DOC is “low molecular weight” (LMW) <1000 Da DOC is composed of many types of molecules, examples: Aminosugars Aromatics Polysaccharides Humic Species

How do you detect marine dissolved organic carbon (DOC)? Not a trivial issue: reasonably accurate part-per-billion level analysis for organic carbon in a high-salt (chloride) matrix. Shimadzu TOC-VCSN high-temperature catalytic oxidation analyzer Sample is acidified to remove inorganic carbon, then combusted over Pt catalyst and CO2 is detected by infrared Many papers, book chapters, and meetings are dedicated to this issue. Perdue at Georgia Tech is one of the well known experts on this.

The issue: recover pure DOC for scientific analysis. The problem: salt Water Process ? Solid DOC sample Salt 35 g/L salts ~ 0.001 g/L=1 ppm DOC

State of the Art Recovery ? Ultrafiltration Adsorption Methods Seawater Tangential Flow Porous Non-polar Resin Salt ~30% DOC Pore Resin Columns Water Salt LMW DOC 100-300 μm Seawater with remaining DOC Only recovers High Molecular Weight DOC (>1000 Da) Salt still present in final sample Only recovers select species (humic, etc) Must use pH or other method to desorb

New Approach: Water Reverse Osmosis Process Electro-dialysis RO removes fresh water concentrating salt and DOC Water Reverse Osmosis Electro-dialysis Process Freeze Dry Solid DOC sample Salt 35 g/L salts ~ 1 ppm DOC ED removes salt with minimal loss of uncharged species

RO ED The Processes Electrodialysis Reverse Osmosis

Electrodialysis Spacers and Membranes Astom AMX/CMX

Electrodialysis - + - - Concentrate + C A Diluate/Feed Diluate Return Na+ Cl- Diluate/Feed Diluate Return Cathode Anode Concentrate Return + C A - Na+ Cl- Diluate/Feed Diluate Return Cathode Anode Concentrate Return + -

Electrodialysis Characterization Limiting Current (Ilim) Limiting Current (Amps) Temperature: 25°C

RO ED The Processes Electrodialysis Reverse Osmosis

Reverse Osmosis ~ Pure Water High Pressure Higher Concentration Retentate Flow Polyamide barrier 0.2 μm Water Salt Water Microporous polysulfone 40 μm Polyester fabric 120 μm Salt Water Discarded Permeate Low Concentration Feed http://www.dow.com/PublishedLiterature/

Spiral Wound RO Module http://www.purewaterplanet.com/images/ROMembrane.jpg

Reverse Osmosis Characterization Permeate Retentate High Feed Flow Rate Low Feed Flow Rate Small Stage Cut Large Stage Cut

Combined Process Operation Unit Electrodialysis Stack

Overall Retrieve seawater sample (200- 400 l) Drive to site Purge ED/RO systems ED/RO 200 l seawater Freeze ~10 l Freeze dry NMR.... Hope for good weather!

Experimentation

Examples: Three shipboard experiments Start with 200 liter seawater

ED: follow the limiting current density

Summary Brackish Seawater 103 7 15 6 3 14 21 26 24 15 6 21 21 2 17 Lab 5-26 6-8 7-3 7-18 7-19 7-20 7-21 7-22 7-24 7-25 7-26 7-27 8-14 8-18 103 7 6 15 Final DOC ppm 3 14 21 26 24 15 6 21 21 2 17 Brackish Seawater Lab

Conclusions ED/RO can recover a significant fraction of DOC from seawater (60%-90%) The process is fast, allowing treatment of large volumes of samples We are able to reduce salt concentration and water volume to make a sample ready for freeze drying Preliminary results by NMR: differences from the high MW fraction that was previously available. Scientists and engineers think differently but can communicate and collaborate successfully

Outlook Examine the impact of temperature Further minimize losses to the ED concentrate, possibly with different membranes Examine modulation of the ED current to optimize DOC recovery Applications for recovery of sensitive molecules (proteins, enzymes)?

Acknowledgements This work is supported by the National Science Foundation, Grants No. 0425624 and 0425603. (Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of NSF) Dr. Mary Rezac who initiated the contact between scientists and engineers that made this work possible. Poulomi Sannigrahi for help at sea and in the laboratory. We would especially like to thank Captain Raymond Sweatte and the excellent crew of the R/V Savannah for two great and productive cruises.