Don Phipps, of the Orange County Water District, asked if the kinetics of absorption were relatively rapid and also wondered if one could consider building or using this material in a pillow-type format that could be put in contact with the sediments. The material would take up PCBs slowly and after a period of time could be replaced and recycled.
Dr. Luthy responded that mats can be put down but he has not investigated this area. In the work that his group has done, modeling shows that there are kinetics of uptake by carbon and diffusion through pore water. When things are well mixed, the release from the particles is limiting. There is a rapid release from the mineral fraction, but there is also coal and other material there. It will transfer to the carbon, like a solid-phase diffusion process.
Mr. Phipps pointed out that it may not be really practical to have this diffusion over a large area, but in a fairly small region the contamination is by diffusion into the water. If contamination is over a large area it may be possible to build a renewable cap and then slowly recover it that way. It would be like digging up the sediments without actually digging them up.
Mark Matsumoto, of the University of California at Riverside, asked why only one concentration of carbon was used and if there is any way to determine the differences in the way the sediment and the activated carbon capture the contaminants..
Dr. Luthy said he started this work with support from DOD and some from Forbes, Stanford, and the Bioex Program. The first part of this work was to show that it is not harmful and that it is effective. For the starting dose of carbon, his group decided to double the total organic content (TOC) of the sediment.
In current work, they are halving and even going to a tenth of what they previously used. What the modeling shows is that if half this dose of carbon is set equal to the TOC, essentially the same results are obtained. When about a tenth of this dose is used with this sediment, higher concentrations result, approximately 1 percent by weight.
Steve Cabaniss, of the University of New Mexico, asked why the modeling shows that going to one-tenth will result in higher aqueous concentration.
Dr. Luthy replied that a higher concentration is being put on the carbon per unit weight. If it is sent to an absorption isotherm, higher concentrations of carbon yield higher aqueous concentrations. The capacity of carbon has not been exceeded, but more is put on the carbon per unit weight.
Jay Means, of Western Michigan University, asked if Dr. Luthy had looked at any change in the efficiency of transfer as a function of fouling with biological films.
Dr. Luthy responded that his group has done studies for as long as six months and found that fouling improved with time. There would be other issues if other materials in the sediment could absorb and displace PCBs. This carbon will probably pick up some humic molecules from the sediment and become sticky or gummy, slowing any further uptake as seen in water treatment. He said it takes a long time for this to appear in water treatment.