Weathering, PCB

Field Evidence of Biochemical Weathering. PCB congener composition variation with depth in sediment was analyzed for evidence of bio-... 

One Submitter noted that the Tryphonas study used doses of unweathered Aroclor 1254, rather than doses of weathered PCBs, like those typically observed in the environment. Speculating that weathered PCBs are more toxic than the unweathered mixtures, this Submitter thought the MRL... 

At a concentration of 5 to 25 wt %, increased the effective kill-life of the lindane spray up to 10 times. May have been used in chlordane and BHC insecticide formulations and in melt coatings for paper and cloth. In various nitrocellulose lacquers to impart weather resistance, luster, adhesion, and decreased burning rate. These lacquers may also contain dibutyl phthalate and/or tricresyl phosphate. A typical paper lacquer may contain acetone, isobutyl acetate, ethanol, toluene and up to 20 wt % PCB-1260 (Monsanto, 1960). 

The development of plasticizers has been plagued with toxicity problems. Thus the use of highly toxic polychlorinated biphenyls (PCBs) has been discontinued. Blood stored in plasticized PVC blood bags and tubing may extract phthalic add esters, such as DOP. These aromatic esters are also distilled slowly from PVC upholstery in closed automobiles in hot weather. These problems have been solved by using oligomeric polyesters as nonfugitive plasticizers instead of DOP. 

The last section deals with several organic contaminants. The first chapter is an analysis of the behavior of surfactants in the context of a comprehensive risk-assessment analysis. The next two chapters examine the fate of PCBs, as influenced by algal uptake in the water column, volatilization, and weathering in sediments. 

DCB, and 2,6-DCB] as the river flowed over contaminated upstream sediments. Thus, they hypothesized that the composition of the PCBs available for deposition in the downstream sediments had already undergone major changes. This theory led to the conclusion that the alterations in PCB residue patterns found in bottom sediments were not likely to be the result of in situ weathering processes. Instead they were seen as caused by the scavenging from the water column of previously altered mixtures of PCBs by suspended particles or water-filtering macrophytes growing in the sediments. 

Steady-state modeling calculations were performed to examine how congener-specific properties (such as sediment-water partition coefficients, Henry s law constants, and molecular diffusion rates) affect the transport and fate of PCBs. A basic description of the model, along with modeling results, is presented here to further explain the importance of physiochemical weathering processes in controlling the fate and distribution of PCB congeners in Twelve Mile Creek and the upper portion of Lake Hartwell. 

In situ reductive dechlorination has been cited as an important pathway for biochemical weathering of PCBs in sediments (2, 5). The reductive dechlorination hypothesis is based on the premise that microbial action on PCB congeners is sterically selective for the chlorines in the meta and para positions on the biphenyl rings (i.e., 3, 3, 4, 4, 5, and/or 5 ). Such selectivity would result in the cleaving of the chlorines in these positions (5). If this process occurs, the terminal dechlorination products of the microbially resistant ortho-substituted congeners in the anaerobic environment should be 2,2 -DCB, 2,6-DCB, 2,2, 6-TrCB, and 2,2, 6,6 -TCB. Thus, the weight... 

Steady-state model calculations were performed to further examine physiochemical weathering behavior. Results were consistent with congener distributions in surficial sediments. In general, they showed that a preferential depletion of the lower chlorinated congeners occurred in the upper portion of Twelve Mile Creek, where volatilization was the only removal mechanism. In the lower portion of Twelve Mile Creek and the upper portion of Lake Hartwell, burial of PCBs in deeper sediments played a more important role. A preferential depletion of higher chlorinated congeners occurs when burial is the dominant process by physiochemical weathering. 

From this initial assessment, approximately 65% of the PCB input is calculated to have been removed from the system by volatilization and 15% of the input by burial in deeper sediments, primarily in the lower portion of Twelve Mile Creek. As indicated most clearly by higher levels of specific ortho-substituted DCBs, TrCBs, and TCBs, PCBs buried in the deeper sediments of the creek have undergone further weathering by reductive dechlorination. 

Dunnivant, F. M. Ph.D. Dissertation ( Congener-Specific PCB Chemical and Physical Parameters for Evaluation of Environmental Weathering of Aroclors ), Clemson University, Clemson, SC, 1988. 

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