Evaluation of sorbing amendments for in-situ remediation of contaminated sediments
Contaminated effluents reaching surface water bodies have led to accumulation of contaminants in sediments. The significance of contaminated sediments has increased as we have cleaned up effluents, often representing the primary human and ecological health risk remaining in surface waters. Contaminants such as polychlorinated-biphenyls (PCBs) and certain metals are bioaccumulative and reach high doses as they travel to higher trophic levels. Monitored natural attenuation and ex-situ treatment are often ineffective and uneconomical, and placing clean inert material over sediments as a cap is sometimes insufficient to adequately reduce risk so sorptive amendments are increasingly being considered to reduce the availability and mobility of contaminants. Laboratory sorption tests with different contaminants and sorbents were conducted to screen potential sorbent materials. Contaminants used in these screening studies included hydrophobic organic and selected inorganic contaminants common to sediments and sorbents included five types of clays and four types of carbon based sorbents. These screening studies were used as a basis to evaluate an in-situ adsorptive remedy for a particular site, the Manistique Harbor and River Superfund site in Michigan. Sorption capacity of five polychlorinated biphenyls (PCBs) on granular activated carbon (GAC) and organophilic clay (OMC) and the influence of natural organic matter (NOM) was evaluated. Results show that NOM fouling reduces performance of GAC by about half an order of magnitude while having minimal effect on OMC. Measured performance of the sorbents was used to evaluate in-situ treatment and capping remedial designs under conditions of weak (1 cm/yr) and rapid (1 cm/day) upwelling flow. Amended capping where the cap material is mixed throughout a thick cap layer provides the best performance, followed by capping in a thin layer followed by use of amendments as an in situ treatment. A Polanyi based adsorption prediction model was developed to predict GAC performance degradation by NOM that requires only micropore volume of GAC and physico-chemical properties of adsorbate for prediction. It accurately predicts adsorption even for a different type of GAC and different source of NOM. Results suggest that NOM-associated reduction of GAC micropore volume is a good indicator of the performance degradation associated with NOM.