In aquatic environments that are impacted by contaminated sediments, risk management strategies focus on interrupting potential exposure pathways by which contaminants might pose an ecological or human health risk. Research has shown that contaminant transport pathways can be interrupted by modifying and enhancing the binding capacity of natural sediments. This is achieved by adding amendments such as activated carbon for persistent organic pollutants; minerals such as apatite for metals or metalloids; ion exchange resins for metals or other inorganic contaminants; or lime for pH control or nitroaromatics degradation. New advances in biomass utilization provide an opportunity to explore the use of specially formulated biochars and activated biochars in the sequestration of organic and metal contaminants in sediments while reducing or even reversing the carbon footprint of sediment remediation efforts.
The primary objective of this SERDP Exploratory Development (SEED) project was to test a range of available biochars and especially formulated biochars that can reduce the bioavailability and leaching of toxic chemicals like polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethanes (DDTs), mercury, and methylmercury in sediments. To address this objective, five key research questions were addressed:
- Are biochars effective sorbents for PAHs, PCBs, DDT, mercury, and methylmercury?
- Do biochars need activation to increase specific surface area and be effective sorbents for PAHs, PCBs, DDT, mercury and methylmercury?
- Can addition of zero valent iron in biochars enhance the dechlorination of chlorinated organic compounds?
- Can incorporation of iron oxide in biochars enhance the metal binding capacity of biochars?
- Can incorporation of iron and iron oxides increase the density of biochars to make them more stable in the sediment environment and allow separation for mass transfer calculations?
A range of biochars made from a number of agricultural residues, phragmites, and hardwoods were evaluated in this research. In addition, the biochars were activated either physically or chemically to enhance their organic contaminant sorption properties, impregnated with zero valent iron to evaluate their potential for the dechlorination of chlorinated compounds, and impregnated with iron oxides to evaluate the enhancement of sorption of mercury and methylmercury. Contaminant sorption to the carbons was evaluated in the aqueous phase by conducting sorption isotherms and pH edge sorption studies, followed by effectiveness testing in the sediment phase. The impregnation of iron/iron-oxides created a denser carbon so the increased stability of iron amended biochars was also assessed. The magnetic properties of these iron amended carbons also allowed for the separation of the carbon after contact with sediment, enabling contaminant mass transfer assessments.
Biochars were able to sorb organic contaminants, mercury, and methylmercury, making them attractive alternatives to activated carbons in sites contaminated with both organic and inorganic contaminants. However, due to their lower surface area, unactivated biochars have a lower affinity for organic contaminants than activated carbons, so activation is necessary for their performance to match that of activated carbons. Unactivated biochars were able to reduce PCB porewater concentration by 18-80%, while the activated carbons and activated biochars consistently reduced organic contaminant porewater concentration by more than 99% in a Department of Defense impacted sediment. Mercury isotherms and pH edge sorption experiments indicate that some of the activated carbons were the most effective in removing mercury from solution at low concentrations. However, they also suggest that these activated carbons could have a limited amount of sorption sites available for inorganic contaminants relative to the biochars as their performance dropped with increasing mercury concentrations. The biochars, particularly poultry litter derived chars, were able to remove more mercury from solution at higher mercury concentrations compared to other carbons (greater than 99% mercury removal in pH edge study). It is possible that the high phosphate content of these poultry litter biochars are responsible for this enhanced mercury sorption. These biochars are therefore attractive from a mercury remediation standpoint, but the stability of the phosphate within the carbon needs to be evaluated before field application. Iron oxide amended chars could be separated magnetically to assess PCB mass transfer from sediment to carbon. The use of iron to impregnate the carbons was effective in improving their density and settling characteristics but had limited success in improving the sorption capacity of the carbons to mercury and methylmercury or in enhancing the dechlorination of chlorinated organic compounds. Refinement of the iron amendment technique and longer-term studies are required to fully explore the potential of iron amended chars.
This study provides the proof-of-concept that can lead to further development of biochars for full-scale sediment remediation through scale-up to large-scale production of the synthesized biochars, evaluation of full-scale economics of the manufacturing, and finally benthic organism bioavailability and toxicity studies to evaluate the impact of the new sorbents in aquatic ecosystems. Activated biochars produced from waste biomass can provide strong sorbents for the remediation of contaminated sediments, reducing treatment costs and possibly reversing the carbon footprint of the remediation strategy. This could be particularly attractive in contaminated wetlands invaded by Phragmites, as the Phragmites itself could be used to produce the activated biochars necessary for sediment remediation on site.