GOALI: Understanding the anomolous adsorption capacity of hydrothermal char

Worcester Polytechnic Institute, Worcester MA

Investigators

Abstract

1605916 Timko Hydrothermal chars are inexpensive carbon-based materials that are synthesized by heating waste feedstock in the presence of liquid water. Hydrothermal chars have recently attracted significant interest because they exhibit promising performance in a variety of applications ranging from soil additives to electrocatalysis. The objective of this GOALI project is to understand and control the unusually high adsorption capacity of hydrothermal chars for inorganic and organic compounds. This adsorption capacity can be exploited to sequester compounds that pose environmental risks. The research will be performed in collaboration with Cabot Corporation, who will provide comprehensive characterizations of complex hydrothermal char materials. Results from the project will be incorporated into Worcester Polytechnic Institute?s student projects curriculum. The research team will partner with the Worcester-based non-profit group, The Toxic Soil Busters, for additional research and mentoring. Hydrothermal chars have remarkable adsorption capacities despite their relatively low surface areas. It has been hypothesized that surface-bound carboxylic acid groups self-associate to mediate an ultramicropore structure that is accessible only in the presence of hydrogen bond donating molecules. Traditional gas sorption measurements that use molecular nitrogen do not open the carboxylic acid gated ultramicropore structure, resulting in low apparent surface areas. However, the carboxylic acids deprotonate in water, and the hydrothermal char structure swells, mimicking the swelling of a hydrogel under appropriate pH conditions. Therefore, the sorption capacity of hydrothermal char is a result of its actual surface area being much greater than measured. To test the hypothesis, two separate ball milling techniques will be used to independently control the surface area and surface chemistry of the hydrothermal chars: (1) a dry-milling technique that is useful for modifying hydrothermal char surface composition, and (2) a solvent-assisted wet-milling technique that increases the surface area of carbonaceous materials. Using these two related techniques, the PI will investigate systematically the adsorption capacity of hydrothermal char to determine why measured adsorption capacities of hydrothermal chars are comparable to those measured for activated carbon, despite the fact that the nitrogen-measured surface areas of hydrothermal chars are less than 1% of those measured for activated carbon. The three aims of the project are: (1) to understand the relationship between surface area and sorption capacity using a reactive gas milling to modify surface composition; (2) to understand the relationship between measured surface area and sorption capacity, using solvent-assisted milling as a technique for increasing measured surface areas of pyrolysis biochar; and (3) to test the carboxylic acid gating of hydrothermal char ultramicropore area by measurement of sorption characteristics for a series of sorbates with varying characteristics and molecular sizes. By disentangling the competing effects of surface area and surface composition, the results will provide a firm understanding of the remarkable sorption capacities of hydrothermal chars.