Catch and Release Chemistry: Reversible Adsorption on Dendrimer and Polymer Functionalized Surfaces

The ability to selectively and reversibly remove large quantities of solutes from solution remains the goal of activities as diverse as environmental remediation, chromatography, and drug delivery. Selectivity requires carefully designed substrates for immobilizing solutes with specific functionality and/or reactivity. Reversibility requires that these same substrates release the bound solutes when the system is subjected to external stimuli such as changes in pH, solvent polarity, temperature, or light. Controlled, reversible adsorption allows substrates to be regenerated and reused, an essential feature of sustainable and environmentally benign separation systems. Advances in host-guest chemistry and molecular recognition address the selectivity issue, but few systems provide for reversible behavior and all are limited to specific support materials. Even when environmental changes can trigger the release of bound solutes, proposed mechanisms describing substrate-solute affinity remain speculative, and in most cases, the functionalized substrates have limited loading capacities. Being able to selectively and reversibly remove solutes from aqueous solution has enormous practical consequences. The United States Environmental Protection Agency recognizes and sets upper concentration limits for more than 90 contaminants in drinking water (including bottled water). More than half of these contaminants are small organic species that are discharged by chemical factories, result from agricultural runoff, or arise from industry-related activities (such as transportation and corrosion). Many of these organic contaminants are known to damage the liver, kidneys and/or central nervous system, and some pose risks as potential carcinogens. Removing these solutes from contaminated sites is expensive. The annual cost of operating pump and treat systems at Superfund sites can exceed $10 million per year per site, and more than 1000 Superfund sites exist across the United States. Similar concerns about water safety have led the European Union to impose strict limits on safe levels and discharges from point sources of organic contaminants.The aim of this project is to design new molecularly-based substrates capable of specific and reversible removal of solutes from aqueous solutions. Two general strategies will be used to functionalize a wide range of high surface area substrates with coatings that target specific solutes for catch and release chemistry ; dendrimers and plasmachemical polymer films. These surfaces will be designed to bind specific solutes and then to release these same solutes following a well-defined trigger. Linear and nonlinear optical methods will be used to determine the kinetics of adsorption and desorption of the targets and to infer the mechanisms of adsorption. The long term objective is to develop practical, cost-effective adsorbents for water treatment that can be regenerated many times, greatly reducing the enviromental impact arising from the manufacture and disposal of current irreversible absorbents.This project is in collaboration with Prof. Robert Walker and Prof. Mary Cloninger of Montana State University, USA.

The long-term beneficiaries of this work will be water authorities and environment agencies tasked with cleaning up polluted water sources and limiting levels of contaminants, such as industrial chemicals, pesticides and antibiotics, in waste water streams and drinking water. This project is aimed at developing new chemistry for better reversible absorbents. While the work is being carried out on a laboratory scale, the plasmachemical methods are inherently scalable. If this project is successful, subsequent scale-up projects and field trials would be carried out in conjunction with Northumbria Water Ltd. (NWL) in the UK and the Department of Natural Resources and Conservation in Montana. In the UK, this would probably be managed through a joint research proposal with NWL to UKWIR, which is the collective research funding body for the water utilities in the UK. We also have contacts with the water and environmental research agency in Jordan which would enable us to address specific problems affecting water purity in desert areas of the Middle East.