When removing very dilute concentrations of pollutants from water, existing separation methods tend to be energy- and chemical-intensive. A new method was developed that could provide a selective alternative for removing even extremely low levels of unwanted compounds. The method relies on an electrochemical process to selectively remove organic contaminants such as pesticides, chemical waste products, and pharmaceuticals, even when these are present in small, yet dangerous concentrations. The approach also addresses key limitations of conventional electrochemical separation methods such as acidity fluctuations and losses in performance that can happen as a result of competing surface reactions.
Current systems for dealing with such dilute contaminants include membrane filtration, which is expensive and has limited effectiveness at low concentrations, and electrodialysis and capacitive deionization, which often require high voltages that tend to produce side reactions. These processes also are hampered by excess background salts.
In the new system, the water flows between chemically treated, or “functionalized,” surfaces that serve as positive and negative electrodes. These electrode surfaces are coated with Faradaic materials that can undergo reactions to become positively or negatively charged. These active groups can be tuned to bind strongly with a specific type of pollutant molecule. This process can effectively remove such molecules even at parts-per-million concentrations.
Previous studies focused on conductive electrodes, or functionalized plates, on just one electrode; these often reach high voltages that produce contaminating compounds. By using appropriately functionalized electrodes on both the positive and negative sides in an asymmetric configuration, the researchers almost completely eliminated these side reactions. Also, these asymmetric systems allow for simultaneous selective removal of both positive and negative toxic ions at the same time.
The same selective process should also be applied to the recovery of high-value compounds in a chemical or pharmaceutical production plant, where they might otherwise be wasted. The system could be used for environmental remediation, for toxic organic chemical removal, or in a chemical plant to recover value-added products.
The system is inherently highly selective, but in practice it would likely be designed with multiple stages to deal with a variety of compounds in sequence, depending on the exact application. Such systems might ultimately be useful for water purification systems for remote areas in the developing world, where pollution from pesticides, dyes, and other chemicals is often an issue in the water supply. The electrically operated system could run on power from solar panels in rural areas.
Unlike membrane-based systems that require high pressures, and other electrochemical systems that operate at high voltages, the new system works at relatively benign low voltages and pressures. In contrast to conventional ion exchange systems where release of the captured compounds and regeneration of the adsorbents would require the addition of chemicals, the same result is achieved by switching the polarity of the electrodes.
The prototype system achieved more than 500 cycles. The researchers have explored a variety of device configurations and contaminants, and have identified general design principles by which to achieve selective removal of contaminants.