When it comes to removing very dilute concentrations of pollutants from water, existing separation methods tend to be energy- and chemical-intensive. Now, a new method developed at Massachusetts Institute of Technology (MIT) could provide a selective alternative for removing even extremely low levels of unwanted compounds.
The new approach is described in the journal Energy and Environmental Science, in a paper by MIT postdoc Xiao Su, Ralph Landau Professor of Chemical Engineering T. Alan Hatton, and five others at MIT and at the Technical University of Darmstadt in Germany.
The system uses a novel method, relying 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 de-ionisation, which often require high voltages that tend to produce side reactions, according to Su. These processes also are hampered by excess background salts.
In the new system, the water flows between chemically treated, or ‘functionalised’, surfaces that serve as positive and negative electrodes. These electrode surfaces are coated with what are known as Faradaic materials, which 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, as the team demonstrated using ibuprofen and various pesticides. The researchers found that this process can effectively remove such molecules even at parts-per-million concentrations.
Previous studies have usually focused on conductive electrodes, or functionalised plates on just one electrode, but these often reach high voltages that produce contaminating compounds. By using appropriately functionalised 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, as the team demonstrated with the herbicides paraquat and quinchlorac.