Traditional methods, such as reverse osmosis, that remove contaminants from water are expensive and energy-intensive. Researchers have developed technology to remove contaminants from water, but only as many as necessary.
A treatment system was built that can be tuned to selectively pull toxins from drinking water and wastewater from factories, sewage systems, and oil and gas wells. The technology will cut costs and save energy compared to conventional systems.
The heart of the system is a set of novel composite electrodes that enables capacitive deionization. The charged, porous electrodes selectively pull target ions from fluids passing through the maze-like system. When the pores get filled with toxins, the electrodes can be cleaned, restored to their original capacity, and reused.
There are many ions in water, but not all are toxic. Sodium chloride (salt), for example, is perfectly benign and does not need to be removed unless the concentration gets too high. In some drinking water wells, however, there is arsenic, and in drinking water pipes, there could be lead or copper. In industrial applications, there are calcium and sulfate ions that form scale — a buildup of mineral deposits that fouls and clogs pipes.
The proof-of-principle system removed sulfate ions, a scale-forming mineral that can give water a bitter taste and act as a laxative. The system's electrodes were coated with activated carbon, which was in turn coated by a thin film of tiny resin particles held together by quaternized polyvinyl alcohol. When sulfate-contaminated water flowed through a channel between the charged electrodes, sulfate ions were attracted by the electrodes, passed through the resin coating, and stuck to the carbon.
Tests showed the positively charged coating on the cathode preferentially captured sulfate ions over salt at a ratio of more than 20 to 1. The electrodes retained their properties over 50 cycles. The system is intended to work with current commercial water-treatment systems. Coatings for other contaminants are being developed.