Researchers eyeing metal removal from wastewater

Engineers and scientists at Brown University are using a combination of two established – but very different – techniques in an effort to efficiently remove heavy metals from industrial wastewater.
The presence of metals in wastewater is a common problem for many industries, including the manufacturing of printed circuit boards, jewelry and electroplating. If not treated correctly, these metals can cause compliance problems with state and federal officials. Some standard treatment methods can be problematic, consuming inefficient amounts of power or generating sludge classified as a hazardous material, requiring further treatment or special disposal.
“The primary target is remediation of wastewater contaminated with a variety of heavy metals present at low concentrations,” said Joseph Calo, research professor at the School of Engineering. “The combination of complex metal mixtures and low concentrations makes this task particularly difficult.”
That is why Calo is developing a new method that collects trace amounts of heavy metals in water by increasing their concentration, so that a proven removal technique can be utilized. Called cyclic electrowinning precipitation (CEP), the method removes up to 99 percent of copper, cadmium and nickel, according to his research.
Once treated, the contaminated water will meet federally accepted standards. The system’s mechanics and results of wastewater-treatment studies have been published in the Chemical Engineering Journal.
Calo recently submitted paperwork to university officials, who have the right of first refusal, in the hopes that the school applies for a patent. Should the school decline he has the option to patent the system himself, as is the standard practice.
“What we’ve found is something that is very promising. It’s a complete system and it has commercial possibilities,” he said. One method to remove metals is to add chemicals to the wastewater to precipitate the metal ions from the water, so they form solids. The problem with this technique is that it generates a sludge laden with metals, requiring hazardous-material treatment and disposal.
Another proven technique is through electrowinning, often called electrolytic removal. In this treatment technique, pairs of cathodes and anodes are set in a reaction chamber containing the wastewater and when the nodes are electrified, metal ions are attracted to the cathodes. The problem with this technique is that there must be a high-enough concentration of metal in the water for it to be effective. If not, it wastes power.
Dr. Eugene Park, associate research professor at the University of Rhode Island’s chemical- engineering program, is familiar with the two processes. A co-founder of the R.I. Department of Environmental Management’s pollution-prevention division, he has assisted many of the state’s electroplaters to achieve pollutant compliance over the last 20 years.
“You have to weigh the advantages both from an economic- and an environmental-impact point of view,” he said in evaluating the potential for Calo’s method. “In this case it could be that the advantages are still there, to recover the metals from the waste stream, but these are the things that need to be examined.”
According to Park, the two treatment options are common but he had never heard of them used in series. “Companies use either one or the other, that’s been standard,” he said.
Seeing disadvantages in both methods, Calo and his associates plan to use one to compliment the other. Their system uses two units, one to concentrate the metals and the other to turn them into stable metals and remove them. In the first stage, chemicals are added to separate the water molecules from the metal precipitate, which settles at the bottom. Clear water is siphoned off and the process is repeated to increase the concentration. The solution is then sent to a second device where the electrowinning takes place, and the metal is chemically changed to a stable, metal solid so it can be easily removed. “The system is automated so that it can be set up and run in various ways,” Calo said, “however, the primary target is for continuous operation.”
“The process combines the more-attractive aspects of chemical precipitation with those of electrowinning in a cyclic process,” Calo said. “The net effect is that contaminated wastewater is fed to the process, along with the particles upon which solid metal is deposited, and what comes out is clean water and the particles with the accumulated deposited metal.
“The water throughput depends primarily on the concentrations of heavy metals present,” he said. “For the pilot scale system that we built and the conditions that we tested, it was about 10 gallons per hour or about 240 gallons per day. Both the spouted particulate electrode and the precipitation/re-dissolution tank can be readily scaled to match the required throughputs.”
According to Calo, the system can be automated to achieve treatment even to levels of federal drinking water standards. The sludge is continuously formed and re-dissolved within the system so that none is left as a hazardous waste.
The process is designed specifically for wastewater treatment but if a particular metal is present in very high amounts, it would be possible to recover it. They could then be sold to be reused, allowing the opportunity to recoup some of the treatment costs.
“Like almost all other remediation processes, there are significant costs involved,” Calo said. “The payback depends on the commodity price of the metals.” &#8226

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