Web Date: August 1, 2013
Circuit Board Waste Mops Up Toxic Metals
Researchers in Hong Kong have found a beneficial new use for the electronic waste from discarded cell phones, computers, and other gadgets. Ground up into a powder, printed circuit boards from these products could sponge up another type of pollution—toxic heavy metals in water (Environ. Sci. Technol. 2013, DOI: 10.1021/es4001664).
About 20 to 50 million tons of electronic waste is produced worldwide each year, and most of it is incinerated or dumped into landfills. Environmental scientists worry about the ecological and human health hazards caused by this e-waste, especially in developing countries that receive the bulk of the waste. Burning the plastic-metal mix in printed circuit boards releases toxic compounds such as dioxins and furans. In landfills, the metals on the boards can eventually contaminate groundwater.
But recycling circuit boards is expensive. Only the metal parts of the boards have reuse value, so the nonmetallic parts must be separated out from the e-waste, which is a costly process.
To make e-waste recycling more economically viable, Gordon McKay and his colleagues at the Hong Kong University of Science & Technology began investigating uses for this nonmetallic fraction, which is made of plastic and aluminosilicates. The team had previously developed adsorbent materials to remove toxic heavy metals from wastewater effluents produced in the microelectronics industry. They thought the aluminosilicate material in the circuit boards would make an effective adsorbent, similar to zeolite materials currently used for this purpose.
To test this idea, McKay and colleagues worked with a powder made by grinding up the nonmetallic fraction of circuit boards. They heated it and treated it with potassium hydroxide, which is a common technique used to increase porosity in carbon-based adsorbents. The team then added the powder to solutions of copper, lead, and zinc.
The researchers found that the metals adsorbed to the treated powder more efficiently than to three commonly used industrial adsorbents. For example, the powder soaked up about 25% more copper than an equivalent amount of commercially available ion-exchange resin did. The ground-up circuit boards also had a higher adsorption capacity for these metals than several low-cost adsorbents, including soybean hulls and blast furnace slag.
Interestingly, the metals did not adsorb to the untreated powder. The team proposes, based on inductively coupled plasma atomic emission spectroscopy, that the heating and chemical treatment opens up the circuit board material’s aluminosilicate lattice. This creates a porous structure that enhances ion exchange with the toxic heavy metals.
Daniel E. Giammar, an environmental engineer at Washington University in St. Louis, says the team has found an intriguing use for this e-waste material. Although the boards can become effective adsorbents, he says the method for making the materials may not be as energy efficient and cost effective as for other adsorbents, such as granular ferric hydroxide, because of all the processing steps needed to produce the treated powder.
The researchers are now conducting a pilot study to make a 10-kg batch of the adsorbent, and they are discussing developing the project on an industrial scale with an electronic waste recycling company in Hong Kong, McKay says. He thinks the ideal application for the new adsorbent would be to treat wastewater effluent from electronics production. “It would be quite a nice complete circle,” he says.
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