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Digging Through New Types Of Waste To Recycle Metals

ACS Meeting News: From sewage sludge to junkyards, scientists explore new ways to mine metals

by Stephen K. Ritter
April 6, 2015 | APPEARED IN VOLUME 93, ISSUE 14


Metals are ubiquitous in our lives. There are the metal objects we can see—paper clips, kitchen utensils, electronics, cars or bicycles, wires and pipes, lightbulbs, hammers and shovels. Most of those are not recycled, although they could be.

More on this story

Wanted: New Ways To Recycle Metals

But then there are personal care products, glass, paint, and tires—things that contain metals but for which there is no ready way to recover the metal, or the metal is abraded and gets scattered in the environment.

Fortunately, some researchers are thinking about the sustainable use of metals. As metal resources are becoming scarce, scientists and engineers are developing strategies and technologies to economically recover metals that go beyond recycling soda cans.

A Yale University analysis of the fate of elements used in commercial products, including automobile parts, illustrates the unsustainability of current metal use. SOURCE: Environ. Sci. Technol.

“If we can recover and reuse chemical elements from waste streams, we have the opportunity to decrease reliance on our remaining natural resources and improve sustainability,” said Kathleen S. Smith, a geochemist at the U.S. Geological Survey. Smith was one of hundreds of scientists who spoke on the theme of “Chemistry of Natural Resources” at the American Chemical Society national meeting in Denver held March 22–26.

Smith and others have been examining metal life cycles, thinking about alternative ways of acquiring metals so they don’t have to be freshly mined. Their work comes at a time when global ore quality is slipping, from high-grade, low-bulk ore to low-grade, high-bulk ore, according to the International Council on Mining & Metals. Some economists estimate that we have now extracted more commercially accessible metals out of Earth than are left in our planet. But that may not be a problem.

Instead, it might be an opportunity. Smith and her colleagues are studying whether it’s feasible to mine metals from sewage sludge that accumulates in wastewater treatment plants. Sewage sludge, also known as biosolids, is rich in nitrogen, phosphorus, and organic matter. It also could literally be a gold mine: Gold and other recoverable metals end up in the sludge from industrial processing and consumer products—things that go down the drain, get flushed, or become dust in the wind.

“There are metals everywhere,” Smith said. “Hair care products, detergents, medicines, even nanoparticles that are put in socks to prevent bad odors.” Whatever their origin, these metals end up being funneled through wastewater treatment plants.

The U.S. has about 16,500 wastewater treatment plants that generate some 7 million dry tons of biosolids per year, Smith said. Biosolids are regulated by the Environmental Protection Agency to ensure they are safe to use. The material is treated to wipe out pathogenic microbes and to reduce the amount of nine metals of concern (As, Cd, Cu, Pb, Hg, Mo, Ni, Se, and Zn). About 60% of the material is deemed safe enough to be applied to agricultural land as fertilizer. The remainder is incinerated or sent to landfills, she noted.

If researchers could find ways to recover valuable metals from biosolids while removing the nuisance ones, it would be a win-win, Smith said.

Gold Dust
Credit: Heather Lowers/USGS
USGS scientists are exploring the distribution of potentially recoverable metals in municipal wastewater sewage sludge samples.
Credit: Heather Lowers/USGS
USGS scientists are exploring the distribution of potentially recoverable metals in municipal wastewater sewage sludge samples.

For now, Smith and her team’s mission is to find out exactly what is in our waste and how much. They are building on EPA’s 2009 Targeted National Sewage Sludge Survey, which surveyed for 28 metals among random samples taken from some 3,300 facilities that treat more than 1 million gal of wastewater per day. The survey found potentially valuable amounts of critical elements in sludge—for example, silver up to about 850 mg/kg, and copper up to about 2,600 mg/kg.

Smith and her group hope to pinpoint which wastewater facilities might be most advantageous for recovering metals. So they are collecting biosolids from a range of facilities: ones in semirural communities, in large towns, and in small mountain towns in the Colorado mineral belt.

The USGS researchers are using scanning electron microscopy to examine small metal particles in these biosolids. They also are testing extraction chemicals similar to those used in the mining industry to determine how effective they are at pulling metals out of the sludge. Their preliminary results suggest that the treated waste is enriched with precious metals such as platinum, silver, and gold. “The gold we found was at the level of a minimal mineral deposit,” Smith said. If that amount were in an ore deposit, it might be commercially viable to mine it.

“The next step is to better characterize the form of the metals in biosolids and examine the ease of their extractability,” Smith cautioned. “The economic and technical feasibility of metal recovery from biosolids needs to be evaluated on a case-by-case basis.”

Smith and her colleagues aren’t working in isolation. A team led by Paul Westerhoff of Arizona State University is also looking at metals in sewage sludge through ASU’s Biodesign Institute and its National Sewage Sludge Repository.

Westerhoff and his colleagues recently analyzed biosolids for 58 elements by SEM, inductively coupled plasma mass spectrometry, and energy-dispersive X-ray spectroscopy (Environ. Sci. Technol. 2015, DOI: 10.1021/es505329q). The researchers came up with a list of the 14 most lucrative elements to recover (Ag, Cu, Au, P, Fe, Pd, Mn, Zn, Ir, Al, Cd, Ti, Ga, and Cr), with a combined estimated value of $280 per ton of sludge at current market prices. They concluded that in communities of 1 million or more people, recovered metals in biosolids could be worth $13 million annually. That’s money that could help fuel local economies.

These sludge studies could also help bring awareness to other efforts for recovering metals from alternative sources, Smith told C&EN. Other researchers are exploring developing sorbent materials that can snatch the most valuable metals right out of wastewater or from storm water from highways, parking lots, and roofs. For example, Ian Sofian Yunus and Shen-Long Tsai of National Taiwan University of Science & Technology recently developed a protein-modified cellulosic sorbent for extracting palladium from water (RSC Adv. 2015, DOI: 10.1039/c4ra16200e).

“Although exploring methods to recover metals accumulated in media such as sewage water is not new, the research is providing better information and more accurate estimates of the potential profitability that may derive from urban deposits of metals,” according to Luca Ciacci, a chemist at Yale University’s Center for Industrial Ecology. The work also highlights major hindrances to recovery.

For example, Ciacci, Thomas E. Graedel, and their Yale colleagues just published a study on metals in manufactured products that they described as “lost by design” (Environ. Sci. Technol. 2015, DOI: 10.1021/es505515z). These are typically metals that are dispersed in the environment as the product is in use or incorporated into products in ways for which no viable recycling approach exists, Ciacci told C&EN. Not surprising, many of these uses are associated with the most common objects on the planet—zinc in tires, platinum-group metals in catalytic converters, and titanium dioxide pigment in paint, to name a few.

Catalytic converters, for example, do a good job of zapping pollutants such as hydrocarbons, CO, and NOx and turning them into more benign chemicals such as CO2, H2, and N2. But as cars putt down the road the converters also slowly disperse platinum, palladium, rhodium, cerium, and osmium into the environment. Researchers who have assessed the abundance of the dissipative metals coming from cars in soils along roadways think there is enough there, or will be over time, to make it worthwhile to scoop up the top layer of dirt to recover the metals.

Agro-mining might also prove useful. This plant-based strategy is being promoted by an international team including Antony van der Ent of the Centre for Mined Land Rehabilitation and the University of Queensland, in Australia (Environ. Sci. Technol. 2015, DOI: 10.1021/es506031u). It takes advantage of the fact that certain plants have the ability to selectively accumulate metals at high amounts from soil or water. Reaping, drying, and incinerating the biomass concentrates the metal in the ash, which in the case of remediating toxic metals at mining or industrial sites is often disposed of in landfills. But for agro-mining, the ash is considered a high-grade “bio-ore.”

Van der Ent and his colleagues suggest that agro-mining could provide local communities in developing regions with an alternative use for degraded land and an additional source of income. The communities would farm not for crops, but for metals such as nickel, manganese, and zinc. Nickel in bio-ore, for example, can range from 10 to 25%, compared with only about 2% in low-grade ores.

In addition to sewage sludge and mining waste, piles of electronics and lightbulbs containing metals could be another source to extract metals. Koen Binnemans of the University of Leuven, in Belgium, and coworkers have been using ionic liquid solvents that selectively dissolve metal oxides to separate rare-earth and transition metals during recycling.

Binnemans and his colleague David Dupont recently reported recovering rare-earth metals from stockpiles of discarded neodymium-rich magnets (NdFeB) used in electric motors and from the yttrium and europium phosphor (Y2O3:Eu3+) in extinguished fluorescent lights (Green Chem. 2015, DOI: 10.1039/c4gc02107j and 2014, DOI: 10.1039/c5gc00155b).

But extracting the metals is just the first step in the recovery process. Researchers must also consider how to turn what they extract into a useful form.

What is needed, Smith told C&EN, is to figure out the best form to recover the metals. “That’s where chemists come in, to use innovative chemistry to get the metal in a form, as pure metal or as a metal salt, that can be used to make new products or in smelters. We need to recover the metals in a usable form, or all bets are off.”  



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Caroline Snyder (April 6, 2015 3:11 PM)
Removing metals from sewage sludge is not a win-win situation. Even if it is possible, at great cost, to remove some metals from sludge, this complex and unpredictable waste mixture could still not be used as a safe fertilizer. Treatment Plants do not wipe out harmful pathogens, as Smith claims. Prions, for example resist all treatment methods and end up in land applied sludges. Also, new research indicates that sewage treatment plants are breeding grounds for super bugs-- which when land applied through biosolids -- affect healthy soil microorganisms through horizontal gene transfer. Worse, every industry connected to a sewer is permitted to discharge its hazardous waste into treatment plants. In addition to metals, PCBs, EDCs and dioxins, this includes thousands of other synthetic chemical compounds, some persistent, highly toxic, and able to enter the food chain.
And shouldn't our diminishing farm land be used to grow food, rather than metal-absorbing plants? For documentation and more information visit
Jusden Messick (April 7, 2015 7:29 PM)
This is a good refutation, but one point: "diminishing" farm land means the lands is unfit to grow food, probably because they have Too High Metal Concentration to produce food suitable for human consumption. agro-mining could become an important step in futuristic crop rotation, where soil metal concentrations are primed for subsequent food production, while extracting other potentially profitable metal crops to be sold in non-food-producing seasons. This will actually take a load off of farmers, who today are constantly pressured to plant food-producing plants to break even, in seasons where, in their crop rotation, they could optimize food-production in later years by planting a no-crop plant. These no-crop plants could be replaced, or even used as metal-crop plants in the future, leading to greater food quality and quantity in later seasons. And since not all farmers operate on the same crop rotation, this would result in an overall increase in food quality and quantity, and universally stronger soil. The metal crops could also serve to lower food prices, as farmers can lower their break-even price for food production with funds from their metal-crop production.

As to your point about prions, this is a recognized problem held in limbo by civic and corporate interests. It is a prime place for research, however. Prions are rogue proteins that are stable in the environment. pH and temperature treatments are known to denature proteins. Perhaps running all wastewater through highly acidic or highly basic solutions at various temperatures could help to alleviate these problems. The same could be said for your superbugs, or any other organic material. Coupled with the addition of proteases, amylases, and lipases, at different stages of treatment, all prions, even in mucal form, could be rendered inert and discarded from waste water, or even harvested and used in polymerization of commercial proteins.

To your point about industries being able to dump anything into treatment plants, that is a fully valid point, and I sympathize. This is why I do not trust government regulation to fully protect the citizenry. In much the same way as environmental activism must be done in a grassroots form to garner any actual change, we must just the same hold all firms accountable for questionable commercial and industrial practices, to protect ourselves.
Steve Ritter (April 9, 2015 12:05 PM)
Thanks for your comment Caroline. You are correct that sewage sludge is a place where many of the by-products of industrial and commercial products end up. EPA has established criteria by which sewage sludge can be safely used, else the sludge is incinerated or sent to a landfill. I think the point of this story and others that report on how metals and natural and synthetic chemicals cycle through the environment is that there are chemistry solutions, such as pulling metals out of sewage sludge, that will help ease environmental impacts and aid sustainable use of natural resources. Pollution is not going away, and there are no one-size-fits-all solutions, so many approaches will be needed.

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