Mercury from underground estuary - American Chemical Society

Environmental t News Mercury from underground estuary gan increasing in the mixing zone of the estuary, reaching a peak of about 200 pM at intermediate salinity. The levels declined slightly in high-salinity areas. Waquoit Bay receives freshwater from the Childs River and meets Joanne Tromp, Woods Hole Oce anogr aphic Institution

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esearchers and lawmakers trying to deal with mercury contamination may soon have to worry about a hidden source for mercury in oceans— saline groundwater from subterranean estuaries. In a new study published in this issue of ES&T (pp 3090–3095), a team of researchers from the Woods Hole Oceanographic Institution reports that a subterranean estuary that drains into the Waquoit Bay in Cape Cod, Mass., delivers high amounts of mercury to it. If confirmed in underground estuaries elsewhere, the findings could help explain how this toxic metal circulates through the aqueous networks of the planet. These results may also shed light on how different water systems respond to efforts to lower atmospheric mercury emissions, thus increasing the efficacy of such measures in the long run. Subterranean estuaries form the mixing zone between two water masses—fresh groundwater and salt groundwater. Fresh groundwater resides beneath beaches, perched slightly above sea level. “Gravity drives that fresh groundwater from the aquifer into the coastal ocean,” says team leader Matthew Charette. Closer to the ocean, this fresh groundwater mixes with salt groundwater moving inland. The mixing drives some brackish water away from the aquifer and back into the ocean. To map the flux of this toxic metal in the estuary, Charette and his team collected groundwater profiles from six different locations in the estuary and analyzed their mercury content. Mercury levels in the fresh groundwater were very low—less than 15 picomolar (pM). But the mercury concentration be-

Two students use a piezometer to draw groundwater profiles of the subterranean estuary at Waquoit Bay, Mass.

the saline waters of the Atlantic Ocean at Vineyard Sound. The researchers discovered that the surface waters of the bay had mercury levels closer to those in the groundwater discharge than to a combination of the mercury in the waters of the Childs River (3.2 pM) and Vineyard Sound (4.4 pM). In addition, the concentration of mercury in the open waters of the bay was highest in the region closest to the underground estuary, leading the team to hypothesize that “subterranean groundwater is delivering mercury to the surface waters of the bay.” This “intriguing” study makes a good case for the importance of groundwater to mercury cycling

3032 n Environmental Science & Technology / MAY 1, 2007

in coastal bays, says geochemist John Reinfelder of Rutgers University. But, he adds, “I am not sure they made the case that the load of the bays from the groundwater is major or comparable to the river.” Reinfelder says rivers usually carry most of their mercury load in sediments and not in the waters. So, to calculate the actual levels of mercury that the Childs River is delivering to Waquoit Bay, Charette and colleagues would need to look at the mercury concentrations in the river sediments, because “that can be 50–90% of mercury coming down the river.” Charette and his team members agree. In fact they did quantify sedimentary mercury, but only in the underground estuary, where most of the load in the freshwater regions was in the sediments. Increasing salinity in the mixing zone (and perhaps other unknown factors) caused the sediments to release most of their mercury content into the water. So “it is definitely possible that riverine [sediment] particles are another important source of mercury to Waquoit Bay,” says coauthor Sharon Bone, currently a graduate student at the University of California, Berkeley. The important “take-home message” from their study is that “a substantial portion of the mercury in the bay can be explained by this groundwater flux,” says Bone. “However, this does not exclude the possibility that there could be other important sources of mercury to the bay.” Another big question that remains to be answered is whether the phenomenon is unique to Waquoit Bay or is “something you would see over a variety of estuarine systems,” says Elsie Sunderland, a physical scientist at the U.S. EPA’s Council for Regulatory Environmental Modeling. —RHITU CHATTERJEE © 2007 American Chemical Society

News Briefs

A recently discovered marine toxin may be made by tiny organisms such as cyanobacteria, which create huge blooms each year in the Baltic Sea.

Swedish Me teorological and Hydrological Institute

The Baltic Sea, long plagued by contaminants and toxic cyanobacteria blooms, may have a new class of marine toxins to add to its environmental problems, according to results published in this issue of ES&T (pp 3069–3074).

Called polybrominated dibenzo-p-dioxins (PBDDs), the compounds have been documented previously in blue mussels and shellfish (Environ. Sci. Technol. 2005, 39, 8235–8242). PBDDs are produced naturally, probably by algae or cyanobacteria, and accumulate in organisms as high on the food chain as fish and shellfish, says lead author Peter Haglund of Umeå University (Sweden). Levels of PBDDs were low in freshwater species but increased farther offshore in the marine environment. In some mussels, concentrations were in the range of nanograms per gram of tissue—greater than levels of the most abundant PCB contaminants. “We’re afraid it might have some ecological impacts on aquatic species, since mussels account for 90% of the animal biomass in some of these ecosystems,” says Haglund. Brominated dioxins act on the

same receptor system as the better-known chlorinated dioxins and could have similarly harmful effects on reproduction, development, and neurology, the authors say. However, the exact nature of PBDDs is important in determining potential health effects. Compounds with fewer bromine atoms are less toxic and are metabolized faster, says Martin van den Berg of Utrecht University (The Netherlands). And when bromine is found in certain positions around the outside of a molecule, it packs more of a toxic wallop. “I don’t think the diand tribrominated dioxins are posing a risk for higher organisms and humans,” van den Berg says, but he adds that certain tetrabrominated compounds, which were not found at high levels in this study, could pose health risks. The researchers measured increasing levels of PBDDs over the past decade. The toxins may be on the rise, the authors say, thanks to global warming and eutrophication (loading of excess nutrients from sewage treatment and agriculture). In a warm, nutrient-rich broth, the organisms at the bottom of the food chain that likely make PBDDs tend to ramp up activity. “It’s alarming that we are adding these on top of the chlorinated dioxins that are already at high levels,” Haglund says. Already, some Baltic Sea fish can’t be sold because they exceed European Commission thresholds for total dioxins, he adds. The toxins may have always been present, but they escaped notice until Haglund and colleagues stumbled across them. In 2000, the group was concerned about polybrominated diphenyl ether (PBDE) flame retardants. “There had been indications that brominated diphenyl ethers might be pyrolyzed to dioxins and could be emitted to

Australia in a new light

Australia will phase out incandescent light bulbs within 3 years in a bid to curb greenhouse gas (GHG) emissions, making it the first country in the world to do so. Environment Minister Malcolm Turnbull announced in February that incandescent bulbs, which lose much of their energy as heat, will be replaced by more efficient compact fluorescent bulbs by 2009. He hopes the ban will help cut 800,000 metric tons by 2012 from Australia’s current GHG emissions and lower household lighting costs by 66%. Per capita, Australians are among the world’s biggest producers of GHGs. In California, state assembly member Lloyd Levine introduced a bill in January that would ban the sale of traditional light bulbs by 2012.

Forcing water through nanotubes

Nanotubes are notorious for their water phobia, but a new study published in Nano Letters (2007, 7, 697– 702) reports using electricity not only to make the nanotubes waterfriendly but also to fine-tune the rate of water flow through them. By applying a small positive potential of 1.7 volts to the membranes of nanotubes while giving the water a negative potential, Nikhil Karotkar of Rensselaer Polytechnic Institute and his team succeeded in changing nanotubes’ water affinity—they now allowed water to pass through. And raising the water’s charge sped up its flow. The ability to micromanipulate water movement through these tiny tubes opens up new avenues for applications, including nanofiltration for supplying clean drinking water.

MAY 1, 2007 / Environmental Science & Technology n 3033

U.S. EPA / Energystar

Another toxin for the Baltic Sea?

Environmentalt News the environment by incinerators for municipal solid waste, so we started a broad screening for brominated dioxins and furans,” Hag lund says. Their results surprised them. Even though fish from only one of nine sites tested positive for the brominated compounds, all eight samples from that site had similar levels of di-, tri-, and tetrabromodioxins. The compounds were inconsistent with breakdown products of PBDEs and seemed to have appeared out of nowhere. The team found that on the ba-

sis of geographical distribution, the Baltic PBDDs are likely to be produced naturally in coastal waters. The next step was to develop a possible pathway for biological synthesis of PBDDs. “Analytically, it’s a slam dunk,” says Christopher Reddy of Woods Hole Oceanographic Institute. “They made a convincing case that these compounds are natural. I’m not surprised that they’re natural, but I’m impressed with their detective work,” he says. The discovery that organisms make PBDDs begs a question, Red-

Scaling up microbial fuel cells The new and improved anode looks like a laboratory bottle brush and provides essentially unlimited space for bacterial attachment to a highly conductive material, notes Logan. It is made by a company

A brush anode and a tube cathode are shown next to an assembled cube microbial fuel cell containing one brush anode and two tube cathodes.

that makes bottle brushes and can be manufactured inexpensively. With a brush anode in a cube reactor, Logan’s group achieved a power density of 2400 milliwatts per square meter, a new high for an air-cathode system. Power density was normalized to the area of the cathode rather than the anode, because “the anode surface area is so large that it no longer would affect power generation,” says Logan. The cathode, likely to remain the most expensive component, “is what is limiting power


Yi Zuo, Pennsylvania State Universit y

Microbial fuel cells (MFCs) remain a laboratory curiosity—bugs powering bug-sized motors by converting sugar or other organic materials into electricity. In a move to scale up the power generated by MFCs, Bruce Logan and colleagues at the Pennsylvania State University developed two new electrodes. The work is described in a companion pair of papers published in this issue of ES&T (pp 3341–3346; 3347–3353). In the first paper, graphite-fiber brushes function as anodes, and in the second, tubular membranes serve as air cathodes. “We can scale from little to big, because we just put in more brushes or we make bigger brushes. We can put in more tubes or make different size tubes. It’s the first time that we conceptually know how to move from the bench to large scale,” explains Logan. “We have followed this work with interest,” says John Hruby, an environmental supervisor with Gerber Products Co. “These two papers are breakthroughs in engineering real systems applicable in today’s world,” he says. Hruby believes that eventually MFCs can help the food processing industry convert high-strength organic wastewater, which is expensive to treat and dispose of, into an asset.

dy says: “Why are they making these things?” Many marine organisms make toxins as chemical defenses, but why algae would produce dioxins is not clear, he adds. Although the research points out a natural source for brominated dioxins that is unassociated with flame retardants, several researchers say that doesn’t let chemical manufacturers off the hook. “In mass-balance terms, we have to keep in mind that industry still delivers a significant amount,” Reddy says. —ERIKA ENGELHAUPT

on a surface-area basis.” The MFC produced 73 watts per cubic meter of its reactor volume; a reactor the size of a typical refrigerator might produce 100 watts. Logan’s group turned a nonconductive ultrafiltration (UF) tube membrane into a tubular cathode by applying a conductive graphite coating and a nonprecious-metal catalyst. This cathode produced “a power density that is comparable to our first-generation cube reactors,” says Logan. Although the UF membrane is not optimal because of high internal resistance, “other membrane materials are available that produce substantially lower internal resistances,” he says. “If these other materials can be made as tubular cathodes, we will have a relatively inexpensive and functioning MFC,” he adds. MFC experts agree that 3D electrodes are the way to go. Uwe Schröder of the University of Greifs wald (Germany) notes that brush anodes should be less susceptible to clogging than packed electrodes. Lars Angenent of Washington University says he is especially excited about scalability of the tubular cathode and notes that both the brushes and tubes should be cheap to manufacture. However, it is still too soon to predict what impact the improved anode and cathode will have in the real world, Schröder says. —BARBARA BOOTH

Courtesy of Willy Verstr ae te

Microbial fuel cells (MFCs), which rely on microbes to generate electricity from organic and inorganic matter, have long held the promise of creating clean energy. But so far, the technology has required expensive inputs to “feed” microbes the electrons they need. Now, researchers have developed the first MFC that can produce power while removing nitrate from wastewater, without the need for additional energy sources.

Microbes sit directly on the graphite anode and cathode in this microbial fuel cell. The colonies can clean water of nitrate almost completely, without extra inputs, while they generate a small amount of electricity.

The new MFC, which is described in this issue of ES&T (pp 3354–3360), relies on microbes sitting on both the anode and cathode without any intermediary. Electrons supplied by microbes at the anode are used by microbes at the cathode for complete denitrification, reducing potentially harmful nitrates to harmless nitrogen gas. Previously, a team led by Kelvin Gregory (now at Carnegie Mel-

lon University) and Derek Lovley of the University of Massachusetts, Amherst, showed in principle that some species, such as Geobacter, can interact directly with both the anode and the cathode, without any mediating materials in between (Environ. Microbiol. 2004, 6, 596–604). Their lab setup still required an outside power source to keep the current going, but it served as the starting point for developing a sustainable biological fuel cell, says Peter Clauwert of the University of Ghent (Belgium), the first author of the new work that proves the concept. Clauwert and his co-workers took soil and sediment samples that contained various microbes and directly incubated them on the surface of graphite beds in both the anode and cathode chambers of an MFC for 1 month. “You really have to have patience with it,” Clauwert says. “If I had stopped at 3 weeks, you would see nothing.” Once the microbial populations equilibrated, they released a steady current in the system. The researchers also observed the almost complete denitrification of a nitrate-laden solution fed through the cathode side of the reactor in an experiment run over 8 months. Although cleaner water is usually a byproduct of MFCs, it is not necessarily the endpoint. But in this totally microbe-powered system, clean water could be the goal, the authors say, with the added benefit of some extra electricity to run it. The team’s attempts to boost the power output of the MFC, however, have led to deterioration in the bugs’ efficacy in denitrifying the water. “In many countries [such as The Netherlands or northern Belgium] where we have to cut down on agricultural nitrate,” the system “could treat groundwater to be used as drinking water,” says coauthor Willy Verstraete, head of the Laboratory of Microbial Ecology and Technology at the University

News Briefs Selenium: a potential mercury shield?

Mercury and selenium concentrations are often strongly correlated in mammalian and fish livers, leading researchers to suspect that selenium buffers the toxic effects of mercury. New evidence supporting this hypothesis comes from a study that traces levels of the two elements over the past 1500 years in seal hairs and bird poop preserved in Antarctic sediments. Various metabolic processes require small amounts of selenium, and its environmental levels have remained fairly constant over the years. Yet levels of selenium and mercury rose and fell roughly in step in seal hairs and bird droppings. This suggests that selenium weeds out free mercury by binding to it (mercury is known to have an affinity for selenium), the authors report in Environmental Toxicology and Chemistry (2007, 26, 381–386).

EPA nanotech report

The U.S. EPA Science Policy Council released a white paper on nanotechnology in February. The document surveys the nanotechnology field and its potential applications for human health and the environment. It also lists potential risks, scientific issues, and areas for future research. The peer-reviewed paper was written by an intra-agency working group formed in December 2004. The group encourages collaborations with other agencies on potential environmental applications and risks of the technology. The authors recommend gaining a better understanding of the structure of nanomaterials, how they are transported through the environment, and their impact on human health and ecosystems. The experts also stress the need for sustainable technologies. To read the paper, go to www. epa.gov/osa/nanotech.htm.


NOAA

Taking out nitrates with a self-sufficient MFC

Environmentalt News of Ghent. But, he cautions, scaling up the technology is far off. “If we ever go industrial, [treatment] would have to be thousands of gallons per hour. It’s now at liters per day.” Other leading MFC researchers call the new work “excellent.” The researchers “really demonstrated the first anaerobic cathode which does not need any external energy input—which is producing, in an unambiguous way, excess energy,” says Gregory. However, scaling up

such a process remains challenging, he says. Bruce Logan of Pennsylvania State University notes the group’s departure from ferricyanide, an expensive component that has boosted power output for other MFCs from Verstraete’s lab. Logan agrees that the power output is relatively low compared with other systems, but he still thinks this is an important step for scaling up such MFCs. One stumbling block, he points out, would be in waste-

water treatments: although the system could get rid of nitrate, ammonia in wastewater would need to be oxidized to nitrate before the MFC bugs could denitrify it. Still, “they’ve gotten rid of a pollutant in water while at the same time generating electricity,” Logan says, “and they’ve accomplished both things with just bacteria catalyzing the reaction. . . . The fact that you can do this, that’s the exciting result.” —NAOMI LUBICK

Models reveal pesticide exposure routes eventually are excreted in urine. Researchers can monitor levels of exposure by analyzing the levels of these metabolites in urine. Understanding the routes of exposure, however, is trickier. McKone e t al .

Thomas McKone and his co-workers set out to answer a simple question: can the levels of pesticides in pregnant women in the Salinas Valley (Calif.) be correlated with the amounts of these chemicals used in surrounding farmlands? A simple statistical model suggested no direct correlation, but McKone and his colleagues at the Lawrence Berkeley National Laboratory and the University of California, Berkeley, didn’t stop there. The team combined multiple fate and exposure models with biomonitoring data. The results, published in this issue of ES&T (pp 3233–3240), show that the study population has a significantly higher intake of organophosphorus (OP) pesticides than the average woman in the same age group. The difference is attributed to exposure through air, water, and soil, but not from food. By using these models with hard data, the researchers could track how pesticides travel from the fields through different channels into the human body. Farmers use large amounts of pesticides in the Salinas Valley, an important agricultural region of California. In 2001, 240,000 kilograms of OP pesticides were applied in and around the valley. Humans metabolize commonly used OP pesticides, such as chlorpyrifos, diazinon, and malathion, into simpler compounds that

Map of the Salinas Valley showing average yearly pounds of OP pesticides applied per section (~1 square mile).

McKone and his team studied OP pesticide exposure in about 600 pregnant Latina women in the Salinas Valley. They were all part of a larger study on women and children’s environmental health, called the Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS) project. The concentration of the pesticide metabolites in the CHAMACOS population was compared with the average levels in women from across the U.S. (from


data collected by the National Health and Nutrition Examination Survey, or NHANES, in 1999–2000). This revealed that the Salinas Valley women are being exposed to “significantly higher” levels of the pesticides, says McKone. The next step was to determine how these women are being exposed. The team predicted that many different pathways were involved. And it was crucial to determine how much each route or source was contributing to the measured levels of pesticide metabolites in the women. First, they adapted the CalTOX fate and exposure model to estimate OP pesticide concentrations in outdoor air and soil near participants’ homes. They then combined the results with an indoor mass-balance model to estimate indoor air, dust, and surface concentrations. The team modeled nondietary exposures through inhalation, nondietary ingestion, and skin contact, and they estimated dietary exposures using the U.S. Food and Drug Administration’s Total Diet Study, which provides information on the levels of pesticide intake through “tableready” foods. When McKone and colleagues combined the results from the models with biomonitoring data, they found that the Salinas Valley population was receiving similar levels of the pesticides from their diet as women elsewhere in the U.S. The real culprit behind the higher levels of OP pesticide in this

Modeling Center at Trent University. Most environmental models merely predict “that if you use this amount of chemicals in this area you probably get this concentration in air, and this in water, and that in fish, and that’s about as far as they go,” he says. McKone’s work goes beyond that and “quantifies the whole journey [of pesticides] from sources into the environments . . . into outdoors, indoors, into [the] human body,” he says. Few studies “include how much [of a contaminant] is actually reaching us and how this results in concentrations in our tissue and fluids,” adds MacKay. This study provides “a shining example of the approach that should be taken for more substances. If that can be done, it will help the whole risk assessment process enormously.” —RHITU CHATTERJEE

News Briefs Toxic waste and race

Environmental laws in the U.S. don’t protect minorities any better than they did 20 years ago, according to the authors of a new report, Toxic Waste and Race at Twenty: 1987– 2007. Using 2000 census data, Robert Bullard, director of the Environmental Justice Resource Center at Clark Atlanta University, and colleagues find that race remains a dominant factor in siting hazardous-waste facilities. The study examines disparities by region and state and separately analyzes cities where most of these facilities are located, according to advance highlights released by the United Church of Christ in February. The full report chronicles environmental justice milestones and includes essays by advocates on topics such as the government’s response to Hurricane Katrina. More information can be found at www.nccecojustice.org/ toxicwasteandrace.htm.

Environmental costs of organic food

Buying organic and local foods is not necessarily better for the environment, according to a life-cycle assessment examining the environmental impacts of food from cultivation to consumption. The report by researchers at the Manchester Business School was conducted for the U.K. environment agency Department for Environment Food and Rural Affairs (Defra). Among its conclusions, the team finds that organic farming can use more land and release more nutrients to local water sources than conventional agriculture, and can have a larger carbon footprint. They add that buying locally isn’t always better than getting food from afar and that shopping by car may have higher environmental impacts than the entire food-distribution network, including air freight.


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population was exposure through air, water, and soil. Interpreting biomonitoring data on chemicals like OP pesticides is difficult because the chemicals don’t persist in the environment for very long and the data have an “inherent uncertainty and variability,” says Dana Barr, the chief of the pesticides laboratory at the Centers for Disease Control and Prevention’s National Center for Environmental Health. “If you take one spot, you may get no exposure where you’ve been exposed previously, and another time you can get peak exposure,” she says. McKone’s approach provides a unique way of getting at the complex mechanism of exposure to pesticides, Barr comments. The study is also very informative in terms of understanding the fate of pesticides, says Don MacKay of the Canadian Environmental

Mercury from underground estuary - American Chemical Society

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