Toxic mercury, accumulating in the Arctic, springs from a hidden source

Harvard study finds circumpolar rivers most responsible for high levels of mercury in the Arctic.

The Lena River delta. The Lena is one of several major rivers that flows northward into the Arctic Ocean.

Cambridge, Mass. - May 21, 2012 - Environmental scientists at Harvard have discovered that the Arctic accumulation of mercury, a toxic element, is caused by both atmospheric forces and the flow of circumpolar rivers that carry the element north into the Arctic Ocean. 

 

While the atmospheric source was previously recognized, it now appears that twice as much mercury actually comes from the rivers.

The revelation implies that concentrations of the toxin may further increase as climate change continues to modify the region's hydrological cycle and release mercury from warming Arctic soils.

"The Arctic is a unique environment because it's so remote from most anthropogenic (human-influenced) sources of mercury, yet we know that the concentrations of mercury in Arctic marine mammals are among the highest in the world," says lead author Jenny A. Fisher, a postdoctoral fellow in Harvard's Atmospheric Chemistry Modeling Group and the Department of Earth and Planetary Sciences (EPS). "This is dangerous to both marine life and humans. The question from a scientific standpoint is, where does that mercury come from?"

The results of the study, which was led jointly by Harvard School of Engineering and Applied Sciences (SEAS) and Harvard School of Public Health (HSPH), appeared in the journal Nature Geoscience on May 20.

Mercury is a naturally occurring element that has been enriched in the environment by human activities such as coal combustion and mining. When converted to methylmercury by microbial processes in the ocean, it can accumulate in fish and wildlife at concentrations up to a million times higher than the levels found in the environment.

"In humans, mercury is a potent neurotoxin," explains co-principal investigator Elsie M. Sunderland, Mark and Catherine Winkler Assistant Professor of Aquatic Science at HSPH. "It can cause long-term developmental delays in exposed children and impair cardiovascular health in adults."

Mercury is considered a persistent bioaccumulative toxin because it remains in the environment without breaking down; as it travels up the food chain, from plankton to fish, to marine mammals and humans, it becomes more concentrated and more dangerous.

"Indigenous people in the Arctic are particularly susceptible to the effects of methylmercury exposure because they consume large amounts of fish and marine mammals as part of their traditional diet," Sunderland says. "Understanding the sources of mercury to the Arctic Ocean and how these levels are expected to change in the future is therefore key to protecting the health of northern populations."

Sunderland supervised the study with Daniel Jacob, Vasco McCoy Family Professor of Atmospheric Chemistry and Environmental Engineering at SEAS, where Sunderland is also an affiliate.

Mercury enters the Earth's atmosphere through emissions from coal combustion, waste incineration, and mining. Once airborne, it can drift in the atmosphere for up to a year, until chemical processes make it soluble and it falls back to the ground in rain or snow. This deposition is spread worldwide, and much of the mercury deposited to Arctic snow and ice is re-emitted to the atmosphere, which limits the impact on the Arctic Ocean.

"That's why these river sources are so important," says Fisher. "The mercury is going straight into the ocean."

The most important rivers flowing to the Arctic Ocean are in Siberia: the Lena, the Ob, and the Yenisei. These are three of the 10 largest rivers in the world, and together they account for 10% of all freshwater discharge to the world's oceans. The Arctic Ocean is shallow and stratified, which increases its sensitivity to input from rivers.

Previous measurements had shown that the levels of mercury in the Arctic lower atmosphere fluctuate over the course of a year, increasing sharply from spring to summer. Jacob, Sunderland, and their team used a sophisticated model (GEOS-Chem) of the conditions in the Arctic Ocean and atmosphere to investigate whether variables like melting ice, interactions with microbes, or the amount of sunlight (which affects chemical reactions) could account for the difference.

Incorporating those variables, however, was not enough.

The GEOS-Chem model, which is backed by rigorous environmental observations and more than a decade of scientific review, quantifies the complex nuances of the ocean-ice-atmosphere environment. It takes into account, for example, ocean mixing at various depths, the chemistry of mercury in the ocean and the atmosphere, and the mechanisms of atmospheric deposition and re-emission.

When the Harvard team adapted it for their Arctic mercury simulations, the only adjustment that could explain the spike in summertime concentrations was the incorporation of a large source to the Arctic Ocean from circumpolar rivers. This source had not been recognized previously.

As it turns out, approximately twice as much mercury in the Arctic Ocean originates from the rivers as from the atmosphere.

The researchers' new model describes the known inputs and outputs of mercury to the Arctic Ocean. (Image courtesy of Jenny Fisher.)

"At this point we can only speculate as to how the mercury enters the river systems, but it appears that climate change may play a large role," says Jacob. "As global temperatures rise, we begin to see areas of permafrost thawing and releasing mercury that was locked in the soil; we also see the hydrological cycle changing, increasing the amount of runoff from precipitation that enters the rivers."

"Another contributing factor," he adds, "could be runoff from gold, silver, and mercury mines in Siberia, which may be polluting the water nearby. We know next to nothing about these pollution sources."

As the contaminated river water flows into the Arctic Ocean, Jacob says, the surface layer of the ocean becomes supersaturated, leading to what scientists call an "evasion" of mercury from the ocean into the lower atmosphere.

"Observing that telltale supersaturation, and wanting to explain it, is what initially motivated this study," says Fisher. "Relating it to Arctic rivers was detective work. The environmental implications of this finding are huge. It means, for example, that climate change could have a very large impact on Arctic mercury, larger than the impact of controlling emissions to the atmosphere. More work is needed now to measure the mercury discharged by rivers and to determine its origin."

source: http://www.seas.harvard.edu/news-events/press-releases/toxic-mercury-accumulating-in-the-arctic-springs-from-a-hidden-source

Oxygen-separation membranes could aid in CO2 reduction

Ceramic membranes may reduce carbon dioxide emissions from gas and coal-fired powerplants.

It may seem counterintuitive, but one way to reduce carbon dioxide emissions to the atmosphere may be to produce pure carbon dioxide in powerplants that burn fossil fuels. In this way, greenhouse gases — once isolated within a plant — could be captured and stored in natural reservoirs, deep in the Earth’s crust.

Such “carbon-capture” technology may significantly reduce greenhouse gas emissions from cheap and plentiful energy sources such as coal and natural gas, and help minimize fossil fuels’ contribution to climate change. But extracting carbon dioxide from the rest of a powerplant’s byproducts is now an expensive process requiring huge amounts of energy, special chemicals and extra hardware.

Now researchers at MIT are evaluating a system that efficiently eliminates nitrogen from the combustion process, delivering a pure stream of carbon dioxide after removing other combustion byproducts such as water and other gases. The centerpiece of the system is a ceramic membrane used to separate oxygen from air. Burning fuels in pure oxygen, as opposed to air — a process known as oxyfuel combustion — can yield a pure stream of carbon dioxide.

The researchers have built a small-scale reactor in their lab to test the membrane technology, and have begun establishing parameters for operating the membranes under the extreme conditions found inside a conventional powerplant. The group’s results will appear in the Journal of Membrane Sciences, and will be presented at the International Symposium on Combustion in August.

Ahmed Ghoniem, the Ronald C. Crane Professor of Engineering at MIT, says ceramic membrane technology may be an inexpensive, energy-saving solution for capturing carbon dioxide.

“What we’re working on is doing this separation in a very efficient way, and hopefully for the least price,” Ghoniem says. “The whole objective behind this technology is to continue to use cheap and available fossil fuels, produce electricity at low price and in a convenient way, but without emitting as much CO₂ as we have been.”

Ghoniem’s group is working with other colleagues at MIT, along with membrane manufacturers, to develop this technology and establish guidelines for scaling and implementing it in future powerplants. The research is in line with the group’s previous work, in which they demonstrated a new technology called pressurized oxyfuel combustion that they have shown improves conversion efficiency and reduces fuel consumption.

Streaming pure oxygen

The air we breathe is composed mainly of nitrogen (78 percent) and oxygen (21 percent). The typical process to separate oxygen from nitrogen involves a cryogenic unit that cools incoming air to a temperature sufficiently low to liquefy oxygen. While the freezing technique produces a pure stream of oxygen, the process is expensive and bulky, and consumes considerable energy, which may sap a plant’s power output.

Ghoniem says ceramic membranes that supply the oxygen needed for the combustion process may operate much more efficiently, using less energy to produce pure oxygen and ultimately capture carbon dioxide. He envisions the technology’s use both in new powerplants and as a retrofit to existing plants to reduce greenhouse gas emissions.

Ceramic membranes are selectively permeable materials through which only oxygen can flow. These membranes, made of metal oxides such as aluminum and titanium, can withstand extremely high temperatures — a big advantage when it comes to operating in the harsh environment of a powerplant. Ceramic membranes separate oxygen through a mechanism called ion transport, whereby oxygen ions flow across a membrane, drawn to the side of the membrane with less oxygen.

A two-in-one solution

Ghoniem and his colleagues built a small-scale reactor with ceramic membranes and studied the resulting oxygen flow. They observed that as air passes through a membrane, oxygen accumulates on the opposite side, ultimately slowing the air-separation process. To avert this buildup of oxygen, the group built a combustion system into their model reactor. They found that with this two-in-one system, oxygen passes through the membrane and mixes with the fuel stream on the other side, burning it and generating heat. The fuel burns the oxygen away, making room for more oxygen to flow through. Ghoniem says the system is a “win-win situation,” enabling oxygen separation from air while combustion takes place in the same space.

 

Members of the Ghoneim lab.

MIT researchers are investigating ceramic membranes as a way to reduce carbon dioxide emissions in powerplants. In their system, the air (red dots) that’s needed for combustion passes over a ceramic membrane (red layer). Only oxygen passes through the membrane, mixing with fuel (black and green dots) to produce a pure stream of carbon dioxide and water. After evaporating water, carbon dioxide can then be captured and stored.

“It turns out to be a clever way of doing things,” Ghoniem says. “The system is more compact, because at the same place where we do separation, we also burn. So we’re integrating everything, and we’re reducing the complexity, the energy penalty, and the economic penalty of burning in pure oxygen and producing a carbon dioxide stream.”

The group is now gauging the system’s performance at various temperatures, pressures and fuel conditions using their laboratory setup. They have also designed a complex computational model to simulate how the system would work at a larger scale, in a powerplant. They’ve found that the flow of oxygen across the membrane depends on the membrane’s temperature: The higher its temperature on the combustion side of the system, the faster oxygen flows across the membrane, and the faster fuel burns. They also found that although the gas temperature may exceed what the material can tolerate, the gas flow acts to protect the membrane.

“We are learning enough about the system that if we want to scale it up and implement it in a powerplant, then it’s doable,” Ghoniem says. “These are obviously more complicated powerplants, requiring much higher-tech components, because they can much do more than what plants do now. We have to show that the [new] designs are durable, and then convince industry to take these ideas and use them.”

The lab work and the models developed in Ghoniem’s group will enable the design of larger combustion systems for megawatt plants. 

Madhava Syamlal, focus area leader for computational and basic sciences at the National Energy Technology Laboratory, says simulations such as Ghoniem’s will help push next-generation technologies such as oxygen-separating membranes into powerplants. “We have seen that in other areas, like aircraft, simulations really improve how the product is developed,” Syamlal says. “You can use simulations and even skip some of the intermediate testing and go directly to designing and building a machine. In the energy industry, these are the pieces we need to increase the scale quite rapidly.”

Ghoniem’s group includes research scientist Patrick Kirchen and graduate students James Hong and Anton Hunt, in collaboration with faculty at King Fahed University of Petroleum and Minerals (KFUPM) in Saudi Arabia. The research was funded by KFUPM and King Abdullah University of Science and Technology.

source: http://web.mit.edu/newsoffice/2012/next-generation-carbon-0521.html