Using Nanotechnology to Measure Mercury

Mercury is a common environmental pollutant with well known bioaccumulative and neurotoxic properties. In the gas phase, elemental mercury has an average atmospheric residence time of 5.7 years before it is absorbed by aquatic life and enters into the food chain. With estimates that ~2400 tons of mercury per year is currently released to the atmosphere as a result of human activity, it is in particular a serious threat to children and pregnant women. According to the US EPA, more than 60,000 babies may be born in the US alone each year at risk of mercury-related learning and developmental problems because pregnant mothers either inhale volatile mercury compounds or ate mercury contaminated fish.

Where is all the mercury coming from and what can be done to ultimately stop it?

A partial answer can be found in the tens of thousands of coal-burning power stations world wide - and this number is growing rapidly. This multi-trillion dollar electricity generation industry and other industries such as alumina refining are the major source of mercury air emissions and are the latest target of US federal and state clean air regulations. Beginning in 2010, the cap-and-trade standards are going to impose the total mercury emission from US power plants to 38 tons annually (a 21% reduction vs. 1999 levels).

RMIT University researchers have used nanotechnology to create a pioneering sensor that can precisely measure one of the world's most poisonous substances, mercury. The mercury sensor developed by RMIT's Industrial Chemistry Group uses nano-engineered gold structures that attract mercury molecules.

The first step of controlling any kind of toxin (including mercury emissions) is to be able to measure them, according to Professor Suresh Bhargava, Dean of the School of Applied Sciences at RMIT University and leader of the Industrial Chemistry Group. "Traditional mercury sensors can be unreliable because industrial chimneys release a complex concoction of volatile organic compounds, ammonia and water vapor, which interfere with their monitoring systems - a significant challenge to overcome", says Bhargava.

"In order to better understand mercury emission sources, migration, and environmental and societal impacts of Hg vapor, continuous emissions monitors (CEMs) located at strategic points within a given process is a must," Bhargava said. Currently, mercury vapor detection is typically performed, mostly in laboratories, by the use of cold vapor atomic absorption (CVAA) or atomic fluorescence (CVAF) spectrometry following Hg capturing procedures.

Although such spectrometry based systems have excellent mercury detection capabilities, industries like the alumina and many of the coal fired power plants are reported to emit high concentrations of Hg vapor in the milligrams rather than micrograms per cubic meter range. Bhargava goes on to say, "combine this with the complex concoction of volatile organic compounds, high costs and delicate nature their incompatibility to function as online CEMs in large industry quickly becomes apparent".

Bhargava and his colleagues have tackled this issue head on by combining the humble quartz crystal microbalance (QCM), a cheap and inexpensive mass sensitive transducer platforms, with nanotechnology principles. The mercury sensor was developed with the use of patented electrochemical processes that enabled the RMIT researchers to alter the surface of the gold, forming hundreds of tiny nano-spikes, each one about 1,000 times smaller than the width of a human hair.

Figure 1. Scanning Electron Micrographs of pure gold surface electro-deposited with gold nanospikes imaged at 0 seconds, 15 seconds, 90 seconds and 150 seconds (clockwise), illustrating the nucleation and nanostructural growth formation in time. Scale bar is 500 nm - 1nm (nano meter) is 10-9 meters or 1 billionth of a meter).

Part of the research published earlier this year in Sensors and Actuators B: Chemical journal indicated that mercury affinity could be increased by forming a surface with an increased number of active sites. "We've known since ancient times that gold attracts mercury, and vice versa, but a regular gold surface doesn't absorb much vapor and any measurements it makes are inconsistent," Professor Bhargava said. "Our nano-engineered gold surfaces are 180 per cent more sensitive than non-modified surfaces when operating at 89°C". "They're finely targeted, so they're unaffected by the usual gases found in effluent gas streams and the sensors we've created using those nano-engineered surfaces have worked successfully at a range of extreme temperatures over many months, just as they'll need to in an industrial location."

Figure 2. Coloured Scanning Electron Micrograph of 150 second electro-deposited gold. Scale bar 500nm. (Image processed colour applied).

The breakthrough is attributed to nano-engineered gold surfaces having a higher affinity for Hg, and "not solely due to increased surface area effects," says Bhargava. His teams results indicate that mercury adsorption on gold varies substantially for different morphologies. The developed nano-spike surfaces retained their affinity for longer periods (time) and large number of Hg monolayer coverage, while decreasing the influence of the contaminant gases present in the industrial effluent streams.

Figure 3. Coloured Scanning Electron Micrograph of evaporated gold thin film before electro-deposition. Scale bar 500nm (Image processed colour applied).

Funded through an Australian Research Council Linkage grant, the project was supported by leading industry partners, who have now engaged RMIT to develop a mercury sensing device for a pilot plant trial at one of their Australian refineries.

Sources and Further Reading

• Mercury Emissions from Electric Power Plants: An Analysis of EPA's Cap-and-Trade Regulations
• Funding struggle for mercury monitoring
• Mercury Rising - Up the Food Web