Heavy metal – in and around the lake

Heavy metal pollution of lakes has a seriously detrimental impact on people and ecosystems that rely on such bodies of water. According to a study published in the current issue of Interdisciplinary Environmental Review, researchers have focused on the physicochemical properties and toxicology of water from and around Thane City of Maharashtra.

Environmental chemist Pravin Singare of Bhavan’s College, in Mumbai, and colleagues highlight the fact that fresh water bodies all over the world are becoming increasingly polluted day by day and that this represents a growing problem in the developing world and beyond. They suggest that regular monitoring is crucial for the well-being and health of the surrounding population and as such, the team has carried out a systematic study to estimate the physico-chemical parameters and level of toxic heavy metal content in the Jail Talav and Kalwa Lakes of Thane City, as perhaps being indicative of similar problems with other bodies of water.

The team’s measurements suggest that the presence of heavy metals such as iron, copper, nickel and zinc, which are essential for life at trace levels are well above permissible concentrations making them a significant threat to ecosystems and a problem for those who rely on the lakes for drinking water or crop irrigation. In addition mercury, arsenic and cadmium were all present at much higher than acceptable concentrations.

South Asia is home to more a fifth of the world’s population, the researchers say, and is facing a serious water crisis. “This region, which is in the grip of flood and drought cycles, needs a long-term strategy for management of its water resources,” the team says. Unfortunately, strategies adopted so far have all failed in India, the team asserts, this is obvious given the poor quality of the water revealed by their measurements of Jail Talav and Kalwa Lakes assuming these are typical of the region as a whole.

Food chain contamination by heavy metals has become an important issue partly because of the potential accumulation in biosystems, through contaminated water, the team adds. “A better understanding of heavy metal sources, their accumulation in water and the effect of their presence in water on plant systems are particularly impertinent in ongoing risk assessments,” the researchers say.

Singare, P., Naik, K., & Lokhande, R. (2011). Impact assessment of pollution in some lake water located at and around Thane City of Maharashtra, India: physico-chemical properties and toxic effects of heavy metal content Interdisciplinary Environmental Review, 12 (3) DOI: 10.1504/IER.2011.041819

The Martian Lake District

Three billion years ago, the red planet, Mars, was warm enough to sustain lakes of liquid water, according to satellite images just published in the journal Geology. Previously, astronomers had assumed that this period was simply too cold and arid for surface water.

Researchers at Imperial College London and University College London now suggest that during the Hesperian Epoch, the Martian surface around the equator was spotted with lakes, each approximately 20 kilometres across, formed from melted ice. Earlier studies had hinted at the warm and wet early history of Mars during the period 4 billion to 3.8 billion years ago, well before the Hesperian Epoch. Detailed images from NASA’s Mars Reconnaissance Orbiter, which is currently circling the planet, suggest that there were later warm and wet periods; age is determined by meteorite crater count. The evidence lies in several flat-floored depressions located above Ares Vallis, a giant gorge that runs 2000 km across the equator of Mars.

Martian lakes
Martian lakes

According to Nicholas Warner, of IC’s Department of Earth Science and Engineering, “Most of the research on Mars has focused on its early history and the recent past. Scientists had largely overlooked the Hesperian Epoch as it was thought that Mars was then a frozen wasteland. Excitingly, our study now shows that this middle period in Mars’ history was much more dynamic than we previously thought.”

Warner and colleagues, Sanjeev Gupta, Jung-Rack Kim, Shih-Yuan Lin, and Jan-Peter Muller, claim that there may have been increased volcanic activity, meteorite impacts or shifts in Mars’ orbit during this period, which could have warmed its atmosphere enough to melt ice. This in turn would have bolstered the greenhouse effect temporarily, trapping more heat from the sun and making the planet warm enough for liquid water to exist on its surface.

Until now, the Ares Vallis depressions have remained a mystery to scientists, although they suspected that their formation was due to sublimation of ice directly to water vapour. The loss of ice would have created cavities between the soil particles, which would have caused the ground beneath to subside.

Martian channels
Martian channels

The researchers have now discovered small, sinuous channels that connect the depressions, which they say could only have been formed by running water, essentially making the sublimation theory redundant. The team also compared the Mars images with images of thermokarst landscapes on Earth in places such as Siberia and Alaska. Thermokarst landscapes are areas where permafrost is melting, creating lakes that are interconnected by the same type of channels the team says exist on Mars. The team says that the melting ice created lakes that may have burst their banks allowing water to carve pathways through the frozen ground from higher lakes into lower-lying lakes.

UCL’s Muller who works at the Mullard Space Science Laboratory who carried out the 3D mapping of the Martian surface, explains how modelling with sub-metre resolution allowed the team to test their hypotheses much more rigorously than ever before.

Topographic image
Topographic image

One thing that the scientists do not yet know is how long the warm and wet period lasted during the Hesperian epoch or how long the lakes remained liquid. Nevertheless, the study may have implications for so-called “astrobiologists” looking for evidence of life on Mars. The team say these lake beds indicate regions on the planet that may have once been suitable for some form of microbial Martian life. As such, they represent good targets for future robotic missions seeking out ancient life on Mars.

LINKS

Geology, 2010, 38, 71-74
http://dx.doi.org/10.1130/G30579.1
Video flypast

Non-carbon nanotubes

Carbon nanotubes rose to prominence on the back of the buckyball chemistry revolution in the 1990s and are now emerging from prototype applications across academic and industrial laboratories. They have potential in microelectronic circuits, novel sensor devices, special light conductors, and light-emitting nanotubes for display technology.

Indeed, applications as diverse as medical technology, for fibres with ultrahigh tensile strength, in hydrogen storage, for rechargeable batteries, in catalysis, and in nanotechnology are being developed. There are even applications for antifouling coatings for ships.

With this in mind, chemists in Germany who work with inorganic materials have now developed an approach to synthesising tin sulfide nanotubes which could expand the nanotube concept much further still and open up yet more avenues for applications. After all, carbon does not have a monopoly on nanotubes. Early in the development of tubular fullerene structures, inorganic chemists opted to make their nanotubes from metals and non-carbon atoms: tungsten sulfide, nickel chloride, vanadium sulfide, titanium sulfide, and indium sulfide. Many others have been produced.

Wolfgang Tremel and colleagues, Aswani Yella, Martin Panthoefer, Helen Annal Therese, Enrico Mugnaioli, Ute Kolb, of the Johannes Gutenberg-Universitaet in Mainz, Germany, have now developed a new process for the production of tin sulfide nanotubes, which they report in the Wiley journal Angewandte Chemie, the researchers found they could “grow” tin sulfide nanotubes from a drop of metal using a bismuth catalyst.

The team were faced with one of the fundamental problems of synthesising sulfidic nanotubes in that they require a high temperature to force the planar layers of material to bend and fuse into tubular structures. For tin sulfide, the situation is complicated still further by an unstable intermediate that is almost impossible to trap because it decomposes at a lower temperature.

The researchers used a different approach. First, they employed a vapour-liquid-solid (VLS) process, a technique borrowed from semiconductor scientists for producing nanowires as opposed to nanotubes. The process involved mixing bismuth metal powder with minute flakes of tin sulfide and heating this mixture in a tube furnace under a stream of the relatively unreactive noble gas argon. The product of the reaction forms a deposit at the cooler end of the tube.

The team explains that tiny droplets on the nanometre scale are form within the oven. These nanodroplets act as local points of contact for the tin so that the reactants become concentrated within the metal droplet and nanotubes can then grow from these seeds.

“In this process, the metal drop is obtained as a sphere at the end of the tube, and the nanotubes grow out of the sphere like a hair out of a follicle,” explains Tremel. “Catalysis by the metal droplet makes growth possible at low temperatures.”

The team has successfully grown nanotubes comprising multiple layers of tin sulfide with few defects. The nanotubes have diameters of between 30 and 40 nanometres and are 100 to 500 nm in length.

Tin sulfide nanotubes grow from droplets (Credit: Tremel et al/Wiley-VCH)

Angewandte Chemie, in press

Group of Prof. Dr. Wolfgang Tremel