Australasian Science: Australia's authority on science since 1938

Toxic Legacy of Volcanic Ash

By Jenny Bennett

Toxic trace elements deposited by volcanic ash can persist in the environment for decades, with implications for water purity.

Explosive volcanic eruptions cause considerable environmental repercussions that affect many of the Earth’s cycles, particularly the water cycle. For example, they can induce climatic effects associated with the injection of sulfur aerosols into the stratosphere on a truly global scale (AS, March 2011, pp.21–23). Other consequences include changes in the evaporation of water from the Earth’s surface due to ashfall deposits, and the input of key micronutrients such as iron into the ocean.

Although eruptions are short-lived, ashfall deposits can remain in the local environment for periods lasting years or even decades. In addition, strong wind storms may resuspend the ash, causing environmental effects similar to those caused by the original eruption.

One of the most important environmental concerns of ashfall is related to the water-soluble constituents in volcanic ash. The formation of these materials results from a complex interaction between particles and volatile compounds in the volcanic plume. They consist of macro- and micronutrients that fertilise continental and ocean ecosystems, but they also contain trace elements that may be toxic.

By analysing the ash from five historical eruptions over 20 years in the Southern Andes, Jose-Luis Fernandez-Turiel of the Institute of Earth Sciences, Barcelona, and co-workers have found that the major potentially toxic trace elements deposited by ashfall were arsenic, copper, fluoride, molybdenum, nickel, lead and zinc. All of these elements are included in drinking water guidelines due to their possible toxicity.

The results are reported in the April issue of Environmental Chemistry.

Chemical Control of Weeds
Weeds are plants that grow where they’re not wanted. They compete with valuable crops for water, nutrients, light and space.

Weed control can be achieved by mechanical (cultivation and moving), biological and chemical methods. The advantages of this last method include rapid treatment time, selectivity and its cost-effectiveness.

A widely used weed control agent that selectively kills grass species is the cyclohexanedione oxime ether class of herbicides. The April issue of the Australian Journal of Chemistry profiles the search by Keith Watson of ICI Australia Research Laboratories for new molecules within this class of herbicide, including the previously untold story of the discovery and development of the herbicide butyroxydim, the most active of all the cyclohexanediones. To date, eight cyclohexane-dione herbicides have been brought to market, including six general grass-killers, one selective compound for grass weed control in wheat and barley, and one rice-selective herbicide for barnyard grass control.

After 20 years of concentrated exploration of the cyclohexanediones, it appears that there has been little exploratory chemistry or patenting activity in this area. Given the enormous scope for structural variation within this class of compound, it seems unlikely that the perfect structure for optimal activity has been discovered yet.

The first X-ray crystal structure of a cyclohexanedione bound to its target enzyme has only recently been solved, and has provided an explanation for some of the observed mutations in certain grasses that are resistant to the cyclohexanedione herbicides. Perhaps this structural information will trigger another wave of cyclohexanedione chemistry, enabling the discovery of more active molecules that are able to control resistant grass weeds.

Jenny Bennett is Publisher – Chemical Sciences at CSIRO Publishing.