Applying black carbon converted from silver-leaf nightshade – a local weed – to soil is a novel, low cost method to "weed out" chromium contamination, Professor Nanthi Bolan and Girish Choppala from CRC CARE and the University of South Australia told the CleanUp 2011 conference in Adelaide this month.
"While one form of chromium (Cr(III)) may be important to human health at low levels, another (Cr(VI)) is highly toxic to plants, marine life and humans," Professor Bolan said.
"Approximately 3400 tonnes of (Cr(VI)) is used to treat poles used in vineyards in Australia – the most common source of this toxic metal.
"The best strategy, we’ve found, is to change the soluble, mobile and toxic (Cr(VI)) form to the insoluble and less toxic (Cr(III))."
The common sources of chromium, Choppala said, are timber treatment sites, vineyards that use treated timber, the leather tanning industry and electroplating workshops, industrial centres and airbases.
He said (Cr(VI)) is commonly used to protect timber and metals, but its mobility means that it can easily dissolve and leak into soil or water, where it can persist for years and get into the food supply.
"For example, timber is often treated with chromated copper arsenate (CCA) as it prevents wood from being invaded by fungus and rotting, thus increasing its lifespan. However, the treated timber releases large quantities of (Cr(VI)) into the environment each year, not to mention when a vineyard replaces its timber trellis posts. We can end up with around 160 000 cubic metres of toxic wood in dumping sites in South Australia alone," Choppala said.
The idea of using silver-leaf nightshade came to Professor Bolan when he was on a bushwalk: "This weed is native to Mexico and the south-west areas of United States, and can be found in many areas, including cultivated land, orchards, managed grasslands, riverbanks and roadsides."
"It is harmful to a wide range of crops, livestock production and the environment, as it competes for moisture and nutrients with other plants. The most affected crops include cotton, sorghum, wheat and maize."
The difficulty of controlling its spread, together with its destructive properties, has resulted in the weed being declared as noxious in certain countries, Professor Bolan said.
"It can be found all over the place in South Australia, and we picked some from the University of SA campus at Mawson Lakes, to test its effectiveness in countering the toxic (Cr(VI))."
The research team found that black carbon produced by incinerating this weed was highly effective in ridding sample soils of (Cr(VI)). Adding black carbon to the soil decreased (Cr(VI)) leaching by up to 10.5 per cent and 22.6 per cent within 3 days in acidic and alkaline soils, respectively.
"The high efficiency of black carbon may be attributed to its oxygen containing groups, which changes (Cr(VI)) to the less mobile and less toxic (Cr(III)) form," Choppala explained.
The researchers recommend spreading the char on vineyard soil before inserting timber, or coating black carbon on the wood before using it in vineyards or parks.
