September 13, 2011:
Using a noxious weed to clean up a toxic metal can rid the
environment of two harmful substances, protecting people, water
life, soil health and food crops, a new study shows.
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 Mr Girish Choppala from CRC CARE and the University of
South Australia will tell the CleanUp 2011 conference in Adelaide
“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,” says Prof. Bolan.
“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
The common sources of chromium, Mr Choppala says, are timber
treatment sites, vineyards that use treated timber, the leather
tanning industry and electroplating workshops, industrial centres
He explains that 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
“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.”
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, Prof. Bolan says.
“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% and 22.6% within 3 days in acidic and alkaline soils,
“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,” Mr Choppala explains.
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.
Prof. Bolan and Mr Choppala will deliver their paper at 4.40pm on
CleanUp 2011 incorporates the 6th International Workshop on
Chemical Bioavailability in the Terrestrial Environment (7–9
September 2011) and the 4th International Contaminated Site
Remediation Conference (11–15 September 2011). It is hosted
by the CRC for Contamination Assessment and Remediation of the
Environment (CRC CARE).
CleanUp 2011 is being held at the Hilton Adelaide hotel in
Adelaide, South Australia.
Mr Girish Choppala, CRC CARE and UniSA, ph +61 8 8302 6291 or 0425
Prof. Nanthi Bolan, CRC CARE and UniSA, ph +61 8 8302 6218 or 0447
Meredith Loxton, Acting Communications Manager, CRC CARE, 08 8302
3925 or 0429 779 228
Sharmin Patard, Communications Officer, CRC CARE, 0437 917 352