A rice variety originally from West Bengal is able to accumulate high concentration of the metal in its grain.
It is a rice variety with a silver touch, literally. Garib-sal one of 505 types of rice plants tested by scientists, is capable of absorbing silver found naturally in soil and accumulating it in the grain to unusually high levels of 15 mg per kg. The rice was able to accumulate high quantities of silver even when the soil contained only about 0.15 mg per kg.
The unusually high accumulation of silver in the grain and other parts of the plant, researchers say, throws open the possibility of commercial extraction of the metal through farming.
Also found in the stem and leaves
The maximum concentration of silver is in the grains while the stem and leaves accumulate it to a lesser extent. Silver accumulation is largely in the bran of the rice grain, and once polished, the silver in the grain is cut significantly.
“We do not advocate consumption of the unpolished rice as staple food. If the rice is polished very well then it may not lead to silver toxicity,” says Prof. T. Pradeep from the Department of Chemistry, IIT Madras and the corresponding author of the paper.
Silver is not known to accumulate in the reproductive tissues of any cereal, and in agricultural crops the amount of silver that gets accumulated is less than 1 mg per kg of dry weight of the plant.
Researchers at the Indian Institute of Technology (IIT) Madras stumbled upon the rice variety while screening for different metal ions in 505 rice varieties. Only nine varieties showed high silver accumulation, with Garib-sal showing the highest concentration.
The rice varieties are maintained by Dr. Debal Deb, head, Centre of Interdisciplinary Studies, Kolkata, as part of rice variety conservation efforts. It is grown in a farm in Bissau Cuttack block, Rayagada district in southern Odisha. Garib-sal used to be grown by farmers in Purulia, West Bengal.
Put to test
The researchers tested Garib-sal’s ability to accumulate silver even when grown in soils with very low silver concentration. When the soil contains only about 0.01 mg of silver per kg , the rice plant was able to concentrate 0.20 mg of silver per kg in the grains.
“The rice variety has the ability to accumulate silver about 100 times more than any other rice,” says Prof. T. Pradeep on the research, which has been peer-reviewed and published in the journal ACS Sustainable Chemistry & Engineering.
The variety was cultivated in the farm for three successive years in soil containing about 0.15 mg per kg and the uptake and accumulation of the noble metal was nearly the same.
To reconfirm the unique property of the rice, the team led by Prof. Pradeep grew it in the lab at IIT Madras along with another, acting as a control; silver was added to the soil. The Garib-sal rice accumulated about 50 times more silver than the other type.
“It is possible to extract 14.60 mg per kg of silver from the rice using a cheap and simple chemical method. This is a unique way of extracting silver through agriculture,” Prof. Pradeep says. “With further research, it may be possible to find better ways of enhancing the bioaccumulation of silver.”