Due to automobile transportation, metallurgy, agrochemistry and other industries, soils get polluted by heavy metals, including lead and copper, which results in the accumulation of toxic compounds in plants and reduced crop yields. Soils can be purified with charcoal (biochar), which absorbs metals due to its porous structure. However, biochar’s absorption capacity is quite low due to the small surface area across which it interacts with pollutants.
These properties can be improved by adding substances to biochar particles so as to form additional pores and actively bind metals. The scientists from Southern Federal University have proposed using metal-organic frameworks for this purpose. Metal-organic frameworks (MOFs) are highly porous polymers consisting of metal ions bound together by organic molecules. Thanks to their high absorption properties, compounds with a framework structure are used to absorb and separate gases, including for the purposes of CO2 capture. One can easily modify the pore size and chemical properties of MOFs by varying their components.
The authors created a composite using a MOF based on iron powder and organic acid, as well as biochar from wheat straw. In order to obtain the nanocomposite, its components were held up and mixed for 20 minutes at a temperature of 120 degrees Celsius. Subsequent analysis of the composite revealed that the MOF increased the surface area of the biochar by six times via additional pores.
The scientists conducted an experiment by introducing the nanocomposite into soil samples contaminated with lead and copper. The control sample consisted of biochar into the structure of which no MOFs were introduced. The authors found that the nanocomposite removes up to 99% of heavy metals at their high concentration, whereas the efficiency of biochar under these conditions stands at just 82%.
Aside from a large number of pores, there are two factors explaining the nanocomposite’s higher efficiency: metals form complexes with oxygen-containing groups in the MOF, and cations (positively charged metal particles) are exchanged between the iron-containing centers in the composite and the contaminated soil. As a result, pollutants get safely secured in the pores of the nanocomposite.
“In the future, we plan to study the effects of long-term use of our nanocomposite. We also want to improve its functionality, namely, to use it as a basis for platforms that could provide plants with various useful bioadditives, such as humic acid or auxins, so as to increase their capacity for survival in drought conditions,” Vladimir Polyakov, candidate of chemical sciences, is quoted as saying by Southern Federal University.
