The heat dried fungal biomass adsorbs toxic hexavalent chromium from untreated tannery waste and converts it into non-toxic trivalent chromium thus solving the problem of disposing toxic chromium waste.
Removing hexavalent chromium [Cr(VI)] from industrial effluents, particularly untreated tannery waste, will become easier and more efficient, thanks to the work by a team of researchers from the Indian Institute of Chemical Biology (CSIR-IICB), Kolkata. Importantly, the heat dried fungal biomass converts Cr(VI), which is neurotoxic, genotoxic and a carcinogen, into a non-toxic trivalent form of chromium thus eliminating the problems of disposing Cr(VI)-containing waste. Cr(VI) is found in very high concentration in tannery waste. The results were published in the journal Scientific Reports.
A team led by Dr. Sucheta Tripathy from the Structural Biology & Bio-Informatics Division at IICB isolated a fungus Arthrinium malaysianum and used the fungus biomass to remove Cr(VI). In experiments carried out in the lab using potassium dichromate solution, the adsorption capacity of the biomass was found to be as high as over 100 mg per gram of dry weight. In the case of untreated tannery waste, the fungal biomass was able to remove over 70% of Cr(VI). In addition to chromium, it can also remove other toxic metals such as lead and arsenic, which are normally found in tannery waste.
“We were working on another fungus (which is a mushroom) that grows in termite mounds. It was then that we noticed this fungus growing as a contaminant on the mushroom,” recalls Dr. Tripathy. “Since the cell wall of fungus is well known for adsorbing heavy metals, we were curious to test the newly found fungus’ ability to adsorb them.” Though the A. malaysianum fungus is not pathogenic, they heat dried before using it.
The positively charged functional groups found on the surface of the fungus bind to Cr(VI) ions, and these functional groups were found intact even after heat drying. “The positively charged functional groups have more ability to adsorb the negatively charged Cr(VI) by strong electrostatic attraction in acidic conditions,” says Dr. Rajib Majumder from the Structural Biology & Bio-Informatics Division at IICB and the first author of the paper. “Though the tannery waste is slightly alkaline, the fungus was still able to adsorb heavy metals.” The adsorption capacity increased nearly eight-fold when the tannery waste was made acidic.
Conversion of Cr(VI) to Cr(III)
Once Cr(VI) gets adsorbed, the reducing functional groups found on the fungus completely converts it to Cr(III), which is not toxic. “The efficiency of conversion of Cr(VI) to Cr(III) was unaffected even when the concentration of Cr(VI) was increased 10 times,” says Dr. Majumder.
While the permissible limit is 2 ppm for chromium, untreated tannery waste contains as high as 10-15 ppm. The efficiency of the fungus biomass to adsorb and convert chromium was unaffected even when concentration was increased to 1,000 ppm.
On testing the biomass using untreated tannery effluent, the researchers were able to remove over 70% of Cr(VI) from the effluent in 16-20 hours. In the lab, the biomass able to remove almost 50% of Cr(VI) from potassium dichromate solution in 4-5 hours and over 90% in 12-15 hours.
Aside from chromium, the fungus biomass can also adsorb and remove other heavy metals such as lead and arsenic, which are found in industrial waste.
Besides adsorbing and converting the toxic form of chromium to a non-toxic form, the fungus biomass can also be reused by removing the adsorbed material. “We were able to reuse the biomass three times efficiently, beyond which it became unusable,” says Dr. Majumder. “We can probably increase the reusability by chemically modifying the biomass.”
Having tested the ability of the heat dried fungus biomass in removing chromium and other heavy metals, the team is working to immobilise the biomass on a glass or ceramic substrate. “We are trying to produce a biomaterial to increase the surface area for real-time applications,” says Dr. Tripathy.