Marine surfactant soaks up heavy metal

Atomic absorption spectroscopy (AAS) and various other techniques have been used to demonstrate the effectiveness of a natural surfactant molecule in removing heavy metals from solutions for potential bioremediation applications.

Palashpriya Das, Soumen Mukherjee, and Ramkrishna Sen of the Department of Biotechnology, at the Indian Institute of Technology, in Kharagpur, have studied a natural product lipopeptide surfactant from a microorganism first isolated from Andaman and Nicobar Islands, India, Bacillus circulans.

Sen and colleagues reported the isolation of a lipopeptide biosurfactant in 2008 in the Journal of Applied Microbiology. "The biosurfactant produced by marine B. circulans has a potent antimicrobial activity against Gram-positive and Gram-negative pathogenic and semi-pathogenic microbial strains including MDR strains," the researchers said, "Only one of the HPLC fractions of the crude biosurfactants was responsible for its antimicrobial action. The antimicrobial lipopeptide biosurfactant fraction was also found to be non-haemolytic in nature."

They now point out how the same surfactant might be used to help solve a modern environmental problem. Widespread industrialization and mining have led to contamination of the environment with heavy metals including arsenic, cadmium, chromium, copper, lead, mercury, nickel, and zinc, they explain. These materials accumulate in soil and water bodies and enter the food chain, thus representing a health risk to people and animals, with birth defects, liver and kidney damage and learning disabilities linked to absorption of toxic quantities of various heavy metals.

The team describes how conventional remediation techniques require inorganic and organic acids, chemical surfactants and metal chelating agents but these do not wholly remove metal ions from soil and also initially leave land unusable. For effective decontamination, a strong metal complexing agent that has solubility, environmental stability and good chelation properties is needed.

Various researchers have investigated surface-active metabolites from bacteria, biosurfactants, that are metal-chelating agents. Such compounds are putatively less toxic than synthetic surfactants, they themselves are biodegradable so once flushed from the soil can be broken down to release the heavy metals readily for further processing. Moreover, these natural surfactants side-step synthetic chemistry and can be produced in large amounts from inexpensive agro-based raw materials and organic wastes. Sen and colleagues also point out that biosurfactants can usually survive extremes of temperature, acidity and alkalinity, and salt concentration.

Until now, the majority of bacterial biosurfactants have been obtained from soil microbes. Sen suggests that the marine environment might offer a vast repository of microbial diversity and so provide a source for novel biosurfactants.

The team has now tested how well the biosurfactant from B. circulans can remove heavy metals from solutions. They used atomic absorption spectroscopy (AAS) to demonstrate that metal removal is even possible at a biosurfactant concentration below the critical micelle concentration (CMC), which contrasts starkly with other studies that suggested micelles were needed for metal removal.

"The metal-biosurfactant co-precipitate could be seen as an off-white precipitate after addition of biosurfactant to metal solutions followed by proper incubation," the researchers explain, "The percentage removal for both the metals varied with the different concentrations of metals and biosurfactants. As a general trend, higher concentration of biosurfactants removed metals in greater amounts." They add that, "At a concentration five times CMC, almost complete removal of 100 ppm of lead and cadmium occurred."

In addition, the researchers used Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM) equipped with energy dispersive X-ray spectroscopy (EDS) to further substantiate their results.