Absorbing work with heavy metal

Raman spectroscopy, Fourier transform infrared spectroscopy and scanning electron microscopy have been used to work out the adsorption mechanism of toxic heavy metals on to magnetic nanoparticles. The particles are being developed as a novel remediation material for metals such as the toxic hexavalent chromium.

Huidong Li, Zhao Li, Ting Liu, Xiao Xiao, Zhihui Peng, and Le Deng of the Department of Microbiology, at Hunan Normal University, in Changsha, People's Republic of China, hope to develop an efficient method for the removal and recovery of heavy metals from waste water. They have now demonstrated that Cr(VI), a toxic and carcinogenic material made infamous by activist Erin Brockovitch in her legal battle against Californian company Pacific Gas and Electric (PG&E), can be adsorbed and recovered by bio-functional magnetic beads.

Deng and colleagues explain how industrial waste streams are increasingly contaminated with metal ions. These pollutants are non-degradable and persistent in nature, they say, and significant among them is chromium(VI), or chromate. This metal is increasingly used in the chemical industry of the developing world for chrome plating, dye production, battery manufacture and other applications, despite regulatory limitations elsewhere. Hexavalent chromium is more toxic than its trivalent form and exposure by inhalation to Cr(VI) is a known carcinogenic route. Ingestion is linked to problems of the digestive tract and lungs, epigastric pain and nausea.

Various techniques have been developed to remove and recover heavy metals from waste water. These include chemical precipitation as insoluble salts, separation using ion-exchange resins, and membrane separation. None of these is especially effective and all can be extremely costly to employ, Deng and colleagues point out in the journal Bioresource Technology.

The team reasoned that an effective and inexpensive approach might involve making nanoscopic, magnetic beads capable of absorbing chromium(VI) and would be useful in the remediation of contaminated waste water. They sugges that the material could then allow the metal, adsorbed on to the magnetic particles, to be extracted from the water with an external magnetic field.

To this end, the researchers produced beads composed of a powder of the zygomecetous mould Rhizopus cohnii, more properly known as R. microsporus var. rhizopodiformis. This biomass material is readily available and cheap. They found that it could be blended with nanostructured particles of iron(III) oxide and the particles coated with alginate, a viscous gum found in algal cell walls, and polyvinyl alcohol (PVA).

"The combined technique of biosorption and magnetic separation holds the advantages of flexibility, eco-friendly characteristics and economic in operational cost. Thus, the current investigation into this technique for the removal and recovery of heavy metals is very important and inspiring," the researchers say.

They have tested their approach and defined the optimal parameters for extraction and separation of Cr(VI) from a waste water stream as being pH 1.0 and an optimum temperature of 28 Celsius. They explain that biosorption of the metal ions takes place through direct adsorption into the fungal biomass of the particles. They used standard kinetics to determine the role of the material in Cr(VI) adsorption and found that the beads were mechanically stable and still functional even after five cycles of adsorption of the chromium(VI) and desorption with sodium hydroxide solution. The mechanism of adsorption was determined using FTIR, Raman, and SEM, the researchers explain.

"It may be concluded that the bio-functional magnetic beads exhibit great potential for the treatment of Cr(VI) in waste water," the researchers add. However, they concede that further research is required to improve the sorption capacity of the beads and to develop the technology for the continuous removal of heavy metals in a large-scale treatment operation.