Surface characterisation of fungi and its implications to metal ion adsorption

Heavy metal pollution of water ways via industrial activities has the potential to have considerable impact on the environment. Specifically, heavy metals tend to accumulate through the food chain, leading to serious environmental and public health problems. Therefore technologies that allow efficient and economic removal of metal contaminates are highly desirable. Biological substrates provide the basis of one such technology and have been shown to remove aqueous heavy metals from effluent streams. Many biological substrates have been studied ranging from fungi, bacteria, seaweed and leaves. Fungal species in particular, have shown promise as effective, efficient and economic adsorbents, comparable to currently used inorganic substrates. Metal removal by biological substrates occurs through a process known as biosorption, which is considered complex, often involving multiple mechanisms such as ion exchange, complex formation and/or precipitation. While the suitability of various substrates has been extensively studied, elucidation of the mechanisms involved has yet to be as thoroughly examined. This study examined three inactive (dead) fungal substrates (Mucor rouxii, Pycnoporus cinnabarinus and Rhizopus stolonifer) for their potential to remove a range of divalent aqueous heavy metals from solution as a function of pH. It was found that Mucor rouxii demonstrated substantially greater adsorption efficiency than the other two fungi for all metals studied (Cd(II), Cu(II), Mg(II), Pb(II), Ni(II) and Zn(II)). Kinetic studies of Cu(II) adsorption show that the biosorption step is a relatively fast step with the majority of adsorption occurring within the first minute. The mechanisms responsible for biosorption were studied through a range of surface characterisation techniques including, analysis of the chemical structure of the substrates (via X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR)), electrophoretic mobility studies and modelling of surface charge titration data. Spectroscopic studies established the importance of both carboxyl and amine functionality, a finding also supported by the surface charge modelling of the three fungi. Adsorption efficiency appears to be directly correlated to the amount of these functional groups present M. rouxii, in particularly, was established as a suitable candidate for removal of heavy metals from solution and worthy of consideration as a biological adsorbent