Fungal biosorption of metal ions

In recent decades, community awareness of the effect of pollution on the environment and on public health has intensified and has now become an important social and political issue. The detrimental effects of pollution and phenomenology are well known. One such example is bioaccumulation; where intractable pollutants circulate throughout the food chain, eventually accumulating to potentially toxic levels (Volesky & Holan, 1995; Williams et al., 1998). Aqueous heavy metals are one such group of intractable pollutants and unless immobilized and/or removed from natural systems, pose a serious pollution threat (Volesky & Holan, 1995). Traditional methods that have been used to remove heavy metals from effluent include chemical precipitation, chemical oxidation/reduction, ion exchange, electrochemical treatment, evaporation and filtration. Many of these methods are, however, ineffective; resulting in low levels of metal ion removal and can also be economically inefficient (Malik, 2004; Volesky, 1990). One removal method that shows potential for being both effective and economic is the adsorption of metals onto a simple, cheap, solid substrate (Burns et al., 1999; Ramalho, 1977), followed by a separation process such as foam flotation. In the last fifteen years or so one adsorption technology known as biosorption has shown considerable promise. In this case, the adsorption of aqueous metals is based on the chemical activity of microbial biomass (Volesky & Holan, 1995). This study carried out aqueous metal ion biosorption studies under carefully controlled conditions on three inactive (dead) fungal substrates and chitin (a constituent of fungal cell walls). A thorough surface characterisation of the four substrates was completed, in conjunction with sophisticated metal-ion adsorption modelling (triple layer surface complexation and FOCUS models). The main focus of this thesis was to determine the mechanism behind divalent metal ion biosorption onto fungal substrates and thus determine why differences in biosorption ability exist between fungal substrates. The mechanism of fast surface biosorption was examined through divalent metal ion removal experiments, surface characterisation and modelling. The divalent metal ion studied included Cd(II), Cu(II), Mg(II), Ni(II), Pb(II) and Zn(II). Through Cu(II) removal studies it was noted that the “fast” metal ion biosorption step occurs in less than 30 seconds and during this step the majority of metal ions were removed. The speed of the Cu(II) uptake, spectroscopic analysis (no evidence of short range chemisorption) and subsequent triple layer surface complexation modelling (with adsorption occuring in the β plane) suggest a biosorption mechanism of long range electrostatics (outer sphere biosorption). The biosorption reaction was found to involve the subsequent displacement of protons from the substrate surface upon uptake of metal ions. This suggests site specific binding such as that represented by the following equations (where L denotes a surface site) [See electronic version of thesis for equation diagram]. Triple layer surface complexation and FOCUS modelling found that one or more of these binding reactions were able to successfully describe most divalent adsorption. Both modelling and analytical studies suggest that the number of surface sites per gram will determine the fungis effectivness at metal ion adsorption. This highlights the importance of the quantity, not necessarily the composition, of fungal surface sites for single divalent aqueous metal ion adsorption. Thus from analytical and modelling analysis it appears that the most appropriate factor for standardising the biosorption effectiveness of fungal substrates is the site density (amount of surface sites per gram). While the specific type of the adsorbing site/functionality may have some impact on adsorption, the number of surface sites appears to have far greater impact. Determining the number of surface sites per gram also readily enables the efficiency (on mass basis) of the fungal substrate to be readily calculated, which is useful when considering industrial applications. Future studies should therefore focus on identifying fungal materials with high densities of surface sites, particularly amongst those fungi produced as waste by-products, as the commercial success of biosorption water treatment technologies will rely on the identification of both efficient and economic substrates.