Sorption of toxic metals by fungi and clay minerals

Removal of divalent metal ions from pH buffered solutions by the soil fungi Rhizopus arrhizus and Trichoderma viride has been examined at Cu²⁺, Cd²⁺ and Zn²⁺ concentrations ranging from 0 to 1000 m, in the presence and absence of the clay minerals montmorillonite and kaolinite. Uptake of the metals was followed, at three different pH values, by analysis of the reaction filtrates using atomic absorption spectrophotometry (AAS) and ion specific electrodes (ISE). The clays showed a more rapid uptake of the metal ions with equilibrium being reached in < 2 min compared to 3–6 h taken by the fungal biomass. Results were plotted and characterized using Langmuir, Freundlich and Brunauer-Emmett-Teller (BET) adsorption isotherm models. Isotherms for the clay minerals conformed to Langmuir and/or Freundlich type models, corresponding to monolayer binding of metal ions on a homogeneous adsorbent surface. Isotherms for the fungi were non-Langmuir types, better fits being obtained using the BET model for multi-layer adsorption onto non-homogeneous surfaces. On a dry weight basis the clay minerals took up greater amounts of metal (up to 435 mol g⁻¹) than the fungi (up to 78 mol g⁻¹). However, when the data were expressed in terms of metal bound per unit surface area, the fungi showed much greater sorption capacities (up to 36·6 mol m⁻²) than the clays (up to 3·1 mol m⁻²). Mixtures of montmorillonite and fungal biomass showed reduced uptake of metals, depressed below calculated values by up to 37% at pH 4, possibly because of masking of exchange sites. Spherical aggregates were visible in the kaolinite and fungal mixtures, but there was no reduction in metal uptake capacity from the predicted uptake values. There was good correlation between values obtained for the sorption of the metal ions onto the sorbent surfaces and the Irving-Williams series for the order of stability of metal-ligand complexes (Cu²⁺ ≫ Zn²⁺ > Cd²⁺). Accumulation of the metals by all of the sorbents was dependent on the external pH and decreased with lower pH: maximum uptakes were achieved at pH 6·5 for Cd²⁺ and Zn²⁺ and at pH 5 for Cu²⁺.