Evaluation of two types of polyurethane for the immobilisation of Rhizopus oryzae for copper uptake

Previous studies have shown that Rhizopus oryzae strain IM 057412 grown in reticulated polyurethane foam demonstrated the same heavy metal adsorption capacity as the free biomass. Immobilisation in other types of polymers was shown to reduce the biomass uptake capacity because of mass transfer limitations due to the restricted porosity of the immobilisation matrices. For practical purposes the growing of biomass in polyurethane support particles to use as a commercial adsorbent is not viable or financially sound. The current work describes a different approach in which dried non-viable cells of R. Oryzae were incorporated into two types of polyurethane carrier matrix during the production process. The polymers used were a conventional hydrophobic polyurethane and a hydrophilic polyurethane, Hypol 2002. Oven-dried and powdered particles (D<150 μm) of R. oryazea were immobilised by mixing the biomass with each of the polymers prior to the reaction in which the polymer was expanded to form a foam: consequently the biomass was uniformly dispersed throughout the porous matrix. The resulting fungi-polyurethane matrices were then cut into cubes (≡4-5 mm dimension) and their adsorptive properties studied with respect to copper. Experiments were conducted in shake flasks to establish the equilibrium time for the reaction for both free and immobilised biomass. The biomass immobilised in Hypol gave the same adsorptive capacity as that of free biomass when compared on a weight basis, but biomass immobilised in conventional polyurethane foam showed no adsorption. To assess fully the effect of pH on copper and to eliminate precipitation as a removal mechanism experiments were conducted at different pHs and different copper concentrations. In each case the solution pH was maintained by acid or base addition in response to measurements using a standard calomel electrode. It was shown that at pH 5 copper concentrations above 100 mg l−1 were likely to precipitate. The amount of precipitation was accounted for within the high concentration adsorption isotherm experiments by using a mass balance approach. Results showed that the adsorption of the Hypol immobilised biomass followed the Langmuir adsorption isotherm model and showed the copper adsorption capacity of the matrix to be between 10 and 13 mg g−1. The copper attached to the immobilised biomass could easily be desorbed by increasing the acidity, allowing the matrix to be used in repetitive sorption-desorption cycles. There was a small decrease in the adsorption capacity after the first desorption cycle that could be explained by a partial loss of biomass as detected by loss of total organic carbon (TOC).