Bench-scale shake flask experiments rarely mimic the complexities encountered in unit operations where large volumes and short reaction times must be employed to ensure the economy of scale. Unit operations are preferred in the chemical industry since the process can usually be subject to computational modeling and thereby more predictable. Also, many industrial wastewaters contain several heavy metal contaminants and therefore competitive adsorption to sites on the surface of fungi has to be accounted for as described in a recent study involving Penicillium chrysogenum and Streptoverticillium cinnamoneum by Puranik and Paknikar (1999). Biomass used in a reactor has to be either inexpensive and disposable or preferably in any case reusable after adsorbed metal ions are eluted. Yeast and fungi present somewhat difficult materials for carrying out column separation studies as the biomass often forms an impervious mat. However, the use of polyurethane foams as supports for biomass may help to overcome this problem (Dias et al. 2002; Tsekova and Ilieva 2001). Reaction times of biomass with metals to achieve sorption of metal ions must be short, yet many reports in the literature allow incubations to run for days to achieve suitable and reportable metal ion uptake. A typical experiment requires a 4-day incubation to prepare sufficient mycelia of A. niger for study and then another 24 hr for adsorption in shake flask following addition of metal ion (Price et al. 2001). However there are report of more rapid equilibration requiring from 10 to 30min of contact (Puranik and Paknikar 1999). Immobilizing A. niger biomass within a polyurethane foam within an adsorption column increased copper uptake threefold compared with free mycelia studied in batch adsorption experiments (Tsekova and Ilieva 2001). Computational methods applied to unit operations permit the economy of a process to be predicted (Al-Asheh and Duvnjak 1992; Sag et al. 2001; Schiewer and Wong 1999). However, commercial scale bioreactors must address the problem of how a particular biotrap can render the influent metal concentration less harmful as the effluent is discharged. If it is to succeed, new biotrap technology must compete economically with those already at hand in the chemical industry. Sophisticated approaches have been reported for mathematical modeling of metal-biotrap sorption and precipitation
(Figueira et al. 1997; Schiewer and Volesky 1996; Veglio and Beolchini 1997). A mathematical Langmuir model has was recently developed for the prediction and performance of the simultaneous biosorption of Cr(VI and Fe(III) by R. arrhizus in a semi-batch reactor. Likewise Sag et al. (2000) have recently reported the theoretical approach to and use of a continuous-flow stirred tank reactor for removing lead, nickel and copper from aqueous solution by nonliving R. arrhizus biomass.
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