Azolla filiculoides has been evaluated for the adsorption of trace toxic metals from aqueous
solution. The adsorption performance of the material was compared with commercial resins
and fitted using the Langmuir and Freundlich models. The Freundlich model described the
adsorption of copper and cadmium. Whilst the Langmuir isotherm had the better fit of the
mercury data. The assumptions of the Freundlich model include multi-layer adsorption and
different functional group binding. Conversely the Langmuir model suggests mono-layer
adsorption and can infer single group reactivity. The pH effect on the uptake of the metals
was investigated and an increase in removal was observed at higher pH with all the metals
The material has been thoroughly characterised using physical methods, such as, scanning
electron microscopy X-ray photoelectron spectroscopy and electrophoretic mobility
measurements. This enabled conclusions to be made regarding the surface functionality of
the solid. Chemical characterisation included direct titrations, revealing a gradual
dissociation of acidic groups as the pH increased within the experimental range. Kjeldahl
nitrogen and amino acid analysis of several biological materials that have been used in metal
sorption experiments showed A. filiculoides as having a large proportion of these cell
constituents. The kinetics of metal ion uptake by the biosorbent was investigated and compared with
commercially available resins. The kinetics are slower than conventional ion exchange resins
and carbon adsorbents but entirely adequate for utilisation in a column process.
The mechanism hypothesized for metal ion removal by the biosorbent is primarily attributed
to ionogenic groups exchanging ions for copper and cadmium removal. Mercury on the other
hand is said to be predominantly involved in a reduction-precipitation reaction on the surface
of the adsorbent.
Regeneration was successfully accomplished for copper and cadmium after minicolumn
trials, with greater than 95 % elution of the metals using 0.1M HCI. The mini column trials
showed a sharp breakthrough for these metals singularly and a dynamic equilibrium was observed during multi-metal processing. Mercury removal was much slower and more
difficult with the same eluant, achieving a maximum of 50% removal.
A method for a semi-continuous biosorbent process has been evaluated and proven to be
successful in processing metal laden solution.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.