Adsorption of trace toxic metals by Azolla filiculoides from aqueous solution
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 studied. 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.