Abstract
Metallic species are non-biodegradable and can only be removed physically, chemically and recently biologically from contaminated wastewater. Among the commonly found toxic heavy metals, thallium (Tl) and cadmium (Cd) are listed as priority pollutants from mostly mining and industrial wastewater effluents. In this study, 4 green algae species of Stichococcus bacillaris, Chloroidium saccharophilum, Desmodesmus multivariabilis and Chlamydomonas reinhardtii were used as biosorbents for Tl and Cd. Equilibrium and kinetic experiments were carried out to determine the sorption capacity ( qmax ) and rate of reaction, respectively. The possible biosorption mechanisms were determined using FTIR on the algal surface. The Langmuir model performed better than the Freundlich model with a correlation co-efficient ( R2 ) of = 0.9 for both metals in free and immobilised algal cells. Chlamydomonas reinhardtii and Chloroidium saccharophilum showed the highestqmax for removal of Tl at 1000mg/g whereas removal of Cd was highest for Chloroidium saccharophilum at 128.21mg/g. The sorption capacity for Tl removal increased 5fold for immobilised Chlamydomonas reinhardtii from 1000 to 5000mg/g and 4fold for Cd removal from 23.31 to 86.21mg/g. The immobilised algae showed potential for re-usability especially for Cd with relative consistency in adsorption for the 3 cycles in the range of 88.7-92.4%. For both metals, the pseudo-second order model performed better than the first order model with R2 of = 0.99 in kinetic studies. The most active functional groups found on all tested algae for the removal of both metals were carboxyl, alkanes and amines. Generally the immobilised algae improved the sorption efficiency of Cd and Tl adsorption/desorption.
