Bioreduction of Thallium and Cadmium Toxicity from Industrial Wastewater using Microalgae

Abstract

Thallium (Tl) and Cadmium (Cd) are listed among priority metallic pollutants known to cause irreversible health effects and easily magnify in the higher trophic levels of the food chain. Micro algae has a diversity of species found in freshwater bodies but only a few have been explored for their biosorption potential as compared to macro algae. The study sourced microalgae from eutrophic freshwater body (Hartbeespoort dam) in South Africa and isolated using streak plating technique. The pure strains were identified using molecular methods of 18S ribosomal RNA gene (rRNA) as Chlorella vulgaris and Scenedesmus acuminutus. The pure strains were then cultured in the laboratory and used to determine the adsorption potential and recovery of Tl and Cd. Equilibrium and kinetic experiments were used to estimate sorption capacity and rate of reaction respectively. The effect of initial concentration on Cd and Tl adsorption was also studied. The algae was characterized to determine the active functional groups on the algal surface wall using Fourier Transform Infra-red spectroscopy (FTIR). From the study, removal efficiency was achieved at 100% for lower concentrations of = 150 mg/L of Tl. At higher concentrations in a range of 250-500 mg/L Tl, the performance of Chlorella vulgaris and Scenedesmus acuminutus was still high with sorption capacity (^q_max) of 1000 and 833.33 mg/g respectively. Cd removal was highest for Chlorella vulgaris at ^q_max of 175.44 mg/g and affinity (_b) of 0.011 L/g. When compared to other studies on Tl and Cd adsorption, the tested algae showed a relatively better q_maxthan most adsorbents. The kinetic studies showed better correlation co-efficient of = 0.99 for Pseudo-second order model (PSOM) than the first order model. Recovery of Tl was achieved highest for Chlorella vulgaris at 93.26% and Cd was highest for Scenedesmus acuminutus at 91.92% using nitric acid. The strongest functional groups responsible for Tl and Cd binding on the algal cell wall were carboxyl and amines. Microalgae from freshwater bodies showed significant potential for Tl and Cd removal/recovery from industrial wastewater.