E.M. Cukrowska,Julien Lusilao-Makiese and Rosamund Tshumah-Mutingwende
Department of Chemistry, University of the Witwatersrand, South Africa
Anthropogenic activities such as mining, coal combustion and the chlor-alkali industry amongst other sources have largely contributed to the increased mercury concentrations in the environment. Current physical and chemical analysis methods commonly used to detect the presence of mercury in the environment have proved to be laborious, cost intensive and only provide information on the current status of the environment. Due to these challenges, the use of algae as heavy metal bioindicators in aquatic environments has received much attention. In this study, the performance of a common freshwater living green alga, Cladophora sp. as a mercury bioindicator and its potential for phytoremediation applications was assessed by various parameters which included the influence of contact time, pH, initial mercury concentration and the presence of competing metal cations. A rapid uptake of mercury by Cladophora sp. was displayed. More than 99% of mercury in solution was removed within the first 5 min of contact and equilibrium was attained after 10 min. High adsorption capacities of 800 mg kg-1, 530 mg kg-1 and 590 mg kg-1 at pH 3, 6.5 and 8.5 respectively were obtained at the optimum mercury concentration of 1.0 mg l-1. Competitive adsorption studies showed that the selectivity of heavy metal cations by Cladophora sp. was in the following order: Hg2+ ˃Fe2+˃Cu2+˃ Zn2+ ˃ Co2+. From the FTIR results obtained, it was assumed that the carboxylic acid (O-H) and v(C=O) functional groups were responsible for high metal adsorption at low pH and at high pH conditions, the presence of the hydroxyl (-OH) functional group facilitated the metal adsorption process. These results indicate that living Cladophora sp. algae are suitable for use as mercury bioindicators in AMD waters and are also suitable for the removal of mercury in AMD conditions.
Keywords: Acid mine drainage, mercury, algae, bioindicators, accumulation, phytoremediation, adsorption.