Effects of pH and biomass concentrations in the removal of selenite from acid mine drainage by Aspergillus niger as biosorbent in Sarcheshmeh porphyry copper mine

The biosorption process of a lypophilised cell suspension of Aspergillus niger was studied for selenite ( − 2 3 SeO ) removal from acid mine drainage in the Sarcheshmeh porphyry copper mine. The biomass was first characterised by the potentiometric titration, and the major ionic content was evaluated. The experimental data suggest that the biomass cell wall contains two main acidic groups with a total amount of 5 meq/g. Equilibrium biosorption trials of selenite were carried out to investigate the effects of two major experimental factors, the pH and the biomass concentrations. Biosorption trials were also performed for selenite concentrations between 0 and 2 mmol/l at different equilibrium pH between 3 and 6. Selenite biosorption extent was repressed by pH decrease. The effect of the biomass concentrations changes both with the equilibrium pH value and the kind of oxyanion adsorbed. In the case of selenite biosorption at pH above 5, the increase of biomass concentration causes a decrease of maximum specific oxyanion uptake most likely due to cell aggregation phenomenon. Experimental data obtained for selenite was fitted using adsorption models including Langmuir and Freundlich isotherms. Analysis of FTIR showed that selenite was adsorbed on extracellular of biomass. INTRODUCTION Acid mine drainage is one of the major environmental problems facing the mining industry and clearly poses a serious unfavourable impacts on receiving waters (Gray, 1998). Acid mine drainage is characterised by low pH, high concentrations of iron, sulphates and variable concentrations of toxic heavy metals. Selenite (SeO3) is considered to be the most important toxic oxyanions which is associated with acid mine drainage. It can also be found in agricultural effluents (Adams et al., 1993). Selenium even with low concentrations is a toxic metal with 0.001 ppm as an acceptable concentration of selenium for drinking water. Selenite is usually dissolved in water and different forms of selenium such as elemental selenium, selenite and selenate have potential for water pollution (Oremland et al., 1990). It is however, necessary for many strains of microorganisms to consume selenium as a nutrient (Bennet et al., 1993). Using microorganisms as biological adsorbents, many oxyanions such as selenite oxyanion can be removed from industrial effluents in particular acid mine drainage, or its concentration reduced to an acceptable level (Schmite & Kavana, 1999). Equilibrium data based on the adsorption isotherms have been used to design adsorption scheme. The adsorption isotherms including the Langmuir and Freundlich models are normally used to describe the equilibrium system between adsorbed oxyanion on fungal cells (qeq) and those oxyanions remained in the solution phase (Ceq) at a constant temperature (Aksu, 2001). Kinetics studies of the biosorption processes is also considered in the present study. Studies by Volesky and Holan (1994) revealed that the second order kinetics is appropriate to describe the rate of sorption. The paper describes the results of an experimental study using the biosorption process of a lypophilised cell suspension of Aspergillus niger for the removal of selenite (SeO3) from acid mine drainage in the Sarcheshmeh porphyry copper mine in Iran. EXPERIMENTAL SETTINGS Microbiological growth media Aspergillus niger stain was extracted from the surface and deeper sediment samples of Shour river in Sarcheshmeh porphyry copper mine. The samples were taken from the discharge of acid mine drainage into the Shour river. The media composition for growth of Aspergillus niger is as follows (all gl): glucose, 2.0; yeast extract, 1.0; peptone, 0.5; FeSO4 . 7H2O, 0.01; MgSO4 . 7 H2O, 0.05; NaSeSO4, 0.1. The pH of the medium was 9 INTERNATIONAL MINE WATER CONGRESS 52 adjusted to 5.0. Inoculation of Aspergillus niger strain was made in the shake flasks containing growth media. These flasks were air dried and incubated at 25 C and spinning at 150 rpm. Solutions containing selenite and effect of Aspergillus niger strain on biosorption system After 4 to 5 days of the Aspergillus niger growth, fungal cells were centrifuged at room temperature and agitated at 400 rpm for 5-6 min. The samples were washed twice and then were dried at 60 °C for 24 hr. In order to perform biosorption study, 7.5 g of fungal strain (dried biomass) was mixed in flask containing 100 ml water. The samples were finally homogenised at 8000 rpm for 30 minutes and were placed in the refrigerator. Solutions containing selenite were in the range of 25-200 mgl. To perform biosorption experiment, the pH of each solution was first adjusted with HNO3 solution before mixing with fungal suspension. Biosorption study in a batch system Biosorption tests were performed in 250 ml flask containing100 ml selenite solution. The solution system within the available flasks was agitated in a shaker equipment at 150 rpm for 240 min. Five ml samples were used to determined the residual concentrations of selenite in the solution phase at various time intervals (0, 5, 10, 15, 30, 60, 120 and 240 min) prior to mixing the samples the biosorbent and selenite solutions. Finally, an atomic adsorption was used to determine the selenite concentration. RESULTS AND DISCUSSION Effect of equilibrium pH on biosorption of selenite Figure 1 shows the quantity of selenite adsorbed by Aspergillus niger as the solution pH changes at a constant temperature of 20 °C and selenite initial concentration of 100 mgl. As Figure 1 shows, biosorption of selenite increased at a steady rate as pH increased up to 5. The biosorption of selenite decreases sharply as pH increases from 5 to 6 and then decreases steadily as the pH increases. The maximum equilibrium sorption of 62.3 mgg was obtained at pH of 5.