Biosorption of lead (II) and cadmium (II) from aqueous solution by Chlorella pyrenoidsa and its influential factors

Abstract

The remains of aquatic algae are widespread and constitute an important organic fraction in natural freshwater environments. Accordingly, their adsorption of heavy metals may influence or determine the environmental processes and behavior of the metals. Conversely, they may be developed into low cost biosorbents for the removal of heavy metals from water, thus serving as green adsorbents for industrial heavy metal wastewater treatment and remediation of metal polluted natural water bodies. This study investigated the adsorption of two toxic heavy metals that are of great concern, Pb(II) and Cd(II), from water by the freshwater alga Chlorella pyrenoidsa. In addition, the factors that influence the adsorption were evaluated. To accomplish this, the alga was cultured for self-reproduction, after which the cells were collected and freeze-dried to obtain the biosorbent. Initial kinetic measurements showed that both Pb and Cd were rapidly adsorbed by the alga with equilibria acquired within 5 minutes to 4 hours, and that these metals resisted desorption. Accordingly, the adsorption of Pb and Cd was conducted using a range of solution concentrations by mixing for 4 hours while maintaining the pH at 5. 0. After attaining equilibrium, both the solution phase and algal phase were analyzed for metal concentrations. The adsorption isotherms obtained for both Pb and Cd followed a Langmuir shape, with the adsorption initially increasing rapidly and then approaching saturation with further increasing metal concentration. The alga significantly adsorbed both metals, apparently in response to an abundance of acidic functional groups such as -COOH and -OH on the algal cell walls that formed complexes with the metal ions. While both metals are divalent, Pb was comparatively more effectively adsorbed than Cd. This may have resulted from the difference in the metal radius caused by Pb being less hydrated and thus more strongly bound to the adsorption sites of the alga. By fitting adsorption data to the Langmuir equation, the obtained maximal adsorptions were 0. 373 and 0. 249 mmol/g for Pb and Cd, respectively. When 400 mg/L of dissolved fulvic acid was present in solution, the adsorption of both metals by the alga was inhibited by up to 34%. This was clearly caused by the competitive binding of metals with fulvic acid. The overall smaller inhibition by fulvic acid that was observed at higher metal concentrations likely resulted from the full occupation of fulvic acid binding sites by the metal ions. The effects of pH, ionic strength and temperature on the adsorption were determined by using a constant initial metal concentration while varying these factors. The adsorption of both metals increased as the pH increased from 2. 0 to 5. 0, reached the maximum value at pH 5. 0-6. 0, and then decreased as the pH increased further. This was due to the progressively increased dissociation of acidic functional groups of the alga with increasing pH in acidic solutions that facilitated the metal complexation but reduced the availability of metal ions via binding with increased -OH in neutral to weakly basic solutions. As the concentration of NaNO3(ionic strength) increased to 0. 15 mol/L, the adsorption of both metals decreased by up to 15% due to increasingly compressed double electric layers leading to charge neutralization and enhanced aggregation of algal particles. By increasing the temperature from 277 K to 323 K, the adsorption of both Pb and Cd by the alga increased, with the extent of the increase for the former metal being much higher than that for the latter. This enhancement of adsorption with temperature may have resulted from the surfacing of existing adsorption sites that were initially concealed and/or the creation of new adsorption sites by elevated temperature. The enhancement also indicated that the adsorption was endothermic. According to the Clausius\Clapeyron equation, the heat of adsorption was calculated to be in the range of 12194-21706 J/mol and 3398-9520 J/molfor Pb and Cd, respectively. In view of its high self -reproduction and thus wide availability, the alga Chlorella pyrenoidsa characterized by strong adsorption of heavy metals as well as resistance to desorption may be applied as an effective biosorbent for the remediation of metal-polluted natural water bodies and the treatment of metal-containing waters under proper manipulation of water pH and temperature.