Heavy metals released by a number of industrial processes are major pollutants in marine, ground, industrial and even treated wastewaters. Lead is widely used in many industrial applications such as storage battery manufacturing, printing, pigments, fuels, photographic materials and explosive manufacturing. Lead is highly toxic as its presence in drinking water above the permissible limit (5 ng/mL) causes adverse health effects such as anemia, encephalopathy, hepatitis and nephritic syndrome (Lo et al. 1999). Conventionally, the following methods are employed for the removal of heavy metals from effluents: oxidation and reduction, precipitation, filtration, electrochemical treatment and evaporation. Physico-chemical methods presently in use have several disadvantages such as unpredictable metal ion removal, high reagent requirements and formation of sludge and its disposal, in addition to high installation and operational costs. Search for newer treatment technologies for removal of toxic metals from wastewaters has directed attention to biosorption.

Biosorption is a passive non-metabolically - mediated process of metal binding by biosorbent. Bacteria, yeasts, fungi and algae have been used as biosorbents of heavy metals. Saccharomyces cerevisiae can remove toxic metals, recover precious metals and clean radio-nuclides from aqueous solutions to various extents. S. cerevisiae not only a by-product of established fermentation processes, but also can be easily obtained in considerably substantial quantities at low costs. The application of S. cerevisiae as a biosorbent not only removes metals from wastewaters but also eases the burden of disposal costs associated with the waste. Metal uptake by biosorption occurs through interactions with functional groups native to the biomass cell wall. Identification of the functional groups would help to determine the mechanisms responsible for the binding of target metals. Pre-treating the biomass can help shed light on the role played by functional groups in biosorption of lead.

Biosorbents have been used to remove metals and other pollutants from laboratory prepared aqueous solutions thus far. Literature on the application of biosorption to ‘real' industrial effluents is scarce, lead-bearing effluents in particular. The present study investigates the potential of waste beer yeast S. cerevisiae in biosorbing lead from battery manufacturing industrial effluent at varying experimental conditions, viz., biosorbent concentration, pH, metal concentration and agitation speed. The roles played by amines, carboxylic acids, phosphates, sulhydryl group and lipids of S. cerevisiae in lead biosorption were also studied.

Potentiometric titration permits the qualitative and semi-quantitative determination of the nature of acidic sites present on the yeast cell wall. From the curve, it was inferred that S. cerevisiae contained the carboxyl, phosphate and amine groups. When these groups were blocked using chemicals, the biosorption of lead decreased.The extent of contribution of the functional groups and lipids to lead biosorption was in the order: carboxyl > lipid > amine > phosphate. Blocking of sulfhydryl group did not have any significant effect on lead uptake. This research showed that the electrostatic attraction and complexation seem to be the most important mechanism of biosorption of metal cation (Pb(II) in this case).

The effluent contained 102 mg lead/L. Lead uptake rose with increase in biosorbent concentration from 0.5% to 2%. It decreased slightly when the biosorbent concentration reached 4%. High biosorbent concentrations are known to cause cell agglomeration leading to ‘protection' of the binding sites from metal ions and consequent reduction in biosorption. Lead uptake increased gradually with rise in initial pH from 1 to 5. At low pH, the cell surface sites are closely linked to the H⁺ ions, thereby making these unavailable for other cations. At pH 6, the lead uptake decreased due to its partial precipitation. A rise in lead concentration from 25 to 100 mg/L resulted in an increase in its uptake by S. cerevisiae by more than 3-fold. Lead uptake increased more than 3-fold with rise in agitation speed from 50 to 150 rpm, beyond which there was no further increase. Lower metal uptake at higher agitation speeds beyond a point is attributed to non-homogeneity of the biosorption mixtures as a result of vortex phenomenon.

The Freundlich and Langmuir adsorption isotherm equations are frequently used to represent the biosorption equilibrium. Values of K_f (0.5149) and 1/n (1.1871) obtained from the Freundlich adsorption isotherm illustrate the separation of metal ions from wastewater and the adsorption capacity of the yeast. Values of Q_max (55.71 mg/g) and b (0.0883 L/mg) obtained from the Langmuir adsorption isotherm indicate high metal uptake by the biosorbent. Comparing these constants with those available in literature for lead biosorption by various biosorbents, it was observed that comparatively lower K_f value was obtained in the present study. This may be due to the presence of ions other than lead in the effluent which decrease the specificity of Saccharomyces cerevisiae for lead. However, the Q_max value of 55.71 mg/g is indicative of high biosorption potential of the biomass.

Biosorption is basically at lab scale in spite of its development for tens of years (Wang and Chen, 2006). Various aspects which shed light on the application of biosorption on an industrial scale include:

• Physicochemical characteristics of ‘real' wastewater on the basis of thermodynamics and reaction kinetics.
• Screening of biosorbents for high metal-binding capacity and selectivity.
• Optimization of parameters.
• Combination of biosorption with physicochemical treatment technologies for ‘complete' wastewater treatment and recovery/reuse of metals.

The present study is a small step towards characterizing the biosorption process on the basis of thermodynamics and reaction kinetics and optimizing the process-affecting parameters. This study showed that waste beer yeast can efficiently remove lead from battery manufacturing industrial effluents. This study also emphasized the importance and need for carrying out extended testing for the compatibility of biosorption to a specific industrial effluent. The findings of the study indicate that biosorption is a promising technology for removal of lead from battery manufacturing effluent. However, further studies with respect to metal-biosorbent specificity, applicability to various other types of metal-laden effluents and large scale studies will help fine-tune the biosorption technology for large-scale application.

LO, Waihung; CHUA, Hong; LAM, Kim-Hung and BI, Shu-Ping. A comparative investigation on the biosorption of lead by filamentous fungal biomass. Chemosphere, December 1999, vol. 39, no. 15, p. 2723-2736. [CrossRef]

WANG, Jianlong and CHEN, Can. Biosorption of heavy metals by Saccharomyces cerevisiae: A review. Biotechnology Advances, September-October 2006, vol. 24, no. 5, p. 427-451. [CrossRef]

Note: Electronic Journal of Biotechnology is not responsible if on-line references cited on manuscripts are not available any more after the date of publication.