Experimental studies and mathematical modeling of biosorption of heavy metals by a gram-negative bacterium

Abstract

The biosorption of lead, copper, nickel, zinc and chromium by four different strains (Pseudomonas pseudoalcaligenes, Bacillus subtilis Mu331 & 168 and Saccharomyces cerevisiae) was investigated. Pseudomonas pseudoalcaligenes exhibited the most effective performance among the strains tested. The lead, copper and nickel removal ability of Pseudomonas pseudoalcaligenes was studied at different experimental conditions. The initial rate of biosorption was very fast with half of the total amount of metal sorbed within the first ten minutes; the biosorption almost reached equilibrium after 200 minutes. The biosorption kinetics was well described by the pseudo-second order Lagergren rate equation. The biosorption capacity of Pseudomonas pseudoalcaligenes was unaffected by the age of the cells (5-24 h old). The biosorption isotherms for all the three heavy metals were well described by the Langmuir and Freundlich adsorption isotherm equations. The maximum biosorption capacities (Qmax) obtained from the Langmuir isotherms were 232.55, 28.17 and 17.48 mg/g dried cell for lead, copper and nickel, respectively. The optimum pH for biosorption of these heavy metals was about 5. Microprecipitation was detected when pH was above 6. In the binary biosorption study, mutual inhibitory effect was observed in lead-copper binary biosorption system as the presence of either one of the ions affected the biosorption capacity of another. However, unequal inhibitions were observed in all nickel binary systems. The presence of either lead or copper significantly reduced the biosorption capacities of nickel, whereas the uptake of lead and copper was not greatly affected by nickel. The modified competitive multicomponent Langmuir model described the binary biosorption systems quite well, whereas the multicomponent Freundlich model was not suitable for our binary biosorption systems. Phosphorus and sulphur were found to be two of the major elements present in the cell wall by the techniques of scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDAX). The technique of SEM coupled with EDAX also showed that lead and copper have exchange with potassium in the cell wall of Pseudomonas pseudoalcaligenes. The analysis of the biomass by FT-IR spectroscopy indicated that carboxyl, phosphate, amino and amide were the major functional groups on the cell surface. The spectrum of biomass after metal biosorption showed no significant change compared to virgin biomass. Potassium, magnesium, calcium and sodium were released from the cell surface when the solution pH varied. These ions were also released during biosorption of copper. The surface acid-base properties of Pseudomonas pseudoalcaligenes were studied by potentiometric titration. Constant capacitance model (CCM) was applied to interpret the results. One site-two pKa model, which assumes the cell surface to be amphoteric interface and subject to protonation and deprotonation, can simulate the experimental results very well. The pKa values obtained were 4.21+-0.045 and 4.85+-0.037, which correspond to protonation and deprotonation of carboxylic group. The CCM was further applied to the biosorption of lead and copper by Pseudomonas pseudoalcaligenes. Two site-one pKa model was the best model to describe the biosorption process, which suggested that the heavy metal ions bound to the deprotonated and neutral sites. The Log K values for these processes were determined as 4.007+-0.04 and 4.713+-0.97 for copper, 4.874+-0.57 and 4.068+-0.62 for lead.