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Pardo et al., 2003

"Milligrams per gram of dry weight sorbent.

"Milligrams per gram of dry weight sorbent.

In the case of P. aeruginosa, the amount of cobalt did not change substantially with the cell growth phase. The quantities of cobalt taken up by the cells were 180.3 ^mol/g dry cells at midex-ponential phase and 178.9 ^mol/g dry cells at stationary phase, respectively (no data shown). From the result, the sorption of cobalt by P. aeruginosa appeared to be independent of the cell growth phase. Chang et al. [51] reported an increase in sorption of lead and cadmium by P aeruginosa with increasing culture age; however, copper uptake was also found to be independent of growth phase.

Numerous studies show that the biosorption of heavy metal from aqueous solution depends strongly on pH. Rao et al. [52] studied Cu2+ biosorption by G. lucidum and A. niger at initial copper concentration of 0.5 mM and found that the metal binding had an increasing trend from pH 2 to 6, with the maximum occurring between pH 5 and 6.

On the other hand, for some biosorbents, pH plays a different role in biosorption when the initial ion concentrations are different. Ke et al. [53] reported results for the biosorption of Ag+ by using Datura cells. The binding was pH independent when the initial concentration was 0.1 mM, but it became strongly pH dependent when the initial concentration increased to 1 mM. These investigations suggested that at least two binding sites are involved: one site is pH independent and displays a greater affinity and lower availability than the other site, which is pH dependent.

Figure 17.5 shows that the adsorption of metal ions by the P. aeruginosa depended highly on solution pH. The adsorption percentage of metals in the lower pH levels (e.g., pH 2 or 3) was significantly low due to competition with the H+ ions for binding sites on the surface of bacteria; the increase in pH favored metal sorption mainly because of the elevated levels of negatively charged groups on the cell surface.

The negatively charged groups existing on the surface of the microbial cell wall may undergo protonation at low pH, leading to an increase in the positive charge density on the cell surface. Such a process results in a competition between H+ and cationic ions for the same binding sites, as suggested by Hu et al. [32]. These coworkers noted that an increased H+ concentration resulted in suppressed uranium uptake by P. aeruginosa CSU biomass. The metal biosorption capacities declined substantially under lower pH condition. They concluded that the reduction in uranium-loading capacities with decreasing pH may be due to protonation of the cell wall, high solubility of uranyl ions, and cell structure damage at very acidic conditions.

17.3.3.3 Competing Cations

Most studies on biosorption using microorganisms have involved the removal of only one kind of metal ion from aqueous solutions. However, the presence of only one kind of heavy metal is a rare situation in nature or in wastewater.

To study the competitive biosorption behaviors between trivalent chromium and bivalent cobalt and nickel ions using P. aeruginosa, mixed metal solution was used in a batch sorption system (Figure 17.6). In a ternary system, cobalt (-93%) and nickel (-91%) uptake was strongly affected by the presence of chromium when compared with uptake in a single system. The decrease in uptake of chromium ion was negligible in the presence of bivalent ions.

At pH 4 of mixture solutions, cobalt, nickel, and chromium ions are present in the positively charged forms. In this state, they can compete with each other for negatively charged surfaces of the biomass. It is well documented that the ionic charge and ionic radii of cations affect the ion exchange as well as adsorption phenomena [42]. Because monovalent or bivalent ionic species are sorbed to a lesser extent than polyvalent ones, cobalt and nickel do not suppress the prevailing affinity of chromium to the binding sites of P. aeruginosa.

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