Biosorption

Scientific and technological developments over the past few decades have made possible the application of biological processes to the solution of complex

Sprinkling system

Leaching solution

Leachate

Bottom liner Drainage/leachate

collecting system Retaining

dam

FIG. 21. A heap leaching system.

environmental problems (Refs [339–341] and the proceedings of the interna-tional biohydrometallurgy symposia). Microorganisms, that is to say single cell organisms such as bacteria or fungi, have been used as minute biological reactors that can efficiently and economically carry out specialized operations.

Microbial biomass, whether living or not, has been shown to selectively sequester and retain elements from dilute aqueous solutions via a process named biosorption.

Through the process of biosorption the biosorbed species are selectively removed from the solution and are retained inside the microbial cells (biomass) in concentrations that are several orders of magnitude higher than those in the original solution. Heavy metals and radionuclides are taken up into cellular components such as the cell walls of certain microorganisms, which then can be harvested, carrying along the sequestered radionuclides. Biosorption is being explored in hydrometallurgy to concentrate metal bearing solutions, for example from heap leaching, and in the treatment of contaminated mining effluents [291, 316, 342, 343].

Engineering developments in the area of biosorption have led to the design of engineered biosorbents, microbial biomass cells or cellular components immobilized on or within various matrices, thus acquiring the form of small particles such as those of conventional adsorbents (e.g. activated carbon) or ion exchange resins. Organic cellular material derived from higher plants or algae have also been proposed as the basic material for manufacturing biosorbents that can be used for the extraction of metals, including radionu-clides [344–354].

Biosorption methods are largely ex situ methods applicable for diluting contaminated solutions such as groundwaters or seepage. The contaminated solution is pumped into engineered reactors, in which it contacts the immobilized microbial biomass under optimized conditions (solution pH, flow rate, etc.). The contaminants are retained in an insoluble form by the biomass and the treated solution is let out of the reactor. The process of biosorption is reversible under certain conditions, which means that after the biosorbent is exhausted it could potentially be used for regeneration, releasing the previously held radionuclides in a small volume of the regenerating solution.

Alternatively the biosorbent could be used once through and then disposed of appropriately.

Biosorption is an equilibrium process, with solution pH playing the role of the master variable, since it defines the speciation of the elements in the solution. This also means that the key driving force that dictates the biosorptive uptake capacity of the biomass in terms of mass of biosorbed species per unit mass of biosorbent (also referred to as the loading capacity) is the residual

concentration of the contaminants after treatment and not the initial contaminant concentration [355].

The optimal biosorption pH depends on the biomass used and on the elements being removed; for example, the biosorption of uranium by the fungus Rhizopus arrhizus appears to be optimal at pH4, with significant reduction of the metal uptake capacity as the pH drops to pH2. The increased concentration of hydrogen ions at the acidic pH along with the chemical effects on the cell walls of the microorganisms are responsible for this reduction in capacity [356, 357]. However, increasing the pH towards neutral values may again create operational problems, depending on the composition of the contact solution. The hydrolysis and subsequent precipitation of ferric ions which adsorb on to (coat) the biosorbent adversely affect the biosorption process [227].

The biosorption of metals by algal biomass is another example in which the sequestering of metals such as lead, zinc or copper by microorganisms such as Chlorella vulgaris, Chlorella regularis or Chlamydomonas sp. is optimal in the range of pH6 to pH8. The biosorption of oxyanions such as chromates or selenates by the same type of algae has an optimal biosorption pH in the acidic range of pH2 to pH3 [358, 359].

Biosorption of ²²⁶Ra by several types of microorganisms, such as Rhizopus arrhizus, Aspergillus niger and Streptomyces niveus, exhibited an optimal contact pH in the neutral to alkaline range, with corresponding radium equilibrium uptake capacities in the range of tens of MBq/g. It is therefore obvious that optimization of biosorption processes should be made on a case by case basis and requires increased care so that the process will perform satisfac-torily.

Considerable efforts have been made to understand the underlying mechanisms of biosorption and to improve the process efficiency. The available information has shown that cell walls are the major biosorption functional sites for heavy metals, uranium and thorium. It has also been shown that EPSs play a significant role in biosorption [360]. The molecular level understanding of the biosorptive processes is still limited to selected pairs of metals and microor-ganisms. The microbial biomass provides ligand groups on to which the metal species bind. In addition, sorptive and hydrolysis processes play a role. Three major classes of microbial biopolymers (proteins, nucleic acids and polysaccha-rides) provide biosorption sites. Different ionic species of a given element might exhibit preference for a different binding site. Should the preference of one metal ion for a ligand be similar to that of another ion, a biosorption competition effect might be observed if both elements are simultaneously present in the contact solution.

A model of bisorption competition effects that is based on Pearson’s classification of metals [361] has been reported as a basic tool for under-standing such effects. On the basis of this model, significant ionic competition effects can be observed for metals belonging to the same Pearson classification class. Elements belonging to different classes demonstrate limited competition, while elements belonging to the Pearson’s classification borderline class are affected by the presence of competing co-ions [362]. Additional systematic work for the mechanistic understanding of biosorptive processes and the associated ionic competition effects is required.

Numerical simulation techniques play an important role in designing and assessing remediation processes, including those using biotechnological methods [363–365]. Although biosorption using inactive microbial biomass has been demonstrated to be effective in substantially removing (and in some cases recovering) targeted radionuclides such as uranium, radium and thorium from contaminated solutions, a full scale commercial application is not yet available.

The use of living microorganisms in innovative reactor configurations has recently been under investigation for the same purposes as conventional biosorption. This approach to the biological sequestering of metals has substan-tially different requirements and operating conditions than conventional inactive biomass biosorption. This alternative biotechnological approach is often referred to as bioaccumulation or bioprecipitation and is showing excellent potential.