**5. Conclusion**

mechanical strength, the small size of individual microbial cells and the difficulty of separating cells once they have been utilised to adsorb metals in liquid effluents. Several processes using biomass immobilisation have been investigated to overcome these disadvantages. Immobili‐ sation of biomass in bio-towers, trickle filters, airlift reactors or rotating systems where microbial biofilms play a key role have been examined [76]. As the immobilised biomass grows and its size increases, there is natural expansion and leakage of the biomass, which can then be collected as a microbial sludge. Provided the metals in the wastewater do not have a deleterious effect on the biofilm or other co- habiting organisms, this system can work well. The advantage of rotating immobilised systems, in the case of cyanobacteria, would be that they can still be exposed to light, as opposed to bio-tower systems. Moving sand bed reactors have also been used [77] to develop consortia to treat mixed metal pollutant effluents, which could also provide enough light for cyanobacterial consortia. Technologies and processes for

Dried or dead cells may absorb more metals than live cells and for this reason encapsulation of biomass may be advantageous [79], which would mean the utilisation of different process configurations. Although dead cells or biomass can be used, there is little data on the relative merits when compared to live cells. Generally, in addition to metallic pollution, natural waste materials may contain other substances that need remediation, and thus having live biomass may, on occasion, be more advantageous. It is envisaged however that should biosoption be employed at scale then some form of continuous flow through system would need to be employed. Many variables need to be considered; including biomass concentration, pollutant metal concentration, pH of the system, and flow rate. As such studies have been carried out at small laboratory scale there is little data available on large scale systems particularly with

Metals absorbed by EPS or biomass are often required to undergo elution in subsequent processes. The nature of such elution processes is dependent on whether the biomass needs to be reused or recycled. Acid or alkali desorption can generally be used for elution [1]. For particular cases, such as precious metal recovery, selective desorption may be used. In the case of radionucleotide recovery, this can occur via combustion and ash removal. In other cases simple liquid extraction may be used on occasion with a variety of solvents. The desorption procedures utilised are thus dependent on the metal, its value and whether the biomass will

**4. Other biodegradative reactions associated with** *Synechocystis*

The genome sequences of a number of *Synechocystis* strains have now been determined but there appears to be few genes associated with biodegradation in this organism. As the organism utilises a phototrophic metabolism, it appears to lacks transport systems for organic materials in general as it synthesises most of what it needs. Interrogation of the KEGG database [80] for metabolic activities associated with *Synechocystis* has revealed that the organism does not possess metabolic pathways to degrade organic pollutants. This may not be a general feature of all *Synechocystis* genera as there have been reports that some species may possess

metal recovery are reviewed in [78].

64 Advances in Bioremediation of Wastewater and Polluted Soil

cyanobacteria.

be reused.

Although there have been many studies on the biosorption potential of cyanobacteria, there remains some way to go before their potential may be realised. Many laboratory based studies do not translate to the field. This may be a factor of the altered physicochemical environment, the competition for binding sites for metals when there are mixed metal species present, or the presence of other competing substances in the polluting water. The debate between use of live and dead cells is also open, with some metals showing both EPS binding and bioaccumulation. Bioaccumulation of the metal species may thus be favoured by the use of live biomass. When live cells are used the uptake tends to be bi-phasic, with initial rapid uptake occurring followed by a slower metabolism driven accumulation [74, 79]. Live cells additionally may display the potential to mutate, become more resistant to the metals and adapt to increase metal loadings in the longer term. Indeed there is a trend to seek out specific metal resistant species as biosorbants, in many cases to verify their potential as a start point for further study [82]. Some studies support the use of dead or dried cells, which often show greater metal binding capacity and may be particularly important if the biomass is to be reused a number of times. There have also been attempts to utilise mixed consortia, using organisms with varying and mixed metal sorption capacities [83]. Such consortia, which may develop naturally in response to the metal loading, are difficult to characterise and members are often transient, making the assignment of roles to particular genera or species difficult.
