Uranium Trappers, a Partial Order Study
199
5 Conclusions
From the biotechnological point of view, it is important to remark that the results
of the application of ranking methodologies and the relations among attributes are
based on the set of 83 microorganisms collected and on the kind of attributes considered. The presence of two components in the HD evidences the incomparability
among the microorganisms: In the rightward component, 5 microorganisms are
characterised by high values in UC and time for removing U input (t ≥ 6 h, i.e.
low eff ), whereas 78 microorganisms in the leftward component evidenced relative
low values in UC and their time for the U removal is ≤ 1 h (i.e., high eff ).
From the application of order theoretical methods, the ordering of 83 microorganisms is shown, being the best one Bacillus licheniformis ATCC 14580 [Quintero
et al. 2017; Quintero et al. 2018]. Although the two ranking approaches yielded
similar results, some ties between the two rankings are found. These ties can be
gathered in equivalence classes as follows: four in LPMO, i.e. [44,75], [12,71],
[20,30,82] and [68,77] and three equivalence classes in the case of LPOMext,
namely [44,75],[13,25] and [68,77].
Regarding partial order, the results could be eminently broadened, if the density
of active sites for U trapping in cell walls of microorganisms could be included as
an attribute in the data matrix. However, until now this information is not available.
From the biological point of view, it has been found that a high uranium
uptake capacity associated to the microorganisms studied does not depend upon
the taxonomic hierarchy, e.g., species, genera or phyla, even strains in the same
species show different ability for U biosorption (Quintero et al. 2017; Happel 1996).
Differences in U uptake capacities may be joined to specific genetic traits of each
microorganism expressed in its chemical composition and in the number of binding
sites for trapping U cations in the cell walls (Borrok et al. 2005).
As an outlook, the following biotechnological perspectives can be desirable:
– The suitability of the U trappers proposed in the current study could be further
tested in studies combining batch experiments at the wet lab.
– Although the extrapolation of knowledge from the laboratory scale to industrial
application has been a slow process, if an industrial application of the optimal
microorganisms here reported is sought for, other features need to be studied,
e.g. economical ones. This would allow obtaining an estimation of the overall
cost of the sorbent and biosorption process to treat large volumes of waste waters
with low U concentrations in nuclear facilities.
– In addition to the aforementioned suggestions, a step towards the practical
application is to study the adsorption equilibria supplemented with adsorption
kinetics. Due to the economic importance of recovering U from the biomasses,
it is recommended to carry out studies focused on the separation of this
actinide from the biomass. Likewise, it is proposed to assess the reuse of the
microorganisms through adsorption-desorption cycles for improving the benefitcost relation inherent to the process at industrial scale.
199
5 Conclusions
From the biotechnological point of view, it is important to remark that the results
of the application of ranking methodologies and the relations among attributes are
based on the set of 83 microorganisms collected and on the kind of attributes considered. The presence of two components in the HD evidences the incomparability
among the microorganisms: In the rightward component, 5 microorganisms are
characterised by high values in UC and time for removing U input (t ≥ 6 h, i.e.
low eff ), whereas 78 microorganisms in the leftward component evidenced relative
low values in UC and their time for the U removal is ≤ 1 h (i.e., high eff ).
From the application of order theoretical methods, the ordering of 83 microorganisms is shown, being the best one Bacillus licheniformis ATCC 14580 [Quintero
et al. 2017; Quintero et al. 2018]. Although the two ranking approaches yielded
similar results, some ties between the two rankings are found. These ties can be
gathered in equivalence classes as follows: four in LPMO, i.e. [44,75], [12,71],
[20,30,82] and [68,77] and three equivalence classes in the case of LPOMext,
namely [44,75],[13,25] and [68,77].
Regarding partial order, the results could be eminently broadened, if the density
of active sites for U trapping in cell walls of microorganisms could be included as
an attribute in the data matrix. However, until now this information is not available.
From the biological point of view, it has been found that a high uranium
uptake capacity associated to the microorganisms studied does not depend upon
the taxonomic hierarchy, e.g., species, genera or phyla, even strains in the same
species show different ability for U biosorption (Quintero et al. 2017; Happel 1996).
Differences in U uptake capacities may be joined to specific genetic traits of each
microorganism expressed in its chemical composition and in the number of binding
sites for trapping U cations in the cell walls (Borrok et al. 2005).
As an outlook, the following biotechnological perspectives can be desirable:
– The suitability of the U trappers proposed in the current study could be further
tested in studies combining batch experiments at the wet lab.
– Although the extrapolation of knowledge from the laboratory scale to industrial
application has been a slow process, if an industrial application of the optimal
microorganisms here reported is sought for, other features need to be studied,
e.g. economical ones. This would allow obtaining an estimation of the overall
cost of the sorbent and biosorption process to treat large volumes of waste waters
with low U concentrations in nuclear facilities.
– In addition to the aforementioned suggestions, a step towards the practical
application is to study the adsorption equilibria supplemented with adsorption
kinetics. Due to the economic importance of recovering U from the biomasses,
it is recommended to carry out studies focused on the separation of this
actinide from the biomass. Likewise, it is proposed to assess the reuse of the
microorganisms through adsorption-desorption cycles for improving the benefitcost relation inherent to the process at industrial scale.