The potential of metal concentration by certain
types of dead biomass has been well established over the last two decades.
This phenomenon can probably make the most significant impact in using
it for removing toxic heavy metals from industrial effluents. An interdisciplinary
approach seems essential for bringing the phenomenon to a successful process
application stage. Challenges in the novel biosorption process development
are briefly summarized here for scientists and entrepreneurs alike.
By far the greatest demand for metal sequestration comes from the need
of immobilizing the metals ‘mobilized’ by and partially lost through human
technological activities. It has been established beyond any doubt that
dissolved particularly heavy metals escaping into the environment
pose a serious health hazard. They accumulate in living tissues throughout
the food chain which has humans at its top. The danger multiplies. There
is a need for controlling the heavy metal emissions into the environment.
The food-chain pyramid receives metals through man’s activities.
On top of the pyramid, man receives pre-concentrated metal toxicity.
Advantages of biosorption would tend to outweigh the very few shortcomings of biosorbents in applications.
|The main attraction of biosorption is its cost effectiveness.
While ion exchange can be considered a ‘mature’ technology, biosorption is in its early developmental stages and further improvements in both performance and costs can be expected.
Yes, biosorption can become a good weapon in the fight against toxic
metals threatening our environment. While the biosorption process could
be used even with a low degree of understanding of its metal-binding mechanisms,
better understanding will make for its more effective and optimized applications.
That poses a scientific challenge and continued R&D
"Treatability studies" which are usually carried out in close cooperation with the client provide the backbone for assessing the optimum treatment sequence.
Biosorption does offer a competitive wastewater treatment alternative,
the basis of which needs to be well understood in order to prevent application
|The potential pitfalls in introducing the new biosorption alternative
are quite similar to those encountered with any other novel technology
close to the application stage.
However, there is little doubt that steadily mounting environmental pressures provide a powerful driving force for new business opportunities.
When it comes to a new "biosorption" enterprise, there are two aspects to such:
1) products: new family of biosorbents;
2) services involved in:
- assessing the effluent problem;
Metal Removal/Recovery "Priorities"
(1) environmental risk (ER);
Environmental risk assessment could be based on a number of different factors which could even be weighed.
The RDR category is used as an indication of probable
future increase in the market price of the metal.
When coupled with the ER in this example there is an indication that Cd,
Pb, Hg, Zn are a high priority. However, the technological uses of Hg and
Pb may be considered declining, while the Cd use is on the increase. These
projections and the degree of risk assessment sophistication could change
the priorities among the metals considered.
Biosorption and entrepreneurial activities
TABLE 1: Ranking of metal interest priorities
A BIOSORPTION PROJECT
With new discoveries of highly metal-sorbing biomass types there is a real potential for the introduction of a whole family of new biosorbent products which are likely to be very competitive and cost-efficient in metal sorption.
As a potential competition for synthetic ion exchange resins, capable of doing the same ‘job’, the costs of biosorbents must be maintained very low. That could be guaranteed by low-cost raw material and minimum of processing.
Some types of industrial fermentation waste biomass
are excellent metal sorbers. It is necessary to realize that some "waste"
biomass is actually a commodity, not a waste: this applies particularly
for ubiquitous brewer’s yeasts sold on the open market for a price, usually
as animal fodder.
Some types of seaweed biomass offer excellent metal-sorbing
properties. Local economies can benefit from turning seaweeds into a resource.
Screening for new biosorbents is essential
As a fall-back, high metal-sorbing biomass could even be specifically
propagated relatively cheaply in fermentors using low-cost or even waste
carbohydrate-containing growth media based on e.g. molasses or cheese whey.
Example of (bio)sorption isotherms affected by the pH of solution
Solution chemistry of the metals to be examined for
biosorption should be well understood for explanation of experimental results.
For this purpose, a widely available computer data-base program MINIQL+
is extremely useful.
| ‘Enough’ time is allowed for equilibrium
contact sorption experiments. Kinetics tests show the time-concentration
profile for sorption. The sorption reaction itself is inherently an extremely
fast one. It is mainly the particle mass transfer which controls the overall
sorption kinetics (sorbent particles size, porosity and mixing in the sorption
Environmental factors such as the solution pH, ionic strength, to a lesser degree temperature, etc. are likely to affect the sorption performance. The range of conditions for biosorbent screening should be carefully selected.
sorption studies are invariably more demanding. The most optimal configuration
for continuous-flow sorption is the packed-bed column which gets gradually
saturated from the feed to the solution exit end. Correct and non-trivial
interpretation of experimental results is important and becomes scientifically
rather involved. However, it is expected.
The possibility of regeneration of loaded biosorbent is crucially important to keeping the process costs down and to opening the possibility of recovering the metal(s) extracted from the liquid phase. The deposited metals are washed out (desorbed) and biosorbent regenerated for another cycle of application. The desorption process should result in:
- high-concentration metal effluent;
- undiminished metal uptake upon re-use;
- no biosorbent physico-chemical damage.
and sorbent regeneration studies might require somewhat different methodologies.
Screening for the most effective regenerating solution is the beginning.
Recovery of the metal from these concentrated desorption
solutions is carried out in a different plant by electrowinning.
Following desorption of the metal(s), the column may still be pre-treated (e.g. pre-saturated with protons, Ca, K, etc.) for optimum operation in the subsequent metal uptake cycle. The types of this pre-treatment may vary and could be used to optimize the column performance.
|Mechanism of metal biosorption:
Adsorption and desorption studies invariably yield
information on the mechanism of metal biosorption: how is the metal bound
within the biosorbent. This knowledge is essential for understanding of
the biosorption process and it serves as a basis for quantitative stoichiometric
considerations which constitute the foundation for mathematical modeling
of the process.
| A number of different metal-binding mechanisms has
been postulated to be active in biosorption such as:
- chemisorption: by ion exchange, complexation,
There are also possible oxidation/reduction reactions taking place in the biosorbent. Due to the complexity of the biomaterials used it is quite possible that at least some of these mechanisms are acting simultaneously to varying degrees depending on the biosorbent and the solution environment.
More recent studies with fungal biomass and seaweed in particular have indicated a dominant role of ion exchange metal binding. Indeed, the biomass materials offer numerous molecular groups which are known to offer ion exchange sites: carboxyl, sulfate, phosphate, amine, could be the main ones.
When the metal - biomass interaction mechanism(s) are reasonably understood, it opens the possibilities of:
- optimizing the biosorption process on
Simple and economically feasible pretreatment procedures for suitable biomaterials may be devised based on better understanding of the metal biosorbent mechanism(s).
Mathematical modeling and computer simulation of biosorption offers an extremely powerful tool for a number of tasks on different levels. It is essential for process design and optimization where the equilibrium and dynamic test information comes together representing a multivariable system which cannot be effectively handled without appropriate modeling and computer-based techniques.The dynamic nature of sorption process applications (columns, flow-through contactors) makes this approach mandatory. When reaction kinetics is combined with mass transfer which is, in turn, dependent on the particle and fluid flow properties only a rather sophisticated apparatus can make sense out of the web of variables.
The mission of biosorption process modeling must be predicting the process performance under different conditions. Computer simulations can then replace numerous tedious and costly experiments.
Advanced sophistication in this area and availability of very powerful computer hardware and software makes contribution of the process modeling/simulation activity very realistic and indispensable indeed.
Advanced scientific approach aids in understanding the phenomenon and in developing biosorption for applications
A whole new area is opening up in modeling of molecules, their parts and interactions. "Seeing" how the biosorbent works on a molecular level would aim at purposefully preparing, ‘engineering’, a ‘better biosorbent’. While significant inroads have been made in revealing protein and nucleic acid structures and their behavior, carbohydrate chemistry which seems to be at the basis of the biosorption behavior still has not significantly benefited from these advanced computer modeling techniques.
Essential process ‘development’ type of work for flow-through sorption applications
The last but not the least area to be developed in the field of biosorption is the granulation of biosorbent materials. It is rather empirically based but without it reliably delivering granulated biosorbents there may not be any scaled-up biosorption applications.
The most effective mode of a sorption process is undoubtedly based on a fixed-bed reactor/contactor configuration. The sorption bed has to be porous to allow the liquid to flow through it with minimum resistance but allowing the maximum mass transfer into the particles as small as practical (0.7-1.5 mm) for a reasonable pressure drop across the bed.
Biosorbents have to be hard enough to withstand the application pressures, porous and/or ‘transparent’ to metal ion sorbate species, featuring high and fast sorption uptake even after repeated regeneration cycles. Considering the vast variety of and differences in the raw biomass materials, this is a tall order.
Conventional granulation technologies are rather advanced and their adaptation(s) will likely yield desirable biosorbent granules. At the same time, the broad variety of biomass types will undoubtedly require extensive experimentation for the purpose. There may be also some ‘logistical’ problems because of transportation of raw biomass. Microbial biomass comes with a high water content and is prone to decay. Its drying may be required if it cannot be processed and/or granulated directly on location in the wet state.
Processing or ‘granulation’ of biomass materials into suitable cost-effective biosorbents is a crucial step for the success of biosorption processes.
Different areas of the project can benefit most from specific scientific disciplines
It is obvious that many different and challenging contributions can be made on the path to developing biosorption from a scientific curiosity to useful applications. There is no doubt that there is a potential in this field. Apart from individual scientific challenges there is a special one in crossing the boundaries of conventional science disciplines to accomplish the goal. Individual projects undertaken best be effectively interdisciplinary.
The two types of backgrounds which might undoubtedly
contribute most in developing the science basis of biosorption in the direction
of its applications are chemistry, including biochemistry, and (chemical
process) engineering. Applied microbiology needs to elucidate the composition
of microbial and algal cell walls which are predominantly responsible for
sequestering the metals.
Following equilibrium sorption and dynamic sorption
studies, the quantitative basis for the sorption process is established,
including process performance models.
The biomass supplies need to be well secured. That, in turn, brings the ‘whole world’ into the picture whereby it may become attractive for developing countries with biomass resources to participate in further development of the new biosorption technology.