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BIOSORPTION FOR THE NEXT CENTURYA part of the Invited Lecture to be presented at the International Biohydrometallurgy Symposium
El Escorial, Spain, June 20-23, 1999
Boya Volesky
Chemical Engineering Department, McGill University,3610 University St., MONTREAL, Canada H3A 2B2
The potential of metal concentration by certain types of dead biomass has been well established over thelast 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 thephenomenon to a successful process application stage. Challenges in the novel biosorption process
development are briefly summarized here for scientists and entrepreneurs alike.
METALS:
ENVIRONMENTAL
THREAT
By far the greatest demand formetal sequestration comes from
the need of immobilizing the
metals mobilized by andpartially lost through human
technological activities. It has
been established beyond anydoubt that dissolved particularly
heavy metals escaping into the
environmentpose a serioushealth hazard. They accumulatein 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.
Environmental Pressures
Stricter regulations withregard to the metal
discharges are being
enforced particularly forindustrialized countries.
The food-chain pyramid receives metals through mans
activities.
On top of the pyramid, man receives pre-concentrated metal
toxicity.
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Toxicology of heavy
metals confirms their
dangerous impacts.
The currently practiced
technologies for removalof heavy metals from
industrial effluentsappear to be inadequate
and expensive. They
often create secondaryproblems with metal-
bearing sludges.
Biosorption is competitive and cheap
Advantages of biosorption would tend to outweigh the very
few shortcomings of biosorbents in applications.
Heavy metals need to best be removed at the source in a
specially designed pre-treatment step which has tofeature low costs to be feasible. The search is on for
efficient and particularly cost-effective remedies.
Biosorption promises to fulfill the requirements.Biosorption uses biomass raw materials which are
either abundant (seaweeds) or wastes from other
industrial operations (fermentation wastes). The metal-
sorbing performance of certain types of biomass can bemore or less selective for heavy metals. That depends
on:
- the type of biomass,
The main attraction of biosorption is its costeffectiveness.
While ion exchange can be considered a ma
technology, biosorption is in its early develo
stages and further improvements in both perand costs can be expected.
Yes, biosorption can become a good weapon
fight against toxic metals threatening our en
While the biosorption process could be useda low degree of understanding of its metal-b
mechanisms, better understanding will make
more effective and optimized applications. Ta scientific challenge and continuedR&D ef
In addition, even the same type of industrial
can produce effluents which differ from eac
great deal. Close collaboration with each clindustrial operation is absolutely essential: a
consulting-engineering type of approach. En
skills become quite important because it is a
operation one is aiming at and dealing with.
"Treatability studies" which are usually carr
close cooperation with the client provide the
for assessing the optimum treatment sequen
Biosorption does offer a competitive wastew
treatment alternative, the basis of which nee
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- the mixture in the solution,
- the type of biomass preparation,
- the chemico-physical environment.It is important to note that concentration of a specific
metal could be achieved either during the sorption
uptake by manipulating the properties of a biosorbent,or upon desorption during the regeneration cycle of the
biosorbent.
Biosorption process of metal removal is capable of aperformance comparable to its closest commercially
used competitors, namely the ion exchange treatment.
Effluent qualities in the order of only ppb (mg/L) of
residual metal(s) can be achieved. While commercialion exchange resins are rather costly, the price tag of
biosorbents can be an order of magnitude cheaper (1/10
the ion exchange resin cost).
well understood in order to prevent applicat
failures.
___________________________________
_
The potential pitfalls in introducing the new biosorption
alternative are quite similar to those encountered withany other novel technology close to the application
stage.
However, there is little doubt that steadily mounting
environmental pressures provide a powerful drivingforce 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;
- assessing biosorption applicability;
- developing customized treatment;- designing and building the plant;
- eventually even operating the effluent
treatment process, and even
- recovering metal(s) for resale/re-use.
Metal Removal/Recovery "Priorities"
An example of the priorization for recovery of tenmetals is in TABLE 1 which may be simplistic but
provides a useful direction by ranking into 3 general
Biosorption and entrepreneurial activities
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priority categories:
(1) environmental risk (ER);(2) reserve depletion rate (RDR);
(3) a combination of the two factors.
Environmental risk assessment could be based on anumber of different factors which could even be
weighed.
The RDR category is used as an indication of
probable futureincrease in the market price of themetal. 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 beconsidered 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.____________________________________________
_
Growth industries and point-source effluents are
concern
TABLE 1: Ranking of metal interest priorities
Relative
Priority
Environ.Risk
ReserveDepletion
Co
Fa
HIGH : Cd Cd C
Pb Pb Pb
Hg Hg H
- Zn Zn
MEDIUM : Cr - -
Co Co C
Cu Cu CNi Ni N
Zn - -
LOW : Al - A
- Cr C
Fe Fe Fe
STRUCTURE OF
A BIOSORPTION PROJECT
With new discoveries of highly metal-sorbing
biomass types there is a real potential for theintroduction 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 exchangeresins, 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 ofprocessing.
Some types of industrial fermentation waste biomass
are excellent metal sorbers. It is necessary to realize
that some "waste" biomass is actually a commodity, nota waste: this applies particularly for ubiquitous
brewers yeasts sold on the open market for a price,
Screening for new biosorbents is essential
As a fall-back, high metal-sorbing biomass c
be specifically propagated relatively cheaply
fermentors using low-cost or even waste car
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usually as animal fodder.
Activated sludge from wastewater treatment plants has
not demonstrated high enough metal-sorbingcapacities.
Some types of seaweed biomass offer excellent
metal-sorbing properties. Local economies can benefit
from turning seaweeds into a resource.
Using waste biomass for preparing new biosorbents is particularly
advantageous. Seaweeds have ready-made macro-structures.
containing growth media based on e.g. mola
cheese whey.
Screening for Adsorption:
Batch equilibrium sorption experiments arscreening for suitable biomass types. Unfor
there are to many errors in the literature betrunderstanding of equilibrium sorption conce
Standard procedure for evaluating simple sorption sy
See details in the biosorption web-site.
Example of (bio)sorption isotherms affected by the pH of solution
Enough time isallowed for equilibrium
contact sorption
experiments. Kineticstests show the time-
concentration profile for
sorption. The sorptionreaction itself is
inherently an extremely
fast one. It is mainly theparticle mass transfer
which controls the overallsorption kinetics (sorbent
particles size, porosityand mixing in the
sorption system).
Environmental factorssuch as the solution pH,
ionic strength, to a lesser
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Fixed-bed column is the most powerful sorption process
arrangement
Solution chemistry of the metals to be examined forbiosorption should be well understood for explanation of
experimental results. For this purpose, a widely available
computer data-base program MINIQL+ is extremely useful.The most appropriate method of assessing the biosorbent
capacity is the derivation of a whole sorption isotherm.
Anything else represents a potentially misleading shortcutwhich may lead to outright erroneous conclusions. While
experimental volume increases almost exponentially with the
number of metallic species present in the solution, evaluation of
multimetal sorption systems offers a special challenge.
degree temperature, etc.
are likely to affect the
sorption performance.The range of conditions
for biosorbent screening
should be carefullyselected.
Dynamic sorption
studies are invariably
more demanding. The
most optimal
configuration forcontinuous-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.
In the sorption column
contactor the saturatedzone is moving along the
column length pushing
the transitional dynamicsorption zone ahead of
itself. With multimetal
sorption systems
featuring differentaffinities of ions toward
the sorbent the whole
system becomes evenmore complex as
chromatographic effects
and simultaneousdisplacement of
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deposited ions take place.
It is obvious that
simplistic observations of
the experimental "break-
through" curve resultingfrom the conventional
operation of a flow-
through sorption column
will not suffice. They are
usually narrowly specific
and cannot be used
elsewhere.
Desorption:
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 resultin:
- high-concentration metal
effluent;
- undiminished metal uptake
upon re-use;
- no biosorbent physico-chemical
damage.
The desorption and sorbent
regeneration studies might
require somewhat different
methodologies. Screening for the
most effective regenerating
solution is the beginning.
Different affinities of metal
ions for the biosorbent result in
certain degree of metal
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selectivity on the uptake.
Similarly, another selectivity
may be achieved upon the
elution-desorption operation
which may serve as anothermeans of eventually separating
metals from one another if
desirable.
The Concentration Ratio (CR) is
used to evaluate the overallconcentration effectiveness of
the whole sorption-desorption
process:
Obviously, the higher the CR isthe better is the overall
performance of the sorption
process making the eventual
recovery of the metal morefeasible with higher eluate
concentrations.
Recovery of the metal from
these concentrated desorption
solutions is carried out in adifferent plant by
electrowinning.
Following desorption of the
metal(s), the column may still be
pre-treated (e.g. pre-saturatedwith protons, Ca, K, etc.) for
optimum operation in the
subsequent metal uptake cycle.
The types of this pre-treatmentmay vary and could be used to
optimize the columnperformance.
Complete biosorbent regeneration may take two or more
operations.
Mechanism of metal biosorption:
Adsorption and desorption studies invariably yield
information on the mechanism of metal biosorption: how is the
A number of different
metal-binding mechanismshas been postulated to be
active in biosorption such
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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 whichconstitute the foundation for mathematical modeling of the
process.
Understanding the mechanism of biosorption is important even for
very practical reasons
While other mechanisms might also contribute, ion exchangeprevails
as:
- chemisorption: by ion
exchange, complexation,coordination,
chelation;- physical adsorption,microprecipitation.
There are also possible
oxidation/reduction
reactions taking place inthe biosorbent. Due to the
complexity of the
biomaterials used it is
quite possible that at least
some of these mechanismsare acting simultaneously
to varying degreesdepending on the
biosorbent and the
solution environment.
More recent studies withfungal biomass and
seaweed in particular have
indicated a dominant role
of ion exchange metalbinding. Indeed, the
biomass materials offernumerous molecular
groups which are known
to offer ion exchange
sites: carboxyl, sulfate,phosphate, amine, could
be the main ones.
When the metal -
biomass interactionmechanism(s) are
reasonably understood, it
opens the possibilities of:
- optimizing the
biosorption process on
the molecular level;
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- manipulating the
biosorption properties of
biomass when it isgrowing;
- developing economicallyattractive
analogous sorbent
materials;
- simplifying and
effectively guiding thescreening process;
- activating biomaterials
low-levelbiosorbent behavior.
Simple and economicallyfeasible pretreatment
procedures for suitablebiomaterials may be
devised based on better
understanding of the metalbiosorbent mechanism(s).
Modeling:
Mathematical modeling and computer simulation ofbiosorption offers an extremely powerful tool for a
number of tasks on different levels. It is essential forprocess design and optimization where the equilibriumand dynamic test information comes together
representing a multivariable system which cannot be
effectively handled without appropriate modeling andcomputer-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 whichis, 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
predictingthe process performance under different
conditions. Computer simulations can then replace
numerous tedious and costly experiments.
Advanced sophistication in this area and availability
Advanced scientific approach aids in understand
phenomenon and in developing biosorption for a
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of very powerful computer hardware and softwaremakes contribution of the process modeling/simulation
activity very realistic and indispensable indeed.
Contemporary molecular modeling software is extremelypowerful can be very useful
Process modeling is sophisticated and should b
pragmatically
A whole new area is opening up in model
molecules, their parts and interactions. "See
biosorbent works on a molecular level woul
purposefully preparing, engineering, a bet
biosorbent. While significant inroads have
in revealing protein and nucleic acid structu
behavior, carbohydrate chemistry which see
the basis of the biosorption behavior still ha
significantly benefited from these advanced
modeling techniques.
Essential process development type of work for flow-through
sorption applications
Granulation:
The last but not the least
area to be developed in thefield of biosorption is the
granulation of biosorbent
materials. It is rather
empirically based but withoutit reliably delivering
granulatedbiosorbents there
may not be any scaled-upbiosorption applications.
The most effective mode of
a sorption process is
undoubtedly based on a fixed-bed reactor/contactor
configuration. The sorption
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Different biomass types require different pre-processing after
which the sorption performance has to be always tested
Establish the overall process feasibility.
bed has to be porous to allow
the liquid to flow through it
with minimum resistance butallowing the maximum mass
transfer into the particles assmall as practical (0.7-1.5mm) for a reasonable pressure
drop across the bed.
Biosorbents have to be hard
enough to withstand theapplication pressures, porous
and/or transparent to metal
ion sorbate species, featuring
high and fast sorption uptake
even after repeatedregeneration cycles.
Considering the vast variety ofand differences in the raw
biomass materials, this is a tall
order.
Conventional granulationtechnologies are rather
advanced and their
adaptation(s) will likely yield
desirable biosorbent granules.At the same time, the broad
variety of biomass types willundoubtedly require extensive
experimentation for the
purpose. There may be also
some logistical problemsbecause of transportation of
raw biomass. Microbial
biomass comes with a highwater content and is prone to
decay. Its drying may berequired if it cannot beprocessed and/or granulated
directly on location in the wet
state.
Processing or granulationof biomass materials into
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suitable cost-effective
biosorbents is a crucial step for
the success of biosorptionprocesses.
Different areas of the project can benefit most from specific scientific disciplines
Challenges for chemistry and biochemistry
Project disciplines:
It is obvious that many
different and challengingcontributions can be made on the
path to developing biosorption
from a scientific curiosity to
useful applications. There is nodoubt that there is a potential in
this field. Apart from individual
scientific challenges there is a
special one in crossing theboundaries of conventional
science disciplines to accomplishthe goal. Individual projects
undertaken best be effectively
interdisciplinary.
The two types of backgroundswhich might undoubtedly
contribute most in developing
the science basis of biosorption
in the direction of its applicationsare chemistry, including
biochemistry, and (chemical
process) engineering. Appliedmicrobiology needs to elucidate
the composition of microbial and
algal cell walls which arepredominantly responsible for
sequestering the metals.
Following equilibriumsorption and dynamic sorption
studies, the quantitative basis for
the sorption process isestablished, including process
performance models.
The biosorption processfeasibility is assessed for well
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Process engineering will have to develop the process with its 2 pilots
selected cases. It is necessary to
realize that there are 2 types of
pilot plants to eventually be runhand in hand:
- Biomass processing pilotplant;
- Biosorption pilot plant.
The biomass supplies need to be
well secured. That, in turn,
brings the whole world into thepicture whereby it may become
attractive for developing
countries with biomass resources
to participate in further
development of the newbiosorption technology.