ion for the Next Century

8/8/2019 ion for the Next Century

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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.

Documents

ion for the Next Century