Detoxification of metal-bearing effluents: biosorption for...

Ž .Hydrometallurgy 59 2001 203–216www.elsevier.nlrlocaterhydromet

Detoxification of metal-bearing effluents: biosorption forthe next century

B. Volesky )

Department of Chemical Engineering, McGill UniÕersity, 3610 UniÕersity Street, Montreal, Quebec, Canada H3A 2B2

Received 20 October 1999; accepted 7 March 2000

Abstract

Metals can be removed and concentrated from solutions by using biomass material. Conservative estimates give newbiosorbents the potential share amounting to US$27 millionryear of the currently existing environmental market in NorthAmerica alone. Very high cost-effectiveness of biosorption technology would tend to open new opportunities currentlyuntapped. Biosorbents can be regenerated for multiple reuse, offering the metal recovery possibility from concentrated washsolutions. Relatively simple metal biosorption processes can meet the progressively stricter environmental discharge criteria.As with any up-start technology, the continuing R&D is crucial. The interdisciplinary nature on both sides, application aswell as R&D, poses quite a challenge. While there are numerous potential industrial clients, a successful biosorptionenterprise will have to have courage, multidisciplinary skills and adequate financing. q 2001 Elsevier Science B.V. Allrights reserved.

Keywords: Metal; Effluent; Biosorption

1. Metals: environmental threat

By far the greatest demand for metal sequestrationcomes from the need of immobilizing the metals‘mobilized’ by and partially lost through growingand ever-intensifying human technological activities.It has been established beyond any doubt that dis-solved particularly heavy metals escaping into theenvironment pose a serious health hazard. Thethreatening presence of heavy metals has even beenlinked to the demise of the Roman empire in the pastw x1 . Nowadays, with the exponentially increasingpopulation the need for controling heavy metal emis-

) Tel.: q1-514-398-4276; fax: q1-514-398-6678.Ž .E-mail address: [email protected] B. Volesky .

sions into the environment is even more pronounced.This is best done right at the source of such emis-sions, before toxic metals enter the complex ecosys-tem. To follow the fate of metallic species after theyenter the ecosystem becomes very difficult and theystart to inflict the damages as they move throughfrom one ecological trophic layer into another. Theyaccumulate in living tissues throughout the food

Ž .chain which has humans at its top Fig. 1 . Thedanger multiplies and humans eventually tend toreceive the problems associated with the toxicity ofheavy metals pre-concentrated and from many differ-ent directions. The resulting health problems demon-strate themselves on the acute as well as chroniclevels and are reflected in the well-being of individu-als and in society’s spiraling health care costs. Con-

0304-386Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0304-386X 00 00160-2

( )B. VoleskyrHydrometallurgy 59 2001 203–216204

Fig. 1. With increasing amounts of metals AunearthedB, amounts of toxic heavy metals entering the environment increases. They threatenhumans as they become pre-concentrated throughout the food chain.

trolling heavy metal discharges and removing toxicheavy metals from aqueous solutions has become achallenge for the 21st century.

1.1. EnÕironmental pressures

v Under the public and media pressure, govern-ments introduce and progressively enforce stricterregulations with regard to the metal discharges par-ticularly for industrial operations.

v The compounding toxic effects of heavy metalsin the environment are being recognized and theirdangerous impacts better understood.

v The currently practiced technologies for re-moval of heavy metals from industrial effluents ap-pear to be inadequate, creating often secondary prob-lems with metal-bearing sludges which are extremelydifficult to dispose of. Due to their classification asAtoxic substancesB they require special handling,disposal methods and sites. Their handling and dis-posal is, in most instances, closely monitored by thegovernments.

v The currently available Abest treatment tech-nologiesB for metal-bearing effluents are either noteffective enough or are prohibitively expensive andinadequate considering the vast wastewater quanti-ties.

1.2. ConÕentional metal-remoÕal technologies

Municipal sewage treatment plants are not de-signed and equipped for handling toxic wastes. Met-als and their toxicity persist even in the sludges andby-product streams of municipal sewage treatmentplants. Heavy metals need to best be removed at thesource in a specially designed ‘pre-treatment’ step.This specific treatment needs to be cheap because itmost often deals with large volumes of effluents.

The conventional approaches to heavy metal re-moval are using technologies listed in Table 1. It isnot within the framework of this text to discuss theseconventional techniques but it would suffice to saythat as the emission standards tighten they are be-coming progressively more inadequate. On the otherhand, better technologies are more costly and oftenjust not feasible. The search is on for efficient andparticularly cost-effective remedies. Biosorptionpromises to fulfill the requirements. This novel ap-proach is competitive, effective and cheap.

1.3. Biosorption

Under the term of metal AbiosorptionB a passiveprocess of metal uptake and sequestering is under-stood whereby the metal is sequestered by chemical

( )B. VoleskyrHydrometallurgy 59 2001 203–216 205

Table 1Conventional metal removal technologies

Method Disadvantages Advantages

Chemical precipitation and filtration for higher concentrations simpledifficult separation cheapNOT effectiveresulting sludges

Ž .Chemical oxidation or reduction chemicals required not universal mineralizationŽ .biological system slow rates mineralization

climate sensitiveElectrochemical treatment for high concentrations metal recovery

EXPENSIVEŽ .Reverse osmosis high pressures pure effluent for recycle

membrane scalingEXPENSIVE

Ion exchange sensitive to particles effectiveEXPENSIVE RESINS pure effluent metal recovery possible

Adsorption not for metals conventionalŽ .sorbents carbon

Ž .Evaporation energy intensive pure effluent for recycleEXPENSIVEresulting sludges

sites naturally present and functional even when thebiomass is dead. The advantage of biosorption is inusing biomass raw materials which are either abun-

Ž .dant seaweeds or wastes from other industrial oper-Ž .ations fermentation wastes . The unique capabilities

of certain types of biomass to concentrate and immo-bilize particularly heavy metals can be more or lessselective. That depends, to a certain degree on:

v the type of biomass,v the mixture in the solution,v the type of biomass preparation,v the chemico-physical process environment.

Broad-range biosorbents can collect all the heavymetals from the solution with a small degree ofselectivity among them. It is important to note thatconcentration of a specific metal could be achievedeither during the sorption uptake by manipulating theproperties of a biosorbent, or upon desorption duringthe regeneration cycle of the biosorbent.

1.4. Metal remoÕalrrecoÕery priorities

An example of the prioritization for recovery of10 metals in Table 2 may be perhaps somewhat

simplistic. It serves as an example of considerationsto take when choosing the metals of interest foreither removal andror recovery. These considera-tions rank the metals into three general prioritycategories:

Ž .1. environmental risk ER ;Ž .2. reserve depletion rate RDR ;

3. combination of the two above factors.

Table 2Ranking of metal interest priorities

Relative priority Environmental Reserve Combinedrisk depletion factors

High Cd Cd CdPb Pb PbHg Hg Hg– Zn Zn

Medium – Al –Cr – –Co Co CoCu Cu CuNi Ni Ni

Ž .Zn – see HighLow Al – Al

– Cr CrFe Fe Fe


Table 3Major target industrial sectors

Industry Metals Possible interferences

Mining operations Cations: Cu, Zn, Pb, Mn, U, . . . Fe, AlAnions: Cr, As, Se, V, . . . sulphates, phosphates

Electroplating operations Cr, Ni, Cd, Zn Fe, surfactantsMetal processing Cu, Zn, Mn, Fe, Al, surfactantsCoal-fired power generation Cu, Cd, Mn, Zn, . . . Fe, AlNuclear industry U, Th, Ra, Sr, Eu, Am, . . . FeSpecial operations Hg, Au and precious metals

The environmental risk assessment could be basedon a number of different factors which could even beweighed. The RDR category is used as an indicatorof probable future increase in the market price of themetal. When coupled with the ER in this examplethere is an indication that Cd, Pb, Hg and Zn are ahigh priority. However, the technological uses of Hgand Pb may be considered declining, while the Cduse is on the increase. These projections and thedegree of risk assessment sophistication could changethe priority sequence among the metals considered.The metallic elements handled by the nuclear indus-try represent a very special category.

Table 3 lists the key industrial sectors that arelikely to have metal-bearing effluent discharge prob-lems and deserve an especially close scrutiny. Natu-rally, they are the most likely potential clients forremoval of metals from their wastewaters.

2. Metal biosorption development areas

With new discoveries of highly metal-sorbingbiomass types there is a real potential for the intro-duction of a whole family of new biosorbent prod-ucts which are likely to be very competitive andcost-efficient in metal sorption. These materials canprovide a basis for a whole new technology of metalremoval and recovery. The search for cheap biosor-bent materials for the purpose focuses on examiningthe potential of waste andror abundant biomasstypes. The discovery of metal biosorption was due torecognizing the fact that metal concentration bybiomass was based on its ‘chemical’ properties ratherthan biological activity. Biosorption research tookoff in the 1980s.

While there are copious quantities of waste acti-vated sludge from conventional biological wastewa-ter treatment plants all over the world, the metal-sorbing capacities of these sludges representing verymixed and heterogeneous microbial populations areusually rather low. There may be some possibilitiesfor improving their metal-sorbing capacity but theheterogeneity of the biomass makes this difficult anddoubtful. Microbial biomass can be grown extremelyfast and in many instances there are large quantitiesof it posing even a serious disposal problem forfermentation industries. These are possibly thecheapest biomass sources. It is necessary to realizethat some AwasteB biomass is actually a commercialcommodity, not a waste: this applies particularly forubiquitous brewer’s yeasts sold on the open marketfor a good price usually as animal fodder. A uniqueand ubiquitous type of macroscopic biomass knownfor its metal-sorbing potential are seaweeds, marinealgae.

For preparation of suitable biosorbent materialsfrom industrial waste biomass for application inlarge-scale sorbing equipment, the consistency of thebiomass will have to be altered. Normally, industrialwaste biomass appears as wet ‘mud’ or dry cake orpowder. It will have to be processed into durablesmall granules to withstand the conditions of thesorption process. Seaweed biomass, on the otherhand, does have a certain rigid macro-structure of itsown and in some instances it has been revealed tooffer excellent metal-sorbing properties. At certainocean locations, seaweeds are plentiful and very fastgrowing. At some locations, they threaten the touristindustry by spoiling pristine environments and foul-ing beaches. Turning seaweeds into a resource canbe quite beneficial to some local economies. From


simple collection of the seaweed biomass the trendpoints at aquaculture methods as the demand may beincreasing.

As a fall-back, high metal-sorbing biomass couldeven be specifically propagated relatively cheaply infermentors using low-cost or even waste carbo-hydrate-containing growth media based on, e.g. mo-lasses or cheese whey.

2.1. Screening

Screening of microbial biomass types for metalbiosorption constitutes an important, albeit tedious,way of identifying the most promising types ofbiomass. In the absence of deeper knowledge withregard to metal biosorption, various materials have tobe tested to assess their metal-sorbing potential. Thisis done mainly based on simple batch equilibriumsorption tests. For the sake of expediency at thisstage of work many errors have been committed andeven reported in the literature by those who do notquite understand equilibrium sorption concepts. Themost appropriate method of assessing the biosorbentcapacity is the derivation of a whole sorption isothermw x2 . Anything else represents a potentially misleadingshortcut, which may lead to outright erroneous con-clusions. A very comprehensive list of metal biosorp-

w xtion test results was assembled in 1995 3 .

2.2. Biosorption uptake studies

Equilibrium sorption studies with simple one-metal sorption systems are usually completed firstand gradually expanded into examining multimetalbiosorbent behavior in conjunction with the selectedbiosorbent. Then good experimental planning is es-sential and result interpretation becomes more in-volved. Kinetics studies yield more accurate informa-tion on sorbent uptake rates which are known to beinherently very fast for sorption reactions and diffi-cult to follow. Rapid sorption reactions are usually

Ž .NOT the rate controlling factor in the bio sorptionprocess. It has been widely recognized and con-firmed that it is actually the intraparticle mass trans-fer rate which represents the bottleneck and is thuscontrolling the rate of the entire sorption process.

Ž .The particle size and its structure s are thus veryimportant.

In the ‘real world’ the solutions processed arerarely pure. The assessment of the effect on thebiosorption performance of ‘impurities’ that may bepresent is of interest. These could be both organicand inorganic, dissolved, colloidal or suspended. Ac-tually, different metal ions could interfere with thesorption process, as might other ionic species. Thechoice of those to be studied has to be made judi-ciously and with regard to the real process condi-tions. At this point, asking a pragmatic questionAwhat is it all good for?B may be very helpful interms of guiding the choices of factors to be studiedand which may eventually be important in the over-all mission of the process itself.

The same applies to the choice of environmentalfactors to be examined some of which tend to influ-ence the sorption process more than others. How-ever, the solution pH and ionic strength are usuallythe most obvious and important ones to look at. Agreat deal of information can be derived from theequilibrium sorption studies that provide the infor-mation basis for the design of the biosorption pro-cess.

Dynamic sorption studies are invariably more de-manding. They involve liquid flow and relativelycomplex mass transfer and reflect more closely theactual configuration of the sorption process. Correctand non-trivial interpretation of experimental resultsis important and becomes technically and scientifi-

w xcally rather sophisticated 4,5 . Particularly consider-ing the contemporary state of the art in sorptionprocesses, advanced sophistication in dynamic sorp-tion studies is expected. It is obvious that simplisticobservations of experimental Abreak-throughB curvesŽ .Fig. 2 resulting from the conventional operation ofa flow-through sorption column will not suffice.These types of results are usually very specific andcannot be used for explaining the behavior of an-other, even similar, sorption system.

The difficulty of analyzing the dynamic sorptionbehavior stems from the fact that such systems in-volve the sorption equilibrium behavior, mass trans-fer, and fluid flow properties all at the same time asthe saturation of the sorbent progresses not only intime but also in space of the sorption column. In thesorption column contactor, the steady state zone is


Fig. 2. When the sorption column becomes saturated, it ceases to function and the metal Abreaks throughB. This situation determines theuseful service time of the column. The size of the still unsaturated column portion at this time determines the degree of column utilization.

moving right along the column length pushing thetransitional sorption zone ahead of itself. In multi-metal sorption systems, where ions of different met-als feature different affinities toward the sorbent thewhole system becomes even more complex as chro-matographic effects due to simultaneous displace-ment of ions already deposited take place.

When compared to the sorption fixed-bed columnreactor, the continuous-flow stirred-tank reactorŽ .CSTR lies on the opposite end of the reactorspectrum. While the flow in the theoretical fixed-bedreactor is considered as non-mixed plug flow, thebasic assumption for the CSTR is complete andinstantaneous mixing. Its contents is thus consideredhomogeneous. Its outflow reflects exactly what isinside. This means that only a very small concentra-tion difference as a driving force may be thus avail-able for the sorption process in the CSTR. Normally,the CSTR would not be the reactor of choice for asorption process unless it is organized as a series ofmixed reactors. Technological process requirementsmay warrant such an operational mode in some casessuch as sorption from particle-containing solutionsŽ .suspensions which cannot be done in a packed-bedcolumn without it becoming rapidly plugged up. Tofluidize the bed in the column may prevent the

Ž .plugging Fig. 3 . Performance estimation of suchhybrid systems becomes especially challenging par-ticularly considering their complex fluid dynamics.

2.3. Desorption

If the biosorption process were to be used as analternative in the wastewater treatment scheme, theregeneration of the biosorbent may be crucially im-portant for keeping the process costs down and to

Ž .opening the possibility of recovering the metal sextracted from the liquid phase. For this purpose it isdesirable to desorb the sorbed metals and to regener-ate the biosorbent material for another cycle of appli-cation. The desorption side of the process should:

v yield the metals in a concentrated form;v restore the biosorbent to close to the original con-

dition for effective reuse with:( undiminished metal uptake;( no physical changes or damage.

Extensive ‘desorption’ work may be necessary forassessing whether this is possible and under whatconditions. The desorption and sorbent regeneration


Fig. 3. The fluidized-bed sorption contactor would feature lower sorption efficiencies. However, it allows for some suspended solids in thefeed stream. The usually practised in-situ regeneration is not quite straightforward for such a system.

studies might require somewhat different methodolo-Žgies particularly for fluidized-bed arrangements Fig.

.3 . While the regeneration of the biosorbent may beaccomplished by washing the metal-laden one withan appropriate solution, the type and strength of thissolution would depend on just how the depositedmetal has been bound. Screening for the most effec-

w xtive regenerating solution is the beginning 6 . Inbatch tests of desorbing solutions, one has to realize

Ž .that the desorbed sorbate metal stays in the solutionand a new equilibrium is established between that

Ž .and the one remaining still fixed on the biosorbent.This leads to the concept of a Adesorption isothermBwhere the equilibrium is strongly shifted towards the

w xsorbate dissolved in the solution 7 . However, someresidual sorbate may still be retained by the biosor-bent to a various degree. The effect of the desorptionwash on the structure of the biosorbent has to be

Žestablished microscopy, structure tests, pressure.drop, etc. .

Due to different affinities of metal ions for theŽpredominant sorption site under the solution condi-

.tions , there will be a certain degree of metal selec-tivity by the biosorbent on the uptake. Similarly,another selectivity may be achieved upon the elu-

w xtion–desorption operation 8,9 . Advantage could betaken of this selectivity on the desorption side of theoperation which can contribute to the separation ofmetals from one another if desirable.

AThe proof of the puddingB, so to speak, for thesorption process is its overall capacity to concentratethe sorbate metal. This is assessed by expressing asimple overall process parameter, the Concentration

Ž .Ratio CR . Obviously, the higher the CR, the betteris the overall performance of the sorption processmaking the eventual recovery of the metal morefeasible as it becomes concentrated in the smallvolume of the elutant solution. Following desorption

Ž .of the metal s , the column may still be pre-treatedby another type of wash solution in order to achieveits optimum performance in the subsequent metaluptake cycle. The types of this treatment may vary.

The potential recovery of the metal from concen-trated desorption solutions is another question. Itwould usually be carried out as an independent metalrecovery operation, in a different plant, by an en-tirely different process or a sequence of operations. Itis most often feasible to use electrowinning proce-dures to recover metals from concentrated solutions.

2.4. Mechanism of metal biosorption

Adsorption and desorption studies invariably yieldsome information on the mechanism of metalbiosorption: how is the metal bound within thebiosorbent. This knowledge is essential for under-standing of the biosorption process and it serves as a


basis for quantitative stoichiometric considerationswhich constitute the foundation for mathematical

w xmodeling of the process 4,10–12 .A number of different metal-binding mechanisms

has been postulated to be active in biosorption metaluptake such as:

v chemisorption by ion exchange, complexation, co-ordination, chelation;

v physical adsorption and microprecipitation.

There are also possible oxidationrreduction reac-w xtions taking place in the biosorbent 13 . Due to the

complexity of biomaterials used it is quite possiblethat at least some of these mechanisms are actingsimultaneously to varying degrees depending on thebiosorbent and the solution environment. More re-cent studies with fungal biomass and seaweed inparticular have indicated a dominant role of ion

w xexchange metal binding 14–17 . Indeed, the biomassmaterials offer numerous molecular groups that areknown to offer ion exchange sites, carboxyl, sulfate,phosphate, and amine, could be the main ones.

When the metal–biomass interaction mechanismsare reasonably understood, the work can begin onoptimizing the biosorption process on the molecularlevel. That could include even manipulation of selec-

Ž .tivity for particular metal s of interest. An intriguig-ing long-range challenge would be to manipulate themetal biosorption properties of the biomass alreadywhen it is being biosynthesized during the growth of

w xthe cell 18 .The knowledge of metal biosorption mechanisms

could lead to developing economically attractivew xanalogous sorbent materials 19,20 . Better under-

standing of the metal biosorption phenomenon wouldsimplify the screening process which could be muchmore focused. The possibility of ‘activating’ thosebiomaterials that do not exhibit apparent biosorbentbehavior is also very attractive. Simple and economi-cally feasible pretreatment procedures for suitablebiomaterials may be devised based on better under-

Ž .standing of the metal biosorbent mechanism s .

2.5. Modeling

Mathematical modeling and computer simulationof biosorption offers an extremely powerful tool for

a number of tasks on different levels. It is essentialfor process design and optimization where the equi-librium and dynamic test information comes togetherrepresenting a multivariable system which cannot beeffectively handled without appropriate modeling andcomputer-based techniques. The dynamic nature of

Žsorption process applications columns, flow-through.contactors makes this approach mandatory. When

reaction kinetics is combined with mass transferwhich is in turn dependent on the particle and fluidflow properties only a reasonably sophisticated appa-ratus can make sense out of the web of variablesw x10,12,15 . The know-how needs to be transferredfrom established areas of sorption processes as de-veloped for activated carbon or ion exchange resins.

Biosorption process modeling is particularly use-ful for predicting the process performance underdifferent conditions. Computer simulations can thenreplace numerous tedious and expensive experimentsto the extent that only the key experiments can beselected and need to be carried out to verify thepredictions. Needless to say, the simulations are onlyas good as the model behind them is. However,advanced sophistication in this area and availabilityof very powerful computer hardware and softwaremakes contribution of the process modelingrsimula-tion activity very realistic and indispensable indeed.

Biosorption process modeling then can:

v guide experimental research;v optimize a given process;v provide basis for process control strategies;v provide a process diagnostic tool.

A whole new area is opening up in modeling ofmolecules, their parts and interactions. ASeeingB howthe biosorbent works on a molecular level would aimat purposefully preparing, ‘engineering’, a ‘betterbiosorbent’. While significant inroads have beenmade in revealing protein and nucleic acid structuresand their behavior, carbohydrate chemistry whichseems to be at the basis of the biosorption behaviorstill has not significantly benefited from these ad-vanced computer modeling techniques. The opportu-nity beckons.


2.6. Granulation

The last but not the least area to be developed inthe field of biosorption is the granulation of biosor-bent materials. It is not necessarily a scientificallyglamorous endeavour since it appears to be ratherempirically based but without it reliably deliveringgranulated biosorbents there may not be any scaled-up biosorption applications. The most effective modeof a sorption process is undoubtedly based on afixed-bed reactorrcontactor configuration. The sorp-tion bed has to be porous to allow the liquid to flowthrough with minimum resistance but allowing themaximum mass transfer into the particles. The parti-cles thus should be as small as practical for thereasonable pressure drop across the bed. This con-cept of this type of a process compromise isschematically depicted in Fig. 4. Ion exchange resinsmanufactured for the same purpose generally featureparticle sizes between 0.7 and 1.5 mm. Biosorbentsshould come in about the same size. Hard enough towithstand the application pressures, porous andror‘transparent’ to metal ion sorbate species, featuringhigh and fast sorption uptake even after repeatedregeneration cycles. Considering the vast variety ofand differences in the raw biomass materials, this isa tall order.

However, conventional granulation technologiesare rather advanced and the chances are very goodthat some applications will yield desirable biosorbent

w xgranules 21–23 . At the same time, the broad vari-ety of biomass types will undoubtedly require exten-sive experimentation for this purpose. There may bealso formidable ‘logistical’ problems because oftransportation of raw biomass. Microbial biomasscomes with a high water content and is prone todecay. Its drying may be required if it cannot beprocessed directly on location in the wet state. How-ever, bulk processing may not add significantly tothe costs. Similarly with the seaweed biomass—fol-lowing the collection, it would have to be processedon location immediately in the wet state or at leastsun dried. The actual granulation procedures coulddiffer substantially depending on whether dry or wetbiomass is to be processed.

In order to make biosorbent granules, the high-sorbing biomass needs to be AimmobilizedB, madeinto suitable solid particles by a using technique suchas:

v entrapment in a strong but permeable matrix;v encapsulation within a membrane-like structure.v Ž .bonding of smaller microscopic particles.

Each of these techniques has certain advantages anddisadvantages, some of which are indicated in Table4.

Most of the methodologies used for particle mak-Ž .ing granulation have been reasonably well devel-

Fig. 4. The best sorption performance would be for small particles with minimized intra-particle mass transfer. The pumping pressure has tobe high to push the liquid through a small-particle bed. A feasible operation compromise has to be devised.


Table 4Biomass immobilizaton

Immobilization Advantages Disadvantages References

w xGel entrapment known cheap limitations: 27w xmass transfer 28w xcatalyst densities 29w xEncapsulation higher catayst fragile capsules 30w xdensities mass transfer 31

barrierw xBiomass cross- increased loss of activity 32

linking strength not universalw xNo treatment cheap irregularities 24w x33

oped for many different types of materials. However,the special nature and widely varied chemistry asso-ciated with different biomass types requires specialefforts and continuing investigations in order to ob-tain the desired granule properties.

While granulation is essential for advancement ofbiosorption as a technology, it is more of a trial-and-error task making it difficult for academic re-searchers to make a significant contribution. Some-what distant from the AresearchB part of the typicalR&D effort it seems to be more of a product Adevel-

opmentB type activity with inherent logistical hurdlesinvolved.

2.7. Biosorption process operation

The most effective configuration of the biosorp-tion process, as the case is for most sorption pro-cesses in general, is the flow-through packed bed

Ž .sorption column Fig. 2 . Its performance is cruciallydependent on the quality of the sorption particles it isfilled with. Multiple reuse of the active sorbentparticles greatly increases the process economy. Thecolumn operation would often consist of two or three

Ž .cycles Fig. 5 : the sorption uptake period followedby desorption of the deposited species and some-times regeneration of the column for the next uptake

w xcycle is necessary 9,24,25 .In general, the biosorption process feasibility de-

pends primarily on the sorption material uptake. Theoverall process performance, in correspondence tothe column operating cycles, is affected by the effi-ciency of the desorption and regeneration cycles.While the biosorption process feasibility rests on theavailabilities and costs of the biomass raw material,the way this raw material is formulated into an activebiosorbent is very important. That refers to its granu-lation and pre-treatment processing which deter-

w xmines the price of the final product 26 .

Fig. 5. The sorption column overall operation cycle consists of the sorption and desorption periods. The number of cycles a column can gothrough depends on many factors.


2.8. Project disciplines

It is obvious that many different and challengingcontributions can be made on the path to developingbiosorption from a scientific curiosity to useful ap-plications. There is no doubt that there is a potentialin this field. Apart from individual scientific chal-lenges, there is a special one in crossing the bound-aries of conventional science disciplines to accom-

Ž .plish the goal: to develop understanding of the metalbiosorption phenomenon that could reliably serve asa base for a successful technological enterprise.

Different science disciplines can make the bestcontribution at different points of the field as theywere discussed above. While a great deal of overlapexists in those crucial areas, no one discipline coulddo it all. The challenge is in developing the individ-ual projects undertaken on an interdisciplinary basisand in coordinating the efforts of professionals withdifferent science backgrounds needed.

The two types of backgrounds which might un-doubtedly contribute most in developing the sciencebasis of biosorption in the direction of its applica-tions are chemistry, including biochemistry, andŽ .chemical process engineering. Their most impor-tant areas of contribution are summarized in Table 5.This does not exclude possible and valuable contri-butions of applied microbiology. However, the latter

Table 5Areas of science discipline contributions

Chemistry

Biosorption mechanismsAnalytical methodologiesInstrumental analysesSolution chemistryCell-wall compositionSorption optimization on a molecular level

Engineering

Sorption equilibriaDynamic flow sorptionProcess optimizationProcess feasibility and scale-up

Ž .Biosorption applications Pilot 1Ž .Biosorbent preparation Pilot 2

could hardly address the sorption process aspects.More needs to be known regarding the compositionof microbial and algal cell walls which are predomi-nantly responsible for sequestering the metals. Inte-rior of cells, due to its very ‘dilute’ nature seems tocontribute very little to the overall metal uptakecapacity of biomass.

Following equilibrium sorption and dynamic sorp-tion studies, the quantitative basis for the sorptionprocess is established, including process performancemodels. The biosorption process feasibility is as-sessed for well-selected cases. It is necessary torealize that there are two types of pilot plants toeventually be run hand in hand:

v Biomass processing pilot plant;v Biosorption pilot plant.

The biomass supplies need to be well secured. That,in turn, brings the ‘whole world’ into the picturewhereby it may become attractive for developingcountries with biomass resources to participate infurther development of the new biosorption technol-ogy.

3. Biosorption enterprise

Biosorption process of metal removal is capableof a performance comparable to its closest commer-cially used competitors, namely the ion exchangetreatment. Effluent qualities in the order of only ppbŽ . Ž .mgrL of residual metal s can be achieved. Whilecommercial ion exchange resins are rather costly, theprice tag of biosorbents can be an order of magni-

Ž .tude cheaper one-tenth the ion exchange resin cost .The main attraction of biosorption is that it offers

a great deal of cost effectiveness. While ion ex-change can be considered a ‘mature’ technology,biosorption is in its early developmental stages andfurther improvements in both performance and costscan be expected.

Yes, biosorption can become a good weapon inthe fight against toxic metals threatening our envi-ronment and our health. It is particularly in environ-mental applications where biosorption can make a


difference due to its anticipated low costs. Needlessto say, there may be many other applications on thehorizon—wherever heavy metals need to be ex-tracted from relatively dilute solutions. The applica-tion aspect is what makes the R&D work in thisnovel area exciting and worthwhile. While thebiosorption process could be used even with a rela-tively low degree of understanding of its metal-bind-ing mechanisms, better understanding will make forits more effective and optimized applications. Thereis a scientific challenge in revealing the basis of themetal biosorption phenomenon.

Despite the relative simplicity of the biosorptionprocess, the technology based on it is as yet un-proven and for its field success it requires continuedR&D efforts. When it comes to client-specific appli-cations, it is essential to realize that industrial efflu-ents can differ from each other a great deal eventhough the technological processes where they origi-nate may be turning out similar end-products. Closecollaboration with each ‘client’ industrial operationis absolutely mandatory, requiring a broader back-ground and understanding of the manufacturing pro-cedures involved upstream. This is a typical consult-ing–engineering type of approach. Engineering skillsbecome quite important because it is a process thatone ends up dealing with. Biosorption becomes onlyone of the technologies potentially available for theeffluent problem remediation.

In order to prevent failures in attempts to intro-duce the biosorption process as a viable and working

alternative clean-up technology, close collaborationwith the client is essential. A thorough test programusing the real effluent becomes mandatory. How-ever, this all should be based on adequate under-standing of biosorption principles, mechanisms andmaterials. Procedures involved in this represent anoutstanding challenge emphasizing the necessity ofinterdisciplinary collaborations.

There is little doubt that the combined environ-mental pressures, reflected in regulatory statutes, andeconomic stimuli make the removal and recovery ofheavy metals from industrial effluents an importantand ever-increasing priority which represents an ex-traordinary business opportunity coupled with anexciting scientific challenge.

When it comes to a new AbiosorptionB enterprise,to grow from the knowledge and know-how acquiredin the process of scientific investigations providingthe basis for the technology development, there aretwo aspects to such:

1. the new family of products: biosorbents;2. the services involved in:

( assessing the type of a problem to be reme-died,

( assessing the applicability of biosorption,( developing a customized biosorption process,( designing and building the plant,( eventually even operating the effluent treat-

ment process, and evenŽ .( recovering the metal s for resalerreuse.

Fig. 6. An enterprise based on the new technology of metal biosorption can cover a wide range of activities from full Apollution controlserviceB on one side to just selling the new family of biosorbents to clients. Metal recoveryrresale constitutes the revenue-generatingside-line activity.


Fig. 7. The immediate potential market niche for biosorbent materials can be estimated as a likely fraction of their penetration into thecurrently existing sales of AspecialtyB ion exchange resins. In addition, competitive low-cost new biosorbents might open up vast new,currently untapped, environmental markets.

Obviously, it is not a small feat to develop abusiness venture along these broad lines. This isperhaps why the commercialization of biosorptiontechnology has been so relatively slow and painfulfor those few who attempted it. However, the poten-tial is undoubtedly there. The schematics of thebiosorption enterprise involvement is seen in Fig. 6.

It is perhaps worthwhile mentioning that somesources put the current and established ion exchangeresin sales in the order of maybe US$2 billionryearin North America alone. While ion exchange resinsare considered a commodity on the market, theactual sales figures are not reliably available.World-wide sales are perhaps approximately quadru-ple the above figure for North America. However,only about 15% of the total ionex resin sales are forthe specialty use such as heavy metal removal. Con-sidering that only 10% of this volume could be‘stolen’, or as the more commercial term goes

Ž .‘penetrated’ by cheaper! biosorbents, one is look-ing at the most conservatively estimated immediateand existing market for new biosorbent materials inthe order of at least US$30 million only in North

Ž .America Fig. 7 .It can easily be envisaged that cheaper biosor-

bents would open up new, particularly environmen-tal, markets so far non-accessible to ion exchangeresins because of their excessive costs which makethem prohibitive for clean-up operation applications.

As with any new technology, the essential needfor continuing and strong R&D work in the field ofbiosorption cannot be overemphasized.

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