A part of the  Invited Lecture to be presented at the International Biohydrometallurgy Symposium
El Escorial, Spain, June 20-23, 1999
                            Boya Volesky              icon
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 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. 

Environmental Pressures 

  • Stricter regulations with regard to the metal discharges are being enforced particularly for industrialized countries. 
  • Toxicology of heavy metals confirms their dangerous impacts. 
  • The currently practiced technologies for removal of heavy metals from industrial effluents appear to be inadequate and expensive. They often create secondary problems with metal-bearing sludges.
The food-chain pyramid receives metals through man’s activities.  
On top of the pyramid, man receives pre-concentrated metal toxicity.  

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 to feature 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 be more or less selective for heavy metals. That depends on: 
    - the type of biomass, 
    - 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 a performance 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 commercial ion exchange resins are rather costly, the price tag of biosorbents can be an order of magnitude cheaper (1/10 the ion exchange resin cost).
  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 efforts. 
In addition, even the same type of industrial activity can produce effluents which differ from each other a great deal. Close collaboration with each ‘client’ industrial operation is absolutely essential: a consulting-engineering type of approach. Engineering skills become quite important because it is a process operation one is aiming at and dealing with. 

"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 failures. 



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; 
- 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 ten metals is in TABLE 1 which may be simplistic but provides a useful direction by ranking into 3 general 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 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 

Growth industries and point-source effluents are of primary concern 

TABLE 1:  Ranking of metal interest priorities 
HIGH : Cd Cd Cd
Pb Pb Pb
Hg Hg Hg
- Zn Zn
MEDIUM : Cr - -
Co Co Co
Cu Cu Cu
Ni Ni Ni
Zn - -
LOW : Al - Al
- Cr Cr
Fe Fe Fe

       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. 
Activated sludge from wastewater treatment plants has not demonstrated high enough metal-sorbing capacities. 

    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.

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. 
Screening for Adsorption:  
 Batch equilibrium  sorption experiments are used for screening for suitable biomass types.  Unfortunately, there are to many errors in the literature betraying little understanding of equilibrium sorption concepts. 

Standard procedure for evaluating simple sorption systems. 
See details in the biosorption web-site. 

Example of (bio)sorption isotherms affected by the pH of solution 

Fixed-bed column is the most powerful sorption process arrangement 

    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. 
    The most appropriate method of assessing the biosorbent capacity is the derivation of a whole sorption isotherm. Anything else represents a potentially misleading shortcut which 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. 

    ‘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 system). 
    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. 

    Dynamic 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. 
    In the sorption column contactor the saturated zone is moving along the column length pushing the transitional dynamic sorption zone ahead of itself. With multimetal sorption systems featuring different affinities of ions toward the sorbent the whole system becomes even more complex as chromatographic effects and simultaneous displacement of deposited ions take place. It is obvious that simplistic observations of the experimental "break-through" curve resulting from the conventional operation of a flow-through sorption column will not suffice. They are usually narrowly specific and cannot be used elsewhere. 

    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. 

    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 selectivity on the uptake. Similarly, another selectivity may be achieved upon the elution-desorption operation which may serve as another means of eventually separating metals from one another if desirable. 
The Concentration Ratio (CR) is used to evaluate the overall concentration effectiveness of the whole sorption-desorption process: 

Obviously, the higher the CR is the better is the overall performance of the sorption process making the eventual recovery of the metal more feasible with higher eluate concentrations. 

    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. 



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 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. 
Understanding the mechanism of biosorption is important even for very practical reasons  

While other mechanisms might also contribute, ion exchange prevails

    A number of different metal-binding mechanisms has been postulated to be active in biosorption such as: 

- chemisorption: by ion exchange, complexation,  
      coordination, chelation; 
- physical adsorption, microprecipitation. 

    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 
       the molecular level; 
- manipulating the biosorption properties of 
       biomass when it is growing; 
- developing economically attractive 
       analogous sorbent materials; 
- simplifying and effectively guiding the 
       screening process; 
- ‘activating’ biomaterials low-level 
       biosorbent behavior. 

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. 

Contemporary molecular modeling software is extremely powerful can be very useful

Advanced scientific approach aids in understanding the phenomenon and in developing biosorption for applications  

Process modeling is sophisticated and should be done very pragmatically  

    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  

Different biomass types require different ‘pre-processing’ after which the sorption performance has to be always tested  

Establish the overall process feasibility.


    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 

Challenges for chemistry and biochemistry 

Process engineering will have to develop the process with its 2 pilots 

Project 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 biosorption process feasibility is assessed for well selected cases. It is necessary to realize that there are 2 types of pilot plants to eventually be run hand in hand: 
    - Biomass processing pilot plant; 
    - Biosorption pilot plant. 

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.