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Introduction Biosorption is the binding and concentration of heavy metals from aqueous solutions (even very dilute ones) by certain types of inactive, dead, microbial biomass6. Pioneering research on biosorption of heavy
metals has led to the identification of a number of microbial biomass types3
that are extremely effective in concentrating metals. Some types of biomass are waste Individuals with different backgrounds, from engineering to biochemistry, can make significant contributions to the understanding of biosorption. Interdisciplinary efforts are essential to exploit this technology commercially. A chemical engineering background is particularly useful for expanding the application of this technology in large-scale process industries.
Threat from the Environment The greatest demand for metal sequestration today comes from the need
to immobilize the metals released to the environment (or mobilized) by and partially lost
through human technological activities. It has been established that dissolved metals
(particularly heavy metals) escaping into the environment pose a serious health hazard11.
They accumulate in living tissues throughout the food chain (Figure 1), which has humans
at its top, multiplying the danger. Thus, it is necessary to control emissions of heavy
metals into the environment. Am example of one method for prioritizing the recovery of ten metals is presented in Table 1. This may be simplistic, but it provides a useful direction by ranking metals into three general priority categories: (1) Environmental Risk (ER) (2) Reserve Depletion Rate (RDR) (3) Combination of ER and RDR. Environmental risk assessment could be based on a number of different factors, which could also be weighted. Table 1: Ranking of Risks Associated with Various Metals
The Need for Novel Technology Conventional techniques to remove toxic metals and radionuclides, such as ion exchange and precipitation, lack specificity and are ineffective at low metal ion concentrations. The need for effective and economically viable technologies is driven by environmental pressures such a: ·
Stricter
regulations with regard to the metal discharges are being enforced, particularly in
industrialized countries. ·
Toxicology
studies confirm the dangerous impacts of heavy metals. · Current technologies for the removal of heavy metals from industrial effluents often create secondary problems with metal-bearing sludge. Biosorption Mechanisms Various metal-binding mechanisms have been postulated to be active in biosorption, such as: · Chemisorption by ion exchange, complexation, coordination and/or chelation · Physical Adsorption · Microprecipitation · Oxidation/Reduction. Due to the complexity of the biomaterials used, it is possible that at least some of these mechanisms are acting simultaneously to varying degrees, depending on the biosorbent and the solution environment.
Ion Exchange Ion exchange is a reversible chemical reaction wherein an ion in a solution is exchanged for a similarly charged ion attached to an immobile solid particle. These solid ion-exchange particles are either naturally occurring inorganic zeolites or synthetically produced organic resins. Synthetic organic resins are the predominant type used today because their characteristics can be tailored to specific applications. Ion exchange reactions are stoichiometric and reversible, and as such they are similar to other solution-phase reactions. For example, in the reaction NiSO4 + Ca(OH)2 ® Ni(OH)2
+ CaSO4 the nickel ions of the nickel sulfate (NiSO4) are exchanged for the calcium ions of the calcium hydroxide Ca(OH)2 molecule.
The word chelation is derived from the Greek word chele, which means claw, and is defined as the firm binding of a metal ion with an organic molecule (ligand) to form a ring structure. The resulting ring structure protects the mineral from entering into unwanted chemical reactions. Examples include the carbonate (CO32–) and oxalate (C2O42–) ions:
Coordination (Complex
Formation) A coordination complex is any combination of cations with molecules or anions containing free pairs of electrons. Bonding may be electrostatic, covalent or a combination of both; the metal ion is coordinately bonded to organic molecules. Example of the formation of a coordination compound are: Cu2+ + 4H2O ® [Cu(H2O)]42+ Cu2+ + 4Cl– ® [CuCl4]2– where coordinate covalent bonds are formed by donation of a pair of electrons from H2O and Cl– (Lewis bases) to Cu2+ (Lewis acid). In general, biosorption of toxic metals and radionuclides is based on non-enzymatic processes such as adsorption. Adsorption is due to the non-specific binding of ionic species to polysaccharides and proteins on the cell surface (Figure 2) or outside the cell4,19. Bacterial cell walls and envelopes, and the walls of fungi, yeasts and algae, are efficient metal biosorbents that bind charged groups. The cell walls of gram-positive bacteria bind larger quantities of toxic metals and radionuclides than the envelopes of gram-negative bacteria. Bacterial sorption of some metals can be described by the linearized Freundlich adsorption equation: log S = log K + n log C where: S is the amount of metal absorbed in µmol/g C is the equilibrium solution concentration in µmol/L K and n are the Freundlich constants. Biomass deriving from several industrial fermentations may provide an economical source of biosorptive materials. Many species have cell walls with high concentrations of chitin, a polymer of N-acetyl-glucosamine that is an effective biosorbent. Biosorption uses biomass raw materials that are either abundant (e.g., seaweeds) or wastes from other industrial operations (e.g., fermentation wastes)9. The metal-sorbing performance of certain types of biomass can be more or less selective for heavy metals, depending on the type of biomass, the mixture in the solution, the type of biomass preparation, and the chemical-physical environment. It is important to note that the concentration of a specific metal in solution can be reduced either during the sorption uptake by manipulating the properties of the biosorbent or upon desorption during the regeneration cycle of the biosorbent.
Sources of Biomass for
Biosorption Sources of biomass include: · Seaweeds · Microorganisms (bacteria, fungi, yeast, molds) · Activated sludge · Fermentation waste · Other specially propagated biomasses. Biosorbents must be hard enough to withstand the application pressures, porous and/or “transparent” to metal ion sorbate species, and have high and fast sorption uptake even after repeated regeneration cycles17. Granulation of biomass materials into suitable cost-effective biosorbents is a crucial step for the successful application of biosorption processes. The objectives of granulation are to: · Establish the behavior of native biomass in a packed-bed reactor · Establish the effectiveness of biomass granulation and reinforcement · Determine the effect of size reduction on sorption capacity · Determine the feasibility of biomass processing. Conventional granulation technologies are rather advanced, and their adaptation will likely yield desirable biosorbent granules5. Because of the wide variety of biomass types, extensive experimentation will undoubtedly be required. The need to transport raw biomass may also present some logistical problems. Microbial biomass has a high water content and is prone to decay, so drying may be required if it cannot be processed and/or granulated directly on location in the wet state. Equilibrium Modeling Biosorption has been studied as simplified sorption systems, usually containing one heavy metal. This is an appropriate simplification for effective experimentation. Table 2 summarizes some of the simple sorption isotherm models that are most frequently applied. A particular model may not apply to a particular situation, and in some cases more than one model may explain the biosorption mechanism. There is no critical reason to use a more-complex model if a two-parameter model (such as the Langmuir and Freundlich isotherm models) can fit the data reasonably well. Table 2. Frequently used single-component adsorption models.11
As a matter of practicality, multi-metal biosorption models such as those in Table 3 must be used judiciously. Table 3. Frequently used multi-component adsorption models.11
where: Ce is the equilibrium solute concentration in the fluid K,n are the Freundlich isotherm constants ai,bi are the Langmuir isotherm parameters e is the column bed porosity; Polanyi’s adsorption potential qm is the Langmuir maximum metal uptake in mg/g Wo,Wm are the initial and final volumes, respectively, in L K(with various subscripts) are the intrinsic equilibrium constants Q is the meta uptake in mg/g b is the Polanyi scaling factor in the Polanyi models. The sorption uptake, q, can be expressed in different units depending on the purpose of the exercise: · For practical and engineering process evaluation purposes eventually concerned with process mass balances, it is customary to use weight per (dry) weight (e.g., mg of metal sorbed per gram of the (dry) sorbent material). · Ultimately, mainly because of reactor volume considerations (e.g., a packed-bed column), the uptake may also be expressed on a per volume basis (e.g., mg/L). However, the porosity may complicate the quantitative comparison of biosorption performance. · Only when working on the stoichiometry of the process and when studying the functional groups and metal-binding mechanisms might it be useful to express q on a molar or charge equivalent basis — again, per unit weight or volume of the sorbent (e.g., mmol/g or mequiv/g).
It is relatively easy to convert among these units; the only problem may arise with the sorbent weight-volume conversions. For scientific interpretations, the sorbent material dry-weight basis is thus preferred. The use of "wet biomass weight" should be discouraged, unless the wet-weight-to-dry-weight conversion is well specified. Different biomass types are likely to retain different moisture contents, intracellular as well as that trapped in the interstitial space between the cells or tissue particles (e.g., seaweed particles). Different types of biomass obviously compact in a different ways. When centrifuging biomass, the g-force and time need to be specified, and even then it is difficult to make any comparisons. All this makes the "wet biomass weight" citation very approximate at best and generally undesirable.
Biomass Types The assessment of the metal-binding capacity of some types of biomass has gained momentum since 198518. Indeed, some biomass types are very effective in accumulating heavy metals. Availability is a major factor to be taken into account to select biomass for clean-up purposes. The economics of environmental remediation dictate that the biomass must come from nature, or even be a waste material. Seaweeds, molds, yeasts, bacteria, and crab shells, among other kinds of biomass (Figure 3), have been tested for metal biosorption with very encouraging results. Biosorbents Some biosorbents can bind and collect a wide range of heavy metals with no specific priority, whereas others are specific for certain types of metals. When choosing the biomass for metal biosorption experiments, its origin is a major factor to be considered. Biomass can come from: · industrial wastes which should be obtained free of charge · organisms that can be obtained easily in large amounts in nature (e.g., bacteria, yeast, algae) ·
fast-growing organisms that are
specifically cultivated or propagated for biosorption purposes (crab shells, seaweeds). Organisms for Biosorption There is a wide variety of microorganisms (Table 4), including bacteria, fungi, yeast, and algae, that can interact with metals and radionuclides and transform them through several mechanisms. Table 4. Examples of toxic heavy metals accumulating microorganisms
Cost-effectiveness is the main attraction of metal biosorption. This cost-effectiveness can be maintained by using the microbial biomass directly where possible. In addition, biosorbents derived from microbial biomass through a simple process are expected to be the lowest-priced and most-economical for metal removal. It has been suggested that numerous chemical groups contribute to biosorption metal binding, by either whole organisms such as algae and bacteria or by molecules such as biopolymers. These include hydroxyl, carbonyl, carboxyl, sulfhydryl, thioether, sulfonate, amine, imine, amide, imidazole, phosphonate, and phosphodiester groups. The importance of any given group for biosorption of a certain metal by a certain biomass depends on such factors as the number of sites in the biosorbent material, the accessibility of the sites, the chemical state of the sites (i.e., availability), and the affinity between the site and the metal (i.e., binding strength). For covalent metal binding, even an occupied site is theoretically available; the extent to which the site can be used by a given metal depends on its binding strength and concentration compared to the metal already occupying the site. 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 to the ubiquitous brewer’s yeasts sold on the open market, 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, and sometimes a local economy can benefit from turning seaweeds into a resource. As a fallback, biomass with a high metal-sorbing capacity can be specifically grown relatively cheaply in fermenters using low-cost or even waste carbohydrate-containing growth media such as molasses or cheese whey.
Experimental Sorption
Isotherms It is relatively easy to obtain equilibrium sorption data for a single sorbate in the laboratory. A small amount of the sorbent is brought into contact with a solution containing the sorbate of interest. The conditions of the sorption system, particularly pH, must be carefully controlled at the required values over the entire period of contact until the sorption equilibrium is reached. This may take a few hours or much longer, depending on the size of the sorbent particles and the time it takes until they attain sorption equilibrium. A simple preliminary sorption kinetics test will establish the exposure time necessary for the given sorbent particles to reach the equilibrium state. The following procedure provides an example for obtaining the experimental sorption equilibrium data points for the isotherm: 1. Prepare the sorbate in solution at the highest concentration of interest. 2. Prepare dilutions covering the entire concentration range, from 0 (blank) to the maximum. 3. Adjust the conditions, e.g., pH, ionic strength, etc. 4. Determine the sorbate initial concentrations (Ci ) in all the liquid samples. 5. Distribute the samples into containers of appropriate volumes (30–150 mL of liquid) such as flasks or test tubes; prepare samples in duplicate, triplicate or as required. 6. Accurately weigh each quantity of the biosorbent solids to be used in the tests and record the weights (S, mg). It may help to be able to roughly estimate the anticipated sorption uptake so that there is an easily detectable final sorbate concentration in each sample solution at equilibrium. If too much sorbent is added, there may be virtually no sorbate left in the solution, precluding a reliable analysis. Varying the initial concentration could cause the sorbent weight to fluctuate, which has to be precisely known for each sample. Metal depletion in the solution must be avoided because it renders such samples useless. 7. Add the sorbent solids into each sample solution and provide rather gentle mixing over the contact period. 8. Make sure the conditions (especially pH) are controlled at constant values during the contact period. Use an appropriate acid or base for this; do not dilute the sorption system by adding excessive volume. 9. At the end of the contact period, separate the solids from the liquid by decantation, filtration, centrifugation, etc. 10. Analyze the liquid portion to determine the residual final sorbate concentration (Cf). 11. Calculate the sorbate uptake: q = V [L] ´ (Ci – Cf) [mg/L] / S [g]. Note that q could also be determined directly by analyzing the separated solids and thus closing the material balance on the sorbate in the system. However, this usually presents analytical difficulties (digestion-liquefaction of solids, and/or very sophisticated analytical methods may be required). 12. Plot the sorption isotherm q vs. (Cf). Comparison of Sorption
Performance The performance of sorbing materials needs to be evaluated and often compared. The simplest situation is when there is only one sorbate species in the system, in which case it is best to base the single-sorbate sorption performance on a complete single-sorbate sorption isotherm curve. To fairly compare two or more sorbents, the comparison must be done under uniform conditions. These may be restricted by the environmental factors under which sorption may have to take place (pH, temperature, ionic strength, etc.), which may not necessarily be easily or widely adjustable. In particular, it is important to compare sorption performance under the same pH conditions, since isotherms can vary with pH. The performance of the sorbent is usually gauged by its uptake (q). Sorbents can be compared based on their respective maximum uptake values (qmax), which can be calculated by fitting the Langmuir isotherm model to the actual experimental data (if it fits). This approach is feasible if qmax reaches a plateau. Some isotherms might not exhibit the asymptotic plateau represented by the Langmuir equation. In general, one is looking for a “good” sorbent with a high sorption uptake capacity (qmax). Surface area in biosorption is not particularly important.
Types of Biosorption Biosortpion can be carried out as a batch process, a continuous process, or a two-stage process with continuous metal recovery. Biomass should be defrosted and washed with deionized water. To ensure equal quality of the biomass during all experiments, different kinds of biomass should be mixed together to obtain a uniform mixture. Batch Process Batch biosorption experiments can be done in a stirred vessel (Figure 4) with a working volume of approximately 100 mL. A minimal amount of concentrated solution of Pb(NO3)2 (metal) can be added into a suspension of fungal pellets in water of various concentrations (25, 50, 100, 150 and 200 g of wet biomass per L of biomass suspension) to produce the desired initial metal concentrations of 10, 20, 50, 100 and 300 mg/L Pb2+ (metal). The decreasing metal concentration can be recorded as a function of the initial metal concentration and the biomass loading. Continuous Process Continuous process experiments can be carried out in a glass column having an inner diameter of 5–8 cm and filled with a packed bed of biomass pellets of varying heights (20, 40 and 55 cm), set with an adjustable plug (Figure 5). The effluent solution of metal ions can be fed from the top of the column with the help of a pump using varying flowrates. An inert bed of glass spheres can be placed at the bottom of the column below the active biomass bed to ensure homogenous distribution of the feed. The remaining metal concentration can be measured online in the effluent at the top of the column. The breakthrough curves can be recorded as a function of the flowrate and bed height. Measurements of metal ion concentrations in the solution can be made online with metal-detecting electrodes or ion-selective electrodes, and may be verified with an atomic absorption spectrometer. Two-Stage Process with
Continuous Metal Recovery Two-stage continuous biosorption and metal recovery can be carried out as shown in Figure 6. This process is similar to continuous biosorption, although the metal solution is adsorbed in two stages. After initial adsorption and filtration in stage one, the effluent is fed with fresh biosorbent into stage two, where further biosorption of the metal ion takes place. The effluent from the second stage is filtered to recover the metal ions and biosorbents. The effluent sample can be analyzed using an ion meter or by adsorption spectroscopy.
Desorption Regeneration of loaded biosorbent is critical to keeping costs down and to recovering the metal(s) extracted from the liquid phase. The deposited metals are washed out (desorbed) and the biosorbent is regenerated for another cycle. The desorption process should result in: · high-concentration metal effluent · undiminished metal uptake upon re-use · no physico-chemical damage to the biosorbent. The desorption and sorbent regeneration studies might require somewhat different methodologies, beginning with screening for the most effective regenerating solution. Because different metal ions have different affinities for the biosorbent, the uptake has some degree of metal selectivity. The selectivity of the elution-desorption operation may be different, 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, the better 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 be further pre-treated (e.g., pre-saturated with protons such as Ca, K, etc.) for optimum operation in the next metal uptake cycle. The specific type of pre-treatment used to optimize the column performance may vary.
Feasibility of Biosorption For successful application on a large scale, any operation needs to be economically viable. The feasibility of a biosorption process depends on such factors as: · biosorbent uptake performance · the source of the raw biomass · biomass granulation and treatment · the desorption and regeneration processes used. Often, the source of the biosorbent has a major impact on the feasibility of the operation. Biosorbents (biomass) should always be obtained from the least-expensive source, such as from the effluent of a fermenter, seaweeds from nearby bodies of water, algae, etc. The spent biosorbents can be regenerated at very low cost using water, so the material can be reused many times. Hence, considering the overall unit operations involved in biosorption, we can conclude that the process is generally economically viable. Advantages of Biosorption Biosorption is highly competitive with the presently available technologies like ion exchange, electrodialysis, reverse osmosis, etc. Some of the key features of biosorption compared to conventional processes include: · competitive performance · heavy metal selectivity · cost-effectiveness · regenerative · no sludge generation. Biosorption is particularly economical and competitive for environmental applications in detoxifying effluents from, for example: · metal plating and metal finishing operations · mining and ore processing operations · metal processing · battery and accumulator manufacturing operations · thermal power generation (coal-fired plants in particular) · nuclear power generation.
Conclusion There appear to be many modes of non-active metal uptake by microbial biomass. Any one or a combination of them can be functional in immobilizing metallic species on biosorbents. A number of anionic ligands participate: phosphoryl, carbonyl, sulfhydryl and hydroxyl groups can all be active to various degrees in binding the metal. Many scientific studies are currently underway to provide a deeper understanding of biosorption and to support its effective application. Some pollution seems inevitable, and one might wonder what should be done to minimize it. Human populations need methods and technologies to clean waters and diminish the environmental dangers related to technological progress. Biosorption can be one such solution to clean up heavy metal contamination.
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