Solvent impregnated resin

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Solvent impregnated resins (SIRs) are commercially available (macro)porous resins impregnated with a solvent/an extractant. In this approach, a liquid extractant is contained within the pores of (adsorption) particles. Usually, the extractant is an organic liquid. Its purpose is to extract one or more dissolved components from a surrounding aqueous environment. The basic principle combines adsorption, chromatography and liquid-liquid extraction.

Resin solid or highly viscous substance of plant or synthetic origin

In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on naturally-occurring resins.

Solvent substance that dissolves a solute (a chemically different liquid, solid or gas), resulting in a solution

A solvent is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. The quantity of solute that can dissolve in a specific volume of solvent varies with temperature. Common uses for organic solvents are in dry cleaning, as paint thinners, as nail polish removers and glue solvents, in spot removers, in detergents and in perfumes (ethanol). Water is a solvent for polar molecules and the most common solvent used by living things; all the ions and proteins in a cell are dissolved in water within a cell. Solvents find various applications in chemical, pharmaceutical, oil, and gas industries, including in chemical syntheses and purification processes.

Adsorption adhesion of atoms, ions, or molecules from a substance to a surface

Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. This process differs from absorption, in which a fluid is dissolved by or permeates a liquid or solid, respectively. Adsorption is a surface phenomenon, while absorption involves the whole volume of the material. The term sorption encompasses both processes, while desorption is the reverse of it.

Contents

History

The principle of Solvent Impregnated Resins was first shown in 1971 by Abraham Warshawsky. [1] This first venture was aimed at the extraction of metals. Ever since then, SIRs have been mainly used for metal extraction, be it heavy metals or specifically radioactive metals. Much research on SIRs has been done by J.L Cortina and e.g. N. Kabay, K. Jerabek or J. Serarols. [2] However, lately investigations also go towards using SIRs for the separation of natural compounds, and even for separation of biotechnological products.

Extractive metallurgy is a branch of metallurgical engineering wherein process and methods of extraction of metals from their natural mineral deposits are studied. The field is a materials science, covering all aspects of the types of ore, washing, concentration, separation, chemical processes and extraction of pure metal and their alloying to suit various applications, sometimes for direct use as a finished product, but more often in a form that requires further working to achieve the given properties to suit the applications.

Heavy metals member of a loosely defined subset of elements that exhibit metallic properties

Heavy metals are generally defined as metals with relatively high densities, atomic weights, or atomic numbers. The criteria used, and whether metalloids are included, vary depending on the author and context. In metallurgy, for example, a heavy metal may be defined on the basis of density, whereas in physics the distinguishing criterion might be atomic number, while a chemist would likely be more concerned with chemical behaviour. More specific definitions have been published, but none of these have been widely accepted. The definitions surveyed in this article encompass up to 96 out of the 118 known chemical elements; only mercury, lead and bismuth meet all of them. Despite this lack of agreement, the term is widely used in science. A density of more than 5 g/cm3 is sometimes quoted as a commonly used criterion and is used in the body of this article.

Basic principle

Figure 1: Basic principle of extraction with SIR. Fig 1 SIR principle.tif
Figure 1: Basic principle of extraction with SIR.

Figure 1 to the right explains the basic principle, in which the organic extractant E is contained inside the pores of a porous particle. The solute S, which is initially dissolved in the aqueous phase surrounding the SIR particle, physically dissolves in the organic extractant phase during the extraction process. Furthermore, the solute S can react with the extractant to form a complex ES. This complexation of the solute with the extractant shifts the overall extraction equilibrium further towards the organic phase. This way, the extraction of the solute is enhanced. [3]

Figure 2: Comparison of emulsification during liquid-liquid extraction and with SIR particles. Fig 2 emulsification.tif
Figure 2: Comparison of emulsification during liquid-liquid extraction and with SIR particles.

While during conventional liquid-liquid extraction the solvent and the extractant have to be dispersed, in a SIR setup the dispersion is already achieved by the impregnated particles. This also prevents an additional phase separation step, which would be necessary after the emulsification occurring in liquid-liquid extraction. In order to elucidate the effect of emulsification, Figure 2 (to the left) compares the two systems of an extractant in liquid-liquid equilibrium with water, left, and SIR particles in equilibrium with water, right. The figure shows that no emulsification occurs in the SIR system, whereas the liquid-liquid system shows turbidity implying emulsification. Also, the impregnation step decreases the solvent loss into the aqueous phase compared to liquid-liquid extraction. [4] This decrease of extractant loss is contributed to physical sorption of the extractant on the particle surface, which means that the extractant inside the pores does not entirely behave as a bulk liquid. Depending on the pore size of the used particles, capillary forces may also play a role in retaining the extractant. Otherwise, van-der-Waals forces, pi-pi-interactions or hydrophobic interactions might stabilize the extractant inside the particle pores. However, the possible decrease of extractant loss depends largely on the pore size and the water solubility of the extractant. Nonetheless, SIRs have a significant advantage over e.g. custom made ion-exchange resins with chemically bonded ligands. SIRs can be reused for different separation tasks by just rinsing one complexing agent out and re-impregnating them with another more suitable extractant. This way, potentially expensive design and production steps of e.g. affinity resins can be avoided. Finally, by filling the whole volume of the particle pores with an extractant (complexing agent), a higher capacity for solutes can be achieved than with ordinary adsorption or ion exchange resins, where only the surface area is available.

A dispersion is a system in which discrete particles of one material are dispersed in a continuous phase of another material. The two phases may be in the same or different states of matter. They are different from solutions, where dissolved molecules do not form a separate phase from the solute.

Capillary action ability of a liquid to flow in narrow spaces

Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity. The effect can be seen in the drawing up of liquids between the hairs of a paint-brush, in a thin tube, in porous materials such as paper and plaster, in some non-porous materials such as sand and liquefied carbon fiber, or in a cell. It occurs because of intermolecular forces between the liquid and surrounding solid surfaces. If the diameter of the tube is sufficiently small, then the combination of surface tension and adhesive forces between the liquid and container wall act to propel the liquid.

Van der Waals force residual attractive or repulsive forces between molecules or atomic groups that do not arise from covalent bonds nor ionic bonds

In molecular physics, the van der Waals force, named after Dutch scientist Johannes Diderik van der Waals, is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and therefore more susceptible to disturbance. The Van der Waals force quickly vanishes at longer distances between interacting molecules.

However, there are possible drawbacks of SIR technology, such as leaching of the extractant or clogging of a fixed bed by attrition of the particles. These might be remedied by choosing the proper particle-extractant-system. This implies selecting a suitable extractant with low water solubility, which is sufficiently retained inside the pores, and selecting mechanically stable particles as a solid support for the extractant. Additionally, SIRs can be stabilized by coating them, as shown by D. Muraviev et al. [5] As coating material, A. W. Trochimczuk et al. used polyvinyl alcohol. [6]

In order to remove or recover the extracted solute, SIR particles can be regenerated using low pressure steam stripping, [7] which is particularly effective for the recovery of volatile hydrocarbons. However, if the vapor pressure of the extracted solute is too low, or if the complexation between solute and extractant is too strong, other techniques need to be applied, e.g. pH swing.

Vapor pressure

Vapor pressure or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to the tendency of particles to escape from the liquid. A substance with a high vapor pressure at normal temperatures is often referred to as volatile. The pressure exhibited by vapor present above a liquid surface is known as vapor pressure. As the temperature of a liquid increases, the kinetic energy of its molecules also increases. As the kinetic energy of the molecules increases, the number of molecules transitioning into a vapor also increases, thereby increasing the vapor pressure.

Preparation techniques

The main impregnation techniques are wet impregnation and dry impregnation. During wet impregnation, the porous particles are dissolved in the extractant and allowed to soak with the respective fluid. [8] In this approach, the particles are either contacted with a precalculated amount of extractant, which completely soaks into the porous matrix, or the particles are contacted with an excess of extractant. After soaking, the remaining extractant, which is not inside the pores, is evaporated.

Figure 3: SIR particles prepared with the wet impregnation method, dispersed in water. Cut-out section shows enlarged segment of SIR particle surface. Fig 3 SIR particle in solution.tif
Figure 3: SIR particles prepared with the wet impregnation method, dispersed in water. Cut-out section shows enlarged segment of SIR particle surface.

If the wet method is used, the extractant is dissolved in an additional solvent prior to impregnation. The porous particles are then dispersed in the extractant-solvent solution. [8] After soaking the particles, the excess solvent can either be filtered off or evaporated. In the first case, an extractant-solvent mixture would be retained within the pores. This would be of interest for extractants which would be solid at design conditions when pure. In the second case, only the extractant would remain inside the pores. Figure 3 shows porous particles dispersed in an aqueous solution after wet impregnation. The cut-out in Figure 3 shows an enlarge segment of the surface of such an impregnated particle. An additional, albeit not so frequently used technique is the modifier addition method. This technique relies on the use of an extractant/solvent/modifier system. The additional modifier is supposed to enhance the penetration of the extractant into the particle pores. [8] The solvent is subsequently evaporated, leaving extractant and modifier in the particle pores.

Furthermore, the dynamic column method can be used. The particles are contacted with a solvent until they are completely soaked. This can be done prior or after packing into the column. The packed bed is then rinsed with the liquid extractant until inlet and outlet concentrations are the same. [8] This approach is particularly interesting when particles are already packed in a column and shall be reused for a SIR application.

Applications

SIRs in Metal Extraction

Mostly, SIRs have been investigated and used for the recovery of heavy metals. [9] [10] [11] Applications include the removal of cadmium, vanadium, copper, chrome, iridium, etc.

Extraction of Organics

Only recently also other extraction applications have been investigated, e.g. the large scale recovery of apolar organics on offshore oil platforms using the so-called Macro-Porous Polymer Extraction (MPPE) Technology. [12] In such an application, where the SIR particles are contained in a packed bed, flow rates from 0.5 m3 h−1 upward without maximum flow restrictions can apparently be treated cost competitive to air stripping/activated carbon, steam stripping and bio treatment systems, according to the technology developer. Additional investigations, mostly done in an academic environment, include polar organics like amino-alcohols, [13] organic acids, [14] [15] amino acids, [16] flavonoids, [17] and aldehydes on a bench-scale or pilot-scale. Also, the application of SIRs for the separation of more polar solutes, such as for instance ethers and phenols, has been investigated in the group of A.B. de Haan. [18]

Applications in Biotechnology

Applications in biotechnology were developed only most recently. This is due to the sensitivity of bioproducts such as proteins towards organic extractants.

One approach by C. van den Berg et al. focuses on the use of impregnated particles for in situ recovery of phenol from Pseudomonas putida fermentations using ionic liquids. [19] Further development led to the use of high capacity polysulfone capsules. [20] These capsules are basically hollow particles surrounded by a membrane. The interior is completely filled with extractant and thus increases the impregnation capacity as compared to classical SIRs.

A completely new approach of using SIRs for the separation or purification of biotechnological products such as proteins is based on the concept of impregnating porous particles with aqueous polymer solutions developed by B. Burghoff. These so-called Tunable Aqueous Polymer-Phase Impregnated Resins (TAPPIR) [21] enhance aqueous two-phase extraction (ATPE) by applying the SIR technology. During classical aqueous two-phase extraction, biotechnological components such as proteins are extracted from aqueous solutions by using a second aqueous phase. This second aqueous phase contains e.g. polyethylene glycol (PEG). On the one hand, a low density difference and low interfacial tension between the two aqueous phases facilitate comparatively fast mass transfer between the phases. On the other hand, PEG appears to stabilize the protein molecules, which results in a comparatively low protein denaturation during the extraction. However, a significant drawback of ATPE is the persistent emulsification, which makes phase separation a challenge. The idea behind TAPPIR is to use the advantages posed by SIRs, namely low extractant loss due to immobilization in the pores and less emulsification than in liquid-liquid extraction. This way, the drawbacks of ATPE could be remedied. The setup would consist of a packed column or a fluidized bed rather than liquid-liquid extraction equipment with additional phase separation steps. Nonetheless, as yet only first feasibility studies are on the way to prove the concept. Adrawback of this method is the non-conitnous working mode. The packed column is run similar as a chromatographic column.

Related Research Articles

Chromatography is a laboratory technique for the separation of a mixture. The mixture is dissolved in a fluid called the mobile phase, which carries it through a structure holding another material called the stationary phase. The various constituents of the mixture travel at different speeds, causing them to separate. The separation is based on differential partitioning between the mobile and stationary phases. Subtle differences in a compound's partition coefficient result in differential retention on the stationary phase and thus affect the separation.

Size-exclusion chromatography size-exclusion chromatography

Size-exclusion chromatography (SEC), also known as molecular sieve chromatography, is a chromatographic method in which molecules in solution are separated by their size, and in some cases molecular weight. It is usually applied to large molecules or macromolecular complexes such as proteins and industrial polymers. Typically, when an aqueous solution is used to transport the sample through the column, the technique is known as gel-filtration chromatography, versus the name gel permeation chromatography, which is used when an organic solvent is used as a mobile phase. The chromatography column is packed with fine, porous beads which are composed of dextran polymers (Sephadex), agarose (Sepharose), or polyacrylamide. The pore sizes of these beads are used to estimate the dimensions of macromolecules. SEC is a widely used polymer characterization method because of its ability to provide good molar mass distribution (Mw) results for polymers.

High-performance liquid chromatography method

High-performance liquid chromatography is a technique in analytical chemistry used to separate, identify, and quantify each component in a mixture. It relies on pumps to pass a pressurized liquid solvent containing the sample mixture through a column filled with a solid adsorbent material. Each component in the sample interacts slightly differently with the adsorbent material, causing different flow rates for the different components and leading to the separation of the components as they flow out of the column.

Pervaporation is a processing method for the separation of mixtures of liquids by partial vaporization through a non-porous or porous membrane.

Gel permeation chromatography (GPC) is a type of size exclusion chromatography (SEC), that separates analytes on the basis of size. The technique is often used for the analysis of polymers. As a technique, SEC was first developed in 1955 by Lathe and Ruthven. The term gel permeation chromatography can be traced back to J.C. Moore of the Dow Chemical Company who investigated the technique in 1964 and the proprietary column technology was licensed to Waters Corporation, who subsequently commercialized this technology in 1964. GPC systems and consumables are now also available from a number of manufacturers. It is often necessary to separate polymers, both to analyze them as well as to purify the desired product.

Activated carbon form of carbon processed to have small, low-volume pores that increase the surface area

Activated carbon, also called activated charcoal, is a form of carbon processed to have small, low-volume pores that increase the surface area available for adsorption or chemical reactions. Activated is sometimes substituted with active.

An artificial membrane, or synthetic membrane, is a synthetically created membrane which is usually intended for separation purposes in laboratory or in industry. Synthetic membranes have been successfully used for small and large-scale industrial processes since the middle of twentieth century. A wide variety of synthetic membranes is known. They can be produced from organic materials such as polymers and liquids, as well as inorganic materials. The most of commercially utilized synthetic membranes in separation industry are made of polymeric structures. They can be classified based on their surface chemistry, bulk structure, morphology, and production method. The chemical and physical properties of synthetic membranes and separated particles as well as a choice of driving force define a particular membrane separation process. The most commonly used driving forces of a membrane process in industry are pressure and concentration gradients. The respective membrane process is therefore known as filtration. Synthetic membranes utilized in a separation process can be of different geometry and of respective flow configuration. They can also be categorized based on their application and separation regime. The best known synthetic membrane separation processes include water purification, reverse osmosis, dehydrogenation of natural gas, removal of cell particles by microfiltration and ultrafiltration, removal of microorganisms from dairy products, and Dialysis.

Hydrometallurgy is a method for obtaining metals from their ores. It is a technique within the field of extractive metallurgy involving the use of aqueous chemistry for the recovery of metals from ores, concentrates, and recycled or residual materials. Metal chemical processing techniques that complement hydrometallurgy are pyrometallurgy, vapour metallurgy and molten salt electrometallurgy. Hydrometallurgy is typically divided into three general areas:

Separatory funnel

A separatory funnel, also known as a separation funnel, separating funnel, or colloquially sep funnel, is a piece of laboratory glassware used in liquid-liquid extractions to separate (partition) the components of a mixture into two immiscible solvent phases of different densities. Typically, one of the phases will be aqueous, and the other a lipophilic organic solvent such as ether, MTBE, dichloromethane, chloroform, or ethyl acetate. All of these solvents form a clear delineation between the two liquids. The more dense liquid, typically the aqueous phase unless the organic phase is halogenated, sinks and can be drained out through a valve away from the less dense liquid, which remains in the separatory funnel.

Ion chromatography

Ion chromatography is a chromatography process that separates ions and polar molecules based on their affinity to the ion exchanger. It works on almost any kind of charged molecule—including large proteins, small nucleotides, and amino acids. However, ion chromatography must be done in conditions that are one unit away from the isoelectric point of a protein.

Liquid–liquid extraction (LLE), also known as solvent extraction and partitioning, is a method to separate compounds or metal complexes, based on their relative solubilities in two different immiscible liquids, usually water (polar) and an organic solvent (non-polar). There is a net transfer of one or more species from one liquid into another liquid phase, generally from aqueous to organic. The transfer is driven by chemical potential, i.e. once the transfer is complete, the overall system of protons and electrons that make up the solutes and the solvents are in a more stable configuration. The solvent that is enriched in solute(s) is called extract. The feed solution that is depleted in solute(s) is called the raffinate. LLE is a basic technique in chemical laboratories, where it is performed using a variety of apparatus, from separatory funnels to countercurrent distribution equipment called as mixer settlers. This type of process is commonly performed after a chemical reaction as part of the work-up, often including an acidic work-up.

Aqueous biphasic systems (ABS) or aqueous two-phase systems (ATPS) are clean alternatives for traditional organic-water solvent extraction systems.

Synthetic resins are industrially produced resins, typically viscous substances that convert into rigid polymers by the process of curing. In order to undergo curing, resins typically contain reactive end groups, such as acrylates or epoxides. Some synthetic resins have properties similar to natural plant resins, but many do not.

Reversed-phase chromatography includes any chromatographic method that uses a hydrophobic stationary phase. RPC refers to liquid chromatography.

Mixer-settler

Mixer settlers are a class of mineral process equipment used in the solvent extraction process. A mixer settler consists of a first stage that mixes the phases together followed by a quiescent settling stage that allows the phases to separate by gravity.

In chemistry, phase-boundary catalysis (PBC) is a type of heterogeneous catalytic system which facilitates the chemical reaction of a particular chemical component in an immiscible phase to react on a catalytic active site located at a phase boundary. The chemical component is soluble in one phase but insoluble in the other. The catalyst for PBC has been designed in which the external part of the zeolite is hydrophobic, internally it is usually hydrophilic, notwithstanding to polar nature of some reactants. In this sense, the medium environment in this system is close to that of an enzyme. The major difference between this system and enzyme is lattice flexibility. The lattice of zeolite is rigid, whereas the enzyme is flexible.

Chloroauric acid chemical compound

Chloroauric acid is an inorganic compound with the chemical formula HAuCl
4. Both the trihydrate and tetrahydrate are known. It is an orange-yellow solid, a common precursor to other gold compounds and an intermediate in the purification of gold metal. Both the trihydrate and tetrahydrate are available commercially.

Leaching is the process of extracting substances from a solid by dissolving them in a liquid, either naturally or through an industrial process. In the chemical processing industry, leaching has a variety of commercial applications, including separation of metal from ore using acid, and sugar from sugar beets using hot water.

Thin layer extraction is a time-periodic reactive liquid extraction process that provides excellent mass transfer while maintaining phase separation. It is performed via a periodic batch production process that controls the time of each chemical reaction..

Extraction (chemistry) separation process consisting in the separation of a substance from a matrix; types: liquid-liquid extraction, solid-phase extraction, solid-phase microextraction, Soxhlet extraction, fizzy extraction

Extraction in chemistry is a separation process consisting in the separation of a substance from a matrix. It includes Liquid-liquid extraction, and Solid phase extraction. The distribution of a solute between two phases is an equilibrium condition described by partition theory. This is based on exactly how the analyte move from the water into an organic layer.

References

  1. ↑ Warshawsky, A. (1971). South African Patent Application 71/5637.
  2. ↑ Kabay, N.; Cortina, J.L.; Trochimczuk, A.; Streat, M. (2010). “Solvent-impregnated resins (SIRs) – Methods of preparation and their applications.” React. Funct. Polym. 70: 484–496.
  3. ↑ Babić, K.; van der Ham, A. G. J.; de Haan, A. B. (2008). “Sorption kinetics for the removal of aldehydes from aqueous streams with extractant impregnated resins.” Adsorption 14: 357-366.
  4. ↑ Babic, K.; van der Ham, L.; de Haan, A. (2006). “Recovery of benzaldehyde from aqueous streams using extractant impregnated resins.” React. Funct. Polym. 66 (12): 1494-1505.
  5. ↑ Muraviev, D.; Ghantous, L.; Valiente, M. (1998). “Stabilization of solvent-impregnated resin capacities by different techniques.” Reactive & Functional Polymers, 38: 259-268.
  6. ↑ Trochimczuk, A. W.; Kabay, N.; Arda, M.; Streat, M. (2004). Stabilization of solvent impregnated resins (SIRs) by coating with water soluble polymers and chemical crosslinking, React. Funct. Polym., 59 (1) 1-7.
  7. ↑ MPPSystems, Macro Porous Polymer Extraction System - Water Purification, Akzo Nobel, Arnhem, p. 1-7.
  8. 1 2 3 4 van Hecke, K.; Goethals, P. (2006). Open Report of the Belgian Nuclear Research Centre: Research on Advanced Aqueous Reprocessing of Spent Nuclear Fuel: Literature Study, ISSN 1379-2407
  9. ↑ Warshawsky, A.; Cortina, J. L.; Aguilar, M.; Jerabek, K. (1999). “New Developments in Solvent Impregnated Resins. An Overview.” International Solvent Extraction Conference 1999, Barcelona, Spain
  10. ↑ Serarols, J.; Poch, J.; Villaescusa, I. (2001). “Expansion of adsorption isotherms into equilibrium surface Case 1: solvent impregnated resins (SIR).” React. Funct. Polym. 48 37-51.
  11. ↑ Wang, Y.; Wang, C.; Warshawsky, A.; Berkowitz, B. (2003). “8-Hydroxyquinoline-5-sulfonic acid (HQS) Impregnated on Lewatit MP 600 for Cadmium Complexation: Implication of Solvent Impregnated Resins for Water Remediation.” Separ. Sci. Technol. 38 (1): 149-163.
  12. ↑ Veolia Water Solutions and Technologies, MPPE Systems, , Date of last access: 18 February 2012
  13. ↑ Babic, K.; Driessen, G. H. M.; van der Ham, A. G. J.; de Haan, A. B. (2007). “Chiral Separation of Amino-Alcohols using Extractant Impregnated Resins.” J. Chromatogr. A 1142: 84-92.
  14. ↑ Juang, R.-S.; Chang, H.-L. (1995). “Distribution Equilibrium of Citric Acid between Aqueous Solutions and Tri-n-octylamine-Impregnated Macroporous Resins.” Ind. Eng. Chem. Res. 34: 1294-1301.
  15. ↑ Traving, M.; Bart, H.-J. (2002). “Recovery of Organic Acids Using Ion-Exchanger-Impregnated Resins.” Chem. Eng. Technol. 25 (10): 997-1003.
  16. ↑ Kostova, A.; Bart, H.-J. (2004). “Reaktivsorption von L-Phenylalanin durch kationentauscherimpraegnierte Polymere (Gleichgewichte).” Chem-Ing-Tech 76 (11): 1743-1748.
  17. ↑ Kitazaki, H.; Ishimaru, M.; Inoue, K.; Yoshida, K.; Nakamura, S. (1996). “Separation and Recovery of Flavonoids by means of Solvent Extraction and Adsorption on Solvent-Impregnated Resin.” International Solvent Extraction Conference 1996, Australia.
  18. ↑ Burghoff, B. (2009). “Solvent Impregnated Resins (SIRs) for the Recovery of Low Concentration Ethers and Phenols from Water.” Dissertation, Technische Universiteit Eindhoven, 153 pages, ISBN   978-90-386-1552-3.
  19. ↑ van den Berg, C.; Wierckx, N.; Vente, J; Bussman, P.; de Bont, J.; van der Wielen, L. (2008). “Solvent-impregnated resins as an in situ product recovery tool for phenol recovery from Pseudomonas putida S12TPL fermentations.” Biotechnol. Bioeng. 2008;100: 466–472.
  20. ↑ van den Berg, C; Roelands, C.P.M.; Bussman, P.; Goetheer, E.L.V.; Verdoes, D.; van der Wielen, L. (2009). “Preparation and analysis of high capacity polysulfone capsules.“ React. Funct. Polym. 69: 766–770.
  21. ↑ Burghoff, B; van Winssen, F.A.; Schembecker, G (2011) “Verfahren zur Trennung/Reinigung von Biomolekülen”, German patent application 10 2011 001 743.7.
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