GB2399344A - Surface-coated solvent impregnated resins - Google Patents

Abstract

A solvent impregnated resin (SIR) is formed from a polymeric adsorbent, typically poly(styrene-co-divinylbenzene) within which a solvent is impregnated. Said solvent is typically a liquid-liquid extractant such as primary, secondary or tertiary amines or phosphorus containing compounds such as tri-n-butyl phosphate. The SIR is then stabilised by forming a surface protective layer or either (i) polyvinyl alcohol with molecular weight 10,000-300,000 and 60-99% degree of hydrolysis, or (ii) linear and/or branched polyamides with molecular weight of 10,000-300,000. The layer is formed by immersing the SIR in the polyvinyl alcohol or polyamide and then immersing in an aqueous solution of cross-linking agent. The cross-linking agent is selected from divinyl sulfone, glutaraldehyde or 1,4-butanediol diglycidyl ether.

Description

SOLVENT IMPREGNATED RESINS WITH IMPROVED OPERATIONAL

STABILITY

THE DESCRIPTION

This invention relates to a process for the preparation of solvent impregnated resins (SIRs) that are stabilized with a protective polymeric barrier on the surface and the ensuing novel absorbents that are derived as a result of this process. The derived absorbents are used as heterogeneous extractants for the removal of trace contaminants from aqueous solution, in particular complex metal ions.

The removal of metal ions from aqueous solution using SIRs is known. Warshawsky, [South African Patent Application 71/5637 (1971)] and independently Grinstead, [Report by the Dow Chemical Co., on contract no 14-12-808 to the Water Quality Office of U.S. Environmental Protection Administration, Jan 1971] described the synthesis and applications of SIRs for this purpose in the early 1970's. Interest in the commercial development of SIRs and their application for the removal of various metal ions from aqueous solutions was stimulated by the fact that these composite materials combine the advantages of both liquid/liquid extraction and ion- exchange into a conventional polymeric adsorbent material. The main disadvantage of SIRs is the loss of the impregnated extractant due to its solubility in the aqueous phase. Leakage of the extractant from the polymeric support leads to a steady loss of adsorptive capacity towards targeted ions in aqueous solution thereby rendering the SIRs ineffective after several cycles of application. Moreover, this leakage is not acceptable from an environmental point of view as the leachate is likely to contain toxic and odorous compounds (amines, thiophosphinic acids and esters, etc.) that will contaminate effluents.

This problem is thought to be the main reason why SIR technology has not evolved into large-scale application.

The operational stability of SIRs can be improved. Muraviev has discussed major factors influencing the stabilization of SIRs, [D. Muraviev, Surface impregnated ion-exchangers: preparation, properties and applications. Solvent Extraction and Ion Exchange, 16, (1998), 381-457]. Two methods are recommended, i.e. to carry out several ion-exchange cycles or alternatively keeping freshly prepared SIRs in boiling water for several hours.

Both of these techniques attempt to remove the unbound portion of extractant that loosely adheres to the structure of the polymeric support.

Alexandratos and Ripperger have also indicated that SIRs are stabilized by post- impregnation formation of a protective barrier [S.D. Alexandratos and K.P. Ripperger, Synthesis and characterization of high-stability solvent impregnated resins. Industrial and Engineering Chemistry Research, 37, (1998), 4756-4760]. They polymerized a mixture of glycidyl methacrylate and N,N-methylenebisacrylamide monomers onto the surface of an SIR containing di(2-ethylhexyl) phosphoric acid as extractant. The double vinylidene bonds, necessary for the chemical attachment of the protective layer, are generated on the polymer surface in a separate reaction of phosphonate ester with phenyllithium.

In another paper, Muraviev et al took a different approach that they referred to as post- impregnation encapsulation [D. Muraviev, L. Ganthous and M. Valiente, Stabilization of solvent-impregnated resin capacities by different techniques. Reactive and Functional Polymers, 38, (1998), 259-268]. This process included precipitation of a linear poly(sulphone) on the surface of SIRs. This method leads to physical sorption of a filmforming polymer i.e. poly(sulphone) on the surface of the bead.

Both these methods suffer from serious disadvantages. The former is a multi-step process and is time consuming. In addition, it requires the use of the phenyllithium (an expensive reagent) and also a low temperature lithiation reaction. The latter method involving post- impregnation encapsulation with poly(sulphone) is much simpler but requires the soaking of the freshly prepared SIR in a dimethylformamide (DMF) solution. DMF is a good solvent for the majority of organic extractants, thus causing an immediate undesirable loss of the extractant phase to the DMF solution during the encapsulation procedure.

The principal aim of the present invention is a process for the preparation of stabilized SIRs. This process is straightforward and capable of being readily commercialized for the production of SIRs in large amount. This process is sufficiently general that it can be applied to a family of absorbents containing a variety of immobilised extractants and it produces SIRs with increased operational stability.

The principal objective of this invention is, therefore, achieved by the formation of a protective polymeric layer on the surface of conventional SIRs and subsequent chemical crosslinking. The process of synthesizing the derived SIR is characterized by the following steps: (1) providing a highly crosslinked polymeric adsorbent impregnated with liquid extractant, where the polymeric adsorbent is typically poly(styrene-codivinylbenzene), (2) formation of the polymeric layer on the surface of a highly crosslinked polymeric adsorbent impregnated with liquid extractant by immersing such material in an aqueous solution of water soluble polymer preferably in the presence of a polyelectrolyte. The water soluble polymer is chosen from the poly(vinyl alcohols) having a degree of hydrolysis in the range of 60-99% and molecular weight in the range of IO, OOO - 300,000 or linear and/or branched polyamines having molecular weight in the range l O. 000 - 300, 000, (3) separating of a highly crosslinked polymeric adsorbent impregnated with liquid extractant with the formed polymeric layer on its surface from polymer solution and immersing it in an appropriate crosslinking agent aqueous solution. The crosslinking agent is chosen from a group of difunctional reagents such as divinyl sulphone, glutaraldehyde or 1,4- butanediol diglycidyl ether.

The following examples are given as illustrations and do not limit the scope of the present invention.

Example 1.

The poly(styrene-co-divinylbenzene) resin used is a porous copolymer resin marketed by Rohm and Haas (USA): Amberlite XAD-4. The characteristics of this highly crosslinked macroporous resin according to manufacturers data are: specific surface area 750 m2/g, average pore diameter 5 nm, pore volume 51 %. This material is impregnated with a liquid extractant (Aliquat 336) in the following way: 3 g of dry resin (XAD-4) is immersed in 50 mL of IM solution of Aliquat 336 in hexane and shaken at ambient temperature for 17 hr. The polymer beads are subsequently separated by filtration using a Buchner funnel and washed with distilled water. Impregnated resin beads are then air- dried for several hours and subsequently vacuum-dried at room temperature. The derived material typically contains about 0.6 - 0.8 mmol of Aliquat 336 per g of dry weight.

Table 1. Average results of N and S analysis of SIRs SIR A A1 A2 R4 ml of vinyl 2 1 0.5 sulphone (crosslinking agent) mmol N / 0.466 0.452 0.532 0.630 g-dry SIR N % 0. 653 0.632 0.745 0.882 mmol S / 2.12 1.02 0.31 g-dry SIR The formation of a protective layer on the surface of impregnated XAD-4 is achieved by immersion of these beads in a 50 mL solution of 3% wt./v 3 l - 50 kD polyvinyl alcohol (PVA) 98-99% hydrolysed and shaken for 17 hr. Thereafter, lO mL of 1 M potassium chloride solution is added and shaking is continued for 24 hr. The solution is removed using a water aspirator and the beads are re- suspended in 20 mL of 1 M sodium carbonate to which is added 2 mL of vinyl sulphone after waiting l h. The contents of the flask are s shaken for a further 24 hr and then the beads are separated using a Buchner funnel and washed with a large excess of distilled water. The derived stabilized solvent impregnated resin typically contains 0.4 - 0.6 mmol of extractant/g of dry weight (designated as sample A). SIRs were also prepared using less crosslinking agent, i.e. l and 0.5 mL of vinylsulphone (designated as samples Al and A2 respectively). The properties of these materials are given in Table 1. Note that uncoated sample R4 is given for comparison.

Example 2.

3 g of dry Amberlite XAD-4 resin is immersed in 50 mL of I M solution of Aliquat 336 in hexane and shaken at ambient temperature for 17 hr. The polymer beads are subsequently separated by filtration using a Buchner funnel and washed with distilled water. Impregnated beads are then air-dried for several hours and subsequently vacuum- dried at room temperature. The derived material typically contains 0.6 - 0.8 mmol of Aliquat 336 per g of dry weight.

The formation of a protective layer on the surface of impregnated XAD-4 is achieved by immersion of these beads in a 50 mL solution of 3% wt./v 31 50 kD PVA 98-99% hydrolysed and shaken for 17 hr. Thereafter, 10 mL of 1 M potassium chloride solution is added and shaking is continued for 24 hr. The solution is removed using a water aspirator and the beads are re-suspended in 20 mL of 1 M sodium carbonate to which is added 2 mL of 1, 4-butanediol diglycidyl ether after waiting for 1 hr. The contents of the flask are shaken for a further 24 hr and then the beads are separated on a Buchner funnel and washed with a large excess of distilled water. The derived stabilized solvent impregnated resin typically contains 0.4 - 0.6 mmol of extractant/g of dry weight.

Example 3.

3 g of dry Amberlite XAD-4 resin is immersed in 50 mL of 1 M solution of Aliquat 336 in hexane and shaken at ambient temperature for 17 hr. The polymer beads are subsequently separated by filtration using a Buchner funnel and washed with distilled water. Impregnated beads are then airdried for several hours and subsequently vacuum dried at room temperature. The derived material typically contains 0.6 - 0.8 mmol of Aliquat 336 per g of dry weight.

The formation of a protective layer on the surface of impregnated XAD-4 is achieved by immersion in a 50 mL solution of 3% wt./v 124 - 186 kD PVA 98-99% hydrolysed and shaken for 17 hr. Thereafter, 10 mL of l M potassium chloride solution is added and shaking is continued for 24 hr. The solution is removed using a water aspirator and the beads are resuspended in 20 mL of 1 M sodium carbonate to which is added 2 mL of vinyl sulphone after waiting for 1 hr. The content of the flask is shaken for a further 24 hr and then the beads are separated on a Buchner funnel and washed with a large excess of distilled water. The derived material typically contains 0.3 - 0.5 mmol of Aliquat 336 per g of dry weight.

Example 4.

Table I summarizes the nitrogen and sulfur contents of coated highly crosslinked (Sample A), low crosslinked (Sample Al), lowest crosslinked (Sample A2) and uncoated (Sample R4) SIRs prepared in this study. The percentages of solid content for swollen Slits are given in Table 2. The water content of SIRs decreased with an increase in the degree of crosslinking of coated resins.

Table 2. Determination of percent solid in wet SIRs SIR Wet weight Dry weight % Solid % H2O (g/mL) (g/mL) A 0.6622 0.5191 78.38 21.62 A1 0.6086 0.4688 77.03 22.97 A2 0.6299 0.4562 72.43 27.58 R4 0.6428 0.4529 70.46 29.54

Example 5.

The resultant SIRs were tested for chromate removal at pH 4. Firstly, the effect of resin concentration on batch-mode removal of chromate from 4x104 M K2CrO4 solution at pH 4.0 was studied using wet weighed SIRs in the concentration range 0.01-0. 5 g dry resin/50 mL K2CrO4 solution by continuous shaking at 25 C for 24 hr. The uptake of chromium increased with an increase in the amount of wet resin. The optimum amounts of SIRs were 1.420, 3.828, 3.894, and 4. 142 g-wet SIRs designated as R4, A, Al, and A2 in Table 1, respectively. According to these results, coated SIRs were able to remove chromate quantitatively and performed as well as uncoated samples.

Example 6.

Optimum amounts of wet-weighed SIRs were immersed into a vessel containing a I L of 4x104 M K2CrO4 solution (pH 4.0) at 25 C. The mixture was stirred mechanically at 220 RPM. 5 mL of the supernatant was withdrawn from the solution at prescribed time intervals to monitor the decrease in chromium concentration. The extraction kinetics is significantly influenced by the extent of coating and the degree of crosslinking. The equilibrium half times for SIRs designated as A, Al, A2 and R4 were 240, 90, 15, and 15 minutes, respectively (Fig 1). Pre-conditioning of samples A, Al, A2 with I M NaOH-I M NaCI solution prior to chromium sorption greatly enhanced the kinetic performance of coated solvent impregnated resins. The equilibrium half-times were decreased to 60, 30, and 5 minutes for samples A, Al, and A2, respectively (Fig 2). The kinetic performance of sample R4 was virtually uninfluenced by pre-conditioning. Some decrease in the capacity of sample R4 was observed due to the small amount of chromate that remained in solution with an increase in time. This could be caused by a small loss of extractant in sample R4 as a function of time.

Example 7.

3 mL of wet-settled SIR was contacted with 100 mL of I M NaOH -I M NaCI mixture by continuous shaking at 25 C for 24 hours. The treated resin was washed with deionized water and then was packed into a plastic minicolumn with an internal diameter of 1.2 cm, fitted with a 20 1lm polyethylene frit as bed support. A solution of K2CrO4 (4x104 M) at pH 4. 0 was passed through the column (downflow) using a peristaltic pump at a flow rate of 10 bed volumes per hour. Elution of chromate from the SIR was performed with 1 M NaOH - 1 M NaCl mixture, at a flow rate of 5 bed volumes per hour in downflow. Effluent (5 bed volumes) and eluate (2 bed volumes) samples were collected continuously by fraction collector. The column-mode recycle performance of uncoated and coated SIRs was investigated using four sorption-washing-elusion-washing cycles.

The degree of crosslinking influenced the breakthrough profiles (Fig 3). The breakthrough capacities of samples R4, A, A1, and A2 were 8.93, 4.93, 6.03, and 8.21 mg/mL-wet resin, respectively. The column capacities of the Samples A and A1 remained almost the same throughout four successive sorption-washing-elusion-washing cycles whilst a small decrease was observed in the case of samples A2 and R4. The chemical stabilities of coated and uncoated SIRs were evaluated by nitrogen analysis.

According to nitrogen data given in Tables 3 and 4 and Figs 4-6, the nitrogen content of sample R4 decreased very significantly after each cycle. However, the nitrogen content of samples A and A1 decreased only very slightly in the initial stages and then it remained virtually constant within the range of standard deviation of the nitrogen analyses.

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Claims (6)

1. A stabilized SIR characterized by the presence of a surface protective layer composed of water soluble polymer crosslinked chemically using a difunctional reactant, where the water soluble polymer is from the group consisting of poly(vinyl alcohols) having degree of hydrolysis in the range of 60-99% and molecular weight in the range of 10,000 - 300,000 or linear and/or branched polyamines having molecular weight in the range of 10, 000 - 300, 000, and crosslinking reagents that are chosen from a difunctional group such as divinyl sulphone, glutaraldehyde or 1,4-butanediol diglycidyl ether.

2. A resin according to claim 1, wherein the water soluble polymer is poly(vinyl alcohol) having molecular weight of 31- 50 kD and degree of hydrolysis 98-99% and the crosslinking agent is vinyl sulphone.

3. A resin according to claim 1, wherein the water soluble polymer is poly(vinyl alcohol) having molecular weight of 31- 50 kD and degree of hydrolysis 98-99% and the crosslinking agent is 1,4-butanediol diglycidyl ether.

4. A resin according to claim 1, wherein the water soluble polymer is poly(ethyleneimine) having molecular weight of 25 kD and the crosslinking agent is glutaraldehyde.

5. A SIR containing Aliquat 336 or other liquid-liquid extractant, e.g., nitrogen- containing reagents such as primary, secondary and tertiary amines, phosphorus- containing reagents such as tri-n-butyl phosphate are typical examples that are stabilized by coating with poly(vinylalcohol) and chemically crosslinked and that exhibit strong afflnity for the removal of metal ions and/or toxic heavy metals from aqueous solutions.

6. A coated and crosslinked SIR that retains virtually constant sorption capacity after four or more successive sorption-washing-elusion-washing cycles and negligible ' leaching of extractant, e.g., Aliquat 336. In comparison, an uncoated SIR containing Aliquat 336 loses sorption capacity for trace metal, e.g. chromium, and increasingly leaches extractant from the polymer support phase into solution as a function of time and as the number of sorption-washing-elusion-washing cycles increases.