Mercury sorption by “non-functional” crosslinked polyacrylamides

REACTIVE

ELSEVIER

Reactive & Functional Polymers 27 (1995) 155-161

FUNCiONAL POLYMERS

Mercury sorption by “non-functional” crosslinked polyacrylamides Niyazi Bicak ‘, David C. Sherrington * Department of Pure and Applied Chemistry, lJniversi& of Strathclyde, 295 Cathedral Street, Glasgow, Gl XL, Received

23 September

1994; revised version

accepted

UK

3 April 1995

Abstract

Three powdered crosslinked “non-functional” polyacrylamide gels and four spherical crosslinked polyacrylamide resins have been prepared by aqueous solution and inverse suspension polymerisation respectively. Methylene bisacrylamide, ethylene bisacrylamide and NJ-bisacryloyl piperazine have been used as crosslinkers. The seven polymers readily and rapidly sorb Hg(II) quantitatively from a 100 ppm solution (pH 6.4), and maximum Hg capacities are -1.5 g per g of dry polymer. Quantitative stripping has been demonstrated using hot acetic acid. Qualitative experiments with linear polyacrylamide show that other metal ions Ni(II), Cu(II), Co(II), Cd(II), Zn(II), Fe(I1) and Fe(II1) are unaffected and so the resins offer the prospect of a very cost effective selective sorbent for Hg removal and recovery. Keywords:

Mercury; Recovery; Polyacrylamides

1. Introduction

The investigation of chelating ion exchange resins is now well developed and the area has been cogently reviewed a number of times [l-3]. The concept of immobilising a particular ligand on a polymer support such that highly selective capture or extraction of one particular metal ion from a mixture of ions in aqueous solution is achieved, remains a highly exciting and potentially very useful one. Generally, however, such chelating ion exchange resins do not display absolute selectivity in this way, nor indeed is such specific selectivity necessarily totally ad* Corresponding author. Fax: +44 (41) 552-5664. 1Permanent address: Department of Chemistry, Technical

University,

Istanbul,

Turkey.

1381-5148/95/$09.50 0 1995 Elsevier SSDI 1381-5148(95)00048-8

Science

Istanbul

vantageous. Thus, the more selective a resin is, the fewer the possibilities for use, hence the smaller the market potential, and therefore the unit cost is likely to be higher. Where the metal ion in question is itself very costly e.g. Au, then in principle the more selective the resin to Au extraction the better. Also, where the metal ion is say a common and highly toxic contaminant of aqueous streams, e.g. Hg, then again the more selective the resin the better. Hg selective resins have been reported before [4-61. These typically involve thioether-containing ligands [4,5] and have been shown to have both good capacity and selectivity. However, it seems that ultimately none of the species have the optimum characteristics, including cost, for technical exploitation. We now report on our preliminary results

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N Bicak, D.C. Sherrington /Reactive & Functional Polymers 27 (1995) 155-161

156

-WH2T~+-C=O

+ Hg(CH3C00)2

+ CH3C02H Scheme

1. Reaction

of polyacrylamide

with Hg(I1) showing

involving the extraction of Hg(I1) with simple “non-functional” polyacrylamide-based sorbents. The reactivity of the amide group towards Hg(I1) has been known for more than 100 years. The reaction is fairly fast in neutral aqueous and organic solutions [7]. The Hg atom appears to become covalently bonded to one or two amide nitrogen atoms [S]. This is extremely interesting because, of course, the lone pair of electrons on N are somewhat delocalised in the amide functional group, such that the donor character of N is reduced to a level where coordination with most metal ions is too weak for complex formation. Also, of possible relevance, is a report in the literature where linear polyacrylamide has been used as a flocculant for Hg-containing mineral particles [9]. We were immediately encouraged by a simple test in which addition of traces of Hg(I1) to an aqueous solution of linear polyacrylamide caused rapid precipitation of the polymer, presumably via crosslinking involving Hg bridges (Scheme 1). This appeared to parallel the behaviour of low molecular weight organic amides. In our hands spot tests showed that dilute solutions of metal ions such as Ni(II), Cu(II), Co(II), Cd(II), Zn(II), Fe(I1) and Fe(II1) do not interact in the same way with an aqueous solution of linear polyacrylamide, although metal ion coordination by amide has been reported before [lo-131.

amide derivatives

and possible

crosslinking

species.

2. Experimental 2.1. Materials Piperazine (BDH) was recrystallised from toluene before use. Methylene bisacrylamide (MBA) (BDH), ethylene bisacrylamide (EBA) (Aldrich), acrylamide (Fluka) Span 80 and 85 (Fluka), maleic anhydride octadecene copolymer (Polysciences) and other analytical grade reagents and solvents were used without further purification. 2.2. Preparation of N,N-bisacryloylpiperazine WP)

Attempts to prepare BAP using a literature procedure [14] in our hands gave only low yields (-15%). As a result the modified procedure below was developed. Acryloyl chloride (20 ml, 0.25 mol) and dichloromethane (50 ml) were placed in a threenecked flask (250 ml) equipped with a reflux condenser, dropping funnel and N2 inlet, the whole being immersed in an ice bath. To the stirred solution was added dropwise piperazine (8.6 g, 0.10 mol) in dichloromethane (60 ml). The addition was completed in -1 h. Stirring was continued for a further 1 h then powdered anhydrous potassium carbonate (15 g, 0.25 mol) was added. A CaC12 guard tube was placed on the reflux condenser

N. Bicak, D.C. Shem’ngon IReactive & Functional Polymers 27 (1995) 155-161

and ethyl acetate (10 ml) added to the mixture which was refluxed for 7 h. While hot, the solids in the flask were filtered off and washed with dichloromethane (20 ml). About two thirds of the solvent in the filtrate was removed on the rotary evaporator, diethyl ether (10 ml) was added to the solution, which was then left overnight to crystallise. The white crystalline product was collected and dried in vacua at 40°C for 8 h. Yield, 12.7 g (-60%); m.pt. 99°C (lit. m.pt. [14], 97-98°C). Elemental microanalytical data, found (calculated for C,oH1402)%: C, 61.6 (61.9); H, 7.4 (7.2); N, 14.4 (14.4). ‘H-nmr (250 MHz, DMSO d-6) 6: 3.6 (8H, s, N-CII#II~-N); 5.6-7 (6H, m, OC-CH=CHz). The product has good solubility in water, methylene chloride, dioxane, ethanol and toluene, but is insoluble in diethyl ether. 2.3. Preparation of crosslinked polyacrylamide powdered gels Rigid gels of polyacrylamide were prepared via solution polymerisation in water. Acrylamide (8.55 g, 0.12 mol) and crosslinking agent (MBA, EBA or BAR 0.3 mmol) were dissolved in distilled water (60 ml) under Nz at 0°C in a 250 ml flask. (NH4)&0s (0.25 g) in water (0.5 ml) was added, and after stirring for 5 minutes, tetramethylethylene diamine (TMEDA) (0.5 ml) was added. Stirring was stopped and a clear rigid gel was obtained very rapidly (< 1 minute). This was left for 5-10 minutes before being broken up and washed with water. The water swollen product was placed in a Soxhlet extractor and the water removed by extraction with acetone for 12 h. The gel was dried at 80°C for 36 h in vacua. The dry product was further powdered in a mortar and pestle, and was re-dried for another 8 h. Elemental analysis data correlated poorly with theoretical expectations, but could be accounted for by the present of -15-18% of tightly bound water, and this was assumed to be the case. Further extended periods of drying failed to reduce the water content. Materials prepared with MBA, Gl, EBA, G2 and BAP, G3 were identical in this respect.

157

2.4. Preparation of spherical resin particles of crosslinked polyaqlamides

Spherical resin particles were prepared by inverse suspension polymerisation methodology. Two procedures were used differing essentially only in the nature of the suspension stabiliser employed. The first was a mixture of Span 80 and Span 85 as previously reported [15], and the second was a maleic acid-octadecene 1: 1 alternating copolymer [16]. 2.4.1. Procedure one Acrylamide (8.55 g, 0.12 mol) and crosslinker (2.0 mmol) were dissolved in cold water (50 ml) under Nz and (N&)&08 (0.25 g) in water (0.5 ml) was added. The resulting solution was then dispersed as droplets in an organic phase (toluene, 290 ml; chloroform, 110 ml) containing the suspension stabiliser (Span 80, 0.85 ml; Span 85, 0.35 ml). The reaction was performed in a cylindrical glass jacketed reactor (500 ml) equipped with a glass stirrer. The suspension was stirred under N2 at 300-350 rpm for 10 minutes at O-PC. Then TMEDA (0.5 ml) was added and stirring continued for 2 h. The spherical particles formed were collected by filtration, washed with alcohol, then dried at 80°C for 36 h in vacua. The yields were essentially quantitative, and the diameter of the resin beads was in the range -8O150 ,um. Resins Rl, R2 and R3 were prepared in this way using MBA, EBA and BAP respectively as the crosslinking agent. 2.4.2. Procedure two This method was employed in order to produce a resin with a larger particle size. Only MBA was employed as the crosslinker in this case yielding resin, R4. The maleic acid-octadecene copolymer suspension stabiliser was prepared by alkaline hydrolysis of the commercially available maleic anhydride-octadecene copolymer. The latter (0.39 g) and NaOH (0.3 g) were dissolved by sonication for 3 h at room temperature in a mixture of water (10 ml) and ethanol (5 ml).

N. Bicak, D.C. Sherrington / Reactive & Functional

158

The mixture was then heated slowly to dryness and the copolymer extracted into cyclohexane (50 ml). This solution was added to a mixture of more cyclohexane (95 ml) and chloroform (65 ml) and the resulting solution charged into the polymerisation reactor. The same polymerisation mixture (using MBA as crosslinker) and procedure were then used as described earlier. The washed and dried resin beads were again obtained in essentially quantitative yield and the particle diameters were in the range 400-600 pm. A small portion of these were also aggregated together into larger clusters. Elemental microanalysis, found (theory)%: C, 52.1 (51.7); H, 7.5 (6.9); N, 20.1 (19.8). In the case of the resins Rl-R4 significant retention of bound water did not appear to be a problem.

Polymers 27 (199.5) 155-161

1.588 g 1-l) were added such that the initial Hg(I1) concentration was 100 ppm at pH 6.4. The mixture was stirred gently and at appropriate times aliquots (10 ml) of solution were transferred through filter paper into volumetric flasks. Sample solutions were made up to 50 ml and the Hg(I1) content assayed by quantitative atomic absorption spectrophotometric (AAS) means (Phillips PU91OOX). To minimise the possibility of reduction to elemental Hg two drops of c.HNOs were added to each sample before analysis [17]. All the crosslinked polyacrylamides (Gl-3, Rl-4) showed rapid up-take of Hg(II) under the conditions of the experiment and a typical curve (for R3) is shown in Fig. 1. Solutions were not buffered and the pH fell typically to -5.2 presumably as protons are released as in Scheme 1.

2.5. Hg uptake experiments 2.6. Hg loading capacities of polyacrylamide gel Simple batch kinetic experiments were performed as follows. A sample of resin (1.5 g) was soaked in distilled water (100 ml) overnight. Then an additional volume of water (350 ml) and a stock solution of Hg(CHsCO& (50 ml,

powders and resins

To estimate the maximum capacity for Hg(I1) of the gel powders and resins each was contacted with an excess (up to -33%) of Hg(II), corre-

25

50 TI# (min)

Fig. 1. Sorption

of Hg(II)

by resin R3 from 100 ppm aqueous

solution

at pH 6.4 (non-buffered)

and ambient

temperature.

N. Bicak, D.C. Shem’ngton I Reactive & Functional Polymers 27 (1995) 155-161

sponding to the content of primary amide groups in each polymer sample. Resin beads (1.5 g) or gel powder (1.1 g) were added to water (500 ml) containing Hg(CH$02)2 (6.55 g for resins, 4.55 g for powders) and the mixtures stirred gently for 18 h. Resins and swollen gels were removed by filtration and a sample of each filtrate (5 ml) made up to 50 ml in a volumetric flask for analysis by AAS as before. Each collected solid was washed with water and ethanol, and then dried in vacua at room temperature for 1 h. The increase in mass of each polymer sample was a factor of -2.5-4, and the full data are reported in Table 1. 3. Discussion Two series of crosslinked polyacrylamides have been prepared, one in the form of powdered gels (Gl-3) and one in the form of spherical resin beads (Rl-4). The former isolated from rigid swollen transparent gels retain relatively large levels of bound water (-15-18 wt%) on drying. The latter were obtained from inverse suspension polymerisations Rl-3 having particle sizes of -80-150 pm, and R4 -400-600 pm. The change in size was achieved by moving from the use of nonionic surfactant stabilisers, Span 80 and 85, to a maleic acid-octadecene alternating 1: 1 copolymer as the suspension stabiliser. This control of particle size should not be regarded as general, and each polymerisation system needs to be investigated in its own right. The spherical resins did not display the same tendency to retain bound water tenaciously and elemental microanalytical data correlated well with the theoretical expectation. The reason for the difference in the retention of water is unclear, but may be associated with the ability of the gels to collapse on drying, and perhaps encapsulate pockets of highly hydrated polymer. Whereas the gels were -0.3 mol% crosslinked the resins were of higher crosslink ratio, -2 mol%. Drying of the latter network would be expected to produce a more rigid expanded network which might allow more complete removal of water on prolonged drying.

159

It must be emphasised, however, that both the gels and the resins described here represent first generation sorbent materials. No attempt has been made yet to optimise the morphology of the crosslinked polyacrylamides. The effectiveness of all the polymers Gl-3 and Rl-4 to sorb Hg(I1) is shown clearly by the batch kinetic experiments, the results being exemplified by the data in Fig. 1. All the polyacrylamides examined yielded similar data. The results are remarkable from two points of view; firstly the essentially quantitative extraction of Hg(I1) from 100 ppm solutions (the limit of detection of the analytical technique used was -2 ppm). Secondly, the extremely rapid rate of removal of Hg(I1). Typically within 10 minutes, >95% extraction was achieved. In terms of the performance under these conditions there is little to choose between the behaviour of all the polymers examined. The data in Table 1 give the capacities for Hg(I1) for the various polymers and here some differences start to emerge. The sorption of Hg(I1) was clearly apparent in the case of the resins, which turned rapidly from transparent beads to opaque ones. Capacities were measured directly as an increase in mass of each loaded polymer, and indirectly by AAS monitoring of the residual Hg(I1) of test solutions. The apparent masses of Hg(I1) loaded onto the polymers are subject to a large error because it was not possible to dry the gels nor the resins properly. Attempts to vacuum dry over prolonged periods resulted in the loss of Hg (by sublimation) which introduced its own error, and more importantly, raised a toxicity hazard in the laboratory. The data in Table 1 are useful, however, because they show dramatically the accumulation of Hg on the polymers, with increases of mass of factors of 2.5-4. Interestingly the resins Rl-4 seem to retain moisture more effectively in this experiment than the gels Gl-3, and the increase in mass displayed by these does not correlate with the AAS data. The latter is almost certainly more accurate and more representative of the differences in the behaviour of the polymers. The

N. Bicak, D.C. Shem’ngton I Reactive & Functional Polymers 27 (1995) 155-161

160 Table 1 Mercury loading

capacities

Resin (R) or gel (G) (% water) a

of polyacrylamide

Polymerisation feed-composition acrylamide

(g)

crosslinker

gel powders

and resins

Polymer sample (wet) (g)

Weight b of loaded sample

SorbedC Hg (g gg’ dry polymer)

Sorbed Hg (mm01 g-r dry polymer)

Primary amide groups (mm01 g-l dry polymer)

d

Gl (14.9)

8.55

MBAM 0.45

1.1

2.8

1.3

6.3

13.4

47

;;.8)

8.55

0.49 EBAM

1.1

3.2

1.4

7.0

13.3

53

G3 (15.6)

8.55

BAP 0.57

1.1

3.3

1.4

7.2

13.2

54

MBAM 3.08 EBAM 3.36 BAP 3.88 MBAM 3.08

1.5

6.8

1.4

7.2

10.9

65

1.5

5.2

0.94

4.7

10.1

47

1.5

5.9

isiripped

5.2

9.7

54

6.4

10.4

62

Rl

10.65

R2

8.55

R3

8.55

R4

8.55

1.1) 1.5

6.7

1.3 (stripped

1.3)

il Water content of the gels calculated from the elementary analysis data. h Mercury loaded samples were only superficially dried due to toxicity of the mercury compounds. ’ From AAS data. d DM: degree of mercurisation, calculated by assuming only primary amide groups react to give 1: 1 binding i.e. @--CONH-Hg(02CCHs) structure.

powdered gels Gl-3 typically sorb -1Sg of Hg gg’ dry gel, and there is little variation with the crosslinker employed. Marginally lower capacities are shown by the resins, Rl-4, but here there is greater variation. The structural variants are not really large enough to draw too hard a conclusion, but the two resins crosslinked with MBA, Rl and R4 both show the best capacity, and essentially no dependence on the particle size of the resin. The larger particles, however, would be more convenient in any technological exploitation. Preliminary attempts to strip Hg-loaded resins using mineral acids showed the resins to be rather chemically unstable. In the case of Rl and R4 the residues from the MBA crosslinker are known to be acid labile, and indeed to generate undesirable formaldehyde in the process. Essentially quantitative removal of Hg was achieved however with hot acetic acid. Typically Hg loaded resin (2 g) was heated at -118°C for 1 h in glacial acetic acid (80 ml) and the solution assayed for Hg. No degradation of polymer was observed and milder stripping conditions may

well be effective as well. In principle therefore it seems that appropriate resins can be designed for recycling or continuous use. Further work is also required to quantify the competitive Hg(I1) binding properties of these resins, and the influence of anions such as Cl-. The latter coordinate strongly to Hg(I1) and are likely constituents of wastewaters. The final fundamental question which remains concerns the nature of the Hg binding to the polymers If it is assumed that a 1: 1 complex with primary amide functions is involved i.e. - CONH- Hg- 02CCHs then a nominal degree of mercurisation (DM) of groups can be estimated as shown in Table 1. Since the primary amide content of all the polymers is similar the DM value parallels closely the quantity of sorbed Hg (g g-l). It is also possible, however, that 2 : 1 complexes are formed as shown in Scheme 1 and the contribution of this species may depend on the detailed structure and morphology of each polymer. Attempts to try to characterise the structure of the complexes have so far not been successful.

h? Bicak, D.C. Sherrington J Reactive & Functional Polymers 27 (1995) 155-161

4. Conclusions and future work Clearly simple “non-functional” polyacrylamide gels and resins are very effective for the sorption of Hg(I1). Further experiments involving the effect of concentration, pH, ionic strength, temperature and polymer structure and morphology are needed, but the system looks a very exciting one, and potentially of low or modest cost. To date we have also not been able to carry out quantitative competitive extraction experiments, but qualitative tests using soluble polyacrylamide indicate that a high or very high level of selectivity is likely. This would also be predicted from the “non-functional” nature of the polyacrylamides involved. Acknowledgement

[2.]

[3.] [4.] [S.] [6.] [7.] [8.] [V.] [lo.] [ll.] [12.] [13.] [14.]

N.B. acknowledges the financial support of the Turkish Education Authorities which enabled him to spend a sabbatical leave in the U.K.

[15.] [16.] [17.]

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A. Warshawsky. (Eds.) Syntheses

In: D.C. Sherrington and P. Hodge and Separations Using Functional Poly-

161

mers. J. Wiley and Sons, Chichester, 1988, Chap. 10, p 325. A. Warshawsky. In: M. Streat and D.A. Naden (Eds.) Ion Exchange and Sorption Processes in Hydrometallurgy. J. Wiley and Sons, Chichester, 1987, Chap. 4, p. 166. K. Dorfner. In: K. Dorfner (Ed.) Ion Exchangers. Walter de Gruyter, Berlin, 1991, Chap. 1.2., p. 189. M. Lauth, Y. Frere, M. Prevost and Ph. Gramain, React. Polym., 13 (1990) 73. K. Yamashita, A. Yamada, T Goto, K. Iida, K. Higashi, M. Nango and K. ‘ISuda, React. Polym., 22 (1994) 65. A. Winston and G.R. McLauchIin, J. Polym. Sci., 14 (1976) 597. A. Strecker, Ann. der Chem., 103 (1857) 324. H. Ley and H. Kissel, Ber., 32 (1899) 1358. T Kidokoro, K. Matsui, S. Ishiwatata and T Sakaki, Bull. Chem. Sot. Jap., 61 (1988) 1505. S. Schlick, NATO AS1 Series, Ser. B (1988) 174. H. Sigel and R.B. Martin, Chem. Rev., 82 (1982) 385. G.C. Rex and S. Schlick, Polymer, 28 (1987) 2134. C. Methenitis, J. Morcellet, and M. Morcellet, Eur. Pol. J., 23 (1987) 287,403. E Danusso, P Ferruti and G. Ferroni, Chim. Ind., 49 (1967) 271 [C.A. 67 (1967) 1180421. R. Nilsson, R. Mosbach and K. Mosbach, Biochim. Biophys. Acta, 268 (1972) 253. 7: Brock and D.C. Sherrington, J. Mater. Chem., 1 (1991) 151. A.M. Ure and C.A. Shand, Anal. Chim. Acta, 72 (1974) 63.