161

Deaalinution, 19 (19’76) 161-167 8 EIsevier Scientific Publishing Company, Amsterdam - Printed in The Netharlands

PHENOL-SELECTIVE. HIGHLY RESISTANT RO-MEMBRANES MADE FROM PVA FOR THE PURIFICATION OF TOXIC INDUSTRIAL UASTES

S. PETER*. N. tiESEt* and R. STEFAN*

+ Snstitut fUr technische Chemie 11. UniversitXt Erlangen. Germany 8RD

++ Schleicher 6 SchUll G&H. Oassel, Germany BRD

Abstract

Asynsnetric RO-mranes made from PVA and from derivatives of PVA can be produced bv two different methods. .._ __ ___. By the ‘classical’ method of phase-inversion membranes with ‘classical’ asymmatry result from Boric-acid-derivatives of PVA. These water-soluble membranes are inso- lubilized in a second step by unidirectional, diffusion-controlled crosslinking with polyvalent metal ions. By this a ‘crosslinking’ or ‘density’ asynxnetry is superimposed. Homogeneous membranes precipitated by acetone from aqueous casting solutions can be rnsolubilized. too. by crosslinking with poly-valent metal ions as described above. These crosslinked membrazc, compared to such made by phase-inversion are exeptionally resistent to solvents, acids and bases in the region between pH 0 and pH 14. They are resistant to aqueous solutions of phenol up to 5 weight % and have a selectivity for phenol.

The followina reoroducible orooerties were achieved: .# . with dP= 50 (bar) driving pressure. T = 25 OC and Feed = 0.2 weight Z Pheno?: Retention at pH 12: ca. 90 - 95 P

pH a: cd. 60 f Product f&x: 150 - 250 (L/&d). Temperature resistance: up to 80 C (continuous operation).

Introduction

The waste-waters discarded. especially those from the chemical industries are so badly polluting the water resources of many countries that grave problems for the future supply of the populations and of industry have already arisen. The costs for the purification of waste waters generally exceed those for the supply of fresh water; in the case of industrial waste waters the costs for puri- fication are much higher. Here membrane processes have come into consideration and on trial. reverse osmosis (RO) as well as ultrafiltration (IJF). While UF is already a standard procedure. for instance in the dairy industry and in the separation of oil and water. promising results from the purification of waste waters by RO have been reported in a very few cases only. The results shown in general concern bina- ry or ternary systems. that is ‘synthetic’ waste waters. Results from a field trial with a ‘live’ waste water have yet to be waited for. A group of compounds extremely problematic in water purification processes are the phenoles. Economically their concentration can be lowered to about 0.02 weight 4 by an extraction process. But even this concentration is highly toxic for the bia- logical steps in waste water treatment. For example, one cubic meter of water con- taining 0.2 Weight X of phenol discarded directly into a reservoir of drinking water pollutes about 200.000 cubic meters to a degree not suitable for human con- sumption and for the use in agriculture as well. ‘Live’ waste waters. however. polluted by phenoles, generally contain a multitude of organic and inorganic com- pounds_ Their pH-values tend to range between pH2 and pH 11. The separation of phenoles and water by RO has been investigated by several anthors (1.2). the results are contradicting because positive and negative retention values have been reported. Our experiments showed that twnbranes made from celluloris ace- tate (CA) were rapidly destroyed by 0.2 weight % of phenol in water.

162 S. PETER, pi. HESE AND R. STEFAN

Keeping all this in mind the problem for the RO-separation of phenoles and water is to pmduce a membrane resistant to organic solvents. pH-changes and phenoles and showing a selectivity for these phenoles. For some years we have been investigating the mechanfsm of water transport through membranes made from polyvinylalcohol (PVA) and its derivatives as welt as the appli- cability of these ~mbranes for desalination purposes (3. 4, 5) and lately for the purification of waste waters. According to our data a solution-diffusion model for the transport of water through membranes made from pure PVA and its derivatives can be assumed.

Experimental

PVA is a polymer 1.3 diglycol with a monomer weight of 44. The conxnercially avai- lable ‘pure’ compound is soluble in water and only in a very few organic solvents. such as dimethylfotnaamide (CJMF), dimethylsulfoxide (BMSO) and formamide (FA) etc. The solubility of PVA in water is dependent on molecular weight. the content of acetyl groups and the temperature of the solvent.

FIG. 1 Polyvinylalcohol (PVA)

When a rising amount of the polymer’s hydroxyle-groups are blocked by ketalization, esterification or etherification the volubility in water decreases and that in or- ganic solvents increases. For a solution-diffusion-membrane, the content of polar groups in the oolymer ma- trix is one of the factors influencing the water transport rate. In the first step of the production of RO-membranes from PVA the polymer has to be insolubilized by one of the reactions memtioned above or some other procedure. For the ‘tailoring’ of nmmbrane properties, however. the ratio of polar groups needed for crosslinking and isolubilization to the one needed for the transport properties must be optimi- zed. From keta’ies and esters of PVA asyimnetric membranes could be produced by the phase- inversion-method. The procedure for the pmduction of asymmetric membranes by phase- inversion has been described explicitly by Strathmann (6.). These films were rather sensitive, chemically as well as mechanically and showed only a moderate reten- tion. By thermal etherification extremely dense, homogenic (isotropic) films with a very low water flux and nearly no retention for sodium chloride at all were ob- tained. The development of the membranes presented here is based on two we11 known pheno- mena : 1. The titration of glycerol becomes possible by adding boric acid to the solu-

tion and thus producing an acidic boric acid-glycerol-complex. The same com- plexation reaction can be observed with PVA ( 7.. 8 )_

2. The viscosity of aqueous PVA solutions iS increased markedly by adding poly- valent metal ions to the solutions (9.).

The combination of those two phenomena led to the crosslinking of two PVA-chains. complexed by boric acid. via a Calcium-ion. 18.) A first type of membranes was produced by complexing PVA in aqueous or organic solution with a calculated amount of boric acid_ After the precipitation of the complex by means of acetone and air-drying. the complex was solved in an appro- priate solvent or a solvent mixture. The solution was drawn to the desired thick- ness on a steel plate and precipitated. after an evaporation period, in acetone. Here the membranes are still soluble in water. In the next step the films, still on thefr drawing p!ates, are immersed into a solution of a polyvalent mtal ion, using for instance Ba (OH)R-BH30. AI(Cl)3-6H3O or K Cr(S04)2-12H20. The diffusion- control1ed crosslinking process begins at the film-solution interface and is there-

RO tlQlBRAN~S FOR PtiRIFICATION OF TOXIC INDUSTRIAL WASTES 163

after slowed considerably by the increasing density of the surface layer. The resulting membranes show the ‘classical asymmetry’ induced by the phase-inver- sion process with a ‘crosslinking-or density-gradient’ superimposed. Fig. 2, shows the crosscut of a membrane. swollen in water, as seen through a microscope. The density gradient is not to be seen here_

FIG. 2 PVA 45000. reacted to 75 X of the equivalent with H3BO . cast from a 10 Z solution in LV4SO: CH30H - FA=60 : 2a: 20. crosslinked by Cr (III)-ions. Membrane swollen in water. Magnification 250 x

A second type of membranes was produced by drawing a film of a defined thickness on steel plates from a concentrated solution of PVA in water. Thereafter the film was precipitated in acetone and crosslinked in a solution of polyvalent metai ions as described above_ By certain procedures before and after crosslinking the diffe- rent properties of the membranes can be improved considerably.

Results

The scanning electron microscope photoes of the membranes mentioned showed no signs of asymmetry. In RO tests, however, the membranes showed the flux increase and re- tention decrease at flux reversal typical for asymmetric membranes. By using Some deeply colored chromium canpounds in the crosslinking bath the asymmetric or anlso- tropic nature of the membranes can be made visible by means of a good light mlcro- scope, as shown in Fig. 3.

FIG. 3 PVA 45000; 97,5- in water. crosslinked by Cr (III)-Ions-

164 s_ pm=. N. HE% AM) R- snzlux

Both types of membranes are insoluble in water, acids. bases and the usual organic solvents and mixtures thereof_ They are resistant to phenol up to 5 weight I and show selectivity for phenol. Retentions between 90 and 95 s can be reproduced. The following table shows the results of the investigations on the chemical resistance of the new mmbranes compared to such made from Potyvinylbutyral (PVB), Polyvinyl- acetatr? (PVAc) and Cellulosisacetate (CA). For the investigation of the chemical stabilaty ready cut membranes uereimnvwsedinto the testing ftuids for two tnzeks at 20 C.

Table 1 Chemical Stability at 20 Of

Type of Uembrane

Reagent Cone _ PH I II III IV v --.

HNfJ3 12 -w.x 0 330 00

H2S04 48 ; 0 3 -3 0 0 0

H Cl

0 = Destruction of membrane

1 = strong suelling, decrease of membrane properties

2.= little SWel1ing. membrane still usable

3 = stable, IIO a!teration of mehdtrane nromrties

Types I = PVA; u3R@S-cyplexed. metal ion crosslinked

II= PkA. metal ion crosslinked

IIl=PVB (Polyvinylbuty~~l) . IV= PVAc (h0iflin.31.3cetatej

V = CA iCellulosisac&ate)-

~0 XEUMWGX FOR PURIFICArION OF TOXIC IR’DUSTRXAL WASTES 165

The mechanical and chemical resistance was tested by alternately driving one membrane of each type for three days with a feed at pH 5. containing 0.2 weight % of phenol at 50 (bar). thereafter with a feed at pH 12. containing 0.2 weight 9: of phenol, too and so on. These cycles were repeated for about twelve days each. It was found, that the re*zntion and flux values did not change practically. Typi- cal results can be seen in Figures 4. and 5. That means, the membranes showed very little compaction. This statement is valid for driving pressures up to 100 (bars).

409

qos

OP’

FIti.

006

002

001

00

d- *Y--A P 12.I:*06.0’1. H P,,l2. Ro EG.l%

-4 -+---Q--O w-0

PRS, R*L7,1% R:&7.0% R=L7.1%

T --

3 6 9 12 Cdl

4 PVA 45000. 97.5-99.5 96 hydrolyzed, H3IIO3-complexed. Cr (III)-crosslinked Feed: 0.2 weight % Phenole AP = SO (bar). 1 = 25 ‘C

I PRtm3/n?d]

P,,lt, R=79.1% u-77.5-1.

-a, - *

PR5. R- 55.5%

-M--Q-4

R&5,1%

P=WCbar3

A

3 6 9 12

FIG. 5 PVA 45000. Cr (III) crosslinked test-conditions as in Fig. 4.

166 s. Pm. H. HESE AND a. STEFM

20 40 6u 8a 100 1

FIG. 6. o = PVA 45000, 97.5-99.5 % hydrotyred. H-$0

a= PVA 45000, Cr (XII)-crosstinted

complexed. Cr I I ? - - I) Feed: 0.2 weight f phenole + 200 pm

cm&link&d . ’ NaCY l pH = 4

LO 60 80. 1130 APfba*J

FIG. 7 0 t PVA 100000, 86-89 % hydrolyzed ti3B03-complexed, Cr (1X-t )-cmssl inked

~1, PVA 100000. Cr (III)-cross- 1 inked

Feed: 0-E weight X Phenole, pH =13

RO WlWWNES FOR PURIFZCATIC711 OF TOXIC IZIDUSTRIAC. WASTES 167

The dependance of water flux and phenol retention as a function of driving pressure was found to be hyperbolic as shown in Fig. soIotion-

8.

6 and 7. This would be expected for a iffusion membrane. (II) The new membranes are resistant to temperatures

up to 80 C at continuO= operation. membranes show a product flux of 150

At 50 (bar) driving pressure and 25 OC the - 250 (L/mtd). Tt should be noted that the

retentionnreasuredwas of&awed with the rebtivefy 0.2 weight Z of phenol _

high feed concentration of

Acknowledgement

This work was supported by a research contract from the Ministerium fiir Ferschung und Technologie (BHFT) of the German Federal gavernment.

bibliography

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