SYNTHESIS AND USE OF POLYDADMAC FOR WATER PURIFICATION W Johna, CA Buckley b, EP Jacobs c and RD Sandersonc a. Umgeni Water, 310 Burger Street, Pieternaritzburg, 3201: [email protected] b. School of Chemic al Engineering, University of Natal, Durban, 4041:[email protected] c. Institute of Polymer Science, University of Stellenbosch, Matieland, 7602:[email protected] and [email protected] ABSTRACT Polydiallyldimethylammonium Chloride (polyDADMAC) is synthesised by free radical addition polymerisation of diallyldimethylammonium chloride using a persulfate initiator. In this study, the exothermic reaction was conducted under controlled conditions of temperature, pressure and monomer concentration. Poor control of the above parameters results in the formation of a waxy solid early in the reaction which was unable to be used for the purposes of water treatment. Under better controlled conditions, polyDADMAC was synthesised and isolated from aqueous solution by precipitation and characterised by spectroscopic techniques (1HNMR, 13C-NMR) and gel permeation chromatography (GPC). The molecular mass distribution (MMD) was determined and based on a calibration using polyethylene oxide (PEO) narrow standards. The effectiveness of the polymer was determined using the standard jar test and bentonite suspensions at a concentration of 70 mg/L. The optimum dose was determined from the residual turbidity measurements of the supernatant liquid after a period of 15 minutes of settling. The results obtained for stability studies of polyDADMAC with water treatment chemicals such as chlorine, different conditions of pH, temperature and UV radiation are presented. Changes in the polymer structure was followed by GPC, HPLC and GC-MS and the results indicate that the product is affected by the above conditions.

Paper presented at the Biennial Conference of the Water Institute of Southern Africa (WISA) www.wisa.co.za CD-ROM produced by: Water Research Commission (WRC), www.wrc.org.za

19 – 23 May 2002, Durban, South Africa ISBN Number: 1-86845-844-X Organised by: Conference Planners

INTRODUCTION PolyDADMAC is a cationic linear polymer used extensively for water purification. It is synthesised by the free radical initiated addition polymerization of diallyldimethylammonium chloride [1], according to Scheme 1. CH2

.

R

CH2

+ CH

CH2 heat vacuum

CH

CH

CH

CH2

CH2

CH2 +

CH2

CH2

N

+

CH3

N CH3

CH3

CH3

n

Scheme 1: The synthesis of polyDADMAC by free radical addition polymerization of DADMAC 13

C NMR spectroscopic studies conducted on the synthesized product, revealed that it consisted of repeating units of pyrolidine rings [2,3]. The polymer falls in the category of low to medium molecular mass range and usually exists as a mixture of the cis and trans isomers [4].

+

CH3

N cis

+

CH3

CH3

N

CH3 trans

The objectives of this research were to synthesise polyDADMAC and acquire some insights into its behavior and properties during a typical water clarification process. Some polymers are known to be affected adversely by treatment chemicals such as chlorine and ozone and one of the major concerns is the potential formation of undesirable by-products as a result of their use. EXPERIMENTAL The apparatus used for the synthesis consisted of a 1 L glass reactor attached to a four neck manifold by an O-ring gasket and clamp system (Figure 1). Two pressure equalizing dropping funnels were used, one for the addition of the free radical initiator and the other to replenish water losses. Vacuum was applied through the condenser outlet and controlled by a bleed valve installed to the main vacuum line using a PVC T-connector. The vacuum was monitored by the use of a vacuum guage (Wika, Instruments, Durban, SA) fitted on the main vacuum line. PVC tubing was used for the water inlet, outlet and vacuum lines. Temperature and stirring of the reaction mixture was achieved with the use of a heater stirrer unit

vacuum guage

needle valve vent

water funnel catalyst funnel

vacuum water outlet

condenser

water inlet

thermometer manifold with O-ring gasket and clamp reactor water bath

heater/stirrer

Figure 1: Schematic diagram of the reactor used for polymer synthesis Chemicals All chemicals were of analytical reagent grade. Ammonium persulfate was purchased from Merck. DADMAC monomer (65%, m/v) was purchased from the Aldrich Chemical Company (Milwaukee, USA). The polyethylene narrow molecular mass standards were obtained from Polymer Standards Service (USA). Potassium dihydrogen phosphate, methanol, and EDTA was purchased from BDH (Poole, UK). All buffers, samples and standards for chromatography were prepared from Milli Q water (Millipore, MA, USA). Milli Q water refers to water passed through the Millipore Gradient A10 water purification system which consists of a series of ion exchange and organic removal resins.

Synthesis The reactor was charged with sufficient monomer and EDTA as described by Hunter et al [1]. The mixture was purged with nitrogen for 20 min. The reactor was then evacuated and heated to the recommended temperature after which slow addition of the initiator commenced. The mixture was stirred gently throughout the exothermic reaction. INSTRUMENTATION Jar Tests A 0.1% (m/v) polymer solution was used as the primary coagulant in flocculation experiments. The test sample to be clarified was a clay suspension containing bentonite at a concentration of 70 mg/L. Aliquots of 800 mL test sample was transferred to each of six 1 L beakers and dosed with increasing amounts (0.21 to 0.77 mg) of polymer. Using a jar stirrer apparatus, the samples were mixed at 300 rpm for 2 min and 40 rpm for 15 min. The supernatant turbidity at a depth of 40 mm was measured after 15 min of settling. Conductivity, colour, TOC and pH were recorded after filtration through Whatman No. 1 filter paper. Gel Permeation Chromatography Gel permeation chromatography (GPC) was conducted on 0.5% (m/v) polymer solutions in water or mobile phase using the Waters Alliance 2690 HPLC system fitted with a Waters 410 refractive index (RI) detector. A Waters Ultrahydrogel 500 column of dimensions 7.8 x 300 mm and 10 µm particle size was used. A 0.25 M phosphate buffer adjusted to pH 2.3 and a flow rate of 0.5 mL/min was used as the mobile phase. The mobile phase was filtered and degassed with a 0.45 µm membrane filter supplied by Waters. Column calibration was achieved using 0.1% (m/v) polyethylene oxide (PEO) narrow molecular mass standards ranging from 25 300 to 850 000 Daltons. All samples and standards were filtered through 0.45 µm syringe type filters. Data acquisition and analysis was accomplished with the use of Millenium 32 software having the GPC option, supplied by Waters. Reversed Phase High Performance Liquid Chromatography Reversed Phase High Performance Liquid Chromatography (RP HPLC) was conducted on a Waters Alliance 2690 system fitted with a Waters 996 photodiode array (PDA) UV detector. A Waters µBondapak C18 column of dimensions 7.8 x 300 mm was used. The mobile phase was methanol:water (50:50) at a flow rate of 1 mL/min. Samples were filtered through 0.45 µm syringe type filters.

Gas Chromatography Gas chromatography (GC) was conducted on a Hewlett Packard (HP) 6890 gas chromatograph fitted with a flame ionization detector (FID). A DB 5 capillary column (J&W Scientific) of dimension 30 m x 0.25 mm x 0.25 µm was used. Data acquisition and analysis were conducted using HP Chemstation software. The instrument conditions were as follows: initial temperature 50oC held for 1 min, ramped at 15 oC/min to 280 oC and held for 5 min. A splitless injection mode was used with the injector temperature set at 250oC and detector temperature at 280oC. The gas flow rate was set at 1 mL/min in the constant flow mode. Sample pretreatment was by solid phase extraction using Waters C 18 Sep-Pak cartridges. Purge and Trap Gas Chromatography Mass Spectroscopy An HP 6890 gas chromatograph (GC) interfaced to an HP 5973 mass selective detector (MSD) was used. Sample introduction for the analysis of volatile organic compounds (VOCs) was achieved with the use of the CDS 6000 Purge and Trap device having a cryogenic focusing facility and liquid nitrogen as the coolant. GC MS conditions were similar to that of the GC FID. The scan range was from m/z 35 to m/z 500 atomic mass units. CHEMICAL REACTIONS Chlorination A stock hypochlorite solution was prepared by diluting aqueous NaOCl (Jik) with Milli Q water and standardized by iodimetry [6]. This solution was used to treat 0.5% polymer solutions to give spike concentrations ranging from 0 mg to 22 mg of free chorine. The chlorinated samples stored in 100 mL volumetric flasks, were sufficiently filled to remove all head space , tightly stoppered and sealed with Parafilm to prevent loss of volatile components. After a 24 h contact period, sodium thiosulfate was added to remove residual free chlorine and the samples were analysed by GPC and GC MS. Temperature Experiments A 0.5% polymer solution was prepared in Milli Q water and subjected to increasing temperatures (room temperature to 80 oC) and held for 30 min at each temperature. The solution was stirred gently during heating. A 10 mL aliquot was removed at each temperature for GPC analysis. pH Effects Polymer solutions (0.5%, m/v) were adjusted with phosphoric acid and sodium hydroxide to range from pH 2 to 12. The samples were allowed to stand for 1 h prior to analysis. UV Experiments A 0.5% polymer solution was prepared and exposed to long wavelength UV radiation (365 nm), using a 9 A UV lamp (Spectroline Model EA-160/FE, Spectronix Corporation, USA) for 24 h.

RESULTS AND DISCUSSION Synthesis Initial experiments were plagued with problems of poor temperature and pressure control and resulted in a rapid increase of the reaction temperature well in excess of the recommended maximum of ca 140oC. This occurred in the early stages of the reaction and resulted in the formation of a waxy solid material. The product did not dissolve readily in water but dissolved partially when allowed to stand overnight. About 10 to 15% of the product remained in the form of a hydrated gel. These findings were consistent with that of Butler and co-workers [5]. The low solubility was attributed to the formation of a highly cross linked product. In subsequent experiments, good control of the reaction rate and temperature was achieved by slow addition of the persulfate initiator as well a decrease in the monomer concentration from 65% to 50% (v/v) with Milli Q water. The reaction proceeded for 3 h without any precipitation of polymer as noted previously. The polymer was purified through precipitation from ethanol, filtered through GF/C filter paper and dried in a desicator for ca 3 h. The product was then characterized by 13C NMR and 1H NMR analysis [2,3]. Gel Permeation Chromatography The GPC chromatogram (Figure 2), show the typical polymer profile in the form of a broad peak eluting in the region 11 to 22 min. Small molecular species including residual monomer, elute as a single peak at 24 min. This region was first established as the useful analytical range (V 0 to Vt) of the column during calibration. 16.00 14.00 12.00

MV

10.00 8.00 6.00 4.00 2.00 0.00 2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

20.00

22.00

24.00

26.00

28.00

30.00

Minutes

Figure 2: GPC chromatogram of the synthesized product with a refractive index detector and an Ultrahydrogel 500 column operating at 0.5 mL/min. The mobile phase was 0.25 M potassium dihydrogen phosphate adjusted with phosphoric acid to pH 2.3

For economic reasons, the molecular mass distribution (MMD) based on a calibration with polyvinylpyridine standards was not possible. PEO standards were used as a substitute because of its relatively low cost and it was readily available. The MMD is presented as a distribution and a cumulative trace in Figure 3. From the peak molecular weight (MP), the product can be categorised as a low molecular weight polymer. 100.00

MP=62098

0.90 0.85 0.80

95.00 90.00 85.00

0.75

80.00

0.70

75.00

0.65

70.00 0.60 65.00

0.45 0.40

55.00 50.00 45.00

0.35

Cumulative %

0.50

60.00

Mn=32977

Mw=382940

d wt/d(logM)

0.55

40.00 0.30

35.00

0.25

0.10 0.05

25.00

Mz=14498670

0.15

30.00

Mz+1=45367052

0.20

20.00 15.00 10.00 5.00

0.00 7.50

7.00

6.50

6.00

5.50

5.00

4.50

4.00

3.50

3.00

2.50

0.00

Slice Log MW

Figure 3: Molecular mass fractions of the polymer shown as a distribution and a cumulative trace. Calibration achieved with the use of PEO narrow standards

Performance Assessment The effectiveness of the product was assessed using the standard jar test on a synthetic sample with a starting turbidity of 20.1 NTU, pH 7.7, conductivity 0.76 mS/m, colour 6.1 oH and TOC 1.64 mg/L. Figure 4, is a plot of the supernatant turbidity as a function of polymer dose. 6

Supernatant Turbidity (NTU)

5 4 3 2 1 0 0

0.2

0.4

0.6

0.8

1

Polymer Dose (mg) Figure 4: Plot of supernatant turbidity as a function of polymer dose during the jar test performed on 800 mL sample volumes.

There is a steady decrease in turbidity as a result of particle destabilisation, with each addition up to 0.49 mg, after which there is an increase. This is consistent with the charge neutralization model which predicts restabilisation and the formation of positively charged colloidal particles due to the excess of positively charged polymer adsorbed onto the particles. No significant trend occurs with pH (Table 1) and the values remain relatively unaffected with increasing dose. This pH stability is very important in water treatment in that there is no need for pH adjustment of the final treated water. The conductivity of the original solution (0.76 mS/m), shows a significant increase with a corresponding increase in the polymer dose. This may be attributed to the increase in the concentration of ionic species during dosing as well as the result of the presence of the charged polymer itself. There is a slight drop in the conductivity leading up to the optimum indicating a removal of ionic components during coagulation and flocculation. The conductivity increases slightly with particle restabilisation. Table 1: Filtrate results for jar test experiment 0.35 0.42 0.49 7.45 7.36 7.26 0.86 0.91 0.91

Dose (mg) pH Conductivity (mS/m) o Color ( H)

0.21 7.10 1.03