Indian Journal of Chemical Technology Vol. 8, September2001, pp. 371-377

Synthesis of crosslinked methacrylic acid-co-N,N'-methylene bis acrylamide sorbents for recovery of heavy metal ions from dilute solutions

H Hari Prasad, Ashrima Senger, Kavita Chauhan, Kirit M Popat & Pritpal Singh Anand*

Separation Technology Discipline, Central Salt & Marine Chemicals Research Institute, Bhavnagar 364 002, India

Received 19 February 200 I; revised 4 June 2001; accepted 26 June 2001

Several crosslinked porous copolymers of methacry lic acid-N,N'-methylene bis acrylamide were synthesized by suspension polymerisation using benzoylperoxide as the initiator. They were characterized for physico-chemical properties like, surface area, porosity and scientific weight capacity. The sorbents were further studied for adsorption of nickel and copper ions from spiked metal ion solutions in static and dynamic conditions. Concentration ratios of I :60 and I :30 have been achieved for nickel and copper ions respectively.

The disposal of sludge from the electroplating industry effluents involves environmental and economic problems. The first problem arises due to limited dissolution of sludge and the latter might result due to the presence of valuable metals in sludge. Thus the conventional method to address this problem is the prevention of sludge formation by adopting a technique which results in the formation of concentrated solutions. The composition of these solutions will be similar to those used m electroplating tanks and can be reused after appropriate modification.

Weakly acidic sodium form of carboxylic acid type cation exchangers are quite appropriate for the removal of metals ions from dilute solutions. It is well-established that the metal ions like Cu(II) or Ni(ll), undergo the following reaction at pH 4 to 5.

Ion-exchange capacity of the order of 3 equivalents per liter of carboxylic resin can be utilized in this processes. The exhausted resin beds can be regenerated by dilute mineral acids resulting into eluates which can be directly fed to electroplating bath after minor composition make-up. This process results into solutions containing slight excess of sodium ions which can be minimized by incorporating a de-ionizer.

A voluminous work has been reported by several authors using different adsorbents such as treated and

*For correspondence E-mail: salt@cscsmcri.rcn.nic.in; Fax: 0278-566970

untreated bituminous coal 1, blast furnace flyash2, supported liquid membranes contmmng trioctylamine3, seed and seedshell of magiteraindica4, congo red attached to poly(ethylene glycol dimethacrylate-hydroxyethy I methacrylate )5, iminodiacetic acid and thiourea based resins6 etc., for the removal of metal ions from waste water. Recently, the removal of various metal ions such as copper and nickel by using polymethylmethacrylate microbeads carrying ethylenediamine7, chelate polymers and ion-exchange resins8, polymer immobilized rhizopus oryzae9, biopolymer chitin 10, low grade phosphate mineral surface 11 , and saw dust12 etc. has been studied.

This paper describes the synthesis and characterization of methacryl ic acid -co-N,N'-methylene bis acrylamide sorbent and their application for the removal of nickel and copper ions from waste water.

Experimental Procedure Materials

Methacry lic acid, N,N'-methylene-bis-acrylamide, benzoyl peroxide, hydrochloric acid, n-heptane, starch, sodium hydroxide, sodium chloride, Fast Green FCF dye, toluene, cyclohexane, nickel ammonium sulphate, and cupric chloride were used as received. Benzoyl peroxide (BPO) was recrystallized from methanol-chloroform mixture ( 1: I) .

Synthesis of porous copolymer The cross-linked copolymers were synthesized by

suspension polymerization accord ing to the reported method 13• In a three neck round bottom flask

372 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2001

equipped with a stirrer, a thermometer and a reflux condenser, was taken a suspension medium composed of saturated solution of sodium chloride containing 2% (w/v) starch as suspension stabilizer. Then, a mixture of methacrylic acid, N,N'-methylene bis acrylamide, n-heptane and benzoyl peroxide (I % of the monomer mixture) was added in required proportion. The ratio of the reaction mixture (monomer plus diluent) to suspension medium was kept at I :4 (w/v). Copolymerization was initially carried out at 80°C for 2 h and then at 1 00°C for 2 h. The copolymer beads thus obtained were washed with hot water several times to remove any adhering stabilizer and then air-dried for 16 h. They were extracted in a soxhelet apparatus with acetone to remove any unreacted monomers and other trapped

Table !- Preparation conditions of methacrylic acid (MAA)-co-N,N'-methylene bis acylamide(N,N'MBA) copolymer

Polymer MAA N,N'MBA 11-Heptane Yield

matrix (g) (g) (g) (%)

I.

2.

3.

4.

5.

6.

7.

8.

9.

10.

II.

12.

66.50

76.00

88.50

95.00

64.75

74.00

83.25

92.50

63.00

72.00

8 1.00

90.00

3.50

4.00

4.50

5.00

5.25

6.00

6.75

7 .50

7.00

8.00

9 .00

10.00

30.00

20.00

10.00

0.00

30.00

20.00

10.00

0.00

30.00

20.00

10.00

0 .00

98.28

98.56

99.32

98.64

98.36

99.23

98.86

98.67

98.21

98.96

98.76

98.75

compounds, and air dried. A sieved fraction between -18, +52 BSS mesh size was selected for further studies. Following this procedure, polymeric sorbents having varying degree of cross-linking and Fm value [Fm value is the ratio of the total quantity of the monomer mixture to the total quantity of monomers plus diluent (w/v)] were synthesized. The polymeric sorbents synthesised were designated as follows:

(1) CPR4-5-0.7-nH (7) CPR4-7.5-0.9-nH

(2) CP~-5-0.8-nH (8) CP~-7 .5-1 .0-GEL

(3) CP~-5-0.9-nH (9) CP~-10-0.7-nH

(4) CP~-5-1.0-GEL (10) CPR4-l 0-0.8-n H (5) CPR4-7.5-0.7-nH (II ) CPR4-l 0-0.9-nH

(6) CP~-7 .5-0.8-nH (12) CPR4- l 0- 1 .0-GEL

In this nomenclature the fi rst numerical indicates the percentage of cross-linking monomer, the second numerical indicates the Fm value, and nH stands for the diluent n-heptane. The letters GEL represent conventional gel-type matrix synthesized in the absence of diluent. The copolymers were evaluated for surface area by p-nitrophenol adsorption method 14, porosity by Fast Green FCF dye uptake 15, bulk density, ion-exchange capacity, solvent uptake and the results are tabulated in Tables 1 and 2. The data on the uptake of metal IOns by the sorbents are presented in Tables 3 and 4. The effect of various parameters such as pH, dissolved salt concentration, time variation and column operations on the sorption of metals was studied for CPP4-10-J.O (GEL) because, it has shown good ion exchange capacity,

Table 2- Characterization of polymeric adsorbents

Po lymer IEC MR BD FGdye Solvent u12take (mU g) S.A matrix (meq/g) (%) (g/mL) uptake(mg/g) Toluene Cyclohexane (m2/g)

I. 11.54 44.29 0.4245 27.54 0.598 1 0.4986 27.54

2. 11.1 9 57. 16 0.4510 62.76 0.31 20 0.2931 28.59

3. 10.75 63.2 1 0.4545 84.51 0.2782 0.2732 28.59

4. 11.08 51.57 0.4959 31.73 0 .2667 0.1668 27.09

5. 11 .09 41.49 0.5129 25.7 1 0.5105 0.4786 27.84

6. 10.83 50.49 0.5634 58.51 0. 1683 0.1567 28.59

7. 10.43 53.30 0 .5829 78.80 0. 1052 0.0992 28.59

8. 11.09 58.10 0.5882 22.54 0.0755 0.0654 27.09

9. 11 .82 49.82 0.4286 17.40 0.3 120 0.3326 28.5 1

10. 10.27 63.89 0.5042 29.94 0.1582 0.1485 54.17

11. 10. 15 54.35 0 .5042 45. 17 0. 1235 0.1123 54.17

12. 11.99 60.22 0.5480 2 1.1 1 O.Q75 1 0 .0594 41.38

IEC: !on-exchange capaci ty; MR: moisture retention; BD: bulk d!!nsity; FG dye: f-ast Green dye; SA: surface area

PRASAD et al.: CROSS-LINKED POROUS COPOLYMERS 373

Table 3-Effect of concentration of copper in solution on its uptake (mg/g).

Polymer matrix

Concentration of Cu(Il) ion solution (M) 0.01 22

20 66 34

28 20 44

66

82 82 24 76

0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 144

142 170 172

176 178

106 206 164 210

160 224

0.1 142

140 170 ISO

178

168 106

I.

2.

3.

4.

5.

6.

7.

8.

9.

10. II.

12.

48 58 76 120 134 134 146

45 62 89 125 138 145 145 88 134 152 176 ISO 174 172

~ % I~ 1% 172 I~ 56 48 62 94 132

120

68 112

98

66 76 130

136 128 106 140

134

98 84 174

186 140 188

148

120 108

138

192 150 230

170 164

11 8 202 138

182 148 248

ISO

170

122

154

200 128 230

168 146

118

154

190 144

226

230 176

232

Table 4-Effect of concentration of nickel in solution on its uptake (mg/g).

Polymer Concentration of Ni(II) ion solution (M) matrix 0.01 O.Q2 0.03 0.04 0.05 0.06 0.07 0.08 0.09

137 147 156 168

0.1

150

152

I. 26 26 55 76 80 89 120 137 2. 22 3. 36 4. 48 5. 34 6. 46 7. 46 8. 56 9. 82

48 78 76 43

58

68 74

62

75 86 88

65 74 76

68

74 76 79

90 92 110

106

70 82 100 120

119 124 134

128 10. II.

12. 54

68 66 84

82 102

98 127

high metal uptake and physical stability compared to others.

Spectral characterization Fr-IR spectra were obtained on a Bio-Rad FfS-

40Ff-IR spectrophotometer as KBr pellet.

Uptake of nickel and copper Static equil ibrium studies were carried out by

equilibrating separately 50 mL of metal io.n solutions { Ni(Il)/Cu(Il)} of predecided concentration (0.0 I M) with about 0.2 g of sorbent with occasional stirring at room temperature for 16 h. The concentration of metal ions was evaluated by EDT A method 16. The same procedure was followed for determining the effect of different parameters such as pH of equilibrating solution, concentration of metal ions in solution and kinetics of metal ion uptake. In the case

105 150

11 0 120 126 124

132 143 147

148

145

160

135 146 156 146 148

156 153

164

168

192

142 152 168 167

186 198 176 179

170 220

188

187 183

171

215

184

190 210 194

184

173 224

of studies involving the effect of dissolved salts in solution on the sorption of metal tons the concentration of metal ion was 0.1 M.

The uptake of nickel and copper ions was also studied under dynamic conditions by percolating I 00 ppm of spiked metal ion solutions through the resin bed at different service flow rates, till e ffluent showed leakage of metal ions of I mg/L. The adsorbed metal ions on the sorbent bed were eluted by passing I N hydrochloric acid for nickel and a mixture of 2 N sulphuric acid +2% sodium chloride for copper ions.

Results and Discussion Polymer synthesis

Crosslinked copolymers of methacrylic acid and N,N'-methylene bis acylamide were prepared by suspension polymerization using benzoylperoxide as

374 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2001

BPO l 80{ 1HJ lHJ

-CHz-r-CHl -

40

..

pH

Fig. 3-Effect of pH on the uptake of metal ions. , Cu(IJ): 0-0 Ni(Il): tl-tl

obtained under the experimental conditions. It is very clear from the data in Tables 1 and 2 that toluene and cyclohexane uptake decreases with increase in Fm value at the same cross-linking density and this value is minimum for gel polymers. For example, the cyclohexane uptake has been decreased from 0.4786 to 0.0654 when Fm value was increased from 0.7 to 1.0 at the cross-linking density of 7.5. The polymers prepared at lower Fm value have low bulk density. This was observed during earlier studies with porous polymers17. Fast Green FCF dye uptake increases with the increase in F m value at a particular cross-linking density and decreases with the increase in cross-linking density at a constant Fm value. It is well-known that sorption of dyes by porous polymers is a surface phenomenon and increased dye sorption is evidently due to increase in surface area CFm 0.7-0.9) which is more pronounced at higher cross-l inking density. Surface area measurements by p -nitrophenol adsorption reveal that there is a considerable increase in surface area in the case of 10% cross-linked polymers than those for 5 and 7.5% sorbents. At higher cross-linking density the surface area is contributed by inter-molecular distances as well.

Spectral characterisation Fig. 2A shows the IR spectrum of copolymer. The

broad band around 3486 is attributed to COOH and amide NH groups. A strong peak at 17 19 em·' is due to amide carbonyl stretchings of carboxylic acid and

PRASAD et al.: CROSS-LINKED POROUS COPOLYMERS 375

., 2.1 0 Ol 0

1.8

1.21-----,.......,---===,..--------.-:::--~ 2-8 3.2 3.7

tog Ce

Fig. 4-Freundlich isotherm plot for the absorption of metal ion. Cu(II): 0 - 0 Ni(II): !:l- !:l

2.0

r:::t 'QJ 1.8

r:::t

01 .9

1.6

1.4'"----~---~r----1:1----r-----,,.---

376 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2001

250 (8)

20

~ 150 a> E

llJ ::.::: ;'! 250 . Q :::>

(A)

150

400 600 CONCE NTRATION OF SALT (ppm)

Fig 7-Effect of dissolved salt concentration on Cu(II) uptake.(A) : Cu(II), (B) : Ni(Il), NaHC03: 0- 0 MgCI2 : 1'1.- 1'1. CaCI2 : *- * NaCI: 0 - 0 , Na2S04 : 0-0

sorbent 18. The experiments with solution pH as a variable were conducted to determine the optimum pH where maximum metal adsorption occurs and the results are shown in Fig. 3. Adsorption of metal ion was increased with increase in pH up to a certain value and thereafter decreased. The maximum adsorption took place around pH 7.94 for Nt2 and 4.68 for Cu+2 ions. The presence of C=O also plays a role in the sorption of metal ions due to the polar attractions between the metal ions and keto group 19•

F > C=O + M +2 ___. R C=O ···M+2

R ) C=O + M (OHt ___. R C=O · ·M(OHt

An increase in p H increases the negatively charged nature of the sorbent surface. This leads to an increase in the electrostatic attraction between positive substrate and negative sorbent and results in increase in the adsorption of metal ions. The decrease in removal of metal ions at lower pH is apparently due to the higher concentration of H+ ions present in the reaction mixture which compete with the metal ions for the sorption sites on the sorbent surface. Decrease in sorption at higher pH is due to the formation of insoluble hydroxy complexes of metal ions.

Sorption dynamics The data on the uptake of metal ions by the sorbent

has been plotted according to Freundlich isotherm equation,

LogQe= log Kt + lin log Ce

where Ce is the equilibrium concentration (mg/L), Qe is the amount adsorbed at equilibrium (mg/g), K1 and lin are Freundlich constants related to sorption capacity and intensity of adsorption. A linear relationship was observed between log Qe and log Ce from the plotted parameters indicating the applicability of Freundlich equation (Fig. 4). Freundlich constants K1 and 1/n were calculated as 1.862 and 0.6 for Ni(II) and 2.188 and 0.625 for Cu(II) ions. Values of l

PRASAD et al.: CROSS-LINKED POROUS COPOLYMERS 377

compared to those of NaCI, MgCI2, and Na2S04. This may be happening due to the precipitation of cu+2

ions at alkaline pH in Ca+2 and HC03. solutions. In the case of Ni(ll), the presence of dissolved salts at lower concentrations (200 to 400 ppm) do not affect the uptake of Ni(II) .

Column operations The performance of sorbent in continuous column

operations was studied by conducting column runs using about 25 mL bed volume of sorbate and an influent having I 00 ppm metal ion concentration. The boundary between the used and unused portions of the bed appears quite sharp as it moves down the column with progressive exhaustion of the bed. The capacity of sorbent for I ppm leakage at different influent flow rates such as 5 B.V/h., 10 B.V/h. and 15 B.V/h. were determined. As the flow rate increases the removal of metal ions decreases because of the lesser contact time with the sorbent.

Desorption The exhausted beds were stripped off by passing

1 N HCI and a mixture of 2 N H2S04+2% NaCI for Ni(II) and Cu(II) respectively. In the case of Ni(II) the metal ions were eluted with 75 mL eluting agent at the flow rate of I B.V /h. About I 00 mL of eluting agent was required to elute the Cu(ll) ions at the rate of 0.5 B.V/h.

Conclusions Ni(II) and Cu(II) at lower concentrations can be

removed from aqueous effluents by using methacrylic acid - N,N'-methylene bis acrylamide sorbent. The adsorption is mainly due to COOH and amide (C=ONH) groups. At lower service flow rates the sorbent can treat more bed volumes of metal ion solutions than at higher flow rates. The loaded Ni(II) and Cu(II) metal ions can be stripped off by mineral acids such as I N HCI and 2N H2S04+ 2% NaCI, respectively , and the bed can be reused for further

operations by regenerating with 1 N HCI and 1 N NaOH. The concentration ratios of nearly 1:60 and I :30 were achieved for Ni(II) and Cu(II) ions, respectively.

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