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Development of Novel Polymeric Flocculant Based onGrafted Sodium Alginate for the Treatment of CoalMine Wastewater

TRIDIB TRIPATHY,1 N. C. KARMAKAR,1,2 R. P. SINGH1

1 Materials Science Centre, IIT, Kharagpur, 721302, India

2 Indian School of Mines, Dhanbad, 826004, Bihar

Received 10 January 2000; accepted 29 November 2000Published online 30 July 2001; DOI 10.1002/app.1861

ABSTRACT: Sodium alginate-g-polyacrylamide (SAG) was synthesized by ceric ion in-duced redox polymerization technique. Six grades of graft copolymers were prepared byvarying as well as monomer concentrations. The graft copolymers were characterizedby intrinsic viscosity measurements, IR, and 13C-NMR spectroscopies. Of the abovegrades, the graft copolymer of grade six (SAG-VI), which has longer polyacrylamidechains, was used for flocculation study. Two coking and noncoking coal fine suspensionswere selected for the flocculation study. The flocculation performance of the graftcopolymer was compared with commercial flocculants. In all cases, it was found that thegraft copolymer showed better performance than the commercial flocculants. © 2001John Wiley & Sons, Inc. J Appl Polym Sci 82: 375–382, 2001

Key words: grafting; flocculation; sodium alginate; sodium alginate-g-polyacryl-amide; polyacrylamide; coking coal; noncoking coal

INTRODUCTION

Organic polymeric flocculants have found wideapplications wherever some form of solid–liquidseparation is involved. The organic flocculantsmay be natural or synthetic. Natural polymershave the advantage of being low cost, nontoxic,and biodegradable. The biodegradability of thenatural polymers acts as a drawback in that itreduces storage life as well as performance. Onthe other hand, synthetic polymers are highlyefficient and can be tailored to the needs of aparticular application, but often suffer from theirshear degradability.

Recently, a new class of flocculating agentsbased on graft copolymers of natural polysaccha-rides and synthetic polymers was developed inthe authors’ laboratory.1–5 These graft copoly-mers essentially combine the best properties ofboth initial components. Thus, the graft copoly-mers are biodegradable to some extent because ofthe presence of polysaccharide backbone and werealso found to be reasonably shear stable becauseof the attachment of flexible synthetic polymersonto rigid polysaccharide backbones.

The study of the flocculation performance ofgraft copolymers of polyacrylamide onto variouspolysaccharides, such as guargum, xanthangum,starch, amylose, and amylopectin, proved thateach of graft copolymers is a better flocculatingagent than polysaccharide and polyacrylamide,respectively. On the basis of the above studies, amodel was proposed by Singh6 that the betterperformance of the graft copolymers compared to

Correspondence to: R. P. Singh.Contract grant sponsor: CSIR New Delhi.

Journal of Applied Polymer Science, Vol. 82, 375–382 (2001)© 2001 John Wiley & Sons, Inc.

375

linear polymers is due to the better approachabil-ity of the branched flexible chains grafted ontorigid polysaccharide backbones to the colloidalparticles. Of the above polysaccharides, it hasbeen found that the performance of amylopectin-g-PAM is the best.7 The largest drawback of amy-lopectin is that it is not readily water soluble andis also costly. In the present investigation, sodiumalginate was chosen for its easy water solubilitybecause of the presence of OCOO2Na1 groupsand its extended structure in aqueous solution.8

Other advantages of sodium alginate include lowcost and easy availability.

For the treatment of coal mines’ wastewater,various kinds of flocculating agents are widely inuse. Recently, some applications of graft copoly-mers for the treatment of coal mine wastewaterwere reported.9–11 It was therefore planned tosynthesize various grades of graft copolymers ofpolyacrylamide onto sodium alginate and to com-pare their performance with respect to some ofthe commercial flocculants for the treatment ofsuspensions containing coal fines.

With respect to coal, literature mostly dealswith the treatment of effluent generated fromwashing of coking coals. Washing of noncokingcoals is a rather recent practice. In the presentinvestigation, emphasis is given to both cokingand noncoking coal fines with respect to the na-ture of suspension and their flocculation. Thepresent article reports synthesis and character-ization of various grades of sodium alginate-g-

polyacrylamide (SAG-g-PAM) and investigationof their performance as flocculants in both cokingand noncoking coal suspensions. A comparison oftheir performance with some commercially avail-able flocculants was also undertaken.

EXPERIMENTAL

Materials

The chemicals used for synthesis are given inTable I. The commercial flocculants used for theflocculation study are given in Table II.

The coal samples, their source along with theircharacteristics, are shown in Table IV. The com-mercial flocculants are polyacrylamide based andtheir structures are totally classified by theirmanufacturers.

Synthesis

The graft copolymers were synthesized by usingceric ion induced12 aqueous polymerization tech-nique. The synthetic details are published else-where.4 The results are shown in Table III.

Characterization of Graft Copolymers

Graft copolymers are characterized by intrinsicviscosity measurement, IR, and NMR spectros-copies. The results were given in our previousarticle.13 It was established by these techniques

Table I Chemicals for Synthesis

Serial No. Material Source

1 Sodium alginate Aldrich Chemicals, USA2 Ceric ammonium nitrate Loba Chemie, Mumbai, India3 Hydroquinone (analar grade) S.D. Fine Chemicals Ltd., Mumbai, India4 Sodium nitrate and acetone E. Merck Ltd., Mumbai, India5 Acrylamide E. Merck, Germany

Table II Characteristics of Commercial Flocculants

Serial No. Flocculants Source Nature Mol. Weight

1 Aquaset AS510 Aquapharm Chemicals Ltd., India Anionic —2 Separan AP273 Dow Chemicals, USA Anionic —3 Superfloc N300 Cyanamide Co., USA Nonionic 4.5 3 106

4 Sufloc Suyog Chemicals Ltd., India Nonionic —5 Magnafloc 1011 Allied Chemicals, UK Anionic 5 3 106

376 TRIPATHY, KARMAKAR, AND SINGH

that there is very little homopolymerization andthe grafting took place on the sodium alginatebackbone.

Characterization of Coal Samples

The coal samples were characterized by the fol-lowing techniques.

Proximate Analysis of Coal Samples

Proximate analysis of coal samples was done asper the method given by the Bureau of IndianStandards (1S: 1350, 1959). For each analysis, 1 gof coal sample of 272 mesh was taken. Moisturewas determined by heating the sample in an ovenat 105°C for 1 h. Volatile matter was determinedby heating the sample at 925 6 10°C for 7 min.Moisture content was subtracted to get the vola-tile matter content of the coal. Ash was deter-mined by heating the sample at 850 6 10°C for1 h. An average of three readings was taken ineach case. The results are shown in Table IV.

Measurement of Zeta Potential

Zeta potential was measured by particle micro-electrophoresis (Apparatus mark II) made in En-gland. The results are shown in Table IV.

Flocculation Study

In our previous study,4 it was found that graftcopolymers having longer polyacrylamide chainsshow better performance in flocculation. Here,SAG-VI, which has longer polyacrylamide chains,is taken for flocculation studies in 1 wt % of 2200mesh coal fine suspension. The standard jar testtechnique was followed for the flocculation study.

Jar Test

Flocculation jar tests were carried out on stan-dard flocculation jar apparatus supplied by M. B.Flocculators, India. Flocculation procedure wasfollowed in the line of Bratby.14 In 1-L flocculationjars, 400 mL of the flocculating suspension was

Table III Synthetic Details of the Graft Copolymers

PolymerPolysaccharide

(g)Acrylamide

(mol)

Amount ofCAN

(mol 3 1024)%

Conversiona

IntrinsicViscosity(dL/gm) Mw 3 106 Mn 3 106

SAG-I 2.5 0.12 1.003 83.76 6.75 1.93 1.19SAG-II 2.5 0.12 2.006 84.58 6.63 1.89 1.15SAG-III 2.5 0.12 3.009 85.88 6.00 1.67 0.99SAG-IV 2.5 0.12 5.015 92.35 5.15 1.37 0.78SAG-V 2.5 0.24 2.006 93.45 7.82 2.32 1.48SAG-VI 2.5 0.30 2.006 95.56 8.20 2.47 1.59

a % of conversion was calculated from the following equation: % Conversion 5 [(wt. of grafted product 2 wt. of polysaccharide)/wt. of acrylamide] 3 100.

Table IV Characteristics of Coal Fines

No. Name Source Coal TypeNatural

pH%M

%Ash

%VM

%FC

ZetaPotential

1. Jamadoba 14 Seam, Jamadoba Colliery,Jhanjra Coal Field,Jharkhand, India

Prime Coking 6.5 1.0 17.5 24.9 56.6 219.4

2. W. Bokaro-5 5 Seam, W. Bokaro Colliery,W. Bokaro Coal Field,Jharkhand, India

Semicoking 6.5 0.2 32.6 18 49.2 219.4

3. Jagannath 2 Seam, Jagannath Colliery,Talchar Coal Field, Orissa,India

Noncoking 6.0 2.6 22.3 34.6 40.5 230.5

4. Lajkura Lajkura Seam, Orient-3Colliery Mahanadi CoalField, Orissa, India

Noncoking 6.3 5.7 35.1 26.5 32.7 247.8

GRAFTED SODIUM ALGINATE WASTEWATER TREATMENT 377

taken. The jars were placed on the flocculatorbench by dipping the stirrer blades in the suspen-sion. Dilute polymer solution was added to eachjar under slow stirring. Immediately after theaddition of polymer to all the jars, the suspen-sions were stirred at a constant high speed of 75rpm for 2 min followed by slow stirring at 25 rpmfor 5 min. The flocs developed during slow stirringwere allowed to settle down for 15 min. Cleansupernatant liquid was drawn and its turbiditywas measured by a digital Nephelo turbiditime-ter. The turbidity was expressed in Nephelo tur-bidity units (NTU). Turbidity versus polymer dos-age graph was then plotted.

RESULTS AND DISCUSSION

Synthesis and Intrinsic Viscosity

Table III shows the synthetic details of graft co-polymers based on sodium alginate. A series of sixgraft copolymers was synthesized with sodiumalginate. In the case of the first four graft copol-ymers (SAG-I to SAG-IV), the catalyst concentra-tion was varied, keeping the concentrations ofacrylamide and sodium alginate fixed. In the sec-ond set of two graft copolymers (SAG-V to SAG-VI), only acrylamide concentration is varied,

keeping the other parameters constant. Themechanism of ceric ion action involves the forma-tion of a chelate complex that decomposes to gen-erate free-radical sites on the polysaccharidebackbone. These active free-radical sites in thepresence of acrylic monomers generate graft co-polymers. A low concentration of catalyst shouldinitiate few grafting sites, resulting in longerpolyacrylamide chains; oppositely, a high concen-tration of catalyst will initiate a larger number ofgrafting sites, thus making the average polyacryl-amide chains shorter for the same acrylamideconcentration.1,3,13 This is reflected in the graftcopolymers in series.

It is known that the intrinsic viscosity of apolymer is a measure of its hydrodynamic volumein solution, which in turn is a function of thepolymer molecular weight, its structure, nature ofsolvent, and the temperature of the medium.Keeping the others factors constant, for two poly-mers of approximately similar molecular weight,a branched polymer will have lower hydrody-namic volume and hence a lower intrinsic viscos-ity than its linear counterpart. Further, along aseries of branched polymers, the longer thebranches, the higher intrinsic viscosity will beand vice versa. This was observed in practice.Thus, SAG-I with 1.003 3 1024 mol of ceric am-

Figure 1 Comparison of flocculation characteristics of SAG-g-PAM with five othercommercial flocculants in the Jamadoba coal fine suspension.


monium nitrate (CAN) has an intrinsic viscosityof 6.75 compared to SAG-IV with 5.015 3 1024

mol of CAN, which has an intrinsic viscosity of5.15. Similarly, a variation of the monomer con-centration, keeping the catalyst concentrationfixed in the second series of two graft copolymers,produces variation in the intrinsic viscosities.

Calculation of the Approximate Molecular Weight

Molecular weight of polymer samples can be esti-mated from the intrinsic viscosity [h] values.Mark–Houwink equation, [h] 5 KMa, is generallyemployed for the estimation of molecular weightof linear polymers, where K and a are constantsfor a given polymer/solvent/temperature system.For polyacrylamide, the values of K and a aregiven as

@h# 5 6.8 3 1024~Mn!0.66 (1)

@h# 5 6.31 3 1025~Mw!0.80 (2)

Graft copolymers are synthesized by opening therings of polysaccharide mers on the backbone andgrafting of polyacrylamide onto the free radicals

so generated. This opening imparts slight flexibil-ity to the backbone. Moreover, the percentage ofpolysaccharide is small in comparison with thepolyacrylamide. Hence, in case of the graftedpolysaccharides, several workers2,15–17 have usedthe Mark–Houwink equation to estimate approx-imate molecular weight, which is applicable forlinear polymers. The same was done in thepresent case. The approximate molecular weightsof the graft copolymers are given in Table III.

Proof of Grafting

Grafting was also supported by IR and 13C-NMRspectroscopies, as reported in our previous arti-cle.13 IR spectrum of sodium alginate shows abroad peak at 3450 cm21 for OOH groups, twopeaks, one at 1618 cm21, and the other peaks at1440 cm21 for the COO2 groups. One sharp peakappears at 1050 cm21, which is for the OCOOgroup, but in the graft copolymer apart from theabove peaks, four additional peaks are present.Two peaks at 3300 and 3100 cm21 are for ONH2groups. The peak at 1685 cm21 is due to theamide carbonyl group and the peak at 1400 cm21

is due to the CON groups. Because acrylamide

Figure 2 Comparison of flocculation characteristics of SAG-g-PAM with five othercommercial flocculants in the West Bokaro-5 coal fine suspension.


and polyacrylamide were removed, the presenceof peaks at 3300, 3100, 1685, and 1400 cm21 inthe graft copolymer is strong evidence for graft-ing.

13C-NMR spectrum of graft copolymer showsfour characteristic peaks at d183, d105, d75, andd45 ppm, which are for the carboxyl carbon, ano-meric carbon, carbon bearing the secondary hy-droxyl groups, andOCH2OCHO groups that areformed during polymerization, respectively. 13C-NMR spectrum of acrylamide shows two peaks atd130 and d133 ppm, which are for two sp2 hybrid-ized carbon atoms Those peaks are absent in graftcopolymer. Moreover, one additional peak ispresent in graft copolymer at d45 ppm. Neithersodium alginate nor acrylamide has this peak.Therefore, absence of peak at d130 and d133 ppmand presence of peak at d45 ppm are clear evi-dence of the grafted polyacrylamide chains ontothe backbone of sodium alginate.

Flocculation Study

The efficiency of SAG-g-PAM was tested in 1 wt %coal suspension. Two samples each of coking andnoncoking coals were taken for the flocculationstudy. The results are shown in Figures 1-4. Ineach case, the turbidity of supernatant liquid af-ter flocculation was plotted against the polymerconcentration. In Figure 1, SAG-g-PAM (SAG-VI)was compared with five commercially availableflocculants in coking coal suspension obtainedfrom the Jamadoba coal mine, whereas it wascompared with these same flocculants in anothercoking coal obtained from West Bokaro-5 coalmine, as shown in Figure 2. Two noncoking coalswere taken for flocculation study, one from theJagannath coal mine and another from the La-jkura coal mine. The results were shown in Fig-ures 3 and 4, respectively. From the flocculationstudy, it was found that SAG-g-PAM showed bet-ter performance than the commercial flocculants,except in Lajkura coal, where it showed betterperformance in higher flocculant doses only. In allcases, the performance of a particular flocculantwas expressed in terms of turbidity. The lower theturbidity, the better its performance will be. Thereason for better performance of graft copolymerwill be as follows.

The major mechanisms of flocculation of poly-electrolytes are surface-charge neutralization andbridging. Surface-charge neutralization occurs ifthe charge of flocculant is opposite in sign to thatof the suspended particles. Addition of such a

polymer to the suspension will result in aggrega-tion caused by specific ion absorption. For neutralflocculants, the major mechanism of flocculationis the polymer bridging. When very long-chainpolymer molecules are absorbed on the surface ofparticles, they tend to form loops that extendsome distance from the surface into the aqueousphase, and their ends may also dangle. Theseloops and ends may come into contact with, andattach to, another particle, forming a bridge be-tween the two particles. This is the bridging modeof flocculation.14 Here, charge of the particles/andor polymer does not play any important role. Es-sentially, polymer bridging occurs because seg-ments of a polymer chain get absorbed on variousparticles, thus linking the particles together. Foreffective bridging to occur, there must be suffi-cient polymeric chain lengths, which extend farenough from the particle surface to attach toother particles. In the case of linear polymers, thepolymer segments attached to the surface intrains, projected into the solution as tails, orformed a part of loops, which linked the traintogether. In this way, they can form bridges be-

Figure 3 Comparison of flocculation characteristicsof SAG-g-PAM with four other commercial flocculantsin the Jagannath coal fine suspension.


tween the colloidal particles to form flocs.18 Thelinear polymers also get coiled in the suspension.The graft copolymer, due to the better approach-ability of the dangling grafted chains onto therigid backbone, can easily bind the colloidal par-ticles through bridging to form flocs. This type ofintense bridging is not possible in the case oflinear polymers. Hence, for graft copolymers,bridging will be better and easier than that oflinear polymers. The commercial flocculants arepolyacrylamide-based linear polymer, hence theirperformance is inferior to the graft copolymers.

The results of flocculation indicate a wide dif-ference in the behavior of coking and noncokingcoals. The blank NTU of two coking coals are 20NTU for Jamadoba coal and 9 NTU for WestBokaro-5 coal, respectively, where it is 670 and720 NTU for two noncoking coals, Jagannath andLajkura, respectively. The wide difference in thebehavior of the coking and noncoking coals cannotbe explained from the particle size and specificgravity of coals. The coking coals, in general, areof higher grade than the noncoking coals, which isthe higher fixed carbon values (Table III). They

are much less oxidized than the noncoking coals.Thus, the noncoking coals are highly hydrophilicbut the coking coals are to some extent hydropho-bic. From literature,19 it is well known that withthe increase in total acidity, which is a measure ofthe carboxylic and phenolic group content, appar-ent contact angle decreases. Because of the hydro-phobic nature of coking coal, the particles have anatural tendency to agglomerate. The nephelo-metric turbidimeter, which is based on the Tyn-dall effect of light, measures the scattered light bythe surface of the particles. Hence, the greater thesurface area per higher unit mass, the greater theturbidity. The agglomeration of small particleswould reduce the effective scattering surfacearea. Thus, coking coal fines give lower turbiditythan a noncoking coal of similar concentration.

CONCLUSION

SAG-g-PAM was synthesized by ceric ion inducedredox polymerization technique in aqueous solu-tion. Variation of the synthetic parameters re-sults in a series of graft copolymers with variationin the number and length of polyacrylamidechains that result in different intrinsic viscositiesas well as different molecular weights. Study ofIR spectra of the graft copolymers after extractingthe homopolymer provides strong proof of graft-ing. 13C-NMR spectra also support the formationof graft copolymers. From the flocculation study,it can be concluded that the graft copolymershowed better performance in both coking andnoncoking coal suspensions. The flocculation effi-ciency of graft copolymer is better in coking coalsuspensions than noncoking coal suspensions.This is attributed to the hydrophobic nature ofthe surfaces of the former. Hence, by graftingpolyacrylamide chains onto sodium alginate, aneffective flocculating agent can be developed forthe treatment of coal mines’ waste water.

The authors are grateful to CSIR New Delhi for thefinancial support to carry out this research.

REFERENCES

1. Rath, S. K.; Singh, R. P. J Appl Polym Sci 1997, 66,1721.

2. Karmakar, G. P.; Singh, R. P. Colloids Surf, A 1998,113, 119.

Figure 4 Comparison of flocculation characteristicsof SAG-g-PAM with four other commercial flocculantsin the Lajkura coal fine suspension.


3. Rath, S. K.; Singh, R. P. Colloids Surf, A 1998, 139,129.

4. Tripathy, T.; Pandey, S. R.; Karmakar, N. C.; Bha-gat, R. P.; Singh, R. P. Eur Polym J 1999, 35, 2057.

5. Tripathy, T.; Singh, R. P. Eur Polym J 200, 36,1471.

6. Singh, R. P. Advanced Turbulent Drag Reducingand Flocculating Materials Based on Polysaccha-rides, Polymers and Other Advanced Materials:Emerging Technologies and Business Opportuni-ties; Prasad, P. N.; Mark, E.; Fai, T. J., Eds.; Ple-num Press: New York, 1995; p 227.

7. Singh, R. P.; Karmakar, G. P.; Rath, S. K.; Kar-makar, N. C.; Tripathy, T.; Pandey, S. R.; Kannan,K.; Jain, S. K.; Lan, N. T. Polym Eng Sci 2000, 40,46.

8. Cottrell, I. W.; Covas, P.; Alginates; In Handbookof Water Soluble Gums and Resins, Davidson,R. L., Ed.; McGraw-Hill: New York, 1980; Chap-ter 2.

9. Karmakar, N. C.; Rath, S. K.; Sastry, B. S.; Singh,R. P. J Appl Polym Sci 1998, 70, 2619.

10. Karmakar, N. C.; Sastry, B. S.; Singh, R. P. IXInternational Conf. of Coal Science (ICCS); Essen,Germany, 1997.

11. Karmakar, N. C.; Sastry, B. S.; Singh, R. P. IUPACInternational Symposium on Advanced PolymerScience Technology, Srinivasan, K. S. P., Ed.; Macro-98; Allied Publishers: New Delhi, 1998; p 114.

12. Mino, G.; Kaizerman, S. J Polym Sci 1958, 31, 242.13. Tripathy, T.; Singh, R. P. J Appl Polym Sci 2001,

81, 3296.14. Bratby, J. Flocculation and Coagulation; Uplands

Press Ltd.: Croydon, England, 1980.15. Fanta, G. F.; Burr, R. C.; Doane, W. M.; Russel,

C. R. J Appl Polym Sci 1966, 10, 929.16. Madelson, M. A. Polym Eng Sci 1969, 9, 350.17. Tripathy, T.; Karmakar, N. C.; Singh, R. P. Int J

Polym Mater 2000, 46, 81.18. Gregory, J. Industrial Water Soluble Polymers;

Finch, C. A., Ed.; The Royal Society of Chemistry:Cambridge, U.K., 1996; p 63.

19. Palmes, J. R.; Laskowski, J. S. Minerals MetallProcess 1993, 10, 218.


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