Preparation,characterization and salt-resistance of a coal based super absorbent composite

railableonlineatwww.sciencedirect.comMINING° Science DirectSCIENCE ANDTECHNOLOGYMining Science and Technology 20(2010)0864-0871ww.elsevier. com/locate/jcumtPreparation, characterization and salt-resistance ofa coal based super absorbent compositeCHU Mo, HUANG Zhanbin, XU Bang, FANG Tao, DUAN HuiSchool of Chemical and Environmental Engineering, China University of Mining Technology, Beijing 100083, ChinaAbstract: Potassium humate was extracted from brown coal. A novel super absorbent composite, poly(acrylic acid-co-acrylamide)/potassium humate(PAA-AM/KHA), was prepared by graft polymerization of acrylic acid, acrylamide and coal based potas-sium humate using N, N,-methylenebisacrylamide as a crosslinker and potassium peroxydisulfate as an initiator. The effects ofeaction temperature, degree of neutralization of the poly(acrylic acid) and the amounts of crosslinker, initiator and potassium hu-mate were investigated. Salt resistance tests were also carried out. The composite prepared under optimal conditions had a potas-um humate content of 10% and exhibited a water absorption of 770 g/g in distilled water, and 349. 286 and 41 g/g in 0.5 mollKCl, MgCl and AlCl solutions respectively. The results indicate that the salt resistance of PAA-AM/KHA was superior to that ofly(acrylic acid-co-acrylamide)because of the collaborative effect of functional groups of the coal based potassium humate. ThePAA-AM/KHA micro powder was characterized by IR spectroscopy and the micrographic surface was characterized by scanninglectron microscopy. Introduction of potassium humate into the poly(acrylic acid-co-acrylamide) structure creates a compositemore suitable for use as a water-managing material in the renewal of arid and desert environments. The salt resisting property of thecomposite is improved, production costs are reduced and the growth stimulant effect is still present.Keywords: brown coal; coal based potassium humate; superabsorbent; salt-resistant property1 Introductionhave been many reports about introducing inorganicclays, such as kaolin, montmorillonite or attapulgite,Super absorbents are hydrophilic crosslinked into pure polymeric super absorbents to improve thepolymers that can imbibe huge amounts of water. swelling properties, in saline solutions and to reduceWorkers at the U.s. Department of Agricultureproduction costs 3-51ported the first super absorbent polymer. Since then Brown coal is a kind of soft, brownish-black coalsuper absorbents have been widely used in many in an early stage of maturity, which not only can befields such as sanitary goods, sealing composites and used for energy generation but also is an importantmedical drug-delivery systems 2. Recent research onsource of chemical raw materials. Potassium humatethe use of super absorbents as water managing mate- can be extracted from brown coal at a very low costrials to renew arid and desert environments has atCoal based potassium humate contains a large num-tracted great attention. Here the water absorbency of ber of hydrophilic functional groups such as car-the super absorbents in saline solution is critically boxylate and phenolate, just as seen in the brownimportant. However, applicationsis field hacoal(6-7. The molecular structure of potassium humateencountered some problems. Because the super ab- causes it to have numerous benefits for crop producsorbent is formed from pure poly (acrylic acid)or tion. Potassium humate helps break up clay andpoly(acrylamide)it is too sensitive to ions and not compacted soils, assists in transferring micro-nutri-suitable for use in saline containing water in soils. ents from the soil to the plant, enhances water reten-Also, it is too expensive for large scale use in agri- tion, increases seed germination rates, penetration andcultural and horticultural situations. Consequently, stimulates the development of micro-flora in the soilresearch on preparing salt resistant and cost efficient Also, it has the ability to act as a cation exchangesuper absorbents has been of wide interest. There agent to form chelates with metallic micro-nutrientssuch中国煤化工 m. The chemical16 January 2010. accepted 19 May 2010ing author. Tel: 86 10 62331863(acrylCNMHGallows the fabrcation of a novel super absorbent composite with sig016s16745264(096029740nificantly improved salt-resistant properties, reducedproduction costs and growth stimulant properties. TheTable 1 Analytical results: brown coalauthors here report the synthesis of coal based potas- M My A, Vu FCA Std Co Has Na O. HAsium humate super absorbent composites and thei462718.1823.65397736580.2849432371.1923.095957properties. More details will be discussed in followng publicationsTable 2 Analytical results: KHAMmol/g)2 ExperimentalTotal acidCarboxylic Hydroxyl Exchange Coagulationcapacityvalue2.1 Materials4.4262.22l1Acrylic Acid (AA, chemically pure, Beijing Yili 2.3 Preparation of poly(acrylic acid-co-acrylamide)/potassium humatepressure before use. Potassium Peroxydisulfate(KPs,alyticalade, supplied by Beijing Chemical Fac-A series of poly(acrylic acid-co-adN'-methylenebisan recrystallized from water. N, tassium humate(PAA-AM/KHA) composites weretory, China)wasynthesized. The reactions were conducted in a flaskShanghai Chemical Reagent Corp, China)was used equipped with a mechanical stirrer and condenser. Aas purchased. The other agents used were all analyti- weighed quantity of Acrylic Acid (AA)was dissolvedcal-grade. All solutions were prepared using distilled in distilled water. The AA was neutralized with potas-watersium hydroxide solution (30% by weight)andThe brown coal sample was taken from the coal- Acrylamide(AM)was added to the partially neutralfield near Huolinhe located in the northeast part of ized AA solution. KHa powder was then dispersedthe Inner mongolia autonomous region, Chinainto this mixture. A cross-linking agent, N, N,-me-thylenebisacrylamide(MBA), was then added to the2.2 Preparation of coal based potassium humateperoxydisulfate(KPS)The extracting reaction was conducted in a beakerto the reaction mixture and the flask was heated in aBrown coal was ground into small grains(<3 mm).A water bath with vigorous stirring. After 5 h the re-weighed quantity of the coal grains and KOH solu- sulting mixture was washed several times with dis-tion(15% by weight)were mixed at room temperatilled water and then dried in a vacuum oven to conture. After 2 h the mixture was removed from the stant weight, at 60C. The super absorbent compositebeaker and separated by filtration. The liquid product was obtained by milling and screening this driedwas dried in an oven. Coal based potassium humate product. All samples used had a particle size in the(KHA)was prepared by milling and screening this range of 40-60 mesh. A typical recipe for preparing adried product. Results from analysis of the brown super absorbent composite is shown in Table 3coal and the coal based Kha are shown in Tables 1by weight) and 2Table 3 Typical conditions for preparing PAA-AM/KHABA): n(AA)(mol/mol) n(KPS): n(AA)(mol/mol) n( KOH): n(AA)(mol/mol) n(AM): n(AA)(mol/mol) w(KHA): w((AA+AM)(%)0.0045Note: The reaction temperature is 80C, and the initial concentration(by weight)of AA(CAA)35%2.4 Preparation of poly (acrylic acid-co-acryla- where mi is the weight of the dry sample and m2 theThe preparation procedure for preparation of poly as grams of water per gram of dry sample. egweight of the water-swollen sample. Q is calculate(acrylic acid-co-acrylamide)(PAA-AM) was similarferent samples were examined and the results aver-to that for the PAA-AM/KHa super absorbent com- agedposite except that no KHa was used.The salt-resistance of the samples wasbyinvestigating their absorbency in various salt con-2.5 Measurement of water absorbencytaining solutions(KClag): Mg Clz(a); AlCl3(aq)) at dif-A weighed quantity (0.05+0.001 g)of the dry sam-ferent concentrationswas immersed in distilled water at room temperature until equilibrium swelling was established. The3 Results and discussionswollen sample was then separated from the unab-sorbed water by filtering through a 100-mesh screen3.1中国煤化工The absorbency of the super absorbents, @, was cal-C N MH Extremely impor-culated bytant part in the water absorbency of various super2(gH. o/sample)=(mr-mi) /mIabsorbents. The effect of the initiator concentrationMining Science and TechnologyVol, 20 No 6(molar ratio of KPS to AA)on water absorbency of At low persulphate ion concentrations fewerthe resulting PAA-AM/KHA is shown in Fig. 1. Thewater absorbency increases as n(KPS): n(AA)increases from 0.001 to 0.009 mol/mol and then deKHAor KHAradicals are producedcreases with further increases in KPS concentration. Then the PAA-AM/KHA polymeric structure cannotThe composite synthesized using 0.009 mol/mol had be formed efficiently and that results in a decrease inthe highest water absorbencywater absorbency. At a high concentration of persul-phate ions the large number of free radicals producedmay cause chain transfer to KHA This would in-troduce more KHa into the polymer structure and theresulting super absorbent composite would have ahigher crosslink density, which decreases the waterabsorbency. The results seen here are consistent withthis interpr00150.0203.2 Effect of potassium humaten(KPS) m(AA)(mol'mPotassium humate is technically known as a lowFig. 1 Effect of initiator level on Q of the resultinrank coal classified between peat and sub-bituminousled water): reaction temperature 800.00045 mol/mol; n (KOH): n (AA), 0.65It is thought to consist of complex aromatic macro-mol/mol: CAA, 35%: weight ratio of KHA, 10%6molecules and to contain free and bound phenolic OHgroups, quinone structures, nitrogen and oxygeWhen heated aqueous persulphate ions(S 0 2 bridges, and -COOH,-COOK and-NH2 groups vari-decompose to sulphate radical ions(SO42) 8. These ously located on aromatic rings"o1.primary radicals react with the monomer(AA or AM)to form free radicals that then propagate a chain reaction to form, after termination, the high molar mass600polymer. It was assumed that phenolic hydroxygroups on the KHa might react with K2S2Og in asimilar way to generate free radicals on the potassiumhumate structure from which graft polymerizationwould take place. This would create PAA-AM50510152025303540branches on the potassium humate backbone andform the PAA-AM/KHA composite. A possible reacFig. 2 Effect of [KHA]on @( distilled water): reactiontion scheme can be formulated asnature, 80C; n(KPS): n (AA), 0.009 mol/mol:K2S2Og-2KSO4(R) initiator radicalsn(MBA): n(AA), 0.000 45 mol/mol; n(KOH): n (AA),monomer radicalomol,0.65;CA,35%R+M→RMRM+nM→RMn+1normal polymerizationThe effect of Kha concentration on water absorbency of the composite is shown in Fig. 2. When thecomposite contains a small amount of KHA(<10KHd+K2S2Og→KHAwater absorbency increases with increasing KHAcontent. This may be attributed to the increasedfree radicals on the potassium humate structurenumber of non-polar groups in the KHA: phenolichydroxyl, ester and acylamino groups. DifferentK+nM→KHnon-polar and polar groups produce a collaborativeeffect to improve water absorbency. A schematic il-graft copolymerizationlustration of the pAa-am/Kha structure is shown inFig 3. Notice that polar-COOH or-COOK and-NE-OH,-COo-or-CO- groups are located on the aro-matic rings in the PAA-AM/KHA structure. Thesegroups can interact with each other and causes a col-chain transferlaborative absorbent effect to produce water absorbekat nf ccl-ted CooH or-COOKo中国煤化工+RM→KHACNMH Gumate content inhigh(>10%)wa-terminationter absorbency decreased with further increases in theamount of potassium humate. This may be attributedCHU Mo et alPreparation, characterization and salt-resistanceto the fact that potassium humate contains condensed diffusion and, consequently, a decrease in the absoraromatic nng structuresMore potassium humate bency. Hence the water absorbency of PAA-densifies (or stiffens) the composite and increases its AM/KHA can be efficiently improved by introducingsistance to expansion and permeation thereby re- a moderate amount of KHa into the polymeric net-ducing the water absorbency. This fits with Flory work. In these experiments the composite synthesizedwhich states that increasing the density of the with a 10% weight ratio of Kha had the highest wa-network enhances the elastic forces between seg- ter absorbencyments of the gel, which leads to a retardation of waterCOoCONHCH-c--aHtratatstoaHrcataCoOHoCOoHNcocc是aa-a+o+a-8-acMucOOK CONHFig 3 Schematic illustration of the structure of PAA-AM/KHA3.3 Effect of reaction temperatureusually fast. a great quantity of polymerization heatFig. 4 shows that the water absorbency of the is generated during the process and explosive polym-erization may occur!. Hence the molecular weightcreases from 60 to 80C but then decreases with fur. decreases and an excess of water-soluble materialther increases in reaction temperature. If the tem- produced resulting in decreased absorbency of theperature is too low the initiator does not decompose product.to sulphate radical ions(SO4)at a sufficient rate3.4 Effect of initial monomer concentrationform a three-dimensional network. That results in thedecrease in water absorbency. If the temperature isThe effect of initial monomer concentration(CAA)too high(95C)the decomposition rate will be un- on water absorbency is shown in Fig. 5. It is obviousthat the water absorbency increases from 576 to 770g/g with an increase of CAA from 24% to 35%: it de-creases with further increases in CAA. This behavior isinterpreted by proposing that the effective crosslinkdensity is less at a lower total monomer concentrationAt higher CAA an explosive polymerization can occurcausing much more water-soluble material. Thissults in the absorbency of the super absorbent com-posite中国煤化工 to obtain a composite%)and the waterFig. 4 Effect of reaction temperature on Q(distilled water):n(KPS): n (AA), 0.009 mol/mol; n(MBA): n(AA), 0.00045CNMH Guse of the largeol/mol: n(KOH): n(AA), 0.65 mol/mol; CAA, 35%amounts of water-soluble material (asot. The influght ratio of KHA, 10%ence of Caa on the amount of water-soluble materialMining Science and Technologyvol 20 No 6and the yield of super absorbent composite is shown 3.6 Effect of neutralizationin Table 4. It is clear that the amount of water-solubleThe effect of neutralization(molar ratio of KOH tomaterial is lower and the yield of super absorbent AA)on the water absorbency is shown in Fig. 7. Thethis range the composite possesses a higher water water absorbency first increases from 25% to 65%composite higher in the range of 35%-40% CAA. Inin the preparation of poly(sodium acrylate)by oth- AA increases. This behavior may be interpreted as aand carboxylate groups that is superior to either of theTable 4 Influence of CaA on the amount of water-solublegroups acting alone. Florys network theory hypothematerial (a mo)and yield of super absorbent composite(%) sizes that the swelling ability of an ionic network is5101524303540attributable to the rubbery elasticity, ionic osmoticaw80972452120.1414.0141051210.68forces and the affinity of the polymer towardwaterYield 19.1 27.4 47.9 78.86 8599 95.90 94.88 8932 When poly(acrylic acid) is neutralized with potas-ote: The reaction conditions: reaction temperature, 80C: n(KPS): n(AA), sium hydroxide the negatively charged carboxylmol/mol; weight ratio of KHA, 10%electrostatic forces. This tends to expand the network.Over a certain range of neutralization the electrostaticrepulsion increases with increases neutralization. Thisresults in the increase in water absorbency. However,further increases in neutralization of the aa resultthe inclusion of more potassium ions, which reducesthe electrostatic repulsion by screening the negativeharges of the carbops. Thiscrease in water absorbency. Under our experimentalconditions a neutralization degree of 65% of the aaFig. 5 Effect of the initial concentration of AA on Q(distilled water): reaction temperature, 80C: n(KPS): n (AA),in the composite gave the highest water absorbency0.009 mol/mol; n(MBA): n(AA). 0.00045 mol/mol;Similar results were reported in previous studiesn(KOH): n(AA), 0.65 mol/mol; weight ratio of KHA, 10%3.5 Effect of crosslinker concentrationThe effect of crosslinker concentration(molar ratioof MBa to aa)on water absorbency is shown in Fig6. The water absorbency increases with an increase incrosslinker content from0.0l×102to0.045×100203040.5060.70.8mol/mol. It decreases with further increases in cross-n(NaOH): /AA)(momlinker concentration. When the crosslinker concentra-Fig 7 Effect of neutralization degree on @(distilled water)tion is <0.045x10--mol/mol the three-dimensional reaction temperature, 80oC; n(KPS): n(AA), 0.009 mo/mol;crosslinked network cannot form so more solublen(MBA): n(AA). 0.00045 mol/mol: CAA, 35material is produced with the result that water absorweight ratio of KHA, 10%bency decreases. On the other hand, higher crosslincer contents result more crosslinks, which causes ad- 3.7 Resistance of PAA-AM/KHA to salt solutionskeditional network structures that decrease the spaceThe super absorbent based on pure poly (acrylleft for adsorbed water. These results conform with acid or acrylamide) is too sensitive to ions5-l.It has arepo'ed by othe rses and similar obserauons were lch lower absorb nc si sabin sr saion than ti dis.water in soil. It is critically important to investigatethe resistance of PAA-am/Kha to saline solutions iftheir applications to agriculture and horticulture willThe absorbency of the PAA-AM/KHA compositesin various salt solutions was measured to determinethe resistance to salts. Some common cations were00020.00040.00060.0008cho中国煤化工ADmFig 6 Effect of n(MBA): n(AA)on Q(distilled water):tratreaction temperature, 80C: n(KPS): n(AA), 0.009 mol/mol;CNMHG super absorbentn(KOH): n (AA), 0.65 mol/mol; CAA, 35%:composite compared to that of PAA-AMeight ratio of KHA, 10%CHU Mo et alparation, characterization and salt-resistance ofIn Kcl solution:AA-Al133 g/g, when the MgClz(ag) concentration increasesfrom 0.005 to 0.1 mol/L. PAA-AM/KHA has higherIn MgCI solution: --PAA-AMwater absorbency in a bivalent salt solution comparedIn AlCl, solutionPAA-AM/KHAto PAA-AM. This behavior is similar to the KckagThe absorbency of PAA-AM/KHA and PAA-AMalso decreases in a trivalent salt solution(AIClyag).PAA-AM/KHA absorbency drops from 128 to 0.7 gg,0.0.20.3040and PAA-AM absorbency drops from 100 too gg, asthe salt concentration increases from 0.005 to 0.1Fig 8 Effect of salt solutions(KCkag): MgCl2(ag): AICImol/L. PAA-AM/KHA also has a higher absorbencyon Q of PAA-AM/KHA and PAA: reaction temperature, 80C;in the trivalent saline solution. This behavior is con-n(KPS): n(AA), 0.009 moV/ mol: n(MBA): n(AA), 0.00045sistent with what is observed in the univalent andmol/ mol; n(KOH): n (AA), 0.65 mol/mol: CAA, 354bivalent salt solutions. It is clear that PAA-AM/KHAweight ratio of KHA, 10%0is less sensitive to various salts when compared toPAA-AM. The PAA-AM/KHA composite possessesIt can be seen from Fig. 8 that in a univalent salt better salt-resistancesolution the water absorbency of both the PAA-There are two possibilities that would account forAM/KHA and PAA-AM decreases with increasing the greater salt resistance of PAA-AM/KHA. On onesalt concentration. When the KCla) concentration hand, the increase may be due to the fact that there isincreases from 0.005 to 0. I mol/L the water absor- a high K* concentration in KHA. The introduction ofbency of the PAa-AM/KHA is reduced from 486a moderate amount of Kha into the PAA-AM net49 gg. For the PAA-AM material the water absorwork increases the charge density of the compositebency is reduced from 358 to 220 g/g. The PAA- network 251. When the PAA-AM/KHA swells in theAM/KHA has a higher water absorbency in univalent salt solutions additional K- in the PAA-AM/KHAsalt solutions compared to PAA-AMnetwork will increase the osmotic pressure differenceThe absorbency of PAA-AM/KHA decreasesbetween the composite and the external aqueous solu-MgClzlag) solution as the salt concentration increases. tion. This results in an increase in absorbency FigPAA-AMKHA absorbency drops from 406 to 286 9 illustrates the structures of PAA-AM/KHA andg/g, but for PAA-AM absorbency drops from 270 to PAA-AM in aqueous solutionsCOOnK+W PAA-AM chainHOoC-nK+IK+ Potassium humate COocOoCOOCONH2COOFCONHz(a)PAA-AM/KHAb)PAA-AMFig 9 Schematic of the structure of PAA-AM/KHA and PAA-AM in aqueous solutionOn the other hand, quinone and ester structures and Bio-Red WIN FTIR using KBr pellets. They showphenolic-OH and-NH2 groups on the aromatic rings peaks corresponding to the functional groups attachedof the PAA-AM/KHa are non-polar hydrophilic to the structural units. The infrared spectrum of thegroups. These are less sensitive to salt solutions PAA-AM/KHA composite is shown inVarious non-polar and polar groups in the PAA- can be compared to the spectra of PAA-AM and KHAAM/KHA structure may interact with each othershown in Figs. 10b and c, respectively. The infraredcreate a collaborative absorbent effect. This results in spectrum of the PAA-AM/KHA composite has peaksan increase in salt solution absorbency: so salt-resat 3300-3500 cm that are attributed to the -OhPAA-AM/KHA is greater than that of groups of acrylate moieties and NH2 groups of thePAA-AMacrylamide unit. The peak at 1375.94 cm is from thearomatic C=C stretching of the KHA. The bands near4 Characterization of PAA-AM/KHA中国煤化工l4 clare attrib-utedtively. the infra-4.1 IRsredCN M H Composite showsthat the characteristic groups of the two raw materialsThe IR spectra of the samples were recorded on a exist in the product. Furthermore, by comparing Figng Science and TechnologyVol 20 No, 610a, b and c one may observe the peak at 1280. 11 KOH to AA), an initial AA concentration of 35% andcharacteristic of an alkyl-aryl-ether, which pro- a reaction temperature of 80C.vides evidence of graft polymerization between 2)The PAA-AM/KHA composite possesses betterPAA-AM and KHAsalt-resistance than PAA-AM in various saline solutions of the same concentration. The composite syn-thesized under optimum conditions absorbed 349,f the solution in 0.5 mol/L KClMgCl2 and AlCl] solutions, respectively. This is muchhigher than the adsorption of PAA-AM3)The infrared spectrum of the PAA-AM/KHAcomposite shows a peak at 1280. ll cm" characteris-tic of alkyl-aryl-ethers. This provides evidence of agraft polymerization between PAA-AM and KHA.The SEM micrograph of PAA-AM/KHA shows a fine,rugged network structure and a higher microporous3500300025002000surface area than that of PAA-Am. these observa-Wavenumber(em)tions are in good agreement with other experimentalFig 10 Infrared spectra of (a)PAA-AM/KHA,result(b)PAA-AM and(c) KHA4)The chemical blending of KHA, AA and AMallys preparation of a coal based4. 2 SEM observationscomposite, PAA-AM/KHA, with improvedSurface morphology is very significant for water sistance, reduced production costs and growth stimuabsorbency. The morphology of the samples waslant properties. This is more suitable as a wa-examined using an AMRAY-1820 SEM instrument ter-managing material for the renewal of arid andafter coating the sample with a gold film. The micro- desert environmentsgraphs of PAA-AM and PAA-AM/KHA are shown inFigs. lla and b, respectively.AcknowledgementsFinancial supports from the fundamental researchfunds for central universities and the National NatureScience Foundation of China (No 2010-2012)aregreatly acknowledged. Thanks are also given to theanonymous referee for a careful review of the manu-script.(a)PAA-AM(b) PAA/PVA/KHAReferencesFig 11 Micrographs of PAA-AM and PAA/PVA/KHA[1] Weaver M O, Bagley E B, Fanta G F, Doane W MThe PAA-AM shows a rugged surface with broadHighly absorbent Starch-Containing Polymeric Composions, US Patent: 3981100 1976network structure. The SEM micrograph of PAA-AM [2] Zhang J P, Li A. Study on superabsorbent composite/KHA, which has a greater absorbency, shows a finesynthesis, swelling behaviors and application of polyrugged network structure and has a more mi-(acrylic acid-co-acrylamide )sodium humate/attapulgitecro-porous surface than the PAA-AM. 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