Psyllium-g-Poly-(acrylamide-co-acrylonitrile) N2 atmosphere in the presence of


Psyllium-g-Poly-(acrylamide-co-acrylonitrile)has been synthesised from psyllium under N2 atmosphere in thepresence of ceric ammonium nitrate and ascorbic acid couple (CAN/AA) initiatorfor adsorption of mercuric ions from synthetic solution of HgCl2.The synthesised samples were optimised by varying various synthetic parameters viz.monomer concentration, reaction time, temperature, initiator concentration etc.

for maximum yield and adsorption of ionic mercuric.  The optimised samplehas been characterized by using FTIR spectroscopy, SEM analysis, X-Raydiffraction and thermal studied (TGA/DTA/DTG). The mercury adsorption wasstudied onto the optimum sample and found maximum at temperature (30°C), dose(30 mg), pH (6), time (60 min) and initial mercury concentration 100 ppm. Equilibriumisotherm data were analyzed using the Langmuir and Freundlich isotherms both.The Langmuir model was more fitted (R2=0.9976) which indicates themonolayer sorption. The kinetics of therate of sorption of mercury (II) were also analysed using the first order (R2=0.9971), second order (R2=0.

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9887), pseudo-first order (R2=0.9971), pseudo-second-order (R2=0.9481), intra-particlediffusion (R2= 0.9958) and Elovich equation (R2=0.9624). Second order rate kinetics has best linearly fitting,which follows chemisorption mechanism.Key words –Psyllium mucilage, Synthesis, Graft copolymer, Mercury, Adsorption.

 1.     INTRODUCTION Mercuryoccurs in liquid state at ambient conditions, and mercuric salts (Hg++)are much common. The toxicity of these salts are due to the capacity ofbio-accumulation in living species and their own compounds to bio-concentratein organisms and even to biomagnifies through the food chain 1. Mercury has very high tendency to bindsprotein and severely disturbs the nervous and renal  system 2. Mercury continuously effluenting inthe aquatic systems and creating long term water contamination problem 3-5.          Removal of mercury from the aqueous solution isgreat challenge for scientific community for few past decades.

Mercury ishighly concern at current time because of its persistence, toxicity andvolatility   into the environment 6-9.  The minorconcentration of the mercuric ions is toxic to the atmosphere and human health(higher permissible limit recommended WHO 2.0 ?g/L) 10, and all species of mercuryare damaging to human beings and can be simply absorbed through human beingsand injury our nervous system, kidneys, other organsand also several environmental pollutions 11-13.  Many methods have been used forwaste water treatment and heavy metal removal from aqueous solutions such ascoagulation, ion exchange, advanced oxidation, reverse osmosis, adsorption, chemicalprecipitation and absorption and biomass adsorption14, 15.

Among of these techniques, an adsorption method is widelyused for the removal of heavy metal ions from aqueous solutions. Many adsorbents including graftcopolymer have been established for heavy metal adsorption, due to their uniquechemical, mechanical, electrical, thermal and rheological properties.The present article deals withthe synthesis and characterization of a binary grafted copolymer on Psyllium witha mixture of acrylamide (Am) and acrylonitrile (An) for efficient mercuryremoval application. The graft copolymers were synthesized with the free radicalpolymerization technique using ceric ammonia nitrate/ascorbic acid (CAN)/AAcouple as the free radical initiator by conventional method. The graftcompound was characterized by FTIR spectroscopy, SEM, XRD and thermal studiesand also investigated the Hg(II) adsorption capability of the adsorbentPsy-g-Poly (Am-co-An) through batch adsorption studies.

2.     EXPERIMENTAL 2.1.

MaterialsPsyllium husks were procured from Sidhpur Sat-Isabgol Factory India and acrylamide (Am),acrylonitrile (An), sodium hydroxide (NaOH), hydrochloric acid (HCl), methylalcohol (MeOH), acetone (MeCoMe), gelatine, mercuric chloride (HgCl2)were supplied by E-Merck Ltd. Mumbai, India. Rhodamine6 G, Potassium iodide (KI) were supplied bySD Fine chem Ltd. Mumbai, India.  Doubledistilled deionised water was used for the synthesis of binary graftedcopolymer and water analysis.2.2.Synthesis ofPsy-g-Poly (Am-co-An)Graftedpsyllium Psy-g-Poly (Am-co-An) was synthesized by using the previouslyreported method.

1.0 g of psyllium mucilage (Psy) was dissolved in doubledistilled water (100 mL) in two necks round bottom flask. Require amount ofacrylamide and acrylonitrile monomers were dissolved in distilled water (10 mL)in conical flask and this solution was added to the Psy solution in the twonecks round bottom flask.

This solution was stirred with the help of magneticstirrer. The round bottom flask wrapped by septum stopper and nitrogen gasflushed into the solution by using a hypodermic needle. Solution wascontinuously stirred, while being N2 bubbled. They require an amountof CAN was injected in solution by the stopper through hypodermic syringe. Thenitrogen gas flushing was continued for again 20 min, and the needle was takenout from the flask and was further wrapped with teflon tape and the reactionmixture was continuously stirred  at 30°Cfor require time followed by adding 0.5 mL of saturated aqueous hydroquinonesolution for reaction termination16, 17 . The reaction product Psy-g-Poly(Am-co-An) was precipitate in methanol and washed with acetone and dry at40°C. The%  was calculated by the equation (1)-            2.

3.CharacterizationsPsyllium,graft Psy-g-Poly (Am-co-An) copolymer was characterized by Fouriertransform infrared (FTIR) (Nicole – 6700). FTIR spectrum offers the necessaryinformation about the molecular structure such as functional group and chemicalbonding in the region between 4000 and 450 cm?1.The Samples were kept absent from light formerlyand dried for 1 hour 50°C temperature in a drying oven. The dried samples wereground with KBr and formed the KBr pellets containing 1 % (w/w) of samplesbefore analysis. X-ray Diffraction analysis was performed using a Bruker D 8advance (Shimadzu, Japan), XRD spectra iswidely used for quantitative analysis and identificationof different crystalline forms of molecules.

The samples were scanned from 2? = 5o to 50owith a steep angle of 6o/min. Thermogravimetricanalysis, differential thermal analysis,and differential thermogravimetric analysis were performed with an SII 6300 EXSTAR TG-DTA (Japan).  5.0 mg Samples were placed in the TGA furnaceand the measurements were carried out under a nitrogenatmosphere with a heating rate of 10°C/min from 0 to 800°C. Weight losses ofthe samples were determined with respect to temperature. Scanning electronmicroscopy (SEM) was used to obtain the morphologicalimages of Psyllium, graft Psy-g-Poly(Am-co-An) copolymer18.The instrument (JSM, 6490) was used to investigate the surface morphology ofPsyllium, graft Psy-g-Poly (Am-co-An) copolymer in powder form at 15 KV and with a resolution of 10nm.

The pH of the solution was measured with Digital pH meter (Globe instrumentauto pH meter). The concentration of Hg (II) was determined by Systronicsdouble beam UV visible spectrophotometer 2203.2.

4.Determinationof pHZPCA solution of 5.0×10-3 molar calcium chloride(CaCl2) was boiled to remove the dissolved carbon dioxide (CO2)and then cooled to room temperature. The pH was set to a value between 2 and 10by the using 2.5 M hydrochloric acid or 5.0 M sodium hydroxide. 20 mg graftedpsyllium was added in 20 mL of the pH solution in conical flask for 60 min andmeasure the final pH, and plotted graph between the initial pH and final pH.

ThepH at which the both curve crosses  toeach other ( pH initial/pH final lines) that point is the pHZPC 19, 20. (Figure. was not given)2.5.Hg (II) adsorption methodA stock solution of 1000 ppm of Hg (II) was prepared bydissolving 1.354 g of HgCl2 in deionized double distilled water.

Allmercury (II) adsorption experiments were investigated at ambient temperature. Theimpact of various factors like adsorbent amount, contact time, pH and Hg (II)concentration were investigated by batch adsorption experiment. The impact of pHon mercury (II) adsorption was investigated at various pH and adjust pH the by 0.1 M HCl or 0.1 M NaOH. 20 mL Hg (II) solution (100 ppm) was taken in 50 mL beaker and addedthe 20 mg adsorbent and stirred with magnetic stirrer for the desired timeperiod, and filtered the solution by a Whatman 0.

45mm filter paper. Afterappropriate dilution, the remaining quantity of Hg (II) was projected by adouble beam UV spectrophotometer (?-575nm) by using  the rhodamine 6G dye and iodine buffersolution20, 21. The quantity of Hg++ adsorbed by graftedcopolymer in ppm was calculated using the following equation (2),Where qe = the amount of the metal adsorbed(ppm) onto the adsorbent, Qo = the initial concentration solution(ppm), Qe = equilibrium concentration of solution (ppm), V= volumeand W= adsorbent weight.2.6.KineticStudiesIn order to investigate kinetic data, the contact timewas various from 10  to 120 min and thekinetic studies were completed the using 100 ppm Hg(II) concentration, 30 mgadsorbent dose at pH 6 and temperature of 25°C.

2.7.Optimizationof Various Adsorption ConditionsThe adsorption Hg (II) by Psy-g-Poly(Am-co-An) has been investigated by varying only one adsorption parameter at atime while others keep the fixed.

Varied adsorption parameters and their range pH ( 4 to 10), adsorbent dose (10 mg-70 mg), temperature (15°C to 50°C),contact time 60 minutes and contact volume 10 mL at 100 ppm mercury(II)concentration were studied.2.8.AdsorptionIsotherm StudiesFor isotherm investigation, the adsorption equilibriumdata were originated at different initial Hg (II) concentrations ranging from50 ppm to 400 ppm using a 30mg adsorbent, pH 6, 10 mL Hg (II) solution, 60 mininteraction time at room temperature. 3.     RESULT ANDDISCUSSION3.1.Effectof various parameters variation onto the grafting3.

1.1.      MonomersconcentrationThe effect the monomerconcentration on tografting was shown in figure1 (a) and it carried out from the binary mixture of vinylmonomers (Am and An). The concentration of Am was varied from 0.07 to 0.

28mol/L in different sets of experiments while keeping the concentration of afixed at 0.01 mol/L. The grafting increased with the increasing in Am (0.

07mol/L to 0.21mol/L), but as Am was increased beyond 0.21mol/L, graftingdecreased. The initial augmentation of the monomer concentration gradually increasedgrafting with the diffusion of Am to the backbone psyllium as acrylonitrile wasfurther increased, grafting decreased due to more homo polymerization 22, 23. 3.1.2.

      Initiator(CAN/AA) ConcentrationThe effect of initiator concentration (CAN/AA)on the percentage of grafting is shown in figure (1b) with increasing theconcentration of initiator (1.8×10-3 to 7.2x 10-3 mol/L), increase the grafting yield due to theavailability of more free radicals to initiate grafting at higher CAN/AAconcentration and further increase initiator concentration decreased graftingyield due to the favoured homo polymerization at high radical availability24.

3.1.3.     ReactionTime The influence of time on grafting is shown infigure (1c).

The grafting enhance with enhance thetime up to 124 min and then a slight decline or constancy in grafting contentwas observed. The quick increase of graftingbetween 60 and 102 min enhances due to rate of initiation and propagation and the decline or constancy of grafting after 120 minis a clear remark on the depletion of monomerfrom the solution 16, 25.3.1.4.     ReactionTemperature Theeffect of reaction temperature on grafting parameters has been studied in thetemperature range of 20–50°C shown in figure (1d) the enhanced in reactiontemperature (up to 50°C) increased the grafting. The increase in the graftingyield with increase in grafting temperature can be attributed to  enhanced mobility of the initiator as well asmonomer and increased the number of reactive sites  26. 3.

2.Characterization3.2.1.     FTIRspectraTheFTIR spectra of pure psyllium and Psy-cl-Poly (Am-co-An) are shown in figure 2aand 2b, respectively.

The FTIR spectra of purified psyllium showscharacteristic peak 3392 cm-1, is due to stretching vibration of OH andSmaller peak at 2923 cm-1 is assigned to the C-H stretchingvibrations. The band at 1043 cm-1 is attributed to the C-O-C stretchingvibrations. Whereas, in case of Psy-cl-Poly(Am-co-An) one peaks at 2240.78 cm-1 (C?N stretching ofnitrile), 1726, 1673.25 cm-1 (C=O stretching of amide-I) 27, 1423 cm-1 (N-H in planebending of amide-II) and 1251 cm-1 (C-N stretching of amide-III)were observed in addition to the peaks observed with IR of psyllium.3.2.

2.     SEMIt is evident from the SEMmicrographs of psyllium figure 3(a) andthat of the best grade of Psy-g-Poly (Am-co-An) figure 3(b) that profound morphological change has taken place. Thehomogeneous surface of psyllium was convert into heterogeneous (fibrillar)surface after the modification. The homogenous morphology of psyllium was lostafter grafting with acrylic acid and acrylamide and converted intoheterogeneous morphology.3.2.

3.      XRD analysis The XRD investigation of any material provides evidenceabout its crystalline nature. The XRD patterns of Psyllium and Psy-g-Poly(Am-co-An) are shown in figure (4a) and(4b), respectively. The XRD patterns of Psyllium demonstrations a broad a peakat 2? = 22°, indicating amorphous nature 28, 29. Whereas, in caseof Psy-cl-Poly(Am-co-An) one peaks at 2? = 22°, also show amorphicity,on studying both XRD spectra it was found that the diffractionpeak intensity of psyllium are not significantly decreased after grafting atall angles but the XRD of grafted spectrum present slightly broad spectrum ascompared to pure psyllium means slightly increase the amorphous nature ofcrystals. 3.2.4.

      Thermal behaviourThermalstudy (TGA/ DTA/DTG) of pure psyllium and Psy-g-Poly (Am-co-An) were displayedin figure (5a-b). TGA curves of psyllium and Psy-g-Poly (Am-co-An) wereachieved to scan the both samples 0°C to 800°C. We previously reported that TGA curves ofpsyllium (figure 5a) show two weight loss step. The initial weight loss is 12.2 % betweentemperatures 30-250°C which is due to the traces of moisture present.

Again weight loss of 83.8% is detected up to 442°C which is due to break of psyllium backbone. Psy-g-Poly(Am-co-An) is also showing two thermal degradation stages. The first stage ranged from 25°C to 250°C (14.5%) corresponds to the remove ofwater. In the second stage, the weight loss is 79.3%in the temperate region of 350-500°C.

Decomposition of Psy-g-Poly (Am-co-An) started at 100°C and 50% weight loss of Psy-g-Poly (Am-co-An) at the temperature 350°C. Finally, the polymer backbone of Psy-g-Poly(Am-co-An) wascompletely degraded at 500°C shown in figure 5b, which is slightly higher comparedto original psyllium (Table 1.) 30. It is also observe formin which DTA and DTG curve, grafted psyllium comparatively more stable to purepsyllium, as one major peak in both the appears at 416 (98.0 uV) in figure 5aand at 413 (71.1 uV) in figure 5b ie the grafted material has enough goodstability up to 400°C while the psyllium itself almost decomposes upto 300°C asshown in figure 5a-b.3.3.

Effect of various parameters ontothe adsorption3.3.1.      Adsorbentdose The effect of adsorbent (Psy-g-Poly (Am-Co-An) dose on the Hg(II) adsorption was studied in the 10 to 50 mg, keeping other parametersaffecting the adsorption and the result is shown infigure 6a. It was observed that removal of Hg (II)increases from 56.5% to 89.

9% with the increase in adsorbent dose from 10 mg to30 mg due to the availability of extra binding sitesat higher doses and further increase the adsorbent dose 30 to 50 mg found theslightly nominal increase removal of Hg (II).30 mg adsorbent dose was designated for further optimization and kineticstudies as there was only slightly increase (in adsorption) beyond 50 mgadsorbent dose.3.

3.1. Effectof pHThe impact of pH on the Hg (II) removal was investigated in the pHrange 2-8 under constant other parameters affecting the adsorption.

The resultis shown in figure 6b. It wasobserved that the percentage removal of Hg (II) increases from 41 % to 92 %with the increase in adsorbent dose from 2 to 6, because at low pH mercuryexist as positive ions (Hg++) further increase the pH 7 to 8, decrease the percentageremoval of Hg(II) due the formation of Hg(OH)2 31 and pH 6 wasselected for further optimization and kinetic studies because at  pH 6 mercury exist as positive ion (Hg+) 32.   3.3.2.     Contact timeThe study of the removal of Hg (II) wasperformed with fixed adsorbent dose at various time intervals (10–120 min). Theresult is shown in figure 6c. It was investigated that thepercentage removal of Hg (II) increases from 53.

5% to 94.9% with the increasein adsorption time from 10 to 60 min due to increase the metal binding timewith vacant adsorbing sites. Further increase the time beyond 60 min did not approvableincrease the adsorption due to deposition of Hg++ ion on theavailable adsorption sites 33.  3.

3.3.      TemperatureThe influence of temperature on the Hg (II)adsorption was performed in the range 20–50 °C underconstant parameters at equilibrium condition shown in figure 6d. The mercury adsorptioncontinuously increased with the increase the temperature 20-30 °C; increase theactive surface centre sites for sorption. Further increase the temperature,decrease the adsorption was observed indicating some desorption taking placeabove 30 °C.3.3.4.

      InitialHg (II) ion concentrationEffect of initial concentration of Hg (II)ion on adsorption when initial concentration of Hg (II) ion was varied from 50to 400 ppm at a particular time, particular pH and at a fixed temperature was shownin figure 6e. With the increasein the initial concentration of Hg (II) from 50 to 400 ppm, then increasemercury adsorption due to the availability of extra mercury (II) for thebinding. 3.4.Adsorption isotherms andmodelsAdsorptions isotherms give theinformation how molecules subjected to adsorption distribute themselves betweenadsorbate and adsorbent phases at equilibrium time{Bao, 2017 #307;Aly, 2013 #798}. They offer some insight into the adsorption mechanism, surfaceproperties and affinities of the adsorbent. The commonly used adsorption modelsare the Langmuir model (corresponding to monolayer homogeneous adsorbent surface)and the Freundlich model (corresponding to heterogeneous adsorbent surface). Theequilibrium sorption of the Hg2+ ions was carried out by contacting30 mg of the Psy-g-Poly (Am-co-An) with 20 mL of 100 ppm of differentconcentrations from 50 to 400 ppm in 25 mL conical flasks for 60 minutes on theorbital shaker.

The mixture was filtered and the filtrate analyzed for metalions concentration using a UV spectrophotometer. The data were fitted into the Langmuiras well as Freundlich adsorption34.3.

4.1.     LangmuirAdsorption IsothermThe Langmuir adsorption isotherm is highly effective formonolayer sorption due to a surface of a finite number of identical sites andexpressed in the linear form as equation (3).Where Ce = Equilibrium concentration Qe = Amount adsorbed at equilibrium Qm = Langmuir constants KL =Heat of adsorption.The vital characteristics ofLangmuir model are explain through means of RL (dimensionlessconstant) and RL is calculated the  equation (4).Where C0 = Hg (II)concentration (mg/L). If RL values lie between 0and 1, the adsorption is favourable.The value of Qm(57.

47 mg/g) was calculated from Langmuir model, indicating that theadsorbent showed high capacity to remove mercuric ions (figure 7a). RL and KL werecalculated to be 0.01493 and 0.01649 ml/mg respectively, thus adsorptionis favourable.3.

4.2.     FreundlichAdsorption IsothermFreundlich isotherm defines the heterogeneous surfaceenergy through multilayer adsorption 35 and indicates the linear form as equation (5).Where Kf = Adsorption capacity of adsorbent  Valueof Freundlich parameters (Kf), Correlation constant (R2)and rate constant were calculated by Freundlich isotherm (figure7b) given in Table-2. Theequilibrium data fitted to Langmuir (R2= 0.9976) model better than Freundlichmodel (R2 = 0.9434) indicating surface homogeneity of adsorbent and monolayeradsorption.

3.5.KineticstudiesThe removedrate of mercuric ions from aquatic system by the adsorbent is a significantvariable for the application of the real process in treatments. In order todetermine the equilibrium time, effect of interaction time for removal of Hg (II)by the using many initial Hg(II) concentrations was illustrated in figure 6c. It is observed that the adsorption reaction initially fast,and at equilibrium time, become slows 36 because great number of unoccupiedsurface sites are presented for adsorption in the initial phase. Six of the most widelyused kinetics models: first order, Elovich equation, second order, pseudo-second-order,intra-particle diffusion and pseudo-first order models 37 were used to evaluate kinetic mechanism ofthe Hg (II) adsorption onto Psy-g- Poly (Am-co-An).3.

5.1.     First-order kinetics equation The linear form of firstorder kinetics equation is givenas equation (6).……………(6)WhereQo (mgL-1) and Qt (mgL-1) areconcentration at the time zero (initial) and a given time ‘t’ concentration ofmetal ions in solution respectively.

K1 (min-1) is thefirst order rate constant. The regression R2 obtained by the linearplot of ln (Qo/Qt) Vs t (figure 8a), is shown in Table 3.For mercury, R2 was more than 0.9, which shows a good fit theexperimental data. 3.5.

2.     Secondorder rate equation Thelinear form second order kinetics eqution is given in equation (7) below:WhereK2 Lmg-1min-1 is the second order rateconstant for the sorption process, determined from the linear plot of (1/Qt– 1/Qo) against t, shown in figure (8b) for mercury see the Table-3above for the value of the constants. 3.

5.3.     Pseudo-first-order kinetic equationLinear formpseudo-first-order equation is given in equation(8).where, Qt, Q0and k1 are adsorbate at time t, adsorption capacity atequilibrium, and rate constant respectively.  All parameters of this kinetic equation were calculatedby figure 8c resultshown in Table 3.

3.5.4.      Pseudo second order kinetics equationThepseudo-second-order kinetic rate was studied by equation (9) 31.

Where k2is the rate constant. The plot for the equation(9) was shownin figure 8d which shows the data was perfectly fitted to the model and valueof all parameters were given in table 3. R2 for the second-orderkinetic model exceeded 0.99 which indicated adsorption system highly followssecond-order kinetic mechanism compare to other kinetic mechanisms. So itsupports the assumption behind the model and suggests that the overall rate ofHg(II) adsorption by psy-g-Poly(Am-co-An) appeared to be controlled byphysicochemical process.

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