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Synthesize and characterization of binary grafted psyllium for removing toxic mercury (II) ions from aqueous solution
Materials Science & Engineering C 104 (2019) 109900
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Synthesize and characterization of binary grafted psyllium for removing toxic mercury (II) ions from aqueous solution
T
⁎
Deepak Kumara,b, , Jyoti Pandeya, Nida Khanb, Pramendra Kumarb, Patit P. Kunduc a
Department of Chemistry, Babasaheb Bhimrao Ambedkar University (A Central University), Lucknow 226025, U.P., India Department of Applied Chemistry, M J P Rohilkhand University, Bareilly 243006, U.P., India c Department of Chemical Engineering, Indian Institute of Technology Roorkee, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Psyllium Graft copolymer Mercury Adsorption Kinetic models
Acrylamide and acrylonitrile were grafted on psyllium employing ceric ammonium nitrate (CAN) as initiator under N2 atmosphere to get an adsorbent of mercury ions. The synthesized adsorbent was optimized by varying synthetic parameters viz. monomer concentration, reaction time, temperature, initiator concentration, etc. to obtain the maximum yield of the grafted product as well as maximum adsorption of ionic mercury. The synthesized adsorbent was characterized by FT-IR, SEM, XRD, zeta potential and thermal techniques. The effect of various process parameters such as pH, time, adsorption dose and temperature on Hg (II) adsorption was investigated. The maximum Hg (II) adsorption (96%) was achieved at temperature (30 °C), dose (30 mg), pH (6), time (60 min) and initial concentration of mercury with 100 ppm. The Hg(II) adsorption on Psy-g-Poly (Am-coAn) was confirmed by XPS study. The isotherm data of the adsorption experiments obeyed the classical Langmuir adsorption isotherm. On the other hand, the kinetic data followed the second-order kinetics, indicating the chemisorption mechanism.
1. Introduction Due to fast industrial development, an increased discharge of metal pollutants in the aquatic system has been observed [1]. Given their liquidity and poisonousness, metal ions, especially heavy metal ions, create significant risk for animals, plants and the environment. Lead, copper and mercury are three common heavy metals having high toxicity, which is prevalent in many industrial effluents, such as electroplating industries, nonferrous metals smelting, and mine tailing [2,3]. Among these toxic metals, mercury, released from the various type of natural sources and habitat, is noted as the greatest noxious and universal heavy-metal of pollutants [4]. Methyl mercury is the most toxic form of mercury (Hg). Mercury can enter the human body through the contaminated food chain [5] and has a high tendency to bind with protein chains through HgeS strong bond with cysteine residue [6]. It is highly harmful to the kidney, cardiovascular system [7], bones and also damages the human nervous system [8,9]. Mercury removal from water has been a challenge for the global scientific community for decades. At present, it is a major concern because of its toxicity and volatility into the environment [10–12]. The Agency for Toxic Substances and Disease Registry (ATSDR) declared that mercury is highly dangerous metal due to its high toxicity, mobility
⁎
and their high resistive tenure in the atmosphere [13]. The minor concentration of the mercuric ions in water is toxic as the maximum permissible limit as recommended by WHO is 0.002 ppm in aquatic life [14,15]. The noxiousness of mercury mainly depends on the valance state of mercury [7]. Mercury, in natural water, exists in 0, +1 and +2 oxidation states and may be present in various other hydrated forms depending on the factors like ionic strength, the dose of the suspended particulate matter (SPM), temperature, pH, salinity and organic solvents [16]. The various mercury forms include organic mercury (CH3Hg and C2H2Hg), inorganic mercury (HgCl2) and elemental mercury [Hg] [17–19]. Mercury vapors on reaching human blood through the respiratory system spread all over the human body and changes to +2 oxidation state [Hg (II)], a highly toxic stale to human beings [9,20]. The main sources of mercury are various chemical industries mainly fertilizers, pulp, paper, battery manufacturing, plastic, paint and oil refining [21] and spread in the environment through the wastewater line of these chemical industries [22–24]. Various techniques such as advanced oxidation, coagulation, reverse osmosis, chemical precipitation adsorption, ion exchange, absorption and biomass adsorption [25] have been performed for wastewater treatment and for removal of hazardous material and
Corresponding author at: Department of Chemistry, Babasaheb Bhimrao Ambedkar University (A Central University), Lucknow 226025, U.P., India. E-mail address: [email protected] (D. Kumar).
https://doi.org/10.1016/j.msec.2019.109900 Received 6 November 2018; Received in revised form 13 June 2019; Accepted 15 June 2019 Available online 17 June 2019 0928-4931/ © 2019 Published by Elsevier B.V.
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contaminants [26,27]. Among these techniques, the adsorption method is found to be a standard, cheap and highly used technique for the removal of toxic metal from the aqueous solution [28]. Many adsorbents including graft copolymer/modified polymers have been established for toxic metal removal purpose, due to their unique chemical, electrical, mechanical, rheological and thermal characteristics. The present work deals with the preparation and characterization of psyllium based binary grafted material via grafting of acrylamide (Am) and acrylonitrile (An) with psyllium for mercury adsorption. The graft copolymers were synthesized via free radical polymerization using ceric ammonium nitrate/ascorbic acid [(CAN)] as the free radical initiator under thermal conditions. The graft copolymer was characterized via FTIR, SEM, XRD and thermal analysis. The Hg (II) adsorption capability of the adsorbent [Psy-g-Poly (Am-co-An)] through the batch adsorption method has also been studied.
Grafting% =
wt.of grafting psyllium × 100 wt.of pure psyllium
2.3. Hg (II) adsorption method A standard solution of 1000 ppm of Hg (II) was obtained by dissolving 1.354 g of HgCl2 in 1 L deionized double distilled water [32]. All mercury (II) adsorption experiments were investigated at room temperature [33]. The effect of various influences like adsorbent amount, contact time, pH and Hg (II) concentration were studied by batch adsorption experiment. The effect of pH on mercury adsorption was investigated at various pH by adjusting pH with 0.1 M HCl or 0.1 M NaOH [34]. 20 mL Hg (II) solution (100 ppm) was taken in 50 mL beaker 20 mg adsorbent was added to and it stirred with a magnetic stirrer for the desired time period, and filtered the solution using Whatman 0.45 mm filter paper. After appropriate dilution, the remaining quantity of Hg (II) was measured by a double beam UV spectrophotometer (λ-575 nm) using the rhodamine 6G dye and iodine buffer solution [35,36]. The quantity of Hg2+ adsorbed by grafted copolymer in ppm was calculated by the following Eq. (2) [37].
2. Experimental 2.1. Materials Psyllium husks were procured from Sidhpur Sat-Isabgol Factory India. Acrylamide (Am), acrylonitrile (An), sodium hydroxide (NaOH), hydrochloric acid (HCl), methyl alcohol (MeOH), acetone (MeCoMe), gelatin, ascorbic acid and mercuric chloride (HgCl2) were supplied by Merck Ltd. Mumbai, India. Rhodamine 6G and potassium iodide (KI) was supplied by SD fine Chem. Ltd. Mumbai, India. Double distilled water was used for synthesis and adsorption analysis. Chemical structures of newly synthesized psyllium and grafted psyllium [Psy-g-Poly (Am-co-An)] were examined by Fourier transform infrared (FTIR) (Nicole - 6700) spectrophotometer with wave number range of 4000 to 500 cm−1. X-ray diffraction analysis was carried out on Bruker-D8 advance diffractometer (Shimadzu, Japan) in 2-theta angle range 5°–40°. The surface morphology of pure psyllium and grafted psyllium were studied by scanning electron microscope (JSM, 6490) [29]. Thermogravimetric analysis (TGA) was performed by using SII 6300 EXSTAR TG-DTA (Japan). All measurements of analysis were carried out under a nitrogen atmosphere [12]. The conductivity of newly grafted psyllium was measured by zeta sizer 1000 HS (Malvern Instruments, Worcestershire, UK) at 25 °C and 90° angle. Zeta potential graph was directly obtained from the zeta sizer software (Malvern, USA). The pH of the solution was measured using a Digital pH meter (Globe instrument auto pH meter). The mercury adsorption capacity of Psy-g-Poly (Am-co-An) was performed by using Systronics double beam UV visible spectrophotometer 2203 [10]. The XPS experiments were performed in an ultrahigh vacuum (UHV) using high-resolution X-ray photoelectron spectrophotometer (PHI 5000 VersaProbe III). All samples were dehydrated under vacuum prior to XPS analysis.
qe =
Qe − Qo ×V W
(2)
where qe = the amount of the metal adsorbed (ppm) onto the adsorbent, Qo = the initial concentration of the solution (ppm), Qe = equilibrium concentration of the solution (ppm), V = Volume and W = adsorbent weight. 2.4. Optimization of various adsorption conditions The mercury adsorption by grafted psyllium was investigated by varying only one adsorption parameter at a time while keeping others fixed [11]. Various adsorption parameters and their range [pH (4 to 10), adsorbent dose (10 mg–70 mg), temperature (15 °C to 50 °C), contact time 60 min and contact volume 10 mL at 100 ppm of mercury (II) concentration] were studied. 2.5. Adsorption isotherm studies For isotherm investigation, the adsorption equilibrium data was originated at different initial concentrations of Hg (II) ranging from 50 ppm to 400 ppm using 30 mg of the grafted psyllium, pH 6, 60 min contact time with 20 mL mercury solution at room temperature. 2.6. Kinetic studies In order to investigate kinetic data, the interaction time was varied from 10 to 120 min and the kinetic studies were completed by using 100 ppm of Hg(II) concentration, 30 mg of adsorbent dose at pH 6 and a temperature of 25 °C [38,39].
2.2. Synthesis of Psy-g-Poly (Am-co-An) Grafted psyllium [Psy-g-Poly (Am-co-An)] was synthesized through our earlier reported protocol [12]. Briefly, 1.0 g of psyllium mucilage was dissolved in double distilled water in a two-necked round bottom flask. The required amount of acrylamide and acrylonitrile monomers were dissolved in distilled water (10 mL) in a conical flask and this solution was added to the psyllium solution. The round bottom flask was wrapped by septum stopper assembly to flush nitrogen gas into the solution by using a hypodermic needle throughout the duration of the reaction. Later on, an essential amount of CAN/ascorbic acid initiator was injected in the solution via hypodermic syringe and reaction mixture was continuously stirred at 30 °C for a required time followed by adding 0.5 mL of saturated aqueous hydroquinone solution to terminate the reaction [30,31]. The reaction product [Psy-g-Poly (Am-co-An)] was allowed to precipitate out in methanol and washed with acetone and dried at 40 °C. The grafting (%) was calculated as follows:
3. Result and discussion 3.1. Effect of various parameters variation onto grafting (%) 3.1.1. Monomers concentration The effect of monomer concentration onto grafting (%) was shown in Fig. 1a and it was obtained from the binary mixture of both monomers (acrylamide and acrylonitrile). The concentration of acrylamide was varied from 0.07 to 0.28 mol/L in different sets of experiments while keeping the concentration of acrylonitrile constant at 0.01 mol/L. The grafting increased with the increase in acrylamide concentration (0.07 mol/L to 0.21 mol/L), but as acrylamide concentration was increased beyond 0.21 mol/L, grafting started to decrease. The initial augmentation of monomers gradually increases the grafting with the 2
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Fig. 1. Effect of various parameters onto the grafting (a) Effect the monomer concentration on to grafting (b) Effect of initiator (CAN) Concentration on to grafting (c) Effect of reaction time on to grafting (d) Effect of reaction temperature on to grafting.
dispersion of acrylamide to the backbone psyllium. As the acrylonitrile content increased, grafting decreased due to the formation of a homopolymer as well as copolymer [29,40]. 3.1.2. Initiator (CAN) concentration The effect of initiator concentration (CAN) on grafting (%) was shown in Fig. 1b. On increasing the concentration of CAN initiator from 1.8 × 10−3 to 5.4 × 10−3 mol/L grafting (%) increased due to increase in the obtainability of more free radicals sites on psyllium backbone. The decrease in the grafting (%) at the higher concentration of initiator (5.4 × 10−3 to 7.2 × 10−3 mol/L) is a well-known phenomenon and ascribed to the increasing participation of the ceric ion in the termination of the growing grafted chains [41,42]. 3.1.3. Reaction time The effect of time on grafting (%) is shown in Fig. 1c. The grafting enhanced with the increase in time from 30 to 124 min and then a slight decline in grafting (%) was observed. The quick increase of grafting between 60 and 102 min is due to the higher rate of initiation and propagation and the decline of grafting after 120 min is a clear indication of depletion of monomer concentration from the solution [30,43]. 3.1.4. Reaction temperature The impact of reaction temperature on grafting yield has been investigated at the temperature range from 20 °C to 50 °C as shown in Fig. 1d. It was found that the grafting yield slightly increased up to 50 °C i.e. the grafting yield increased with the increase in temperature due to the increase in the number of reactive sites of monomer [44].
Fig. 2. (a)FT-IR spectra of the Psyllium and Psy–g-Poly (Am-co-An) (b) XRD spectra of the Psyllium and Psy-g-Poly (Am-co-An).
stretching vibrations. The FTIR spectrum of grafted psyllium showed additional peaks at 2240.78 cm−1 (C^N [45] nitrile stretching), 1726, 1673.25 cm−1 (C]O stretching of amide-I) [43], 1423 cm−1 (NeH inplane bending of amide-II) and 1251 cm−1 (CeN stretching of amideIII), which were absent in the IR spectrum of pure psyllium. This clearly indicates the formation of graft polymer.
3.2. Characterization 3.2.1. FTIR spectra The FTIR spectra of psyllium and Psy-g-Poly (Am-co-An) were shown in Fig. 2a, respectively. The FTIR spectra of purified psyllium showed a characteristic peak at 3392 cm−1, due to stretching vibration of OeH, whereas smaller peak at 2923 cm−1 is assigned to the CeH stretching vibrations. The peak at 1043 cm−1 is due to the CeOeC
3.2.2. XRD analysis The XRD pattern of psyllium and grafted psyllium are shown in Fig. 2b. In the XRD pattern of pure psyllium mucilage, a broad 3
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Fig. 3. SEM images of the (a) Psyllium (b) Psy-g-Poly (Am-co-An).
characteristic peak was observed at 2θ of 20.3° which indicated that pure psyllium mucilage was amorphous in nature polysaccharides [46,47]. The XRD pattern of grafted psyllium also had a broad characteristic peak at 2θ of 22.4° which also indicated that grafted psyllium was also amorphous in nature. Thus, the amorphous nature of psyllium mucilage was remained unchanged after the grafting [48]. 3.2.3. Scanning electron microscopy The surface morphology of pure psyllium and binary grafted psyllium [Psy-g-Poly (Am-co-An)] has been studied by SEM and the surface morphology of psyllium and grafted psyllium were shown in Fig. 3. It is observed from the SEM that the psyllium has smooth and homogeneous surface morphology, whereas, the modified psyllium has roughness and structural heterogeneity. The homogeneous surface of pure psyllium was vanished after grafting and converted into heterogeneous morphology.
Fig. 5. Zeta potential of Psy-g-Poly (Am-co-An).
three distinguished steps (Fig. 5b). The first weight loss (14.4%) occurred in the temperature range of 18°–250 °C, referred to the loss of water. The second step involves the weight loss of 34.5% in the temperature range of 250°–300 °C. The third stage between 300°–500 °C corresponds to a weight loss of about 44.2%. This involves the complete decomposition of the grafted psyllium chain, indicating better thermal stability of grafted psyllium in comparison to pure psyllium. The increase in thermal stability was due to the presence of acrylamide and acrylonitrile moieties which were successfully grafted on psyllium leading to the strengthening of its molecular structure [49]. These findings were also further supported by DTA and DTG curves (Fig. 4).
3.2.4. Thermal analysis Thermal degradation of psyllium and grafted psyllium were studied by means of TGA, DTG and DTA as shown in Fig. 4. The TGA curve of psyllium was divided into three stages (Fig. 5a). The first weight loss stage (12.16%) ranges between 30°–250 °C is attributed to the loss of surface absorbed moisture and softening of the amorphous structure of psyllium. The second stage of weight loss (45.2%) was in the range of 250°–300 °C due to the dehydration of saccharide rings and depolymerization of psyllium [12]. The third stage of weight loss (38.6%) between 300°–442 °C clearly indicates the complete degradation of psyllium. TGA curve of binary grafted psyllium was also degraded in
3.2.5. Zeta potential The surface properties
of
Fig. 4. TGA, DTA and DTG curves of (a) Psyllium (b) Psy-g-poly (Am-co-An). 4
newly
modified
psyllium
were
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3.3.2. Effect of pH on Hg (II) adsorption The effect of pH on the Hg (II) adsorption was investigated in the pH range 2–8 keeping other parameters constant which affect the adsorption and outcome was presented in Fig. 7b. It was observed that the percentage of Hg (II) adsorption increases from 41% to 92% with increase in pH from 2 to 6 because at low pH mercury exists as Hg2+. Whereas on further increase in the pH from 7 to 8, decrease in Hg (II) adsorption (%) was observed due to the formation of Hg(OH)2 [51] and therefore, pH 6 was selected for kinetic studies, as at pH 6 mercury exists as singular positive ion (Hg+) [52].
Table 1 Measurement of zeta potential. Zeta potential: −16.8 (mV) Mobility: −2.0–4 (cm2/Vs) Conductivity: 0.133 (mS/cm)
Doppler shift: 15 (Hz) Base frequency: 123 (Hz) Conversion equation: smoluchowski
50000
Intensity
40000
3.3.3. Contact time Hg (II) removal study was performed with a fixed adsorbent dose at several time intervals (10–120 min). The result was shown in Fig. 7c. It was observed that of Hg (II) adsorption (%) increases from 53.5% to 94.9% with the increase in adsorption time from 10 to 60 min. This is due to the increase in the metal binding time with vacant adsorbing sites [53]. Further increase in time beyond 60 min did not lead to any significant increase in the adsorption due to the optimum capacity of adsorption sites [9].
30000 20000 10000 0 0
100
200
300
400
500
600
700
3.3.4. Temperature Temperature effect on Hg (II) adsorption was investigated in the range of 20–50 °C under other constant parameters and the results were shown in Fig. 7d. The mercury adsorption constantly increased with the increase in the temperature from 20 to 30 °C. This is due to the increase in the active surface centre site for sorption. Further increase in the temperature, decrease in the adsorption due to some desorption phenomenon was taking place above 30 °C.
Binding Energy (eV) Fig. 6. XPS survey AlKα PES and high-resolution XPS spectrum of binary grafted psyllium with mercury loaded.
investigated and determined by zeta potential. The zeta potential of Psy-g-Poly (Am-co-An) was observed to the negative charge with less mobility as given in Table 1, which clearly indicates their considerable stability in the mercury removal from its solution (Fig. 5).
3.3.5. Initial Hg (II) ion concentration The concentration effect on mercury (II) ion adsorption was shown in Fig. 7e. It was found that the mercury (II) concentration was increased from 50 to 400 ppm. Mercury adsorption was also increased from 48 to 240 ppm due to the increase in the number of available mercuric ion for adsorption. The mercury adsorption was sharply increased with increase in the mercury (II) concentration.
3.2.6. XPS study XPS technique was applied to recognize the interaction between Hg (II) ions and grafted psyllium [Psy-g-Poly (Am-co-An)]. The XPS spectra of the binary grafted psyllium with mercury loaded show a characteristic band with binding energy at 102 eV (Hg4d), 200 eV (Cl2p,) 284 eV (C1s,), 399 eV (N1s) and 531.3 eV (O1s) (Fig. 6). The characteristic peak with binding energy at 102 eV is quite a resemblance with earlier reported XPS data of Hg (II)-loaded polymeric materials [50] and hence clearly indicates the Hg(II) adsorption on Psy-g-Poly (Am-co-An). XPS results of grafted psyllium [Psy-g-Poly (Am-co-An)] with mercury loaded are summarised in Table 2.
3.4.1. Langmuir adsorption isotherm Langmuir adsorption isotherm is highly effective for monolayer sorption because the surface has a finite number of identical sites and expressed in the linear form as Eq. (3) [54].
3.3. Effect of various parameters onto the adsorption
Ce K C = L + e Qe Qm Qm
3.4. Adsorption isotherms and models
where
3.3.1. Adsorbent dose The effect of adsorbent dose on Hg (II) adsorption was studied from 10 to 50 mg, keeping other parameters constant and the result was presented in Fig. 7a. It was observed that the removal of Hg (II) increased from 56.5% to 89.9% with increase in the adsorbent from 10 mg to 30 mg. This is due to the availability of more binding sites at higher doses and the further increase in adsorbent dose from 30 to 50 mg results in nominal increase in elimination of Hg (II). Therefore, 30 mg adsorbent dose was selected for further optimization and kinetic studies.
Ce = Equilibrium concentration Qe = Amount adsorbed at equilibrium Qm = Langmuir constants KL = Heat of adsorption. The vital characteristics of the Langmuir model are explained through means of RL (dimensionless constant) and RL is calculated from the Eq. (4).
RL = Table 2 XPS data of binary grafted psyllium with mercury loaded. Name
Position
FWHM
C1s O1s Hg4d
284.00 532.00 102.00
3.66 2.79 4.43
(3)
1 (1 + KL Co)
(4)
where
Area
at.%
213.79 232.56 250.54
70.33 26.11 3.56
C0 = Hg (II) concentration (mg/L). Adsorption is favorable when the RL value is between 0 and 1. The value of Qm (57.47 mg/g) was calculated from the Langmuir 5
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Fig. 7. Effect of various parameters onto the Hg adsorption (a) Effect of adsorbent dose on to Hg sorption (b) Effect of pH variation on to Hg sorption (c) Effect of time variation on to Hg sorption (d) Effect of temperature variation on to Hg sorption (e) Effect of initial Hg (II) ion concentration on to Hg sorption. Table 3 Correlation coefficients and constant parameters calculated for Langmuir Adsorption Isotherm and Freundlich adsorption models for Hg (II). Langmuir adsorption Isotherm
Freundlich adsorption isotherm
Qm = 57.47 Kl = 0.1649 Rl = (0.1081–0.0149) R2 = 0.9976
Kf = 3.432 n = 4.0883 R2 = 0.9434
3.5. Kinetic studies Kinetic study of mercury adsorption onto Psy-g-Poly (Am-co-An) was investigated by fitting the experimental data in the Elovich equation, pseudo-second, pseudo-first, second, first order and intra-particle diffusion models and were presented in Fig. 9.
Fig. 8. (a) Langmuir (b) Freundlich adsorption model for the adsorption of Hg.
3.5.1. First -order kinetics equation The linear form of first-order kinetics equation is given as Eq. (6).
model, indicating that the adsorbent showed high capacity to remove mercuric ions (Fig. 8a). RL and KL were determined as 0.01493 and 0.01649 mL/mg, respectively, leading to favorable adsorption.
ln
Qo = k1t Qt
(6) −1
where Qo (mgL ) and Qt (mgL ) are the respective concentration of metal ions at the time zero (initial) and a given time ‘t’. K1 (min−1) is the first order [56,57] rate constant and the regression R2 obtained by the linear plot of ln (Qo/Qt) vs t (Fig. 9a), is shown in Table 4. For mercury, R2 was more than 0.9, showing a good fit of the experimental data.
3.4.2. Freundlich adsorption isotherm Freundlich isotherm defines the heterogeneous surface energy through multilayer adsorption and indicates the linear form as Eq. (5) [54,55].
ln qe = ln Kf + n ln Ce
−1
(5)
where
3.5.2. The second order rate equation The linear form second-order kinetics equation is given in Eq. (7) below:
Kf = Adsorption capacity of the adsorbent
1 = k2t (Qo − Qt )
Value of Freundlich parameters (Kf), correlation constant (R2) and rate constant was calculated by Freundlich isotherm (Fig. 8b) given in Table 3. The equilibrium data fitted the Langmuir (R2 = 0.9976) model better than the Freundlich model (R2 = 0.9434), indicating the surface homogeneity of adsorbent and monolayer adsorption.
(7) −1
−1
where K2 [Lmg min ] is the second order [58] rate constant and calculated from Fig. 9b and the value of the constants are given in Table 4. 6
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Fig. 9. Kinetic models for the adsorption of Hg(II) by Psy-g-poly(Am-co-An) (a) first order (b) second order (c) pseudo-first order (d) pseudo-second-order (e) intraparticle diffusion (f) Elovich model.
t t t = + Qt k2Qe 2 Qe
3.5.3. Pseudo-first-order kinetic equation Linear form pseudo-first-order equation is given in Eq. (8) [59].
log
(Qo − Qt ) kt = log Qo − 1 Qo 2.303
(9)
where k2 represents rate constant. The plot for the Eq. (9) was demonstrated in Fig. 9d, on this, shows the data was perfectly fitted to the model and the value of all parameters were given in Table 4. R2 is 0.99 which indicated that the adsorption system was highly in accordance with the kinetic mechanism compared to other kinetic mechanisms [60]. Therefore, it supports the assumption behind the model and suggests that the overall rate of Hg (II) adsorption by Psy-g-Poly (Amco-An) appeared to be controlled by the physicochemical process.
(8)
where, Qt, Q0 and k1 are adsorbate at time t, adsorption ability at equilibrium, and rate constant respectively. All parameters of this equation were calculated by using data in Fig. 9c (the result is shown in Table 4).
3.5.4. Pseudo-second order kinetics equation The pseudo-second-order kinetic rate was studied by Eq. (9) [51].
3.5.5. Intraparticle diffusion Eq. (10) is intraparticle diffusion kinetic equation [61].
Table 4 Comparison of the first order, second order, pseudo-first order, pseudo-second-order, intra-particle diffusion and elovich equation models parameters for the sorption by Psy-g-poly (Am-co-An) at initial Hg (II) concentration of 100 mg/ L, dose 30 mg, contact volume 10 mL and temperature 25 °C. S.N.
Kinetic model
Linear form
1
First-order rate equation
ln
2
Second order rate equation
3
Pseudo first-order equation
Qo Qt
Plot
= k1t
1 (Qo − Qt)
ln
vs t
1 (Qo − Qt)
= k2t
log (Qo − Qt)/Qo = log Qo −
log t Qt
vs t
(Qo − Qt) Qo
4
Pseudo-second-order rate equation
5
Intraparticle diffusion
Qt = kid t0.5 + C
Qt vs t.5
6
Elovich equation model
Qt = αlog(aα) + α ln (t)
Qt vs ln (t)
k2Qe2
+
t Qe
k1t 2.303
t Qt
=
t
Qo Qt
Parameter
7
vs t
vs t
K1 = 0.0014 R2 = 0.9971 K2 = 4 × 10−5 R2 = 0.99877 K1 = 2.3 × 10−5 R2 = 0.9947 Qo = 77.51 K2 = 9.9 × 10−5 R2 = 0.9481 Kid = 2.76 C = 9.16 R2 = 0.9958 α = 16.11 a = 0.982 R2 = 0.9624
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Qt = Kid t0.5 + C
(10)
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Namasivayam, Removal of heavy metals from industrial wastewaters by adsorption onto activated carbon prepared from an agricultural solid waste, Bioresour. Technol. 76 (1) (2001) 63–65. [21] J. Benoit, R.P. Mason, C.C. Gilmour, G.R. Aiken, Constants for mercury binding by dissolved organic matter isolates from the Florida Everglades, Geochim. Cosmochim. Acta 65 (24) (2001) 4445–4451. [22] K. Nahar, M.K. Hossain, T.A. Khan, Alginate and its versatile application in drug delivery, Journal of Pharmaceutical Sciences and Research 9 (5) (2017) 606. [23] U. Remminghorst, B.H. Rehm, Bacterial alginates: from biosynthesis to applications, Biotechnol. Lett. 28 (21) (2006) 1701–1712. [24] W. Chen, J. Mo, X. Du, Z. Zhang, W. Zhang, Biomimetic dynamic membrane for aquatic dye removal, Water Res. 151 (2019) 243–251. [25] T.A. Saleh, A. Sarı, M. Tuzen, Optimization of parameters with experimental design for the adsorption of mercury using polyethylenimine modified-activated carbon, Journal of Environmental Chemical Engineering 5 (1) (2017) 1079–1088. [26] A.M. Aljeboree, A.N. Alshirifi, A.F. Alkaim, Kinetics and equilibrium study for the adsorption of textile dyes on coconut shell activated carbon, Arab. J. Chem. 10 (2017) S3381–S3393. [27] M. Ghaedi, B. Sadeghian, A.A. Pebdani, R. Sahraei, A. Daneshfar, C. Duran, Kinetics, thermodynamics and equilibrium evaluation of direct yellow 12 removal by adsorption onto silver nanoparticles loaded activated carbon, Chem. Eng. J. 187 (2012) 133–141. [28] P. Askari, A. Faraji, G. Khayatian, S. Mohebbi, Effective ultrasound-assisted removal of heavy metal ions As (III), Hg (II), and Pb (II) from aqueous solution by new MgO/ CuO and MgO/MnO2 nanocomposites, J. Iran. Chem. Soc. 14 (3) (2017) 613–621. [29] D. Kumar, J. Pandey, P. Kumar, Synthesis and characterization of modified chitosan via microwave route for novel antibacterial application, Int. J. Biol. Macromol. 107 (2018) 1388–1394. [30] A. Mishra, J.H. Clark, S. Pal, Modification of Okra mucilage with acrylamide: synthesis, characterization and swelling behavior, Carbohydr. Polym. 72 (4) (2008) 608–615. [31] D. Bhatia, N.R. Sharma, J. Singh, R.S. Kanwar, Biological methods for textile dye removal from wastewater: a review, Crit. Rev. Environ. Sci. Technol. 47 (19) (2017) 1836–1876. [32] B. Singha, S.K. Das, Removal of Pb (II) ions from aqueous solution and industrial effluent using natural biosorbents, Environ. Sci. Pollut. Res. 19 (6) (2012) 2212–2226. [33] C. Zhou, H. Zhu, Q. Wang, J. Wang, J. Cheng, Y. Guo, X. Zhou, R. Bai, Adsorption of mercury (ii) with an Fe3O4 magnetic polypyrrole–graphene oxide nanocomposite, RSC Adv. 7 (30) (2017) 18466–18479. [34] Q. Li, L. Sun, Y. Zhang, Y. Qian, J. Zhai, Characteristics of equilibrium, kinetics studies for adsorption of Hg(II) and Cr(VI) by polyaniline/humic acid composite, Desalination 266 (1) (2011) 188–194. [35] S. Mallakpour, S. Rashidimoghadam, 9 - Carbon nanotubes for dyes removal, in: G.Z. Kyzas, A.C. Mitropoulos (Eds.), Composite Nanoadsorbents, Elsevier, 2019, pp. 211–243. [36] V. Katheresan, J. Kansedo, S.Y. Lau, Efficiency of various recent wastewater dye removal methods: a review, J. Environ. Chem. Eng. 6 (2018) 4676–4697. [37] C.W. Phetphaisit, S. Yuanyang, W.C. Chaiyasith, Polyacrylamido-2-methyl-1-propane sulfonic acid-grafted-natural rubber as bio-adsorbent for heavy metal removal from aqueous standard solution and industrial wastewater, J. Hazard. Mater. 301 (2016) 163–171. [38] A. M. Awwad, A. M. Farhan, Equilibrium, kinetic and thermodynamics of biosorption of lead (II) copper (II) and cadmium (II) ions from aqueous solutions on to olive leaves powder, American Journal of Chemistry 2 (4) (2012) 238–244. [39] K. Jainae, N. Sukpirom, S. Fuangswasdi, F. Unob, Adsorption of Hg(II) from aqueous solutions by thiol-functionalized polymer-coated magnetic particles, J. Ind. Eng. Chem. 23 (2015) 273–278.
−0.5
) and C is intraparticle diffuwhere Kid is rate constant (mg/ g min sion constant (mg/ g). Fig. 9e shows the intraparticle diffusion kinetic curve for mercury adsorption and the calculated value of intraparticle diffusion parameters were given in Table 4. It was observed that the rate constant enhanced with enhanced mercury concentration. 3.5.6. Elovich rate equation The Elovich equation [62] is given in Eq. (11)
Qt = αlog(aα) + αln(t)
(11)
where Q is the amount adsorbed at t time and α (g/mg) and a (mg/ g−1 min−1) are the Elovich constants [63]. The values of α and a were calculated with the help of linear plot of Qt Vs lnt (Fig. 9f) and presented in Table 4. 4. Conclusion The synthesis of a binary grafted copolymer of Psyllium with acrylamide and acrylonitrile [Psy-g-poly (Am-co-An)] was successfully achieved by using the conventional route in the presence of CAN initiator under the N2 atmosphere. In this experiment, the prominent effect of monomer concentration, initiator concentration, temperature and different reaction time interval were investigated to achieve maximum yield. The synthesized grafted psyllium exhibited the high efficiency for mercury ions adsorption. The adsorption of mercuric ions through Psy-g-poly (Am-co-An) was found to be pH dependent and pH 6 was found to be highly suitable for the adsorption. The adsorption followed kinetic of the second order, which indicated the chemisorption mechanism and the adsorption isotherm was following the Langmuir model, indicating monolayer formation. In this regard, we opined that Psy-g-Poly (Am-co-An) may serve as a milestone in the path of the environmental field as a super adsorbent for effective toxic metal removal from the aquatic system. Acknowledgments The authors are thankful to University Grants Commission, New Delhi, India for financial assistance and of the author Dr. Pramendra Kumar is thankful to coordinator TEQIP-III FET MJPRU Bareilly to provide the partial financial support, and USIC, BBAU, Lucknow for characterization of grafted polymer, and IIT Roorkee, India for XRD, XPS and thermal study. References [1] P.K. Rai, Heavy metal pollution in aquatic ecosystems and its phytoremediation using wetland plants: an ecosustainable approach, International journal of phytoremediation 10 (2) (2008) 133–160. [2] M. Tuzen, A. Sari, D. Mendil, M. Soylak, Biosorptive removal of mercury (II) from aqueous solution using lichen (Xanthoparmelia conspersa) biomass: kinetic and equilibrium studies, J. Hazard. Mater. 169 (1–3) (2009) 263–270. [3] Z. He, M. Zhang, X. Yang, P. Stoffella, Release behavior of copper and zinc from sandy soils, Soil Sci. Soc. Am. J. 70 (5) (2006) 1699–1707. [4] D. Shi, F. Yan, M. Wang, Y. Zou, T. Zheng, X. Zhou, L. Chen, Rhodamine derivative functionalized chitosan as efficient sensor and adsorbent for mercury (II) detection and removal, Mater. Res. Bull. 70 (2015) 958–964. [5] H. Sadegh, G.A.M. Ali, A.S.H. Makhlouf, K.F. Chong, N.S. Alharbi, S. Agarwal, V.K. Gupta, MWCNTs-Fe3O4 nanocomposite for Hg(II) high adsorption efficiency, J. Mol. Liq. 258 (2018) 345–353. [6] A. Sari, M. Tuzen, Removal of mercury (II) from aqueous solution using moss (Drepanocladus revolvens) biomass: equilibrium, thermodynamic and kinetic studies, J. Hazard. Mater. 171 (1–3) (2009) 500–507. [7] K. Li, Y. Wang, M. Huang, H. Yan, H. Yang, S. Xiao, A. Li, Preparation of chitosangraft-polyacrylamide magnetic composite microspheres for enhanced selective removal of mercury ions from water, J. Colloid Interface Sci. 455 (2015) 261–270. [8] I. Ghodbane, O. Hamdaoui, Removal of mercury (II) from aqueous media using eucalyptus bark: kinetic and equilibrium studies, J. Hazard. Mater. 160 (2) (2008) 301–309. [9] H. Kolya, S. Das, T. Tripathy, Synthesis of Starch-g-Poly-(N-methylacrylamide-coacrylic acid) and its application for the removal of mercury (II) from aqueous
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D. Kumar, et al. [40] D. Kumar, P. Kumar, J. Pandey, Binary grafted chitosan film: synthesis, characterization, antibacterial activity and prospects for food packaging, Int. J. Biol. Macromol. 115 (2018) 341–348. [41] J.C. Sousa, A.R. Ribeiro, M.O. Barbosa, M.F.R. Pereira, A.M. Silva, A review on environmental monitoring of water organic pollutants identified by EU guidelines, J. Hazard. Mater. 344 (2018) 146–162. [42] D. Kumar, S. Kumar, Grafting of acrylic acid on to Plantago psyllium mucilage, IOSR J, Appl. Chem. (IOSR-JAC) 7 (2014) 76–82. [43] S. El Harfi, A. El Harfi, Classifications, properties and applications of textile dyes: a review, Applied Journal of Environmental Engineering Science 3 (3) (2017) 311–320 00000-3 N° 3. [44] S.A. Jadhav, H.B. Garud, A.H. Patil, G.D. Patil, C.R. Patil, T.D. Dongale, P.S. Patil, Recent advancements in silica nanoparticles based technologies for removal of dyes from water, Colloid and Interface Science Communications 30 (2019) 100181. [45] B.S. Kaith, R. Jindal, H. Mittal, Superabsorbent hydrogels from poly (acrylamide-coacrylonitrile) grafted Gum ghatti with salt, pH and temperature responsive properties, Der Chemica Sinica 1 (2) (2010) 92–103. [46] M.N.R. Kumar, A review of chitin and chitosan applications, React. Funct. Polym. 46 (1) (2000) 1–27. [47] N.M. Alves, J.F. Mano, Chitosan derivatives obtained by chemical modifications for biomedical and environmental applications, Int. J. Biol. Macromol. 43 (5) (2008) 401–414. [48] S.K. Bajpai, N. Chand, A. Agrawal, Microwave-assisted synthesis of carboxymethyl psyllium and its development as semi-interpenetrating network with poly (acrylamide) for gastric delivery, J. Bioact. Compat. Polym. 30 (3) (2015) 241–257. [49] F. Okieimen, Preparation, characterization, and properties of cellulose–polyacrylamide graft copolymers, J. Appl. Polym. Sci. 89 (4) (2003) 913–923. [50] S. Huang, C. Ma, Y. Liao, C. Min, P. Du, Y. Jiang, Removal of mercuryII from aqueous solutions by adsorption on poly1-amino-5-chloroanthraquinone nanofibrils, J. Nanomater. 2016 (2016) 53. [51] H.K. Moghaddam, M. Pakizeh, Experimental study on mercury ions removal from aqueous solution by MnO2/CNTs nanocomposite adsorbent, J. Ind. Eng. Chem. 21 (2015) 221–229. [52] Z. Li, F. Yang, R. Yang, Synthesis and characterization of chitosan derivatives with dual-antibacterial functional groups, Int. J. Biol. Macromol. 75 (2015) 378–387.
[53] M. Aslam, S. Rais, M. Alam, A. Pugazhendi, Adsorption of Hg (II) from aqueous solution using Adulsa (Justicia adhatoda) leaves powder: kinetic and equilibrium studies, Journal of Chemistry 2013 (2013). [54] Q. Li, Z. Wang, D.-M. Fang, H.Y. Qu, Y. Zhu, H.J. Zou, Y.R. Chen, Y.-P. Du, H.-l. Hu, Preparation, characterization, and highly effective mercury adsorption of L-cysteine-functionalized mesoporous silica, New J. Chem. 38 (1) (2014) 248–254. [55] M. Vhahangwele, G.W. Mugera, N. Tholiso, Defluoridation of drinking water using Al3+-modified bentonite clay: optimization of fluoride adsorption conditions, Toxicol. Environ. Chem. 96 (9) (2014) 1294–1309. [56] W. Gin, A. Jimoh, A. Abdulkareem, A. Giwa, Kinetics and isotherm studies of heavy metal removals from electroplating wastewater using cassava peel activated carbon, Int. J. Eng. 3 (3) (2014). [57] M. Abuh, G. Akpomie, N. Nwagbara, N. Abia-Bassey, D. Ape, B. Ayabie, Kinetic rate equations application on the removal of copper (II) and zinc (II) by unmodified lignocellulosic fibrous layer of palm tree trunk-single component system studies, International Journal of Basic and Applied Science 1 (4) (2013) 800–809. [58] M. Matouq, N. Jildeh, M. Qtaishat, M. Hindiyeh, M.Q. Al Syouf, The adsorption kinetics and modeling for heavy metals removal from wastewater by Moringa pods, Journal of Environmental Chemical Engineering 3 (2) (2015) 775–784. [59] A. Okewale, P. Igbokwe, J. Ogbuagu, Kinetics and isotherm studies of the adsorptive dehydration of ethanol-water system with biomass based materials, Int. J. Eng. Innov. Technol 2 (9) (2013) 36–42. [60] S. Rout, A. Kumar, P.M. Ravi, R.M. Tripathi, Pseudo-second order kinetic model for the sorption of U (VI) onto soil: a comparison of linear and non-linear methods, Int. J. Environ. Sci. 6 (1) (2015) 145. [61] A. Itodo, F. Abdulrahman, L. Hassan, S. Maigandi, H. Itodo, Intraparticle diffusion and intraparticulate diffusivities of herbicide on derived activated carbon, Researcher 2 (2) (2010) 74–86. [62] M.K. Pati, P. Nayak, Grafting vinyl monomers onto chitosan: IV: graft copolymerized of acrylicacid onto chitosan using ceric ammonium nitrate as the initiator—characterization and antimicrobial activities, Mater. Sci. Appl. 2 (12) (2011) 1741. [63] R.-S. Juang, M.-L. Chen, Application of the Elovich equation to the kinetics of metal sorption with solvent-impregnated resins, Ind. Eng. Chem. Res. 36 (3) (1997) 813–820.
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Materials Science & Engineering C journal homepage: www.elsevier.com/locate/msec
Synthesize and characterization of binary grafted psyllium for removing toxic mercury (II) ions from aqueous solution
T
⁎
Deepak Kumara,b, , Jyoti Pandeya, Nida Khanb, Pramendra Kumarb, Patit P. Kunduc a
Department of Chemistry, Babasaheb Bhimrao Ambedkar University (A Central University), Lucknow 226025, U.P., India Department of Applied Chemistry, M J P Rohilkhand University, Bareilly 243006, U.P., India c Department of Chemical Engineering, Indian Institute of Technology Roorkee, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Psyllium Graft copolymer Mercury Adsorption Kinetic models
Acrylamide and acrylonitrile were grafted on psyllium employing ceric ammonium nitrate (CAN) as initiator under N2 atmosphere to get an adsorbent of mercury ions. The synthesized adsorbent was optimized by varying synthetic parameters viz. monomer concentration, reaction time, temperature, initiator concentration, etc. to obtain the maximum yield of the grafted product as well as maximum adsorption of ionic mercury. The synthesized adsorbent was characterized by FT-IR, SEM, XRD, zeta potential and thermal techniques. The effect of various process parameters such as pH, time, adsorption dose and temperature on Hg (II) adsorption was investigated. The maximum Hg (II) adsorption (96%) was achieved at temperature (30 °C), dose (30 mg), pH (6), time (60 min) and initial concentration of mercury with 100 ppm. The Hg(II) adsorption on Psy-g-Poly (Am-coAn) was confirmed by XPS study. The isotherm data of the adsorption experiments obeyed the classical Langmuir adsorption isotherm. On the other hand, the kinetic data followed the second-order kinetics, indicating the chemisorption mechanism.
1. Introduction Due to fast industrial development, an increased discharge of metal pollutants in the aquatic system has been observed [1]. Given their liquidity and poisonousness, metal ions, especially heavy metal ions, create significant risk for animals, plants and the environment. Lead, copper and mercury are three common heavy metals having high toxicity, which is prevalent in many industrial effluents, such as electroplating industries, nonferrous metals smelting, and mine tailing [2,3]. Among these toxic metals, mercury, released from the various type of natural sources and habitat, is noted as the greatest noxious and universal heavy-metal of pollutants [4]. Methyl mercury is the most toxic form of mercury (Hg). Mercury can enter the human body through the contaminated food chain [5] and has a high tendency to bind with protein chains through HgeS strong bond with cysteine residue [6]. It is highly harmful to the kidney, cardiovascular system [7], bones and also damages the human nervous system [8,9]. Mercury removal from water has been a challenge for the global scientific community for decades. At present, it is a major concern because of its toxicity and volatility into the environment [10–12]. The Agency for Toxic Substances and Disease Registry (ATSDR) declared that mercury is highly dangerous metal due to its high toxicity, mobility
⁎
and their high resistive tenure in the atmosphere [13]. The minor concentration of the mercuric ions in water is toxic as the maximum permissible limit as recommended by WHO is 0.002 ppm in aquatic life [14,15]. The noxiousness of mercury mainly depends on the valance state of mercury [7]. Mercury, in natural water, exists in 0, +1 and +2 oxidation states and may be present in various other hydrated forms depending on the factors like ionic strength, the dose of the suspended particulate matter (SPM), temperature, pH, salinity and organic solvents [16]. The various mercury forms include organic mercury (CH3Hg and C2H2Hg), inorganic mercury (HgCl2) and elemental mercury [Hg] [17–19]. Mercury vapors on reaching human blood through the respiratory system spread all over the human body and changes to +2 oxidation state [Hg (II)], a highly toxic stale to human beings [9,20]. The main sources of mercury are various chemical industries mainly fertilizers, pulp, paper, battery manufacturing, plastic, paint and oil refining [21] and spread in the environment through the wastewater line of these chemical industries [22–24]. Various techniques such as advanced oxidation, coagulation, reverse osmosis, chemical precipitation adsorption, ion exchange, absorption and biomass adsorption [25] have been performed for wastewater treatment and for removal of hazardous material and
Corresponding author at: Department of Chemistry, Babasaheb Bhimrao Ambedkar University (A Central University), Lucknow 226025, U.P., India. E-mail address: [email protected] (D. Kumar).
https://doi.org/10.1016/j.msec.2019.109900 Received 6 November 2018; Received in revised form 13 June 2019; Accepted 15 June 2019 Available online 17 June 2019 0928-4931/ © 2019 Published by Elsevier B.V.
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contaminants [26,27]. Among these techniques, the adsorption method is found to be a standard, cheap and highly used technique for the removal of toxic metal from the aqueous solution [28]. Many adsorbents including graft copolymer/modified polymers have been established for toxic metal removal purpose, due to their unique chemical, electrical, mechanical, rheological and thermal characteristics. The present work deals with the preparation and characterization of psyllium based binary grafted material via grafting of acrylamide (Am) and acrylonitrile (An) with psyllium for mercury adsorption. The graft copolymers were synthesized via free radical polymerization using ceric ammonium nitrate/ascorbic acid [(CAN)] as the free radical initiator under thermal conditions. The graft copolymer was characterized via FTIR, SEM, XRD and thermal analysis. The Hg (II) adsorption capability of the adsorbent [Psy-g-Poly (Am-co-An)] through the batch adsorption method has also been studied.
Grafting% =
wt.of grafting psyllium × 100 wt.of pure psyllium
2.3. Hg (II) adsorption method A standard solution of 1000 ppm of Hg (II) was obtained by dissolving 1.354 g of HgCl2 in 1 L deionized double distilled water [32]. All mercury (II) adsorption experiments were investigated at room temperature [33]. The effect of various influences like adsorbent amount, contact time, pH and Hg (II) concentration were studied by batch adsorption experiment. The effect of pH on mercury adsorption was investigated at various pH by adjusting pH with 0.1 M HCl or 0.1 M NaOH [34]. 20 mL Hg (II) solution (100 ppm) was taken in 50 mL beaker 20 mg adsorbent was added to and it stirred with a magnetic stirrer for the desired time period, and filtered the solution using Whatman 0.45 mm filter paper. After appropriate dilution, the remaining quantity of Hg (II) was measured by a double beam UV spectrophotometer (λ-575 nm) using the rhodamine 6G dye and iodine buffer solution [35,36]. The quantity of Hg2+ adsorbed by grafted copolymer in ppm was calculated by the following Eq. (2) [37].
2. Experimental 2.1. Materials Psyllium husks were procured from Sidhpur Sat-Isabgol Factory India. Acrylamide (Am), acrylonitrile (An), sodium hydroxide (NaOH), hydrochloric acid (HCl), methyl alcohol (MeOH), acetone (MeCoMe), gelatin, ascorbic acid and mercuric chloride (HgCl2) were supplied by Merck Ltd. Mumbai, India. Rhodamine 6G and potassium iodide (KI) was supplied by SD fine Chem. Ltd. Mumbai, India. Double distilled water was used for synthesis and adsorption analysis. Chemical structures of newly synthesized psyllium and grafted psyllium [Psy-g-Poly (Am-co-An)] were examined by Fourier transform infrared (FTIR) (Nicole - 6700) spectrophotometer with wave number range of 4000 to 500 cm−1. X-ray diffraction analysis was carried out on Bruker-D8 advance diffractometer (Shimadzu, Japan) in 2-theta angle range 5°–40°. The surface morphology of pure psyllium and grafted psyllium were studied by scanning electron microscope (JSM, 6490) [29]. Thermogravimetric analysis (TGA) was performed by using SII 6300 EXSTAR TG-DTA (Japan). All measurements of analysis were carried out under a nitrogen atmosphere [12]. The conductivity of newly grafted psyllium was measured by zeta sizer 1000 HS (Malvern Instruments, Worcestershire, UK) at 25 °C and 90° angle. Zeta potential graph was directly obtained from the zeta sizer software (Malvern, USA). The pH of the solution was measured using a Digital pH meter (Globe instrument auto pH meter). The mercury adsorption capacity of Psy-g-Poly (Am-co-An) was performed by using Systronics double beam UV visible spectrophotometer 2203 [10]. The XPS experiments were performed in an ultrahigh vacuum (UHV) using high-resolution X-ray photoelectron spectrophotometer (PHI 5000 VersaProbe III). All samples were dehydrated under vacuum prior to XPS analysis.
qe =
Qe − Qo ×V W
(2)
where qe = the amount of the metal adsorbed (ppm) onto the adsorbent, Qo = the initial concentration of the solution (ppm), Qe = equilibrium concentration of the solution (ppm), V = Volume and W = adsorbent weight. 2.4. Optimization of various adsorption conditions The mercury adsorption by grafted psyllium was investigated by varying only one adsorption parameter at a time while keeping others fixed [11]. Various adsorption parameters and their range [pH (4 to 10), adsorbent dose (10 mg–70 mg), temperature (15 °C to 50 °C), contact time 60 min and contact volume 10 mL at 100 ppm of mercury (II) concentration] were studied. 2.5. Adsorption isotherm studies For isotherm investigation, the adsorption equilibrium data was originated at different initial concentrations of Hg (II) ranging from 50 ppm to 400 ppm using 30 mg of the grafted psyllium, pH 6, 60 min contact time with 20 mL mercury solution at room temperature. 2.6. Kinetic studies In order to investigate kinetic data, the interaction time was varied from 10 to 120 min and the kinetic studies were completed by using 100 ppm of Hg(II) concentration, 30 mg of adsorbent dose at pH 6 and a temperature of 25 °C [38,39].
2.2. Synthesis of Psy-g-Poly (Am-co-An) Grafted psyllium [Psy-g-Poly (Am-co-An)] was synthesized through our earlier reported protocol [12]. Briefly, 1.0 g of psyllium mucilage was dissolved in double distilled water in a two-necked round bottom flask. The required amount of acrylamide and acrylonitrile monomers were dissolved in distilled water (10 mL) in a conical flask and this solution was added to the psyllium solution. The round bottom flask was wrapped by septum stopper assembly to flush nitrogen gas into the solution by using a hypodermic needle throughout the duration of the reaction. Later on, an essential amount of CAN/ascorbic acid initiator was injected in the solution via hypodermic syringe and reaction mixture was continuously stirred at 30 °C for a required time followed by adding 0.5 mL of saturated aqueous hydroquinone solution to terminate the reaction [30,31]. The reaction product [Psy-g-Poly (Am-co-An)] was allowed to precipitate out in methanol and washed with acetone and dried at 40 °C. The grafting (%) was calculated as follows:
3. Result and discussion 3.1. Effect of various parameters variation onto grafting (%) 3.1.1. Monomers concentration The effect of monomer concentration onto grafting (%) was shown in Fig. 1a and it was obtained from the binary mixture of both monomers (acrylamide and acrylonitrile). The concentration of acrylamide was varied from 0.07 to 0.28 mol/L in different sets of experiments while keeping the concentration of acrylonitrile constant at 0.01 mol/L. The grafting increased with the increase in acrylamide concentration (0.07 mol/L to 0.21 mol/L), but as acrylamide concentration was increased beyond 0.21 mol/L, grafting started to decrease. The initial augmentation of monomers gradually increases the grafting with the 2
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Fig. 1. Effect of various parameters onto the grafting (a) Effect the monomer concentration on to grafting (b) Effect of initiator (CAN) Concentration on to grafting (c) Effect of reaction time on to grafting (d) Effect of reaction temperature on to grafting.
dispersion of acrylamide to the backbone psyllium. As the acrylonitrile content increased, grafting decreased due to the formation of a homopolymer as well as copolymer [29,40]. 3.1.2. Initiator (CAN) concentration The effect of initiator concentration (CAN) on grafting (%) was shown in Fig. 1b. On increasing the concentration of CAN initiator from 1.8 × 10−3 to 5.4 × 10−3 mol/L grafting (%) increased due to increase in the obtainability of more free radicals sites on psyllium backbone. The decrease in the grafting (%) at the higher concentration of initiator (5.4 × 10−3 to 7.2 × 10−3 mol/L) is a well-known phenomenon and ascribed to the increasing participation of the ceric ion in the termination of the growing grafted chains [41,42]. 3.1.3. Reaction time The effect of time on grafting (%) is shown in Fig. 1c. The grafting enhanced with the increase in time from 30 to 124 min and then a slight decline in grafting (%) was observed. The quick increase of grafting between 60 and 102 min is due to the higher rate of initiation and propagation and the decline of grafting after 120 min is a clear indication of depletion of monomer concentration from the solution [30,43]. 3.1.4. Reaction temperature The impact of reaction temperature on grafting yield has been investigated at the temperature range from 20 °C to 50 °C as shown in Fig. 1d. It was found that the grafting yield slightly increased up to 50 °C i.e. the grafting yield increased with the increase in temperature due to the increase in the number of reactive sites of monomer [44].
Fig. 2. (a)FT-IR spectra of the Psyllium and Psy–g-Poly (Am-co-An) (b) XRD spectra of the Psyllium and Psy-g-Poly (Am-co-An).
stretching vibrations. The FTIR spectrum of grafted psyllium showed additional peaks at 2240.78 cm−1 (C^N [45] nitrile stretching), 1726, 1673.25 cm−1 (C]O stretching of amide-I) [43], 1423 cm−1 (NeH inplane bending of amide-II) and 1251 cm−1 (CeN stretching of amideIII), which were absent in the IR spectrum of pure psyllium. This clearly indicates the formation of graft polymer.
3.2. Characterization 3.2.1. FTIR spectra The FTIR spectra of psyllium and Psy-g-Poly (Am-co-An) were shown in Fig. 2a, respectively. The FTIR spectra of purified psyllium showed a characteristic peak at 3392 cm−1, due to stretching vibration of OeH, whereas smaller peak at 2923 cm−1 is assigned to the CeH stretching vibrations. The peak at 1043 cm−1 is due to the CeOeC
3.2.2. XRD analysis The XRD pattern of psyllium and grafted psyllium are shown in Fig. 2b. In the XRD pattern of pure psyllium mucilage, a broad 3
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Fig. 3. SEM images of the (a) Psyllium (b) Psy-g-Poly (Am-co-An).
characteristic peak was observed at 2θ of 20.3° which indicated that pure psyllium mucilage was amorphous in nature polysaccharides [46,47]. The XRD pattern of grafted psyllium also had a broad characteristic peak at 2θ of 22.4° which also indicated that grafted psyllium was also amorphous in nature. Thus, the amorphous nature of psyllium mucilage was remained unchanged after the grafting [48]. 3.2.3. Scanning electron microscopy The surface morphology of pure psyllium and binary grafted psyllium [Psy-g-Poly (Am-co-An)] has been studied by SEM and the surface morphology of psyllium and grafted psyllium were shown in Fig. 3. It is observed from the SEM that the psyllium has smooth and homogeneous surface morphology, whereas, the modified psyllium has roughness and structural heterogeneity. The homogeneous surface of pure psyllium was vanished after grafting and converted into heterogeneous morphology.
Fig. 5. Zeta potential of Psy-g-Poly (Am-co-An).
three distinguished steps (Fig. 5b). The first weight loss (14.4%) occurred in the temperature range of 18°–250 °C, referred to the loss of water. The second step involves the weight loss of 34.5% in the temperature range of 250°–300 °C. The third stage between 300°–500 °C corresponds to a weight loss of about 44.2%. This involves the complete decomposition of the grafted psyllium chain, indicating better thermal stability of grafted psyllium in comparison to pure psyllium. The increase in thermal stability was due to the presence of acrylamide and acrylonitrile moieties which were successfully grafted on psyllium leading to the strengthening of its molecular structure [49]. These findings were also further supported by DTA and DTG curves (Fig. 4).
3.2.4. Thermal analysis Thermal degradation of psyllium and grafted psyllium were studied by means of TGA, DTG and DTA as shown in Fig. 4. The TGA curve of psyllium was divided into three stages (Fig. 5a). The first weight loss stage (12.16%) ranges between 30°–250 °C is attributed to the loss of surface absorbed moisture and softening of the amorphous structure of psyllium. The second stage of weight loss (45.2%) was in the range of 250°–300 °C due to the dehydration of saccharide rings and depolymerization of psyllium [12]. The third stage of weight loss (38.6%) between 300°–442 °C clearly indicates the complete degradation of psyllium. TGA curve of binary grafted psyllium was also degraded in
3.2.5. Zeta potential The surface properties
of
Fig. 4. TGA, DTA and DTG curves of (a) Psyllium (b) Psy-g-poly (Am-co-An). 4
newly
modified
psyllium
were
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3.3.2. Effect of pH on Hg (II) adsorption The effect of pH on the Hg (II) adsorption was investigated in the pH range 2–8 keeping other parameters constant which affect the adsorption and outcome was presented in Fig. 7b. It was observed that the percentage of Hg (II) adsorption increases from 41% to 92% with increase in pH from 2 to 6 because at low pH mercury exists as Hg2+. Whereas on further increase in the pH from 7 to 8, decrease in Hg (II) adsorption (%) was observed due to the formation of Hg(OH)2 [51] and therefore, pH 6 was selected for kinetic studies, as at pH 6 mercury exists as singular positive ion (Hg+) [52].
Table 1 Measurement of zeta potential. Zeta potential: −16.8 (mV) Mobility: −2.0–4 (cm2/Vs) Conductivity: 0.133 (mS/cm)
Doppler shift: 15 (Hz) Base frequency: 123 (Hz) Conversion equation: smoluchowski
50000
Intensity
40000
3.3.3. Contact time Hg (II) removal study was performed with a fixed adsorbent dose at several time intervals (10–120 min). The result was shown in Fig. 7c. It was observed that of Hg (II) adsorption (%) increases from 53.5% to 94.9% with the increase in adsorption time from 10 to 60 min. This is due to the increase in the metal binding time with vacant adsorbing sites [53]. Further increase in time beyond 60 min did not lead to any significant increase in the adsorption due to the optimum capacity of adsorption sites [9].
30000 20000 10000 0 0
100
200
300
400
500
600
700
3.3.4. Temperature Temperature effect on Hg (II) adsorption was investigated in the range of 20–50 °C under other constant parameters and the results were shown in Fig. 7d. The mercury adsorption constantly increased with the increase in the temperature from 20 to 30 °C. This is due to the increase in the active surface centre site for sorption. Further increase in the temperature, decrease in the adsorption due to some desorption phenomenon was taking place above 30 °C.
Binding Energy (eV) Fig. 6. XPS survey AlKα PES and high-resolution XPS spectrum of binary grafted psyllium with mercury loaded.
investigated and determined by zeta potential. The zeta potential of Psy-g-Poly (Am-co-An) was observed to the negative charge with less mobility as given in Table 1, which clearly indicates their considerable stability in the mercury removal from its solution (Fig. 5).
3.3.5. Initial Hg (II) ion concentration The concentration effect on mercury (II) ion adsorption was shown in Fig. 7e. It was found that the mercury (II) concentration was increased from 50 to 400 ppm. Mercury adsorption was also increased from 48 to 240 ppm due to the increase in the number of available mercuric ion for adsorption. The mercury adsorption was sharply increased with increase in the mercury (II) concentration.
3.2.6. XPS study XPS technique was applied to recognize the interaction between Hg (II) ions and grafted psyllium [Psy-g-Poly (Am-co-An)]. The XPS spectra of the binary grafted psyllium with mercury loaded show a characteristic band with binding energy at 102 eV (Hg4d), 200 eV (Cl2p,) 284 eV (C1s,), 399 eV (N1s) and 531.3 eV (O1s) (Fig. 6). The characteristic peak with binding energy at 102 eV is quite a resemblance with earlier reported XPS data of Hg (II)-loaded polymeric materials [50] and hence clearly indicates the Hg(II) adsorption on Psy-g-Poly (Am-co-An). XPS results of grafted psyllium [Psy-g-Poly (Am-co-An)] with mercury loaded are summarised in Table 2.
3.4.1. Langmuir adsorption isotherm Langmuir adsorption isotherm is highly effective for monolayer sorption because the surface has a finite number of identical sites and expressed in the linear form as Eq. (3) [54].
3.3. Effect of various parameters onto the adsorption
Ce K C = L + e Qe Qm Qm
3.4. Adsorption isotherms and models
where
3.3.1. Adsorbent dose The effect of adsorbent dose on Hg (II) adsorption was studied from 10 to 50 mg, keeping other parameters constant and the result was presented in Fig. 7a. It was observed that the removal of Hg (II) increased from 56.5% to 89.9% with increase in the adsorbent from 10 mg to 30 mg. This is due to the availability of more binding sites at higher doses and the further increase in adsorbent dose from 30 to 50 mg results in nominal increase in elimination of Hg (II). Therefore, 30 mg adsorbent dose was selected for further optimization and kinetic studies.
Ce = Equilibrium concentration Qe = Amount adsorbed at equilibrium Qm = Langmuir constants KL = Heat of adsorption. The vital characteristics of the Langmuir model are explained through means of RL (dimensionless constant) and RL is calculated from the Eq. (4).
RL = Table 2 XPS data of binary grafted psyllium with mercury loaded. Name
Position
FWHM
C1s O1s Hg4d
284.00 532.00 102.00
3.66 2.79 4.43
(3)
1 (1 + KL Co)
(4)
where
Area
at.%
213.79 232.56 250.54
70.33 26.11 3.56
C0 = Hg (II) concentration (mg/L). Adsorption is favorable when the RL value is between 0 and 1. The value of Qm (57.47 mg/g) was calculated from the Langmuir 5
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Fig. 7. Effect of various parameters onto the Hg adsorption (a) Effect of adsorbent dose on to Hg sorption (b) Effect of pH variation on to Hg sorption (c) Effect of time variation on to Hg sorption (d) Effect of temperature variation on to Hg sorption (e) Effect of initial Hg (II) ion concentration on to Hg sorption. Table 3 Correlation coefficients and constant parameters calculated for Langmuir Adsorption Isotherm and Freundlich adsorption models for Hg (II). Langmuir adsorption Isotherm
Freundlich adsorption isotherm
Qm = 57.47 Kl = 0.1649 Rl = (0.1081–0.0149) R2 = 0.9976
Kf = 3.432 n = 4.0883 R2 = 0.9434
3.5. Kinetic studies Kinetic study of mercury adsorption onto Psy-g-Poly (Am-co-An) was investigated by fitting the experimental data in the Elovich equation, pseudo-second, pseudo-first, second, first order and intra-particle diffusion models and were presented in Fig. 9.
Fig. 8. (a) Langmuir (b) Freundlich adsorption model for the adsorption of Hg.
3.5.1. First -order kinetics equation The linear form of first-order kinetics equation is given as Eq. (6).
model, indicating that the adsorbent showed high capacity to remove mercuric ions (Fig. 8a). RL and KL were determined as 0.01493 and 0.01649 mL/mg, respectively, leading to favorable adsorption.
ln
Qo = k1t Qt
(6) −1
where Qo (mgL ) and Qt (mgL ) are the respective concentration of metal ions at the time zero (initial) and a given time ‘t’. K1 (min−1) is the first order [56,57] rate constant and the regression R2 obtained by the linear plot of ln (Qo/Qt) vs t (Fig. 9a), is shown in Table 4. For mercury, R2 was more than 0.9, showing a good fit of the experimental data.
3.4.2. Freundlich adsorption isotherm Freundlich isotherm defines the heterogeneous surface energy through multilayer adsorption and indicates the linear form as Eq. (5) [54,55].
ln qe = ln Kf + n ln Ce
−1
(5)
where
3.5.2. The second order rate equation The linear form second-order kinetics equation is given in Eq. (7) below:
Kf = Adsorption capacity of the adsorbent
1 = k2t (Qo − Qt )
Value of Freundlich parameters (Kf), correlation constant (R2) and rate constant was calculated by Freundlich isotherm (Fig. 8b) given in Table 3. The equilibrium data fitted the Langmuir (R2 = 0.9976) model better than the Freundlich model (R2 = 0.9434), indicating the surface homogeneity of adsorbent and monolayer adsorption.
(7) −1
−1
where K2 [Lmg min ] is the second order [58] rate constant and calculated from Fig. 9b and the value of the constants are given in Table 4. 6
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Fig. 9. Kinetic models for the adsorption of Hg(II) by Psy-g-poly(Am-co-An) (a) first order (b) second order (c) pseudo-first order (d) pseudo-second-order (e) intraparticle diffusion (f) Elovich model.
t t t = + Qt k2Qe 2 Qe
3.5.3. Pseudo-first-order kinetic equation Linear form pseudo-first-order equation is given in Eq. (8) [59].
log
(Qo − Qt ) kt = log Qo − 1 Qo 2.303
(9)
where k2 represents rate constant. The plot for the Eq. (9) was demonstrated in Fig. 9d, on this, shows the data was perfectly fitted to the model and the value of all parameters were given in Table 4. R2 is 0.99 which indicated that the adsorption system was highly in accordance with the kinetic mechanism compared to other kinetic mechanisms [60]. Therefore, it supports the assumption behind the model and suggests that the overall rate of Hg (II) adsorption by Psy-g-Poly (Amco-An) appeared to be controlled by the physicochemical process.
(8)
where, Qt, Q0 and k1 are adsorbate at time t, adsorption ability at equilibrium, and rate constant respectively. All parameters of this equation were calculated by using data in Fig. 9c (the result is shown in Table 4).
3.5.4. Pseudo-second order kinetics equation The pseudo-second-order kinetic rate was studied by Eq. (9) [51].
3.5.5. Intraparticle diffusion Eq. (10) is intraparticle diffusion kinetic equation [61].
Table 4 Comparison of the first order, second order, pseudo-first order, pseudo-second-order, intra-particle diffusion and elovich equation models parameters for the sorption by Psy-g-poly (Am-co-An) at initial Hg (II) concentration of 100 mg/ L, dose 30 mg, contact volume 10 mL and temperature 25 °C. S.N.
Kinetic model
Linear form
1
First-order rate equation
ln
2
Second order rate equation
3
Pseudo first-order equation
Qo Qt
Plot
= k1t
1 (Qo − Qt)
ln
vs t
1 (Qo − Qt)
= k2t
log (Qo − Qt)/Qo = log Qo −
log t Qt
vs t
(Qo − Qt) Qo
4
Pseudo-second-order rate equation
5
Intraparticle diffusion
Qt = kid t0.5 + C
Qt vs t.5
6
Elovich equation model
Qt = αlog(aα) + α ln (t)
Qt vs ln (t)
k2Qe2
+
t Qe
k1t 2.303
t Qt
=
t
Qo Qt
Parameter
7
vs t
vs t
K1 = 0.0014 R2 = 0.9971 K2 = 4 × 10−5 R2 = 0.99877 K1 = 2.3 × 10−5 R2 = 0.9947 Qo = 77.51 K2 = 9.9 × 10−5 R2 = 0.9481 Kid = 2.76 C = 9.16 R2 = 0.9958 α = 16.11 a = 0.982 R2 = 0.9624
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Qt = Kid t0.5 + C
(10)
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−0.5
) and C is intraparticle diffuwhere Kid is rate constant (mg/ g min sion constant (mg/ g). Fig. 9e shows the intraparticle diffusion kinetic curve for mercury adsorption and the calculated value of intraparticle diffusion parameters were given in Table 4. It was observed that the rate constant enhanced with enhanced mercury concentration. 3.5.6. Elovich rate equation The Elovich equation [62] is given in Eq. (11)
Qt = αlog(aα) + αln(t)
(11)
where Q is the amount adsorbed at t time and α (g/mg) and a (mg/ g−1 min−1) are the Elovich constants [63]. The values of α and a were calculated with the help of linear plot of Qt Vs lnt (Fig. 9f) and presented in Table 4. 4. Conclusion The synthesis of a binary grafted copolymer of Psyllium with acrylamide and acrylonitrile [Psy-g-poly (Am-co-An)] was successfully achieved by using the conventional route in the presence of CAN initiator under the N2 atmosphere. In this experiment, the prominent effect of monomer concentration, initiator concentration, temperature and different reaction time interval were investigated to achieve maximum yield. The synthesized grafted psyllium exhibited the high efficiency for mercury ions adsorption. The adsorption of mercuric ions through Psy-g-poly (Am-co-An) was found to be pH dependent and pH 6 was found to be highly suitable for the adsorption. The adsorption followed kinetic of the second order, which indicated the chemisorption mechanism and the adsorption isotherm was following the Langmuir model, indicating monolayer formation. In this regard, we opined that Psy-g-Poly (Am-co-An) may serve as a milestone in the path of the environmental field as a super adsorbent for effective toxic metal removal from the aquatic system. Acknowledgments The authors are thankful to University Grants Commission, New Delhi, India for financial assistance and of the author Dr. Pramendra Kumar is thankful to coordinator TEQIP-III FET MJPRU Bareilly to provide the partial financial support, and USIC, BBAU, Lucknow for characterization of grafted polymer, and IIT Roorkee, India for XRD, XPS and thermal study. References [1] P.K. Rai, Heavy metal pollution in aquatic ecosystems and its phytoremediation using wetland plants: an ecosustainable approach, International journal of phytoremediation 10 (2) (2008) 133–160. [2] M. Tuzen, A. Sari, D. Mendil, M. Soylak, Biosorptive removal of mercury (II) from aqueous solution using lichen (Xanthoparmelia conspersa) biomass: kinetic and equilibrium studies, J. Hazard. Mater. 169 (1–3) (2009) 263–270. [3] Z. He, M. Zhang, X. Yang, P. Stoffella, Release behavior of copper and zinc from sandy soils, Soil Sci. Soc. Am. J. 70 (5) (2006) 1699–1707. [4] D. Shi, F. Yan, M. Wang, Y. Zou, T. Zheng, X. Zhou, L. Chen, Rhodamine derivative functionalized chitosan as efficient sensor and adsorbent for mercury (II) detection and removal, Mater. Res. Bull. 70 (2015) 958–964. [5] H. Sadegh, G.A.M. Ali, A.S.H. Makhlouf, K.F. Chong, N.S. Alharbi, S. Agarwal, V.K. Gupta, MWCNTs-Fe3O4 nanocomposite for Hg(II) high adsorption efficiency, J. Mol. Liq. 258 (2018) 345–353. [6] A. Sari, M. Tuzen, Removal of mercury (II) from aqueous solution using moss (Drepanocladus revolvens) biomass: equilibrium, thermodynamic and kinetic studies, J. Hazard. Mater. 171 (1–3) (2009) 500–507. [7] K. Li, Y. Wang, M. Huang, H. Yan, H. Yang, S. Xiao, A. Li, Preparation of chitosangraft-polyacrylamide magnetic composite microspheres for enhanced selective removal of mercury ions from water, J. Colloid Interface Sci. 455 (2015) 261–270. [8] I. Ghodbane, O. Hamdaoui, Removal of mercury (II) from aqueous media using eucalyptus bark: kinetic and equilibrium studies, J. Hazard. Mater. 160 (2) (2008) 301–309. [9] H. Kolya, S. Das, T. Tripathy, Synthesis of Starch-g-Poly-(N-methylacrylamide-coacrylic acid) and its application for the removal of mercury (II) from aqueous
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