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Synthesis of nanocomposites from polyacrylamide and graphene oxide: Application as flocculants for water purification
Accepted Manuscript Synthesis of Nanocomposites from Polyacrylamide and Graphene Oxide: Application as Flocculants for Water Purification M. Manafi, P. Manafi, Shilpi Agarwal, Arvind Kumar Bharti, M. Asif, Vinod Kumar Gupta PII: DOI: Reference:
S0021-9797(16)30981-X http://dx.doi.org/10.1016/j.jcis.2016.11.096 YJCIS 21824
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
9 October 2016 21 November 2016 26 November 2016
Please cite this article as: M. Manafi, P. Manafi, S. Agarwal, A.K. Bharti, M. Asif, V.K. Gupta, Synthesis of Nanocomposites from Polyacrylamide and Graphene Oxide: Application as Flocculants for Water Purification, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.11.096
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Synthesis of Nanocomposites from Polyacrylamide and Graphene Oxide: Application as Flocculants for Water Purification M. Manafi a,*, P. Manafib, Shilpi Agarwalc, Arvind Kumar Bhartic, M. Asifd, Vinod Kumar Guptac** a
Department of Applied Chemistry, South Tehran Branch, Islamic Azad University, Tehran, Iran, Email: [email protected] (M. Manafi)
b
Department of Polymer Engineering & Color Technology, Amir kabir University of
Technology (Tehran Polytechnic), Mahshahr Campus, Mahshahr, P.O. Box 415, Email: [email protected] (P. Manafi) c
Department of Applied Chemistry, University of Johannesburg, Johannesburg, South Africa, Email:[email protected] (S. Agarwal) d
Chemical Engineering Department, King Saud University Riyadh, Saudi Arabia
* Corresponding author **Correspondence to V.K. Gupta, Department of Chemistry, Indian Institute of Technology Roorkee-247667, India
Tel.: +91 1332285801; fax:
1
+91 1332273560,
E-mail:
[email protected], [email protected] (V.
K.
Gupta):
[email protected]
(M.
Manafi) Abstract Polyacrylamide/graphene (PAM/GO) based nanocomposites were synthesized and applied as flocculating agent for cleansing the solvent phase. In order to obtain the better dispersion of the graphene nanoplatelets in matrix the functionalization was carried out using acid and upon analytical characterization the successful fine dispersion of FGNp was found in the PAM matrix. The influence of GO concentrations on viscosity, charge demand, flocculating and dewatering with ambient micrometer-sized ground calcium carbonate (GCC) was well evaluated and elucidated. It was found that on increasing the GO content in the PAM/GO nanocomposites, the filtered weight of GCC suspensions also increases, and the filtrate turbidity decreased and it was also observed that on adding the GO both
and supernatant
turbidity reduced: by growing GO concentrations, there was a fall in
and turbidity of
cleaned water. The retention mechanism proceeds through a parallel method to microparticle retention system which is done via bridge/patch among pulp and filler.
Keywords: Nanocomposites, Graphene oxide, Polyacrylamide, Flocculation and Retention
2
1. Introduction Water pollution by industrial and agricultural waste systems is rapidly becoming one of the major challenges in environmental science [1]. As to name a few water impurities, one could mention organic matter and heavy metals ions which are the causes of major health issues in many organisms [2]. For instance, various health issue may arise due to them; namely: nausea, coma, convulsions, renal failure, cancer, and detrimental effects on metabolism and intelligence, hemochromatosis, spasm in the calves area, skin dermatitis brass chills, gastrointestinal catarrh and Wilson’s disease [3-6]. Developments in techniques or methods and remedies such as membrane processes, chemical precipitation, electrolysis, adsorption procedures and ion exchange have led to new opportunities to eliminate the contaminations from wastewater [7-10]. Regrettably, some obstacles such as expenses, level of complication, yield, as well as by secondary waste, prohibit the enhancement in the usages [2]. To compare these methods, one of the best methods which have common usage is the exclusion of organic matter and heavy metals via adsorptive removal. This could be attributed to several prominent properties of this method: relative low cost, easy handling, and being effective at low concentration [11]. Particularly, the improvements of nano science and biology for organic matter and heavy metal removal have been the subject of much debate in multiple researches due to their distinctive properties [12, 13]. After fullerene and carbon nanotubes were created and in past twenty five years, graphene with its repertoire of specific physical properties has created interesting new understanding in two dimensional (2D) nanomaterials science and technology[14]. After 3
much research, it has been revealed that graphene shows a more proper adsorption behavior for heavy metal ions and fluoride as compared to CNTs [15,16]. However, nanotubes need special oxidation processes for hydrophilic groups to be added on them for extensive sorption of heavy metal ion; yielding graphene oxide (GO) nanosheets from graphite using Hummers method results in multiple oxygen functional groups such as –COOH, –C = O, and –OH which are vital if one pursues proper heavy metal sorption. As an appropriate replacement for conductive carbon filler in nanocomposites, GNp could be considered candidates which is due to its elevated aspect ratio, facile synthesis and high conductivity. The method of improving mechanical, physical, thermal and barrier characteristics in nanocomposites containing graphene are found in the literatures [17-20]. Enhancements in physical and mechanical characteristics are mainly done by uniform and proper dispersion of fillers. Mismatch and how graphene blends with most polymers has prevented the achievement of a good dispersion, thus functionalization is an inevitable process. Compared to non-functionalized GNp, functionalized graphene nanoplatelets (FGNp) have multiple groups on their surface which are oxygen-containing. These functional groups are the reason for high dispersion of FGNp in polymers and they also make the interaction between the matrices and FGNp easier through covalent bonds. Nevertheless, carbon based nano materials such as CNTs and GO have opposing impacts on activated sludge mechanism, thus it is reasonable to believe that GO will lead to toxic issues to the wastewater microbial colony. Also, it has been mentioned in multiple studies that proper dispersion and higher time of contact between carbon-based nanomaterials and pure bacterial cultures, enhance their antimicrobial response [21-23]. As a comparison between GO and CNTs, GO is of a more hydrophilic nature and has a better 4
dispersion in organic solutions as compared to CNTs. Thus, these substances could very well lead to higher contact times between GO and microbial colonies and and result in more vigorous opposing effects to the wastewater treatment as compared to CNTs. Several biomaterials such as Cholerra vulgaris,
Scenedesmus quadricaud,
Saccharomyces cerevisiae strains, [24-26] etc are applied as flocculating materials for the remediation and adsorption of noxious impurities from the water. In the present study, the impact of graphene oxide concentration on the flocculation and retention properties was well studied and investigated. The surface modification of the graphene with acid and subsequently polyacrylamide/GO (PG) nanocomposites preparation and characterization was carried out with the help of various analytical techniques. Hence in the present study, we discuss the effect of GO concentrations impact on viscosity, charge demand, flocculating and dewatering with ambient micrometer-sized ground calcium carbonate (GCC). 2. Experimental 2.1. Materials A modified Hummers route was utilized to yield graphene nanoplatelets. Acrylamide and acrylic acid monomers, H2SO4 (98%), HNO3 (68%), DMF and THF were procured from Sigma-Aldrich and used as received; however, so as to have a more proper efficiency, water exclusion was done on THF by sodium benzophenone purged with nitrogen and it was distilled exactly before use. Bleached eucalyptus pulp (Aracruz, Brazil). GCC (diameter = 1.4 µm, Omiya, Korea) served as pulp in this research and no purification was carried out on it. In all experiments, we used ultra-pure water (resistivity >18.2 MΩ/ cm). 5
2.2. Graphene Functionalization To exclude any previously adsorbed humidity, GNp (0.2 g) was stored in a vacuum oven for approximately 48 h at 80°C. In order to oxidize the GNp, a mixture of H2SO4/HNO3 (3:1, v/v) was incorporated for 2 h by ultrasonication at 60°C. Polycarbonate filtration (0.45 μm pore diameter) was used on the oxidized GNp, and the bulk was then diluted with plentiful amounts of distilled water until there was no more trace of acid (pH=7) and vacuum dried for 12 hours in an oven at 80°C. 2.3. Synthesis of polyelectrolyte samples For preparation of polyelectrolyte samples, 1.25 M solutions of acrylamide and acrylic acid monomers in pure water by chain polymerization were synthesized. Acrylamide and acrylic acid solutions according to the desired ratios in Table 1 were added to the balloon with two openings one of which was for nitrogen gas inlet and the other opening to reflux, thus the solution volume was 100 ml and also total concentration of acrylamide and acrylic acid monomers was 1.25 M. The system was then purged with nitrogen gas for about 10 min for thorough removal of oxygen gas. Then, 1 ml of K2S2O8 initiator (10 mmol) was added and reaction temperature reached 60 °C, which continued for 4 h at this temperature. Afterwards, the mixture was stirred in ethanol and then was precipitated. The prepared sample was vacuum dried in an oven at 60 °C. Accordingly, other samples with different mole fractions were prepared by the same method. It should be noted that the sample was a viscous and transparent liquid which precipitated thoroughly and turned white after the addition of ethanol. The mass of initiator and reaction time were kept constant and had no effect on sample’s appearance. Due to the 6
best surface charge gained (1.09 mequiv/g), PA10 was chosen as sample for nanocomposites.
2.4. Synthesis of Nanocomposites First, desired amounts of 1 mg/ml GO were added into distilled water. The suspension was sonicated for 2 h in order to carry out aggregations to collapse and yield a uniform dispersion. Simultaneously, a 100 mg/ml solution of PAM in water was prepared at 80 °C. Subsequently, a mixture of PAM with GO was produced by mixing the two for 4 h at 80 °C. Further sonication for another 10 min, the solution was heated for 1 hour to evaporate the solvent yielding the nanocomposites. Afterward, to complete evaporation of any residual solvent, the prepared sample was vacuum-dried at 80 °C for 4 days. As the final stage, PAM/GOx nanocomposites were prepared by the addition of x wt% nanoparticles to PAM; wherein, x stands for 0.1, 0.3, 0.5 and 0.7 w.t % nanoparticles content. 2.5. Flocculate material characterization The surface functionlization reaction was carried out using FTIR and Raman analysis was carried out at 514.5 nm excitation. The elemental composition of the synthesized material was analyzed using the Elementar Vario EL III. A Philips EM208 microscope Transmission Electron Microscope (Netherlands) operating at 100 kV was utilized to study the dispersion quality of the nanoparticles within the matrix and the nanostructures of the nanocomposites. The samples were microtome at -160 °C. Polyelectrolyte titration (PCD 03, Mutek, Germany) was used to assess the surface charge of the nanocomposites. Samples intrinsic viscosity ([ ]) in aqueous solution were compared 7
via capillary viscometric measurements for which an Ubbelohde viscometer was utilized. The measurements were repeated five times for each sample at 25 °C, the [ ] values are as follows: (1)
where
is the viscosity of the solution,
is the viscosity of water, and c is the
concentration of the solution. Then, 1 wt% GCC suspensions were prepared by dispersing GCC particles in 300 mL of pure water at room temperature, in order to carry out for the flocculation and retention analyses. After storing the mixtures jars, adding the PAM/GO nanocomposites, stirring the resulting solution for about 3 min at a low velocity and filtering the floccules through a 325-mesh screen was done, then the material was dried and the resulting solid filtrate was measured. As for this specimen, the nanocomposites loading ratio to the filtered weight of the fillers was reported. The same route (1.0 g pulp and 0.5 g filler suspension in 750 mL of water) was utilized to assess the flocculation impact of mixtures. For the current flocculation research, after adding 75 mg of the PAM/GO nanocomposites, the filtrates turbidity analysis (measured with a HACH tubidimeter, USA) was carried out with respect to the PAM/GO nanocomposites GO concentration. A dynamic drainage analyser (DDA, AKRIBI, Sweden) with 73-mesh screen was utilized to assess the GCC retention on the pulp with the PAM/GO nanocomposites. An 800 mL pulp suspension of 0.5% was added to the DDA vessel at low velocity; thereby, GCC (10 wt% in respect to pulp) and the pre-determined weight of the PAM/GO nanocomposites were added. The dewatering time and turbidity of the filtrate liquid were monitored. 8
3. Results and discussion 3.1. Characterization of Graphene Functionalization The results of the FTIR analysis of non-functionalized and functionalized GNp have been presented in Fig.1. In oxidized GNp spectra, the appearance of the peaks at 1707, 1580 and 1130 cm-1 are respectively due to the stretching vibrations of the formation of carbonyl groups from carboxylic acids (–COOH–), asymmetric –COO– stretch and symmetric –COO– stretch. Higher absorption intensity of the band at 3448 cm-1 for functionalized GNp compared to unmodified GNp resulted from the hydroxyl stretching vibration of the carboxylic groups (COOH) formed through oxidation process. There have been other reports regarding the functionalization of CNT [27, 28]. As a result, it is confirmed by FTIR spectra that acid treatment places oxygen-containing functional groups have been formed on graphene surface. Raman spectroscopy could serve as yet another efficient characterization method to ascertain the presence of chemical functional groups of GNp. Pristine and functionalized GNp’s Raman spectra have been presented in Fig. 2. As it was anticipated, two major peaks are evident in the spectrum of carbon allotropes; D and G bands. The former is generally found at approximately 1340-1360 cm-1 which is due to the ring-breathing modes of sp2 atoms in rings while the latter is an indication of bond stretching of all pairs of sp2 atoms in both rings and chains and is located circa 1585 cm-1 [27]. Referring to Fig. 2, disordered pattern found in graphene samples alter the intensity ratio of D and G band (ID/IG) and are due to the formation of defects in FGNp, as it differs for pristine and functionalized graphene. For FGNP, the ID/IG ratios (1.39) were higher compared to that of GNP (1.15); 9
the higher value of ID/IG ratio in FGNp can be robust evidence to the thorough accomplishment of the functionalization process. TGA was then utilized to study GNp functionalization, Fig. 3 represents the TGA of GNp and FGNp. It was observed that as the temperature increases, two major steps in decomposition behavior for non-functionalized GNp sample are visible. The mass loss around 100 °C is considered to be due to the exclusion of absorbed water and moisture emission, and the shoulder at around 200 °C is ascribed to pyrolysis of the respective oxygen-containing functional groups [28]. In the range of 150 º C to 230º C, the mass loss for GNp was 3% while the mass loss for FGNp was 8 wt%. The decomposition behavior of the specimen varies for which the oxygen containing groups on the surface of FGNp are presumably liable, i.e. in this temperature range there is a more major mass loss as compared to non-functionalized GNp which is ascribed to the existence of more organic groups created through oxidation process. As it is evident from the results of Elemental analysis in the Fig. 3, due to the formation of oxygen containing groups by functionalization process, the oxygen percentage of FGNp is of higher value. Like other polymer nanocomposites, the mixing condition of graphene layers by matrix and their interfacial bonding have the largest impact on the efficacy of nanocomposites. TEM was done on the ultrathin section of the specimen for a careful analysis of the morphology of nanocomposites. In Fig. 4(a, b), one can observe typical micrographs of PAM/GNp0.7 and PAM/FGNp0.7 where the impact of graphene functionalization is obviously seen; whereas the rather large agglomerate is seen in PAM/GNp 0.7 sample (Fig. 4a). After functionalization however, that nanoparticles 10
uniform dispersion in all of the parts of the matrix are denoted by the flexible and craggy graphene sheets which were thoroughly mixed in the PAM matrix. 3.2. Characterization of copolymer Fig. 5 shows the FTIR spectra of chemical structure of prepared samples. Peaks at 1320 and 1455 cm-1 are respectively due to the C–O and C-N groups stretching vibrations. The band at 1670 cm-1 is attributed to the presence of CONH2 group and also the appearance of the peak at 3455 cm-1 is related to stretching vibration of hydroxyl groups. The band at 1716 cm-1 confirms the presence of CH groups in backbone. Absence of a band at the range of 1630 cm-1 proves the completion of the reaction, i.e. all the monomers have been consumed and thus there is no C=C band. After evaluation of all results, the synthesis of acrylamide and acrylic acid copolymer is confirmed. After copolymerization of acrylamide and nanocomposites preparation, we found that by increasing of GO content, the nanocomposites became more viscose as well, while the surface charge of the nanocomposites underwent a reduction. It can be seen that by increasing GO content, the nanocomposites viscosity rose due to proper dispersion of graphene oxide; also because of the interaction between the hydroxyl groups of the GO and the amide and carboxylic groups on the polymer, surface charge of the nanocomposites underwent a reduction. In Table 2, nanocomposites solutions intrinsic viscosity, and surface charge have been reported. 3.3. Flocculation and retention of nanocomposites 3.3.1 Effect of PAM/GOs concentration on flocculation The filtered weights of solid containing the PAM/GO nanocomposites throughout the flocculation test with 1 wt% GCC suspension is shown in Fig. 6. From the it is clear 11
that, as the concentration of PAM/GO nanocomposites increased, the floccules weight increases, this is due to the adsorption ability the sample surfaces; the flocculation between filler particles and retention which is dictated by the particle size and surface charge of the filler [29, 30]. Hence, on increasing the GO concentration, the floccules weight increases, which depend on the size and charge. Indeed, adding PG0 (0.1 wt%, without GO) leads to the flocculation of GCC particles and the concentration effect of GO nanocomposites for suspension is solely overwhelmed by PAM. By increasing PAM/GO nanocomposites concentration, the filtered weight of GCC suspensions also increases. 3.3.2. Effect of PAM/GO nanocomposites concentration on turbidity PAM/GO nanocomposites concentration plays the vital role in affecting the turbidity of the pulp/filler mixture suspensions in the 0.5 wt% PAM/GO nanocomposites emulsion. As shown in Fig. 7, on increasing the GO content in the PAM/GO nanocomposites, the filtrate turbidity decreases [31-34]. 3.3.3. Effect of PAM/GO nanocomposites concentration on dewatering time The concentration of the nanocomposites and GO affect the dewatering time and turbidity of phase separation as shown in Fig. 8. It is found that on adding GO both
and
supernatant turbidity reduced: by growing GO concentrations, there was a fall in
and
turbidity of cleaned water. Likewise, the turbidity reduced as the concentration of nanocomposites increases (Fig. 8B). On the contrary,
improved with increasing the
nanocomposites concentration, the rate of water removal was thus reduced by increasing the density of the filtered mass as the nanocomposites concentration rises. Finally, all evidence suggests that PAM itself is less effective than nanocomposites for retention. The microparticles size and shape are of major roles in retention mechanism and drainage 12
enhancement; particles with high aspect ratio have superior bridging ability [35-38]. As a result, via bridge/patch among pulp and filler, a parallel route applies to the retention mechanism of microparticle retention system [39-53]. 4. Conclusion Solution route was used to make samples based on PAM / graphene oxide nanoplatelets. Copolymers based on acrylamide and acrylic acid was synthesized through chain polymerization in water system. Upon experimentation it was observed that the synthesized material can be actively used as flocculent due its specific properties like excellent cleansing properties and it was observed that on increasing the GO content in the PAM/GO nanocomposites, the filtered weight of GCC suspensions also increases, and the filtrate turbidity decreased and it was also observed that on adding the GO both
and supernatant
turbidity reduced: by growing GO concentrations, there was a fall in
and turbidity of
cleaned water, these above observation clearly depicts that its excellent flocculent property. Acknowledgments The efforts of Islamic Azad University, South Tehran Branch are appreciated for having financially supported the research. M. Asif appreciates the support of the Deanship of Scientific Research at King Saud University for the Prolific Research Group PRG-1437-31.
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Figure captions Fig. 1 FTIR spectra of GNp and FGNp. Fig. 2 Raman spectra of GNp and FGNp. Fig. 3 TGA thermogram and elemental analysis of GNp and FGNp. Fig. 4 TEM images showing the GNp and FGNp dispersion from the ultrathin section of (a) PAM/GNp1 and (b) PAM/FGNp1 nanocomposites. Fig. 5 FTIR spectra of PAM Fig. 6 The filtered weights of 1 wt% GCC suspension in the flocculation test, after mixing with nanocomposites Fig.7 Concentration effect of GOs in the 0.5 wt% PAM/GO nanocomposites on turbidity of the suspensions. Fig. 8 (A) dewatering time and (B) turbidity of the supernatants for the suspensions for different nanocomposites.
19
Table 1 Compositions of copolymer samples prepared in aqueous solution 1.25 M acrylamide
1.25 M acrylic acid
solution (ml)
solution (ml)
PA10
10
90
PA20
20
80
PA30
30
70
PA40
40
60
PA50
50
50
sample
20
Table 2 Intrinsic viscosity, and surface charge of PAM and nanocomposites Samples
FGNP (wt%)
Intrinsic viscosity
Surface charge
(cm3/g)
(mequiv/g)
PG0
0
328
1.09
PG0.1
0.1
880
1.02
PG0.3
0.3
915
0.94
PG0.5
0.5
1225
0.51
PG0.7
0.7
1351
0.40
21
Fig.1 FTIR spectra of GNp and FGNp.
22
Fig. 2 Raman spectra of GNp and FGNp.
23
Fig. 3 TGA thermogram and elemental analysis of GNp and FGNp.
24
Fig. 4 TEM images showing the GNp and FGNp dispersion from the ultrathin section of (a) PAM/GNp1 and (b) PAM/FGNp1 nanocomposites.
25
Fig. 5 FTIR spectra of copolymer( PAM)
26
Fig. 6 The filtered weights of 1 wt% GCC suspension in the flocculation test, after mixing with nanocomposites
27
Fig. 7 Concentration effect of GOs in the 0.5 wt% PAM/GO nanocomposites on turbidity of the suspensions.
28
Fig. 8 (A) dewatering time and (B) turbidity of the supernatants for the suspensions for different nanocomposites.
29
Graphical abstract
Presentation of developed flocculent in the form of bars for the flocculation test
30
S0021-9797(16)30981-X http://dx.doi.org/10.1016/j.jcis.2016.11.096 YJCIS 21824
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
9 October 2016 21 November 2016 26 November 2016
Please cite this article as: M. Manafi, P. Manafi, S. Agarwal, A.K. Bharti, M. Asif, V.K. Gupta, Synthesis of Nanocomposites from Polyacrylamide and Graphene Oxide: Application as Flocculants for Water Purification, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.11.096
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Synthesis of Nanocomposites from Polyacrylamide and Graphene Oxide: Application as Flocculants for Water Purification M. Manafi a,*, P. Manafib, Shilpi Agarwalc, Arvind Kumar Bhartic, M. Asifd, Vinod Kumar Guptac** a
Department of Applied Chemistry, South Tehran Branch, Islamic Azad University, Tehran, Iran, Email: [email protected] (M. Manafi)
b
Department of Polymer Engineering & Color Technology, Amir kabir University of
Technology (Tehran Polytechnic), Mahshahr Campus, Mahshahr, P.O. Box 415, Email: [email protected] (P. Manafi) c
Department of Applied Chemistry, University of Johannesburg, Johannesburg, South Africa, Email:[email protected] (S. Agarwal) d
Chemical Engineering Department, King Saud University Riyadh, Saudi Arabia
* Corresponding author **Correspondence to V.K. Gupta, Department of Chemistry, Indian Institute of Technology Roorkee-247667, India
Tel.: +91 1332285801; fax:
1
+91 1332273560,
E-mail:
[email protected], [email protected] (V.
K.
Gupta):
[email protected]
(M.
Manafi) Abstract Polyacrylamide/graphene (PAM/GO) based nanocomposites were synthesized and applied as flocculating agent for cleansing the solvent phase. In order to obtain the better dispersion of the graphene nanoplatelets in matrix the functionalization was carried out using acid and upon analytical characterization the successful fine dispersion of FGNp was found in the PAM matrix. The influence of GO concentrations on viscosity, charge demand, flocculating and dewatering with ambient micrometer-sized ground calcium carbonate (GCC) was well evaluated and elucidated. It was found that on increasing the GO content in the PAM/GO nanocomposites, the filtered weight of GCC suspensions also increases, and the filtrate turbidity decreased and it was also observed that on adding the GO both
and supernatant
turbidity reduced: by growing GO concentrations, there was a fall in
and turbidity of
cleaned water. The retention mechanism proceeds through a parallel method to microparticle retention system which is done via bridge/patch among pulp and filler.
Keywords: Nanocomposites, Graphene oxide, Polyacrylamide, Flocculation and Retention
2
1. Introduction Water pollution by industrial and agricultural waste systems is rapidly becoming one of the major challenges in environmental science [1]. As to name a few water impurities, one could mention organic matter and heavy metals ions which are the causes of major health issues in many organisms [2]. For instance, various health issue may arise due to them; namely: nausea, coma, convulsions, renal failure, cancer, and detrimental effects on metabolism and intelligence, hemochromatosis, spasm in the calves area, skin dermatitis brass chills, gastrointestinal catarrh and Wilson’s disease [3-6]. Developments in techniques or methods and remedies such as membrane processes, chemical precipitation, electrolysis, adsorption procedures and ion exchange have led to new opportunities to eliminate the contaminations from wastewater [7-10]. Regrettably, some obstacles such as expenses, level of complication, yield, as well as by secondary waste, prohibit the enhancement in the usages [2]. To compare these methods, one of the best methods which have common usage is the exclusion of organic matter and heavy metals via adsorptive removal. This could be attributed to several prominent properties of this method: relative low cost, easy handling, and being effective at low concentration [11]. Particularly, the improvements of nano science and biology for organic matter and heavy metal removal have been the subject of much debate in multiple researches due to their distinctive properties [12, 13]. After fullerene and carbon nanotubes were created and in past twenty five years, graphene with its repertoire of specific physical properties has created interesting new understanding in two dimensional (2D) nanomaterials science and technology[14]. After 3
much research, it has been revealed that graphene shows a more proper adsorption behavior for heavy metal ions and fluoride as compared to CNTs [15,16]. However, nanotubes need special oxidation processes for hydrophilic groups to be added on them for extensive sorption of heavy metal ion; yielding graphene oxide (GO) nanosheets from graphite using Hummers method results in multiple oxygen functional groups such as –COOH, –C = O, and –OH which are vital if one pursues proper heavy metal sorption. As an appropriate replacement for conductive carbon filler in nanocomposites, GNp could be considered candidates which is due to its elevated aspect ratio, facile synthesis and high conductivity. The method of improving mechanical, physical, thermal and barrier characteristics in nanocomposites containing graphene are found in the literatures [17-20]. Enhancements in physical and mechanical characteristics are mainly done by uniform and proper dispersion of fillers. Mismatch and how graphene blends with most polymers has prevented the achievement of a good dispersion, thus functionalization is an inevitable process. Compared to non-functionalized GNp, functionalized graphene nanoplatelets (FGNp) have multiple groups on their surface which are oxygen-containing. These functional groups are the reason for high dispersion of FGNp in polymers and they also make the interaction between the matrices and FGNp easier through covalent bonds. Nevertheless, carbon based nano materials such as CNTs and GO have opposing impacts on activated sludge mechanism, thus it is reasonable to believe that GO will lead to toxic issues to the wastewater microbial colony. Also, it has been mentioned in multiple studies that proper dispersion and higher time of contact between carbon-based nanomaterials and pure bacterial cultures, enhance their antimicrobial response [21-23]. As a comparison between GO and CNTs, GO is of a more hydrophilic nature and has a better 4
dispersion in organic solutions as compared to CNTs. Thus, these substances could very well lead to higher contact times between GO and microbial colonies and and result in more vigorous opposing effects to the wastewater treatment as compared to CNTs. Several biomaterials such as Cholerra vulgaris,
Scenedesmus quadricaud,
Saccharomyces cerevisiae strains, [24-26] etc are applied as flocculating materials for the remediation and adsorption of noxious impurities from the water. In the present study, the impact of graphene oxide concentration on the flocculation and retention properties was well studied and investigated. The surface modification of the graphene with acid and subsequently polyacrylamide/GO (PG) nanocomposites preparation and characterization was carried out with the help of various analytical techniques. Hence in the present study, we discuss the effect of GO concentrations impact on viscosity, charge demand, flocculating and dewatering with ambient micrometer-sized ground calcium carbonate (GCC). 2. Experimental 2.1. Materials A modified Hummers route was utilized to yield graphene nanoplatelets. Acrylamide and acrylic acid monomers, H2SO4 (98%), HNO3 (68%), DMF and THF were procured from Sigma-Aldrich and used as received; however, so as to have a more proper efficiency, water exclusion was done on THF by sodium benzophenone purged with nitrogen and it was distilled exactly before use. Bleached eucalyptus pulp (Aracruz, Brazil). GCC (diameter = 1.4 µm, Omiya, Korea) served as pulp in this research and no purification was carried out on it. In all experiments, we used ultra-pure water (resistivity >18.2 MΩ/ cm). 5
2.2. Graphene Functionalization To exclude any previously adsorbed humidity, GNp (0.2 g) was stored in a vacuum oven for approximately 48 h at 80°C. In order to oxidize the GNp, a mixture of H2SO4/HNO3 (3:1, v/v) was incorporated for 2 h by ultrasonication at 60°C. Polycarbonate filtration (0.45 μm pore diameter) was used on the oxidized GNp, and the bulk was then diluted with plentiful amounts of distilled water until there was no more trace of acid (pH=7) and vacuum dried for 12 hours in an oven at 80°C. 2.3. Synthesis of polyelectrolyte samples For preparation of polyelectrolyte samples, 1.25 M solutions of acrylamide and acrylic acid monomers in pure water by chain polymerization were synthesized. Acrylamide and acrylic acid solutions according to the desired ratios in Table 1 were added to the balloon with two openings one of which was for nitrogen gas inlet and the other opening to reflux, thus the solution volume was 100 ml and also total concentration of acrylamide and acrylic acid monomers was 1.25 M. The system was then purged with nitrogen gas for about 10 min for thorough removal of oxygen gas. Then, 1 ml of K2S2O8 initiator (10 mmol) was added and reaction temperature reached 60 °C, which continued for 4 h at this temperature. Afterwards, the mixture was stirred in ethanol and then was precipitated. The prepared sample was vacuum dried in an oven at 60 °C. Accordingly, other samples with different mole fractions were prepared by the same method. It should be noted that the sample was a viscous and transparent liquid which precipitated thoroughly and turned white after the addition of ethanol. The mass of initiator and reaction time were kept constant and had no effect on sample’s appearance. Due to the 6
best surface charge gained (1.09 mequiv/g), PA10 was chosen as sample for nanocomposites.
2.4. Synthesis of Nanocomposites First, desired amounts of 1 mg/ml GO were added into distilled water. The suspension was sonicated for 2 h in order to carry out aggregations to collapse and yield a uniform dispersion. Simultaneously, a 100 mg/ml solution of PAM in water was prepared at 80 °C. Subsequently, a mixture of PAM with GO was produced by mixing the two for 4 h at 80 °C. Further sonication for another 10 min, the solution was heated for 1 hour to evaporate the solvent yielding the nanocomposites. Afterward, to complete evaporation of any residual solvent, the prepared sample was vacuum-dried at 80 °C for 4 days. As the final stage, PAM/GOx nanocomposites were prepared by the addition of x wt% nanoparticles to PAM; wherein, x stands for 0.1, 0.3, 0.5 and 0.7 w.t % nanoparticles content. 2.5. Flocculate material characterization The surface functionlization reaction was carried out using FTIR and Raman analysis was carried out at 514.5 nm excitation. The elemental composition of the synthesized material was analyzed using the Elementar Vario EL III. A Philips EM208 microscope Transmission Electron Microscope (Netherlands) operating at 100 kV was utilized to study the dispersion quality of the nanoparticles within the matrix and the nanostructures of the nanocomposites. The samples were microtome at -160 °C. Polyelectrolyte titration (PCD 03, Mutek, Germany) was used to assess the surface charge of the nanocomposites. Samples intrinsic viscosity ([ ]) in aqueous solution were compared 7
via capillary viscometric measurements for which an Ubbelohde viscometer was utilized. The measurements were repeated five times for each sample at 25 °C, the [ ] values are as follows: (1)
where
is the viscosity of the solution,
is the viscosity of water, and c is the
concentration of the solution. Then, 1 wt% GCC suspensions were prepared by dispersing GCC particles in 300 mL of pure water at room temperature, in order to carry out for the flocculation and retention analyses. After storing the mixtures jars, adding the PAM/GO nanocomposites, stirring the resulting solution for about 3 min at a low velocity and filtering the floccules through a 325-mesh screen was done, then the material was dried and the resulting solid filtrate was measured. As for this specimen, the nanocomposites loading ratio to the filtered weight of the fillers was reported. The same route (1.0 g pulp and 0.5 g filler suspension in 750 mL of water) was utilized to assess the flocculation impact of mixtures. For the current flocculation research, after adding 75 mg of the PAM/GO nanocomposites, the filtrates turbidity analysis (measured with a HACH tubidimeter, USA) was carried out with respect to the PAM/GO nanocomposites GO concentration. A dynamic drainage analyser (DDA, AKRIBI, Sweden) with 73-mesh screen was utilized to assess the GCC retention on the pulp with the PAM/GO nanocomposites. An 800 mL pulp suspension of 0.5% was added to the DDA vessel at low velocity; thereby, GCC (10 wt% in respect to pulp) and the pre-determined weight of the PAM/GO nanocomposites were added. The dewatering time and turbidity of the filtrate liquid were monitored. 8
3. Results and discussion 3.1. Characterization of Graphene Functionalization The results of the FTIR analysis of non-functionalized and functionalized GNp have been presented in Fig.1. In oxidized GNp spectra, the appearance of the peaks at 1707, 1580 and 1130 cm-1 are respectively due to the stretching vibrations of the formation of carbonyl groups from carboxylic acids (–COOH–), asymmetric –COO– stretch and symmetric –COO– stretch. Higher absorption intensity of the band at 3448 cm-1 for functionalized GNp compared to unmodified GNp resulted from the hydroxyl stretching vibration of the carboxylic groups (COOH) formed through oxidation process. There have been other reports regarding the functionalization of CNT [27, 28]. As a result, it is confirmed by FTIR spectra that acid treatment places oxygen-containing functional groups have been formed on graphene surface. Raman spectroscopy could serve as yet another efficient characterization method to ascertain the presence of chemical functional groups of GNp. Pristine and functionalized GNp’s Raman spectra have been presented in Fig. 2. As it was anticipated, two major peaks are evident in the spectrum of carbon allotropes; D and G bands. The former is generally found at approximately 1340-1360 cm-1 which is due to the ring-breathing modes of sp2 atoms in rings while the latter is an indication of bond stretching of all pairs of sp2 atoms in both rings and chains and is located circa 1585 cm-1 [27]. Referring to Fig. 2, disordered pattern found in graphene samples alter the intensity ratio of D and G band (ID/IG) and are due to the formation of defects in FGNp, as it differs for pristine and functionalized graphene. For FGNP, the ID/IG ratios (1.39) were higher compared to that of GNP (1.15); 9
the higher value of ID/IG ratio in FGNp can be robust evidence to the thorough accomplishment of the functionalization process. TGA was then utilized to study GNp functionalization, Fig. 3 represents the TGA of GNp and FGNp. It was observed that as the temperature increases, two major steps in decomposition behavior for non-functionalized GNp sample are visible. The mass loss around 100 °C is considered to be due to the exclusion of absorbed water and moisture emission, and the shoulder at around 200 °C is ascribed to pyrolysis of the respective oxygen-containing functional groups [28]. In the range of 150 º C to 230º C, the mass loss for GNp was 3% while the mass loss for FGNp was 8 wt%. The decomposition behavior of the specimen varies for which the oxygen containing groups on the surface of FGNp are presumably liable, i.e. in this temperature range there is a more major mass loss as compared to non-functionalized GNp which is ascribed to the existence of more organic groups created through oxidation process. As it is evident from the results of Elemental analysis in the Fig. 3, due to the formation of oxygen containing groups by functionalization process, the oxygen percentage of FGNp is of higher value. Like other polymer nanocomposites, the mixing condition of graphene layers by matrix and their interfacial bonding have the largest impact on the efficacy of nanocomposites. TEM was done on the ultrathin section of the specimen for a careful analysis of the morphology of nanocomposites. In Fig. 4(a, b), one can observe typical micrographs of PAM/GNp0.7 and PAM/FGNp0.7 where the impact of graphene functionalization is obviously seen; whereas the rather large agglomerate is seen in PAM/GNp 0.7 sample (Fig. 4a). After functionalization however, that nanoparticles 10
uniform dispersion in all of the parts of the matrix are denoted by the flexible and craggy graphene sheets which were thoroughly mixed in the PAM matrix. 3.2. Characterization of copolymer Fig. 5 shows the FTIR spectra of chemical structure of prepared samples. Peaks at 1320 and 1455 cm-1 are respectively due to the C–O and C-N groups stretching vibrations. The band at 1670 cm-1 is attributed to the presence of CONH2 group and also the appearance of the peak at 3455 cm-1 is related to stretching vibration of hydroxyl groups. The band at 1716 cm-1 confirms the presence of CH groups in backbone. Absence of a band at the range of 1630 cm-1 proves the completion of the reaction, i.e. all the monomers have been consumed and thus there is no C=C band. After evaluation of all results, the synthesis of acrylamide and acrylic acid copolymer is confirmed. After copolymerization of acrylamide and nanocomposites preparation, we found that by increasing of GO content, the nanocomposites became more viscose as well, while the surface charge of the nanocomposites underwent a reduction. It can be seen that by increasing GO content, the nanocomposites viscosity rose due to proper dispersion of graphene oxide; also because of the interaction between the hydroxyl groups of the GO and the amide and carboxylic groups on the polymer, surface charge of the nanocomposites underwent a reduction. In Table 2, nanocomposites solutions intrinsic viscosity, and surface charge have been reported. 3.3. Flocculation and retention of nanocomposites 3.3.1 Effect of PAM/GOs concentration on flocculation The filtered weights of solid containing the PAM/GO nanocomposites throughout the flocculation test with 1 wt% GCC suspension is shown in Fig. 6. From the it is clear 11
that, as the concentration of PAM/GO nanocomposites increased, the floccules weight increases, this is due to the adsorption ability the sample surfaces; the flocculation between filler particles and retention which is dictated by the particle size and surface charge of the filler [29, 30]. Hence, on increasing the GO concentration, the floccules weight increases, which depend on the size and charge. Indeed, adding PG0 (0.1 wt%, without GO) leads to the flocculation of GCC particles and the concentration effect of GO nanocomposites for suspension is solely overwhelmed by PAM. By increasing PAM/GO nanocomposites concentration, the filtered weight of GCC suspensions also increases. 3.3.2. Effect of PAM/GO nanocomposites concentration on turbidity PAM/GO nanocomposites concentration plays the vital role in affecting the turbidity of the pulp/filler mixture suspensions in the 0.5 wt% PAM/GO nanocomposites emulsion. As shown in Fig. 7, on increasing the GO content in the PAM/GO nanocomposites, the filtrate turbidity decreases [31-34]. 3.3.3. Effect of PAM/GO nanocomposites concentration on dewatering time The concentration of the nanocomposites and GO affect the dewatering time and turbidity of phase separation as shown in Fig. 8. It is found that on adding GO both
and
supernatant turbidity reduced: by growing GO concentrations, there was a fall in
and
turbidity of cleaned water. Likewise, the turbidity reduced as the concentration of nanocomposites increases (Fig. 8B). On the contrary,
improved with increasing the
nanocomposites concentration, the rate of water removal was thus reduced by increasing the density of the filtered mass as the nanocomposites concentration rises. Finally, all evidence suggests that PAM itself is less effective than nanocomposites for retention. The microparticles size and shape are of major roles in retention mechanism and drainage 12
enhancement; particles with high aspect ratio have superior bridging ability [35-38]. As a result, via bridge/patch among pulp and filler, a parallel route applies to the retention mechanism of microparticle retention system [39-53]. 4. Conclusion Solution route was used to make samples based on PAM / graphene oxide nanoplatelets. Copolymers based on acrylamide and acrylic acid was synthesized through chain polymerization in water system. Upon experimentation it was observed that the synthesized material can be actively used as flocculent due its specific properties like excellent cleansing properties and it was observed that on increasing the GO content in the PAM/GO nanocomposites, the filtered weight of GCC suspensions also increases, and the filtrate turbidity decreased and it was also observed that on adding the GO both
and supernatant
turbidity reduced: by growing GO concentrations, there was a fall in
and turbidity of
cleaned water, these above observation clearly depicts that its excellent flocculent property. Acknowledgments The efforts of Islamic Azad University, South Tehran Branch are appreciated for having financially supported the research. M. Asif appreciates the support of the Deanship of Scientific Research at King Saud University for the Prolific Research Group PRG-1437-31.
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Figure captions Fig. 1 FTIR spectra of GNp and FGNp. Fig. 2 Raman spectra of GNp and FGNp. Fig. 3 TGA thermogram and elemental analysis of GNp and FGNp. Fig. 4 TEM images showing the GNp and FGNp dispersion from the ultrathin section of (a) PAM/GNp1 and (b) PAM/FGNp1 nanocomposites. Fig. 5 FTIR spectra of PAM Fig. 6 The filtered weights of 1 wt% GCC suspension in the flocculation test, after mixing with nanocomposites Fig.7 Concentration effect of GOs in the 0.5 wt% PAM/GO nanocomposites on turbidity of the suspensions. Fig. 8 (A) dewatering time and (B) turbidity of the supernatants for the suspensions for different nanocomposites.
19
Table 1 Compositions of copolymer samples prepared in aqueous solution 1.25 M acrylamide
1.25 M acrylic acid
solution (ml)
solution (ml)
PA10
10
90
PA20
20
80
PA30
30
70
PA40
40
60
PA50
50
50
sample
20
Table 2 Intrinsic viscosity, and surface charge of PAM and nanocomposites Samples
FGNP (wt%)
Intrinsic viscosity
Surface charge
(cm3/g)
(mequiv/g)
PG0
0
328
1.09
PG0.1
0.1
880
1.02
PG0.3
0.3
915
0.94
PG0.5
0.5
1225
0.51
PG0.7
0.7
1351
0.40
21
Fig.1 FTIR spectra of GNp and FGNp.
22
Fig. 2 Raman spectra of GNp and FGNp.
23
Fig. 3 TGA thermogram and elemental analysis of GNp and FGNp.
24
Fig. 4 TEM images showing the GNp and FGNp dispersion from the ultrathin section of (a) PAM/GNp1 and (b) PAM/FGNp1 nanocomposites.
25
Fig. 5 FTIR spectra of copolymer( PAM)
26
Fig. 6 The filtered weights of 1 wt% GCC suspension in the flocculation test, after mixing with nanocomposites
27
Fig. 7 Concentration effect of GOs in the 0.5 wt% PAM/GO nanocomposites on turbidity of the suspensions.
28
Fig. 8 (A) dewatering time and (B) turbidity of the supernatants for the suspensions for different nanocomposites.
29
Graphical abstract
Presentation of developed flocculent in the form of bars for the flocculation test
30