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Superabsorbent polymer hydrogels with good thermal and mechanical properties for removal of selected heavy metal ions
Accepted Manuscript Superabsorbent polymer hydrogels with good thermal and mechanical properties for removal of selected heavy metal ions Luqman Ali Shah, Majid Khan, Rida Javed, Murtaza Sayed, Muhammad Saleem Khan, Abbas Khan, Mohib Ullah PII:
S0959-6526(18)32368-0
DOI:
10.1016/j.jclepro.2018.08.035
Reference:
JCLP 13819
To appear in:
Journal of Cleaner Production
Received Date: 27 April 2018 Revised Date:
10 July 2018
Accepted Date: 4 August 2018
Please cite this article as: Shah LA, Khan M, Javed R, Sayed M, Khan MS, Khan A, Ullah M, Superabsorbent polymer hydrogels with good thermal and mechanical properties for removal of selected heavy metal ions, Journal of Cleaner Production (2018), doi: 10.1016/j.jclepro.2018.08.035. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Superabsorbent polymer hydrogels with good thermal and mechanical properties for removal of selected heavy metal ions Luqman Ali Shah1,*, Majid Khan1, Rida Javed1, Murtaza Sayed1, Muhammad Saleem Khan1, Abbas Khan2 and Mohib Ullah3
2
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National Centre of Excellence in Phys ical Chemistry, University of Peshawar, 25120, KPK, Pakistan Department of Chemistry, Abdulwali Khan University, KPK, Pakistan
3
Institute of Chemical Sciences, Gomal University D.I. Khan, Pakistan
* Corresponding author
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1
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Email: [email protected]
[email protected] Tel: (9291)9216766
Abstract
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Fax: (9291)9216671
This article reports an efficient removal of selected heavy metal ions using a low-cost superabsorbent polymer hydrogel (SPH) composed of acrylic acid and acrylamide in different
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compositions which were prepared by single step free radical polymerization technique using
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ammonium persulphate and N, N-methylene bis-acrylamide as an initiator and cross-linker respectively. The morphological, thermal and mechanical properties were assessed for superabsorbent polymer hydrogels. The effect of pH and monomer content on the swelling behaviour of SPH was studied in detail and a maximum swelling capacity of 1841% was found for composition having maximum acrylic acid (AA) content. All the samples were highly effective in the removal of Cd2+, Ni2+, Cu2+ and Co2+ from aqueous medium at pH range of 2-10 following pseudo second order kinetics and Freundlisch adsorption model. Also, the removal capacity was greater at pH 7 and materials showed high selectivity towards
ACCEPTED MANUSCRIPT Co2+ and Cu2+ in competitive removal process. The high removal ability >75% for each metal ion, make these materials as an efficient, easily obtainable, and environmental friendly product. Keywords: competitive removal; heavy metal ions; polymer hydrogel; superabsorbent;
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swelling capacity 1. Introduction
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Superabsorbent polymer hydrogels (SPH), are class of polymeric hydrogels that are gently cross-linked systems having unique ability to imbibe water hundred times of their dried
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weight (Chang et al., 2010; Khodadadi Dehkordi). Due to their extraordinary water retention ability such as excellent hydrophilic properties, high swelling ratio, biocompatibility and abundance in availability (Argin et al., 2014), they have been widely used in many fields, like agriculture and horticulture (Ibrahim et al., 2015), drug delivery systems (Singh and Lee,
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2014), sanitary (Ma et al., 2015), tissue engineering (Sayyar et al., 2015), immobilization of protein and cells (Li et al., 2014), (Gawande and Mungray, 2015), and for waste water treatment (Javed et al., 2018). Due to extensive use these materials were industrialized in
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USA and Japan at the start of 1980’s for hygienic applications such as baby napkins and
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diapers (Kabiri et al., 2011).
The global demand for superabsorbent hydrogels is increasing and reaches 1.9 million
metric tons in 2015 (Peng et al., 2016). However, lack of biocompatibility and biodegradability is the major hassle for the present superabsorbent materials, which can be overcomes by introducing natural polymers in the synthetic materials, which would produce biocompatible and bio-degradable SPH, but they have inferior mechanical properties (Tse and Engler, 2010). The remedy for this problem is to design the hydrogels by using petroleum based monomers like acrylic acid (AA) and acrylamide (AM), which have high
ACCEPTED MANUSCRIPT thermal, mechanical properties and swelling capacity compared to the natural polymers (Spagnol et al., 2012). The high swelling capacity, easy handling, synthesis with different compositions to tune the properties and its applicability makes these SPH a good candidate for the removal of heavy metal ions from contaminated water, compared to other
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conventional adsorbents i.e. activated carbon (Ahmad and Hameed, 2010), zeolites (Wang and Peng, 2010), charcoal (Pap et al., 2017), plants (Syukor et al., 2016) etc.
The removal of heavy metal ions is of prime importance from waste water before its
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discharge into fresh water bodies, because these ions produce a number of health related
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problems in human beings and aquatic animals directly or indirectly when come in contact (Anastopoulos et al., 2016; Liu et al., 2013; Zhang et al., 2016). For instance Cd is a toxic heavy metal which accumulates and retained in the body, causing the demineralization of bones, lung cancer and kidney damage (Johri et al., 2010). Similarly, the large quantity of Ni causes more serious effects on human bodies like lung embolism, heart disorders, respiratory
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failure, birth defects (Schaumlöffel, 2012) etc. Too much intake of Co by human body produces pneumonia, asthma and wheezing (Finley et al., 2012). Similarly, exposure of
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human body to higher doses of Cu is harmful and may cause nausea, diarrhea, dizziness and headaches (Stern, 2010). Although, these metals are very much toxic in high concentration,
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but the use of these metals on other side is also very essential for many processes that takes place in routine life (Duruibe et al., 2007). Therefore, a proper simple and sophisticated method is required to be adopted for the removal of inorganic heavy metal ions from waste water (Lam et al., 2018; Liu et al., 2008). Currently, some of materials and methods have been designed for the removal of toxic inorganic ions from waste water like ion exchange resins, electrochemical, ultrafiltration membranes etc. But the most expensive and time consuming nature of these materials limits their applications in water treatment. To overcome these difficulties different superabsorbent systems have been used for multiple purposes like
ACCEPTED MANUSCRIPT diapers applications (Cordella et al., 2015), removal of organic dyes (Lai et al., 2017), water treatment (Ahmad et al., 2016) etc. But to best of our knowledge there is no serious attempt adopted for the removal of heavy metal ions from aqueous medium using superabsorbent polymer hydrogels (SPH).
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In this work we have prepared SPH with different composition of AA and AM, crosslinked with N, N-methylene bis-acrylamide (MBA) by simple free radical polymerization method, and applied these materials for the removal of selected heavy metal ions i.e. Cd2+,
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Ni2+, Cu2+ and Co2+ from waste water in different range of environmental conditions. The AA
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and AM moieties introduce both pH and temperature sensitive behaviour in SPH. The materials show high thermal stability and second order phase transition. Effect of monomer composition on the swelling ratio, removal capacity, % removal and removal selectivity of SPH towards different heavy metal ions have been studied in detail.
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2. Experimental 2.1 Chemicals and reagents
Except monomers acrylicacid (AA, 99.5%; ACROS) and acrylamide (AM,
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ORGANICS) which were recycled from a mixture of n-hexane and toluene (v:v=3:1) and
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recrystallized before use, all other reagents N, N-methylene bis-acrylamide (MBA, Sigam Aldritch), ammonium peroxosulphate (APS, Sigma), copper chloride, nickel chloride, cobalt chloride and cadmium chloride from Scharlau were of analytical grade and used as received without any further treatment. The solutions throughout the experimental work were prepared in pre-cleaned pyrex vessels using Milli-Q distilled water as a solvent. 2.2 Synthesis of superabsorbent polymer hydrogels (SPH) A series of SPH with different mole ratios of AA and AM were synthesized by slight modification in free radical polymerization protocol previously adopted (ur Rehman et al.,
ACCEPTED MANUSCRIPT 2015). The required amount of monomers for each composition (given in Table 1) was added to a flask containing 8 mL of Milli-Q distilled water, both monomers are soluble in the water and results a clear solution with a slight mixing. Nitrogen gas was bubbled into this solution for 45 minutes to knock out the dissolved oxygen. After being purged with nitrogen the
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reaction mixture was then heated at 60 °C, followed by addition of freshly prepared mixture of 51.2 mg (0.12 mmol) of N, N-methylene bis-acrylamide (MBA) as cross linker and 27 mg (0.12 mmol) of ammonium peroxodisulphate (APS) as initiator prepared in 1 mL of distilled
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water. The introduction of this mixture causes polymerization of monomers, and SPH formed within 5 minutes, the samples were cooled at room temperature, cut into small pieces, washed
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five times continuously with pure distilled water, and then dried in vacuum oven at 80 oC and kept for further study and characterizations.
Table 1: Prepared samples with different monomer concentrations moles of AA
A1M3
0.08
A1M2 A1M1
amount of AM (g)
5.76
0.12
8.52
0.08
5.76
0.10
7.1
0.08
5.76
0.08
5.68
0.10
7.2
0.08
5.68
0.12
8.64
0.08
5.68
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A3M1
moles of AM
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A2M1
amount of AA (g)
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Sample
2.3 Swelling ratio measurement The swelling behaviour of synthesized samples was investigated by immersion of 1g of the SPH in 1000mL Milli-Q distilled water at room temperature and allows the samples to reach equilibrium swelling for 24h (Alla et al., 2012). For pH adjustment a dilute solutions of HCl and NaOH was used. The amount of water entrapped by SPH was weighted by
ACCEPTED MANUSCRIPT SHIMADZU AY220 electronic balance after removing the surface water by tissue paper. The percent swelling ratios were determined by applying Equation1; M t - Mo ×100 Mo
Eq.1
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%Swelling Ratio =
Where Mt is the weight of swollen and Mo is the weight in collapsed dry state of SPH. 2.4 Absorption experiment
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The absorption experiment was performed by immersing 0.1g of SPH in 50mL of selected metal salt solutions with different concentrations (conditions dependent) and then
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allowed to shake in Shel Lab shaker at the speed of 80 oscillations per minute for 24h to reached equilibrium swelling at room temperature. The SPH with metal ions entrapped were collected by centrifugation and metal contents were found by atomic absorption spectrometer (AAS) in remaining solutions. The percent removal capacity of ions was calculated by
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Equation 2;
PercentRemoval =
Co - Cf ×100 Co
Eq.2
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Where Co is the initial and Cf is the equilibrium final concentration of metal ions
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respectively. The equilibrium removal efficiency was calculated using Equation 3;
qe =
Co - Cf ×V M
Eq.3
Where, qe (mg/g) represents the equilibrium removal efficiency of SPH, Co(mg/L) is
the initial and Cf(mg/L) is the equilibrium concentrations of metals in liquid phase, V (L) the volume of metal solution and M (g) the mass of SPH used. The effect of initial concentration, adsorbent dose etc were elaborated successfully.
ACCEPTED MANUSCRIPT 2.5 Absorption selectivity The competitive absorption capacity of SPH for the selected heavy metal ions i.e. Cd2+, Ni2+, Cu2+ and Co2+ was performed by immersion of 0.1 g SPH in 50 mL of solution having 100ppm concentration equally contributed by all salt ions with 25ppm each. The
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absorption condition was same as applied for separate removal of metal ions. 2.6 Characterization
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The formation of SPH was confirmed by FT-IR, detecting the functional groups in it. The prepared SPH were analysed using Shimadzu (IR Prestige-21) spectrometer (Japan). The
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samples were first mixed with dried KBr pallets before analysis and spectra of each sample was obtained in the range of 500-4000 cm-1. JEOL Scanning Electron Microscope (SEM) Model JSM-5910 (Japan), with current voltage applied in the range of 5-20 kV was used to study the surface morphology of the prepared SPH. Proper mass of hydrogel was taken on
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specified aluminium stubs and images were recorded at suitable resolution. Mechanical properties of SPH were determined by universal testing machine 300 series (Zwick Roell Z010), dual column which can exert up to 600 KN force. The specified load was applied with
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the help of hydraulic pressure on the sample and results were recorded. TGA and DSC were used to study the thermal and phase transition properties of SPH. TGA was done by operating
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Diamond TG/DTA (Perkin Elmer USA) analyser, taking 6 - 8 mg of sample. The experiment was performed in nitrogen atmosphere to avoid oxidation of the hydrogels. For accuracy in operation the analysed samples were kept closely connected to the thermocouple. Each sample was held at 40 ºC in nitrogen atmosphere for one minute and then temperature was increased from 40 to 700 ºC at the rate of 10 ºC rise per minute. The spectra recorded the weight loss in the sample versus rise in temperature. The DSC analysis were performed in nitrogen atmosphere using a DSC-7 (Mettler Toledo Inc.) system. The prepared sample and
ACCEPTED MANUSCRIPT reference were kept in the sample holder. The nitrogen flow was continued throughout the operation to avert the oxidation. The sample was kept for one minute at 50 ºC and then ramped to 600 ºC at the rate of 10 °C/min. Atomic absorption spectrometer (AAS) was used to determine the amount of metal
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ions in solutions using the absorption of optical radiation (light) by free atoms in the gaseous state. The sample to be analysed was diluted to the detectable limit before analysis. Perkin Elmer Analyst 800 system with acetylene flame was used to find the concentration of
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solutions. The solution was measured in triplicate and a mean concentration was recorded.
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3. Results and discussion 3.1 Synthesis of SPH
The SPH were synthesized by ordinary free radical addition polymerization in aqueous medium. Ammonium peroxodisulphate (APS) was used as free radical initiator,
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which decomposes at about 58 ºC and furnish sulphate anion radical in the system which initiate the polymerization process. The illustration for the synthesis of SPH by free radical
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polymerization is given in Figure 1.
Fig. 1. Illustration of SPH formation
ACCEPTED MANUSCRIPT Initially the persulphate results sulphate anion radical upon heating. This free radical then approached to the terminal double bond of the AA and AM monomers in the system which results a free radical active centre in the solution. In the system, the nearby monomers became the acceptor for free radical active centre and results the formation of polymers. The
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multifunctional MBA combines the polymer chains in a 3D network structure. The MBA acts like a hook in the chain propagation which attaches the monomers on its terminal ends and results in multifunctional hard brittle 3D network of SPH. The synthesis of samples was
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further confirmed by FTIR spectroscopy and results are explored in Figure 2.
Figure 2a represents the FT-IR spectra of pure PAM and AM functionalized with different
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content of AA. The spectrum of pure PAM shows no peak in the region of 3600 cm-1 due to the lack of –OH group, but after the incorporation of AA monomer the samples give a characteristic peak at ~3600 cm-1 clearly indicates the attachment of AA with AM and further increase in AA content causes the increase in intensity of this peak in SPH. The peak
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appeared in the 3180cm-1 is the characteristic stretching vibration of hydroxyl (OH) group of AA. The shifting of peak at 1658 cm-1 also confirms the successful occurrence of addition
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polymerization. The disappearance of broad and intense peaks in the range of 500-900 cm-1 in pure PAM indicates the formation of copolymers, because no such peaks observed in the
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FTIR spectra for SPH (Sohail et al., 2015). The functionalization of AA with AM is also confirmed by FTIR spectra and results are given in Figure 2b. All the characteristic peaks related to functional groups in PAA were present in FTIR spectrum of pure PAA. The carboxyl peaks appeared in the range of 2500-2700cm-1 and 1200-1300cm-1. The FTIR spectra for functionalized PAA represent shifting as well as appearance of new peaks in the range of 3000-3400cm-1. The appearance of new peaks is due to amine groups and confirms the successful fabrication of AA with AM. The shifting in peaks was thought due to intra Hbonding generated between carboxyl and amine groups of polymer network.
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Fig. 2 FTIR spectra for prepared SPH (a) pure AM and functionalized with AA and (b) pure AA and functionalized with AM 3.2 Morphology and physical appearance of SPH
ACCEPTED MANUSCRIPT The morphology of SPH was analysed by using scanning electron microscopy (SEM) and results are explored in Figure 3. The SEM micrographs reveal that hydrogels of both pure AA and AM are with almost uniform and smooth surface (Figure 3 (a and e)), but after the functionalization of monomers with one another produced SPH with rough surface by
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producing pores and cracks in the smooth surface of template material, which grows more and more with the incremental addition of monomer content. It was further confirmed that porosity and cracks appearance increases more and changed to tube like structures with the
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addition of AA content keeping AM content constant in SPH compositions as shown in Figure 3 (b, c and d). the addition of AM in composition of SPH also produces some cavities
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and pores (Figure 3f), but in this case the porosity was not greater as compared to porosity produced by increase in AA content in SPH. This result was further confirmed by highest
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values of swelling ratio for SPH with more AA content.
ACCEPTED MANUSCRIPT Fig. 3 SEM images of SPH (a) PAM, (b) A1M1, (c) A2M1 (d) A3M1, (e) PAA and (f) A1M2; Scale bar is in nm. The physical appearance of SPH was confirmed by taking photographs before and after complete swelling for one day in aqueous medium; both forms are quite different from each
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other shown in Figure S1, after absorbing water the weight and volume of SPH significantly
3.3 Composition effect on swelling ratio
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increases to a maximum values which confirms their super-absorbing ability.
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Water sensitivity is one of the significant properties that must be taken into consideration before practical applications of SPH. The swelling behaviour of synthesized materials was investigated by calculating the % swelling ratio using Equation 1 at equilibrium and results are unveiled in Figure 4.
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The results indicate that swelling ratio/water absorbency of SPH increases with the competitive addition of both AA and AM in polymer composition up to some optimum values. A greater value of swelling ratio i.e 1841% obtained for A3M1 sample due to the high
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content of AA in its composition, similar trend was observed by increasing AM content in
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SPH, but this increase is less than the AA. When the polymer composition approached towards two extremes (having one monomer almost), a decrease in swelling ratio was observed and reached to the swelling ratio of pure component i.e 1131 and 1092% for PAA and PAM respectively as shown in Figure 4. The drastic increase in the swelling rations is might be due to the fact that AA and AM monomers integrate in the polymer and result in elongation of chain in the polymer structure. The chain of monomers enlarges and folds upon each other resulting the formation of intra and inter hydrogen bonding among carboxylic and amide groups(Mekewi and Darwish, 2015). These established hydrogen bonding in the
ACCEPTED MANUSCRIPT polymeric structure starts breaking by entrapping water when exposed to aqueous media. This splitting of hydrogen bonding in the structure push the amide and carboxylic groups far away and create space for the entrapped water and led to higher swelling ratio(Mahdavinia et al., 2006). These hydrogels when entrap water in their structure swell in size to much extent.
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The more increase swelling behaviour for PAA compared to PAM was due to the presence of carboxyl moieties which further apart polymer chains, enhances the porosity due to internal osmotic pressure generated by repulsive forces inside SPH network(Bao et al., 2011)
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(confirmed from SEM images).
Fig. 4 Swelling ratio as a function of composition for SPH at pH 4.8
3.4 pH effect on swelling ratio The pH of solution is one of the key factors that affect the swelling ratio of superabsorbens. Swelling behavior of the synthesized A1M1 was measured at different pH values ranging from 2.0 to 8.0 (±0.2) as shown in Figure 5. It can be seen that the equilibrium swelling ratio for A1M1 increases as pH of the medium increases and this behaviour is
ACCEPTED MANUSCRIPT interpreted as a buffer action of —COOH and —COO- moieties. When the pH of the medium is in between 2 to 5, the swelling capability of SPH is only due to hydrophilic groups in polymer backbone because at pH ˂ 5, AA exist in fully protonated form (—COOH) and repulsion between polymer chains disappears, which decreases the swelling ability of SPH
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compared at pH > than 5 (Shah et al., 2016a; Shah et al., 2015; Shah et al., 2016b). The increase in pH of the medium causes de-protonation of AA which produces internal osmotic pressure in polymer network due to repulsive forces among —COO- groups and increases the
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swelling ratio.
Fig. 5 Swelling ratio as a function of pH for A1M1
3.5 Composition effect on mechanical property The effect of composition change on mechanical properties of SPH was studied and results are explored in Figure 6. The results clearly indicate that the compressive gel strength decreases as the amount of AA in polymer composition increases. The lowest value of compressive strength (14.6kPa) was obtained for A3M1 due to more AA content. It could be
ACCEPTED MANUSCRIPT explained that the swelling stress could accumulate among the swelling behaviours, while AA took up a large proportion, which weakened the compressive strength. The swelling stress of hydrogel gradually augmented, while the polymer molecule did not relax fast and water rapidly entered as existing abundant –COO- groups during the water uptake. The
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swollen hydrogels with higher swelling ratio were easier to crack and resulted in lower compressive strength. But different increase in trend was obtained when we enhances the AM content in polymer composition, this can be explained due to the intra-hydrogen bonding in
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AM, which resist to the applied force and improve the compressive gel strength. Thus, the
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higher content of AM led to the stronger mechanical strength of the hydrogel.
Fig. 6 Compressive strength as a function of polymer composition for SPH
3.6 Thermal properties
Thermal study was carried out by analysing TGA and DSC calculations for all the synthesized SPH samples. As SPH contain three types of water i.e free water (no interaction with polymer chains), bound water (directly attached with hydrophilic groups of polymer
ACCEPTED MANUSCRIPT back bone through mostly H-bonding) and half bound water (attached with bound water), therefore different transition stages observed in TGA curves. The TGA thermograms of SPH are shown in Figure 7a. Total weight loss for all the samples at 600 °C is almost same. As depicted from the results, all the samples show similar first weight loss (22%) at 146-261°C
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due to the removal of free water from SPH network. The second weight loss is due to the removal of half-bound water form SPH network. As the amount of half-bound water in A3M1 is greater, therefore, it shows less weight loss compared to other samples at same
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temperature initially. However, at above 378 °C, the weight loss is due to bound water. The third stage shows a complete thermal destruction of amide and carboxylic groups and
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splitting of backbone and crosslinking of SPH network.
Glass transition is associated with a base line shift in heat flow for the materials. A clear second order phase transition occurs for all samples of SPH(Grinberg et al., 2014), indicating a change in heat capacity only, keeping enthalpy constant. All SPH show
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approximately same value for glass transition temperature (Tg, 198 °C), except A3M1 which
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has low value (191 °C) due to more hydrophilic nature as shown in Figure 7b.
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Fig. 7 (a) TGA thermograms and (b) DSC curve for synthesized SPH 3.7 Absorption study of SPH
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3.7.1 Removal absorption capability
The removal absorption capability of the synthesized SPH was examined for the selected heavy metal ions i.e. Cd2+, Ni2+, Co2+ and Cu2+ in batch experiments. The removal
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process strongly depends on ion exchange and chelation properties of SPH, which are linked
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further with the network composition and most important factor in determination of absorption capacity. The penetration of metal ions in SPH is due to diffusion (with water molecules) and attraction (with polymer backbone) among opposite charged species. It was observed that with the increase of the AA concentration the absorption ability of the hydrogels increases, this may be because of carboxyl group. This behaviour to the —COOH is rationalized by the disparate electronegativity’s of oxygen and hydrogen (Jin and Bai, 2002). The percent removal capacity of ions was calculated by Equation 2 and is shown in Figure 8. From Figure 8, it was concluded that uptake of metal ions strongly depends on
ACCEPTED MANUSCRIPT composition of SPH and type of metal ions. An increase in % removal was obtained for all metal ions, when the concentration of both monomers in polymer composition was enhanced. This shows that the uptake of metal ions is associated with swelling property of SPH, which also gave the same trend (Figure 4). SPH having high amount of AA (A3M1 and A2M1)
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shows highest removal of metal ions than other samples having large amount of AM (A1M3 and A1M2) except Co and Cu, which shows the reverse trend. The greater % removal values for A1M3 and A1M2 compared to A3M1 and A2M1 towards Cu and Co is due to its strong
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complex formation properties. Many studies have been reported in literature regarding the attachment of metal ions to polymer network. For SPH used in this study, it may be expected
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that the only attractive sites for metals are the nitrogen atom of amino group in AM and the oxygen atom of the hydroxyl group in AA. Both nitrogen and oxygen atoms have lone pair of electrons that can bind a metal ion through an electron pair sharing to form a complex(Li et al., 2013). The negative charge on carboxyl group further enhances the attractive affinity
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towards metal ions and show high removal capacity. However, the nitrogen atom has higher complex formation ability compared to oxygen atom, similarly, Cu and Co have greater tendency towards complexation compared to Cd and Ni. Therefore, after incorporation of
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AM content in polymer composition, the amount of nitrogen enhances, which were
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favourable for removal of Cu and Co more as compared to Cd and Ni. The percent removal of selected heavy metal ions by synthesized SPH with different composition is tabulated in Table 2.
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Fig. 8 Percent removal of selected heavy metals by SPH at pH 4.8
Sample
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Table 2. % removal data for different SPH towards heavy metal ions at pH 4.8 % Removal
Ni
Cu
Cd
A1M3
85.08
69.16
84
76.17
A1M2
78.11
67
75.88
70.27
A1M1
66.68
64.66
71.94
61.83
A2M1
70.55
68.82
73.89
80.34
A3M1
74.05
75.55
79.14
84.39
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Co
3.7.2 Effect of pH on absorption The presence of stimulus responsive monomers in the matrix of SPH greatly affects the properties of the hydrogel composite. Out of these environmental stimuli pH is an essential parameter for investigating the absorption capability of meterials. The equilibrium
ACCEPTED MANUSCRIPT removal efficiency was calculated using Equation (A.3) and plot against pH of the solution. As discussed earlier, the swelling behaviour of SPH is strongly depends on pH of the medium, so, the removal efficiency also correspond to initial pH. The SPH used in this investigation composed of AM and AA, having functional amine and carboxyl groups. These
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functional groups are greatly affected by the pH of the solutions. The protonation and deprotonation of carboxyl group at low pH< 4.4 and high pH > 4.8 respectively, changes the removal efficiency of SPH towards the heavy metal ions. At high pH, both the electrostatic
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attraction between —COO- and metal ions, and diffusion of metal ions favours the removal
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process, compared to low pH where only diffusion process with slow rate exist. Figure 9 shows the removal effeciency and % removal of Cd ions at different initial pH ranging from 1 to 10 (±0.5), at constant initial ion concentration 100 mg/L, at room temperature for A1M1. Highest removal percentage was observed at pH 7 with maximum removal percentage of 78%. The high removal percentage was attributed to the high amount of de-protonated —
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COO- groups activated at this pH, thereby increasing binding of the cations to the polymer backbone and large size of SPH. The cations removal percentage then decreased to 14.79%
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with reducing pH from 7 to 2. This decrease was attributed to the gradual reduction in the amount of de-protonated carboxyl groups on AA surface, thereby decreasing electrostatic
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interaction between SPH and metal ions. However, the decrease in %R above pH 7 is due to the precipitation of metal salt, which restricts the availability of metal ions for SPH absorption. The obtained %R values are correlated with other reported values and tabulated in Table 3.
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Fig. 9 Effect of initial pH on removal efficiency and % removal of Cd2+ by A1M1. Salt
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concentration 100 mg/L and Temperature 25 °C
Table 3 Comparison of maximum % removal capacity of adsorbents towards heavy metal ions in aqueous medium
Metal ions
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Applied Adsorbent
Magnetic PEI-rGO-loaded
% Removal
pH Time (h)
Only Cr6+
99
4
12
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hydrogels
Gum tragacanth/graphene
Ref.
(Halouane et al., 2017)
Pb2+, Cd2+, Ag+
90, 75, 80
6
3
oxide composite hydrogel
(Sahraei and Ghaemy, 2017)
0.2Fe/RGO/Polyacrylamide
Ce3+, Cd2+, +
Ag , Cu Acrylic acid-acrylamide
66, 53, 52, 35
4.5
48
2+
Cd2+, Ni2+,
(Dong et al., 2018)
84, 74, 79, 76
4.8
24
Our study
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superabsoebent polymer hydrogels
3.7.3 Effect of Cd2+ initial dosage
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Figure 10a shows the influence of Cd2+ initial concentration ranging from 20 to 140 mg/L. When the initial concentration is increases from lower to higher value, the removal percentage also increases up to 100 ppm this was due to increase of ions for the available
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active site of A1M1 hydrogel. Further increase of concentration decrease the removal efficiency, because all available sites of hydrogel were filled with ions no vacant site is
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available for more ions to entrap into it, but the absorption uptake capacity is still maintained at 78%. Initial concentration is a crucial factor, which directly affects the absorption behaviour of metal ions.
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3.7.4 Effect of hydrogel dosage
The dose of A1M1 hydrogel as a decisive factor is estimated and the results are given in Figure 10b. The absorption capacity of Cd ions rises expressively as the rise in the absorbents
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quantity. Subsequently, absorption reaches saturated state when A1M1 dose ranging from 0.1–0.2. There is no significant change in absorption capacity at this concentration series.
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Initially, all Cd ions were not absorbed by absorbent due to insufficient absorption sites because of less absorbent dose. With increase in the dosage of the absorbent, additional absorption sites are available hence absorption capacity rises.
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Fig. 10 (a) Effect of initial concentration on removal of Cd ions by A1M1 hydrogel, (b) Effect of A1M1 hydrogel amount on removal of Cd ions at initial concentration of 100 mg/L. Temperature: 25 °C; contact time: 24 hrs; pH = 7 3.7.5 Absorption isotherms
ACCEPTED MANUSCRIPT To study the type of absorption two models Langmuir and Freundlich were applied. The Langmuir isotherm is effective for monolayer absorption of metal ions on regular surface, which means equal absorption energy is for every binding site on the absorbent surface, resultant each site can be occupied just one metal ion, while Freundlich isotherm is an
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empirical model, which shows that the absorption process occurs on an actively heterogeneous surface in the way of multilayers absorption. The two models are presented in
Eq.4
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Ce C 1 = + e Q e K L .Q m Q m
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Equations 4 and 5;
1 lnQe = lnK F + lnCe Eq.5 n
Where Ce (mg/L) and Qe (mg/g) are the equilibrium concentration and equilibrium adsorption capacity respectively, For Langmuir expression (Eq.4), KL (L/mg) is the Langmuir absorption
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constant and Qm (mg/g) is the maximum monolayer concentration of absorbate, whereas KF and n in Eq.5 are the Freundlich constants, KF correlating to the quantity of metal ions entrapped in the hydrogel network and n reflecting suitability of the desorption process. All
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the mentioned parameters of the models were calculated and given in Table 3.
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Comparison of the linear regression from the Freundlich and Langmuir isothermal models (Figure 11) indicate that the Freundlich model yields better fits with a higher R2 value (0.999) than Langmuir (0.895), so the absorption of Cd ions by A1M1 is multilayer physical absorption.
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Fig. 11 absorption models of (a) Freundlisch and (b) Langmuir for absorption of Cd+2 by A1M1 hydrogel 3.7.6 Kinetics study
ACCEPTED MANUSCRIPT To elaborate the mechanism and rate of absorption of Cd2+ by A1M1 hydrogel, the kinetic study was performed by applying pseudo-first order and pseudo-second order kinetic models as expressed in Equation 6 and 7 respectively.
t 1 1 = + 2 Q t k2 .Qe Qe .t
Eq.6
Eq.7
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ln(Qe - Q t ) = lnQe − k1t
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where k1 and k2 are the pseudo-first and pseudo-second order rate constants, Qe and Qt (mg/ g) are the sorption capacities at equilibrium and at time t respectively. Linearized model of
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first and second order for absorption of Cd2+ by A1M1 hydrogel is illustrated in Figure 12. The results indicate that the absorption of Cd2+ by A1M1 SPH in aqueous medium obey pseudo second order (Figure 1b) because of higher R2 value (0.963) compared to pseudo first order (Figure 12a) having R2 0.938 as shown in Table 4. So, absorption of metal ions by SPH
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is multistep process, at first, metal ions accumulates on the outer surface of A1M1 hydrogel
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from aqueous solution of salts; and then diffuses rapidly in the interior of SPH network.
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Fig. 12 Plots of (a) pseudo-first-order kinetics and (b) pseudo-second-order kinetics for the absorption of Cd2+ by A1M1 hydrogel
Isotherms/Models
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Table 4 Isotherms and kinetic parameters for absorption of Cd2+ by A1M1 hydrogel parameters KL=2438.2
isotherm
Qm=1979.6 mg/g
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Langmuir absorption
Freundlich absorption
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isotherm
Pseudo first order
Pseudo second order
R2 0.895
KF = 1.792 1/n = 0.9045
0.999
n = 1.105 k1 = -15.306 g/mg. min Qe = 420.91 mg/g k2= 0.0176 g/mg. min Qe= 523 mg/g
0.938
0.963
3.8 Competitive removal and recycling of SPH The competitive removal ability of SPH for selected heavy metals i.e. Cd2+, Co2+,
ACCEPTED MANUSCRIPT Cu2+ and Ni2+ from the mixture solution was conducted to demonstrate their selective behaviour. Known amount of SPH was put in the solution having the same concentration of the four ions. The results obtained are given in Figure 13a, which show that all the heavy metal ions can be removed by SPH, and all samples exhibited more selective behavior
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towards Co2+ and Cu2+ compared to Ni2+ and Cd2+ ions. Perhaps, it may be due to the fact that each metal interacts and adsorb in a different way and affect the removal efficiency of SPH. The metals having less size can easily be penetrated in the polymer matrix and then adsorb on
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the vacancy functionalized in the polymer chain.
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The recycling performance of SPH was tested in absorption and de-sorption process for five continuous cycles and there is no countable change in decrease absorption efficiency was observed. Figure 13b shows the recycling % removal performance of A3M1 composition towards selected heavy metal ions at mentioned conditions, which indicate no change in removal capability of SPH when used upto five continuous cycles. The SPH with absorbed
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heavy metal ions was recycled and used again by dipping into 0.1 M NaOH solution with
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thoroughly stirring for 2 hrs.
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Fig. 13 (a) A competitive % removal ability of SPH and (b) Recycling ability of loaded A3M1 towards Cu2+, Cd2+, Ni2+ and Co2+ ions. Concentration 100 mg L-1; Temperature 25 °C; Time 24h; and pH=4.8
4. Conclusion
ACCEPTED MANUSCRIPT Superabsorbent polymer hydrogels (SPH) composed of AA and AM with different mole rations were synthesized and characterized successfully. The thermal properties confirm that the materials show second order phase transition and can be utilized at high temperature upto 300 °C easily. The surface morphology has a porous and crack structure which enable them a
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good candidate for water retention and removal of heavy metal ions. The increase in swelling ratio and decrease in tensile strength was found with the enhancement of both AA and AM monomers in polymer composition respectively. The integrated —NH and —OH groups in
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the superabsorbent polymer hydrogels successfully entrap the metal ions, which strongly depends on pH of the medium. The entrapment process supports the Freundlisch adsorption
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model following pseudo second order kinetics. All the samples were found more selective towards better removal of Co2+ and Cu2+ ions in competitive removal process compared to Cd2+ and Ni2+ ions. It is concluded that the method followed is easy, simple and can be
Acknowledgment
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applied in the running waste water treatment on a large scale.
Dr. Luqman Ali Shah is highly thankful to Higher Education Commission (HEC) of
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Pakistan for financial support under the Startup Research Grant Program (SRGP) No:21-718/ SRGP/R&D/HEC/2016.
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•
Synthesis of superabsorbent polymer hydrogels in different ratio Swelling property is considered for removal of heavy metal ions Replacement of classical adsorbents with superabsorbent polymer hydrogels for removal of metal ions Optimum pH 7 was found for enhanced removal of metal ions
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• • •
S0959-6526(18)32368-0
DOI:
10.1016/j.jclepro.2018.08.035
Reference:
JCLP 13819
To appear in:
Journal of Cleaner Production
Received Date: 27 April 2018 Revised Date:
10 July 2018
Accepted Date: 4 August 2018
Please cite this article as: Shah LA, Khan M, Javed R, Sayed M, Khan MS, Khan A, Ullah M, Superabsorbent polymer hydrogels with good thermal and mechanical properties for removal of selected heavy metal ions, Journal of Cleaner Production (2018), doi: 10.1016/j.jclepro.2018.08.035. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Superabsorbent polymer hydrogels with good thermal and mechanical properties for removal of selected heavy metal ions Luqman Ali Shah1,*, Majid Khan1, Rida Javed1, Murtaza Sayed1, Muhammad Saleem Khan1, Abbas Khan2 and Mohib Ullah3
2
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National Centre of Excellence in Phys ical Chemistry, University of Peshawar, 25120, KPK, Pakistan Department of Chemistry, Abdulwali Khan University, KPK, Pakistan
3
Institute of Chemical Sciences, Gomal University D.I. Khan, Pakistan
* Corresponding author
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1
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Email: [email protected]
[email protected] Tel: (9291)9216766
Abstract
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Fax: (9291)9216671
This article reports an efficient removal of selected heavy metal ions using a low-cost superabsorbent polymer hydrogel (SPH) composed of acrylic acid and acrylamide in different
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compositions which were prepared by single step free radical polymerization technique using
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ammonium persulphate and N, N-methylene bis-acrylamide as an initiator and cross-linker respectively. The morphological, thermal and mechanical properties were assessed for superabsorbent polymer hydrogels. The effect of pH and monomer content on the swelling behaviour of SPH was studied in detail and a maximum swelling capacity of 1841% was found for composition having maximum acrylic acid (AA) content. All the samples were highly effective in the removal of Cd2+, Ni2+, Cu2+ and Co2+ from aqueous medium at pH range of 2-10 following pseudo second order kinetics and Freundlisch adsorption model. Also, the removal capacity was greater at pH 7 and materials showed high selectivity towards
ACCEPTED MANUSCRIPT Co2+ and Cu2+ in competitive removal process. The high removal ability >75% for each metal ion, make these materials as an efficient, easily obtainable, and environmental friendly product. Keywords: competitive removal; heavy metal ions; polymer hydrogel; superabsorbent;
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swelling capacity 1. Introduction
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Superabsorbent polymer hydrogels (SPH), are class of polymeric hydrogels that are gently cross-linked systems having unique ability to imbibe water hundred times of their dried
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weight (Chang et al., 2010; Khodadadi Dehkordi). Due to their extraordinary water retention ability such as excellent hydrophilic properties, high swelling ratio, biocompatibility and abundance in availability (Argin et al., 2014), they have been widely used in many fields, like agriculture and horticulture (Ibrahim et al., 2015), drug delivery systems (Singh and Lee,
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2014), sanitary (Ma et al., 2015), tissue engineering (Sayyar et al., 2015), immobilization of protein and cells (Li et al., 2014), (Gawande and Mungray, 2015), and for waste water treatment (Javed et al., 2018). Due to extensive use these materials were industrialized in
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USA and Japan at the start of 1980’s for hygienic applications such as baby napkins and
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diapers (Kabiri et al., 2011).
The global demand for superabsorbent hydrogels is increasing and reaches 1.9 million
metric tons in 2015 (Peng et al., 2016). However, lack of biocompatibility and biodegradability is the major hassle for the present superabsorbent materials, which can be overcomes by introducing natural polymers in the synthetic materials, which would produce biocompatible and bio-degradable SPH, but they have inferior mechanical properties (Tse and Engler, 2010). The remedy for this problem is to design the hydrogels by using petroleum based monomers like acrylic acid (AA) and acrylamide (AM), which have high
ACCEPTED MANUSCRIPT thermal, mechanical properties and swelling capacity compared to the natural polymers (Spagnol et al., 2012). The high swelling capacity, easy handling, synthesis with different compositions to tune the properties and its applicability makes these SPH a good candidate for the removal of heavy metal ions from contaminated water, compared to other
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conventional adsorbents i.e. activated carbon (Ahmad and Hameed, 2010), zeolites (Wang and Peng, 2010), charcoal (Pap et al., 2017), plants (Syukor et al., 2016) etc.
The removal of heavy metal ions is of prime importance from waste water before its
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discharge into fresh water bodies, because these ions produce a number of health related
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problems in human beings and aquatic animals directly or indirectly when come in contact (Anastopoulos et al., 2016; Liu et al., 2013; Zhang et al., 2016). For instance Cd is a toxic heavy metal which accumulates and retained in the body, causing the demineralization of bones, lung cancer and kidney damage (Johri et al., 2010). Similarly, the large quantity of Ni causes more serious effects on human bodies like lung embolism, heart disorders, respiratory
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failure, birth defects (Schaumlöffel, 2012) etc. Too much intake of Co by human body produces pneumonia, asthma and wheezing (Finley et al., 2012). Similarly, exposure of
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human body to higher doses of Cu is harmful and may cause nausea, diarrhea, dizziness and headaches (Stern, 2010). Although, these metals are very much toxic in high concentration,
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but the use of these metals on other side is also very essential for many processes that takes place in routine life (Duruibe et al., 2007). Therefore, a proper simple and sophisticated method is required to be adopted for the removal of inorganic heavy metal ions from waste water (Lam et al., 2018; Liu et al., 2008). Currently, some of materials and methods have been designed for the removal of toxic inorganic ions from waste water like ion exchange resins, electrochemical, ultrafiltration membranes etc. But the most expensive and time consuming nature of these materials limits their applications in water treatment. To overcome these difficulties different superabsorbent systems have been used for multiple purposes like
ACCEPTED MANUSCRIPT diapers applications (Cordella et al., 2015), removal of organic dyes (Lai et al., 2017), water treatment (Ahmad et al., 2016) etc. But to best of our knowledge there is no serious attempt adopted for the removal of heavy metal ions from aqueous medium using superabsorbent polymer hydrogels (SPH).
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In this work we have prepared SPH with different composition of AA and AM, crosslinked with N, N-methylene bis-acrylamide (MBA) by simple free radical polymerization method, and applied these materials for the removal of selected heavy metal ions i.e. Cd2+,
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Ni2+, Cu2+ and Co2+ from waste water in different range of environmental conditions. The AA
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and AM moieties introduce both pH and temperature sensitive behaviour in SPH. The materials show high thermal stability and second order phase transition. Effect of monomer composition on the swelling ratio, removal capacity, % removal and removal selectivity of SPH towards different heavy metal ions have been studied in detail.
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2. Experimental 2.1 Chemicals and reagents
Except monomers acrylicacid (AA, 99.5%; ACROS) and acrylamide (AM,
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ORGANICS) which were recycled from a mixture of n-hexane and toluene (v:v=3:1) and
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recrystallized before use, all other reagents N, N-methylene bis-acrylamide (MBA, Sigam Aldritch), ammonium peroxosulphate (APS, Sigma), copper chloride, nickel chloride, cobalt chloride and cadmium chloride from Scharlau were of analytical grade and used as received without any further treatment. The solutions throughout the experimental work were prepared in pre-cleaned pyrex vessels using Milli-Q distilled water as a solvent. 2.2 Synthesis of superabsorbent polymer hydrogels (SPH) A series of SPH with different mole ratios of AA and AM were synthesized by slight modification in free radical polymerization protocol previously adopted (ur Rehman et al.,
ACCEPTED MANUSCRIPT 2015). The required amount of monomers for each composition (given in Table 1) was added to a flask containing 8 mL of Milli-Q distilled water, both monomers are soluble in the water and results a clear solution with a slight mixing. Nitrogen gas was bubbled into this solution for 45 minutes to knock out the dissolved oxygen. After being purged with nitrogen the
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reaction mixture was then heated at 60 °C, followed by addition of freshly prepared mixture of 51.2 mg (0.12 mmol) of N, N-methylene bis-acrylamide (MBA) as cross linker and 27 mg (0.12 mmol) of ammonium peroxodisulphate (APS) as initiator prepared in 1 mL of distilled
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water. The introduction of this mixture causes polymerization of monomers, and SPH formed within 5 minutes, the samples were cooled at room temperature, cut into small pieces, washed
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five times continuously with pure distilled water, and then dried in vacuum oven at 80 oC and kept for further study and characterizations.
Table 1: Prepared samples with different monomer concentrations moles of AA
A1M3
0.08
A1M2 A1M1
amount of AM (g)
5.76
0.12
8.52
0.08
5.76
0.10
7.1
0.08
5.76
0.08
5.68
0.10
7.2
0.08
5.68
0.12
8.64
0.08
5.68
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A3M1
moles of AM
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A2M1
amount of AA (g)
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Sample
2.3 Swelling ratio measurement The swelling behaviour of synthesized samples was investigated by immersion of 1g of the SPH in 1000mL Milli-Q distilled water at room temperature and allows the samples to reach equilibrium swelling for 24h (Alla et al., 2012). For pH adjustment a dilute solutions of HCl and NaOH was used. The amount of water entrapped by SPH was weighted by
ACCEPTED MANUSCRIPT SHIMADZU AY220 electronic balance after removing the surface water by tissue paper. The percent swelling ratios were determined by applying Equation1; M t - Mo ×100 Mo
Eq.1
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%Swelling Ratio =
Where Mt is the weight of swollen and Mo is the weight in collapsed dry state of SPH. 2.4 Absorption experiment
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The absorption experiment was performed by immersing 0.1g of SPH in 50mL of selected metal salt solutions with different concentrations (conditions dependent) and then
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allowed to shake in Shel Lab shaker at the speed of 80 oscillations per minute for 24h to reached equilibrium swelling at room temperature. The SPH with metal ions entrapped were collected by centrifugation and metal contents were found by atomic absorption spectrometer (AAS) in remaining solutions. The percent removal capacity of ions was calculated by
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Equation 2;
PercentRemoval =
Co - Cf ×100 Co
Eq.2
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Where Co is the initial and Cf is the equilibrium final concentration of metal ions
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respectively. The equilibrium removal efficiency was calculated using Equation 3;
qe =
Co - Cf ×V M
Eq.3
Where, qe (mg/g) represents the equilibrium removal efficiency of SPH, Co(mg/L) is
the initial and Cf(mg/L) is the equilibrium concentrations of metals in liquid phase, V (L) the volume of metal solution and M (g) the mass of SPH used. The effect of initial concentration, adsorbent dose etc were elaborated successfully.
ACCEPTED MANUSCRIPT 2.5 Absorption selectivity The competitive absorption capacity of SPH for the selected heavy metal ions i.e. Cd2+, Ni2+, Cu2+ and Co2+ was performed by immersion of 0.1 g SPH in 50 mL of solution having 100ppm concentration equally contributed by all salt ions with 25ppm each. The
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absorption condition was same as applied for separate removal of metal ions. 2.6 Characterization
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The formation of SPH was confirmed by FT-IR, detecting the functional groups in it. The prepared SPH were analysed using Shimadzu (IR Prestige-21) spectrometer (Japan). The
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samples were first mixed with dried KBr pallets before analysis and spectra of each sample was obtained in the range of 500-4000 cm-1. JEOL Scanning Electron Microscope (SEM) Model JSM-5910 (Japan), with current voltage applied in the range of 5-20 kV was used to study the surface morphology of the prepared SPH. Proper mass of hydrogel was taken on
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specified aluminium stubs and images were recorded at suitable resolution. Mechanical properties of SPH were determined by universal testing machine 300 series (Zwick Roell Z010), dual column which can exert up to 600 KN force. The specified load was applied with
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the help of hydraulic pressure on the sample and results were recorded. TGA and DSC were used to study the thermal and phase transition properties of SPH. TGA was done by operating
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Diamond TG/DTA (Perkin Elmer USA) analyser, taking 6 - 8 mg of sample. The experiment was performed in nitrogen atmosphere to avoid oxidation of the hydrogels. For accuracy in operation the analysed samples were kept closely connected to the thermocouple. Each sample was held at 40 ºC in nitrogen atmosphere for one minute and then temperature was increased from 40 to 700 ºC at the rate of 10 ºC rise per minute. The spectra recorded the weight loss in the sample versus rise in temperature. The DSC analysis were performed in nitrogen atmosphere using a DSC-7 (Mettler Toledo Inc.) system. The prepared sample and
ACCEPTED MANUSCRIPT reference were kept in the sample holder. The nitrogen flow was continued throughout the operation to avert the oxidation. The sample was kept for one minute at 50 ºC and then ramped to 600 ºC at the rate of 10 °C/min. Atomic absorption spectrometer (AAS) was used to determine the amount of metal
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ions in solutions using the absorption of optical radiation (light) by free atoms in the gaseous state. The sample to be analysed was diluted to the detectable limit before analysis. Perkin Elmer Analyst 800 system with acetylene flame was used to find the concentration of
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solutions. The solution was measured in triplicate and a mean concentration was recorded.
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3. Results and discussion 3.1 Synthesis of SPH
The SPH were synthesized by ordinary free radical addition polymerization in aqueous medium. Ammonium peroxodisulphate (APS) was used as free radical initiator,
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which decomposes at about 58 ºC and furnish sulphate anion radical in the system which initiate the polymerization process. The illustration for the synthesis of SPH by free radical
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polymerization is given in Figure 1.
Fig. 1. Illustration of SPH formation
ACCEPTED MANUSCRIPT Initially the persulphate results sulphate anion radical upon heating. This free radical then approached to the terminal double bond of the AA and AM monomers in the system which results a free radical active centre in the solution. In the system, the nearby monomers became the acceptor for free radical active centre and results the formation of polymers. The
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multifunctional MBA combines the polymer chains in a 3D network structure. The MBA acts like a hook in the chain propagation which attaches the monomers on its terminal ends and results in multifunctional hard brittle 3D network of SPH. The synthesis of samples was
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further confirmed by FTIR spectroscopy and results are explored in Figure 2.
Figure 2a represents the FT-IR spectra of pure PAM and AM functionalized with different
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content of AA. The spectrum of pure PAM shows no peak in the region of 3600 cm-1 due to the lack of –OH group, but after the incorporation of AA monomer the samples give a characteristic peak at ~3600 cm-1 clearly indicates the attachment of AA with AM and further increase in AA content causes the increase in intensity of this peak in SPH. The peak
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appeared in the 3180cm-1 is the characteristic stretching vibration of hydroxyl (OH) group of AA. The shifting of peak at 1658 cm-1 also confirms the successful occurrence of addition
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polymerization. The disappearance of broad and intense peaks in the range of 500-900 cm-1 in pure PAM indicates the formation of copolymers, because no such peaks observed in the
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FTIR spectra for SPH (Sohail et al., 2015). The functionalization of AA with AM is also confirmed by FTIR spectra and results are given in Figure 2b. All the characteristic peaks related to functional groups in PAA were present in FTIR spectrum of pure PAA. The carboxyl peaks appeared in the range of 2500-2700cm-1 and 1200-1300cm-1. The FTIR spectra for functionalized PAA represent shifting as well as appearance of new peaks in the range of 3000-3400cm-1. The appearance of new peaks is due to amine groups and confirms the successful fabrication of AA with AM. The shifting in peaks was thought due to intra Hbonding generated between carboxyl and amine groups of polymer network.
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Fig. 2 FTIR spectra for prepared SPH (a) pure AM and functionalized with AA and (b) pure AA and functionalized with AM 3.2 Morphology and physical appearance of SPH
ACCEPTED MANUSCRIPT The morphology of SPH was analysed by using scanning electron microscopy (SEM) and results are explored in Figure 3. The SEM micrographs reveal that hydrogels of both pure AA and AM are with almost uniform and smooth surface (Figure 3 (a and e)), but after the functionalization of monomers with one another produced SPH with rough surface by
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producing pores and cracks in the smooth surface of template material, which grows more and more with the incremental addition of monomer content. It was further confirmed that porosity and cracks appearance increases more and changed to tube like structures with the
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addition of AA content keeping AM content constant in SPH compositions as shown in Figure 3 (b, c and d). the addition of AM in composition of SPH also produces some cavities
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and pores (Figure 3f), but in this case the porosity was not greater as compared to porosity produced by increase in AA content in SPH. This result was further confirmed by highest
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values of swelling ratio for SPH with more AA content.
ACCEPTED MANUSCRIPT Fig. 3 SEM images of SPH (a) PAM, (b) A1M1, (c) A2M1 (d) A3M1, (e) PAA and (f) A1M2; Scale bar is in nm. The physical appearance of SPH was confirmed by taking photographs before and after complete swelling for one day in aqueous medium; both forms are quite different from each
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other shown in Figure S1, after absorbing water the weight and volume of SPH significantly
3.3 Composition effect on swelling ratio
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increases to a maximum values which confirms their super-absorbing ability.
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Water sensitivity is one of the significant properties that must be taken into consideration before practical applications of SPH. The swelling behaviour of synthesized materials was investigated by calculating the % swelling ratio using Equation 1 at equilibrium and results are unveiled in Figure 4.
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The results indicate that swelling ratio/water absorbency of SPH increases with the competitive addition of both AA and AM in polymer composition up to some optimum values. A greater value of swelling ratio i.e 1841% obtained for A3M1 sample due to the high
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content of AA in its composition, similar trend was observed by increasing AM content in
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SPH, but this increase is less than the AA. When the polymer composition approached towards two extremes (having one monomer almost), a decrease in swelling ratio was observed and reached to the swelling ratio of pure component i.e 1131 and 1092% for PAA and PAM respectively as shown in Figure 4. The drastic increase in the swelling rations is might be due to the fact that AA and AM monomers integrate in the polymer and result in elongation of chain in the polymer structure. The chain of monomers enlarges and folds upon each other resulting the formation of intra and inter hydrogen bonding among carboxylic and amide groups(Mekewi and Darwish, 2015). These established hydrogen bonding in the
ACCEPTED MANUSCRIPT polymeric structure starts breaking by entrapping water when exposed to aqueous media. This splitting of hydrogen bonding in the structure push the amide and carboxylic groups far away and create space for the entrapped water and led to higher swelling ratio(Mahdavinia et al., 2006). These hydrogels when entrap water in their structure swell in size to much extent.
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The more increase swelling behaviour for PAA compared to PAM was due to the presence of carboxyl moieties which further apart polymer chains, enhances the porosity due to internal osmotic pressure generated by repulsive forces inside SPH network(Bao et al., 2011)
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(confirmed from SEM images).
Fig. 4 Swelling ratio as a function of composition for SPH at pH 4.8
3.4 pH effect on swelling ratio The pH of solution is one of the key factors that affect the swelling ratio of superabsorbens. Swelling behavior of the synthesized A1M1 was measured at different pH values ranging from 2.0 to 8.0 (±0.2) as shown in Figure 5. It can be seen that the equilibrium swelling ratio for A1M1 increases as pH of the medium increases and this behaviour is
ACCEPTED MANUSCRIPT interpreted as a buffer action of —COOH and —COO- moieties. When the pH of the medium is in between 2 to 5, the swelling capability of SPH is only due to hydrophilic groups in polymer backbone because at pH ˂ 5, AA exist in fully protonated form (—COOH) and repulsion between polymer chains disappears, which decreases the swelling ability of SPH
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compared at pH > than 5 (Shah et al., 2016a; Shah et al., 2015; Shah et al., 2016b). The increase in pH of the medium causes de-protonation of AA which produces internal osmotic pressure in polymer network due to repulsive forces among —COO- groups and increases the
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swelling ratio.
Fig. 5 Swelling ratio as a function of pH for A1M1
3.5 Composition effect on mechanical property The effect of composition change on mechanical properties of SPH was studied and results are explored in Figure 6. The results clearly indicate that the compressive gel strength decreases as the amount of AA in polymer composition increases. The lowest value of compressive strength (14.6kPa) was obtained for A3M1 due to more AA content. It could be
ACCEPTED MANUSCRIPT explained that the swelling stress could accumulate among the swelling behaviours, while AA took up a large proportion, which weakened the compressive strength. The swelling stress of hydrogel gradually augmented, while the polymer molecule did not relax fast and water rapidly entered as existing abundant –COO- groups during the water uptake. The
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swollen hydrogels with higher swelling ratio were easier to crack and resulted in lower compressive strength. But different increase in trend was obtained when we enhances the AM content in polymer composition, this can be explained due to the intra-hydrogen bonding in
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AM, which resist to the applied force and improve the compressive gel strength. Thus, the
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higher content of AM led to the stronger mechanical strength of the hydrogel.
Fig. 6 Compressive strength as a function of polymer composition for SPH
3.6 Thermal properties
Thermal study was carried out by analysing TGA and DSC calculations for all the synthesized SPH samples. As SPH contain three types of water i.e free water (no interaction with polymer chains), bound water (directly attached with hydrophilic groups of polymer
ACCEPTED MANUSCRIPT back bone through mostly H-bonding) and half bound water (attached with bound water), therefore different transition stages observed in TGA curves. The TGA thermograms of SPH are shown in Figure 7a. Total weight loss for all the samples at 600 °C is almost same. As depicted from the results, all the samples show similar first weight loss (22%) at 146-261°C
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due to the removal of free water from SPH network. The second weight loss is due to the removal of half-bound water form SPH network. As the amount of half-bound water in A3M1 is greater, therefore, it shows less weight loss compared to other samples at same
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temperature initially. However, at above 378 °C, the weight loss is due to bound water. The third stage shows a complete thermal destruction of amide and carboxylic groups and
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splitting of backbone and crosslinking of SPH network.
Glass transition is associated with a base line shift in heat flow for the materials. A clear second order phase transition occurs for all samples of SPH(Grinberg et al., 2014), indicating a change in heat capacity only, keeping enthalpy constant. All SPH show
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approximately same value for glass transition temperature (Tg, 198 °C), except A3M1 which
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has low value (191 °C) due to more hydrophilic nature as shown in Figure 7b.
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Fig. 7 (a) TGA thermograms and (b) DSC curve for synthesized SPH 3.7 Absorption study of SPH
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3.7.1 Removal absorption capability
The removal absorption capability of the synthesized SPH was examined for the selected heavy metal ions i.e. Cd2+, Ni2+, Co2+ and Cu2+ in batch experiments. The removal
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process strongly depends on ion exchange and chelation properties of SPH, which are linked
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further with the network composition and most important factor in determination of absorption capacity. The penetration of metal ions in SPH is due to diffusion (with water molecules) and attraction (with polymer backbone) among opposite charged species. It was observed that with the increase of the AA concentration the absorption ability of the hydrogels increases, this may be because of carboxyl group. This behaviour to the —COOH is rationalized by the disparate electronegativity’s of oxygen and hydrogen (Jin and Bai, 2002). The percent removal capacity of ions was calculated by Equation 2 and is shown in Figure 8. From Figure 8, it was concluded that uptake of metal ions strongly depends on
ACCEPTED MANUSCRIPT composition of SPH and type of metal ions. An increase in % removal was obtained for all metal ions, when the concentration of both monomers in polymer composition was enhanced. This shows that the uptake of metal ions is associated with swelling property of SPH, which also gave the same trend (Figure 4). SPH having high amount of AA (A3M1 and A2M1)
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shows highest removal of metal ions than other samples having large amount of AM (A1M3 and A1M2) except Co and Cu, which shows the reverse trend. The greater % removal values for A1M3 and A1M2 compared to A3M1 and A2M1 towards Cu and Co is due to its strong
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complex formation properties. Many studies have been reported in literature regarding the attachment of metal ions to polymer network. For SPH used in this study, it may be expected
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that the only attractive sites for metals are the nitrogen atom of amino group in AM and the oxygen atom of the hydroxyl group in AA. Both nitrogen and oxygen atoms have lone pair of electrons that can bind a metal ion through an electron pair sharing to form a complex(Li et al., 2013). The negative charge on carboxyl group further enhances the attractive affinity
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towards metal ions and show high removal capacity. However, the nitrogen atom has higher complex formation ability compared to oxygen atom, similarly, Cu and Co have greater tendency towards complexation compared to Cd and Ni. Therefore, after incorporation of
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AM content in polymer composition, the amount of nitrogen enhances, which were
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favourable for removal of Cu and Co more as compared to Cd and Ni. The percent removal of selected heavy metal ions by synthesized SPH with different composition is tabulated in Table 2.
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Fig. 8 Percent removal of selected heavy metals by SPH at pH 4.8
Sample
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Table 2. % removal data for different SPH towards heavy metal ions at pH 4.8 % Removal
Ni
Cu
Cd
A1M3
85.08
69.16
84
76.17
A1M2
78.11
67
75.88
70.27
A1M1
66.68
64.66
71.94
61.83
A2M1
70.55
68.82
73.89
80.34
A3M1
74.05
75.55
79.14
84.39
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Co
3.7.2 Effect of pH on absorption The presence of stimulus responsive monomers in the matrix of SPH greatly affects the properties of the hydrogel composite. Out of these environmental stimuli pH is an essential parameter for investigating the absorption capability of meterials. The equilibrium
ACCEPTED MANUSCRIPT removal efficiency was calculated using Equation (A.3) and plot against pH of the solution. As discussed earlier, the swelling behaviour of SPH is strongly depends on pH of the medium, so, the removal efficiency also correspond to initial pH. The SPH used in this investigation composed of AM and AA, having functional amine and carboxyl groups. These
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functional groups are greatly affected by the pH of the solutions. The protonation and deprotonation of carboxyl group at low pH< 4.4 and high pH > 4.8 respectively, changes the removal efficiency of SPH towards the heavy metal ions. At high pH, both the electrostatic
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attraction between —COO- and metal ions, and diffusion of metal ions favours the removal
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process, compared to low pH where only diffusion process with slow rate exist. Figure 9 shows the removal effeciency and % removal of Cd ions at different initial pH ranging from 1 to 10 (±0.5), at constant initial ion concentration 100 mg/L, at room temperature for A1M1. Highest removal percentage was observed at pH 7 with maximum removal percentage of 78%. The high removal percentage was attributed to the high amount of de-protonated —
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COO- groups activated at this pH, thereby increasing binding of the cations to the polymer backbone and large size of SPH. The cations removal percentage then decreased to 14.79%
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with reducing pH from 7 to 2. This decrease was attributed to the gradual reduction in the amount of de-protonated carboxyl groups on AA surface, thereby decreasing electrostatic
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interaction between SPH and metal ions. However, the decrease in %R above pH 7 is due to the precipitation of metal salt, which restricts the availability of metal ions for SPH absorption. The obtained %R values are correlated with other reported values and tabulated in Table 3.
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Fig. 9 Effect of initial pH on removal efficiency and % removal of Cd2+ by A1M1. Salt
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concentration 100 mg/L and Temperature 25 °C
Table 3 Comparison of maximum % removal capacity of adsorbents towards heavy metal ions in aqueous medium
Metal ions
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Applied Adsorbent
Magnetic PEI-rGO-loaded
% Removal
pH Time (h)
Only Cr6+
99
4
12
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hydrogels
Gum tragacanth/graphene
Ref.
(Halouane et al., 2017)
Pb2+, Cd2+, Ag+
90, 75, 80
6
3
oxide composite hydrogel
(Sahraei and Ghaemy, 2017)
0.2Fe/RGO/Polyacrylamide
Ce3+, Cd2+, +
Ag , Cu Acrylic acid-acrylamide
66, 53, 52, 35
4.5
48
2+
Cd2+, Ni2+,
(Dong et al., 2018)
84, 74, 79, 76
4.8
24
Our study
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superabsoebent polymer hydrogels
3.7.3 Effect of Cd2+ initial dosage
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Figure 10a shows the influence of Cd2+ initial concentration ranging from 20 to 140 mg/L. When the initial concentration is increases from lower to higher value, the removal percentage also increases up to 100 ppm this was due to increase of ions for the available
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active site of A1M1 hydrogel. Further increase of concentration decrease the removal efficiency, because all available sites of hydrogel were filled with ions no vacant site is
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available for more ions to entrap into it, but the absorption uptake capacity is still maintained at 78%. Initial concentration is a crucial factor, which directly affects the absorption behaviour of metal ions.
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3.7.4 Effect of hydrogel dosage
The dose of A1M1 hydrogel as a decisive factor is estimated and the results are given in Figure 10b. The absorption capacity of Cd ions rises expressively as the rise in the absorbents
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quantity. Subsequently, absorption reaches saturated state when A1M1 dose ranging from 0.1–0.2. There is no significant change in absorption capacity at this concentration series.
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Initially, all Cd ions were not absorbed by absorbent due to insufficient absorption sites because of less absorbent dose. With increase in the dosage of the absorbent, additional absorption sites are available hence absorption capacity rises.
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Fig. 10 (a) Effect of initial concentration on removal of Cd ions by A1M1 hydrogel, (b) Effect of A1M1 hydrogel amount on removal of Cd ions at initial concentration of 100 mg/L. Temperature: 25 °C; contact time: 24 hrs; pH = 7 3.7.5 Absorption isotherms
ACCEPTED MANUSCRIPT To study the type of absorption two models Langmuir and Freundlich were applied. The Langmuir isotherm is effective for monolayer absorption of metal ions on regular surface, which means equal absorption energy is for every binding site on the absorbent surface, resultant each site can be occupied just one metal ion, while Freundlich isotherm is an
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empirical model, which shows that the absorption process occurs on an actively heterogeneous surface in the way of multilayers absorption. The two models are presented in
Eq.4
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Ce C 1 = + e Q e K L .Q m Q m
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Equations 4 and 5;
1 lnQe = lnK F + lnCe Eq.5 n
Where Ce (mg/L) and Qe (mg/g) are the equilibrium concentration and equilibrium adsorption capacity respectively, For Langmuir expression (Eq.4), KL (L/mg) is the Langmuir absorption
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constant and Qm (mg/g) is the maximum monolayer concentration of absorbate, whereas KF and n in Eq.5 are the Freundlich constants, KF correlating to the quantity of metal ions entrapped in the hydrogel network and n reflecting suitability of the desorption process. All
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the mentioned parameters of the models were calculated and given in Table 3.
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Comparison of the linear regression from the Freundlich and Langmuir isothermal models (Figure 11) indicate that the Freundlich model yields better fits with a higher R2 value (0.999) than Langmuir (0.895), so the absorption of Cd ions by A1M1 is multilayer physical absorption.
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Fig. 11 absorption models of (a) Freundlisch and (b) Langmuir for absorption of Cd+2 by A1M1 hydrogel 3.7.6 Kinetics study
ACCEPTED MANUSCRIPT To elaborate the mechanism and rate of absorption of Cd2+ by A1M1 hydrogel, the kinetic study was performed by applying pseudo-first order and pseudo-second order kinetic models as expressed in Equation 6 and 7 respectively.
t 1 1 = + 2 Q t k2 .Qe Qe .t
Eq.6
Eq.7
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ln(Qe - Q t ) = lnQe − k1t
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where k1 and k2 are the pseudo-first and pseudo-second order rate constants, Qe and Qt (mg/ g) are the sorption capacities at equilibrium and at time t respectively. Linearized model of
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first and second order for absorption of Cd2+ by A1M1 hydrogel is illustrated in Figure 12. The results indicate that the absorption of Cd2+ by A1M1 SPH in aqueous medium obey pseudo second order (Figure 1b) because of higher R2 value (0.963) compared to pseudo first order (Figure 12a) having R2 0.938 as shown in Table 4. So, absorption of metal ions by SPH
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is multistep process, at first, metal ions accumulates on the outer surface of A1M1 hydrogel
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from aqueous solution of salts; and then diffuses rapidly in the interior of SPH network.
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Fig. 12 Plots of (a) pseudo-first-order kinetics and (b) pseudo-second-order kinetics for the absorption of Cd2+ by A1M1 hydrogel
Isotherms/Models
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Table 4 Isotherms and kinetic parameters for absorption of Cd2+ by A1M1 hydrogel parameters KL=2438.2
isotherm
Qm=1979.6 mg/g
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Langmuir absorption
Freundlich absorption
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isotherm
Pseudo first order
Pseudo second order
R2 0.895
KF = 1.792 1/n = 0.9045
0.999
n = 1.105 k1 = -15.306 g/mg. min Qe = 420.91 mg/g k2= 0.0176 g/mg. min Qe= 523 mg/g
0.938
0.963
3.8 Competitive removal and recycling of SPH The competitive removal ability of SPH for selected heavy metals i.e. Cd2+, Co2+,
ACCEPTED MANUSCRIPT Cu2+ and Ni2+ from the mixture solution was conducted to demonstrate their selective behaviour. Known amount of SPH was put in the solution having the same concentration of the four ions. The results obtained are given in Figure 13a, which show that all the heavy metal ions can be removed by SPH, and all samples exhibited more selective behavior
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towards Co2+ and Cu2+ compared to Ni2+ and Cd2+ ions. Perhaps, it may be due to the fact that each metal interacts and adsorb in a different way and affect the removal efficiency of SPH. The metals having less size can easily be penetrated in the polymer matrix and then adsorb on
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the vacancy functionalized in the polymer chain.
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The recycling performance of SPH was tested in absorption and de-sorption process for five continuous cycles and there is no countable change in decrease absorption efficiency was observed. Figure 13b shows the recycling % removal performance of A3M1 composition towards selected heavy metal ions at mentioned conditions, which indicate no change in removal capability of SPH when used upto five continuous cycles. The SPH with absorbed
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heavy metal ions was recycled and used again by dipping into 0.1 M NaOH solution with
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thoroughly stirring for 2 hrs.
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Fig. 13 (a) A competitive % removal ability of SPH and (b) Recycling ability of loaded A3M1 towards Cu2+, Cd2+, Ni2+ and Co2+ ions. Concentration 100 mg L-1; Temperature 25 °C; Time 24h; and pH=4.8
4. Conclusion
ACCEPTED MANUSCRIPT Superabsorbent polymer hydrogels (SPH) composed of AA and AM with different mole rations were synthesized and characterized successfully. The thermal properties confirm that the materials show second order phase transition and can be utilized at high temperature upto 300 °C easily. The surface morphology has a porous and crack structure which enable them a
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good candidate for water retention and removal of heavy metal ions. The increase in swelling ratio and decrease in tensile strength was found with the enhancement of both AA and AM monomers in polymer composition respectively. The integrated —NH and —OH groups in
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the superabsorbent polymer hydrogels successfully entrap the metal ions, which strongly depends on pH of the medium. The entrapment process supports the Freundlisch adsorption
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model following pseudo second order kinetics. All the samples were found more selective towards better removal of Co2+ and Cu2+ ions in competitive removal process compared to Cd2+ and Ni2+ ions. It is concluded that the method followed is easy, simple and can be
Acknowledgment
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applied in the running waste water treatment on a large scale.
Dr. Luqman Ali Shah is highly thankful to Higher Education Commission (HEC) of
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Pakistan for financial support under the Startup Research Grant Program (SRGP) No:21-718/ SRGP/R&D/HEC/2016.
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Ibrahim, M.M., Abd‐Eladl, M., Abou‐Baker, N.H., 2015. Lignocellulosic biomass for the preparation of cellulose‐based hydrogel and its use for optimizing water resources in
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Synthesis of superabsorbent polymer hydrogels in different ratio Swelling property is considered for removal of heavy metal ions Replacement of classical adsorbents with superabsorbent polymer hydrogels for removal of metal ions Optimum pH 7 was found for enhanced removal of metal ions
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