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Novel polyacrylamide-based solid scale inhibitor
Journal of Hazardous Materials 334 (2017) 1–9
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Novel polyacrylamide-based solid scale inhibitor Ahmed A. Younes a,∗ , Heba H. El-Maghrabi b , Hager R. Ali c a
Department of Chemistry, Faculty of Science, Helwan University, Cairo, Egypt Petroleum Refining Department, Egyptian Petroleum Research Institute (EPRI), Cairo, Egypt c Analysis and Evaluation Department, Egyptian Petroleum Research Institute (EPRI), Cairo, Egypt b
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Novel polyacrylamide-based solid • • • •
scale inhibitor is synthesized and characterized. Sorption behavior toward divalent earth metal ions was studied. The optimum sorption conditions were determined. Thermodynamic, kinetic and isotherm of the sorption procedure were evaluated. Polymer regeneration for reuse was also explored.
a r t i c l e
i n f o
Article history: Received 16 January 2017 Received in revised form 23 March 2017 Accepted 24 March 2017 Available online 29 March 2017 Keywords: Scale inhibitor Scale inhibition Sorption Metal ion removal Regeneration
a b s t r a c t Novel solid-state phosphorus-containing polymer was synthesized, characterized and investigated as anti-scaling agent for the removal of alkaline earth metals ions from water. An optimization protocol for the sorption process of the metal ions on the polymer surface was proposed and executed. The protocol involved parameters such as pH, contact time, polymer dose, and the initial concentration of the metal ion. The optimum pH was found to be around seven for all of the tested metal ions. The maximum sorption capacities of the prepared polymer were 667, 794, 769 and 709 (mg/g) for Mg, Ca, Sr and Ba ions, respectively. Evaluation of the sorption process from the isotherm, kinetic and thermodynamic points of view was also studied. The experimental evidence revealed that the sorption obeys Langmuir isotherm model and follows a pseudo-second order mechanism. Moreover, the sorption process is exothermic. Possibility of polymer reuse was also investigated. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Scaling is the phenomenon of deposition or simple adhesion of mineral salts on processing equipment. The deposition of these salts, especially on heat exchanger surfaces in cooling, boiler, geothermal, and distillation systems, does cause technical and economic problems such as total or partial obstruction of pipes and pumps, inefficient water treatment chemical usage, increased
∗ Corresponding author. E-mail address: [email protected] (A.A. Younes). http://dx.doi.org/10.1016/j.jhazmat.2017.03.052 0304-3894/© 2017 Elsevier B.V. All rights reserved.
operation costs, lost production due to system downtime and reduction of heat transfer efficiency in cooling systems [1]. Therefore, understanding this phenomenon and finding effective ways to retard or prevent it is a pressing demand. The precipitation of scale forming salts on processing equipment surfaces is markedly retarded by either water acidification or the use of chemical additives (polymeric and non-polymeric). The acidification method has the disadvantage of exposing metal surfaces to a corrosive environment. The use of chemical additives such as scale inhibitors is the most popular and operative method of scale inhibition [2–6]. Scale inhibitors cut out crystal growth or slow the nucleation process, which in turn spoils the scale formation process.
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Most inhibitors for mineral scales are phosphorous compounds, e.g. inorganic polyphosphates, organic phosphate esters, organic phosphonates, and organic aminophosphates [6]. The scale inhibition process in industrial cooling water is of great economic significance because of the high costs of treating undesirable results of scale formation, high cost of water in certain arid areas and losses because of shut-down of operations [7]. In the oil and gas field, polymeric scale inhibitors are widely used because of their enhanced thermal stability and better environmental compatibility. Synthesis of functionalized polymers that pre-concentrate and remove metal ions can be done by two ways. The first way involves the polymerization of a monomer with a specific active ligand to target metal ions, while the second modifies the polymeric matrix, after the polymerization, by an active and specific ligand to one or more different metal ions [8]. In order to extract, determine, pre-concentrate, and remove some metal ions, a variety of new types of functional resins has been developed and intensively studied [9–15]. Although how to make polymeric scale inhibitors solid is still challenging, crosslinked-polymers could provide an effective approach to produce such solid scale inhibitors. The approach of using solid scale inhibitors is much better than conventional chemical treatment methods, such as a squeeze, continuous injection, or batch treatment with liquid chemicals. This also is more effective than pumping liquid inhibitors with a stimulation treatment. The slow dissolution of the solid scale inhibitor allows longer scale protection in both the wellbore area and tubing, and will not contribute to chemical runoff if spilled. Moreover, Reduce downtime caused by scale buildup in the near well-bore. In this context, chelating polymers have received a great priority because of their outstanding simplicity, high efficiency and low cost during the ion exchange, physical adsorption, and chelation [16]. In this article, a crosslinked polyacrylamide-based polymer bearing phosphonate groups was synthesized, characterized and investigated, by batch-equilibrium procedure, as a scale inhibitor for barium, calcium, magnesium and strontium ions. 2. Experimental 2.1. Materials and instruments All chemicals used were of analytical reagent grade unless otherwise stated; and bidistilled water was used throughout. N,N methylene bis-acrylamide (NMBA) and acrylamide (AAm) monomers were obtained from Merck Chemical Co. Nitrate or chloride salts were used to prepare the stock solutions of the investigated metal ions. Working standard metal ion solutions were prepared by appreciate dilution. Jenway pH meter (model 3310) was used in the pH measurements. The infrared spectra were recorded in the wave number range 400–4000 cm−1 on a PerkinElmer IR spectrophotometer-3100. Metal ion concentration was determined according to ASTM D 4327 using Dionex IC model DX 600 equipped with high capacity columns. 2.2. Synthesis of the chelating resin 2.2.1. Synthesis of crosslinked polyacrylamide (CPAAm) Crosslinked polyacrylamide (CPAAm) was synthesized by addition copolymerization. A 50 mg of potassium persulfate as initiator and monomer acrylamide with 5% mole ratio of N,N methylene bisacrylamide were dissolved in 100 mL of bidistilled water/ethanol (70:30, v/v) mixture at 50 ◦ C until the polymer is formed. Additional amount of bidistilled water (50 mL) was added and the temperature was raised to 80 ◦ C with stirring. A solid precipitate is formed as the reaction proceeded which was finally filtrated, washed with water then methanol and allowed to dry at 80 ◦ C [17].
2.2.2. Incorporation of amino groups on CPAAm The attachment of amino groups onto the crosslinked polymer was achieved by a reaction between the CPAAm and ethylenediamine (EDA). A 10 g of the crosslinked polyacrylamide was added to 100 mL of ethylenediamine. The mixture was heated at 110 ◦ C with stirring. After 9 h, the mixture was poured onto cold water. The obtained polymer was filtered and washed with sodium chloride solution until filtrate be free from the ethylenediamine. The presence of the ethylenediamine can be detected using ninhydrin reagent as it gives deep blue color with it. The polymer was washed with water then methanol and dried at 70 ◦ C [18]. 2.2.3. Phosphorylation of CPAAm polymer The incorporation of phosphonic acid groups into the aminografted CPAAm was easily achieved using a straightforward Mannich type reaction of phosphorous acid with formaldehyde and primary amine compounds at low pH in the presence of about 2–3 mol of concentrated hydrochloric acid per mole of an amine. The polymer incorporated with the amine group (0.2 mol amine groups), crystalline phosphorous acid (0.4 mol), and concentrated hydrochloric acid (0.6 mol) were dissolved in a total volume of 200 mL with water, and the mixture was heated to reflux in a threenecked flask fitted with thermometer, condenser, and dropping funnel. Over the course of about 1 h, 60 mL of a 40% (w/v) aqueous formaldehyde solution was added drop by drop, and the reaction was kept at the reflux temperature for an additional hour. The water was evaporated using a rotary evaporator, and the concentrate was neutralized with concentrated ammonia solution. Finally, the concentrated solution was precipitated in methanol to get a white powdered product. 2.3. Sorption studies Sorption, power and capacity, depends not only on the adsorbents’ physical and chemical properties, such as type and structure of the functional group introduced into the polymer matrix, but also on the sorption conditions including the solution’s pH, contact time, metal ion concentration, and presence of interfering ions [19]. The batch equilibration technique was utilized to investigate the sorption tendency of the synthesized chelating polymer and to determine the optimum sorption conditions, such as the pH, contact time, temperature, initial concentration of metal ion, and dose of the polymer. Judicious constant weight of dried polymer beads was used in all batch experiments. All experiments were performed at room temperature by using a mixture of 10 mg beads and metal ion solution (50 mL, 500 mg/L) in 50 mL flasks which were stirred in a temperature controlled shaker at a constant speed of 200 rpm. The sorption ability for Mg(II), Ca(II), Sr(II) and Ba(II), under noncompetitive conditions, was determined as a function of the pH. The suspensions were brought to the desired pH (2, 7, 10) by adding HCl (0.1 M) and NaOH (0.1 M). The contents of the flasks were equilibrated on the shaker at 30 ◦ C for 4 h. After equilibration, the residual concentration of metal ion was determined by Dionex IC equipped with high capacity columns. The sorption capacity (Q, in mg/g polymer) was calculated on the basis of Eq. (1): Q =
(C0 − CA )V W
(1)
where C0 and CA are the concentration (mg/L) of metal ion in the initial solution and in the aqueous phase after sorption, respectively, V is the volume of the aqueous phase (L) and W is the weight of the adsorbent (0.01 g). The sorption efficiency for ions from the solution (R [%]) was calculated using Eq. (2): R=
C − C 0 A C0
× 100
(2)
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Scheme 1. Synthetic pathway of the polymer (a) polymerization reaction, (b) transamination reaction and (c) phosphorylation of the polymer.
Influence of contact time (30–240 min), temperature (30–70 ◦ C), amount of polymer (5–50 mg) and concentration of metal ion (250–1000 mg/L) on sorption behavior of the chelating polymer were performed at the optimum pH of the corresponding metal ion, as described under noncompetitive conditions. 2.4. Sorption isotherms Equilibrium isotherms are explored to determine the capacity of the prepared polymer toward the tested metal ions. The most common types of models describing this type of system are the Langmuir and Freundlich models. The Langmuir sorption model assumes that the maximum sorption capacity is due to a saturated monolayer of solute molecules on the adsorbent surface. The Langmuir equation is described with the following linearized form [20]
C e
qe
=
1 KL qmax
C e +
qmax
(3)
where qe is the amount of sorption per unit mass of the polymer at equilibrium (mg/g), Ce is the equilibrium concentration (mg/L), qmax is the maximum theoretical saturation sorption capacity, and KL is the energy of sorption. The Freundlich sorption isotherm describes the relationship between the quantity of metal adsorbed per unit mass of the adsorbent (qe ) and the concentration of the metal ions in solution at equilibrium (Ce ). The mathematical equation is [26] 1/n
qe = KF Ce
(4)
where KF and n are Freundlich constants. The equation can be linearized if we take logarithms to find the values of KF and n: log qe = log KF +
1 n
log Ce
(5)
The total sorption capacities of the prepared polymer was determined via the shaking of 50-mL solutions with different concentrations of the metal ions with 10 mg of the polymer for 4 h at the optimum sorption pH at room temperature in a thermostatic water bath shaker to ensure complete equilibration. The polymer was filtered off, and the concentration of metal ions in the filtrate was determined using Dionex IC equipped with high capacity columns. 2.5. Thermodynamic studies Thermodynamic studies on the sorption of the metal ions of interest and the chelating polymer were performed in order to determine parameters such as free energy of sorption (Go ), the heat of sorption and standard entropy changes (Ho and So ). 50 mL of metal ion solution (500 mg/L) was stirred with 10 mg of the polymer in a mechanical shaker at different temperatures, from 25 to 70 ◦ C. The polymer was filtered off and the concentration of metal ion in the filtrate was determined using Dionex IC equipped with high capacity columns. The following equations were used to evaluate such parameters. Kd =
qe Ce
(6)
where Kd is sorption distribution coefficient, qe (mmol g−1 ) is the amount of sorption per unit mass of resin at equilibrium, and Ce
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(mmol l−1 ) is the equilibrium concentration. The Kd values are used in following equation to determine the Gibbs free energy of sorption process at different temperatures. Go = −RT ln Kd
(7) (kJ mol−1 ),
Go
where is the free energy of sorption T is the temperature in Kelvin and R is the universal gas constant (8.314 J mol−1 K−1 ). The sorption distribution coefficient may be expressed in terms of enthalpy change (Ho ) and entropy change (So ) as a function of temperature: ln Kd = −
Go =− RT
H o So RT
+
R
(8)
where Kd is the distribution coefficient, So , Ho and Go are the changes of entropy, enthalpy and the Gibbs free energy and T is the temperature (K). 3. Results and discussion 3.1. Polymer characterization The synthetic pathway of the solid scale inhibitor is shown in Scheme 1. The structure of the synthesized anti-scaling crosslinked polymer was confirmed by IR analysis. Fig. 1 shows the FT-IR spectral of the phosphorylated CPAAm polymer. In Fig. 1(a), bands appeared at 3429 and 1650 cm−1 are related to the N H stretching vibration in NH-group of N,N-methylene bis(acrylamide) or CONH2 groups of acrylamide. The C H stretching band is characterized by the peak at 2930 cm−1 due to symmetric or asymmetric stretching vibration of the CH2 groups of acrylamide or N,N methylene bis(acrylamide). After the transamidation reaction (Fig. 1(b)), the aminofunctionalized polymer has a new peak at 3600 cm−1 , related to primary amino group. Appearance of such peak proves the perfect anchoring of the amino group in to the CPAAm moiety. A new set of bands are observed in Fig. 1(c). The absorption bands at 1231, 1071, and 903 cm−1 for (P O), (P OH), and (P O) groups, respectively [21]. The very broad band extending from 3600 cm−1 to as low as 2500 cm−1 is likely to be due to the absorption characteristic of the phosphonic acid hydroxyl group (OH). In addition, The N H stretching band at 3600 cm−1 in phosphorylated CPAAm markedly decreased, reflecting its conversion by the Mannich reaction. Thereby, FTIR results confirm the successful replacement of the amine groups by the phosphonic acid groups.
Fig. 1. IR charts for (a) CPAAM, (b) amino-grafted CPAAM and (c) phosphorylated polymer.
workers for metal ions removal on different adsorbents [23,24]. The maximal sorption removal of Calcium, Magnesium, Barium and Strontium on polymer adsorbent was 76.2%, 66%, 69.7% and % 70.7 at pH 6, which is considered as the optimum pH for sorption. Subsequent reduction in sorption capacities was observed at pH higher than 6. This is probably due to the partial hydrolysis of metal ions. Furthermore, the low solubility of hydrolyzed metals species would have resulted into precipitation of metals at pH above 6, thereby reducing sorption capacities of the polymers. 3.2.2. Effect of contact time After finding out the optimum pH, determining the time required to reach equilibrium is the target. Fig. 3 shows the effect
3.2. Sorption studies 3.2.1. Effect of pH Solution’s pH has a significant impact on metal ions sorption, since it influences both the adsorbent surface chemistry and solution chemistry of soluble metal ions. Changes in pH do affect the polymer surface charge, the degree of ionization of metal ions and, in turn, the amount of adsorbed metal ions [22]. The dependency of sorption capacity of the synthesized polymer on the pH value is related to the functional groups of adsorbent and type of metal ion. Fig. 2 shows the uptake percentage of the investigated metal ions as a function of pH. The results showed the sorption process to be more favorable near neutral conditions and reflected the relatively low acidity of the chelating polymer. At low pH, the functional groups on the polymer surface bear positive charges because its protonation. In turn, the sorption of cationic species becomes unfavorable. As the pH increases, the sorption surfaces become less positive and enough phosphoric acid and amine groups become available to complex metal ions. This would explain the higher uptake observed at the higher pHs. A similar theory was proposed by several earlier
Fig. 2. Effect of pH on sorption.
A.A. Younes et al. / Journal of Hazardous Materials 334 (2017) 1–9
Fig. 3. Effect of contact time on sorption.
of contact time on the sorption capacity of the chelating polymer. It was observed that 180 min is enough to reach an equilibrium state for all metal ions; further increase in time did not bring any further improvement in the sorption capacity. The removal percentages were 79% for Ca(II), 72% for Sr(II), 70% for Ba(II) and 66% for Mg(II). 3.2.3. Effect of polymer dose The data obtained in this section reflected the direct proportionality between the amount of polymer used and the sorption capacity. The maximum removal percentage of Ca jumped from 35 to 99% when the amount of polymer increased from 5 to 50 mg. Similar trend was observed for the other metal ions (Fig. 4). It is apparent that the increase in the percentage removal with increase in the adsorbent dosage is due to the increase in the number of sorption sites. These results were anticipated because increasing adsorbent dose could provide a great of surface area or ion-exchange sites for a fixed initial solute concentration [25].
5
Fig. 5. Effect of initial M(II) concentration on the sorption.
on the availability of active sites on polymer surface. At low adsorbate concentration, the ratio of surface active sites to total metal ion is high. Hence the metal ions could interact with the sorbent to occupy the active sites on the polymer surface sufficient and can be removed from the solution. But with the increase in adsorbate concentration, the number of active adsorption sites is not enough to accommodate metal ions and this agrees with the literatures.
3.3. Thermodynamic evaluation of the process
3.2.4. Effect of initial concentration of metal ion The influence of metal ion initial concentration of the sorption efficiency was explored by shaking different initial concentrations of the metal ions (250, 500, 700 and 1000 mg/L) with constant polymer dose (50 mg/500 mL) at pH 6 for 180 min. Based on the result shown in Fig. 5, it is possible to conclude that percentage sorption of metal ions was found to decrease with increase in initial metal concentration. But the actual amount of metal ion adsorbed per unit mass of polymer increased with increasing the initial concentration (Fig. 6). Basically this phenomenon can be explained based
The free energy of sorption (Go ), the heat of sorption (Ho ) and standard entropy changes (So ) were evaluated using Eqs. (6)–(8). The Gibbs free energy values serve as an indication of the degree of spontaneity of the pertinent chemisorption process. Negative values of Go mean an energetically favorable sorption of the tested metal ions by the prepared polymer. With temperature increasing, Go values become more positive. This reveals the low sorption tendency at high temperatures, e.g. the removal percentage decreases with an increase in temperature (Fig. 7). Ho and So , see Table 1, were obtained from the slope and intercept of a plot of ln Kd against 1/T (Fig. 8). The negative values of Ho give clear evidence about the exothermic nature of the sorption process. Association, fixation or immobilization of alkaline earth metal ions on the polymer surface results in a decrease in the degree of freedom of adsorbate ions which, in turn, gives rise to a negative entropy change (So ).
Fig. 4. Effect of adsorbent dosage on sorption.
Fig. 6. Effect of initial concentration on uptake amount.
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Table 1 Thermodynamic parameters. Go (kJ mol−1 ) at different T (K)
M(II)
Mg Ca Sr Ba
So
298
313
323
333
343
(kJ mol
−3630 −5016 −4536 −4288
−2625 −3906 −3561 −3283
−1955 −3166 −2911 −2613
−1285 −2426 −2261 −1943
−615 −1686 −1611 −1273
−67 −74 −65 −67
Ho −1
−1
K
)
(kJ mol−1 ) −23,596 −27,068 −23,906 −24,254
3.4. Sorption isotherms
3.5. Sorption kinetics
The sorption capacity of a sorbent is an essential parameter that should be considered in the determination of the amount of the sorbent required to remove, quantitatively, a specific amount of a metal ion from a solution. In general, sorption isotherms are utilized to determine the sorption capacity of an adsorbent. The Freundlich and Langmuir sorption isotherms equations have been extensively considered for this purpose [26]. Plots and experimental data obtained by applying these two isotherm models are given in Fig. 9 and Table 2, respectively. The fitting curves and regression coefficient showed the obedience of the sorption behavior to the Langmuir model. The maximum sorption capacity (qmax ) of the prepared polymer toward each metal ion was determined at the optimum pH, and the results, expressed in mg/g, are listed in Table 2.
Two kinetic models, pseudo-first-order and pseudo-secondorder, were investigated to predict the sorption mechanism of the tested metal ions on the prepared polymer. Eqs. (9) and (10) represent the pseudo-first-order and pseudo-second-order, respectively. ln(qe − qt ) = ln(qe ) − k1 t 1 t = + qt k2 q2e
1 qe
t
(9) (10)
where k1 and k2 are the pseudo first- and second-order rate constant (1/min), qe and qt are the amounts of metal ions adsorbed at equilibrium (mg/g) and time t (min), respectively. The value of ln(qe − qt ) can be calculated from the experimental results and plotted against t (min). The slope and the intercept of each linear plot in Fig. 10(a and b) are used to calculate the sorption rate constants and the amount of sorption in equilibrium (qe ). The calculated kinetics parameters for the sorption of divalent metal ions onto the polymer are tabulated in Table 3. The experimental results indicated that sorption behavior is well presented by the pseudo-second order model with high correlation coefficient (R2 ) (Table 3). In recent years, the pseudo-second-order rate expression has been widely applied to the sorption of pollutants from aqueous solutions [27]. 3.6. Polymer regeneration Polymer regeneration and reusability is a vital parameter from the economical point of view. Although the main contributor to the sorption capacity of the prepared polymer is the complexation between the phosphonate groups [ PO(OH)2 ] on the polymer surface and the alkaline earth metal ions, the amide groups [ CO NH ] on the polymer surface as well as the cavities of the polymer network also contribute. Simplified sorption and desorption chemical equation is given below. Regeneration of the polymer was carried out by desorbing the metal ions from polymer surface using 0.1 M HCl at room temperature. After 1 h of desorption, the polymer beads were filtered, washed, with deionized water, and dried. The dried polymer was then used for the sorption of metal ions under the predefined optimum sorption conditions. The decrease in the sorption capacity of the reused polymer was found to be around 1% for all tested metal ions, see Table 4. In order to evaluate the reusability of the polymer, sorption-desorption cycle was repeated two more times by using the same polymer beads. After three times of reuse, the sorption capacity of the solid scale inhibitor decreased by 4.2% for both Mg(II) and Sr(II), by 3.3% for Ba(II) and by 2.5% for Ca(II) (Table 4).
Fig. 7. The influence of temperature on sorption.
2.5
Ln(K d)
2 1.5
Ca
y = 3255.6x - 8.8984 R² = 0.9978
Mg
y = 2838x - 8.1266 R² = 0.9902
Sr
y = 2875.2x - 7.8696 R² = 0.9398
Ba
y = 2917.1x - 8.1118 R² = 0.9431
1 0.5 0 0.0028
0.0029
0.003
0.0031
0.0032
0.0033
1/T (K) Fig. 8. Sorption thermodynamics, Van’t Hoff plot of ln Kd versus 1/T.
0.0034
A.A. Younes et al. / Journal of Hazardous Materials 334 (2017) 1–9
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Fig. 9. Langmuir and Freundlich isotherm models.
Table 2 Sorption isotherm parameters. M(II)
Mg Ca Sr Ba
Langmuir equation model
Freundlich equation model −1
qm (mg/g)
KL (min
666.6 793.6 769 709
0.12 0.116 0.05 0.21
)
2
R
KF
n
R2
0.999 0.999 0.999 0.999
304.9 221.18 275.8 317.3
9.6 6.25 6.06 8.3
0.34 0.77 0.82 0.61
Fig. 10. The sorption kinetics
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Table 3 Sorption kinetic parameters. M(II)
Mg Ca Sr Ba
qe,exp (mg/g)
Pseudo-first-order kinetics model
663 789 725 705
Pseudo-second-order kinetics model
qe (mg/g)
k1 (min−1 )
R2
qe (mg/g)
k2 (mg/(g min)) × 10−5
R2
1275.3 1237 1096 1808
0.0229 0.0258 0.0258 0.0296
0.90 0.94 0.96 0.95
666.6 769.2 724.6 704.2
1.8 3.3 3.1 1.9
0.98 0.99 0.99 0.99
Table 4 Sorption capacities after regeneration. Reuse run
0 1 2 3
Sorption capacity (mg/g) Mg(II)
Ca(II)
Sr(II)
Ba(II)
663 656 647 634
789 784 776 767
725 716 706 692
705 696 688 679
4. Conclusion This article presented a new chelating polymer based on crosslinked polyacrylamide and containing phosphonic acid as functional groups. The polymer synthesis went through three main steps, preparation of the crosslinked polymer, transamination of the crosslinked polymer, and finally functionalization of the polymer. FTIR charts confirmed the success of the three steps. The sorption tendency of the prepared polymer for alkaline metal ions such as Mg, Ca, Ba and Sr, was studied. An optimization protocol for the sorption process was proposed and executed. The protocol involved parameters such as pH, contact time, polymer dose, and the initial concentration of the metal ion. The optimum pH was found to be around seven for all of the tested metal ions. After complete optimization, neutral pH, 150 min contact time, and 50 mg of the polymer were selected as the optimum conditions for the sorption process. Application of these optimum conditions enabled the polymer to have a maximum removal percentage of 667, 794, 769 and 709 (mg/g) for Mg, Ca, Sr and Ba ions, respectively. The sorption process was thermodynamically evaluated. The obtained negative values of Go and Ho indicated that the sorption process is energetically favorable and exothermic in nature. Association, fixation or immobilization of alkaline earth metal ions on the polymer surface results in a decrease in the degree of freedom of adsorbate ions which, in turn, gives rise to a negative entropy change (So ). Sorption isotherm and kinetic studies on the sorption behavior of the prepared polymer revealed that the sorption follows a pseudo-second order mechanism and obeys Langmuir isotherm model. Regeneration experiments proved the efficient reusability of the polymer several times with almost the same sorption efficiency.
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[5] Y. Zhao, L. Jia, K. Liu, P. Gao, H. Ge, L. Fu, Inhibition of calcium sulfate scale by poly (citric acid), Desalination 392 (2016) 1–7. [6] A. Rochdi, O. Kassou, N. Dkhireche, R. Touir, M. El Bakri, M. Ebn Touhami, M. Sfaira, B. Mernari, B. Hammouti, Inhibitive properties of 2,5-bis (n-methylphenly)-1,3,4-oxidiazole and biocide on corrosion, Corros. Sci. 80 (2014) 442–452. [7] K.D. Demadis, E. Mavredaki, A. Stathoulopoulou, E. Neofotistou, C. Mantzaridis, Industrial water systems: problems, challenges and solutions for the process industries, Desalination 213 (2007) 38–46. [8] C. Kavakli, S. Malci, S.A. Tuncel, B. Salih, Selective adsorption and recovery of precious metal ions from geological samples by 1,5,9,13-tetrathiacyclohexadecane-3,11-diol anchored poly(p-CMS-DVB) microbeds, React. Funct. Polym. 66 (2006) 275–285. [9] Z. Guan, J. Lv, P. Bai, X. Guo, Boron removal from aqueous solutions by adsorption, Desalination 383 (2016) 29–37. [10] H. Can, G. Üner, Water-soluble anhydride containing alternating copolymers as scale inhibitors, Desalination 355 (2015) 225–232. [11] J. Kim, S. Kwak, Efficient and selective removal of heavy metals using microporous layered silicate AMH-3 as sorbent, Chem. Eng. J. 313 (2017) 975–982. [12] A.F. El-Kafrawy, S.M. El-Saeed, R.K. Farag, H.A. El-Saied, M.E. Abdel-Raouf, Adsorbents based on natural polymers for removal of some heavy metals from aqueous solution, Egypt. J. Pet. 26 (2017) 23–32. [13] R. Hasanzadeh, P.N. Moghadam, N. Bahri-Laleh, M. Sillanpää, Effective removal of toxic metal ions from aqueous solutions: 2—Bifunctional magnetic nanocomposite base on novel reactive PGMA-MAn copolymer@Fe3 O4 nanoparticles, J. Colloid Interface Sci. 490 (2017) 727–746. [14] M.G. Kiran, K. Pakshirajan, G. Das, Heavy metal removal from multicomponent system by sulfate reducing bacteria: mechanism and cell surface characterization, J. Hazard. Mater. 324 (2017) 62–70. [15] M.A. Sharaf, S.A. Sayed, A.A. Younes, A.B. Farag, Separation and preconcentration of some heavy-metal ions using new chelating polymeric hydrogels, J. Appl. Polym. Sci. 113 (2009) 1335–1344. [16] M.A. Riswan Ahamed, R.S. Azarudeen, R. Subha, A. Burkanudeen, Sorption behavior of ion-exchange terpolymer resin with environmental impact: synthesis, characterization and isotherm models, Polym. Bull. 71 (2014) 3209–3235. [17] E.O. Augustine, Sorption dynamics and isotherm studies of methylene blue uptake on to palm kernel fibre, Chem. Eng. 126 (2007) 35–43. [18] E.R. Kenawy, F.I. Abdel-Hay, A.A. El-Magd, Y. Mahmoud, Synthesis and antimicrobial activity of some polymers derived from modified amino polyacrylamide by reacting it with benzoate esters and benzaldehyde derivatives, J. Appl. Polym. Sci. 99 (2006) 2428–2437. [19] D. Chandra, S. Kumar Das, A. Bhaumik, A fluorophore grafted 2D-hexagonal mesoporous organosilica: excellent ion exchanger for the removal of heavy metal ions from wastewater, Microporous Mesoporous Mater. 128 (2010) 34–40. [20] S. Rengaraj, C.K. Joo, C.K. Kim, J. Yi, Kinetics of removal of chromium from water and electronic process wastewater by ion exchange resins: 1200H, 1500H and IRN97H, J. Hazard. Mater. B 102 (2003) 257–275. [21] K.D. Demadi, S.D. Katarachia, Phosphorus, sulfur silicon, Relat. Elem. 179 (2004) 627–684. [22] M. Arshadi, M.J. Amiri, S. Mousari, Kinetic, equilibrium and thermodynamic investigation of Ni(II), Cd(II), Cu(II) and Co(II) adsorption on barley straw ash, Water Resour. Ind. 6 (2014) 1–17. ˘ Ozcan, Immobilization of 2,20-dipyridyl [23] B. Erdema, A. Ozcan, O. Gök, A.S.O. onto bentonite and its removal behavior of copper(II) ions, J. Hazard. Mater. 163 (2009) 418–426.
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[26] S. Schiewer, Volesky, in: D.R. Lovely (Ed.), Environmental Microbe–Metal Interactions, ASM Press, Washington, DC, 2000 (Chapter 14). [27] Y.S. Ho, Review of second-order models for adsorption systems, J. Hazard. Mater. 136 (2006) 681–689.
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Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Novel polyacrylamide-based solid scale inhibitor Ahmed A. Younes a,∗ , Heba H. El-Maghrabi b , Hager R. Ali c a
Department of Chemistry, Faculty of Science, Helwan University, Cairo, Egypt Petroleum Refining Department, Egyptian Petroleum Research Institute (EPRI), Cairo, Egypt c Analysis and Evaluation Department, Egyptian Petroleum Research Institute (EPRI), Cairo, Egypt b
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Novel polyacrylamide-based solid • • • •
scale inhibitor is synthesized and characterized. Sorption behavior toward divalent earth metal ions was studied. The optimum sorption conditions were determined. Thermodynamic, kinetic and isotherm of the sorption procedure were evaluated. Polymer regeneration for reuse was also explored.
a r t i c l e
i n f o
Article history: Received 16 January 2017 Received in revised form 23 March 2017 Accepted 24 March 2017 Available online 29 March 2017 Keywords: Scale inhibitor Scale inhibition Sorption Metal ion removal Regeneration
a b s t r a c t Novel solid-state phosphorus-containing polymer was synthesized, characterized and investigated as anti-scaling agent for the removal of alkaline earth metals ions from water. An optimization protocol for the sorption process of the metal ions on the polymer surface was proposed and executed. The protocol involved parameters such as pH, contact time, polymer dose, and the initial concentration of the metal ion. The optimum pH was found to be around seven for all of the tested metal ions. The maximum sorption capacities of the prepared polymer were 667, 794, 769 and 709 (mg/g) for Mg, Ca, Sr and Ba ions, respectively. Evaluation of the sorption process from the isotherm, kinetic and thermodynamic points of view was also studied. The experimental evidence revealed that the sorption obeys Langmuir isotherm model and follows a pseudo-second order mechanism. Moreover, the sorption process is exothermic. Possibility of polymer reuse was also investigated. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Scaling is the phenomenon of deposition or simple adhesion of mineral salts on processing equipment. The deposition of these salts, especially on heat exchanger surfaces in cooling, boiler, geothermal, and distillation systems, does cause technical and economic problems such as total or partial obstruction of pipes and pumps, inefficient water treatment chemical usage, increased
∗ Corresponding author. E-mail address: [email protected] (A.A. Younes). http://dx.doi.org/10.1016/j.jhazmat.2017.03.052 0304-3894/© 2017 Elsevier B.V. All rights reserved.
operation costs, lost production due to system downtime and reduction of heat transfer efficiency in cooling systems [1]. Therefore, understanding this phenomenon and finding effective ways to retard or prevent it is a pressing demand. The precipitation of scale forming salts on processing equipment surfaces is markedly retarded by either water acidification or the use of chemical additives (polymeric and non-polymeric). The acidification method has the disadvantage of exposing metal surfaces to a corrosive environment. The use of chemical additives such as scale inhibitors is the most popular and operative method of scale inhibition [2–6]. Scale inhibitors cut out crystal growth or slow the nucleation process, which in turn spoils the scale formation process.
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Most inhibitors for mineral scales are phosphorous compounds, e.g. inorganic polyphosphates, organic phosphate esters, organic phosphonates, and organic aminophosphates [6]. The scale inhibition process in industrial cooling water is of great economic significance because of the high costs of treating undesirable results of scale formation, high cost of water in certain arid areas and losses because of shut-down of operations [7]. In the oil and gas field, polymeric scale inhibitors are widely used because of their enhanced thermal stability and better environmental compatibility. Synthesis of functionalized polymers that pre-concentrate and remove metal ions can be done by two ways. The first way involves the polymerization of a monomer with a specific active ligand to target metal ions, while the second modifies the polymeric matrix, after the polymerization, by an active and specific ligand to one or more different metal ions [8]. In order to extract, determine, pre-concentrate, and remove some metal ions, a variety of new types of functional resins has been developed and intensively studied [9–15]. Although how to make polymeric scale inhibitors solid is still challenging, crosslinked-polymers could provide an effective approach to produce such solid scale inhibitors. The approach of using solid scale inhibitors is much better than conventional chemical treatment methods, such as a squeeze, continuous injection, or batch treatment with liquid chemicals. This also is more effective than pumping liquid inhibitors with a stimulation treatment. The slow dissolution of the solid scale inhibitor allows longer scale protection in both the wellbore area and tubing, and will not contribute to chemical runoff if spilled. Moreover, Reduce downtime caused by scale buildup in the near well-bore. In this context, chelating polymers have received a great priority because of their outstanding simplicity, high efficiency and low cost during the ion exchange, physical adsorption, and chelation [16]. In this article, a crosslinked polyacrylamide-based polymer bearing phosphonate groups was synthesized, characterized and investigated, by batch-equilibrium procedure, as a scale inhibitor for barium, calcium, magnesium and strontium ions. 2. Experimental 2.1. Materials and instruments All chemicals used were of analytical reagent grade unless otherwise stated; and bidistilled water was used throughout. N,N methylene bis-acrylamide (NMBA) and acrylamide (AAm) monomers were obtained from Merck Chemical Co. Nitrate or chloride salts were used to prepare the stock solutions of the investigated metal ions. Working standard metal ion solutions were prepared by appreciate dilution. Jenway pH meter (model 3310) was used in the pH measurements. The infrared spectra were recorded in the wave number range 400–4000 cm−1 on a PerkinElmer IR spectrophotometer-3100. Metal ion concentration was determined according to ASTM D 4327 using Dionex IC model DX 600 equipped with high capacity columns. 2.2. Synthesis of the chelating resin 2.2.1. Synthesis of crosslinked polyacrylamide (CPAAm) Crosslinked polyacrylamide (CPAAm) was synthesized by addition copolymerization. A 50 mg of potassium persulfate as initiator and monomer acrylamide with 5% mole ratio of N,N methylene bisacrylamide were dissolved in 100 mL of bidistilled water/ethanol (70:30, v/v) mixture at 50 ◦ C until the polymer is formed. Additional amount of bidistilled water (50 mL) was added and the temperature was raised to 80 ◦ C with stirring. A solid precipitate is formed as the reaction proceeded which was finally filtrated, washed with water then methanol and allowed to dry at 80 ◦ C [17].
2.2.2. Incorporation of amino groups on CPAAm The attachment of amino groups onto the crosslinked polymer was achieved by a reaction between the CPAAm and ethylenediamine (EDA). A 10 g of the crosslinked polyacrylamide was added to 100 mL of ethylenediamine. The mixture was heated at 110 ◦ C with stirring. After 9 h, the mixture was poured onto cold water. The obtained polymer was filtered and washed with sodium chloride solution until filtrate be free from the ethylenediamine. The presence of the ethylenediamine can be detected using ninhydrin reagent as it gives deep blue color with it. The polymer was washed with water then methanol and dried at 70 ◦ C [18]. 2.2.3. Phosphorylation of CPAAm polymer The incorporation of phosphonic acid groups into the aminografted CPAAm was easily achieved using a straightforward Mannich type reaction of phosphorous acid with formaldehyde and primary amine compounds at low pH in the presence of about 2–3 mol of concentrated hydrochloric acid per mole of an amine. The polymer incorporated with the amine group (0.2 mol amine groups), crystalline phosphorous acid (0.4 mol), and concentrated hydrochloric acid (0.6 mol) were dissolved in a total volume of 200 mL with water, and the mixture was heated to reflux in a threenecked flask fitted with thermometer, condenser, and dropping funnel. Over the course of about 1 h, 60 mL of a 40% (w/v) aqueous formaldehyde solution was added drop by drop, and the reaction was kept at the reflux temperature for an additional hour. The water was evaporated using a rotary evaporator, and the concentrate was neutralized with concentrated ammonia solution. Finally, the concentrated solution was precipitated in methanol to get a white powdered product. 2.3. Sorption studies Sorption, power and capacity, depends not only on the adsorbents’ physical and chemical properties, such as type and structure of the functional group introduced into the polymer matrix, but also on the sorption conditions including the solution’s pH, contact time, metal ion concentration, and presence of interfering ions [19]. The batch equilibration technique was utilized to investigate the sorption tendency of the synthesized chelating polymer and to determine the optimum sorption conditions, such as the pH, contact time, temperature, initial concentration of metal ion, and dose of the polymer. Judicious constant weight of dried polymer beads was used in all batch experiments. All experiments were performed at room temperature by using a mixture of 10 mg beads and metal ion solution (50 mL, 500 mg/L) in 50 mL flasks which were stirred in a temperature controlled shaker at a constant speed of 200 rpm. The sorption ability for Mg(II), Ca(II), Sr(II) and Ba(II), under noncompetitive conditions, was determined as a function of the pH. The suspensions were brought to the desired pH (2, 7, 10) by adding HCl (0.1 M) and NaOH (0.1 M). The contents of the flasks were equilibrated on the shaker at 30 ◦ C for 4 h. After equilibration, the residual concentration of metal ion was determined by Dionex IC equipped with high capacity columns. The sorption capacity (Q, in mg/g polymer) was calculated on the basis of Eq. (1): Q =
(C0 − CA )V W
(1)
where C0 and CA are the concentration (mg/L) of metal ion in the initial solution and in the aqueous phase after sorption, respectively, V is the volume of the aqueous phase (L) and W is the weight of the adsorbent (0.01 g). The sorption efficiency for ions from the solution (R [%]) was calculated using Eq. (2): R=
C − C 0 A C0
× 100
(2)
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3
Scheme 1. Synthetic pathway of the polymer (a) polymerization reaction, (b) transamination reaction and (c) phosphorylation of the polymer.
Influence of contact time (30–240 min), temperature (30–70 ◦ C), amount of polymer (5–50 mg) and concentration of metal ion (250–1000 mg/L) on sorption behavior of the chelating polymer were performed at the optimum pH of the corresponding metal ion, as described under noncompetitive conditions. 2.4. Sorption isotherms Equilibrium isotherms are explored to determine the capacity of the prepared polymer toward the tested metal ions. The most common types of models describing this type of system are the Langmuir and Freundlich models. The Langmuir sorption model assumes that the maximum sorption capacity is due to a saturated monolayer of solute molecules on the adsorbent surface. The Langmuir equation is described with the following linearized form [20]
C e
qe
=
1 KL qmax
C e +
qmax
(3)
where qe is the amount of sorption per unit mass of the polymer at equilibrium (mg/g), Ce is the equilibrium concentration (mg/L), qmax is the maximum theoretical saturation sorption capacity, and KL is the energy of sorption. The Freundlich sorption isotherm describes the relationship between the quantity of metal adsorbed per unit mass of the adsorbent (qe ) and the concentration of the metal ions in solution at equilibrium (Ce ). The mathematical equation is [26] 1/n
qe = KF Ce
(4)
where KF and n are Freundlich constants. The equation can be linearized if we take logarithms to find the values of KF and n: log qe = log KF +
1 n
log Ce
(5)
The total sorption capacities of the prepared polymer was determined via the shaking of 50-mL solutions with different concentrations of the metal ions with 10 mg of the polymer for 4 h at the optimum sorption pH at room temperature in a thermostatic water bath shaker to ensure complete equilibration. The polymer was filtered off, and the concentration of metal ions in the filtrate was determined using Dionex IC equipped with high capacity columns. 2.5. Thermodynamic studies Thermodynamic studies on the sorption of the metal ions of interest and the chelating polymer were performed in order to determine parameters such as free energy of sorption (Go ), the heat of sorption and standard entropy changes (Ho and So ). 50 mL of metal ion solution (500 mg/L) was stirred with 10 mg of the polymer in a mechanical shaker at different temperatures, from 25 to 70 ◦ C. The polymer was filtered off and the concentration of metal ion in the filtrate was determined using Dionex IC equipped with high capacity columns. The following equations were used to evaluate such parameters. Kd =
qe Ce
(6)
where Kd is sorption distribution coefficient, qe (mmol g−1 ) is the amount of sorption per unit mass of resin at equilibrium, and Ce
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(mmol l−1 ) is the equilibrium concentration. The Kd values are used in following equation to determine the Gibbs free energy of sorption process at different temperatures. Go = −RT ln Kd
(7) (kJ mol−1 ),
Go
where is the free energy of sorption T is the temperature in Kelvin and R is the universal gas constant (8.314 J mol−1 K−1 ). The sorption distribution coefficient may be expressed in terms of enthalpy change (Ho ) and entropy change (So ) as a function of temperature: ln Kd = −
Go =− RT
H o So RT
+
R
(8)
where Kd is the distribution coefficient, So , Ho and Go are the changes of entropy, enthalpy and the Gibbs free energy and T is the temperature (K). 3. Results and discussion 3.1. Polymer characterization The synthetic pathway of the solid scale inhibitor is shown in Scheme 1. The structure of the synthesized anti-scaling crosslinked polymer was confirmed by IR analysis. Fig. 1 shows the FT-IR spectral of the phosphorylated CPAAm polymer. In Fig. 1(a), bands appeared at 3429 and 1650 cm−1 are related to the N H stretching vibration in NH-group of N,N-methylene bis(acrylamide) or CONH2 groups of acrylamide. The C H stretching band is characterized by the peak at 2930 cm−1 due to symmetric or asymmetric stretching vibration of the CH2 groups of acrylamide or N,N methylene bis(acrylamide). After the transamidation reaction (Fig. 1(b)), the aminofunctionalized polymer has a new peak at 3600 cm−1 , related to primary amino group. Appearance of such peak proves the perfect anchoring of the amino group in to the CPAAm moiety. A new set of bands are observed in Fig. 1(c). The absorption bands at 1231, 1071, and 903 cm−1 for (P O), (P OH), and (P O) groups, respectively [21]. The very broad band extending from 3600 cm−1 to as low as 2500 cm−1 is likely to be due to the absorption characteristic of the phosphonic acid hydroxyl group (OH). In addition, The N H stretching band at 3600 cm−1 in phosphorylated CPAAm markedly decreased, reflecting its conversion by the Mannich reaction. Thereby, FTIR results confirm the successful replacement of the amine groups by the phosphonic acid groups.
Fig. 1. IR charts for (a) CPAAM, (b) amino-grafted CPAAM and (c) phosphorylated polymer.
workers for metal ions removal on different adsorbents [23,24]. The maximal sorption removal of Calcium, Magnesium, Barium and Strontium on polymer adsorbent was 76.2%, 66%, 69.7% and % 70.7 at pH 6, which is considered as the optimum pH for sorption. Subsequent reduction in sorption capacities was observed at pH higher than 6. This is probably due to the partial hydrolysis of metal ions. Furthermore, the low solubility of hydrolyzed metals species would have resulted into precipitation of metals at pH above 6, thereby reducing sorption capacities of the polymers. 3.2.2. Effect of contact time After finding out the optimum pH, determining the time required to reach equilibrium is the target. Fig. 3 shows the effect
3.2. Sorption studies 3.2.1. Effect of pH Solution’s pH has a significant impact on metal ions sorption, since it influences both the adsorbent surface chemistry and solution chemistry of soluble metal ions. Changes in pH do affect the polymer surface charge, the degree of ionization of metal ions and, in turn, the amount of adsorbed metal ions [22]. The dependency of sorption capacity of the synthesized polymer on the pH value is related to the functional groups of adsorbent and type of metal ion. Fig. 2 shows the uptake percentage of the investigated metal ions as a function of pH. The results showed the sorption process to be more favorable near neutral conditions and reflected the relatively low acidity of the chelating polymer. At low pH, the functional groups on the polymer surface bear positive charges because its protonation. In turn, the sorption of cationic species becomes unfavorable. As the pH increases, the sorption surfaces become less positive and enough phosphoric acid and amine groups become available to complex metal ions. This would explain the higher uptake observed at the higher pHs. A similar theory was proposed by several earlier
Fig. 2. Effect of pH on sorption.
A.A. Younes et al. / Journal of Hazardous Materials 334 (2017) 1–9
Fig. 3. Effect of contact time on sorption.
of contact time on the sorption capacity of the chelating polymer. It was observed that 180 min is enough to reach an equilibrium state for all metal ions; further increase in time did not bring any further improvement in the sorption capacity. The removal percentages were 79% for Ca(II), 72% for Sr(II), 70% for Ba(II) and 66% for Mg(II). 3.2.3. Effect of polymer dose The data obtained in this section reflected the direct proportionality between the amount of polymer used and the sorption capacity. The maximum removal percentage of Ca jumped from 35 to 99% when the amount of polymer increased from 5 to 50 mg. Similar trend was observed for the other metal ions (Fig. 4). It is apparent that the increase in the percentage removal with increase in the adsorbent dosage is due to the increase in the number of sorption sites. These results were anticipated because increasing adsorbent dose could provide a great of surface area or ion-exchange sites for a fixed initial solute concentration [25].
5
Fig. 5. Effect of initial M(II) concentration on the sorption.
on the availability of active sites on polymer surface. At low adsorbate concentration, the ratio of surface active sites to total metal ion is high. Hence the metal ions could interact with the sorbent to occupy the active sites on the polymer surface sufficient and can be removed from the solution. But with the increase in adsorbate concentration, the number of active adsorption sites is not enough to accommodate metal ions and this agrees with the literatures.
3.3. Thermodynamic evaluation of the process
3.2.4. Effect of initial concentration of metal ion The influence of metal ion initial concentration of the sorption efficiency was explored by shaking different initial concentrations of the metal ions (250, 500, 700 and 1000 mg/L) with constant polymer dose (50 mg/500 mL) at pH 6 for 180 min. Based on the result shown in Fig. 5, it is possible to conclude that percentage sorption of metal ions was found to decrease with increase in initial metal concentration. But the actual amount of metal ion adsorbed per unit mass of polymer increased with increasing the initial concentration (Fig. 6). Basically this phenomenon can be explained based
The free energy of sorption (Go ), the heat of sorption (Ho ) and standard entropy changes (So ) were evaluated using Eqs. (6)–(8). The Gibbs free energy values serve as an indication of the degree of spontaneity of the pertinent chemisorption process. Negative values of Go mean an energetically favorable sorption of the tested metal ions by the prepared polymer. With temperature increasing, Go values become more positive. This reveals the low sorption tendency at high temperatures, e.g. the removal percentage decreases with an increase in temperature (Fig. 7). Ho and So , see Table 1, were obtained from the slope and intercept of a plot of ln Kd against 1/T (Fig. 8). The negative values of Ho give clear evidence about the exothermic nature of the sorption process. Association, fixation or immobilization of alkaline earth metal ions on the polymer surface results in a decrease in the degree of freedom of adsorbate ions which, in turn, gives rise to a negative entropy change (So ).
Fig. 4. Effect of adsorbent dosage on sorption.
Fig. 6. Effect of initial concentration on uptake amount.
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Table 1 Thermodynamic parameters. Go (kJ mol−1 ) at different T (K)
M(II)
Mg Ca Sr Ba
So
298
313
323
333
343
(kJ mol
−3630 −5016 −4536 −4288
−2625 −3906 −3561 −3283
−1955 −3166 −2911 −2613
−1285 −2426 −2261 −1943
−615 −1686 −1611 −1273
−67 −74 −65 −67
Ho −1
−1
K
)
(kJ mol−1 ) −23,596 −27,068 −23,906 −24,254
3.4. Sorption isotherms
3.5. Sorption kinetics
The sorption capacity of a sorbent is an essential parameter that should be considered in the determination of the amount of the sorbent required to remove, quantitatively, a specific amount of a metal ion from a solution. In general, sorption isotherms are utilized to determine the sorption capacity of an adsorbent. The Freundlich and Langmuir sorption isotherms equations have been extensively considered for this purpose [26]. Plots and experimental data obtained by applying these two isotherm models are given in Fig. 9 and Table 2, respectively. The fitting curves and regression coefficient showed the obedience of the sorption behavior to the Langmuir model. The maximum sorption capacity (qmax ) of the prepared polymer toward each metal ion was determined at the optimum pH, and the results, expressed in mg/g, are listed in Table 2.
Two kinetic models, pseudo-first-order and pseudo-secondorder, were investigated to predict the sorption mechanism of the tested metal ions on the prepared polymer. Eqs. (9) and (10) represent the pseudo-first-order and pseudo-second-order, respectively. ln(qe − qt ) = ln(qe ) − k1 t 1 t = + qt k2 q2e
1 qe
t
(9) (10)
where k1 and k2 are the pseudo first- and second-order rate constant (1/min), qe and qt are the amounts of metal ions adsorbed at equilibrium (mg/g) and time t (min), respectively. The value of ln(qe − qt ) can be calculated from the experimental results and plotted against t (min). The slope and the intercept of each linear plot in Fig. 10(a and b) are used to calculate the sorption rate constants and the amount of sorption in equilibrium (qe ). The calculated kinetics parameters for the sorption of divalent metal ions onto the polymer are tabulated in Table 3. The experimental results indicated that sorption behavior is well presented by the pseudo-second order model with high correlation coefficient (R2 ) (Table 3). In recent years, the pseudo-second-order rate expression has been widely applied to the sorption of pollutants from aqueous solutions [27]. 3.6. Polymer regeneration Polymer regeneration and reusability is a vital parameter from the economical point of view. Although the main contributor to the sorption capacity of the prepared polymer is the complexation between the phosphonate groups [ PO(OH)2 ] on the polymer surface and the alkaline earth metal ions, the amide groups [ CO NH ] on the polymer surface as well as the cavities of the polymer network also contribute. Simplified sorption and desorption chemical equation is given below. Regeneration of the polymer was carried out by desorbing the metal ions from polymer surface using 0.1 M HCl at room temperature. After 1 h of desorption, the polymer beads were filtered, washed, with deionized water, and dried. The dried polymer was then used for the sorption of metal ions under the predefined optimum sorption conditions. The decrease in the sorption capacity of the reused polymer was found to be around 1% for all tested metal ions, see Table 4. In order to evaluate the reusability of the polymer, sorption-desorption cycle was repeated two more times by using the same polymer beads. After three times of reuse, the sorption capacity of the solid scale inhibitor decreased by 4.2% for both Mg(II) and Sr(II), by 3.3% for Ba(II) and by 2.5% for Ca(II) (Table 4).
Fig. 7. The influence of temperature on sorption.
2.5
Ln(K d)
2 1.5
Ca
y = 3255.6x - 8.8984 R² = 0.9978
Mg
y = 2838x - 8.1266 R² = 0.9902
Sr
y = 2875.2x - 7.8696 R² = 0.9398
Ba
y = 2917.1x - 8.1118 R² = 0.9431
1 0.5 0 0.0028
0.0029
0.003
0.0031
0.0032
0.0033
1/T (K) Fig. 8. Sorption thermodynamics, Van’t Hoff plot of ln Kd versus 1/T.
0.0034
A.A. Younes et al. / Journal of Hazardous Materials 334 (2017) 1–9
7
Fig. 9. Langmuir and Freundlich isotherm models.
Table 2 Sorption isotherm parameters. M(II)
Mg Ca Sr Ba
Langmuir equation model
Freundlich equation model −1
qm (mg/g)
KL (min
666.6 793.6 769 709
0.12 0.116 0.05 0.21
)
2
R
KF
n
R2
0.999 0.999 0.999 0.999
304.9 221.18 275.8 317.3
9.6 6.25 6.06 8.3
0.34 0.77 0.82 0.61
Fig. 10. The sorption kinetics
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A.A. Younes et al. / Journal of Hazardous Materials 334 (2017) 1–9
Table 3 Sorption kinetic parameters. M(II)
Mg Ca Sr Ba
qe,exp (mg/g)
Pseudo-first-order kinetics model
663 789 725 705
Pseudo-second-order kinetics model
qe (mg/g)
k1 (min−1 )
R2
qe (mg/g)
k2 (mg/(g min)) × 10−5
R2
1275.3 1237 1096 1808
0.0229 0.0258 0.0258 0.0296
0.90 0.94 0.96 0.95
666.6 769.2 724.6 704.2
1.8 3.3 3.1 1.9
0.98 0.99 0.99 0.99
Table 4 Sorption capacities after regeneration. Reuse run
0 1 2 3
Sorption capacity (mg/g) Mg(II)
Ca(II)
Sr(II)
Ba(II)
663 656 647 634
789 784 776 767
725 716 706 692
705 696 688 679
4. Conclusion This article presented a new chelating polymer based on crosslinked polyacrylamide and containing phosphonic acid as functional groups. The polymer synthesis went through three main steps, preparation of the crosslinked polymer, transamination of the crosslinked polymer, and finally functionalization of the polymer. FTIR charts confirmed the success of the three steps. The sorption tendency of the prepared polymer for alkaline metal ions such as Mg, Ca, Ba and Sr, was studied. An optimization protocol for the sorption process was proposed and executed. The protocol involved parameters such as pH, contact time, polymer dose, and the initial concentration of the metal ion. The optimum pH was found to be around seven for all of the tested metal ions. After complete optimization, neutral pH, 150 min contact time, and 50 mg of the polymer were selected as the optimum conditions for the sorption process. Application of these optimum conditions enabled the polymer to have a maximum removal percentage of 667, 794, 769 and 709 (mg/g) for Mg, Ca, Sr and Ba ions, respectively. The sorption process was thermodynamically evaluated. The obtained negative values of Go and Ho indicated that the sorption process is energetically favorable and exothermic in nature. Association, fixation or immobilization of alkaline earth metal ions on the polymer surface results in a decrease in the degree of freedom of adsorbate ions which, in turn, gives rise to a negative entropy change (So ). Sorption isotherm and kinetic studies on the sorption behavior of the prepared polymer revealed that the sorption follows a pseudo-second order mechanism and obeys Langmuir isotherm model. Regeneration experiments proved the efficient reusability of the polymer several times with almost the same sorption efficiency.
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