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Kinetics and thermodynamics modeling of Nd(III) removal from aqueous solution using modified Amberlite XAD7
Journal Pre-proof Kinetics and thermodynamics modeling of Nd(III) removal from aqueous solution using modified Amberlite XAD7 Petru Negrea, Andreea Gabor, Corneliu Mircea Davidescu, Mihaela Ciopec, Adina Negrea, Narcis Duteanu PII:
S1002-0721(18)31007-X
DOI:
https://doi.org/10.1016/j.jre.2019.04.023
Reference:
JRE 624
To appear in:
Journal of Rare Earths
Received Date: 7 December 2018 Revised Date:
15 April 2019
Accepted Date: 17 April 2019
Please cite this article as: Negrea P, Gabor A, Davidescu CM, Ciopec M, Negrea A, Duteanu N, Kinetics and thermodynamics modeling of Nd(III) removal from aqueous solution using modified Amberlite XAD7, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.04.023. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
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Kinetics and thermodynamics modeling of Nd(III) removal from aqueous solution using modified Amberlite XAD7
Petru Negrea1, Andreea Gabor1,†, Corneliu Mircea Davidescu1,†, Mihaela Ciopec1,*, Adina Negrea1,†, Narcis Duteanu1,* 1
Politehnica University of Timisoara, Faculty of Industrial Chemistry and Environmental Engineering, 2 Piata Victoriei, RO 300006 Timisoara, Romania † Authors with equal contributions
Corresponding authors: Faculty of Industrial Chemistry and Environmental Engineering, Politehnica University of Timisoara, 2 Piata Victoriei, RO 300006 Timisoara, Romania, E-mail addresses: [email protected]; [email protected]
ABSTRACT. New adsorbent material was obtained by modification of commercially Amberlite XAD7 with thiourea that represents a non-toxic, cheap and environmentally friendly extractant. Prepared adsorbent was used for removal of neodymium ions from aqueous solutions. Thiourea modified Amberlite involved in this study show good adsorption capacities (74.3 mg/g) and excellent efficiency during Nd removal process. In order to elucidate the mechanism of the Nd adsorption process kinetic, thermodynamic and equilibrium studies were performed, establishing this way which kinetic model is better describing the Nd adsorption process. Moreover the thermodynamic studies prove that the Nd adsorption on thiourea modified Amberlite XAD7 is an endothermic and spontaneous process. Keywords: Neodymium; Amberlite XAD7; adsorption mechanism; adsorption isotherm; thiourea 1.
Introduction Extensive development of human society during last decades involves usage of special materials into different applications such as superconductors, optical technology, telecommunications, nuclear and solar energy, photocatalysis, bisosciences, ceramics, alloys, fertilizers and security protection systems.[1-4] Because their unique properties rare earth elements are widely used in all these industrial fields [2], some of their applications are represented by the production of strong permanent magnets, automotive catalytic converters, laser, superconductors and supercapacitors [5]. Increase of computers market leads to increase of sintered magnets which represent an indispensable piece in construction of high performance electronic devices, especially of hard disks used in computers construction.[6] Last decades researches show the possibility to use Nd3+ ions as dopant in some hosts materials used to produce laser sources in near infrared spectral region.[7, 8] Neodymium is a common uranium fission product, which is also important in the medical field
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because in form of Nd3+, it represents a reasonable radiochemical source analog to the trivalent actinides such as: Am3+, Cm3+, Pu3+[2, 9]. Latest years have increased popularity and demand for green technologies, such as hybrid or electric cars, wind turbines, and compact lamps leads at higher demand of rare earth elements. This demand is of course boosting the prices of such elements, for example in case of neodymium the price increase at 94 $ per kg in 2014 from 16 $ per kg in 2001.[10] Increased demand for rare earth elements lead at an increased demand for lanthanides ores which lead at higher increase of public exposure at lanthanides from different sources: from different commercial products and also from production wastes and effluents.[11] During fabrication process and at end of life cycle large quantities of scrap and waste materials are generated, thus the need for a cheap and effective recovery technology. For the moment RRE recovery technologies are not standard, in reality less than 1% of RREs were recovered worldwide during 2013 [1, 2, 10] in comparison with the recycling rate for copper, chromium and gold which is higher than 50% [12]. According to data from EU the amount of electric and electronic equipment waste (EEEW) is around 12 million tons per year, from which only 2.2 million tons were treated in 2014, suggesting that the REEs recycling level is really limited.[13]It is expected that by 2020, the European market of waste electrical and electronic equipment recycling increase by 1.5 times, reaching a total turnover of 1.5 billion.[13] The present paper is dealing with the preparation of new adsorbent materials obtained by modification of commercial polymers with active groups containing S and N atoms, used for neodymium removal from aqueous solutions. This specific functional groups are introduced on the adsorbent surface with the aim to improve the adsorption properties of commercial resin [14]. It is well known from the large number of research papers dealing with removal of REEs that several removal techniques are widely spread, such as chemical precipitation [15, 16], ionic exchange [15, 17], and adsorption onto the different adsorbents.[15,18-22]Investigations carried out into the field of adsorptions during last decades lead at development and intensive usage of new unconventional materials such as activated graphite [23], bio-adsorbents [15, 24],new synthesized materials [25-28], superficial chemically modified or functionalized polymers.[29-33] During the years, many different types of ion exchange resins and chelated polymers where developed and used for removal of metallic ions from different wastewaters. Ion exchange resins present higher removal efficiencies but they have a major disadvantage represented by low mechanical resistance due to the swelling of the polymeric skeleton in the presence of water.[29] Chelated polymers can be used with good results during the removal and recovery or REEs but due to their low hydrophilic properties, low specific surface, the adsorption process needs very long contact time. A possible solution for this problem could be the modification of such polymers by functionalization with different pendant groups that lead at increase of material active surface and thus the material adsorption capacity. Different research papers proved that is possible to use Amberlite resins for removal of rare earth elements, for example Amberlite XAD7 resin was used as adsorbent after the functionalization with di-2-etyl-hexyl-phosphoric acid (DEHPA) [34]. Also, Amberlite XAD 4 resin was used for REEs removal after functionalization with n,n-bis-2hydroxyethyl-glycine[30] or with functional groups containing N atoms.[35]
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Accordingly to Liu et al., [36] thiourea is a really strong chelating agent for most of the metals, and its nitrogen and sulfur groups are responsible for coordination of heavy metal ions, forming in this way chelate compounds. Different solid adsorbents containing thiourea functional groups were used for adsorption of cadmium, silver [14, 37], lead [38], europium and lanthanum.[39] All above considered, present study aims to obtain an adsorbent material with higher adsorption capacity by modification of Amberlite XAD7 polymer with thiourea, followed by material characterization. After that, the modified material was used in the adsorption studies in order to establish the adsorption mechanism of Nd ions by using kinetics, thermodynamic and equilibrium studies. 2. Experimental 2.1. Amberlite XAD7 modification Amberlite XAD7 was modified by mixing 5 g of Amberlite resin (560–710µm, SigmaAldrich) with a solution obtained by dissolution of 0.5 g thiourea (purity 99.0%, from SigmaAldrich) into 25 mL of pure ethanol (Merck). Amberlite resin surface modification was obtained by using the technique of low-pressure concentration, using a Heidolph vacuum rotavapor, at pressure of 2 Pa for a concentration time of 10 minutes at 323 K. After impregnation, the concentration of thiourea into the residual solution was determined. By making the difference between the initial concentration of thiourea and the residual concentration after impregnation, was determined the quantity of thiourea impregnated onto the Amberlite XAD7 resin. No other process than impregnation was observe while contacting the polymer with the extractant solution. Based on that can conclude that each gram of Amberlite XAD7 resin was impregnated with 946 µmoles of thiourea (or each gram of Amberlite XAD7 was impregnated with 0.072 g of thiourea). 2.2. Characterization of chemically modified Amberlite resin Modified Amberlite XAD7 resin was characterized by using two different methods: Energy Dispersive X-ray Analysis (EDX) spectra and Fourier Transform Infrared Spectroscopy (FTIR). EDX spectra were obtained by using Quanta FEG 250 scanning electron microscopy and the FTIR spectra were recorded on Bruker Platinum ATR-QL FTIR spectrometer into the range 4000 – 400 cm–1 in order to distinguish the presence of N and S active groups onto the modified polymer surface. 2.3. Sorption experiments All experiments carried out in order to establish the optimum conditions regarding the pH, contact time and initial concentration of Nd ions for the adsorption onto the thiourea modified Amberlite XAD7 were made using a Julabo SW23 mechanical shaker bath at 200 rpm. All neodymium solutions were prepared from a stock solution with concentration of 1000 mg/L obtained by dissolution of Nd (purity 99%, purchased from Sigma-Aldrich) in HNO3 63% purchased from Merch. In all experiments, the neodymium residual concentration was measured using inductively coupled plasma mass spectrometer – ICPMS Bruker Aurora M90.
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First it was evaluated the influence of solution pH on the maximum adsorption capacity by measuring the neodymium residual concentration obtained after keeping in contact 0.1 g of adsorbent material with 25 mL solution containing 50 mg Nd(III) per L at 298 K. This study was made by modifying the solution pH between 2 and 7. In second stage was studied the influence of contact time and temperature on the maximum adsorption capacity of the modified polymer. In this stage samples of 0.1 g of adsorbent material were kept in contact with 25 mL solutions containing 50 mg Nd(III) per L for different times (15, 30, 45 and 60 min) and at different temperatures (298, 308 and 318 K) into a thermostatic bath at 200 rpm. After that, all the samples were filtered and in the filtered solution was determined the residual concentration of Nd ions. The influence of initial concentration over the maximum adsorption capacity was evaluated by variation of neodymium initial concentration in the solutions used during adsorption experiment from 10 to 400 mg/L (10, 50, 100, 150, 200, 300, 350 and 400 mg/L) obtained from previously prepared stock solution. During this stage, all the experiments were carried out at optimum pH, time and temperature obtained in the previous stages. 2.4. Adsorption isotherm modeling, kinetics and thermodynamics Evaluation of a solid - liquid adsorption system is done by performing equilibrium tests and dynamic adsorption studies [40]. During adsorption of metallic cations, their adsorption in formation of physical or physicochemical interaction between adsorbent and metallic ions until the equilibrium is reached. Equilibrium adsorption capacity can be evaluated by using equation: ( C − Ce )V (1) qe = 0 m Where, qe – equilibrium adsorption capacity (mg/g), C0 – initial concentration of the metallic ions in solution (mg/L), Ce – concentration of metallic ions in solution when the equilibrium is reached (mg/L), V – volume of the solution used in adsorption experiment (L), m – mass of adsorbent material used in adsorption experiment (g). In order to elucidate the adsorption mechanism of neodymium on thiourea modified Amberlite XAD7 was obtained the experimental adsorption isotherm. This experimental data were fit using three non-linear isotherm models: Langmuir, Freundlich and Sips. Langmuir isotherm consider that the adsorption is taking place in a monolayer on the surface of the adsorbent material, the Freundlich isotherm was developed in order to explain the adsorption onto the heterogeneous surfaces and Sips model represent a combination between Langmuir and Freundlich adsorption isotherms.[41, 42] Langmuir model takes into account three hypotheses: (i) adsorption is taking place into a monolayer, (ii) all the surface gaps are identical and can accommodate a single metallic ion, and (iii) the ability of a molecule to be adsorbed on the surface is independent on the occupancy of the neighboring sites.[15] Non-linear form of Langmuir isotherm is described by equation:[43] q K C (2) qe = L L e 1 + K L Ce
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where: qe – equilibrium adsorption capacity (mg/g), Ce – concentration of metallic ions in solution when the equilibrium is reached (mg/L), qL – Langmuir maximum adsorption capacity (mg/g), and KL – Langmuir constant (L/mg). Starting from Langmuir non-linear isotherm characteristics a dimensionless constant RL named separation factor or equilibrium parameter can be calculated. This dimensionless parameter can be evaluated by using Eq.(3): 1 RL = (3) 1 + K L Co Where, RL – separation factor, KL – Langmuir constant (L/mg), and C0 – initial concentration of metallic ions (initial concentration of Nd ions, mg/L). Freundlich isotherm represents an empirical isotherm describing the adsorption process by using the equation:[44]
qe = K F Ce1 nf
(4)
Where, qe – equilibrium adsorption capacity (mg/g), Ce – metallic ion concentration when the equilibrium is reached (mg/L), KF and nF – represent characteristic constants which can be associated with adsorbent relative adsorption capacity and with adsorption intensity. Sips model represent a combination of Langmuir and Freundlich isotherms and is described by relation:[45] qs K S Ce1 nS (5) qe = 1 + K S Ce1 nS Where, qs – maximum adsorption capacity (mg/g), Ks – constant linked with adsorbent adsorption capacity, and ns – heterogeneity factor. Starting from Sips isotherm was calculated the separation factor that represents a dimensionless parameter, defined by relation: 1 RS = (6) 1 + KS C01 nS Where, Rs – separation factor (which is a constant linked to the adsorption capacity), ns – heterogeneity factor, and C0 – metallic ions initial concentration. Essential characteristics of Sips isotherm are described by the value of separation factor. So, if the separation factor is > 1 the adsorption process is not a favorable one, when separation factor is one the adsorption isotherm have a linear form, the adsorption is favorable when the separation factor have a value between zero and one, and adsorption process is an irreversible one when the separation factor is zero. Based on experimental data obtained for Nd adsorption on thiourea modified Amberlite was obtained a separation factor between 0 and 1, which means that the neodymium adsorption on studied adsorbent is favorable. Kinetic models are used in order to identify the adsorption mechanism for the studied system and in order to identify the limiting stage, including in that the mass transport and chemical reactions [46]. Most used kinetics models are pseudo-first order kinetic model (Lagergren model) [47] and pseudo-second-order model (Ho and McKay model).[48, 49] Pseudo-first-order model is described by equation:
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ln ( qe − qt ) = lnqe − k1 t
(7)
where: qe – equilibrium adsorption capacity (mg/g), qt – adsorption capacity at time t (mg/g), k1 – pseudo-first-order rate constant (min–1), t – contact time (min). The pseudo-second-order model (Ho and McKay) is described by equation: t 1 t = + (8) 2 qt k2 qe qe Where, qe- adsorption capacity at equilibrium (mg/g), qt – adsorption capacity at time t (mg/g), k2 – pseudo-second-order rate constant (g/(mg min)), t – contact time (min). Pseudo-first-order rate constant and also equilibrium adsorption capacity can be evaluated from the linear dependence of ln(qe–qt) versus t. Similarly it is possible to evaluate the pseudosecond-order rate constant and equilibrium adsorption capacity form linear dependence of t/qt versus t. Based on the kinetics parameters obtained for each used kinetic model can decide which model is better describing the neodymium ions adsorption onto the thiourea modified Amberlite XAD polymer. Based on rate constant evaluated from pseudo-second-order model, which is a specific constant for the Nd adsorption process, is possible to obtain the value of activation energy using Arrhenius equation: E (9) ln k 2 = lnA − a RT where: k2 – rate constant obtained from pseudo-second-order model (g/(mg min)), A – Arrhenius constant (g min/mg), Ea – activation energy (kJ/mol), T – absolute temperature (K), and R – ideal gas constant (8.314 J/(mol·k)). Is possible to evaluate the activation energy for the Nd(III) adsorption process from the graphical representation of ln k2 versus 1/T. In order to elucidate the Nd(III) adsorption mechanism on the modified Amberlite surface is essential to calculate the value of free Gibbs energy, based on Gibbs-Helmholtz equation:[50] ∆G o = ∆H o − T ⋅ ∆S o (10) Standard entropy variation and standard enthalpy variation are evaluated from the linear dependence of ln Kd versus 1/T:
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∆S o ∆H o − (11) R RT where: Kd – equilibrium constant, ∆G0 – standard free Gibbs energy variation (kJ/mol), ∆H0 – standard enthalpy variation (kJ/mol), ∆S0 – standard entropy variation (J/(mol K)), T – absolute temperature (K), R – ideal gas constant (J/(mol·K)). Equilibrium constant represents the ratio between equilibrium adsorption capacity and equilibrium concentration.
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2.5. Desorption and repeated use experiments In order to establish the maximum number of adsorption/desorption cycles the neodymium ions were adsorbed and desorbed until desorption was not possible any more. Nd(III) ions adsorbed onto the modified Amberlite surface were desorbed by mixing the polymer containing adsorbed ions with 250 mL HCl 15%. Reaction mixture was shaken for 6 h at 300 r/min at room temperature.
224
ln Kd =
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After that, the filtered adsorbent material was rinsed with distilled water and dried at room temperature. This step was repeated until the Nd ions were irreversibly fixed onto the adsorbent material surface, this way establishing the maximum number of usage cycles. Adsorption/desorption process efficiency was establish by counting the adsorbed/desorbed of Nd ions quantity.
3. Results and discussions 3.1. Characterization of thiourea modified Amberlite XAD 7 polymeric support Fig.1 depicts the EDX spectra recorded for thiourea modified Amberlite XAD7. From data depicted in figure 1 can observe the presence of the peaks associated with presence of C and O atoms from polymeric chain and the peaks of nitrogen and sulfur atoms specific for functional groups used for Amberlite XAD7 modification. Presence of these two specific peaks prove that the Amberlite XAD7 commercial polymer was modified with thiourea. Figure 1. EDX spectra of thiourea modified Amberlite XAD7 polymer After that, the modified polymer was characterized by using FTIR spectroscopy, spectra depicted into the figure 2. Figure 2. FTIR spectra of thiourea modified Amberlite XAD7 From the FTIR spectra recorded for modified Amberlite XAD7 can observe the presence of O-H group specific bands located between 3640 and 3620 cm–1 and at 1700 cm–1 the peak associated with the presence of O–H group from water. Also in the wavelength interval located between 1720 – 1715 cm–1, and 1300 – 1100 cm–1 can observe the presence of bands associated with the vibration of C=O bond. The bands located at 1470 and 2925 cm–1 are specific for the vibrations of C–H bonds. Bands located between 1560 – 1650 cm–1 and 3400 – 3500 cm–1 are associated with the vibrations of alkyl – NH2 bonds. Bands located between 650 – 700 cm–1 and at 950 cm–1 are associated with the vibrations characteristic for group NH–C=S. Based on presented data can conclude that analyzed material is represented by thiourea modified Amberlite XAD7.[39, 51]
3.2. Nd (III) ions adsorption studies 3.2.1. pH effect on the Nd(III) adsorption A major role during the metal ions adsorption is played by the solution pH value that can influence the form of the metallic ions and also the adsorbent proprieties. Into the specific case of Nd(III) ions adsorption pH value represent an important factor which can control the performance of the adsorption process on the thiourea modified Amberlite XAD7 polymer. pH value affects the behaviour of the adsorbent material and also the chemical behaviour of the Nd(III) ions into the aqueous medium.[15, 27, 52, 53]Adsorption of Nd(III) positive ions occurs on the negatively charged groups found on the adsorbent surface. At low pH values is expected that the higher concentration of H+ ions leads at some competitive adsorption; a large number of active adsorption sites are
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blocked due to the protonation. It’s expected that by increasing the pH value the protonation reaction speed is decreased due to the decrease of the number of protons into the solution, so is increasing the number of active groups available for neodymium ion adsorption. Further increase of solution pH lead at the competitively adsorption of OH– groups on the active adsorption sites, so the Nd adsorption becomes an electrostatic one.[27, 54] In order to study the effect of pH value on the neodymium ions adsorption onto the thiourea modified Amberlite XAD7 a series of experiments were carried out using aqueous solution with different pH. Obtained experimental data are depicted in Fig. 3. Fig. 3 pH effect on the Nd(III) adsorption onto the thiourea modified Amberlie XAD7 Based on experimental data presented in Fig. 3 can observe that maximum adsorption capacity increase when the solution pH is increasing. Maximum adsorption capacity (11.75 mg Nd(III) per g of adsorbent) is reached when the pH solution is 6, further increase of pH values lead at no increase of the maximum adsorption capacity, results which are in concordance with the data presented by other research papers [27, 29]. When the pH value is higher than 6 the neodymium ions can precipitate as Nd(OH)3 [54], so we evaluate the value of pH from which the precipitation process begins. Neodymium hydroxide presents a low solubility in water (4.8×10–6 moles of Nd(OH)3per L of water [55, 56]), and its dissociation equilibrium is presented in chemical equation (12):[56, 57] Nd(OH)3(s) Nd3+(aq) + 3 OH−(aq) (12) In order to calculate the pH at which the precipitation is started must take into account that the Nd(III) complexation moves the equation balance to the right this way increasing the neodymium hydroxide solubility. Based on Eq.(12) can evaluate the Nd(OH)3 solubility product using equation: Ks = [Nd3+][HO–]3, where [Nd3+] and [HO–] represent the equilibrium concentrations of neodymium and hydroxyl ions. From this equation is possible to evaluate equilibrium concentration of HO- ions taking into account that the Ks is 3.10×10–22 mol/L [55, 56]. During experimental study we used solutions with a maximum concentration of 50 mg Nd(III) per L, which is equal with a concentration of Nd(III) ions of 3.46×10–4 mol/L. For this particular case, was calculated the pH at which is beginning the Nd(OH)3 precipitation, having a value of 7.99, means that during the experimental study pH should not exceed this value.
3.2.2. Effect of contact time and temperature onto the Nd(III) adsorption on thiourea modified Amberlite XAD7 Contact time and temperature influence were studied as described earlier, and from obtained data (figure not show) was observed that the concentration profile of Nd ions during the process shown a typical profile for an adsorption process. Adsorption rate is fast in the first 25 min (as long as active centers are fully available), then the process is evolving slower rate. The equilibrium state being attained after 30 min, when the maximum adsorption capacity was observed. In meanwhile can observe that the increase of temperature have only a relatively small influence onto the adsorption capacity. For example when the working temperature increase from 298 at 318 K the
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maximum adsorption capacity increases from 12.41 at 12.45 mg/g. Based on these experimental data we choose for all further studies the contact time of 30 min. This behavior can be explained when taking in account that at the beginning of adsorption process the active sites of produced adsorbent are available, so the neodymium ions can reach it easily. When the contact time increases the number of free adsorption sites decreases rapidly so the remaining active sites presented onto the surface of adsorbent materials cannot be easily occupied by Nd (III) ions.
3.2.3. Adsorption mechanism for Nd(III) uptake onto thiourea modified Amberlite XAD7 adsorption kinetics Adsorption kinetics was studied in order to better describe the adsorption process rate and efficiency [27].In order to evaluate the kinetic mechanism responsible for Nd ions adsorption on thiourea modified Amberlite XAD7 the experimental data were modeled using pseudo-first-order and pseudo-second-order kinetics models. Plots of the two used models are depicted in figure 4 and the calculated parameters of the Nd(III) adsorption kinetics are presented in table 1.
Figure4. Plots of pseudo-first-order (a) and pseudo-second-order (b) models for the Nd(III) ions adsorption on thiourea modified Amberlite XAD7 Values of constant k1 and qe used in description of pseudo-first-order model are obtained from the slope and from the intercept of linear plot ln(qe–qt) versus time. Similar for the pseudosecond-order model the value of constant k was evaluated from the slope of linear representation of t/qt versus t. From data presented in Table 1 can observe that the correlation coefficient R2 obtained when the pseudo-second-order was used have a higher value compared with that obtained when the pseudo-first-order model is used. At used temperatures, the correlation coefficient obtained when the pseudo-second-order model was used shows values located between 0.9938 and 0.9959, compared with the values obtained from pseudo-first-order model that are located between 0.6231 and 0.6655. These values indicate that the pseudo-second-order model describes better the Nd(III) ions adsorption onto the thiourea modified Amberlite XAD7. This correlation is in concordance with the literature data showing that the Nd(III) ions adsorption is influenced by the solution pH and temperature. In addition, these data proved that the chemical reactions represent the limiting factor which control the adsorption speed.[49, 58-60] Table 1. Kinetic parameters associated with Nd(III) adsorption on thiourea modified Amberlite XAD7 Pseudo-first-order kinetic model Temperature (K) 298
qe, exp (mg/g) 12.42
k1 (min–1) 0.0368
9
qe, calc (mg/g) 3.83
R2 0.666
308
12.45
0.0361
3.51
0.662
318
12.45
0.0343
3.11
0.623
R2
Pseudo-second-order kinetic models Temperature (K)
351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371
298
qe, calc (mg/g) 12.42
k (g/(mg·min)) 483.225
qe, exp (mg/g) 13.72
0.994
308
12.45
535.304
13.61
0.995
318
12.45
598.623
13.46
0.996
Adsorption isotherm models In present work the equilibrium adsorption studies were conducted using a fixed quantity of 0.1 g adsorbent material and a variable initial concentration situated between 10 and 400 mg Nd(III) per L of solution. In all adsorption studies the pH value was adjusted at 6, contact time was 30 minutes and temperature was 298 K. In order to understand the interaction between adsorbent surface and Nd(III) ions were calculated the parameters used to explain the adsorption process by modeling the experimental data with Langmuir, Freundlich and Sips isotherms (depicted in figure 5). The model that is better describing the Nd (III) adsorption process was designed by calculating the correlation coefficient R2. Fig. 5. Adsorption isotherm of Nd(III) ions onto thiourea modified Amberlite XAD7 Correlation coefficients and relative parameters determined from used adsorption isotherms are shown in Table 2. By comparing the values of correlations coefficients obtained in these three cases, can observe that the correlation coefficient obtained in case of Sips isotherm is much better in comparison with that obtained for Langmuir and Freundlich. So, based on that can conclude that the Sips model is the most suitable to describe the Nd(III) ions adsorption onto the thiourea modified Amberlite XAD7 polymer. This conclusion is in accordance with the literature data proving that the increase of initial concentration have a positive effect on the Nd adsorption.[27] Table 2. Parameters calculated for Langmuir, Freundlich and Sips isotherms models Parameter XAD7-thiourea Experimental values qm,exp (mg/g)
74.30
Isotherm model Langmuir
qL (mg/g)
10
79.45
372 373 374 375 376 377 378 379 380 381 382 383 384 385 386
387 388 389 390
KL (L/mg) R2 KF (mg/g)
2.253 0.910 45.608
Freundlich
1/nF R2 qs (mg/g)
0.125 0.651 74.94
Sips
Ks 1/ns
6.421 2.155
R2
0.996
Analyzing data presented in Table 2 can observe that the maximum adsorption capacity calculated by using Sips isotherm model is 74.9 mg/g, value very close to the experimental value of 74.3 mg/g. Because the value of coefficient n is higher than 2 can say that the Nd removal on thiourea modified Amberlite XAD7 is favorable one [27]. Based on obtained data can conclude that the Nd(III) adsorption on used modified polymeric substrate is a multi-layer adsorption on an inhomogeneous surface. In addition, the adsorption mechanism is controlled by chemisorption that occurs as a consequence of chelating processes between Nd (III) ions and OH groups, or between Nd(III) ions and the free electrons located over the sulfur and nitrogen atoms from the surface of thiourea modified Ambrelite XAD7. In table 3is presented the comparison between the maximum adsorption capacities obtained when as adsorbents were used different synthesized or modified polymers. Analyzing this data can observe that thiourea modified Amberlite XAD7 can represent an useful adsorbent for Nd(III) ions removal. Table 3. Comparison between different materials used for Nd(III) removal Adsorption Adsorbent Reference capacities (mg/g) [34] XAD7-DEHPA 50.0–55.0 [61] MCM resin 72.00 [62] 8-Hydroxy-2-quinolinecarboxaldehyde functionalized 70.70 Amberlite XAD-4 [63] Chemically modified Amberlite XAD-4 resin with azacrown – ether Present XAD7-Thiourea 74.30 paper
Adsorption thermodynamics Main factor influencing the adsorption processes is temperature. In order to investigate if the Nd (III) adsorption is a spontaneous process and to investigate the thermal properties of studied
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adsorption was investigated temperature influence on the Nd (III) adsorption on thiourea modified Amberlite XAD7 polymer, and the linearized form is presented in Fig 6. Fig. 6. Linear plot of ln K versus 1/T Based on data depicted in Fig. 6 were evaluated the thermodynamic parameters associated with adsorption process (free Gibbs energy ∆G0, enthalpy ∆H0, and entropy ∆S0). Simultaneously was evaluated the value of regression coefficient R2 (data presented in table 4). ∆H0 and ∆S0 were obtained from slope and intercept of linear plot depicted in figure 6. Positive value of enthalpy variation proved that the energy needed for Nd (III) adsorption process exceeded the energy released when the Nd (III) ions are attached on the thiourea modified Amberlite, meaning that higher temperature make the adsorption easier [27, 64]. By dispersing the thiourea modified Amberlite XAD7 the hydrogen interactions between water molecules and free electrons presents on sulfur and nitrogen atoms becomes intermolecular hydrogen bonds, so the presence of water molecules represent a possible barrier for further neodymium adsorption [27], explaining why the adsorption is enhanced when the temperature increases. Table 4. Thermodynamic parameters obtained for the adsorption of Nd(III) on thiourea modified Amberlite XAD7 ∆S° (J/(mol·K)) ∆G° (kJ/mol) ∆H° R2 Thiourea modified (kJ/mol) 298 K 308 K 318 K 298 K 308 K 318 K Amberlite XAD7 49.99 201.84 201.85 201.86 –10.16 –12.18 –14.20 0.9970 Negative value of ∆G0 means that the adsorption process is a spontaneous process. When the experimental temperature increases, the free Gibbs energy becomes more negative, that can be associated with the increase of active free sites onto the adsorbent surface. Positive values of the entropy suggest that by increasing the temperature increases the disorder degree at interface solution / adsorbent corresponding to some changes in the adsorbent surfaces.[27, 64] Reason for the binding of multivalent ions onto the studied adsorbent is not the electrostatic force, being the entropic effect which keep the process free energy negative.[65] Based on all these can conclude that the neodymium adsorption on thiourea modified Amberlite XAD7 is an endothermic and spontaneous process. Also was evaluated the value of activation energy which have a value of 8.08 kJ / mol. Such value of activation energy means that during the neodymium adsorption are involved physical and chemical interactions.
3.3. Desorption and recycling experiments Usage of adsorbent materials depends not only on his adsorbent capacity but also is dependent of his regenerative capacity and his reuse. For real application of adsorbents is necessary that the regenerative process occurs easy and the adsorbed metallic ions are desorbed in higher quantity, so the reuse of adsorbents becomes economically feasible. In present paper it was investigated the possibility of adsorbent reuse by establishing the number or adsorption / desorption
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cycle. The desorption process occurs with good results when 15% HCl is used, and this process has been repeated 11 times (Fig. 7). The maximum adsorption and desorption quantities of Nd (III) ions are presented in table 5. Fig. 7. Nd(III) adsorption/desorption cycles on thiourea modified Amberlite XAD7 As is presented in Fig. 7 can observe that the adsorption efficiency decreases gradually starting with first cycle, until the reuse of adsorbent material becomes inefficient. Table 5 Regeneration of adsorbent material by consecutive adsorption/desorption cycles Cycle Uptake Desorption 85.60% 1 98.70% 2
90.80%
71.20%
3
84.20%
65.20%
4
78.20%
59.20%
5
67.00%
51.20%
6
59.20%
40.80%
7
45.00%
35.00%
8
32.10%
27.60%
9
35.40%
21.20%
10
20.70%
19.20%
4. Conclusions We obtained a new adsorbent material by modification of Amberlite XAD7 with thiourea that represents a cheap and environmental friendly extractant, having a spread usage in agriculture, medicine, industry. Simultaneous presence of nitrogen and sulfur atoms onto the polymer surface after ` and implicit the presence of free electrons represents a plus which transforms such materials in suitable adsorbents used for removal of neodymium ions from aqueous solutions. Presence of functional groups containing nitrogen and sulfur atoms onto the thiourea modified Amberlite XAD7 was evidenced by characterizing the obtained material using EDX and FTIR techniques. Novel modified Amberlite XAD7 polymer was used as adsorbent for Nd(III) removal from aqueous solutions. After that were carried out kinetic, thermodynamic and equilibrium studies in order to establish the Nd(III) adsorption mechanism on thiourea modified Amberlite XAD7. Experimental results proved that the Nd(III) adsorption on thiourea modified Amberlite XAD7 is an endothermic and spontaneous process. Based on calculated value of activation energy can conclude that the interactions between adsorbent and Nd (III) ions can be represented by physical one, hydrogen bonds or electrostatic attractions, but they can be also due to the
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complexation of metallic ions. Nd(III) adsorption on modified Amberlite can be modeled by using pseudo-second-order model, and the process is better described by Sips adsorption isotherm. In order to scale up the removal of Nd(III) ions using thiourea modified Amberlite XAD7 were performed desorption studies followed by reuse of adsorbent material. Based on that was found that thiourea modified Amberlite XAD7 can be used with good efficiency for 11 adsorption / desorption cycles. Can resume that the thiourea modified Amberlite XAD7 represent a potentially adsorbent material used for removal of neodymium ions, due to his good removal efficiency, easy way of preparation and his good stability in aqueous solutions.
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[53] Aksu Z, Isoglu I A. Removal of copper(II) ions from aqueous solution by biosorption onto agricultural waste sugar beet pulp. Process Biochem 2005, 40: 3031. [54] Palmieri M C, Garcia O, Melnikov P. Neodymium biosorption from acidic solutions in batch system. Process Biochem 2000, 36(5): 441. [55] Russell S, Garrett A B. The Thermodynamic Properties of Neodymium Hydroxide Nd(OH)3, in Acid, Neutral and Alkaline Solutions at 25°; the Hydrolysis of the Neodymium and Praseodymium Ions, Nd3+, Pr3+. J Am Chem Soc 1958, 80(14): 3532. [56] Russell S T. The thermodinamic properties of neodymium hydroxide in acid, neutral and alkaline solutions at 25 °C: an interprettion of the ionic species present in aqueous solutions of neodymium salts: The Ohio State University, 1956. [57] Harvey D. Modern analytical chemistry. Boston: McGraw-Hill Higher Education, 2000. [58] Plaza Cazón J, Viera M, Donati E, Guibal E. Zinc and cadmium removal by biosorption on Undaria pinnatifida in batch and continuous processes. J Environ Manage 2013, 129(Supplement C): 423. [59] Ho Y S, Ng J C Y, McKay G. REMOVAL OF LEAD(II) FROM EFFLUENTS BY SORPTION ON PEAT USING SECOND-ORDER KINETICS. Sep Sci Technol 2001, 36(2): 241. [60] Ramos S N d C, Xavier A L P, Teodoro F S, Elias M M C, Gonçalves F J, Gil L F, et al. Modeling mono- and multi-component adsorption of cobalt(II), copper(II), and nickel(II) metal ions from aqueous solution onto a new carboxylated sugarcane bagasse. Part I: Batch adsorption study. Ind Crops Prod 2015, 74(Supplement C): 357. [61] Siva Kesava Raju C, Subramanian M S. Sequential separation of lanthanides, thorium and uranium using novel solid phase extraction method from high acidic nuclear wastes. J Hazard Mater 2007, 145(1): 315. [62] Karadaş C, Kara D. Determination of Rare Earth Elements by Solid Phase Extraction Using Chemically Modified Amberlite XAD-4 Resin and Inductively Coupled Plasma-Optical Emission Spectrometry. Water Air Soil Pollut 2014, 225(11): 2192. [63] Dave S R, Kaur H, Menon S K. Selective solid-phase extraction of rare earth elements by the chemically modified Amberlite XAD-4 resin with azacrown ether. React Funct Polym 2010, 70(9): 692. [64] Zhang C, Zhang F, Li L, Zhang K. Adsorption Rare Earth Metal Ions from Aqueous Solution by Polyamidoamine Dendrimer Functionalized Soy Hull. Waste Biomass Valori 2016, 7(5): 1211. [65] Sinn C G, Dimova R, Antonietti M. Isothermal Titration Calorimetry of the Polyelectrolyte/Water Interaction and Binding of Ca2+: Effects Determining the Quality of Polymeric Scale Inhibitors. Macromolecules 2004, 37(9): 3444.
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Neodymium adsoption / desorption was studied onto Amberlite XAD7 functionalized with thiourea. A maximum adsorption capacity of 74.3 mg of Nd per g of adsorbent was obtained.
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Fig. 1. EDX spectra of thiourea modified Amberlite XAD7 polymer
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Fig. 2. FTIR spectra of thiourea modified Amberlite XAD7
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Fig. 3. pH effect on the Nd(III) adsorption onto the thiourea modified Amberlie XAD7
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a) b) Fig. 4. Plots of pseudo-first-order (a) and pseudo-second-order (b) models for the Nd(III) ions adsorption on thiourea modified Amberlite XAD7
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Fig. 5. Adsorption isotherm of Nd(III) ions onto thiourea modified Amberlite XAD7
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Fig. 6. Linear plot of lnK versus 1/T
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Fig. 7. Nd(III) adsorption / desorption cycles on thiourea modified Amberlite XAD7
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Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled
S1002-0721(18)31007-X
DOI:
https://doi.org/10.1016/j.jre.2019.04.023
Reference:
JRE 624
To appear in:
Journal of Rare Earths
Received Date: 7 December 2018 Revised Date:
15 April 2019
Accepted Date: 17 April 2019
Please cite this article as: Negrea P, Gabor A, Davidescu CM, Ciopec M, Negrea A, Duteanu N, Kinetics and thermodynamics modeling of Nd(III) removal from aqueous solution using modified Amberlite XAD7, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.04.023. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
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Kinetics and thermodynamics modeling of Nd(III) removal from aqueous solution using modified Amberlite XAD7
Petru Negrea1, Andreea Gabor1,†, Corneliu Mircea Davidescu1,†, Mihaela Ciopec1,*, Adina Negrea1,†, Narcis Duteanu1,* 1
Politehnica University of Timisoara, Faculty of Industrial Chemistry and Environmental Engineering, 2 Piata Victoriei, RO 300006 Timisoara, Romania † Authors with equal contributions
Corresponding authors: Faculty of Industrial Chemistry and Environmental Engineering, Politehnica University of Timisoara, 2 Piata Victoriei, RO 300006 Timisoara, Romania, E-mail addresses: [email protected]; [email protected]
ABSTRACT. New adsorbent material was obtained by modification of commercially Amberlite XAD7 with thiourea that represents a non-toxic, cheap and environmentally friendly extractant. Prepared adsorbent was used for removal of neodymium ions from aqueous solutions. Thiourea modified Amberlite involved in this study show good adsorption capacities (74.3 mg/g) and excellent efficiency during Nd removal process. In order to elucidate the mechanism of the Nd adsorption process kinetic, thermodynamic and equilibrium studies were performed, establishing this way which kinetic model is better describing the Nd adsorption process. Moreover the thermodynamic studies prove that the Nd adsorption on thiourea modified Amberlite XAD7 is an endothermic and spontaneous process. Keywords: Neodymium; Amberlite XAD7; adsorption mechanism; adsorption isotherm; thiourea 1.
Introduction Extensive development of human society during last decades involves usage of special materials into different applications such as superconductors, optical technology, telecommunications, nuclear and solar energy, photocatalysis, bisosciences, ceramics, alloys, fertilizers and security protection systems.[1-4] Because their unique properties rare earth elements are widely used in all these industrial fields [2], some of their applications are represented by the production of strong permanent magnets, automotive catalytic converters, laser, superconductors and supercapacitors [5]. Increase of computers market leads to increase of sintered magnets which represent an indispensable piece in construction of high performance electronic devices, especially of hard disks used in computers construction.[6] Last decades researches show the possibility to use Nd3+ ions as dopant in some hosts materials used to produce laser sources in near infrared spectral region.[7, 8] Neodymium is a common uranium fission product, which is also important in the medical field
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because in form of Nd3+, it represents a reasonable radiochemical source analog to the trivalent actinides such as: Am3+, Cm3+, Pu3+[2, 9]. Latest years have increased popularity and demand for green technologies, such as hybrid or electric cars, wind turbines, and compact lamps leads at higher demand of rare earth elements. This demand is of course boosting the prices of such elements, for example in case of neodymium the price increase at 94 $ per kg in 2014 from 16 $ per kg in 2001.[10] Increased demand for rare earth elements lead at an increased demand for lanthanides ores which lead at higher increase of public exposure at lanthanides from different sources: from different commercial products and also from production wastes and effluents.[11] During fabrication process and at end of life cycle large quantities of scrap and waste materials are generated, thus the need for a cheap and effective recovery technology. For the moment RRE recovery technologies are not standard, in reality less than 1% of RREs were recovered worldwide during 2013 [1, 2, 10] in comparison with the recycling rate for copper, chromium and gold which is higher than 50% [12]. According to data from EU the amount of electric and electronic equipment waste (EEEW) is around 12 million tons per year, from which only 2.2 million tons were treated in 2014, suggesting that the REEs recycling level is really limited.[13]It is expected that by 2020, the European market of waste electrical and electronic equipment recycling increase by 1.5 times, reaching a total turnover of 1.5 billion.[13] The present paper is dealing with the preparation of new adsorbent materials obtained by modification of commercial polymers with active groups containing S and N atoms, used for neodymium removal from aqueous solutions. This specific functional groups are introduced on the adsorbent surface with the aim to improve the adsorption properties of commercial resin [14]. It is well known from the large number of research papers dealing with removal of REEs that several removal techniques are widely spread, such as chemical precipitation [15, 16], ionic exchange [15, 17], and adsorption onto the different adsorbents.[15,18-22]Investigations carried out into the field of adsorptions during last decades lead at development and intensive usage of new unconventional materials such as activated graphite [23], bio-adsorbents [15, 24],new synthesized materials [25-28], superficial chemically modified or functionalized polymers.[29-33] During the years, many different types of ion exchange resins and chelated polymers where developed and used for removal of metallic ions from different wastewaters. Ion exchange resins present higher removal efficiencies but they have a major disadvantage represented by low mechanical resistance due to the swelling of the polymeric skeleton in the presence of water.[29] Chelated polymers can be used with good results during the removal and recovery or REEs but due to their low hydrophilic properties, low specific surface, the adsorption process needs very long contact time. A possible solution for this problem could be the modification of such polymers by functionalization with different pendant groups that lead at increase of material active surface and thus the material adsorption capacity. Different research papers proved that is possible to use Amberlite resins for removal of rare earth elements, for example Amberlite XAD7 resin was used as adsorbent after the functionalization with di-2-etyl-hexyl-phosphoric acid (DEHPA) [34]. Also, Amberlite XAD 4 resin was used for REEs removal after functionalization with n,n-bis-2hydroxyethyl-glycine[30] or with functional groups containing N atoms.[35]
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Accordingly to Liu et al., [36] thiourea is a really strong chelating agent for most of the metals, and its nitrogen and sulfur groups are responsible for coordination of heavy metal ions, forming in this way chelate compounds. Different solid adsorbents containing thiourea functional groups were used for adsorption of cadmium, silver [14, 37], lead [38], europium and lanthanum.[39] All above considered, present study aims to obtain an adsorbent material with higher adsorption capacity by modification of Amberlite XAD7 polymer with thiourea, followed by material characterization. After that, the modified material was used in the adsorption studies in order to establish the adsorption mechanism of Nd ions by using kinetics, thermodynamic and equilibrium studies. 2. Experimental 2.1. Amberlite XAD7 modification Amberlite XAD7 was modified by mixing 5 g of Amberlite resin (560–710µm, SigmaAldrich) with a solution obtained by dissolution of 0.5 g thiourea (purity 99.0%, from SigmaAldrich) into 25 mL of pure ethanol (Merck). Amberlite resin surface modification was obtained by using the technique of low-pressure concentration, using a Heidolph vacuum rotavapor, at pressure of 2 Pa for a concentration time of 10 minutes at 323 K. After impregnation, the concentration of thiourea into the residual solution was determined. By making the difference between the initial concentration of thiourea and the residual concentration after impregnation, was determined the quantity of thiourea impregnated onto the Amberlite XAD7 resin. No other process than impregnation was observe while contacting the polymer with the extractant solution. Based on that can conclude that each gram of Amberlite XAD7 resin was impregnated with 946 µmoles of thiourea (or each gram of Amberlite XAD7 was impregnated with 0.072 g of thiourea). 2.2. Characterization of chemically modified Amberlite resin Modified Amberlite XAD7 resin was characterized by using two different methods: Energy Dispersive X-ray Analysis (EDX) spectra and Fourier Transform Infrared Spectroscopy (FTIR). EDX spectra were obtained by using Quanta FEG 250 scanning electron microscopy and the FTIR spectra were recorded on Bruker Platinum ATR-QL FTIR spectrometer into the range 4000 – 400 cm–1 in order to distinguish the presence of N and S active groups onto the modified polymer surface. 2.3. Sorption experiments All experiments carried out in order to establish the optimum conditions regarding the pH, contact time and initial concentration of Nd ions for the adsorption onto the thiourea modified Amberlite XAD7 were made using a Julabo SW23 mechanical shaker bath at 200 rpm. All neodymium solutions were prepared from a stock solution with concentration of 1000 mg/L obtained by dissolution of Nd (purity 99%, purchased from Sigma-Aldrich) in HNO3 63% purchased from Merch. In all experiments, the neodymium residual concentration was measured using inductively coupled plasma mass spectrometer – ICPMS Bruker Aurora M90.
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First it was evaluated the influence of solution pH on the maximum adsorption capacity by measuring the neodymium residual concentration obtained after keeping in contact 0.1 g of adsorbent material with 25 mL solution containing 50 mg Nd(III) per L at 298 K. This study was made by modifying the solution pH between 2 and 7. In second stage was studied the influence of contact time and temperature on the maximum adsorption capacity of the modified polymer. In this stage samples of 0.1 g of adsorbent material were kept in contact with 25 mL solutions containing 50 mg Nd(III) per L for different times (15, 30, 45 and 60 min) and at different temperatures (298, 308 and 318 K) into a thermostatic bath at 200 rpm. After that, all the samples were filtered and in the filtered solution was determined the residual concentration of Nd ions. The influence of initial concentration over the maximum adsorption capacity was evaluated by variation of neodymium initial concentration in the solutions used during adsorption experiment from 10 to 400 mg/L (10, 50, 100, 150, 200, 300, 350 and 400 mg/L) obtained from previously prepared stock solution. During this stage, all the experiments were carried out at optimum pH, time and temperature obtained in the previous stages. 2.4. Adsorption isotherm modeling, kinetics and thermodynamics Evaluation of a solid - liquid adsorption system is done by performing equilibrium tests and dynamic adsorption studies [40]. During adsorption of metallic cations, their adsorption in formation of physical or physicochemical interaction between adsorbent and metallic ions until the equilibrium is reached. Equilibrium adsorption capacity can be evaluated by using equation: ( C − Ce )V (1) qe = 0 m Where, qe – equilibrium adsorption capacity (mg/g), C0 – initial concentration of the metallic ions in solution (mg/L), Ce – concentration of metallic ions in solution when the equilibrium is reached (mg/L), V – volume of the solution used in adsorption experiment (L), m – mass of adsorbent material used in adsorption experiment (g). In order to elucidate the adsorption mechanism of neodymium on thiourea modified Amberlite XAD7 was obtained the experimental adsorption isotherm. This experimental data were fit using three non-linear isotherm models: Langmuir, Freundlich and Sips. Langmuir isotherm consider that the adsorption is taking place in a monolayer on the surface of the adsorbent material, the Freundlich isotherm was developed in order to explain the adsorption onto the heterogeneous surfaces and Sips model represent a combination between Langmuir and Freundlich adsorption isotherms.[41, 42] Langmuir model takes into account three hypotheses: (i) adsorption is taking place into a monolayer, (ii) all the surface gaps are identical and can accommodate a single metallic ion, and (iii) the ability of a molecule to be adsorbed on the surface is independent on the occupancy of the neighboring sites.[15] Non-linear form of Langmuir isotherm is described by equation:[43] q K C (2) qe = L L e 1 + K L Ce
4
161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197
where: qe – equilibrium adsorption capacity (mg/g), Ce – concentration of metallic ions in solution when the equilibrium is reached (mg/L), qL – Langmuir maximum adsorption capacity (mg/g), and KL – Langmuir constant (L/mg). Starting from Langmuir non-linear isotherm characteristics a dimensionless constant RL named separation factor or equilibrium parameter can be calculated. This dimensionless parameter can be evaluated by using Eq.(3): 1 RL = (3) 1 + K L Co Where, RL – separation factor, KL – Langmuir constant (L/mg), and C0 – initial concentration of metallic ions (initial concentration of Nd ions, mg/L). Freundlich isotherm represents an empirical isotherm describing the adsorption process by using the equation:[44]
qe = K F Ce1 nf
(4)
Where, qe – equilibrium adsorption capacity (mg/g), Ce – metallic ion concentration when the equilibrium is reached (mg/L), KF and nF – represent characteristic constants which can be associated with adsorbent relative adsorption capacity and with adsorption intensity. Sips model represent a combination of Langmuir and Freundlich isotherms and is described by relation:[45] qs K S Ce1 nS (5) qe = 1 + K S Ce1 nS Where, qs – maximum adsorption capacity (mg/g), Ks – constant linked with adsorbent adsorption capacity, and ns – heterogeneity factor. Starting from Sips isotherm was calculated the separation factor that represents a dimensionless parameter, defined by relation: 1 RS = (6) 1 + KS C01 nS Where, Rs – separation factor (which is a constant linked to the adsorption capacity), ns – heterogeneity factor, and C0 – metallic ions initial concentration. Essential characteristics of Sips isotherm are described by the value of separation factor. So, if the separation factor is > 1 the adsorption process is not a favorable one, when separation factor is one the adsorption isotherm have a linear form, the adsorption is favorable when the separation factor have a value between zero and one, and adsorption process is an irreversible one when the separation factor is zero. Based on experimental data obtained for Nd adsorption on thiourea modified Amberlite was obtained a separation factor between 0 and 1, which means that the neodymium adsorption on studied adsorbent is favorable. Kinetic models are used in order to identify the adsorption mechanism for the studied system and in order to identify the limiting stage, including in that the mass transport and chemical reactions [46]. Most used kinetics models are pseudo-first order kinetic model (Lagergren model) [47] and pseudo-second-order model (Ho and McKay model).[48, 49] Pseudo-first-order model is described by equation:
5
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ln ( qe − qt ) = lnqe − k1 t
(7)
where: qe – equilibrium adsorption capacity (mg/g), qt – adsorption capacity at time t (mg/g), k1 – pseudo-first-order rate constant (min–1), t – contact time (min). The pseudo-second-order model (Ho and McKay) is described by equation: t 1 t = + (8) 2 qt k2 qe qe Where, qe- adsorption capacity at equilibrium (mg/g), qt – adsorption capacity at time t (mg/g), k2 – pseudo-second-order rate constant (g/(mg min)), t – contact time (min). Pseudo-first-order rate constant and also equilibrium adsorption capacity can be evaluated from the linear dependence of ln(qe–qt) versus t. Similarly it is possible to evaluate the pseudosecond-order rate constant and equilibrium adsorption capacity form linear dependence of t/qt versus t. Based on the kinetics parameters obtained for each used kinetic model can decide which model is better describing the neodymium ions adsorption onto the thiourea modified Amberlite XAD polymer. Based on rate constant evaluated from pseudo-second-order model, which is a specific constant for the Nd adsorption process, is possible to obtain the value of activation energy using Arrhenius equation: E (9) ln k 2 = lnA − a RT where: k2 – rate constant obtained from pseudo-second-order model (g/(mg min)), A – Arrhenius constant (g min/mg), Ea – activation energy (kJ/mol), T – absolute temperature (K), and R – ideal gas constant (8.314 J/(mol·k)). Is possible to evaluate the activation energy for the Nd(III) adsorption process from the graphical representation of ln k2 versus 1/T. In order to elucidate the Nd(III) adsorption mechanism on the modified Amberlite surface is essential to calculate the value of free Gibbs energy, based on Gibbs-Helmholtz equation:[50] ∆G o = ∆H o − T ⋅ ∆S o (10) Standard entropy variation and standard enthalpy variation are evaluated from the linear dependence of ln Kd versus 1/T:
225 226 227 228
∆S o ∆H o − (11) R RT where: Kd – equilibrium constant, ∆G0 – standard free Gibbs energy variation (kJ/mol), ∆H0 – standard enthalpy variation (kJ/mol), ∆S0 – standard entropy variation (J/(mol K)), T – absolute temperature (K), R – ideal gas constant (J/(mol·K)). Equilibrium constant represents the ratio between equilibrium adsorption capacity and equilibrium concentration.
229 230 231 232 233 234
2.5. Desorption and repeated use experiments In order to establish the maximum number of adsorption/desorption cycles the neodymium ions were adsorbed and desorbed until desorption was not possible any more. Nd(III) ions adsorbed onto the modified Amberlite surface were desorbed by mixing the polymer containing adsorbed ions with 250 mL HCl 15%. Reaction mixture was shaken for 6 h at 300 r/min at room temperature.
224
ln Kd =
6
235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275
After that, the filtered adsorbent material was rinsed with distilled water and dried at room temperature. This step was repeated until the Nd ions were irreversibly fixed onto the adsorbent material surface, this way establishing the maximum number of usage cycles. Adsorption/desorption process efficiency was establish by counting the adsorbed/desorbed of Nd ions quantity.
3. Results and discussions 3.1. Characterization of thiourea modified Amberlite XAD 7 polymeric support Fig.1 depicts the EDX spectra recorded for thiourea modified Amberlite XAD7. From data depicted in figure 1 can observe the presence of the peaks associated with presence of C and O atoms from polymeric chain and the peaks of nitrogen and sulfur atoms specific for functional groups used for Amberlite XAD7 modification. Presence of these two specific peaks prove that the Amberlite XAD7 commercial polymer was modified with thiourea. Figure 1. EDX spectra of thiourea modified Amberlite XAD7 polymer After that, the modified polymer was characterized by using FTIR spectroscopy, spectra depicted into the figure 2. Figure 2. FTIR spectra of thiourea modified Amberlite XAD7 From the FTIR spectra recorded for modified Amberlite XAD7 can observe the presence of O-H group specific bands located between 3640 and 3620 cm–1 and at 1700 cm–1 the peak associated with the presence of O–H group from water. Also in the wavelength interval located between 1720 – 1715 cm–1, and 1300 – 1100 cm–1 can observe the presence of bands associated with the vibration of C=O bond. The bands located at 1470 and 2925 cm–1 are specific for the vibrations of C–H bonds. Bands located between 1560 – 1650 cm–1 and 3400 – 3500 cm–1 are associated with the vibrations of alkyl – NH2 bonds. Bands located between 650 – 700 cm–1 and at 950 cm–1 are associated with the vibrations characteristic for group NH–C=S. Based on presented data can conclude that analyzed material is represented by thiourea modified Amberlite XAD7.[39, 51]
3.2. Nd (III) ions adsorption studies 3.2.1. pH effect on the Nd(III) adsorption A major role during the metal ions adsorption is played by the solution pH value that can influence the form of the metallic ions and also the adsorbent proprieties. Into the specific case of Nd(III) ions adsorption pH value represent an important factor which can control the performance of the adsorption process on the thiourea modified Amberlite XAD7 polymer. pH value affects the behaviour of the adsorbent material and also the chemical behaviour of the Nd(III) ions into the aqueous medium.[15, 27, 52, 53]Adsorption of Nd(III) positive ions occurs on the negatively charged groups found on the adsorbent surface. At low pH values is expected that the higher concentration of H+ ions leads at some competitive adsorption; a large number of active adsorption sites are
7
276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315
blocked due to the protonation. It’s expected that by increasing the pH value the protonation reaction speed is decreased due to the decrease of the number of protons into the solution, so is increasing the number of active groups available for neodymium ion adsorption. Further increase of solution pH lead at the competitively adsorption of OH– groups on the active adsorption sites, so the Nd adsorption becomes an electrostatic one.[27, 54] In order to study the effect of pH value on the neodymium ions adsorption onto the thiourea modified Amberlite XAD7 a series of experiments were carried out using aqueous solution with different pH. Obtained experimental data are depicted in Fig. 3. Fig. 3 pH effect on the Nd(III) adsorption onto the thiourea modified Amberlie XAD7 Based on experimental data presented in Fig. 3 can observe that maximum adsorption capacity increase when the solution pH is increasing. Maximum adsorption capacity (11.75 mg Nd(III) per g of adsorbent) is reached when the pH solution is 6, further increase of pH values lead at no increase of the maximum adsorption capacity, results which are in concordance with the data presented by other research papers [27, 29]. When the pH value is higher than 6 the neodymium ions can precipitate as Nd(OH)3 [54], so we evaluate the value of pH from which the precipitation process begins. Neodymium hydroxide presents a low solubility in water (4.8×10–6 moles of Nd(OH)3per L of water [55, 56]), and its dissociation equilibrium is presented in chemical equation (12):[56, 57] Nd(OH)3(s) Nd3+(aq) + 3 OH−(aq) (12) In order to calculate the pH at which the precipitation is started must take into account that the Nd(III) complexation moves the equation balance to the right this way increasing the neodymium hydroxide solubility. Based on Eq.(12) can evaluate the Nd(OH)3 solubility product using equation: Ks = [Nd3+][HO–]3, where [Nd3+] and [HO–] represent the equilibrium concentrations of neodymium and hydroxyl ions. From this equation is possible to evaluate equilibrium concentration of HO- ions taking into account that the Ks is 3.10×10–22 mol/L [55, 56]. During experimental study we used solutions with a maximum concentration of 50 mg Nd(III) per L, which is equal with a concentration of Nd(III) ions of 3.46×10–4 mol/L. For this particular case, was calculated the pH at which is beginning the Nd(OH)3 precipitation, having a value of 7.99, means that during the experimental study pH should not exceed this value.
3.2.2. Effect of contact time and temperature onto the Nd(III) adsorption on thiourea modified Amberlite XAD7 Contact time and temperature influence were studied as described earlier, and from obtained data (figure not show) was observed that the concentration profile of Nd ions during the process shown a typical profile for an adsorption process. Adsorption rate is fast in the first 25 min (as long as active centers are fully available), then the process is evolving slower rate. The equilibrium state being attained after 30 min, when the maximum adsorption capacity was observed. In meanwhile can observe that the increase of temperature have only a relatively small influence onto the adsorption capacity. For example when the working temperature increase from 298 at 318 K the
8
316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350
maximum adsorption capacity increases from 12.41 at 12.45 mg/g. Based on these experimental data we choose for all further studies the contact time of 30 min. This behavior can be explained when taking in account that at the beginning of adsorption process the active sites of produced adsorbent are available, so the neodymium ions can reach it easily. When the contact time increases the number of free adsorption sites decreases rapidly so the remaining active sites presented onto the surface of adsorbent materials cannot be easily occupied by Nd (III) ions.
3.2.3. Adsorption mechanism for Nd(III) uptake onto thiourea modified Amberlite XAD7 adsorption kinetics Adsorption kinetics was studied in order to better describe the adsorption process rate and efficiency [27].In order to evaluate the kinetic mechanism responsible for Nd ions adsorption on thiourea modified Amberlite XAD7 the experimental data were modeled using pseudo-first-order and pseudo-second-order kinetics models. Plots of the two used models are depicted in figure 4 and the calculated parameters of the Nd(III) adsorption kinetics are presented in table 1.
Figure4. Plots of pseudo-first-order (a) and pseudo-second-order (b) models for the Nd(III) ions adsorption on thiourea modified Amberlite XAD7 Values of constant k1 and qe used in description of pseudo-first-order model are obtained from the slope and from the intercept of linear plot ln(qe–qt) versus time. Similar for the pseudosecond-order model the value of constant k was evaluated from the slope of linear representation of t/qt versus t. From data presented in Table 1 can observe that the correlation coefficient R2 obtained when the pseudo-second-order was used have a higher value compared with that obtained when the pseudo-first-order model is used. At used temperatures, the correlation coefficient obtained when the pseudo-second-order model was used shows values located between 0.9938 and 0.9959, compared with the values obtained from pseudo-first-order model that are located between 0.6231 and 0.6655. These values indicate that the pseudo-second-order model describes better the Nd(III) ions adsorption onto the thiourea modified Amberlite XAD7. This correlation is in concordance with the literature data showing that the Nd(III) ions adsorption is influenced by the solution pH and temperature. In addition, these data proved that the chemical reactions represent the limiting factor which control the adsorption speed.[49, 58-60] Table 1. Kinetic parameters associated with Nd(III) adsorption on thiourea modified Amberlite XAD7 Pseudo-first-order kinetic model Temperature (K) 298
qe, exp (mg/g) 12.42
k1 (min–1) 0.0368
9
qe, calc (mg/g) 3.83
R2 0.666
308
12.45
0.0361
3.51
0.662
318
12.45
0.0343
3.11
0.623
R2
Pseudo-second-order kinetic models Temperature (K)
351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371
298
qe, calc (mg/g) 12.42
k (g/(mg·min)) 483.225
qe, exp (mg/g) 13.72
0.994
308
12.45
535.304
13.61
0.995
318
12.45
598.623
13.46
0.996
Adsorption isotherm models In present work the equilibrium adsorption studies were conducted using a fixed quantity of 0.1 g adsorbent material and a variable initial concentration situated between 10 and 400 mg Nd(III) per L of solution. In all adsorption studies the pH value was adjusted at 6, contact time was 30 minutes and temperature was 298 K. In order to understand the interaction between adsorbent surface and Nd(III) ions were calculated the parameters used to explain the adsorption process by modeling the experimental data with Langmuir, Freundlich and Sips isotherms (depicted in figure 5). The model that is better describing the Nd (III) adsorption process was designed by calculating the correlation coefficient R2. Fig. 5. Adsorption isotherm of Nd(III) ions onto thiourea modified Amberlite XAD7 Correlation coefficients and relative parameters determined from used adsorption isotherms are shown in Table 2. By comparing the values of correlations coefficients obtained in these three cases, can observe that the correlation coefficient obtained in case of Sips isotherm is much better in comparison with that obtained for Langmuir and Freundlich. So, based on that can conclude that the Sips model is the most suitable to describe the Nd(III) ions adsorption onto the thiourea modified Amberlite XAD7 polymer. This conclusion is in accordance with the literature data proving that the increase of initial concentration have a positive effect on the Nd adsorption.[27] Table 2. Parameters calculated for Langmuir, Freundlich and Sips isotherms models Parameter XAD7-thiourea Experimental values qm,exp (mg/g)
74.30
Isotherm model Langmuir
qL (mg/g)
10
79.45
372 373 374 375 376 377 378 379 380 381 382 383 384 385 386
387 388 389 390
KL (L/mg) R2 KF (mg/g)
2.253 0.910 45.608
Freundlich
1/nF R2 qs (mg/g)
0.125 0.651 74.94
Sips
Ks 1/ns
6.421 2.155
R2
0.996
Analyzing data presented in Table 2 can observe that the maximum adsorption capacity calculated by using Sips isotherm model is 74.9 mg/g, value very close to the experimental value of 74.3 mg/g. Because the value of coefficient n is higher than 2 can say that the Nd removal on thiourea modified Amberlite XAD7 is favorable one [27]. Based on obtained data can conclude that the Nd(III) adsorption on used modified polymeric substrate is a multi-layer adsorption on an inhomogeneous surface. In addition, the adsorption mechanism is controlled by chemisorption that occurs as a consequence of chelating processes between Nd (III) ions and OH groups, or between Nd(III) ions and the free electrons located over the sulfur and nitrogen atoms from the surface of thiourea modified Ambrelite XAD7. In table 3is presented the comparison between the maximum adsorption capacities obtained when as adsorbents were used different synthesized or modified polymers. Analyzing this data can observe that thiourea modified Amberlite XAD7 can represent an useful adsorbent for Nd(III) ions removal. Table 3. Comparison between different materials used for Nd(III) removal Adsorption Adsorbent Reference capacities (mg/g) [34] XAD7-DEHPA 50.0–55.0 [61] MCM resin 72.00 [62] 8-Hydroxy-2-quinolinecarboxaldehyde functionalized 70.70 Amberlite XAD-4 [63] Chemically modified Amberlite XAD-4 resin with azacrown – ether Present XAD7-Thiourea 74.30 paper
Adsorption thermodynamics Main factor influencing the adsorption processes is temperature. In order to investigate if the Nd (III) adsorption is a spontaneous process and to investigate the thermal properties of studied
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408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427
adsorption was investigated temperature influence on the Nd (III) adsorption on thiourea modified Amberlite XAD7 polymer, and the linearized form is presented in Fig 6. Fig. 6. Linear plot of ln K versus 1/T Based on data depicted in Fig. 6 were evaluated the thermodynamic parameters associated with adsorption process (free Gibbs energy ∆G0, enthalpy ∆H0, and entropy ∆S0). Simultaneously was evaluated the value of regression coefficient R2 (data presented in table 4). ∆H0 and ∆S0 were obtained from slope and intercept of linear plot depicted in figure 6. Positive value of enthalpy variation proved that the energy needed for Nd (III) adsorption process exceeded the energy released when the Nd (III) ions are attached on the thiourea modified Amberlite, meaning that higher temperature make the adsorption easier [27, 64]. By dispersing the thiourea modified Amberlite XAD7 the hydrogen interactions between water molecules and free electrons presents on sulfur and nitrogen atoms becomes intermolecular hydrogen bonds, so the presence of water molecules represent a possible barrier for further neodymium adsorption [27], explaining why the adsorption is enhanced when the temperature increases. Table 4. Thermodynamic parameters obtained for the adsorption of Nd(III) on thiourea modified Amberlite XAD7 ∆S° (J/(mol·K)) ∆G° (kJ/mol) ∆H° R2 Thiourea modified (kJ/mol) 298 K 308 K 318 K 298 K 308 K 318 K Amberlite XAD7 49.99 201.84 201.85 201.86 –10.16 –12.18 –14.20 0.9970 Negative value of ∆G0 means that the adsorption process is a spontaneous process. When the experimental temperature increases, the free Gibbs energy becomes more negative, that can be associated with the increase of active free sites onto the adsorbent surface. Positive values of the entropy suggest that by increasing the temperature increases the disorder degree at interface solution / adsorbent corresponding to some changes in the adsorbent surfaces.[27, 64] Reason for the binding of multivalent ions onto the studied adsorbent is not the electrostatic force, being the entropic effect which keep the process free energy negative.[65] Based on all these can conclude that the neodymium adsorption on thiourea modified Amberlite XAD7 is an endothermic and spontaneous process. Also was evaluated the value of activation energy which have a value of 8.08 kJ / mol. Such value of activation energy means that during the neodymium adsorption are involved physical and chemical interactions.
3.3. Desorption and recycling experiments Usage of adsorbent materials depends not only on his adsorbent capacity but also is dependent of his regenerative capacity and his reuse. For real application of adsorbents is necessary that the regenerative process occurs easy and the adsorbed metallic ions are desorbed in higher quantity, so the reuse of adsorbents becomes economically feasible. In present paper it was investigated the possibility of adsorbent reuse by establishing the number or adsorption / desorption
12
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436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452
cycle. The desorption process occurs with good results when 15% HCl is used, and this process has been repeated 11 times (Fig. 7). The maximum adsorption and desorption quantities of Nd (III) ions are presented in table 5. Fig. 7. Nd(III) adsorption/desorption cycles on thiourea modified Amberlite XAD7 As is presented in Fig. 7 can observe that the adsorption efficiency decreases gradually starting with first cycle, until the reuse of adsorbent material becomes inefficient. Table 5 Regeneration of adsorbent material by consecutive adsorption/desorption cycles Cycle Uptake Desorption 85.60% 1 98.70% 2
90.80%
71.20%
3
84.20%
65.20%
4
78.20%
59.20%
5
67.00%
51.20%
6
59.20%
40.80%
7
45.00%
35.00%
8
32.10%
27.60%
9
35.40%
21.20%
10
20.70%
19.20%
4. Conclusions We obtained a new adsorbent material by modification of Amberlite XAD7 with thiourea that represents a cheap and environmental friendly extractant, having a spread usage in agriculture, medicine, industry. Simultaneous presence of nitrogen and sulfur atoms onto the polymer surface after ` and implicit the presence of free electrons represents a plus which transforms such materials in suitable adsorbents used for removal of neodymium ions from aqueous solutions. Presence of functional groups containing nitrogen and sulfur atoms onto the thiourea modified Amberlite XAD7 was evidenced by characterizing the obtained material using EDX and FTIR techniques. Novel modified Amberlite XAD7 polymer was used as adsorbent for Nd(III) removal from aqueous solutions. After that were carried out kinetic, thermodynamic and equilibrium studies in order to establish the Nd(III) adsorption mechanism on thiourea modified Amberlite XAD7. Experimental results proved that the Nd(III) adsorption on thiourea modified Amberlite XAD7 is an endothermic and spontaneous process. Based on calculated value of activation energy can conclude that the interactions between adsorbent and Nd (III) ions can be represented by physical one, hydrogen bonds or electrostatic attractions, but they can be also due to the
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complexation of metallic ions. Nd(III) adsorption on modified Amberlite can be modeled by using pseudo-second-order model, and the process is better described by Sips adsorption isotherm. In order to scale up the removal of Nd(III) ions using thiourea modified Amberlite XAD7 were performed desorption studies followed by reuse of adsorbent material. Based on that was found that thiourea modified Amberlite XAD7 can be used with good efficiency for 11 adsorption / desorption cycles. Can resume that the thiourea modified Amberlite XAD7 represent a potentially adsorbent material used for removal of neodymium ions, due to his good removal efficiency, easy way of preparation and his good stability in aqueous solutions.
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Graphical abstract
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Neodymium adsoption / desorption was studied onto Amberlite XAD7 functionalized with thiourea. A maximum adsorption capacity of 74.3 mg of Nd per g of adsorbent was obtained.
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Fig. 1. EDX spectra of thiourea modified Amberlite XAD7 polymer
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Fig. 2. FTIR spectra of thiourea modified Amberlite XAD7
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Fig. 3. pH effect on the Nd(III) adsorption onto the thiourea modified Amberlie XAD7
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a) b) Fig. 4. Plots of pseudo-first-order (a) and pseudo-second-order (b) models for the Nd(III) ions adsorption on thiourea modified Amberlite XAD7
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Fig. 5. Adsorption isotherm of Nd(III) ions onto thiourea modified Amberlite XAD7
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Fig. 6. Linear plot of lnK versus 1/T
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Fig. 7. Nd(III) adsorption / desorption cycles on thiourea modified Amberlite XAD7
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Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled