Separation of Formic Acid from Aqueous Solutions onto Anion

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

pubs.acs.org/jced

Separation of Formic Acid from Aqueous Solutions onto Anion Exchange Resins: Equilibrium, Kinetic, and Thermodynamic Data Hani Zeidan and Mustafa Esen Marti*

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Department of Chemical Engineering, Konya Technical University, Konya 42075, Turkey ABSTRACT: Wastewaters containing organic compounds such as acids, ketones, phenols, and amines can cause critical environmental problems depending on their concentration and composition. Efficient and low-cost separation of these components may also bring their reuse in industry while cleaning the aqueous streams. Formic acid (FA) is an important raw material in industry and is widely seen in wastewaters. In this study, two different anion exchangers were compared for the separation of FA from aqueous solutions. Effects of process parameters and the mechanism were discussed. Kinetic, equilibrium, and thermodynamic studies were performed, and the data were interpreted using the relevant isotherm and kinetic models. The results showed that Lewatit MP-64 was more efficient than Amberlite IRA-96 in the range of the parameters studied. The maximum adsorption capacity obtained with Lewatit MP-64 (442.75 mg/g) was significantly higher than that were previously reported in the literature for FA separation. The data were well explained by the Langmuir isotherm model. However, relatively high R2 values were obtained with Temkin and Freundlich isotherms. Therefore, further studies are ongoing to clarify the mechanism.

1. INTRODUCTION Wastewaters containing toxic substances such as pesticides, herbicides, heavy metals, textile dyes, and persistent organic pollutants need to be cleaned or treated before their discharge to the environment or re-use in the process. Carboxylic acids (CAs) are among the toxic organic chemicals and usually seen in wastewater streams because of their huge amount of use in several types of industries such as pharmaceutical, food, cosmetic, agriculture, and so forth.1−5 The type and quantity of CAs in waste streams change according to the raw materials, production methods, and process variables. Depending on their composition and concentration, CAs can be poisonous to humans and other living things. In addition, because they are reactive materials, CAs such as formic acid (FA), acetic acid (AA), and so forth can react with other chemicals in the nature to form much more toxic substances than CAs. Therefore, their removal from wastewater streams is required prior to their discharge to the environment or transfer to another process unit.2,6,7 Their efficient, low-cost, and selective separation will also enable their reutilization in the industry. However, recovery of CAs from dilute aqueous solutions such as fermentation broths and wastewaters is still a challenging separation problem.8−11 FA (HCOOH) or methanoic acid is the simplest CA and one of the significant raw materials used in several industries. It is extensively employed in food preservatives, pharmaceutical, dye, tanning, antibacterial agent, rubber, leather, and textile industries.7,12,13 It is a commonly seen intermediate product in the oxidation of organic wastes, which are generally diluted in aqueous streams. Thus, FA is present in several types of industrial wastewaters such as textile, pharmaceutical, leather, and so forth.7 Moreover, it is encountered in several types of fermentative productions as a byproduct.14 Recently, its efficient removal and recovery from aqueous-based solutions © XXXX American Chemical Society

has gained a great interest in industry because of its increasing usage amounts and also the production costs.8,13−16 Various methods and strategies have been developed and evaluated for the recovery of CAs from dilute aqueous solutions. Solvent and reactive extraction, distillation, membrane filtration, electrodialysis, crystallization, ion exchange, and adsorption are the techniques that have been tested and applied to the recovery of CAs.10,11 However, most of these were not appropriate to be used in the industrial productions because of the ecological, environmental, and economic disadvantages. Among them, ion exchange and adsorption have been shown as promising techniques because of their high selectivity, low cost, ease of operation, and environmentally friendly nature. Another important advantage of these techniques is their availability to be integrated with other systems. Even though there are some papers on the ion exchange and adsorption of low molecular weight CAs from various types of aqueous solutions in the literature, studies are still required on the optimization of the process conditions and selection of the most effective resin, which are the key elements of these separation processes. In the literature, several researchers studied about the recovery of CAs from fermentation broth such as lactic acid,17−20 succinic acid,21 and butyric acid20 using various types of anion-exchange resins (AERs). In addition, separation of acetic,22,23 levulinic,24 fumaric,25 lactic,26,27 itaconic,28 citric,29,30 and succinic23,26,31 acids from aqueous solutions using commercially available resins produced by different companies were reported. Some researchers also studied the recovery of acids from multiacid aqueous solutions.20,23 Received: February 4, 2019 Accepted: May 17, 2019

A

DOI: 10.1021/acs.jced.9b00128 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Chemicals Utilized in This Study compound formic acid sodium hydroxide hydrochloric acid Amberlite IRA-96 Lewatit MP-64

IUPAC name methanoic acid sodium hydroxide hydrochloric acid

source VWR Chemicals Merck Merck Sigma-Aldrich Sigma-Aldrich

However, there is limited number of studies on the recovery of FA using AERs in the literature. Bhandari et al. (1992) studied the sorption equilibria and batch dynamics of FA and monochloroacetic acid on different AERs. The authors evaluated the appropriateness of the reversible sorption model based on the double-layer theory for weak acid adsorptions on weak basic resins. They successfully correlated the effective ionic pore diffusion coefficient using the experimental results.13 Uslu (2009) used a weakly basic geltype polyacrylic resin, Amberlite IRA-67, for the removal of FA from aqueous solutions. The researcher studied in the concentration range of 23−115 g/L and reached the highest adsorption capacity as about 239 mg/g according to our calculations using the relevant data.2 Liu et al. (2009) investigated the suitability of several isotherm models for single-component and binary-competitive adsorption of levulinic/FAs using polymeric adsorbents 335, D301 and D315, purchased from Shanghai Huazhen Science and Technology Co. Ltd. The highest adsorption capacity obtained in the study was about 130 mg/g and with 335, which has tertiary amines and epoxyamine as functional groups and matrix, respectively.6 Lin et al. (2016) studied the removal of FA from aqueous solutions using D1107, a weak basic AER, synthesized by Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences. Trends obtained in the study were consistent with those previously reported in the literature. Maximum adsorption capacity obtained with the resin was about 140 mg/g for FA adsorption.7 Hence, comparative and detailed studies on the separation of FA by ion exchange or adsorption are still needed for the integration of the process with industrial wastewater and production units. The first goal of this study was to compare the efficiencies of Lewatit MP-64 and Amberlite IRA-96 for the separation of FA from aqueous solutions. Moreover, the results were also compared with those previously reported in the literature. Effects of several process parameters such as pH, acid concentration, contact time, resin dose, and temperature were investigated. Furthermore, kinetic models and isotherms were used to interpret the data and understand the mechanism. Thermodynamic parameters were also calculated using the related data.

CAS no. 64-18-6 1310-73-2 7647-01-0 39409-19-3 39433-45-9

assay (%) 99−100 ≥99 37

Table 2. Characteristic Properties of Amberlite IRA-67 and Lewatit MP-64 resin atrix structure functional group physical form total capacity (equiv/L) specific gravity beads mean size (mm) uniformity coefficient ionic form moisture holding capacity (%) maximum temperature (°C) operating pH

Amberlite IRA-96 styrene divinylbenzene tertiary amine opaque ≥1.25 1.04−1.06 0.55 ± 0.20 ≤1.8 free base 57−63

Lewatit MP-64 cross-linked polystyrene tertiary/quaternary amine opaque ≥1.3 1.04 0.59 ± 0.05 1.1 free base/Cl− 61−66

60

70

0−6

0−7

whereas Lewatit MP-64 has both quaternary and tertiary amines. Both resins were made of polymeric matrices that contain styrene monomers. Amberlite IRA-96 has a styrene divinylbenzene (DVB) copolymer matrix while Lewatit MP-64 has a cross-linked polystyrene matrix. In addition, every resin producer has its own resin production technique, which makes the resins different from others, even though similar functional groups and matrix types were used in the fabrication. 2.2. Assay Method. The concentration of FA in the aqueous phase before and after the separation process was determined by HPLC (Agilent LC 1220) equipped with a UV detector and a C18 column (ACE). The column temperature was maintained at 30 °C and the detection of FA was carried out at 210 nm. The mobile phase was pH 2.8−0.05 (mol/L) potassium dihydrogen phosphate (KH2PO4) + 1% acetonitrile solution and the flow rate was 1.25 mL/min. All experiments and analyses were performed at least twice. The relative uncertainty of the data was 0.1% and that of replicated experiments was less than 1%. Averages of the concentration values were used in the calculation of the process parameters. 2.3. Equilibrium Methods. Five FA solutions with different initial concentrations (0.05−2.0 mol/L) were prepared. Each aqueous solution (10 mL) was contacted with the AERs at the predetermined dose levels (0.1 g/10 mL−0.5 g/10 mL or 10−50 g/L) in a 50 mL erlenmeyer. A constant temperature shaker bath was used for the contact of the AERs and FA solutions. The experiments were carried out by shaking the phases at 150 rpm, 298 K, and 101 325 Pa for 2 h, which was sufficient to reach the equilibrium. Following that, the mixture was settled for 5 min to separate the phases. After reaching a significant phase separation, the sample was carefully taken from the aqueous phase and analyzed for the concentration of FA. The amount of acid adsorbed on the resin at time t, qt, was calculated using eq 1

2. MATERIALS AND METHODS 2.1. Materials. The chemicals utilized in this study are listed in Table 1. FA (99% purity) was obtained from Van Waters and Rogers (VWR) and used without any treatment. Ultrahigh pure (UHP) water obtained from Millipore was employed in all experimental studies. Aqueous FA solutions were prepared using the FA and UHP water. Both macroporous basic anion exchangers, Amberlite IRA-96 and Lewatit MP-64 that were manufactured by Rohm Haas and Lanxess, respectively, were supplied by Sigma. The physical characteristics of the AERs are given in Table 2. According to the table, the resins mainly differed in their functional group types. Amberlite IRA-96 has tertiary amines as functional groups, B


Journal of Chemical & Engineering Data qt =

C0 − Ct ·V ·M m

Article

changes depending on the resin basicity.8,22,31,33 With a weak basic AER, the mechanism is based on the adsorption of the undissociated forms of acid on the resin. On the other hand, with a strong basic AER, it becomes an anion-exchange process between the dissociated form of the target acid in the solution and the functional group of the resin. Amberlite IRA-96 has tertiary amines as functional groups. Thus, the first mechanism was expected to occur in the process at low pH values (pH < 3.75) because most or all of the FA molecules were undissociated (eqs 4 and 5). On the other hand, Lewatit MP-64 has both tertiary and quaternary amines, and both mechanisms can be valid depending on the initial pH of the FA solution.

(1)

where C0 and Ct represent the initial and final amounts of FA, respectively, qt is the adsorption capacity of the resin at time t, V is the volume of the solution, M is the molecular weight of FA, and m is the amount of the AER. To calculate the adsorption capacity at equilibrium (qe), Ct is replaced with the FA concentration at the equilibrium, Ce, in the same equation. Separation efficiency was calculated using eq 2. Separation efficiency (%) =

(C0 − Ce) × 100 C0

(2)

The initial pH of 0.1 mol/L FA solution was measured to be about 2.39. To study the effect of pH on FA adsorption onto Amberlite IRA-96 and Lewatit MP-64, the solution pH was adjusted between pH = 1.57 and pH = 7.0 using the aqueous solutions of HCl and NaOH. Then, the resins were added to the solutions, and the mixtures were shaken at 150 rpm and 298 K for 2 h and likewise done at all experiments except kinetics. The samples were removed from aqueous phases and analyzed for FA concentration as previously explained. Adsorbed amount of FA was calculated by material balance. The results of the equilibrium studies were used on the investigation of the effects of resin dose and FA concentration. The data were also employed in plotting the graphs of the isotherm models (Langmuir, Freundlich and Temkin) to understand the mechanism of the process. 2.4. Kinetic Experiments. The studies on the adsorption rate (kinetic experiments) were carried out using 10 mL of aqueous solution (0.1 mol/L FA) and 0.2 g (resin dose = 20 g/ L) of each AER in a 50 mL conical flask. Each experiment was simultaneously started but continued for different time intervals. At time t, each sample was quickly removed from the shaker and the FA concentration of the aqueous phase was measured. Hence, the volume of the aqueous solution did not change during the sampling. Kinetic data were interpreted using several kinetic models to determine the rate of the process. 2.5. Thermodynamics. Thermodynamic studies were conducted to observe the effect of temperature on the separation process and for the calculation of thermodynamic parameters. Experiments were carried out by using the FA solutions having concentrations that varied between 0.05 and 2.0 mol/L. The resin dose was chosen as 20 g/L, while temperature was changed between 298 and 318 K. Aqueous samples were analyzed for the residual amount of FA in the solution as previously described.

pH = pK a + log

[F−] [HF]

(3)

R + H δ + ↔ RH δ +

(4)

RH δ + + Fδ − ↔ RHF

(5)

Figure 1 shows the effect of the pH on the separation of FA from aqueous solutions using Amberlite IRA-96 or Lewatit

Figure 1. Effect of solution pH on the separation of FA using Amberlite IRA-96 and Lewatit MP-64.

MP-64. The study was performed in the pH range of pH = 1.57−7.0 because the resins are not available to be used above neutral pH (Table 2). Solutions of HCl and NaOH were used to reduce and increase the initial pH of the FA solutions, respectively. In these experiments, the FA concentration and resin dose were chosen as 0.1 mol/L and 20 g/L, respectively. Even though the types of the functional groups of the resins were not completely same, the trends observed with the pH were almost similar for the two AER types tested. The highest efficiency was obtained at the natural pH value of the FA solution among the ones studied. Even though there are more undissociated FA molecules in the solution at pH = 1.57, the efficiencies obtained at pH = 2.39 with the two resins were higher than the ones at that pH. Interactions between the HCl−FA molecules and HCl−resin were the most probable reasons for the lesser efficiency at the lower pH value. This trend is consistent with the reports previously published in the literature.20,21,23,31 Lin et al. (2016) also stated that the authors reached the maximum adsorption capacity at the natural pH of the FA solution.7 At pH = 3, the separation efficiency decreased by almost 6.6% for Amberlite IRA-96 and 13% for Lewatit MP-64. Figure 1 clearly shows the negative influence of the pH increase on the separation process, especially at pH > 4. This is most probably due to the decrease in the undissociated acid

3. RESULTS AND DISCUSSION 3.1. Effect of Solution pH. The pH of an aqueous solution can directly affect the dissociation of the solute and charge of the surface of the adsorbent. Therefore, ion-exchange and adsorption processes can be significantly influenced by the pH of the solution. Because FA is a monocarboxylic acid, it has only one dissociation constant, which is Ka = 1.77 × 10−4, in other words, pKa = 3.75. According to the Henderson− Hasselbach equation (eq 3), the pH of the solution and pKa value are the main parameters that influence the ratio of dissociated and undissociated acids.32 This means that below pH = 3.75, most of the FA molecules in the solution are undissociated, whereas above that value, they are dissociated. As it is known, the mechanism for the adsorption process C



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Table 3. Kinetic Data for the Adsorption of FA onto Amberlite IRA-96 and Lewatit MP-64 Amberlite IRA-96 (20 g/L)

Lewatit MP-64 (20 g/L)

time (min)

C0 (mol/L)

Ce (mol/L)

qt (mg/g)

sep. eff. (%)

std. dev. (%)

C0 (mol/L)

Ce (mol/L)

qt (mg/g)

sep. eff. (%)

std. dev. (%)

2 5 10 20 30 40 50 60 90 120

0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100

0.069 0.060 0.056 0.053 0.051 0.049 0.048 0.046 0.046 0.046

71.3 92.0 101.2 108.1 112.7 117.3 119.6 124.2 124.2 124.2

31.0 40.0 44.0 47.0 49.0 51.0 52.0 54.0 54.0 54.0

0.33 0.65 0.65 0.98 0.65 0.33 0.00 0.00 0.00 0.00

0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100

0.0585 0.0545 0.0505 0.0485 0.0450 0.0430 0.0405 0.0395 0.0395 0.0395

95.45 104.65 113.85 118.45 126.50 131.10 136.85 139.15 139.15 139.15

41.50 45.50 49.50 51.50 55.00 57.00 59.50 60.50 60.50 60.50

0.65 0.98 0.33 0.33 0.65 0.98 0.33 0.00 0.00 0.00

concentration with pH in the aqueous solution. At pH = 3.75, half of the FA molecules are undissociated in the aqueous solution and above this pH value, amount of them dramatically decreased. This can be a valid explanation for the decrease observed in the performance of the Amberlite IRA-96. Husson and King (1999) reported that maximum efficiency of the separation of acetic, formic, lactic, and succinic acids were obtained at the pH values lower than the pKa of the associated acid.23 A similar trend was also reported by Yousuf et al. (2016) as the researchers stated that adsorption efficiencies of lactic, butyric, and acetic acids with Amberlite IRA-67 were significantly high at the pH values lower than the pKa of these three acids.20 On the other hand, Lewatit MP-64 was expected to separate FA anions at higher pH values, especially between pH = 4 and pH = 7. However, it acts like a weak AER, even though it has quaternary amine functional groups with tertiary amines on the surface (Table 2 and Figure 1). This may show that tertiary amines are the dominant functional groups on the surface of the Lewatit MP-64 or quaternary amine functional groups on this resin cannot serve, as they are supposed to be in this pH range for FA separation. Because the highest process efficiency was achieved at pH = 2.39, the pH of the aqueous FA solutions was not adjusted at the following experiments and the solutions at their natural pH values were used in equilibrium, kinetic, and thermodynamic studies. 3.2. Equilibration Time and Kinetic Models. Kinetic studies are required to understand the rate of the process, which is one the most important parameter for the design of a process. The effect of contact time on the separation of FA using Amberlite IRA-96 and Lewatit MP-64 was studied for a period of 120 min (Table 3). The FA concentration and resin dose were chosen to be 0.1 mol/L and 20 g/L, respectively. Several researchers showed that the influences of the temperature and pH on the adsorption rate of CAs were not significant.7,21,24 Therefore, the kinetic studies were investigated at the temperature (298 K) and pH value (pH = 2.39) where the highest separation efficiency was obtained in this study. Figure 2 shows that the adsorption of FA on the AERs was extremely quick at the initial phase of the contact. This is expected because all of the surface sites were initially available for the uptake of the solute molecules. However, after a certain time, reaching the remaining free surface sites was more difficult for FA molecules because of repelling forces between the solute molecules on the resin. Hence, the process slowed down with time and finally reached the equilibrium. The trend is similar to those previously reported in the literature.2,7,13,23−25,28,33 Kinetic data showed that 60 min was sufficient to reach the equilibrium for the system (Figure 2).

Figure 2. Influence of the contact time on the separation of FA using Amberlite IRA-96 and Lewatit MP-64.

This is a shorter period compared to that previously reported by Uslu (2009) for the system which includes FA and Amberlite IRA-67, as it reached the equilibrium by 90 min.2 Several kinetic models such as pseudo-first-order (PFO), pseudo-second-order (PSO), Elovich, and intraparticle diffusion (ID) models were utilized during the analysis of the kinetic data for the adsorption of FA onto Amberlite IRA-96 and Lewatit MP-64. The properties of these models are given in the related literature in detail.34−37 Equations of the models (eqs 6−9) are given below Elovich

dqt dt

= α ·exp( −β ·qt )

(6)

Pseudo‐first order (PFO) dqt /dt = k1·(qe − qt )

(7) 2

Pseudo‐second order (PSO) dqt /dt = k 2·(qe − qt )

(8)

Intraparticle diffusion (ID) qt = k id·t 1/2 + I

(9)

where qt and qe are the amount of FA adsorbed per gram resin at time t and equilibrium, respectively, t is the contact time, α is the Elovich initial adsorption rate, and β is the Elovich desorption constant. Moreover, k1 is the PFO kinetic model rate constant, k2 is the PSO kinetic model rate constant, kid is the ID model rate constant, and I is the boundary layer diffusion effects. Appropriate linearizations of these equations and their graphs provide linear relationships between the parameters of the models and the determination coefficient (R2) values, which represent the appropriateness of the model for the data.24,25,27,31,37 D



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The constants of the kinetic models and the R2 values are listed in Table 4. As can be seen in the table, the highest R2 Table 4. Constants and R2 Values of the Kinetic Models for the Separation of FA Using Amberlite IRA-67 and Lewatit MP-64 kinetic models Elovich Model

PFO model

PSO model

intraparticle diffusion (ID) model

constants

Amberlite IRA-96

Lewatit MP-64

α β R2 qe k1 R2 qe k2 R2 kid

7.790 14.147 0.978 44.29 0.045 0.968 126.58 3.2 × 10−3 0.998 7.088

60.350 12.735 0.978 50.45 0.054 0.938 142.85 2.9 × 10−3 0.997 6.667

I R2

73.080 0.901

89.164 0.984

Figure 3. Effect of temperature on the adsorption of FA using (A) Amberlite IRA-96 and (B) Lewatit MP-64.

for both AERs studied. The trend indicated the exothermic nature of the process and is consistent with the several studies previously published in the literature.2,7,19,24 González et al. (2006) mentioned that lactic acid recovery efficiencies from fermentation broth at 298 K were slightly higher than those at 313 K; however, selectivity values at 313 K were higher than those at 298 K.19 If we compared the two resins, the temperature effect was clearer for Amberlite IRA-96 compared to Lewatit MP-64. Especially at low FA concentrations, the efficiency obtained at 298 K was significantly higher than that obtained at 308 and 318 K with Amberlite IRA-96. In other words, the extent of the decrease in the efficiency with the temperature was higher with Amberlite IRA-96. Moreover, for all temperature levels studied, the efficiency consistently reduced with the increase in initial acid concentration. Thermodynamic parameters such as Gibbs free energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°) were calculated using the thermodynamic data and the following equations (eqs 10−12)

values were obtained with the PSO kinetic model for both resins. The R2 values with PSO kinetic model were 0.998 and 0.997 for Amberlite IRA-96 and Lewatit MP-64, respectively, whereas they were 0.968 and 0.938 with the PFO kinetic model. The R2 values belonging to the PFO model might not be so low; however, the qe values with this model were much lower than those obtained at the experimental studies were. Thus, it can be said that the PFO model is not appropriate to explain the mechanism of FA adsorption onto Amberlite IRA96 and Lewatit MP-64. On the other hand, the results showed that the PSO model was fitted to the kinetic data obtained in this study very well. Besides the relatively high R2 values, the calculated qe values were very close to the experimental results, which supports the conclusion (126.58 mg/g vs 125.35 mg/g for Amberlite IRA-96 and 142.85 mg/g vs 139.15 mg/g for Lewatit MP-64). The results also indicated that chemical and physicochemical interactions might occur between FA and the resins during the process.25 Moreover, the results denoted that k2 value might be influenced by some other process parameters, which were not investigated in this study.36,37 Another outcome could be the probable compatibility of the data with the Langmuir isotherm model.24,33 Zhang and Yang (2016) mentioned that the boundary layer of the AER reduces with the decrease of external mass-transfer coefficient in a well-stirred system and consequently, ID may become the rate-limiting step.25,38 In the present study, ID can be one of the significant steps in the process because of the relatively high R2 values, especially for Lewatit MP-64. However, it can be deduced from the Table 4 that ID was not the only rate-limiting step because the linear relationship did not pass through the origin and thickness of the boundary layer was different from zero.25 3.3. Effect of Temperature and Thermodynamics. Figure 3 shows the effect of temperature on the separation of FA from aqueous solutions using Amberlite IRA-96 and Lewatit MP-64 at the values of 298, 308, and 318 K. The resin dosage was chosen to be 20 g/L as done at pH and kinetic studies. Experiments were carried out with aqueous solutions having FA concentrations that changed between 0.05 and 2.0 mol/L (Table 5). Figure 3 shows that separation efficiency or adsorption capacity decreased with the increase in temperature

ΔG° = ΔH ° − T ΔS°

(10)

ΔG° = −R ·T ·ln KL

(11)

ΔH ° ΔS° + (12) R· T R where R is the gas constant, T is the temperature, and KL is the Langmuir equilibrium constant. Using the equations, relevant data and the plot of log KL versus 1/T, thermodynamic parameters, and the KL values were calculated and presented in Table 6. As can be seen, KL decreased with the increase in temperature for both resins. The negative ΔG° values revealed the spontaneous behavior of the FA separation processes with Amberlite IRA-96 and Lewatit MP-64. Moreover, the extents of the ΔG° values were in the range of 0−20 kJ/mol, which showed that the process might occur due to the interactions of physical forces. The ΔH° values were calculated to be −10.432 and −26.190 kJ/mol for Amberlite IRA-96 and Lewatit MP-64, respectively. The negative values of ΔH° exhibited the exothermic nature of the adsorption process with these resins in this temperature range. In addition, negative ΔS° values indicated the decreased randomness at the interface between the resin and solution during the sorption of FA. 3.4. Effects of Acid Concentration and Resin Dose. The isotherm curves obtained with the AERs studied at the initial dosages of 10 and 50 g/L are exhibited in Figure 4. The plots drawn for both Lewatit MP-64 and Amberlite IRA-96 showed type 1 adsorption isotherm. The figure described a monolayer sorption and could be directly related to Langmuir ln KL = −

E



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Table 5. Thermodynamic Data for the Adsorption of FA onto Amberlite IRA-96 and Lewatit MP-64 Amberlite IRA-96

Lewatit MP-64

resin amount (g)

T (K)

C0 (mol/L)

Ce (mol/L)

qe (mg/g)

sep. eff. (%)

std. dev. (%)

Ce (mol/L)

qe (mg/g)

sep. eff. (%)

std. dev. (%)

0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

298 308 318 298 308 318 298 308 318 298 308 318 298 308 318

2.000 2.000 2.000 1.100 1.100 1.100 0.510 0.510 0.510 0.250 0.250 0.250 0.050 0.050 0.050

1.921 1.964 1.972 1.030 1.066 1.074 0.450 0.480 0.489 0.195 0.228 0.235 0.022 0.031 0.036

182.74 82.57 63.71 160.66 78.09 60.38 137.89 69.58 48.88 126.16 50.14 34.73 64.29 43.70 33.35

3.98 1.80 1.39 6.35 3.09 2.39 11.76 5.93 4.17 21.94 8.72 6.04 55.90 38.00 29.00

0.49 0.33 0.98 0.49 0.49 0.49 0.16 0.49 0.49 0.49 0.65 0.33 0.16 0.16 0.33

1.8501 1.8542 1.8693 0.9902 1.0033 1.0158 0.4104 0.4303 0.4438 0.1651 0.1912 0.1952 0.0196 0.0225 0.0245

344.77 335.34 300.70 252.54 222.53 193.77 229.20 183.41 152.26 195.39 135.27 126.04 69.92 63.37 58.79

7.50 7.29 6.54 9.98 8.80 7.66 19.54 15.64 12.98 33.98 23.53 21.92 60.80 55.10 51.12

0.33 0.65 0.46 0.65 0.16 0.50 1.14 0.51 0.33 0.16 0.37 0.65 0.33 0.16 0.13

with the AER dose when the acid concentration was kept constant. The increased number of available sites for the separation process was the most probable reason for this trend. These tendencies are consistent with the reports previously published.2,7,27 At the lowest concentration level (0.05 mol/L), increasing the dose from 10 to 50 g/L increased the acid removal efficacy from 41% (94.3 mg/g) to 74% (34 mg/g) for Amberlite IRA96 and 54% (124.2 mg/g) to 92.7% (42.6 mg/g) for Lewatit MP-64. The values were from 2.5% (230 mg/g) to 8.5% (156.4 mg/g) for Amberlite IRA-96 while 4.8% (442.75 mg/g) to 16% (294.4 mg/g) for Lewatit MP-64 when the initial FA concentration was 2.0 mol/L. At all resin dosages and initial concentration levels investigated, the efficiencies with Lewatit MP-64 were higher than those obtained with Amberlite IRA96. The extent of the superiority varied with the initial acid concentration. At the lowest FA concentration studied (0.05 mol/L), the efficiencies with Lewatit MP-64 were only 20− 31% higher than those with Amberlite IRA-96. About 25% increase was observed when the initial FA amount was 0.25 mol/L at the highest three dosages. However, at the other two doses and at 0.5 and 1.0 mol/L concentration levels, the difference in the efficiency values changed between 40 and 67%. At 2.0 mol/L level, Lewatit MP-64 almost doubled the Amberlite IRA-96 for FA separation from aqueous solutions. Maximum adsorption capacity values with Amberlite IRA-96 and Lewatit MP-64 were obtained at 50 g/L dosage and 0.05 mol/L FA concentration level to be 230 and 442.75 mg/g, respectively. The value reached with Amberlite IRA-96 was very close to the highest value reported in the literature so far, which was by Uslu (2009) with Amberlite IRA-67 (239 mg/g). However, Lewatit MP-64 almost doubled these capacity values. This might partially be due to the difference in the relative basicity of the resins caused by the matrix type on the sorption capacity.8,22,26 Even though they are both styrene-based matrices, Lewatit MP-64 has a cross-linked polystyrene matrix, while Amberlite IRA-96 has a styrene DVB copolymer matrix. On the other hand, Amberlite IRA-67 is a gel-type AER and made of a polyacrylic matrix. Besides, another reason could be the synergy obtained by containing two different types of functional groups (quaternary and tertiary amine functional groups) on the Lewatit MP-64 surface. This might positively affect the interactions between the solute and resin functional

Table 6. Thermodynamic Parameters for the Separation of FA Using Amberlite IRA-96 and Lewatit MP-64 resin name

T (K)

KL

ΔG°

ΔH°

ΔS°

Amberlite IRA-96

298 308 318 298 308 318

8.67 7.70 6.70 5.60 3.11 2.80

−5.397 −5.227 −5.029 −4.268 −2.905 −2.722

−10.432

−16.94

−26.190

−74.36

Lewatit MP-64

Figure 4. Adsorption isotherms for the separation of FA using Amberlite IRA-96 and Lewatit MP-64.

adsorption isotherm. The trend also gave information on the microporous structure of the resin pores. The effects of the AER dose and FA concentration on the process efficiency were studied at 298 K for 120 min, and the initial pH values of the aqueous phases were not adjusted. The impact of the resin dosage on the separation of FA from aqueous solutions was investigated between 10 and 50 g/L levels. Initial FA concentrations of the solutions were changed between 0.05 and 2.0 mol/L (Table 7). Figure 5 shows the influences of these two parameters on the adsorption efficiency. As can be seen in the figure, the trends obtained with the AERs were similar. At a constant resin dosage, the FA concentration negatively affected the sorption efficiency for both resins studied. The effect was clearly seen at all resin dosages and FA concentration levels. This was most probably due to the saturation of the available sites on the resin surface for the operation. As expected, adsorption efficiency increased F



Article

Table 7. Equilibrium Data for the Adsorption of FA onto Amberlite IRA-96 and Lewatit MP-64 Amberlite IRA-96

Lewatit MP-64

resin amount (g)

C0 (mol/L)

Ce (mol/L)

qe (mg/g)

sep. eff. (%)

std. dev. (%)

Ce (mol/L)

qe (mg/g)

sep. eff. (%)

std. dev. (%)

0.1 0.2 0.3 0.4 0.5 0.1 0.2 0.3 0.4 0.5 0.1 0.2 0.3 0.4 0.5 0.1 0.2 0.3 0.4 0.5 0.1 0.2 0.3 0.4 0.5

2.000 2.000 2.000 2.000 2.000 1.100 1.100 1.100 1.100 1.100 0.510 0.510 0.510 0.510 0.510 0.250 0.250 0.250 0.250 0.250 0.050 0.050 0.050 0.050 0.050

1.950 1.920 1.891 1.861 1.831 1.060 1.031 1.001 0.970 0.940 0.481 0.451 0.430 0.410 0.380 0.220 0.200 0.170 0.150 0.131 0.030 0.022 0.021 0.019 0.004

229.77 183.66 167.90 160.37 155.85 182.85 159.97 152.34 149.28 147.02 134.09 136.97 122.28 114.54 119.33 137.08 114.89 122.44 114.83 109.76 91.08 63.71 44.85 35.61 42.32

2.50 4.00 5.48 6.98 8.48 3.62 6.33 9.04 11.80 14.53 5.72 11.68 15.64 19.53 25.43 11.92 19.98 31.94 39.94 47.72 39.60 55.40 58.50 61.92 92.00

0.33 0.16 0.43 0.57 0.39 0.33 0.49 0.33 0.16 0.26 0.98 0.16 0.33 0.33 0.26 0.65 0.16 0.33 0.08 0.13 0.65 0.33 0.11 0.05 0.00

1.904 1.850 1.791 1.741 1.680 1.010 0.991 0.960 0.911 0.871 0.461 0.410 0.380 0.340 0.300 0.200 0.170 0.150 0.131 0.10 0.023 0.020 0.014 0.010 0.004

442.75 344.31 321.08 298.49 294.22 412.39 251.74 214.21 217.76 211.05 227.24 229.20 198.95 195.45 192.97 229.77 183.66 153.11 137.20 137.68 123.51 69.00 55.20 45.83 42.28

4.82 7.49 10.48 12.98 16.00 8.15 9.95 12.70 17.22 20.86 9.69 19.54 25.44 33.32 41.13 19.98 31.94 39.94 47.72 59.86 53.70 60.00 72.00 79.70 91.90

0.33 0.65 0.65 0.41 0.13 0.98 0.49 0.43 0.57 0.78 0.65 0.16 0.11 0.24 0.33 0.33 0.49 0.11 0.16 0.07 0.33 0.00 0.00 0.24 0.07

solute molecules on a heterogeneous adsorbent surface.41 In addition, Temkin isotherm model was applied to the equilibrium data. It is generally used for the systems with heterogeneous surfaces and nonuniform adsorption heat distributions. It assumes that the heat of sorption of the solute molecules reduces linearly with the coverage of the surface due to the resin-solute interactions.27,42 Equations of the isotherm models (eqs 13−15) are given below Langmuir qe = qmax ·

KL ·Ce 1 + KL ·Ce

Freundlich qe = KF·[Ce]1/ n

Figure 5. Variation of separation efficiency with the resin dose for various initial FA concentration levels: (A) Amberlite IRA-96 and (B) Lewatit MP-64.

Temkin qe =

R·T · ln(KT·Ce) B

(13) (14) (15)

where qe is the weight of the FA adsorbed per unit weight of resin at equilibrium, qmax is the maximum adsorption capacity, KL is the Langmuir equilibrium constant, and Ce is the FA concentration in the bulk solution at equilibrium. KF is the Freundlich adsorption capacity, while n is the Freundlich adsorption intensity. Finally, B and KT are the Temkin equilibrium-binding constant and Temkin constant related to the adsorption heat, respectively. Another characteristic of Langmuir isotherm model is the dimensionless separation factor (RL) that gives information on the favorability of the process (eq 16). The process is said to be favorable if the RL value is between 0 and 1.7

groups. Further experimental and theoretical studies might be required by using other parameters such as porosity, pore size, agitation speed, and so forth to better understand the behavior of the resin and causes of the higher efficiency obtained with Lewatit MP-64. 3.5. Isotherm Models. Langmuir, Freundlich, and Temkin isotherm models were used to understand the mechanism of the adsorption process.39−42 Langmuir isotherm model assumes a monolayer adsorption on the homogenous resin surface and negligible interactions between the adsorbed solute molecules. According to the model, only one molecule can adsorb onto a specific site and unlike the Freundlich isotherm model and only one adsorption event can occur on this available site. The model supposes that the energy between the adsorbate and adsorbent is uniform at each adsorption site.7,23,28,39,40 On the other hand, the Freundlich isotherm model assumes a multilayer and nonuniform adsorption of the

RL = 1/(1 + (KL ·C0))

(16)

Likewise done at kinetics, appropriate linearizations of these isotherm equations provided the linear plots of the models belonging to the equilibrium data (Figures 6−8). The intercept G



Article

Table 8. Isotherm Model Constants for the Adsorption of FA on Amberlite IRA96 and Lewatit MP-64 at 298 K isotherm model

constants

Amberlite IRA-96

Lewatit MP-64

Langmuir

qmax KL R2 RL n Kf R2 B KT R2

192.31 8.67 0.994 0.05−0.70 3.55 165.88 0.942 29.47 15.14 0.986

357.14 5.6 0.966 0.08−0.78 3.00 289.07 0.949 55.23 14.00 0.956

Freundlich

Temkin

consistent with many reports on the adsorption of FA or other CAs using different types of AERs in the literature.2,7,21,23,25,27,28 Differently, relatively high R2 values, which were close to those reached with Langmuir isotherm, were obtained with Temkin and Freundlich isotherms for Lewatit MP-64 and with Temkin isotherm for Amberlite IRA-96 in this study. According to the Langmuir model, adsorption energy is same at all adsorption sites while Temkin suggested that the heat of adsorption decreases during the coverage of the solute on the surface. Moreover, as previously mentioned, Freundlich proposed a multilayer adsorption on a heterogeneous surface while Langmuir assumed a monolayer sorption on a homogenous surface. These results showed that further studies are needed to understand and clarify the mechanism. Hence, plans on the new experimental and theoretical studies are ongoing.

Figure 6. Langmuir isotherm for the adsorption of FA at different temperatures with (A) Amberlite IRA-96 and (B) Lewatit MP-64.

4. CONCLUSIONS A comparative study was performed for the separation of FA from aqueous solutions. Two macroporous basic AERs were compared for the purpose. Amberlite IRA-96 has tertiary amine functional groups and was produced by Rohm & Haas. On the other hand, Lewatit MP-64 has both quaternary and tertiary functional groups and was manufactured by Lanxess. Matrices of both AERs include styrene monomers, as Lewatit MP-64 has a cross-linked polystyrene matrix, whereas Amberlite IRA-96 has a styrene DVB copolymer matrix. The pH of the aqueous solution had a significant influence on the sorption capacity with both resins. Highest separation efficiency was obtained at the natural pH value of the FA solutions (pH ≈ 2.39) for both AER studied. Thus, all experiments except for pH studies were carried out without any pH adjustment. Equilibration times for both systems were same and about 60 min, again for both resins. The PSO was the most appropriate kinetic model to interpret the kinetic data because it provided the highest R2 values as 0.998 and 0.997 for Amberlite IRA-96 and Lewatit MP-64, respectively. Thermodynamic results showed that temperature decreased the separation efficiency and the effect was clearer with Amberlite IRA-96 compared with Lewatit MP-64. The negative ΔG° and ΔH° values indicated that the adsorption of FA with these AERs were spontaneous and exothermic. Moreover, calculated ΔS° values were also negative, which revealed that randomness at the interface decreased during the separation process. Highest FA separation efficiencies with Amberlite IRA-96 and Lewatit MP-64 were 74 and 92.7%, whereas maximum adsorption capacities were 230 and 442.75 mg/g, respectively. The results showed that Lewatit MP-64

Figure 7. Freundlich isotherm for the adsorption of FA using Amberlite IRA-96 and Lewatit MP-64 at 298 K.

Figure 8. Temkin isotherm for the adsorption of FA using Amberlite IRA-96 and Lewatit MP-64 at 298 K.

and slope of these graphs gave model constants and R2 values for each isotherm model and AER. All of the model parameters and R2 values are listed in Table 8. According to this, the highest R2 values (0.994 and 0.966) were obtained with the Langmuir isotherm for both AER types studied, which means that the experimental data were well described with this model. All RL values were found to be between 0 and 1, showing that the separation of FA using Amberlite IRA-96 and Lewatit MP64 were favorable. The results obtained by isotherm studies are H



Article

qmax

was more efficient compared to Amberlite IRA-96 in the range of the parameters investigated. The highest adsorption capacity value obtained with Lewatit MP-64 (442.75 mg/g) was significantly higher than those previously reported in the literature for FA separation. The synergistic effect of having both quaternary and tertiary amine functional groups on the surface might be the cause of high efficiency as well as the matrix type. The results demonstrated that the data were well fitted by Langmuir isotherm model. In addition, the dimensionless separation factors were found to be in between 0 and 1, showing that the process was favorable. However, relatively high R2 values were obtained with Temkin and Freundlich isotherms for Lewatit MP-64 and with Temkin for Amberlite IRA-96. This necessitated additional experimental and theoretical studies to be carried out to understand the mechanism better.

■

R RL R2 t T V α β [] ΔG° ΔH° ΔS°

■

maximum amount of FA adsorbed per gram resin (mg/g) gas constant (8.314 J/mol/K) dimensionless separation factor determination coefficient time (minute) temperature (K or °C) volume of the aqueous solution (L) Elovich initial adsorption rate (mg/g/min) Elovich desorption constant (g/mg) concentration of the species specified, (mol/L) change in Gibbs free energy, kJ/mol change in enthalpy, kJ/mol change in entropy, kJ/mol/K

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Phone: +90-507-148-9978, +90-332-223-2304. Fax: +90-332241-0635. ORCID

Mustafa Esen Marti: 0000-0002-9829-0602 Funding

The authors wish to acknowledge Konya Technical University for the funding through the Scientific Research Projects (BAP) Coordination Unit under grant numbers, 14401102, 16201042, and 16201115. Notes

The authors declare no competing financial interest.

■

NOMENCLATURE AND ABBREVIATIONS AER anion-exchange resin B Temkin equilibrium-binding constant (J/mol) C0 initial concentration of acid (mol/L) Ce equilibrium concentration of acid (mol/L) Ct concentration of acid at time t (mol/L) F− dissociated formic acid FA formic acid HF undissociated formic acid HCl hydrochloric acid HCOOH formic acid I boundary layer diffusion effects (external film resistance) (mg/g) k1 pseudo-first-order rate constant (min−1) k2 pseudo-second-order rate constant (g/mg/min) kid intraparticle diffusion rate constant (mg/g/min−0.5) Ka acid dissociation constant, (m3/kmol) KF Freundlich adsorption capacity (L/mg) KL Langmuir equilibrium constant (L/mg) KT Temkin constant related to the adsorption heat (L/ mg) m mass of anion-exchange resin (g) M molecular weight of formic acid (g/mol) n Freundlich adsorption intensity NaOH sodium hydroxide qe amount of FA adsorbed per gram resin at equilibrium (mg/g) qt amount of FA adsorbed per gram resin at time t (mg/g) I



Article

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