Synthesis of Layered Double Hydroxides Containing a Biodegradable

Jan 1, 2015 - In this work, we report ultrasonic-assisted synthesis of layered double hydroxides (LDHs) containing biodegradable amino acid derivative...
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Synthesis of Layered Double Hydroxides Containing a Biodegradable Amino Acid Derivative and Their Application for Effective Removal of Cyanide from Industrial Wastes Kiomars Zargoosh,* Sara Kondori, Mohammad Dinari, and Shadpour Mallakpour Department of Chemistry, Isfahan University of Technology, Isfahan, 84156-83111 Isfahan, Iran S Supporting Information *

ABSTRACT: In this work, we report ultrasonic-assisted synthesis of layered double hydroxides (LDHs) containing biodegradable amino acid derivative, N,N′-(pyromellitoyl)-bis-L-isoleucine diacid, and their application for removal of cyanide ions from industrial wastes. The structure of the modified Mg−Al LDHs was studied using transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM), FT-IR spectroscopy, and X-ray diffraction (XRD) techniques. The ability of the prepared LDHs for adsorption of cyanide ions from aqueous solutions was examined, and the effects of the different parameters were investigated. It was found that, under optimum conditions, the proposed LDHs adsorbent can remove 98.7% of cyanide content of the aqueous solution in less than 35 min. Compared to the recently reported methods for removal of cyanide; the proposed method has several advantages. First, the adsorbent particles (LDHs) are synthesized via an environmentally acceptable route; because water is used as solvent under ultrasonic irradiation. Second, cyanide ions are exchanged with biodegradable anions, thus the proposed method does not produce toxic waste. Third, the prepared adsorbent has high adsorption capacity and the fast adsorption kinetics, thus it can be used for lowering the concentration of cyanide in the wastes to below regulatory limits in practically acceptable times. particles,13 and adsorption onto powdered pistachio hull14 have been proposed by researchers for removal of cyanide from industrial wastes. However, some of the proposed methods have serious drawbacks that limited their applicability for treatment of the real wastes. For example, most of the proposed microorganisms need nutritionally balanced wastewaters and cannot be used for removal of cyanide from samples such as cassava wastewater with low nitrogen (N) and high chemical oxygen demand (COD) content.10 In addition, wastes containing high concentration of cyanide are toxic to the microorganisms, thus most microorganisms can be used for treatment of wastes with limited cyanide content.8 Some of the proposed methods are reagent intensive (e.g., 10 g L−1).12 The slow kinetics of the reported methods together with their inability for lowering the concentration of cyanide to below regulatory limits are other major problems. For example a method that uses azotobactor vinelandii bacterium for cyanide detoxification needs 66 h to reduce the cyanide content of the cassava wastewater from 50 to 22 mg L−1.10 Due to high concentration of cyanide in the treated waste (22 mg L−1), it cannot be discharged to water resources, because the World Health Organization (WHO) has proposed a guideline value of 0.07 mg L−1 for drinking water.15,16 Based on the above-mentioned facts, it is relevant to explore and develop rapid and effective methods for removal of cyanide from industrial effluents. In the present work, we report ultrasonic-assisted synthesis of modified layered double

1. INTRODUCTION Cyanide anion (CN−) is known as a primary toxic agent with both acute and chronic toxicity. When cyanide salts such as potassium and sodium cyanide are ingested, free cyanide ion can rapidly bind hydrogen ion to form hydrogen cyanide in the acidic medium of the stomach. Essentially all cyanide ingested as cyanide salts will form hydrogen cyanide and will be quickly absorbed.1 Although cyanide ion has adverse chronic effects on several organs of the human body, high acute toxicity of the cyanide ion (or hydrogen cyanide) is due to its primary toxic effect of inhibiting cytochrome c oxidase, the terminal enzyme of the mitochondrial electron transport chain. Due to inhibition of cytochrome c oxidase, the cells of the tissues are unable to use oxygen, and, hence, a state of histotoxic anoxia occurs.2 Due to cessation of aerobic metabolism, oxygen-sensitive organs such as the central nervous system and the heart are primary targets for cyanide toxicity. Based on case report studies, the following acute median lethal exposure levels for humans were estimated: an LC50 of 524 ppm for a 10 min inhalation exposure to hydrogen cyanide, an LD50 of 1.52 mg/kg for the oral route, and an LD50 of 100 mg/kg for the dermal route, assuming that CN− is readily released from the compound.3 On the other hand, cyanide has widespread applications in different industrial processes such as electroplating, casehardening of metals, extraction of gold, coal gasification, base metal flotation, fumigation of ships, and production of organic chemicals.4−7 Therefore, cyanide must be removed from industrial wastes before being discharged into the environment. In recent years, different methods such as biological treatment systems,8 plasma discharge technology,9 degradation by mesophilic microorganisms,10,11 oxidation with hydrogen peroxide,12 photocatalytic removal using TiO2−SiO2 nano© XXXX American Chemical Society

Received: October 14, 2014 Revised: December 25, 2014 Accepted: January 1, 2015

A

DOI: 10.1021/ie504064k Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 1. Synthesis of N,N′-(Pyromellitoyl)-bis-L-isoleucine Diacid via Reaction of PMDA and L-Isoleoucine

copy (FE-SEM) S-4160 Hitachi (Japan) was used to investigate the morphology and size distribution of the prepared LDHs. Low temperature N2-adsorption measurements were recorded on a Quantachrome Autosorb-1 system. Samples pretreated by heating at 353 K under vacuum for 2 h were used to determine the surface areas at 77 K using N2 as the adsorbate and He as the carrier gas. 2.3. Synthesis of Mg−Al LDHs Containing Biodegradable Amino Acid Derivative. In the first step, Mg−Al LDHs containing CO32− anion was prepared using an ultrasonic assisted method reported in our previous work.17 As depicted in Scheme 1, in the second step, potentially biodegradable N,N′-(pyromellitoyl)-bis-L-isoleucine diacid (3) was synthesized via reaction of PMDA (1) and L-isoleoucine (2) according to our previous study.18 In the third step, modification of the synthesized Mg−Al LDHs with the above diacid was carried out by the ion exchange method under N2 atmosphere as follows: A mixture of 1.0 g of MgAl-CO3 LDHs powder and 50 mL of N,N′(pyromellitoyl)-bis-L-isoleucine diacid aqueous solution (0.2 M) was vigorous stirred at 80 °C for 24 h under N2 atmosphere. This solution was added dropwise to an alkaline solution (100 mL aqueous solution containing 2.0 g of NaOH). The resulting white precipitate was aged for 2 h at 65 °C, and then it was sonicated for 2 h under N2 atmosphere. Finally it was filtered until all of the supernatant liquids were removed. The obtained precipitate was washed several times with large amounts of distilled water and then dried at 70 °C in a vacuum oven to giving the diacid-modified LDHs. Compositional information on the pristine LDHs and modified LDHs were obtained using inductively coupled plasma emission spectroscopy (ICP), spectrophotometric titration (see section 3.3.6), and elemental analysis. The chemical formula of the pristine LDHs and modified LDHs were found to be Mg2.01Al0.98(OH)2(CO3)0.54·1.7H2O and Mg1.98Al0.97(OH)2(C22H22N2O82−)1.08·1.05H2O, respectively. 2.4. Determination of Cyanide. A previously reported method with some modifications was used for determination of cyanide in aqueous solution.19 Nickel (Ni2+) ions react with 2(5-bromo-2-pyridylazo)-5-(diethylamino)phenol (5-BrPADAP) to form a water-soluble red complex (Ni-5-BrPADAP). Addition of cyanide ions to the solution containing constant concentration of Ni-5-Br-PADAP inhibits the red color of the solution, because cyanide ions react with Ni2+ ions to form a colorless Ni(CN)42− complex. The overall reaction can be concluded as below.19

hydroxides (LDHs) containing biodegradable anion and their application for removal of cyanide ions from industrial wastes. Compared to the recently reported adsorbents for removal of cyanide, the proposed method has several advantages. First, the adsorbent particles (modified LDHs) are synthesized via an environmentally acceptable route; because water is used as solvent and consumed chemicals are not toxic for environment. Second, cyanide ions are exchanged with biodegradable anions, thus the proposed method does not produce toxic waste. Third, the interaction between LDHs particles and cyanide ions is fast; therefore, cyanide ions can be removed from aqueous solutions in practically acceptable times. Fourth, the prepared adsorbent has high adsorption capacity, thus it can be used for lowering the concentration of cyanide in the wastes to below regulatory limits.

2. EXPERIMENTAL SECTION 2.1. Reagents and Solutions. All chemicals and reagents were analytical grade. Hydrochloric acid, aluminum nitrate, magnesium nitrate, sodium cyanide, sodium hydroxide, nickel chloride, sodium tetraborate decahydrate, ammonium acetate, sodium thiocyanate, sodium carbonate monohydrate, disodium hydrogen phosphate, sodium fluoride, sodium bromide, methanol and 2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol (5-Br-PADAP) were purchased from Sigma-Aldrich. Mg(NO3)2·6H2O, Al(NO3)3·9H2O, Na2CO3, L-isoleucine, and pyromellitic dianhydride (PMDA) were purchased from Merck Chemical Co. Doubly distilled water was used throughout. 2.2. Apparatus. All absorbance measurements were carried out on a Scinco UV−vis 2100 spectrophotometer (UK). A Jenway (USA) model 3020 pH meter with a combined glass electrode was used after calibration against standard Merck buffers for pH determinations. A totally glass Fisons (UK) double distiller was used for preparation of doubly distilled water. Fourier transform infrared (FT-IR) spectra for pristine LDHs and modified LDHs were recorded for wave numbers 4000−400 cm−1 using a Jasco-680 FT-IR spectrometer (Japan). X-ray diffraction (XRD) measurements were carried using a Siemens D-5000 X-ray diffractometer (Germany) with Cu Kα radiation. Compositional analysis was performed by inductively coupled plasma emission spectroscopy (ICP) using a Shimadzu ICPS-7500 instrument. C, H, and N microanalysis was carried out Vario EL elemental analyzer. The reaction occurred on a MISONIX ultrasonic liquid processor, XL-2000 SERIES. Ultrasonic irradiation was carried out with the horn probe of the ultrasonic immersed directly in the solution mixture with frequency 2.25 × 104 Hz and power 100 W. A transmission electron microscopy (TEM) analyzer, Philips CM 120, was used for investigating the morphology and size distribution of the prepared LDHs. Field-emission scanning electron micros-

Ni‐5‐Br‐PADAP + 4CN−→ Ni(CN)4 2 − + 5‐Br‐PADAP red B

colorless DOI: 10.1021/ie504064k Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. Scheme for the preparation of diacid modified LDHs and a possible mechanism for adsorption of CN− ions on it.

molecules, thus LDHs can be used as anion exchangers for removal of different organic and inorganic pollutants.20−22 In this study L-isoleucine as a natural amino acid was used as building blocks for efficient synthesis of novel potentially biodegradable and chiral nanohybrid material of LDHs/ dicarboxylate via a simple ion exchange reaction. At first, the optically active diacid monomer was synthesized starting from PMDA and L-isoleucine as shown in Scheme 1.17 In vitro toxicity and biodegradability behavior of the synthetic diacid, which was investigated in culture media in our previous study,23,24 showed that the synthesized compound is biologically active and biodegradable in natural environment. Results showed the fungal growth on diacid cocultivated with Aspergillus and Penicillium spores on Potato Dextrose Agar culture media. Since the synthesized diacid was completely invaded and colonized by fungal mycelium, it seems that this compound does not have any toxicity for saprophytic activity of fungi. It also contains naturally occurring amino acids which makes it structurally close to natural polypeptides. Then, this compound was used for the preparation of modified LDHs. The dicarboxylate ion was considered to be arranged horizontally to the LDHs basal layer. Figure 1 shows the schematic representation of preparation steps of diacid modified LDHs and its ability for adsorption of CN− ions from aqueous solution by anion exchange mechanism.25 Several techniques such as FT-IR spectroscopy, XRD analysis, Brunauer−Emmett−Teller (BET) method, and FESEM and TEM microscopy were used for structural and morphological characterization of the diacide-modified LDHs. The specific surface areas of pristine Mg−Al−CO32− LDHs and diacid modified LDHs were 78.0 and 69.7 m2 g−1, respectively, according to BET measurements. FT-IR spectroscopy was used for confirmation of the intercalation of dicarboxylate anion within the interlamellar region of LDHs. Figure 2 shows the FT-IR spectra of pristine LDHs, N,N′-(pyromellitoyl)-bis-Lisoleucine diacid, and hybrid of LDHs/diacid. For pristine LDHs, the broad band in the range of 3200−3700 cm−1 is due to the O−H stretching vibration of the metal hydroxide layer and interlayer water molecules. A shoulder present around 3000−3100 cm−1 is caused by the interaction between the CO32− and H2O present in the interlayer region, which involves mostly hydrogen bonds.23 The bending vibration of the interlayer H2O is also reflected in the broad bands around 1620 cm−1. Two bands at about 1543 and 1409 cm−1 can be assigned to the asymmetric and symmetric stretching vibration modes of carboxylate groups. The band characteristic to metal− oxygen bond stretching appears below 700 cm−1. The sharp bands around 780, 554 and 440−450 cm−1 are caused by various lattice vibrations associated with metal hydroxide sheets. The FT-IR spectrum of L-isoleucine containing dicarboxylic acid shows a broad and strong peak between 800 and 3500 cm−1, which can be assigned to the COOH groups.

For determination of cyanide concentration in solutions, 4 mL of methanolic solution of 5-Br-PADAP (5 × 10−5 mol L−1), 6 mL of methanol (for adjusting the polarity of solution), 0.3 mL of Ni2+ solution (3 × 10−4 mol L−1), 3 mL of sodium tetraborate buffer solution (% 2 w/v), and appropriate volume of the cyanide solution were sequentially introduced into a 25 mL volumetric flask. Then, the volume was adjusted to 25 mL using doubly distilled water. After 30 min, the absorbance of the final solution was measured at 560 nm against the blank reference. 2.5. Adsorption Measurement. The adsorption characteristics of the diacid-modified LDHs for cyanide ion were studied in batch experiments. To obtain the optimum conditions for removal of cyanide, the effects of pH (8.5−11.5), kinetics time (5−180 min), temperature (298−318 K), and adsorption isotherm (initial concentration 0.1−200 mg L−1) of the cyanide were studied. In addition, the effects of possible interfering anions over a concentration range of 0−0.3 mol L−1 were investigated. Analyzing adsorption behavior of the diacidmodified LDHs involved adding 0.05 g of diacid-modified LDHs to 20 mL of solution of cyanide at different concentrations at room temperature. The pH was maintained at a constant value during adsorption. The equilibrium time was less than 35 min. When the adsorption behavior reached equilibrium, the adsorbent particles were separated by filtration. The concentrations of the cyanide in aqueous phase were determined using the procedure described in section 2.4. Cyanide concentrations adsorbed per unit mass of the LDHs adsorbent (mg cyanide per g dry diacid-modified LDHs) were calculated by using eq 1. Removal efficiencies (%Re) were calculated by using eq 2 qe =

(C 0 − Ce) × v m × 1000

%Re =

(C 0 − Ce) C0

× 100

(1)

(2)

where C0 and Ce are the concentrations (mg L−1) of the cyanide in the aqueous phase before and after the adsorption period, respectively; v is the volume of the aqueous phase (mL); and m is the amount of dry LDHs adsorbent used (g).

3. RESULTS AND DISCUSSION 3.1. Characterization of the Modified LDHs. LDHs are anionic clays with the general formula of [M1−x2+Mx3+(OH)2][Ax/nn‑·yH2O], where M2+, M3+, and An‑ are divalent metal ions, trivalent metal ions, and exchangeable anions with charge n, respectively.20,21 LDHs have brucite (Mg(OH)2)-like lamellar structures with positive structural charge on the metal hydroxide layers.17,22 Positive charges on the LDHs surface are neutralized by exchangeable interlayer anions and water C

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intercalation of diacid anions (Figure 3). To investigate the orientation of the diacid ion in the interlayer space of the modified LDHs the size of the guest diacid was estimated using ChemOffice software and compared to the basal spacing obtained via XRD patterns. Approximate length and width of the diacid molecule were about 0.6 and 1.48 nm, respectively (Figure 3). Based on the XRD data the total interlayer distance (including thickness of one Mg−Al hydroxide sheet and interlayer distance) was 1.65 nm. According to previous reports,17 the thickness of Mg−Al hydroxide sheets is about 0.48 nm. Thus, the net distance between layers of the modified LDHs is 1.17 nm. By comparison of this distance with the dimensions of the guest diacid, two diacid molecules can be horizontally oriented in the interlayer space of the modified LDHs. Figure 4 shows the FE-SEM micrograph of the pristine LDHs (a and b) and diacid modified LDHs (c and d). The FEFigure 2. From top to bottom, FT-IR spectra of pristine LDHs, modified LDHs, and N,N′-(pyromellitoyl)-bis-L-isoleucine diacid.

Absorption bands at 1780 and 1700 cm−1 are characteristic peaks for imide rings and acidic groups, respectively. The absorbance of the CH2 group in the chiral dicarboxylate molecule can be observed at 2930−3100 cm−1. In the FT-IR spectrum of the diacid modified LDHs, some new peaks other than unmodified LDHs were observed. Stretching vibrations of CO imide at 1718 and 1774 cm−1 and the asymmetric and symmetric stretching vibrations of the COO− are observed at 1562 and 1424 cm−1. Absorption peaks of the imide ring confirm the presence of the diacid molecules between LDHs layers. Figure 3 shows the XRD patterns of the pristine Mg−Al LDHs and diacid modified LDHs. Before diacid modification,

Figure 4. FE-SEM images of CO32−/LDHs (a and b) and diacidmodified LDHs (c and d).

SEM image reveals the feature of LDHs particles, which roughly consists of platelike shapes stacked on top of each other with lateral dimensions ranging over a few micrometers and thickness over a few hundred nanometers. Figure 5 shows the TEM images of diacid modified LDHs. The results show that the particles have a hexagonal shape with

Figure 5. TEM micrographs of the diacid-modified LDHs with different magnifications.

Figure 3. XRD patterns of the synthesized Mg−Al LDHs and diacidmodified LDHs and molecule dimensions of the diacid anion.

rounded corners. There are no signs of aggregation visible in the micrographs. 3.2. Calibration Curve for Determination of CN−. A procedure described in section 2.4 was used for determination of cyanide concentration in solutions. Figure 6 shows the typical UV−vis spectra of the proposed system in the presence of different concentrations of cyanide in ammonium acetate buffer solution of pH 8.5. It was found that the absorbance of the system decreases linearly with the concentration of cyanide, at a concentration range of 3.80 × 10−6 to 7.7 × 10−4 mol L−1

the basal spacing of the pristine LDHs was 0.76 nm, which corresponds to the (003) diffraction peak with 2θ = 11.4°. After modification of Mg−Al LDHs with diacid ion, this diffraction peak was shifted to the lower angle. The position of the basal reflections of modified LDHs was shifted to a higher d value, indicating the expansion in the interlayer distance. In the XRD pattern of diacid modified LDHs, the d003 has shifted to 5.26°, corresponding to an interlayer distance of 1.65 nm. This result shows that the basal spacing d003 is expanded by 0.90 nm after D

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Figure 8. Effect of time on the adsorption of CN−; LDHs: 0.05 g; concentration of CN−: 100 mg L−1; volume of test solution: 20 mL; pH: 11; temperature: 298 K. Figure 6. Absorbance spectra of the proposed method in the presence of the varying concentrations of CN− ion. The CN− concentrations are from 3.80 × 10−6 mol L−1 (top) to 7.7 × 10−4 mol L−1 M (bottom) in an ammonium acetate buffer solution of pH 8.5.

ions by the diacid-modified LDHs adsorbent. As is clear, the cyanide ions rapidly reached equilibrium in less than 35 min. Compared with other reported adsorbents, the diacid-modified LDHs shows fast adsorption.26,27 This fast adsorption could be attributed to the larger surface area of the proposed adsorbent or due to its differences with other reported adsorbents in removal mechanism of cyanide. As shown in Figure 1 LDHs adsorbent removes the cyanide ions from solution via an ion exchange mechanism, that is intrinsically faster than other adsorption mechanisms such as chemical bonding and photocatalytic oxidation. Lagergren pseudo-first-order (eq 3) and pseudo-secondorder (eq 4) models were used to investigate the adsorption kinetics of cyanide by the proposed LDHs adsorbent:28

(0.1 to 20 ppm). Beyond this concentration range, there was no linear relation between CN− concentrations and UV−vis absorbance of the system. 3.3. Adsorption Properties of the Diacid-Modified LDHs for CN− Ions. 3.3.1. Effect of pH on the Adsorption Efficiency of CN−. Both target pollutant (CN−) and diacidmodifided LDHs adsorbent have the structural ability to interact with H+ ions, thus, the concentration of H+ ions must be optimized to obtain maximum adsorption capacity for cyanide ions. Figure 7 shows the effects of solution pH on the

ln(qe − qr ) = ln(qe) − k1t

(3)

⎛ ⎞ ⎛1⎞ t 1 ⎟ = ⎜⎜ ⎟⎟t + ⎜⎜ 2⎟ qt ⎝ qe ⎠ ⎝ k 2 × qe ⎠

(4) −1

In these equations qt (mg g ) demonstrates the adsorption at time t (min); qe (mg g−1) shows the adsorption capacity at equilibrium time; k1 (min−1) is the rate constant for the pseudo-first-order model, and k2 (g mg−1 min−1) is the rate constant for the pseudo-second-order model. The kinetic adsorption data were fitted to eqs 3 and 4, and the obtained results are shown in Figure 9 and Table 1. As can be seen from Figure 7. Effect of pH on the adsorption of cyanide, adsorbent: 0.05 g; initial concentration of cyanide: 100 mg L−1; volume of cyanide solution: 20 mL; time: 3 h, temperature: 298 K.

adsorption of cyanide ions by diacid-modifided LDHs adsorbent in the pH range of 8.5−11.5. The pH of the solutions was adjusted by the use of sodium tetraborate buffer. As is clear from Figure 7, the removal efficiency of cyanide increases with increasing pH values of the test solution from 8.5 to about 11. This effect is due to deprotonation of HCN and production of free CN− ions. At pH values higher than 11, the removal efficiency of cyanide remains constant, because all of the cyanide content of the sample presents as free CN−. For adsorption of cyanide by diacid-modified LDHs pH 11 was selected as optimum pH. 3.3.2. Adsorption Kinetics. The rate of the adsorption is one of the most important characteristics of an adsorbent. Figure 8 shows the effects of contact time on the adsorption of cyanide

Figure 9. Fitting of the experimental results in Lagergren pseudo-firstorder model (A) and pseudo-second-order model (B).

Table 1, the correlation coefficient (R) for the second-order kinetic model is about unity (0.9999) which confirms the second-order nature of the adsorption of cyanide ions on the surface of the LDHs adsorbent layers. This observation is in agreement with the ion exchange mechanism proposed in Figure 1 for adsorption of cyanide ions by the modified LDHs. E

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Industrial & Engineering Chemistry Research Table 1. Characteristics of the Applied Kinetics Models for Fitting of the Experimental Results in Figure 8 first-order

a

second-order

R2

qe (mg g−1)

k1

R2

qe (mg g−1)

k2

q (mg g−1)a

0.899

0.985 ± 0.35

0.024

0.9999

40.00 ± 2.04

0.089

39.5 ± 2.1

−1

q (mg g ) = qexperimental.

3.3.3. Adsorption Isotherm of Modified LDHs Adsorbent for Cyanide. Generally, the adsorption capacities of the adsorbents depend on the concentration of the adsorbate in solution. The effects of the cyanide concentration on the adsorption capacity of the proposed adsorbent were studied under batch condition at optimum pH values 11 and temperature 298 K. The mass of the modified LDHs adsorbent was 0.05 g, and the cyanide concentration was changed over the concentration range of 0.1 to 200 mg L−1. The contact time was 35 min. After equilibrium establishment between modified LDHs adsorbent and cyanide ion solution, the remaining concentration of the cyanide ion in the solution was determined using the method described in section 2.4. When the remaining cyanide concentration in the solution was higher than 20 mg L−1, the solution was diluted before determining its cyanide content. Finally, the equilibrium isotherm for the adsorption of the cyanide ion on the modified LDHs adsorbent was analyzed using the Langmuir model (eq 5) and Freundlich model (eq 6). ⎞ ⎛ 1 ⎞ ⎛ Ce 1 ⎟⎟ ⎟⎟Ce + ⎜⎜ = ⎜⎜ qe ⎝ qmax ⎠ ⎝ KL × qmax ⎠

(5)

qe = KF × Ce(1/ n)

(6)

Figure 11. Fitting of the experimental results in Langmuir model (A) and Freundlich model (A).

indicate that adsorption isotherm data for cyanide are clearly in better agreement with the Langmuir model rather than the Freundlich model. Appropriate fitness of the experimental results to the Langmuir model indicates that cyanide ions have been adsorbed as a monolayer onto the adsorbent layers. Again, this observation confirms our proposed mechanism for exchange of cyanide ions with diacid ions in the interlayer space of the LDHs adsorbent. As is clear from Table 2, the maximum adsorption capacity (qm) for cyanide calculated by the Langmuir equation is 71.43 mg g−1. High adsorption capacity of the prepared LDHs adsorbent makes it possible to treat wastes containing a high concentration of cyanide and reduces the concentration of cyanide in them to below regulatory limits. 3.3.4. Effects of Temperature. Generally, adsorbents with practical applications in industrial wastes treatment must have temperature-independent adsorption characteristics, because temperature-sensitive adsorbents need additional equipment for temperature conditioning of the wastes. Figure 12 shows the effects of the solution temperature on the maximum adsorption capacity of the diacid-modified adsorbent for removal of cyanide. As can be seen from Figure 12, the maximum adsorption capacity of the proposed adsorbent is almost constant with only 3% reduction in capacity during changing temperature from 298 to 318 K. This temperature-insensitivity of the proposed adsorbent implies that cyanide containing wastes can be treated by this adsorbent at room temperature without any temperature conditioning process. 3.3.5. Effects of Possible Interfering Ions. Real samples such as environmental waters and industrial wastes include not only target pollutant but also possible interfering species, thus suitable adsorbents must be able to remove the target pollutant in the presence of coexisting interfering species. Figure 13 shows the effects of different possible interfering anions in the concentration range of 5.0 × 10−4 to 2.1 × 10−2 mol L−1 on the adsorption of cyanide by the proposed adsorbent. As can be seen from Figure 13, with increasing concentration of the interfering species from 5.0 × 10−4 to 2.1 × 10−2 mol L−1 the adsorption capacity of the LDHs adsorbent decreases less than 7.5%. However, compared to the other studied ions, F− and SCN− have a higher impact on the adsorption capacity of the prepared LDHs. There are two reasons for this observation, first the ligating ability of these ions is notable, thus, they may

In these equations, qe, Ce, qm, KL, KF, and n are the adsorption capacity of cyanide on the adsorbent (mg g−1) at equilibrium conditions, the equilibrium cyanide concentration in solution (mg L−1), the maximum adsorption capacity of the adsorbent (mg g−1), the Langmuir constant (L mg−1), the Freundlich constant (L mg−1), and the heterogeneity factor, respectively.29 The results are depicted in Figure 10, Figure 11, and Table 2. As is obvious from Figure 10, Figure 11, and Table 2, the equilibrium capacity (qe) of cyanide was increased with increasing the concentration of cyanide in solution. Moreover, correlation coefficients (R) values for Langmuir model are higher than those of the Freundlich model. These results

Figure 10. Equilibrium isotherm of cyanide by diacid-modified LDHs, performed in batch mode; adsorbent: 0.05 g, volume of test solution: 20 mL; initial concentration of CN−: 0.1−200 mg L−1; temperature: 298 K; pH: 11; time: 35 min. F

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Industrial & Engineering Chemistry Research Table 2. Langmuir and Freundlich Isotherm Constants, Correlation Coefficients, and Adsorption Capacities Langmuir

Freundlich

KL (L mg−1)

qm (mg g−1)

R2

KF (mg1‑(1/n) L1/n g−1)

n

R2

4.67 ± 0.23

71.43 ± 3.24

0.990

3.365 ± 0.92

0.7818

0.413

Figure 12. Effects of temperature on the maximum adsorption capacity of the diacid-modified LDHs. Adsorbent: 0.05 g; volume of test solution: 20 mL; initial concentration of CN−: 100 mg L−1; temperature: 298−318 K; pH: 11; time: 35 min.

Figure 13. Effects of possible interfering anions on the adsorption capacity of the diacid-modified LDHs. Adsorbent: 0.05 g; volume of test solution: 20 mL; initial concentration of CN−: 100 mg L−1; temperature: 298 K; pH: 11; time: 35 min; concentration range of the studied ions (5.0 × 10−4 to 2.1 × 10−2 mol L−1).

Figure 14. Absorption spectra of the diacid molecule at different concentration (A) and calibration curve for determination of diacid in solution (B).

be adsorbed via inner-sphere (complexation) interactions with metal ions of the LDHs layers. Second, they can be adsorbed via hydrogen bonding. Other studied anions are weaker in these issues. In all cases the modified LDHs save more than 92.5% of their adsorption capacity, thus, the proposed adsorbent can be used for removal of cyanide from samples containing considerable amounts of possible interfering anions. 3.3.6. Determination of Total Diacid Intercalated between the LDHs Layers. Fortunately, there is a simple method for quantitative determination of the diacid loaded in the interlayer of the modified LDHs. As can be seen in Figure 14 diacid anion has a strong absorption peak in the wavelength range of 250− 370 nm. A calibration curve between diacid concentration and its absorbance at 320 nm (λmax) was obtained. There was a nice linear relation between absorbance and diacid concentration (absorbance = 366 [diacid] + 0.1636). Our previous experience demonstrated that under optimum conditions for adsorption of cyanide, diacid loaded in the interlayer of the LDHs can be quantitatively replaced with cyanide ions. However, to ensure the quantitative replacement, modified LDHs must be treated with an excess amount of cyanide ions under alkaline conditions. To obtain the total

diacid loaded between layers, 0.05 g of the diacid loaded LDHs was treated with 20 mL of cyanide solution containing an excess amount of cyanide (200 mg) for 1 h, and the released diacid concentration in the solution was calculated using the calibration curve. It was found that each g of modified LDHs contains 0.777 ± 0.005 g of diacid ion. After determining the total diacid loaded between the interlayer spaces of the LDHs, it is interesting to determine how much of them can be replaced with cyanide ions. To investigate this issue 0.1 g of the modified LDHs was dispersed in 100 mL of cyanide solution (100 mg L−1) under optimum conditions (pH 11, time 35 min, and temperature 298 K). After 35 min, the concentration of released diacid ions was measured by recording their absorbance at 320 nm. It was found that the obtained solution contains 0.0756 ± 0.004 g of diacid ions. This observation shows that, under optimum conditions, up to 97.2% of the total diacid loaded between the interlayer spaces can be replaced with cyanide ions. 3.3.7. Long-Term Stability of the Modified LDHs. To investigate the long-term stability of the modified LDHs, especially about the diacid ions in the interlayer space, two types of experiments were performed. In the first type of experiments, the maximum adsorption capacity of the fresh G

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Industrial & Engineering Chemistry Research LDHs was compared with the maximum adsorption capacity of the 6 months old LDHs; it was found that the maximum adsorption capacity of the modified LDHs remains almost constant (71 ± 3.3 mg g−1) after 6 months. This observation confirms that the prepared LDHs save their ion exchange ability during 6 months. In the second type of experiments, 0.1 g of fresh LDHs or 6 months old LDHs was added to 100 mL of aqueous solution containing an excess amount of cyanide (400 mg). The mixture was stirred for 1 h. Then, the UV−vis absorbance of the released diacid ions (exchanged by cyanide ions) was recorded in solution against water as blank. The recorded spectra are shown in Figure S1. As can be seen from Figure S1 in the Supporting Information at 320 nm (λmax) the absorbances of the released diacid ions from fresh LDHs or 6 months old LDHs are 0.798 and 0.799, respectively. Again, this observation confirms that the modified LDHs are completely stable. 3.3.8. Effects of Chemical Composition on the Adsorption Capacity of the Proposed LDHs. To investigate the effects of the diacid anion on the removal performance of the prepared LDHs, the adsorption capacity of the pristine LDHs (Mg2-AlCO32−) and modified LDHs (Mg2-Al-diacid) was measured under optimum conditions. The adsorption capacities of the pristine LDHs and modified LDHs were 1.90 and 71.43 mg g−1, respectively. In addition, to study the effects of the Mg:Al molar ratio on the adsorption capacity of the modified LDHs, three types of modified LDHs containing Mg:Al molar ratios of 1:1, 2:1, and 3:1 were prepared, and their adsorption capacities were determined. The adsorption capacities of the modified LDHs containing Mg:Al molar ratios of 1:1, 2:1, and 3:1 were 52.74. 71.43, and 37.25 mg g−1, respectively. Thus, the modified LDHs containing Mg:Al molar ratios of 2:1 was selected for removal of cyanide from solution in all experiments. 3.3.9. Confirmation the Ion Exchange Mechanism of the Cyanide Adsorption. To confirm the ion exchange nature of the cyanide adsorption, 0.4 g of the modified LDHs was placed in a 100 mL volumetric flask, and 100 mL of the cyanide solution (100 mg L−1) was added to the flask (under optimum conditions for removal of cyanide ion). Then, the remaining cyanide concentration in the solution and the released diacid concentration into solution were simultaneously determined by two analysts with two spectrophotometers for 50 min (at 5 min intervals). Cyanide concentration was determined using the method described in section 2.4, and diacid concentration was determined using the method described in section 3.3.6. The results are shown in Figure 15. If the mechanism of the adsorption of cyanide includes the ion-exchange process, it will be logical to expect simultaneous decreasing in cyanide concentration and increasing in diacid concentration in solution. As can be seen from Figure 15 reduction in cyanide concentration is simultaneous with rising in the diacid concentration. This observation directly confirms the ion exchange mechanism proposed for adsorption of cyanide ions by the modified LDHs. Interestingly, it can be seen from Figure 15 that each diacid ion is exchanged with two cyanide ions, confirming the complete deprotonation of carboxyl functional groups of the diacid molecule under optimum conditions for removal of cyanide. 3.4. Removal of CN− from Industrial Wastes. To examine the applicability of the diacid- modified LDHs for removal of CN− ions from real samples, the wastes of the Mozafari Silver Company (Isfahan, Iran) were treated by the LDHs adsorbent. Based on the standard addition method, the

Figure 15. Simultaneous changes in the cyanide concentration and diacid concentration in the solution during cyanide adsorption process. Adsorbent: 0.4 g; volume of test solution: 100 mL; initial concentration of CN−: 100 mg L−1; temperature: 298 K; pH: 11.

initial cyanide concentration of the wastes was found to be 300 mg L−1. The samples were 3 times diluted using a solution of NaOH 10−3 mol L−1. Then 0.05 g of the dry LDHs adsorbent was added to the 20 mL of the diluted waste. After 35 min, the adsorbent was separated, and the remaining cyanide concentration in the solution was determined. The remaining cyanide concentration in the solution was 2.3 mg L−1. This result indicates that diacid-modified LDHs adsorbent can remove 98.7 ± 0.1% of the cyanide content of the waste. Table 3 compares the performance characteristics of the diacid-modified LDHs adsorbent with recently reported Table 3. Comparison of the Performance Characteristics of Recently Reported Methods for Removal of Cyanide applied method sequencing batch reactor system plasma discharge technology azotobactor vinelandii bacterium TiO2−SiO2 nanoparticles powdered pistachio hull polymeric adsorbent iron composites coagulation by ferric chloride adsorption and biodegradation pulse current electrolysis electrochemical oxidation degradation with ozone Ni−Zn−Cr LDHs LDHs with biodegradable anion

removal efficiency (%) 97.7 92

required time (h) 240 1.5

ref 8 9

90

360

10

99 99 93 95 39

1 1 2 5 1

13 14 15 26 27

70

30

96.7 96.9 99 96.4 60 98.7

3 5 0.7 2 0.6

31 32 33 34 this work

methods for removal of cyanide from aqueous solutions. As seen, among these methods, removal of cyanide with diacidmodified LDHs is not only the fastest one but also is one of the most efficient methods. Although the removal performance of this work seems to be at a similar level with those obtained in some methods cited in Table 3, this work shows significant advantages over them. For example, the method based on the photocatalytic removal of cyanide with cobalt doped TiO2−SiO2 nanoparticles13 needs H

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(7) Botz, M.; Mudder, T.; Akcil, A. Cyanide treatment: physical, chemical and biological processes. In Advances in Gold Ore Processing; Adams, M., Ed.; Elsevier Ltd.: Amsterdam, 2005. (8) Sirianuntapiboon, S.; Chairattanawan, K.; Rarunroeng, M. Biological removal of cyanide compounds from electroplating wastewater (EPWW) by sequencing batch reactor (SBR) system. J. Hazard. Mater. 2008, 154, 526. (9) Hijosa-Valsero, M.; Molina, R.; Schikora, H.; Müller, M.; Bayona, J. M. Removal of cyanide from water by means of plasma discharge technology. Water Res. 2013, 47, 1701. (10) Kaewkannetra, P.; Imai, T.; Garcia-Garcia, F. J.; Chiu, T. Y. Cyanide removal from cassava mill wastewater using Azotobactor vinelandii TISTR 1094 with mixed microorganisms in activated sludge treatment system. J. Hazard. Mater. 2009, 172, 224. (11) Dash, R. R.; Gaur, A.; Balomajumder, C. Cyanide in industrial wastewaters and its removal: A review on biotreatment. J. Hazard. Mater. 2009, 163, 1. (12) Yeddou, A. R.; Nadjemi, B.; Halet, F.; Ould-Dris, A.; Capart, R. Removal of cyanide in aqueous solution by oxidation with hydrogen peroxide in presence of activated carbon prepared from olive stones. Miner. Eng. 2010, 23, 32. (13) Baeissa, E. S. Photocatalytic removal of cyanide by cobalt metal doped on TiO2−SiO2 nanoparticles by photo-assisted deposition and impregnation methods. J. Ind. Eng. Chem. 2014, 20, 3761. (14) Moussavi, G.; Khosravi, R. Removal of cyanide from wastewater by adsorption onto pistachio hull wastes: Parametric experiments, kinetics and equilibrium analysis. J. Hazard. Mater. 2010, 183, 724. (15) Zheng, W.; Wang, Y.; Yang, L.; Li, X.; Zhou, L.; Li, Y. Novel adsorbent of polymeric complex derived from chaleting resinwith Cu(II) and its removal properties for cyanide in aqueous solution. Colloids Surf., A 2014, 455, 136. (16) Chemical safety of drinking-water: assessing priorities for risk management; WHO: 2007. http://apps.who.int/iris/bitstream/10665/ 43285/1/9789241546768_eng.pdf?ua=1 (accessed Dec 21, 2014). (17) Mallakpour, S.; Dinari, M.; Behranvand, V. Ultrasonic-assisted synthesis and characterization of layered double hydroxides intercalated with bioactive N,N′-(pyromellitoyl)-bis-L-α-amino acids. RSC Adv. 2013, 3, 23303. (18) Mallakpour, S.; Dinari, M. Progress in synthetic polymers based on natural amino acids. J. Macromol. Sci., Part A: Pure Appl. Chem. 2011, 48, 644. (19) Fu-Sheng, W.; Yu-Qin, L.; Fang, Y.; Nai-Ku, S. Determination of cyanide by an indirect spectrophotometric method using 5-Br-PADAP. Talanta 1981, 28, 694. (20) Kameda, T.; Takeuchi, H.; Yoshioka, T. Ni−Al layered double hydroxides modified with citrate, malate, and tartrate: Preparation by coprecipitation and uptake of Cu2+ from aqueous solution. J. Phys. Chem. Solids 2011, 72 (6), 846. (21) Greenwell, H. C.; Jones, W.; Rugen-Hankey, S. L.; Holliman, P. J.; Thompson, R. L. Efficient synthesis of ordered organo-layered double hydroxides. Green Chem. 2010, 12, 688. (22) Ai, L.; Zhang, C.; Meng, L. Adsorption of Methyl Orange from Aqueous Solution on Hydrothermal Synthesized Mg−Al Layered Double Hydroxide. J. Chem. Eng. Data 2011, 56 (11), 4217. (23) Mallakpour, S.; Tirgir, F.; Sabzalian, M. R. Synthesis, characterization and in vitro antimicrobial and biodegradability study of pseudo-poly(amino acid)s derived from N,N-(pyromellitoyl)-bis-Ltyrosine dimethyl ester as a chiral bioactive diphenolic monomer. Amino Acids 2011, 40, 611. (24) Mallakpour, S.; Iderli, M.; Sabzalian, M. R. In vitro studies on biodegradable chiral nanostructure poly(amide-imide)s containing different natural amino acids in green medium. Des. Monomers Polym. 2013, 16, 509. (25) Kloprogge, J. T.; Hickey, L.; Frost, R. L. FT-Raman and FT-IR spectroscopic study of synthetic Mg/Zn/Al-hydrotalcites. J. Raman Spectrosc. 2004, 35, 967. (26) Do, S. H.; Jo, Y. H.; Park, H. D.; Kong, S. H. Synthesis of iron composites on nano-pore substrates: Identification and its application to removal of cyanide. Chemosphere 2012, 89, 1450.

toxic solvents such as acetylacetone and aqueous ammonia for preparation of the catalyst. In addition, this method needs H2 gas (as explosive gas) for drying the obtained catalyst. In contrast, in the proposed method, aqueous solution of NaOH is used for preparation of modified LDHs. In addition, the proposed method does not include any explosive chemical. The method based on the powdered pistachio hull14 needs more than 1 h for removal of cyanide. In addition, powdered pistachio hull may introduces secondary pollutants in environment, because the surface of this material is full of phenolic species.14 In contrast, in this LDHs-based system the required time for removal of cyanide is less than 36 min and a biodegradable amino acid is introduced in wastes.

4. CONCLUSION This study shows that N,N′-(pyromellitoyl)-bis-L-isoleucine diacid-modified LDHs can be used as selective adsorbent for fast removal of cyanide ions from real samples. The maximum adsorption capacity of the proposed adsorbent was 71.43 mg g−1. The required time for equilibrium establishment between cyanide ions and LDHs adsorbent was less than 35 min. It was found that the proposed adsorbent can remove 98.7% of the cyanide content of the silver industrial wastes. Compared to the recently reported methods for removal of cyanide from aqueous samples, the proposed method has several advantages such as high selectivity, high adsorption capacity, temperature− independent adsorption characteristics, and fast kinetics. Finally, cyanide ions are exchanged with a biodegradable amino acid derivative, thus modified LDHs adsorbent does not produce toxic wastes.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: +98 3133913287. Fax: +98 3133912352. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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