Synthesis and characterization of ZnAl-layered double hydroxide and organo-K10 montmorillonite for the removal of diclofenac from aqueous solution

Abstract

Two prepared adsorbents, namely, calcined ZnAl-layered double hydroxide and K10 montmorillonite intercalated with cetyltrimethyl-ammonium bromide cations are used in diclofenac sodium adsorption under the batch reactor operations. The pseudo-second-order model describes better the results of the kinetics. Fitting parameters revealed that the rate of adsorption increased with the increase in diclofenac sodium concentration and decrease in background electrolyte concentration and temperature, while the solution pH did not have a significant effect. The maximum adsorption capacities of diclofenac obtained by Langmuir model are found to be 55.46 and 737.02 mg/g for K10 montmorillonite intercalated with cetyltrimethyl-ammonium bromide cations and ZnAl-C LDH, respectively. The results of adsorption–desorption cycles revealed that ZnAl-C LDH and K10 montmorillonite intercalated with cetyltrimethyl-ammonium bromide cations have an excellent potential to be used as an economical adsorbent for the removal of diclofenac sodium from water.

Keywords

Adsorption diclofenac sodium calcined layered double hydroxides organo-K10 montmorillonite kinetics

Introduction

Drug residues are classified as micro-contaminants as they are often detected in very small amounts, ranging from microgram to nanogram per liter (Kümmerer, 2001). Despite this presence in small amounts, their potential effects on the environment should not be neglected, because they are molecules designed to act on living organisms (Heath, 2010). A recent survey in European river waters revealed that diclofenac, which is a non-steroidal anti-inflammatory drug (NSAID), is detected in 83% of collected samples (Loos, 2009). Therefore, diclofenac is one of the top 10 compounds most commonly found in aquatic environments, due to its high level of consumption. It has been detected even in drinking water in many countries at µg–ng L⁻¹ concentration levels (Terns, 1998). Continued intake of diclofenac shows several adverse biochemical effects (Bort et al., 1998; Haap et al., 2008; Martínez et al., 2011; Oaks et al., 2004; Zhang et al., 2011). However, the most important fact about diclofenac is that its structure contains a toxic halogenated aliphatic, which could cause an inhibitory effect upon the oxidation of any member of Krebs cycle; without this cycle, there would be no available energy to maintain our vital process. Adsorption is one of the several processes used to remove diclofenac from water; it is preferable in order to remove toxic compounds due to the production of high-quality effluents, do not add undesirable by-products and is cheap to perform using different adsorbents such as activated carbon and clays.

Clay minerals and clays, also called cationic clays, present very important properties such as high specific surface area, important chemical and mechanical stability and excellent adsorptive capacity. These properties are related to their colloidal size and crystalline structure (Del Hoyo, 2007). Montmorillonite (Mt) is one of the smectite group, composed of two silica tetrahedral sheets layered between an alumina octahedral sheets, known as T-O-T sheets. Water molecules are allocated in the interlayer sheet and are coordinated by the exchangeable cations. The size of the cations influences the (001) basal space of the structure (Sciascia et al., 2011). The inorganic–organic ion exchange between the desired organic cations and exchangeable inorganic cations in the interlayers of clays allows to obtain organoclays, which are effective adsorbents for organic and aromatic contaminants in pollution treatment (Meng et al., 2009).

Layered double hydroxides (LDH), often named anionic clays, display physical and chemical properties close to those of clay minerals and possess a low toxicity and good biocompatibility (Perioli et al., 2011). LDH can be easily and inexpensively synthesized. Synthetic LDH, after thermal decomposition, present “memory effect” properties, which allows reconstruction under mild conditions of the original structure by contact with solutions containing various anions (Del Hoyo, 2007).Up to now, cationic clays had been more widely used and studied in the pollution field than LDH. Furthermore, the available literature data on LDH are less extended than that for clay minerals.

Previously, several materials were used for the attenuation of diclofenac from aqueous medium. Granular activated carbon in the fixed bed column is found to be effective in the removal of diclofenac from aqueous solution as studied under column reactor operations (Sotelo et al., 2012). The hybrid material precursor to the carbon nanotubes/alumina is found to be effective materials in the removal of diclofenac from aqueous solutions (Wei et al., 2013). Hexagonal mesoporous silicate (HMS) and amine mercapto-functionalized HMS are employed in the adsorptive removal of diclofenac (Suriyanon et al., 2013). Recently, Thanhmingliana and Tiwari (2015) demonstrated that bentonite intercalated with cetyltrimethyl-ammonium bromide (C16) displayed good adsorbent properties for diclofenac sodium (DS) in aqueous solution.

The aim of this work was to prepare and characterize an organo-K10 Mt, consisting of K10 montmorillonite (K10Mt) intercalated with cetyltrimethyl-ammonium bromide cations (K10Mt-C16), and a calcined ZnAl-layered double hydroxides (ZnAl LDH). Once synthesized, the most novel aspect of this work lie in the investigation of DS adsorption results using the synthesized materials. However, to our knowledge, there is no published information about the use of these two materials as adsorbents of DS from water.

Materials and methods

Materials

K10Mt, C16 (CMC: 0.92 mM (20–25℃)) and DS (M = 318.13 g/mol) were supplied by Sigma–Aldrich. ZnCl_2, 6H₂O (99%) is obtained from Biochem, AlCl₃, 6H₂O (95%) is procured from PRS Panreac, and Na₂CO₃ (99%) is obtained from Flucka. NaOH (99%) is obtained from Rectapur.

Methodology

Preparation of uncalcined and calcined ZnAl LDH

ZnAl (molar ratio = 2) layered double hydroxides containing carbonate as the interlayer anion were obtained using a conventional coprecipitation method according to You et al. (2002a, 2002b). The obtained ZnAl LDH was calcined at 500℃ with the same rate of heating (1℃/min) for 4 h in the absence of air and oxygen.

Preparation of organo-K10 montmorillonite

The organo-K10 montmorillonite is obtained by cation exchange in an acidic medium. Ten grams of K10Mt dispersed in a 10³ ml solution 10⁻² M HCl containing 3.64 g of C16 placed under magnetic stirring at T = 80℃ for 3 h in order to protonate the amine function. The reaction is carried out at 80℃ for 3 h. Finally, the precipitate formed was washed with distilled water and dried at T = 80℃ for 24 h.

Characterization

X-ray diffraction (XRD) data are recorded using an XRD system (Bruker Advanced X-ray). The Cu Kα radiations having wavelength of 1.5418 Å are used for XRD analysis. Fourier transform-infra red (FT-IR) data are obtained for these materials using an FT-IR spectrophotometer (FTIR 8400S Shimadzu by KBR disk method). The specific surface area measurements are obtained by the Micromeritics Gemini VII instrument at 77 K. The pH_pzc (point of zero charge) is used to define a state of the surface of a dispersed solid phase. The pH_pzc of ZnAl LDH, ZnAl-C LDH, K10Mt and K10Mt-C16 is obtained by pH titration procedures according to Ofrao et al. (2006).

Batch reactor experiments

A solution of DS is prepared by dissolving 1 g of DS into 1 L of distilled water. The solubility is enhanced with the sonication of the solution for 24 h. The calibration curve is obtained by using prepared standard diclofenac solutions having concentrations from 2 to 20 mg/L. Different diclofenac concentrations (from 10 to 500 mg/L) are obtained by dilution of 1 g/L solution. Fifty milligrams of solid sample is introduced with these solutions at pH 7 for 2 h at 22 ± 1℃. After centrifugation and filtration of these solutions, the filtrates are subjected to its bulk diclofenac concentration using UV-Visible spectrophotometer (Model: UV PharmaSpec-1700 UV-Visible spectrophotometer Shimadzu). The absorbance is recorded at 276 nm. Time dependence adsorption of diclofenac by these materials is obtained at different time intervals from 5 to 120 min. The diclofenac concentration (from 5 to 100 mg/L) with solid dose of 1 g/L is taken as constant, and the adsorption experiments are conducted at constant pH 7.0 and temperature 22 ± 1℃. Results are reported as adsorption capacity (mg/g) as a function of time (min). The effect of pH on adsorption is studied by varying the pH from 4 to 10 for ZnAl-C LDH and from 5 to 9 for K10Mt-C16. The effect of temperature on adsorption is studied by varying the temperature intervals from 25 to 55℃.

Effect of background electrolyte concentration on adsorption is studied by varying the background electrolyte concentrations from 0.005 to 0.1 mol/L NaCl. The diclofenac concentration of 50 mg/L with solid dose of 1 g/L is taken as constant.

Results and discussion

Characterization of materials

The FT-IR data are collected for ZnAl LDH, ZnAl-C LDH, K10Mt and K10Mt-C16 in Figure 1(a) to (d), respectively. The band at 3500–3600 cm⁻¹ in the IR spectra of ZnAl LDH can be ascribed to the H-bonding stretching vibrations of the OH group in the brucite-like layer (Cavani et al., 1991). The bending vibration of the interlayer water (δ_H2O) is recorded at 1600–1650 cm⁻¹. For the common inorganic anions, the band at 1350–1380 (ν₃), 770–800 (ν₂) and 550–630 (ν₄) cm⁻¹ are indicative of ${CO}_{3}^{2 -}$ . These bands become less intense when LDH is calcined, but calcination does not cause the complete loss of interlayer carbonate anions and bound water. This is in agreement with previous reports that have pointed out that the complete removal of the anionic species occurs only at temperatures higher than 700℃ (Reichle, 1986; Sing et al., 1985).

Figure 1.

FT-IR spectra of (a) Zn-Al LDH, (b) Zn-Al-C LDH, (c) K10Mt and (d) K10Mt-C16.

In the FT-IR spectra of K10Mt and K10Mt-C16, the absorption at 3620 cm⁻¹ was attributed to the stretching band of the inner OH unit within the Mt structure, and the bands at 3436 cm⁻¹ are related to the OH vibrations of water molecules (Farmer, 1974). The bending vibration of the water occurs at 1638 cm⁻¹. Other characteristic vibrations of hydroxyl groups, the silicate anion and the octahedral cations are present in the IR spectrum of K10Mt and K10Mt-C16 in Figure 1(c) and (d), respectively. The band at 1047 cm⁻¹ was attributed to the Si–O stretching vibrations of the tetrahedral sheet and the bands at 527 and 465 cm⁻¹ are due to Si–O–Al_oct and Si–O–Si bending vibrations. The band at 800 cm⁻¹ indicated the presence of quartz. For the organo-K10 montmorillonite (Figure 2(d)), new absorption bands were detected at 2924 and 2851 cm⁻¹, which were assigned as the asymmetric and symmetric stretching of methyl groups (CH₃) and the methylene (CH₂) of the aliphatic chain of the amine (Madejova, 2003). The bending vibration of the methylene groups can be observed at 1475 cm⁻¹ verifying the intercalation of surfactant molecules between the silicate sheets.

Figure 2.

XRD pattern of Zn-Al LDH, Zn-Al-C LDH, K10Mt and K10Mt-C16.

Further, the XRD analysis is conducted for these materials. The results are presented graphically in Figure 2 for ZnAl LDH, ZnAl-C LDH, K10Mt and K10Mt-C16. The X-ray pattern of ZnAl LDH contains sharp reflections at low 2θ values corresponding to successive orders of basal space ć. On the other hand, relatively weak non-basal reflections are observed at higher values of 2θ. The interlayer space is estimated to be at 2.89 Å; it is the difference between basal space ć (7.67 Å) and the brucite-like sheet thickness (4.78 Å). The interlayer space is dependent on the size and the orientation of the charge-balancing anion ${CO}_{3}^{2 -}$ . The parameter a of the unit cell value, calculated by the formula a = 2 d110, is estimated to be at 3.07 Å, confirming that M²⁺/M³⁺ ≈ 2. The X-ray pattern of ZnAl-CLDH shows that LDH structure was destroyed and converted to an amorphous material after calcinations at 500℃. The spectrum shows formation of metal oxides ZnO and ZnAl₃O₄ corresponding to 200 and 220 reflections. The X-ray pattern of K10Mt exhibited a typical reflection of Mt that resulted in a 001 basal reflection at 9.9 Å. Further, the over visible reflections are, perhaps, due to the presence of some impurity. The X-ray pattern of K10Mt-C16 is almost identical to its virgin K10Mt with slight change in intensities of reflections.

The specific surface area measurements of samples were determined using the N₂ adsorption–desorption isotherms. The method involves degassing the samples under vacuum at T = 120℃ for 2 h. The various parameters calculated by the Brunauer, Emmett and Teller (BET) method of samples are summarized together for comparison in Table 1. The specific surface area of ZnAl LDH increased from 35.92 to 77.04 m²/g. The removal of water and carbon dioxide during calcination can lead to the formation of channels and pores (Yu et al., 2008), which are accessible to the N₂ molecules and could increase the specific surface area of ZnAl LDH. For K10Mt and K10Mt-C16, there is a diminution of the surface area from 270 to 180 m²/g due to the presence of C16, which occupies the adsorption sites.

Table 1.

Experimental data textural studies of actual samples.

samples	ZnAl LDH	ZnAl-C LDH	K10Mt	K10Mt-C16
S_BET (m²/g)	35.9	77.0	270.0	180.0

Batch reactor operations

Effect of pH

The pH dependence adsorption of DS by these materials is carried out at room temperature for 120 min. The pH_PZC value of these solids is found to be 7.80, 10.50, 3.50 and 3.40 for the samples ZnAl LDH, ZnAl-C LDH, K10Mt and K10Mt-C16, respectively. It is assumed that the solid carries net positive charge below this pH and beyond this pH the surface carries a net negative charge. On the other hand, the diclofenac is having low dipole moment and acid dissociation constant value of pKa = 4.21 (Meloun et al., 2007). This implies that diclofenac carries negative charge beyond pH 4.2. Theoretically, adsorption is favorable at pH beyond diclofenac pKa and below pH_PZC of these materials. The average adsorption capacities are found to be 44.78 and 38.67 mg/g for ZnAl-C LDH and K10Mt-C16, respectively. The final pH does not have a large variation for the two samples. The results showed that DS adsorption by these solids is independent of the change in solution pH, suggesting that the strong affinity towards diclofenac is due to its high octanol–water partition coefficient (log K_ow = 0.7 (Sotelo et al., 2012); in the case of ZnAl-C LDH, the organophilic behaviour of K10Mt-C16 surface has a strong hydrophobic core (Alexander et al., 2000; Li and Bowman, 1997; Zanetti et al., 2000). Similar results are found for the adsorption of DS by modified organo-bentonite (Thanhmingliana and Tiwari, 2015).

Effect of initial DS concentrations

The concentration dependence removal of diclofenac is studied for a range of initial diclofenac concentration from 5 to 100 mg/L at constant pH 7.0. The initial concentration of DS has significant effect on the DS adsorption. The amount of DS adsorbed increased from 3.28 to 96.18 mg/g and from 3.55 to 58.73 mg/g for ZnAl-C LDH and K10Mt-C16, respectively (Figure 3).

Figure 3.

Effect of initial DS concentrations on adsorption capacity of ZnAl-C LDH and K10Mt C16.

Effect of contact time on DS adsorption

According to Figure 4, the adsorption amounts of DS increased rapidly in the initial 10 min using ZnAl-C LDH samples, it becomes slow to attained equilibrium time which is 20 min. For K10Mt-C16, the equilibrium is instantaneous (5 min). The difference between the equilibrium time obtained with ZnAl-C LDH and K10Mt-C16 can be explained by the fact that DS anions adsorption onto ZnAl-C LDH occurs by surface and ion exchange phenomena by reconstruction (memory effect), the DS anions diffuse in the interlayer of LDH and long time is needed to obtained saturation, whereas the uptake of DS by K10Mt-C16 is due to the strong repulsive force seemingly restricts the partitioning of diclofenac within the modified K10Mt samples.

Figure 4.

Effect of time and initial concentration on the adsorption of DS onto the two samples (V_sol = 50 mL; adsorbent mass = 50 mg; T = 22 ± 1℃, pH = 7.0).

The kinetic data are further utilized in the kinetic modeling to estimate the rate constants along with the removal capacity of these solids. Pseudo-first-order (PFO) expressed by equation (1) (Zhu et al., 2010) and pseudo-second-order (PSO) expressed by equation (2) (McKay and Ho, 1999) kinetic models are used

Ln (q e - q t) = Ln q e - k 1 t

(1)

\frac{t}{q t} = \frac{1}{q_{e}^{2} k 2} + \frac{1}{q e} t

(2)

where q_t and q_e are the amount of diclofenac removed at time ‘t’ and removal capacity at equilibrium, respectively. k₁ and k₂ are the PFO and PSO rate constants, respectively. The PFO kinetic model, which is theoretically derived by the Lagergren, could show the properties of the Langmuir rate at initial times of adsorption or close to equilibrium (Azizian, 2004; Liu and Shen, 2008). The estimated values of q_m, k₁ and k₂ are tabulated in Table 2. Results are best fitted to the PSO model comparing to the PFO model. For the kinetic model of the PSO, it can be noticed that the adsorption capacity q_m increases with the increase of DS initial concentration from 5 to 100 mg/L. Furthermore, we note the decrease in k₂ values; adsorption takes place in two stages: the first is fast, then the second is slow (Hibino et al., 1995). The values of the correlation coefficients are 0.99, superior to those obtained upon application of the first-order kinetic model. Thus, most likely, the adsorption of DS onto ZnAl-C LDH and K10Mt-C16 follows a rate expression PSO (Figure 5). Moreover, the applicability of PSO model indicates that the adsorbate species are bound onto the surface of solids by strong forces.

Table 2.

Kinetic parameters estimated using PFO and PSO model for the removal diclofenac by ZnAl-C LDH and K10Mt-C16 solids.

			Pseudo-first-order			Pseudo-second-order
Adsorbent	C	q_exp	q_e	K₁.10²	R²	q_e	K₂.10³	h	R²
ZnAl-C LDH	5	3.3	3.7	2.3	0.96	7.9	0.8	0.05	0.88
	10	7.2	5.7	4.0	0.96	8.3	7.0	0.48	0.99
	20	17.7	5.5	7.4	0.54	17.4	29.0	8.76	0.99
	50	45.7	3.9	6.7	0.68	45	22.0	44.6	0.99
	100	95.3	10.3	9.3	0.67	96.2	4.0	37.0	0.99
K10Mt-C16	5	4.7	3.8	4.0	0.54	5.6	0.013	0.400	0.97
	10	8.3	1.4	2.0	0.07	7.7	0.076	4.494	0.99
	20	17.4	2.32	2.2	−0.40	16.7	0.081	22.48	0.99
	50	34.4	5.12	1.6	0.15	34.5	0.027	32.09	0.99
	100	58.7	9.3	2.3	0.24	58.8	0.023	79.57	0.99

Note: C (mg/L), K₁ (L/min), K₂ (g/mg.min), q_e (mg/g), h (mg/g.min). q_ex (mg/g).

Figure 5.

Pseudo-second-order model of DS adsorption onto K10Mt-C16 and ZnAl-C LDH.

Effect of background electrolyte concentration

The adsorption of diclofenac by ZnAl-C LDH and K10Mt-C16 is assessed by varying the background electrolyte concentrations from 0.005 to 0.1 mol/L NaCl at an initial diclofenac concentration of 50 mg/L and at constant pH 7. Increasing in the background electrolyte concentrations from 0.005 to 0.1 mol/L leads to the decrease of DS adsorbed amount from 42.9 to 27 mg/g and from 48.6 to 36.9 mg/g for ZnAl-C LDH and K10Mt-C16, respectively. The batch adsorption studies lead to lower adsorption capacities for the system NaCl–DS, showing the competitive effect for the available adsorption sites. On the other hand, the presence of salt, even at low concentrations, affects the composition of the organic phase of the water–octanol–salt system. This phase becomes richer in octanol when the salt concentration is increased, which could decrease the octanol–water partition coefficient of diclofenac and explains the lower DS adsorption.

Effect of temperature

The effect of temperature on the DS adsorption was investigated under isothermal conditions in the temperature range of 25–55℃. The DS adsorption capacity onto these materials decreases with increasing in temperature, suggesting the exothermic nature of adsorption reaction. The change in standard free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) of adsorption was calculated from the following equation (Önal et al., 2007)

Δ G^{0} = - RT \ln K c

(3)

where R is gas constant, T the temperature in K and K_c is the equilibrium constant calculated from equation (4)

K c = \frac{C A e}{C S e}

(4)

where C_Ae is the equilibrium concentration of DS on adsorbent (mg/L) and C_Se is the equilibrium concentration of DS in the solution (mg/L).

Standard enthalpy (ΔH°) and entropy (ΔS°) of adsorption can be estimated from Van't Hoff equation given in

Ln K C = \frac{- Δ H^{0}}{RT} + \frac{Δ S^{0}}{R}

(5)

The slope and intercept of the Van't Hoff plot (Figure not shown) are equal to −ΔH°/R and ΔS°/R, respectively.

Thermodynamic parameters obtained are summarized in Table 3. The ΔG° values at different temperatures are negative and decreased with an increase in temperature, indicating that adsorption process is spontaneous and inversely proportional to the temperature. Negative values of ΔH° indicate the exothermic nature of the process. The standard enthalpy change is lower than the value of 40 kJ/mol; it indicates the physical nature of adsorption (Gereli et al., 2006; Karaca et al., 2004). The negative value of ΔS° suggests that the adsorption process involves an associative mechanism and that there is no significant change occurring in the internal structures of the adsorbent during the adsorption process.

Table 3.

Thermodynamic parameters for the adsorption of DS onto ZnAl-C LDH and K10Mt-C16.

T (K)	K _c		ΔG° (kJ/mol)		ΔH° (kJ/mol)		ΔS° (J/mol K)
T (K)	K10Mt-C16	ZnAl-C LDH	K10Mt-C16	ZnAl-C LDH	K10Mt-C16	ZnAl-C LDH	K10Mt-C16	ZnAl-C LDH
298	2.49	11.13	−2.25	−5.96	−12.36	−13.11	−33.72	−23.25
308	1.90	9.42	−1.64	−5.73
318	1.76	9.00	−1.49	−5.79
328	1.56	6.62	−1.21	−5.17

Adsorption isotherm

Several models have been used in the adsorption studies to describe the experimental data adsorption isotherms. Langmuir and Freundlich isotherms are the most frequently used models. Both models were investigated in this work. Using a method of non-linear regression, the adsorption data have been fitted to Langmuir adsorption model to describe the adsorption processes between solid–liquid interface. The equation is given as (Langmuir, 1916)

q e = \frac{q m K L C e}{1 + K L C e}

(6)

where q_e (mg/g) is amount of DS adsorbed/weight of adsorbent, K_L and q_m are the Langmuir coefficients and the maximum adsorption capacity corresponding to complete monolayer coverage on the adsorbent surface.

Freundlich isotherm is more general than Langmuir isotherm, because it does not assume a homogenous surface or constant sorption potential (Weber and Chkraborti, 1974). Freundlich (1960) adsorption isotherm can be expressed as

q e = K F C_{e}^{1 / n}

(7)

where the intercept log K_F is a measure of adsorbent capacity and the slope 1/n is the adsorption intensity.

According to Giles classification of adsorption isotherms, the DS adsorption onto these two materials forms an L-shape curve, which indicates that the adsorbents have a high affinity for DS in the initial stages of the isotherm. As adsorption sites are filled, the DS molecules have increasing difficulty in finding vacant sites and the slope of the curve decreases. The equilibrium adsorbate concentrations are important parameters, which can affect the adsorption process considerably. On reviewing the results reported in Figure (6), an increase in DS concentration leads to increase in its uptake. The maximum monolayer capacity values of adsorbent (q_m), Freundlich constant K_F, 1/n and R² are listed in Table 4. The value of the constant 1/n is calculated to be 0.66 and 0.265 for ZnAl-C LDH and K10Mt-C16, respectively. Since the value of 1/n is less than 1, it indicates a favorable adsorption, and also implies minimum interactions between the adsorbed molecules (Fytianos et al., 2000; Tsai et al., 2004). Both Freundlich and Langmuir model fit the experimental data well. The Langmuir maximum adsorption capacities were 55.46 and 737.02 mg/g for K10Mt-C16 and ZnAl-C LDH, respectively, which are very promising compared with other adsorbents for DS adsorption described in Table 5. Results show that adsorption capacity for K10Mt-C16 and ZnAl-C LDH is higher than that of pristine K10Mt and ZnAl LDH at the same equilibrium concentration of DS. The difference is due to the increase of specific surface area of ZnAl LDH by calcination treatment and to the organophilic behavior of K10Mt-C16 surface.

Figure 6.

Freundlich and Langmuir isotherm model of DS adsorption onto different samples.

Table 4.

Langmuir and Freundlich isotherm model constants and correlation coefficients for adsorption of DS on samples.

Model	Parameters	ZnAl-C LDH	ZnAl LDH	K10Mt-C16	K10Mt
Langmuir	q_m (mg/g)	737.02	94.32	55.46	33
	R²	0.99	0.84	0.897	0.92
	K_L (dm³/mg)	0.0141	0.0034	0.1825	0.0157
Freundlich	1/n	0.66	0.675	0.265	0.40
	K_F (mg/g (dm³/mg)^1/n)	21.41	0.996	14.41	2.83
	R²	0.98	0.80	0.898	0.87

Table 5.

Comparison of maximum adsorption of DS onto various adsorbents.

Adsorbents	Maximum monolayer adsorption capacities (mg/g)	References
Montmorillonite	680.27	Kaur M Datta (2014)
ZnAl-C LDH	737.02	This work
K10Mt-C16	55.46	This work
Granular activated carbon	331	Sotelo et al. (2012)
Organobentonite (BH)	18.99	Thanhmingliana and Tiwari (2015)
Microwave-assisted activated carbon	63.47	Saucier et al. (2015)
Isabel grape bagasse	76.98	Márjore et al. (2012)
Activated carbon (AC) Oxidized activated carbons (OAC)	83 490	Bhadra et al. (2016) Bhadra et al. (2016)

Desorption and reusability studies

The adsorption–desorption cycles were used to determine the reusability of the samples for the adsorptive removal of diclofenac from aquatic environment. The concentration of DS solution used was 50 mg/L. Hundred milligrams of the samples were introduced with 100 mL of the solution (pH = 7) at room temperature for 120 min. Ethanol is used as a desorbing agent. The used samples were recycled and used as regenerated adsorbent for another three adsorption–desorption cycles. The percentages of desorbed DS in the four adsorption–desorption cycles for ZnAl-C LDH and K10Mt-C16 are depicted in Figure 7. The percentage desorption decreases during the four adsorption–desorption cycles; however, the reduction was not too significant, and this result demonstrates the high reusability of the samples, which ascertains its applicability for the removal of DS from aquatic environment.

Figure 7.

The desorption capacity on recycling ZnAl-C LDH and K10Mt-C16.

Conclusion

Equilibrium adsorption studies have been carried out in order to investigate the adsorption capacity of ZnAl-C LDH and K10Mt-C16 to remove DS micro-contaminant from aqueous solution. The uptake of diclofenac by these solids is extremely efficient as within 2 min of contact. An apparent equilibrium is achieved by ZnAl-C LDH and K10Mt-C16 within 20 min and 5 min of contact, respectively. The obtained results indicated that the PSO model fits the experimental data suitably well. The batch data imply that a very high uptake of diclofenac by these materials is almost unaffected with the increase in adsorptive pH and much affected with an increase in DS concentration, background electrolyte concentration and temperature. The batch adsorption studies lead to lower adsorption capacities of DS using K10Mt-C16, showing the efficiency of ZnAl-C LDH for diclofenac adsorption, with a strong affinity due, perhaps, to the higher values of octanol–water partition coefficient of DS molecule for the adsorbents surface. Langmuir adsorption isotherm model showed the best fit with the experimental adsorption data. The Langmuir monolayer adsorption capacities were 55.46 and 737.02 mg/g for K10Mt-C16 and ZnAl-C LDH, respectively. Results of adsorption–desorption studies show the efficiency of K10Mt-C16 and ZnAl-C LDH after four regeneration cycles.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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