Fabrication of Novel Al(OH)3/CuMnAl-Layered Double Hydroxide for Detoxification of Organic Contaminants from Aqueous Solution.

A novel lamellar Al(OH)3/CuMnAl-layered double hydroxide (LDH) nanocomposite was successfully synthesized via the hydrothermal method and tested as a highly efficient adsorbent for the removal of Congo red (CR) dye from aqueous solution. Structural, morphological, and spectroscopic characterization of the Al(OH)3/CuMnAl-LDH nanocomposite were studied by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, photoluminescence (PL) analysis, and UV-visible spectroscopy analysis techniques. The CR dye adsorption performance of the prepared materials increased with an increase in functionality. The adsorption capacity of the Al(OH)3/CuMnAl-LDH nanocomposite (172 mg/g, pH 7, temp 30 °C) was found to be higher than that of pure Al(OH)3 (32 mg/g, pH 7, temp 30 °C) and CuMnAl-LDH (102 mg/g, pH 7, temp 30 °C). The results revealed that anion exchange and hydrogen bonding are mainly responsible for the adsorption of CR onto the Al(OH)3/CuMnAl-LDH nanocomposite. Moreover, the adsorption of CR in the presence of Cu(II) and NaCl salt showed a synergistic and antagonistic effect while the presence of anionic Cr(VI) ions had no significant effect. The adsorption thermodynamics, isotherm, and kinetics modeling analyses were also conducted to study the interactions between CR molecules and the Al(OH)3/CuMnAl-LDH nanocomposite. The adsorption of CR was found to be endothermic and followed by the pseudo-second-order kinetics and the Langmuir adsorption isotherm model. The developed nanocomposite showed excellent potential for treating industrial wastewater.

Introduction

Wastewater effluents, which contain dyestuff and heavy metals from a variety of industries, are often discharged into water bodies, and it may cause harmful effects to aquatic life and the environment. Among the significant polluters, especially with effluents containing both dyes and heavy metals, textile industries are well known. Because of the complex structures of dyes, they are not readily degradable, and hence concerted human-made efforts are required to reduce the toxicity of CR dye in aqua systems.[1] There are over 10 000 available commercial dyes, and the primary consumers are the textile, pulp, and tannery industries.[2] Congo red (CR) dye is highly soluble in water and has a very high affinity for cellulose fiber, owing to which it is preferably used in textile industries.[3] For its toxic effect on human health and aquatic life,[4] the use of CR dye is already banned in few countries. However, many countries still overlook these health concerns and other environmental hazards of CR.[5] There is now a rising consensus that due to their hazardous nature, these pollutants needs to be eliminated with effective techniques before the effluents discharged into the ecosystem.

Several techniques such as flocculation, photocatalysis, coagulation, biological degradation, membrane filtration, adsorption, and so forth have been widely employed to eliminate toxic dyes from polluted wastewater.[6−8] Amongst these, the application of the adsorption technique has become dominant in the use of wastewater treatment because of its peculiar properties such as high efficiency in removing pollutants when compared to other conventional methods[9] and its low cost, ease of setup, and recyclability. Recently, the use of the adsorption technique in treating wastewater has got a renewed boost with the synthesis of newer novel materials in order to solve the defects of the traditional sorbents.

Over other conventional materials, nanomaterials have shown a very high prospect in treating wastewater because of their exceptional properties which include high surface area, possibilities of fabrication into different morphological forms, possibilities of customized functionalization, and higher efficiencies of adsorption with a fast adsorption rate.[10] Several nanomaterials and their composites, such as S-doped Fe2O3/C nanocomposite,[11] zinc peroxide (ZnO2),[3] and Maghemite nanoparticles,[5] have been successfully utilized for the adsorption of CR dye from water. Aluminum oxides/hydroxides are the most favorable adsorbent material because of their excellent physical and textural characteristics. Aluminum oxides/hydroxides also possess the ability to alter the mechanical strength, porosity, hydrophilicity, and sorption capacity of nanocomposite materials.[12] Aluminum hydroxide-based materials such as Al(OH)3,[13,14] Fe3O4@Al(OH)3,[15] and F–Al hydroxides[16] have shown excellent adsorption property for different types of adsorbates including heavy metal organics, mineral ions, and so forth in the treatment of wastewater.

Recently, layered double hydroxide (LDH) has shown remarkable properties to combat environmental challenges owing to its superior characteristics in efficiently removing pollutants from wastewater. This property is attributed to their unique two-dimensional (2D) nanostructure anionic clays with a very large surface area and high anion exchange property. The interlayer is easily reachable to anionic dyes, which is influenced by the charged balancing anions and layer charge density.[17] LDH has a general formula expressed as [M1–2+M3+(OH)2](A)·mH2O where M2+ represents the divalent metal cations such as Ni2+, Cu2+, and so forth, M3+ represents the trivalent metal cations such as Mn3+ and Al3+, A is the anion OH–, and X is the molar ratio of the trivalent metal ion to total metal ions. By considering the characteristics and the adsorption strength of aluminum hydroxide and LDH, a new nanocomposite material having the properties of both can be a good material for the environmental remediation applications.

This study focuses on the synthesis of aluminum hydroxide [Al(OH)3], copper manganese aluminum-LDH (CuMnAl-LDH), and Al(OH)3/CuMnAl-LDH nanocomposite as a superior material for scavenging of CR dye anions. Furthermore, the adsorption kinetics of CR dye onto Al(OH)3, CuMnAl-LDH, and Al(OH)3/CuMnAl-LDH nanocomposite has been investigated. The equilibrium data were applied to the isotherm models, and the co-adsorption of heavy metals and sodium salt with CR onto the Al(OH)3/CuMnAl-LDH nanocomposite was investigated.

Results and Discussion

Characterization

The scanning electron microscopy (SEM) images of the synthesized materials are shown in Figure . The SEM images of Al(OH)3 (Figure a,b) show a sheetlike porous structure. A typical characteristic SEM image of LDH is the formation of clustered platelike particles or a layered structure, which looks cloudy and is connected at the edges at different angles[18] as evident from the SEM images of CuMnAl-LDH shown in Figure c,d. The Al(OH)3/CuMnAl-LDH nanocomposite images shown in Figures e,f and 2a possess the characteristic nanosize pores with a large surface area nucleated in the agglomerated vertical sheet, which are connected at the edges at different angles. The transmission electron microscopy (TEM) image shown in Figure b buttresses our SEM image shown in Figure a; it reveals that CuMnAl-LDH is deposited over Al(OH)3 sheets. Moreover, to confirm the elemental composition and distribution of the elements in the composite, energy-dispersive spectrometry (EDS) and elemental (Figure S1) analysis of the Al(OH)3/CuMnAl-LDH nanocomposite were performed. EDS analysis showed the presence of O, Al, Mn, and Cu with 68.12, 27.36, 3.19, and 1.32 atomic weight percentages, respectively. Elemental mapping analysis clearly showed well-distributed O, Al, Mn, and Cu in the Al(OH)3/CuMnAl-LDH nanocomposite.

Figure 1

SEM images (using an SEI detector) of (a,b) Al(OH)3, (c,d) CuMnAl-DLH, and (e,f) Al(OH)3/CuMnAl-LDH nanocomposite.

Figure 2

(a) SEM image (using an LEI detector) and (b,c) TEM images of the Al(OH)3/CuMnAl-LDH nanocomposite.

The X-ray diffraction (XRD) patterns of Al(OH)3, CuMnAl-LDH, and Al(OH)/CuMnAl-LDH nanocomposite are shown in Figure . Al(OH)3 shows six major peaks located at 17.8°, 29.8°, 39.0°, 47.6°, 55.6°, and 52.2° 2θ which can be indexed to 001, 111, 132, 141, 024, and 153 planes of nordstrandite [Al(OH)3]. The observed diffraction peak reflects the XRD characteristics for aluminum hydroxide.[19] The diffractogram of CuMnAl-LDH shows sharp peaks, which are indicative of the characteristic pattern of LDH. The intense peaks at 24.2°, 31.4°, 37.4°, 41.4°, 45.2°, 49.8°, 51.8°, and 63.9° correspond to the 006, 015, 107, 111, 220, 116, 0111, and 0015 planes, suggesting the formation of the CuMnAl-LDH pattern.[17,18] Also, the XRD pattern of the Al(OH)3/CuMnAl-LDH nanocomposite (Figure ) follows a similar pattern as Al(OH)3 and CuMnAl-LDH with the peak diffraction of the crystalline structure and mesostructured Al(OH)3 with corresponding peaks at 24.3°, 28.1°, 31.4°, 37.6°, 49.1°, 51.8°, and 64.5° 2θ.

Figure 3

XRD patterns of (a) Al(OH)3, (b) CuMnAl-LDH, and (c) Al(OH)3/CuMnAl-LDH nanocomposite.

Photoluminescence (PL) and UV–visible (UV–vis) absorbance analysis were carried out to determine the optical properties of the adsorbents. The PL emission spectra of Al(OH)3, CuMnAl-LDH, and Al(OH)3/CuMnAl-LDH nanocomposite are shown in Figure a. Three emission peaks are seen on the emission spectra of Al(OH)3, the strongest peak is seen at 476 nm, and the weak peaks are seen at 390 and 361 nm. CuMnAl-LDH shows two peaks at 371 and 471 nm, while slight peak shifts were observed in the Al(OH)3/CuMnAl-LDH nanocomposite with a sharp emission peak at 367 nm and a weak peak at 472 nm. The PL intensity of the materials is in the order Al(OH)3/CuMnAl-LDH nanocomposite < CuMnAl-LDH < Al(OH)3. In general, the weaker the PL intensity, the better are the optical properties.[8] The Al(OH)3/CuMnAl-LDH nanocomposite showed the weakest PL intensity in comparison with the other materials; it is expected to possess the best optical properties. These results are further confirmed by the UV–vis absorbance analysis as shown in Figure b. The visible light absorption properties of the Al(OH)3/CuMnAl-LDH nanocomposite is much higher than those of Al(OH)3 and CuMnAl LDH. This may be attributed to the combined effect of the optically active surface generated from the mixing of Al(OH)3 and CuMnAl-LDH.

Figure 4

(a) PL analysis and (b) UV–visible absorption spectra of Al(OH)3, CuMnAl-LDH, and Al(OH)3/CuMnAl-LDH nanocomposite.

Adsorption Studies

Effect of pH

CR is an anionic dye, and initial solution pH influences its molecular structure.[20] Therefore, the effect of the solution pH on the removal of CR by Al(OH)3, CuMnAl-LDH, and Al(OH)3/CuMnAl-LDH nanocomposite was investigated at varying pH from 4 to 10 at 30 °C, and the results are shown in Figure . It was observed that the adsorption of CR decreases as the solution pH increases from 4 to 10, and maximum adsorption was attained at pH 4 for all the studied adsorbents. Moreover, the Al(OH)3/CuMnAl-LDH nanocomposite showed the highest adsorption capacity for CR dye in comparison with CuMnAl-LDH and Al(OH)3 at all the studied pH values. A similar trend for adsorption of the CR dye at different pH has been reported.[21,22] This behavior can be explained based on the functional groups and surface charge on the adsorbents and the CR. At the acidic pH, there are more H+ ions in the solution and the adsorbent material becomes positively charged by absorbing the H+ ions, which interact electrostatically with the SO3– functional group of the anionic CR dye molecules. As the solution pH shifts from acidic to basic, more OH– in the solution gets adsorbed on the surface of the materials and thereby generates a negative charge on the adsorbent surface; which does not favor the adsorption of the anionic CR dye molecule because of electrostatic repulsion. Thus, the lowest adsorption capacity was recorded at pH 10.[23]

Figure 5

Effect of pH on the removal of CR dye onto Al(OH)3, CuMnAl-LDH, and Al(OH)3/CuMnAl-LDH nanocomposite.

Effect of Contact Time and Adsorption Kinetics

The effect of contact time on the adsorption of CR dye onto Al(OH)3, CuMnAl-LDH, and Al(OH)3/CuMnAl-LDH nanocomposite was investigated at an initial CR dye solution of 250 mg/L concentration and the contact time ranging from 15 to 300 min, and the results are depicted in Figure . The sorption of CR dye onto Al(OH)3/CuMnAl-LDH nanocomposite, CuMnAl-LDH, and Al(OH)3 was found to increase with an increase in the contact time until equilibrium was attained. The adsorption capacity increased from 52 to 172 mg/g, 13 to 102 mg/g, and 5 to 32 mg/g for the Al(OH)3/CuMnAl-LDH nanocomposite, CuMnAl-LDH, and Al(OH)3, respectively, as the reaction times varied from 15 to 300 min. The curves for CuMnAl-LDH and the Al(OH)3/CuMnAl-LDH nanocomposite showed that sorption occurred in three steps. The adsorption equilibrium for Al(OH)3 was reached within 90 min. Although adsorption of CR dye on CuMnAl-LDH and the Al(OH)3/CuMnAl-LDH nanocomposite showed saturation in 210 min. The process was rapid and produced more than half of the sorption within the initial 60 min in the first step. As the contact time increased to 120 min, the sorption rate became slow for the second step until equilibrium was attained at the final step after 210 min. These results suggest that there was availability of more active sites (vacant) on the materials for sorption, which resulted in a fast adsorption process in the first step. After that, in the second step, adsorption became slower as the active sites got saturated, and thus the adsorption reached to equilibrium.[3]

Figure 6

Effect of contact time on the removal of CR dye onto Al(OH)3, CuMnAl-LDH, and Al(OH)3/CuMnAl-LDH nanocomposite.

A kinetic study was performed to determine the rate of dye removal on the synthesized adsorbent materials. The data obtained were fitted to pseudo-first-order and pseudo-second-order kinetic models. The pseudo-first-order model[24] was based on the assumptions that the rate of change of solute uptake with time is directly proportional to the difference in saturation concentration and the amount of solid uptake with time. The linear equation is denoted aswhere qe and q (mg/g) are the CR adsorption capacity at equilibrium and at time t, respectively, and k1 (1/min) is the pseudo-first-order rate constant. A linear plot log(qe – q) against t is shown in Figure S2a.

The linear equation for the pseudo-second-order model proposed by Ho and Mckay[25] is denoted aswhere k2 (g/min) is the pseudo-second-order rate constant. The values of the kinetic parameters were calculated from a linear plot of t/q versus t, as shown in Figure S2b. The values obtained from kinetic models are shown in Table . The regression coefficient R2 and qe(cal) are important values that can be used to illustrate a good agreement with these models. The R2 values for Al(OH)3 (R2 – 0.9145) and the Al(OH)3/CuMnAl-LDH nanocomposite (R2 – 0.9929) obtained from the pseudo-second-order are higher than R2 values obtained from the pseudo-first-order model. Moreover, the qe(cal) values calculated from the pseudo-second-order kinetic equation are much closer to the experimental qe(exp), which indicates that the adsorption of CR dye on Al(OH)3 and the Al(OH)3/CuMnAl-LDH nanocomposite is best described using the pseudo-second-order model. Similar results for the adsorption kinetics of CR dye have also been reported elsewhere.[21,23] These results suggested that the adsorption process is supposed to be chemisorption.[3] On the other hand, the adsorption of CR dye on the CuMnAl-LDH material best fitted into the pseudo-first-order model because of the higher R2 (0.9635) than the R2 (0.9425) value obtained from the pseudo-second-order model. Moreover, calculated adsorption qe(cal) (94.27 mg/g) is much closer to the experimental qe(exp) (102 mg/g) for the pseudo-first-order kinetic model, indicating adsorption of CR dye onto CuMnAl-LDH to be physisorption.[20,39]

Table 1

Adsorption Kinetic Constant for Adsorption of CR on Al(OH)3, CuMnAl-LDH, and Al(OH)3/CuMnAl-LDH Nanocomposite

		pseudo-first-order			pseudo-second-order
	qe(exp) (mg/g)	R2	K1 (1/min)	qecal	R2	K2 (g/mg/min)	qecal (mg/g)
Al(OH)3/CuMnAl-LDH	172	0.8196	0.021	247.8	0.9929	0.00008	208.3
CuMnAl-LDH	102	0.9635	0.014	94.27	0.9425	0.00007	140.8
Al(OH)3	32	0.9105	0.035	65	0.9145	0.00024	45.05

Adsorption Isotherm and Thermodynamics

The effect of initial CR concentrations ranging from 100 to 500 mg/L and temperatures between 30 and 50 °C on the adsorption process are presented in Figure . The results revealed that the adsorption is highly dependent on the initial CR concentration and temperature. The adsorption of CR onto the Al(OH)3/CuMnAl-LDH nanocomposite increases with the increase in initial CR concentrations at all the studied temperatures, which may be because of the increase in the driving force for mass transfer of the CR dye molecules onto the surface of the adsorbent at a high CR dye concentration.[3,11] Moreover, higher adsorption (208 mg/g) at 50 °C revealed that the reaction is endothermic in nature.[26] It is proposed that with an increase in the solution temperature, there is an increase in the kinetic mobility of the adsorbent and adsorbate molecules as they gain energy leading to the improvement of CR dye accessibility to the active site on the surface of the adsorbent molecules.[27]

Figure 7

Effect of temperature on the removal of CR dye onto the Al(OH)3/CuMnAl-LDH nanocomposite.

To investigate the distribution of CR molecules onto the Al(OH)3/CuMnAl-LDH nanocomposite, Langmuir and Freundlich isotherm models are applied to the equilibrium data. The Langmuir isotherm is based on the assumptions that there is an equal affinity for the adsorbate molecules by all the sites on the adsorbent and there is little or no interaction among the adsorbate molecules, which form a monolayer at the adsorbent surface.[28] The linearized equation of the Langmuir model is represented aswhere Ce (mg/L) and qe (mg/g) are the CR concentration at equilibrium and the adsorption capacity at equilibrium, respectively. qm (mg/g) is the measure of the theoretical maximum adsorption capacity for CR dye onto Al(OH)3/CuMnAl-LDH and KL (L/g) represents the Langmuir constant which measures the energy of adsorption.

Unlike the Langmuir isotherm, the Freundlich isotherm assumes that the affinity for adsorbate molecules by all the sites on the adsorbent is not equal and there is interaction among the adsorbate molecules forming a multilayer on the adsorbent surface. The linearized equation of the Freundlich model is represented aswhere Ce and qe are the concentration at equilibrium and adsorption equilibrium, respectively. KF and 1/n represent the Freundlich constants. KF [mg/g (L/mg)1/] is the measure of the adsorption capacity of the adsorbent, that is, the greater the KF, the greater the adsorption, and (1/n) is the indicator of adsorption effectiveness. The favorable adsorption of the Langmuir isotherm can be expressed in terms of an equilibrium parameter or separation factor RL (mg/L).[29,30]RL is a basic characteristic of the Langmuir model.[1] It gives information about the nature of the CR dye adsorption and is represented aswhere b is the Langmuir constant and Ci is the initial concentration of the dye. For a favorable process, RL must be greater than 0 and less than 1 (0 < RL < 1), and the RL values for adsorption of CR dye onto the Al(OH)3/CuMnAl-LDH nanocomposite were well within the favorable adsorption range (Table ). Figure S3 a,b depicts the Langmuir and Freundlich isotherm curves for adsorption of CR onto the Al(OH)3/CuMnAl-LDH nanocomposite at 30, 40, and 50 °C, respectively. Langmuir and Freundlich constants were calculated from the slopes and the intercept of the plot Ce/qe versus Ce and qe versus Ce, respectively. Table shows the values of Langmuir and Freundlich constants. On comparing the values of R2, it has been observed that the Langmuir adsorption model is best fitted to the adsorption data than the Freundlich model. This may be due to homogenous sorption of CR dye on all the sites of the Al(OH)3/CuMnAl-LDH nanocomposite.[20] Furthermore, the RL values obtained for the adsorption of CR dye onto the Al(OH)3/CuMnAl-LDH nanocomposite have the values between 0.007 and 0.036 which are in the range of 0–1, revealing a favorable adsorption process.

Table 2

Langmuir and Freundlich Isotherm Parameters for Adsorption of CR on the Al(OH)3/CuMnAl-LDH Nanocomposite

	Langmuir				Freundlich
temp (°C)	qm mg/g	KL (L/g)	RL	R2	n	KF [mg/g (L/mg)1/n]	R2
30	136.99	0.041	0.009–0.043	0.9957	0.1118	94.60	0.8639
40	163.93	0.152	0.007–0.036	0.9975	0.0923	100.25	0.9097
50	212.97	0.026	0.007–0.036	0.9990	0.0655	112.75	0.9146

The effect of temperature was studied at different dye concentrations, and the related thermodynamic parameters: standard free energy change (ΔG°), standard enthalpy change (ΔH°), and standard entropy change (ΔS°) were calculated from the following equationswhere Kc represents the equilibrium constant, T (K) is the temperature in kelvin, Ce (mg/L) and qe (mg/g) are the equilibrium concentration and adsorption capacity, respectively, and R represents the universal gas constant. The (ΔH°) and (ΔS°) values were obtained from the slope and intercept of the thermodynamic plots of ln(qe/Ce) versus 1/T, and the calculated values of the thermodynamic parameters are shown in Table . At all the temperatures, the values of ΔG° are negative, which indicate a spontaneous nature of the adsorption process; the more negative value of ΔG° with an increase in temperature shows that the process becomes more favorable with an increase in temperature. The positive values of (ΔH°) and (ΔS°) indicate that the reaction is endothermic and there is an increase in randomness in the adsorbent/adsorbate interface during the adsorption process which is consistent with the effect of temperature.

Table 3

Thermodynamics Parameters on Adsorption of CR Dye over the Al(OH)3/CuMnAl-LDH Nanocomposite

T (K)	ΔG° (J/mol)	ΔH° (KJ/mol)	ΔS° (J/mol)
303	–503.7	3.30	0.282
313	–1379.2
323	–2014.06

Co-Adsorption of CR

The adsorptive removal of CR onto the Al(OH)3/CuMnAl-LDH nanocomposite in the presence of Cu(II) ions, Cr(VI) ions, and NaCl salt has been performed to investigate the effect of co-ions on the adsorption process. As shown in Figure a, the co-existence of Cu(II) ions with CR increases the adsorption capacity of the Al(OH)3/CuMnAl-LDH nanocomposite for CR dye. These results suggest that a more positive charge was created by the Cu(II) ion complexes formed on the surface of the composite to create an electrostatic force leading to the more active site for adsorption of the anionic CR dye. However, no significant effect on the adsorption capacity of CR dye onto the Al(OH)3/CuMnAl-LDH nanocomposite in coexistence with Cr(VI) ions was observed. This may be due to the little or no interaction between the Cr(VI) ions and the surface of the Al(OH)3/CuMnAl-LDH nanocomposite. The presence of NaCl salt hindered the adsorption capacity of CR dye on the Al(OH)3/CuMnAl-LDH nanocomposite in an antagonistic effect, causing a reduction of the adsorption capacity at all the studied concentrations. This lead to a hypothesis that there is an exchange of ions between Na+ and the proton of surface OH– groups on the Al(OH)3/CuMnAl-LDH nanocomposite,[31] thereby decreasing the active sites for adsorption of CR dye.

Figure 8

(a) Adsorption of CR on the Al(OH)3/CuMnAl-LDH nanocomposite in a binary mixture of Cu(II), Cr(VI), and NaCl (b) adsorption of Cu(II) and Cr(VI) onto the Al(OH)3/CuMnAl-LDH nanocomposite.

Moreover, to find the interaction between the metal ions and Al(OH)3/CuMnAl-LDH nanocomposite, the adsorption of Cu(II) and Cr(VI) was performed in the absence of CR. The result depicted in Figure b shows complete removal of Cu(II) at a concentration of 10 mg/L, until equilibrium was reached at 50 mg/L concentration. The maximum adsorption capacity recorded was 18.85 mg/L. However, there was no adsorption of Cr (VI) over the Al(OH)3/CuMnAl-LDH nanocomposite. The sorption of the Cu(II) ions on the Al(OH)3/CuMnAl-LDH nanocomposite may be explained using the formation of complexes between the exchangeable anions, that is, NO3–, SO42–, and Cl– in the interlayer of the Al(OH)3/CuMnAl-LDH nanocomposite and the Cu(II) ions. Also, the hydroxide precipitation of Cu(II) ions via chemical bonding with the hydroxyl groups present on the surface of the Al(OH)3/CuMnAl-LDH nanocomposite is another possibility.[32,33] On the other hand, Cr(VI) ions are anionic species and their removal on the Al(OH)3/CuMnAl-LDH nanocomposite mainly depends on the interaction with the protonated sites of the surface of the composite.[34] Therefore, Cr(VI) ions are noninteractive with the Al(OH)3/CuMnAl-LDH nanocomposite, which led to no sorption of Cr(VI) ions. The result seen in this study agrees to our observation in the binary adsorption studies that the precipitated copper complexions on the surface of the Al(OH)3/CuMnAl-LDH nanocomposite protonated the surface of the Al(OH)3/CuMnAl-LDH nanocomposite adsorbent which increases the electrostatic strength for CR dye by providing more active sites for adsorption. Also, Cr(VI) noninteraction with the Al(OH)3/CuMnAl-LDH nanocomposite in a single batch experiment agrees that there is no effect of the sorbates on the adsorption of CR dye onto the Al(OH)3/CuMnAl-LDH nanocomposite in a mixture.

Adsorption Mechanism

The adsorption mechanism is controlled by the nature of the adsorbent and the pollutant.[32] Aluminum trihydrate is characterized as a metal hydroxide-based adsorbent material having an octahedral structure made up of double layers of hydroxyl ions containing two-thirds of aluminum ions within the layers. The hydroxyl ions are arranged in a sequential layer directly opposite to each other in a cubic packing, and the availability of hydroxyl functional groups is of high possibility. These hydroxyl groups may interact electrostatically with the CR dye molecules.[5,35] The anionic nature of the dye suggests sorption of the CR dye by an electrostatic attraction with the positive surface of CuMnAl-LDH. The presence of hydroxyl ions on CuMnAl-LDH may also initiate hydrogen bonding between the OH– group of LDH and the O atoms of the CR dye molecules.[36] The most dominant adsorption mechanism in LDH can be attributed to anionic exchange. The interlayer anions (NO3–, SO42–, and Cl–) present in CuMnAl-LDH are easily replaced by the anionic CR molecules.[37] The Al(OH)3/CuMnAl-LDH nanocomposite is characterized as CuMnAl-LDH having a deposit on Al(OH)3; these Al(OH)3 sheets increase the oxygen functional group in the Al(OH)3/CuMnAl-LDH nanocomposite which provides more vacant active sites, thereby enhancing the adsorption of CR molecules via hydrogen or chemical bonding and electrostatic interaction is formed between the surface of the positively charged Al(OH)3/CuMnAl-LDH nanocomposite and the anionic CR dye under the acidic conditions as discussed in Effect of pH . The number of functional groups and better distribution of CuMnAl-LDH in the Al(OH)3/CuMnAl-LDH nanocomposite are mainly responsible for the higher adsorption of CR onto the Al(OH)3/CuMnAl-LDH nanocomposite in comparison with Al(OH)3 and CuMnAl-LDH. Fourier transform infrared (FTIR) spectroscopy analysis further revealed the adsorption mechanism of the sample before and after adsorption of CR (Figure ). A series of bands in the low-frequency region 1000–400 cm–1 can be ascribed to the vibration of lattice metal–oxygen (Cu–O, Mn–O, and Al–O), and the band at 661 cm–1 confirms the presence of Al–OH. The peaks at 1384 and 934 cm–1 are mainly attributed to the NO3– and SO42– interlayer ions in LDH, respectively. The noticeable changes in the Al(OH)3/CuMnAl-LDH CR spectrum at the bands corresponding to the metal lattice indicate adsorption of CR dye by electrostatic attraction between the metal ions and the CR dye molecules. The disappearance of peaks at 1384 and 934 cm–1 is due to the loss of NO3– and SO42– interlayer ions because of anionic exchange. The shift of hydroxyl groups and interlayer water molecule peak in the Al(OH)3/CuMnAl-LDH CR spectrum demonstrates surface adsorption and hydrogen bonding between the adsorbent and the CR dye molecules. After the reaction, the new peaks at 1578 and 1611 cm–1 represent the −N=N– stretching which is a typical of CR. Other peaks seen at 3475, 1045, and 1174 cm–1 represent the N–H group from the parent dye structure and the SO3– stretching of the sulfonate group of CR dye.[42,43] From these findings, it is presumed that CR dye was incorporated into the sorbent with the active functional group.

Figure 9

FTIR spectrum of the Al(OH)3/CuMnAl-LDH nanocomposite before and after CR adsorption.

Moreover, XPS analysis has been performed to confirm the adsorption of CR onto the Al(OH)3/CuMnAl-LDH nanocomposite. XPS spectra of the Al(OH)3/CuMnAl-LDH nanocomposite before and after CR adsorption are shown in Figure . The survey scan (Figure a) of the Al(OH)3/CuMnAl-LDH nanocomposite shows the presence of Al 2p, C 1s (background carbon), O 1s, Cu 2p, and Mn 2p at their respective binding energies. The survey scan of the CR-adsorbed Al(OH)3/CuMnAl-LDH nanocomposite (Figure a) shows the presence of two extra peaks for N 1s and S 2p, which mainly belong to CR. The high-resolution XPS spectra of all the elements in the Al(OH)3/CuMnAl-LDH nanocomposite are shown in Figure b–e. The high-resolution spectra of Al 2p shown in Figure b show the major peak centered at 74.24 eV mainly because of Al(OH) and a small peak at a binding energy (BE) of 72.99 eV belonging to the metallic Al. The peaks appearing at a BE of 933 eV (Figure c) belong to Cu 2p3/2, which indicates the presence of bivalent Cu in the Al(OH)3/CuMnAl-LDH nanocomposite. The deconvoluted spectra for Mn 2p show two peaks belonging to Mn 2p3/2 and Mn 2p1/2 at 640.96 and 652.56 eV, respectively, with a separation of 11.60 eV, an indication of the presence of Mn2+.[44] The peaks for O 1s at a BE of 530.09 eV belong to the M–O (M = Al, Cu, and Mn) bond and 531.61 eV are assigned to the hydroxyl group of the adsorbed oxygen water.

Figure 10

XPS analysis of the Al(OH)3/CuMnAl-LDH nanocomposite before and after CR adsorption (a) survey scan, (b–e) before CR adsorption, and (f–l) after CR adsorption.

The high-resolution spectra of the Al(OH)3/CuMnAl-LDH nanocomposite after CR adsorption are shown in Figure f–l. The deconvoluted peaks for Al 2p, C 1s, O 1s, Cu 2p, and Mn 2p appeared at the same position with the slight change in the BE because of interaction of the functional groups with CR dye molecules. The presence of the two new peaks for N 1s and S 2p confirms the adsorption of CR.[45] The high-resolution spectra of N 1 s (Figure l) show the peaks at 398.87, 400, 401, and 403 eV corresponding to the C–N, N–H, N=N, and NH2 groups present in the CR molecules, respectively. The deconvoluted spectra of S 2p (Figure k) show the peaks at 162.82, 165.06, and 167.44 eV belonging to the C–S and SO3– groups of CR dye molecules. The appearance of the new peaks for N and S confirms the successful adsorption of the CR dye onto the Al(OH)3/CuMnAl-LDH nanocomposite.

Desorption Studies

In order to evaluate the reusability of the material, the regeneration properties of the Al(OH)3/CuMnAl-LDH nanocomposite were studied in a sorption–desorption cycle. A preliminary desorption study was conducted using solutions of sodium hydroxide (NaOH), hydrogen chloride (HCl), carbon tetrachloride (CCl4), acetone (C3H6O), ethanol (C2H5OH), propanol (C3H8O), acetonitrile (C2H3N), and dimethyl formaldehyde (DMF) (C3H7NO), and the solution with the best performance (DMF) was chosen for the study. The saturated Al(OH)3/CuMnAl-LDH nanocomposite from adsorption was desorbed in DMF solution and shaken for 3 h. The solid Al(OH)3/CuMnAl-LDH nanocomposite particles were washed with deionized water, alcohol, and acetone. After that, it was centrifuged to recover the solid material which was dried at 105 °C for 2 h. The recovered adsorbent was used for the next adsorption cycle. Three cycles were completed (Figure S4), and throughout these cycles, the concentration of CR to the adsorbent was kept constant. The result shows that the adsorption capacity of 65.88 mg/g was sustained after three cycles. The low desorption may be as a result of the chemisorption between the CR molecules and Al(OH)3/CuMnAl-LDH nanocomposite molecules during the adsorption process. After the first use of the Al(OH)3/CuMnAl-LDH nanocomposite, all the exchangeable anions were replaced by the CR molecules. Therefore, the exchangeable anions NO3, SO4, Cl were absent in the layers of the Al(OH)3/CuMnAl-LDH nanocomposite during the regeneration process, and adsorption of CR relied on the hydroxyl groups. Hence, physical adsorption, hydrogen bonding, and electrostatic forces were supposed to be responsible for the adsorption of CR during the reusability study.[38]

Comparison of the Adsorption Capacity of the Various Adsorbents

The adsorption capacity of various adsorbents used for the removal of CR from aqueous solution has been compared with the Al(OH)3/CuMnAl-LDH nanocomposite and is shown in Table . The comparison reveals that the Al(OH)3/CuMnAl-LDH nanocomposite shows better adsorption for the CR molecules, which suggests that the Al(OH)3/CuMnAl-LDH nanocomposite is a suitable adsorbent and can be applied for wastewater purification applications.

Table 4

Comparison of the Adsorption Capacity of Various Adsorbents

adsorbent	CR adsorption capacity (mg/g)	references
ZnFe2O4/SiO2/tragacanth gum magnetic nanocomposite	159.9	(40)
Bael shell carbon	98.03	(41)
ZnO nanoparticles	71.4	(21)
cattail root	38.79	(48)
Mg–Al-LDH	65	(42)
hierarchical NiO nanosheets	152	(46)
carbon fibers	167	(47)
commercial MgO powders	105	(48)
F-MWCNT	148	(49)
Ca-bentonite	107.4	(50)
PS–N+ microspheres	18	(51)
MgO-GO microspheres	237	(52)
NiCo2O4 hollow spheres	366	(53)
hierarchical vaterite spherulites	86	(54)
Al(OH)3/CuMnAl-LDH	175	this work

Conclusions

In this study, Al(OH)3, CuMnAl-LDH, and Al(OH)3/CuMnAl-LDH nanocomposite were synthesized and characterized by various techniques. Based on the result, the sorbents showed a high adsorption capacity for CR dye in the order Al(OH)3/CuMnAl-LDH > CuMnAl-LDH > Al(OH)3. The presence of more functional groups and layer anions contributed to the superior adsorption capability of the Al(OH)3/CuMnAl-LDH nanocomposite. The adsorption reaction was endothermic, and the maximum adsorption capacity of CR dye was attained at pH 4. In the binary systems of combined pollutants with CR dye, Cu(II) ions had a synergistic effect, Cr(VI) ions had an antagonistic effect, and NaCl salt had no effect on CR dye adsorption by the Al(OH)3/CuMnAl-LDH nanocomposite. Besides, the Al(OH)3/CuMnAl-LDH nanocomposite was more effective in removing Cu(II) ions from aqueous solution than Cr(VI) ions. The kinetic and isotherm adsorption studies of CR dye onto the Al(OH)3/CuMnAl-LDH nanocomposite revealed that the experimental model fitted well with the pseudo-second-order and Langmuir model which indicates chemisorption and homogeneity of sorption on all the sites of the Al(OH)3/CuMnAl-LDH nanocomposite. These results revealed that the Al(OH)3/CuMnAl-LDH nanocomposite is an efficient adsorbent which can be used for the removal of the contaminants from the wastewater.

Materials and Methods

Materials

CR dye (C32H22N6Na2O6S2) and urea were procured from Techno Pharmchem, Haryana, India. CuSO4·5H2O was procured from Chadwell Heath, Essex, England, and BDH Chemicals Ltd. Sigma-Aldrich Co., USA, supplied sodium hydroxide, and Al2(NO3)3·9H2O was obtained from Panreac Quimica SAU, Spain. Scharlab S.L., Spain, supplied sodium dodecyl sulfonate (SDS). MnCl2·4H2O, K2Cr2O7,, and ammonia solution were supplied by BDH Chemicals Ltd., Poole, England.

Aluminum Hydroxide Preparation

Al(OH)3 was synthesized via a hydrothermal method. The solutions were prepared by mixing 2.8 g SDS and 22.9 g Al2(NO3)3·9H2O in 70 and 40 mL of deionized water, respectively, at 50 °C. After that, the SDS solution was added slowly to the Al2(NO3)3·9H2O solution under stirring at 50 °C. After that, 12 mL of ammonia (25%) was added dropwise into the mixture and stirred for 2 h. The obtained white precipitate was transferred to a hydrothermal reactor and heated for 90 h at 120 °C in the oven. The reactor was allowed to cool down to room temperature after the completion of the reaction, and the resulting solution was centrifuged to collect the solid product which was thoroughly washed with deionized water, ethanol, and acetone and subsequently dried at 105 °C to get Al(OH)3.

Synthesis of CuMnAl-LDH and the Al(OH)3/CuMnAl-LDH Nanocomposite

A typical procedure for the synthesis of the Al(OH)3/CuMnAl-LDH nanocomposite is described as follows. Initially, 0.31 g CuSO4·5H2O, 0.33 g Al2(NO3)3·9H2O, and 0.25 g MnCl2·4H2O were dissolved into 50 mL of deionized water. Thereafter, 0.5 g Al(OH)3 powder was added to the metal ion solution and stirred for 2 h. After that, 1.9 g urea and 0.02 g NaOH were added to the solution under stirring conditions. After 2 h of stirring, the solution was transferred to the hydrothermal reactor and heated in the oven for 20 h at 130 °C. After cooling of the reactor, the precipitate was filtered, thoroughly washed with deionized water, ethanol, and acetone, and dried at 105 °C. CuMnAl-LDH was synthesized using the same method in the absence of Al(OH)3.

Adsorption Experiment

The experimental adsorption procedure for the removal of CR dye and heavy metals [Cu(II) and Cr(VI) ions] onto the synthesized materials was performed using the batch method. A constant mass of 0.02 g of each adsorbent was added to 20 mL of the CR dye and/or heavy metal solution of known concentration in a conical flask under constant agitation at 150 rpm and fixed pH and temperature. Standard solutions of HCl and NaOH (0.1 M) were used to adjust the pH of the dye solution in the range 4–10. The adsorption equilibrium was studied using the CR dye concentrations between 100 and 500 mg/L and Cu(II) and Cr(VI) of concentrations from 10 to 50 mg/L. The samples were drawn after a fixed time interval from conical flasks into the vials and centrifuged on the TG16- WS centrifuge system at 4000 rpm to separate the adsorbent from the solution. The Cu(II) and Cr(VI) concentrations remaining in solution after adsorption was analyzed using the powder pillow bicinchoninate method (HACH—cuVer 1 copper reagent) and 1,5-diphenylcarbohydrazide method (HACH—chromaVer 3 chromium reagent), respectively, while the CR dye concentration was analyzed by a UV–visible spectrophotometer (HACh DR 6000) at 495 nm wavelength. The adsorption capacity of dye removal was calculated using the following formulawhere qe is the adsorption capacity at equilibrium, Ci is the initial dye concentration (mg/L), Ce is the dye concentration at adsorption equilibrium (mg/L), V is the volume of dye solution (L), and m is the weight of the adsorbent (g).

Instrumentation

The XRD was carried out using the XRD Ultima IV X-ray diffractometer using Cu Kα X-ray radiation (λ = 1.5418 Å) at 40 kV and a current of 40 mA, in the scan range of 5–80°. The surface morphologies of the adsorbents were determined using the scanning electron microscope(JEOL-JSM7600 F) and high-resolution transmission electron microscope (Tecnai G2 F20, FEI, USA), and UV–visible absorbance analysis was recorded on the Lambda 900 (PerkinElmer UV WinLab 6.0.4.0738/1.61.00 Lambda 900), while the study of PL was carried out using the PerkinElmer 750, USA.