Itaconic acid-modied layered double hydroxide as a novel adsorbent for effective removal of Congo red from aqueous solutions

21 Herein, we report the synthesis of Cu-Ca-Al/NO 3 -based layered double hydroxide through 22 co-precipitation methodology. The prepared layered double hydroxide was then modified 23 with itaconic acid. The physicochemical properties of the prepared materials were studied 24 using Fourier transform-infrared spectroscopy, scanning electron microscopy, X-ray 25 diffraction analysis, thermogravimetric analysis, and nitrogen adsorption/desorption 26 technique. The prepared materials were then applied as novel adsorbents for the removal of 27 Congo red as a model of an anionic dye from aqueous media. To reach maximum adsorption, 28 the effect of parameters including sample solution pH, adsorbent amount, contact time, and 29 initial concentration of Congo red on the adsorption process was investigated. Kinetic studies 30 were also conducted to study the mechanism of adsorption. In this regard, the kinetic models 31 of pseudo-first-order, pseudo-second-order, Elovich, and intra-particle diffusion were studied. 32 The results showed that the adsorption of Congo red onto Cu-Ca-Al-LDH and LDH-ITA 33 adsorbents followed the pseudo-second-order kinetic model. To evaluate the equilibrium 34 adsorption data, different isotherms including Langmuir, Freundlich, and Dubinin- 35 Radushkevich were also applied. The data revealed that the Freundlich isotherm provided the 36 best fit with the equilibrium data of both adsorbents. Maximum adsorption capacities of 81 37 and 84 mg g -1 were obtained using Cu-Ca-Al-LDH and LDH-ITA adsorbents, respectively. 38 39 40


Introduction
As a growing trend, nanotechnology has attracted huge interest among scientists in 47 various functionalization strategies have been used for the modification of LDHs. Some 71 common functionalization strategies included intercalation, hybrid assembly, surface 72 modification, size and morphology regulation, layer composition tuning, and defect 73 introduction which are discussed in detail in a reported review by Laipan et al [27]. 74 As an important environmental concern in both developing and industrial countries, the 75 lack of clean water due to the disposal of various toxic compounds into the environment 76 caused drastic concerns about the creature's health. As an unpleasant result of the 77 industrialization process, water pollution is an international problem, and nowadays 78 scientists, and policymakers are beginning to take more notice of this vital problem to find a 79 suitable solution. Among the well-known pollutants, synthetic organic dyes are one of the 80 important groups of water pollutants. The release of these contaminants is an increasing and 81 serious global challenge from the environmental point of view. These relatively complex 82 organic molecules are widely used in different industries such as leather tanning, food 83 processing, paper making, cosmetics, textile, and plastics. As a water-soluble member of 84 synthetic dyes, Congo red is used in a huge quantity in textile and biochemistry based 85 industries [28]. It is a benzidine-based anionic dye that was discovered by Paul Bottinger in 86 the adsorption strategy provided a simple, low cost, and relatively fast methodology with 96 high efficiency. This conventional strategy is used in advanced wastewater treatment due to 97 its ease of operation, and flexibility in adsorbent design. This method also produces no 98 harmful by-products and prevents sludge formation during the removal process [15]. Until 99 now, various synthetic and natural adsorbents have been used for the adsorption of organic 100 dyes which is discussed in some useful review reports [31][32][33]. Notably, developing 101 adsorbents with improved characteristics to be used for the adsorption process is an ongoing 102 trend. 103 Herein, we present the synthesis and characterization of a novel adsorbent of itaconic 104 acid-modified Cu-Ca-Al layered double hydroxide for adsorption purposes. The Cu-Ca-Al 105 layered double hydroxide was synthesized at first and then modified with itaconic acid 106 through a simple approach. To study the applicability of the prepared materials for adsorption 107 purposes, Congo red as an anionic dye was selected to be removed from the aqueous solution.

108
Kinetic studies were also conducted for the prepared materials. To the best of our knowledge, 109 this is the first report to study the potential application of itaconic acid-modified Cu-Ca-Al 110 layered double hydroxide for adsorption purposes.   mmol L -1 of Na2CO3 was prepared. Then, the two prepared solutions were slowly added to a 143 500 mL round bottom glass flask. The pH of the solution was kept between 10 and 11 during the addition. After the addition, the mixture was stirred at room temperature for 60 min. 145 Then, the temperature of the solution was enhanced to 60 °C and maintained for 18 h. 146 Afterward, the slurry was cooled and filtered. The prepared material (Cu-Ca-Al-LDH) was 147 washed several times with deionized water and dried in an oven at 85 °C.
In this equation, W is the adsorbent amount (mg) and V is the solution volume (mL).

194
The FT-IR spectra of the prepared Cu-Ca-Al-LDH and LDH-ITA are shown in Fig. 1.

195
In the case of Cu-Ca-Al-LDH, the broadband located at 3435 cm -1 is associated with the  The XRD patterns of the prepared Cu-Ca-Al-LDH and LDH-ITA are shown in Fig. 2a.

214
In the case of Cu-Ca-Al-LDH, it exhibited characteristic peaks located at 2θ = 23. In contrast, some diffraction peaks became stronger or new peaks appeared. These

248
The N2 adsorption/desorption isotherms of the prepared Cu-Ca-Al-LDH and LDH-ITA 249 are shown in Fig. 4. As can be seen in Fig. 4, Cu-Ca-Al-LDH showed a combination of type 250 III and IV isotherms with H3-type hysteresis loops according to the IUPAC classification.

350
(L mg -1 ) 1/n ), B (mol 2 kJ -2 ), R (j mol -1 K -1 ), and T (K) are the concentration of Congo red at equilibrium, adsorption capacity at equilibrium, the maximum adsorption capacity of 352 adsorbent, the Langmuir constant, the Freundlich isotherm constants for adsorption capacity 353 and adsorption intensity, the Dubinin-Radushkevich isotherm constant, the universal gas 354 constant, and temperature, respectively. Figure 7 and Table 3 show the adsorption isotherms and Dubinin-Radushkevich (d) for adsorption of Congo red by the prepared adsorbents. 363 Table 3 The parameters obtained by isotherm models for the adsorption of   Table 4 compares the adsorption capacities of several adsorbents as well as 371 corresponding optimal adsorption conditions. As shown in Table 4, the highest adsorption 372 capacities were observed at acidic pHs for all adsorbents. Also, compared to the adsorbents 373 reported in Table 4, adsorbents synthesized in this work have a shorter adsorption time and a 374 higher adsorption capacity, which indicates the acceptable performance of these adsorbents.

375
One possible reason for observing this acceptable performance is the adsorption mechanism,    initial concentration of the dye) was studied. The kinetic studies showed that the best fit was achieved with the pseudo-second-order model for the adsorption of Congo red using the two 390 prepared adsorbents. Three isotherm models were used to investigate the equilibrium 391 adsorption studies. The data showed that the Freundlich model showed the best fit with R 2 392 values of 0.9411 and 0.9320 for Cu-Ca-Al-LDH and LDH-ITA adsorbents, respectively.

393
Maximum adsorption capacities of 81 and 84 mg g -1 were obtained using Cu-Ca-Al-LDH and 394 LDH-ITA adsorbents, respectively. Finally, the prepared adsorbents showed acceptable 395 characteristics to be considered as effective adsorbents for anionic dye (e.g. Congo red) 396 removal from aqueous media.       The XRD patterns (a) and the TGA thermograms (b) of the prepared Cu-Ca-Al-LDH and LDH-ITA.  The N2 adsorption/desorption isotherms of Cu-Ca-Al-LDH (a), LDH-ITA (b), and the BJH pore size distribution curves of the samples (c).

Figure 5
The effect of pH (a), adsorbent amount (b), and contact time (c) on the adsorption of Congo red by the prepared adsorbents.

Figure 6
The kinetic adsorption models of pseudo-rst-order (PFO), pseudo-second-order (PSO), Elovich, and intraparticle diffusion (IPD) for adsorption of Congo red by the prepared adsorbents. The equilibrium isotherm (a) and the isotherm models of Langmuir (b), Freundlich (c), and Dubinin-Radushkevich (d) for adsorption of Congo red by the prepared adsorbents.

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