Organic dye removal from aqueous solutions by hierarchical calcined Ni-Fe layered double hydroxide: Isotherm, kinetic and mechanism studies

Accepted Manuscript Organic dye removal from aqueous solutions by hierarchical calcined Ni-Fe layered double hydroxide: Isotherm, kinetic and mechanism studies Chunsheng Lei, Meng Pi, Panyong Kuang, Yingqing Guo, Fenge Zhang PII: DOI: Reference:

S0021-9797(17)30192-3 http://dx.doi.org/10.1016/j.jcis.2017.02.025 YJCIS 22052

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

17 December 2016 8 February 2017 12 February 2017

Please cite this article as: C. Lei, M. Pi, P. Kuang, Y. Guo, F. Zhang, Organic dye removal from aqueous solutions by hierarchical calcined Ni-Fe layered double hydroxide: Isotherm, kinetic and mechanism studies, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis.2017.02.025

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Organic dye removal from aqueous solutions by hierarchical calcined Ni-Fe layered double hydroxide: Isotherm, kinetic and mechanism studies Chunsheng Leia,b,*,[email protected], Meng Pia,b, Panyong Kuangb, Yingqing Guoa, Fenge Zhanga a College of Environmental and Safety Engineering, Changzhou University, Changzhou 213164, PR China b

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P. R. China *

Corresponding author.

Graphical abstract

ABSTRACT Hierarchically porous nickel–iron-layered double hydroxide (NiFe-LDH) with a Ni2+/Fe3+ molar ratio of 3 was successfully synthesised through a simple hydrothermal route. After calcination at 400 °C, NiFe-LDH transformed into nickel–iron-layered double oxides (NiFe-LDO). The as-prepared samples were characterised through X-ray powder diffraction, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and nitrogen adsorption. The calcined and uncalcined NiFe-LDH was used as adsorbents to remove Congo red (CR) dye in an aqueous 1 / 24

solution. The equilibrium adsorption data of NiFe-LDH and NiFe-LDO samples were well fitted to Langmuir model and were characterised by excellent adsorption capacities of 205 and 330 mg/g, respectively. Pseudo-second-order kinetic and intra-particle diffusion models indicated that CR was well adsorbed on the adsorbent. The underlying adsorption mechanism was investigated and observed as anion exchange and reconstruction.

Keywords: Hierarchical porous structures; LDH; Congo red; Adsorption; Mechanism

1. Introduction Organic dyes have been widely used in various industrial processes, including textiles, leather, printing, dyes and plastics [1-3]. Dye pollutants discharged into water released from these industries present a serious threat to human health and the ecosystem because of their high toxicity, carcinogenicity and mutagenicity [4-6]. Therefore, rational and efficient methods should be developed to remove dyes from wastewater. Various wastewater treatment techniques, such as adsorption [7], coagulation [8], flocculation [9], electrochemical method [10], photocatalysis [11], ion exchange [12], and biodegradation [13], have been applied to eliminate dye effluents from water. Among these techniques, adsorption is the most promising strategy because of its high efficiency, simple operation and low energy requirement [14]. An appropriate adsorbent is central to the adsorption method. Various materials, such as activated carbon [15], fly ash [16], clay minerals [17], silicon nanomaterials [18], and metal oxides [7,19], have been used as adsorbents. However, the application

2 / 24

of these adsorbents is limited by different factors, such as low adsorption capability, high cost and difficult regeneration. Therefore, new adsorbents with excellent adsorption capability and low production costs should be developed for theoretical research and practical applications. Layered double hydroxides (LDHs, [M1−x2+Mx3+(OH)2Ax/nn−·mH2O]) as a class of anionic clays [20] have been extensively investigated because of their tunable charge density and wide application prospects [21,22], such as catalysts [23-25], biological agents [26], energy storage and conversion [27-29]. LDHs have been considered excellent adsorbent materials for wastewater treatment because of their layered structure, high surface area and interlayer ion exchange [22,30-32]. Easily prepared LDHs, such as ZnAl-LDH [22], MgAl-LDH [4], MgFe-LDH [32] and CuAl-LDH [33], have been applied to remove dyes. These LDHs provide advantages over traditional adsorbents in terms of high adsorption capacity, low cost and non-toxicity. Thus, the utilisation of LDHs can provide substantial economic and environmental benefits to wastewater treatment. LDH microspheres with hierarchically porous structures have also been widely explored because of their excellent surface area and structural stability. LDH microspheres have also been utilised for adsorption. For instance, Lin et al. [30] synthesised ZnAl-LDH microspheres through co-precipitation and compared their adsorptive and removal properties for methyl orange. Ahmed et al. [32] prepared Mg–Fe–CO3-LDH to adsorb anionic reactive dye. Sun et al.[34] synthesised hierarchically porous NiAl-LDHs via a hydrothermal method and investigated their capacity to adsorb p-nitrophenol from water. 3 / 24

Congo red (CR) as a typical anionic diazo dye is usually chosen as a model pollutant because of its toxicity, carcinogenicity and poor degradation. In this work, a simple hydrothermal method was used to prepare nickel–iron-layered double hydroxide and oxide with a hierarchical porous structure and a large surface area. These compounds were utilised as adsorbents to remove CR dye from water. 2. Experimental 2.1. Materials All reagents, such as Ni(NO3)2·6H2O, FeSO4·7H2O, urea and CR (Shanghai Chemical Industrial Company) were of analytical grade and used as received without further purification. Distilled water was utilised in synthesis and treatment processes. 2.2. Preparation of samples Hierarchical porous nickel–iron-layered double hydroxide (NiFe-LDH) was prepared hydrothermally. In a typical synthesis process, 3 mmol Ni(NO3)2·6H2O, 1 mmol FeSO4·7H2O and 12 mmol urea were dissolved in 60 mL of deionised water. The resulting solution was magnetically stirred for 30 min, transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 24 h. The precipitate was centrifuged, washed several times with deionised water and absolute ethanol and dried at 80 °C for 12 h to obtain the NiFe-LDH. The NiFe-LDH sample was calcined in air at 400 °C for 4 h in a muffle furnace to obtain nickel–iron-layered double oxides (NiFe-LDO). 2.3. Characterization X-ray powder diffraction (XRD) measurements were performed using an X-ray 4 / 24

diffractometer (type HZG41B-PC) with Cu Kα irradiation at 40 kV, 80 mA and 0.05° 2θ s−1 scanning rate. The morphological characteristics of the samples were determined through field-emission scanning electron microscopy (FE-SEM, JSM-7500F Hitachi, Japan) at an accelerating voltage of 15 kV (linked with an Oxford Instruments X-ray analysis system). The Brunauer–Emmett–Teller (BET) surface area (SBET) of the powders was measured through nitrogen adsorption in a Micromeritics TriStar П 3020 nitrogen adsorption apparatus (USA) at a relative pressure (P/P0) range of 0.05–0.3. The adsorption branches of the nitrogen adsorption–desorption isotherm were utilised to determine pore size distribution through Barrett–Joyner–Halenda method. Pore volume and average pore size were obtained according to the nitrogen-adsorption volume at P/P0 = 0.97. The Fourier transform infrared (FTIR) spectra of the samples were obtained by using a Shimadzu IR Affinity-1 spectrometer with KBr pellets as a support within a wavenumber range of 4000–400 cm−1. X-ray photoelectron spectroscopy (XPS) measurements were performed using an ultra-high vacuum VG ESCALAB 210 electron spectrometer equipped with a multichannel detector. Binding energy was referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon.

2.4. Kinetic adsorption of experiments Kinetic tests were conducted to evaluate the effect of time on adsorption and to quantify the adsorption rate. The adsorption kinetics of CR was investigated at 30 °C in a thermostatic shaker. In a typical procedure, 10 mg of the sample powder was added to 100 mL of CR solution (50 mg/L). Analytical samples were extracted at 5 / 24

pre-set time intervals and separated through centrifugation. The concentration of CR was measured on the basis of the maximum adsorption peak of CR (497 nm) by using a UV/Vis spectrophotometer (Shimadzu UV/Vis 1240 spectrophotometer, Japan). The amount of CR adsorbed at time t [qt (mg/g)] was calculated on the basis of the following equation:

qt 

(C0  Ct )V W

(1)

where C0 and Ct (mg/L) are the initial concentration of CR and the concentration of CR at specified time t, respectively, V (L) is the volume of the CR solution and W (g) is the mass of the adsorbent used. 2.5. Equilibrium adsorption experiments CR adsorption isotherms were obtained by adding a defined amount of the as-prepared adsorbents (10 mg) to a series of 250 mL conical flasks containing 100 mL of diluted CR solutions (10–100 mg/L). The flasks were sealed and kept at a thermostatic shaker and shaken at 150 rpm for 15 h at 30 °C. Afterwards, the solutions were separated from the samples through centrifugation. The amount of CR adsorbed on the sample at equilibrium qe (mg/g) was calculated by using the following equation:

qe 

(C0  Ce )V W

(2)

where C0 and Ce (mg L−1) are the initial and equilibrium concentrations of CR, respectively, V (L) is the volume of the CR solution and W (g) is the mass of the adsorbent used. 6 / 24

3. Results and discussion 3.1. Phase structure and morphology Fig. 1 demonstrates the XRD patterns of the NiFe-LDH and NiFe-LDO samples. The NiFe-LDO samples after CR was adsorbed (Fig. 1). Nine major peaks of the NiFe-LDH sample were located at approximately 11.4°, 23.0°, 34.4°, 39.0°, 45.98°, 59.9°, 61.3°, 65.1° and 71.3°. These peaks can be assigned to the (003), (006), (012), (015),

(018),

(110),

(113),

(116)

and

(119)

planes

of

Ni0.75Fe0.25(CO3)0.125(OH)20.38H2O (JCPDS Data Card No. 40-0215) [35]. After calcination occurred, the layered structure of NiFe-LDH material was destroyed and changed into bimetallic oxide NiFe-LDO. The NiFe-LDO sample exhibited three weak diffraction peaks located at approximately at 30.3°, 35.7° and 57.4° that corresponded to the (220), (311) and (511) planes of spinel NiFe2O4 (JCPDS Data Card No. 54-0964) [36]. The four obvious peaks located at 37.3°, 43.3°, 62.9° and 75.4° could be indexed to the (111), (200), (220) and (311) planes of bunsenite syn NiO (JCPDS 47-1049) [37,38]. After CR was adsorbed, the primary peaks of NiFe-LDO remained the same. A broad peak centred at approximately 23.0° appeared, and this peak was indexed to the (006) plane of NiFe-LDH. The NiFe-LDO samples underwent rehydration and structural reconstruction through "memory effect" [39, 40].

Fig. 2a presents the FE-SEM images of the NiFe-LDH samples. The NiFe-LDH samples exhibited a hierarchical porous structure containing microspheres with an 7 / 24

average diameter of approximately 7 µm. The high-magnification FE-SEM image (inset in Fig. 2a) shows that the microspheres were assembled from two-dimensional (2D) nanosheets. The NiFe-LDO microspheres were also obtained after the NiFe-LDH samples were calcined at 400 °C in air for 4 h. The morphological characteristics of the NiFe-LDO particles are illustrated in Fig. 2b. The overall structure was retained after the samples were calcined. This finding indicated that the material was thermally stable. Compared with the NiFe-LDH samples, the NiFe-LDO microspheres possessed an improved pore structure (inset in Fig. 2b) possibly because of the loss of volatile gases, such as H2O and CO2, during thermal treatment. The energy dispersive X-ray spectroscopy (EDS) spectrum in Fig. 2c revealed that the NiFe-LDO sample was mainly composed of O, Ni and Fe. The peak of Al originates from the Al base. Fig. 2d, e and f show the element mapping images of the NiFe-LDO sample. O, Ni and Fe are distributed homogeneously in the microspheres. Consistent with the XRD data, these results indicated that bimetallic oxide NiFe-LDO was successfully prepared.

3.2. Textural properties Fig. 3 illustrates the nitrogen isotherms and the corresponding pore size distribution curves of the NiFe-LDH and NiFe-LDO samples. The isotherms of NiFe-LDH and NiFe-LDO can be defined as type IV with H3 hysteresis loops, which are characteristics of mesoporous materials. Further observation revealed that the features of the hysteresis loops of the two samples are different from each other. This 8 / 24

attribute indicated the variation of pore structures. The hysteresis loops of NiFe-LDH at a relative pressure range of 0.45–1 suggested the presence of mesoporous materials in the hierarchal spheres and slit-like pores because of nanosheet aggregation. The hysteresis loops of NiFe-LDO at a relative pressure range of 0.8–1 implied the existence of large mesopores and macrospores. The power spectral density curves of the samples showed wide pore size distributions from 2 nm to more than 100 nm with a small peak at 3 nm and a broad peak centred at ca. 45 nm. This observation further confirmed the presence of mesopores and macropores. Table 1 lists the pore structure parameters of the NiFe-LDH and NiFe-LDO samples. These parameters indicated that the NiFe-LDO sample possessed a relatively large specific surface area (121 m2 g−1). The NiFe-LDH sample exhibited a low surface area (60 m2 g−1). This finding could be attributed to the loss of interlayer water during thermal treatment. High surface area and hierarchical pore structure were conducive to CR diffusion and adsorption. Thus, adsorption capacity was increased.

3.3. FTIR and XPS analysis Fig. 4 shows the FTIR spectra of the NiFe-LDH and NiFe-LDO samples and the samples after CR was adsorbed. The intense and broad adsorption bands at approximately 3439 cm−1 are associated with the stretching vibration of O–H arising from the hydroxyl groups in the interlayer water molecules [4]. The band at 1640 cm−1 is ascribed to the bending vibration of water [4,23]. The peaks occurring at 1116 cm−1 are attributed to the vibration of carbonate species [32]. The peaks at approximately 1360 and 1040 cm−1 are associated with CO32− in the interlayer [41,42], 9 / 24

which became relatively strong after CR was adsorbed because the part of the CO32− was replaced by SO32−. The two peaks did not appear in the FTIR spectrum of the NiFe-LDO sample, whereas two new peaks at 1365 and 1045 cm−1 were observed after CR was adsorbed. This result suggested that SO32− were successfully intercalated into the interlayer spaces of NiFe-LDO during adsorption. Consistent with XRD analysis results, these findings corresponded to the memory effect of NiFe-LDH. Furthermore, the band at ca. 628 cm−1 is attributed to the Fe–O bond [43]. The peaks occurring at ca. 551 and 510 cm−1 are ascribed to the bonding between Ni and O [44,45]. The detailed chemical composition and chemical status of the NiFe-LDO samples were further determined through XPS. The survey spectrum (Fig. 5a) indicated that the NiFe-LDO sample was mainly composed of Ni, Fe, O and C. In Fig. 5b, the XPS spectra of Fe species present two dominant peaks at approximately 712.2 and 724.7 eV, which are ascribed to the binding energies of Fe 2p 3/2 and Fe 2p 1/2 of Fe3+, respectively [46,47]. In Fig. 5c, the high-resolution XPS spectrum of Ni 2p displays four peaks at 855.7, 873.3, 861.7 and 879.7 eV, which can be attributed to Ni 2p 3/2 and Ni 2p 1/2 of Ni2+ and their corresponding satellites, respectively [46,48]. The O 1s spectrum is shown in Fig. 5d. The peak fitting of the O 1s spectra is located at 530.1 and 532.6 eV corresponding to O–Ni and O–Fe bonding, respectively [49-51]. The last peak at 531.5 eV is attributed to the surface hydroxyl oxygen [52]. The result of the XPS analysis conforms to the previous outcomes of EDS and FTIR. This finding significantly demonstrated the bimetallic mixed oxides in the NiFe-LDO 10 / 24

sample.

3.4. Adsorption kinetic and isotherm Adsorption kinetics is essential for adsorption investigation because this process can predict the adsorption rate and elucidate the adsorption mechanism of sorption reactions [53]. In our study, the effect of contact time on CR removal by NiFe-LDH and NiFe-LDO samples is presented in Fig. 6. The two samples exhibited high adsorption rates at an initial time of 50 min. Afterwards, the adsorption rates of the samples gradually decelerated over time, and the final quasi-equilibrium was achieved within ca. 250 min. Under the same experimental conditions, the NiFe-LDO sample presented high adsorption rate and capacity because of its large surface areas and pore volumes (Table 1). Moreover, the strong adsorption affinity of NiFe-LDO sample was attributed to its unusual structural reconstruction [42].

The kinetics of CR adsorption on the NiFe-LDH and NiFe-LDO samples were fitted by various kinetic models, including pseudo-first-order, pseudo-second-order kinetic and intraparticle diffusion, to explore the adsorption mechanism [32,33,54]. The linear forms of these equations are expressed respectively as follows:

log(qe  qt )  log qe 

k1 t 2.303

(3)

t 1 t   2 qt k2 qe qe

(4)

qt  kdi t  Ci

(5)

11 / 24

where qe and qt (mg·g−1) are the amounts of CR adsorbed at equilibrium and at a specified time t (min), respectively. k1 (min−1) and k2 (g mg−1 min−1) are the pseudo-first-order and pseudo-second-order rate constants, respectively. Kdi (mg g−1 min−1/2) and Ci are the intra-particle diffusion rate constant and intercept of stage i, respectively. The calculated kinetic parameters and correlation coefficients (R2) of the kinetic model are listed in Table 2. All of the correlation coefficients (R2) of pseudo-second order were greater than 0.996, which was higher than that of pseudo-first order. The calculated qe of pseudo-second order was close to the experimental qe. The normalised standard deviation (S.D., %) of the samples in the pseudo-second-order model was much lower than that of the pseudo-first-order model (Table 2). These results indicated that the pseudo-second-order kinetic model is suitable for the characterisation of CR adsorption on the samples. Fig. 7a and b present the linear plots of log (qe − qt) versus t and t/qt versus t for the adsorption of CR on the prepared samples, respectively. Fig. 8 illustrates the linear fitting of the intraparticle diffusion model of CR adsorption on the NiFe-LDH and NiFe-LDO samples. The kinetics for the transport of CR from the solution to the surface of the prepared samples can be controlled by three processes. In the first sharp region, CR molecules diffuse through the solution to the external surface of the samples. This process labelled instantaneous or external surface adsorption is completed within the first 6 min. This portion is caused by high initial CR concentrations and numerous active adsorption sites [55]. The adsorption rates gradually decline in the second portion referred to as the gradual or slow 12 / 24

adsorption stage. This stage is due to intraparticle diffusion [56]. The last portion is the final equilibrium stage, in which intraparticle diffusion further slows down until it reaches equilibrium because of the extremely low adsorbate concentrations retained in the solutions [57]. Table 3 displays the parameters of intraparticle diffusion during adsorption. The adsorption of CR into the prepared samples followed the intraparticle diffusion model.

The adsorption isotherms varying with the initial dye concentration were examined to investigate the adsorption capacity of CR on the NiFe-LDH and NiFe-LDO samples. Fig. 9 shows the adsorption isotherms of CR on the prepared samples at 30 °C. This observation suggested the efficient removal of CR from the aqueous solution by NiFe-LDH and NiFe-LDO. Langmuir and Freundlich models were used to describe adsorption. According to Langmuir isotherm theory, no interaction is observed between adsorbed molecules, and adsorption occurs in a monolayer [58]. Freundlich isothermal equation is an empirical model, which assumes that adsorption takes place on heterogeneous surfaces or different surface supporting sites [59]. The model assumes that strong binding sites are occupied primarily, and binding strength decreases as the degree of site occupation increases. Langmuir and Freundlich equations are expressed as follows [31,42]:

Ce C 1  e  qe qmax K L qmax 13 / 24

(6)

log qe  log K F 

1 log Ce n

(7)

where Ce is the equilibrium concentration of CR in the solution; qe is the amount of CR adsorbed at equilibrium (mg g−1), qmax and KL are the theoretical maximum adsorption capacity (mg g−1) and Langmuir adsorption equilibrium constant (L mg−1), respectively, and 1/n and KF are the heterogeneity factor and Freundlich constant [(mg/g)(L/mg)1/n], respectively. The linear plots of Langmuir and Freundlich models of the CR adsorption on the NiFe-LDH and NiFe-LDO samples are shown in Fig. 10. The fitted constants of the two models and the regression coefficients are listed in Table 4. In Fig. 10a, the Langmuir isotherm presented a straight line, and the correlation coefficients of all samples reached 0.999. Fig. 10b represents the plot of the experimental data based on Freundlich isotherm model with correlation coefficients of 0.876 and 0.961 for NiFe-LDH and NiFe-LDO, respectively. The adsorption of CR on the prepared samples fitted the Langmuir isotherm reasonably well. This finding indicated that the adsorption of CR dye molecules was represented well by a monolayer-localised adsorption model. In Table 4, the theoretical maximum adsorption capacities of NiFe-LDH and NiFe-LDO for CR calculated from the Langmuir isotherm model are 205 and 330 mg/g, respectively. This value is larger than that of the other adsorbents containing iron or nickel. Table 5 shows the results of the as-prepared samples compared with other adsorbents for CR removal.

3.5. Adsorption mechanism The NiFe-LDH and NiFe-LDO samples show excellent adsorption performance. Thus, the adsorption mechanisms of NiFe-LDH and NiFe-LDO samples should be investigated. The NiFe-LDH samples might remove CR dye from a solution via two 14 / 24

steps: (i) adsorption occurred on the external surface through electrostatic attraction [65], and (ii) interlayer anions in LDH structures were replaced by SO 32− in the CR dye via anion exchange (Path 1 in Fig. 11) [4]. For NiFe-LDO samples, the layered structure was destroyed because of interlayer carbonate decomposition and water release when the NiFe-LDH samples were calcined at 400 °C for 4 h. Moreover, the BET surface area of the NiFe-LDO samples increased (Table 1). This result was conducive to the adsorption of CR. After the NiFe-LDO samples were reconstructed in CR aqueous solution with the intercalation of CR and CO 32−/HCO3−/OH− in water into the layered structure, the LDH structure reconstruction was stimulated by the ‘memory effect’ (Path 2 in Fig. 11) [48]. Therefore, CR dye can be removed through this method. 4. Conclusions

In summary, NiFe-LDH with hierarchical porous structures was successfully synthesised through a facile hydrothermal route. After the sample was calcined at 400 °C, NiFe-LDH transformed into NiFe-LDO. The BET surface area increased from 60 m2/g to 121 m2/g. The maximum adsorption capacities of the NiFe-LDH and NiFe-LDO samples of CR calculated from the Langmuir isotherm model were 205 and 330 mg g−1, respectively. The experimental data of the samples well fitted to the Langmuir isotherm model and pseudo-second-order model. The intra-particle diffusion models demonstrated that adsorption was controlled by three steps. Ion exchange and reconstruction can explain the adsorption mechanism of the NiFe-LDH and NiFe-LDO samples. Therefore, the products exhibit a considerable potential in 15 / 24

wastewater treatment because of their pore structures and excellent adsorption capacity.

Acknowledgements This study was partially supported by the NSFC (51208068), Social Development Project of Jiangsu Province (BE2015670, BY2016029-20).

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Fig. 1. XRD patterns of the NiFe-LDH and NiFe-LDO samples, and the NiFe-LDO samples after adsorption of CR. Fig. 2. FESEM images of the NiFe-LDH (a) and NiFe-LDO (b); EDS spectrum (c) and element mapping images of the NiFe-LDO (d, e, f). Fig. 3. Nitrogen adsorption–desorption isotherms and corresponding pore-size distribution curves of the NiFe-LDH and NiFe-LDO samples. Fig. 4. FTIR spectra of the NiFe-LDH and NiFe-LDO samples and the samples after CR is adsorbed. Fig. 5. XPS survey spectra (a) and high resolution XPS spectra for Fe 2p (b), Ni 2p (c), and O 1s (d) of the NiFe-LDO sample. Fig. 6. Effect of contact time on the adsorption capacity of CR on the NiFe-LDH and NiFe-LDO samples (T = 30 °C; adsorbent dose = 100 mg/L; CR concentration = 50 mg/L) Fig. 7. Pseudo-first-order kinetics (a) and pseudo-second-order kinetics (b) for CR adsorption on 21 / 24

the prepared samples (T = 30 °C; adsorbent dose = 100 mg/L; and initial concentration = 50 mg/L). Fig. 8. Intraparticle diffusion kinetics for CR adsorption on the prepared samples (T = 30 °C; adsorbent dose = 100 mg/L, adsorbent dose; initial concentration = 50 mg/L). Fig. 9. Adsorption isotherms of CR on the NiFe-LDH and NiFe-LDO samples (T = 30 °C; adsorbent dose = 100 mg/L; CR concentration = 10–100 mg/L). Fig. 10. Langmuir (a) and Freundlich (b) isotherms for CR adsorption on the NiFe-LDH and NiFe-LDO samples at 30 °C. Fig. 11. A proposed mechanism for CR onto NiFe-LDH and NiFe-LDO samples Table 1. Pore structure parameters of the samples

SBET (m2/g) 60 121

Samples NiFe-LDH NiFe-LDO

VP (cm3/g) 0.30 0.41

dp (nm) 19.6 13.4

Notes: SBET: BET specific surface area; Vp: pore volume; dp: average pore size.

Table 2. Pseudo-first-order and pseudo-second-order kinetic model constants of the as-prepared samples Pseudo-first-order model

qe,exp

Samples

(mg/g)

NiFe-LDH NiFe-LDO

195.6 302.2

qe,cal

Pseudo-second-order model

−3

k1 (10 −1

(mg/g)

min )

76.01 112.65

9.235 8.037

R

SD

2

0.881 0.827

qe,cal

k2 (×10−3 g mg−1

(%)

(mg/g)

min )

18.44 18.91

198.8 307.7

0.06 0.04

SD

R2

−1

(%)

0.998 0.996

0.493 0.549

Table 3. Intraparticle diffusion model constants and correlation coefficients for CR adsorption on all samples.

Samples NiFe-LDH NiFe-LDO

C0 (mg L-1)

kd1 (mg/g min-1/2)

kd2 (mg/g min-1/2)

kd3 (mg/g min-1/2)

C1

C2

C3

(R1)2

(R2)2

(R3)2

50

44.49

8.13

0.96

0

93.4

168.5

1

0.970

0.940

50

69.13

11.88

1.37

0

150.3

257.8

1

0.941

0.914

Table 4. Adsorption isotherm parameters of the NiFe-LDH and NiFe-LDO samples. Samples

NiFe-LDH

Langmuir isotherm model

Freundlich isotherm model 2

qmax (mg/g)

KL (L/mg)

R

205

1.33

0.999 22 / 24

KF (mg/g)·(L/mg)1/n

n

R2

120.75

7.34

0.876

NiFe-LDO

330

0.46

0.999

145.01

4.78

Table 5. Adsorption capacities of CR on the prepared samples and other reported materials.

qmax (mg/g)

References

Hierarchical porous NiFe-LDH Hierarchical porous NiFe-LDO Hierarchical NiO

205 330 250

This work This work [60]

NiO nanosheets Fe3O4@SiO2 nanospheres a-Fe2O3 nanoplates Mg–Fe–CO3-LDH NiFe2O4/ZnO hybrids

168 55 252 105 222

[61] [62] [63] [32] [36]

Nanocrystalline NiFe2O4 Nanocrystalline CoFe2O4

97 245

[64] [64]

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0.961

TOC/Graphic Abstract

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