Benzene Sulfonate Modified ZnCr-LDH and Its Enhanced Adsorption Properties for Anionic Dyes


 A benzene sulfonate modified hydrotalcite (SO3-LDH) was synthesized by a facile one-pot hydrothermal technique, which can efficiently remove methyl orange (MO), Congo red (CR) and orange II (OII) from aqueous solution. After modified by benzene sulfonate, the microstructure of hydrotalcite changes obviously, from the cellular structure to the stacking structure formed by the face-face contact of hydrotalcite nanosheets, which resulted in much more exchangeable nitrate ions to remain in the interlayer space. The pre-insertion of benzene sulfonate as a pillar expanded the interlayer gallery, which facilitated the pollutant anions (MO, CR and OII) into the interlayer of LDH in the subsequent adsorption process. The maximum adsorption capacity of SO3-LDH for MO, CR and OII was 4200.8 mg/g, 1252.0 mg/g and 1670.6 mg/g respectively, which is approximately 1.86 times, 1.8 times and 2.32 times that of the pristine NO3-LDH, respectively. The removal mechanism of anionic dyes was determined as anion exchange between NO3− ions and dye molecules. The adsorption behavior for MO and OII is multilayer adsorption, while the adsorption behavior for CR is monolayer adsorption. The adsorption process mainly was controlled by the chemical bonding between the dye molecules and adsorbent active sites. The benzene sulfonate modified LDH has a great potential to be used as a high-efficient adsorbent to remove anionic dyes from aqueous solution.


Introduction 31
With the widespread application of dyes in textile, leather, papermaking and other chemical 32 industries, a large amount of polluted water containing poisonous and hardly-degradable organic dyes 33 is discharged (Tao et al. 2017). Undoubtedly, these dye wastewater will lead to serious environmental 34 issues such as color pollution, light penetration interference, damage aquatic organisms, even harm to 35 human health. Therefore the effective treatment of dyeing wastewater is one of the most serious 36 concerns of the current era. So far, many techniques have been used to treatment dyeing wastewater, 37 such as adsorption methods (Zheng et  polymeric resins, transition metal composite (Zheng et al. 2017) and nanocomposites (Zheng et al. 45 2019). However, the adsorption capacity is limited. Therefore, efficient adsorbents with high adsorption 46 capacity and fast adsorption rate are urgently needed. 47 temperature until adsorption equilibrium. After MO, OII or CR solution adsorbed by the SO3-LDH 119 samples, it was respectively named as SO3-LDH-MO, SO3-LDH-OII or SO3-LDH-CR. Similarity, After 120 MO, OII or CR solution adsorbed by the NO3-LDH samples, it was respectively named as 121 NO3-LDH-MO, NO3-LDH-OII or NO3-LDH-CR. Then, the suspensions were centrifuged at 4000 rpm 122 for 20 min. The supernatant concentration was determined using a PERSEE TU-1810 UV-vis 123 spectrometer. The MO, OII and CR removal performance was evaluated by calculating the adsorbed 124 capacity at equilibrium (Qe, mg/g) and at any time (Qt, mg/g), and is given as Eq. (1) and (2): 125 where C0 (mg/L) corresponds to the dye initial concentration, Ce (mg/L) is the dye equilibrium 128 concentration, Ct (mg/L) is the dye concentration in the aqueous solution at any time t (min), V (L) is 129 the solution volume, and m (g) is the adsorbent mass. materials. The interlayer distance of (003) basal plane (d003) of the pristine NO3-LDH is 8.269 Å. For 139 the modified sample (SO3-LDH), the d003 value is 8.654 Å, and larger than that of NO3-LDH, a new 140 peak appears at 19.698 o , and the diffraction peak of (006) basal plane is absent, which indicates that 141 some benzene sulfonate anions enters the interlayer space of (003), (006) and (009)

Adsorption performance of unmodified and modified samples 185
In order to compare the adsorption performance of unmodified and modified LDH for different 186 dyes. The 10 mg adsorbent was dispersed in 100 mL MO, OII and CR solutions with an initial 187 concentration of 1000 mg/L. The pH was adjusted to 5, stirred the suspension at room temperature. The 188 adsorption capacity of NO3-LDH and SO3-LDH for dyes are shown in Fig.4a. The maximum 189 adsorption capacity of MO by SO3-LDH was about 4200.8 mg/g, and significantly higher than that of 190 NO3-LDH (2252.8 mg/g). The maximum adsorption capacity of CR and OII by SO3-LDH was about 191 1252.0 mg/g and 1670.6 mg/g respectively, and higher than that of NO3-LDH (695.4 mg/g and 719.8 192 mg/g). Therefore, SO3-LDH has better adsorption performance for MO, CR and OII dyes. In addition, the stability of the SO3-LDH as adsorption material was also studied. The three dye 197 solutions before and after adsorbed by SO3-LDH were diluted 20 times, respectively, and then 198 spectroscopically scanned by UV-vis spectrophotometer. As can be seen from Fig.4b, after SO3-LDH 199 adsorbed MO, the intensity of characteristic peak of methyl orange was significantly reduced.  Fig.4 d, the intensity of CR and OII characteristic peak was also reduced after 202 adsorbed by SO3-LDH, but the location and number of the peak were not changed. Therefore, during 203 the adsorption process, the dye molecules intercalation into the interlayer of LDH will not be 204

Adsorption kinetics 208
To further investigate the adsorption rate of the three anionic pollutants by the SO3-LDH sample. The pseudo-first-order: The pseudo-second-order: Where k1 (min −1 ) and k2 (g/mg/min) are the rate constant of two models. 214

Adsorption isotherms 232
Equilibrium data, generally known as sorption isotherms are elementary necessity to comprehend   relatively easy to happen and belongs to multi-layer adsorption with strong adsorption affinity (Kausar 263 et al. 2018). For CR, the R 2 for the Langmuir isotherm is larger than the Freundlich isotherm models. 264 Langmuir isotherm model greatly predicted the CR adsorption process, the adsorption occurs on a 265 homogeneous adsorbent surface and belongs to single-layer adsorption. 266 show that the experimental Qm of SO3-LDH in this work is significantly superior to all of the reported 270 LDHs containing different interlayer anions, and shows very high adsorption capacity. In general, the 271 as-synthesized SO3-LDH is a promising adsorbent for the removal of anions from dye containing 272 effluents. 273

XRD analysis 275
XRD patterns of different dyes adsorbed by SO3-LDH are shown in Fig. 7a and Fig. 7b of OII also inserted into the interlayers of (006) parallel crystal plane in LDH host structure, the 291 intercalated peak of (006) basal plane is located at 7.901 o , and the corresponding interlayer distance 292 (d006) is about 11.180 Å. After the SO3-LDH adsorbed by 1000 mg/L CR solution (SO3-LDH-1000CR), 293 the peak intensities of (003) and (006) basal reflection reduce, along with a new weak peak ascribed to 294 the intercalated peak of (006) basal plane appeared at the position of 8.13 o , and the corresponding d006 295 value is about 10.863 Å, which indicates sulfonate anions of CR also entered into the interlayer of (006) 296 parallel crystal plane. 297

FTIR analysis 298
FTIR spectroscopy was conducted to investigate the interaction between the SO3-LDH and the 299 dye molecules. In order to identify the adsorption mechanisms, the FTIR spectra of SO3-LDH adsorbed 300 by different dyes are shown in Fig.7c. As the increasing MO solution concentration from 100mg/L to 301 1000mg/L, the intensities of the adsorption bands at 1605 cm −1 ascribed to aromatic C−C stretching 302 vibration, at 1520 cm −1 assigned to N=N bands, at 1365 and 1119 cm −1 attributed to the C−N bands, at 303  (Fig.8b). After the SO3-LDH adsorbed more MO − anions, the LDH sheets in the sample of 325 SO3-LDH-500 becomes loose (Fig.8c), and that of SO3-LDH-1000 become much looser and smaller 326 (Fig.8d).

Schematic illustration 331
The schematic illustration of the synthetic process of benzene sulfonate modified and unmodified 332 hydrotalcites by one-pot hydrothermal method and the adsorption process of MO by SO3-LDH were 333 shown in Fig.9. Modified hydrotalcite (SO3-LDH) was prepared using methyl orange as a soft template 334 agent. Compared with unmodified hydrotalcite (NO3-LDH), the insertion of benzene sulfonate anions 335 into the hydrotalcite layer resulted in the increase of the interlayer Spacing from 8.269 Å to 8.654Å. 336 The LDH host structure pre-intercalated by benzene sulfonate anions evolved into pillared layered 337 materials, benzene sulfonate anions as a column expanded the interlayer spacing of (003) base plane,  Adsorption of dyes by NO3-LDHs and SO3-LDH (a) ; Spectral scanning curve of (b) MO, (c) CR and (d) OII before and after SO3-LDH adsorption  The XRD patterns (a, b) and FTIR spectra (c) of different dyes adsorbed by SO3-LDH Schematic illustration of the synthetic process of LDH materials and adsorption process of MO by SO3-LDH

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