2.2.4. FTIR Spectroscopy

To exclude the possible incorporation during the synthesis of undesirable anions or other species in the interlayer (e.g., CO2 from the atmosphere), FTIR spectroscopy was performed using a Spectrum 65 FT-IR Spectrometer (PerkinElmer, Waltham, MA, USA) equipped with a KBr beamsplitter and a DTGS detector by use of an ATR accessory with a diamond crystal. All spectra were recorded from 4000 to 600 cm<sup>−</sup>1.

#### 2.2.5. Inductively Coupled Plasma Optical Emission Spectroscopy

The chemical analyses were performed by ICP-OES after the dissolution of the samples in the concentrated nitric acid solution.

#### *2.3. Pollutant Removal from Wastewater*

Based on the results reported in the literature about the adsorption of anions and cations [7,11], in this work the adsorption e fficiency of di fferent pollutants (cationic and anionic) was tested on a real industrial wastewater sample. The sample available was analyzed by means of ICP-OES in order to determine the chemical composition of the dissolved elements. The results are the following: 127 ppm Cu(II), 460 ppm Fe(III), and 8780 ppm Cr(VI). The pH value of the wastewater was about 3. Owing to the very high metals concentration, the sample was diluted to 1/100 with water, before the adsorption tests. The water pH was corrected at a value of about 5 after dilution, to avoid both the LDH dissolution and the iron hydroxide precipitation. For both the LDHs, a weight of 0.5 g of the compound was added to a volume of 100 mL wastewater, and the mixture was shaken for 24 h. The solid and the liquid phases were separated throughout centrifugation at 7000 rpm 10 min−1, then the solid was repeatedly washed with deionized water. The residual wastewater and the solid phase (after dried and acid dissolution) were analyzed by ICP-OES. After the batch equilibration procedure, the pH value of the residual wastewater did not change significantly, and no precipitation of iron hydroxide was observed.

### **3. Results and Discussions**

The compounds were characterized after synthesis and in Figure 1a,b, their PXRD patterns are reported.

Both diagrams had peculiar features of the presence of the LDH structure, such as the strong basal reflections (003 and 006) around 12◦ and 24◦ 2θ, respectively. X-ray data confirm the presence of carbonate and nitrate in the two LDHs. The MgAl-CO3 LDH has an interplanar distance associated with the main basal reflection of 7.54 Å, which is comparable to those of similar natural and synthetic compounds [1,21], whereas the NiAl-NO3 LDH has an interplanar distance for the (003) reflection of 7.91 Å, as other similar compounds [22]. Comparing the two diagrams, it is also possible to state that the synthesis of the MgAl-CO3 LDH yielded a material with a crystallite size significantly higher than the NiAl-NO3 LDH. In fact, the NiAl-NO3 LDH has broader reflections. Moreover, the PXRD pattern of the NiAl-NO3 LDH shows a doublet in the (006) reflection, coupled with a di ffuse and broad (003) reflection. This is probably due to the di fferent hydration state of this LDH, which leads to the presence of crystallites with small di fferences in the interlayer distance. Water content is well known to

influence the basal reflection of LDHs, but also the different orientation of nitrate in the interlayer and M<sup>2</sup>+/M3<sup>+</sup> ratio could explain this effect [23].

**Figure 1.** PXRD patterns of (**a**) NiAl nitrate and (**b**) MgAl carbonate layered double hydroxides (LDHs). On the graph, the hkl indexes of the main reflections are superimposed.

In Figure 2a,b, the FTIR spectra obtained for the synthesized compounds, the NiAl-NO3 LDH and MgAl-CO3 LDH, respectively, are shown, and the presence of the desired functional groups was confirmed.

**Figure 2.** FTIR spectra of (**a**) the NiAl nitrate LDH, (**b**) the MgAl carbonate LDH.

Sample (a): A 3600 cm<sup>−</sup><sup>1</sup> OH group stretching, 3410 cm<sup>−</sup><sup>1</sup> hydrogen bond stretching in coordination with the cations, and 1632 cm<sup>−</sup><sup>1</sup> bending of the H2O bond into the interlayer and at 1348 cm<sup>−</sup><sup>1</sup> can be found in the nitrate group stretching; the two slight bands at 749 and 652 cm<sup>−</sup><sup>1</sup> together with the one at 1440 cm<sup>−</sup><sup>1</sup> partially overlapped with the nitrate bending are related to the undesired interlayer carbonate groups (probably from the atmospheric CO2). Sample (b): the broad band at 3386 cm<sup>−</sup><sup>1</sup> concerns the stretching of the OH groups in coordination with the cations, at 2979 cm<sup>−</sup><sup>1</sup> there is the large band related to the H-bonding between the water and carbonate anions in the interlayer, the 1576 cm<sup>−</sup><sup>1</sup> band originates from the H2O bending in the interlayer, and the main carbonate anion adsorption band at 1440 cm<sup>−</sup><sup>1</sup> is overlapped with the 1347 cm<sup>−</sup><sup>1</sup> band related to the nitrate group

stretching; ongoing to the lower frequency at 1063 cm<sup>−</sup>1, there is the signal due to the carbonate vibration and the two bands at 769 and 654 cm<sup>−</sup><sup>1</sup> are related to the interlayer carbonate groups.

From these spectra, it seems that in the MgAl carbonate LDH there are some impurities of the nitrate anions, while the NiAl LDH is affected by a little impurity of carbonate.

The FESEM images are shown in Figure 3a,b, and from the micrographic appearance, the two structures appear similar in morphology, showing the typical hydrotalcite structure, and the thickness of the constituent lamellae was estimated between 10 and 20 nm.

**Figure 3.** FESEM images of (**a**) the NiAl-NO3 LDH, (**b**) the MgAl-CO3 LDH.

For the NiAl-NO3 LDH the specific surface area, calculated by the Brunauer–Emmett–Teller (BET) method, has been measured, resulting in 46.8504 m<sup>2</sup> g<sup>−</sup>1.

The thermogravimetric analysis, whose results are reported in Figure 4a,b, revealed that the NiAl-based LDH (Figure 4a) loses 2.7 mass% due to humidity at about 150 ◦C, and has a second mass decreasing at 309.3 ◦C, involving both the interlayer water and the nitrogen oxide for a whole amount of 24.7 mass%. The MgAl-based LDH (Figure 4b) loses mass in three steps. The first, due to humidity, at 231 ◦C with a mass decrease of 10 mass%, the second ascribable to the removal of the interlayer water (6.5 mass% loss at 328 ◦C), and the third of about 16 mass% was due to the CO2 loss, which started at about 400 ◦C.

**Figure 4.** TG thermograms of (**a**) the NiAl nitrate LDH, (**b**) the MgAl carbonate LDH.

As the pollutant removal, both the samples have been kept to a batch equilibration with the wastewater, in the experimental conditions previously described. In Tables 1 and 2, the results obtained for the extraction of copper, iron, and chromium, the three pollutants of the industrial wastewater, are reported.


**Table 1.** Pollutant amount in the diluted wastewater before and after treatment with the NiAl-NO3 LDH.



In the tables, the concentration of the three species investigated in the wastewater, before and after the adsorption procedure, and in the solid LDH used to extract the pollutants are shown. The recovery efficiency value expresses the ratio of the element concentration in the LDH to the element concentration in the medium, calculated in each test [24].

The NiAl-NO3 compound demonstrated greater affinity for the CrO4<sup>2</sup>− anion than for the Fe(III) and Cu(II) cations. On the contrary, the MgAl-CO3 structure did not adsorb significantly the CrO4<sup>2</sup>− anion, while it seemed more useful for the two cations.

The chromium adsorption is related to the different exchange capacity of the two anions (CO3<sup>2</sup>− and NO3−) in the interlayers, where nitrate can be easily substituted by chromate, as confirmed from the PXRD. The comparison between the position of the three main reflections of the compound before and after the chromium extraction from the water are reported in Figure 5. The shift of the main basal reflections of the NiAl LDH toward lower 2θ values (Figure 5a) indicates an enlargement of the interlayers due to the substitution of NO3– with CrO4<sup>2</sup><sup>−</sup>. This shift is reflected in a change in the cell parameter *c* (calculated as *c* = 3 × d(003)), which changed from 23.85 to 24.99 Å, caused by the swelling of the interlayer due to the NO3– with CrO4<sup>2</sup>− substitution. The low crystallinity of the samples prevented to meaningfully discuss the cell parameter *a*, calculated as *a* = 2 × d(110).

The values of the cell parameter *c* of the MgAl-CO3 LDHs before and after the experiment are 22.71 and 22.56 Å, respectively. This fact suggests that little changes took place in the interlayer of these LDHs, as confirmed by Figure 5b. The high affinity of the MgAl-CO3 LDH for the two cations is worthy of further study, but as for copper, it seems to be imputable to an exchange between the copper and magnesium divalent cations due to the similar ionic radius values of the two elements in octahedral coordination (72 and 73 pm, respectively). No significant change in the *a* parameter before and after the experiment is observed, as its value remains constant at 3.04 Å. Probably, the disordered nature of the cation arrangemen<sup>t</sup> in the brucite-like layers prevents the observation of the changes in the *a* parameter [2]. The Cu-Mg substitution has already been proposed for MgAl LDHs in contact with solutions in which bivalent cations are dissolved [25], even if sorption on the OH- functional groups, surface complexation, and/or precipitation of small amounts of Me(OH)2 on the surface of the MgAl LDH could not be completely excluded [9]. Regarding iron adsorption, the characterization analysis did not provide precise information on where it might have accumulated It is possible that Fe has been removed via interaction with the functional groups on the surface of the mineral.

**Figure 5.** PXRD for (**a**) the NiAl nitrate LDH before (dark grey) and after (light grey) pollutant removal from the wastewater and (**b**) the MgAl carbonate LDH before (dark grey) and after (light grey) pollutant removal from the wastewater.
