Layered Double Hydroxides for Remediation of Industrial Wastewater from a Galvanic Plant

: Owing to their structure, layered double hydroxides (LDHs) are nowadays considered as rising materials in di ﬀ erent ﬁelds of application. In this work, the results obtained in the usage of two di ﬀ erent LDHs to remove, by adsorption, some cationic and anionic pollutants from industrial wastewater are reported. The two compounds MgAl-CO 3 and NiAl-NO 3 have been prepared through a hydrothermal synthesis process and then characterized by means of PXRD, TGA, FESEM, and FTIR spectroscopy. The available wastewater, supplied by a galvanic treatment company, has been analyzed by inductively coupled plasma-optical emission spectrometry (ICP-OES), resulting as being polluted by Fe(III), Cu(II), and Cr(VI). The water treatment with the two LDHs showed that chromate is more e ﬃ ciently removed by the NiAl LDH through an exchange with the interlayer nitrate. On the contrary, copper and iron cations are removed in higher amounts by the MgAl LDH, probably through a substitution with Mg, even if sorption on the OH − functional groups, surface complexation, and / or precipitation of small amounts of metal hydroxides on the surface of the MgAl LDH could not be completely excluded. Possible applications of the two combined LDHs are also proposed.


Introduction
Layered double hydroxides (LDHs) belong to a family of minerals, the so-called hydrotalcite supergroup, whose crystal structure consists of brucite-type layers, in which a trivalent cation partially substitutes a divalent cation [1,2].
This substitution produces a net positive charge balanced by the entrance of an anionic species in the interlayer, giving as a general formula M 2+ 1−x M 3+ x (A z− ) x/z (OH) 2 ·nH 2 O. Their capacity to easily exchange the interlayer anions makes LDHs attractive as carriers or scavengers of potential toxic anions [3][4][5][6]. Furthermore, their flexible structure, which can be reproduced using several bivalent and trivalent cations, suggests the possibility to use LDHs as a getter of pollutant cations [7,8]. For all of the above, the knowledge of the relationships between metals and LDHs is fundamental to allow the use of these minerals [9].
In a previous study, the relationships between lanthanides metals and a woodwardite (CuAl-SO 4 LDH) structure were investigated [10], with the aim to use these materials for the recovery of the rare earth elements from waste electric and electronic equipment for both georemediation and georecovery exploitation. The aim of this work is the synthesis and characterization of two different LDHs to experimentally investigate their capability to recover pollutants from wastewater. Such wastewater has been taken from a galvanic plant. Its composition is enriched in environmentally hazardous anions Crystals 2020, 10, 443 2 of 9 and metal cations, which derive from the treatment process. This problem is widespread [11]. Many different treatments have been proposed to solve this contamination issue [12,13]. In this study, the exchange capability of the two compounds is tested directly on this real wastewater, based on the results previously obtained in a laboratory-prepared Cr 2 O 7 2− solution [14]. The selected compounds are LDHs, as they represent a cheap and effective method to remove pollutants from aqueous solutions. The two compounds investigated are a NiAl-NO 3 and a MgAl-CO 3 LDHs. The choice of these compositions is based on the fact that previous works have shown the chromate uptake capacity by similar LDHs both intercalated with other sorbents [15] and through an ion exchange reaction with the interlayer anion [16,17].

Samples Synthesis
The compounds were synthesized via the co-precipitation route (direct method), followed by hydrothermal treatment obtaining nanoscopic crystallites with a partially disordered (turbostratic) structure. The addition of urea to the reactants helps to keep the pH to the desired value and the development of CO 2 allows to obtain a high exchange surface structure [15]. Both the LDHs were with the M 2+ /M 3+ ratio = 2.
The NiAl-NO 3 LDH was synthesized following the pathway suggested by [18], starting from Ni(NO 3 ) 2 ·6H 2 O (99.0% purity, supplied by Merck KGaA, Darmstadt, Germany), Al(NO 3 ) 3 ·9H 2 O (98.8% purity, supplied by VWR CHEMICALS, Leuven, Belgium), and urea (99.8mass% purity, supplied by CARLO ERBA, Milan, Italy). The two salts and urea, in a proper stoichiometric amount, were dissolved in 200 mL of deionized water under magnetic stirring, then the solution was transferred in a Teflon vessel autoclave and kept at 100 • C for 24 h. After heating, the system was cooled at room temperature naturally, and the green solid compound obtained was vacuum filtered and washed with water and ethanol. Subsequently, it was dried in a stove at 60 • C for 24 h. To synthesize the MgAl-CO 3 LDH, as suggested by [19], the reagents Al(NO 3 ) 3 ·9H 2 O (98.8% purity, supplied by VWR CHEMICALS, Leuven, Belgium), Mg(NO 3 ) 2 ·9H 2 O (98.9% purity, supplied by VWR CHEMICALS, Leuven, Belgium), and urea (99.8 mass% purity, supplied by CARLO ERBA, Milan, Italy) were used. After the dissolution of the due amount of the reagents, as a function of the Mg/Al desired ratio, the reaction was continued in a Teflon vessel autoclave at 180 • C temperature for one hour. The white compound obtained was separated from the solution by centrifugation at 7000 rpm 10 min −1 , repeatedly washed with water, and dried in a stove at 60 • C for 24 h.

Samples Characterizations
The compounds were characterized by means of the following techniques: thermo gravimetric analysis (TGA), X-ray diffraction analysis on powders (PXRD), field emission scanning electron microscope analysis (FESEM), Fourier transform infrared spectroscopy (FTIR), and inductively coupled plasma optical emission spectroscopy (ICP-OES) chemical analysis.

Thermo Gravimetric Analysis
In order to perform the TGA, about 30-50 mg of the dried powdered LDH was placed in alumina open crucibles; the measurements were carried out by means of a H/LABSYSEVO-1A SETARAM apparatus (Setaram, Caluire, France), at a heating rate of 5 • C min −1 under argon flux of 30 mL min −1 .

Field Emission Scanning Electron Microscope Analysis
To investigate the morphology and the structure of the synthesized compounds, an FESEM analysis has been performed. The samples were adhered on a conductive resin support, then analyzed by applying an acceleration voltage of 5 kV for 50 s, and a cobalt standard was used for the calibration.

Powder X-ray Diffraction
To determine the crystal structures and to calculate the lattice parameters of the phases, PXRD analysis was carried out by the vertical diffractometer X'Pert MPD (Philips, Almelo, The Netherlands) equipped with a Cu tube (Kα1 wavelength: 1.5406 Å). The samples were grounded in an agate mortar. The patterns were collected between 10 • and 100 • 2θ with a step of 0.001 • and measuring time of 50 s/step. The indexing of the obtained diffraction data was performed by a comparison with the literature or calculated data (the program Powder Cell-version1999 [20]).

FTIR Spectroscopy
To exclude the possible incorporation during the synthesis of undesirable anions or other species in the interlayer (e.g., CO 2 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 −1 .

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.

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 efficiency of different 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.

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-CO 3 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-NO 3 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-CO 3 LDH yielded a material with a crystallite size significantly higher than the NiAl-NO 3 LDH. In fact, the NiAl-NO 3 LDH has broader reflections. Moreover, the PXRD pattern of the NiAl-NO 3 LDH shows a doublet in the (006) reflection, coupled with a diffuse and broad (003) reflection. This is probably due to the different hydration state of this LDH, which leads to the presence of crystallites with small differences 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 2+ /M 3+ ratio could explain this effect [23]. 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 diffuse and broad (003) reflection. This is probably due to the different hydration state of this LDH, which leads to the presence of crystallites with small differences 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 2+ /M 3+ ratio could explain this effect [23].
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.  In Figure 2a,b, the FTIR spectra obtained for the synthesized compounds, the NiAl-NO 3 LDH and MgAl-CO 3 LDH, respectively, are shown, and the presence of the desired functional groups was confirmed. stretching; ongoing to the lower frequency at 1063 cm −1 , there is the signal due to the carbonate vibration and the two bands at 769 and 654 cm −1 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. For the NiAl-NO 3 LDH the specific surface area, calculated by the Brunauer-Emmett-Teller (BET) method, has been measured, resulting in 46.8504 m 2 g −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 CO 2 loss, which started at about 400 • C. 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. 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  Figure 5b. The high affinity of the MgAl-CO 3 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 arrangement 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 OHfunctional 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. 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 2− . 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 2− substitution. The low crystallinity of the samples prevented to meaningfully discuss the cell parameter a, calculated as a = 2 × d(110).

Conclusions
The experimental results obtained lead to the following conclusions: • The LDHs' syntheses are relatively easy to prepare, leading to the required products as shown by the characterization analyses conducted; • As the tested LDHs: • The NiAl-NO 3 compound demonstrates greater affinity for the CrO 4 2− anion than for the Fe(III) and Cu(II) cations; • The MgAl-CO 3 structure does not significantly adsorb the CrO 4 2− anion, while it seems more effective for the two cations; • By comparing the residual concentration of the three elements studied with the Italian legal limit for industrial wastewater, reported in Table 3, with respect to the disposal in superficial water or in the drainage system: The NiAl-NO 3 compound reduces the Cr(VI) content at a value close to the legal limit, but is not sufficiently low. Some improvements of the methods (reaction time, sorbent/wastewater ratio) could help in reaching the goal.
The MgAl-CO 3 LDH demonstrates to be very effective in lowering the Cu(II) and Fe(III) concentration to below the legal limit.
For all of the above, the next step will be to test the effectiveness of a combined use of the two metal getters and their transformation into a spinel-like structure after heating for a potential reuse.

Conflicts of Interest:
The authors declare no conflict of interest.