Coprecipitation Preparation of Cu / Zn / Al-Hydrotalcite-Like Compound for Copper Removal from Electroplating Wastewater

Cu/Zn/Al-hydrotalcite-like compound (Cu/Zn/Al-HTlc) was prepared by the coprecipitation method with Zn, Al cations solution, and electroplating wastewater containing Cu cation. 0e preparation conditions of Cu/Zn/Al-HTlc were optimized. 0e metal ion pollutants removal effect and iodide maximum adsorption capacity of Cu/Zn/Al-HTlc were also studied. 0e physicochemical properties of the Cu/Zn/Al-HTlc were analyzed by X-ray diffraction, FTIR, SEM, N2 adsorption-desorption isotherms, and TG-DTG.0e results showed that Cu/Zn/Al-HTlc should be prepared with the Zn-Al molar ratio of 1.5 :1, pH � 11, and aged at room temperature for 0.5 d. Structural analysis showed that the Cu/Zn/Al-HTlc was a layered compound. Cu and other metal ion pollutants can also be successfully removed from electroplating wastewater in the preparation process of Cu/ Zn/Al-HTlc and reached Chinese National Emission Standard (GB 21900-2008). 0e optimal adsorption condition of calcinated Cu/Zn/Al-HTlc for iodide was as follows: the solid-liquid ratio was 1 : 250, the pH value was 8, and the adsorption process was carried out at 25°C for 30min. 0e saturated adsorption capacity reached 1000mg·g at the optimal adsorption condition. 0e main reason for this high-saturated adsorption capacity of Cu/Zn/Al-HTlc was that iodide penetrated into the layered structure of Cu/Zn/Al-HTlc by physical adsorption and Cu undergoes a specific redox reaction, producing CuI. Hence, coprecipitation synthetic technology and prepared Cu/Zn/Al-HTlc could be potentially used for electroplating wastewater treatment.


Introduction
Heavy metal ions and organic substances are discharged by industrial activities, which could be consumed and accumulated in tissues of living organisms through the food chain [1][2][3][4][5].Heavy metal ions entered waterbody through industrial wastes [6], among which electroplating wastewater needs the most attention [7].Some heavy metal ions, such as copper, were necessary elements for human body, but excessive intake of Cu II may bring about chronic copper poisoning [8,9].e Chinese National Electroplating Wastewater Discharge Standard (GB21900-2008) made clear regulations that Cu II concentration must be less than 0.5 mg•L −1 [10].erefore, the electroplating wastewater which can lead to health lesion must be properly treated and must be less than 0.5 mg•L −1 prior to discharge.ere are a lot of methods for heavy metal ions removal from electroplating wastewater [11], such as precipitation [12], ion exchange [13], membrane separation [14], trapping, and electrochemical reduction [15].However, these methods need high cost and may produce the secondary pollution.erefore, efficient, low cost, and easy-to-operate electroplating wastewater treatment methods need to be urgently developed.
Iodine is one of the nuclides causing most concern among radioactive anions. 129I was especially considered as one of the key radionuclides that dominates the long-term radiation in underground radioactive.
e radioactive iodine can be introduced into the environment via accidents in nuclear facilities and was then absorbed by humans.Adsorption has been the most competitive method for removal of iodide in aqueous solution because of its simplicity, high efficiency, and availability [16,17].In recent years, various adsorbents capable of isolating iodide pollutants from aqueous solution, such as zeolite-based materials, minerals, and layered double hydroxide, have been investigated [18][19][20][21].
e hydrotalcite (HT) and hydrotalcite-like compounds (HTlcs) are double hydroxide-layered structure compounds [22].Due to the layered structure, they have good adsorption property.As adsorbents, they have been used to remove some metal ions and inorganic and organic anionic pollutants in recent years [23][24][25][26][27].In addition, calcinated HTlcs have larger surface areas and alkaline centers.ey also can reconstruct the layered structure by incorporating with available ions in the solution [28].So a rational preparation technology should be developed, the technology is essential to reduce heavy metal ions concentration of electroplating wastewater, and the product HTlcs could be applied to iodide pollutants adsorption.
In this study, the preparation of Cu/Zn/Al-HTlc, optimum conditions, heavy metal ions removal capability, and iodide adsorption capacity were first reported.

Experimental
2.1.Materials.All chemicals were pure commercial products obtained from Aladdin.Deionized water was used in the experiment.Electroplating wastewater was obtained from electroplating enterprises.

Preparation of HTlc.
e HTlc was prepared by the coprecipitation method.Firstly, different metal ion systems (Zn II , Ca II , Fe III , Zn II -Al III , and Cd III -Al III ) were selected together with electroplating wastewater (15.0 mL), and deionized water (30.0 mL) was placed in a beaker.en, mixed solution was added dropwise into an aqueous solution of NaCO 3 (0.25 mol•L −1 , 90.0 mL) under vigorous stirring, and the pH value was adjusted to 10 by adding NaOH (0.1 mol•L −1 ).e mixture solution was stirred for 30 min and left to age for 24 h at 60 °C.e mixture was filtered, and the precipitate was washed with deionized water.Finally, the residue was dried and calcinated at 400 °C for 3 h in a muffle furnace.

Iodide Adsorption.
Calcinated HTlc (0.1 g) was placed in a 50 mL volumetric flask, and then different concentrations of KI solution (25.0 mL) were added.
e mixture was shaken for 60 min and centrifuged.Subsequently, the supernatant (1.0 mL), deionized water (10.0 mL), and bromine water (2 drops) were added in a 50 mL volumetric flask.e mixed solution was shaken for 5 min.Formic acid (1.0 mL) was added in the mixture and shaken until the solution was colorless.Phosphate (2.0 mL) was then added and shaken up again.Finally, the aqueous solution of KI (1.0 mL, 200 mg• L −1 ) was added and diluted to deionized water (50.0 mL).e reaction was proceeded for 30 min and then was measured by a UV-Vis spectrophotometer at 350 nm.

Characterization of HTlc. X-ray diffraction (XRD)
patterns of the products were recorded by the X'pert PRO Xray diffractometer at a scanning range of 3-80 °min −1 , operated at 40 kV and 40 mA.e Brunauer-Emmett-Teller (BET) surface area was measured by the N 2 adsorptiondesorption method using NOVA 3000 specific surface area analyzers.Scanning electron microscopy (SEM) was performed using a ZEISS Ultra55 emission-scanning electron microscope scanned at an acceleration voltage of 15 kV.Infrared spectrum was obtained using a TENSOR27 Fourier transform infrared spectrometer in the range of 400-4000 cm −1 employing the KBr dilution technique (1.5%, w/ w).ermogravimetric analysis was carried out at a heating rate of 15 °C min −1 up to 600 °C using a SDTQ600 thermal analyzer.
e metal ions concentration was determined directly by inductively coupled plasma optical emission spectrometry (ICP-OES 710ES, Varian).

Results and Discussion
3.1.Preparation of HTlc.In this study, Zn II , Ca II , Fe III , Zn II -Al III , and Cd III -Al III were added in electroplating wastewater for HTlc preparation with the coprecipitation method, respectively.All coprecipitation products were characterized by X-ray diffraction (XRD), as shown in Figure 1.HTlcs have an X-ray diffraction peak at low angle 2θ [29].However, coprecipitation products of electroplating wastewater or electroplating wastewater with Zn II , Ca II , Fe III , and Cd III -Al III did not show the characteristic peak of HTlcs in corresponding XRD.In contrast, the coprecipitation product of electroplating wastewater and Zn II -Al III showed a sharp diffraction peak at low angle 2θ, and its interplanar distance was 0.7514 nm, which was attributed to the typical HTlcstratified structure [27,29].e sharp and intense diffraction peak and corresponding interplanar distance were similar with standard Zn/Al-HT.At the same time, HTlcs was an important layered matrix represented by the general formula , where M 2+ was a divalent cation such as Mg II , Ni II , Zn II , Cu II , or Co II ; M 3+ is a trivalent cation such as Al III , Cr III , Fe III , or Ga III ; and A n− is an anion of charge n, and they maintain charge neutrality by positively charged layers and exchangeable interlayer anions [30][31][32].Consequently, the results of the XRD indicated that Cu II exchanged parts of Zn II together with Zn II and Al III constitute the layered Cu/Zn/Al-HTlc.It also shows that exogenous Zn II -Al III provides the good route to Cu II of electroplating wastewater removal via the formation of Cu/ Zn/Al-HTlc with layered structures.

Effect of the Zn II /Al III Molar Ratio.
e XRD pattern of the three products, which were prepared by coprecipitation of electroplating wastewater and Zn II -Al III solution (molar ratio � 5 : 1, 2 : 1, 1.5 : 1), is shown in Figure 2(a).e XRD pattern result indicated that the diffraction angle of the three prepared products have sharp and symmetric peaks at low 2θ angles and broad and asymmetric peaks at higher 2θ angles which are characteristic of the layered structure, and they were similar to Zn/Al-HT [29].It also can be seen that three products have intense diffraction peak, which indicated they have good degree of crystallization.All diffraction peak intensity and interplanar distance have no significant 2 Journal of Chemistry difference in the XRD pattern of three products (Figure 2(a)).At low molar ratio, good degree of crystallization can also be obtained.erefore, the Zn II -Al III molar ratio of 1.5 : 1 was used for Cu/Zn/Al-HTlc preparation.

Effect of the pH Value.
e coprecipitation preparation of HTlcs needs to be achieved at a constant pH [33].Consequently, different pH values (pH � 7, 8, 9, 10, 11, and 12) were chosen to evaluate the effect of the pH value to  Cu/Zn/Al-HTlc and then determined the optimal pH value.Figure 2(b) showed the effect of the pH value on Cu/Zn/Al-HTlc preparation.With the initial pH ranging from 7 to 10 in the preparation process, the yield of Cu/Zn/Al-HTlc increased and reached constant until pH 10.And it can be observed that their products have a weak diffraction peak, which indicated that they have low crystallinity at pH between 7 and 10.But two products have a sharp diffraction peak at the pH value of 11 and 12, and the crystallinity of product was better at the pH value of 11, which attributed to its higher diffraction peak.erefore, the pH value of 11 was an ideal parameter for preparation Cu/Zn/Al-HTlc.

Effect of Aging
Temperature.e three temperatures (room temperature, 40 °C, and 60 °C) were used to investigate the effect of the aging temperature for Cu/Zn/Al-HTlc preparation.
e XRD pattern of products at the corresponding temperature was shown in Figure 2(c).It can be seen that three products have a high diffraction peak, and their intensity have no obvious difference in the XRD pattern, which indicated products have good crystallinity at three aging temperatures.At the same time, it was also found that their yield was almost the same.Considering the convenience of the operation, the optimum preparation temperature was set at room temperature.

Effect of Aging Time.
e aging time periods (0.5 d, 1 d, 2 d, and 3 d) were researched for Cu/Zn/Al-HTlc preparation.Figure 2(d) showed the XRD pattern of the corresponding product of four aging time periods.e diffraction peak intensity of all products was high and has no obvious difference, which demonstrated the aging time has not a certain effect on the crystallinity of Cu/Zn/Al-HTlc.In order to rapidly obtain Cu/Zn/Al-HTlc, 0.5 d was selected as the optimum aging time.
erefore, the results of metal ions molar ratio, pH value, aging temperature, and aging time indicated that Cu/Zn/Al-HTlc can be prepared with the Zn II /Al III molar ratio of 1.5 : 1, pH � 11, and aging at room temperature for 0.5 d.

Characterization of Cu/Zn/Al-HTLc.
e structure and thermal stability of Cu/Zn/Al-HTlc were confirmed by XRD, FTIR, SEM, EDS, and TG. e XRD diffraction parameters of   [29].e FTIR spectrum of Cu/Zn/Al-HTlc was shown in Figure 3.As shown in the FTIR spectrum, the broad band is observed at 3353 cm −1 , which was ascribed to the O-H stretching vibration of interlayer water molecules.ere was also a vibration band at 1632 cm −1 , assigned to the O-H bending vibration.e strong vibration band at 1353 cm −1 was ascribed to interlayer CO 3 2-anions.e sharp bands at around 772, 552, and 428 cm −1 were caused by various lattice vibrations associated with metal hydroxide sheets [34].
e porous structures of calcinated Cu/Zn/Al-HTlc was analyzed by N 2 adsorption-desorption curves in Figure 4(a).According to the IUPAC classification, the isotherm of calcinated Cu/Zn/Al-HTlc is classified as the type IV adsorption isotherm, indicating that it is a mesoporous material [35,36].e Brunauer-Emmett-Teller (BET) surface area of Cu/Zn/Al-HTlc reached 60.16 m 2 •g −1 , which was closed to calcinate Mg/Al-HT [25].e big surface area was crucial for the adsorption capacity because of more active sites.To further analyze the microstructural features of the material, the SEM image of Cu/Zn/Al-HTlc was studied.As shown in Figure 4(b), the expected platelet-like morphology of Cu/Zn/Al-HTlc was observed, which possess a relatively small average platelet size.e layered structure of Cu/Zn/ Al-HTlc was in line with the typical hydrotalcite morphology with irregular edges.
Furthermore, the energy dispersive X-ray spectra (EDS) of Cu/Zn/Al-HTlc demonstrated that the prepared Cu/Zn/ Al-HTlc contained Mg, Al, Cu, Zn, Cd, and Cr elements, which indicated the layered structure of prepared Cu/Zn/Al-HTlc was also composed of Mg, Cd, and Cr elements of the electroplating wastewater.
Here, TG-DTG analyses were performed to evaluate thermal stability of Cu/Zn/Al-HTlc.e TG-DTG curve is shown in Figure 5. e results indicated that the Cu/Zn/Al-HTlc decomposed mainly in three stages.e first stage with 8.18% weight loss at about 92.2 °C corresponded to the loss of physical adsorption water.
e second stage with 7.43% weight loss centering at 170.6 °C was due to the loss of interlayer water molecules without collapse of the hydrotalcite structure.And the last stage with 10.24% weight loss after 227.1 °C was due to the removal of the interlayer anions and hydroxyl groups [25,37].

Cu II Removal from Electroplating Wastewater.
e Cu II was removed from electroplating wastewater by preparation of Cu/Zn/Al-HTlc.And electroplating wastewater containing Cu II together with exogenous Zn II and Al III formed the layered Cu/Zn/Al-HTlc by the coprecipitation method.At the same time, it was found that the best condition of Cu II removal was the same with the optimal preparation condition.To evaluate the Cu II removal effect from the electroplating wastewater, the metal ion content of the Cu/Zn/ Al-HTlc, electroplating wastewater, and residual liquid were determined by ICP-OES.e results of these measurements are presented in Tables 2 and 3. Table 2 showed that Cu/Zn/ Al-HTlc contains Zn II , Cu II , Ni II , Mg II , Al III , Cd II , Fe III , and Cr III metal ion.Its mass ratio was identified as 1.442 : 0.129 : 0.103 : 0.007 : 1 : 0.082 : 0.002 : 0.038 by calculating metal ion content.erefore, the chemical molecular formula of the Cu/Zn/Al-HTlc can be represented as Zn 0.515 Cu 0.046 Ni 0.037 Mg 0.002 Cd 0.029 Fe 0.001 Al 0.357 Cr 0.013 (OH) 2 (CO 3 ) 0.185 •0.5H 2 O.
Table 3 showed the concentrations of remaining Zn II and Al III of residual liquid were high. is result can be attributed to exogenous Zn II and Al III in the preparation process of Cu/ Zn/Al-HTlc.But compared with electroplating wastewater, the concentrations of Zn II and Al III were greatly reduced, which indicated exogenous Zn II and Al III have not brought about secondary pollution.And the residual Mg II , Al III , Cu II , Zn II , Cd III , Fe III , and Ni II concentrations have reached the national emission standards.e calculating result of chromium was little lower than electroplating wastewater, which due to Cr VI exists in electroplating wastewater and have not participated in the preparation process of Cu/Zn/Al-HTlc.e analysis result indicated Cu II and other metal ion pollutant can be successfully removed by Cu/Zn/Al-HTlc preparation, and all

Iodide Adsorption of Cu
/Zn/Al-HTlc.Generally, Cu/Zn/ Al-HTlc has good adsorption property.Firstly, the paper researched the adsorption condition (adsorption time, pH, solid-liquid ratio, and adsorption temperature) for iodide.
en, the maximum adsorption capacity was determined by the Langmuir model.

Effect of the Adsorption Condition.
e adsorption time effect was evaluated from 0 to 120 min.e effect of solution pH on iodide adsorption was monitored at pH 4, 5, 6, 7, 8, 9, and 10, respectively.e effect of the solid-liquid ratio on iodide adsorption was evaluated by using different ratios (1 : 100, 1 : 150, 1 : 200, 1 : 250, and 1 : 300).e adsorption temperature effect was studied at 25 °C, 35 °C, 45 °C, 55 °C, and 65 °C, respectively.e results were shown in Figure 6.As shown in Figure 6(a), when Cu/Zn/Al-HTlc was mixed with   6(c), with the increasing solid-liquid ratio, the adsorption rate increased.When the solid-liquid ratio was 1 : 250, the adsorption rate reached 72.9%.e adsorption rate was decreased when solid-liquid ratio was more than 1 : 250.So the optimal solid-liquid ratio was chosen to be 1 : 250.As shown in Figure 6(d), the adsorption rate curve indicated that the adsorption temperature has no obvious influence on adsorption rate.e iodide adsorption can proceed at room temperature.Consequently, the iodide adsorption experiment can be completed on the condition of 25 °C, pH � 8, solidliquid ratio of 1 : 250, and adsorption time of 30 min.

Adsorption Capacity.
e adsorption capacity of Cu/ Zn/Al-HTlc for different concentration iodide solutions was measured.e results were shown in Figure 7.As shown in Figure 7, the iodide concentration was 200 to 1400 mg•L −1 , and the iodide adsorption capacity of Cu/Zn/Al-HTlc was increased with the increasing iodide concentration.When iodide concentration reached 1400 mg•L −1 , adsorption capacity did not reach equilibrium, which indicated Cu/Zn/Al-HTlc has an excellent adsorption capacity.
In order to measure specific adsorption capacity, saturated adsorption capacity of Cu/Zn/Al-HTlc to iodide should be determined.Freundlich and Langmuir equations are common models to analyze the adsorption isotherm [22].In this study, the two models were used to fit the experimental data.
e Freundlich equation can be expressed in a linear form as shown in equation (1), where q e (mg•g −1 ) is the equilibrium adsorption capacity, C e (mg•L −1 ) is the equilibrium concentration, and k f (L•mg −1 ) and 1/n are the Freundlich constant and heterogeneity factor: e Langmuir isotherm model can be expressed in a linear form as shown in equation (2), where b (mg•L −1 ) is the Langmuir adsorption constant and q m (mg•g −1 ) is the Journal of Chemistry maximum adsorption capacity.By using equations ( 1) and ( 2), the iodide-saturated adsorption experiment results were fitted.e fitted results indicated the correlation coefficient of the Langmuir model and Freundlich model was 0.9990 and 0.9959, respectively.e results indicated that the Cu/ Zn/Al-HTlc adsorption with iodide was effective monolayer sorption.e maximum adsorption capacity of iodide calculated by the Langmuir model for Cu/Zn/Al-HTlc was 1000 mg•g −1 .It was higher than the reported iodide adsorbent [20,21], which was attributed to iodide penetrated into the layered structure of Cu/Zn/Al-HTlc, and Cu II undergoes a specific redox reaction, producing CuI and indirectly enhancing the adsorption capacity for iodide.So Cu/Zn/Al-HTlc has better iodide adsorption property [38].Hence, Cu/Zn/Al-HTlc could effectively treat iodide pollution in wastewater.

Conclusions
Cu/Zn/Al-HTlc was successfully prepared by the coprecipitation method, and Zn II , Al III , and Cu II of electroplating wastewater contributed to the formation of the layered structure.Cu II pollutant was successfully removed from electroplating wastewater in the coprecipitation preparation process and reached Chinese National Emission Standard (GB 21900-2008).And the prepared Cu/Zn/Al-HTlc has good adsorption capacity for iodide.
e maximum iodide adsorption capacity reached 1000 mg•g −1 at the optimized condition by the Langmuir model.Consequently, it was a good technology for removal of the metal ion pollutant of the electroplating wastewater by using electroplating wastewater to prepared Cu/Zn/Al-HTlc.And the Cu/Zn/Al-HTlc also could be potentially used as an effective adsorbent of anion pollutants for electroplating wastewater.

Figure 1 :
Figure 1: XRD pattern of precipitation of (a) Zn II and wastewater, (b) Ca II and wastewater, (c) Fe III and wastewater, (d) Zn II -Al III and wastewater, (e) Cd III -Al III and wastewater, and (f ) wastewater.

Figure 6 :
Figure 6: Effect of (a) adsorption time, (b) pH value, (c) solid-liquid ratio, and (d) adsorption temperature on the iodide adsorption.
HTlc and Zn/Al-HT were shown in Table1.It can be observed that XRD diffraction parameters of Cu/Zn/Al-HTlc and Zn/Al-HT were similar, which could be attributed to similar ion radius of Cu II and Zn II .At the same time, Cu II together with Zn II constructed layers structure of the Cu/Zn/ Al-HTlc, which leads to smaller change of interplanar distance, layer distance, and crystal cell size.It can be seen that d 003 , d 006 , and d 012 have good multiple relationship for the Cu/Zn/Al-HTlc, which indicated it has an excellent layer structure

Table 3 :
e metal ions concentration of the electroplating wastewater and remaining metal ions concentration of residual liquid.