The Application of Response Surface Methodology for 2,4,6 ‐ Trichlorophenol Removal from Aqueous Solution Using Synthesized Zn 2+ ‐ Al 3+ ‐ Tartrate Layered Double Hydroxides

: Trichlorophenols are on the US environmental protection agency’s list of priority pollutants due to their serious damage to water safety. With the aim of adsorbing the 2,4,6 ‐ trichlorophenol (2,4,6 ‐ TCP), Zn 2+ ‐ Al 3+ ‐ tartrate layered double hydroxides (Zn 2+ ‐ Al 3+ ‐ C 4 H 4 O 62 − ‐ LDHs) adsorbent was synthesized via homogeneous precipitation method. X ‐ ray powder diffraction (XRD), Fourier infrared spectroscopy (FT ‐ IR), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were used to characterize Zn 2+ ‐ Al 3+ ‐ C 4 H 4 O 62 − ‐ LDHs. The concentration of 2,4,6 ‐ TCP was determined using gas chromatography–mass spectrometry (GC ‐ MS). Zn 2+ ‐ Al 3+ ‐ C 4 H 4 O 62 − ‐ LDHs exhibited a good adsorption performance of 2,4,6 ‐ trichlorophenol, since a bigger layer spacing of Zn 2+ ‐ Al 3+ ‐ C 4 H 4 O 62 − ‐ LDHs was obtained than that in Zn 2+ ‐ Al 3+ ‐ CO 32 − ‐ LDHs. Adsorption parameters of adsorption temperature, contact time, adsorbent dosage, and solution pH were investigated, the initial concentration of 2,4,6 ‐ TCP was 2.0 g/L. Response surface methodology (RSM) was employed to provide an investigative approach towards optimization of the adsorption process. The highest removal rate of 89.94% and the average removal rate of 88.74% were achieved under a temperature of 20.0 °C, a contact time of 2.5 h, an adsorbent dosage of 0.15 g, and a solution pH of 3. the capacity of the adsorbent is 599.6 mg/g. Meanwhile, the reusable properties of Zn 2+ ‐ Al 3+ ‐ C 4 H 4 O 62 − ‐ LDHs were evaluated by the same adsorption system, and the removal rate of 2,4,6 ‐ TCP was 85.57% at the fifth regeneration. The obtained results confirmed that the Zn 2+ ‐ Al 3+ ‐ C 4 H 4 O 62 − ‐ LDHs can be used as a potential introduction in practical applications for the removal of 2,4,6 ‐ TCP.


Introduction
Water safety plays such an important role in human life and the ecosystem, and the economic growth of a country depends on a healthy and disease-free environment, since sewage treatment has become a hot issue in the world along with the increasingly serious environmental pollution problems today. Chlorophenol (CP) contamination can deteriorate water quality, posing a threat to the health of human beings with the potential of cancer, deformity, and mutation, as well as to the survival of aquatic organisms [1][2][3]. The phenol 2,4,6-trichlorophenol (2,4,6-TCP), which is often used in dyeing, textile, wood and leather industries, and paper industries [4][5][6][7], is also used as a by-product in the process of greenhouse gas emissions from fossil fuels such as petrochemicals, oil and plastic, municipal solid waste combustion, and chlorination of phenol-contaminated waters [8,9]. Due to the special chemical structure of 2,4,6-TCP, it can exist perpetually with high toxicity, carcinogenicity, bioaccumulation capacity, long-distance migration and ease of producing hazardous secondary pollution during treatment [10][11][12], which makes it one of the most difficult organic pollutants to degrade [13].Therefore, it must be treated strictly before being discharged into the water circulatory system. Currently, several strategies have been extensively used to remove TCP, including heat treatment, adsorption, advanced oxidation, biological degradation [14][15][16][17][18], photocatalytic degradation, ion exchange, and solvent extraction [19][20][21][22][23][24][25][26][27][28]. However, some of these have limitations due to the complexity of treatment processes, production of toxic by-products, high cost, high energy consumption, incomplete mineralization, or specific conditions, which have led to the choice of adsorption process in most studies [29][30][31][32]. The technology of adsorption has the characteristics of environmental compatibility, mild reaction conditions, flexibility and simplicity of design, insensitivity to toxic pollutants, green environmental protection, and no secondary pollution, which therefore explains the extensive application of the adsorption method [33,34].
Selecting the appropriate adsorbent has a fundamental role in the efficiency of the adsorption process in TCP treatment. Among the various natural and biological adsorbents, hydrotalcite-like compounds have been considered an important adsorbent. Hydrotalcite-like compounds are called layered double hydroxides (LDHs), and are a form of 2D anionic clay with significant features, such as high porosity arranged with a specific surface area and pore size distribution; resistance to thermal changes and pH variations; tunable interior architecture; high surface area; and great anion-exchange capacity, dehydration, ion-exchange capacity, and low density; as well as the ability to remove organic and inorganic contaminants [20,24,35]. In recent years, as shown in Figure  1, publications concerning applications of LDHs in adsorption have gradually increased in number, which indicates that the potential of LDHs and their derivatives in adsorption have raised the attention of researchers. LDHs and their derivatives may become a promising adsorbent because of the following merits: (i) low cost and simple process of production; (ii) flexible chemical composition. For example, the anion layer of CO3 2− can be replaced by larger anions such as C4H4O6 2− or MoO4 2− , leading to an increase in saturated adsorption capacity. A variety of transition metals and functional anions also could be employed to prepare LDHs to enhance catalytic performance; (iii) a favorable fixation effect for toxic ions due to the stabilized structure of LDHs [36].
Therefore, a novel material of Zn 2+ -Al 3+ -C4H4O6 2− -LDHs was prepared via homogeneous precipitation method with raw materials of sodium tartrate (C4H4O6Na2), zinc nitrate (Zn (NO3)2), and aluminum nitrate (Al(NO3)3). The objectives of this work are: (i) to synthesize and characterize the Zn 2+ -Al 3+ -C4H4O6 2− -LDHs; (ii) to explore the removal of operation parameters of adsorption temperature, contact time, adsorbent dosage, and solution pH, as well as the optimization of removal operation for 2,4,6-TCP using a mathematical and statistical technique of response surface methodology (RSM). RSM can provide an investigative approach towards optimization. It was used in the significance of several affecting factors in an optimum manner, and to determine the optimum operational conditions for the process [37,38]; (iii) to evaluate the regeneration property of Zn 2+ -Al 3+ -C4H4O6 2− -LDHs.

Synthesis of Zn
The mixture of Al(NO3)3•9H2O (7.0300 g), Zn(NO3)2·6H2O (16.7300 g) and CO(NH2)2 (11.7100 g) was dissolved with 500.0 mL of secondary distilled water, and this prepared solution was sonicated for 5 min in order to mix evenly. It was then immediately transferred into two round-bottom flasks, sealed, and placed in a constant-temperature water bath at a temperature of 98 °C. After being refluxed for 8.0 h under the condition of uniform stirring (500 rpm), the precipitate was separated, washed, filtered, and dried at room temperature to obtain Zn 2+ -Al 3+ -CO3 2− -LDHs product. Then, NaCl (29.2200 g), HCl (0.15 mL) and Zn 2+ -Al 3+ -CO3 2− -LDHs powder (0.5000 g) were mixed with ultrasonic stirring (500 rpm) for 5 min before diluting to volume of 500 mL, then ventilated for 30 min (the flow rate was 10.00 L/h) and magnetically stirred (500 rpm, room temperature of 20.0 °C) for 8.0 h. The following processes are precipitation, filtration, washing and drying at room temperature, and finally the obtention of Zn 2+ -Al 3+ -Cl − -LDHs. Next, Zn 2+ -Al 3+ -Cl − -LDHs (0.3000 g) and C4H4O6Na2·2H2O (6.9024 g) were mixed to make a uniform solution (300.00 mL), which was transferred into a single-port round-bottom flask (500.0 mL), ventilated (nitrogen, N2) for 30.0 min, then sealed before the reaction took place on a magnetic stirrer 8.0 h later. Finally, the Zn 2+ -Al 3+ -C4H4O6 2− -LDHs were obtained after cleaning with water-free ethanol which aimed to prevent the entry of CO2.

Field Experiment
Simulated aqueous solution with 2,4,6-TCP (the initial concentration is 2.0 g/L, the volume is 50.0 mL) was employed as the processing target, and the adsorption performance of Zn 2+ -Al 3+ -C4H4O6 2-LDHs will be evaluated as adsorbent. The solution was stirred constantly via magnetic stirring (800 rpm) at a certain temperature for a period of time before being separated into solid and liquid phases via centrifugal technique, thus detecting the concentration of 2,4,6-TCP in liquid phase.
The aqueous solution samples (50.0 mL) were extracted by the mixture of NaCl (15.0000 g) and CH2Cl2 (25.0 mL); this operation was repeated twice. Then, the organic phases were merged together and residual moisture was removed by Na2SO4. The following work was conducted to concentrate the extract until its volume was reduced to 5.0 mL, and the continuous concentration process via nitrogen-blowing instrument remained necessary before the solution's volume reached 0.5 mL. Finally, the volume was filled to 1.0 mL with CH2Cl2 in order to prepare for GC-MS test.
During the GC-MS test, the carrier gas was high-purity helium (constant flow mode, flow rate with 1.0 mL/min), and the injection volume was 1.0 uL. Column heating mode was adopted from an initial temperature of 80.0 °C to final temperature of 230.0 °C, with a heating rate of 5.0 °C min, and kept at the final temperature for 5.0 min. The detector temperature was 260.0 °C, the sample inlet temperature was 220.0 °C, and ion source temperature was 260.0 °C. The external standard method was used to draw the standard working curve of 2,4,6-TCP. At a concentration range of 0.5～10.0 mg/L, the concentration (Ci) was linear-fitting, plotted by 2,4,6-TCP peak area (Ai), and the obtained equation was Ai = 37921Ci-2 348, (R 2 = 0.9997). According to general requirements for quantitative detection limits (S/N > 3 in Total Ions Chromatograph), under the optimized chromatographic conditions, the limit detection of 2,4,6-TCP by GC-MS was 0.4 mg/L, the GC-MS analysis retention time graph is represented in Figure 2. The removal rate © of 2,4,6-TCP can be calculated by the Equation (1) below.
where C0 is the initial concentration of 2,4,6-TCP in simulated aqueous solution; C1 is the concentration of 2,4,6-TCP in aqueous solution after adsorption process.

FT-IR Analysis of Adsorbents
The FT-IR analysis of Zn 2+ -Al 3+ -CO3 2− -LDHs (a), Zn 2+ -Al 3+ Cl --LDHs (b) and Zn 2+ -Al 3+ -C4H4O6 2− -LDHs (c) are shown in Figure 3, which can reflect the anion intercalation between these materials. Clearly, the adsorption peak at a wavenumber of 1355 cm −1 is recognized as CO3 2− of sample (a), whereas it nearly disappeared on the curve of sample (b), indicating that Clreplaced the CO3 2− after the Clintercalation process. Furthermore, a telescopic vibration peak at a wavenumber of 1385 cm −1 and a bending vibration peak at a wavenumber of 605 cm −1 can be observed on sample (c), which are attributed to C4H4O6 2− according to the infrared spectrum; hence, a signal was sent that the C4H4O6 2− were intercalated into the LDH structure.

BET Analysis of Adsorbents
The two key properties of adsorbents applied in wastewater treatment are large surface area and uniform pore-size distribution. These properties were examined by N2adsorption/desorption measurements. As show in Figure 6, this structural type does not exhibit limiting adsorption at high P/P0; meanwhile, a broad pore size distribution between 2 and 100 nm was observed. The BET surface area is 162 m 2 /g and the pore diameter is 13.58 nm.

Factors of Adsorption Process
The single-factorial experiment was first carried out in this study in order to obtain the range of process parameters for optimizing adsorption conditions. Four factors of adsorption temperature (A), contact time (B), adsorbent dosage (C), and solution pH (D) were mainly investigated; the experimental results are shown in Figure 7 as below. The volume of 2,4,6-TCP in Figure 7 exhibits the effect of contact time, dosage of adsorbent, adsorption temperature, and solution pH on removal rate of 2,4,6-TCP in aqueous solution. The solution volume was 50.0 mL and the initial concentration was 2.0 g/L. The adsorption experiment was repeated three times under each condition. According to the results of the highest removal rate in each group, an optimal parameter was determined; that the contact time was 2.0 h, the dosage of adsorbent was 0.15 g, the adsorption temperature was 20 °C, and the solution pH was 3.

Design and Result
An experimental design for the removal efficiency of 2,4,6-TCP was required to reduce the experimental cost. Factors adopted here are displayed in Section 3.2; each factor has three levels, as shown in Table 1. The orthogonal array designed by RSM method and the results are listed in Table 2.

Analysis of Model
The stepwise regression method was adopted to analyze the experimental data in Table 2. Analysis of variance and significance tests for the regression model are listed in Table 3 and Table 4, respectively.  According to Table 3, the R 2 is 0.990126 and the p value is 0.0001, which indicates that the stepwise regression model is suitable for the analysis of experimental data. Hence, the model can be used to analyze and optimize the adsorption process. When the p index <0.0100, the factor can be interpreted as an extremely significant factor. From Table 4, the significant factors are A and D, as well as A × A, B × D, A × D, and D × D, which means the interaction of four factors among them was not a simple linear relationship. The sequence for these key factors was A/D > B > C.

RSM Optimization and Verification
The RSM method was employed to analyze the experimental data in Table 2 The quadratic regression surface model was established after eliminating the non-significant items, and the model for removal rate can be calculated by the obtained quadratic regression equation given by Equation (2). In order to achieve the optimization conditions for adsorption, the interaction effect among four factors was explored via three-dimensional RSM of the quadratic polynomial regression equation, as shown in Figure 8.  The R under any condition can be calculated by the predictive expression of the Equation (2). Moreover, with the RSM graph, the optimal adsorption process conditions can be determined as an adsorption temperature of 20 °C, a contact time of 2.5 h, a dose of adsorbent of 0.15 g, and a solution pH of 3. Meanwhile, three parallel adsorption experiments (No.22,No.23,and No.24) were performed in a laboratory with the optimal parameters, and the R was 88.92%, 89.94%, and 87.37%, respectively. The average R was 88.74%, which is close to the prediction value. According to the highest R of 89.94%, the capacity of the adsorbent is 599.6 mg/g.

Regeneration
The regeneration of adsorbent was obtained by dissolving the adsorbate with NaOH solution (4 Wt%), and the regeneration reaction is shown in Equation (3).
The adsorption property of the regenerated Zn 2+ -Al 3+ -C4H4O6 2− -LDHs after each recycling were also evaluated by R, and the value of each cycle was 88.24%, 88.69%, 88.01, 87.53%, and 85.57% (Figure 9), respectively. There is a slight decrease in R after each regeneration, indicating the excellent recycling performance of Zn 2+ -Al 3+ -C4H4O6 2− -LDHs. The structure of hydrotalcite laminates is relatively stable, and the tartrate structure is not easily damaged due to the effect of interlayer limitation, and can still maintain a good laminate structure after multiple applications. In a nutshell, Zn 2+ -Al 3+ -C4H4O6 2− -LDHs are a kind of material with good performance of 2,4,6-TCP adsorption and regeneration.

Conclusions
Zn 2+ -Al 3+ -C4H4O6 2− -LDHs were prepared by homogeneous precipitation and ionexchange technology with urea as precipitant. The content ratio of Zn 2+ and Al 3+ was 2:1. The FT-IR and XRD analysis confirmed that the C4H4O6 2− was successfully implanted. SEM images showed that it had an obvious lamellar structure. In the adsorption application, single-factorial experiments and RSM analysis were adopted to find out the optimum conditions for the adsorption of 2,4,6-TCP. The conditions for optimization are that the contact time is 2.5 h, the adsorption temperature is 20.0 °C, the dosage of adsorbent is 0.15 g, and the solution pH is 3. The highest removal rate was 89.94% and the average removal rate was 88.74%, and the capacity of the adsorbent was 599.6 mg/g. Regeneration experiment results showed that the removal rate could still be 85.57% after the fifth regeneration. Zn 2+ -Al 3+ -C4H4O6 2− -LDHs are good adsorbents for 2,4,6-TCP, with good regeneration properties.