Noncrystalline, high surface area magnesium silicate gel was successfully prepared by hydrothermal method. Such product was characterized by BET and XRD to determine surface area 576.4 m2·g−1, average pore width 2.76 nm, and amorphous surface. The adsorption behaviors of Basic Brown and Chrysophenine on magnesium silicate gel were investigated through changing initial concentration, adsorbent dosage, solution pH, contact time, and temperature. The experimental data was analyzed by the adsorption isotherms and kinetics. The results showed the adsorption progress was fast for Basic Brown, and the adsorption equilibrium was finished in 2 h, while the adsorption equilibrium of Chrysophenine was finished in 7 h. Freundlich isotherm model and second-order kinetic models described the adsorption process very well.
At present, dyes have been widely used in many fields, such as printing and dyeing, papermaking, textile, and food [
A survey revealed that many materials including chitosan, bagasse pith, peat, rice husk, fly ash, wood, and some natural minerals, such as bentonite, montmorillonite, alunite, sepiolite, zeolite, and diatomite, have been used as adsorbents to remove the dyes from solution [
Herein, we synthesized a kind of new, high special surface area and multiporous magnesium silicate gel through hydrothermal method. The adsorption experiments were performed by investigating the removal behavior of Basic Brown and Chrysophenine on the as-prepared magnesium silicate gel from water solution. This study provides the theory evidence and practice support for industrial wastewater treatment.
All chemical reagents were of analytical grade purity and used as received without further purification. All dyes were dried at 110°C for 2 h before using. All solutions were prepared with distilled water.
Basic Brown, industrial grade, is a dark brown-red powder with Color Index number 1 (21000) and CAS 8052-76-4. Its structure is shown in Figure
Molecular structure of Basic Brown (a) and Chrysophenine (b).
The surface area and pore-size distribution of magnesium silicate gel were determined by N2 adsorption/desorption analysis using ASAP 2020, Micromeritics. The surface area was evaluated by Brunauer-Emmett-Teller (BET) equation, and the pore-size distribution was determined by Barrett-Joyner-Halenda (BJH) equation. The finial equilibrium concentration was measured by 722 UV-Vis spectrophotometer. Solution pH was determined through pHS-3C meter equipped with a combined pH electrode.
Magnesium chloride hexahydrate and sodium silicate nonahydrate (Na2O/SiO2 module = 1) were mixed with the mole ratio of 2 : 1 at room temperature. The resultant white precipitate appeared immediately. Keeping the precipitate stirring for 5 h at room temperature, then the turbid liquid was transited into reaction kettle for 24 h reaction at 120°C. Finally, the reaction was cooled to room temperature, and the resultant samples were washed using heat water until without Cl−. The prepared adsorbent was dried in an air oven at 110°C for 10 h and allowed to cool naturally.
Adsorption experiments of Basic Brown and Chrysophenine on magnesium silicate gel were carried out through changing dyes initial concentration, adsorbent dosage, solution pH, contact time, and temperatures. Solution pH was adjusted by adding 0.1 mol·L−1 NaOH or HCl solution and determined by using a pHS-3C meter equipped with a combined pH electrode. The pH-meter was standardized with normal buffer solution (NBS) before measurement. After adsorption equilibrium was established, the residual concentration was measured by using a 722 UV-Vis spectrophotometer at the corresponding maximum wavelength. The percent of removal and adsorption capacity of the two dyes on magnesium silicate gel were calculated as the following equations:
N2 adsorption/desorption isotherms and pore-size distribution are shown in Figure
N2 adsorption/desorption isotherms and pore-size distribution (inset).
The X-ray powder diffraction (XRD) of magnesium silicate gel is shown in Figure
The X-ray powder diffraction (XRD) of magnesium silicate gel.
The effect of initial concentration of the two dyes on magnesium silicate gel can be investigated when the mass of adsorbent used was 2 g·L−1. The removal ratio of the two dyes on magnesium silicate gel as a function of dyes initial concentration was given in Figure
The adsorption effect of the two dyes on magnesium silicate gel as a function of initial concentration (mass of magnesium silicate gel: 2 g⋅L−1; adsorption time: 2 h for Basic Brown and 7 h for Chrysophenine; pH: 7.1).
The mass of magnesium silicate gel was in the range of 0.5–5 mg·L−1, and the concentration of dyes was 200 mg·L−1. The removal ratio of the two dyes on magnesium silicate gel was shown in Figure
The effect of mass of magnesium silicate gel on the removal of Basic Brown and Chrysophenine (concentration of dyes: 200 mg⋅L−1; adsorption time: 2 h for Basic Brown and 7 h for Chrysophenine; pH: 7.1).
The removal effect of Basic Brown and Chrysophenine on magnesium silicate gel as a function of solution pH was shown in Figure
The effect of solution pH on the removal of the two dyes on magnesium silicate gel (concentration of dyes: 100 mg⋅L−1; mass of adsorbent: 2 g⋅L−1; adsorption time: 2 h for Basic Brown and 7 h for Chrysophenine; pH: 7.1).
At 298 K, the adsorption effect of Basic Brown and Chrysophenine on magnesium silicate gel as a function of different contact time was shown in Figure
The effect of contact time on the removal of the two dyes on magnesium silicate gel (concentration of dyes: 100 mg⋅L−1; mass of adsorbent: 2 g⋅L−1; pH: 7.1).
The effect of adsorption temperature on the removal of Basic Brown and Chrysophenine is shown in Figure
Adsorption isotherms of Basic Brown (a) and Chrysophenine (b) at different adsorption temperatures.
The Langmuir and Freundlich adsorption models have been used to fit the experimental data at different adsorption temperatures:
The analysis revealed that the adsorption isotherms did not meet the Langmuir adsorption model, which indicated the adsorption of Basic Brown and Chrysophenine on magnesium silicate gel was not simple single-molecule adsorption. The Freundlich adsorption model can describe the adsorption process of the two dyes very well, and the square correlation coefficient value of Basic Brown approached 0.97, while the coefficient value of Chrysophenine was 0.94. The corresponding parameters are given in Table
The relative parameters of Freundlich adsorption isotherm model.
Dyes |
|
|
Freundlich model | ||
---|---|---|---|---|---|
mg⋅L−1 |
|
|
| ||
Basic Brown | 200 | 298 | 0.5629 | 1.2849 | 0.9888 |
200 | 308 | 0.2473 | 1.49 | 0.9895 | |
200 | 318 | 0.1802 | 1.565 | 0.9876 | |
|
|||||
Chrysophenine | 200 | 298 | 0.3574 | 1.5201 | 0.9742 |
200 | 308 | 0.2998 | 1.5327 | 0.9450 | |
200 | 318 | 0.0738 | 1.8268 | 0.9556 |
At different adsorption temperatures, the removal of Basic Brown and Chrysophenine on magnesium silicate gel at different adsorption time was shown in Figure
The adsorption kinetics of Basic Brown (a) and Chrysophenine (b) at different adsorption time.
In order to investigate the adsorption kinetics behavior, the first-order kinetic and second-order kinetic models were used to discuss the adsorption mechanism.
The first-order kinetics equation is presented as
The second-order kinetics equation is given as
The experimental data was analyzed through the first-order kinetics model and second-order kinetics model, respectively. The corresponding kinetics parameters were obtained from (
Second-order kinetics values calculated for the adsorption of the two dyes on magnesium silicate gel.
Dyes |
|
|
First-order | Second-order | ||||
---|---|---|---|---|---|---|---|---|
mg⋅g−1 |
|
|
|
|
|
|
||
Basic Brown | 298 | 0.0187 | 0.951 | 76.6 | 76.92 | 0.0180 | 1.000 | |
308 | 0.0377 | 0.973 | 76.4 | 76.92 | 0.0133 | 1.000 | ||
318 | 0.0043 | 0.944 | 76.1 | 74.07 | 0.0190 | 1.000 | ||
|
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Chrysophenine | 298 | 0.0092 | 0.972 | 56.8 | 60.61 | 5.44 × 10−4 | 0.9975 | |
308 | 0.0079 | 0.994 | 76.4 | 58.82 | 5.47 × 10−4 | 0.9980 | ||
318 | 0.009 | 0.993 | 76.1 | 57.47 | 4.90 × 10−4 | 0.9974 |
A kind of high special surface area adsorbent, magnesium silicate gel, was prepared through hydrothermal synthesis. The calculated special surface area was 576.4 m2·g−1, and the average pore width was 2.76 nm. The adsorption experiments showed that the removal ratio gradually increased with the increase of initial concentration, mass of adsorbent, solution pH, and contact time as well as the decrease of the temperature. The adsorption quantity of Basic Brown was 76.5 mg·g−1 and the adsorption quantity of Chrysophenine was 57.8 mg·g−1 with the initial concentration of 200 mg·L−1 for both dyes, respectively. The adsorption equilibriums were finished in a short time for removal of Basic Brown, and the adsorption process of the two dyes performed physical adsorption. Freundlich adsorption isotherm and second-order kinetics models well described the adsorption behavior of the two dyes on magnesium silicate gel.
The authors declare that there is no conflict of interests regarding the publication of this paper.
Zhenhua Li and Zhun Zhao contributed equally to this study.
The authors are grateful for financial support of this work from the Science and Technology Project in Colleges and Universities in Shandong Province (J10LB61).