Preparation of new diatomite–chitosan composite materials and their adsorption properties and mechanism of Hg(II)

A new composite absorbent with multifunctional and environmental-friendly structures was prepared using chitosan, diatomite and polyvinyl alcohol as the raw materials, and glutaraldehyde as a cross-linking agent. The structure and morphology of the composite absorbent, and its adsorption properties of Hg(II) in water were characterized with Fourier transform infrared (FT-IR) spectra, scanning electron microscope (SEM), X-ray diffraction (XRD), Brunauer Emmett Teller (BET) measurements and ultraviolet–visible (UV–Vis) spectra. The effect of the pH value and contact time on the removal rate and absorbance of Hg(II) was discussed. The adsorption kinetic model and static adsorption isotherm and regeneration of the obtained composite absorbent were investigated. The results indicated that the removal of Hg(II) on the composite absorbent followed a rapid adsorption for 50 min, and was close to the adsorption saturation after 1 h, which is in accord with the Langmuir adsorption isotherm model and the pseudo-second-order kinetic model. When the pH value, contact time and the mass of the composite absorbent was 3, 1 h and 100 mg, respectively, the removal rate of Hg(II) on the composite absorbent reached 77%, and the maximum adsorption capacity of Hg(II) reached 195.7 mg g−1.


Introduction
As is known, water is one of the basic necessities required for the sustenance and continuation of life. It is, therefore, important that good quality water should be available for various activities. in 2θ range (10°-80°). The Brunauer-Emmett-Teller (BET) surface area and the pore size distribution were measured using N 2 adsorption and desorption (Quadrasorb SI, Quantachrome, USA) at 77 K. The mercury ion concentration was measured by a TU-1901 dual-beam UV-Vis spectrophotometer (Beijing Purkinje General Instrument Co., Ltd., China) with a range of 450-600 nm.

Preparation of new composite adsorbent
The diatomite was treated with 40% sulfuric acid at a solid-liquid ratio of 1 : 4, after which the mixture was stirred at 80°C for 8 h, filtered, washed several times with distilled water and dried at 60°C for 24 h. The dried diatomite was placed in a muffle furnace and calcined at 450°C for 6 h, and then sealed for storage.
In a 1000 ml three-neck flask, a mixture of 350 ml of 2% chitosan acetic acid solution, 27 ml of 10% polyvinyl alcohol solution and 3 g purified diatomite was stirred vigorously. After the mixture was maintained for 4 h at room temperature, 7.5 ml of 1% glutaraldehyde was added dropwise to the mixture and continuously stirred for 4 h. The crude product was placed in a freezer for 24 h at −18°C, and then 12 h at room temperature. The process was repeated three times. Finally, the product was placed in a diluted sodium hydroxide solution for 24 h and washed to neutral with deionized water. The cross-linking diatomite/chitosan powder CA was obtained by drying until a constant sample mass was obtained.

Adsorption experiments
The adsorption experiments were performed by using CA as a new adsorbent. First, the pH value of mercury nitrate solution was adjusted by adding a small amount of 0.1 M HCl or NaOH solution. Secondly, a certain amount of adsorbent was added to the mercury nitrate solution and adsorption equilibrium was obtained through stirring. Finally, the concentration of Hg(II) in water was detected with dithizone spectrophotometry.

Structural characterization
The structure of the obtained CA was characterized by FT-IR, SEM, XRD and Brunauer Emmett Teller (BET) measurement. The FT-IR spectrum of CA, shown in figure 1, exhibited characteristic bands at 3435, 1650 and 1108 cm −1 , attributed to −OH, −NH 2 , acid amides and Si−O stretching vibration peaks, respectively. In addition, the bank appearing at 1560 cm −1 belonged to a C=N stretching vibration peak, which showed the formation of a Schiff base in CA due to the cross-linking reaction of glutaraldehyde with amino groups of chitosan.
The SEM image of CA is shown in figure 2. As can been clearly seen, the particles of chitosan were loaded on the surface of the diatomite particles. To further identify the structure of CA, XRD studies were carried out. Figure 3 shows the XRD patterns of CA powder at room temperature. Two peaks at 2θ of 20.6°and 26.4°belonged to typical SiO 2 diffraction peaks, and a characteristic peak at 8.8°was attributed to a chitosan diffraction peak. This is the result of the formation of strong molecular and intermolecular hydrogen bonding between −OH and −NH 2 in chitosan molecules; this cross-linking reaction makes the structure of the diatomaceous soil layer relatively stable. Specific surface area and pore structure of CA and diatomite were measured and shown in table 1. Immobilizing chitosan on diatomite blocked the channels of some of the diatomite, resulting in reduction of the pore volume and pore size of CA. However, the specific surface area of CA was larger than diatomite because chitosan was cross-linked with glutaraldehyde to form some new micropores.

Adsorption properties
To investigate the effect of CA on the adsorption properties of Hg(II), 100 ml of standard solution of mercury ions (200 mg l −1 ) was added to two small beakers with 100 mg CA and 100 mg diatomite for 1 h at 20°C, respectively. Figure 4 shows the effect of CA and diatomite on the adsorption of Hg(II). The adsorptive capacity of CA for Hg(II) was more than that of the diatomite. So the efficiency of modified diatomite/chitosan CA was very high.            almost no change. The results were explained as follows: as adsorption proceeded and the surface of CA tended to saturate, mercury ions could further spread inside the adsorbent, but because of the existence of diffusion resistance, the removal rate by CA of Hg(II) was obviously less than the initial surface adsorption rate. Figure 6 shows the effect of pH on the adsorption of Hg(II). It can be seen from figure 6 that CA had a relative good adsorption capacity for Hg(II) when the pH value of the solution was in the range from 1 to 5. The maximum removal rate of CA achieved 77% when the pH value was 3. When the pH value was more than 7, white flocculation appeared due to the generation of Hg(OH) 2 precipitation.

Regeneration of adsorbent
To confirm a reversible process and re-use of CA, an experiment was performed as follows. First, 100 mg CA was placed in 200 mg l −1 of mercury nitrate solution, adsorbed and then separated. Second, this CA was added to 50 ml of saturated EDTA solution as a desorbent, and the mixture was stirred for 3 h. Finally, the concentration of mercury ions was detected by dithizone spectrophotometry. Figure 7 shows the reuse effect of CA.  After performing the cycle five times, the removal rate of mercury ions decreased from 79% to 62%. CA acts as an adsorbent at pH 3; in this condition, a small amount of chitosan is dissolved in solution, resulting in the loss of associated adsorbent material during the repeated cycles, which is one of the main reasons for the decrease of recyclability. The results illuminate that the adsorbents could be effectively regenerated by HNO 3 , indicating that CA had a better reusable performance.

Kinetic model analysis
In order to describe the adsorption mechanism and the speed steps of the adsorption process, the pseudo-first-order kinetic model and the pseudo-second-order kinetic model were adopted based on the experimental data.
The equation of the pseudo-first-order kinetic model was as follows: where t is contact time, q e (mg g −1 ) and q t (mg g −1 ) are the adsorption capacity at equilibrium and at contact time t and k 1 (min −1 ) represents the pseudo-order reaction rate constant. The equation of the pseudo-second-order kinetic model was described as follows: where k 2 (g mg −1 min −1 ) is the pseudo-second-order reaction rate constant. Figure 8 shows the fitting results of the first-and second-order kinetic models. It could be seen that the adsorption of mercury ions to CA was in agreement with the pseudo-second-order kinetics model.   As shown in table 2, the adsorption equilibrium capacity value of the first-order kinetic model was 58.32 mg g −1 , while the experimental adsorption balance capacity value was 155 mg g −1 . However, for the second-order kinetic model, the capacity value of adsorption equilibrium was 160.5 mg g −1 , and it had only a difference of 5.5 mg g −1 from the balance capacity value. Thus, this indicated that the adsorption of mercury ions to CA was well in agreement with the second-order kinetic model.

Adsorption isotherms
The isothermal adsorption equation for metal ions is described by the Langmuir model and Freundlich model. Both can be used for physical and chemical adsorption. Their adsorption isotherm equations is described as follows: and lg q e = lg K F + 1 n lg C e (3.4) where C e (mg l −1 ) is the equilibrium concentration for heavy metal ions, q e (mg g −1 ) is the adsorption capacity, q m (mg g −1 ) is the maximum adsorption capacity, K F (l mg −1 ) and K L (l mg −1 ) represent the Freundlich and Langmuir adsorption constant, respectively, and n is the adsorption intensity. Figures 9 and 10 show linear regression for the Langmuir equation C e /q e− C e and Freundlich equation lgq e − lgC e . According to Freundlich adsorption, K F and 1/n were obtained by the intercept and slope of the straight line of lgq e versus lgC e , respectively. The corresponding Langmuir and Freundlich parameters of CA for the adsorption of Hg(II) are shown in table 3.
Comparing figures 9 and 10, the Langmuir adsorption isotherm can better describe the adsorption behaviour of CA towards Hg(II). The CA obtained in this study for the adsorption of Hg(II) was in the form of a monolayer adsorption.
As shown in      Table 4 compares the best adsorption capacity of CA with different CS composite sorbents for the removal of Hg(II) ions. It can be clearly seen in the table that the adsorption capacity of CA is higher than that of most adsorbents. In short, considering the process characteristics of this work, CA as a lowcost sorbent not only decreases the amount of chitosan needed using cheap diatomite but also has high adsorption capacity and good mechanical stability, which indicates that the CA will be a good candidate for applications in heavy metal removal from wastewater.

Conclusion
In this study, a new composite absorbent was prepared by a cross-linking reaction of chitosan, diatomite and glutaraldehyde. When the pH-value, contact time and mass of the composite absorbent was 3, 1 h, and 100 mg, respectively, the removal rate of Hg(II) on the composite absorbent reached 77%.  of the static non-equilibrium adsorption isotherm showed that the removal of Hg(II) on the composite absorbent followed a rapid adsorption for 50 min, and was close to adsorption saturation after 1 h, which followed the pseudo-second-order kinetic model. The results of the static equilibrium adsorption isotherm showed that the adsorption of Hg(II) was according to the Langmuir adsorption isotherm model, and the maximum adsorption capacity of Hg(II) reached 195.7 mg g −1 at the same conditions. In addition, it was found that the adsorption capacity of Hg(II) decreased little after four times re-use of the composite absorbent, which indicated that this composite absorbent had good regeneration behaviour.
Data accessibility. This paper has no additional data. Authors' contributions. J.H. and Q.C. designed the study. Y.F. prepared all samples for analysis. Y.F., Y.H. and X.X. collected and analysed the data. Y.F. and Y.W. interpreted the results and wrote the manuscript. All authors gave final approval for publication.