| Mercury can transport and transform in environment through dissolving, volatilization, sedimentation, methylation and some other processes. Also, it can endanger human health through food chain and bioaccumulation. Currently, the main removal methods of mercury in wastewater are: chemical precipitation, ion exchange, adsorption, photocatalysis and so on. For wastewater, which contains low concentration of mercury, these methods have some defects, such as, slow reaction rate, high energy consumption, low removal efficiency, or large dosage of chemical reagents. Zeolite is a natural silicate mineral, which has abundant reserves, low cost of processing and mining, larger specific surface area and strong ability of ion exchange and adsorption. Because of these, it can effectively remove mercury in soil, water, and atmosphere, therefore, it become topics in the study of pollution control in China and abroad.Removal effects of nature zeolite on wastewater, which contains low concentration of mercury, are not ideal. In order to improve its adsorption ability, this study proposed to use cationic surfactant CPB (cetyl pyridine bromide) to modify clinoptilolite. Static adsorption and desorption, dynamic adsorption and desorption, structure changes of natural and CPB modified zeolite, the adsorption mechanism were discussed in this paper. Therefore, it can provide part of basis for improving adsorption capacity of Hg (Ⅱ) on zeolite. The results were all as follows:(1) Structures and surface properties of natural zeolite were changed by CPB, and the adsorption capacity of Hg(Ⅱ) on zeolite were improved. LSD difference comparison found, compared to other modification methods in the test, the adsorption ability of CPB modified zeolite had significantly increased(p<0.01)). The best modification conditions were:activation temperature:550℃, activation time:2h, dose of CPB:80g/kg zeolite and contact time:48h. The organic carbon of natural zeolite increased from4.73g/kg to36.69g/kg, cation exchange capacity increased from19.12cmol (+)/kg to80.70/kg (+), and the surface Zeta potential changed from30.27mv to7.80mv after CPB modified. The surface Zeta potential of natural zeolite had basically no change after the adsorption of Hg(Ⅱ). Meanwhile, the surface Zeta potential of CPB modified zeolite changed from negative to positive after the adsorption of Hg(Ⅱ), electric potential changed obviously, which showed more Hg(Ⅱ) were combined to CPB modified zeolite.(2) Adsorption process of natural and CPB modified zeolite fit quite well with Langmuir equation. In the same condition, the adsorption of Hg(Ⅱ) on CPB zeolite was easier than that of natural zeolite, and the adsorption capacity of CPB zeolite was much bigger. According to the result of isothermal adsorption, the adsorption capacity of CPB zeolite for Hg(Ⅱ) increased from0.521mg/g to3.07mg/g, the adsorption rate increased from67.5%to98.9%. Kinetics fitting results showed that, the adsorption of Hg(Ⅱ) on both kinds of zeolite fit well with the second-order kinetics dynamic equation, and Elovich equation fitting reached significant level. Test results showed that, chemical adsorption was an important limiting step of the adsorption of Hg(Ⅱ) for both kinds of zeolite, and the ion exchange plays an important role in the adsorption. Dynamic boundary model fitting found that, liquid film diffusion was the main limiting step of the adsorption for both kinds of zeolite.(3) CPB modified zeolite was more stable to the change of pH and temperature. The adsorption rate of Hg(Ⅱ) on CPB zeolite remained above98%under different temperature and pH. Meanwhile, the adsorption rate of Hg(Ⅱ) on natural zeolite was more changeable with temperature and pH. Under the coexistence condition of heavy metal ions or anions with Hg(Ⅱ), the adsorption rate of Hg(Ⅱ) on CPB zeolite was stable, but the existence of Cl-had a strong inhibitory effect on the adsorption of natural zeolite. Specific adsorption ability of both kinds of zeolite was similar, but the electrostatic adsorption (ion exchange adsorption) capacity of CPB zeolite was much stronger than that of natural zeolite. Dynamics equation fitting, D-R fitting and thermodynamic study showed that, in the process of adsorption for Hg(Ⅱ) on two kinds of zeolite, there existed not only chemical adsorption, but also physical adsorption. Gibbs free energy change (ΔG0), enthalpy (ΔH0), and entropy change (ΔS0) of the adsorption of Hg(Ⅱ) on both kinds of zeolite were less than zero. Which indicated the adsorption was spontaneous exothermic reaction; low temperature was conducive to the adsorption.(4) The desorption order of natural zeolite by different desorption agen was:deionized water> NaNO3>HNO3>Na2S, the desorption order of CPB modified zeolite was:NaNO3>HNO3> deionized water>Na2S. CPB zeolite+Na2S treatment and natural zeolite+HNO3treatment were the best among eight kinds of desorption treatments in the regeneration experiments. There was no significant difference between CPB zeolite+Na2S treatment and natural zeolite+HNO3treatment, but both of them had an extremely significant difference (p<0.01) with other treatments in the test. Desorption quantity of Hg(Ⅱ) on zeolite increased with the increase of the adsorption quantity. Relationship between adsorbance and desorption quantity of Hg (Ⅱ) on CPB zeolite conformed well to power equation and polynomial equation, which was meaningful for the forecast of desorption quantity while the adsorption was known.(5) Result of dynamic adsorption test showed that the penetration time and saturate time of CPB zeolite were both longer than that of natural zeolite, dynamic adsorption ability of CPB modified zeolite were stronger than that of natural zeolite under the condition of different column height, different flow rate and different initial concentration. Thomas model and Yoon-Nelson model can describe the dynamics adsorption of Hg (Ⅱ) on natural zeolite better. Using Na2S as stripping liquid, natural zeolite can hardly be reused, while CPB zeolite can be used repeatedly. The adsorption ability of CPB zeolite remained relatively stable after being reused three times.(6) XRD, FT-IR and SEM analysis showed:the crystalline structure, molecular groups and surface structure of natural zeolite all had some changes after the modification of CPB. But after the adsorption of Hg (Ⅱ), the crystalline structure, molecular groups and surface structure all had little changes on both kinds of zeolite. |
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