Modification Of Magnetic Mesoporous Silica Using Rice Husk And Study On Adsorption Effect Of Cadmium

The contamination of cadmium in the aquatic environment poses a serious threat to ecosystems and the health of the public.Cadmium pollution is particularly remarkable.There has been considerable interest in the development of adsorption materials which are renewable,inexpensive and have high adsorption rate.In this study,the magnetic mesoporous silica（MMS）was prepared by using Fe3O4 as the magnetic material and the rice husk ash as the silica source.The main emphasis of the research was to synthesize the adsorption material that could efficiently adsorb Cd²⁺.In order to analyze the adsorption mechanism of MMS and TMMS,the effects of the adsorption condition on MMS and TMMS,the adsorption kinetic models,isothermal adsorption models and the results of structural characterization were all studied.The desorption behavior of MMS and TMMS was studied to improve the reusability of the two materials.This research would be of great importance in efficient utilization of rice husk ash and removal of Cd²⁺ from wastewater.TMMS had better adsorption effect than MMS modified by amine.The results showed that the value of surface thiohydroxy concentration was 0.131 mmol/g,and TMMS had the best adsorption effect when the reaction condition was 80 oC,the vibrating time was 8 h and the amount of ammonia was 750 μL.The characterization of MMS and TMMS was researched by magnetic separation,nitrogen adsorption,XRD,FTIR and Zeta potential.The results of magnetic separation showed that the magnet could easily enrich the samples dispersing in distilled water.The results of nitrogen adsorption showed that the average pore sizes of MMS and TMMS were 4.58 nm and 4.86 nm,and the BET surface area was 581.77 m2/g and 367.67 m2/g,respectively.The BET surface area reduced because of the grafting of thiohydroxy.The results of XRD showed that MMS and TMMS had uniform pores orderly arranged with hexagonal,Fe3O4 was standard spinel structure.The results of FTIR displayed the characteristic peaks of SiO2,Fe3O4 and-CH2-CH2-CH2-SH,indicating the success of the grafting of thiohydroxy.The results of Zeta potential showed that the isoelectric points of MMS and TMMS were 3.4 and 3.2,respectively.It was found that the adsorption of Cd²⁺ on MMS could reach adsorption equilibrium within 5 hours at 30 oC when pH value was 5.At this time,the adsorption capacity was 2.71 mg/g and cadmium removal rate was 79.46%.It was found that pseudo-second-order kinetic model and intraparticle diffusion model could better describe the dynamic process of adsorption.The equilibrium adsorption capacity was 2.81 mg/g.The isotherm of MMS had good correlation with Langmuir isotherm model and Freundlich isotherm model,which suggested that both chemical and physical forces existed in the adsorption process.The saturated adsorption capacity was 17.86 mg/g.It was found that the adsorption of Cd²⁺ on TMMS could reach adsorption equilibrium within 4 hours at 30 oC when pH value was 5.At this time,the adsorption capacity was 2.88 mg/g and cadmium removal rate was 95.12%.The pseudo-2nd-order kinetic model could better describe the dynamic process of adsorption,which indicated that chemisorption might dominate the adsorption process.The equilibrium adsorption capacity was 2.91 mg/g.The adsorption rate was bigger than that of MMS,indicating that the adsorption speed of TMMS was faster.The isotherm of TMMS had good correlation with Langmuir isotherm model rather than Freundlich isotherm model,which suggested that the adsorption was monolayer adsorption.The saturated adsorption capacity was 33.33 mg/g,which was twice as high as that of MMS.By studying the desorption behavior of MMS and TMMS,we could draw the conclusion that the process of MMS could reach desorption equilibrium within 4 hours in hydrochloric acid.On this condition,desorption rate of MMS was 96.58%.Comparing with the first adsorption,the fifth adsorption capacity was only 29.18%.The process of TMMS could reach desorption equilibrium within 5 hours in hydrochloric acid of the same concentration.At this time,the desorption rate of TMMS was 96.10%.Comparing with the first adsorption,the fifth adsorption capacity was more than 70%.The pseudo-2nd-order kinetic model could better describe the dynamic process of desorption of MMS and TMMS,which indicated that chemisorption might dominate the desorption process.The desorption rate constant k2 of TMMS was smaller than that of MMS,indicating that the desorption speed of MMS was faster.It was probable because the binding force between TMMS and Cd²⁺ was stronger than the binding force between MMS and Cd²⁺.Maybe it was because adsorption condition might have great impacts on the nature and the structure of MMS.On the optimum adsorption conditions,the removal rate of Cd²⁺ in wastewater(the concentration of Cd²⁺ was 1.36 mg/L)which had been used to soak the cadmium rice was 78.80% by using TMMS as the adsorbent.