Adsorption Of Phenol And Cadmium Onto Amphotericmodified Magnetic Bentonites

A variety of contaminants including organic contaminants and heavy metals had been released into the environment,which makes severe environmental pollution.Their interactions posed a serious threat to the ecological environment and health of human.Therefore,the development of novel adsorbents with easy separation and effective removal of combined pollutants was urgently needed.Bentonite（Bt）is a low-cost and eco-friendly adsorbent for environmental applications.Because of its large specific surface area,high cation exchange capacity,as well as physical and chemical stability,bentonite had been applied in the remediation of polluted environment.In this dissertation,a series of organic-magnetic bentonites（OMBts）,i.e.,amphoteric modified MBt（BS-MBt）,amphoteric-cationic modified MBt（BS-CT-MBt）and amphoteric-anionic modified MBt（BS-SDS-MBt）were prepared by two-step adapting method that magnetic bentonite（MBt）was primarily synthesized,and followed by modifying with amphoteric surfactant（BS-12）,cationic surfactant（CTMAB）and anionic surfactant（SDS）.The surface characterization,structure,functional group and specific surface area of the materials were characterized by Scanning Electron Microscope（SEM）,X-ray diffraction（XRD）,Fourier Transform Infrared Spectroscopy（FT-IR）,Thermogravimetric Analysis（TGA）.The content of C,H,and N were also analyzed with elemental analysis.The modification mechanism of magnetic bentonite modified with different modifiers was investigated.The adsorption characteristics and affect factors of phenol and Cd²⁺on various adsorbents with single and combined pollution conditions were explored.The adsorptive performance and mechanism of the adsorbents were revealed.The magnetic separation,stability,regeneration performance of the adsorbents and the performance of application in adsorption-magnetic separation device were clarified.The main conclusions are as follows:1.The Fe₃O₄ prepared by co-precipitation method were successfully introduced on the interlayer and surface of magnetic uniformly,which did not destroy the primary structure of bentonite.With the increasing of the amounts of Fe₃O₄ added,Fe₃O₄ nanoparticles agglomerated gradually and blocked the interlayer pore structure of bentonite,and the layer spacing decreased first and then increases,and the specific surface area increased first and then decreased,and the CEC decreased gradually,while the saturation magnetization increases gradually.The magnetic bentonite with Fe₃O₄ content of 0.01 mol·g^-1 was selected as the adsorption matrix for subsequent experiments according the magnetic separation performance and structural characteristics.The selected magnetic bentonite was modified by BS-12 with different modification ratios.And the BS-12 loaded on the interlayer and surface of MBt did not change the crystal structure of MBt.The lamellar structure of bentonite in BS-MBt was loose and fragmented gradually,the content of C and N increased significantly,the surface area decreased,the hydrophobicity increased,and the thermal stability decreased slightly with the increase of modification ratio.The CEC and AEC of BS-MBt were higher than those of MBt,and they decreased first and then increased with the increase of modification ratio.The crystal structure of BS-CT-MBt and BS-SDS-MBt adsorbents modified by BS-12and CTMAB/SDS did not change significantly.Compared with BS-MBt,the content of C and N in BS-CT-MBt increased,the specific surface area decreased,hydrophobicity increased,the thermal stability of the composite modifier loaded on BS-CT-MBt surface decreased,while that of the modifier loaded on the interlayer increased.With the content of modifier increase,the organic carbon chain arrangement on BS-CT-MBt became relatively orderly.With the increase of the modification ratio,the total modifier content of BS-CT-MBt increased first and then remained in certain content.Compared with BS-MBt,the content of C and N of BS-SDS-MBt decreased,the specific surface area increased,the modifier became disordered and had low density,and the hydrophobicity decreased.With the increase of BS-12 modification ratio,the content of modifier on BS-SDS-MBt increased,and with the increase of SDS modification ratio,the content of modifier decreased.MBt was negatively charged in the whole pH range.The isoelectric points of 50BS-MBt,100BS-MBt,150BS-MBt,100BS-100CT-MBt and 100BS-100SDS-MBt were 3.4,3.7,3.7,8.8 and 3.7,respectively.The saturation magnetization of BS-MBt,BS-CT-MBt and BS-SDS-MBt all reached the criterion of separation by external magnetic field.2.The modification mechanism of BS-12 on MBt was as follows:BS-12 was mainly introduced on magnetic bentonite by ion exchange and electrostatic attraction when the modification ratio of BS-12 was less than or equal to 100%CEC;BS-12 was mainly introduced on magnetic bentonite by combining hydrophobic bond with modified BS-12 when the modification ratio exceeded 100%CEC.The modification mechanism of CTMAB on BS-MBt was as follows:the positive charge group of CTMAB could combine with the negative charge group of BS-12 and the negative charge point on the surface of magnetic bentonite which was not covered by BS-12 through ion exchange and electrostatic attraction;and the CTMAB could also combine with the organic surface of BS-12 modified Bt through hydrophobic interaction,and also it could replace the BS-12 modified on the surface of magnetic bentonite,furthermore,part of the released BS-12could be reloaded on the magnetic bentonite through hydrophobic bond.The modification mechanism of SDS on BS-MBt was that the negative charge of Fe₃O₄on SDS could combine with the positive charge groups of BS-12 through electrostatic attraction,and which could also combine with the organic group of BS-12 which was formed on the surface of magnetic bentonite by hydrophobic interaction,and SDS had a certain elution effect on the modified BS-12.3.The adsorption of phenol by Bt,MBt,BS-MBt,BS-CT-MBt and BS-SDS-MBt reached equilibrium in about 20 minutes.The adsorption capacity of MBt to phenol was lower than that of Bt.The amount of phenol adsorbed by BS-MBt was higher than that by MBt.With the increase of BS-12 modification ratio,the amount of phenol adsorbed by BS-MBt increased.The adsorption of phenol on BS-CT-MBt was higher than that on BS-MBt.When the BS-12modification ratio was 50%and 100%of CEC,the adsorption of phenol increased with the increase of BS-12 and CTMAB modification ratio.However,when the BS-12 modification ratio was 150%of CEC,the adsorption of phenol decreased with the increase of CTMAB modification ratio.The adsorption of phenol by BS-SDS-MBt was lower than that by BS-MBt.The adsorption capacity decreased with the increase of SDS modification ratio,but which increased first and then decreased with the increase of BS-12 modification ratio.Cd²⁺inhibited the adsorption of phenol on Bt,MBt and BS-MBt,and which inhibited the adsorption of phenol on BS-CT-MBt and BS-SDS-MBt when increase the proportion of modification BS-12.The pseudo-second-order kinetic model and Henry model could describe the adsorption characteristics of phenol on various adsorbents.The adsorption of phenol on all adsorbents showed a negative temperature-increasing effect,which showed the characteristics of physical adsorption.The increase of pH would not benefit the adsorption of phenol.The increase of ionic strength promoted the adsorption of phenol on Bt,MBt,BS-MBt and BS-CT-MBt,but which had little effect on the adsorption of phenol on BS-SDS-MBt.The hydrophobic partition due to the function of hydrophobic organic phases of surfactants was the key mechanism of phenol sorption on BS-MBt,BS-CT-MBt and BS-SDS-MBt.4.Bt,MBt,BS-MBt,BS-CT-MBt and BS-SDS-MBt could adsorb Cd²⁺rapidly and reach the adsorption equilibrium within 20 minutes.The amount of Cd²⁺adsorbed on MBt was less than that on Bt,furthermore,which on BS-MBt was more than that on MBt,and which had negatively correlation with BS-12 modification ratio.The adsorptive capacity of BS-CT-MBt and BS-SDS-MBt to Cd²⁺was lower than that of BS-MBt.The adsorptive capacity of BS-CT-MBt to Cd²⁺decreased with the increase of modification ratio modified by CTMAB and BS-12.The adsorptive capacity of BS-SDS-MBt to Cd²⁺increased with the increase of modification ratio modified by SDS and decreased with the increase of modification ratio modified by BS-12.The phenol had little effect on the adsorption of Cd²⁺on Bt,MBt,BS-MBt and BS-CT-MBt,but which had inhibitory effect on the adsorption of Cd²⁺on BS-SDS-MBt.The pseudo-second-order kinetic model and Langmuir isothermal adsorption model could describe the adsorption characteristics of phenol on adsorbents.The adsorption of Cd²⁺by adsorbents showed a positive effect of increasing temperature.The adsorptive capacity of adsorbents for Cd²⁺increased slightly with the increase of pH,and decreased with the increase of ionic strength.The adsorption mechanism of Cd²⁺on MBt was possibly attributed to the electrostatic attraction and ion exchange reaction between bentonite and Cd²⁺,and Cd²⁺might be adsorbed on the surface of Fe₃O₄ by electrostatic attraction.In addition,the electrostatic attraction and chelation of BS-12 molecule on Cd²⁺were the reasons for the increase of the adsorption of Cd²⁺on BS-MBt.Compared with BS-MBt,the chelation of Cd²⁺with BS-12 in BS-CT-MBt and BS-SDS-MBt was weakened.The positively charged CTMAB in BS-CT-MBt increased the exclusion of Cd²⁺on adsorbents,which was not conducive to the adsorption.The group of SO₃^-on SDS in BS-SDS-MBt could increase the electrostatic adsorption of adsorbents to Cd²⁺.5.Fe₃O₄ and modifier loaded on the adsorbents were stable in the experimental range.The magnetic separation rate of adsorbent material with magnetic condition was significantly higher than that of natural sedimentation.50BS-MBt,100BS-MBt,150BS-MBt,100BS-100CT-MBt and 100BS-100SDS-MBt could be separated quickly in 3 minutes.The adsorbents could be regenerated by desorbing Cd²⁺and phenol with 10%NaCl and heating treatment respectively.In a self-designed adsorption-magnetic separation device,a mixture solution(phenol 50mg·L^-1)and(Cd²⁺1000 mg·L^-1)was treated with the papered composites.After treatment,the residual concentration of phenol in the solution reached the emission standards by using the composites of 100BS-100CT-MBt,150BS-MBt,100BS-MBt and 50BS-MBt after 4,5,8 and9 cycles of treatments,respectively.Meanwhile,by using composites of 50BS-MBt,100BS-MBt,150BS-MBt,MBt and 100BS-100CT-MBt,the residual concentration of Cd²⁺in the solution reaches the emission standard after 4,4,4,5 and 6 cycles of treatments,respectively.150BS-MBt is suitable for simultaneous adsorption of phenol and Cd²⁺.From the above conclusions,BS-MBt,BS-CT-MBt and BS-SDS-MBt could be separated easily by an external magnet.BS-MBt revealed better adsorption ability for Cd²⁺and phenol than MBt.Compared with BS-MBt,the amounts of loaded modifiers on BS-CT-MBt increased,and the adsorption capacities of phenol increased,while the adsorption capacities of Cd²⁺decreased.However,the total surfactant content of BS-SDS-MBt was lower than that of BS-MBt,and BS-SDS-MBt showed lower adsorption capacity for Cd²⁺and phenol than BS-MBt,suggesting that combined BS-12 with SDS were not effective in MBt modification.