Adsorption Of Phenol And Cadmium Onto Dual-amphoteric Modified Bentonite

In this work,mono-amphiphilic modified bentonite?BS12Bt,BS18Bt?and dual-amphiphilic modified bentonite?BS12-BS18Bt?were prepared using dodecyl dimethyl betaine?BS-12?and octadecyl dimethyl betaine?BS-18?.The materials were characterized by infrared spectroscopy?FTIR?,X-ray diffraction?XRD?and specific surface analysis.Molecular simulations of the modification and adsorption processes were performed.The adsorption characteristics and influence factors of each adsorption material on phenol and Cd²⁺were investigated to reveal their adsorption mechanisms.The main findings are as follows:1. The TC values of BS12Bt and BS18Bt were significantly higher than that of Bt, and increased with the proportion of modifications.The carbon content of dual-amphoteric modified bentonite increased with BS-18 compounding proportion,but the increasing trend was slowing.The actual modification rates for the soil samples were smaller than the setting modification ratios,and the higher the modification ratio,the slower the increasing trend.The results of XRD and FTIR indicated that the loading of the surface modifiers in bentonite is successful,and the functional group absorption peaks of the surface modifier were observed in the FTIR profiles,which gradually moved towards lower frequencies with the increasing proportion of total modifications.The loading of BS-12 and BS-18 increased the layer spacing of the bentonite.The layer spacing sizes of BS18Bt were higher than BS12Bt with the same modifier ratios.The layer spacing of dual-amphoteric modified bentonite increased with the increasing BS-18proportion.Different spacing sizes lead to different arrangements of modifier molecules.Molecular simulations showed that the N and O elements of the modifier molecules were mainly distributed near the bentonite layer,and the electrostatic gravitational effect of the hydrophilic end of the modifier molecule is observed.The main mechanisms of modification of bentonite by BS-12 and BS-18 included electrostatic gravity between N⁺of the surface modifier molecules and the negatively charged bentonite layers and hydrophobic binding between the carbon chains of the modifier molecules.2. For phenol adsorption,BS12Bt and BS18Bt showed a significant increase in adsorption amount compared to Bt,and both the adsorption amount and the adsorption affinity increased with the increasing proportion of modification.The adsorption amount of BS18Bt is higher than BS12Bt with the same modification ratios.When the BS-12modification ratio was 25%CEC,the amount of phenol adsorbed by dual-amphoteric modified bentonite increased with the BS-18 modification ratio.When the BS-12modifications were 50%CEC and 100%CEC,the adsorption amount of phenol tended to increase and then decrease with the increase of the BS-18 compound modification.When the BS-12 modification was 150%CEC,the adsorption amount of phenol decreased with the increasing proportion of BS-18 modification.BS-18 had a more significant effect on phenol adsorption than BS-12.The best compounded modified bentonite was150BS12-25BS18Bt,with an adsorption amount of 279.11mmol·kg^-1in the 500mg·L^-1concentration range,a 13.54-fold improvement over Bt.Bt,50BS12Bt,50BS18Bt,50BS12-25BS18Bt,50BS12-50BS18Bt,50BS12-100BS18Bt,50BS12-150BS18Bt were selected for kinetic and isothermal adsorption models fitting and thermodynamic study.The kinetic results showed that the adsorption of phenol reached adsorption equilibrium within 20 min,and the quasi second order dynamic equation was able to describe the adsorption process better.The adsorption rate of mono-amphoteric modified bentonite was significantly higher than Bt,and the rate of50BS18Bt was higher than 50BS12Bt.The adsorption rates of dual-amphoteric modified bentonites decreased as the BS-18 compounding proportion increased.When the BS-18compounding ratio reached 100%CEC,the adsorption rates were lower than that of mono-amphoteric modified bentonite.Intra-particle diffusion was involved in the adsorption process.Langmuir,D-R,and Sips models had significant fitting effects.The order of adsorption amount obtained from the Sips model was 50BS12-100BS18B>50BS12-50BS18B>50BS12-150BS18Bt>50BS12-25BS18Bt>50BS18Bt>50BS12Bt>Bt.The phenol adsorption amount of 50BS12-100BS18Bt was 1810.64 mmol·kg^-1,which was 26.61,2.82,1.93 times higher than that of Bt,50BS12Bt and 50BS18Bt.The D-R model results indicated that the adsorption process was mainly physical adsorption.The adsorption amount of dual-amphiphilic bentonite for phenol decreased slightly with increasing temperature and p H values,and was slightly facilitated by increasing background ion concentration.Thermodynamic parameters indicated that phenol adsorption processed were spontaneous,exothermic,entropy-increasing,and molecular simulations results indicated that phenol was mainly adsorbed in the hydrophobic phase region formed by the aggregation of surface modifier carbon chains between bentonite layers.Phenol adsorption was determined by hydrophobic interactions and spatial dislocation.3. For Cd²⁺adsorption,the adsorption amount and adsorption strength of mono-amphiphilic modified bentonite with a modification ratio below 100%%CEC were improved compared to Bt.When the modification ratio reached 150%CEC,the adsorption amount was lower than that of Bt.When the BS-12 modification ratio was less than 50%CEC,the Cd²⁺adsorption amount of dual-amphiphilic bentonite increased and then decreased with the BS-18 modification ratio.When the BS-12 modification ratio was?100%CEC,the Cd²⁺adsorption amount of dual-amphiphilic bentonite decreased continuously with the increasing BS-18 compounded ratio.BS-18 had a greater effect on Cd²⁺adsorption than BS-12 at the 25%?50%?150%CEC modification ratios,but the effect of BS-12 was more pronounced at the 100%CEC modification ratio.50BS12-50BS18Bt was the best modification ratio for Cd²⁺adsorption with an adsorption amount of 199.95 mmol·kg^-1.Bt,50BS12Bt,50BS18Bt,50BS12-25BS18Bt,50BS12-50BS18Bt,50BS12-100BS18Bt,50BS12-150BS18Bt were selected for kinetic and isothermal adsorption model fitting and thermodynamic studies.The kinetic results showed that the Cd²⁺adsorption reaches adsorption equilibrium within 20 min.The adsorption process was well described by the quasi second order dynamic equation.The adsorption rates of 50BS12Bt and50BS18Bt were significantly higher than Bt.Adsorption rate of dual-amphiphilic bentonite increased and then decreased with increasing BS-18 compounding ratio.Intra-particle diffusion participated in the adsorption process.Langmuir,Freundlich,D-R and Sips models were suitable for the description of the Cd²⁺adsorption process,and the order of q_mvalues obtained by Sips model was 50BS12-50BS18Bt>50BS12-25BS18Bt>50BS18Bt>50BS12Bt>50BS12-100BS18Bt>Bt>50BS12-150BS18Bt.The maximum adsorption amount of 50BS12-50BS18 Bt to Cd²⁺reached 270.13 mmol·kg^-1.The D-R model results indicated that the Cd²⁺adsorption process was mainly physical adsorption.The values of 1/n less than 1 in the Freundlich model indicated that the Cd²⁺adsorption processed on the amphiphilic modified bentonite samples were readily occurring.The adsorption of Cd²⁺was facilitated by the elevated temperature and p H values,but was inhibited by the increasing background ions concentration.The Cd²⁺adsorption processes on amphoteric modified bentonite were spontaneous,heat-absorbing,entropy-increasing.Combined with the results of molecular simulations,Cd²⁺ions was mainly distributed in the region close to the bentonite layers,and the adsorption mechanisms included electrostatic gravitation between the negatively charged bentonite layer and Cd²⁺,and electrostatic gravitation between COO^-and Cd²⁺.The above studies showed that the reasonable modification ratios made the adsorption amount of phenol and Cd²⁺on the dual-amphiphilic modified bentonite significantly better than that of mono-amphiphilic modified bentonite,but high modification ratios were detrimental to the their adsorption.