Studies On The Equilibrium Adsorption Of Amphoteric-cationic Modified Bentonites To Cd²⁺, CrO₄^2- And Phenol

The organic-heavy metal complex contaminants from the sewage, waste water andpesticide in the process of industrial and agricultural production pouring into the soil/waterenvironment, has become the main form of environmental pollution. Currently, adopting theclay minerals modifed by different types and composition of the surfactants as adsorbentmaterial, is widely used in repairing water and soil environment, which has become one ofhot topics on the study of environmental field.Based on the high cation exchange capacity of the bentonite as substrate, aftermodifying bentonite with a mixture of amphoteric modifier, dodecyldimethylbetaine (BS-12),and cationic modifier, hexadecylrimethylammonium bromide(CTMAB), the structure ofamphoteric-cationic modified bentonites were characterized by the organic carbon contents,X-ray diffraction, FIRT and specific surface area analysis. The adsorption mechanisms ofphenol, Cd2+and CrO42-on amphoteric-cationic modified bentonites and its affecting factors,i.e. modified ratio, ionic strength, temperature and pH, were investigated, along with itsadsorption isotherms and thermodynamics were also discussed in order to achieve atheoretical basis for the research and application of the pollutants adsorption onamphoteric-cationic modified adsorbents. The main conclusions of this paper are as follows:1. Under the single BS-12modification treatments equivalent to50%、100%of thebentonite’s CEC (50BS、100BS), the results in organic carbon contents, layer spacing andspecific surface area of amphoteric-cationic modified bentonites demostrated that with anincrease of CTMAB modified ratio, the organic carbon contents of the amphoteric-cationicmodified bentonites gradually increased, while the specific surface area decreased. The layerspacing value d of BS+CT modified bentonites rised with an increase of CTMAB modifiedratio as less than150%CEC in the combination ratio of BS and CT, while the external layerspacing would appeared in the outer surface of BS+CT modified bentonites when comlexmodified ratio were higher than150%CEC, which manifested that the bentonite adsorptionmechanisms modified by two complex surfactants started to transform from the intercalationto external surface adsorption. The layer spacing value d and organic carbon contents ofbentonites modified with BS+CT complex surfactants were higher than the original bentoniteand the single BS modification bentonites. 2. The equilibrium sorption amount of phenol experiment showed that the complexmodification with the amphoteric surfactant (BS-12) and cationic surfactant (CTMAB) wasan effective way to enhance phenol adsorption capacity on the basis of single50、100BSmodified bentonites, and BS+CT modified bentonites to the maximum adsorption of singlephenol was31.03times that of the original soil, was the50BS9.47times, about100BS3.76times; The equilibrium adsorption amounts were releated to amphoteric-cationic surfactantsmodified ratio: with an increase of CT modified ratio, the phenol adsorption onamphoteric-cationic modified bentonites rised in the region of0~100%CEC, while decreasedslightly when CT modified ratio was higher than100%CEC. The coexistence of Cd2+coulddemote the adsorption of phenol on modified bentonites except100BS-12treatment; Thephenol adsorption decreased with an increase of temperature and pH, but increased with ionicstrength rise in low concentration range. The adsorption thermodynamic parameters of phenolon BS+CT amphoteric-cationic modified bentonites was a spontaneous physical adsorptionprocess with the characteristics of both enthalpy decrement and entropy increment. Henryequation is the best model to describe the phenol isotherms.3. The equilibrium adsorption amount of Cd2+on the amphoteric bentonites was muchhigher than the unmodified bentonite, and its Cd2+adsorption rised with an increase of BS-12modified ratio; The adsorption of Cd2+on amphoteric bentonites could be decreased byCTMAB complex modification, which showed the negative decrement with an increase ofCT modified ratio; Under the Cd2++phenol complex conditions, the coexistence of phenolwas unfavorable for the adsorption of Cd2+in low complex modified ratio, while better forthe Cd2+adsorption in higher modified ratio; The adsorption isotherms can be described byLangmuir equation; The change of temperature effect on the BS+CT amphoteric-cationicmodified bentonites was positive with an increase of temperature, and the equilibriumsorption amount increased with an increase of pH, but bad for the rise of ionic strength; Theadsorption thermodynamic parameters of Cd2+on BS+CT modified bentonites was aspontaneous physical adsorption controlled by the characteristics of entropy increment, andthe transformation of adsorption process from endothermic on CK, amphoteric bentonites toexothermic on the BS+CT modified bentonites was observed, along with the characteristicsof chaotic degree falling down.4. The equilibrium adsorption amount of CrO42-on the amphoteric bentonites increasedcompared with the unmodified bentonite, while its adsorption kept steady with an increase ofBS-12modified ratio; The adsorption of CrO42-on amphoteric bentonites could be increasedsignificantly by CTMAB complex modification, meanwhile, BS+CT modified bentonites tothe maximum adsorption of CrO42-was12.25times that of the original soil, was3.64times of the50BS, about100BS3.94times, and the CrO42-adsorption amount on BS+CT modifiedbentonites rised with an increase of CT modified ratio; The adsorption isotherms weresuitable to be described by Langmuir equation; The adsorption of CrO42-on the modifiedbentonite was not sensitive to temperature, and the CrO42-adsorption decreased with anincrease of pH, and unfavourable for the rise of ionic strength. The adsorptionthermodynamic parameters on BS+CT modified bentonites was a spontaneous physicaladsorption process, which was transformed from endothermic on CK, amphoteric bentonitesto exothermic on the BS+CT bentonites along with the characteristics of chaotic degree goingdown.