Adsorption Characteristics Of Cd (Ⅱ), Cr (Ⅵ) And Phenol On Complex Modified Bentonites With BS-12and SDS

At present, oganic-heavy metal complex contamination has become a hot issue inenvironmental science field. It is important for environmental protection and human health tostudy for a method which can remove organic-heavy metal complex pollutantssimultaneously.Bentonite was modified with different proportions combining amphoteric modifierdodecyl dimethyl betaine (BS-12) and anionic modifier sodium dodecyl sulfonate (SDS)according to cation exchange capacity. The structure of amphoteric and amphoteric-anioniccomplex modified bentonites were characterized by X-ray diffraction (XRD), infraredspectroscopy analysis (FTIR), the organic carbon contents (TOC), and determination ofspecific surface area (BET), the adsorption regularity of Cd2+, Cr (VI) and phenol on modifiedbentonites and its affecting factors, i.e. modified ratio, temperature, pH and ionic strength,were studied, and the adsorption mechanisms were discussed also. This master’s thesis aimedto provide a theoretical basis for application of amphoteric-anionic complex modified organicclay in environmental pollution remediation.The results achieved in this paper are as follows:(1) The organic carbon contents of modified bentonites showed a positive correlationwith modified ratio, and the specific surface area of modified bentonites showed a negativecorrelation with modified ratio. The adsorption mechanism of amphoteric modifier ontobentonite was that the amphoteric modifier entered into the layers by means of intercalation inthe region of0～100%CEC ratio. Anionic modifier modified amphoteric-bentonite includedtwo adsorption mechanisms, which was through electrostatic interaction combination onamphoteric-bentonite in the region of0～100%CEC ratio, while instead by hydrophobicbond in higher than100%CEC when the adsorption point reached saturation.(2) The equilibrium adsorption amount of Cd2+on modified bentonite samples showed apositive correlation with modified ratio of amphoteric and anionic modifier. Either50%or100%modified ratio of BS-12, Cd2+adsorption decreased as follow: BS+150SDS＞BS+100SDS＞BS+50SDS>BS+25SDS＞BS＞CK, indicated that BS and SDS modificationcan significantly enhance the adsorption of Cd2+on bentonites. The adsorption isotherms can be described by Langmuir equation. The adsorption of Cd2+on modified bentonites dependedon ionic exchange adsorption and electrostatic interaction of surface negative charge.(3) The change of temperature effect from positive on CK and amphoteric-bentonites tonegative on BS+150SDS bentonites was observed with an increase of SDS modified ratio. pHhad little influence on Cd2+adsorption on bentonites. The adsorption of Cd2+on bentonitesdecreased with ionic strength rise, but the effect of ionic strength can be reduced with anincrease of BS and SDS modified ratio also. The coexistence of phenol was favorable for theadsorption of Cd2+on amphoteric-bentonites, while it was unfavorable for the adsorption ofCd2+on amphoteric-anionic-bentonites. The adsorption thermodynamic parametersdemonstrated that the adsorption of Cd2+on modified bentonites was spontaneously controlledby entropy increment. When the SDS modified ratio was lower than100%CEC, theadsorption of Cd2+on modified bentonites was a process with the characteristics of bothenthalpy increment and entropy increment, while the SDS modified ratio was equal to orhigher than100%CEC, instead by the process of enthalpy decrement.(4) Compared with the amphoteric-bentonites, the equilibrium adsorption amount ofphenol on amphoteric-anionic-bentonites decreased. Phenol adsorption isotherms can bedescribed by Henry equation. The adsorption ability of phenol decresed with an increase ofSDS modification. The adsorption of phenol on the modified bentonites depended mainly onpartition effect, and the equilibrium adsorption amount decreased with an increase oftemperature. The effect of pH on the adsorption of phenol was different in different ranges,when pH was less than9.95, the adsorption amount of phenol increased with an increase ofpH, while decreased when pH was higher than9.95. As ionic strength increased, theadsorption of phenol increased and then decreased, meanwhile the adsorption capacity ofphenol was highest when the concentration of KNO3was0.1mol/L. The coexistence of Cd2+was beneficial for the adsorption of phenol on100BS modified bentonite and BS+SDScomplex modified bentonites when SDS modified ratio was equal to or higher than100%CEC. The phenol adsorption thermodynamics demonstrated that the phenol adsorption onmodified bentonites was a physical adsorption, which was a spontaneous process with theexothermic character.(5) The adsorption isotherms of chromium can be described by Langmuir equation.Bentonite could adsorb CrO42-by specific adsorption via a covalent bond, but its negativecharge was unfavorable for the adsorption of CrO42-. The equilibrium adsorption amount ofCrO42-on amphoteric-bentonites increased compared with unmodified bentonite, while itsCrO42-adsorption decreased with an increase of BS-12modified ratio. The adsorption ofchromium on amphoteric bentonites can be increased significantly by SDS complex modification, and the adsorption amount of chromium increased with an increase of SDSmodified ratio.