Studies On The Sorption Of Pb²⁺ And P-nitrophenol On Montmorillonite, Kaolinite And MgAl-layered Double Hydroxides

With the development of industry and agriculture,the emission of pollutants is increasing which causes serious harm to ecological safety and human health.Heavy metal pollution is one of the environmental problems to be urgently resolved.The remediation of heavy metal contaminated soil and the treatment of waste water have become hot topics of environmental science and engineering.The adsorption/desorption behavior of heavy metal at the solid liquid interface is the fundamental problem which determines the geochemistry process of heavy metal and environmental ecological effect.The scientific knowing of the basic phenomena is the basis of understanding the speciation distribution,transport and fate of heavy metal pollutants and techonolgy on remediation of contaminated soil and waste water treatment.A great deal of studies have been done on the adsorption of heavy metal at the solid liquid interface and an overall understanding to adsorption law has been reached, however,the adsorption mechanism is not very clear due to the complexity of adsorption systems.Moreover,the adsorbents studied are usually negative charged.Rare studies have been done about positive charged heavy metal adsorbents.We choose structurally negative charged montmorillonite,structurally no charged kaolinite(or small quantity of negative structural charge)and structurally positive charged layered double hydroxides as model adsorbents and study the adsorption behavior of Pb²⁺ on them.The influence of various factors were investigated to obtain the influence law of structural charge(or interface phase microenvironmental acidity induced by it)on heavy metal adsorption and deepen the cognition to the adsorption mechanism.Adsorption of p-nitrophenol and Pb²⁺on MgAl-CLDHs,adsorption of Pb²⁺on LDHs- EDTA were also discussed to supply information to the development of the high- efficiency sorbent.The contents include the followings:Section1.Study on adsorption behavior and adsorption mechanism of Pb²⁺on montmorillonite and kaoliniteSorption properties of Pb²⁺on montmorillonite and kaolinite were studied,the influence of temperature,solution pH,ion strength and sorbent content on the sorption of Pb²⁺were investigated,and the sorption mechanism was discussed in combination with the results of XPS,XRD and specific surface area(SSA)experiments.The sorption kinetics can be well described by the pseudo-second-order kinetics and the Elovich kinetic equation and the sorption isotherms can be described by Langmuir isotherm.The sorption amount in mg/g of Pb²⁺on montmorillonite is obviously higher than that on kaolinite,however that in mg/m² on montmorillonite is very close with that on kaolinite.Adsorption amount is mainly determined by the SSA while the adsorption ability is determined by structural negative charge density of clays.The sorption mechanism of Pb²⁺on the montmorillonite and kaolinite may be distinguished to the chemi-bonded adsorption(or inner-sphere surface complexation adsorption)and electrostatic-bond adsorption(or outer-sphere surface complexation adsorption).The electrostatic-bond adsorption is mainly negative structural charge sites adsorption.The chemi-bonded adsorption is irreversible,strong selective and the ionic strength has no effect on it.The electrostatic-bond adsorption is reversible,no selective and the ionic strength has a significant effect on it.The relative content of chemi-bonded adsorption and the electrostatic-bond adsorption is dependent of pH,and the relative content of the electrostatic-bond adsorption in the pH range of 4～8 is higher than in the pH ranges of lower than 4 and higher than 8.The increases of temperature,ion strength and sorbent content would induce the decrease of the adsorption amounts of Pb²⁺on montmorillonite and kaolinite while the sorption amount increases with increasing pH.Pb²⁺can be intercalated between the layers of the montmorillonite while it can not be intercalated between the layers of kaolinite.Portion of Pb²⁺can be fixed by enterring in the micropore of clay particles.EDTA can significantly decrease the adsorption of Pb²⁺on montmorillonite,hardly affects the adsorption of Pb²⁺on kaolinite.Section2.Adsorption of Pb²⁺on MgAl Layered double hydroxides and their calcined productsLayered double hydroxides with an Mg/Al molar ratio of 1:1(MgAl-LDH),2:1 (Mg₂Al-LDH)and EDTA-LDH nanohybrids were synthesized by using a co-precipitation method and calcined LDHs(CLDHs)were obtained by calcination LDHs at 500℃.The sorption removal of Pb²⁺by them was investigated.It was found that both Mg-Al-LDHs and Mg-Al-CLDHs show good sorption ability and they could be a new type of environmental sorbent for the removal of Pb²⁺.The sorption kinetics and the sorption isotherms of Pb²⁺on both the Mg-Al-LDHs and the Mg-Al-CLDHs can be described by the pseudo-second order kinetics and Freundlich isotherm,respectively.The sorption amounts of Pb²⁺are independent of the pH in the range of about 3～10.The presence of electrolyte inhibited the sorption of Pb²⁺on the Mg-Al-LDHs while not on the Mg-Al-CLDHs.The sorption mechanism of Pb²⁺may be contributed to the surface complexation.The adsorption of Pb²⁺on the Mg-Al-LDHs may be contributed to both the chemi-bonded adsorption(or inner-sphere surface complexation adsorption)and the electrostatic-bond adsorption(or outer-sphere surface complexation adsorption),while that on the Mg-Al-CLDHs mainly to the chemi-bonded adsorption.The adsorption ability of Pb²⁺on the Mg-Al-CLDHs is higher than that on the Mg-Al-LDHs.The adsorption ability of Pb²⁺on the EDTA-LDH nanohybrids is also higher than that on the Mg-Al-LDHs which indicates the intercalation of EDTA had synergy.Section3.Adsorption of p-nitrophenol on Mg₂Al-LDH and Mg₂Al-CLDHThe sorption removal of p-nitrophenol on Mg₂Al-LDH and Mg₂Al-CLDH were investigated.The influence of pH,temperature on the adsorption was investigated.The results showed that p-nitrophenol could be adsorbed on both Mg₂Al-LDH and Mg₂Al-CLDH.Adsorption isotherms could be fitted with the Freundlich equation,the sorption kinetics of p-nitrophenol on Mg₂Al-LDH could be well described by the two-constant equation while the sorption kinetics of p-nitrophenol on Mg₂Al-CLDH could be well described by pseudo-first-order kinetics and Elovich equation.The sorption rate and sorption amount of p-nitrophenol on the Mg₂Al-CLDH was higher than that on the Mg₂Al-LDH.The sorption amounts of p-nitrophenol on Mg₂Al-LDH and Mg₂Al-CLDH were independent of the initial pH in the range of about 3～10.The increases of temperature,the adsorption amounts decrease.CLDH was promising to be a new kind of high effective adsorbents for phenols.