The Investigation Of Isothermal Adsorption And Mechanism Of Heavy Metals Adsorbed Onto Modified Lignites

As the processing of industrialization in our country,toxic metals pollution has been more and more serious,due to their long-term environmental persistency with a consequent accumulation in both aquatic and terrestrial ecosystems.These pollutants,when discharged into waterways without any type of treatment,may harm aquatic life and at the same time make these waters inappropriate for human intake.Heavy metals can accumulate in the human body through the role of the food chain,causing serious harm to the health of the human body.Lignite resources are widely distributed in China.As a kind of high quality humic acid resource,the content of humic acid in young lignite is more than 80%.Lignite has a large specific surface area,rich in humic acid,and contains carboxyl,quinone,carbonyl,methoxy and other active groups.It can be chelated or complexed with heavy metal ions,and it is a good adsorbent.However,the complexity of natural lignite and the single structure of voids make the adsorption capacity of natural lignite to heavy metals is limited,and the stability and selectivity of natural lignite are relatively poor.The high dosage of lignite in practical application makes a higher cost of the lignite.In order to solve these problems,the physicochemical method is usually used to modify the adsorbents.In this paper,raw lignite was purchased from Zhaotong,Yunnan,China.The surface of lignite was modified by different methods,and five different adsorbents were prepared.The physico-chemical structure of these adsorbents was characterized by SEM-EDS,TG-DTG/DSC,FTIR 13 C NMR and XPS.Adsorption kinetics,adsorption isotherms,the effects of initial Cd(II)and Pb(II)ions concentrations,p H and temperatures on the adsorption process were studied.The results of the study are as follows:1)The surface morphology and main element distribution of lignite were analyzed by scanning electron microscopy energy dispersive spectroscopy(SEM-EDS).The SEM images of the samples show that the surface structure of MT and MX is relatively loose and contains more pore structure,which is conducive to provide adsorption sites for heavy metal ions adsorption and increases the physical adsorption capacity of materials.The decomposition rule of materials is similar before and after adsorption of metal ions,indicating that adsorption does not change the carbon skeleton structure of lignite adsorbent.The results of 13 C NMR show that the proportion of carbonyl carbon in different lignite adsorbents is MX>MT>MHA>MQ>M,and the increase of carbonyl carbon is beneficial to the adsorption of metal cations.The results of FTIR and XPS analyses showed that the adsorption process of Cd(II)and Pb(?)ions is directly bonded to O atoms of the carboxyl and hydroxyl groups.2)The adsorption amount of Pb(?)ions increases with the increase of solution p H,the sorption process is p H-dependent.In competitive adsorption,the presence of Cd(II)can only slightly reduce the adsorption of Pb(II)onto lignite materials,while the presence of Pb(II)ions decreases Cd(II)sorption significantly,indicating that Pb(II)has much higher competitive power than Cd(II),and has higher adsorption priority.3)It is apparent that the initial adsorption rate of Cd(II)and Pb(II)ions adsorption are all really fast and then gradually become slower.The rapid initial adsorption rate could be related with the high concentration gradient between the adsorbent and adsorbate in the solution and abundance of vacant adsorption sites being available.With the adsorption process,the adsorption sites on the surface of adsorption materials gradually decrease,and ions begin to diffuse into the adsorbent,and the adsorption rate gradually decreases.With the increase of adsorption concentration,the adsorption amount of Pb(II)and Cd(II)ions increased by lignite adsorbents.This may be due to the higher mass transfer driving force under high concentration,which promotes the entry of metal ions into the adsorbent and increases the utilization of the adsorption site.The pseudo-second-order kinetic model could provide a better interpretation of the Cd(II)and Pb(II)ions adsorption behaviors on lignite materials.It is suggested that there are strong chemical forces caused by exchanging electrons at the solid/solution interface.The Weber-Morris intraparticle diffusion fitting curve of Cd(II)and Pb(II)ions adsorption by lignite materials missed the original point(i.e.,C?0),indicating that the adsorption process was also controlled by extra-particle diffusion(e.g.,surface adsorption and liquid film diffusion).4)The adsorption of Cd(II)and Pb(II)ions by lignite adsorbents showed that the adsorption capacity of MX and MHA for Cd(II)ions increased by 8.33% ~ 22.62% and 15.42% ~ 21.97% than M,and the adsorption capacity of MX for Pb(II)ions increased by 20.58% ~ 35.24% than that of M.The negative Gibbs free energy change(?G)and positive enthalpy change(?H)confirmed that the sorption process was spontaneous and endothermal.The positive entropy change(?S)reflects the increasing randomness at the solid-liquid interface due to the adsorption process.The maximum adsorption amount of Pb(II)on lignite adsorbents increase with temperature increase.The possible reason for this trend is that increasing temperature promotes the extension of organic molecules on the surface of adsorbent.In addition,the temperature increases reduce the interaction between adsorbents and solvent solid surfaces,thus exposing more adsorption sites and improving adsorption of adsorbents on the surface of adsorbents.