Preparation Of Sulfhydryl Attapulgite And Its Adsorption Application To Cadmium And Lead

In recent decades,with the rapid development of human society,environmental pollution is becoming increasingly serious.As the ultimate receptor for environmental pollutants,soil is polluted by heavy metals such as cadmium and lead.According to National Soil Pollution Survey bulletin in 2014,the total over standard rate of heavy metals in soils is 16.1%,in which over standard rate of cadmium is the first with7.0%,and the over standard rate of lead is 1.5%.Therefore,it is urgent to develop remediation materials for heavy metal contaminated soil.Attapulgite?ATP?is a natural clay mineral which has good adsorption properties for heavy metals,that is low-cost,abundant in reserves,and environmental friendliness.However,the natural ATP often contains impurities,the adsorption capacity to heavy metals is not high,and the adsorption stability and selectivity are poor.So it needs to be modified to meet the application needs.In this paper,3-mercaptopropyltrimethoxysilane and polyethylene glycol 20000were selected as modifiers to prepare sulfhydryl attapulgite?ATP-SH?.Then,the preparation process of ATP-SH,the adsorption process of ATP-SH for cadmium and lead and the adsorption mechanism of ATP-SH for cadmium and lead were studied.Finally,ATP-SH was applied to the remediation of cadmium contaminated soil.The research results provide effective passivation materials for the heavy metals contaminated soil,and offer theoretical and technical supportion for the application of passivation materials in the field.The main research works and results are as follows:1.ATP-SH was prepared by solution method,and its structure was characterized.Based on the adsorption rate of Cd on ATP-SH,the best preparation process of ATP-SH is as follows:the amount of 3-mercaptopropyltrimethoxysilane is 5%of the weight of ATP,the surfactant is polyethylene glycol 20000,and the amount of surfactant is 0.1%?2%of the weight of ATP,the ethanol concentration is 25%of the modifical solution,the solid-liquid ratio?g/m L?of the modifical solution is 1:50,its p H is 8.0,then,the modifical solution is stirred at 25°C for 7 h.Structural characterization of ATP and ATP-SH were researched by FT-IR,XRD,TG,etc.The results show that the layered crystal structure and crystallinity of ATP-SH are basically unchanged.3-mercaptopropyltrimethoxysilane is successfully grafted onto ATP-SH.The thermal stability of ATP-SH is enhanced,the surface structure,specific surface area and pore size distribution have some changes.2.The adsorption process and stability of ATP and ATP-SH to Cd and Pb were studied.The effects of p H,ionic strength,solid-liquid ratio,time and temperature on the adsorption were investigated.For ATP and ATP-SH adsorbing Cd and Pb,When the p H of adsorbed Cd and Pb are 6.5 and 6.0,respectively,the adsorption medium is0.1 mol/L KNO₃,the solid-liquid ratio is 1:100,the time is 120 min,and the temperature is 25°C.The saturated adsorption capacities of ATP and ATP-SH for Cd and Pb are 17.21,22.71mg/g and 41.57,65.57 mg/g,respectively.Compared with ATP,the saturated adsorption capacities of ATP-SH for Cd and Pb are increased by 31.96%and 57.74%,respectively.The adsorption stability mainly investigated the effects of acid rain p H,ionic strength,temperature and time.The results show that the adsorption of Cd and Pb on ATP-SH is very stable,and it is difficult to desorb under natural conditions.3.The adsorption and desorption mechanism of Cd and Pb on ATP-SH were discussed by experiments of adsorption kinetics,adsorption thermodynamics and desorption kinetics.The kinetic behavior of the adsorption of Cd and Pb on ATP-SH is most consistent with Lagergren second-order equation,and the correlation coefficients are all above 0.99,indicating that the adsorption of Cd and Pb on ATP-SH is dominated by chemical adsorption.The thermodynamic behavior of the adsorption of Cd and Pb on ATP-SH is in accordance with Langmuir equation,D-R equation and tempkin equation.It is shown that the adsorption process of Cd and Pb on ATP-SH is a single molecular layer adsorption,the surface of the adsorbent is inhomogenous,and its surface coverage increases with the increase of adsorption heat.The calculated results of thermodynamic parameters??H>0,?G<0,?S>0?show that the adsorption process of Cd and Pb on ATP-SH is endothermic and spontaneous,increasing temperature is favorable for adsorption.The adsorption forces of ATP-SH to Cd and Pb include electrostatic attraction,ion exchange,surface and interlayer hydroxyl complexation,and sulfhydryl complexation.When the concentration of Cd and Pb are low,the adsorption processes are mainly based on physical adsorption such as electrostatic interaction and ion exchange,which is mainly reacted on the surface of ATP-SH.With the increase of Cd and Pb concentration,the surface adsorption sites are completely occupied,Cd and Pb diffuse to the layer of ATP-SH,and further the ion exchange and chemical actions such as hydroxyl and sulfhydryl complexation take place.Simulated acid rain is mainly desorption of Cd and Pb which are adsorbed by physical action,but most of the Cd and Pb adsorbed by chemical action are very stable and difficult to be desorbed.However,the desorption process of simulated acid rain to Pb is relatively complex,and the proportion of Pb adsorbed by material through physical action is large,so the desorption rate of simulated acid rain to Pb is larger.4.We collected cadmium contaminated soil in Danling County,Sichuan Province,and added cadmium solution to prepare different series of cadmium contaminated soils.Then,we added different amounts of ATP-SH as passivator to carry out rape experiment.Through the determination results of cadmium content in different parts of rape and available cadmium content in soil,we evaluated the remediation effect of ATP-SH on cadmium contaminated soil.The results show that after adding ATP-SH,the content of Cd in different parts of rape and the content of available Cd in soil decreased,the more the amount add,the greater the reduction is.Compared with blank,the maximum reduction rate of available cadmium is 72.29%,and the maximum reduction rate of cadmium in rape root,stem,limb and rapeseed are75.54%,88.45%,95.89%and 95.81%respectively.The correlation analysis shows that there is a significant negative correlation between the amount of ATP-SH and the content of available cadmium in soils,the content of cadmium in all parts of rape.ATP-SH can immobilize a large amount of cadmium through electrostatic attraction,ion exchange,and hydroxyl and sulfhydryl complexation,reduce the activity of cadmium in soils,and effectively prevent the migration of cadmium from soil to plants.