Preparation Of Hydroxyapatite/attapulgite Composite And Its Passivation Properties On Heavy Metal Contaminated Soil

Heavy metals pollution such as lead and cadmium in soil has become one of the main environmental problems in China.Lead and Cadmium in soil mainly come from industrial,mining,agricultural and other production activities.It enters the soil through different ways,It poses a serious threat to human health and the quality of the soil environment.Heavy metals in soil are characterized by high toxicity,difficulty in degradation,and easy migration.In-situ passivation has good technical advantages in controlling the bioloxicity,morphological transformation and migration of heavy metals in soils.It has been widely used in remediation of heavy metal contaminated soils.Hydroxyapatite is an environmentally friendly passivation material,which has good adsorption capacity for heavy metals and has a good application prospect in heavy metal soil remediation.Hydroxyapatite is easy to agglomerate,and proper addition of matrix can improve the agglomeration effect.The special properties of attapulgite are used as the matrix of synthetic hydroxyapatite to prepare hydroxyapatite/attapulgite by coprecipitation method,in order to alleviate the agglomeration and enhance the adsorption performance.The preparation conditions and physicochemical properties of HA/A composites were studied.The effects of HA/A composites on the adsorption of lead and cadmium were explained.The influence of HA/A composites on the adsorption of lead and cadmium was discussed.The passivation effect of cadmium contaminated soil.The main research results are as follows:?1?The preparation conditions and physicochemical properties of HA/A composites were studied.The effects of preparation temperature,attapulgite dosage,initial calcium concentration,initial phosphate concentration and calcination temperature on the adsorption properties were investigated by batch experiments.The HA/A was characterized by X-ray diffraction?XRD?,Fourier Transform Infrared?FTIR?,Scanning Electron Microscopy?SEM?and BET?Brunauer-Emmett-Teller?.The preparation results showed that the optimum suitable preparation were as follows:the preparation temperature is 30?,the dosage of attapulgite is 1.0 g,the initial concentration of calcium salt and phosphate is 8.23 mg/L and 11.62 mg/L,respectively.The material is not calcined.The characterization results showed that hydroxyapatite is successfully loaded on the surface of attapulgite,and the problem of easy agglomeration of hydroxyapatite is improved.The specific surface area of HA/A composite was much larger than that of hydroxyapatite and attapulgite.?2?The factors affecting the adsorption of lead and cadmium on HA/A composites were studied.The effects of dosage,reaction time,initial solution concentration,reaction temperature,solution pH and cation coexistence on adsorption performance were investigated by batch experiments.The results showed that the increase of HA/A dosage could increase the removal rate;the equilibrium time of adsorption of cadmium and lead by HA/A composites was 1 h and 2 h,respectively;The adsorption capacity of HA/A composites increased with the initial concentration of cadmium and lead.When the concentration reached a certain level,the adsorption amount was basically unchanged;Increase of the reaction temperature and pH can promote the adsorption of cadmium and lead by HA/A composites;The coexistence of four kinds of cations had an inhibitory effect on adsorption,and the effect size was Cu²⁺>Zn²⁺>Ca²⁺>Na⁺.?3?The adsorption mechanism of HA/A composites on cadmium and lead was explained by kinetic model,isothermal adsorption model,thermodynamic analysis and XPS,FTIR and XRD characterization before and after reaction.The results show that the adsorption process of cadmium and lead by HA/A composites accords with quasi-secondary kinetics,indicating that the adsorption process is mainly chemical adsorption;The adsorption isotherms of cadmium and lead in HA/A composites are more in line with the Langmuir model,indicating that the adsorption belongs to monolayer adsorption;The adsorption of cadmium and lead by HA/A composites is a spontaneous,endothermic process??H⁰>0,?S⁰>0?.The characterization results showed that the mechanism of adsorption of cadmium by HA/A composites was mainly ion exchange,complexation with-OH in the material and electrostatic adsorption.The mechanism of adsorption of lead by HA/A composites was mainly ion exchange,precipitation and surface adsorption.?4?The effect of HA/A composites on passivation of lead and cadmium contaminated soil was investigated.By changing the dosage of HA/A composites in the soil,the effects on soil pH,morphological transformation,and effective state were evaluated and their passivation effects on lead and cadmium were evaluated.After applying HA/A composites in soil,the pH of cadmium and lead contaminated soils increased obviously.The effective content of cadmium and lead in contaminated soil decreased with the increase of material application amount;the main conversion of acid-dissolved and reduced states of cadmium In the residual state,the acid-soluble state of lead is mainly converted into a reduced state and a residual state;the application of HA/A composite material can improve the passivation effect of cadmium and lead in heavy metal contaminated soil.