Adsorption Mechanism Of Heavy Metals On Serpentine And Its Effects On Heavy Metals Formations In Contaminated Soil

With the rapid development of industry and agriculture,heavy metal pollution of farmland soil is becoming more and more serious.The content of heavy metals in soil exceeds a certain limit,which will lead to soil degradation,lower crop yields and lower quality of agricultural products,and then resulting in eventually poisoning the plants or entering human body through the food chain.Finally it would endanger the health of the human body.Therefore,it attracted extensive attention to carry out the remediation of soil contaminated by heavy metals.In this paper,the nature serpentine was used as tested materials,and its structure and composition were adjusted by thermal activation.The adsorption properties of nature and modified serpentine to heavy metals(Cd²⁺,Pb²⁺,Zn²⁺)and the influence mechanism of the serpentine on the form of heavy metals in soil were investigated.The work and main conclusions are summarized as follows:1.Thermal activation modification of serpentine and characterizationThe natural serpentine was activated at different temperatures to obtain a series of thermally activated serpentine.The natural and thermally activated serpentine were characterized by XRD,FT-IR,TG-DSC,low temperature nitrogen adsorption and SEM?EDS?.The results showed that dehydroxylation reaction of serpentine occured during the process of thermal activation.The structure,composition,and surface properties of the thermally activated serpentine were obviously changed compared to the natural serpentine.The serpentine remained still layered structure at lower activation temperatures,the layer structure began to collapse when the temperature reached 550 ^oC,and the lamellar structure was destroyed when the activation temperature was over 700 ^oC.Moreover,the surface area increased greatly.2.Adsorption properties of natural and thermally activated serpentine to heavy metal ionsThe adsorption of heavy metals(Cd²⁺,Pb²⁺and Zn²⁺)were investigated by isothermal adsorption experiments using natural and thermally activated serpentine as adsorbent.The results showed that the adsorption isotherm of Langmuir was more suitable to describe the adsorption process of Cd²⁺on natural and thermally activated serpentine than Freundlich isotherm,and the adsorption process of Pb²⁺and Zn²⁺on different serpentine was more suitable to describe by Freundlich isotherm adsorption model.It demonstrated that the adsorption of Cd²⁺on serpentine surface was similar to monolayer adsorption,while Pb²⁺and Zn²⁺were multilayer adsorption on serpentine surface.The adsorption process of Cd²⁺,Pb²⁺and Zn²⁺on different serpentine could meet the pseudo-second-order kinetic model.The adsorption capacity of thermally activated serpentine to heavy metals(Cd²⁺,Pb²⁺and Zn²⁺)would be increased,and the content of which depended on the thermal activation temperature.The serpentine activated at 700 ^oC exhibited maximum adsorption capacity to Cd²⁺,Pb²⁺and Zn²⁺,i.e.,15.21 mg·g^-1,67.19 mg·g^-11 and 101.49 mg·g^-1,respectively.The analysis of thermodynamic parameters showed that the adsorption processes of Cd²⁺,Pb²⁺and Zn²⁺on different serpentine were spontaneous endothermic processes.Based on the XRD,SEM and XPS characterization,it could be found that the morphology and structure of serpentine in the adsorption process did not change significantly.Cd²⁺was uniformly adsorbed on the serpentine surface in the form of precipitation of CdCO₃ and Cd?OH?₂,and the amount of CdCO₃ is greater than that of Cd?OH?₂.Pb²⁺was adsorbed on the serpentine surface by the precipitation of Pb?OH?Cl and Pb?OH?₂.And Zn²⁺mainly existed in the form of Zn₅?NO₃?₂?OH?₈·2H₂O and Zn?NO₃??OH?·H₂O in the surface of serpentine.3.Remediation of heavy metals in soil by natural and thermally activated serpentineSoil incubation experiments were conducted to investigate the effects of adding natural and thermally activated serpentine on soil pH,the content of TCLP-Cd,-Pb and-Zn,and the content of different forms and distributions of heavy metals?Cd,Pb and Zn?in soil.The results showed that adding different serpentine could significantly increase soil pH.The improvement of pH value depended on the thermal activation temperature of serpentine and the soil incubation time.The increase of soil pH value was the most obvious after adding 5%dose serpentine activated at 700 ^oC for incubating 15 days.After adding different serpentine,the contents of TCLP-Cd,-Pb and-Zn in soil decreased significantly,especially,adding serpentine activated at 700 ^oC.Through association analysis,it could be found that the soil pH was significantly negatively correlated with the content of TCLP-Cd,-Pb and-Zn,which showed that soil pH was an important factor affecting the content of soil TCLP-Cd,-Pb and-Zn.The different serpentine exhibited different passivation capabilities for Cd,Pb and Zn contaminated soils.After adding different serpentine,the formations of heavy metals in soil was changed.The exchangeable form of Cd content in Cd contaminated soil decreased,while the content of Fe-Mn oxide form of Cd and residual form of Cd increased.For Pb contaminated soil,the exchangeable form of Pb content and carbonate form of Pb content decreased,while the content of Fe-Mn oxide form of Pb and residual form of Pb increased.The exchangeable form of Zn content and carbonate form of Zn content in Zn contaminated soil decreased,while the content of residual form of Zn increased obviously.The soil pH increased after adding different serpentine,which was conducive to the heavy metal affiliated with OH^-and CO₃^2-in soil,and then form hydroxide precipitation and carbonate precipitation of heavy metals.Besides,natural and thermally activated serpentine promoted the transformation from exchangeable form of Cd,Pb,Zn in soil to low bioavailability of Fe-Mn oxides form and residual form by surface adsorption and surface precipitation,and then reduced the bioavailability of Cd,Pb,Zn in soil.