Study On Adsorption Behavior And Soil Remediation For Heavy Metals By Modified Attapulgite Materials

The increasing pollution of Cd and pb in soil poses a direct threat to food security and human health.Therefore,the development of safe,efficient and economical remediation technology is very important.In recent years,clay minerals have received considerable attention because of their low cost and good effect on heavy metal treatment.Attapulgite(ATP)is a crystalline hydrated aluminum magnesium silicate mineral with fiber morphology that exists in nature.Because of its large surface area and a large of silanol groups on the surface,its use as an adsorbent to remove pollutants in water and soil is a current research hotspot.However,the adsorption capacity of natural ATP is very limited due to low purity,so it’s necessary to use modified methods to increase its adsorption capacity and selectivity for heavy metals.Incorporating inorganic substances into ATP is a method to improve the adsorption performance.Magnesium oxide(MgO)is a natural substance with less environmental impact and low solubility and has a good application prospect in pollution control and remediation of heavy metal contaminated soil.However,MgO particles are easy to agglomerate due to their size and high surface energy,which limits functional applications in practical applications.The method of modifying ATP with MgO can not only increase the stability of MgO,but also enhance the adsorption capacity of ATP.In this paper,A-ATP was obtained by acid treatment and activation,and then was calcined at high temperature in a muffle furnace to obtain high-temperature modified T-ATP or MgO-ATP by high-temperature hydrothermal method of reactor.A series of physical and chemical characterization,were used to analyze the effects of different modification methods on the physical and chemical properties and surface structure of ATP.Different batches of water adsorption experiments were carried out on Cd(Ⅱ)and Pb(Ⅱ)polluted wastewater to investigate their adsorption properties.Finally,the combination of X-ray diffraction(XRD)and X-ray photoelectron spectroscopy(XPS)was used to discuss the adsorption mechanism of the removal of Cd(Ⅱ)and Pb(Ⅱ)on MgO-ATP.Then the effect of modified ATP on the immobilization of heavy metal contaminated soil was explored.Modified ATPs were applied to simulated Cd and Pb contaminated soil and actual Cd and Pb contaminated farmland soil to explore the bioavailability to Cd and Pb,p H,CEC and organic matter content on the influence of modified ATP,and further studied the changes of soil N,P and K content and the form of Cd and Pb in soil.The performance of modified ATPs on heavy metal contaminated soil was evaluated.The mainly conclusions were as follows:(1)The characterization analysis confirmed that MgO was successfully loaded onto the surface of ATP.Despite high temperature and loading modification,but the internal structure,the crystal structure and characteristic groups of ATP were still maintained.(2)When the reaction temperature was 298 K,the adsorption of A-ATP,T-ATP and MgO-ATP on Cd(Ⅱ)and Pb(Ⅱ)was in accordance with the Langmuir model.When the reaction temperature was higher than 298 K,the adsorption of Cd(Ⅱ)and Pb(Ⅱ)on T-ATP and MgO-ATP was still in accordance with the Langmuir model.A little bit different,the adsorption of Cd(Ⅱ)on A-ATP was in accordance with the Freundlich model,but the adsorption of Pb(Ⅱ)on A-ATP agreed with the Langmuir model.Pseudo-second-order kinetic equations better described the adsorption of A-ATP,T-ATP and MgO-ATP to Cd(Ⅱ)and Pb(Ⅱ);In the adsorption thermodynamic experiment,the adsorption of A-ATP and T-ATP to Pb(Ⅱ)and the adsorption of A-ATP,T-ATP and MgO-ATP to Cd(Ⅱ)were all endothermic reactions,but the adsorption of Pb(Ⅱ)was an exothermic reaction;The coexisting ions inhibit each other’s adsorption.A-ATP,T-ATP and MgO-ATP all showed preferential adsorption for Pb(Ⅱ),and Pb(Ⅱ)inhibited Cd(Ⅱ),Cu(Ⅱ)and Ni(Ⅱ)adsorption;MgO-ATP adsorption mechanism for Cd(Ⅱ)and Pb(Ⅱ)involves chemical precipitation,complexation,ion exchange and physical adsorption.(3)A-ATP,T-ATP and MgO-ATP were applied to simulated Cd and Pb contaminated soil at 1%,3% and 5% dosage,respectively,and cultured for 56 days.The results showed that: The p H and CEC value of modified ATP on simulated soil increased with the increase of dosage and immobilization time.All three amendments had a certain passivating effect on the bioavailablility of Cd and Pb in the soil,and the passivation efficiency was higher with the increase of the dosage.The immobilization effect on soil was MgO-ATP> T-ATP> A-ATP.There were different Cd and Pb immobilization mechanisms among MgO-ATP,A-ATP and T-ATP.MgO-ATP could form a stable complex with Cd and Pb in the soil,and it was not easy to lose efficacy over time.The loaded MgO was the rate-limiting step for MgO-ATP to passivate heavy metals in the soil.The form analysis of Cd and Pb in soil showed that modified ATPs reduced the bioavailability risk of Cd and Pb.The effect of MgO-ATP was the most obvious.With the addition of 5% MgO-ATP,acid extractable Cd and Pb reduced by 18.89% and 5.80% respectively.(4)Adding 1% and 3% of A-ATP,T-ATP and MgO-ATP to actual Cd and Pb contaminated soil respectively,and cultivating for 28 d.3% A-ATP,T-ATP and MgO-ATP for Cd were 16.8%,24.4% and 29.5%,and for Pb were 27.7%,37.8% and 43.3%.Modified ATPs had good retention of organic matter,soil ammonium nitrogen,available potassium and available phosphorus nutrients.According to the immobilization effect,soil p H and soil nutrient analysis,it was recommended to select MgO-ATP as the farmland amendments.The widely-sourced natural clay material ATP would not cause secondary pollution,the loss of soil nutrients and the soil to be more alkaline,which could help restore heave metal polluted farmland soil.