Application And Research Of Nano-Hydroxyapatite In Heavy Metal Polluted Soil Remediation

With the increasing of heavy metal contamination of soil,the pollution of heavy metal in the soil has become a hot spot in current research.Soil pollution have a high cumulative and irreversible, so it is difficult to carry out a thorough restoration only to rely on methods to cut off the pollution sources.At present,there are some contaminated soil treatment methods,but in the view of development and the demands,we must develop some more effective treatment technology to improve it.As we know,nano-materials has high surface area,high reactivity and strong adsorption characteristics,so it have been used for water pollution treatment,and the world take more and more attention on this technology.However,the application in the soil environmental governance,this technology have both challenging and opportunities for development.Calcium hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂(HAP),is the main inorganic component of hard tissues of vertebrates such as bones and teeth,which plays an important role in the purification of wastewaters and the remediation of heavy metal contaminated soils.The possible reaction mechanisms for the immobilization of heavy metals on HAP include:ion exchange processes;surface complexation;precipitation as new metal phosphates.There are some literatures studying the immobilization of heavy metals by HAP in water,but little is known about their adsorption and desorption on soil amended-with nano-HAP.Firstky the basic properties of nano-HAP which was composed by myself were characterized. Secondly the adsorption and desorption of Cu(â…¡),Zn(â…¡),Fb(â…¡) and Cd(â…¡) on soils amended with nano-Hap were studied by batch experiments.We choiced two kinds different soil-a Red soil(Udic Ferrosols) and a Wushan soil(Anthrosol).Then we started the culture experiment after nano-HAP and were employed into contaminate soil.We concentrated on studying the correlativity between the nano-HAP amounts,the cultivated time and effective contents of heavy metal.At the same time,we compared the different effection of nano-HAP and average AP to heavy metal ion.Basing the culture experinment's result,we choice rygrass as testing plant and studied how nano-HAP and AP effected on rygrass's growth.Because Low-molecular-weight organic acids are natural products of root exudates,microbial secretions,and plant and animal residue decomposition in soils.Little molecule organic acid strongly affected adsorption of heavy metal on HAP.The main objective of our study was to investigate factors influencing Cu²⁺ immobilization by low-crystalline HAP powder such as: contact time,aqueous Cu²⁺ concentration,solution pH,presence of some little molecule organic acid, as well as to determine the main sorption mechanisms.The results are as follows:(1) the synthesized nano-HAP was rod-shaped and its mean sizes were 20 nm in width and 120 nm in length,the Ca/P molar ratio was 1.65.BET specific surface area is 49.7m²g^-1.As XRD and SEM analyse display,the nano-HAP is zeolite HAP.(2) Freundlich model describes the adsorption isotherms of heavy metals on the two soils amended with nano-HAP very well,and the adsorption of heavy metals on the RS was lower than that on the WS.The affinities of heavy metals on the two soils follow this order:Cu(â…¡)＞Pb(â…¡)＞Cd(â…¡)＞Zn(â…¡).The presence of nano-HAP decreased heavy metal desorption from the soils, suggesting that the nano-HAP could significantly reduce the mobilization of heavy metals and increase the geochemical stability of soil Cu(â…¡),Zn(â…¡),Pb(â…¡) and Cd(â…¡) in contaminated soils.(3) In cultivation experiment,the nano-HAP and AP significantly reduced the bioavailability of soil Cu and Zn,compared with the control.The more nano-HAP and AP added,the more the increasing equilibrium time,and the more the decreased bioavailability of soil Cu and Zn,since soil pH was significantly increased after the addition of nano-HAP and AP,and heavy metals could adsorb on nano-HAP and AP.But the treatment of nano-HAP was better than AP.The treatment of packet application was not better than mixed application and needed more time for equilibrium,but it could be fetched out from siol easily.(4)After nano-HAP and AP mixed applicated in soil,the rygrass's growth becomed better.The nano-HAP mixed application was better than the AP mixed application.As nano-HAP used, rygrass's ground part grew to be more exuberant and the root part be more developed.The using Quantity of nano-HAP was bigger,ygrass's biomass was bigger and the heavy metal concents of rygrass's ground part changed small.The nano-HAP mixed application can decreased exchangeable heavy metal concents and increased residual heavy metal concents.(5)The adsorption kinetics of Cu on HAP were best described by the pseudo-second-order model,the quantity adsorbed at equilibrium q_e experimental and calculated is 420.7and 412.4mmol kg^-1,respectively.The value of pseudo-second-order rate constant k₁ is 0.000897 kg mmol^-1 min^-1, while the value of initial sorption rate v₀ is 152.6 mmol kg^-1 min^-1.Adsorption isotherms of Cu on HAP fit Dubinin-Radushkevich model very well with high correlation coefficient(R²=0.97-0.99) The mean free energy E(kJ mol^-1) of Cu sorption on HAP were 7.76 and 8.41 KJ mol^-1 at pH 4.5 and 5.5.The presence of organic acid(malic acid,succinic acid,citric acid,acetic acid,oxalic acid, tartaric acid and EDTA) decreased adsorption quantity of Cu,which ascribe to the coordination complex between Cu and organic acid with lower sorption affinities than Cu on HAP.