The Effect Of Crystallinity Or Low-Molecular-Weight Organic Acids On The Environmental Application Of Nanosized Hydroxyapatite

Bulk apatite can be used to immobilize Pb, Cd and F in water and soil. Previous results indicate that lowering soil pH value or applying sufficient amount of bulk apatite could improve the efficiency and shorten the time of immobilization. But several disadvantages should be noticed when optimizing amendment quantity and pH so that adverse environmental effects are not a by-product, i.e., water entrophication and soils acidifying. Compared with the apatite of normal size, nanosized hydroxyapatite (nHAP) represents a promising application in a variety of areas, due to their higher surface area and reactivity. Therefore, the enhancement of Pb, Cd and F immobilization, worked by the addition of nHAP, could be realized in practice. However, there are some literatures studying the immobilization of Pb, Cd and F by bulk apatite in water, but litter is known about their immobilization by nHAP. In particular, there are no reports about the effect of nHAP crystallinity or low molecular weight organic acids (LMWOAs) on the immobilization of Pb, Cd and F by nHAP. In this study, the main objectives of this work are (1) to investigate whether the decrease of nHAP crystallinity can promote the immobilization of Pb and Cd by nHAP, (2) to evaluate whether the LMWOAs can promote defluoridation capacity of nHAP.Main results are as follows:(1) Sample preparation and their characterizationThe nHAPs were synthesized by the precipitation and sol-gel method. XRD, TEM, IR were used to characterize these nHAPs. Results showed that the samples were pure HAP and crystallinities of samples increased with annealing temperature. In addition, determination of dissolution rates for nHAP was conducted at the controlled pH condition. Results showed that the dissolution was inversely related to pH and the solubility of HAPs increased with decreasing HAP crystallinity. Within 2 h reaction time, the complete dissolution was obtained at pH<5 for the least crystallinity of HAP. However, there were not complete dissolution at pH<3 for the highest crystallinity of HAP.(2) The effect of nHAP crystallinity on the immobilization of anglesite and cerrusite in aqueous solutionIn this section, the effect of nHAP crystallinity on the immobilization of anglesite and cereusite was investigated at various conditions, such as pH, adsorbent dose, contact time. Results showed that the main mechanism involved HAP dissolution, followed by phosphate reacted with dissolved Pb and precipitation of chloropyromorphite. Under the same conditions (pH, contact time or sorbent dose), with the decrease of HAP crystallinity, the complete immobilization was achieved at higher pH, especially at high P/Pb ratio. As sorbent dose and contact time increased, the immobilization of Pb by nHAP was increased. For the reaction between anglesite and nHAP, the soluble Pb level was controlled by the solubility of chloropyromorphite during the entire reaction process. For the reaction between cerrusite and nHAP, the soluble Pb level was controlled by the solubility of cerrusite and chloropyromorphite during the entire reaction process. In the dynamic pH system, a complete transformation of Pb from anglesite or cerrusite to chloropyromorphite was achieved due to the complete dissolution of nHAP and anglesite/cerrusite at the initial low pH.(3) The effect of nHAP crystallinity on the immobilization of CdCO3 in aqueous solutionIn this section, the effect of nHAP crystallinity on the immobilization of Cd from CdCO3 was investigated at various conditions, such as pH, adsorbent dose, contact time. Results showed that the main mechanism involved adsorption. Under the same conditions (pH, contact time or sorbent dose), with the decrease of HAP crystallinity, the percentage of Cd removal from CdCO3 by nHAP was increased. As pH, sorbent dose and contact time increased, the percentage of Cd removal from CdCO3 by nHAP was also increased. The soluble Cd level was controlled by the solubility of CdCO3 and nHAP during the entire reaction process. In the dynamic pH system, a complete immobilization of Cd was achieved due to the complete dissolution of nHAP and CdCO3 at the initial low pH. The main mechanism involves ion-exchange between Cd2+ and Ca2+ in the dynamic pH system.(4) The effect of LMWOAs on the defluoridation capacity of nHAPIn this section, oxalic, citric and malic acids were selected as the representative of LMWOAs. Bath adsorption experiments were carried out to investigate the effects of contact time, initial fluoride concentration, solution pH, and LMWOAs on fluoride adsorption onto nHAP, and to explore the mechanisms of fluoride adsorption onto the nHAP. Results indicated the defluoridation capacity of nHAP was enhanced by the addition of LMWOAs. The nHAP adsorbed LMWOAs on its surface, and the LMWOAs on nHAP were considered to be the newly formed active sites for fluoride adsorption. In addition, the order of the ability of LMWOAs to enhance the defluoridation capacity of nHAP was oxalic acid> citric acid> malic acid, which was the same order as the amounts of LMWOAs adsorbed onto nHAP. In the presence/absence of LMWOAs, the defluoridation capacity increased with increasing the contact time, initial fluoride concentration and contact temperature, while decreased with the increase of the sorbent dose and solution pH. The adsorption isotherms in the presence/absence of LMWOAs were well fitted by the Freundlich model (R2>0.98), and the sorption kinetics were well described by the pseudo-second-order model. Moreover, thermodynamic parameters indicated that the adsorption was spontaneous and endothermic.