Evaluation of Influence of Nano-Hydroxyapatite Application on the Cadmium Immobilization in Contaminated Calcareous Soil

Cadmium (Cd) is one of the most common soil pollutants that can adversely affect all living organisms Therefore, proper remediation is necessary to reduce metal availability in soil. Owing to nano-material with higher reactivity and adsorption capacity than ordinary-sized materials, with the development of various nanotechnology applications in human life, the evaluation of the effectiveness of nanoparticles in remediation of polluted soils has been considered by researchers. However up to now it has been not reported information about heavy metal immobilization by nHAP in calcareous soils. This experiment was conducted to investigate efficiency of nHAP on stabilization of Cd in polluted soil. This experiment was conducted in a completely randomized design with factorial arrangement and three replications. First the soils were contaminated with Cd at three levels (0, 20 and 40 mg kg-1 using CdCl2) and then were incubated for one month in 70% field capacity. Then nHAP was applied to soils at three levels (0, 0.25 and 1%). After 30 days incubation, Cd was fractionated by sequential extraction and analyzed for DTPA extractable form. To quantify the effect of nHAP in binding intensity and mobility of loaded Cd, the reduced partition index (IR) and mobility factor were used. The results of sequential extraction showed that nHAP application significantly reduced concentration of Cd in the exchangeable and organic fraction and increased in the carbonate fraction. But there was no change for Cd in the residual fraction. The results of extraction with DTPA experiment indicated that at 40 mg kg-1 of Cd both of the levels of nHAP decreased DTPA-extractable Cd, but at 20 mg kg-1 of Cd, addition of nHAP at 0.25 % level did not significantly reduced concentration of DTPA extractable Cd. It was also found that efficiency of nHAP increased when Cd loading quantities to soils increased. However the role of nHAP in the reducing of bioavailability was not very large. The results also illustrated that the IR value increased when Cd and nHAP loading quantities to soils increased, demonstrating a decrease in the mobility of Cd in mobile fractions. Application of nHAP caused reduction of the mobility factor of Cd indicating decreasing availability and environmental risk of Cd. On the basis of results obtained in this study, it can be stated that although the nHAP addition has somewhat stabilized and reduced the mobility of Cd in the soil, it seems that the effect of nHAP on decreasing of Cd was not considerable. Therefore, further studies should be carried out more on the feasibility of application of nHAP at the widespread level and the economic status of its application.