Synthesis and characterization of a new class of stabilized apatite nanoparticles and applying the particles to in situ Pb immobilization in a fire-range soil

Abstract

Phosphate compounds and the related materials are effective agents for in situ immobilization of heavy metals in contaminated soils. Problems associated with using these phosphate materials include difficulties in delivering the solid phosphate minerals to the deep contaminated zones or risks of eutrophication with applying soluble phosphates. Therefore, a new class of apatite nanoparticles was synthesized using carboxymethyl cellulose as a stabilizer in order to increase the dispersion rate of phosphate in soils but without introducing significant amount of soluble phosphate into the environment. The product was confirmed by XRD as chlorapatite (Ca5(PO4)3Cl) with poor crystallinity. TEM and SEM revealed that the particles were spherical or irregular in shape with sizes spanning from a few nm to around 200nm. FTIR spectra suggested that Ca(II) cations formed outer-sphere bonds with carboxyl and hydroxyl groups in cellulose molecules, thus inhibiting further agglomeration of the particles. Dry combustion data supported a formula of [C6H7O2(OH)2OCH2COOCa5(PO4)3Cl]n for the nano-apatite composite.Laboratory tests showed that the nanoparticles could effectively decrease the TCLP-leachable Pb fraction in a Pb-contaminated soil from 66% to 10% after one-month amendment with a ratio of 2mL solution to 1g soil and the resultant Pb content in the TCLP solution was reduced to 12 from 94mgL−1. When the amendment ratio was increased by 5 times, the leachable Pb was further reduced to 3.8mgL−1 with only about 3% of the soil Pb leachable. The soil sample, containing an average of 2.7×103mg Pbkg−1, was taken from a shooting-range in Southern USA.