Lead and zinc removal from aqueous solutions by aminotriphosphonate-modified converted natural phosphates

Apatite particles prepared from natural phosphate rock and grafted with nitrilotris(methylene)triphosphonate (NTP) were evaluated for Pb²⁺ and Zn²⁺ sorption from aqueous solutions. Sorption capacities as high as 640 mg.g^-1 and 300 mg.g^-1 could be obtained for the highest organic content (10 wt%). Analysis of the sorption isotherms using Langmuir, Freundlich and Dubinin-Kaganer-Radushkevich models revealed that Pb²⁺ ions have a larger affinity for apatite (sorption energy ≈ 8 kJ.mol^-1) than for NTP so that organo-modified surfaces led to a heterogenous adsorption process. In contrast, Zn²⁺ interacts weakly (sorption energy ≈ 1 kJ.mol^-1) and similarly with the mineral surface and the organic moieties following a homogenous sorption process. Such an association of organic metal ligands with reactive apatite surfaces within porous materials appears as a promising strategy to obtain efficient adsorbents at low cost and limited environmental impact.