This study was undertaken to investigate the long-term performance of hydroxyapatite (HAP) as reactive material for the removal of uranium in passive groundwater remediation systems. 237U used as a radioindicator enabled tracking the movement of the contamination front through a test column without taking samples or dismantling the apparatus. The stoichiometric ratio between uranium and HAP was found to be 1:(487±19). Uranium removal by HAP is of pseudo first-order kinetics and the rate constant was measured to be (1.1±0.1)×10-3 s-1. HAP can sorb more than 2900 mg/kg uranium. Possible reaction pathways of uranium and HAP are discussed. The data obtained enable the calculation of ideal lifetime for permeable reactive barriers (PRB) using HAP for uranium removal neglecting hydrological factors that may impair the function of PRBs.
The present work shows that U can be effectively removed from groundwater using permeable reactive barriers with hydroxyapatite (HAP) as reactive material. The main factor influencing the removal processes is the composition of the groundwater, namely the concentration of Ca and carbonate. Sorption of U onto the HAP surface seems to be the dominant removal process with the possibility of remobilisation. Newly formed U-phosphate minerals were detected by ESEM/EDX and XRD in samples with high U content indicating either a dissolution-precipitation mechanism or sorption onto the apatite surface followed by alteration of the structure. The formed U-phosphate minerals are stable under common groundwater conditions and can be remobilised only at high pH-values and high carbonate concentrations.
Permeable reactive barriers
(2005)