Modification of hydroxyapatite (HAP) with 1-hydroxyethane-1, 1-diphosphonic acid (HEDP) for the selective sorption of metal ions from water

The United States Center for Disease Control and Prevention (CDC) has linked several diseases to water pollution. Metal ions such as lead, cadmium and copper are among the biggest contributors to water pollution and can leach into water from corroded plumbing, battery and paint discharges, glass and electronic production wastes and natural deposits. Toxic amounts of these metals have been shown to cause kidney failure, liver damage, developmental delay and several forms of cancer. The effect of groundwater pollution on disease has created a dire need to safely remove toxic metal ions from groundwater systems. The application of organophosphate-modified hydroxyapatite (mHAP) for water remediation is presented in this study. Hydroxyapatite (HAP) was modified with complexant 1-hydroxyethane 1, 1-diphosphonic acid (HEDP) and its effect on metal ion affinity and selectivity was determined. HAP was synthesized from calcium hydroxide and phosphoric acid, calcined at 700°C and modified by a post-functionalization method with HEDP. The influence of time and temperature on the extent of modification was investigated. An increasing concentration of HEDP (0.01 to 0.50 M) resulted in more modification. Both untreated and HEDP-treated HAP were characterized using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), ICP-OES and specific surface area (SSA) measurements. Ca/P ratios, acid capacities and phosphorus elemental analyses provide information about the effect of modification on mHAP composition, surface characteristics and interactions. A high reaction temperature during modification produced new, distinct phosphate bands (993cm-1, 1082cm-1, 1144cm-1) indicating the presence of HEDP. This was independent of reaction time. HAP modified at both high temperature and long reaction time had the highest HEDP loading and revealed the sharpest XRD peaks. The emergence of new HAP-HEDP strands was observed in SEM images for treated samples while EDS and ICP showed high phosphorus contents in these strands. HAP modified at high temperature for the long reaction also had a high acid capacity and more organic phosphorus than the controls, resulting from the presence of additional P-OH groups in HEDP. Selectivity for Pb(II) (90%) and Cd(II) (80%) over Zn(II) (25%), Ni(II) (18%) and Cu(II) (0%) from 10-4 N nitrate solutions was high for this sample compared to the metal ion sorption of unmodified HAP and HAP modified for the short reaction time at low temperature. The newly synthesized mHAP was not only selective for Pb(II) and Cd(II) but had unique chemical properties that were different from HAP-HEDP salts prepared by alternative pre-functionalization methods and different from Ca-HEDP salts.