MECHANISM OF ENHANCED HUMIC ACID REMOVAL FROM AQUEOUS SOLUTION USING POORLY CRYSTALLINE HYDROXYAPATITE NANOPARTICLES

Poorly crystalline hydroxyapatite (HAP) nanoparticles synthesized by an improved neutralization method were used as novel adsorbents for removing humic acid (HA) from aqueous solution. Surface functionality, crystallinity, and morphology of the synthetic adsorbents were studied by FT-IR spectroscopy, powder X-ray diffraction and transmission electron microscopy. Characterization results showed that phase-pure, poorly crystalline and well dispersed HAP nanoparticles with average diameter of 30 nm were successfully obtained. Adsorption behaviors of HAP for HA were investigated by batch experiments and adsorption kinetic tests. HA adsorption process on the adsorbent obeyed pseudo-second-order kinetics and the adsorption rates decreased with increasing initial HA concentration. HA adsorption amount on the adsorbent decreased with increasing solution pH and the presence of alkali-earth metal ions resulted in the enhanced HA adsorption. HA adsorption on poorly crystalline HAP could be well described by Sips model and the maximum adsorption amount of the adsorbent for HA at 298 K was 123.44 mg/g. The FT-IR results revealed that HA adsorption over the adsorbent could be attributed to the surface complexation between the oxygen atoms of functional groups of HA and calcium ions of HAP. HA loaded adsorbent could be regenerated easily by using neutral phosphate buffer. Findings of the present work highlight the potential for using poorly crystalline HAP nanoparticles as effective and recyclable adsorbents for HA removal from aqueous solution.