Evaluation of the effects of treatment factors on the properties of bio-apatite materials

Animal bones have been studied as a resource of low-crystalline apatite for immobilization of toxic heavy metals. Chemical and thermal treatments can be applied for selective removal of bone organic constituents. In this study, the impact of different treatment factors was determined by experimental design methodology, and comparison of the estimated effects was performed in order to optimize the treatment conditions. Fractional factorial design with five variables at two levels was applied for factors screening. The type of chemical reagent (H₂O₂ or NaOH), its concentration (0.1 or 2 mol/L), reaction temperature (20 or 60 °C), contact time (1 or 3 h) and sample annealing (without or at 400 °C) were considered as process variables. Structural properties of bone sorbents, loss of ignition, point of zero charge, surface functional groups, specific surface area, chemical composition, and Cd²⁺ sorption efficiency, were selected as system responses. Samples significantly differed in respect to the organic phase content and composition, thus mechanisms of Cd²⁺ sorption were complex. Statistical analysis has revealed that, between all studied factors, temperature treatment was the most important for sorbents increased specific surface area, porosity and cation sorption. Furthermore, NaOH was more effective than H₂O₂, while the effects produced by variation of other factors were less pronounced. The highest proportion of bio-apatite with desired low crystallinity and high specific surface area was achieved by synergetic effects of chemical treatment with NaOH and thermal treatment at 400 °C.