Hydroxyapatite from waste materials for treatment of heavy metals in aqueous medium

Abstract

Hydroxyapatite (HAP) is an important biomaterial which could be a promising adsorbent for removal of heavy metals from aqueous medium besides its wide application in biomedical field. Flexible hexagonal structure of HAP allows incorporation of wide range of different ionic substitution which helps to make changes in HAP properties in a synergistic way for environmental remediation and biomedical applications. This thesis was focused on synthesis and characterization of pure and doped (Fe and Cu) HAP by wet chemical precipitation method using eggshell as calcium source which was followed by investigation on their applications in treating heavy metals from aqueous system. Doping of HAP was done with various Fe(III) and Cu(II) contents and the products were calcined at different temperatures (100°, 300° and 600°C). Physical and chemical characterization of synthesized HAPs have been carried out using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX), DLS particle size analysis and BET surface area analysis. FT-IR spectra and XRD patterns of synthesized HAPs including doped and nano HAP confirmed the formation of single HAP phase in the samples. XRD patterns showed decrease in crystalline size from 34 nm to 14-16nm for Fe-HAP and 15-12 nm for Cu-HAP was observed upon doping of HAP. BET surface area of Fe and Cu doped HAP calcined at 300°C increased from 66.80 m2g-1 for pure HAP to 122.29 and 153.39 m2g-1 respectively. Furthermore, pure and doped HAP calcined at 300°C showed highest surface area in comparison to HAPs both oven dried at 100°C and thermally treated at 600°C. For the first time nano HAP was synthesized from eggshell derived calcium precursor using microemulsion method. Phase purity of synthesized nano HAP was confirmed by FT-IR spectra and XRD patterns of the samples. BET surface area was measured 79.23 m2g-1 for nano HAP calcined at 600°C which was 22.40 m2g-1 for synthesized pure HAP calcined at same temperature. Analysis of particle size by DLS technique was found to be 12 nm for as prepared nano HAP. After successful synthesis of HAPs from eggshell, these were applied to remove As(V), Cr(VI) and Pb(II) from aqueous system. Removal of As(V) was given priority as arsenic contamination in Bangladesh is a major concern now. Adsorption equilibrium was established at pH 9, contact time 60 min, As(V) concentration 100 µgL-1 and experiment was done at room temperature (30C). Arsenic (V) removal efficiency was investigated in detail and it was observed that the efficiency of HAPincreased from 22% to 48% and 50% upon doping with Cu(II) and Fe(III) respectively keeping the equilibrium conditions same. The capacity enhanced to 59% and 74% while adsorbent dosage of copper and iron increased from 1 gL-1 to 10gL-1. Moreover the results show that the arsenate removal efficiency for Fe-HAP and Cu-HAP is twofold higher than that for pure HAP. Higher Cr(VI) removal efficiency of 6%Cu-HAP calcined at 300°C indicated the superiority of Copper doped HAP over pure HAP sample as a promising adsorbent. 2%Fe-HAPcalcined at 300°C showed highest Pb(II)removal efficiency which was 93.14%. Adsorption equilibrium was attained very fast at 20 and 30 mins in removing Cr(VI) and Pb(II) from aqueous system respectively.Arsenic removal efficiency of nano HAP was tested at pH 7 and 9, which shows a significant increase in arsenic removal efficiency in comparison to synthesized HAP micro particles. The equilibrium data fitted well with Langmuir model for 2 and 6% Fe doped HAP while 4%Fe-HAP followed Freundlich isotherm. However, negative values for theconstants of Langmuir, Freundlich and Temkin isotherms for As (V) adsorption for pure and copper doped HAP implied the inadequacy of the isotherm model to explain the adsorption process. First order and Pseudo second order kinetic models well describe the adsorption kinetics and probably both physical and chemical adsorptions were involved in adsorption of As (V) ion by pure and doped HAP.HAPs were further used as adsorbents in treating real waste waterandresults were found quite promising. The potential of the pure, copper and iron doped and also nano HAP synthesized from eggshells were explored to create a new dimension in waste management system. This work will be a significant step towards environmental remediation in two ways- first, utilization of the waste eggshells for synthesis of value added products HAP and secondly these products will be applied in the removal of heavy metals from aqueous medium.