| Summary: | In this study, two different methods were used to synthesis hydroxyapatite (HA,
Ca10(PO4)6(OH)2). Fibrous HA was synthesised by hydrothermal reaction and HA powders
were obtained from high temperature solid state reaction. Kinetics and thermodynamics of
lead sorption from aqueous lead nitrate solutions with initial lead concentration of 2590 ppm
by fibrous HA and HA powders were investigated for different durations, between 1-336
hours at various temperatures of 23°C, 50°C and 80°C.
It was deduced by Rietveld analysis that the sorption capacities of both HA materials
increased with the increase in temperature through the growing proportions of
hydroxypyromorphite (HP), Pb10(PO4)6(OH)2. Fibrous HA displayed better performance in
better lead uptake capabilities due to their higher solubility, lower crystallinity and oriented
crystals as compared to HA powders. The maximum sorption capacity of fibrous HA for Pb2+
ions at 80°C for 240h was 2.42 mmol/g relative to 0.67 mmol/g for the HA powders at the
same temperature, with 2.50 mmol/g being the maximum possible amount of lead sorbed by
HA.
Fresh HA materials and samples previously exposed to lead were also analyzed via scanning
electron microscopy (SEM) and the detection of HP reinforced the hypothesis that its
formation is the end of a kinetic process in which the HP crystals are continuously dissolved
and recrystallised in order to form more stable structures with higher lead content.
The first-order kinetic model was used to describe the kinetic data, and the data constants
were evaluated. The activation energy was also calculated from fitting the Arrhenius equation
with the graph of ln K against temperature inverse. Fibrous HA has activation energy of
54.92kg/mol.
Hence, fibrous HA can be considered as a potential adsorbent for the remediation of lead ions
in contaminated industrial wastewaters.
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