Results and discussion
3.1. Properties of prepared magnetic adsorbents
The scanning electron microscope (SEM)micrograph of MNHAP
is shown in Fig. 1. The result showed that the synthesized MNHAP
were spherical shape with the diameter of about 28nm and were
aggregated withmanynanoparticles, which resulted in a rough surface
and porous structure. Fig. 2 shows the EDAX spectra of MNHAP
adsorbents before and after loaded with Cd2+ and Zn2+ respectively.
The Ca/P molar ratio of the MNHAP adsorbents is 1.65, which is less
than the ideal stoichiometric apatite (1.67). Table 1 indicates the
presence of iron and oxygen in addition to major constituents – calcium
and phosphorous in sample of MNHAP adsorbent. Comparing
the spectra of the MNHAP loaded with Cd2+ and Zn2+ with that of
unloaded one, the cadmium peak and zinc peak could be observed.
It was suggested that heavy metals including Cd2+ and Zn2+ had
been adsorbed on the surface of MNHAP successfully. Moreover,
after loading with heavy metal, a distinct decrease of calcium peak
intensity could be found. This phenomenon might be derived from
the participation of certain degree of interchange in the Cd2+ and
Zn2+ adsorption. In addition, the diminution of the phosphate peak
could be observed after adsorption (shown in Fig. 2b and c). The reason
may be caused by the fact that the adsorption of heavy metal
such as cadmium or zinc on the surface of MNHAP adsorbent lead-
Fig. 1. SEM micrograph of the synthesized MNHAP adsorbents.
ing to the increase of total amount of MNHAP adsorbent and the
decrease of phosphate proportion.
The BET surface area of MNHAP adsorbent was 142.5m2 g−1,
which was higher than that of HAP previously reported
[18,23,31–33]. Fig. 3 shows XRD pattern of the prepared MNHAP
adsorbent. It was observed that the principal components of
MNHAP adsorbent included hydroxyapatite (HAP), magnetite
(Fe3O4) and maghemite (Fe2O3). Among them, the magnetite and
maghemite were magnetic.
The room-temperature magnetization curve of the MNHAP
(Fig. S1) showed that the saturation magnetization is 59.4emug−1
indicating a relatively strong magnetic response to amagnetic field.
Saturation magnetization, used to measure themaximummagnetic
strength, is a crucial parameter for successful magnetic separation.
Ma et al. [34] found that a saturation value of 16.3emug−1 was sufficient
for magnetic separation with a conventional magnet. Thus,
the saturation magnetization value of MNHAP was high enough for
magnetic separation. Fig. S2 shows that MNHAP adsorbent suspensions
in aqueous solution can be separated from the solutions by
an external magnetic field conveniently. Therefore, theMNHAPcan
be used as magnetic adsorbents for the removal of Cd2+ and Zn2+
from aqueous solution.