Environmental Engineering Reference
In-Depth Information
9.6
Conclusions
Based on the experimental findings, the following major conclusions on the removal and
recovery of Cr(VI) by magnetic nanoparticles are made:
1)
The -Fe
2
O
3
nanoparticles were applied successfully for the selective removal
and recovery of Cr(VI), Cu(II) and Ni(II) from wastewater. Adsorption of these
heavy metals by -Fe
2
O
3
nanoparticles reached equilibrium within 10 minutes
and the removal efficiency was highly pH-dependent, which also governed the
selective adsorption of metals from solution. The optimal pH for selective
removal of Cr, Cu and Ni was found to be 2.5, 6.5 and 8.5, respectively.
Adsorption data for Cr, Cu and Ni were fitted well by the Langmuir isotherm.
3)
Adsorption mechanisms were studied using TEM, XRD and XPS. The uptakes of
Cr(VI) and Cu(II) were proposed to be due to electrostatic attraction and ion
exchange, while the adsorption of Ni(II) was seen predominately controlled by
electrostatic attraction. Moreover, only adsorption occurred for Cu and Ni at their
optimal pH without a surface precipitation reaction.
4)
Desorption studies showed that 0.01 M of NaOH solution was an effective eluent
for desorption of adsorbed-Cr with an efficiency of 92%; 0.05 M of HCl eluent
was effective for desorption of adsorbed-Cu/Ni with an efficiency of around 94%.
Regeneration and re-adsorption studies demonstrated that the -Fe
2
O
3
nanoparticles could be recovered efficiently for the re-adsorption of the metal
ions and metals could be highly concentrated and possibly considered for
recycling.
5)
Further research was carried out on the removal of Cr(VI) involving the effects
of particle size, pretreatment ways, temperature, shaking speed, and the
competition studies from common coexisting ions and ligands. For given values
of the initial concentration, pH and temperature, the removal of Cr(VI) decreased
with the increase of pH and temperature. The adsorption data for Cr(VI) onto -
Fe
2
O
3
nanoparticles at different pH or temperature were fitted to the Langmuir
isotherm. Competitive effects from coexisting cations (Cu
2+
, Ni
2+
, Na
+
, Ca
2+
and
Mg
2+
) can be ignored. The inner-sphere complex-forming PO
4
3-
, SO
4
2-
and F
-
competed strongly with Cr(VI) for sorption sites, whereas NO
3
-
, Cl
-
and CN
-
did
not compete effectively due to the weak adsorption via outer-sphere
complexation. CrO
4
2-
showed higher affinity with -Fe
2
O
3
nanoparticles
compared to SO
4
2-
in the binary system.
6)
To search for cheaper and more effective magnetic adsorbents for Cr removal,
and also to extend their applications from information storage or electronic
devices areas to wastewater treatment, MeFe
2
O
4
nanoparticles were successfully
produced by the co-precipitation method and considered for removal of Cr(VI).
The TEM studies confirmed that the MeFe
2
O
4
nanoparticles prepared have fairly
uniform structure and a mean particle size of around 20 nm. Adsorption of Cr(VI)
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