The
extremely high adsorption efficiency of malachite green (MG)
was examined through a series of batch experiments by using Fe
3+
-doped Mg/Al layered double hydroxides (LDHs). The incorporation
of iron into Mg/Al LDH with varying Al + Fe molar ratio of 4 + 1,
3 + 2, 2 + 3, and 1 + 4 increased the adsorption capacity with respect
to time. The spectral analysis and N
2
sorption studies
showed that there was retention of surface morphology in all of the
iron-modified LDH samples. The experimental evidences showed that
the adsorbent Mg/(Al + Fe) with a molar ratio of 10:2 + 3 had a significant
removal, i.e., 99.94% for MG with the initial concentration of 1000
mg L
–1
at pH ∼ 9 and at room temperature
in 5 min. With further increase in iron loading (at ratio 10:1 + 4),
there was a decrease in the removal of MG due to the agglomeration
of Fe
2
O
3
on the surface. The adsorption process
was best fitted to the Freundlich isotherm followed by the pseudo-second-order
model. The standard thermodynamic parameters (Δ
H
°, Δ
S
°, and Δ
G
°) were obtained over the temperature range of 20–50
°C. It was observed that the adsorption of MG onto Mg/(Al + Fe)
LDH was spontaneous, exothermic, and enthalpy driven in the physisorption
mode. A worthy desorption efficiency was achieved by using ethanol
and water, which was more than 90% in the three cycles. Maintaining
almost the same removal efficiency of MG even after three cycles indicated
Mg/(Al + Fe) LDH as a promising material for wastewater treatment.
This work was anticipated to open up new possibilities in dealing
with anionic dye pollutants.
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