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Adsorption of aqueous Pb(II), Cu(II), Zn(II) ions by amorphous tin(VI) hydrogen phosphate: an excellent inorganic adsorbent
Zhu, C.; Dong, X.; Chen, Z. & Naidu, R.
Abstract
Amorphous tin(VI) hydrogen phosphate
(ATHP) was synthesized using the liquid phase precipitation
method and served as an adsorbent to remove
Pb(II), Cu(II), and Zn(II) from aqueous solutions. The
ATHP was characterized by powder X-ray diffraction
(XRD), transmission electron microscopy (TEM), scanning
electron microscopy (SEM), X-ray photoelectron
spectroscopy (XPS), and nitrogen adsorption–desorption
techniques. Adsorption properties were evaluated as a
function of pH, reaction time, concentration of reactants,
and salinity. Their equilibrium adsorption data were
modeled using Freundlich, Langmuir, and Dubinin–Kaganer–
Radushkevich isotherms, respectively. The results
revealed that adsorption equilibrium reached within
180 min. ATHP indicated good adsorption even below
the pHZPC, and best adsorption at pH 5 for Pb(II) and
Cu(II) and at pH 5.5 for Zn(II) was observed. Equilibrium
data fitted better to the Langmuir model for Pb(II)
and Cu(II) and fitted better to the Freundlich model for
Zn(II). The saturated adsorption capacities deduced from
the Langmuir model were 2.425, 1.801, and 0.600 mmol/
g for Cu(II), Pb(II), and Zn(II), respectively, indicating
an adsorption affinity order of Cu > Pb > Zn. There is a
negative correlation between the concentration of NaCl
and adsorption capacity of ATHP, yet ATHP still
exhibited excellent adsorption having an adsorption
capacity of 19.35, 15.16, 6.425 mg/g when the concentration
of NaCl was 0.6 mol/L. The free energy (E) was
12.33, 10.70, and 14.74 kJ/mol for Pb(II), Cu(II), and
Zn(II), respectively. An adsorption mechanism based on
ion exchange between heavy metal ions and H+ in the
ATHP is proposed. Furthermore, the used ATHP was
regenerated by HCl solution and the adsorbent was used
repeatedly.
Keywords
Adsorption preference; Amorphous tin(VI) hydrogen phosphate; Potentiometric titration curve; Recyclability; Saline wastewater
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