Application of magnetic iron oxides modified by phosphonic chelating agent for adsorption of heavy metals

Abstract

This work describes the synthesis of magnetic iron oxide (MIO) – magnetite (Fe3O4) and maghemite
(γ-Fe2O3) chemically modified with one of the most widespread phosphonic chelating agent –
nitrilotris(methylenephosphonic) acid (NTMP) and the sorption of heavy metal (HM) cations (Cu2+, Cd2+ ,
Pb2+ , Ni2+) from aqueous solutions by initial and modified sorbents. MIO were obtained according to
procedures described in the literature and modified by chemisorption of NTMP from the solution
(20 mmol/dm3, pH=5-6, 30 min). Initial and NTMP-modified MIO were characterized by BET method,
FTIR-spectroscopy, thermal analysis and acid-base titration. Sorption of HM cations was studied in the
presence of a supporting electrolyte (KNO3, I=0.1) in the pH range of 1-9 for the initial concentrations of HM
from 0.1 to 5 mmol/dm3
.
For non-electrostatic model of oxide surface stepwise dissociation constants of magnetite OHgroups
(pKa1=4.4; pKa2=10.9) and functional groups of magnetite- attached NTMP (pKa1=7.7; pKa2=10.0;
pKa3=10.7) were defined. For all sorbents with increasing pH sorption of HM cations is increased. Sorption
on unmodified MIO was described by equilibrium: ≡FeOH + M2+ ↔ ≡FeOM+ + H+ with the following
constants (lgKp) for magnetite/maghemite: 3.9 and -0.4 (Pb2+); -0.9/-0.1 (Cu2+); -1.1 (Ni2+); -2.0/-1.3 (Cd2+).
For NTMP-modified MIO there is a significant increase in the stability of the sorbed state, moreover, the
sorption of Cd2+ increases more than that of Cu2
+, and the influence of surface modification on HM sorption
is more pronounced for maghemite compared to magnetite. Possible structure for sorption complexes of
surface-bound phosphonic chelator and metal cation was suggested.The possibility of using the obtained
NTMP-modified MIO for efficient removal of HM from aqueous solutions in the presence of Ca2+/Mg2+ was
shown. Thus, the chemical modification of MIO surface by various phosphonic chelating agents, includingNTMP, is a promising method to obtain new chelating sorbents for removal (concentration, support) of
different metal ions