Effect of competing ligands on the sorption/desorption of arsenite on/from Mg-Fe layered double hydroxides (Mg-Fe-LDH)

Layered double hydroxides (LDHs) show excellent capacity to sorb and exchange anions. Although arsenic (As) has a high affinity for sorbents containing Fe, only few studies have been carried out on the sorption of arsenite [As(III)] onto a Fe-based LDH. In this work we studied the sorption of As(III) onto a LDH containing Mg and Fe (Mg-Fe-LDH), as affected by: (i) pH; (ii) the presence of increasing concentrations of organic [citrate (CIT) and oxalate (OX)] and inorganic [phosphate (PO4), selenite (SeO3) and sulphate (SO4)] ligands, (iii) the effect of residence time on the desorption of As(III) by these ligands, and; (iv) the kinetics of desorption of As(III) by PO4.The As(III) sorption isotherms, carried out at different pHs, indicated a good affinity of the harmful oxyanion for the sorption sites of the Mg-Fe-LDH. This material was able to remove efficiently As(III) from contaminated solutions over a wide range of pH, but more in acidic than in alkaline systems. Competing ligands differently prevented the As(III) sorption on Mg-Fe-LDH, according to the following order: SO4<OX<<SeO3<CIT<PO4. The desorption of As(III) by these anions decreased with increasing As(III) residence time on LDH surfaces. A comparison between the sorption/desorption of As(III) and As(V) on/from Mg-Fe-LDH in the presence or absence of anions highlights that less As(III) than As(V) is sorbed, whereas more As(III) than As(V) is desorbed by all the selected organic and inorganic ligands, but PO4.