Iron Humate as a Low-Cost Sorbent for Metal Ions

Abstract

Iron humate produced as a waste by-product during an industrial manufacture of humic substances from low-rank brown coals was tested as a sorbent for the removal of metal cations – Cd (II), Cu (II), Co (II), Ni (II), Zn (II), Tl (I), Eu (III), Cr (III) - as well as hexavalent chromium from waters. The Langmuir-type isotherms were used to describe the metal sorption; the respective equations may be derived and interpreted on the basis of a concept of surface–complexation reactions. Parameters of the sorption isotherms were estimated from experimental dependencies measured in a batch arrangement. The sorption capacities ranged from 0.024 mmol g−1 for Co (II) to 0.324 mmol g−1 for Tl (I). The metal uptake was affected by the presence of complexing agents (EDTA, salicylate, citrate), but this effect depends strongly on pH and the kind of complexing agent. The sorption of Cu (II) ions was suppressed in the presence of EDTA in an almost whole examined pH range (ca. 1 – 6.5) and in the presence of citrate at higher pH values (above pH 4). In the presence of salicylate, on the other hand, a slight sorption enhancement was observed. The metal sorption was suppressed also in the presence of anionic surfactant (sodium dodecylsulfate). Both in the presence as well as in the absence of complexing agents, the metal sorption showed a strong dependence on pH – the sorption was reduced considerably at low pH values below ca. 2. A typical working range for iron humate as a sorbent is ca. pH 3 – 5, where it exhibits a high buffering capacity, a sufficient stability and metal-binding capability.