Chapter 5 - Effects of organic ligands on the adsorption of trace elements onto metal oxides and organo-mineral complexes

Abstract

Biomolecules released by plants and microorganisms have an important effect on the mobility of trace elements in soil environments. It has been found that in the presence of low-molecular-mass organic ligands (e.g., oxalate (OX) and citrate), the adsorption of heavy metals on short-range-ordered oxides increased with an increase in the initial organic ligand/Me molar ratio up to 10, whereas on well-crystallized Al- or Fe-oxides, the adsorption initially increased up to 2.5 (goethite) or 5 (bayerite) and then decreased. The TL value, the nature of the organic ligands and heavy metals, and the surface properties of the sorbents are critical in determining whether metal adsorption is enhanced or inhibited. Organic ligands inhibit the adsorption of trace elements in anionic form (arsenate (As), arsenite, and selenite). The adsorption of heavy metals and metalloids on variable-charge minerals is influenced not only by the presence, nature, and concentration of organic ligands but also by the sequence of addition of trace elements on the sorbents. Trace elements in cationic and anionic form show a different adsorption capacity onto metal oxides, organic matter, and organomineral complexes. Heavy metals also compete for sorption sites on these soil components. However, it was found that Cu inhibits the sorption of Zn more on ferrihydrite than on an organomineral complex or a humic acid-like material.