Extraction of phytoavailable trace metals from tropical soils by mixed ion exchange resin modified with inorganic and organic ligands

Chelating agents and organic acid ligands readily solubilize trace and heavy metals in soils. We compared Fe, Mn, Cu and Zn extracted from 16 soils by a mixed ion exchange resin (Amberlite IRA-120+ Amberlite IRA-400) modified with six simple inorganic and organic acid ligands with a conventional chelating agent (DTPA) and bioassay. The mixed cation–anion exchange resins were saturated with Na-salts of chloride (R–CHL), bicarbonate (R–BIC), fluoride (R–FLU), acetate (R–ACT), citrate (R–CIT) and tartrate (R–TAR). Solubilization and adsorption of Fe and Cu from the soils by resin–ligand systems decreased in the order R–CIT≥R–FLU≥R–TAR=R–ACT>R–CHL=R–BIC reflecting differential stability of Fe- and Cu–organic/inorganic complexes. The R–CHL solubilized and adsorbed more Mn and Zn than other resin–ligand systems because of the relatively low pH maintained by the R–CHL-soil suspension. Extraction with DTPA gave similar amounts of Fe and Mn as resin–ligand systems, but DTPA extracted two to three times more Cu and Zn than resin. Resin-extractable Fe, Mn and Cu correlated with the corresponding DTPA-extractable metal contents, while both resin- and DTPA-extractable Cu correlated with the bioassay results. The R–ACT and R–TAR extractable Zn correlated more strongly with the bioassay results than DTPA-extractable Zn. Mixed ion exchange resin is apparently feasible for characterizing plant available Fe, Mn, Cu and Zn in soils.