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

Chelating agents and organic acid ligands readily solubilize trace and heav y 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) modifie d with six simple inorganic and organic acid ligands with a conventional ch elating agent (DTPA) and bioassay. The mixed cation-anion exchange resins w ere saturated with Na-salts of chloride (R-CHL), bicarbonate (R-BIC), fluor ide (R-FLU), acetate (R-ACT), citrate (R-CIT) and tartrate (R-TAR). Solubil ization and adsorption of Fe and Cu from the soils by resin-ligand systems decreased in the order R-CIT greater than or equal to R-FLU greater than or equal to R-TAR = R-ACT > R-CHL = R-BIC reflecting differential stability o f Fe- and Cu-organic/inorganic complexes. The R-CHL solubilized and adsorbe d 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 s imilar 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 bo th 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 fe asible for characterizing plant available Fe, Mn, Cu and Zn in soils. (C) 1 999 Elsevier Science B.V. All rights reserved.