Speciation of exchangeable and crystalline Fe oxide bound Zn, Cu, Fe and Mn ions from calcareous soils during sequential fractionation

Abstract

The use of 1 M neutral NH4OAc has been criticized for extraction of exchangeable ions in calcareous soils as it dissolves free CaCO3 from soils. As a result the metal ions (i) occluded on CaCO3 and (ii) by their additional exchange from solid phases owing to an increased concentration of free Ca2+ ions in solution, are released during extraction. In order to develop a suitable substitute of 1 M neutral NH4OAc, it was hypothesized that the extraction of soils with Pb(NO3)2 at a concentration <0.05 M for a finite time, prior to their extraction with frequently used 0.05 M Pb(NO3)2 (pH 6.0) for extraction of specifically adsorbed cations, should be able to extract the exchangeable metals from soils. Of the various concentrations (0.0025, 0.005, 0.01, 0.02 and 0.04 M) of Pb(NO3)2 and shaking intervals (15, 30, 60, 90 and 120 min) tested, the extraction of soils with 0.005 M Pb(NO3)2 (pH 6.8) for 15 min was adjudged to be satisfactory for estimation of exchangeable Zn, Cu, Fe and Mn. The efficacy in the extractability by 0.005 M Pb(NO3)2 (pH 6.8) was compared with that of 1M neutral NH4OAc. Compared to 0.005 M Pb(NO3)2 (pH 6.8), a significant increase in the extraction of exchangeable Zn (16.2%), Cu (70.9%), Fe (62.4%) and Mn (76.1%) with 1 M neutral NH4OAc was observed. However in subsequent steps, the extraction of Zn (47.5%), Cu (49.1%) and Mn (22.9%) associated with free CaCO3 of soils decreased significantly when exchangeable metals were extracted with 1 M neutral NH4OAc. Thus, a considerable part of these metals associated with free CaCO 3 in soils may be considered in exchangeable form estimated with 1 M neutral NH4OAc. These findings were further substantiated by the detection of a significantly higher mean concentration of 130 me HCO3- and 32.2 me Ca2 + + Mg2 + ions L- 1 in 1M neutral NH4OAc extracts compared to 1.66 me HCO3- and 16.2 me Ca2 + + Mg2 + ions L-1 in 0.005 M Pb(NO3)2 (pH 6.8) extracts. The use of 0.1 M ascorbic acid as a reducing agent was tested with 0.25M NH2OH.HCl + 0.25M HCl to extract Zn, Cu, Fe and Mn associated with crystalline Fe oxides.