SELECTIVE SEQUENTIAL DISSOLUTION TECHNIQUES FOR TRACE-METALS IN ARID-ZONE SOILS - THE CARBONATE DISSOLUTION STEP

Dissolution capacity and kinetics of carbonates by sodium acetate (NaO Ac)-acetic acid (HOAc) at various pHs were studied. A comparative stud y of the selectivity, specificity, and effectivity of NaOAc-HOAc solut ion on carbonate bound fraction during the sequential selective dissol ution procedure was conducted by comparing the dissolution of major an d trace elements from arid zone soils by this buffer solution at vario us pHs. The effect of the pH of NaOAc-HOAc solution on the following f ractions in the sequential selective dissolution procedure was also st udied. NaOAc-HOAc solution at pH 5.5 at a soil to solution ratio of 1: 25, can dissolve all the carbonate from calcareous soils with .10-20% of carbonate; at pH 5.0 it can dissolve all the carbonate in soils wit h about 30-50% calcium carbonate (CaCO3). A second era-action with fre sh buffer solution at pH 5.0 is required for soils with more than 50% of carbonate. Six hours of extraction time is generally sufficient for complete carbonate dissolution. For most of agricultural soils in ari d and semi-arid zones, the attack of the buffer solution at pH 5.0 on other solid-phases seems to be limited. But the buffer solution at pH 5.5 would be better for some forest soils with low carbonate content a nd high organic matter content. The part of carbonate fraction not be dissolved in this step is released in the following steps: easily redu cible oxides fraction (ERO), organic matter fraction (OM), and reducib le oxides fraction (RO), leading to gross misinterpretation of the ele mental partitioning in arid zone soils.