Studies on trace and major elements association in soils using continuous-flow leaching in rotating coiled columns

The potential of continuous-flow leaching in rotating coiled columns (RCC) for studies on elements association and binding in soils has been evaluated. Soddy–podzolic, anthropogenic transformed urbo soils and floodplain soils were under study. A particulate solid sample (about 0.5 g) was retained in a PTFE rotating column as the stationary phase whereas different aqueous eluents were continuously pumped through. The eluents were applied in correspondence with a selective extraction procedure (SEP) for trace metals addressing exchangeable, specifically sorbed, bound to Mn oxides, bound to organic complexes, bound to amorphous Fe and Al oxides fractions extractable by Ca(NO3)2, CH3COOH, NH2OH·HCl, K4P2O7, NH4-oxalate buffer (Tammʹs reagent), respectively. Elements were determined in the effluent off-line and on-line by inductively coupled plasma atomic emission spectrometry (ICP–AES). Apart from the assessment of risk to biota, the position and the shape of peaks for exchangeable metals may help to evaluate whether the soil under study is subjected to an active source of metal contamination. Interrelation of elution profiles for acid soluble (specifically sorbed) and pyrophosphate extractable (bound to organic complexes) metals as a function of organic carbon content allowed us to study the competitive binding of metals (Cu, Pb, Zn, Ni) with organic matter and mineral structures of soil. It has been found that the sharper the peak of acid soluble metal, the lower the content of this metal subsequently recovered in pyrophosphate extractable fraction, and vice versa. It has been also shown that the dissolution of amorphous Al and Fe oxides under the action of Tammʹs reagent can be time-resolved. Cu is eluted like Al whereas Zn, Pb, and Ni are released like Fe.