Chemical Speciation of Rare Metals in Three Natural Soils of China

Soil profile samples of isohumosol,primosol,and ferrosol were obtained in relatively clean areas of China.Concentrations and chemical speciation of rare metals [e.g.,Ag(Silver),Bi(Bismuth),In(Indium) and Sn(Tin)]were determined,in which sequential extraction was used to fraction metals.Vertical distributions of elemental enrichment ratios(ERs) were used to study the anthropogenic influence.The mobility of metals was characterized by the sum of exchangeable form and carbonate-bound form.Average concentrations of Ag,Bi,In and Sn in the A horizon were(0.14 ± 0.01),(0.49 ± 0.09),(0.050 ± 0.01),and(2.8 ± 0.9) mg.kg-1,respectively.The ERs were close to 1.Average distributions of metal speciation were: Ag,residual(65.8%) H2 O2-extractable organically bound(H2 O2-Org)(25.9%) Amorphous metal oxide-bound(am-MeOx)(4.2%) Metal-organic complex-bound(Me-Org) ≈ Crystalline Fe oxide-bound(cr-FeOx)(2.0%);Bi,residual(29.2%) H2 O2-Org(27.1%) cr-FeOx(22.7%) am-MeOx(18.8%) Me-Org(1.0%) carbonate-bound(0.9%) easily reducible metal oxide-bound(re-MeOx)(0.3%);In,residual(67.8%) cr-FeOx(17.0%) am-MeOx(6.8%) H2O2-Org(6.0%) carbonate-bound(1.4%) Me-Org(1.0%);Sn,residual(58.1%) am-MeOx(23.0%) cr-FeOx(15.6%) H2 O2-Org(1.8%) Me-Org(1.1%) exchangeable ≈carbonate-bound(0.2%).the results indicated that the exogenous elements enrichment was deficiency to minimal,and the concentrations of the rare metals were close to background values of soil in studied areas.Rare metals were stable in soils.The mobility of metals was in the order In Bi Sn Ag.