| Reaction rates govern element behavior in the environment, yet little attention has been given to kinetic processes in whole soil samples. Lack of progress is largely due to the slow development of sensitive, interference-free methods. Thus, Cd reactivity and pool dynamics was investigated in long-term (59 wk) incubation studies using a stable isotope (111Cd) tracer - sequential extraction procedure and four diverse soils. Short-term (14 d) studies were also conducted. Tracer-spiked samples were periodically extracted into five fractions and the 111Cd:110Cd isotope ratio (IR) was measured by inductively coupled plasma mass spectrometry. The extraction scheme was: 0.1M Sr(NO3)2 (x2, F1); 1M NaOAc (x1, F2); 5% NaOCl (x3, F3); 0.4M oxalate + 0.1M ascorbate (x3, F4); and 3 HNO3:1 HCl (x2, F5). Repeat treatments at specified steps improved Cd extraction efficiency. On average, 11, 32, 40, 8 and 6% of the Cd was found in the five respective fractions. Good reagent selectivity with minimal Cd redistribution was achieved. Interferences (high total dissolved solids, F1 and F2; high Zr, F4 and F5) compromised IR measurement accuracy and precision, and were eliminated using Chelex-100, ion-exchange chromatography. Tracer distribution among the fractions was used to estimate half-lives and rate coefficients, k. Results showed that no soil fraction had reached the new equilibrium IR by wk 59 and that the plateau commonly ascribed to equilibrium in short-term studies instead represented sluggish isotopic exchange (IE). Inert pools reduced the amount of native Cd available for IE thus IRs converged on "apparent" rather than true isotopic equilibrium values. Across all samples, IE was rapid (k > 102 wk-1) for F2. Different IE rates appeared to indicate different forms of Cd (organic vs. inorganic) in F3. Samples contaminated by sewage-sludge, smelter emissions, or derived from Cd-rich shale showed rapid IE for F3 (k ∼ 101 wk-1 ) and slow IE for F4 and F5 (k < 10-2 wk-1). Conversely for the mine-spoil soil, IE was slow for F3 (k ∼ 10-4 wk -1) and fast for F4 and F5 (k ∼ 10 0 wk-1). Overall, Cd reactivity was: sorbed/carbonate >> organic matter > secondary-oxides ∼ aluminosilicates >> inorganic sulfides. |
