Exploring Cd, Cu, Pb, and Zn dynamic speciation in mining and smelting-contaminated soils with stable isotopic exchange kinetics. (January 2016) - Ren, Zong-ling
Abstract: The exchange kinetics of Cd, Cu, Pb, and Zn in seven mining and smelting-contaminated soils and the other two anthropogenically contaminated soils was investigated by using multi-elementary stable isotopic exchange kinetic (SIEK) method, and the experimental results were successfully interpreted by modelling using a sum of pseudo first order kinetics equations. SIEK results show that in the studied soils the isotopic exchange of Cd is a relatively fast process, and the exchange almost reaches an apparent plateau after 3-d equilibration; whereas for Cu, Pb, and Zn, the exchange is more sluggish, suggesting that it is important to understand the time-dependent metal mobility for risk assessment and management of contaminated soils. In most of the soils, the total isotopically exchangeable pool is divided, for all the metals, into two distinct pools: a fast exchangeable pool ( E 1 ) with a kinetic rate constant k 1 having values around 1 min −1 and a much slower exchangeable pool ( E 2 ) with k 2 ranging from 0.0001 min −1 to 0.001 min −1 . The distribution of the two exchangeable pools varies significantly among metals. The amount of isotopically exchangeable Cd related to the fast pool is dominant, accounting for on average 60% of total isotopically exchangeable pool in the soils; whereas this pool is smaller for Cu, Zn, and Pb. The sequence of average k 1 values is Cd > Pb ≈ Zn > Cu, consistent with the reported sequence of stability constants of metal-humic substances (HS) complexes while the average k 2 values follow the order: Cd > Pb > Cu > Zn, probably controlled by the slow desorption of metal ions associated with soil organic matter (SOM) fraction. Our results imply that further study on the exchange kinetics of metals on each individual sorption surface in soils, especially SOM, is critical to help understanding the overall exchange kinetics of heavy metals in whole soils. Highlights: In contaminated soils, the isotopic exchange of Cd is faster than Cu, Pb, and Zn. A sum of pseudo first order kinetics equations can be used to interpret SIEK data. A fast exchangeable pool and a slow rate-limited exchangeable pool were found. The kinetic parameters for each pool were related to the metal bearing phases in soils.