Effect of EDTA on the Self‐Diffusion of Zinc in Aqueous Solution and in Soil

Abstract

Abstract Self‐diffusion coefficients for Zn and ZnEDTA (zinc ethylenediaminetetraacetic acid) were determined in aqueous solution and in soil. Those in solution were estimated from diffusion measurements in agar‐agar gel and were found to be 8.8 × 10 ‐6 cm 2 sec ‐1 for Zn and 6.8 × 10 ‐6 cm 2 sec ‐1 for ZnEDTA. The apparent diffusion coefficient of Zn in soil increased from 0.36 × 10 ‐9 cm 2 sec ‐1 to 3.0 × 10 ‐9 cm 2 sec ‐1 with addition of 1.53 × 10 ‐8 mole of EDTA/g soil and to 9.0 × 10 ‐9 cm 2 sec ‐1 with addition of 1.53 × 10 ‐8 mole of ZnEDTA/g soil. The true diffusion coefficient D p of Zn in soil was fairly constant at different levels of EDTA, averaging 1.81 × 10 ‐7 cm 2 sec ‐1 . Addition of EDTA to soils increased the diffusion of Zn; this increase was explained by the transformation of the solid phase soil Zn into soluble ZnEDTA complexes, thereby increasing the concentration gradient of total diffusible Zn. Chelating agents were shown to play an important role in overcoming the rate‐limiting steps of solution and diffusion which are largely responsible for the movement of micronutrient cations in soils.