solubility of Cu minerals in soils
The solubilities of various Cu minerals in soils are given by reaction 1 through 15 of Table 14.1. and the solubilities of CU oxides. Hydroxides, carbonates, cupric ferrite, and soil-Cu are plotted in Fig 14.1 Reaction 7 of this table gives the solubility expression for soil Cu which is taken from the experimental measurements of Norvell and Lindsay. In there studies CuEDTA and CuDTPA were reacted with soils of different pH.The Chelated Cu remaining in solution after 30 days was measured and used to estimate an equilibrium constant for the reaction
soil-Cu + 2H+ .__-;~ Cuz* log K? = 2-8 (14.1)
which gives
log Cu” = 2.8 + 2pH (14.2)
Equation 14.2 is plotted as the soil-Cu line in Fig. 14.1 and is used as the reference solubility for Cuz* in soils. The activity of Cu” maintained by soil-Cu is about 10`3 that of Zn” in equilibrium with soil-Zn (Eq. 13.4).
All minerals included in Fig. 14.1 are more soluble than soil-Cu. The order of decreasing solubility is
CuCO3(c) > Cu3(OH)2(CO3)2(azurite) > Cu(OH)2(c)
> Cu2(OH)2CO3(malachite) > CuO(tenorite)
> CuFe2O4(cupric ferrite) > soil-Cu.
These minerals have a solubility range of 109. Increasing CO2 decreases the solubility of the carbonate minerals. The solubility of cupric ferrite is in fluenced by Fe and lies close to that of soil-Cu. It is possible that soil-Cu may indeed be cupric ferrite. The solubility relationships upon which the