where ZnE(ads) is the labile concentration of Zn in the soil solid phase
(quantity; mol kg−1) and (Zn2+) is the free ion activity in the solution
phase (intensity; mol L−1); k1 and k2 are the regression coefficients. For
Model II, it was assumed that the free ion activity of Zn2+ was controlled
by dissolution of Zn carbonate solid phases based on a normal solubility
product (Ks)with an activity of 1.0 assumed for the solid phase smithsonite
(Eqs. (3a) and (3b)) or hydrozincite (Eqs. (4a) and (4b)).