Why can acidification of solution lead to selective precipitation of ions such as Fe2+ and Zn2+? Given $K_\textrm{sp}(\ce{ZnS})= \pu{2.5E-22}$ and $K_\textrm{sp}(\ce{FeS})= \pu{3.7E-19}$, why would $\ce{Fe^{2+}}$ ions remain in solution but not $\ce{Zn^{2+}}$ with acidification of the solution?
I have little understanding of this, so I don't have a great attempt at the solution.
FeS has 1500 times the solubility of ZnS.  Given the slow addition of a small molar fraction of acid that solubilizes both Zn(II) and Fe(II), that fraction will retain at equilibrium.  It gets complicated if soluble polysufides exist. Iron gives pyrite and marcasite ($\ce{FeS2}$) via mackinawite ($\ce{FeS}$) and greigite ($\ce{Fe3S4}$), but they are insoluble.