Resolving the mystery of the second charge reversal on solid surfaces in the presence of divalent heavy metal ions

Understanding adsorption of heavy metal ions onto solid surfaces is critical to many areas but remains due to complicated surface charge reversals (CRs). Experiments show three distinctive pHs values of curring during heavy metal ion interactions with oxide minerals like silica. CR1 and CR3 are well understood. Mechanisms for CR2 are mysteriously controversial. Here we have resolved the mystery by investigating adsorption and surface precipitation on the CR2 formation. We determined electrokinetics of silica versus pH and surface coverage of adsorbed species in the presence of Pb(II), Cu(II), Zn(II), Mg(II), and and quantified the adsorbed chemical species by X-ray photoelectron spectroscopy. For Pb(II), polynuclear complexes like Pb-3(OH)(4)(2+) are critical to the CR2 formation at full surface coverage. The polynuclear + complexes (large and less hydrated) with strong affinity to the silica surface penetrate the Stern layer to inner-sphere complexation with the surface and reverse its charge. The adsorption of unhydrolyzed cations their first hydrolysis product compress electrical double layer and neutralize the charge but cannot sign. Our results shed light on the critical role of polynuclear complexes of heavy metal ions for the mation in many practical applications.