Revised aqueous solubility product constants and a simple laboratory synthesis of the Pb(II) hydroxycarbonates: Plumbonacrite and hydrocerussite

摘要

Plumbonacrite [Pb-10( OH)(6)O( CO3)(6)] is a lead(II) hydroxycarbonate mineral that has received scarce attention regarding its geochemical formation and behavior. Previous methods to synthesize it were based on complex experimental conditions that frequently yielded this mineral mixed with the more common hydrocerussite [Pb-3(CO3)(2)(OH)(2)] and Pb(II) oxide, preventing its study in pure form. In this work, we present very simple and novel methods to separately synthesize pure plumbonacrite and hydrocerussite in an aqueous medium under ambient conditions starting from Pb(ClO4)(2) and Pb(NO3)(2) salts, respectively. We studied their long-term stability and determined their thermodynamic solubility product constants. This yielded values that are more than 24 orders of magnitude lower (log K-sp = -65.92) than the latest report for plumbonacrite (and 57 orders of magnitude lower than the value used in current geochemical speciation programs). This new value predicts a much higher geochemical formation potential for plumbonacrite under lower than ambient CO2 partial pressures. The Ksp value for hydrocerussite was also found to be 5-7 orders of magnitude lower than previous reports. The improved Pb(II) solubility measurements were achieved by effectively dialyzing out all solids formed from the aqueous phase analyzed as a significant proportion of small Pb(II) hydroxycarbonate nanoparticles are formed during precipitation. The considerably lower solubilities of the Pb(II) hydroxycarbonates obtained will help correct geochemical speciation databases and predict their potential formation more accurately under varying geochemical conditions. Further, the synthesis methods presented will be very useful for geochemists attempting to study the details of their formation, structure, and environmental behavior.