Dissolution - crystallization reactions can control the uptake of heavy metal such as lead, which is harmful to human health if inhaled or ingested. When Pb-bearing aqueous solution interacts with carbonates and sulfates, the formation of secondary phases at the interface mineral-water plays an important role on both the migration and accumulation of this pollutant in earth surface systems. The interaction of lead dissolved with calcium bearing minerals has been previously studied at pH between 4.75 and 6.8 [1-2]. However, less attention has been paid to the crystallization of Pb-bearing minerals in high acidic environments, in which the availability and mobility of dissolved Pb increase. Here, we have studied the interaction of Pb dissolved with both calcite and gypsum at highly acidic and atmospheric conditions. The main goals of this work were (a) to analyse the physicochemical evolution of the system and (b) to study the nature and morphology of the neo-formed phases. With these aims, a set of macroscopic experiments has been carried out by placing 1 g of crystals of a mix of calcite and gypsum (3.0 – 4.0 mm) in batch reactors containing 100 mL of Pb-bearing aqueous solution at pH= 2.5. Then, the vessels were closed with parafilm to minimize evaporation during reaction times (from 1 hour to 48 hours). The parent solution was prepared using reagent-grade Pb(NO3)2 and high-purity deionized water to yield [Pbaq]initial of 500 mg/L. After given period of time, the solid phases were analysed by glancing incident X-ray diffraction (GIXRD), Raman spectroscopy (RS) and Scanning Electron Microscopy (SEM). The initial and final pH were monitored and Ca2+ and Pb2+ in the aqueous solutions were analysed by ICP-OES.
Results show that dissolution of calcite and gypsum is simultaneous to elimination of the almost Pb2+. Anglesite (PbSO4) were identified using RS and GIRDX in all interaction experiments with gypsum. SEM reveals the heterogeneous nucleation of anglesite crystals, which are distributed randomly without covering all the gypsum surface. Different morphologies of anglesite are identified, including small rhombus-shaped single crystals, aggregates of identical crystals growing in parallel and contact twins. On the contrary, calcite is dissolving during the entire period of reaction and lead-bearing carbonates are not detected on the calcite surfaces. These data show that anglesite is a stable phase responsible of the Pb elimination under acidic conditions. The evolution of the anglesite morphologies after given reaction periods are discussed.