Coupled feldspar dissolution-clay precipitation kinetics and lead sorption onto ferrihydrite nano-particles.

One of the fundamental problems in modern geochemistry is the significant discrepancy between laboratory-measured and field derived feldspar dissolution rates. Zhu et al. (2004) proposed a new hypothesis for explaining the laboratory--field discrepancy wherein the slow kinetics of secondary clay precipitation is the rate limiting step and thus controls the overall feldspar dissolution rate. We conducted new feldspar dissolution batch experiments and performed geochemical modeling to test this hypothesis. The experimental results show that partial equilibrium was not attained between secondary minerals and aqueous solutions for the feldspar hydrolysis batch systems. Modeling results show that a quasi-steady state was reached. At the quasi-steady state, dissolution reactions proceeded at rates that are orders of magnitude slower than the rates measured at far-from-equilibrium. Results reported in this dissertation lend support to Zhu et al. (2004) hypothesis and showed how the slow secondary mineral precipitation provides a regulator to explain why the systems are held close to equilibrium and show how the most often-quoted "near equilibrium" explanation for an apparent field-lab discrepancy can work quantitatively.

The second topic of this dissertation is Pb sorption onto ferrihydrite nano-particles. The differences of adsorption and coprecipitation of Pb with iron oxyhydroxide are studied with sorption edge measurements, High Resolution Transmission and Analytical Electron Microscopy (HR TEM-AEM), and geochemical modeling. Coprecipitation of Pb2+ with ferric oxyhydroxides occurred at ∼ pH 4, about 0.5-1.0 pH unit higher than Fe3+ precipitation. Coprecipitation is more efficient than adsorption in removing Pb2+ from aqueous solutions at similar sorbate/sorbent ratios. X-ray Diffraction and HRTEM shows Pb-Fe coprecipitates are 2-line ferrihydrite (2LFh) and lepidocrocite. Geochemical modeling shows that a surface complexation model can explain adsorption experimental data well. In contrast, a solid solution model or a model of combined solid solution formation and surface complexation can fit coprecipitation experimental data sets well. Hence, coprecipitation and adsorption experiments resulted in different Pb2+ incorporation mechanisms, which could result in different mobility, bioavailability, and long-term stability of Pb2+ in the environment.