pyromorphite and lead-phosphate

Inorganic binder-based stabilization/solidification (S/S) of Pb-contaminated soil is a commonly used remediation approach. This paper investigates the influences of soluble Pb species on the hydration process of two types of inorganic binders: ordinary Portland cement (OPC) and magnesium potassium phosphate cement (MKPC). The environmental leachability, compressive strength, and setting time of the cement products are assessed as the primary performance indicators. The mechanisms of Pb involved in the hydration process are analyzed through X-ray diffraction (XRD), hydration heat evolution, and thermogravimetric analyses. Results show that the presence of Pb imposes adverse impact on the compressive strength (decreased by 30.4%) and the final setting time (prolonged by 334.7%) of OPC, but it exerts much less influence on those of MKPC. The reduced strength and delayed setting are attributed to the retarded hydration reaction rate of OPC during the induction period. These results suggest that the OPC-based S/S of soluble Pb mainly depends on physical encapsulation by calcium-silicate-hydrate (CSH) gels. In contrast, in case of MKPC-based S/S process, chemical stabilization with residual phosphate (pyromorphite and lead phosphate precipitation) and physical fixation of cementitious struvite-K are the major mechanisms. Therefore, MKPC is a more efficient and chemically stable inorganic binder for the Pb S/S process.

Topics: Calcium Compounds; Construction Materials; Gels; Lead; Magnesium Compounds; Minerals; Phosphates; Potassium Compounds; Silicates; Soil Pollutants; Water Pollutants, Chemical; X-Ray Diffraction

Topics: Bacillus; Bacteria; Biodegradation, Environmental; Biomass; Culture Media; Immobilization; Lead; Mexico; Minerals; Mining; Nitrates; Phosphates; Siderophores; Soil Pollutants; Spectroscopy, Fourier Transform Infrared; Staphylococcus; Wastewater; X-Ray Diffraction

Phosphate addition to lead-contaminated soils can immobilize lead in situ through the formation of lead phosphate minerals such as chloropyromorphite (Pb5(PO4)3Cl). The long-term stability of lead immobilized in lead phosphate precipitates depends on the equilibrium solubility and dissolution rates of the lead phosphate solids. The equilibrium solubility and dissolution kinetics of chloropyromorphite were quantified in a series of batch and flow-through reactors. Both equilibrium solubility and dissolution rates were strongly affected by pH. Synthetic chloropyromorphite was more soluble than was predicted using a widely cited solubility product for pure chloropyromorphite, an observation that is consistent with several other recent studies. A trace amount of a more soluble lead solid, such as lead hydroxide, in chloropyromorphite could significantly increase dissolved Pb at neutral pH. The pH-dependence of the dissolution rate was examined in flow-through experiments. A dissolution rate law with a single rate constant for synthetic chloropyromorphite and a constant reaction order of 0.65 with respect to [H+] has been determined. The slow dissolution rate of chloropyromorphite relative to other lead minerals and its low solubility can result in its long-term stability in soils.

Topics: Algorithms; Hydrogen-Ion Concentration; Kinetics; Lead; Microscopy, Electron, Scanning; Minerals; Phosphates; Soil Pollutants; Solubility