Abstract
Anthropogenic lead pollution is an environmental problem that threatens the quality of soils and waters and endangers living organisms in numerous surface and subsurface habitats. Lead coprecipitation on mineral surfaces through dissolution-recrystallization processes has long-term effects on lead bioavailability. Gypsum and calcite are among the most abundant and reactive rock forming minerals present in numerous geological settings. In this work, we studied the interaction of slightly acidic (pHi = 5.5) Pb-bearing aqueous solutions ([Pb]i = 1 and 10 mM) with crystals of gypsum and/or calcite under atmospheric conditions. This interaction resulted in a reduction of the concentration of lead in the liquid phase due to the precipitation of newly formed Pb-bearing solid phases. The extent of this Pb removal mainly depended on the nature of the primary mineral phase involved in the interaction. Thus, when gypsum was the only solid phase initially present in the system, the Pb-bearing liquid-gypsum interaction resulted in Pb removals in the 98–99.8% range, regardless of [Pb]i. In contrast, when the interaction took place with calcite, Pb removal strongly depended on [Pb]i. It reached 99% when [Pb]i = 1 mM, while it was much more modest (~13%) when [Pb]i = 10 mM. Interestingly, Pb-removal was maximized for both [Pb]i (99.9% for solutions with [Pb]i = 10 mM and 99.7% for solutions with [Pb]i = 1 mM) when Pb-polluted solutions simultaneously interacted with gypsum and calcite crystals. Despite the large Pb removals found in most of the cases studied, the final Pb concentration ([Pb]f) in the liquid phase was always well above the maximum permitted in drinking water (0.01 ppm), with the minimum ([Pb]f = 0.7 ppm) being obtained for solutions with [Pb]i = 1 mM after their interaction with mixtures of gypsum and calcite crystals. This result suggests that integrating the use of mixtures of gypsum-calcite crystals might help to develop more efficient strategies for in-situ decontaminating Pb-polluted waters through mineral coprecipitation processes.
Highlights
The quantity and the quality of available freshwater have been steadily declining over the last century in both industrialized and developing countries
The processes that take place during the interaction between Pb-bearing aqueous solutions and gypsum, calcite, or mixtures of gypsum and calcite crystals were studied by monitoring both chemical changes in the liquid phase and mineralogical changes in the solid phase in macroscopic batch experiments
Under the mildly acidic conditions in this work, the interaction of aqueous solutions bearing 10 mM ≥ [Pb] ≥ 1mM with gypsum, calcite, or mixtures of crystals or 50:50 mixtures of gypsum and calcite crystals resulted in a Pb removal that could be fast and extensive when gypsum was present in the system, but progressed at a comparably extremely slow rate in the absence of this phase
Summary
The quantity and the quality of available freshwater have been steadily declining over the last century in both industrialized and developing countries. Human activities like farming, mining, or manufacturing, which provoke the continuous rise of salinity and pollutant contents of freshwater in many sites worldwide overworld, are the main factors of this decline [1,2,3,4], as well as the adoption of new lifestyles by large groups of citizens in densely populated cities, which is responsible for the general increase of the drug and pharmaceutic contents in wastewaters. Pb is a most hazardous heavy metal whose concentration in the atmosphere, soils, and waters underwent a striking worldwide increase during the 20th century associated with the development of the automobile industry and the generalized use of leaded gasolines until their banning in the 1980s [6,7]. Mining activities have contributed to the long-term release of Pb as a result of the interaction of large volumes of fine-grained
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