Mining-impacted waters contain unsafe levels of toxic elements like arsenic, zinc, cadmium, and lead. Such metals and metalloids can be incorporated in the solid phase via inclusion of foreign ions in the crystalline structure (coprecipitation). In this work, we focus on coprecipitation for simultaneous heavy metal(loid) and CO<sub>2</sub> removal, motivated by naturally observed and engineered process of carbonation in mine tailings for the purpose of water treatment. This approach can be considered effective if it can be applicable to commonly found heavy metals, is permanent, and mitigates risk of rerelease into the environment. Therefore, quantifying the degree of uptake, and mapping the path of elements is critical since mineral precipitates of different crystalline structure or composition exhibit varying degrees of reactivity and stability. Carbonates, along with sulfates and oxides were precipitated in a series of synthetic mine water-based experiments to explore elemental distributions and uptake mechanisms. ICP-MS data of the treated solution indicated As and Zn to be most effectively taken up in the solid phase, but Cd and Pb concentrations in the treated solution still reached levels below U.S. EPA national primary drinking water standards. Experiments conducted at acidic conditions showed abundance of calcium sulfate (gypsum) precipitates, and S/TEM/EDS analyses showed homogeneous distributions of the four elements, strongly suggesting incorporation of the cations and/or anion in the crystalline structure. Experiments conducted at near neutral to basic conditions revealed metal(loid)-rich calcites and Fe-oxides, and stabilization of vaterites, a phase that is commonly overlooked due to its metastability. Results from this work are especially useful as it provides evidence for coprecipitated metal(loid)s in the form of stable carbonate and/or commonly found secondary minerals. We show that with pH adjustments and CO<sub>2</sub> addition, carbonate precipitation can be promoted utilizing existing cations in mine waters for effective water treatment.

Treating mining-impacted waters for heavy metals using coprecipitation in carbonates and other mineral precipitates

ACS Spring Meeting 2021

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