Carbonate minerals, such as CaCO<sub>3</sub>, can sequester CO<sub>2</sub> thereby mitigating this important greenhouse gas. In this work, we studied the additional benefit of carbonates as mineral hosts to sequester toxic heavy metals. These coprecipitation reactions can be enabled via mineral carbonation of fly ash, desalination brines, subsurface produced waters, and mine wastes. An important feature of coprecipitation reactions is that trace elements can precipitate even when the aqueous solution is undersaturated with respect to the pure endmember containing that element. In this work, we studied these reactions using batch experiments, porous media columns, and synthetic sodium silicate gels. We used ICP-MS to observe metal removal from water. To observe the solid phase, we employed scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction. We also used x-ray fluorescence nanospectroscopy at the HXN beamline at NSLS-II synchrotron for observation of metal incorporation patterns. Experiments conducted at near neutral to basic conditions revealed metal(loid)-rich calcites as well as the polymorph vaterite, which was found to be stabilized by the addition of foreign ions (Figure 1). In one set of experiments, As and Zn were very effectively coprecipitated, but Cd and Pb concentrations in the treated solution still reached levels below U.S. EPA drinking water standards. In some cases, trace element incorporation was consistently higher than predicted by theory, with incorporation of Cr being 3000 times more than theoretical estimates.

Simultaneous mitigation of greenhouse gases and heavy metal pollution with carbonate mineral precipitation

ACS Fall 2021

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