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. 2022 Sep 6;56(17):12325-12335.
doi: 10.1021/acs.est.2c04142. Epub 2022 Aug 19.

Molybdenum Release Triggered by Dolomite Dissolution: Experimental Evidence and Conceptual Model

Sarah Koopmann  1 , Henning Prommer  2   3 , Thomas Pichler  1
Affiliations

Molybdenum Release Triggered by Dolomite Dissolution: Experimental Evidence and Conceptual Model

Sarah Koopmann et al. Environ Sci Technol. .

Abstract

The injection of oxygenated water into anoxic aquifers during managed aquifer recharge (MAR) can cause the mobilization of metal(loid)s. Here, we study the processes controlling MAR-induced molybdenum (Mo) release in dolomitic aquifers. Sequential chemical extractions and energy dispersive X-ray spectroscopy combined with scanning electron microscopy point to an association of Mo with easily soluble sulfurized organic matter present in intercrystalline spaces of dolomites or directly incorporated within dolomite crystals. The easily soluble character was confirmed by a batch experiment that demonstrated the rapid mobilization of Mo, dissolved organic carbon, and sulfur. The type and time of batch solution contact with the sulfurized organic matter impacted the release of Mo, as demonstrated by a 36% increase in Mo concentrations when shaking was intensified. Based on the experimental results, a conceptual model for the release of Mo was formulated, where (i) the injection of oxygenated water causes the oxidation of pyrite in the aquifer matrix, and (ii) the associated release of protons (H+) induces the dissolution of dolomite as a buffering reaction, which (iii) enhances the accessibility of the injectant to intercrystalline and incorporated sulfurized organic matter within dolomite, causing the release of Mo.

Keywords: aquifer storage and recovery; dolomite; molybdenum; sulfurized organic matter.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Backscatter images of the samples 55 OC and 63 OC. (A) Transition from dolomite to a mixture of dolomite and OM. (B) OM inclusion in the dolomite matrix. (C) Pyrite framboids. (D) Powellite next to OM.
Figure 2
Figure 2
Portions of Ca, Mg, Sr, Mo, As, S, and Fe in steps 1 to 5 relative to the sum of all steps during the sequential extraction with the 5 times (A) and 1 time (B) execution of extraction step 2.
Figure 3
Figure 3
Concentrations of S and Mo under different redox conditions, pH values, solvents, and filter sizes determined in the repeated extraction step 1. The numbers above the columns indicate the relative standard deviation to the conditions during the initial sequential extractions (oxic, NH4 acetate, pH 8.2, 0.45 μm filter size).
Figure 4
Figure 4
(A) Concentration of dissolved OM (DOC), sulfur (S), and molybdenum (Mo) during the first 24 h of the batch experiment when intermittently hand-shaken and constantly shaken at 3 rpm. (B) Linear correlation of Mo with DOC and S in intermittently and constantly shaken samples during the first 24 h.
Figure 5
Figure 5
Linear correlation of TOC and Mg (left) and TOC and Ca (right) between 387 and 415 days since the start of the ASR operation. Data from FDEP.
Figure 6
Figure 6
Conceptual model for the release of Mo in Orange County. Pyrite oxidation led to a decrease in oxygen and pH (A). The reduction in pH was followed by the dissolution of dolomite and the release of intercrystalline and incorporated sulfurized OM and Mo (B).
See this image and copyright information in PMC

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