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

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.

Molybdenum Mobility During Managed Aquifer Recharge in Carbonate Aquifers

“…Here, our reaction network description is focused on the key components that were hypothesized to play a role for the mobility of Mo. The mechanism for the initial release of Mo follows the conceptual model proposed by Koopmann et al, where (i) pyrite oxidation is the primary driver for redox reactions that increase acidity, (ii) dolomite acts as a buffer in response to the release of acidity, and (iii) triggers the release of Mo. Molybdenum was assumed to be associated with highly soluble sulfurized organic matter present in intercrystalline spaces in the dolomite matrix or directly incorporated in dolomite crystals .…”

“…Additional geochemical analysis of the aquifer material indicated sulfurized organic matter and only occasional abundance of powellite. 42 The Orange County ASR site includes one ASR well (ASR-1) and two monitoring wells, LFMW-1 and LFMW-2, at a distance of 30.5 and 152.4 m from ASR-1, respectively. 43 While the screen of ASR-1 covers the entire depth of the storage zone (Figure 1), LFMW-1 and LFMW-2 are screened from 318.5 to 365.8 m and from 335.3 to 365.8 m, respectively.…”

“…Arthur et al identified dolomite, calcite, trace amounts of clay, natural organic matter, and pyrite framboids in the aquifer matrix. Additional geochemical analysis of the aquifer material indicated sulfurized organic matter and only occasional abundance of powellite . The Orange County ASR site includes one ASR well (ASR-1) and two monitoring wells, LFMW-1 and LFMW-2, at a distance of 30.5 and 152.4 m from ASR-1, respectively…”

“…The mechanism for the initial release of Mo follows the conceptual model proposed by Koopmann et al, where (i) pyrite oxidation is the primary driver for redox reactions that increase acidity, (ii) dolomite acts as a buffer in response to the release of acidity, and (iii) triggers the release of Mo. Molybdenum was assumed to be associated with highly soluble sulfurized organic matter present in intercrystalline spaces in the dolomite matrix or directly incorporated in dolomite crystals . Hence, dolomite dissolution was proposed to facilitate the access of the injectant to the sulfurized organic matter and, therefore, Mo release occurred proportionally to dolomite dissolution.…”

“…Here, we use reactive transport modeling to integrate and interpret the data collected at an aquifer storage and recovery (ASR) site in Orange County (Florida), where the cyclic injection of oxygenated water into an anoxic dolomite aquifer of the Floridan Aquifer System resulted in Mo concentrations of up to 853 μg/L . Laboratory-scale experiments with sediment material from the Floridan Aquifer System indicated that Mo release could be closely linked to dolomite dissolution because Mo was associated with easily soluble sulfurized organic matter in intercrystalline spaces or incorporated in dolomite crystals . In this study, we explore an additional line of evidence for the release of Mo by performing a detailed model-based interpretation of a comprehensive data set collected during the ASR operations that triggered highly dynamic and spatially varying flow, transport and geochemical reaction patterns.…”