Laboratory and initial field testing of the Min‐Trap™ for tracking reactive iron sulfide mineral formation during in situ remediation

Abstract

The Mineral Trap, or Min‐Trap™, is a monitoring well‐based sampler designed to collect direct physical evidence of reactive mineral formation in situ without collecting soil or rock core samples. The Min‐Trap consists of a nonreactive granular medium (e.g., silica sand) within water‐permeable mesh pillows that are supported inside a slotted polyvinyl chloride housing that is incubated within a conventional monitoring well. The primary objective of the Min‐Trap in this application is to collect reactive minerals that are forming in the aquifer in a retrievable format that can be submitted for laboratory analysis. To evaluate the capability of Min‐Traps to capture reactive iron minerals, both a laboratory tank test and a field test were conducted. Both tests confirmed that iron sulfide minerals form in the Min‐Trap under sulfate reducing conditions within several weeks. Analysis of the precipitated minerals via the AMIBA analysis suite showed that they almost entirely consisted of weak acid soluble (biogenic, microcrystalline) ferrous iron‐based minerals, and at least two thirds of the sulfur‐containing minerals were monosulfides (i.e., mackinawite) at the end of each test. Scanning electron microscopy confirmed the colocation of iron and sulfur in the mineral masses. The dominance of the ferrous iron and reduced sulfur verifies that little to no oxidation of the captured minerals occurred between sample collection and analysis. A subsurface soil core was collected during the field test next to the Min‐Trap‐containing well. AMIBA results were consistent between the native soil and the Min‐Trap except for much higher strong acid soluble (crystalline) ferric iron in the native soil when compared to the silica sand of the Min‐Trap, as expected. This work shows that Min‐Traps are useful for documenting the formation of reactive iron sulfides (FeSx) that can form during in situ anaerobic biostimulation and can drive complementary abiotic treatment of chlorinated volatile organic compounds. Mineralogical data obtained from Min‐Traps can be applied to assess remedial objectives at several stages of the remedial program, including initial characterization, alternatives evaluation, feasibility testing, remedy optimization, and transition from active treatment to passive remedial methods.

Viewing the End from the Beginning: Designing for the Transition to Long‐Term Passive Phases of In Situ Chlorinated Solvent Treatment

“…Although the increase in ferrous (dissolved) iron and decrease in sulfate concentrations, as well as the increased abundance of iron‐reducing bacteria and sulfate‐reducing bacteria, that occur following organic carbon amendment injections may be used as a proxy for the potential presence of reactive iron sulfide minerals, conclusive evidence of the formation of said minerals (i.e., direct mineralogical data) is generally lacking from performance monitoring programs. In a previous column entitled New Tools for Assessing Reactive Mineral‐Mediated Abiotic Contaminant Transformation (Horst et al 2019; Ulrich et al 2021), we describe the Min‐Traps, a specialized sampling device that can be deployed in conventional monitoring wells (with minimum well diameter of 2 in.) for the collection of reactive minerals for analysis.…”

“…For example. Ulrich et al (2021) evaluated the Min-Trap technology in highly controlled sand tank tests where lactate was continuously injected for 175 days. AMIBA analyses of Min-Trap samples deployed in this tank confirmed the formation of substantial amounts of FeSx minerals (in excess of about 200 mg/kg in most locations).…”

Passive sampler designed for per‐ and polyfluoroalkyl substances using polymer‐modified organosilica adsorbent

“…A simulated aquifer tank was used for additional laboratory testing of the passive samplers. The tank test setup is described in detail in previous work (Ulrich et al, 2021), and was modified slightly from a single-pass flowthrough design to recirculation. The simulated aquifer tank consisted of a glass aquarium tank (46 cm wide, 122 cm long, and 41 cm tall), filled with 10 lateral centimeters of gravel (Imagitarium Nutmeg Aquarium Gravel) on each end and Ottawa sand in the body of the tank (Humboldt Manufacturing, Chicago, Illinois) mixed with 0.5 weight percent sand-sized limestone chips.…”

“…The tank was designed to accommodate five 2-inch diameter slotted PVC well screens. Ulrich et al (2021) operated the tank in a single-pass flow-through design; however, for this study, the effluent was collected and recirculated via a peristaltic pump. A three-pronged injection apparatus was built from 1/8-inch stainless steel Swagelok ® tubing, a four-way tee fitting, and two 90 elbow fittings.…”

Field methods and example applications for the Min‐Trap® mineral sampler

“…The recently developed mineral trap (known commercially as a Min-Trap ® ; Ulrich, Tilton, Ford, et al, 2021;…”