Kinetics of Hydroxyl Radical Production from Oxygenation of Reduced Iron Minerals and Their Reactivity with Trichloroethene: Effects of Iron Amounts, Iron Species, and Sulfate Reducing Bacteria

Abstract

Reactive oxygen species generated during the oxygenation of different ferrous species have been documented at groundwater field sites, but their effect on pollutant destruction remains an open question. To address this knowledge gap, a kinetic model was developed to probe mechanisms of •OH production and reactivity with trichloroethene (TCE) and competing species in the presence of reduced iron minerals (RIM) and oxygen in batch experiments. RIM slurries were formed by combining different amounts of Fe(II) and sulfide (with Fe(II):S ratios from 1:1 to 50:1) or Fe(II) and sulfate with sulfate reducing bacteria (SRB) added. Extents of TCE oxidation and •OH production were both greater with RIM prepared under more reducing conditions (more added Fe(II)) and then amended with O2. Kinetic rate constants from modeling indicate that •OH production from free Fe(II) dominates •OH production from solid Fe(II) and that TCE competes for •OH with Fe(II) and organic matter (OM). Competition with OM only occurs in experiments with SRB, which include cells and their exudates. Experimental results indicate that cells and/or exudates also provide electron equivalents to reform Fe(II) from oxidized RIM. Our work provides new insights into mechanisms and environmental significance of TCE oxidation by •OH produced from oxygenation of RIM. However, further work is necessary to confirm the relative importance of reaction pathways identified here and to probe potentially unaccounted for mechanisms that affect abiotic TCE oxidation in natural systems.

Water Vapor Condensation on Iron Minerals Spontaneously Produces Hydroxyl Radical

“…The hydroxyl radical ( • OH) is undoubtedly among the most powerful oxidants in nature, which can drive oxidative transformation of elements in the atmosphere, water, , and soils. , Further, • OH is deemed as a natural detergent because of its high capacity to degrade aromatic carbons, hydrochlorofluorocarbons, and other pollutants. − The natural source of • OH is frequently linked to photochemical processes. For instance, natural organic matter or black carbon can be activated under sunlight irradiation and transfer electrons to oxygen for • OH production. − In addition, • OH production could also occur under dark conditions triggered by reaction of microbe-excreted electrons with O 2 in soils and sediments, where iron minerals play important roles in • OH production by shuttling electrons and catalyzing Fenton-like reactions. , …”

“…11−14 In addition, • OH production could also occur under dark conditions triggered by reaction of microbe-excreted electrons with O 2 in soils and sediments, 15 where iron minerals play important roles in • OH production by shuttling electrons and catalyzing Fenton-like reactions. 16,17 Recent advances suggest that water microdroplets present unique physicochemical properties, and chemical reactions that are typically unfavorable in bulk phase can spontaneously proceed at the microdroplet interface. 18−21 For instance, hydrogen peroxide (H 2 O 2 ) and • OH can be spontaneously produced at the micromolar level by atomizing bulk water 22 or condensing water vapor to microdroplets on solid surfaces.…”

Unrecognized Role of Organic Acid in Natural Attenuation of Pollutants by Mackinawite (FeS): The Significance of Carbon-Center Free Radicals

“…The redox oscillations of chemically active minerals are intrinsically involved in various geochemical-biological cycles on Earth due to their crucial role in redox reactions. − Particularly, the conversion of transition-metal ions, such as Fe­(II)/Fe­(III) and Mn­(III)/Mn­(IV), plays a pivotal role in other elemental cycles and organic matter turnover. , The past few decades have witnessed an unprecedented booming scene in the study of subsurface geochemical processes driven by iron-bearing minerals, focusing on pollutant transformation in both fundamental research and practical applications. , Among numerous iron-bearing minerals, the reduced Fe­(II) minerals are the most common and exhibit outstanding pollutant-transforming activity, partly due to their inherent surface reactivity. − Mackinawite (FeS), a quintessential reduced Fe­(II) mineral, plays a fundamental role in the global Fe cycles and exhibits significant reactivity in biogeochemical processes. − The pioneering work on using FeS for pollutant removal can be historically traced back to the 1990s . Since then, numerous studies have utilized FeS as an efficient reductant for the elimination of reducible organic compounds and heavy metals in natural anaerobic settings. , However, the oxygen-depleted settings often transition to oxygen-enriched environments due to various natural disturbances (e.g., groundwater fluctuations) and human interventions (e.g., remediation efforts). , Consequently, recent research on the transport, fate, and orientation of pollutants influenced by FeS in the presence of O 2 has attracted considerable attention.…”

Mineral-mediated stability of organic carbon in soil and relevant interaction mechanisms

“…The production of ROS with minerals is mainly achieved via water or O 2 reaction at the defects sites, or through interactions with mineral-adsorbed Fe(II)/Mn(III) biologically or non-biologically [ 76 , 78 , 83 , 85 , [89] , [90] , [91] , [92] , [93] , [94] , [95] , [96] , [97] , [98] , [99] , [100] , [101] ], or via the photochemical redox reaction [ [102] , [103] , [104] , [105] , [106] , [107] ] ( Fig. 2 ).…”