Impact of Divalent Cations on Dark Production of Hydroxyl Radicals from Oxygenation of Reduced Humic Acids at Anoxic–Oxic Interfaces

Abstract

Humic acids (HAs) are redox-active and can serve as either electron acceptors or electron donors to participate in multiple redox reactions. In nature water, HA can be intimately associated with divalent cations, such as Fe 2+ , Ca 2+ , and Mg 2+ , through a series of reactions that may in turn affect the redox reactivity of HA. Recent advances have demonstrated that the oxygenation of reduced HA in the dark can produce •OH at anoxic−oxic interfaces. However, little is known about the roles of the divalent cations complexed with HA on the production of •OH from reduced HA. This study provides new knowledge regarding the impact of Fe 2+ , Ca 2+ , and Mg 2+ ions on the dark production of •OH from oxygenation of reduced HA at anoxic−oxic interfaces over a wide range of environmentally relevant conditions. Results show that the rates and yields of •OH production increase with increasing Fe 2+ concentration (0.18−0.89 mM). This is largely attributed to the formation of complexed Fe(II) with HA, which increases the number of Fe(II)/Fe(III) cycles and enhances the decomposition of formed H 2 O 2 , accelerating the rates of Fenton reactions under circumneutral conditions. However, the promotional effect of Fe 2+ on •OH formation is greatly suppressed in the coexistence of high Ca 2+ /Mg 2+ concentration (5−20 mM), likely due to the retarded formation of HA-Fe(II) complexes and competition of HA surface reactive sites by Ca 2+ /Mg 2+ ions. Findings improve the current understanding of the dark production of •OH from reduced HA and provide valuable insights toward understanding of carbon cycling and contaminant fate at anoxic−oxic interfaces.

Determination of Hydroxyl Radical Production from Sulfide Oxidation Relevant to Sulfidic Porewaters

“…H 2 O 2 was verified as an intermediate, and CO 2 increases upon oxidation of DOC were measured . Recent work by Liao et al revealed that ·OH production in the presence of reduced humic acids is dramatically increased in the presence of Fe­(II), supporting Fe­(II) being the primary driver of ·OH formation in these systems …”

“…17 Recent work by Liao et al revealed that •OH production in the presence of reduced humic acids is dramatically increased in the presence of Fe(II), supporting Fe(II) being the primary driver of •OH formation in these systems. 18 In other natural systems, the relative roles of Fe(II), DOM, and other potential reducing agents has also been studied. Aeration of anoxic lake water, as would occur during turnover cycles, caused light-independent formation of •OH up to 0.2− 4.5 μmol L −1 , which degraded the fungicide dimethomorph by 24−42% per turnover.…”

Formation and Transport of Cr(III)-NOM-Fe Colloids upon Reaction of Cr(VI) with NOM-Fe(II) Colloids at Anoxic–Oxic Interfaces

“…A well-characterized Aldrich HA was selected as a model NOM compound. Because this particular HA was used in numerous other studies, ,,− it was selected so that the findings of our experiments could be directly compared with previous observations. Chemically reduced HA prepared by a Pd–H 2 method described previously was used for all experiments to emulate the reduced state of NOM present at anoxic conditions.…”

Effect of NOM on copper sulfide nanoparticle growth, stability, and oxidative dissolution

“…96 In addition, several studies have identified oxicanoxic interfaces in natural environments as hotspots for dark HO˙formation when DOM and metal-based reductants (such as CuS) are present. 97,98 A recent study even observed the formation of ROS in the dark in oxic solutions containing Cl − ions, SRFA and CuIJII), thus interfering in the redox cycle of such systems. 99 Furthermore, the presence of different NOM types that exhibit increased binding affinities towards CuIJII), such as humic acids, 40,42,100 could enhance Cu x S dissolution.…”