Sunlight-Induced Interfacial Electron Transfer of Ferrihydrite under Oxic Conditions: Mineral Transformation and Redox Active Species Production

Abstract

Fe­(II)-catalyzed ferrihydrite transformation under anoxic conditions has been intensively studied, while such mechanisms are insufficient to be applied in oxic environments with depleted Fe­(II). Here, we investigated expanded pathways of sunlight-driven ferrihydrite transformation in the presence of dissolved oxygen, without initial addition of dissolved Fe­(II). We found that sunlight significantly facilitated the transformation of ferrihydrite to goethite compared to that under dark conditions. Redox active species (hole–electron pairs, reactive radicals, and Fe­(II)) were produced from the ferrihydrite interface via the photoinduced electron transfer processes. Experiments with systematically varied wet chemistry conditions probed the relative contributions of three pathways for the production of hydroxyl radicals: (1) oxidation of water (5.0%); (2) reduction of dissolved oxygen (40.9%); and (3) photolysis of Fe­(III)-hydroxyl complexes (54.1%). Results also showed superoxide radicals as the main oxidant for Fe­(II) reoxidation under acidic conditions, thus promoting the ferrihydrite transformation. The presence of inorganic ions (chloride, sulfate, and nitrate) did not only affect the hydrolysis and precipitation of Fe­(III) but also the generation of radicals via photoinduced charge transfer reactions. The involvement of redox active species and the accompanying mineral transformations would exert a profound effect on the fate of multivalent elements and organic contaminants in aquatic environments.

Transformations of Ferrihydrite–Extracellular Polymeric Substance Coprecipitates Driven by Dissolved Sulfide: Interrelated Effects of Carbon and Sulfur Loadings

“…This experiment used deionized water (Milli-Q, Millipore, 18.2 MΩ•cm) to prepare all solutions. Following the previously reported procedures, 56 2-line ferrihydrite and ferrihydrite−OM coprecipitation with initial C/Fe molar ratios of 0−3 in the solution were synthesized. Pure ferrihydrite was synthesized by vigorously stirring (600 rpm) while adding 0.1 M NaOH to a 10 mM Fe(NO 3 ) 3 solution until a pH of 7 was reached.…”

“…This experiment used deionized water (Milli-Q, Millipore, 18.2 MΩ•cm) to prepare all solutions. Following the previously reported procedures, 56 S1). S(-II)/Fe molar ratios of 0.1, 0.5, and 1.0 are hereinafter referred to as low sulfide, medium sulfide, and high sulfide conditions, respectively.…”

Sunlight-Mediated Reductive Transformation of Thallium(III) in Acidic Natural Organic Matter Solutions: Mechanisms and Kinetic Modeling

“…Furthermore, Fe is one of the most important accompanying elements of Tl, 65 with dozens to thousands of times higher concentrations than Tl in Tl-polluted waters. 9,66,67 The Fe(III)/Fe(II) cycle, which is well reported impacted by light, 19,68 can also influence Tl speciation in acidic sunlit environments. 3,10,17 As we previously reported that the generation of • OH from the Fe(III) photoreduction can induce Tl(I) oxidation under irradiated conditions, 17 we provided further insights here that the concentration ratios of Fe(III) and SRFA will significantly influence the redox transformations of Tl, with the rate and extent of Tl(I) photooxidation enhanced and Tl(III) photoreduction inhibited with increasing Fe(III) concentration (Figure S20).…”

Solar Photocatalysis of A Pesticide in A Tubular Reactor on Titaniferous Sand as A New Semi-Conductor

“…As a result, the formation of hydroxyl radicals (˚OH) and other reactive species, responsible for the degradation of the pollutant, will be improved. Also, photoelectrons can reduce dissolved oxygen and thus lead to the formation of radical superoxide ions 2 O −  according to Equation (4) [23].…”