Abiotic photomineralization and transformation of iron oxide nanominerals in aqueous systems

Abstract

The formation and transformation of iron oxide nanominerals in water environments control the migration and conversion of essential and toxic elements and organic pollutants. This study demonstrates the formation of iron oxide nanominerals through the oxidation of Fe²⁺_aq by hydroxyl radicals (OH˙) and superoxide radicals (O₂˙⁻) generated from the photolysis of nitrate. The mineral compositions were affected by the anion species and pH. In the photochemical system, schwertmannite was formed in 5.0 mmol L⁻¹ SO₄²⁻ solution with the initial pH of 6.0, and a mixture of goethite and lepidocrocite was formed when the SO₄²⁻ concentration decreased to 0.1 mmol L⁻¹. The particle size of schwertmannite increased with decreasing initial pH from 6.0 to 3.0. When Cl⁻ was used instead of SO₄²⁻, single-phase lepidocrocite was formed with the initial pH of 6.0. When the initial pH decreased to 4.5 and 3.0, a mixture of goethite and lepidocrocite was formed, and the relative content of lepidocrocite decreased with decreasing initial pH. Under anoxic conditions, Fe²⁺_aq promoted the transformation of the photochemically synthesized schwertmannite to goethite and lepidocrocite by dissolution–recrystallization. The present work expands our understanding of the generation and transformation of iron oxide nanominerals in nitrate-rich supergene environments.