Supercritical CO₂-assisted regulation of FeOOH hydrophilicity for Fe^IV=O deployment in aqueous waste disintegration

FeOCl and FeOOH are iso-structural and impart defective [Fe³⁺(O^2–)₄(Cl^–)₂]^7– and [Fe³⁺(O^2–)₄(OH^–)₂]^7- sub-units on their surfaces, respectively, where Fe^δ+ and Fe³⁺ defects can evolve ^•OH/Fe^IV=O and ^•OOH/O₂^•–/¹O₂ in H₂O₂-containing aqueous media, respectively. FeOCl surface can incur marked Fe^δ+/3+ leaching or rigid coordination to aqueous organics, thereby limiting its use as a reactive oxygen species (ROS) evolver and/or reservoir in aqueous phases. Conversely, FeOOH surface can avoid Fe^δ+/3+ leaching by eliminating OH^– ligands or their protonated analogs (H₂O/H₃O⁺) from Fe^δ+/3+ in defective [Fe³⁺(O^2–)₄(OH^–)₂]^7– sub-units. This was enabled by extensive exposure of FeOOH to supercritical CO₂, producing hydrophobic FeOOH-CO₂ that offered bunched merits over hydrophilic FeOOH. The number of Fe^δ+ defects and their electron affinity were greater and lower, respectively, in FeOOH-CO₂ than in FeOOH, resulting in higher ^•OH productivity for the former and higher ¹O₂ productivity for the latter, alongside with Fe^IV=O evolution for FeOOH-CO₂. Moreover, FeOOH and FeOOH-CO₂ exploited ^•OH/¹O₂ and Fe^IV=O as their primary ROS for disintegrating aqueous organics, respectively, whose inclusion of e^--donating groups made Fe^IV=O particularly effective in initiating or propagating the destabilization of hard-to-dissociate organics via electron transfer. Furthermore, despite minimized Fe^δ+/3+ leaching across FeOOH/FeOOH-CO₂ surfaces, FeOOH-CO₂ outcompeted FeOOH in providing higher H₂O₂/^•OOH/O₂^•- accessibility to Fe^δ+/3+ defects and in avoiding their coordination to fragmented organics in H₂O₂- and organic-containing aqueous media. Consequently, FeOOH-CO₂ was more active in, selective to, and reusable for degrading e^--donating group-bearing contaminant or mineralizing textile wastewater than FeOOH, Fe₂O₃/Fe₃O₄, and FeOCl.