Formation of Fe(IV) over a wide pH range via iron-carbon composite-catalyzed persulfate activation

Fe(IV) is typically formed under acidic or neutral conditions while its formation mechanism and impacts on pollutant decomposition have been overlooked, particularly in persulfate (PS) systems. In this study, an iron-carbon composite (Fe-BC250) was prepared at a low-temperature (250 °C) to explore the PS activation and Fe(IV) formation processes under a wide range of pH conditions. Fe(IV) was the main reactive species in the iron-carbon composite/PS system, and Fe species promoted the graphitization of biochars. The existence of Fe(IV) in the Fe-BC250/PS system was identified by in situ Raman, Mössbauer spectrum, and methyl phenyl sulfoxide probe (PMSO) experiments. Under alkaline conditions, the solution pH decreased because hydrolyzed Fe2+ generated H+, and surface Fe(II) reacted with PS to form Fe(IV). The Fe-BC250/PS system was superior in removing contaminants under alkaline conditions compared to the Fe2+/PS system, due to the slow release of Fe2+ from Fe-BC250. This study offers new insights into the Fe(IV) formation mechanism in biochar/PS systems under a wide range of pH conditions and provides a theoretical basis for applying low-cost iron-carbon composites in water treatment.