Phosphate-Induced Acidic Microenvironment for Improved Contaminant Removal during FeS Oxygenation

The coexistence of mackinawite (FeS) and phosphate is widely observed in natural systems. However, the underlying mechanism regarding how phosphate influences the environmental behavior of FeS, especially during the FeS oxygenation in aquatic systems, remains in its fancy. This study for the first time reported that the presence of phosphate, even at a low concentration of 0.3 mM, significantly promoted the FeS-mediated O₂ activation and thus the pollutant degradation. The enhancement was attributed to a substantial increase in the generation of •OH, as evidenced by the electron paramagnetic resonance tests and the identification of the probing products. A combination of experiments and theoretical calculations revealed that phosphate adsorbed onto the FeS surface via a monodentate mononuclear configuration, establishing an acidic microenvironment on the FeS surface. Such acidic microenvironment not only increased the utilization efficiency of Fe(II) toward H₂O₂ generation (i.e., <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>+</mml:mo><mml:mn>2</mml:mn><mml:msup><mml:mrow><mml:mi>H</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup><mml:mo>+</mml:mo><mml:mn>2</mml:mn><mml:mi>F</mml:mi><mml:mi>e</mml:mi><mml:mrow><mml:mo>(</mml:mo><mml:mi>I</mml:mi><mml:mi>I</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mo>→</mml:mo><mml:msub><mml:mrow><mml:mi>H</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>+</mml:mo><mml:mn>2</mml:mn><mml:mi>F</mml:mi><mml:mi>e</mml:mi><mml:mrow><mml:mo>(</mml:mo><mml:mi>I</mml:mi><mml:mi>I</mml:mi><mml:mi>I</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:math>), but also prevented the subsequent side reaction of H₂O₂ self-decomposition (i.e., <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mi>H</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>+</mml:mo><mml:mi>O</mml:mi><mml:msup><mml:mrow><mml:mi>H</mml:mi></mml:mrow><mml:mrow><mml:mo>-</mml:mo></mml:mrow></mml:msup><mml:mo>→</mml:mo><mml:mi>H</mml:mi><mml:msubsup><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow><mml:mrow><mml:mo>-</mml:mo></mml:mrow></mml:msubsup><mml:mo>+</mml:mo><mml:msub><mml:mrow><mml:mi>H</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mi>O</mml:mi></mml:math>). The results highlight the beneficial role of commonly encountered phosphate in FeS-based systems, which has profound implications for the degradation of waterborne contaminants.