Photocatalytic Degradation of Organic Pollutants by Sulfate Radicals Activated by Defective NH₂-MIL-88B

Photocatalysis has been recognized as a viable technology for pollutant degradation in wastewater, owing to its ability to generate reactive radicals under photoirradiation. Among these, sulfate radicals <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>(</mml:mo><mml:mi>S</mml:mi><mml:msubsup><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow><mml:mrow><mml:mrow><mml:mo>·</mml:mo></mml:mrow><mml:mrow><mml:mo>-</mml:mo></mml:mrow></mml:mrow></mml:msubsup><mml:mo>)</mml:mo></mml:math> have been attracting significant attention due to their strong oxidizing properties; yet the specific mechanism of action has remained elusive thus far. In this study, defective NH₂-MIL-88B (DNMB) is prepared via a facile hydrothermal procedure in the presence of potassium sodium tartrate and found to facilitate the production of sulfate radicals from sulfate anions under visible light irradiation, due to partial reduction of Fe³⁺ to Fe²⁺ in the NMB skeleton by the added tartrate that enriches the Fe³⁺/Fe²⁺ redox couples, in addition to other reactive species like superoxide radicals and hydroxy radicals. This effectively improves the degradation efficiency toward a variety of organic pollutants, including antibiotics such as tetracycline (TC), sulfamethoxazole (SMX), and levofloxacin (LEV), as well as common organic contaminants like bisphenol A (BPA) and rhodamine B (RhB), as compared to pristine NMB. Specifically, after 40 minutes of visible light irradiation, the degradation rate increases from 61.5% to 92.1% for TC, from 76.1% to 89.4% for SMX, from 60.5% to 75.2% for LEV, from 61.7% to 91.2% for BPA, and from 78.4% to 94.8% for RhB. The primary active species are identified to be sulfate radicals, with minor contributions from holes, superoxide radicals, and hydroxyl radicals, as demonstrated in quenching experiments and electron spin resonance measurements, and further confirmed by theoretical studies. Degradation pathways for the various pollutants are then proposed based on results from Fukui index calculations and liquid chromatography-mass spectrometry analysis. These results underscore the crucial role of structural engineering in driving the advancement of green and sustainable technologies for environmental engineering.