Effect of adsorption on ferrihydrite on the photoreactivity of dissolved black carbon for photodegradation of sulfadiazine

The selective adsorption of dissolved black carbon (DBC) on inorganic minerals is a widespread geochemical process in the natural environment, which could change the chemical and optical properties of DBC. However, it remains unclear how selective adsorption affects the photoreactivity of DBC for photodegradation of organic pollutants. This paper was the first to investigate the effect of DBC adsorption on ferrihydrite at different Fe/C molar ratios (Fe/C molar ratios of 0, 7.50 and 11.25, and marked as DBC0, DBC7.50 and DBC11.25) on the photoproduction of reactive intermediates generated from DBC and their interaction with sulfadiazine (SD). Results showed that UV absorbance, aromaticity, molecular weight and contents of phenolic antioxidants of DBC were significantly decreased after adsorption on ferrihydrite, and higher decrease was observed at higher Fe/C ratio. Photodegradation kinetics experiments showed that observed photodegradation rate constant of SD (kobs) increased from 3.99 × 10−5 s−1 in DBC0 to 5.69 × 10−5 s−1 in DBC7.50 while decreased to 3.44 × 10−5 s−1 in DBC11.25, in which 3DBC* played an important role and 1O2 played a minor role, while ·OH was not involved in the reaction. Meanwhile, the second-order reaction rate constant between 3DBC* and SD (kSD, 3DBC*) increased from 0.84 × 108 M−1 s−1 for DBC0 to 2.53 × 108 M−1 s−1 for DBC7.50 while decreased to 0.90 × 108 M−1 s−1 for DBC11.25. The above results might be mainly attributed to the fact that the decrease of phenolic antioxidants in DBC weakened the back-reduction of 3DBC* and reactive intermediates of SD as the Fe/C ratio increased, while the decrease of quinones and ketones reduced the photoproduction of 3DBC*. The research revealed adsorption on ferrihydrite affected the photodegradation of SD by changing the reactivity of 3DBC*, which was helpful to understand the dynamic roles of DBC in the photodegradation of organic pollutants.