Unveiling the impact of initial pH on humic acid degradation using ultraviolet light-activated peroxydisulfate in landfill leachate treatment

The poor biodegradability of humic acid (HA) in landfill leachate has posed significant challenges for effective leachate treatment. The ultraviolet light-activated peroxydisulfate (UV/PDS) system has shown potential in removing HA, with its performance being highly dependent on the initial pH values. However, the mechanisms by which initial pH influences HA degradation within UV/PDS systems remain unclear. This study investigated the effects and underlying mechanisms of different initial pH values (2, 4, 7, 9, 12) on HA degradation using the UV/PDS system. Results indicated that initial pH values of 4 and 12 were more conducive to the degradation of organic pollutants in HA solutions. In contrast, initial pH 4 was more favorable for organic matter degradation in leachate compared to initial pH 12. The removal rates of UV 254 and COD at initial pH 4 were 87.7 % and 90.0 %, respectively, while at initial pH 12, they were 69.2 % and 86.8 %. Both SO 4 -• and •OH were generated during the reaction, with SO 4 -• playing a predominant role in HA degradation. Moreover, the effect of initial pH on HA degradation was multifaceted. In early stages, it regulated the types of free radicals and reaction selectivity, thereby determining the formation of intermediate products. At pH 4 and 9, intermediate products were predominantly long-chain alkanes, whereas those at pH 12 were primarily amides and acid anhydrides. In later stages, dynamic pH changes led to the generation of stable small molecule carboxylic acids. This study provides insights into the application of UV/PDS systems for the HA treatment.