Urban stormwater, a potential freshwater source, often carries persistent and mobile trace organic contaminants (TrOCs) that traditional treatment methods struggle to remove. A recent study published in Water Research by Yiling Zhuang, Stefan B. Haderlein, Holger V. Lutze, Chen Sun, Friedrich Fink, Andrea Paul, and Stephanie Spahr explores a novel approach: activating persulfate with biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More to break down these stubborn pollutants.

The research highlights that while biochar is an effective adsorbent and persulfate is a chemical oxidant, neither is particularly efficient on its own for widespread TrOC removal from stormwater. However, when combined, the biochar and persulfate system proved significantly more effective. The study found that 1,3-diphenylguanidine (DPG) and 2-hydroxybenzothiazole (OHBT) were transformed by over 90%, and 1H-benzotriazole (BTA), 5-methyl-benzotriazole (MBTA), and diuron saw 30-40% transformation when treated with biochar and peroxydisulfate (PDS). DPG, in particular, was efficiently transformed with a half-life of 9.0 ± 0.1 minutes, followed by OHBT at 18 ± 1.3 minutes.

One of the key findings was the minimal impact of the water matrix on treatment performance. Experiments using street runoff and synthetic water, containing up to 7.5 mg/L dissolved organic carbon (DOC) and 100 mM chloride, showed little effect on the formation of reactive species or the transformation of contaminants. This is a crucial advantage, as stormwater quality can fluctuate greatly depending on land use and season. The ability of the biochar-persulfate system to maintain effectiveness despite varying chloride concentrations, often high due to de-icing activities, is particularly beneficial for real-world applications.

The study delved into identifying the reactive species responsible for this contaminant breakdown. Through a series of scavenger and probe compound experiments, the researchers provided strong evidence that singlet oxygen (1O2​) is the predominant reactive species in the biochar/PDS system. This is significant because, unlike highly reactive and less selective radical species like sulfate radicals (SO4⋅−​) and hydroxyl radicals (⋅OH), singlet oxygen is known to selectively target electron-rich organic compounds. The measured steady-state concentrations of 1O2​ in the experiments were remarkably high—1.38×10−8 M in pure water and 4.73×10−9 M in street runoff—which are four to five orders of magnitude greater than those typically observed in photooxidation processes. These elevated concentrations explain the effective degradation of compounds previously considered recalcitrant to 1O2​ attack.

The mechanism for 1O2​ formation is attributed to the reaction between persulfate and ketonic groups on the biochar surface. The shrimp shell biochar used in this study, produced through pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More, showed the presence of these crucial C=O bonds. This non-radical, singlet oxygen-dominated pathway offers practical advantages for stormwater treatment due to its consistent performance across different water matrices.

This research lays the groundwork for future investigations into real-world applications. Further studies will need to evaluate different biochar types, assess the formation of undesired transformation products, and conduct laboratory-scale column studies to simulate flow-through biofilter systems. Optimizing parameters like persulfate dosing, contact time, and managing sulfate emissions are also crucial steps toward implementing this promising technology for enhanced stormwater treatment at contamination hotspots.

Source: Zhuang, Y., Haderlein, S. B., Lutze, H. V., Sun, C., Fink, F., Paul, A., & Spahr, S. (2025). Persulfate activation by biochar for trace organic contaminant removal from urban stormwater. Water Research.