Cleaning contaminated water often involves powerful oxidation processes, like Fenton-like systems using persulfate ) activated by iron ions. These systems generate highly reactive species that can break down stubborn organic pollutants. However, the efficiency can be limited because the activating iron (II) quickly turns into less reactive iron (III). Adding redox-active biochar can help by reducing Fe(III) back to Fe(II), keeping the reaction going. But biochars vary widely depending on what they are made from (feedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More) and how they are made (pyrolysis conditionsThe conditions under which pyrolysis takes place, such as temperature, heating rate, and residence time, can significantly affect the properties of the biochar produced.   More). Identifying the best biochar for this job is still an open question. Researchers Yiling Zhuang, Stefan B. Haderlein, and colleagues investigated this by testing different biochars’ ability to activate persulfate with Fe(III) to degrade the common insect repellent DEET. Their study was published in Separation and Purification Technology.

The team systematically compared eight different biochars. Four were made from beech wood pyrolyzed at different temperatures (450°C, 600°C, 750°C) and under different nitrogen gas flow rates (low vs. high flow at 450°C). The other four were made from softwood sawdust pyrolyzed at 500°C, but with varying amounts of wood ash (0%, 9%, 16%, or 43% by weight) mixed in before 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. They then tested how quickly each biochar, in combination with Fe(III) and persulfate, could break down DEET in water at a controlled pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More of 2.5. They also characterized the biochars extensively to understand which properties were responsible for better performance, looking at things like electron donating capacity (EDC), surface functional groups (via FTIR), persistent free radicals (PFRs, via ESR), electrical conductivity, and mineral content (via XRD) .

The results showed a clear impact of pyrolysis temperature. Beech wood biochar produced at the lowest temperature (BC450, 450°C) was the most effective, transforming 88% of the DEET within two hours, with a half-life of just 39 minutes. Increasing the pyrolysis temperature drastically reduced efficiency; BC600 (600°C) resulted in a much slower half-life of 227 minutes, and BC750 (750°C) showed no DEET transformation at all. Interestingly, while the high-temperature BC750 had the highest overall electron donating capacity (EDC) and electrical conductivity, it failed to activate the system effectively . The superior performance of the low-temperature BC450 was attributed to its higher concentration of specific redox-active moieties: surface oxygen functional groups (like phenolic and carboxyl groups, detected by FTIR) and persistent free radicals (PFRs) . These features appear crucial for facilitating the Fe(III)/Fe(II) cycling needed to activate persulfate, correlating well with the observed DEET transformation rates. The high nitrogen flow rate during pyrolysis (BC450HF vs BC450) had only a minor effect on performance.

Amending the softwood feedstock with wood ash before pyrolysis dramatically improved performance. All ash-amended biochars outperformed the non-amended control (BC-ash0). The biochar with 16 wt% ash amendment (BC-ash16) showed the best results, achieving a DEET half-life of just 27 minutes—10 times faster than the non-ash amended biochar (BC-ash0) produced under the same conditions. This enhanced performance correlated strongly with increased EDC and a greater ability to reduce Fe(III) to Fe(II)^.. Characterization revealed that ash amendment led to the formation of crystalline iron minerals like hematite and magnetite within the biochar structure. These minerals, especially the mixed-valent magnetite, likely provide additional redox-active sites that boost Fe(III) reduction. Furthermore, the type of PFRs shifted slightly in ash-amended biochars towards potentially more reactive semiquinone radicals. The top performance of BC-ash16 is hypothesized to result from an optimal combination of these newly formed iron minerals, abundant PFRs, and remaining oxygen functional groups, creating a highly effective catalyst for the Fe(III)/persulfate system.

This study highlights that simply maximizing bulk properties like EDC or conductivity isn’t enough; the type and abundance of specific redox-active moieties, heavily influenced by pyrolysis conditions and feedstock amendments, are key. Low pyrolysis temperatures favor the retention of crucial oxygen functional groups and PFRs. Furthermore, adding wood ash before pyrolysis is a highly effective strategy to create biochars with enhanced redox properties, likely due to the in-situ formation of reactive iron minerals. These findings provide valuable insights for producing “designer” biochars with tailored properties optimized for enhancing Fenton-like water treatment processes.

Source: Zhuang, Y., Haderlein, S. B., Hagemann, N., Grafmüller, J., Gogler, K., Paul, A., Fink, F., & Spahr, S. (2025). Activation of persulfate by biochar and iron: Role of biochar pyrolysis conditions and ash amendments. Separation and Purification Technology, 374, 133634. https://doi.org/10.1016/j.seppur.2025.133634