Insight into the effect of pyrolysis temperature on photoreactivity of biochar-derived dissolved organic matter: Impacts of aromaticity and carbonyl groups.

Practical application of biochar may result in more biochar-derived dissolved organic matter (denoted as BDOM) inevitably release into surface waters by infiltration and surface runoff. The photochemical reaction of BDOM has gained intense attention, which played a key role in the fate of organic contaminants. However, the relationships between specific characteristics of BDOM and its photoreactivity are still uncertain. In this study, the characteristics of BDOM pyrolyzed from rice husk derived biochar at different temperature (from 400 °C to 700 °C) and their effect on the photodegradation of oxytetracycline (OTC) were carefully investigated. The C NMR and EEM results indicated the dominated component of BDOM was gradually turned from humic acid like substances with low aromaticity to high aromaticity with abundant oxygen-containing functional groups as pyrolytic temperature increases. Experimental results showed that the apparent rate constants (k) of BDOM700 (4.53 × 10 min) for OTC photodegradation was approximately one order of magnitude higher than BDOM400 (4.52 × 10 min), which was closely correlated with their aromaticity (R = 0.944). It was found that BDOM* rather than O played the major role in BDOM mediated photodegradation of OTC (80.13 % vs 14.34 %), and the carbonyl-containing group was identified as the main BDOM* precursor by NaBH reduction experiment. This work highlighted both aromaticity and carbonyl group contents were critical indicators for assessing the potential to generate BDOM* and corresponding photoreactivity.