Organic matter conversion and contributors to bioavailability in biogas slurry treated by biochar/persulfate during the oxidation pond process

Abstract

Bioavailability is the major basis to judge whether biogas slurry should be used in field application. Methods such as treatment using a biochar/persulfate oxidation pond have proven to improve bioavailability. However, the effectiveness of biochar/persulfate oxidation on dissolved organic matters (DOM) and effect of DOM on the bioavailability of biogas slurry are poorly understood. In this study, biogas slurry was firstly treated with biochar and persulfate for 24 h, then treated using the oxidation pond process. Results from this study suggested that biochar/persulfate dominates the transformation from particulate organic matter to dissolved organic matter (DOM). Persulfate works preferentially to mineralize small molecular weights of DOM and higher persulfate concentrations might lead to over-mineralization of DOM. During the oxidation pond process, hydrophobic substances were converted to hydrophilic substances, and biochar and persulfate significantly accelerated this conversion. Analysis of the kinetics of the fluorescence component showed that biochar and persulfate dominated the oxidation of organic matter more than microbial activities in biogas slurry. Hydrophilic substances and humic substances were major contributors to bioavailability, while hydrophobic microbial metabolites were the major restrictors. The improvement to bioavailability by biochar and persulfate could be due to chemical oxidation and bacteriostasis. Findings from this study revealed the transformation path of organic matter in biogas slurry treated in an oxidation pond and how this process is promoted with the addition of biochar/persulfate, a practical technology for circular agriculture.