Periodical changes of dissolved organic matter (DOM) properties induced by biochar application and its impact on downward migration of heavy metals under flood conditions

The migration of heavy metals has strong correlation with dissolved organic matters (DOM) in the soil while biochar contains abundant DOM, so this study aimed to find out how soil DOM properties changed triggered by biochar and whether it would promote the migration of heavy metals under flood conditions. In this work, a 90-day soil column incubation experiment was conducted to simulate the migration of heavy metals in actual flood conditions environment. Different depth of porewater was collected every 30 days. The sampled pore water was analyzed by spectroscopic analysis (PARAFAC, UV-VIS) to observe DOM property changes and then NICA-Donnan model was used to illustrate heavy metals complexation with soil DOM. Results showed that DOM release in biochar-added soil was 57.5% higher than that in biochar-free soil after 90 days and the DOM characteristics were different from those in biochar-free soil. It was also found that DOM continuously released in biochar-added soil was easily complexed with metals. According to two first-order reactions (TFOR), DOM fraction of Ni in biochar addition treatment was 33.3% higher compared to the biochar free treatment after 90 days, and therefore resulted in more significant downward migration trend of Ni. This is also supported by the fact that Ni concentration in the subsoil of biochar addition treatment increased nearly 9 fold after 60 days. Moreover, labile Cd and Ni fraction increased in biochar-added soil, from 16.2% to 28.7% and from 22.2% to 57.2%, respectively. These results suggest a possible risk in wetlands soil and for groundwater pollution by biochar under flood conditions.