Coupling Anaerobic Digestion with Pyrolysis for Phosphorus-Enriched Biochar Production from Constructed Wetland Biomass

Abstract

Coupling anaerobic digestion with pyrolysis (AD–pyrolysis) has drawn increasing attention for the treatment of organic solid wastes. To evaluate the feasibility of AD–pyrolysis to reclaim phosphorus (P) from wetland plants, pyrolysis and AD–pyrolysis were used to dispose of wetland plants at different temperatures (300, 500, and 700 °C). The biochars derived from pyrolysis (TBs) showed a high pH and abundant functional groups, while the biochars derived from AD–pyrolysis (ATBs) contained high P (19.52–27.53 mg g–1) and ash (35.17–49.88%) contents. Both pyrolysis and AD–pyrolysis effectively reduced the risk of P leaching loss by transforming P into a stable pool. In comparison with pyrolysis, AD–pyrolysis enhanced the formation of the medium-term potential plant-available P (NaOH-P) in biochars (21.50–71.69%) and reduced the residual-P content. The diffusive gradients in thin films (DGT) results confirmed that the addition of AD–pyrolysis-derived biochars greatly increased the available P content in the soil, and DGT-P was positively correlated with the NaOH-P content (r = 0.919, P < 0.01) and the total contents of NaOH-P and NaHCO3-P (r = 0.937, P < 0.01). Therefore, AD–pyrolysis is a promising treatment technique to achieve the upgraded recovery of P in biochar from wetland plants.