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

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 degrees 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.