Phosphorus recovery from biogas slurry by ultrasound/H2O2 digestion coupled with HFO/biochar adsorption process

Phosphorus (P) recovery from biogas slurry has recently attracted considerable interest. In this work, ultrasound/H2O2 digestion coupled with ferric oxide hydrate/biochar (HFO/biochar) adsorption process was performed to promote P dissolution, release, and recovery from biogas slurry. The results showed that the optimal total phosphorus release efficiency was achieved at an inorganic phosphorus/total phosphorus ratio of 95.0% at pH 4, 1 mL of added H2O2, and ultrasonication for 30 min. The P adsorption by the HFO/biochar followed pseudo second-order kinetics and was mainly controlled by chemical processes. The Langmuir-Freundlich model matched the experimental data best for P adsorption by HFO/biochar at 298 and 308 K, whereas the Freundlich model matched best at 318 K. The maximum amount of P adsorbed was 220 mg/g. The process was endothermic, spontaneous, and showed an increase in disorder at the solid-liquid interface. The saturated adsorbed HFO/biochar continually releases P and is most suitable for use in an alkaline environment. The amount of P released reached 29.1 mg/g after five extractions. P mass balance calculation revealed that 11.3% of the total P can be made available.