Pyrolysis of waste biomass is a promising technology to produce sterile and renewable organic phosphorus fertilizers. Systematic studies are necessary to understand how different pyrolysis platforms influence the chemical speciation of dissolved (bioavailable) phosphorus. This study employed solution-phase 31P NMR analyses on slow pyrolysis, fast pyrolysis, and flash carbonization charcoals. Dissolved P speciation of ash-rich (15-62 wt %) biochars produced from manures, sewage sludge, and corn stover were compared with low ash (2-5 wt %) pecan shell biochars. Each biochar was sequentially extracted to investigate the strongly complexed (by NaOH-EDTA; 250 mM NaOH+5 mM EDTA for 16 h) and acid-extractable (by acidic oxalate; 200 mM oxalate at pH 3.5 for 4 h) P fractions. In NaOH-EDTA extracts, P concentration correlated (p < 0.0005) with Zn (r = 0.89), Mn (r = 0.90), and Mg (r = 0.98) concentrations. A strong correlation between orthophosphate and Mg (r = 0.98, p < 0.0005; n = 13) indicated the presence of Mg orthophosphate (and struvite or whitlockite) in all biochars. Only in acidic oxalate extracts, P concentration correlated (p < 0.0005) with Al (r = 0.87) and Fe (r = 0.92) concentrations. Pyrophosphate (P2O74-) persisted (23-52% of total P in NaOH-EDTA extracts) in low-ash pecan shell 300-700°C slow pyrolysis biochars. In contrast, ash-rich biochars were primarily (≥90%) composed of inorganic orthophosphate (PO43-), except 350°C slow pyrolysis swine manure biochar (26% pyrophosphate) and sewage sludge-derived flash carbonization charcoal (14% pyrophosphate). Solid-state 13C cross-polarization and magic angle spinning NMR analyses of bulk aromaticity indicated partially carbonized (aliphatic) nature of 350°C swine manure biochar. Surface functional groups of swine manure and sewage sludge biochars could stabilize pyrophosphate by (i) utilizing bridging cations (Al3+, Fe3+, and Mg2+) to form stable six-membered ring complexes, and (ii) direct hydrogen bonding. (Chemical Equation Presented).
All Science Journal Classification (ASJC) codes
- Environmental Chemistry
- Chemical Engineering(all)
- Renewable Energy, Sustainability and the Environment