Oxalic Acid Boosts Phosphorus Release from Sewage Sludge Biochar: A Key Mechanism for Biochar-Based Fertilizers

• Published in: Agriculture (Basel) Vol. 14; no. 9; p. 1607
• Main Authors: Santos, Marcela Granato Barbosa dos, Costa, Camila Rodrigues, Mendes, Gilberto de Oliveira, Blasi Paiva, Andressa, Peixoto, Ludmila Soares, Costa, Jéssica da Luz, Marchi, Giuliano, Ma, Figueiredo, Cícero Célio de
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 01-09-2024
• Subjects: Acidic soils; Apatite; biosolids; Charcoal; Chemical compounds; Fertilizers; Heavy metals; Minerals; Organic acids; Organic soils; Oxalates; Oxalic acid; Pathogens; Phosphorus; Pollutants; Product introduction; Purification; pyrolysis; Sewage; Sewage sludge; Sludge; Soil treatment; Solubilization; Speciation; Temperature; thermal treatment; Brazil
• ISSN: 2077-0472; 2077-0472
• DOI: 10.3390/agriculture14091607

Sewage sludge biochar (SSB) exhibits higher phosphorus (P) concentrations than the original sewage sludge (SS) and can be used as a P fertilizer. However, SSB-associated P is strongly retained in chemical compounds, which hinders its release and subsequent plant uptake. The use of organic acids facilitates P solubilization from SSB. Herein, we evaluated the effect of oxalic acid on P release from SSB applied to soil over time. Biochar was produced at 300 °C (SSB300) and 500 °C (SSB500). P release from SSB increased with an increasing concentration of oxalic acid in the SSB incubation solution and in SSB-treated soil. P speciation in SSB showed that P was predominantly inorganic (P[sub.i]), which represented 81% and 92% of the total P in SSB300 and SSB500, respectively. P[sub.i] in SSB consisted mainly of non-apatite P, accounting for 91% and 96% of all P[sub.i] in SSB300 and SSB500, respectively. Because SSB is predominantly insoluble in water, oxalic acid is crucial for the release of P from SSB. Oxalic acid increased P release from SSB300 and SSB500 by 103- and 600-fold, respectively, compared to the control, from which P was extracted with water. Oxalic acid enhancement of P release from SSB increases the possibility of using SSB as a sustainable source of P for agriculture.