Water Hyacinth (WH) is a plague that occurs frequently in rivers and estuaries along the Brazilian Coast, hampering port activities and navigation. WH may contain a significant phosphorus (P) concentration, and that makes it a potential soil P fertilizer. Our work aimed to determine chemical and molecular composition of WH biochars (WHBC) produced in different temperatures in order to identify adequate conditions for soil conditioners production. WH samples from Sergipe (BR) were air-dried and submitted to pyrolysis (5 L N2 min-1) at 400 (WHB4), 500 (WHB5) and 600°C (WHB6). Contents of C and N were determined and bulk chemical composition was investigated by FTIR, 13C and 31P NMR CP/MAS spectroscopies. Extractable P forms were quantified by exchange resin method (Pexc) and by liquid 31P NMR spectroscopy after extraction with NaOH + EDTA solution. C content ranged from 37 to 40 %, while that of N decreased steadily up to 600°C, indicating a continuous N loss with temperature. C/N ratio ranged from 13 to 18, and these values are relatively low in comparison BC¿s from other sources. With increasing temperature, proportion of O/N-alkyl C groups decreased from 15.8 to 6.4 % while that of aromatic C gradually increased reaching 66% in WHBC600. Up to 500°C, alkyl C increased from 31 to 38% and decreased to 20% at 600°C. It seems that heating up to 500°C, carbohydrates are consumed preferentially with a relative enrichment of alkyl and aromatic structures. Further temperature rise concentrates aromatic C at the expenses of alkyl C consume. Increasing pyrolysis temperature also caused a gradual change from P-mono/diester groups towards phosphate in the bulk sample and a decrease of Pexc (60 to 10 mg P kg-1). Our results show that 400ºC is the most suitable temperature to produce WHB to be used as soil P fertilizer.

Póster presentado en el 7th International Symposium on Soil Organic Matter “Soil Organic Matter (SOM) in a Stressed World” 6-11 Octubre 2019, Adelaide, South Australia