Sorption-desorption of some transition metals, boron and sulphur in a multi-ionic system onto phyto-biochars prepared at two pyrolysis temperatures.

The sorption-desorption of transition metals, B and S onto phyto-biochars prepared from lantana, pine needles and wheat straw by pyrolysis at 300 °C and 450 °C were studied using the batch method. Their sorption-desorption onto phyto-biochars conformed to Freundlich isotherms. Phyto-biochars pyrolyzed at 450 °C had higher sorption capacity for transition metals (Zn, Cu, Fe, and Mn) but lower sorption capacity for S as compared to those pyrolyzed at 300 °C. The desorption capacity of phyto-biochars pyrolyzed at 450 °C for transition metals, B and S was also higher than that of phyto-biochars pyrolyzed at 300 °C except for S in pine needle biochar. Percent desorption of all transition metals, B and S was lower for phyto-biochars pyrolyzed at 450 °C compared to those pyrolyzed at 300 °C; however, an opposite trend was noted for Mn and S in the case of pine needle and wheat biochars, respectively. Simple correlation analysis of Freundlich model constants, desorption index and percent desorption values of transition metals, B and S with the properties of phyto-biochars and changes in Fourier transform infra-red spectra after sorption revealed that several conjunctive mechanisms such as cation exchange, complexation and co-precipitation for the sorption of transition metals, H-bonding/ligand exchange for B and H-bonding/cation bridging for S were operative in phyto-biochars. Phyto-biochars produced from plant biomass wastes by pyrolysis at 300 °C, which have been enriched with Zn, Cu, Fe, Mn, B and S may serve as a potential slow-release nutrient carrier in agriculture.