Potassium-iron rice straw biochar composite for sorption of nitrate, phosphate, and ammonium ions in soil for timely and controlled release.

In this study, potassium-iron rice straw biochar composite (KRSB) was produced and compared with rice straw biochar (RSB) for the sorption of NO3-, PO43-, and NH4+ in aqueous medium and soil column. RSB was produced by pyrolyzing rice straw at 400 and 600 °C in a slow pyrolysis unit. KRSB was produced through chemical and hydrothermal treatments of rice straw biochar produced at 400 and 600 °C. Batch experiment results indicate that the KRSB showed better sorption capacity for nitrate, phosphate, and ammonium ions compared to pristine RSB. The sorption isotherms of all three nutrients (NO3-, PO43-, and NH4+) were better-explained by the Langmuir-Freundlich isotherm model. The column leaching experiment showed that the KRSB loaded soil reached maximum sorption capacity for PO43- and NO3- within six and eight days, respectively but, it showed poor sorption capacity for NH4+. The soil fertility index in the 400 and 600 KRSB amended soils were significantly increased by 50.68 and 52.85%, respectively compared to the control. Results indicated that KRSB could be utilized in the soil in two ways: first, to keep the nutrients attached to its surface and second, to release the nutrients in a phased and timely manner to increase their availability for plants.