Changes in structural and chemical properties of rice husk biochar co-pyrolysed with Eppawala rock phosphate under different temperatures

Although properties of rice husk biochar (RHB) can be changed by altering pyrolysis temperature, they still remain as a poor-qualitysoil amendment. We hypothesised that addition of phosphate rock powder during the pyrolysis process will enhance structural and chemical properties of RHB. This experiment was conducted to determine the effect of temperature on properties of RHB co-pyrolysed with Eppawala Rock Phosphate (ERP). Rice husk was slow-pyrolysed at 350°C, 500°C and 650°C, with and without 4.2% ERP (w/w), and produced six different types of RHB. Scanning electron micrographs showed an improved pore structure in RHB with increasing pyrolysis temperature, however co-pyrolysis with ERP did not show any further improvements. Raman spectroscopy provided evidence for increased aromatisation with the increase in temperature and formation of –PCH 3 (sp 2 bond) in RHB co-pyrolysed with ERP at 350°C. Iodine number in RHB increased by 140 mg/g when the temperature increased from 350°C to 650°C, however it has increased more in the presence of ERP at 350°C. The pH, EC, and available nutrients (P, K and Mg) in RHB increased with increasing temperature. Addition of ERP increased available P by 3-fold at all temperatures and K by 13% in RHB under 500°C. We conclude that chemical properties of RHB can be improved by co-pyrolysing rice husk with ERP. When all characteristics were assessed together, biochar pyrolysed at 500°C appears to be the most suitable soil amendment to improve soil fertility among conditions tested.