Abstract
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 –PCH3 (sp2 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.
Highlights
Rice husk is an agricultural waste which accounts for 20% of the grain weight (Takita, 1987)
With the increase in pyrolysis temperature, the I2 number was increased by 270% suggesting a large improvement in the micropore structure and surface area of the Rice husk biochar (RHB)
Spectroscopic analysis confirmed that the impact on aromatisation, drop in certain aliphatic and –CO2 compounds and enhanced microporosity with increasing pyrolysis temperatures
Summary
Rice husk is an agricultural waste which accounts for 20% of the grain weight (Takita, 1987). Its inherent physical and chemical characteristics depend on the quality of the raw material used and pyrolysis environment (Lehmann, 2007; Enders et al, 2012). Rice husk biochar (RHB) is a material which inherently composed of a poor porous structure due to presence of high amount of silica. Previous studies have tried to improve the porous structure by chemical means and applying high temperatures, around 600 °C during pyrolysis (Oh and Park, 2002; Ahiduzzaman and Islam, 2016). Only a few studies have researched the changes in structural and chemical properties of rice husk biochar co-pyrolysed with mineral matter
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