Characterization of Biochar Prepared from Three Different Feed Stocks

Abstract

Biochar-based soil management strategies in Ghana are new and are now being evaluated in the context of the country’s agricultural system. Biochar produced from organic materials such as saw dust, rice husk and saw dust are being used in Ghana. These different feedstocks may have different physico-chemical properties which will influence the quality of the biochar produced when the feedstocks are carbonized and in turn govern their suitability for use in agriculture. A detailed characterization of biochar produced from rice straw, rice husk and saw dust was carried out in a bid to document the basic features of the products to ensure their safety and suitability for use as soil amendments. Rice husk and straw from the same plant material and saw dust from a saw mill were pyrolysed at 350 o C in a kiln at the Soil Research Institute of the Council of Science and Industrial Research, Kwadaso, Kumasi. These samples were air dried and parameters such as particle size distribution, bulk density, available water, electrical conductivity, pH in water and KCl, total oxidizable organic carbon, total nitrogen, total phosphorus, available phosphorus, and exchangeable basic cations and total elemental analysis were carried out. Over 70% of the rice straw and the rice husk biochar types were in the very small size fraction of between 63 μm and 250 μm. The saw dust biochar, however, had 63.7% of its size fraction in the coarse size regime of between 500 μm and 2500 μm. The bulk density values of the three biochar types were very low ranging between 0.19 Mg/m 3 and 0.23 Mg/m 3 . Moisture content at field capacity was in the order of saw dust (9.55%) > rice straw (8.92%) > rice husk (7.70%). The rice straw biochar had the highest available moisture content of 3.39% which was almost 1.7 times higher than that of the saw dust biochar type. The rice straw had the highest pH of 10.5 as a result of its very high contents total Ca (10.44 mg/kg) and exchangeable Ca (7.63 cmol/kg) in addition to a high Si concentration of 170.8 mg/kg. The rice straw biochar also had the highest EC of 3.57 dS/m due to its high exchangeable Na concentration. The total oxidisable organic carbon of 191.97 g/kg was highest in the rice straw biochar with the rice husk sequestering 136.23 g/kg and saw dust 115.8 g/kg. The saw dust biochar type was the lowest in N content. On account of the C:N ratios of 166.9 for the rice straw, 142 for the rice husk biochar and 156 for the saw dust biochar, the rice straw biochar type would be the most stable and hence sequester more carbon. Available P was very high in the rice straw as a result of the high Si content of the material and was found to be 2.5 and almost 3.8 times more than the total P in the rice husk and saw dust biochar types, respectively. The concentration of heavy metals Cu, Zn and more importantly Co and Pb were very low in all the three samples with concentrations below 0.5 mg/kg due to the near neutral to strongly alkaline pH regime of all the three biochar types. These very low levels of the heavy metals make the three biochar materials very safe for use as soil amendments without any toxicity hazards. The study has identified at a charring temperature of 350 o C, the rice straw biochar has high concentrations of Ca (10.44 mg/kg,) Mg (1.61mg/kg) and Si (170.8 mg/kg) and a high pH of 10.5 in both water and KCl and therefore has the potential of being used as an agricultural liming material.