Effects of pyrolysis temperatures on some properties of Biochar of pistachio waste

Biochar which is used as a soil amendment, is defined as a stable-carbon-rich product, and can endure in soil for thousands of years. Biochar is produced from biomass such as wood, manure, leaves, and other agricultural waste via pyrolysis, by heating at temperatures 300-1000°C in a closed container with little or no available air. Biochar, because of its potential to improve the physical and chemical properties of soil, is known as an effective soil amendment. Different types of organic waste particularly the residual of plants can be used as feedstock to produce biochar, but it's important to assess the Biochar properties to apply it as a soil amendment. In this study, we investigated some physicochemical properties of biochar of pistachio ́s waste, produced in different pyrolysis temperatures. The pistachio harvesting waste was collect from pistachio orchards in Zarand city, which mainly consisted of the green husk, pistachio cluster, leaves and small amounts of nut and woody shell and thin wood waste. A series of biochar were produced from pistachio waste by slow pyrolysis at different temperatures (300, 450, 600 and 750◦C, for 2 h) to find out the best temperature for the preparation of biochar from this matter. After preparation of biochars, their physicochemical properties including pH, electrical conductivity (EC), bulk density, particle density, biochar yield (mass of the biochar to dry mass of feedstock), ash content, water holding capacity (WHC) and stable-OC, were measured. In general, optimal biochar is the one that its yield, water holding capacity and stable organic carbon (OC) are higher and its electrical conductivity is lower. The results showed that as temperature increased from 300 to 750◦C, biochar yield and bulk density of the biochar decreased. In contrast, with increasing the temperature, pH, EC, particle density, ash content and stability of OC were increased. The electrical conductivity (EC) in the feedstock material was about 5.8 dS/m and their conversion to biochar and the increasing of pyrolysis temperature, increased the salinity of this material. The highest of EC was observed at 750◦C which was more than 2.5 and 6 times than in at temperature 300◦C and in the feedstock, respectively. The biochars produced at all temperatures have a high pH which it may be considered as an amendment for the reclamation of acidic soils, however the high salinity of the biochars could be a negative factor for plant growth. Also, as the pyrolysis temperature increased, the amount of ash in the biochar increased. The highest of ash content was observed at the highest temperature (58.3%) which was about 460% more than in the feedstock. Stable organic carbon in biochars produced at temperatures of 300, 450, 600 and 750◦C was about 49, 206, 227 and 227% higher than that of raw pistachio residue, respectively; and the percentage of yield of biochar at 300◦C was more than 65% higher than that of 750◦C. Although the WHC of biochars in different temperatures had no clear trend; it was slightly lower at a temperature of 450◦C in compared to the other temperatures. Also, in a trend, the biochars prepared at the higher temperatures showed higher stable-OC but lower yield. The temperature of the pyrolysis process is a key factor in the yield, quality, and physicochemical properties of the pistachio’s waste biochar. In the context of carbon sequestration as an environmental aspect and more yield of biochar as an economic aspect in the production of biochar and application of this matter in the soil, our results recommend the preparation of biochar from pistachio ́s waste, at temperature 450◦C or 600◦C, or a temperature in between. In the previous studies it has also been shown, the biochars produced at temperatures of 450◦C or higher was most likely to improve soil drainage and make more water available to plants but it needs more energy in the production procedure, while ones produced at lower temperatures could induce soil water repellency. Temperature of pyrolysis process is a key factor on yield, quality, and physicochemical properties of the pistachio’s waste biochar. In context of carbon sequestration as an environmental aspect and more yield of biochar as an economic aspect in production of biochar and application of this matter in soil, our results recommend the preparation of biochar from pistachio ́s waste, at temperature 450◦C or 600◦C, or a temperature in between. In the previous studies it has also been shown, the Biochars produced at temperatures of 450◦C or higher was most likely to improve soil drainage and make more water available to plants but it needs more energy in production procedure, while ones produced at lower temperatures could may induce soil water repellency.