Biochar Produced from Corn Stover Affects Water Retention and Mechanical Properties of Soil

Biochar can be mixed with soil to increase the water absorption capacity and decrease the evaporation rate, which could in turn increase crop yields particularly under prolonged drought conditions. In this project, the biochar was produced by pyrolyzing a sample of corn stover, a widely available resource in the U.S. The torrefaction procedure applied consisted of heating at 350 ºC for 130 minutes in a nitrogen atmosphere. At the end of the pyrolysis, the biochar mass produced was 34% of the initial stover mass. The absorption and emission spectra corresponding to the corn stover pyrolysis product collected from the long quartz tubing by washing it using 95% ethanol indicates that a variety of (aromatic, potentially useful) compounds has been produced.

To assess the hygroscopicity, biochar sample was left to equilibrate in water for 2 weeks; the amounts of water absorbed was determined gravimetrically to be 72%. A TGA experiment was carried out at heating rates of 5, 10, 15, 20, and 25 ºC/min on the sample hydrated for two weeks. The activation energy for the desorption of water was determined to be 54 kJ/mol using the Friedman method and 52 kJ/mol by using the Kissinger-Akahira-Sunose method. Another TGA experiment was performed to assess the effect of biochar on the dehydration rate at the temperatures relevant for agriculture. For example, at 35 ºC, the presence of biochar results in more than tenfold decrease in the dehydration rate in comparison to that of free/bulk water. To further assess the potential use of biochar in agriculture, the dehydration rate of soil was compared to that of the soil and biochar mixture using a TGA technique. Due to a small sample size and variation in the soil particle size the data variance was very large. To overcome this problem, the hydrated 50 weighed percent biochar/soil mixture was placed in a cylindrical vessel with an open top and its mass is being measured as a function of time in order to obtain the dehydration rate. The results to be obtained will be compared to that corresponding to the hydrated soil. During the course of the experiment it was noted that the addition of biochar improves the mechanical properties of hydrated soil and the quantification of the effect observed is in progress.