Material Characterization of Cotton Gin Waste Biochar for Use in Panhandle Soils

Cotton gin waste (CGW) is a large quantity byproduct from cotton fiber and cotton seed oil production in the Texas Panhandle (700 lbmass/bale, 1.7 million tonnes/year of CGW) and many large cotton growing regions globally. As there is little economic value for CGW, it is can be churned into soils to increase organic carbon, composted, fed as a low nutrition supplement for animals, burned or gasified for energy/heat, or simply landfilled as a waste. In general, most “beneficial†uses are more properly described a more elaborate form of disposal. We sought to examine CGW for biochar production (CGW-BC) on a small scale with an eventual application for soil amendment. As a soil amendment, many plant-based biochars have the potential advantage of acting as slow release fertilizers, aiding soil health, increasing soil water holding capacity (WHC), increasing long-term soil organic matter, and sequestering carbon which would other be mineralized to CO2 in a short timeframe. Biochar is highly variable in its production yield and quality according to production and post-production decisions. Using previous experience with cotton seed waste biochar, we determined to produce sixteen (16) CGW-BC variants according to the two temperatures (450°C, 600°C), four pyrolysis times (10, 20, 40, 60 min), and two types of post-treatment (crush-sieve with mild acid wash, crush-sieve with DI wash only, 2 x 4 x 2 = 16). We made all CGW-BC in small batches of approximately 15 g dry initial dry mass which results in nominal final mass of 5 g dry mass biochar. We then examined these biochar variants using the material characterization techniques that will have import for CGW-BC use in soil—XRD, SEM-EDS, TGA and surface characterization by a Micromeritics 3Flex physisorption analyzer. The use of XRD reveals the amorphous nature of biochar, and the SEM-EDS reveals surface morphology and the predominant presence of carbon (>75%) and TGA data demonstrates the thermal stability of biochar. The use of the 3Flex allows us to use multiple adsorptive gases. We used CO2 at a range of low relative pressure (P/Po = 0.00-0.30) and cold temperature (T = 0°C) to determine total and pore-size dependent surface area (m2/g biochar) and volumes (cm3/g biochar) in biochar at the micropore scale of 3.30-7.70 Å. We also used water vapor isotherms (adsorption-desorption) to examine the potential for attraction and retention of water when CGW-BC is deployed in soil environments. The results of this work are on-going. The current range of total micropore surface areas found are on the order of 200-400 m2/g, a relatively high surface area considering the modest amount of energy and materially used to create the biochar. On-going work will suggest the general performance of biochar when added to soils and will provide more optimal conditions for producing biochar according to that which has the lowest bulk density, greater surface area, increased microporosity, or enhanced mineral/nutrient solid phase concentrations. This early work in CGW-BC material characterization will provide promising candidates for inclusion in soil+biochar mix and incubation experiments in root and non-root systems.