Title: Groundwater pollution potential and greenhouse gas emission from soils amended with different swine biochars Author
Submitted to: World Congress of Soil Science
Publication Type: Abstract Only
Publication Acceptance Date: February 13, 2014
Publication Date: N/A
Technical Abstract: Although there exist numerous research studies in the literature on greenhouse gas emission and groundwater pollution potentials of soils amended with plant-based biochar made from traditional dry pyrolysis (hereafter referred as pyrochar), a very few such studies exist for hydrochar made from hydrous pyrolysis or hydrothermal carbonization (HTC). HTC decomposes the organic matter in the biomass by heating anaerobically in the presence of subcritical, liquid water. HTC can convert wet feedstock biomass into carbonaceous solids at relatively high yields without the need for an energy-intensive drying before or during the process. Potential HTC feedstocks include wet animal manures, sewage sludges, and municipal solid waste streams, as well as aquaculture residues. While these feedstocks represent large, renewable residual streams that require some degree of treatment to protect environment, HTC may be used to eliminate or at least reduce the burden of the treatment while producing value-added hydrochar. The objectives of the present paper were to 1) compare physicochemical characteristics of swine manure-based hydrochar and pyrochar and 2) investigate carbon dioxide (CO2) and nitrous oxide (N2O) emissions and nutrients leaching from soils amended with these biochars. Dewatered swine solids were obtained from a 5,600-head finishing swine operation in Sampson County, North Carolina, which were further dried and stored in a refrigerator until experiments. Swine pyrochar was prepared using a skid-mounted pyrolysis system which heated the dried swine solids to 620 degree Celsius (oC) for two hours. Swine hydrochar was prepared by hydrothermally carbonizing the swine solids at 250 oC for 20 hours. Some of the hydrochars were mixed with 200 milliliter acetone and agitated for 2 hours in order to remove labile compounds adsorbed on the hydrochar surface. Two soil pot incubation experiments were conducted approximately 4 months apart. Hydrochar or pyrochar was mixed with a 50/50 mixture of Norfolk Ap and E horizon at a rate of 20 gram per kilogram (g kg-1). Sufficient deionized water (H20) was added periodically so that each pot would be maintained at 10 % moisture content. Triplicate pots containing soil without biochar served as controls. During the incubation periods of 42 to 127 days, greenhouse gas (CO2 and N2O) emission fluxes were measured by nonlinearly regressing time-series headspace gas concentrations. Volatile matter (VM) of the raw swine solids (69.5%) decreased to 60.3% (hydrochar) and 14.1% (pyrochar). When the hydrochar was washed with acetone, its VM further decreased from 60.3% to 49.9%. The opposite trends were observed for fixed carbon and ash contents. When the swine solids were pyrolyzed at 620 oC, the oxygen content was decreased to negligible amounts. Mehlich 1 extraction of the initial soils amended with both pyrochar and hydrochar showed significant increase in nutrients such as Potassium (K), Phosphorous (P), Calcium (Ca), Magnesium (Mg), Zinc (Zn) and Manganese (Mn). As a result, the soils amended with pyrochar leached significantly higher amounts of nutrients than control soils, suggesting potential groundwater pollution from the pyrochar-amended soils. In contrast, the hyrochar-amended soils leached very little of these nutrients probably due to high surface functionalities binding these nutrients. Throughout the incubation period, the control soils produced 0.16 g CO2 meter square per day (m-2d-1). The hydrochar-amended soil, however, emitted 3.0 g CO2 m-2d-1, much higher than 0.32 g CO2 m-2d-1 from the pyrochar-amended soils. The control soils produced 0.31 mg N2O m-2d-1, while the pyrochar-amended soils did not produce any N2O. The hydrochar-amended soil produced 0.24 mg N2O m-2d-1. Addition of swine solid hydrochar dramatically increased soil CO2 emission; however, N2O emission was not significantly affected.