Abstract:Poplar wood biochar was added as adsorbent into two types of northeast region black soil (loamy and sandy) to investigate the effects of poplar wood biochar on black soil adsorbing ammonium and nitrate nitrogen in pig biogas slurry, and the adsorption mechanism was studied by using oscillation method. Commercial activated carbon was used as a standard for comparison. The effects of the particle sizes (0.25mm, 0.5mm and 1mm) and addition proportions (2%, 5% and 10%) of commercial activated carbon and poplar wood biochar, the initial mass concentration of pig biogas slurry, the oscillation time and the oscillation temperature on black soil adsorbing ammonium and nitrate nitrogen in pig biogas slurry were studied systematically. The adsorption mechanism was analyzed by different isothermal and kinetic adsorption models. The results showed that the smaller particle sizes and the more addition proportions of commercial activated carbon and poplar wood biochar led to better ammonium and nitrate nitrogen adsorption capacity of black soil in pig biogas slurry. And 0.25mm particle size and 10% solid weight ratio were optimum addition treatment for commercial activated carbon and poplar wood biochar. Under optimum addition treatment, the ammonium and nitrate nitrogen amount adsorbed by black soil with poplar wood biochar addition were higher than that by pure black soil by 388.7%, 296.3% (black loamy soil) and 453.13%, 333.33% (black sandy soil), and were lower than those by back soil with commercial activated carbon addition by 19.71%, 10.08% (black loamy soil) and 12.38%, 7.14% (black sandy soil), respectively. However, the effects of poplar wood biochar and commercial activated carbon on the ammonium and nitrate nitrogen concentration of pig biogas slurry after adsorptive stability were not significantly different, and all the differences were less than 2.5%. The ammonium nitrogen amount adsorbed by the pure black soil, the pure commercial activated carbon and pure poplar wood biochar and the black soil with commercial activated carbon and poplar wood biochar addition were all increased as the increase of temperature, posing endothermic reaction. While the nitrate nitrogen adsorption amount showed a decreasing trend as temperature was increased, posing spontaneous exothermic reaction. Furthermore, the ammonium and nitrate adsorption processes showed three stages of fast adsorption, slow adsorption and balance. The adsorption models of Freundlich, Langmuir and Pseudosecond order described the isothermal adsorption characteristics and kinetic adsorption characteristics of the black soil added with commercial activated carbon and poplar wood biochar adsorbing ammonium and nitrate nitrogen in pig biogas slurry precisely. However, Freundlich was superior to Langmuir. Therefore, there were multilayers uneven physisorption reaction and monolayer chemisorption reaction during the ammonium and nitrate adsorption process, concurrently. The more adsorption amount was, the more desorption amount was, but the desorption amount was much less than the effective adsorption amount. The effective ammonium and nitrate nitrogen adsorption amount of black soil with poplar wood biochar addition were lower than those of black soil with commercial activated carbon addition by 14.57%, 9.19% (black loamy soil) and 5.34%, 5.74% (black sandy soil). Poplar wood biochar was favorable for improving the nitrogen adsorption capacity of northeast region black soil and decreasing the nitrogen loss of pig biogas slurry. The results provided good insights into improving the northeast region black soil by using poplar wood biochar and pig biogas slurry.
