Effects of Feedstock and Pyrolysis Temperature on Biochar Adsorption of Ammonium and Nitrate

Biochar produced by pyrolysis of biomass can be used to counter nitrogen (N) pollution. The present study investigated the effects of feedstock and temperature on characteristics of biochars and their adsorption ability for ammonium N (NH4+-N) and nitrate N (NO3−-N). Twelve biochars were produced from wheat-straw (W-BC), corn-straw (C-BC) and peanut-shell (P-BC) at pyrolysis temperatures of 400, 500, 600 and 700°C. Biochar physical and chemical properties were determined and the biochars were used for N sorption experiments. The results showed that biochar yield and contents of N, hydrogen and oxygen decreased as pyrolysis temperature increased from 400°C to 700°C, whereas contents of ash, pH and carbon increased with greater pyrolysis temperature. All biochars could sorb substantial amounts of NH4+-N, and the sorption characteristics were well fitted to the Freundlich isotherm model. The ability of biochars to adsorb NH4+-N followed: C-BC>P-BC>W-BC, and the adsorption amount decreased with higher pyrolysis temperature. The ability of C-BC to sorb NH4+-N was the highest because it had the largest cation exchange capacity (CEC) among all biochars (e.g., C-BC400 with a CEC of 38.3 cmol kg−1 adsorbed 2.3 mg NH4+-N g−1 in solutions with 50 mg NH4+ L−1). Compared with NH4+-N, none of NO3−-N was adsorbed to biochars at different NO3− concentrations. Instead, some NO3−-N was even released from the biochar materials. We conclude that biochars can be used under conditions where NH4+-N (or NH3) pollution is a concern, but further research is needed in terms of applying biochars to reduce NO3−-N pollution.以生物质经热解（pyrolysis）制备的生物炭（biochar）可用于缓解氮（N）污染。本研究探讨了原料（feedstock）与热解温度对生物炭特性及其吸附铵态氮（ammonium N, NH4+-N）、硝态氮（nitrate N, NO3−-N）能力的影响。本研究以小麦秸秆（wheat-straw, W-BC）、玉米秸秆（corn-straw, C-BC）及花生壳（peanut-shell, P-BC）为原料，在400、500、600、700℃的热解条件下制备了12份生物炭样品。对各生物炭的理化性质进行了测定，并开展了氮吸附实验。结果显示，当热解温度从400℃升至700℃时，生物炭产率及氮、氢、氧含量均呈下降趋势，而灰分含量、pH值与总碳含量则随热解温度升高而上升。所有生物炭均可吸附大量铵态氮，其吸附行为均能较好地拟合弗里德里希等温吸附模型（Freundlich isotherm model）。生物炭对铵态氮的吸附能力排序为：C-BC > P-BC > W-BC，且吸附量随热解温度升高而降低。其中玉米秸秆生物炭（C-BC）的铵态氮吸附能力最强，因其在所有样品中拥有最高的阳离子交换量（cation exchange capacity, CEC）：例如，CEC为38.3 cmol·kg−1的C-BC400，在50 mg NH4+·L−1的溶液中，每克生物炭可吸附2.3 mg铵态氮。与铵态氮不同，无论硝态氮浓度如何，所有生物炭均未吸附硝态氮；反之，部分生物炭甚至会向溶液中释放硝态氮。综上，生物炭可应用于铵态氮（或氨，NH3）污染管控场景，但在利用生物炭缓解硝态氮污染方面仍需开展进一步研究。