Pyrolysis temperature is known to affect the effect of biochars as a nutrient-holding soil amendment, yet the mechanisms are unclear. In this study, raw switchgrass, water oak, biosolids, and the biochars of these feedstocks produced at different pyrolysis temperatures were characterized for their physiochemical properties and sorption of nitrogen nutrients (i.e., NH4+, NO3-, and NO2-). With increased temperature, decreased mass yield, increased pH and specific surface area were observed. Volatile matter was found to decrease while ash content and fixed carbon were found to increase due to increased pyrolytic decomposition at elevated temperatures. Meanwhile, significant diminishment of H and O content and gradual increase of C content occurred as pyrolysis temperatures were raised to over 600oC. Notably, the changes of cation exchange capacity (CEC) and morphological characteristics (e.g., total pore volumes, mesopore volumes, micropore volumes, and average pore sizes) varied among biochars produced from different feedstocks. Isotherm sorption experiments showed different sorption behaviors of NH4+, NO3-, and NO2- on these biochars. The biochars were found to have good adsorption capacity for NH4+ (0.57 – 43.29 mg g-1) but limited adsorption or even release of NO3- and NO2-. The raw feedstocks and their biochars produced at different pyrolysis temperatures exhibited different NH4+ adsorption capacities because of the different physiochemical properties as evidenced by the scanning electronic microscopy (SEM) analysis. CEC and micropore volume were found to be the two decisive factors co-affecting the sorption capacity. It was discovered that when CEC of the biochars was low, micropore volume was the dominating factor through physisorption; when CEC was high, a combination of physisorption and electrostatic sorption was the mechanism for NH4+ sorption.

Nitrogen sorption by biochars derived from different feedstocks and at different pyrolysis temperatures: Affecting factors and mechanisms

255th National Meeting (2018)

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