Effects of biochar particle size on sorption and desorption behavior of NH4+-N

Abstract

Although optimizing of particle size was a simple and low-cost method for biochar modification, the insufficient understanding of biochar particle size on sorption and desorption performance has hindered its application. In this study, apple branch biochar was mechanically grinded and dry sieved into different particle sizes to investigate the sorption mechanism, sorption and desorption characteristics at various temperature, NH 4 + -N concentrations, contact time, biochar dosages and regeneration times. The particle size modification affected the physical properties of biochar but not the sorption mechanism, which was a comprehensive process involving physicochemical sorption. The result indicated that, for all three particle sizes, the sorption process was exothermic and the Langmuir, pseudo first and second order kinetic models can provide satisfactory descriptions. The sorption capacity increased with the rising of NH 4 + -N concentrations and contact time. The decreasing biochar particle size significantly increased the sorption capacity, desorption capacity and efficiency in the first 24 h, indicating that large particle size biochar had a better slow-release effect. Moreover, both sorption capacity and slow-release effect were also affected by the ratio of NH 4 + -N to biochar. High NH 4 + -N-biochar ratios were beneficial for increasing sorption and desorption capacity of biochar, while small NH 4 + -N-biochar ratios were beneficial for increasing the slow-release effect. Considering the nutrient supply and slow-release performance, the recommended NH 4 + -N-biochar ratios for fine, medium and coarse biochar were 0.013–0.016, 0.013–0.034, and 0.009–0.124, respectively. At the same slow-release effect, coarse biochar had better nutrient supply capacity, indicating that a greater application efficiency. Compared with traditional slow-release fertilizers, biochar had excellent regeneration performance, which could be enhanced by particle size modification. In conclusion, our research provides a theoretical basis for the necessity of biochar particle size optimization. • Fine biochar (＜0.25 mm) enhances sorption capacity but lowers slow-release effect. • NH 4 + -N-biochar ratio affects the biochar sorption and desorption performance. • The recommended NH 4 + -N-biochar ratio varies with the biochar particle size. • Coarse biochar (1–2 mm) is preferred for better efficiency and regeneration.