Preparation and properties of slow-release fertilizer containing urea encapsulated by pinecone biochar and cellulose acetate.

Biochar (BC) has been utilized in the production of carbon-based slow-release fertilizers owing to its extensive specific surface area and abundant functional groups. However, conventional biochar-based fertilizers still exhibit inadequate slow-release performance, necessitating further modification. In this paper, pinecone biochar (PB) was prepared by pyrolysis of pinecone, an agricultural waste, at different temperatures. Among all samples synthesized at different temperatures, the PB prepared at 700 °C exhibited the largest specific surface area, denoted as 700 PB, reaching 305.35 m/g. Compared with biochar-coated urea prepared at other temperatures, the PB-coated urea produced at 700 °C demonstrates the optimal controlled-release performance. Cellulose acetate (CA) represents a cost-effective and naturally occurring biodegradable material. Therefore, in this study, urea was encapsulated with 700 PB and CA through physical coating and impregnation, leveraging CA's cost-effectiveness and inherent biodegradability, resulting in the successful preparation of a slow-release nitrogen fertilizer (PBUA). The sustained release performance and mechanism of different compositions of PBUA under different sustained release conditions (pH and temperature) were investigated. Among them, the composition was optimized, yielding a coated PBUA with a mass ratio of 700 PB to urea of 1.5: 1 and a CA mass fraction of 9 wt%, referred to as PBUA. The release performance of this coated PBUA is demonstrated to be the most optimal. Under room temperature and neutral conditions, the cumulative release amount of PBUA within 24 h is 4.37 %, which increases to 55.63 % after 28 days and further rises to 72.56 % after 60 days. The release results of PBUA are in strict conformity with the national standards. The results suggest that the release mechanism of PBUA adheres to the Weibull model. This work offers an effective approach for the preparation of slow-release nitrogen fertilizers.