Developing slow-release pesticides and reducing the migration of pesticides are the keys to improving the utilization efficiency of pesticides. Here, phosphorus-modified (P-WBC) and subsequent ammonia-modified biochar (NP-WBC) were prepared using pine tree sawdust at different pyrolysis temperatures (300 °C, 500 °C, and 700 °C), while the adsorption and desorption capacities of P-WBC and NP-WBC on 2,4-dichlorophenoxyacetic acid (2,4-D) were compared, as well as their intrinsic mechanism disparity. It was found that pyrolysis temperature, phosphorus- and ammonia-modification all improved the adsorption and slow-release of 2,4-D from biochar significantly through enriching the surface morphology, pore structure and optimizing the composition of functional groups of biochar. The adsorption mechanisms involved pore filling, hydrogen bonding, π-π interactions and phosphorus/nitrogen (P/N)-containing groups bonding, while the release behavior was predominantly governed by Fickian diffusion. The main active reaction sites of P-WBC and NP-WBC were the edge-located phosphate-like functional groups (C-O-P1) and pyridinic N, respectively, and C-O-P1 had the highest affinity for 2,4-D, as corroborated by density functional theory (DFT) calculation. Compared to NP-WBC, P-WBC exhibited greater 2,4-D adsorption and slow-release capacities, which was attributed to the stronger hydrogen bonding and P-containing functional groups bonding of C-O-P1 with 2,4-D. In particular, 500 °C P-WBC demonstrated exceptional thermal stability, great 2,4-D adsorption capacity (75.27 mg/g), and the most effective slow-release performance (9.34%). In addition, further germination experiments showed that 500 °C P-WBC loaded with 2,4-D (500 °C P-WBC@2,4-D) exhibited equivalent herbicidal efficacy to commercially available 2,4-D while also reducing the toxicity of the pesticide to maize to a certain extent. This work offers an avenue for the synthesis to prepare biochar as a carrier for slow-release pesticides under suitable pyrolysis temperature and modification method, improve pesticide efficiency, and reduce environmental pollution.
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