Synergistic effect of zinc-chitosan co-modified rice husk biochar on nutrient retention and controlled release of fertilizer.

The key challenges in the application of biochar in slow-release fertilizers due to the defects of poor pores and few functional groups. In this study, a chitosan-modified biochar-based infiltration nitrogen fertilizer (ZBC@CS-BUF) was synthesized using zinc chloride-chitosan co-modified biochar (ZBC@CS) as a fertilizer carrier with molten urea. The results showed that ZnCl2 promoted the formation of micropores in biochar (SSA is 306.04 m2/g), and the introduction of chitosan increased the abundance of functional groups containing nitrogen (-NH2) and oxygen (-CO, -OH). The fitting results of the pseudo-second-order, Langmuir, and Temkin models indicated that the adsorption by ZBC@CS is controlled by physical-chemical processes, and electrostatic attraction, physical adsorption, hydrogen bonding, and dipole-dipole interactions are the adsorption mechanisms that promote its adsorption capacity of urea up to 144.54 mg g-1. ZBC@CS-BUF exhibited the best sustained-release performance, with a cumulative release rate of urea nitrogen within 72 h of only 56.3%, and the overall release cycle was 5.5 times that of pure urea. The results of the Weibull release kinetics demonstrated that the nutrient release belonged to the slow-release mechanism of Fick diffusion and skeleton dissolution. In addition, the wheat seedlings pot experiment showed that ZBC@CS-BUF significantly promoted the growth of plants at the seedling stage, and the plant height, stem diameter, chlorophyll content, dry weight, and fresh weight increased by 59.23%, 5%, and 7%, respectively. Therefore, this study provides a potential method for developing a straw return system using chitosan-modified biochar derivatives.