Investigation of biomass waste biochar production to act as matrix for urea

In this work, the synthesis of biochar from several biomass wastes to act as matrix for urea was investigated. The objective was to select the most promising biochar synthesis condition that result in a matrix with the highest urea retention and subsequent slow release as well as to understand the urea–biochar interaction. A quite simple urea impregnation method was proposed and investigated for biuret formation; its experimental conditions were optimized to increase final urea retention performance. The results pointed out that the urea–biochar interaction has chemical nature, in which the lower presence of mineral elements associated with an acid surface seemed to favor the urea retention. The material impregnated at 133 °C for 1 h with the straw biochar synthesized at 400 °C for 0.5 h showed the lowest initial release of urea (≈ 17 wt. %). However, it presented a high content of biuret (≈ 8 wt. %). Thus, an alternative material was produced applying a shorter period of urea impregnation (10 min); it also maintained the initial release of urea ≈ 17 wt. %.