Synergetic effect of heating rate, temperature, and residence time of modified agro-industrial waste biochars on phosphate adsorption

Phosphorus is a non-renewable resource and essential for humanity. Due to its discharge in treated and untreated sewage, it became an aquatic pollutant, leading to eutrophication. Exploring the potential approach for phosphate recovery species, such as biochar, is indispensable to ensure food and water security. Therefore, this study investigates the synergic effects of heating rate (5 °C min⁻¹, 10 °C min⁻¹, and 15 °C min⁻¹), temperature (350 °C, 450 °C, and 550 °C) and residence time (0, 30, and 60 min) using a Box–Behnken experimental design in the production of a sawdust calcium-biochar and how the factors affect the phosphate removal. The phosphate removal ranged from 7.6 to 93%, with eleven products above 70%. All the independent variables were statistically significant, highlighting temperature, in linear and quadratic terms. The regression significance and lack of fit tests indicated that the model was applicable to the observed data, with total R² of 97% and adjusted R² of 92%, indicating that the model was suitable to evaluate the percentage of phosphate removal. The results were confirmed by the characterization techniques, such as BET, moisture, volatile matter, ash content and FT–IR, where pore size and functional groups on the surface of the biochar explained the influence of the variables. And the XRD characterization showed the calcium species in the biochar surface. Thus, using a Box–Behnken design allied to the response surface methodology allows to explore the biochar as a promising alternative for phosphate recovery for the first time in literature.