From waste to adsorbent: Properties of CO2-activated biochars from pistachio hulls and walnut shells for advanced water remediation

This study investigates the properties of CO₂-activated biochars derived from second-generation agricultural waste, specifically pistachio hulls and walnut shells. The biochars were prepared using varying pyrolysis temperatures of 500 °C and 750 °C, followed by physical activation with CO₂ at 850 °C and, in some cases, acid washing. A range of characterization techniques were used to assess the effects of biomass type, pyrolysis temperature, and acid-washing on the biochars' properties. This comprehensive analysis provided insights into the surface chemistry and structural characteristics of biochars. The results demonstrate the significant versatility of pyrolysis in modulating the properties of the resultant biochars. Sorption experiments with atrazine showed high removal efficiencies (94-95 %), comparable to commercial active carbon. Notably, the acid washing treatment did not affect pesticide sorption. These findings indicate the potential of using waste-derived biochars as high-performance sorbents for sustainable water remediation and circular economy initiatives.