Temperature-Induced Structural Phase Transitions in Nano-Fe-Modified Sustainable Biochar for Efficient Sb(III) Adsorption

Nano-iron-modified biochar (Fe-SC) has emerged as a promising adsorbent for pollutant removal. However, how pyrolysis temperature changes its iron phases and affects Sb(III) removal remains unclear. In this work, Fe-SC was prepared at three pyrolysis temperatures (400 °C, 600 °C, and 800 °C) to investigate how structural phase transitions affect Sb(III) adsorption performance. XRD analysis suggests a possible evolution of Fe phases with increasing temperature, from an Fe$_3$O$_4$ crystalline phase at 400 °C to an Fe crystalline phase at 600 °C, and subsequently to an Fe$_3$C crystalline phase with pronounced graphitization at 800 °C. These structural changes, together with the specific surface area and graphitization degree, strongly influenced Sb(III) adsorption. Fe-SC600 had the highest specific surface area (217.73 m$^2$/g) and achieved nearly 80% Sb(III) removal, the highest among the three materials. Fe-SC also maintained effective Sb(III) adsorption over a broad pH range (3.0–9.0), and the presence of S$^{2−}$ and HCO$_3$$^-$ ions enhanced Sb(III) removal under the tested conditions. These findings highlight the importance of optimizing pyrolysis conditions to enhance the adsorption performance of Fe-SC, with Fe-SC600 demonstrating the most promising balance of structural properties and adsorption efficiency for environmental remediation applications.