Improved ferrous ions removal from water using freeze-thaw treated biochar

This study investigates the effects of freeze-thaw pretreatment on corn stover to modify porous structure of the resulting biochar for enhancing its ferrous ion (Fe²⁺) removal from water. Corn stover was subjected to freeze–thaw cycling at −10 °C and −80 °C for 1–15 cycles prior to pyrolysis at 500 °C, while 20 °C was used as a control. Comprehensive characterization, including pH, yield, elemental composition, BET surface area, FTIR, XRD, SEM, and adsorption tests, was conducted. Results showed that freeze–thaw cycling significantly altered biomass ultrastructure and biochar properties. Pretreatment at −10 °C for 15 cycles, increased specific surface area to 16.13 m²/g, a 7.03-fold improvement over the untreated control (2.29 m²/g), with enhanced micropore density and honeycomb-like structures, with enhanced micropore density by 37 %, while preserving structural integrity. In contrast, ultralow temperature treatment at −80 °C caused pore collapse and crystallite size reduction. Freeze–thaw treatment also improved carbonization efficiency, raising carbon content from 74.34 % (control) to 80.62 % (N15), while reducing the O/C ratio from 0.15 to 0.13. Adsorption experiments demonstrated that Fe²⁺ removal efficiency increased by 42 % after five cycles at −10 °C, reaching a Langmuir maximum capacity of ∼91 mg/g with strong model fitting (R² > 0.99). Kinetic analysis confirmed pseudo-second-order behavior, with 85 % Fe 2+ removal completed within 30 min. This process also promoted corn stover acidification through cryo-mechanical lignin degradation, accompanied by a transition in biochar surface chemistry from alkaline to acidic at cycle 12. Overall, freeze–thaw pretreatment at −10 °C enhances biochar porosity, surface chemistry, and adsorption performance, offering a simple, low-temperature pretreatment strategy for improving ferrous ions removal from water.