## 2 Materials and Methods  
Pine woodchips were gasified in a TLUD gasifier to generate biochar at different airflow rates (8, 12, 16, and 20 L/min) as described by [PERSON] [16]. Briefly, the gasification unit mainly consisted of a 152-cm-high black iron tube with 10.1-cm diameter, which was loaded and ignited from the top and air was injected from the bottom, and the pyrolysis front started moving downward leaving biochar on the top and producing syngas. Once the peak reaction temperature was sensed by the bottom thermocouple, the reaction was complete and stopped. Then, biochar was then collected and analyzed after cooling. Biochar produced at 20 L/min under different biomass physical properties was also evaluated, e.g., with varying particle sizes (from 2 to 30 mm), moisture contents (from 10 to 22%), and biomass compactness (from 0 to 3 kg), as presented in [PERSON] [19]. The pH of the biochar samples was determined by mixing 0.4 g of biochar in 20 mL of de-ionized water for 8 h [14; 20]. The resulting solution was filtered with Whatman(r) filter paper (Qualitative #1) to remove large particles of biochar. Then, the final solution was filtered with nylon filters (0.2 um). The pH of the solution was measured using a bench pH meter (Model: UltraBasic U-10, Denver Instruments, Bohemia, NY, USA). The Boehm titration method was implemented to determine the surface functional groups of the biochar [21]. Four solutions (NaHCO\({}_{3}\), Na\({}_{2}\)CO\({}_{3}\), NaOH, and HCl) of 0.05 M were prepared. Fifty ml of each solution was mixed individually with 1 g of biochar for 24 h. The final solutions were filtered using the same filtration method as the pH analysis. Finally, the solutions were titrated with HCl (bases) or NaOH (acid) solutions using methyl orange and phenolphthalein indicators, respectively. The elemental analysis of biomass and biochar samples was determined using a CHNS/O (carbon, hydrogen, nitrogen, sulfur and oxygen) elemental analyzer (PerkinElmer 2400, Branford, CT, USA). Capsules of two grams of each sample were prepared with a tin capsule standard (PerkinElmer Mod. Pk96 N2411255). The resulting capsules were placed in the analyzer by the autosampler. The elemental composition of the pine woodchips was 47.90% carbon, 1.70% hydrogen, 0.30% nitrogen, 0.20% sulfur, and 49.90% oxygen (calculated by difference) with an ash content of 0.57% [19]. The specific surface area of the biochars was calculated by the BET ((Brunauer-Emmett-Teller)) analysis technique that evaluates the isothermal nitrogen adsorption/desorption at -196 \({}^{\circ}\)C [22]. The samples were degassed for 12 h at 250 \({}^{\circ}\)C under vacuum before BET analysis. The anion exchange capacity (AEC) of the biochar samples was determined according to Lawrinenko [23]. One gram of biochar was mixed with 40 mL of de-ionized water and 2 mL of KBr (1 M) for 48 h. Then, the biochar residue was filtered with [PERSON](r) filter paper (Qualitative #1) and rinsed to achieve 5 uS conductivity (Model: HI 9813-6, Hanna Instruments, Ann Arbor, MI, USA). Next, 2 mL of CaCl\({}_{2}\) (2.5 M) was added to the biochar slurry and mixed for 48 h. The concentrated biochar was then diluted with 200 mL of de-ionized water; 10-mL of this solution was then filtered with Target2(r) nylon filters (0.2 m) and further diluted with 100 mL of di-ionized water. Bromide was detected from the final solution using an Ion Chromatograph (Dionex 500, Thermo Fischer Scientific, Sunnyvale, CA, USA). The cation exchange capacity (CEC) was measured based on [PERSON] [24] and [PERSON] [25]. Initially, 2 g of biochar and 40 mL of de-ionized water were mixed overnight. Then, the samples were filtered with Whatman(r) filter paper (Qualitative #1). The washed biochar was placed in a flask with 20 mL of BaCl2 (0.2 M) and mixed for 2 h. The final solution was filtered with Target2(r) nylon filters (0.2 m). The concentrations of Na, K, Mg, Ca, Al, Fe and Mn were measured via Inductively-coupled Plasma-Optical Emission Spectrometer (PerkinElmer 8000, Waltham, MA, USA).