*2.4. Physicochemical Properties of Resulting Biochar*

In this work, an accelerated surface area and porosimetry system (ASAP 2020; Micromeritics Co., USA) was used to determine the pore properties of the BRH products, including the Brunauer–Emmett–Teller (BET) surface area, pore volume and pore size distribution. Herein, the calculation of specific surface area was performed using the BET equation, using a range of relative pressure from 0.05 to 0.30. The total pore volume was taken as the nitrogen liquid volume adsorbed at a relative pressure of ca. 0.995. Using the *t*-plot method, the data on micropore area and micropore volume were obtained by the Halsey equation [41]. According to the definition by the International Union of Pure and Applied Chemistry (IUPAC) [41], micropores refer to pores with an internal diameter or width of less than 2 nm. Mesopores are defined as pores with an internal diameter or width between 2 and 50 nm. Regarding the pore size distributions of the BRH products, the Barrett–Joyner–Halenda (BJH) method was employed to calculate them in the range of mesopores and small macropores from experimental N<sup>2</sup> isotherms (desorption branch) using the Kelvin model of pore filling [41].

The porosity of a material is defined as the ratio of the total pore volume to the apparent volume of the particle or powder (excluding inter-particle voids). Therefore, this property can be estimated by subtracting the ratio of particle (apparent or skeletal) density to true density from 1 [42,43]. In this work, a gas (helium) pycnometer (AccuPyc 1340; Micromeritics Co., USA) was used to determine the true density. Although mercury (Hg) porosimetry is often used to measure particle density due to the high surface tension of Hg, this property was estimated by using the measured data on the true density and total pore volume [43].

The textural morphology on the surface of the BRH product was observed by the SEM system (S-3000N; Hitachi Co., Japan). Prior to the SEM analysis, the BRH sample was coated by a conductive gold film using an ion sputter (E1010; Hitachi Co., Japan). An accelerating potential of 15.0 kV (electron beam) in a vacuum chamber was applied to the specimen surface during the SEM analysis. In addition, EDS analysis was used to quantify the elemental compositions rapidly and simply when scanning the BRH sample during the SEM analysis.

The oxygen-containing functional groups on the surfaces of the BRH products were analyzed by FTIR (FT/IR-4600; JASCO Co., Japan). Prior to the FTIR analysis, the dried BRH sample was mixed with spectroscopic-grade potassium bromide (KBr) powder, and then ground in an agate mortar. The finely uniform mixture (around 1 wt% BRH) was pressed in a hydraulic machine to form a disc with a diameter of ca. 1.2 cm and thickness of ca. 1 mm. The FTIR spectra were obtained by scanning with a wave number range of 4000–400 cm−<sup>1</sup> and a resolution of 4 cm−<sup>1</sup> .
