## 3 Results and Discussion  
### The Effect of Airflow Rate on Biochar Surface Properties  
Biochars produced at airflow rates ranging from 8 to 20 L/min were found to be mostly basic (pH \(>\) 7.0). As a result, no acidic functional groups were detected on the surface by Boehm titration characterization [19]. Table 1 presents the results of the pH and the basic functional groups of the biochar when the airflow rate increased in the TLUD gasifier. In general, lower airflow rates resulted in biochar with lower pH. However, no statistical difference in pH was found when analyzing all the biochar samples. The basic functional group of the biochar increased with the airflow rate, which significantly increased from 0.0175 to 0.115 mmol g\({}^{-1}\). The increase in the basic functional group of the biochar was correlated with the peak temperature of the gasification layer that increased from 661 to 840 \({}^{\circ}\)C (R\({}^{2}\) = 0.78). With increasing airflow, the O/C atomic ratio of biochar was significantly reduced from 0.14 to 0.1 which induced carbonization due to aromatization in which polycyclic aromatic structures were formed leading to an increase in aromatic carbon concentration [27]. Similarly, [PERSON] [1] performed microwave pyrolysis experiments of corn stover by increasing the reaction temperature from 550 to 650 \({}^{\circ}\)C. The increase in temperature caused reduction in the O/C ratio of the biochar from 0.06 to 0.04 while the H/C ratio did not change. This suggested a more aromatic-based biochar due to the increase in carbon content and reduction of oxygen (O) [4]. As a result, the basic functional groups of biochar increased. The characterization of the biochar surface functional groups usually presents a combination of carboxylic, lactone, and phenolic acidic groups [14; 20]. However, in the analysis of biochar produced at different airflow rates via TLUD gasification, no acidic functionalities were detected. This could be attributed to the fact that the carbonization of biomass in a controlled oxygen-rich atmosphere usually leads to the absence of acidic components on the surface of biochar when the reaction temperature increases [21; 28]. Contrary to this, [PERSON] et al. [29] and [PERSON] et al. [30] reported that  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & & & **AEC** & **CEC** & \begin{tabular}{c} **BET Surface** \\ **Area m\({}^{2}\) g\({}^{-1}\)** \\ \end{tabular}  & **Atomic Ratio** \\
**(L/min)** & **Temperature (\({}^{\circ}\)C)** & **pH Of** & **Bagic Functional** & **Groups mmol g\({}^{-1}\)** & **cmol** & **cmol** & **H/C** & **O/C** \\  & & & & **kg\({}^{-1}\)** & **kg\({}^{-1}\)** & & \\ \hline
8 & 661.33  & 0.0175  & 3.43  & 0.14 [FOOTNOTE:]Footnote : Different superscripts represent significant differences in the order of a \(>\) b \(>\) c \(>\) d.[ENDFOOTNOTE] \\
12 & 743.00  & 0.02  & 1.03  & 239.2 & 0.26  \\
16 & 798.33  & 0.0625  & 1.01  & 319.4  & 0.10 [FOOTNOTE:]Footnote : Different superscripts represent significant differences in the order of a \(>\) b \(>\) c \(>\) d.[ENDFOOTNOTE] \\
200 mL of de-ionized water; 10-mL of this solution was then filtered with Target2® 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® 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® 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). AEC and CEC calculations were performed following the methods described by [PERSON] [26].