## 3 Results and Discussion
### Effect of Pyrolysis Temperature on Biochar Yield and Physicochemical Characteristics
#### 3.1.2 pH, pH\({}_{\text{PZC}}\), EC, and CEC of the Biochar Samples  
The pH, pH\({}_{\text{PZC}}\), and EC of the studied biochar samples increased with the pyrolysis temperature, as shown in Table 1. This agrees with reports that correlated the ash content with the pH and EC values [7; 16]. For EC, values between 25.7 and 28.1 dS m\({}^{-1}\) were registered (Table 1), which are, as expected, considerably higher than for other types of biochar [12].  
All studied biochar samples had a basic pH (9.8-11.8) (Table 1). This agrees with previous reports showing that the pH of biochar derived from seaweeds varied from 7.6 to 13.7 [26]. The pH increased with the pyrolysis temperature and the content of typical alkaline salts of seaweeds [31]; in the same context, pH\({}_{\text{PZC}}\) also increased with the pyrolysis temperature (Figure S4). In addition, above 300 \({}^{\circ}\)C, alkaline salts began to separate from the organic matrix and were converted into hydroxides, increasing the pH [12; 17; 26]. Conversely, under severe pyrolysis conditions (related to high treatment temperature and residence time), the amount of carboxyl groups in BCs decreases and/or the acidic groups were deprotonated to the conjugated bases, generating a more alkaline pH [11].  
Interchangeable cations, such as Ca\({}^{2+}\), Mg\({}^{2+}\), K\({}^{+}\), and Na\({}^{+}\), were found in the seaweed biochar, which may confer an enhanced adsorption capacity, thus playing a fundamental role in the adsorption mechanism of biochar on pollutants [16]. For the biochar samples in this study, BC450 had the highest CEC value (40.2 cmol kg\({}^{-1}\)), followed by BC600 (35.0 cmol kg\({}^{-1}\)) and BC300 (34.7 cmol kg\({}^{-1}\)) (Table 1). This indicates that BC450 could have a higher presence of functional groups that contain oxygen (such as carboxylic, lactonic, and phenolic groups), which could increase the adsorption capacity [12]. The presence of certain interchangeable cations (e.g., Ca\({}^{2+}\)) was corroborated with the representative peaks found in the FTIR for all BCs (see below). The types of functional group (carboxylic, lactonic, or phenolic groups) can be determined by a titration method [32], which was not considered in this work.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Parameter \({}^{*}\)** & **Unit** & **BC300** & **BC450** & **BC600** \\ \hline Yield & \% & 62.10 \({}^{\text{a}}\) & 53.30 \({}^{\text{b}}\) & 48.80 \({}^{\text{b}}\) \\ Ash & \% & 57.50 \({}^{\text{a}}\) & 69.90 \({}^{\text{b}}\) & 71.10 \({}^{\text{b}}\) \\ pH & & 9.80 \({}^{\text{a}}\) & 11.00 \({}^{\text{b}}\) & 11.80 \({}^{\text{b}}\) \\ pH\({}_{\text{PZC}}\) & & 9.13 \({}^{\text{a}}\) & 10.70 \({}^{\text{b}}\) & 11.30 \({}^{\text{c}}\) \\ EC & dS m\({}^{-1}\) & 25.70 \({}^{\text{a}}\) & 27.40 \({}^{\text{b}}\) & 28.10 \({}^{\text{b}}\) \\ CEC & cmol kg\({}^{-1}\) & 34.70 \({}^{\text{a}}\) & 40.20 \({}^{\text{b}}\) & 35.00 \({}^{\text{a}}\) \\ C & \% & 28.20 \({}^{\text{a}}\) & 23.90 \({}^{\text{a}}\) & 25.50 \({}^{\text{a}}\) \\ H & \% & 1.95 \({}^{\text{a}}\) & 3.01 \({}^{\text{b}}\) & 0.87 \({}^{\text{c}}\) \\ N & \% & 3.15 \({}^{\text{a}}\) & 2.00 \({}^{\text{b}}\) & 1.58 \({}^{\text{c}}\) \\ H:C & & 0.82 \({}^{\text{a}}\) & 1.51 \({}^{\text{b}}\) & 0.41 \({}^{\text{c}}\) \\ OM & \% & 40.00 \({}^{\text{a}}\) & 29.50 \({}^{\text{b}}\) & 28.00 \({}^{\text{b}}\) \\ \hline \end{tabular}
\end{table}
Table 1: Yield and physicochemical characteristics of biochar samples from _Macrocystis pyrifera_ produced at different pyrolysis temperatures (300, 450, and 600 \({}^{\text{c}}\)C). \({}^{\text{a}-\text{c}}\) Different letters indicate significant differences among biochar samples with \(p<0.001\) obtained by ANOVA with a posteriori Tukey test, \(n=9\)–12. \({}^{\text{*}}\) EC, electric conductivity; CEC, cation exchange capacity; OM, organic matter.