# Commercially Biochar Applied for Tartrazine Removal from Aqueous Solutions
## 3 Results and Discussion
### Adsorption Isotherm  
Adsorption isotherms are used to illustrate the relationship between the material's capability for removal and the concentration of the contaminated solution, as well as to characterize the pollutant-sorbent interaction [49].  
The adsorption isotherm studies were carried out by agitating 5 mg of biochar with 5 mL of tartrazine solutions at 30 \({}^{\circ}\)C for 20 min at 400 rpm. The starting concentrations of the tartrazine solutions ranged from 1 to 20 mg L\({}^{-1}\).  
The experimental data obtained were evaluated using the linearized forms of Langmuir, Freundlich, and Temkin isotherm models from Table 1.  
The most popular isotherm for physisorption data is the Langmuir isotherm [53], non-ideal sorption on heterogeneous surfaces can be addressed with the Freundlich isotherm model [54], and abnormal adsorbate/adsorbate communication during adsorption is taken into consideration by the Temkin model [55].  
The best model is the one in which the difference between the experimental and theoretical estimates are the smallest. For this purpose, the coefficient of determination R\({}^{2}\) is used; the closer its value is to unity, the more suitable the model.  
Table 2 summarizes the parameters derived from the isotherms models that might shed light on the adsorption mechanism, surface characteristics, and adsorbent affinity.  
\begin{table}
\begin{tabular}{c c c} \hline
**Isotherm Model** & **Linear Form** & **Reference** \\ \hline [PERSON] & \(\frac{C_{t}}{q_{e}}=\frac{1}{K_{t}q_{m}}+\frac{C_{e}}{2n}\) & [50] \\ [PERSON] & \(logq_{e}=logK_{t}+\frac{1}{n!}logC_{e}\) & [51] \\ [PERSON] & \(q_{e}=BlnK_{T}+BinC_{e}\) & [52] \\ \hline \end{tabular}
\end{table}
Table 1: The linearized forms of isotherm models.  
\begin{table}
\begin{tabular}{c c c} \hline
**Isotherm Model** & **Constants** & **Values** \\ \hline Langmuir & \(\begin{array}{c}\text{q}_{\text{m}}\text{ [mg g}^{-1}]\\ K_{\text{L}}\text{ [L g}^{-1}]\\ R^{2}\end{array}\) & \(\begin{array}{c}3.2841\\ 17.4000\\ 0.9936\end{array}\) \\ \hline [PERSON] & \(\begin{array}{c}\text{K}_{\text{F}}\text{ [L mg}^{-1}]\\ 1/n\\ R^{2}\end{array}\) & \(\begin{array}{c}2.6878\\ 0.1207\\ 0.9451\end{array}\) \\ \hline Temkin & \(\begin{array}{c}\text{K}_{\text{T}}\text{ [L mg}^{-1}]\\ \text{p}_{\text{T}}\text{ [J mol}^{-1}]\\ R^{2}\end{array}\) & \(\begin{array}{c}1.0486\\ 1046.5724\\ 0.3315\end{array}\) \\ \hline \end{tabular}
\end{table}
Table 2: Isotherm constants for the adsorption of tartrazine on biochar.  
The Langmuir isotherm produced the highest correlation coefficient (R\({}^{2}\)) value, indicating that tartrazine was adsorbed on the biochar's monolayer surface. This indicates that every molecule that has been adsorbed is in contact with the adsorbent's surface layer. The constant n was used in order to determine the kind of adsorption: if n = 1, then adsorption is linear; if n < 1, then adsorption is chemical; and if n > 1, then adsorption is physical. In this case, n is greater than 1, so that the adsorption of tartrazine on biochar is physical.  
Furthermore, it is evident from the comparison of the three models with the experimental results (Figure 6) that the Langmuir isotherm best describes the tartrazine adsorption on biochar. Similar results in which the Langmuir isotherm was adapted to the adsorption of tartrazine on bentonite modified with octa-decyltrimethylammonium bromide [56], raw sawdust and activated sawdust [10], and microcline/MWCNTs nanocomposite [57] were also obtained by other authors.  
We evaluated the tartrazine adsorption ability of commercial biochar by comparation with other adsorbent reports. Table 3 provides a summary of the maximal adsorption capacity of this and other adsorbents used to remove tartrazine.