## 2 Materials and Methods
### Adsorption Experiments  
Cr(VI) adsorption by the obtained activated carbon was performed via batch experiments, using a 250-mL glass reactor provided, mechanical shaking, and a 2.3-cm diameter four-blade glass impeller. The metal concentration of the solution was analyzed by flame atomic absorption spectrometry (AAS) using a Perkin Elmer 1100B spectrophotometer. The adsorption percentages were calculated by Equation (1):  
\[\text{{\rm Adsorption}}(\%)=\frac{[\text{Cr}]_{\text{aq},0}-[\text{Cr}]_{\text {aq},t}}{[\text{Cr}]_{\text{aq},0}}\text{x}100 \tag{1}\]  
where [Cr]\({}_{\text{aq},0}\) and [Cr]\({}_{\text{aq},t}\) (mg/L) are the initial concentration and concentration at time t of Cr(VI) in the solution, respectively. The associated analytical error was within \(\pm 2\%\). Different experiments were carried out to analyze several parameters which affect the adsorption process. Stirring speed was modified in the range 250-1500 min\({}^{-1}\) in 200 mL of Cr(VI) solution (10 mg/L concentration), at pH 4, with 10 mg of the activated carbon. The solution pH was adjusted by adding suitable HCl and NaOH solutions until the desirable pH value was achieved using buffer solutions in calibrating pH meter. The temperature of the experiments was modified (in 10 \({}^{\circ}\)C incrememnts) between 20 and 60 \({}^{\circ}\)C. The temperature was controlled using a Selecta Termotronic thermostat-controlled bath. The effect of the ionic strength was investigated by adding different amounts of lithium chloride to the solutions of Cr(VI) of concentration 10 mg/L and 1 mg of the activated carbon. Finally, the adsorption equilibrium isotherms were analyzed varying the activated carbon amount between 0.5 and 100 mg using the Type-I Langmuir [16; 17] (Equation (2)), Type-II Langmuir [18; 19] (Equation (3)), Freundlich [20; 21] (Equation (4)), and Temkin [22; 23] (Equation (5)) linear forms using Origin Pro v.2019 software:  
\[\frac{\left[\mathrm{Cr}\right]_{\mathrm{aq,e}}}{\left[\mathrm{Cr}\right]_{ \mathrm{c,e}}}=\frac{1}{\left[\mathrm{Cr}\right]_{\mathrm{c,m}}\mathrm{K_{L}}}+ \frac{1}{\left[\mathrm{Cr}\right]_{\mathrm{c,m}}\mathrm{x}\left[\mathrm{Cr} \right]_{\mathrm{aq,e}}} \tag{2}\]  
\[\frac{1}{\left[\mathrm{Cr}\right]_{\mathrm{c,e}}}=\frac{1}{\left[\mathrm{Cr} \right]_{\mathrm{c,m}}}+\frac{1}{\mathrm{K_{L}}\left[\mathrm{Cr}\right]_{ \mathrm{c,m}}\mathrm{x}}\frac{1}{\left[\mathrm{Cr}\right]_{\mathrm{aq,e}}} \tag{3}\]  
\[\ln\left[\mathrm{Cr}\right]_{\mathrm{c,e}}=\ln\mathrm{K_{F}}+\frac{1}{n}\ln \left[\mathrm{Cr}\right]_{\mathrm{aq,e}} \tag{4}\]  
\[\left[\mathrm{Cr}\right]_{\mathrm{c,e}}=\frac{\mathrm{RT}}{\mathrm{b_{T}}} \ln\mathrm{A_{T}}+\frac{\mathrm{RT}}{\mathrm{b_{T}}}\ln\left[\mathrm{Cr} \right]_{\mathrm{aq,e}} \tag{5}\]  
where \(\left[\mathrm{Cr}\right]_{\mathrm{aq,e}}\) (mg/L) is the concentration of the metal in solution at equilibrium, \(\left[\mathrm{Cr}\right]_{\mathrm{c,e}}\) (mg/g) is the adsorbed metal amount per gram of activated carbon at equilibrium, \(\left[\mathrm{Cr}\right]_{\mathrm{c,m}}\) is the maximum adsorbed metal amount per gram of activated carbon, \(\mathrm{K_{L}}\) (L/mg) and \(\mathrm{K_{F}}\) (L/g) are the corresponding Langmuir and Freundlich constants, 1/n (non-dimensional) is the adsorption intensity, \(\mathrm{A_{T}}\) (L/g) is the Temkin isotherm equilibrium binding constant, and \(\mathrm{b_{T}}\) (non-dimensional) is the Temkin isotherm constant.