# Effective Adsorption of Phenoxyacetic Herbicides by Tomato Stem-Derived Activated Carbons
## 2 Experimental Section
### Batch Adsorption Experiments  
Adsorption experiments were conducted in Erlenmeyer flasks by adding 0.005 g of TS-AC to 20 mL of herbicide solutions of varying concentrations. The mixtures were shaken at 150 rpm and 23 \({}^{\circ}\)C. After a predetermined time, the solutions were filtered, and the filtrate was analyzed using a spectrophotometer (Cary 3E UV-Vis, Varian Medical Systems, Inc., Palo Alto, CA, USA). The equilibrium herbicide concentration (\(C_{\rm e}\), mmol/L) was used to calculate the adsorption efficiency (\(q_{\rm\%}\)) and adsorption capacity (\(q_{\rm e}\), mmol/g) according to the following equations:  
\[q_{\rm\%}=\frac{\left(C_{0}-C_{e}\right)}{C_{0}}.100\% \tag{4}\]  
\[q_{e}=\frac{\left(C_{0}-C_{e}\right)V}{m} \tag{5}\]where \(C_{0}\) and \(C_{\rm e}\) are the initial and final (equilibrium) concentrations of 2,4-D or MCPA in solution (mmol/L), respectively; \(V\) is the volume of solution (L); and \(m\) is the mass of activated carbon (g).  
In an analogous procedure, the amount of herbicide that was adsorbed after time \(t\) (\(q_{\rm t}\), mmol/g) was calculated:  
\[q_{t}=\frac{(C_{0}-C_{\rm f})V}{m} \tag{6}\]  
where \(C_{\rm f}\) (mmol/L) is the herbicide concentration at time \(t\).  
Kinetic studies were performed using an initial 1 mmol/L herbicide concentration. Adsorption isotherms were determined from a series of 2,4-D and MCPA solutions with different initial concentrations ranging from 0.3 to 1 mmol/L. Both kinetic and isotherm experiments were conducted at the solutions' natural pH (-3.3). The influence of pH and ionic strength on herbicide adsorption was also investigated. Initial herbicide concentrations were maintained at 1 mmol/L for these experiments. Solutions were adjusted to pH values between 2.5 and 10 using 0.1 mol/L HCl or NaOH, monitored with an Elmetron CP-550 pH meter (Zabrze, Poland). Immediately following the pH adjustment, 0.005 g of activated carbon (AC) was added to each solution. NaCl was added to 20 mL of herbicide solutions to assess the effect of ionic strength to achieve final salt concentrations of 0.01, 0.1, and 0.5 mol/L. All adsorption experiments were duplicated, and the average value was used for subsequent calculations.  
The regeneration of activated carbons studied was evaluated using methanol elution. The TS-AC was separated from the solution after adsorbing herbicides (1 mmol/L). Herbicide-loaded TS-AC (20 mg) was then agitated in 20 mL of methanol for 8 h using a laboratory shaker. Following filtration, the TS-AC was dried at 120 \({}^{\circ}\)C for 4 h and weighed. Subsequently, 20 mg of the regenerated TS-AC was added to 20 mL of 1 mmol/L 2,4-D or MCPA solution. This adsorption/desorption cycle was repeated three times, and the regeneration efficiency (\(R\), %) was calculated using the following equation:  
\[R=\frac{q_{n}}{q_{0}}\cdot 100\% \tag{7}\]  
where \(q_{0}\) is the initial adsorption capacity (mmol/g), and \(q_{\rm n}\) is the reuse adsorption capacity (mmol/g) after the next cycle.  
Herbicide concentrations in solutions were determined spectrophotometrically by measuring the absorbance at analytical wavelengths of 283 nm and 278 nm, corresponding to the absorption maximum of 2,4-D and MCPA, respectively. Calibration curves for their quantification, prepared in the range of 0.05 to 0.8 mmol/L, were linear (\(R^{2}\geq 0.996\)) and described by the following equations: \(y=1.448x+0.021\) (for 2,4-D) and \(y=1.342x+0.041\) (for MCPA).