# Adsorption of Metolachlor and Its Transformation Products, ESA and OXA, on Activated Carbons
## Materials and Methods
### Adsorption Experiments  
In order to determine the equilibrium time, 100 mL of a solution of the compound--with an initial concentration of 25 mg/L for MET and 8 mg/L for OXA and ESA--was mixed with a certain amount of adsorbent (5 mg for MET and 4 mg for OXA and ESA, corresponding to 0.5 and 0.2 g compound/g adsorbent, respectively) in glass flasks and continuously stirred (400 rpm) on a multi stirring plate (IKA brand) at 25 \({}^{\circ}\)C in a thermostatic water bath. Aliquots were measured at specific time intervals until equilibrium was reached. The solution volume was great enough to consider the volume changes negligible after aliquot withdrawal. The samples were filtered through a Nylon syringe filter (0.45 \(\upmu\)m) and the concentration was measured on a Cary 60 UV-Vis spectrophotometer (Agilent Technologies) at the corresponding wavelength (230 nm). The composition of solutions with a low concentration was also determined by reverse phase HPLC in an apparatus equipped with a photodiode array detector (Shimadzu Nexera XR). The separation was conducting using a C18 column (Restek) with a particle size of 2.7 \(\upmu\)m (3 mm \(\times\) 100 mm). For the quantification of OXA and ESA, a binary mixture was employed, consisting of a 25 mM dipotassium hydrogen phosphate solution at pH 7 and acetonitrile (80:20 \(v/v\)) as the mobile phase. The column was thermostated at 60 \({}^{\circ}\)C and OXA and ESA were detected at a wavelength of 210 nm after ca. 1.7 and 2.2 min, respectively. For detection of MET, the mobile phase employed was water:acetonitrile (40:60 \(v/v\)) and the column was thermostated at 30 \({}^{\circ}\)C. The compound was detected at 230 nm after 5.5 min. In both cases, the mobile phase flow was set at 0.8 mL/min and the sample injection volume was 50 \(\upmu\)L. LabSolutions software from Shimadzu was used for the identification and quantification of the molecules. The concentration of the compounds was calculated based on the relative peak areas using standards of known concentrations. Equilibrium adsorption isotherms were also carried out in 25 mL of solutions--with an initial concentration ranging from ca. 5 to 150 mg/L--containing 2.5 mg of adsorbent to obtain a range of 0.05-1.5 g compound/gadsorbent. The isotherms were carried out over the course of 3 h (equilibrium time based on the kinetic study).  
Regarding the determination of the adsorption capacity at lower concentrations (imposed by the EU legislation [16]), 500 mL of ESA and OXA aqueous solutions (0.001 and 0.01 mg/L) were stirred at 400 rpm in glass flasks with a mass of 1 mg of adsorbent. After 3 h, an aliquot was taken and filtered with a Nylon syringe filter (0.45 \(\upmu\)m). The concentration of OXA and ESA was analyzed with an SPE on-line ultra-high-performance liquid chromatograph with a tandem mass spectrometer (UPLC-MS-MS, Waters Quattro-Premier XE/Q). More details on the analytical method developed for off-line extraction were described in detail in [11].  
All adsorption assays were performed in duplicate and a blank solution (without adsorbent) was used to check the adsorption of the sorbate on the walls.  
The amount of compound adsorbed was determined by the equation:  
\[q_{t}=\frac{C_{0}-C_{t}}{W}\times V \tag{1}\]  
where \(q_{t}\) is the amount adsorbed in mg/g at time \(t\), \(C_{0}\) and \(C_{t}\) are the concentrations of the compound in mg/mL in the initial solution and at \(t\) time, respectively, \(W\) is the weight of adsorbent in g, and \(V\) is the volume of the solution in mL.