## 2 Materials and Methods
### Third Phase: Characterization of Leachates and Effluents  
The American Standard Test Methods (ASTM) were used to determine the DOC and color. Standardized APHA (American Public Health Association) methods were used for pH, COD, BOD\({}_{5}\), Cl\({}^{-}\), electrical conductivity (ELC), oxidation/reduction potential (Eh), and NH\({}_{3}\)-N. The methods used to characterize the leachates and effluents are listed in Table 1. \begin{table}
\begin{tabular}{l l l} \hline \hline
**Parameter** & **Method** & **Description** \\ \hline Chemical oxygen demand & 5220D & Closed-reflux colorimetric method. \\ Biochemical oxygen demand & 5210 & Incubation for five days with oxygen monitoring. \\ Dissolved organic carbon & ASTM D7573 09 & Catalytic combustion 950 \({}^{\circ}\)C. \\ Ammoniacal nitrogen & 4500 C & Volumetric method with H\({}_{2}\)SO\({}_{4}\) predistillation. \\ pH & 4500 H & Electromagnetic method for measuring the electromotive force. \\ Electrical conductivity & 2510 & Electromagnetic method using an electric field. \\ Oxidation/reduction potential & 2580 B & Potentiometric determination using an indicator electrode. \\ Chlorides & 4500 B & Argentometric method with precipitation of AgCL \\ Color & ASTM 1209-05 19 & Percentage transmittance of light. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Analytical methods used for characterization of leachates and effluents [49; 50; 51]. Figure 1: Study area. Bordo Poniente landfill, Mexico City. The inert or non-biodegradable COD fraction (CODnb) was determined by a aerating a 1.0 L sample, keeping the initial volume constant by adding distilled water. CODb was determined by the difference in CODt [23; 40]. The CODs were determined using ZnSO4 as a coagulant and passing the sample through a 0.45 \(\upmu\)m membrane filter. Particulate COD (CODp) was obtained by subtracting CODt [23; 38]. CODb was reported as CODb, whereas non-biodegradable soluble COD (CODsnb) was determined by the difference between CODs and CODsb [22]. In addition, the percentage of each COD fraction (in relation to CODt or CODb) was calculated to identify variations in the data during the treatment process. The humic fractions of organic matter in the leachates and effluents (HA, FA, and HyL) were determined according to their solubilities in acidic and basic media [35]. HA is only soluble in alkaline media, whereas FA can solubilize under any pH condition [24]. The methodology used to determine the fractions of humic substances is presented in detail in Figure 2.  
Equipment and Materials  
The total COD (CODt) of leachates and samples was determined by a closed-reflux method using a portable DR1900 Hach spectrophotometer(Hach(r), Loveland, CO, USA). In this study, mercuric sulfate (HgSO4) was used to reduce Cl- interference when the concentration exceeded 2000 ppm, as recommended by APHA method 5220D [49]. DOC was determined using a Shimadzu TOC-NT CSH analyzer(Shimadzu(r), Tokyo, Japan),  
Figure 2: Procedure developed for fractionation of humic substances. HA—humic acids; FA—fulvic acids; HyL—hydrophilic fraction; DAX 8—polymethylmethacrylate resin; DOC—dissolved organic carbon; COD—chemical oxygen demand. Adapted from [PERSON] et al. [35]. equipped with a self-sampler. The ELC and pH were measured using a Hach Sension+ MM374 pH meter(r) (Hach(r), Loveland, CO, USA) equipped with pH and conductivity probes. All the chemicals used were of laboratory grade (Merck KGaA(r), Darmstadt, Germany). The DAX 8 (Supelite(r) DAX-8) resin was purchased from Sigma-Aldrich(Merck KGaA(r), Darmstadt, Germany). It is a non-functionalized resin with strong hydrophobic organic matter, endorsed by humic and fulvic acids. MilliQ(Merck KGaA(r), Darmstadt, Germany) water was used for all dilutions of the reagents and samples. Before use, all glass articles were cleaned with 1% HCl solution and rinsed with Milli Q water. All analyses were performed in triplicates. The characteristics of the organic matter in the samples were studied using two spectroscopic methods: UV-VIS and FTIR.