## 3 Results and Discussion
### Extraction Efficacy  
Temporal variation of extraction efficacy was studied under optimized conditions. As Figure 1 shows, the extraction efficacy increased with the extraction time, although after 9 h this increase was almost negligible. A control sample after 24 h showed an extraction efficacy of 35.41 \(\pm\) 4.05%, close to the value obtained after 9 h (30.15 \(\pm\) 1.90%). Then, an extraction time of 9 h was selected with the aim of reducing extraction time as much as possible without loss of efficacy. Under these conditions, procedure blanks were performed, obtaining a nickel concentration of 0.41 \(\pm\) 0.26 ug\(\cdot\)L\({}^{-1}\) in the receiving solution, low enough to allow the application of the system to real natural waters. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Vertex**} & \multirow{2}{*}{**Type**} & **HCl** & **1,2-CHBBH** & **P\(\cdot\)103** \\  & & **(mol\(\cdot\)L\({}^{-1}\))** & **(mmol\(\cdot\)L\({}^{-1}\))** & **(cm\(\cdot\)min\({}^{-1}\))** \\ \hline
1 & I & 0.50 & 1.00 & 1.7 \\
2 & I & 1.00 & 1.00 & 2.5 \\
3 & I & 0.75 & 1.44 & 2.0 \\
4 & R & 1.25 & 1.44 & 1.8 \\
5 & C & 1.06 & 1.33 & 2.9 \\
6 & R & 1.31 & 0.89 & 2.1 \\
2\({}^{\prime}\) & I\({}^{\prime}\) & 1.00 & 1.00 & 2.0 \\
7 & R & 1.38 & 1.22 & 1.9 \\
8 & C & 1.09 & 1.05 & 2.1 \\
5\({}^{\prime}\) & R\({}^{\prime}\) & 1.06 & 1.33 & 1.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simplex optimization of chemical variables. Figure 1: Temporal variation of extraction efficacy. Aqueous solution: 100 ug\(\cdot\)L\({}^{-1}\) Ni\({}^{2+}\), pH = 8, 35 g\(\cdot\)L\({}^{-1}\) NaCl. Receiving solution: 1.3 mol\(\cdot\)L\({}^{-1}\) HCl. Organic solution: 0.89 mmol\(\cdot\)L\({}^{-1}\) 1,2-CHBBH. Stirring rate: 500 rpm. Effect of Dissolved Organic Carbon  
The study of temporal variation in the extraction efficacy was also carried out for a real sample with different DOC concentrations in the range of 0 to 12.58 mg\(\cdot\)L\({}^{-1}\), added as humic acid. The results are shown in Figure 2, where a decrease of E\({}_{\text{e}}\) can be observed with the increase of DOC concentration in the source solution. Nevertheless, temporal behaviour was similar for the different DOC concentrations, allowing the use of an extraction time of 9 h regardless of the DOC content in the sample. The values of E\({}_{\text{e}}\) obtained at this time are shown in Table 2 for each DOC concentration in addition to the theoretical major species distribution. From these data, a negative linear correlation can be established between the experimental values of E\({}_{\text{e}}\) and the organic complexes (Ni-HA) formed in the source solution with a Spearman correlation coefficient of \(-1.0\). These results show the existence of a nickel fraction in the sample related to the organic complex formation, which cannot be transported through the liquid membrane. As a previous work showed, organic nickel complexes remain in the unextracted fraction when 1,2 CHBBH is used to extract nickel in the presence of humic acids [28]. In addition, a linear relationship between the E\({}_{\text{e}}\) and the DOC concentration could be established by Equation (3):  
\[\text{E}_{\text{e}}=-1.15\times[\text{DOC}]+30.88 \tag{3}\]  
Then, in the presence of organic matter, the extraction efficacy for 9 h can be calculated for each sample using the DOC concentration, representing the fraction of total nickel transported through the membrane. To validate this relationship between DOC concentration and E\({}_{\text{e}}\), analysis of nickel fractions in real water samples was performed.