# Evaluation of Agricultural Soil-Improving Zeolite for Improving Irrigation Water Quality
## 3 Results and Discussion
### Central Composite Rotatable Design (CCRD)
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Run** & **X\({}_{1}\)** & **X\({}_{2}\)** & **Zeolite Content** & **Contact Time** & **Na Removal** & **B Removal** \\  & & & **(g/L)** & **(min)** & **(\%)** & **(\%)** \\ \hline
1 & \(-\)1 & \(-\)1 & 12.43 & 194.33 & 16\% & 67\% \\ \hline
2 & 1 & \(-\)1 & 54.84 & 194.33 & 6\% & 67\% \\ \hline
3 & \(-\)1 & 1 & 12.43 & 765.67 & \(-\)147\% & 89\% \\ \hline
4 & 1 & 1 & 54.84 & 765.67 & 17\% & 67\% \\ \hline
5 & 0 & 0 & 33.64 & 480.00 & 10\% & 100\% \\ \hline
6 & 0 & 0 & 33.64 & 480.00 & 22\% & 100\% \\ \hline
7 & 0 & 0 & 33.64 & 480.00 & 15\% & 100\% \\ \hline
8 & 0 & 0 & 33.64 & 480.00 & 11\% & 100\% \\ \hline
9 & \(-\)1.4142 & 0 & 3.64 & 480.00 & 15\% & 94\% \\ \hline
10 & 0 & \(-\)1.4142 & 33.64 & 76.00 & 24\% & 67\% \\ \hline
11 & 1.4142 & 0 & 63.63 & 480.00 & 16\% & 98\% \\ \hline
12 & 0 & 1.4142 & 33.64 & 884.00 & 14\% & 95\% \\ \hline \end{tabular}
\end{table} 
Table 5: Factorial design (2\({}^{2}\)) results for sodium and boron removal (%) in irrigation water through zeolite treatment. For both species, negative and positive response relationships with the variables were observed. Specifically, zeolite content and contact time, both in their quadratic forms (Q12 and Q22), exhibited a negative relationship with the variable. In linear forms, contact time (Q2) for sodium and zeolite content (Q1) for boron also displayed a negative relationship. The interaction of other responses with the variables was positive (Table 6). Positive-effect values mean that an increase in their levels leads to an increase in ionic sorption, while negative values result in a decrease in the response as their levels increase [61]. Based on the significant variables, an analysis of variance (ANOVA) for the entire statistical model was conducted (Table 7). It was necessary to adjust confidence values when evaluating both independent variables simultaneously to confirm that FCal. > FTab. In this regard, the ANOVA results indicated that for sodium removal, the confidence interval was 75%, while for boron, it remained at 90% (Table 7). Based on the linear coefficients, quadratic coefficients, and their interactions, it is possible to conduct the response surface analysis, which helps identify the most appropriate conditions to maximize the removal of the studied cations using the desirability function for multiple response optimization [35; 61; 63]. The 3D response surface plots are shown in Figure 1, once again highlighting the differentiated response of each cation. The maximum sodium removal values were established at around 50% with a zeolite content of up to 10 g/L or between 60-70 g/L, and a contact time of up to 100 min or between 800-1000 min, respectively. However, with a zeolite content of up to 35 g/L and a contact time exceeding 500 min, potential desorption and/or release of sodium by the zeolite was observed, resulting in negative removal values. This situation was observed again with a shorter contact time, up to 200 min, but a higher zeolite content (<55 g/L). Removal of boron, on the other hand, showed a higher response when longer contact times were applied (between 600 and 800 min), displaying a central zone of high removal (> 95%), with a zeolite content of up to 50 g per liter. However, according to Figure 1B, contents \(\leq\) 20 g of zeolite/L allowed for maximum boron removal (100% removal). In all scenarios, with low contact times (\(\leq\)300 min), the removal efficiency was significantly impacted. Based on the obtained results, the statistically defined critical values for the maximum efficiency of the cation removal process from irrigation water using zeolite were determined. Due to their differentiated behavior, the critical values were chosen based on the experimental design for boron, confirmed by ANOVA of the model at a 90% confidence interval and presenting the highest sorption percentages with zeolite. Thus, 4.93 g of zeolite/L and 723 min were determined as the best theoretical conditions for the studied process.