# Sustainable Removal of Cr(VI) by Line Peel and Pineapple Core Wastes
## 3 Results and Discussion
### Experimental Design  
Based on the methodology of design of experiments, a series of experiments were completed according to the three-level Box-Behnken matrix. In this study design, while one of the items was fixed at its center level, combinations of the different levels of the remaining factors were put into practice [23].  
Cr removal/adsorption is related to solution pH because pH exerts influence on Cr speciation and also affects active functional groups (-OH, -COOH, -NH\({}_{2}\)) influencing their dissociation [12]. Moreover, the temperature of the wastewater containing Cr may also vary depending on the industrial procedure and it is a factor to be considered in the efficiency of the process because it has great influence both in the redox reaction and in the adsorption process [24; 25].  
Accordingly, temperature, pH and biosorbent dosage were selected as independent variables, and Cr(VI) and total Cr removal were established as responses of the 17 runs performed (Table 1). Cr(VI) removal values close to 90% were obtained in runs 4 and 12, when the lowest pH was used and operating at medium and highest values of the biosorbent dosage, as well as temperature. Therefore, it was confirmed the great influence of the initial pH on the Cr(VI) removal. This fact is in agreement with the indirect and direct mechanisms previously described in Section 3.1, since protons participate in the Cr(VI) to Cr(III) redox reaction and at low pH the amino and carboxyl groups turn out to be protonated, increasing the rate of anionic Cr(VI) elimination from aqueous phase [26]. On the other hand, as it is shown in Table 1 the removal of Cr(VI) enhanced with an increase of temperature and biosorbent dosage. It is in concordance with the fact previously reported by [PERSON] and [PERSON] [27] where the endothermic nature of the Cr(VI) removal procedure was established.  
Following the tendency detected in the initial experiments (Section 3.1) the percentages of total Cr removal onto the PP were lower than Cr(VI) removal percentages. The highest adsorptions (around 35%) were obtained in runs 2 and 4, when the highest biomass dosage was used. According to [PERSON] and [PERSON] [12], this trend indicates a first-order kinetic model adsorption. In addition, the total Cr removal is clearly favored at high temperatures and low pHs (Table 1). These facts corroborate the endothermic essence of adsorption procedure and the great influence of pH in the removal process.  
Apart from the selected responses, adsorption capacity and distribution coefficient were calculated to confirm the attained results (Table 1). Similarly to the selected responses, the uptake increased operating at more acidic pH value and reaching the highest value at medium and highest values of  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Wavelength (cm\({}^{-1}\))** & **Bond Vibration** & **Related Structure** \\ \hline
3311 & O-H stretching & lignin [22] \\
2900–3000 & C-H stretching & lignin [22] \\
1720–1730 & C=O stretching & carbonyl group \\
1624 & O-H bending & Polysaccharides \\
1600 & N-H bending & amine amide \\
1420 & C-H in-plane deformation & lignin [22] \\
1370 & C-H bending & cellulose, hemicellulose, lignin [22] \\
1247 & O-H bending & cellulose, hemicellulose [22] \\
1030 & C-O, C=C, and C-C-O stretching & cellulose, hemicellulose, lignin [22] \\
960 and 730 & Glycosidic linkage & cellulose, hemicellulose [22] \\ \hline \hline \end{tabular}
\end{table}
Table 2: FTIR analysis of functional groups.  
temperature. On the other hand, \(K_{D}\) presented the highest values operating at the maximum value of temperature. Those results confirm the influence of the selected variables in the performance of the biosorbent.