# Sustainable Removal of Cr(VI) by Line Peel and Pineapple Core Wastes
## 3 Results and Discussion
### Adsorption in the Optimized Conditions  
Finally, the scale-up of the adsorption system was studied through the simulation by using FAST v2.1 software. This software was developed according to the homogeneous surface diffusion model (HSDM) under the following assumptions: (i) one-dimensional plug-flow, (ii) the adsorbent particles behave as a pseudo-homogeneous medium wherein the pollutant diffuses, (iii) external mass-transfer limitation is accounted for, (iv) adsorption equilibrium prevails at the fluid-solid external surface [36]. The usage of this software requires the input of several parameters: (i) operational conditions (working volume, influent concentration, and flow rate), (ii) adsorbent characteristics (bed density, particle density, and diameter), (iii) mass transfer coefficient and equilibrium parameters (Freundlich or Langmuir constants) obtained in the adsorption isotherm assays. Initially, the experimental determination of the required parameters was performed. Thus, adsorption equilibrium isotherms at optimal conditions (pH 2.01, PP dosage 1.50 g, and 30.05 \({}^{\circ}\)C) were evaluated with concentration ranging from 50-500 mg/L of Cr(VI) for 24 h (Figure 5). The maximum experimental uptake reached (8.8 mg/g) for the Cr adsorption was compared with the maximum uptake those reported in the literature for different adsorbents and shown in Table 4. This value was much the same to that determined by [PERSON] et al. [37] (approximately 10 mg/g) using the by-product of _Lentius codes_ and by [PERSON] et al. [38] using reed mat as a biosorbent (7.18 mg/g). Notable enhance those reported by [PERSON] et al. [39] (3.15 mg/g) using untreated rice straw. This fact demonstrated the reliability of using this untreated material for the remediation of Cr(VI) that is present in many affluents. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**adsorbent**} & \multicolumn{3}{c}{**Optimum Adsorption Conditions**} & \multicolumn{1}{c}{**Maximum**} & \multicolumn{1}{c}{**Maximum**} & \multirow{2}{*}{**Estimated**} \\  & \multicolumn{1}{c}{**pH**} & **Dosage (g/L)** & **Temp.** & & & **Cr(VI)** & **Cr Uptake** \\  & \multicolumn{1}{c}{**(g/L)**} & \multicolumn{1}{c}{**(\({}^{\circ}\)C)**} & **Removal (\%)** & **(mg/g)** & & & \\ \hline Chitosan & 3 & 13 & 24.85 & - & 7.94 & 82.04 & [5] \\ Pineapple core & 2 & 30 & 30.05 & 92.39 & 8.8 & 99.29 & This work \\ _Lentius codes_ & 3.9-4.4 & 20 & 25 & - & 10 & - & [37] \\ Reed mat & 4.5–5.5 & 10 & 30 & 52.83 & 7.18 & 25.18 & [38] \\ Mangrove leaves & 4.5–5.5 & 10 & 30 & 99.35 & 8.87 & 189.02 & [38] \\ Water lily & 4.5–5.5 & 10 & 30 & 96.49 & 8.44 & 126.44 & [38] \\ Untreated rice straw & 2.0 & 10 & 27 & 69.25 & 3.15 & 45.49 & [39] \\ Lignocellosic & & & & & & & \\ substrate from & 3.1 & 8 & 25 & - & 9.9 & - & [40] \\ wheat bran & & & & & & & \\ Activated carbon & 3.72 & 10 & 45 & 65.7 & 5.09 & 191.55 & [17] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Adsorption test conditions and uptake capacities of various adsorbents for Cr(VI) removal. Figure 5: Isotherms models: **(a)** Langmuir; and **(b)** Freundlich. Operational conditions: Cr(VI) concentration 50–500 mg/L, PP dosage 1.50, pH 2 and temperature 30.05 \({}^{\circ}\)C during 24 h.  
Langmuir and Freundlich, two commonly used models, were applied to the experimental data (Figure 5). The two equations below (Equations (9) and (10)) determine Langmuir and Freundlich models, respectively:  
\[\mathrm{q_{e}}=k_{L}\times b\times C_{e}/(1+b\times C_{e}) \tag{9}\]  
\[\mathrm{q_{e}}=k_{F}\times C_{e}^{1/n} \tag{10}\]  
where \(k_{L}\) (mg/g) and \(b\) (L/ mg) are the Langmuir constants which reflect the adsorption capacity and rate of adsorption, respectively, and the Freundlich constants are \(k_{F}\) (mg/g)(L/mg)\({}^{n}\), the adsorption capacity, and \(n\) is the adsorption intensity. According to the correlation coefficient (inset Figure 5), the adsorption of Cr on PP showed good correspondence to Freundlich isotherm.