## 5 Water Treatment Employing Bioadsorbents
### Bioadsorbents Based on Fruits and Vegetables  
#### 5.2.1 Heavy Metals  
Biosorbents have proven to be a cheap and effective alternative for the removal of heavy metals in polluted water thanks to their high capacity and selectivity for metal ions [102]. Removal efficiencies for Cu(II) and other heavy metals employing adsorbents derived from fruits and vegetables are presented in Table 4, including the most suitable kinetic and isothermal models. [PERSON] et al. [3] reviewed watermelon finds as a copper adsorbent, reporting removal efficiencies between 58.4% and 88%. An increase in adsorption capacity was observed as the temperature approached the optimal drying temperature (120 \({}^{\circ}\)C), which increased the presence of active sites. Another fruit-based adsorbent that showed a high removal percentage of copper ions (78.8%) was pomergnate peel [103]; four models were considered for the analysis of its adsorption behavior, Langmuir, Freundlich, Dubinin-Radushkevic, and Temkin. These indicated the maximum pomergnate peel adsorption capacity at 30.12 mg/g; the heavy metal was indirectly adsorbed as a mono- or multilayer on a heterogeneous and porous surface [103]. AC derived from coconut buttons efficiently retained Cu(II), Pb(III), and Hg(II), with adsorption capacities (Q\({}^{\circ}\)) from 76.3 to 97.7 mg/g under batch conditions at two pH values (6 and 7) [37]. [PERSON] et al. [104] obtained removal percentages of 99.7% for Cu(II) from industrial wastewater by implementing raw corn cob at pH 5 and 40 \({}^{\circ}\)C with a contact time of 60 min. The maximum adsorption capacity was 6.24 mg/g (Langmuir isotherm) following pseudo-second order kinetics. \begin{table}
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Cr(III) and Pb(II) are other metals that commonly appear in water matrix analyses. Cr(III) was removed by watermelon finds and coconut residues with an efficiency and maximum adsorption capacity up to 98.2% and 172.6 mg/g, respectively, under acidic conditions (pH 3) [3; 59]. Kiwi and tangerine peels exhibited notable adsorbing efficiencies (98% and 96% respectively) for Cr(III), reaching equilibrium in 60 min at pH 6.0 [77; 94]. As for Cr(VI), banana peels adsorbed it following the Langmuir isotherm with a q\({}_{\rm m}\) of 131.56 mg/g after 30 min of contact time [61]. Watermelon rinds sequestered Pb(II), reaching removal percentages between 72% and 99.9% within a temperature range of 20-30 \({}^{\circ}\)C [3]. When an acidic and alkaline treatment was carried out on the watermelon rinds it was possible to capture from 52.4 to 90.3% of Zn(II); although the lowest removal percentages were observed in the presence of other metals in solution, this demonstrated that the simultaneous bioadsorption of multiple pollutants is possible. Similarly, tangerine and kiwi peels achieved a Zn(II) adsorption efficiency of approximately 52% and 57%, respectively, at pH 6.0, respective with q\({}_{\rm m}\) values of 38.4 and 37.0 mg/g [77]. After identifying the optimal adsorption conditions, papaya peels displayed a removal efficiency of 93.2% for Pb(II), and according to the Langmuir and kinetic models the adsorptive capacity ranged from 38.31 to 42.5 mg/g [39]. Another fruit peel that presented good removal efficiency  
\begin{table}
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**Biowaste** & **Contaminant** & \begin{tabular}{c} **Isothermal** \\ **Model Adjusted** \\ \end{tabular}  & \begin{tabular}{c} **Kinetic** \\ **Model Adjusted** \\ \end{tabular}  & \begin{tabular}{c} **q\({}_{\rm m}\)/Q\({}^{\circ}\)** \\ **Efficiency (\%)** \\ \end{tabular}  & **Ref. ** \\ \hline \ \begin{tabular}{c} Pineapple waste \\ (ZnCl\({}_{2}\)-activated) \\ \end{tabular}