## 3 Obtaining Waste-Based Bioadsorbents
### Raw Wastes as Bioadsorbents
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Biowaste** & **Pretreatment** & **Characterization** & **Contaminant** & **Ref.** \\ \hline Banana peel & No treatment, \begin{tabular}{c} Electrification \\ (MeOH-HCl) \\ \end{tabular} & FTIR, BET, SEM, PZC, EDX & MO, MB, RB, CR, MV, AB-10, F, Cd(II), Cr(VI) & [60; 61; 67; 68] \\ \hline Barley straw & \begin{tabular}{c} No treatment, Citric \\ acid–NaOH treatment, \\ Magnetic modification \\ \end{tabular} & FTIR, SEM & BBY, CV, MB, SO & [69] \\ \hline Cauliflower leaf & No pretreatment & FTIR, SEM & MB & [70] \\ \hline Coffee husk & No pretreatment & BET, FTIR, PZC, SEM-EDS & NFX & [71] \\ \hline Coconut coir dust & No pretreatment & FTIR & MB & [72] \\ \hline Coir pith & No pretreatment & N.R & Co(II), Cr(III), Ni(II) & [59] \\ \hline Corn cob and stalk & \
\begin{tabular}{c} No treatment, Formaldehyde, \\ NaOH-H\({}_{2}\)SO\({}_{4}\) treatment \\ \end{tabular} & N.R & Cu, Ni, Cd, Pb & [73] \\ \hline Durian peels & HCl treatment & N.R & AG25 & [74] \\ \hline Graprefruit peel & No pretreatment & FTIR & CV & [75] \\ \hline Jackfruit peel & No pretreatment & N.R & MB & [76] \\ \hline Kiwi and Tangerine peels & NaOH treatment & N.R & Cd(II), Cr(III), Zn(II) & [77] \\ \hline Mango peel & No pretreatment & PZC, FTIR, SEM, EDX & Cd(II), Pb(II) & [78] \\ \hline Mangosteen pericarps & No Pretreatment & SEM, FTIR, EDX, XPS, XAS & I\({}^{-}\) & [79] \\ \hline _Moringa oleifera_ seeds & No pretreatment & FTIR, SEM & RR-120, Ag(I) & [56; 57] \\ \hline Orange peel & No pretreatment & BET, SEM & CR, PO, RB, MO, MB, MV, AB-10, DR23, DR80 & [67; 80; 81] \\ \hline Passion fruit rinds & No Pretreatment & SEM, FTIR, EDX, XPS, XAS & I\({}^{-}\) & [79] \\ \hline Peanut husk &
\begin{tabular}{c} Chemical modification \\ (Fe\({}_{3}\)O\({}_{4}\)-IA-Zr) \\ \end{tabular} & BET, SEM, FTIR, XPS, VSM, XRD & Phosphates & [82] \\ \hline Pomelo peel & No pretreatment & ZP, FESEM, FTIR, BET & RB-114 & [83] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Pretreated and non-treated raw biowastes implemented as bioadsorbents to remove a wide variety of heavy metals, industrial contaminants, ions, and pharmaceuticals. Mango is another abundant fruit grown in tropical and subtropical regions across the world; unfortunately, the peel constitutes 7-24% of the fruit's weight and is not a recyclable byproduct. For this reason, mango peel waste was evaluated as an adsorbent for Cd(II) and Pb(II) ions. It was oven-dried at 70 \({}^{\circ}\)C, ground and sieved until obtaining a particle size of 0.85-1.0 mm [78]. Another interesting fruit waste used to remove Cd(II), Cr(III), and Zn(II) from water was the kiwi peel, ground to two particle sizes of 1 mm and 2 mm and treated with NaOH [77]. Tangerine peel processed in the same way proved to be an excellent source of biomass for the removal of these metals from surface and groundwater due to its chemical composition, which is rich in cellulose and other polysaccharides [77; 94]. [PERSON], et al. [73] evaluated the efficiency of soybean straw, wheat straw, corn stalks, and corn cobs for the adsorption of Cu, Ni, Cd, and Pb from wastewater both with formaldehyde, NaOH and H\({}_{2}\)SO\({}_{4}\) modification and without chemical modification. In another study, [PERSON] et al. [16] dried and ground rice straw into different particle sizes and found the 75-150 um size to be the most optimal for Pb removal. Sugar cane baggage (SCB) and beet pulp have been tested as effective adsorbents for other heavy metals such as Fe(III), Zn(II), Co(II), and Mn(II). These biowastes were soaked in NaOH and CH\({}_{3}\)COOH to remove hydroxide traces before being dried, powdered, and sieved to an average particle size of 0.75 mm [86; 88]. SCB was evaluated in raw conditions and after chemical pre-treatment with hydrochloric acid; the latter had a significant positive effect on its capacity to remove Mn(II) [87].