## 3 Obtaining Waste-Based Bioadsorbents
### Raw Wastes as Bioadsorbents
Other studies have evaluated black tea and mixed tea wastes as low-cost adsorbents for the removal of several heavy metals; for instance, sulphonate-treated tea waste was an effective biosorbent for the removal of Cr(VI), MB, and tetracycline [90; 91; 92; 93]. Different amounts of Fe, Mn, Zn, Ni, Cu, and Cd were removed from aqueous solutions by employing a biosorbent from sunflower stalks and leaves, both without pretreatment and activated with NaOH to improve Ni adsorption [89]. \begin{table}
\begin{tabular}{c c c c c} \hline
**Biowaste** & **Pretreatment** & **Characterization** & **Contaminant** & **Ref.** \\ \hline Potato leaf/stem & No pretreatment & FTIR, SEM & MB, MG & [84] \\ \hline Red onion and Red dragon fruit peels & No Pretreatment & SEM, FTIR, EDX, XPS, XAS & I\({}^{-}\) & [79] \\ \hline Rice husk & No Pretreatment & BET, FTIR, PZC, SEM-EDS & NFX & [71] \\ \hline Rice straw & No Pretreatment & FTIR, BET, FTIR, SEM, EDX & CFA, CBZ, Pb(II) & [16; 85] \\ \hline Soybean and wheat straw & No treatment, Formaldehyde, NaOH and H\({}_{2}\)SO\({}_{4}\) treatment. & N.R & Cu, Ni, Cd, Pb & [73] \\ \hline Spent ground coffee & No treatment, Magnetic modification (Fe\({}_{3}\)O\({}_{4}\) NPs) & BET, XRD, FTIR, SEM-EDX, ZP & Hg(II), Pb(II) & [28; 55] \\ \hline Sugarcane baggase (SCB) & No treatment, NaOH, HCl treatments & FTIR, SEM & Fe(III), Cu(II), Pb(II), Zn(II), Cd(II), Co(II), Mn(II) & [86; 87] \\ \hline SCB and beet pulp & NaOH treatment & FTIR, SEM & Mn(II) & [88] \\ \hline Sunflower leaf/stalk & No treatment, NaOH activation (Ni removal) & PZC, SEM & Fe, Mn, Zn, Ni, Cu, Cd & [89] \\ \hline Tamarind seeds & H\({}_{2}\)SO\({}_{4}\) treatment, Ultrasonic modification & FTIR, SEM & Pb(II) & [32] \\ \hline Tea waste (Black tea) & No treatment, & BET, BSILD, SEM-EDX, FTIR, & \\  & Sulfonation (H\({}_{2}\)SO\({}_{4}\)) & TGA, Raman, XPS, ESR, SAXS & Cu, Pb, MB, Tet, Cr(VI) & [90; 91] \\ \hline Tea waste (mixed tea) & No pretreatment & SEM, TEM, EDS, BET, FTIR, XPS & Mn(II), Zn(II), Cr(VI) & [92; 93] \\ \hline Watermelon finds & No treatment, Ca(OH)\({}_{2}\), Citric acid treatments & SEM-EDX, BET, MP, PZC, FTIR & Zn, Pb, Cu(II), Cr(III) & [62; 63; 64; 65; 66] \\ \hline N.R, not reported; MeOH, Methanol; Fe\({}_{3}\)O\({}_{4}\) NPs, Fe\({}_{3}\)O\({}_{4}\) nanoparticles; MO, Methyl orange; MB, Methylene blue; RB, Rhodamine B; CR, Congo red; MV, Methyl violet; AB-10, Amido black 10B; F, Fluoride; BBY, Bismark brown Y; CV, Crystal violet; SO, Safrani O; NY, Nyrkorica; AG52, Acid Green 25; RR-120, Reactive red 120; PO, Procion orange; DR23, Direct red 23; DR80, Direct red 80; RB-114, Reactive blue 114; MG, Malachite green; CEA, Clofibric acid; CBZ, Carbanazepine; Tet, tetracycline; PZC, point of zero charge; EDS, Energy dispersive \(X\)-ray spectroscopy; XPS, X-ray photoelectron spectroscopy; XAS, X-ray absorption spectroscopy; XRD, X-ray diffraction; ZP, zeta potential; FESEM, Field Emission Scanning Electron Microscope; BSILD, back-scattered laser light diffraction; TGA, Thermogravimetric analysis; ESR, Electron spin resonance spectroscopy; SAXS, Small-angle X-ray scattering; TEM, Transmission electron microscopy; MP, mercury porosimetry. \\ \hline \end{tabular}
\end{table} 
Table 2: _Cont._Bioadsorbents have been implemented as one possible solution to solve the problem of colored wastewater discharges from the textile industry. For example, durian peel was collected as solid waste and used directly for the uptake of acid green 25. Durian peels were ground and screened to a particle size range of 351 to 589 \(\upmu\)m before being treated with HCl and dried at 80 \({}^{\circ}\)C [74]. Another excellent adsorbent to remove dyes such as crystal violet (CV) is non-pretreated grapreruit peel, which is first separated from leaves, twigs, and other debris that interfere into the adsorption process and then dried at 70 \({}^{\circ}\)C, ground, and sieved to a particle size of 0.85-1.0 mm [75]. [PERSON] et al.