| Summary: | Cellulose substrate waste has demonstrated great potential as a biosorbent of pollutants from contaminated water. In this study, <i>Nerium</i><i>oleander</i> fruit, an agricultural waste biomaterial, was used for the biosorption of methylene blue from synthetic solution. Fourier-transform infrared (FTIR) spectroscopy indicated the presence of the main absorption peak characteristics of cellulose, hemicellulose, and lignin compositions. X-ray diffraction (XRD) pattern exhibited peaks at 2θ = 14.9° and 2θ = 22°, which are characteristics of cellulose <b>I</b>. Scanning electron microscopy (SEM) showed a rough and heterogeneous surface intercepted by some cavities. Thermogravimetric analysis (TGA) showed more than a thermal decomposition point, suggesting that <i>Nerium</i> fruit is composed of cellulose and noncellulosic matters. The pH<sub>pzc</sub> value of <i>Nerium</i> surface was experimentally determined to be 6.2. <i>Nerium</i> dosage, pH, contact time, dye concentration, and temperature significantly affected the adsorption capacity. The adsorption capacity reached 259 mg/g at 19 °C. The mean free energy ranged from 74.53 to 84.52 KJ mol<sup>−1</sup>, suggesting a chemisorption process. Thermodynamic parameters define a chemical, exothermic, and nonspontaneous mechanism. The above data suggest that <i>Nerium</i> fruit can be used as an excellent biomaterial for practical purification of water without the need to impart chemical functionalization on its surface.
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