Adsorption Behavior and Mechanism of Methylene Blue, Crystal Violet, Eriochrome Black T, and Methyl Orange Dyes onto Biochar-Derived Date Palm Fronds Waste Produced at Different Pyrolysis Conditions

This study investigated the impact of pyrolysis conditions on physical characteristics of date palm fronds biochars and their performances for aqueous uptake of anionic dyes—methyl orange (MO) and Eriochrome Black-T (EBT)—and cationic dyes—methylene blue (MB) and crystal violet (CV). Detailed characterization of the biochars revealed the formation of oxygen functionalities (C=O, C-O-C, and C-O), improved surface characteristics (surface area and pore volume) and high ash content at higher pyrolysis temperature and time. Biochar-derived date palm with a high surface area of 431.82 m²/g and a pore volume of 0.134 cm³/g was obtained at pyrolysis temperature and time 700 °C and 4 h, respectively. For all the four investigated dyes, the adsorption isotherm was mainly described by Redlich–Peterson isotherm model (R² > 0.95), while the adsorption kinetics well-fitted the pseudo-second order model. The biochar yielded fast dyes adsorption with maximum adsorption capacity for MB, EBT, MO, and CV dye of up to 206.61, 309.59, 163.132, and 934.57 mg/g at 200 mg/L dye concentration, respectively. The adsorption of cationic dyes was pH-independent indicating the involvement of pi–pi and chemical interactions. However, the uptake of the anionic dye was favorable at acidic conditions and was dominated by electrostatic interactions involving ion exchange and chemical reactions. The produced biochars exhibited excellent surface characteristics and enhanced adsorptive performance relative to other similar adsorbents. Thus, the direct pyrolysis of date palm fronds’ waste is a sustainable and economical approach of converting a huge quantity of wastes into excellent adsorbent for effective remediation of dye-contaminated water and wastewater.