Valorization of Tomato Stems into Biochar for Efficient Adsorptive Removal of Cationic and Anionic Dyes from Aqueous Solutions

The biochars obtained by pyrolyzing tomato stems at temperatures of 400, 500, 600, and 700 °C were characterized, and their ability to absorb anionic (Direct Orange 26, DO26) and cationic (Rhodamine B, RhB) dyes from aqueous solutions was investigated. The effects of solution pH and ionic strength were studied. It was found that the adsorption process of both dyes was pH-dependent, but no effect of ionic strength was observed. The kinetics of dye adsorption on biochars were well described by the pseudo-second-order model. The equilibrium adsorption data were analyzed using the Freundlich, Langmuir, and Temkin isotherms. All three equations described dye adsorption on biochars quite well, although a slightly better fit was observed for the Freundlich model. The maximum adsorption capacities of BCs ranged from 54.44 mg/g (BC400) to 108.1 mg/g (BC700) for DO26 and from 4.483 mg/g (BC700) to 8.887 mg/g (BC400) for RhB. The study reveals that biochars derived from tomato stems can be used as efficient, low-cost adsorbents for the removal of anionic and cationic dyes from water.