Evaluation of waste tomato stems in the removal of safranin O and methylene blue dyes from aqueous solution with a zero waste approach

In this study, the removal of two cationic dyes Safranin O (SO) and Methylene Blue (MB) from aqueous solutions was investigated using waste tomato stems (TS) as a novel, natural bio-adsorbent within a zero-waste approach. The effects of pH, adsorbent amount, particle size, initial dye concentration, and contact time were systematically optimized to determine the most effective adsorption conditions. The optimum conditions for SO dye were determined as pH 10, adsorbent dosage 1.5 g/L, adsorbent size 35 mesh, initial dye concentration 25 mg/L, and contact time 45 min. For MB dye, the optimum conditions were pH 6, adsorbent dosage 2.0 g/L, adsorbent size 35 mesh, initial dye concentration 25 mg/L, and contact time 45 min. Adsorption/desorption experiments confirmed that the TS could be reused for at least five cycles with high efficiency, demonstrating its potential for sustainable wastewater treatment. Among the adsorption models tested, the pseudo-second-order kinetic model and Temkin isotherm provided the best fit, suggesting chemisorption on a heterogeneous surface. Thermodynamic analyses revealed that the adsorption processes for both dyes were exothermic and spontaneous. This study highlights waste tomato stems as a low-cost, eco-friendly, and reusable bio-adsorbent for dye-contaminated wastewater, offering a new route toward sustainable resource utilization and circular economy practices.