Waste-derived carbon nano-onions for the mitigation of Coomassie brilliant blue dye: statistical optimization and phytotoxicity assessment of treated water

Abstract

Herein, we have synthesized carbon nano-onions (CNOs) via a simple wick pyrolysis method and used them as an adsorbent material to remove Coomassie brilliant blue (CBB) dye from an aqueous medium. CNOs were found to be quasi-spherical in morphology with a diameter ranging from 20 to 30 nm. BET analysis of CNOs showed a specific surface area of ∼124.22 m² g⁻¹ and a pore radius of 1.69 nm. The as-synthesised CNOs were used here to remove the CBB dye via systematic batch adsorption experiments. A CBB dose of 20 mg L⁻¹ yielded a removal efficiency of ∼97.8% with 5 mg of CNOs at pH ∼ 7 in 30 min at room temperature. The adsorption by CNOs follows pseudo-second-order (PSO) kinetics, which best fits the Langmuir adsorption isotherm model. The statistical analysis of the dye adsorption process using analysis of variance (ANOVA) with the Box–Behnken design model (BBDM) identified key parameter values that closely match those from the batch experiments. The adsorption process shown here is endothermic and proceeds spontaneously, as indicated by thermodynamic characteristics. After adsorption, the recovered and desorbed CNOs were recycled for five sequential adsorption–desorption cycles. Moreover, to address the water reuse issue, Vigna radiata (V. radiata) seeds were germinated with the treated wastewater, demonstrating better growth than with untreated dye wastewater and comparable growth to the control water sample, supporting the environmental applicability of the described procedure.