Greener synthesized P-doped carbon for dual applications: selective cationic dye removal with phytotoxicity assessment and industrial effluent treatment

Abstract

This study presents a novel approach to cationic dye remediation through the strategic valorization of onion peel (OP) waste—a precursor uniquely rich in organosulfur and flavonoid compounds into a phosphorus-doped activated carbon (OPP-1) via one-step H₃PO₄ activation. Unlike conventional lignocellulosic wastes, the inherent heteroatom content and biochemical complexity of OP synergize with phosphoric acid to produce a mesoporous carbon (535.5 m² g⁻¹) with enhanced surface functionality, hydrophilicity (contact angle: 10.1°), and strong negative surface charge (−14.4 mV). OPP-1 exhibits exceptional selectivity for cationic dyes, achieving >90% removal of safranin-O (99.99%), methylene blue, brilliant green, and methyl violet, with significantly lower uptake of anionic dyes. Safranin-O adsorption followed Langmuir and pseudo-second-order kinetics, indicative of monolayer chemisorption driven by electrostatic, π–π, and hydrogen-bonding interactions. The synthesized nanocomposite was tested using two textile effluents (IW-1 and IW-2) by time-dependent UV-visible spectroscopy. Dye adsorption was confirmed by a constant decrease in absorbance at 512 and 510 nm. It was able to remove 95.80% of IW-1 within 50 minutes and 98.75% of IW-2 within 60 minutes. Notably, phytotoxicity assays using Vigna radiata demonstrated substantial detoxification, with root and shoot growth recovering to 91% and 82% of control levels, respectively. The adsorbent also maintained 58% efficiency over 10 regeneration cycles. This work highlights a sustainable, waste-to-resource strategy for designing highly selective adsorbents, positioning OP-derived phosphorous-doped carbon as an effective and eco-friendly solution for cationic dye contamination.