Efficient Adsorption of Anionic Azo Dye in Wastewater with Medulla tetrapanacis Modified by Cationic Surfactant

In this study, Medulla tetrapanacis (MT), a kind of herbaceous plant, was modified by phosphoric acid (H3PO4) and cetyltrimethylammonium bromide (CTAB) to obtain the adsorbent MT-H3PO4-CTAB. It could quickly and efficiently remove the anionic azo dyes, Congo Red (CR) and Ponceau S (Po.S), from solution. For 500 mg/L CR and 150 mg/L Po.S dye solution, the removal efficiency could reach 84.52% and 95.29% within 10 min, and the maximum adsorption capacities of MT-H3PO4-CTAB towards CR and Po.S were 719.42 mg/g and 230.94 mg/g at 303 K, respectively. The effects of initial pH of the solution, temperature, initial dye concentration, and contact time were optimized. The adsorption process followed the pseudo-second-order kinetic model, and the equilibrium adsorption data fitted the Langmuir isotherm well. Moreover, thermodynamic studies indicated that the adsorption of two anionic azo dyes was spontaneous. The adsorption of CR onto MT-H3PO4-CTAB was endothermic while the adsorption of Po.S was exothermic. Molecular interactions, such as electrostatic attraction and hydrogen bonding between dyes molecules and adsorbent, play an important role in the adsorption process. This study demonstrates that acid-activated and surfactant-modified herbaceous plant adsorbent has a simple preparation process and exhibits excellent adsorption properties for anionic azo dyes, which could extend the potential applicability for biomass.