Exploitation of an agro-industrial waste in eco-friendly clean-up: adsorptive removal of Congo red dye from aqueous media using chemically treated olive pomace

In this work, olive pomace, a lignocellulosic solid residue generated in olive oil production was chemically treated using sodium hexametaphosphate (SHMP) and utilised as an effective adsorbent (SHMP-OP) for removing anionic Congo red dye (CR) from aqueous solutions. Characterisation studies were undertaken on the prepared adsorbent using Fourier-transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), scanning electron microscope (SEM), energy-dispersive X-ray (EDX) and zero-point charge (pH_zpc). The zero-point charge was found to be 5.4. The FTIR findings showed that the surface of the prepared adsorbent contains mainly (–OH, C=O, and –NH) functional groups which are mainly responsible for the adsorption of CR dye. The SEM analyses for the prepared adsorbent showed a number of pores, highlighting its potential adsorption capacity. In batch method, the influence of different parameters as: initial pH, adsorbent dose, initial concentration of adsorbate, temperature, contact time, ionic strength, and presence of surfactant on the percentage removal of studied dye from aqueous media using SHMP-OP was investigated. The outcomes displayed that highest rate of adsorptive removal process for CR dye was found at pH 4 after 60 min equilibration time with 200 mg of SHMP-OP dose. Freundlich and Langmuir models were applied for the clarification of experimental data. Langmuir model showed that the maximum adsorption capacity equal 90.58 mg/g. Also, 0.08 mol/L of NaOH was used for successful regeneration study. The CR adsorption mechanism mainly involved electrostatic interaction and hydrogen bonding. Compared with some other adsorbents, SHMP-OP showed better removal efficiency of CR dye, suggesting possible and beneficial use of olive pomace waste residues to effectively decontaminate CR-contaminated wastewater. These obtained results showed that SHMP-OP is an effective, eco-friendly, and a low-cost adsorbent which can be used for the adsorptive removal of CR dye from aqueous solution.