Characterization of sulfonated raw coal products as enhanced adsorbents for toxic methyl parathion pesticide: Advanced equilibrium investigation and effect of acid concentration

Raw coal (R.C) was treated with different H2SO4 concentrations (75, 85, and 95 %) via sulfonation modification, and the sulfonated products (SC-75, SC-85, and SC-95, respectively) were used as adsorbents for the toxic methyl parathion (MPn) pesticide. The XRD investigation reflected increase in the amorphization degree with increasing the acid concentrations as well as the surface area (2.8 mmol/g (SC-75) and 8.4 mmol/g (SC-95)) and acid density value (6.4 m2/g (SC-75) and 26.4 m(2)/g (SC-95)). With increasing acid concentration, sulfonation increasingly functionalised coal with active oxygenated chemical groups (-COOH and -S3OH); this was reflected by MPn adsorption capacities at R.C, SC-75, SC-85, and SC-95 saturation states of 210.2, 321.4, 392.5, and 560 mg/g, respectively. The sulfonation and acid concentration effects were assessed considering steric and energetic mathematical parameters of the monolayer model with one energy site as an advanced isotherm model. The adsorption site densities of R.C, SC-75, SC-85, and SC-95 at 83.45, 120.4, 136.77, and 165.4 mg/g, respectively, demonstrated that sulfonation and high acid concentration provide additional active functional groups in the coal structure. Considering the steric n parameter, each active site on the SC-95 surface adsorbs approximately four MPn molecules, in contrast to three and two molecules on SC-85 and SC-75, respectively. Energetic assessment [Gaussian (<8 kJ/mol) and adsorption (<40 kJ/mol) energies] demonstrated MPn uptake of MPn by sulfonated coal samples via different processes such as electrostatic attractive forces, van der Waals forces, dipole bonding, p-p interactions, and hydrogen bonding. Thermodynamic functions demonstrated MPn uptake via endothermic and spontaneous reactions.