Natural coal (N.C) was sulfonated with sulfuric acid
by normal
stirring (MS.C) and sonication waves (SS.C) to obtain −SO3H functionalized coal as enhanced adsorbents of malachite
green dye (MG). The sulfonated products exhibit enhanced surface area
(MS.C (27.2 m2/g) and SS.C (45.8 m2/g)) as compared
to N.C. SS.C achieved higher acid density (14.2 mmol/g) and sulfur
content (13.2 wt. %) as compared to MS.C. The impact of the sulfonation
processes on the adsorption of MG was assessed based on the monolayer
isotherm model of one energy. The MG Q
sat of N.C (121.3 mg/g), MS.C (226.3 mg/g), and SS.C (296.4 mg/g) validate
the significant effect of the sulfonation processes by the sonication
waves. This is in agreement with the active site densities that reflect
the saturation of SS.C by more active sites (180.74 mg/g) than MS.C
(120.38 mg/g) and N.C (70.84 mg/g). The MS.C and SS.C can adsorb three
MG molecules as compared to two molecules per site of N.C. The Gaussian
energy (<8 kJ/mol) and adsorption energy (<40 kJ/mol)) reflects
the physisorption of MG involving van der Waals forces, hydrogen bonding,
and dipole bonding forces. The thermodynamic functions demonstrate
the uptake of MG by exothermic, spontaneous, feasible reactions.
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