Insight into the removal process mechanism of pharmaceutical compounds and dyes on plasma-modified biomass: the key role of adsorbate specificity

The reason that some molecules, but not others, are easily adsorbed was not fully understood. In order to spotlight the effect of morphological structure and surface functional composition of adsorbate rather than focusing only on the nature of adsorbent as in most of literature reports, this work reports the biosorption of two dyes (anionic Orange G and cationic malachite green) and two pharmaceuticals (Ibuprofen and ampicillin) as target representative contaminants onto plasma-modified cocoa shell (CPHP) used as alternative low-cost adsorbent. As results, when molecules were mixed in solution and then exposed to a substrate, the factors that affect adsorption include the relative solvation of the adsorbates, the ability of each molecule to adhere to the surface, and the degree of interaction between the molecules once they were adsorbed. The maximum adsorbed amounts at 298 K of OG, MG, IBP, and AMP were 23.96, 14.65, 13.99, and 06.66 mg/g, respectively. The most solvated molecules may not adsorb rapidly to the surface, while comparatively, the less soluble molecules will aggregate, so as to maximize self-interactions via Van der Waals, hydrogen bonds or other interactions. This work demonstrated that the adsorbate intrinsic properties could play a significant role in the adsorption process. Hence, properties such as functional active groups, dimensions, and hydrophobicity were the determining parameters in the adsorption process mechanism. Accordingly, the pharmaceuticals biosorption mechanism involved π-π bonding, hydrophobic effect, electrostatic interaction, and van der Waals forces whereas the dye biosorption mechanism was dominated as well known by electrostatic attraction and hydrogen bonding phenomenon. Experimental parameters such as initial pH of solution and contact time were optimized. The optimum pH values were 2.0 for ibuprofen (IBP) and 7.0 for ampicillin (AMP). The kinetics of adsorption and the experimental isotherms data were analyzed using non-linear models. Results indicated that Avrami fractional order was the most successfully fitted model for pharmaceutical biosorption and based on the statistical values of SD and R²ₐdⱼ parameters, Liu isotherm was the most successfully fitted model.