Adsorption of Five Model Organic Compounds on a Peat at Different Stages of Drying

Abstract

Acridine orange (AO), dinitrobenzoic acid (DNB), bromocresol green (BCG), bromophenol blue (BPB), and methylene blue (MB) were chosen as model aromatic compounds of different polarity, charge, and solubility in water to examine the effects of solute properties on hydrophobic adsorption. These compounds show strict structural similarities to some herbicides and other potential xenobiotic pollutants and exhibit distinct absorption maxima in the visible region, which allows for their easy determination. A well-decomposed peat (medisaprist) at four different stages of drying was used to determine compound adsorption/desorption influences based on the degree of hydrophobicity and charge density of an organic surface. Adsorption and desorption isotherms were investigated using the batch equilibration method and determining the concentration of free chemicals by UV-Vis spectrophotometry. AO had a high tendency of adsorption and was strongly sorbed on peat samples that had been air-dried for 12 months. The lower Freundlich coefficient values found for MB when compared with AO at all the drying stages of the peat indicated that electrostatic attraction has a secondary contribution to sorption. On the contrary, the higher energy that must be spent to break solute-solvent interactions in the case of charged or polar molecules is one of the main factors in determining the position of the equilibrium. For a given solute, Kf values varied with the degree of hydrophobicity and the charge density of the surface, but again solute-solvent interactions appear to be much more important in the overall energy balance of hydrophobic pollutants than the electrostatic sorbate-sorbent interactions. A change in the solution pH does not improve the adsorption of the relatively polar DNB molecule, but sorption increases strongly for BCG and BPB when these molecules are in non-dissociated forms. The larger increase in BPB sorption observed on H+ saturated peat suggests that the degree of interaction increases with the suppression of the negative charge, but charge repulsion has a small effect in preventing adsorption of molecules bearing hydrophobic groups such as BCG. Desorption results differed depending on the chemical structure of the compound examined. For example, with AO there was no desorption from the more hydrophobic peat surfaces. A negative hysteresis was observed for DNB; the magnitude of hysteresis, evaluated using the ratio of Freundlich coefficients for adsorption and desorption, increased with the drying stage of the sorbent and was larger on oven-dried samples.