Adsorption of five model organic compounds on a peat at different stages of drying

Acridine orange (AO), dinitrobenzoic acid (DNB), bromocresol green (BCG), b romophenol blue (BPB), and methylene blue (MB) were chosen as model aromati c compounds of different polarity, charge, and solubility in water to exami ne the-effects of solute properties on hydrophobic adsorption. These compou nds show strict structural similarities to some herbicides and other potent ial xenobiotic pollutants and exhibit distinct absorption maxima in the vis ible region, which allows for their easy determination. A well-decomposed p eat (medisaprist) at four different stages of drying was used to determine compound adsorption/desorption influences based on the degree of hydrophobi city and charge density of an organic surface. Adsorption and desorption is otherms were investigated using the batch equilibration method and determin ing 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 ind icated that electrostatic attraction has a secondary contribution to sorpti on. On the contrary the higher energy that must be spent to break solute-so lvent interactions in the case of charged or polar molecules is one of the main factors in determining the position of the equilibrium. Fora given sol ute, K-f values varied with the degree of hydrophobicity and the charge den sity of the surface, but again solute-solvent interactions appear to be muc h more important in the overall energy balance of hydrophobic pollutants th an the electrostatic sorbate-sorbent interactions. A change in the solution pH does not improve the adsorption of the relatively polar DNB molecule, b ut sorption increases strongly for BCG and BPB when these molecules are in non-dissociated farms. The larger increase in BPB sorption observed on H+ s aturated peat suggests that the degree of interaction increases with the su ppression of the negative charge, but charge repulsion has a small effect i n preventing adsorption of molecules bearing hydrophobic groups such as BCG . Desorption results differed depending on the chemical structure of the co mpound 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 coefficie nts for adsorption and desorption, increased with the drying stage of the s orbent and was larger on oven-dried samples.