| Two SiO2 and three Al2O3 adsorbents with varying degrees of intraparticle mesoporosity (2–50 nm pore diameter) were reacted with the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) and the antibiotic 9-fluoro-2,3-dihydro-3-methyl-10-(4-methyl-l-piperanzinyl)-7-oxo-7H-pyrido [1,2,3-de]-1,4-benzoxazine-6-carboxylic acrid (ofloxacin) to investigate the effects of mesopores on adsorption/desorption.; Adsorbents reacted with anionic 2,4-D did not adsorb to either SiO 2 solid, presumably, because of electrostatic repulsion, but it did adsorb to positively-charged Al2O3 adsorbents resulting in concave isotherms. The Al2O3 adsorbent of highest mesoporosity adsorbed more 2,4-D than did non-porous and less mesoporous Al 2O3 over a range of initial 2,4-D concentrations (0.025–2.5 mM) and reaction times (30 min–55 d), this despite equivalent adsorbent surface site densities. Absence of hysteresis between adsorption/desorption isotherms indicates that adsorption is reversible when pores are significantly larger than the adsorbate. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy experiments confirm that 2,4-D is bound as an outer-sphere complex, likely through anion exchange.; Ofloxacin experiments conducted over a range of pH values and initial concentrations demonstrate that maximum ofloxacin adsorption to SiO2 surfaces occurs slightly below the pKa2 (pH 8.28) of the antibiotic and sorption diminishes rapidly at pH > pKa2. For Al 2O3, maximum ofloxacin sorption is observed at pH values slightly higher than the adsorbent point of zero net charge (p.z.n.c.) and less than midway between the pKa values of ofloxacin. Adsorption effects on pH suggest that ofloxacin adsorbs to SiO2 solids through the protonated N4 in the piperazinyl group and to Al2O 3 solids through the dissociated 3-carboxyl group, the latter via a ligand exchange mechanism. Sorption edge and isotherm experiments show that ofloxacin exhibits a higher affinity for mesoporous SiO2 and non-porous Al2O3, relative to their counterparts. It is hypothesized that decreased ofloxacin sorption to mesoporous Al2O3 occurs due to competition between aqueous and surface Al for ligand complexation.; Overall, the results signify that adsorbent intraparticle mesopores can result in consistently greater 2,4-D and ofloxacin adsorption, but the degree of enhancement may be reversed by other factors, such as the amount of adsorbent mesoporosity, adsorbate and adsorbent charge properties, and competition between aqueous and surface metals/metalloids for organic compound complexation. |
