Influence of mesoporosity on the sorption of 2,4-dichlorophenoxyacetic acid onto alumina and silica

Abstract

Two SiO 2 and three Al 2O 3 adsorbents with varying degrees of mesoporosity (pore diameter 2–50 nm) were reacted with 2,4-dichlorophenoxyacetic acid (2,4-D) at pH 6 to investigate the effects of intraparticle mesopores on adsorption/desorption. Anionic 2,4-D did not adsorb onto either SiO 2 solid, presumably because of electrostatic repulsion, but it did adsorb onto positively charged Al 2O 3 adsorbents, resulting in concave isotherms. The Al 2O 3 adsorbent of highest mesoporosity consistently adsorbed more 2,4-D per unit surface area than did the nonporous and less mesoporous Al 2O 3 adsorbents over a range of initial 2,4-D solution concentrations (0.025–2.5 mM) and reaction times (30 min–55 d). Differences in adsorption efficiency were observed despite equivalent surface site densities on the three Al 2O 3 adsorbents. Hysteresis between the adsorption/desorption isotherms was not observed, indicating that adsorption is reversible. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy studies confirm that 2,4-D adsorption does not occur via ligand exchange, but rather via electrostatic interaction. The results indicate that adsorbent intraparticle mesopores can result in consistently greater 2,4-D adsorption, but the amount adsorbed is dependent upon surface charge and the presence of adsorbent mesoporosity. The data also suggest that when mineral pores are significantly larger than the adsorbate, they do not contribute to diffusion-limited adsorption/desorption hysteresis. Adsorbent transformations through time are discussed.