The Adsorption of 2,4-D on Anatase Nanoparticles: The Effect of Concentration

TAUCHERT E.; MENDIVE C.; BAHNEMANN D.

Nevada

Congreso; 21th Ozone World Congress and 7th Ultraviolet World Congress; 2013

International Ozone Association and International Ultraviolet Association

2,4-Dichlorophenoxyacetic acid (2,4-D) is one of the most commonly used herbicides worldwide. Due to its environmental importance the investigation of new degradation processes for the elimination of 2,4-D has been intense in the last decades. Advanced Oxidation Processes (AOPs) appear to be one of the most studied methods for its removal. In particular, the use of nanoparticulate TiO2 for photocatalysis is many times the material of choice. From there arises the importance of the true comprehension of all types of mechanisms occurring at such complex systems. Studying them involves a challenging experimental set-up, which should be able to properly access events occurring at the solid-liquid interface. Since adsorption, as a first step, and subsequent reactions depending on the adsorbed pollutant are unequivocally a crucial factor, we have chosen the Attenuated Total Reflection (ATR) technique by means of Fourier Transformed Infrared (FTIR) spectroscopy to assess the systems. We have performed the investigation of a commercial TiO2 material (anatase, PC500 from Millennium Inorganic Chemicals) in contact with low concentration aqueous solutions of 2,4-D. Concentrations between 0.3 mmol·L-1 and 2.7 mmol·L-1 of 2,4-D have been employed. The number of adsorption sites occupied by the 2,4-D molecules on the first adsorption layer, estimated by adsorption isotherms, is 30 mmol per gram TiO2. The number of molecules per square nanometer is 0.06 which corresponds to a small portion of the available sites at the PC500 anatase surface. Vibrational modes attributed to the carbonyl (1420-1280 cm-1) group, to the aromatic ring as well as to the phenol group (directly attached to the ring) (1590-1480 cm-1 and 1265-1070 cm-1) have been observed. At low concentrations not all adsorption sites are occupied by 2,4-D molecules, leaving empty spaces which enable the rings to rotate, for instance, around the Cl-O axis. These rotations change the stability of the slightly adsorbed structure to some extent, observed in the intensity decrease of the bands, except from those involving Cl atoms. With increasing adsorbate coverage, i.e., using 2.7 mmol·L-1, the adsorption sites at the anatase surface are occupied by more molecules. The adsorbed molecules distribute, therefore, in a closed-packed manner stabilized by ring-ring interactions. The surface complexes thus formed are laterally adsorbed to guarantee these interactions. Free rotations of the rings are therefore sterically hindered, and almost unchanged spectral signal are observed.