Photoactivity of organic xerogels and aerogels in the photodegradation of herbicides from waters

Abstract

The objective of this study was to analyze the influence of the presence of organic xerogels and aerogels on the degradation of herbicide amitrole (AMT) by UV radiation. Organic gels were prepared by polycondensation of resorcinol and formaldehyde in water, using Fe, Ni, and Co acetates as catalytic precursors. Materials were characterized by N2 adsorption at −196°C, pH at the point of zero charge (pHpzc), X-ray diffraction, and X-ray photoemission spectroscopy. The band-gap of these materials was calculated by diffuse reflectance spectra, applying the Kubelka–Munk theory. Results show that the porous texture of gels depends on the salt used, which affects the polymerization rate. Thus, nickel xerogel (X-Ni) has the highest surface area and mesopore volume values. The surface area of Ni aerogel (A-Ni) is four-fold higher than that of its corresponding xerogel, indicating that the drying process also plays a very important role in the textural properties of the material. According to the band-gap values obtained, the gels behave as semiconductor materials, because these values are < 4eV, ranging between 3.81eV for X-Na and 3.66eV for X-Ni. Band-gap values are closely related to the presence of CO/OH groups in the structure of the gels. X-Ni gel has the highest content of CO/OH groups (24.2%). These groups introduce energy bands in the Fermi level, reducing the energy of LUMO–HOMO orbitals. Given the photoactivity of the study materials, AMT removal in the presence of organic xerogels can be attributed to the combination of three factors: direct photolysis, adsorption, and synergic effects. The synergic degradation rate constant (kSE) was found to decrease in the order X-Ni>X-Fe>X-Co>X-Na, passing from 18.7×10−3min−1 for X-Ni to 6.0×10−3min−1 for X-Na. Results obtained indicate that the positive holes generated in the valence band and electrons, which promote the conduction band, play an essential role in the mechanism by which xerogels enhance AMT photodegradation.