Synthesis of titania/activated carbon composites for the synergistic adsorption and photocatalysis of lindane in aqueous solutions.

Abstract

Lindane (LIND) is an organochlorine pesticide, belonging to the persistent organic pollutants (POPs), which are bioaccumulative with detrimental effects on human health and environment. In the present work, activated carbon (AC) produced from coffee waste after activation with sodium hydroxide (NaOH) and pyrolysis at 800 °C (CWAC-NaOH-800) is tested as adsorbent of LIND, and as substrate for the synthesis of hybrid adsorbent/photocatalytic materials of titanium oxide with activated carbon (TiO₂/AC) via the sol-gel method, aiming at the synergistic adsorption and photocatalysis of LIND. Three different TiO₂/AC materials were synthesized at mass ratio of TiO₂ to AC equal to 2.0 (material TiO₂/AC-0.2), 4.0 (material TiO₂/AC-0.1), and 8.0 (material TiO₂/AC-0.05). As the TiO₂ to AC mass ratio increases, the specific surface area and total pore volume of the composite materials decrease, the meso-/macro-porosity are reduced and a pore-and-throat microporous network is created within the agglomerates of TiO₂ nanoparticles. The adsorption of LIND onto the CWAC-NaOH-800 is pH-independent and follows the Langmuir model with a maximum adsorption capacity equal to 9.74 mg/g. The adsorption mechanism is likely to be the hydrophobic interactions as generally ACs tend to repel water molecules and preferentially adsorb non-polar or hydrophobic compounds such as LIND. The mass transfer multi-compartment model was used to describe the LIND sorption dynamics in CWAC-NaOH-800 and TiO₂/AC-0.1, by accounting for the film/pore/surface diffusion and instantaneous linear sorption, and it was found that the contribution fraction of surface diffusion and microporosity to the total LIND sorption was enhanced with the presence of TiO₂, due to the decrease of meso-/macro-porosity. For the study of the adsorptive-photocatalytic capacity of composite materials, two experimental setups were tested and compared: an ultraviolet-A (UVA) oven with LEDs (power = 22 W, λ = 375 nm) radiating from the periphery to the center of the reactor, and a UVA lamp (power = 6 W, λ = 375 nm) radiating radially from the center to the reactor walls. The material TiO₂/AC-0.1 showed LIND sorption capacity comparable to that of CWAC-NaOH-800 and photocatalytic performance better than that of TiO₂ nanoparticles, due to the presence of AC and the "co-adsorption" effect. During the synergistic adsorption-photocatalysis process, the TOC removal efficiency reached 90% after 3 h of photocatalysis, which is indicative of the in situ regeneration of the material. From the inverse modeling of the adsorption-photocatalysis experiments, it was found that the kinetic constant of TiO₂/AC-0.1 photocatalyst was highest in the experimental setup of the UVA lamp, showing a good balance between adsorptive and photocatalytic capacity.