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Photocatalytic degradation (PCD) of aqueous paraquat was accelerated by the addition of either phosphate or sulfate salt. Attachment of these anions to the TiO2 surface possibly results in increased adsorption of the cationic paraquat species and in turn its photocatalysis rate. The same effect was obtained more consistently using the Nafion (an anionic polymer)-coated TiO2. Enhanced PCD of paraquat and some amine compounds was noted. However the anionic and neutral compounds were not affected significantly. Nafion proved to be stable against photocatalysis. It has been suggested that the degradation rate is larger for the cationic compounds with higher pK(B). For a phenol-paraquat-TiO2 system, paraquat degradation did not begin till near-complete phenol removal. Using the Nafion-coated TiO2, both phenol and paraquat degradations started simultaneously. Nevertheless, complete paraquat removal still took longer than phenol.

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Photocatalytic degradation (PCD) of tetramethylammonium (TMA) in water was studied using both pure TiO 2 and silica-loaded TiO 2 (Si-TiO 2 ). Use of Si-TiO 2 catalyst prepared from commercial TiO 2 powder by a simple method developed in this work enhanced the PCD rate of TMA considerably. The Si/Ti atomic ratio of 18% was found to be an optimum in photoactivity and the calcined sample was more efficient than the uncalcined one. Several factors were noted to be responsible for the higher photoefficiency of Si-TiO 2 catalyst. Si-TiO 2 calcined at 700°C did not show any sign of change in the crystalline structure from that of uncalcined pure TiO 2 . The increased thermal stability of Si-TiO 2 enabled the bulk defects to be removed at high temperatures without forming the inactive rutile phase, which may partly contribute to the higher photoactivity. The most outstanding characteristics of Si-TiO 2 is its surface charge modification. Loading silica on to a titania surface made the surface charge highly negative, which was confirmed by zeta potential measurements. The enhanced electrostatic attraction of cationic TMA onto the negatively charged Si-TiO 2 surface seems to be the main reason for the enhanced photoactivity of Si-TiO 2 . As a result of this surface charge change, the TMA PCD rate with Si-TiO 2 exhibited a maximum around pH 7 whereas the PCD with pure TiO 2 was minimized at pH 7. The X-ray photoelectron spectroscopic analysis showed the formation of SiO x on the TiO 2 surface but the diffuse reflectance UV spectra indicated no significant difference in the band gap transition between pure TiO 2 and Si-TiO 2 . In addition, the diffuse reflectance IR spectra showed the presence of more surface OH groups on Si-TiO 2 than on pure TiO 2 , which may also contribute to the higher photoactivity of Si-TiO 2 through generating more OH radicals upon UV illumination.

The Kingdom of Saudi Arabia is one of the leading countries in date fruit farming, and by-products from date palm trees can be used for several important applications, including pollution control. This study successfully employed date palm pit-based granular activated carbon (GAC) to adsorb gas-phase benzene under dynamic flow conditions. The percent carbon content (w/w) for raw date palm pits and produced GAC samples was found to be 47 and 82 %, respectively. Furthermore, the specific surface area (SSA BET ) of the produced GAC was 822 m 2 /g, and the tPlot micropore area and t-Plot external surface area were 734.99 and 87.26 m 2 /g, respectively. The BJH graph for the pore size distribution also indicated a mesoporous structure. The use of date palm pit-based GAC for gas-phase benzene adsorption under dynamic continuous-flow conditions showed high efficiency with breakthrough points for different systems ranging from several hours to several days. The role of surface functional groups and their interactions with the benzene rings during the adsorption process were also explored, and surface oxygen-based groups may initiate an electron donor-acceptor mechanism with the benzene's aromatic ring π electrons. The findings confirmed that GAC produced from date palm pits can be successfully used for gas-phase benzene adsorption under various conditions. It is hoped that countries with large-scale date fruit farming, such as the Kingdom of Saudi Arabia, will be able to utilize this rich resource for environmental applications.