The uptake of tartrazine from its aqueous solution by powdered activated carbon prepared from cola nut shells chemically activated with potassium hydroxide (ACK) and phosphoric acid (ACP) has been investigated using kinetics models. Batch isotherm data were analysed with the pseudo-first order, pseudo-second order model as well as the intraparticle diffusion model. For structural elucidation, the materials were characterized using FTIR, XRD and SEM. These analyses revealed that the activated carbons (ACK and ACP) were predominantly mesoporous with several oxygen-containing functional groups dispersed on their surface. The reaction was systematically investigated under various experimental conditions such as contact time, adsorbent dose and pH. For the two adsorbents, the quantity adsorbed of 19.256 mg/g and 18.196 mg/g respectively for ACP and ACK at respective contact times of 5 and 10 min were obtained. The adsorption data were tested with the Langmuir, Freundlich models. Langmuir model was found to best describe the adsorption of tartrate ions with maximum monolayer adsorption capacities of 24.57 and 21.59 mg/g for ACP and ACK, respectively. Results analysis indicated clearly that the pseudo-second order kinetic rate model best fitted the experimental data and therefore was the adsorption controlling mechanism for both adsorbents. Thermodynamic studies revealed that the adsorption process was spontaneous and exothermic for ACP with increased randomness at the solid solution interface, then exothermic but non-spontaneous for ACK. The results show that these activated carbons could be an alternative for more costly adsorbents for the purpose of tartrate ions elimination.

TiO2/SiO2 composites were synthesized via a simple sol gel method by surface reduction of Ti4+ ions to Ti3+ using titanium isopropoxide as a TiO2 precursor and rice husks (RHA) as a SiO2 source. The silica content and calcination temperature of the materials were evaluated. Thermal, crystallographic and physicochemical aspects suggest that biogenic silica (SiO2) can improve the thermal stability of the anatase phase of TiO2, when the SiO2 content reaches 20%. The N2 adsorption-desorption isotherms showed that the SiO2-modified samples have uniform pore diameters and a large specific surface area. The XPS analysis showed the surface reduction of Ti4+ ions to Ti3+ within the TiO2 network via oxygen vacancies after SiO2 introduction, which is beneficial for the photocatalytic reaction. Photocatalytic degradation of sodium diclofenac (SDFC) shows that TiO2/SiO2 composites have better activity compared to commercial P25. Mesoporous TiO2 composite modified with 20 wt% SiO2 showed better photocatalytic mineralization than P25 (83.7% after 2 h instead of 57.3% for P25). The excellent photocatalytic mineralization of the photocatalysts can be attributed to the high anatase crystallinity exhibited by XRD analysis, high specific surface area, surface hydroxyl groups, and the creation of oxygen vacancy, as well as the presence of Ti3+ ions.

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Catalyst surface characterisations have been carried out to investigate the role of dispersion on catalyst activity and to probe the occurrence of oscillations in coking levels with cycle number generally observed during multiple deactivation and regeneration schemes. The titrations were done, cycle by cycle, at 430°C after oxidation and at the same temperature (430°C) after reduction at 500°C. Results show the usually observed trend -that the dispersions after oxidation are higher than those after reduction. The average decline in dispersion from oxidation to reduction was calculated to be 39.25%. It was observed that the cycles with high toxic coke removal were characterised by high deactivation times. The deactivation times were still high even for cycles subsequent to those with low dispersion. At high dispersions the catalyst had short deactivation times, that is the small crystallites deactivate faster than large ones. The nature of reduceable coke and the efficiency of its removal is a much more determinant factor of catalyst activity than the level of metal dispersion. Thus prolonged toxic coke reduction at the high temperature of 500 "C, though resulting in an apparent lowering of dispersion, does not affect the quality of the catalyst. The dispersion before reduction could be reattained on oxidation. Hence reduction at 500°C did not introduce sintering.

The present study is based on the adsorption of cadmium (II) ions on rice husk and egussi peeling, unmodified and modified with nitric acid in aqueous solution, using batch technique. It was carried out as a function of contact time, dosage, pH and initial concentration. The equilibrium time was achieved within 25 minutes for unmodified rice husk (Glu NT) and 20 minutes for unmodified egussi peeling (Cuc NT) with an adsorbed quantity of 13.18 mg/g. In the case of modified materials, we obtained 15 minutes for modified rice husk (Glu HNO3) and 10 minutes for modified egussi peeling (Cuc HNO3) with an adsorbed quantity of 18.77 mg/g. The maximum biosorption occurred at pH 5.5 for all biosorbents. The adsorbent mass for maximum adsorption was 0.4 g giving an adsorption capacity of 62.02 % for unmodified adsorbents. In the case of modified adsorbents, the minimal mass at which maximum adsorption occurred was 0.4 g giving an adsorption capacity of 98.33 % and 0.6 g giving an adsorption capacity of 98.33 % for modified rice husk and egussi peeling respectively. The adsorbent/adsorbate equilibrium was well described by the pseudo-second order kinetic model and by Langmuir's and Freundlich adsorption model. This models showed that the adsorption of cadmium (II) is a chemisorption process.

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