The occurrence of contaminants in wastewaters, and their behavior during wastewater treatment and production of drinking water are key issues to re-use water resources. The present research aims to remove caffeine from aqueous solutions via adsorption technique, using Multi-Wall Carbon Nanotubes (MWCNTs) as an adsorbent under different experimental conditions. The processing variables such as pH (2-12), contact time (1-30 min), initial concentration of caffeine (2-314 mg/L), temperature (25, 50, 80 °C), and adsorbent mass (0.02-0.15 g) have been investigated with equilibrium and kinetic studies on adsorption of caffeine onto MWCNTs being also developed. Maximum caffeine removal has been obtained at pH=7 and adsorption equilibrium has been achieved in 5 min. The use of pseudo second-order kinetic model with determination coefficient of 99.3% (R2=0.993), has made the adsorption kinetics to be well fitted. The caffeine equilibrium adsorption data have been best fitted to Langmuir-Freundlich Model with a relatively high determination coefficient of 96.5% (R2=0.965) and maximum adsorption capacity of 35.61 mg/g of caffeine on MWCNTs. The thermodynamic parameters display that the adsorption of caffeine onto MWCNTs has been non-spontaneous and endothermic in nature.

This research evaluates dibenzothiophene (DBT) adsorptive removal from the liquid stream on the graphitic carbon nitride (GCN) as a synthesized adsorbent at 25 0C with 3 g for 600 min. The morphological properties of GCN have been investigated by Brunauer–Emmett–Teller (BET), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD). The study of the characteristic properties of nano adsorbent proves the suitability of the synthesized GCN in mercaptan adsorption process with the obtained data showing a good agreement with Freundlich model. The equilibrium capacity of DBT adsorption has been calculated at about 39.1 mg/g. This has also been 25.8 mg/g for TBM (tertiary butyl mercaptan). The adsorption capacity has increased by adding to the adsorbent dosage. Thermodynamic studies expose the negative values for ΔS0 (-8.99 kJ/mol. K), ΔH0 (-21.05 kJ/mol), and ΔG0 (8.91 kJ/mol), which demonstrate that DBT adsorption has been a natural exothermic process. In addition, this experiment verifies that the substitution of N into the carbon structure improves the DBT removal efficiency in comparison with pristine CNT as an adsorbent. The removal efficiency of DBT onto GCN has been approximately 80%, i.e. 20% higher than that of pure CNT. Results show that the adsorption capacity of DBT as a cyclic source of mercaptan has been higher than Tertiary butyl mercaptan (TBM) as a liner one. The DBT adsorption mechanism is done by π–π electron interactions between the aromatic structures of DBT, lone-pair electrons of the S atoms, and the pyridinic GCN planes band.

Ten sets of experiments with different settling times were conducted to investigate the effect of settling time on the adsorption of ¹³⁷Cs in seawater onto ammonium phosphomolybdate (AMP). The weight yields of AMP and ¹³⁷Cs yields in all groups were generally higher than 90%. The average weight yields of AMP in each group varied from 91.8 ± 0.5 to 95.9 ± 0.6% (1 SD), and the average ¹³⁷Cs yields in each group varied from 88.3 ± 3.0 to 97.8 ± 3.7% (1 SD). The results showed that equilibrium between Cs and AMP is established immediately after the addition of stable Cs carrier and AMP, implying that the solution could be filtered immediately after the coprecipitation forms. IAEA seawater proficiency test exercises also confirmed that the AMP precipitate does not need to be treated statically in the case of 2 g AMP and 3.7 mg Cs carrier in a seawater sample solution. The modified AMP preconcentration method simultaneously meets the requirements of routine and nuclear emergency monitoring of ¹³⁷Cs in seawater.

The use of passive sampling methods for monitoring hydrophobic organic chemicals frequently requires the determination of equilibration times and partition coefficients in the laboratory. These experiments are often carried out by exposing passive samplers in a finite water volume, and errors are easily made when the obtained results are applied to the field, where water volumes are essentially infinite. The effect of water volume on the equilibration rate constant is discussed, using a mechanistic model. Application of this model to two literature reports illustrates that aqueous concentrations in the field may be underestimated by a factor of 10 or more, when the water volume effect is neglected. Finally, it is shown that the concept of “sorption capacity” (sampler mass times partition coefficient) allows for a more intuitive understanding of the passive sampling process in small and large water volumes, which may reduce the risk of laboratory-field extrapolation errors.