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Nickel oxide-based catalysts currently represent the state of the art in electrochemical water oxidation in alkaline pH. However, much of their functionality remains poorly understood, particularly regarding catalytically active sites...

This study investigated the applicability of natural Egyptian Na-montmorillonite (Na-MMT) to remove Pb 2+ , Cd 2+ and Ni 2+ under acidic conditions that mimic industrial wastewater acidity. Cation exchange capacity of Na-MMT was found 91 meq/100 g and the specific surface area 42 m 2 g À1 . The adsorbent was characterized using X-ray fluorescence, scanning electron micrograph that showed significant morphological changes after adsorption and Fourier transform infrared spectroscopy, which confirmed that the adsorption occurred mainly in the lattice region. Removal efficiency was evaluated as a function of pH, contact time, initial concentration, and adsorbent mass. Acidic pH value was chosen for the following experiments. Langmuir and Freundlich isotherm models were applied. Freundlich model showed better fitting suggesting heterogeneous adsorption scenario. Freundlich capacity values decreased in the order of Pb 2+ (3.71 mg g À1 ) > Cd 2+ (2.45 mg g À1 ) > Ni 2+ (1.76 mg g À1 ). Kinetic data were accurately fitted to pseudo-second-order, indicating the adsorption occurrence in the interior surface of Na-MMT and the contribution of internal diffusion mechanism was significant. Intraparticle diffusion model gave multi-linear curves so more than onestep controlled the adsorption process. Under temperature range 290-328 K, thermodynamic parameters revealed that adsorption of Pb 2+ and Cd 2+ was spontaneous but Ni 2+ non-spontaneous. Adsorption was exothermic for Pb 2+ and Ni 2+ but endothermic for Cd 2+ . Arrhenius activation energy values were 5.78, 8.51 and 11.45 kJ mol À1 for Pb 2+ , Cd 2+ and Ni 2+ respectively stating the physical adsorption. Na-MMT reusability was confirmed by regeneration experiments. Application study showed excellent efficiency of Na-MMT within range (23.4-81.2%) removal for Pb 2+ , Cd 2+ and Ni 2+ from textile dyeing and tannery wastewater. *Effect of pH was studied in the pH range 2-7 while keeping other parameters constant at 't = 60 min; C o = 10 mg L À1 ; adsorbent mass = 0.25 g; agitation speed = 100 rpm and T = 298 K'.A. A. TAHA ET AL. Figure 6. Effect of adsorbent mass 'adsorbent mass (0.12-0.500) g, t = 60 min, pH = 3.8, C o = 10 mg/L, agitation speed = 100 rpm and T = 298 K'.Figure 7. Linearized isotherm models (a) Langmuir model and (b) Freundlich model. 'C o = (5-30) mg/L, t = 60 min, pH = 3.8, adsorbent mass = 0.25 g, agitation speed = 100 rpm and T = 298 K'.Figure 8. The linear plots of the pseudo-second-order model for (a) Pb +2 , (b) Cd +2 and (c) Ni +2 . 'C o = (5-20) mg/L, t = (1-60) min, pH = 3.8, adsorbent mass = 0.25 g, agitation speed = 100 rpm and T = 298 K'.Figure 9. Intraparticle diffusion plots for (a) Pb +2 , (b) Cd +2 , and (c) Ni +2 . 'C o = (5-20) mg/L, t = (1-60) min, pH = 3.8, adsorbent mass = 0.25 g, agitation speed = 100 rpm and T = 298 K'.

Understanding the controls of the oxidation rate of iron (Fe) in oxygenated aquatic systems is fundamental for students of the Earth and Environmental Sciences as it defines the bioavailability of Fe, a trace metal essential for life. The laboratory experiment presented here was successfully developed and used during a third-year undergraduate lab course at Imperial College London for several years. It employs ultraviolet−visible (UV−vis) spectroscopy calibrated externally with 0 to 50 μM Fe 2+ standards created in a 492 μM ferrozine and 0.43 M acetate matrix. The students conducted the oxidation experiments in stirred batch reactors at equilibrium with atmospheric oxygen. The solution contained 40.5 μM initial Fe 2+ concentration and a 5.1 mM imidazole buffer. The pH was adjusted to values between 7.22 and 7.77. The students observed a pseudo-first-order reaction with respect to Fe 2+ concentration. Plotting the logarithms of the apparent rate constants (k′) at different pH values leads to a gradient of 2.2 ± 0.2 min −1 pH −1 , indicating a second-order reaction with respect to OH − concentration, in agreement with published literature. The oxidation reaction occurred rapidly (tens of seconds to tens of minutes) indicating that in oxygenated aquatic systems, Fe 3+ will be the dominant oxidation state, significantly reducing the bioavailability of Fe. The simple laboratory experiment presented here allows the students to learn about kinetic parameters for a fundamental chemical reaction. It allows the students to explore the significant implications this has for aquatic ecosystems.

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Composite photocatalyst-adsorbents such as TiO 2 / Fe 2 O 3 are promising materials for the one-step treatment of arsenite contaminated water. However, no previous study has investigated how coupling TiO 2 with Fe 2 O 3 influences the photocatalytic oxidation of arsenic(III). Herein, we develop new hybrid experiment/modelling approaches to study light absorption, charge carrier behaviour and changes in the rate law of the TiO 2 /Fe 2 O 3 system, using UV-Vis spectroscopy, transient absorption spectroscopy (TAS), and kinetic analysis. Whilst coupling TiO 2 with Fe 2 O 3 improves total arsenic removal by adsorption, oxidation rates significantly decrease (up to a factor of 60), primarily due to the parasitic absorption of light by Fe 2 O 3 (88 % of photons at 368 nm) and secondly due to changes in the rate law from disguised zero-order kinetics to first-order kinetics. Charge transfer across this TiO 2 -Fe 2 O 3 heterojunction is not observed. Our study demonstrates the first application of a multi-adsorbate surface complexation model (SCM) towards describing As(III) oxidation kinetics which, unlike Langmuir-Hinshelwood kinetics, includes the competitive adsorption of As(V). We further highlight the importance of parasitic light absorption and catalyst fouling when designing heterogeneous photocatalysts for As(III) remediation.