Adsorption of arsenate on Fe-(hydr)oxide

Adsorption using metal oxide materials has been demonstrated to be an effective technique to remove hazardous materials from water, due to its easy operation, low cost, and high efficiency. The high number of oxyanions in aquatic ecosystems causes serious pollution problems. Removal of arsenate (H2AsO4 -), is one of the major concerns, since it is a highly toxic anion for life. Within the metal oxides, the iron oxide is considered as a suitable material for the elimination of oxyanions. The adsorption of H2AsO4 - on Fe-(hydr)oxide is through the formation of inner or outer sphere complexes. In this work, through computational methods, a complete characterization of the adsorbed surface complexes was performed. Three different pH conditions were simulated (acidic, intermediate and basic), and it was found that, the thermodynamic favourability of the different adsorbed complexes was directly related to the pH. Monodentate complex (MM1) was the most thermodynamically favourable complex with an adsorption energy of -96.0kJ/mol under intermediate pH conditions. © Published under licence by IOP Publishing Ltd

Dyes removal from water using low cost absorbents

In this study, the removal capacity of low cost adsorbents during the adsorption of Methylene Blue (MB) and Congo Red (CR) at different concentrations (50 and 100mg•L-1) was evaluated. These adsorbents were produced from wood wastes (cedar and teak) by chemical activation (ZnCl2). Both studied materials, Activated Cedar (AC) and activated teak (AT) showed a good fit of their experimental data to the pseudo second order kinetic model and Langmuir isotherms. The maximum adsorption capacities for AC were 2000.0 and 444.4mg•g-1 for MB and CR, respectively, while for AT, maximum adsorption capacities of 1052.6 and 86.4mg•g-1 were found for MB and CR, respectively. © Published under licence by IOP Publishing Ltd

Microsolvation of NO3 -: Structural exploration and bonding analysis

Exploration of the potential energy surfaces (PESs) of various microsolvated species associated with the microsolvation of the nitrate anion using density functional theory methods uncovers a rich and complex structural diversity previously unnoticed in the scientific literature for the [NO3(H2O)n]−, n = 1–6 clusters. Two types of interactions are at play in stabilizing the clusters: traditional water to water and charge assisted nitrate to water hydrogen bonds (HBs). The formal negative charge on oxygen atoms in nitrate strengthens hydrogen bonding among water molecules. There is outstanding agreement between available experimental data (sequential hydration enthalpies, IR spectra, and vertical detachment energies) and the corresponding expectation values obtained from our structures. Each PES is heavily populated in the vicinities of the corresponding global minimum with multiple structures contributing to the experimental properties. The last two statements, in conjunction with results from other works (see for example Phys. Chem. Chem. Phys. 2014, 16, 19241) place a warning on the generalized and naive practice of assigning experimental observations to individual structures

Utilization of water hyacinth (Eichhornia crassipes) rejects as phosphate-rich fertilizer

Phosphorus (P) recovery from water hyacinth is one of the promising sources to recovery and recycle P to alleviate P supply shortage in the future. The result of calcination temperature during the thermal treatment of calcium (Ca) and P-rich water hyacinth is presented here. Results showed a Ca/P molar ratio of 5.07 in water hyacinth ashes and, that with the increase in calcination temperature, P and Ca are transformed into hydroxyapatite. The amount of hydroxyapatite increased until 34.0 %, while other Ca phases such as CaO, CaCO3, and Ca(OH)2 were obtained in 6.1 %, 3.9 %, and 18.0 %, respectively. The bioavailability test showed that the material produced at 700 °C (hydroxyapatite and other Ca-rich phases) could be used as a fertilizer, with P slow release in aqueous solutions, giving up 3.7 % and 29.3 % of P release in deionized water and formic acid, respectively. Besides, CaO and Ca(OH)2 are used for soil neutralization as their disposition can help the crops. © 2020 Elsevier Ltd

Preparación de carbón activado a partir de residuos de palma de aceite y su aplicación para la remoción de colorantes

The production of activated carbon from residual biomass generated in the production of oil palm, fiber (F) and shell (C) was studied. The chemical activation was done using ZnCl2 and the adsorption capacity of methylene blue (AM) at different concentrations (50, 100, and 150 mg/L) was evaluated. Results showed a good development of pore with surface areas of 835.3 m2/g for activated fiber (FA) and 575.1 m2/g for activated shell (CA). A good fit of the experimental data with the pseudo second order kinetic model and with Langmuir and Freundlich isotherms models was found. In addition, maximum adsorption capacities of 763.4 and 724.6 mg/g for FA and CA were found, respectively. © 2017, Universidad Nacional de Colombia. All rights reserved

Phosphate removal from aqueous solutions by heat treatment of eggshell and palm fiber

Phosphate removal from wastewater and the correspondent sustainable technologies are urgent issues to solve since phosphorous causes eutrophication of water sources. The effect of heat treatment on the mixture eggshell (ES), rich in calcium carbonate, with fiber palm (F), rich in carbonaceous material for phosphate removal is here reported focusing on the effects of temperature and F/ES ratio. The gases obtained from the F pyrolysis process help to improve the Ca(OH)2 formation. In samples with a ES/F ratio of 1/10 (ESF-1:10) the CaCO3 is mainly transformed into Ca(OH)2 (83 %) at 600 °C instead of 800 °C. The obtained solids were employed for phosphate removal from aqueous solutions and characterized before and after P removal. The ESF-1:10 sample pyrolyzed at 600 °C exhibited the best adsorption performance (48.3 %) at 2 h while ES showed 6.5 % at the same experimental conditions. The pseudo-second-order model kinetic and Langmuir model isotherm provided better-fitting models for the adsorption behavior of P. The adsorption capacity using Langmuir model was 72.0 mg g-1, and the pseudo-second-order kinetic model assumes that the removal process of adsorbate is controlled by chemical adsorption. These results show that the Ca(OH)2 is responsible for the phosphate removal by ligand exchange followed by precipitation mechanism leading to the formation of apatite. © 2020 Elsevier Ltd

Microsolvation of F-

A staggering structural diversity for the microsolvation of F- with up to six water molecules is uncovered in this work. Given the structural variety and the proximity in energy among several local minima, we show here that in order to match available experimental data, statistical averages over contributing structures are needed, rather than assigning experimental values to isolated structures. Our results suggest that the formal charge in F- is strong enough as to induce partial and total dissociation of water molecules and to alter the nature of the surrounding network of water to water hydrogen bonds. We provide an extensive analysis of bonding interactions under the NBO and QTAIM formalisms, our main results suggest a complex interplay between ionic and covalent characters for the F?H interactions as a function of the separation between the atoms. © 2018 the Owner Societies

Microsolvation of small cations and anions

Recent advances in the theoretical treatment of microsolvation of small ions, a problem with practical implications in chemistry, physics, and biology, are exposed. In particular, we discuss sound stochastic approaches to sample complex energy landscapes and delve into the nature of bonding interactions that dictate both structural and energetical preferences. An in-depth analysis of the effect of formal charges in the surrounding network of solvent to solvent hydrogen bonds is also presented. The problem, as expected, is more complicated than simple definitions may forecast. © 2018 Wiley Periodicals, Inc

A Comprehensive Picture of the Structures, Energies, and Bonding in [SO4(H2O)n]2-, n = 1-6

Two stochastic methods in conjunction with ab initio computations were used to explore the potential energy surfaces for the microsolvation of SO4 2- with up to six explicit water molecules. At least three water molecules are needed to stabilize the Coulomb repulsion that prevents the existence of isolated SO4 2-. The formal charge in SO4 2- is strong enough to induce water dissociation and subsequent microsolvation of the resulting HSO4 -, OH- ionic pair. Hydrogen bonds characterized as having complex contributions from covalency and from ionicity are at play stabilizing [SO4(H2O)n]2- clusters. Ionicity and covalency act concomitantly rather than opposedly to strengthen both intermolecular interactions and the resulting O-H bond in HSO4 - after proton abstraction. Copyright © 2019 American Chemical Society

Removal of Cr (VI) from an aqueous solution using an activated carbon obtained from teakwood sawdust: Kinetics, equilibrium, and density functional theory calculations

Because of its acute toxicity and high mobility, the hexavalent chromium [Cr (VI)] found in wastewater is a risk to the environment. In this study, activated carbon was produced from teakwood sawdust, which was chemically modified using ZnCl2 (AT) as an efficient adsorbent for Cr (VI) removal from aqueous systems. Batch experiments were conducted to identify kinetic, diffusional, and equilibrium parameters. In addition, to better understand the adsorption process, computer calculations were conducted based on the density functional theory (DFT). A maximum adsorption capacity of 72.46 mg g-1 was achieved by adapting experimental data to the Langmuir isotherm model. Intraparticle diffusion was further identified through a three-dimensional diffusion model, which revealed that it was ruled by intraparticular diffusion based on surface diffusion, with surface diffusion coefficient (Ds) values ranging from 1.29 × 10-10 to 0.78 × 10-10 cm2 s-1. Finally, computational chemistry calculations and an FTIR analysis determined that oxygenated functional groups, lactone, semiquinone, phenols, and carboxylic acids were involved in the process of Cr (VI) adsorption on AT. Moreover, the main adsorption mechanisms were found to be complexation, electrostatic interaction, and reduction of Cr (VI) to Cr (III). © 2020 Elsevier Ltd