RESUMO

Studying the adsorption-desorption kinetics of ions and molecules is crucial to understand the mobility of nutrients and pollutants in the environment. This article reports the adsorption-desorption kinetics of phosphate on goethite, as measured by ATR-FTIR spectroscopy at pH 4.5, 7.0 and 9.5. The system phosphate-goethite has become a model system to test new experimental setups and theories to understand the behavior of pollutants with phosphonic or phosphinic moieties such as glyphosate or glufosinate. One of the main difficulties in the analysis of ATR-FTIR spectra in adsorption-desorption kinetics is to calibrate the equipment to convert absorbance vs. t curves into adsorption vs. t curves, and thus the methodology to achieve a good calibration using spectroscopic data in combination with adsorption isotherms is clearly described. The time evolution of the different surface species was monitored simultaneously during adsorption and desorption at different pH, showing the advantages of this spectroscopy over traditional adsorption methods that only quantify total adsorption. Results were analysed in terms of a simple adsorption-desorption model that takes into account transport, attachment, detachment and surface transformation of the adsorbed species. The same rate parameters at a given pH could predict well the adsorption-desorption kinetics of the two formed surface species and the corresponding adsorption isotherm, giving new insights into the dynamics of phosphate on the surface of goethite. It was found that phosphate desorbed faster from goethite at low pH than at high pH, which is counterintuitive, but has good practical and environmental applications.

Assuntos

RESUMO

Magnetic-clay (MtMag) and magnetic-organoclay (O100MtMag) nanocomposites were synthesized, characterized and evaluated for arsenic adsorption. Batch arsenic adsorption experiments were performed varying pH conditions and initial As(V) concentration, while successive adsorption cycles were made in order to evaluate the materials reuse. The highest As(V) removal efficiency (9 ± 1 mg g-1 and 7.8 ± 0.8 mg g-1 for MtMag and O100MtMag, respectively) was found at pH 4.0, decreasing at neutral and alkaline conditions. From As(V) adsorption isotherm, two adsorption processes or two different surface sites were distinguished. Nanocomposites resulted composed by montmorillonite or organo-montmorillonite and magnetite as the principal iron oxide, with saturation magnetization of 8.5 ± 0.5 Am2 Kg-1 (MtMag) and 20.3 ± 0.5 Am2 Kg-1 (O100MtMag). Thus, both materials could be separated and recovered from aqueous solutions using external magnetic fields. Both materials allowed achieving arsenic concentrations lower than the World Health Organization (WHO) recommended concentration limit after two consecutive adsorption cycles (2.25 and 4.5 µg L-1 for MtMag and O100MtMag, respectively).

Assuntos

RESUMO

Phosphate adsorption at the metal oxide-water interface has been intensely studied, and the system phosphate-goethite in aqueous media is normally used as a model system with abundant information regarding adsorption-desorption under very different conditions. In spite of this, there is still discussion on whether the main inner-sphere surface complexes that phosphate forms on goethite are monodentate or bidentate. A new spectroscopic technique, InfraRed Surface Titration (IRST), is presented here and used to systematically explore the surface speciation of phosphate on goethite in the pH range 4.5-9.5 at different surface coverages. IRST enabled to construct distribution curves of surface species and distribution curves of dissolved phosphate species. In combination with the CD-MUSIC surface complexation model it was possible to conclude that surface complexes are monodentate. Very accurate distribution curves were obtained, showing a crossing point at pH5.5 at a surface coverage of 2.0µmolm-2, with a mononuclear monoprotonated species predominating at pH>5.5 and a mononuclear diprotonated species prevailing at pH<5.5. On the contrary, at the low surface coverage of 0.7µmolm-2 there is no crossing point, with the mononuclear monoprotonated species prevailing at all pH. IRST can become a powerful technique to investigate structure, properties and reactions of any IR-active surface complex at the solid-water interface.

RESUMO

HYPOTHESIS: The environmental mobility and bioavailability of Picloram (PCM) are determined by the amine and carboxylate chemical groups interaction with the soils mineral phases. Clay particles, such as montmorillonite (Mt), and the pH value of the media could play an important role in adsorption processes. Thus, the study of the role of soil components other than organic matter deserves further investigation for a more accurate assessment of the risk of groundwater contamination. EXPERIMENTS: Samples with PCM adsorbed on Mt dispersions were prepared at pH 3-9. Subsequently, the dispersions were separated, washed, centrifuged and stored at room temperature. Picloram (PCM) herbicide interaction with surface groups of montmorillonite (Mt) was studied using XRD, DTA, FTIR and XPS techniques. FINDINGS: The entrance of PCM into the Mt basal space, in two different arrangements, perpendicular and planar, is proposed and the final arrangement depends on PCM concentration. The interaction of PCM with Mt surface sites through the nitrogen of the pyridine ring and carboxylic group of PCM, forming bidentate and bridge inner-sphere complexes was confirmed by FTIR and XPS analysis. The acidity constant of the PCM adsorbed on the Mt surface was calculated.

Assuntos