Ammonium sorption from aqueous solutions by the natural zeolite Transcarpathian clinoptilolite studied under dynamic conditions

The scope of this study is ammonium-ion uptake from synthetic aqueous solutions onto raw and pretreated forms of the natural zeolite Transcarpathian clinoptilolite under dynamic conditions. Hydrogen ions displaced exchangeable cations on the clinoptilolite in distilled water (sodium ions) and hydrochloric acid (sodium, potassium, and calcium ions) and destroyed the zeolite framework structure in the last case. Ammonium uptake onto the zeolite occurs by exchange with Na+, Ca2+, and K+ ions. Although Na+ ions were observed to be more easily exchanged for both hydrogen and ammonium ions, the role of Ca2+ ions increased with zeolite saturation by NH+4 ions. The maximum sorption capacity of the clinoptilolite toward NH+4 ions, estimated under dynamic conditions, is significantly higher than that measured under static conditions; proximity of the values of a distribution coefficient and a retardation factor for different conditions (215-265 dm3/kg and 979¿1107, respectively) allows us to use these parameters to model ammonium uptake onto the clinoptilolite. Slowing down or interruption in filtration resulted in the improvement of ammonium sorption properties of the zeolite. The ammonium removal improves with use of the finer fractions of the clinoptilolite up to 0.35 mm. A recycling study results confirmed the importance of external diffusion for ammonium sorption by the clinoptilolite. Preliminary treatment of the sorbent confirmed the predominant importance of the ion-exchange mechanism. The advantage of prior NaCl treatment of the clinoptilolite in improvement of ammonium removal over the other techniques was shown

Structural Evolution of Polyimide-Derived Carbon during Phosphoric Acid Activation

Carbon adsorbents were obtained by carbonization of polyimide polymer with and without the presence of phosphoric acid at temperatures in the range of 400–1000 °C. Carbons produced in the presence of phosphoric acid have been demonstrated to contain up to 13.2% phosphorus. The structure of phosphorus-containing compounds was investigated by XPS and 31P MAS NMR methods. Deconvolution of the P 2p peak with variable binding energy showed the presence of only phosphates/polyphosphates. However, a low value of the O/P ratio is an indirect indication of the possible presence of phosphonates. A 31P MAS NMR study revealed the existence of several kinds of phosphates as well as a minor quantity (1–9%) of phosphonates. All discovered phosphorus-containing compounds are acidic and therefore give carbon the ability to absorb metal cations. The study of copper ion adsorption demonstrated that phosphorus-containing carbon shows a significant adsorption capability even in extremely acidic conditions. At pH 3–6, phosphorus-containing carbon may completely remove copper from the aqueous solution. Phosphorus-containing carbon has a higher adsorption capacity for copper ions than ion exchange resins with carboxyl or sulfo groups

Study on zinc ions binding to the individual casein fractions: αS1-, β- and κ-casein.

The presented work is focused on the isotherm study on the Zn2+ ions binding to the individual casein fractions: αS1-, β- and κ-casein (αS1CN, βCN and κCN). The experimental isotherms were evaluated using Freundlich and Langmuir models. Subsequently, the stability of the obtained complexes in the respective Zn2+ ion concentrations (120, 350, 600 mg/L) was determined by carrying out zeta potential measurements. Capillary electrophoresis combined with inductively coupled plasma mass spectrometry (CE-ICP-MS) confirmed the occurring binding process. Additionally, physicochemical characteristics of the obtained metal-protein complexes was performed including scanning electron microscopy (SEM) in two modes (SE and Z-contrast) and the binding sites of caseins to Zn2+ ions were indicated using attenuated total reflectance infrared spectroscopy (FTIR-ATR) and Raman analysis as well as mass spectrometry technique (MALDI-TOF MS). Isothermal studies indicated a heterogeneously complex zinc ion adsorption process, and a stability study showed that the zeta potential is strongly related to the hydrophobicity, size and structure of the casein isoforms studied. Electron microscopy confirmed the modification of casein surfaces due to the addition of Zn2+ ions. Spectroscopic techniques indicated the interaction of zinc ions with polar amino acids of casein, such as glutamic acid (Glu) and aspartic acid (Asp), but also His, Cys. The influence of phosphate groups was also observed. Finally, the study culminated in a molecular docking study of the Zn2+ ion binding process, which confirmed the presence of the listed amino acids responsible for the binding process