Adsorption of Cu2+ ions onto silica surface from aqueous solutions containing organic substances

Sol-Gel Derived Adsorbents with Enzymatic and Complexonate Functions for Complex Water Remediation

Sol-gel technology is a versatile tool for preparation of complex silica-based materials with targeting functions for use as adsorbents in water purification. Most efficient removal of organic pollutants is achieved by using enzymatic reagents grafted on nano-carriers. However, enzymes are easily deactivated in the presence of heavy metal cations. In this work, we avoided inactivation of immobilized urease by Cu (II) and Cd (II) ions using magnetic nanoparticles provided with additional complexonate (diethylene triamine pentaacetic acid or DTPA) functions. Obtained nanomaterials were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). According to TGA, the obtained Fe₃O₄/SiO₂-NH₂-DTPA nanoadsorbents contained up to 0.401 mmol/g of DTPA groups. In the concentration range Ceq = 0-50 mmol/L, maximum adsorption capacities towards Cu (II) and Cd (II) ions were 1.1 mmol/g and 1.7 mmol/g, respectively. Langmuir adsorption model fits experimental data in concentration range Ceq = 0-10 mmol/L. The adsorption mechanisms have been evaluated for both of cations. Crosslinking of 5 wt % of immobilized urease with glutaraldehyde prevented the loss of the enzyme in repeated use of the adsorbent and improved the stability of the enzymatic function leading to unchanged activity in at least 18 cycles. Crosslinking of 10 wt % urease on the surface of the particles allowed a decrease in urea concentration in 20 mmol/L model solutions to 2 mmol/L in up to 10 consequent decomposition cycles. Due to the presence of DTPA groups, Cu2+ ions in concentration 1 µmol/L did not significantly affect the urease activity. Obtained magnetic Fe₃O₄/SiO₂-NH₂-DTPA-Urease nanocomposite sorbents revealed a high potential for urease decomposition, even in presence of heavy metal ions.

Binding of transition metals to monosilicic acid in aqueous and xylem (Cucumis sativus L.) solutions: a low-T electron paramagnetic resonance study

The supplementation of monosilicic acid [Si(OH)4] to the root growing medium is known to protect plants from toxic levels of iron (Fe), copper (Cu) and manganese (Mn), but also to mitigate deficiency of Fe and Mn. However, the physicochemical bases of these alleviating mechanisms are not fully understood. Here we applied low-T electron paramagnetic resonance (EPR) spectroscopy to examine the formation of complexes of Si(OH)4 with Mn(2+), Fe(3+), and Cu(2+) in water and in xylem sap of cucumber (Cucumis sativus L.) grown without or with supply of Si(OH)4. EPR, which is also useful in establishing the redox state of these metals, was combined with measurements of total concentrations of metals in xylem sap by inductive coupled plasma. Our results show that Si(OH)4 forms coordination bonds with all three metals. The strongest interactions of Si(OH)4 appear to be with Cu(2+) (1/1 stoichiometry) which might lead to Cu precipitation. In line with this in vitro findings, Si(OH)4 supply to cucumber resulted in dramatically lower concentration of this metal in the xylem sap. Further, it was demonstrated that Si(OH)4 supplementation causes pro-reductive changes that contribute to the maintenance of Fe and, in particular, Mn in the xylem sap in bioavailable 2+ form. Our results shed more light on the intertwined reactions between Si(OH)4 and transition metals in plant fluids (e.g. xylem sap).

A DFT study of the interaction between olefins and Cu2+ on silica and MCM-41 model surfaces

The interaction between ethylene and Cu(2+) on a silica model surface was studied by density functional theory (DFT) with nine popular functionals. It is found that B3LYP with BSSE correction is the best method by comparing the calculated results with reported experimental data. This method was also used to study the interactions of Cu(2+) with β-carotene, 1,3,5,7,9,11,13-tetradecaheptaene and ethylene on a MCM-41 model surface. The relationship between the reorganization energy of an olefin and its conjugation length was studied, and the roles of the electrostatic interaction between the olefin and the Cu(2+) were investigated. It is also found that the different environments of Cu(2+) affect the Cu(2+)-olefin interaction significantly.

Probing the surface structure of divalent transition metals using surface specific solid-state NMR spectroscopy

Environmental and geochemical systems containing paramagnetic species could benefit by using nuclear magnetic resonance (NMR) spectroscopy due to the sensitivity of the spectral response to small amounts paramagnetic interactions. In this study, we apply commonly used solid-state NMR spectroscopic methods combined with chemometrics analysis to probe sorption behavior of the paramagnetic cations Cu(2+) and Ni(2+)at the amorphous silica surface. We exploit the unique properties of paramagnets to derive meaningful structural information in these systems at low, environmentally relevant cation surface loadings by comparing the NMR response of sorption samples to paramagnetic free samples. These data suggest that a simple sorption model where the cation sorbs as inner sphere complexes at negatively charged, deprotonated silanol sites is appropriate. These results help constrain sorption models that are used to describe metal fate and transport.

Formation of Ternary Surface Complexes by Adsorption of 2,2'-Bipyridine onto Silica Modified with Copper Ions

The adsorption of 2,2'-bipyridine on a copper-containing silica surface has been studied. The equilibrium of bipyridine binding to surface copper ions are well reproduced by the constants for the formation of ternary surface complexes from an aqueous solution containing copper ions and bipyridine. This indicates clearly that under experimental conditions all reactions are reversible and speciation is controlled by thermodynamics. Copyright 2001 Academic Press.

Effect of 2,2'-Bipyridine on the Adsorption of Zn(2+) Ions onto Silica Surface

The influence of 2,2'-bipyridine (bipy) on adsorption of zink ions onto a highly dispersed silica surface has been studied. The enhanced adsorption of zink ions onto silica surface from the solution containing 2,2'-bipyridine is explained by ternary surface complex formation, { identical withSiOH(-m)Zn(bipy)(n)((2-m)+)}. The adsorbed Zn(2+) and bipy concentrations were measured at the adsorption from solutions with different ratios of Zn : bipy. The equilibrium reaction constants of binary and ternary complexes have been calculated using the constant capacitance model. The potentiometric titration data were used in order to determine the H(+)/Zn(2+) exchange stoichiometry. The composition of formed surface complexes has been confirmed with their UV absorption spectra. Copyright 2001 Academic Press.