Spas D. Kolev

Metal ion ligands in hyperaccumulating plants

Metal-hyperaccumulating plants have the ability to take up extraordinary quantities of certain metal ions without succumbing to toxic effects. Most hyperaccumulators select for particular metals but the mechanisms of selection are not understood at the molecular level. While there are many metal-binding biomolecules, this review focuses only on ligands that have been reported to play a role in sequestering, transporting or storing the accumulated metal. These include citrate, histidine and the phytosiderophores. The metal detoxification role of metallothioneins and phytochelatins in plants is also discussed.

The extraction of cadmium(II) and copper(II) from hydrochloric acid solutions using an Aliquat 336/PVC membrane

Abstract Poly(vinyl chloride) (PVC)-based membranes containing Aliquat 336 as the extractant are used in a study of the extraction of Cd(II) and Cu(II) from HCl solutions. Factors influencing the extraction with these membranes such as the thickness, the composition and the nature of the surface are studied. A mathematical model for the extraction of Cd(II) is used which is based on an extraction mechanism governed by both the chemical kinetics and the membrane diffusion processes. Fitting the model to the extraction data provides values of the diffusion coefficient and the kinetic rate constants of the complexation reaction between the extractant and the extracted species.

The study of a membrane for extracting gold(III) from hydrochloric acid solutions

A membrane is described consisting of Aliquat 336 chloride immobilized in poly(vinyl chloride) (PVC) which extracts gold(III) selectively from hydrochloric acid solutions in the presence of a 500-fold higher concentration of copper(II). Gold is recovered from the membrane by stripping with a thiourea solution. The stability of the membrane is reported in terms of the extent of leaching of the reagent from the membrane in aqueous solutions.

Microfluidic paper-based analytical device for the determination of nitrite and nitrate.

A low-cost disposable colorimetric microfluidic paper-based analytical device (μPAD) was developed for the determination of nitrite and nitrate. Nitrite is determined directly by the Griess reaction while nitrate is first reduced to nitrite in a hydrophilic channel of the μPAD with immobilized zinc microparticles. This μPAD is fabricated by a simple and inexpensive inkjet printing method. Under optimal conditions, the limits of detection and quantification for nitrite are 1.0 and 7.8 μM, respectively, while the corresponding values for nitrate are 19 and 48 μM, respectively. The repeatability, expressed as relative standard deviation (RSD), is less than 2.9% and 5.6% (n ≤ 8) for the determination of nitrite and nitrate, respectively. This μPAD was successfully applied to the determination of nitrate and nitrite in both synthetic and natural water samples. It is user and environmentally friendly and suitable for on-site measurement of the analytes mentioned above in environmental and drinking waters.

Measuring Donnan-related phenomena using a solid-state ion sensor and a concentration-step method

Measurements are performed with a device consisting of an ISFET pH-sensor in the middle of a Ag/AgCl electrode, on top of which a microporous composite membrane is deposited. A sudden change of the salt concentration in the bathing electrolyte causes a transient change in the electrical potential of these sensors when measured vs. a reference electrode in the bathing electrolyte. The potential transient is modulated by adsorption of protein to the membrane. To explain the measured transients, a model is presented for the measuring device describing the ion transport by the Nernst-Planck and Poisson equations, incorporating the different proton-dissociation reactions occurring in the system, and the sensor responses to their potential determining ions (the proton or the Cl− ion). A finite-difference solution method is presented to solve the resulting differential equations. Measurements are performed before and after the adsorption of the model protein lysozyme to the membrane. Analysis of the measurement results indicates that the measured potential transient is caused by a change of the Donnan potential of the membrane, followed by a compensating change in the concentration of the potential determining ion. It is proven that no diffusion potential is generated. In addition, it is shown that an interlayer of electrolyte between membrane and measuring electrode will not influence the measured response. The potential transients measured by the ISFET have a larger amplitude and a longer duration than the Ag/AgCl-measured transients. An analysis shows that this is caused by the buffering action of the proton-dissociating membrane groups. The longer duration results from the release of a large amount of protons from binding to fixed groups, while chloride ions are not bound. The larger amplitude can be explained by refining the Donnan model to account for the inhomogeneous charge distribution in the membrane. The proton-dissociating groups reside at the surface of the polystyrene beads, at which place the potential change on an ion step is larger than the average in the membrane pore solution. This surface-potential change can be measured by the pH-sensitive ISFET because a proton release occurs from the surface-bound groups into the membrane pores, changing the pore pH.

Transport of Ferrocyanide by Two Eucalypt Species and Sorghum

The wastes from some industrial processes and the tailings from gold mining contain elevated concentrations of cyanide, which reacts with iron in the media to form iron cyanide complexes. This research examined the transport and possible metabolism of ferrocyanide by two native Australian trees, blue mallee and sugar gum, and by sorghum. Hydroponic studies using 15N-labeled ferrocyanide showed that both tree species transported ferrocyanide into roots and displayed significant increases in 15N enrichment and concentration with no evidence of phytotoxicity. A subsequent experiment with blue mallee and membrane-transport inhibitors showed that 15N enrichment was significantly inhibited in the presence of the protonophore carbonyl cyanide m-chlorophenylhydrazone, suggesting that ferrocyanide uptake is mediated partly by H+-symporters. A study of the time dependence of 15N translocation showed a rapid equilibration of 15N from ferrocyanide in the root of blue mallee, accompanied by a slow increase in shoot 15N, suggestive of the metabolism of ferrocyanide in plant roots. A similar experiment with sorghum showed a more rapid translocation of 15N, suggesting that the transport and/or metabolism of ferrocyanide by roots of this species may differ. The results offer additional incentive for the use of these species as vegetative cover over cyanidation wastes and for cyanide phytoremediation.

Solid phase extraction of zinc(II) using a PVC-based polymer inclusion membrane with di(2-ethylhexyl)phosphoric acid (D2EHPA) as the carrier

A polymer inclusion membrane (PIM) is reported consisting of 45% (m/m) di(2-ethylhexyl)phosphoric acid (D2EHPA) immobilized in poly(vinyl chloride) (PVC) for use as a solid phase absorbent for selectively extracting Zn(II) from aqueous solutions in the presence of Cd(II), Co(II), Cu(II), Ni(II) and Fe(II). Interference from Fe(III) in the sample is eliminated by precipitation with orthophosphate prior to the extraction of Zn(II). Studies using a dual compartment transport cell have shown that the Zn(II) flux (2.58 x 10(-6)mol m(-2)s(-1)) is comparable to that observed for supported liquid membranes. The stoichiometry of the extracted complex is shown to be ZnR(2).HR, where R is the D2EHPA anion.

Theoretical and experimental study of palladium(II) extraction from hydrochloric acid solutions into Aliquat 336/PVC membranes

Abstract The extraction of palladium(II) from its hydrochloric acid solutions into poly(vinyl chloride) (PVC)-based membranes containing Aliquat 336 chloride as both the extractant and the plasticizer was experimentally studied. The results showed that these membranes extracted palladium(II) rapidly and to a very high degree. This property can be used either for the analytical determination of trace levels of palladium(II) or for the recovery of this precious metal from industrial wastewaters and hydrometallurgical solutions. A mathematical model for the palladium(II) extraction into an Aliquat 336/PVC membrane which takes into account the interfacial palladium(II)–Aliquat 336 chloride reaction and the diffusion mass transport through the membrane was developed. The model was fitted to the experimental extraction data to determine the values of the relevant physicochemical constants (i.e. diffusion coefficients and kinetic rate constants).

Mathematical modelling of membrane extraction of gold(III) from hydrochloric acid solutions

Abstract A mathematical model describing the selective extraction of gold(III) from hydrochloric acid solution into a membrane consisting of Aliquat 336 chloride immobilized in poly(vinyl chloride) (PVC) is developed. The extraction rate is assumed to be governed by both the rate of chemical reactions occurring at the interface and the diffusion of the Aliquat 336 chloride species within the membrane. The fairly good agreement between the model and experimental results confirms its validity and shows that the model can be used successfully for the mathematical description and optimisation of membrane extraction utilised in analytical and process systems as well as in environmental chemistry.

Separation of cobalt(II) from nickel(II) by solid-phase extraction into Aliquat 336 chloride immobilized in poly(vinyl chloride)

A solid-phase absorbent obtained by the immobilization of Aliquat 336 chloride in poly(vinyl chloride) is reported to extract preferentially Co(II) from its 7M hydrochloric acid solutions containing Ni(II). Under the experimental conditions there was no extraction of Ni(II) which allowed the complete separation of these two ions. Co(II) was rapidly and quantitatively back-extracted with deionised water. A mechanism for the extraction of Co(II) is proposed based on the formation of the ion-pair A(+)[HCoCl(4)](-) where A(+) is the Aliquat 336 cation. Fe(III) and Cd(II), usually present in Co(II) and Ni(II) samples, were also extracted into the solid-phase absorbent though at a slower rate than Co(II) and they did not interfere with the separation of Co(II) from Ni(II). It was also demonstrated that this approach allowed the complete separation of Ni(II) from the other metal ions mentioned above.