Selective recovery of gold(III) using cotton cellulose treated with concentrated sulfuric acid

Novel utilization of cotton cellulose was developed by chemically modifying with concentrated sulfuric acid to prepare a novel kind of adsorption gel for gold. The adsorption behaviors of the gel were investigated for various metals from acidic chloride media. The gel was found to be highly selective for Au(III) over other precious and base metals tested over the whole concentration range of hydrochloric acid. The maximum adsorption capacity for Au(III) was evaluated as 6.21 mmol/g at 303 K. The amount of adsorbed Au(III) was increased with increasing temperature. A kinetic study for the adsorption of Au(III) at various temperatures confirmed the endothermic adsorption process following pseudo-first order kinetics. The activation energy was evaluated as 78.8 kJ/mol. Interestingly, it was found that the adsorbed Au(III) was reduced to elemental form which was evidenced by the clearly visible elemental gold particles which was further confirmed by means of the X-ray diffraction spectrum and optical microscope image of the gel after the adsorption of Au(III). The mechanism of Au(III) adsorption on the cotton gel and its reduction to elemental gold was proved from the results of IR-spectra. The main mechanism could be explained in terms of the coordination of Au(III) to oxygen atom of C–O–C linkage of cross linked cotton gel and to the oxygen atoms of the hydroxyl groups at C₂ and C₃ atoms of pyranose ring of cotton cellulose followed by the reduction to metallic gold. The adsorbed gold in the cotton gel can easily be recovered by incineration.     

Characteristics of a new catalytic hydrogen current observed with albumin in the presence of cobalt(III) or (II) at the hanging mercury drop electrode

When the hanging mercury drop electrode (HMDE) is placed in a solution which is 0.1 M in ammonia and 0.1 M in ammonium chloride and about 5 to 10×10^?4M in cobalt(III)-hexamine or cobalt(II) chloride and in very small concentrations of bovine serum albumin (BSA), the protein is slowly adsorbed. When the adsorption is highly incomplete and the HMDE is kept for 30 s at about ?1.05 V vs. SCE, “active cobalt’ is deposited as a complex (Co(0)BSA). This is anodically oxidized at about 0.0 V to unstable Co(I)BSA). When the electrode is then rapidly (500 mV s^?1) cathodized, a catalytic hydrogen current (i_c) with peak at circa ?1.45 V is observed. In this way it is even possible to detect and estimate BSA in concentrations of the order of 10^?12M. A detailed study has been made of the characteristics of i_c under several conditions. “Active cobalt” on the HMDE does not affect Brdi?ka currents. Cystine and cysteine also yield the catalytic hydrogen current i_c under the same conditions as does BSA.     

Catalytic currents of albumin at the hanging mercury drop electrode

Bovine serum albumin (BSA), as well as completely reduced BSA denoted by P (SH)₃₅, are adsorbed on the hanging mercury drop electrode (HMDE) from alkaline buffer solutions. When time is allowed, a monolayer is adsorbed from very dilute (10^?9M) BSA solutions in ammoniacal and borate buffers. With a monolayer of adsorbed protein the voltammograms at the HMDE are then identical in a given ammoniacal or borax buffer containing cobalt(III) or (II) and different BSA concentrations. Voltammograms of P (SH)₃₅ are virtually identical with those of native BSA. At the HMDE the second Brdi?ka current is proportional to concentration of cobalt(III) or (II) and the first current nearly so. Incompletely or completely adsorbed BSA or P (SH)₃₅ is not desorbed on keeping the HMDE for one hour in ammonia buffers. An incomplete layer of adsorbed BSA or P (SH)₃₅ is relatively rapidly desorbed at ?1.6 V (vs. SCE) and a complete film at ?1.65 V, some desorption occurring at ?1.6 V. Upon desorption, the second Brdi?ka current decreases faster than the first one; this is particularly striking in 1 M ammonia buffer. The rate of desorption is increased by calcium chloride, but the rate of adsorption is not, or only slightly, increased in the presence of calcium. Incomplete adsorption occurs at ?1.60 V (vs. SCE) and no adsorption at ?1.65 V. Indications are obtained that “presodium currents” yield a slight plateau at ?1.67 to ?1.70 V, the plateau currents being attributed to adsorbed BSA, while unadsorbed BSA yields catalytic currents without a plateau, the currents merging with the residual one of the buffer. Calcium chloride greatly increases the presodium currents. From many kinetic data obtained at the dropping mercury electrode (DME) and from results at the HMDE it is concluded that, depending on the BSA concentration, Brdi?ka currents at the DME are partly of a kinetic and partly of a surface adsorption nature and partly diffusion-controlled. Adsorption equilibrium is not attained at the DME at 25° at concentrations of BSA smaller than 10^?6M.     

铂电极上苯和苯磺酸吸附定向的现场红外反射吸收光谱电化学方法研究

本文用现场红外反射吸收光谱电化学方法和循环伏安法研究铂电析上苯和苯磺酸的吸附定向。对于苯/铂势系, 电势在-0.6至0.0V(相对饱和甘汞电极)内, 苯主要以垂直方式吸附; 在0.0至0.8V内则主要以平躺方式吸附。对于苯磺酸/铂体系,电势在-0.4V至0.0V内, 苯磺酸分子中的苯环主要呈垂直吸附且SO~3H基团远离电极表面; 在0.0至1.0V内则主要以倾斜平躺方式吸附, SO~3H基团通过其中的两个氧原子吸附于电极表面上。     

Voltammetric determination of cysteine at a graphite electrode modified with gold nanoparticles

The electrochemical behaviour of cysteine (Cys) at a graphite electrode modified with gold nanoparticles (G-Au_nano electrode) was studied by cyclic voltammetry. It was found that the graphite electrode-Au nanoparticles show an electrocatalytic activity towards the oxidation of Cys in 0.1?M NaOH. At 0.05?V, there is an “inverse” maximum in the cathodic voltammogram of Cys. Using a G-Au_nano electrode, the dependence of the peak current of the “inverse” maximum on Cys concentration was linear in the range from 1 to 14?pM, and the detection limit was 0.6?pM. The proposed analytical method is simple, rapid and sensitive.     

Electrocatalytic Reduction of Oxygen at Multi‐Walled Carbon Nanotubes and Cobalt Porphyrin Modified Glassy Carbon Electrode

The multi‐walled carbon nanotubes (MWNTs) modified glassy carbon electrode exhibited electrocatalytic activity to the reduction of oxygen in 0.1 M HAc‐NaAc (pH 3.8) buffer solution. Further modification with cobalt porphyrin film on the MWNTs by adsorption, the resulted modified electrode showed more efficient catalytic activity to O₂ reduction. The reduction peak potential of O₂ is shifted much more positively to 0.12 V (vs. Ag/AgCl), and the peak current is increased greatly. Cyclic voltammetry (CV), transmission electron microscopy (TEM), X‐ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), were used to characterize the material and the modified film on electrode surface. Electrochemical experiments gave the total number of electron transfer for oxygen reduction as about 3, which indicated a co‐exist process of 2 electrons and 4 electrons for reduction of oxygen at this modified electrode. Meanwhile, the catalytic activities of the multilayer film (MWNTs/CoTMPyP)_n prepared by layer‐by‐layer method were investigated, and the results showed that the peak current of O₂ reduction increased and the peak potential shifted to a positive direction with the increase of layer numbers.     

Speciation of Arsenic by Potentiometric Stripping Analysis Using Gold(III) Solution as Chemical Reoxidant and a Wall‐Jet Flow Cell

A simple procedure is described for the potentiometric stripping of arsenic with a wall‐jet cell by means of potentiostatic co‐deposition of gold and arsenic at a glassy‐carbon electrode and subsequent chemical stripping with Au(III). Optimum medium containing 160 mg L^?1 of Au(III) in HCl 0.1 M, where it is possible to speciate As(III) and As(V). As(V) was electrodeposited directly without prior chemical reduction at working electrode. As(III) was first determined at an electrodeposition potential of ?0.1 V. Afterwards, total arsenic was determined by an electrodeposition potential of ?0.7 V, from the area of peak obtained of the differential stripping potentiogram by using the standard addition method. The original As(V) concentration in the sample was calculated by difference. The possibilities of the optimized method were demonstrated by determinations of As(III), As(V) and total arsenic in samples of polluted water.     

Mechanism of nitrate electroreduction on Pt(100)

Kinetics and mechanism of nitrate anion reduction on the Pt(100) electrode in perchloric and sulfuric acid solutions are studied. Analysis of the results of electrochemical measurements (combination of potentiostatic treatment and cyclic voltammetry) and the data of in situ IR spectroscopy allow suggesting the following scheme of the nitrate reduction process on Pt(100) differing from that in the literature. If the potential of 0.85 V is chosen as the starting potential for a clean flame-annealed electrode surface and negativegoing (cathodic) potential sweep is applied, then an NO adlayer with the coverage of about 0.5 monolayer is formed on Pt(100) in the nitrate solution already at 0.6 V. The further decrease in the potential results in NO reduction to hydroxylamine or/and ammonia, desorbing products vacate the adsorption sites for nitrate and hydrogen adatoms. At E < 0.1 V, adsorbed hydrogen is mostly present on the surface. During positive-going (anodic) potential sweep, the process of nitrate reduction starts after partial hydrogen desorption, the cathodic peak of nitrate reduction to hydroxylamine or ammonia is observed at 0.32 V on cyclic voltammograms. The process of nitrate anion reduction continues up to 0.7 V; at higher potentials, the surface redox process with participation of hydroxylamine or ammonia (the anodic peak at 0.78 V) and nitrate (the cathodic peak at 0.74 V is due to nitrate reduction to NO on the vacant adsorption sites) occurs.     

Electropolymerization of iron tetra(<Emphasis Type="Italic">o</Emphasis>-aminophenyl)porphyrin from aqueous solution and the electrocatalytic behavior of modified electrode

Stable electroactive iron tetra(o-aminophenyl)porphyrin (FeTAPP) films are prepared by electropolymerization from aqueous solution by cycling the electrode potential between −0.4 and 1.0 V vs Ag/AgCl at 0.1 V s⁻¹. The cyclic voltammetric response indicates that polymerization takes place after the oxidation of amino groups, and the films could be produced on glassy carbon (GC) and gold electrodes. The film growth of poly(FeTAPP) was monitored by using cyclic voltammetry and electrochemical quartz crystal microbalance. The cyclic voltammetric features of Fe(III)/Fe(II) redox couple in the film resembles that of surface confined redox species. The electrochemical response of the modified electrode was found to be dependent on the pH of the contacting solution with a negative shift of 57 mV/pH. The electrocatalytic behavior of poly(FeTAPP) film-modified electrode was investigated towards reduction of hydrogen peroxide, molecular oxygen, and chloroacetic acids (mono-, di-, and tri-). The reduction of hydrogen peroxide, molecular oxygen, and dichloroacetic acid occurred at less negative potential on poly(FeTAPP) film compared to bare GC electrode. Particularly, the overpotential of hydrogen peroxide was reduced substantially. The O₂ reduction proceeds through direct four-electron reduction mechanism.     

Potential-induced structure changes of oligopyridine adlayers on Au(111) electrodes

The structure of a bisterpyridine-like oligopyridine (abbreviated as 2,4'-BTP) monolayer on Au(111), adsorbed from an acetone solution, was studied by in situ scanning tunneling microscopy and cyclic voltammetry in aqueous 0.1 M H2SO4. Short-range ordered adsorption with an average distance between the individual molecules of about 2 nm was observed only at electrode potentials positive of +0.4 V vs SCE, whereas at more negative potentials, no order could be found. With the help of Cu underpotential deposition, a potential-induced, fast, and fully reversible structure transition within the organic monolayer was identified at about +0.4 V vs SCE. At negative potentials the molecules apparently cluster together and consequently current-potential curves resemble those for a bare gold surface, whereas for E>+0.4 V vs SCE the molecules are spread over the entire surface in a hexagonal, close-packed fashion. This may have interesting consequences for switching between different template structures.     

Surface poisoning during electrocatalytic monosaccharide oxidation reactions at gold electrodes in alkaline medium

In the present study, the surface poisoning of electrocatalytic monosaccharide oxidation reactions at gold electrodes were investigated. In the cyclic voltammetric studies, the electrocatalytic oxidation of aldohexose and aldopentose type monosaccharides, aminosugars, acetyl-glucosamine and glucronamide were observed at gold plate electrodes in alkaline medium. However, in controlled-potential electrolytic studies ranging −0.3 to −0.2 V in reaction solutions, current flows during electrolyses decreased quickly with time, except when glucosamine was used as a substrate.Results from surface enhanced infrared adsorption (SEIRA) spectroscopic measurements at an evaporated gold electrode for the electrocatalytic oxidation of glucose in 0.1 mol dm⁻³ NaOH at −0.3 V and Gaussian simulated spectra indicated that the gluconic acid as a 2-electron oxidation product and/or its analogs adsorbed onto the gold surface. Electrochemical quartz crystal microbalance (EQCM) measurement results, along with surface adsorption results from surface poisoning at the gold electrode during electrolytic reactions, suggested that gluconic acid and/or its analogs adsorbed vertically onto electrode surfaces in a full monolayer packing-like conformation. In the case of the electro oxidation of glucosamine in 0.1 mol dm⁻³ NaOH at −0.2 V, the obtained SEIRA spectra and EQCM results, clearly indicated that the glucosaminic acid as a 2-oxidation glucosamine product did not strongly bind onto the gold electrode surface.     

碱性介质中L-赖氨酸在纳米金膜电极上的吸附和氧化过程

采用原位红外反射光谱(in situ FTIRS)和循环伏安法(CV)研究了碱性介质中L-赖氨酸在纳米金膜电极(nm-Au/GC)上的解离吸附和氧化过程. 研究结果表明, 在碱性溶液中以阴离子形式存在的赖氨酸[-OOC—CH—NH2—(CH2)4—NH2]在低电位区间(-0.95~-0.80 V, vs. SCE)发生部分解离, 生成AuCN-物种(约2110 cm-1), 同时赖氨酸阴离子的羧基侧还可通过两个氧原子与金电极表面相互作用. 随着电位的升高, 吸附态CN-氧化产生NCO-, OCN-和AuCN, 其对应的红外吸收峰分别位于2254, 2168和2226 cm-1附近.     

The electrochemical reduction reaction of dissolved oxygen on Q235 carbon steel in alkaline solution containing chloride ions

Cyclic voltammetry, electrochemical impedance spectroscopy, and rotating disk electrode voltammetry have been used to study the effect of chloride ions on the dissolved oxygen reduction reaction (ORR) on Q235 carbon steel electrode in a 0.02 M calcium hydroxide (Ca(OH)₂) solutions imitating the liquid phase in concrete pores. The results indicate that the cathodic process on Q235 carbon steel electrode in oxygen-saturated 0.02 M Ca(OH)₂ with different concentrations of chloride ions contain three reactions except hydrogen evolution: dissolved oxygen reduction, the reduction of Fe(III) to Fe(II), and then the reduction of Fe(II) to Fe. The peak potential of ORR shifts to the positive direction as the chloride ion concentration increases. The oxygen molecule adsorption can be inhibited by the chloride ion adsorption, and the rate of ORR decreases as the concentration of chloride ions increases. The mechanism of ORR is changed from 2e⁻ and 4e⁻ reactions, occurring simultaneously, to quietly 4e⁻ reaction with the increasing chloride ion concentration.     

Spectroelectrochemical examination of the interaction between bacterial cells and gold electrodes

The interaction between bacterial cells of Pseudomonas fluorescens (ATCC 17552) and gold electrodes was analyzed by cyclic voltammetry (CV) and attenuated total reflection-surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). The voltammetric evaluation of cell adsorption showed a decrease in the double-layer capacitance of polyoriented single-crystal gold electrodes with cell adhesion. As followed by IR spectroscopy in the ATR configuration, the adsorption of bacterial cells onto thin-film gold electrodes was mainly indicated by the increase in intensity with time of amide I and amide II protein-related bands at 1664 and 1549 cm(-1), respectively. Bands at 1448 and 2900 cm(-1) corresponding to the scissoring and the stretching bands of CH2 were also detected, together with a minor peak at 1407 cm(-1) due to the vs COO- stretching. Weak signals at 1237 cm(-1) were due to amide III, and a broad band between 1100 and 1200 cm(-1) indicated the presence of alcohol groups. Bacteria were found to displace water molecules and anions coadsorbed on the surface in order to interact with the electrode intimately. This fact was evidenced in the SEIRAS spectra by the negative features appearing at 3450 and 3575 cm-1, corresponding to interfacial water directly interacting with the electrode and water associated with chloride ions adsorbed on the electrode, respectively. Experiments in deuterated water confirmed these assignments and allowed a better estimation of amide absorption bands. In CV experiments, an oxidation process was observed at potentials higher than 0.4 V that was dependent on the exposure time of electrodes in concentrated bacterial suspensions. Adsorbed bacterial cells were found to get closer to the gold surface during oxidation, as indicated by the concomitant increment in the main IR bacterial signals including amide I, a sharp band at 1240 cm(-1), and a broad one at 1120 cm(-1) related to phosphate groups in the bacterial membranes. It is proposed to be due to the oxidation of lipopolysaccharides on the outermost bacterial surface.     

硝基化合物的吸附伏安法研究 I: 5-硝基-1, 10-邻菲咯啉的吸附伏安特性

本文用线性扫描技术对氨性溶液中, 低浓度, 5-硝基-1,10-邻菲咯啉的吸附伏安特性进行了研究.     

XPS studies on the gold oxide surface layer formation

The initial stage of gold oxide layer formation on the gold electrode surface was investigated in 0.5 M H₂SO₄. X-ray photoelectron spectroscopy (XPS) spectra of pure gold and the anodically polarized gold electrode surface were compared quantitatively. It was found that gold anodic polarization in the E range from ∼1.3 to 2.1 V causes increase in intensity of the XPS spectra at an electron binding energy ε_b=85.9 eV for gold and at ε_b=530 eV for oxygen. These ε_b values correspond to Au³⁺ and O²⁻ oxidation states in hydrous or anhydrous gold oxide. The larger the amount of the anodically formed surface substance the higher is the intensity of the spectrum at the ε_b values mentioned above. It was concluded that gold anodic oxidation, yielding most likely an Au(III) hydroxide surface layer, takes place in the E range of the anodic current wave beginning at E≈1.3 V. At E_B=1.7 V (the potential of the Burshtein minimum) the stationary surface layer consists of 2.5 to 3 molecular layers of Au(OH)₃. The theoretical amount of charge required for the reduction of one molecular layer of Au(OH)₃ is ∼0.15 mC cm⁻², since the Au(OH)₃ molecule is planar and occupies about four atomic sites on the electrode surface.     

Mechanism of electrochemical reactions of polyaniline films formed under the conditions of cathodic oxygen reduction

Polyaniline films (further, CPANI) were obtained under the conditions of oxygen cathodic reduction in the aniline-containing solution on the electrodes of mixed indium tin oxide (ITO), graphite, and gold. CPANI films are characterized by redox processes in the potential ranges of 0.1–0.2 V and ～0.4 V (SCE). These processes are caused by the polymer chain fragments of different structure and the ratio between the peaks corresponding to these processes varies significantly as dependent on the synthesis conditions (electrode material, stirring, etc). The mechanism of electrode processes on CPANI is studied using the methods of cyclic voltammetry and quartz microgravimetry. It is found that only hydrogen cations and supporting electrolyte anions participate in the electrode process at the potentials of 0.1–0.2 V. The mechanism of redox processes on the obtained polymer films is discussed.     

Adsorptive stripping voltammetric determination of chromium in gallium

The electroanalytical chemistry of trace metals has progressed strongly with the development of cathodic stripping voltammetry (CSV) preceded by adsorption collection of organic metal complexes. A sensitive method for the determination of trace amount of chromium in gallium is described. Gallium is dissolved in sodium hydroxide containing hydrogen peroxide. The method is based on the catalytic activity of nitrate ions on the reduction of Cr(III)TTHA (triethylene tetramine-N,N,N',N',N',N'-hexaacetic acid) complex. The sensitivity of this method is further improved by adsorption preconcentration of Cr(III)TTHA complex at a hanging mercury drop electrode (HMDE). The Cr(III) formed at the electrode surface by the reduction of Cr(VI), which is present in the bulk solution, is immediately complexed by TTHA. The adsorbed complex is then reduced at a peak potential of - 1.26 V, and the peak height of Cr(III) reduction is measured. The determination limit was restricted by the amount of chromium present in the reagent blank solution. The method is suitable for the determination of chromium at level as low as 0.2 mug g(-1) (with about 50 mg of sample) and a relative standard deviation of 15%.     

Direct and alternating current voltammetry at an HMDE in solutions containing activated charcoal suspension

Advantage is taken of the property that activated charcoal particles dispersed in a solution accumulate on the HMDE. Alternating current voltammetry is used to determine the surface area of carbon sticking to the electrode on which redox processes may proceed. The surface area of charcoal determined in this way allowed us to estimate the contents of electroactive oxygen in three types of carbon samples. It has been found that the capability of carbon particles to stick to the mercury surface increases with growing oxygen content.The electroactive forms of oxygen bonded to the carbon surface undergo reduction in a peak at about −0.5 V. The second reduction peak, at about −1.6 V, is connected with hydrogen evolution on the charcoal-covered part of the electrode. Detachment of the charcoal particles from the mercury surface results in decay of the H₃O⁺ reduction current. Photographs provide evidence of the efficiency of accumulation of the carbon particles in dependence on the type of sample tested and the HMDE potential as well as phenomena accompanying “desorption”.     

A detailed analysis of the polarographic behaviour of methylene blue in phosphate buffer on mercury

Polarographic current-potential characteristics and current-time curves for the reduction of methylene blue (MB) to methylene blue leucoform (MBL) in a pH 7.9 aqueous phosphate buffer have been examined in detail over a wide concentration range. It has thus been shown that the so called “normal” or “main” reduction wave of MB actually consists of two separate steps, the former with a half-wave potential practically coinciding with the formal potential E₀=?0.250 V/SCE of the MB/MBL couple and the latter with a half-wave potential of about ?0.310 V/SCE. As soon as the well-known MB adsorption prewave has attained its maximum height (which occurs at a MB bulk concentration c₀^*?5×10^?5M), a further slight increase in c₀^* causes the appearance of the wave with E_1/2=?0.310 V. The height of the linear potential-sweep voltammetric peak corresponding to the latter wave increases proportionally to the sweep rate, thus revealing the “adsorption” nature of this wave. A comparison with a previous chronocoulometric investigation of the MB/MBL system has permitted us to conclude that the wave with E_1/2=?0.310 V is due to reduction of the MB molecules which, after having reached the surface of the dropping electrode by diffusion, are adsorbed at the top of the adsorbed monolayer of MBL in direct contact with the electrode and remain in this adsorbed state after reduction. For c₀^*>7×10^?5M the wave with E_1/2?E₀=?0.250 V starts to develop. This wave is due to the electroformation of MBL molecules which diffuse back into the solution. The shape of polarographic current-potential characteristics and current-time curves has been accounted for semiquantitatively through an approximate solution of the corresponding diffusional problem.