Use of ligand buffers for improved selectivity in the polarographic determination of metals Polarographic determination of zinc in the presence of a large amount of cadmium

The use of ligand buffers composed of a chelating agent and an excess of polarographically inert metal is recommended for the selective polarographic determination of a metal in the presence of a second metal, which has a polarographic wave at a less negative potential than the metal to be determined. Theoretical considerations on the optimum ligand concentration and the optimum composition of the ligand buffer are followed by the determination of zinc in cadmium as an example.     

Determination of heavy metal concentration in feed and permeate streams of polymer enhanced ultrafiltration process

Polymer enhanced ultrafiltration (PEUF) is a newly developed method for the removal of heavy metals from aqueous solutions. This method was applied for the removal of mercury and cadmium with the presence of polyethyleneimine (PEI) as a water soluble polymer. After ultrafiltration experiments for metal–polymer mixtures, two separate streams, namely, retentate and permeate, former of which contains mainly metal–polymer complex and free polymer molecules while latter of which mainly contains free metal ions, were obtained. At the end of PEUF experiments, performance of operation was determined by concentration analyses which was achieved by atomic absorption spectroscopy (AAS) applied in a different way for permeate and retentate streams considering the effect of presence of polymer. For mercury analysis, cold vapor AAS was applied. It was observed that the presence of PEI did not affect the atomic absorption signal when 10% HCl was added to the sample solutions. For calcium and cadmium, flame AAS was used. It was observed that change in PEI concentration results in change in measured concentration of calcium and cadmium. Therefore, two new approaches were developed for accurate measurement of concentrations of calcium and cadmium. It was also observed that presence of other metals did not affect the accuracy of the measurement of a particular metal in the concentration range studied.     

Characteristics of Subtractive Anodic Stripping Voltammetry of Lead,Cadmium and Thallium at Silver‐Gold Alloy Electrodes

Silver‐gold alloy electrodes have been studied for the purpose of the quantitative determination of heavy metals by subtractive anodic stripping voltammetry, (SASV). The results have been compared with those obtained with the silver and gold electrodes. The 50/50 a/o Ag/Au alloy electrode is the most suitable for quantifying thallium in the presence of lead and cadmium. The separation of its peak from those of lead and cadmium is 200 mV, which is about twice the separation obtained on the pure metal electrodes and is also better than on mercury. The silver electrode is suitable for the simultaneous determination of thallium, lead and cadmium. The peaks of lead and cadmium overlap on the 50/50 alloy. Pure silver or pure gold can be used for simultaneous quantification of these two metals. The use of gold for quantifying lead and cadmium is more limited because the peak potential of cadmium is shifted in the negative direction as its concentration increases and at [Cd²⁺]>200 nM, the two peaks merge. SASV enables correction for background currents and is of utmost importance for obtaining well‐defined peaks. The peaks of lead, cadmium and thallium appear over a relatively narrow potential range (ca. 200 mV) on all the electrodes presented in this work. For this reason, the quantifying of a peak is based on the derivative at the inflection point of only one of its branches (ascending or descending). All SASV measurements were carried out without removal of oxygen.     

纳米氧化铁修饰玻碳电极检测痕量镉离子

制备了一种纳米氧化铁修饰玻碳电极,并研究了镉离子在该修饰电极上的溶出伏安行为。结果表明,纳米氧化铁颗粒能有效促进镉离子的溶出伏安响应。在pH 6.0的磷酸缓冲溶液中,镉离子能有效吸附在纳米氧化铁表面并在-1.0 V时被还原。被还原的镉在正向扫描过程中可以重新氧化,并在-0.85 V处出现一明显的溶出伏安氧化峰。该峰电流随镉离子浓度的增大而增大,可用于对镉离子的检测。在最佳检测条件(pH 6.0,富集时间350 s,富集电位-1.0 V)下,镉离子的响应电流与其浓度在6.0×10-10～1.0×10-8mol/L以及1.0×10-8～1.0×10-5 mol/L范围内呈良好线性,检出限(S/N=3)为1.0×10-10 mol/L。干扰实验结果表明,一些常见的阳离子以及阴离子对镉离子的检测无明显干扰。将该方法用于实际样品的检测,回收率良好。     

Coadsorption of Cd(II) and oxalate ions at the TiO2/electrolyte solution interface

The study of the adsorptions of cadmium and oxalate ions at the titania/electrolyte interface and the changes of the electrical double layer (edl) structure in this system are presented. The adsorption of cadmium or oxalate ions was calculated from an uptake of their concentration from the solution. The concentration of Cd(II) or oxalate ions in the solution was determined by radiotracer method. For labeling the solution 14C and 115Cd isotopes were used. Coadsorption of Cd(II) and oxalic ions was determined simultaneously. Besides, the main properties of the edl, i.e., surface charge density and zeta potential were determined by potentiometer titration and electrophoresis measurements, respectively. The adsorption of cadmium ions increases with pH increase and shifts with an increase of the initial concentration of Cd(II) ions towards higher pH values. The adsorption process causes an increase of negatively charged sites on anatase and a decrease of the zeta potential with an increase of initial concentration of these ions. The adsorption of oxalate anions at the titania/electrolyte interface proceeds through the exchange with hydroxyl groups. A decrease of pH produces an increase of adsorption of oxalate ions. The processes of anion adsorption lead to increase the number of the positively charged sites at the titania surface. However, specific adsorption of bidenate ligand as oxalate on one surface hydroxyl group may form inner sphere complexes on the metal oxide surface and may overcharge the compact part of the edl. The presence of oxalate ions in the system affects the adsorption of Cd(II) ions on TiO2, increasing the adsorption at low pH range and decreasing the adsorption at high pH range. Using adsorption as a function of pH data, some characteristic parameters of adsorption envelope were calculated.     

Effect of operating parameters on selective separation of heavy metals from binary mixtures via polymer enhanced ultrafiltration

Performance of continuous polymer enhanced ultrafiltration (PEUF) method was investigated for removal of mercury and cadmium from binary mixtures. This method includes the addition of polyethyleneimine (PEI) as a water soluble polymer to bind the metals, which was followed by ultrafiltration operation performed on both laboratory and pilot scale systems. The influence of various operating parameters such as temperature, metal/polymer ratio, presence of calcium ions and pH on retention of metals and permeate flux was investigated. To investigate the possibility of selective separation of mercury and cadmium, experiments were conducted for binary solutions at different pH and loading ratios. It was seen that the retention of mercury decreased and permeate flux increased when the temperature increased. The increased pH and decreased metal/polymer ratio, loading (L), resulted in higher retention of both metals. Shapes of retention vs. pH or L curves were very similar for both metals. Retentions stay almost constant at a value very close to unity until a critical L or pH value was reached, then, R decreases almost linearly with L or pH. However, retention of cadmium was affected more than that of mercury as the pH decreased and L increased. This leads to the selective separation of mercury and cadmium. At low pH values (about 5) and at high L values (about 0.3), mercury was removed by ultrafiltration operation while almost all cadmium passed through the membrane. At pH 5.5 and cadmium/polymer ratio about 0.35 and mercury/polymer ratio about 0.39, the highest separation factor was obtained as 49.     

Determination of trace metals by anodic stripping voltammetry using a bismuth-modified carbon nanotube electrode

A bismuth-modified carbon nanotube electrode (Bi-CNT electrode) was employed for the determination of trace lead, cadmium and zinc. Bismuth film was prepared by in situ plating of bismuth onto the screen-printed CNT electrode. Operational parameters such as preconcentration potential, bismuth concentration, preconcentration time and rotation speed during preconcentration were optimized for the purpose of determining trace metals in 0.1M acetate buffer solution (pH 4.5). The simultaneous determination of lead, cadmium and zinc was performed by square wave anodic stripping voltammetry. The Bi-CNT electrode presented well-defined, reproducible and sharp stripping signals. The peak current response increased linearly with the metal concentration in a range of 2-100 microg/L. The limit of detection was 1.3 microg/L for lead, 0.7 microg/L for cadmium and 12 microg/L for zinc (S/N=3). The Bi-CNT electrode was successfully applicable to analysis of trace metals in real environments.     

Determination of cadmium and lead in the low ng/l range by stripping potentiometry employing medium exchange in batch mode and multiple stripping in a hanging stripping medium drop

A novel three-in-one electrode assembly, permitting medium exchange under a controlled potential in batch mode by exploiting electrolytic conductance through a drop hanging under the electrode, has been used for the simultaneous determination of cadmium(II) and lead(II) at trace levels. Potentiostatic reduction and amalgamation of these ions are carried out in the sample and the subsequent stripping, in the stripping potentiometry mode, in a drop of a suitable medium. Several such media have been investigated, a mixture of 2.5M acetic acid and 7.5M ammonium acetate having been found to be the most suitable with respect to sensitivity, potential resolution and trace metal purity. The quiescent conditions in the drop of medium facilitated multiple stripping and thus increased sensitivity. Detection limits for cadmium(II) and lead(II) were found to be around 0.5 ng/l. (5 and 2.5pM) for an electrolysis time of 10 min. The relative precision at the concentration level 20 ng/l. was 6.4% for cadmium and 5.4% for lead. The procedure has been used for the determination of cadmium(II) and lead(II) in reference seawater samples.     

Removal of heavy metals and bacteria from aqueous solution by novel hydroxyapatite/zeolite nanocomposite,preparation, and characterization

In this study, a HAp/NaP nanocomposite was prepared by adding a synthesized nano-hydroxyapatite to zeolite NaP gel in the hydrothermal condition and used for the removal of lead(II) and cadmium(II) ions from aqueous solution. HAp/zeolite nanocomposite was then characterized by Fourier transform infrared spectroscopy, X-ray diffraction and Rietveld method, scanning electron microscope, energy-dispersive X-ray analysis, and surface area and thermal analyses. Results suggested that the nanocomposite crystals of HAp were dispersed onto the zeolite external surface and/or encapsulated within the zeolite channels and pores. The potential of the composite in adsorption of heavy metals was investigated by using batch experiment. The metal concentration in the equilibrium C _e (mg/g) after adsorption with nanocomposite of HAp/NaP was analyzed using flame atomic adsorption spectrometry. The adsorption experiments were carried out at pH of 3–9. The influences of contact time, initial concentration, dose, and temperature on the adsorption of lead and cadmium ions were also studied. Results show that these nanocomposites have further adsorption related to NaP and HAp. They have great potential (about 95 %) for Pb(II) and Cd(II) adsorption at room temperature. The equilibrium process was described by Frendlich, Langmuir, Temkin, and Dubinin–Radushkevich (D-R) models. The kinetics data were successfully fitted by a pseudo-second-order model. The in vitro antibacterial activity of these composites was evaluated against Bacillus subtilis (as Gram-positive bacteria) and Pseudomonas aeruginosa (as Gram-negative bacteria) and compared with standard drugs that show inhibition on bacterial growth.     

Biosorption of Cadmium by Phanerochaete Chrysosporium

The biosorption properties of cadmium(Ⅱ) by pre-treated biomass of Phanerochaete chrysosporium in the form of pellet were investigated. It was found that formaldehyde cross-linking and subsequent alkaline treatment could significantly improve the adsorption capacity of the biomass compared to other sorts of treatments, such as calcium chloride treatment, HCl treatment and,acetone treatment. Biosorption capacity of cadmium was examined as a function of physical and chemical factors including the pH of the metal solution pellet size, temperature and biomass concentration. The cadmium removal efficiency was strongly affected by pH. The maximal adsorption occurred around pH4.5. The pellet size also had a marked influence on the cadmium removal efficiency and the optimum size was the diameter range of 1.5-2.0 mm. The effect of biosorption temperature on cadmium uptake was inconspicuous between 25℃ and 35℃, but there was a notable decrease in cadmium uptake when the temperature reached 40℃. The cadmium removal efficiency increased as the biomass concentration when the initial cadmium ion concentration was 10 mg/L. When the biomass concentration was 2 g/L,the removal efficiency was 99.56%. However, the augment of the. removal efficiency was not obvious when the biomass concentration was more than 2 g/L. On the optimum conditions mentioned above,cadmium concentration could be reduced from 10 ppm down to 0.04 ppm that was below the Chinese National Waste Water Integrated Discharge Standard. In the biosotption process, most of the metal uptake happened during a short period immediately after the adsorption process started. It was observed that the biomass pellets had already adsorbed 83.36% of the total amount of cadmium finally adsorbed within the initial 10 minutes. The cadmium uptake rate decreased gradually afterwards. Sorption equilibrium could almost be established in 12 hours. This indicated that biosorption might consist of two processes:a fast surface binding process opcurring first and a slow membrane diffusion process taking place subsequently.     

Three-way multivariate analysis of metal fractionation results from sediment samples obtained by different sequential extraction procedures and ICP-OES

Three typical schemes for metal fractionation were applied to analyse coastal surface sediment samples from Bahía Blanca estuary, where an important industrial emplacement is located. Also, three certified reference materials for total metal concentrations were analysed. The studied metals were cadmium, chromium, copper, lead and zinc because of their hazardous potential and related abundance in the estuary. The concentration of metals was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES). A three-way multivariate analysis was performed in order to obtain a better visualization of the experimental data. The extracted information was used to evaluate the equivalence among the results obtained by the three sequential extraction schemes. The data were analysed by parallel factor analysis (PARAFAC). PARAFAC models with two factors describe appropriately the data sets (explained variance about 54% and core consistency of 100%). The multivariate decomposition showed that the three applied schemes are able to describe equally well the behaviour of the metals in the different sediment fractions.     

Analysis of Tea for Metals by Flame and Graphite Furnace Atomic Absorption Spectrometry with Multivariate Analysis

The concentrations of iron, zinc, manganese, copper, chromium, nickel, lead, arsenic, and cadmium were determined using flame and graphite furnace atomic absorption spectrometry in nine herbal tea samples. Hawthorn, yarrow, elderflower, and bearberry are herbal teas for which the metal content has been rarely determined. The concentration of cadmium in St John’s wort exceeded the maximum permissible limit of 0.3?mg/kg. The metals were also determined in aqueous extracts following 5, 10, and 20?min of boiling and using different preparations, including acidification with lemon juice. The influence of these factors on metal release was demonstrated using multivariate analyses by redundancy analysis and principal component analysis. The metal release was not considerably affected by the boiling time, while the acidity of the medium was generally positively correlated. For the infusions, arsenic was detected only in acidified mint tea. The extraction efficiencies of metals were determined and classified as highly, moderately, and poorly extractable, with chromium showing largest variations.     

Development of highly sensitive cadmium iondashselective electrodes by titration method and its application to cadmium ion determination in industrial waste water

Characteristics of cadmium iondashselective electrode made cadmium sulphide (CdS)-silver sulphide (Ag(2)S) mixture were studied. CdS-Ag(2)S mixtures were obtained by gas/solid-phase reaction between silver-cadmium mixed powder and hydrogen sulphide gas (dry method) and by ionic reaction between cadmium-silver mixed ions and sulphide ion (wet method). As a result, it was found that the CdS-Ag(2)S mixture had to be made in the condition of excess existence of sulfur and had better regulate the excess sulfur quantity minimum, for the CdS-Ag(2)S pressed membrane gave a good Nernstian response against the cadmium ion concentration change. As the best way, CdS-Ag(2)S mixture was obtained by adding sulphide ion solution to 5 mol% cadmium ion and 95 mol% silver ion mixed solution while measuring silver sulphide (Ag(2)S) electrode potential as an indicator electrode. According to the reaction was stopped when the potential variation from the initial potential in the sulphide ion solution reached at 87-116 mV which the sulphide ion concentration became 10(-3) - 10(-4) of the initial concentration, the cadmium ion membrane pressed diameter of 8 mm and thickness of 2 mm showed a Nernstian response from 10(-8) to 10(-1) M of cadmium ion concentration. Furthermore, aiming to its application for industrial waste water, masking buffer for interfering metal ions such as lead ion (Pb(2+)) and copper ion (Cu(2+)), which were possibly coexisted and to adjust total ionic strength and pH of sample was developed. The present Cd(2+) iondashselective electrode was applied to the determination of Cd(2+) in the industrial waste water. The good regression line with correlation factor of 0.984 was obtained compared with the conventional atomic absorption spectroscopy.     

Sub-picomole high-performance liquid chromatographic/mass spectrometric determination of glutathione in the maize (Zea mays L.) kernels exposed to cadmium

Changes in fresh weight, total protein amounts (Bradford’s method), cadmium concentration (DPASV) and glutathione content (HPLC/MS) were studied in maize kernels cultivated for 5 days at three different cadmium concentrations (0, 10 and 100 μmol l⁻¹ CdCl₂). A highly sensitive HPLC/MS method was used for the determination of glutathione on a reversed-phase Atlantis dC₁₈ chromatographic column (150 mm×2.1 mm, 3 μm particle size). An isocratic mode with acetonitrile–0.01% TFA (5:95, flow rate 0.1 ml min⁻¹ and 30 °C) was applied. The m/z spectra and the data for the selected ion monitoring (SIM) mode were recorded at m/z for glutathione 308→179. Cadmium concentration was measured by a differential pulse adsorptive stripping voltammetry (DPASV) after deposition on a hanging mercury drop electrode (HMDE) at potential −0.7 V (accumulation time 180 s, acetate buffer of pH 3.6, 22 °C). An AUTOLAB with a VA-Stand 663 and a three-electrode system consisting of the HMDE as a working electrode with area 0.4 mm², an Ag/AgCl/3 mol l⁻¹ KCl as a reference electrode and a Pt-wire as an auxiliary electrode was employed. The maize kernels exposed to the highest cadmium concentration (100 μmol l⁻¹) germinated formerly and much better. A rapid increase of the fresh weight probably relates with more intensive uptake of water in order to decrease cadmium concentration. An intensive preservation of homeostasis of Cd²⁺ ions in the germinating plants by defending mechanisms might explain differences of uptake rate of cadmium. The linear increase of GSH content with the exposure time at all studied concentration suggests the defending mechanisms might be triggered by concentrations of a heavy metal.     

Kinetic, equilibrium and thermodynamic studies of cadmium (II) adsorption by modified agricultural wastes

Agricultural wastes have great potential for the removal of heavy metal ions from aqueous solution. The contamination of water by toxic heavy metals is a worldwide environmental problem. Unlike organic pollutants, the majority of which are susceptible to biological degradation, heavy metals do not degrade into harmless end products. Discharges containing cadmium, in particular, are strictly controlled because of the highly toxic nature of this element and its tendency to accumulate in the tissues of living organisms. This work aims to develop inexpensive, highly available, effective metal ion adsorbents from natural wastes as alternatives to existing commercial adsorbents. In particular, Tamrix articulata wastes were modified chemically by esterification with maleic acid to yield a carboxyl-rich adsorbent. The adsorption behavior of treated Tamrix articulata wastes toward cadmium ions in aqueous solutions in a batch system has been studied as a function of equilibration time, adsorbent dose, temperature and pH. Results showed that the maximum adsorption capacity was 195.5 mg/g in a pH 4 solution at 30 °C with a contact time of 120 min, an initial concentration of 400 mg/L and an adsorbent dose of 0.3 g/L. The kinetic data were analyzed using pseudo-first-order and pseudo-second-order kinetic models. It was shown that the adsorption of cadmium could be described by a pseudo-second-order equation. The experimental data were also analyzed using the Langmuir and Freundlich models of adsorption. Thermodynamic parameters such as ΔG°, ΔH° and ΔS° have been evaluated and it has been found that the sorption process was spontaneous and exothermic in nature. From all of our data, we conclude that the treated Tamrix articulata wastes investigated in this study showed good potential for cadmium removal from aqueous solutions.     

Determination of cadmium(II) at a gold electrode in the presence of selenium(IV) by anodic-stripping voltammetry with enhancement by iodide ion

Anodic-stripping voltammetry (ASV) has been used in the derivative mode for the determination of cadmium, with a gold electrode in sulphuric add medium containing seleniurn(IV). The peak height for cadmium is enhanced by the presence of iodide. The sensitivity for cadmium is very high, with a peak in the stripping voltamperogram at -0.27V (vs. Ag/AgCl). The peak height for cadmium is not affected by over a 100-fold level of lead in the presence of selenium(IV). The dependence of peak height on the cadmium concentration is linear in the range 0.05-10ng ml .     

Development of peptoid-based ligands for the removal of cadmium from biological media

Cadmium poisoning poses a serious health concern due to cadmium''s increasing industrial use, yet there is currently no recommended treatment. The selective coordination of cadmium in a biological environment—i.e. in the presence of serum ions, small molecules, and proteins—is a difficult task. To address this challenge, a combinatorial library of peptoid-based ligands has been evaluated to identify structures that selectively bind to cadmium in human serum with minimal chelation of essential metal ions. Eighteen unique ligands were identified in this screening procedure, and the binding affinity of each was measured using metal titrations monitored by UV-vis spectroscopy. To evaluate the significance of each chelating moiety, sequence rearrangements and substitutions were examined. Analysis of a metal–ligand complex by NMR spectroscopy highlighted the importance of particular residues. Depletion experiments were performed in serum mimetics and human serum with exogenously added cadmium. These depletion experiments were used to compare and demonstrate the ability of these peptoids to remove cadmium from blood-like mixtures. In one of these depletion experiments, the peptoid sequence was able to deplete the cadmium to a level comparable to the reported acute toxicity limit. Evaluation of the metal selectivity in buffered solution and in human serum was performed to verify minimal off-target binding. These studies highlight a screening platform for the identification of metal–ligands that are capable of binding in a complex environment. They additionally demonstrate the potential utility of biologically-compatible ligands for the treatment of heavy metal poisoning.     

Fast and simultaneous detection of heavy metals using a simple and reliable microchip-electrochemistry route: An alternative approach to food analysis

This paper reports, for the first, the fast and simultaneous detection of prominent heavy metals, including: lead, cadmium and copper using microchip CE with electrochemical detection. The direct amperometric detection mode for microchip CE was successfully applied to these heavy metal ions. The influences of separation voltage, detection potential, as well as the concentration and pH value of the running buffer on the response of the detector were carefully assayed and optimized. The results clearly show that reliable analysis for lead, cadmium, and copper by the degree of electrophoretic separation occurs in less than 3min using a MES buffer (pH 7.0, 25mM) and l-histidine, with 1.2kV separation voltage and -0.8V detection potential. The detection limits for Pb(2+), Cd(2+), and Cu(2+) were 1.74, 0.73 and 0.13microM (S/N=3). The %R.S.D. of each peak current was <6% and migration times <2% for prolonged operation. To demonstrate the potential and future role of microchip CE, analytical possibilities and a new route in the raw sample analysis were presented. The results obtained allow the proposed microchip CE-ED acts as an alternative approach for metal analysis in foods.     

A Potentiometric Stripping Analyzer for Multianalyte Screening

The paper describes a simple and low‐cost multisensing instrument based on neural network to monitor multiple environmentally significant metals in solution by implementing the potentiometric stripping analysis technique. Our analyzer consists of a measuring unit, implemented on a lap‐top/notebook computer equipped with a simple electronic circuitry, and a reusable sensor, based on glassy‐carbon technology. The current system has been characterized with synthetic samples containing different concentrations of lead, copper, cadmium and zinc, showing detection limits suitable for the first monitoring of metal traces. A linear relationship between peak area and concentration has been observed for all tested metals. Therefore, a simple linear regression can be used by the analyzer to determine the actual metal concentration from the PSA peak area. The presence of a second metal or of a real matrix, often, interferes with stripping measurements in different ways: compounds with oxidation potentials close to that of the analytes of interest may interfere if the instrument does not have sufficient resolution to resolve the overlapping PSA peaks; species which form intermetallic compounds with the analytes may result in erroneously low analyte concentration readings, since the oxidation potential of the intermetallic compound is rarely near that of the original analyte. In order to overcome these problems, the described analyzer has been integrated with a Neural Network algorithm, able to separate the different contributions, after a proper training.     

Novel determination of cadmium ions using an enzyme self-assembled monolayer with surface plasmon resonance

The activity of the enzyme urease is known to be inhibited by the heavy metal cadmium. The binding of cadmium to urease and the consequent changes of the enzyme structure are the basis of the surface plasmon resonance (SPR) biosensing system reported herein. To facilitate the formation of a self-assembled monolayer (SAM) of the urease on gold-coated glass SPR sensor disks, the enzyme has been modified with N-succinimidyl 3-(2-pyridyldithiol) propionate (SPDP). The urease monolayer was exposed to trace levels of cadmium ions and monitored by SPR. From circular dichroism (CD) data, it is believed that the conformation of the active nickel site of the urease changes upon binding of the cadmium ions. It is this change of the enzyme monolayer, measured by SPR, which has been related to the cadmium ion concentration in the range of 0–10 mg l⁻¹. These data are the first report of a SPR biosensor capable of detecting metal ions.