LIVE AND DEAD SPIRULINA SP. TO REMOVE ARSENIC (V) FROM WATER

Arsenic (As)-contaminated water is a grave health hazard and its removal from water poses a great challenge. Conventional methods are associated with many shortcomings. Biosorption of arsenic using blue-green algae is an interesting alternative to conventional methods. In this article, the results of the biosorption of As(V) as AsO₄ ^{? 3} by live and dead Spirulina sp. are reported. The sorption of arsenic could be explained satisfactorily both by the Freundlich and the Langmuir isotherms. The maximum sorption capacities of live and dead Spirulina were estimated to be 525 and 402mg/g, respectively. These values are high in comparison with those reported for other arsenic sorbents. The sorption kinetics of arsenic by both live and dead Spirulina sp. could be well modeled by Lagergrens pseudosecond order-rate equation. Infrared spectra have been employed to understand how Spirulina sp. binds with arsenate. Scanning electron micrography and fluorescent microscopic images are used to discuss the extent of uptake. Preferential uptake of Cu(II), Ni(II), Cd(II), and AsO₄ ^?3 by live Spirulina sp. was investigated and explained with the help of rate constants for sorption.     

Bioremediation Potential of Chlorella: Spectroscopic,Kinetics, and Sem Studies

A dead dried alga, Chlorella sp., was used for the uptake of Cr⁺³, Cr₂O₇ ^?2, Cu⁺², and Ni⁺² from the aqueous solutions of these metal ions. The equilibrium data were fitted using the Langmuir and Freundlich isotherm model and the maximum uptakes for Cr⁺³, Cr₂O₇ ^?2, Ni⁺², and Cu⁺² were 98, 104, 108, and 183 mg/g, respectively. The Freundlich model, in comparison to the Langmuir model, better represented the sorption process. The kinetics of metal ions uptake by Chlorella sp. was best described by a pseudo-second order rate equation. Infrared spectroscopic data were employed to identify the site(s) of bonding in Chlorella sp. A scanning electron microscopic (SEM) study of pure dead Chlorella sp. and the species treated with different metal ions provided an idea of the extent of metal uptake by this species. The dead Chlorella sp took up maximum Cu(II). The size of the cell of the metal-treated Chlorella sp. obtained from SEM data is in agreement with the extent of metal uptake.     

Bioremediation potential of live and dead Spirulina: spectroscopic, kinetics and SEM studies

Metal binding by algae has enormous potential for environmental bioremediation targeting towards detoxification of water bodies. The present work reports the use of live and dead Spirulina sp. for sorption of metals like Cr(3+), Ni(2+), Cu(2+), and Cr(6+) in form of Cr(2)O(7)(2-). Preliminary investigation shows that this biomass takes up substantial amount of metal ions indicated above. IR spectroscopic study, kinetics models, Langmuir and Freundlich adsorption isotherms, SEM analysis, and fluorescence microscopic study of Spirulina sp. and the Spirulina sp. treated with different metal ions have been employed to understand the sorption mechanism. It is hoped that live Spirulina sp. will be a strong candidate for management of industrial wastewater.     

Multi-metal biosorption and bioaccumulation by Exiguobacterium sp. ZM-2

The present work deals with the biosorption performance of dried and non-growing biomasses of Exiguobacterium sp. ZM-2, isolated from soil contaminated with tannery effluents, for the removal of Cd²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ from aqueous solution. The metal concentrations studied were 25 mg/l, 50 mg/l, 100 mg/l, 150 mg/l and 200 mg/l. The effect of solution pH and contact time was also studied. The biosorption capacity was significantly altered by pH of the solution. The removal of metal ions was conspicuously rapid; most of the total sorption occurred within 30 min. The sorption data have been analyzed and fitted to the Langmuir and Freundlich isotherm models. The highest Q_max value was found for the biosorption of Cd²⁺ at 43.5 mg/g in the presence of the non-growing biomass. Recovery of metals (Cd²⁺, Zn²⁺, Cu²⁺ and Ni²⁺) was found to be better when dried biomass was used in comparison to non-growing biomass. Metal removal through bioaccumulation was determined by growing the bacterial strain in nutrient broth amended with different concentrations of metal ions. This multi-metal resistant isolate could be employed for the removal of heavy metals from spent industrial effluents before discharging them into the environment.     

Sorption of Heavy Metal Cations by Corn Mitochondria and the Effects on Electron and Energy Transfer Reactions

The passive sorption of Pb⁺², Cd⁺², Zn⁺², Co⁺², Ni⁺², and Mn⁺² by isolated corn mitochondria was determined, and, except for Pb⁺², the maximum sorption for each cation was about 58 nmol per milligram of protein. Sorption of Pb⁺² was apparently ten times greater, but precipitation may have been the cause of this larger value. The effects of Pb⁺², Cd⁺², Zn⁺², Co⁺², and Ni⁺² on acceptorless rates of electron transport for three substrates were determined. Greater than 50% inhibitions of oxidation were observed for succinate after additions of >0.1 mM Cd⁺², Zn⁺², or Pb⁺²: for NADH after additions of >0.5 mM Cd⁺² or Zn⁺²; and for malate + pyruvate after additions of >0.1 mM Cd⁺². Some inhibition of the rate of substrate oxidation was observed for most cations at higher concentrations. Coupling, as measured by ADP/O ratios, was inhibited at lowest concentrations by Cd⁺² or Zn⁺² and at higher concentrations by Co⁺² or Ni⁺². Substantial swelling of mitochondria oxidizing succinate was observed following additions of O.1 mM Cd⁺² or Pb⁺², Correlations are drawn between the effects of Pb⁺², Cd⁺², Zn⁺², Co⁺², and Ni⁺² and their sorption to mitochondrial membranes.     

Toxicity of heavy metals to Asellus aquaticus (L.) (Crustacea,Isopoda)

Effects of heavy metals on the isopod Asellus aquaticus (L.) are studied by static toxicity tests. Results demonstrate that the species is sensitive to Cd⁺², Cr⁺⁶, Cu⁺², Fe⁺³, Hg⁺², Ni⁺² Pb⁺² and Zn⁺², but the toxicity of each metal is different. Differences are also found between adults and between adults and juveniles. The comparative analysis of all data on the toxicity has been performed on the concentrations of metal ions and not on metal compound concentrations.Criteria for establishing water quality in order to guarantee protection of the environment are discussed.     

Characterization of cadmium removal by Rhodotorula sp. Y11

Experiments were conducted studying the removal of Cd²⁺ from water via biosorption using Rhodotorula sp. Y11. The effects of temperature and initial pH of the solution on biosorption were studied. Caustic and heat treatments showed different influences on the biosorption capacity, and the highest metal uptake value (19.38 mg g⁻¹) was obtained by boiling treated yeast cells. The presence of competing cations, such as Ag⁺, Cu²⁺, and Mg²⁺, except Na⁺ ions, significantly interfered with the metal uptake. Results indicate that the Langmuir model gave a better fit to the experimental data than the Freundlich equation. The q ₁₀ value was 11.38 mg g⁻¹ for Cd²⁺ uptake by Y11. Chemical modifications of the biomass demonstrated that carboxyl and amide groups play an important role in Cd²⁺ biosorption.     

Characterization of a salt resistant bacterial strain <Emphasis Type="Italic">Proteus</Emphasis> sp. NA6 capable of decolorizing reactive dyes in presence of multi-metal stress

Microbial biotechnologies for the decolorization of textile wastewaters have attracted worldwide attention because of their economic suitability and easiness in handling. However, the presence of high amounts of salts and metal ions in textile wastewaters adversely affects the decolorization efficiency of the microbial bioresources. In this regard, the present study was conducted to isolate salt tolerant bacterial strains which might have the potential to decolorize azo dyes even in the presence of multi-metal ion mixtures. Out of the tested 48 bacteria that were isolated from an effluent drain, the strain NA6 was found relatively more efficient in decolorizing the reactive yellow-2 (RY2) dye in the presence of 50 g L^?1 NaCl. Based on the similarity of its 16S rRNA gene sequence and its position in a phylogenetic tree, this strain was designated as Proteus sp. NA6. The strain NA6 showed efficient decolorization (>90 %) of RY2 at pH 7.5 in the presence of 50 g L^?1 NaCl under static incubation at 30 °C. This strain also had the potential to efficiently decolorize other structurally related azo dyes in the presence of 50 g L^?1 NaCl. Moreover, Proteus sp. NA6 was found to resist the presence of different metal ions (Co⁺², Cr⁺⁶, Zn⁺², Pb⁺², Cu⁺², Cd⁺²) and was capable of decolorizing reactive dyes in the presence of different levels of the mixtures of these metal ions along with 50 g L^?1 NaCl. Based on the findings of this study, it can be suggested that Proteus sp. NA6 might serve as a potential bioresource for the biotechnologies involving bioremediation of textile wastewaters containing the metal ions and salts.     

Use of Raman spectroscopy and chemometrics for the quantification of metal ions attached to Lactobacillus kefir

Aims: To set‐up an experimental and analytical methodology to evaluate the feasibility of developing simple, accurate and quantitative models based on Raman spectroscopy and multivariate analysis for the quantification of metal ions adsorbed to the bacterial surface of Lactobacillus kefir. Methods and Results: One millilitre cultures from two strains of Lact. kefir in the stationary phase were harvested and washed twice with ultra pure water. The bacterial pellets were resuspended into 1 ml solutions of Pb⁺², Cd⁺² or Ni⁺² ranging from 0 to 0·9 mmol l^?1. The suspensions were further incubated for 1 h at 30°C at pH 5·5. After centrifugation, the pellets were kept to register the Raman spectra and the supernatants were used for the analytical determination of Pb⁺², Cd⁺² and Ni⁺². Micro‐organisms nontreated with metal ions were used as controls. Principal component analysis (PCA) was performed over the preprocessed Raman spectra to evaluate whether the clusters obtained could be correlated with the concentration of metal ions attached to the bacterial biomass. After that, partial least squares (PLS) models were calibrated with the aim of quantifying the metal ions adsorbed to the bacterial surface. According to the analytical determinations, the maximum binding capacity of all the metals (q_max) attained values that are comparable with those observed for other lactic acid bacteria (ca. 0·200 mmol g^?1). The spectral analysis revealed that the main functional groups involved in the bacteria/metal interaction are carboxylates, phosphates and polysaccharides. In PCA, the first two principal components explain more than 72% variance of the spectral data set contained in the data structure, allowing a clear discrimination among samples of different concentrations. Based on this information and using as reference the results obtained by analytical methods, PLS prediction models were successfully defined for the quantification of Pb⁺², Cd⁺² and Ni⁺² attached to the bacterial surface. Conclusions: The calibration and validation of methods based on multivariate analysis allowed the definition of models for the quantification of Pb⁺², Cd⁺² and Ni⁺² attached to bacterial surfaces. The high percentages of explained variances in PCA gave a strong support to calibrate the prediction models, depicting very good correlations with the reference method (correlations ～0·90 in all cases). Significance and Impact of the study: Lactobacillus kefir CIDCA 8348 and JCM 5818 bind Pb⁺², Cd⁺² and Ni⁺² in an efficient way. This fact gives support for their potential use as sequestrants of traces of these metals in products addressed to human and animal consume. The prediction models developed would be useful for the determination of the investigated metal ions in unknown samples giving at the same time, structural information about this interaction. This is certainly the most important contribution of this work.     

Overexpression of AtMHX in tobacco causes increased sensitivity to Mg<Superscript>2+</Superscript>, Zn<Superscript>2+</Superscript>, and Cd<Superscript>2+</Superscript> ions,induction of V-ATPase expression,and a reduction in plant size

AtMHX is a vacuolar transporter encoded by a single gene in Arabidopsis. Electrophysiological analysis showed that it exchanges protons with Mg²⁺, Zn²⁺, and Fe²⁺ ions. The physiological impact of AtMHX was examined so far only in tissue-culture grown seedlings of tobacco plants overexpressing this transporter. Here we investigated the impact of AtMHX on growth, response to different metals, and metal accumulation of mature tobacco plants, as well as Arabidopsis plants in which we overexpressed this transporter. The analyses were carried out in hydroponic growth-systems, in which the mineral composition could be effectively controlled, and the metal content of roots could be examined. Transformed tobacco plants showed necrotic lesions and apical burnings upon growth with increased levels of Mg²⁺, Zn²⁺, and Cd²⁺ ions. This suggested that AtMHX can carry in planta not only Mg²⁺ and Zn²⁺ ions, as previously deduced based on observations in tissue-culture, but also Cd²⁺ ions. Transformed plants of both tobacco and Arabidopsis showed a reduction in plant size. However, the overall response of Arabidopsis to AtMHX overexpression was minor. No change was detected in the mineral content of any organ of the transgenic tobacco or Arabidopsis plants. The necrotic lesions in tobacco resembled those seen in plants with perturbed proton balancing, raising the assumption that AtMHX can affect the proton homeostasis of cells. In agreement with this assumption, the transformed tobacco plants had increased expression and activity of the vacuolar H⁺-ATPase. The relative significance of AtMHX for metal and proton homeostasis still has to be elucidated.     

Functional comparision between truncated MTT1 and truncated MTT2 from Tetrahyemna thermophila

Metallothioneins (MTs) are low-molecular-weight proteins with high Cys content and high metal-chelating ability. CdMT and CuMT subfamilies present different characteristics in Tetrahymena. To explore the effect of the cysteine arrangement and sequence length of MTs for binding different metal ions, MTT1, truncated MTT1 (TM1), MTT2, and truncated MTT2 (TM2) were expressed in E. coli. The half-maximal inhibiting concentrations (IC₅₀) of Cd²⁺ and Cu⁺ for the recombinant strains were different. Furthermore, E. coli cells expressing MTT1 and TM1 exhibited higher accumulating ability for Cd²⁺ than cells expressing MTT2 and TM2. However, the opposite is true for Cu⁺. The binding ability of the different recombinant proteins to Cd²⁺ and Cu⁺ were also different. MTT1 and truncated mutant TM1 were the preference for Cd²⁺, whereas MTT2 and truncated mutant TM2 were the preference for Cu⁺ coordination. These results showed that metal ion tolerance and accumulation ability not only depended on cysteine arrangement pattern but also on sequence length of MT in Tetrahymena.     

Sorption of Methylene Blue by an Oscillatoria sp.–Dominated Cyanobacterial Mat

The ability of an Oscillatoria sp.–dominated cyanobacterial mat in sorbing methylene blue (MB), a cationic dye, was investigated using the batch contact method. The sorption of MB onto the powdered biomass was not significantly influenced by initial pH (2–10) and temperature (5–45°C) of the solution. MB sorption occurred slowly, requiring 1–8 h for the establishment of equilibrium. A slow attainment of equilibrium seems to be related with the large size of MB ions. The isotherm data of MB sorption by the mat biomass could effectively fit to Langmuir and Freundlich models. The maximum MB sorption capacity (q _max) of the test biomass was 78.43 mg g^?1, which changed little with variation in biomass concentration. Moreover, the test biomass could efficiently sorb MB from solution in presence of Na⁺, K⁺, and Ca²⁺, which usually occur at high concentrations in natural waters, and also in presence of Cd²⁺. These particular characteristics together with pH and temperature independence of the sorption process make the mat biomass an ideal MB sorbent.     

Cd2+和Pb2+对花翅摇蚊(Chironomus kiiensis)幼虫生长发育的影响及富集效应

河流、湖泊等水生环境中普遍存在的重金属污染破坏水生生态系统并间接威胁人类健康。为探究重金属胁迫下水生昆虫花翅摇蚊（Chironomus kiiensis）生态毒理,测定了重金属Cd²⁺和Pb²⁺胁迫对花翅摇蚊化蛹率和羽化率的影响,检测了摇蚊的口器致畸与富集效应。研究结果表明,Cd²⁺和Pb²⁺影响摇蚊幼虫化蛹和羽化过程,单一重金属离子处理14 d Pb²⁺处理组的化蛹率和羽化率分别为22.22%和8.89%,低于Cd²⁺的化蛹率（25.56%）和羽化率（11.11%）,表现出更强的抑制效应。混合离子1：2和2：1配比处理组化蛹率和羽化率均为11.11%和4.44%,显著低于单一重金属离子胁迫下的化蛹率和羽化率。单一重金属离子及混合离子处理均能导致花翅摇蚊幼虫口器致畸,表现为上颚前齿断裂,中齿和基齿磨损、缺失,下唇板齿部不规则,下唇板边缘齿与中央齿磨损、断裂、增生、缺失。不同重金属离子处理下幼虫口器致畸率不同,并与暴露时间呈正相关,其中1：2配比处理14 d致畸率达到40.61%。重金属离子在摇蚊幼虫体内产生生物富集效应,单一重金属离子处理下的Pb²⁺富集含量7 d至14 d由11.46 mg/kg上升至31.32 mg/kg,不同配比混合离子处理下Pb²⁺富集含量均呈增加趋势,其中1：2配比处理组由15.48 mg/kg上升至42.50 mg/kg,而Cd²⁺在单一重金属与1：1混合离子处理组7 d至14 d的富集含量无显著性变化,2：1配比处理组由14.20 mg/kg下降至9.52 mg/kg,1：2配比由5.85 mg/kg上升至20.99 mg/kg。这些研究结果表明Cd²⁺和Pb²⁺胁迫影响花翅摇蚊幼虫生长发育且口器出现畸型,与重金属在幼虫体内的富集密切相关,为研究重金属对水生生态系统多重效应提供了理论依据。     

Biosorption of Chromium,Cadmium, and Cobalt from Aqueous Solution by Immobilized Living Cells of Chryseomonas luteola TEM 05

Abstract

In this work, the potential use of the immobilized cells of Chryseomonas luteola TEM 05 for the removal of Cr⁺⁶, Cd⁺² and Co⁺² ions from aqueous solutions was investigated. The living cells of C. luteola TEM 05 were firstly entrapped both in carrageenan and chitosan coated carrageenan gels and then used in biosoption of the metal ions in batch reactors at pH 6.0, 25°C, in 100 mg L^?1 of each metal solution. Besides this, a process of competitive biosorption of these metal ions was also described and compared to single metal ion adsorption in solution. According to the immobilization results, the replacement of KCl by KCl-chitosan as gelling agent improved the mechanical strength and thermal stability of the gel. In addition, the C. luteola TEM 05 immobilized carrageenan-chitosan gel system was quite more efficient for the fast adsorption of metal ions from aqueous solution than the carrageenan gels without biomass.     

Biosorption of long lived radionuclides using immobilized cells of Saccharomyces cerevisiae

The efficacy of immobilized Saccharomyces cerevisiae (biomatrix) for the sorption of different metal ions and its potential applications in nuclear waste treatment were investigated. The sorption of radionuclides such as ²³³U, ²⁴¹Am, ¹⁴⁴Ce, ¹³⁷Cs and ⁹⁰Sr was studied under different experimental conditions. More than 95% sorption of UO₂ ²⁺, Pu⁴⁺, Am³⁺ and Ce³⁺ could be obtained in the pH range 1 to 2 of the aqueous solutions. However the sorption of Cs⁺ and Sr²⁺ were negligible under the similar experimental conditions. The infrared spectra and scanning electron microscopic images of the control and uranium-bearing biomatrix were studied to understand the chemistry of metal uptake by this biomatrix.     

In vivo and in vitro effects of copper and cadmium on the plasma membrane H+-ATPase from cucumber (Cucumis sativus L.) and maize (Zea mays L.) roots

The plasmalemma vesicles isolated from cucumber and maize roots were used to study the effect of Cu²⁺ and Cd²⁺ on the hydrolytic and proton pumping activities of ATPase. In vivo application of metal ions to the plant growth solutions resulted in stimulation of the proton transport in maize. In cucumber roots the action of metals was not the same: cadmium stimulated the H⁺ transport through plasmalemma whereas Cu²⁺ almost completely inhibited it. Copper ions decreased the hydrolytic activity of H⁺-ATPase in cucumber, without any effect on this activity in membranes isolated from maize roots. The effect of cadmium on the hydrolytic activities was opposite: ATP-hydrolysis activity in plasmalemma was not altered in cucumber, whereas in maize its stimulation was observed. The amount of accumulated metals was not the main reason of different influence of metals on H⁺-ATPase activity in tested plants. In in vitro experiments Cu²⁺ inhibited H⁺ transport in the cucumber, to a higher degree than Cd²⁺ and both metals did not change this H⁺-ATPase activity of plasmalemma isolated from corn roots. Cu²⁺ added into the incubation medium reduced the hydrolytic activity of ATPase in the plasma membrane isolated from cucumber as well as from corn roots. Cd²⁺ diminished the hydrolytic activity of ATPase in cucumber, and no effect of Cd²⁺ in the plasmalemma isolated from corn roots was found. Our results indicated different in vitro and in vivo action of both metals on H⁺-ATPase and different response of this enzyme to Cu²⁺ and Cd²⁺ in maize and cucumber.     

Bioaccumulation of Heavy Metals by Green Algae

The biosorption of metal ions (Cr⁺³, , Cu⁺², and Ni⁺²) on two algal blooms (designated HD-103 and HD-104) collected locally was investigated as a function of the initial metal ion concentration. The main constituent of HD-103 is Cladophora sp., while Spirulina sp. is present significantly in the bloom HD-104. Algal biomass HD-103 exhibited the highest Cu⁺² uptake capacity (819 mg/g). This bloom adsorbed Ni⁺² (504 mg/g), Cr⁺³ (347 mg/g), and (168 mg/g). Maximum of Ni⁺² (1108 mg/g) is taken by HD-104. This species takes up 306, 202, and 576 mg/g Cr⁺³, , and Cu⁺², respectively. Equilibrium data fit very well to both the Langmuir and the Freundlich isotherm models. The sorption process followed the Freundlich model better. Pseudo-first-order kinetic model could describe the kinetic data. Infrared (IR) spectroscopic data were employed to identify the site(s) of bonding. It was found that phosphate and peptide moieties participate in the metal uptake by bloom HD-103. In the case of bloom HD-104, carboxylate and phosphate are responsible for the metal uptake. The role of protein in metal uptake by HD-103 was investigated using polyacrylamide gel electrophoresis.     

Some metals inhibit the glutathione S‐transferase from Van Lake fish gills

Glutathione S‐transferases (GSTs) are the superfamily of multifunctional detoxification isoenzymes and play important role cellular signaling. The present article focuses on the role of Cd²⁺, Cu²⁺, Zn²⁺, and Ag⁺ in vitro inhibition of GST. For this purpose, GST was purified from Van Lake fish (Chalcalburnus tarichii Pallas) gills with 110.664 EU mg^?1 specific activity and 79.6% yield using GSH‐agarose affinity chromatographic method. The metal ions were tested at various concentrations on in vitro GST activity. IC₅₀ values were found for Cd⁺², Cu⁺², Zn⁺², Ag⁺ as 450.32, 320.25, 1510.13, and 16.43 μM, respectively. K _i constants were calculated as 197.05 ± 105.23, 333.10 ± 152.76, 1670.21 ± 665.43, and 0.433 ± 0.251 μM, respectively. Ag⁺ showed better inhibitory effect compared with the other metal ions. The inhibition mechanisms of Cd²⁺ and Cu²⁺ were non‐competitive, whereas Zn²⁺ and Ag⁺ were competitive. Co²⁺, Cr²⁺, Pb²⁺, and Fe³⁺ had no inhibitory activity on GST.     

Competitive adsorption of Pb, Cd and Zn ions onto Eichhornia crassipes in binary and ternary systems

A batch sorption technique was used to study the biosorption of Pb²⁺, Cd²⁺ and Zn²⁺ ions onto the vastly abundant water hyacinth weed, Eichhornia crassipes biomass in binary and ternary systems at a temperature of 30 °C and pH 4.84. Mutual interference effects were probed using equilibrium adsorption capacity ratios, , where the prime indicates the presence of one or two other metal ions. The combined action of the metals was found to be antagonistic, and the metal sorption followed the order Pb²⁺  Cd²⁺  Zn²⁺. The behaviour of competitive biosorption for Pb–Cd and Pb–Zn combinations were successfully described by the Langmuir Competitive Model (CLM), whilst the model showed poor fitting to the Cd–Zn data. In conclusion, Pb²⁺ ions could still be effectively removed from aqueous solution in the presence of both Cd²⁺ and Zn²⁺ ions, but removal of the Cd²⁺ and Zn²⁺ ions would be suppressed in the presence of Pb²⁺.     

Luminescent sensor for Cd2+, Hg2+ and Ag+ in water based on a sulphur‐containing receptor: quantitative binding–softness relationship

A water‐soluble, high‐output fluorescent sensor, based on a lumazine ligand with a thiophene substituent for Cd²⁺, Hg²⁺ and Ag⁺ metal ions, is reported. The sensor displays fluorescence enhancement upon Cd²⁺ binding (log  β = 2.79 ± 0.08) and fluorescence quenching by chelating with Ag⁺ and Hg²⁺ (log β = 4.31 ± 0.15 and 5.42 ± 0.1, respectively). The mechanism of quenching is static and occurs by formation of a ground‐state non‐fluorescent complex followed by rapid intersystem crossing. The value of the Stern–Volmer quenching rate constant (k_q) by Ag⁺ ions is close to 6.71 × 10¹² mol/L/s at 298 K. The thermodynamic parameters (ΔG, ΔH and ΔS) were also evaluated and indicated that the complexation process is spontaneous, exothermic and entropically favourable. The quantitative linear relationship between the softness values of Klopman (σ_K) or Ahrland (σ_A) and the experimental binding constants (β) being in the order of Hg²⁺ > Ag⁺ > Cd²⁺ suggests that soft–soft interactions are the key for the observed sensitivity and selectivity in the presence of other metal ions, such as: Pb²⁺, Ni²⁺, Mn²⁺, Cu²⁺, Co²⁺, Zn²⁺ and Mg²⁺ ions. Copyright © 2008 John Wiley & Sons, Ltd.