Mechanism Of Chromium Research Articles

Biosorption and bioreduction of aqueous chromium (VI) by different Spirulina strains.

Spirulina has emerged as a promising microorganism for the treatment of industrial heavy metal ions in wastewater due to their simplicity of cultivation and harvesting, rich functional binding groups and high bioreductive activity during the uptake process. While the capacities of biosorption and bioreduction for heavy metal ions differs significantly among various algal strains. Therefore, the physiological characteristics were investigated to identify the different Spirulina strains and the chromium (VI) adsorption capacities of the algal strains were also evaluated. In this study, it was found that algal strains YCX2643 and CLQ1848 performed higher removal efficiency (86.5% and 83.7%) than other four Spirulina strains (59.4%, 56.3%, 65.6% and 66.5%, respectively). Moreover, the mechanisms of chromium (VI) ions binding and biotransformation in the Spirulina cell were scrutinize by FT-IR spectroscopy and scanning electron microscopy(SEM), and it indicated that the varieties of cellular components involved in high binding affinity may cause the higher biosorption and bioreduction of aqueous chromium (VI) in algal strains YCX2643 and CLQ1848, which could be used as promising biosorbents in the removing heavy metal pollutants from wastewaters.

Adsorption performance and mechanism of chromium on β-cyclodextrin-modified molybdenum disulfide

Adsorption performance and mechanism of chromium on β-cyclodextrin-modified molybdenum disulfide

Mechanisms of chromium(VI) removal from solution by zeolite and vermiculite modified with iron(II).

Mechanisms of Cr(VI) reduction by Fe(II) modified zeolite (clinoptilolite/mordenite) and vermiculite were evaluated. Adsorbents were treated with Fe(SO4)·7H2O to saturate their exchange sites with Fe(II). However, this treatment decreased their CEC and pHPZC, probably due to the dealumination process. Vermiculite (V-Fe) adsorbed more Fe(II) (21.8mgg-1) than zeolite (Z-Fe) (15.1mgg-1). Z-Fe and V-Fe were used to remove Cr(VI) from solution in a batch test to evaluate the effect of contact time and the initial concentration of Cr(VI). The Cr(VI) was 100% reduced to Cr(III) by Z-Fe and V-Fe in solution at 18mg L-1 Cr(VI) after 1min. Considering that 3mol of Fe(II) are required to reduce 1mol of Cr(VI) (3Fe+2 + Cr+6 → 3Fe+3 + Cr+3), the iron content released from Z-Fe and V-Fe was sufficient to reduce 100% of the Cr(VI) in solutions up to 46.8mg L-1 Cr(VI) and about 90% (V-Fe) and 95% (Z-Fe) at 95.3mg L-1 Cr(VI). The Fe(II), Cr(III), Cr(VI), and K+ contents of the adsorbents and solutions after the batch tests indicated that the K+ ions from the [Formula: see text] solution were the main cation adsorbed by Z-Fe, while vermiculite did not absorb any of these cations. The H+ of the acidic solution (pH around 5) may have been adsorbed by V-Fe. The release of Fe(II) from Z-Fe and V-Fe involved cation exchange between K+ and H+ ions from solution, respectively. The reduction of Cr(VI) by Fe(II) resulted in the precipitation of Cr(III) and Fe(III) and a decrease in the pH of the solution to < 5. As acidity limits the precipitation of Cr(III) ions, they remained in solution and were not adsorbed by either adsorbent (since they prefer to adsorb K+ and H+). To avoid oxidation, Cr(III) can be removed by precipitation or the adsorption by untreated minerals.

XPS study of oxide nucleation and growth mechanisms on a model FeCrNiMo stainless steel surface

High resolution X-ray photoelectron spectroscopy analysis has been used to investigate, in-situ, the mechanisms for oxide nucleation and growth on a FeCrNiMo single crystal alloy. Ultra-high vacuum conditions provided the ideal environment in which an oxide-free surface could be oxidised in a controlled manner, using extremely low pressures and step-wise exposures of oxygen. In-situ snapshot measurements, which records an XPS spectrum every 10 s, provided insight into the mechanisms of chromium and nitrogen segregation during annealing of the sample. High resolution XPS spectra was used to follow the evolution of the key elements at step-wise intervals, allowing for detailed insights into the nucleation and growth mechanisms. Angular XPS analysis provided further information on the in-depth distribution and stratification of the oxide film.

Sustainable Chromium (VI) Removal from Contaminated Groundwater Using Nano-Magnetite-Modified Biochar via Rapid Microwave Synthesis.

This study developed a nano-magnetite-modified biochar material (m-biochar) using a simple and rapid in situ synthesis method via microwave treatment, and systematically investigated the removal capability and mechanism of chromium (VI) by this m-biochar from contaminated groundwater. The m-biochar was fabricated from reed residues and magnetically modified by nano-Fe3O4. The results from scanning electron microscopy (SEM) and X-ray diffraction (XRD) characterisations confirmed the successful doping of nano-Fe3O4 on the biochar with an improved porous structure. The synthesised m-biochar exhibited significantly higher maximum adsorption capacity of 9.92 mg/g compared with that (8.03 mg/g) of the pristine biochar. The adsorption kinetics followed the pseudo-second-order model and the intraparticle diffusion model, which indicated that the overall adsorption rate of Cr(VI) was governed by the processes of chemical adsorption, liquid film diffusion and intramolecular diffusion. The increasing of the pH from 3 to 11 significantly affected the Cr(VI) adsorption, where the capabilities decreased from 9.92 mg/g to 0.435 mg/g and 8.03 mg/g to 0.095 mg/g for the m-biochar and pristine biochar, respectively. Moreover, the adsorption mechanisms of Cr(VI) by m-biochar were evaluated and confirmed to include the pathways of electrostatic adsorption, reduction and complexation. This study highlighted an effective synthesis method to prepare a superior Cr(VI) adsorbent, which could contribute to the effective remediation of heavy metal contaminations in the groundwater.

Comparative Assessment of Toxic Metals Bioaccumulation and the Mechanisms of Chromium (Cr) Tolerance and Uptake in Calotropis procera.

Progressive pollution due to toxic metals significantly undermines global environmental sustainability efforts. Chromium (Cr) is one of the most dangerous to human health. The use of plants to rid the environment of such pollutants “phytoremediation” proves to be a promising alternative to the current remediation methods. In the present study, inductively coupled plasma optical emission spectroscopy (ICP-OES) determined Cadmium (Cd), Chromium (Cr), Copper (Cu), Nickel (Ni), and Lead (Pb) concentrations in the soil, and plants (Atriplex leucoclada, Calotropis procera, Salsola imbricata, Typha augustifolia, and Phragmites australis) root and shoots. Results showed that compared to other studied metals, Cr concentration was the highest in the soil at 111.8 mg/kg, whereas Cd records the least concentration of 0.04 mg/kg. Cr also accumulated in higher concentration in C. procera than in the soil and other plants, with up to 188.2 and 68.2 mg/kg concentration in the root and shoot, respectively. In order to understand the mechanism of Cr tolerance and uptake in C. procera, germinated seeds were irrigated with 20 mg/kg Cr and control treatment (no Cr applied) for six (6) weeks under greenhouse conditions. Fourier transformed infrared spectroscopy (FTIR) results showed high Cr complexation and binding to C. procera tissues via hydroxyl and carboxylic groups. Enzymatic assay reveals increased activities of superoxide dismutase (SOD), catalase (CAT), and glutathione reductase (GR) in Cr treated C. procera than in the control. SOD activity increased by up to six (6) folds. Therefore, we conclude that C. procera is suitable for the phytoremediation of Cr polluted arid soil. Additionally, regulation of cellular homeostasis via redox signaling is essential to the Cr tolerance and detoxification mechanism.

Leaching of elements from cement activated fly ash and slag amended soils

Very few studies have investigated the leaching characteristics of cement activated fly ash and slag treated soils, although the inclusion of cement significantly enhances the material pH and may alter the leachability of elements. In this study the leaching behavior and mechanisms of chromium (Cr), copper (Cu), iron (Fe) and sulfur (S) from cement activated fly ash and slag stabilized soils were evaluated. An array of synthetic precipitation leaching procedure (SPLP), batch water leach test (WLT), toxicity characteristic leaching procedure (TCLP) and pH-Static leach tests were conducted. A geochemical equilibrium model Visual MINTEQ was implemented to identify the leaching controlling mechanisms of the metals. Results indicated that, the leached concentrations of Cr, Cu, Fe and S in SPLP, WLT and TCLP effluents were in the range of 0.016–0.74 mg/L, 0.013–0.17 mg/L, 0.019–0.27 mg/L and 1.78–234 mg/L, respectively. Quantitative comparisons between the standard test procedures suggested the necessity of multiple test methods for a comprehensive leaching assessment. Cr and Cu showed amphoteric leaching behaviors, whereas Fe and S followed cationic leaching patterns. According to the geochemical analyses, amorphous Cr(OH)3; tenorite and Cu(OH)2; ferrihydrite and goethite; gypsum and anhydrite; could control the leaching of Cr, Cu, Fe and S, respectively. The effluent Cr concentrations frequently exceeding the U.S. EPA specified maximum contaminant level of 0.1 mg/L. Yet, the use of cement activated fly ash and slag mixed soils could be beneficial, since less toxic trivalent Cr (III) was identified through geochemical modeling.

Performances of purified indigenous Leifsonia sp. and its mechanism in the removal of Cr(VI) under shaking condition

The increasing discharge of wastewater with chromium(VI) plays adverse impacts on the environment in recent years. This study purposes to investigate the adsorption efficiency and removal mechanism of chromium(VI) via a purified indigenous bacteria (Leifsonia sp.). The effect of varying pH, initial chromium(VI) concentration, dosage of dry biosorbent, contact time and temperature on the adsorption efficiency of chromium(VI) was studied via single-factor experiment. Our results demonstrate that the strain has high efficiency of adsorbing chromium(VI) from aqueous solution. The maximum removal rate of chromium(VI) was 95.4% when pH was 2 (range 1.5–9.0), contact time was 4 h, the initial chromium(VI) concentration was 10 mg/L and the temperature was 30 °C (10–50 °C). Freundlich isotherms fit the adsorption process well, and the adsorption kinetics is consistent with the pseudo-secondary kinetic model. Furthermore, the adsorption mechanism was elucidated via FTIR and XPS, indicating that chromium precipitates on the biosorbent surfaces are a mixture of chromium(VI) and chromium(III). It is implied that physisorption and chemisorption proceed at the same time during the chromium(VI) adsorption process. These findings show that Leifsonia sp. as a biosorbent displays excellent adsorption efficiency, having the potential to remove chromium(VI) from aqueous solutions.

Migration and enrichment of chromium and silicon element in glass coating at high temperature

The formation and mechanism of chromium and silicon enriched glass coating on carbon steel substrate were investigated in this paper, which was based on the protection properties of the glass coating to improve the anti-oxidation ability of carbon steel. The starting reaction temperature between the glass coating and steel substrate was 800 °C, which was approved by TG-DTA test. The diffusion of chromium and silicon element in glass coating was investigated at 800 °C and 1000 °C by WDS-EPMA and SEM-EDS. It indicated that chromium element began to migrate to interface between glass coating and carbon steel at 800 °C and a chromium enriched layer formed when the sample was heated at 800 °C for 60 min. Meanwhile, a silicon enriched layer started to form at the interface between glass coating and carbon steel at 1000 °C. The content of silicon element in the scale of coated sample increased with increasing heating time and which could reach up to 8.08% when the sample was heated at 1000 °C for 30 min. The sample was also characterized by XRD, SEM-EDS and Raman to investigate the possible phases formed during high temperature treatment.

Equilibrium, Kinetic and Adsorption Mechanisms of Chromium (VI) on Characterized Activated Carbon Synthesized from Phosphoric Acid Activation of Coconut Shells

Over the years, water pollution due primarily to the discharge of toxic heavy metals from industrial activities has served as a major challenge in our quest to provide clean drinking water to millions of people across the world. Numerous cheap and environmentally friendly methods and technologies have been developed for the treatment of wastewater contaminated with heavy metals. Key among these technologies is the use of adsorbent as it is the most economical and efficient. In this present study, coconut shells were used to develop microporous adsorbent (activated carbon) through chemical activation by phosphoric acid (H3PO4). An analysis of the effect of various process parameters such as pH, temperature, initial metal ion concentration, adsorbent dose and contact time was conducted through batch adsorption of hexavalent chromium [Cr (VI)] on prepared AC sample. Initial Cr (VI) concentration was investigated through a range of 10 – 50 mg/L with the study showing an optimum concentration for AC of 20 mg/L for percentage removal (93.3%) but adsorption capacity (Qe) was highest for 50 mg/L (4.512 mg/g). The optimum conditions for adsorbent dose, contact time and temperature were determined as 6 g/L, 100 minutes and 30°C respectively for the prepared AC. Maximum adsorption was recorded for pH (2) at 88.2 5% (removal) and 4.41 mg/g (adsorption capacity) for AC. The experimental data obtained were modelled using various isotherms, including adsorption equilibrium isotherms, adsorption kinetic study and adsorption mechanisms with positive correlations (better fit) obtained for Freundlich isotherm, D-R isotherm (slightly), pseudo-second-order kinetic and Boyd models.

Preparation and Utilization of the Precursor Activated Carbon from Carbon Enriched Fuel Oil Fly Ash: Part II Removal and Speciation of Chromium(III and VI) from Wastewater

Activated carbon (AC) prepared from residual fly (FA) was activated by combined CO2-steam gasification and physically modified with procaine hydrochloride (PQ+Cl-). Modified AC was used as low cost and effective solid phase extractor (SPE) for removal, total determination and chemical speciation of chromium(III&amp;VI) in wastewater. Pseudo-second order rate equation is proposed for assigning the kinetics of chromium(VI) sorption in aqueous HCl (1.0 mol L-1) by PQ+Cl-- modified AC . The data were correlated with surface structure of the SPE. A dual separation mechanism of chromium(VI) as a binary complex ion associate [PQ+.CrO3Cl-] in/on the modified AC via "weak base anion exchanger" and an added component of “surface adsorption” is proposed. At 2 mL min-1 flow rate Sorption of spiked chromium(VI) at known concentrations (0.5-20 mg mL-1) from water onto modified AC packed columns at 2 mL min-1 was achieved. The sorbed chromium(VI) species were successfully recovered (106.0±2- 110 ±3%, n=3) from AC packed column with NaOH (10 mL, 1.0 mol L-1). Chromium(III) after oxidation to chromium(VI) was also enriched and eluted from the AC packed column and analyzed. Modified PQ+Cl- AC packed column was also used for total determination and speciation of chromium(III &amp; VI) species at trace levels in environmental water samples. Good apparent recovery percentage (94.6±6.3-102.0± 8.5 %) of chromium species was achieved confirming the analytical utility of the modified AC-packed column for removal, total determination and speciation of chromium(III &amp; VI) species in environmental water samples.

A membrane electroflotation process for recovery of recyclable chromium(III) from tannery spent liquor effluent

A two-compartment membrane electroflotation reactor has been demonstrated for recovery of recyclable chromium(III) from tannery spent liquor effluent. Dimensionally stable RuO2/TiO2-Ti and Ti were used as anode and cathode, respectively. The spent liquor effluent was used as catholyte and 0.01 N H2SO4 used as anolyte which was separated by Nafion 117 membrane. About 98% of chromium(III) was recovered and the removal efficiency correlated with the presence of organics in the effluent. The advantage of two-compartment membrane electroflotation process is capable of removing chromium(III) without oxidising it into chromium(VI) in chloride containing tannery spent liquor effluent. The mechanism of chromium(III) removal has been discussed. The recovered Cr(OH)3 was successfully demonstrated for tanning of cowhide.

Electrochemical Mechanism of Cr(III) Reduction for Preparing Crystalline Chromium Coatings Based on 1-Ethyl-3-methylimidazolium Bisulfate Ionic Liquid

The electrochemical mechanism of Cr(III) reduction and chromium nucleation/growth process on a glassy carbon (GC) electrode in 1-ethyl-3-methylimidazolium bisulfate ([EMIM]HSO4) ionic liquid are studied. These results from the cyclic voltammetry (CV) show that the reduction of Cr(III) occurs by a two-step process, Cr(III) to Cr(II) and Cr(II) to Cr(0), respectively, the first step is an irreversible process with a diffusion coefficient of Cr(III) in solution of 2.60 × 10−7 cm2 s−1 at 303 K and 15.99 × 10−7 cm2 s−1 at 358 K. The reduction has been verified to be a two-step process by electrochemical impedance spectroscopy (EIS) study. In the EIS measurements, the preferable equivalent circuit is also made to fit the experimental data. These effects of electrodeposition potential, current density and electrolyte temperature on the coating thickness are studied. These results show that the electrodeposition potential, current density and temperature have great effect on the coating thickness. The chromium deposits are studied by SEM, EDS and XRD. XRD pattern of the electrodeposited layer reveals the characteristic peak of crystal Cr. Chronoampererometry experiments reveal that the growth/nucleation mechanism of chromium in [EMIM]HSO4 on GC electrode is more progressive in character at lower overpotentials and a three-dimensional instantaneous nucleation behavior under diffusion controlled growth process at higher overpotentials.

Mechanisms of chromium and arsenite adsorption by amino-functionalized SBA-15

The adsorption of Cr(VI) and As(III) by amino-functionalized SBA-15 (NH2-SBA-15) from single and binary systems were investigated in this work. The effects of pH and temperature on the adsorption of NH2-SBA-15 were studied. Adsorption kinetics, isotherm model, and thermodynamics were studied to analyze the experimental data. pH 2 was the optimum condition for the adsorption of Cr(VI) and pH 4 for As(III) adsorption. Increasing temperature had a positive effect on the removal of both Cr(VI) and As(III). The Freundlich isotherm model can depict the adsorption process best. The pseudo-second-order kinetic model fitted well with the kinetic data of Cr(VI) and As(III) in the single-component system. In the binary system, the adsorption of As(III) by NH2-SBA-15 was slightly enhanced with the presence of Cr(VI); however, As(III) had no obvious effect on the removal of Cr(VI). Regeneration experiments indicated that 0.1 mol/L NaHCO3 was an efficient desorbent for the recovery of Cr(VI) and As(III) from NH2-SBA-15; the desorption rates for Cr(VI) and As(III) were 91.6 and 33.59 %, respectively. After five recycling cycles, the removal rates were 88 and 7 % for Cr(VI) and As(III) adsorption by NH2-SBA-15, respectively.

Effects and Mechanisms of Chromium on Structural Transformation of Heavy Rail Steel

The different dilatometric curves of continuous cooling transformation with the different cooling rates were determined by means of Gleeble-2000 thermal simulation machine. The CCT curve of heavy rail steel was obtained by measuring the dilatometric curves and metallographic analysis. And the effects and mechanisms of chromium on the phases transformation of heavy rail steel were investigated. It is found that, the heavy rail steel only takes place pearlite and martensite transformation during continuous cooling. The CCT curve of heavy rail steel is moved right with increasing chromium content, which indicates that chromium can obviously improve the stability of super cooled austenite and delay the pearlite transformation. When the content of chromium is increased from 0.21% to 0.40%, the shortest incubation time of pearlite transformation is increased from 25s to 52s. When the cooling rate ≤ 5°C s-1, with increasing chromium content, both starting and finishing temperatures of pearlite transformation are decreased at different extent, meanwhile the pearlite is refined. In addition, when the content of chromium is increased from 0.21% to 0.40%, the critical cooling rate of quenching is decreased from 15°C s-1 to 7°C s-1.

Interaction of chromium(III) complexes with model lipid bilayers: Implications on cellular uptake

To understand molecular cytotoxicity of chromium(III) and how it affects the stability of biological membranes, studies on the interaction of chromium(III) complexes aquapentaminechromium complex (complex I) and trans- [Cr(5-methoxysalcyclohex) (H2O) 2] ClO4 (complex II) with model biomembranes have been carried out. Langmuir films of dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidic acid (DPPA), dioctadecyldimethylammoniumbromide (DOMA) at air/water interface interacting with the chromium(III) complexes have been characterized using the surface pressure-molecular area (π-A) isotherms. Initial surface pressures changes for the two complexes show that the chromium(III) complexes inserted in the Langmuir films and complex I interacted strongly compared to complex II. Supported bilayers (SB) of the lipids on solid substrates formed by hydrating their Langmuir-Blodgett films (LB films) have been characterized using linear dichroic spectra, low angle X-ray diffraction and steady state fluorescence anisotropy. Depending on the geometry of the ligands and concentration, the complexes either insert in the alkyl or in the head group region of the SB and sometimes in both regions. The Supported lipid bilayers are well-layered and at low concentration, the metal complexes are incorporated near the head group region. Order and increase in lamellar spacing show stronger interaction of complex I with the lipids compared with complex II. This study provides some insights into the mechanism of chromium(III) toxicity and uptake of chromium(III) by the cells.

Molecular mechanism of chromium (VI) Induced Cytotoxicity and Apoptosis in L929 Mouse Fibroblasts

Molecular mechanism of chromium (VI) Induced Cytotoxicity and Apoptosis in L929 Mouse Fibroblasts

Mechanisms of chromium (VI)-induced apoptosis in anterior pituitary cells

Hexavalent chromium (Cr (VI)) is a highly toxic metal. Exposure to Cr (VI) compounds may affect reproductive functions. Due to the importance of anterior pituitary hormones on reproductive physiology we have studied the effects of Cr (VI) on anterior pituitary. We previously demonstrated that, after in vivo Cr (VI) administration, Cr accumulates in the pituitary gland and affects prolactin secretion. In vitro, Cr (VI) causes apoptosis in anterior pituitary cells due to oxidative stress generation. To better understand the mechanisms involved in Cr (VI)-induced apoptosis we studied: (a) whether Cr (VI) affects the intracellular antioxidant response and (b) which of the apoptotic factors participates in Cr (VI) effect. Our results show that Cr (VI) treatment induces a decrease in catalase and glutathione peroxidase (GPx) activity but does not modify glutathione reductase (GR) activity. Cr (VI) exposure causes an increase of GSH levels. p53 and Bax mRNA are also upregulated by the metal. Pifithrin α, a p53 transcriptional inhibitor, increases Cr (VI) cytotoxicity, suggesting a role of p53 as a survival molecule. The antioxidant N-acetyl-cysteine (NAC) could prevent Bax mRNA increase and caspase 3 activation, confirming that Cr (VI)-induced apoptosis involves oxidative stress generation.

Liquid-liquid extraction of chromium (VI) from sulfuric acid solutions using tri-n-dodecylamine/kerosene

Extraction of chromium(VI) from sulfuric acid solutions with tri-n-dodecylamine containing octanol-1 as a modifier in kerosene was investigated. All parameters influencing the extraction of chromium(VI) (time of agitation, concentrations of chromium(VI), sulfuric acid, tri-n-dodecylamine and temperature) were studied. The mathematical treatment of the obtained data enabled us to propose a mechanism of chromium(VI) extraction by tri-n-dodecylamine as follows: 2‾ TDAHHSO 4 + Cr 2 O 7 2- ↔ ‾ (TDAH)_2 Cr 2 O 7 + 2 HSO 4 - The logarithm of the equilibrium constant of the extraction reaction was found to be 10.6. Stripping of more than 99% of chromium(VI) from the organic phase of tri-n-dodecylamine/kerosene can be easily achieved in two stages using 0.05 M sodium carbonate solution. The synergistic effect of tri-n-butylphosphate and tri-n-octylphosphine oxide on the extraction of chromium(VI) were also studied.

Kinetics and Mechanism of Chromium(III)-oxalates-2-hydroxynicotinate and Chromium(III)-oxalates-3-hydroxypicolinate Aquation in HClO4 Solutions

New chromium(III) complexes, [Cr(C2O4)2(2-hnic)]2− and [Cr(C2O4)2(3-hpic)]2− (where 2-hnic = O,O′-bonded 2-hydroxynicotinic acid and 3-hpic = N,O-bonded 3-hydroxypicolinic acid), were obtained and characterized in solution. The acid-catalyzed aquation of the both complexes leads to liberation of the appropriate pyridinecarboxylic acid and formation of cis-[Cr(C2O4)2(H2O)2]−. Kinetics of these reactions were studied spectrophotometrically in the 0.1–1.0 M HClO4 range, at I = 1.0 M. In the case of [Cr(C2O4)2(2-hnic)]2−, a slow chelate-ring opening at the Cr–O (phenolate) bond is followed by a fast Cr–O (carboxylate) bond breaking. The rate law: kobs = kHQH[H+] was established, where kH is the acid-catalyzed rate constant and QH is the protonation constant of the coordinated phenolate oxygen atom. In the case of [Cr(C2O4)2(3-hpic)]2−, the reversible chelate-ring opening at Cr–N bond is followed by the rate determining step – the one-end bonded ligand liberation. The rate law for the first step was determined: kobs = k1+k−1/Q1[H+], where k1 and k−1 are the rate constants of the chelate-ring opening and closure and Q1 is the protonation constant of the pyridine nitrogen atom. The aquation mechanisms are proposed and the effect of ligand coordination mode on complex reactivity is discussed.