Techno-economic evaluation of simultaneous arsenic and fluoride removal from synthetic groundwater by electrocoagulation process: optimization through response surface methodology

<p>In the present work, electrocoagulation process has been used to treat arsenic and fluoride containing synthetic water using aluminium electrode. Box–Behnken design, a subnet of response surface methodology, was employed to fix the experimental conditions and Design Expert software was used to evaluate the interaction and effects of different process parameters such as initial pH, current density and run time on removal of arsenic and fluoride as well as the operating cost. Initial concentration of arsenic and fluoride was fixed at 550 μg/l and 12 mg/l, respectively, for all the experiments. High <i>R</i>² values of three responses (arsenic removal: 0.998, fluoride removal: 0.984 and operating cost: 0.996) ensures a satisfactory adjustment of developed quadratic model with the experimental data. Under the optimum conditions, initial pH: 7, current density: 10 A/m² and run time: 95 min, the predicted arsenic and fluoride removal is found to be 98.64 and 84.80%, respectively, whereas the operating cost is found to be 0.354 USD/m³. Further, the experimental values of arsenic removal (98.51%), fluoride removal (88.33%) and operating cost (0.343 USD/m³) are found to be in good agreement with the predicted values. The present electrocoagulation process is able to reduce the arsenic and fluoride concentration below 10 μg/l and 1.5 mg/l, respectively, which are maximum contaminant level of these elements in drinking water according to WHO. EDX analysis of sludge confirms the occurrence of arsenic and fluoride in produced sludge and FTIR spectra suggest that arsenic is also removed in the form of As(III). Real groundwater sample collected from Kaudikasa Village, Rajnandgaon District, Chhattisgarh, India and having As: 512 μg/l, F: 6.3 mg/l was also treated under optimum conditions of the present study and the concentration of arsenic and fluoride became below WHO drinking water norms.</p

Competitive Adsorption of Arsenic and Fluoride onto Economically Prepared Aluminum Oxide/Hydroxide Nanoparticles: Multicomponent Isotherms and Spent Adsorbent Management

The present study deals with adsorptive removal of arsenic and fluoride in single as well as bicomponent system using aluminum oxide/hydroxide nanoparticles (AHNP). For single component system, the Langmuir maximum adsorption capacity of the adsorbent is found as 833.33 μg/g for arsenic and 2000 μg/g for fluoride at optimum conditions. The adsorption process is well explained by Langmuir isotherm and pseudo-second-order kinetic models for both arsenic and fluoride. A real groundwater sample, having arsenic 512 μg/L and fluoride 6300 μg/L along with other ions, has also been treated successfully. Among different isotherms, the modified competitive Langmuir isotherm is found to be most suitable to represent the bicomponent system. Solidification of the spent adsorbent through brick formation is investigated, and this process is found to be an effective option for its management. Through economic evaluation, the adsorbent and treatment costs are found as ∼86.89 INR/kg and 0.36 INR/L, respectively

The Electron-Rich {Ru(acac)₂} Directed Varying Configuration of the Deprotonated Indigo and Evidence for Its Bidirectional Noninnocence

This article highlights the hitherto unexplored varying binding modes of the deprotonated natural dye indigo (H₂L) and its bidirectional noninnocent potential. The reaction of H₂L with the selective metal precursor Ru^II(acac)₂(CH₃CN)₂ (acac^– = acetylacetonate) leads to the simultaneous formation of paramagnetic Ru^III(acac)₂(HL^–) (<b>1</b>; blue solid) and diamagnetic Ru^II(acac)₂(L) (<b>2</b>; red solid), which have been characterized by standard analytical, spectroscopic, and structural analysis. Crystal structures establish that the usual <i>trans</i> configurated and twisted monodeprotonated HL^– and unprecedented <i>cis</i> configurated nearly planar dehydroindigo (L) bind to the {Ru­(acac)₂} metal fragment via the N^–,O and N,N donors, forming six- and five-membered chelates, respectively. It also reveals the existence of intramolecular N–H···O hydrogen-bonding interaction between the NH proton and CO group at the back face of the coordinated HL^–, in addition to an intermolecular N–H···O hydrogen bonding between the NH proton of HL^– of Molecule B and oxygen atom of the nearby acac of the second molecule (Molecule A) in the asymmetric unit of <b>1</b>. The specific role of the electron-rich {Ru­(acac)₂} metal fragment in stabilizing the <i>cis</i>-configuration of the electron-deficient L in <b>2</b> has been pointed out. Both <b>1</b> and <b>2</b> exhibit reversible one-electron oxidation and successive three reductions with varying <i>K</i>_c (comproportionation constant) values in the range of 10¹⁸–10⁶. The potentials for the redox processes of <b>2</b> are positively shifted with respect to those of <b>1</b>. The involvement of the metal or HL^–/L or mixed metal-HL^–/L-based orbitals in the accessible redox processes of <b>1</b>^<i>n</i> and <b>2</b>^<i>n</i> has been analyzed by spectroelectrochemistry, EPR at the paramagnetic states, and DFT calculated MO compositions/spin density distributions. The collective consideration of the experimental results and DFT/TD-DFT data has ascertained the participation of both the metal fragment {Ru­(acac)₂} and the HL^–/L in the redox processes, which in effect result in mixed electronic structural forms of <b>1</b>^<i>n</i> and <b>2</b>^<i>n</i> (<i>n</i> = +1, 0, −1, −2, −3)

The Electron-Rich {Ru(acac)₂} Directed Varying Configuration of the Deprotonated Indigo and Evidence for Its Bidirectional Noninnocence

This article highlights the hitherto unexplored varying binding modes of the deprotonated natural dye indigo (H₂L) and its bidirectional noninnocent potential. The reaction of H₂L with the selective metal precursor Ru^II(acac)₂(CH₃CN)₂ (acac^– = acetylacetonate) leads to the simultaneous formation of paramagnetic Ru^III(acac)₂(HL^–) (<b>1</b>; blue solid) and diamagnetic Ru^II(acac)₂(L) (<b>2</b>; red solid), which have been characterized by standard analytical, spectroscopic, and structural analysis. Crystal structures establish that the usual <i>trans</i> configurated and twisted monodeprotonated HL^– and unprecedented <i>cis</i> configurated nearly planar dehydroindigo (L) bind to the {Ru­(acac)₂} metal fragment via the N^–,O and N,N donors, forming six- and five-membered chelates, respectively. It also reveals the existence of intramolecular N–H···O hydrogen-bonding interaction between the NH proton and CO group at the back face of the coordinated HL^–, in addition to an intermolecular N–H···O hydrogen bonding between the NH proton of HL^– of Molecule B and oxygen atom of the nearby acac of the second molecule (Molecule A) in the asymmetric unit of <b>1</b>. The specific role of the electron-rich {Ru­(acac)₂} metal fragment in stabilizing the <i>cis</i>-configuration of the electron-deficient L in <b>2</b> has been pointed out. Both <b>1</b> and <b>2</b> exhibit reversible one-electron oxidation and successive three reductions with varying <i>K</i>_c (comproportionation constant) values in the range of 10¹⁸–10⁶. The potentials for the redox processes of <b>2</b> are positively shifted with respect to those of <b>1</b>. The involvement of the metal or HL^–/L or mixed metal-HL^–/L-based orbitals in the accessible redox processes of <b>1</b>^<i>n</i> and <b>2</b>^<i>n</i> has been analyzed by spectroelectrochemistry, EPR at the paramagnetic states, and DFT calculated MO compositions/spin density distributions. The collective consideration of the experimental results and DFT/TD-DFT data has ascertained the participation of both the metal fragment {Ru­(acac)₂} and the HL^–/L in the redox processes, which in effect result in mixed electronic structural forms of <b>1</b>^<i>n</i> and <b>2</b>^<i>n</i> (<i>n</i> = +1, 0, −1, −2, −3)

Revelation of Varying Bonding Motif of Alloxazine, a Flavin Analogue, in Selected Ruthenium(II/III) Frameworks

The reaction of alloxazine (L) and Ru^II(acac)₂(CH₃CN)₂ (acac^– = acetylacetonate) in refluxing methanol leads to the simultaneous formation of Ru^II(acac)₂(L) (<b>1</b> = bluish-green) and Ru^III(acac)₂(L^–) (<b>2</b> = red) encompassing a usual neutral α-iminoketo chelating form of L and an unprecedented monodeprotonated α-iminoenolato chelating form of L^–, respectively. The crystal structure of <b>2</b> establishes that N5,O4^– donors of L^– result in a nearly planar five-membered chelate with the {Ru^III(acac)₂⁺} metal fragment. The packing diagram of <b>2</b> further reveals its hydrogen-bonded dimeric form as well as π–π interactions between the nearly planar tricyclic rings of coordinated alloxazine ligands in nearby molecules. The paramagnetic <b>2</b> and one-electron-oxidized <b>1</b>⁺ display ruthenium­(III)-based anisotropic axial EPR in CH₃CN at 77 K with ⟨<i>g</i>⟩/Δ<i>g</i> of 2.136/0.488 and 2.084/0.364, respectively (⟨<i>g</i>⟩ = {1/3­(<i>g</i>₁² + <i>g</i>₂² + <i>g</i>₃²)}^1/2 and Δ<i>g</i> = <i>g</i>₁ – <i>g</i>₃). The multiple electron-transfer processes of <b>1</b> and <b>2</b> in CH₃CN have been analyzed by DFT-calculated MO compositions and Mulliken spin density distributions at the paramagnetic states, which suggest successive two-electron uptake by the π-system of the heterocyclic ring of L (L → L^•– → L^2–) or L^– (L^– → L^•2– → L^3–) besides metal-based (Ru^II/Ru^III) redox process. The origin of the ligand as well as mixed metal–ligand-based multiple electronic transitions of <b>1</b>^<i>n</i> (<i>n</i> = +1, 0, −1, −2) and <b>2</b>^<i>n</i> (<i>n</i> = 0, −1, −2) in the UV and visible regions, respectively, has been assessed by TD-DFT calculations in each redox state. The p<i>K</i>_a values of <b>1</b> and <b>2</b> incorporating two and one NH protons of 6.5 (N3H, p<i>K</i>_a1)/8.16 (N1H, p<i>K</i>_a2) and 8.43 (N1H, p<i>K</i>_a1), respectively, are estimated by monitoring their spectral changes as a function of pH in CH₃CN–H₂O (1:1). <b>1</b> and <b>2</b> in CH₃CN also participate in proton-driven internal reorganizations involving the coordinated alloxazine moiety, i.e., transformation of an α-iminoketo chelating form to an α-iminoenolato chelating form and the reverse process without any electron-transfer step: Ru^II(acac)₂(L) (<b>1</b>) → Ru^II(acac)₂(L^–) (<b>2</b>^–) and Ru^III(acac)₂(L^–) (<b>2</b>) → Ru^III(acac)₂(L) (<b>1</b>⁺)

Revelation of Varying Bonding Motif of Alloxazine, a Flavin Analogue, in Selected Ruthenium(II/III) Frameworks

The reaction of alloxazine (L) and Ru^II(acac)₂(CH₃CN)₂ (acac^– = acetylacetonate) in refluxing methanol leads to the simultaneous formation of Ru^II(acac)₂(L) (<b>1</b> = bluish-green) and Ru^III(acac)₂(L^–) (<b>2</b> = red) encompassing a usual neutral α-iminoketo chelating form of L and an unprecedented monodeprotonated α-iminoenolato chelating form of L^–, respectively. The crystal structure of <b>2</b> establishes that N5,O4^– donors of L^– result in a nearly planar five-membered chelate with the {Ru^III(acac)₂⁺} metal fragment. The packing diagram of <b>2</b> further reveals its hydrogen-bonded dimeric form as well as π–π interactions between the nearly planar tricyclic rings of coordinated alloxazine ligands in nearby molecules. The paramagnetic <b>2</b> and one-electron-oxidized <b>1</b>⁺ display ruthenium­(III)-based anisotropic axial EPR in CH₃CN at 77 K with ⟨<i>g</i>⟩/Δ<i>g</i> of 2.136/0.488 and 2.084/0.364, respectively (⟨<i>g</i>⟩ = {1/3­(<i>g</i>₁² + <i>g</i>₂² + <i>g</i>₃²)}^1/2 and Δ<i>g</i> = <i>g</i>₁ – <i>g</i>₃). The multiple electron-transfer processes of <b>1</b> and <b>2</b> in CH₃CN have been analyzed by DFT-calculated MO compositions and Mulliken spin density distributions at the paramagnetic states, which suggest successive two-electron uptake by the π-system of the heterocyclic ring of L (L → L^•– → L^2–) or L^– (L^– → L^•2– → L^3–) besides metal-based (Ru^II/Ru^III) redox process. The origin of the ligand as well as mixed metal–ligand-based multiple electronic transitions of <b>1</b>^<i>n</i> (<i>n</i> = +1, 0, −1, −2) and <b>2</b>^<i>n</i> (<i>n</i> = 0, −1, −2) in the UV and visible regions, respectively, has been assessed by TD-DFT calculations in each redox state. The p<i>K</i>_a values of <b>1</b> and <b>2</b> incorporating two and one NH protons of 6.5 (N3H, p<i>K</i>_a1)/8.16 (N1H, p<i>K</i>_a2) and 8.43 (N1H, p<i>K</i>_a1), respectively, are estimated by monitoring their spectral changes as a function of pH in CH₃CN–H₂O (1:1). <b>1</b> and <b>2</b> in CH₃CN also participate in proton-driven internal reorganizations involving the coordinated alloxazine moiety, i.e., transformation of an α-iminoketo chelating form to an α-iminoenolato chelating form and the reverse process without any electron-transfer step: Ru^II(acac)₂(L) (<b>1</b>) → Ru^II(acac)₂(L^–) (<b>2</b>^–) and Ru^III(acac)₂(L^–) (<b>2</b>) → Ru^III(acac)₂(L) (<b>1</b>⁺)

Crafting of Neuroprotective Octapeptide from Taxol-Binding Pocket of β‑Tubulin

Microtubules play a crucial role in maintaining the shape and function of neurons. During progression of Alzheimer’s disease (AD), severe destabilization of microtubules occurs, which leads to the permanent disruption of signal transduction processes and memory loss. Thus, microtubule stabilization is one of the key requirements for the treatment of AD. Taxol, a microtubule stabilizing anticancer drug, has been considered as a potential anti-AD drug but was never tested in AD patients, likely because of its’ toxic nature and poor brain exposure. However, other microtubule-targeting agents such as epothilone D (BMS-241027) and TPI-287 (abeotaxane) and NAP peptide (davunetide) have entered in AD clinical programs. Therefore, the taxol binding pocket of tubulin could be a potential site for designing of mild and noncytotoxic microtubule stabilizing molecules. Here, we adopted an innovative strategy for the development of a peptide based microtubule stabilizer, considering the taxol binding pocket of β-tubulin, by using alanine scanning mutagenesis technique. This approach lead us to a potential octapeptide, which strongly binds to the taxol pocket of β-tubulin, serves as an excellent microtubule stabilizer, increases the expression of acetylated tubulin, and acts as an Aβ aggregation inhibitor and neuroprotective agent. Further, results revealed that this peptide is nontoxic against both PC12 derived neurons and primary cortical neurons. We believe that our strategy and discovery of peptide-based microtubule stabilizer will open the door for the development of potential anti-AD therapeutics in near future

Synthesis, Characterization, and Reactivity of High-Valent Carbene Dicarboxamide-Based Nickel Pincer Complexes

High-valent metal–fluoride complexes are currently being explored for concerted proton–electron transfer (CPET) reactions, the driving force being the high bond dissociation energy of H–F (BDEH–F = 135 kcal/mol) that is formed after the reaction. Ni(III)–fluoride-based complexes on the pyridine dicarboxamide pincer ligand framework have been utilized for CPET reactions toward phenols and hydrocarbons. We have replaced the central pyridine ligand with an N-heterocyclic carbene carbene to probe its effect in both stabilizing the high-valent Ni(III) state and its ability to initiate CPET reactions. We report a monomeric carbene-diamide-based Ni(II)–fluoride pincer complex that was characterized through 1H/19F NMR, mass spectrometry, UV–vis, and X-ray crystallography analysis. Although carbenes and deprotonated carboxamides in the Ni(II)–fluoride complex are expected to stabilize the Ni(III) state upon oxidation, the Ni(III)/Ni(II) redox process occurred at very high potential (0.87 V vs Fc+/Fc, dichloromethane) and was irreversible. Structural studies indicate significant distortion in the imidazolium “NCN” carbene plane of Ni(II)–fluoride caused by the formation of six-membered metallacycles. The high-valent NiIII–fluoride analogue was synthesized by the addition of 1.0 equiv CTAN (ceric tetrabutylammonium nitrate) in dichloromethane at −20 °C which was characterized by UV–vis, mass spectrometry, and EPR spectroscopy. Density functional theory studies indicate that the Ni-carbene bond elongated, while the Ni–F bond shortened upon oxidation to the Ni(III) species. The high-valent Ni(III)–fluoride was found to react with the substituted phenols. Analysis of the KIE and linear free energy relationship correlates well with the CPET nature of the reaction. Preliminary analysis indicates that the CPET is asynchronous and is primarily driven by the E0′ of the Ni(III)–fluoride complex

Hydrogen Atom Transfer by a High-Valent Nickel-Chloride Complex

Oxo-metal-halide moieties have often been implicated as C–H bond activating oxidants with the terminal oxo-metal entity identified as the electrophilic oxidant. The electrophilic reactivity of metal-halide species has not been investigated. We have prepared a high-valent nickel-halide complex [Ni^III(Cl)­(L)] (<b>2</b>, L = <i>N</i>,<i>N</i>′-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamide) by one-electron oxidation of a [Ni^II(Cl)­(L)]⁻ precursor. <b>2</b> was characterized using electronic absorption, electron paramagnetic resonance, and X-ray absorption spectroscopies and mass spectrometry. <b>2</b> reacted readily with substrates containing either phenolic O–H or hydrocarbon C–H bonds. Analysis of the Hammett, Evans–Polanyi, and Marcus relationships between the determined rate constants and substrate p<i>K</i>_a, X–H bond dissociation energy, and oxidation potential, respectively, was performed. Through this analysis, we found that <b>2</b> reacted by a hydrogen atom transfer (HAT) mechanism. Our findings shine light on enzymatic high-valent oxo-metal-halide oxidants and open new avenues for oxidative halogenation catalyst design

Noninnocence of Indigo: Dehydroindigo Anions as Bridging Electron-Donor Ligands in Diruthenium Complexes

Complexes of singly or doubly deprotonated indigo (H₂Ind) with one or two [Ru­(pap)₂]²⁺ fragments (pap = 2-phenylazopyridine) have been characterized experimentally [molecular structure, voltammetry, electron paramagnetic resonance (EPR), and UV–vis–near-IR spectroelectrochemistry] and by time-dependent density functional theory calculations. The compound [Ru­(pap)₂(HInd^–)]­ClO₄ ([<b>1</b>]­ClO₄) was found to contain an intramolecular NH---O hydrogen bond, whereas [{Ru­(pap)₂}₂(μ-Ind^2–)]­(ClO₄)₂ ([<b>2</b>]­(ClO₄)₂), isolated as the meso diastereoisomer with near-IR absorptions at 1162 and 991 nm, contains two metals bridged at 6.354 Å distance by the bischelating indigo dianion. The spectroelectrochemical study of multiple reversible reduction and oxidation processes of <b>2</b>^<i>n</i> (<i>n</i> = 4+, 3+, 2+, 1+, 0, 1–, 2–, 3–, 4−) reveals the stepwise addition of electrons to the terminal π-accepting pap ligands, whereas the oxidations occur predominantly at the anionic indigo ligand, producing an EPR-identified indigo radical intermediate and revealing the suitability of deprotonated indigo as a σ- and π-donating bischelating bridge