Influence of Water Hardness and Complexing Agents on Electrochemical Hydrogen Peroxide Generation

Recently, many studies have been published regarding electrochemical oxygen reduction reaction for generating hydrogen peroxide (H2O2) using gas diffusion electrodes (GDEs) for various applications. Sodium salts solved in deionized water were usually used as supporting electrolytes. In technical applications, however, tap water-based electrolytes with hardeners are particularly relevant and have only been considered in a few studies to date. In this work, we investigated the influence of hardeners on H2O2-generation at 150 mA cm-2 and were able to show that scaling occurs predominantly on the GDE-surface and not in its pore structure. With the novel method in electrochemical synthesis by using complexing agents to bind hardeners, we were able to significantly reduce the scaling. Even after 10 h of operation, the reactor still achieves a faradaic efficiency (FE) of above 70 % (>67 mg h-1 cm-2), comparable to the experiments without hardeners and complexing agents in the electrolyte. Furthermore, we demonstrate that the complexing agents are not electrochemically converted at the carbon-based GDE and behave inertly. If the cell is operated with complexing agents and rinsed with acidic liquid (anolyte) between batches, scaling can be completely avoided.

A critical role for iron and zinc homeostatic systems in the evolutionary adaptation of Escherichia coli to metal restriction

Host nutritional immunity utilizes metal deprivation to help prevent microbial infection. To investigate bacterial adaptation to such restrictive conditions, we conducted experimental evolution with two metal sequestering agents. Ethylenediaminetetraacetic acid (EDTA) and diethylenetriamine pentamethylene phosphonic acid (DTPMP) were selected as ligands because they differentially affect cellular levels of iron, manganese and zinc in Escherichia coli. Mutants of E. coli strain BW25113 were isolated after cultivation at sub-minimum inhibitory concentration (MIC) chelant levels and genetic changes potentially responsible for tolerance were identified by whole-genome sequencing. In EDTA-selected strains, mutations in the promoter region of yeiR resulted in elevated gene expression. The yeiR product, a zinc-specific metallochaperone, was confirmed to be primarily responsible for EDTA resistance. Similarly, in two of the DTPMP-selected strains, a promoter mutation increased expression of the fepA-entD operon, which encodes components of the ferric-enterobactin uptake pathway. However, in this case improved DTPMP tolerance was only detectable following overexpression of FepA or EntD in trans. Additional mutations in the cadC gene product, an acid-response regulator, preserved the neutrality of the growth medium by constitutively activating expression of the cadAB regulon. This study uncovers specific resistance mechanisms for zinc and iron starvation that could emerge by selection against host nutritional immunity or competition with heterologous metallophores. It also provides insight into the specific metals affected by these two widely used chelators critical for their antibacterial mode of action.

Degradation of phosphonates in Co(II)/peroxymonosulfate process: Performance and mechanism

The increased release of phosphonates to natural waters causes global concern due to their potential threat to the aquatic environment. It is curial to mineralize phosphonates to orthophosphate (PO₄^3-) before they are thoroughly removed from wastewater via conventional biological treatment. In this study, we systematically investigated the performance and mechanism of degradation of phosphonates in Co(II)-triggered peroxymonosulfate (PMS) activation process. The degradation efficiency of various phosphonates is highly dependent on their coordination with Co(II). Using 1-hydroxyethane 1,1-diphosphonic acid (HEDP) as a target pollutant, the Co(II)/PMS process is effective in a broad solution pH range from 5.0 to 10.0. Multiple experimental results imply that Co(II)-PMS complex is the primary reactive species, while hydroxyl radicals (HO^•), sulfate radicals (SO₄^•-), singlet oxygen (¹O₂) and Co(III) play as the secondary reactive species for the degradation of HEDP. The presence of Cl^-, HCO₃^-, and natural organic matters (NOM) inhibits the degradation of HEDP. However, in real water samples, the selectivity and efficiency for HEDP removal in the Co(II)/PMS process are higher than that in free radicals-mediated advanced oxidation processes. This study not only sheds new lights on the mechanism of Co(II)-triggered PMS activation process, but also provides feasible technology for the degradation of phosphonates in wastewater.

NH3/H2O-mediated proton conductivity and photocatalytic behaviour of Fe(ii)-hydroxyphosphonoacetate and M(ii)-substituted derivatives

Multifunctional Fe(ii)-hydroxyphosphonoacetate and M(ii)-substituted derivatives exhibit ammonia-tunable proton conductivity and photo-Fenton catalytic properties.

Exploring various ligand classes for the efficient sequestration of stannous cations in the environment

Metal pollution, coming from both natural and anthropogenic sources, has become one of the most serious environmental problems. Various strategies have been tested with the aim of removing heavy metals from environment. In this contribution, containing a robust experimental work together with a critical literature analysis, the sequestering ability of a variety of ligands towards Sn²⁺ cation will be evaluated in the conditions of several natural fluids, i.e. sea water, fresh water, human blood plasma, urine and saliva. 13 structural and 11 thermodynamic descriptors will be selected for a total of thirty-eight molecules belonging to different classes (carboxylic acids, amines, amino acids, phosphonates, polyelectrolytes etc. …). For the filling of those missing data relative to the 11 thermodynamic descriptors, different strategies will be adopted, including simple correlations and Nipals algorithm. The evaluation of the sequestering ability of the ligands is assessed in terms of estimation of pL_0.5 (total concentration of ligand required to bind the 50% of metal in solution), an empirical parameter that takes into account all the side reactions in solutions and does not depend on the speciation scheme. Partial least square calculations were performed to model the pL_0.5 and to determine its correlation with the abovementioned descriptors. The possibility to design and build up new tailor-made molecules capable of effectively sequester Sn²⁺ in various conditions is crucial for practical applications in biosphere, hydrosphere and lithosphere.

High-Throughput Synthesis of Pillared-Layered Magnesium Tetraphosphonate Coordination Polymers: Framework Interconversions and Proton Conductivity Studies

Novel pillared-layered framework materials were synthesized by high-throughput or microwave-assisted methodology that contain Mg2+ and the zwitterionic linker HDTMP (hexamethylenediamine-N,N,N′,N′-tetrakis(methylenephosphonic acid)). Three compounds were structurally characterized by X-ray powder diffraction. In the compound {Mg[(HO3PCH2)2N(CH2)6N(CH2PO3H2)2]·(H2O)}n(1), obtained at 140 °C by hydrothermal or microwave-assisted reaction, the layers are built by isolated Mg2+ octahedra coordinated by oxygen atoms from six different zwitterionic HDTMP ligands. Each amino-bis(methylenephosphonate) moiety links three Mg2+ ions, bridging two of them through one phosphonate group and connecting the third polyhedron in a monodentate fashion. In Compound 2, {Mg[(HO3PCH2)2N(CH2)6N(CH2PO3H2)2]}n, hydrothermally synthesized at 180 °C, the layers are composed of bidentate amino-bis(methylenephosphonate) moieties connected to three Mg2+ ions, with one of the phosphonate groups acting as a bridging ligand. Various subtle structural changes are noted for the other two compounds. Thermodiffraction of 1 reveals that a crystalline-to-crystalline phase transformation occurs concomitantly with its dehydration, leading to a new anhydrous phase, namely, {Mg[(HO3PCH2)2N(CH2)6N(CH2PO3H2)2]}n(1deh). This process is fully reversible upon equilibrating the solid at room temperature. The reported compounds can adsorb ammonia and CO2. Compound 1 exhibits a moderate proton conductivity, ~1.5 × 10−5 S·cm−1 at 80 °C and 95% RH, that increases a half order of magnitude after experiencing a complete dehydration/rehydration process, 1→1deh→1.

Phytate–molybdate(vi) interactions in NaCl(aq)at different ionic strengths: unusual behaviour of the protonated species

Stepwise stability constants of phytate/molybdate(vi) complexes regularly increase with the number of protons in the species, affecting their speciation and sequestration.

Thermodynamic study on 8-hydroxyquinoline-2-carboxylic acid as a chelating agent for iron found in the gut of Noctuid larvae

8-HQA is a good sequestering agent towards Fe²⁺ and Fe³⁺ over a wide pH range.