Effects Of Water Constituents Research Articles

Effects of water constituents on the stability of gas diffusion electrode during electrochemical hydrogen peroxide production for water and wastewater treatment

Electrochemically producing hydrogen peroxide (H2O2) from oxygen reduction reaction (ORR) with natural air diffusion electrode (NADE) is an attractive way to supply H2O2 for decentralized water treatment. In this study, the stability of NADE during H2O2 electroproduction in varying water matrices were evaluated, including synthetic electrolyte solutions (0.05 M Na2SO4) with or without calcium ions (Ca2+, 200 mg/L) and/or humic acid (HA, 40 mg/L), as well as a selected municipal wastewater (92.7 mg/L Ca2+, 3.6 mg/L Mg2+, and 23.9 mg/L total organic carbon). The results show that NADEs maintained a good stability during H2O2 electroproduction in Na2SO4 solutions regardless of the presence of HA. However, Ca2+ (and Mg2+) could form significant amounts of mineral precipitates on the surface and in the internal pores of NADEs during H2O2 electroproduction. These mineral precipitates can negatively influence H2O2 production by impeding the oxygen, electron, and proton transfer processes involved in ORR to H2O2. Moreover, the mineral precipitates shifted the NADEs from hydrophobic to hydrophilic, which may promote H2O2 reduction to H2O at the NADEs. Consequently, the apparent current efficiencies of H2O2 production decreased substantially from initially ∼90% to 50%–70% as the NADEs were continuously used for 60 h in the Ca-containing solutions and selected wastewater. These results indicate that water constituents that are commonly present in real water matrices, especially Ca2+, can cause serious deterioration of NADE stability during H2O2 electroproduction. Therefore, proper strategies are needed to mitigate electrode fouling during H2O2 electroproduction with NADEs in practical water and wastewater treatment.

Photodegradation of hydroxyfluorenes in ice and water: A comparison of kinetics, effects of water constituents, and phototransformation by-products

The photochemical behavior of organic pollutants in ice is poorly studied in comparison to aqueous photochemistry. Here we report a detailed comparison of ice and aqueous photodegradation of two representative OH-PAHs, 2-hydroxyfluorene (2-OHFL) and 9-hydroxyfluorene (9-OHFL), which are newly recognized contaminants in the wider environment including colder regions. Interestingly, their photodegradation kinetics were clearly influenced by whether they reside in ice or water. Under the same simulated solar irradiation (λ > 290 nm), OHFLs photodegraded faster in ice than in equivalent aqueous solutions and this was attributed to the specific concentration effect caused by freezing. Furthermore, the presence of dissolved constituents in ice also influenced photodegradation with 2-OHFL phototransforming the fastest in ‘seawater’ ice (k = (11.4 ± 1.0) × 10−2 min−1) followed by ‘pure-water’ ice ((8.7 ± 0.4) × 10−2 min−1) and ‘freshwater’ ice ((8.0 ± 0.7) × 10−2 min−1). The presence of dissolved constituents (specifically Cl−, NO3−, Fe(III) and humic acid) influences the phototransformation kinetics, either enhancing or suppressing phototransformation, but this is based on the quantity of the constituent present in the matrixes, the specific OHFL isomer and the matrix type (e.g., ice or aqueous solution). Careful derivation of key photointermediates was undertaken in both ice and water samples using tandem mass spectrometry. Ice phototransformation exhibited fewer by-products and ‘simpler’ pathways giving rise to a range of hydroxylated fluorenes and hydroxylated fluorenones in ice. These results are of importance when considering the fate of PAHs and OH-PAHs in cold regions and their persistence in sunlit ice.

The degradation of enrofloxacin by a non-metallic heptazine-based OCN polymer: Kinetics, mechanism and effect of water constituents

Antibiotics are widespread in the environment with notable ecological risk, for which efficient and green removal technologies are demanded. As a kind of g–C3N4–based material with remarkable photocatalytic property, OCN is an oxygen- and nitrogen-linked carbon nitride organic polymer which can be synthesized through a single-step thermal polymerization method. In this study, OCN was applied for the visible-light-driven photocatalytic degradation of a typical fluoroquinolone (FQ) antibiotics enrofloxacin (ENR). The photocatalysis process achieved over 97% ENR removal within 60 min with 0.4 mg/L OCN and 4 mg/L ENR at pH 8.2. The photocatalytic mechanism of OCN at different pH was studied for the first time. It was shown that O2⋅−, 1O2 and h+ made contributions at neutral or basic pH and 1O2 contributes the most (57.6% at pH 8.2), while ⋅OH played a role only under acidic condition with a contribution rate of 23.8% at pH 3.2. The cleavage of the piperazine ring and the quinolone ring were two main degradation pathways. The common water constituents humic acid and NO3− showed a dual effect, but HCO3− and Cl− inhibited the degradation. The effect of different water matrices was tested under natural sunlight and it was only a tiny disturbance to the degradation rates. The biotoxicity test conducted using Vibrio fischeri indicated that the toxicity of degradation products became negligible after 3 h. This study demonstrated that OCN is a promising candidate for the advanced treatment and in-situ remediation.

Origin of the enhanced photocatalytic activity of graphitic carbon nitride nanocomposites and the effects of water constituents

A metal-free heterostructure system composed of graphitic carbon nitride (g-C3N4), reduced graphene oxide (rGO), and cyclooctasulfur (α-S) is developed as a facile route for establishing efficient photocatalysts. We aim to identify the governing factors that contributed to the photochemical performance of g-C3N4/rGO/α-S nanocomposites, which are important to advance potential applications but remain unexplored. The results indicate that the constituent ratio of each component in g-C3N4/rGO/α-S composites leads to varying surface microstructure, band alignment, and photochemical properties. The enhanced visible-light photocatalytic activity originates from an upward shift of the conduction bands toward higher energies, higher content of sp2-hybridized pyridine nitrogen in triazine rings (CN–C), and a lower amount of hydrogen bonds. The established structural integration informs a guiding framework for the design of emerging g–C3N4–based nanocomposites. Additionally, the influence of humic acid (HA) on photocatalytic decontamination was studied and shows an overall detrimental effect on the photocatalytic activity of g-C3N4 nanocomposites. Although HA advanced the photoexcited electrons, and therefore the reactive oxygen species, as well as enhanced the adsorption of the pollutants onto g-C3N4, especially at lower pH, attenuation of oxygen transfer as a result of active site competition between oxygen and target pollutants was found.

Transformation of atenolol by a laccase-mediator system: Efficiencies, effect of water constituents, and transformation pathways

In this study, we investigated the transformation of atenolol (ATL) by the naturally occurring laccase from Trametes versicolor in aqueous solution. Removal efficiency of ATL via laccase-catalyzed reaction in the presence of various laccase mediators was examined, and found that only the mediator 2, 2, 6, 6-tetramethyl-1-piperidinyloxy (TEMPO) was able to greatly promote ATL transformation. The influences of TEMPO concentration, laccase dosage, as well as solution pH and temperature on ATL transformation efficiency were tested. As TEMPO concentrations was increased from 0 to 2000 μM, ATL transformation efficiency first increased and then decreased, and the optimal TEMPO concentration was determined as 500 μM. ATL transformation efficiency was gradually increased with increasing laccase dosage. ATL transformation was highly pH-dependent with an optimum pH of 7.0, and it was almost constant over a temperature range of 25–50 °C. Humic acid inhibited ATL transformation through competition reaction with laccase. The presence of anions HCO3− and CO32− reduced ATL transformation due to both anions enhanced solution pHs, while Cl−, SO42−, and NO3− at 10 mM showed no obvious influence. The main transformation products were identified, and the potential transformation pathways were proposed. After enzymatic treatment, the toxicity of ATL and TEMPO mixtures was greatly reduced. The results of this study might present an alternative clean strategy for the remediation of ATL contaminated water matrix.

Effect of water constituents on the degradation of sulfaclozine in the three systems: UV/TiO2, UV/K2S2O8, and UV/TiO2/K2S2O8.

Bicarbonate, phosphate, chloride ions, and humic substances are among the constituents most widely present in natural waters. These non-target constituents can greatly affect the efficiency of advanced oxidation processes used for water decontamination due to their capacity to interfere with the adsorption of the target compounds on the surface of TiO2, absorb photons, scavenge hydroxyl radicals (·OH), and generate photochemical reactive intermediates. In this work, the effect of these constituents on the degradation of sulfaclozine (SCL) was monitored in three different AOPs systems: UV/TiO2, UV/K2S2O8, and UV/TiO2/K2S2O8. It was shown that bicarbonate (HCO3-) and phosphate (HPO42-) ions enhanced the degradation of SCL in UV/TiO2 and UV/TiO2/K2S2O8 systems whereas the addition of humic substances influenced these rates with a much smaller extent. On the other hand, the degradation rate of SCL in the UV/K2S2O8 system was not affected by the presence of HCO3- and HPO42- but was inhibited in the presence of humic substances. In addition, the different mechanisms that can take place in the presence of these constituents were discussed and the degradation rate enhancement in presence of HCO3- and HPO42- was attributed to the formation of new reactive species such as carbonate (CO3·-) and hydroxyl (·OH) radicals activated by TiO2 holes (h+). In the presence of chloride (Cl-) and nitrate (NO3-) ions, an enhancement of SCL adsorption on the surface of TiO2 was observed. Finally, a comparative study of the degradation of SCL in river water and ultrapure water was reported.

Degradation of ciprofloxacin using α-MnO2 activated peroxymonosulfate process: Effect of water constituents, degradation intermediates and toxicity evaluation

Nanoscale α-MnO2 with different morphologies (nanoparticles, nanoflowers and nanorods) were synthesized via a facile hydrothermal method and tested in peroxymonosulfate (PMS) activation for ciprofloxacin (CIP) degradation. The catalytic activity of α-MnO2 mainly relied on the specific surface area and crystallinity, and followed the order of α-MnO2 nanoflowers>α-MnO2 nanorods>α-MnO2 nanoparticles>commercial MnO2. Acidic condition was more beneficial to CIP removal than neutral condition, and the degradation would be greatly accelerated in alkaline circumstance. The introduction of chloride and bicarbonate displayed concentration-dependent dual effect. The presence of natural organic matter (NOM) exerted the detrimental effect on CIP degradation. Eleven oxidation products were identified by UPLC–MS/MS and the changes of their concentrations were monitored, five products showed monotonous uptrend while four intermediates present monotonous downtrend once they were generated. Piperazine ring cleavage, hydroxylation and F/OH substitution were found to be the important oxidation pathways. The acute toxicity of resulted solution gradually decreased with the lapse of time, and higher PMS concentration favored the detoxification. The CIP removal in α-MnO2/PMS process was also efficient under the actual water background, which was testified by the results of fluorescence excitation-emission matrix (EEM).

Degradation of fluoroquinolone antibiotics by ferrate(VI): Effects of water constituents and oxidized products

The degradation of five fluoroquinolone (FQ) antibiotics (flumequine (FLU), enrofloxacin (ENR), norfloxacin (NOR), ofloxacin (OFL) and marbofloxacin (MAR)) by ferrate(VI) (FeVIO42−, Fe(VI)) was examined to demonstrate the potential of this iron-based chemical oxidant to treat antibiotics in water. Experiments were conducted at different molar ratios of Fe(VI) to FQs at pH 7.0. All FQs, except FLU, were degraded within 2 min at [Fe(VI)]:[FQ] ≤ 20.0. Multiple additions of Fe(VI) improved the degradation efficiency, and provided greater degradation than a single addition of Fe(VI). The effects of anions, cations, and humic acid (HA), usually present in source waters and wastewaters, on the removal of FLU were investigated. Anions (Cl−, SO42−, NO3−, and HCO3−) and monovalent cations (Na+ and K+) had no influence on the removal of FLU. However, multivalent cations (Ca2+, Mg2+, Cu2+, and Fe3+) in water decreased the efficiency of FLU removal by Fe(VI). An increase in the ionic strength of the solution, and the presence of HA in the water, also decreased the percentage of FLU removed by Fe(VI). Experiments on the removal of selected FQs, present as co-existing antibiotics in pure water, river water, synthetic water and wastewater, were also conducted to demonstrate the practical application of Fe(VI) to remove the antibiotics during water treatment. The seventeen oxidized products (OPs) of FLU were identified using solid phase extraction-liquid chromatography-high-resolution mass spectrometry. The reaction pathways are proposed, and are theoretically confirmed by molecular orbital calculations.

Detection of Suspended-Matter Concentrations in the Shallow Subtropical Lake Taihu, China, Using the SVR Model Based on DSFs

Accurate detection of suspended-matter concentrations in water columns is an important task in remotely sensing water color. This letter aims to identify an optimal model for estimating suspended-matter concentration in the optically complex Lake Taihu of China. Remote sensing reflectance R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rs</sub> (λ), inherent optical properties, and constituent concentrations of the Lake water were synchronously measured in November of 2007. After the effects of water constituents on R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rs</sub> (λ) were analyzed, the definitive spectral factors were determined, which are indicative primarily of total suspended matter (TSM). Several methods were compared in modeling the relationship between R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rs</sub> (λ) and TSM. Results show that the support vector regression (SVR) model performs best with a root-mean-square error of 4.7 mg · l <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> (R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> = 0.968). Its predictive errors in four seasons were also assessed with the mean absolute percentage errors varying in the range of 22.0%-60.0%. Thus, the SVR model can be used to reliably retrieve TSM concentrations in Lake Taihu. This finding offers new insights into the optical signals of in-water constituents in optically complex lakes.

Quantum Chemical Investigation and Experimental Verification on the Aquatic Photochemistry of the Sunscreen 2-Phenylbenzimidazole-5-Sulfonic Acid

For ecological risk assessment of the large and ever-increasing number of chemical pollutants, it is of importance to develop computational methods to screen or predict their environmental photodegradation behavior. This study developed a computational method based on the density functional theory (DFT) to predict and evaluate the photodegradation behavior and effects of water constituents, taking a sunscreen and personal care product 2-phenylbenzimidazole-5-sulfonic acid (PBSA) as a model compound. Energy and electron transfer reactions of excited state PBSA (PBSA*) with (3)O(2) and water constituents were evaluated. The computational results indicated that PBSA* could photogenerate (1)O(2) and O(2)(-)·, triplet excited state humic/fulvic acid analogs could not photosensitize the degradation, and the anions (Cl(-), Br(-), and HCO(3)(-)) could not quench PBSA* or its radical cation chemically. Experiments employing simulated sunlight confirmed that PBSA photodegraded via the direct and self-sensitization mechanism involving O(2)(-)·. The photodegradation was pH-dependent. The direct and self-sensitized photodegradation was inhibited by fulvic acid. The main photodegradation products were identified, and the pathways were clarified. These results indicate that the DFT-based computational method can be employed to assess the environmental photochemical fate of organic pollutants.