Oxidative removal of aromatic amino acids by manganese dioxide: kinetic modeling and effect of co-solutes.

The degradation of aromatic amino acids, including L-tryptophan, L-tyrosine, and L-phenylalanine, by δ-MnO2 was investigated. L-tryptophan and L-tyrosine underwent rapid oxidative degradation in the δ-MnO2 suspension solution. In contrast, L-phenylalanine was not oxidized by δ-MnO2. The degradation of L-tryptophan was faster than that of L-tyrosine. The oxidation rates of amino acids increased with increasing concentration of δ-MnO2. The increase of concentrations of substrates and pH disfavored the degradation. The kinetics modeling showed that the reaction orders of amino acids, MnO2, and H+ were 0.67, 1.24, 0.72 and 0.32, 1.20, 1.05 for L-tryptophan and L-tyrosine, respectively. The coexistence of metal ions markedly inhibited the degradation in the order of Al3+ > Mn2+ > Zn2+ > Ca2+ > Mg2+. The inhibition effect of metal ions was related to the stability constant and hydration radius of metal ions. The addition of fulvic acid to the reaction solutions did not affect the degradation of substrate. This finding suggests that δ-MnO2 selectively oxidize aromatic amino acids and it can be used as an efficient oxidant for the removal of certain aromatic acid pollutants.

Photodegradation of amitriptyline in Fe(III)-citrate-oxalate binary system: Synergistic effect and mechanism.

Fe(III) and carboxylic acids are ubiquitous in surface water and atmospheric water droplets. Numerous documents have reported the photochemistry of Fe(III)-carboxylate complexes, typically including Fe(III)-oxalate and Fe(III)-citrate. Our previous study preliminarily showed that oxalate enhances the photoreactivity of Fe(III)-citrate system. Here, we further investigate the synergistic effect of Fe(III)-citrate-oxalate binary system at different conditions with pharmaceutical amitriptyline (AMT) as the model pollutant. In the Fe(III)-oxalate system, the photodegradation of AMT decreased with increasing pH from 3.0 to 8.0. In the Fe(III)-citrate system, the optimal pH for AMT degradation is around 5.0 in the same pH range. For the Fe(III)-citrate-oxalate system, the photodegradation of AMT decreased with increasing pH, indicating the combined effect of both oxalate and citrate on the photoreactivity. The addition of oxalate to the Fe(III)-citrate system markedly accelerated the photodegradation of AMT. The Fe(III)-carboxylate binary system exhibited excellent photoreactivity and up to 90% AMT was removed after 30 min at pH 6.0 with Fe(III)/citrate/oxalate ratio of 10:150:500 (µM). Synergistic effect was observed in Fe(III)-citrate-oxalate binary system in the pH range of 5.0-8.0. The presence of oxalate promoted the depletion of citrate in the Fe(III)-citrate system. The higher concentration ratios of oxalate to citrate facilitated the synergistic effect in the Fe(III)-citrate-oxalate system. By LC-MS analyses, a possible pathway of AMT degradation was proposed based on hydroxyl radicals (OH) mechanism. This finding could be helpful for the better understanding of synergistic mechanism of Fe(III)-citrate-oxalate binary complexes, which will be of great potential application in environmental photocatalysis at near neutral pH.

Vertical-algal-biofilm enhanced raceway pond for cost-effective wastewater treatment and value-added products production.

A win-win strategy by the integration of wastewater treatment with value-added products production through a vertical-algal-biofilm enhanced raceway was investigated in the present study. Raceway pond was enhanced by vertically setting the biofilm in the system with a certain interval distance that could be adjusted for different light conditions and wastewater types. Two types of synthetic wastewater were treated with suitability-proven materials as biofilm carriers under four operation distances. Composition of the harvested algal biomass was analyzed. Coral velvet with 5-8 mm length villus was the optimal carrier, since it was durable and with high biomass productivity (6.95-8.11 g m-2·day-1). Nutrients in the wastewaters were efficiently removed with the COD, TN and TP reduction of over 86.61%, 73.68% and 89.85%, respectively. Wastewater with the low nutrients concentration experienced lower biomass and lipid productivity but larger biodiesel productivity and higher nutrient removal efficiency. In addition, as the operation distance increased, wastewater treatment efficiency was first increased but then decreased, while algal biomass footprint production was decreased. Differences in nutrients removal efficiencies were mainly due to the distance difference, which caused different biofilm culture surface areas and light regimes. The optimal operation distance as a function of the efficient nutrient removal and biodiesel production in this study was 6 cm.

Role of humic substances in the photodegradation of naproxen under simulated sunlight.

Humic substances (HS) including humic acid (HA) and fulvic acid (FA) are ubiquitous in the natural waters. Although numerous studies documented their role in photodegradation of organic pollutants, the competitive effects of photosensitization and light-screening of HS on the photodegradation of pollutants are not yet clear. In this work, the role of HS in the photodegradation of the pharmaceutical naproxen (NP) was studied under simulated sunlight. The direct photodegradation quantum yield of NP in deionized water was 2.1 × 10-2, and the apparent quantum yields for photosensitized degradation of NP in the presence of FA and HA were 2.3 × 10-4 and 2.6 × 10-5, respectively. Both direct and photosensitized photodegradation decreased with increasing pH, consistent with the trend of singlet oxygen (1O2) reaction rate constants of NP. HA inhibited the photodegradation of naproxen thoroughly. In contrast, FA accelerated the photodegradation of NP at lower substrate concentration and light intensity, and vice versa. Direct photodegradation of NP declined sharply with spectral radiation attenuation of UV region, when HS-mediated photosensitization predominantly accounted for the photodegradation. The direct photodegradation was ascribed to decomposition of excited triplet state of naproxen (3NP∗) and self-sensitization effect involving 1O2. The FA-mediated photodegradation was mainly attributed to 1O2 oxidation in aerated solution. These findings are important for assessing the competitive effects of humic substances on the photodegradation of pollutants under various conditions in natural waters.

Self-Driven Desalination and Advanced Treatment of Wastewater in a Modularized Filtration Air Cathode Microbial Desalination Cell.

Microbial desalination cells (MDCs) extract organic energy from wastewater for in situ desalination of saline water. However, to desalinate salt water, traditional MDCs often require an anolyte (wastewater) and a catholyte (other synthetic water) to produce electricity. Correspondingly, the traditional MDCs also produced anode effluent and cathode effluent, and may produce a concentrate solution, resulting in a low production of diluate. In this study, nitrogen-doped carbon nanotube membranes and Pt carbon cloths were utilized as filtration material and cathode to fabricate a modularized filtration air cathode MDC (F-MDC). With real wastewater flowing from anode to cathode, and finally to the middle membrane stack, the diluate volume production reached 82.4%, with the removal efficiency of salinity and chemical oxygen demand (COD) reached 93.6% and 97.3% respectively. The final diluate conductivity was 68 ± 12 µS/cm, and the turbidity was 0.41 NTU, which were sufficient for boiler supplementary or industrial cooling. The concentrate production was only 17.6%, and almost all the phosphorus and salt, and most of the nitrogen were recovered, potentially allowing the recovery of nutrients and other chemicals. These results show the potential utility of the modularized F-MDC in the application of municipal wastewater advanced treatment and self-driven desalination.

Removal of Zn(II) from simulated wastewater using an algal biofilm.

An algal biofilm was employed as a novel kind of adsorbing material to remove Zn(II) from simulated wastewater. The algal biofilm system formed by Oedogonium sp. was operated in a dynamic mode for a period of 14 days with an initial Zn(II) concentration of 10 mg/L. The average effluent Zn(II) concentration was 0.247 mg/L and the average removal efficiency reached 97.7%. The effects of Zn(II) on key algal physiological and biochemical indices such as chlorophyll content, nitrate reductase and superoxide dismutase activity, extracellular polysaccharides (EPS), and soluble protein levels were studied. Our results showed that the algal biofilm could adapt to the simulated wastewater containing Zn(II). Scanning electron microscope and Fourier transform infrared spectroscopy analyses of algal biofilm revealed the presence of carboxyl, amino, and sulphonate groups, which were the main functional groups of EPS and proteins, and these were likely responsible for biosorption of the Zn(II) ions.

Study on two operating conditions of a full-scale oxidation ditch for optimization of energy consumption and effluent quality by using CFD model.

The operating condition of an oxidation ditch (OD) has significant impact on energy consumption and effluent quality of wastewater treatment plants (WWTPs). An experimentally validated numerical tool, based on computational fluid dynamics (CFD) model, was proposed to optimize the operating condition by considering two important factors: flow field and dissolved oxygen (DO) concentration profiles. The model is capable of predicting flow pattern and oxygen mass transfer characteristics in ODs equipped with surface aerators and submerged impellers. Performance demonstration and comparison of two operating conditions (existing and improved) were carried out in two full-scale Carrousel ODs at the Ping Dingshan WWTP in Henan, China. A moving wall model and a fan model were designed to simulate surface aerators and submerged impellers, respectively. Oxygen mass transfer in the ditch was predicted by using a unit analysis method. In aeration zones, the mass inlets representing the surface aerators were set as one source of DO. In the whole straight channel, the oxygen consumption was modeled by using modified BOD-DO model. The following results were obtained: (1) the CFD model characterized flow pattern and DO concentration profiles in the full-scale OD. The predicted flow field values were within 1.98 ± 4.28% difference from the actual measured values while the predicted DO concentration values were within -4.71 ± 4.15% of the measured ones, (2) a surface aerator should be relocated to around 15m from the curve bend entrance to reduce energy loss caused by fierce collisions at the wall of the curve bend, and (3) DO concentration gradients in the OD under the improved operating condition were more favorable for occurrence of simultaneous nitrification and denitrification (SND).