N₂O is a GHG of environmental concern. It is generated from the nitrous material contained in wastewater and is the sixth most important contributor to N₂O emissions. There is a great variety of methods to quantify the emission of N₂O in a wastewater treatment plant (WWTP), which present variants among them, such as predetermined values and operational data of the plants. In this paper, we compared three different methods to quantify the N₂O emission in 2015 from WWTP in two metropolitan areas with high population density: Mexico City and the Metropolitan Area of Barcelona (MAB). MAB has advanced treatment plants that remove nutrients from wastewater, and Mexico City has only traditional treatment plants. The N₂O emission/inhabitant from WWTPs in MAB (3,214,211 inhabitants served) was 40% lower than the plants in Mexico City (1,806,440 inhabitants served). The MAB emission was 0.009 tCO₂e/inhabitant and 0.013 tCO₂e/inhabitant in Mexico City; these emission values could be considered statistically different with a risk error of 5%. This difference could be due to the fact that MAB has nutrient removal (42% of inhabitants served), and Mexico City has only traditional treatment plants. The results obtained may be influenced by the default emission factors of each methodology. In addition, per capita protein consumption and water consumption per inhabitant are different parameters that must be considered between these zones to quantify and compare the emission of N₂O. The integral methods are closer to the reality of the N₂O emission when the operating parameters of each plant and wastewater are considered. There should be more research on the reduction of this GHG in wastewater treatment for a correct quantification of these emissions, and more especially in the estimation of N₂O emission factors suitable for each treatment plant and study area.

Total (all forms of inorganic and organic) concentrations of mercury (Hg) and selenium (Se) were measured in dorsal muscle and eggs of wild fishes from two shallow lakes in China: Tai Lake (Ch: Taihu; TL) and Baiyangdian Lake (BYDL). Hazard quotients (HQs) were calculated by dividing concentrations of Se or Hg in muscle or eggs of fishes by threshold concentrations for effects expressed as tissue residue toxicity reference values (TR-TRVs). Concentrations of Hg in whole bodies of fishes were estimated by concentrations in muscle. Based on concentrations of Hg in whole body, HQs for fishes in TL and BYDL were less than 1.0, which suggests little to moderate potential for effects on these fishes and unaccepted adverse effects of Hg are unexpected for adult fishes. HQs of Se in muscle of common carp from TL were closed to 1.0, and 27% of HQs based on concentrations of Hg in eggs of fishes from BYDL exceeded 1.0. Potential hazard due to Hg on common carp in TL and reproductive effects of Se on fishes from BYDL exhibited need for concern. Ratios of molar concentrations of Se to Hg were greater than 1.0. Thus, there might be some protective effects of Se on effects of Hg on fishes in TL and BYDL.

Long-term agricultural development has led to agricultural non-point source (NPS) pollution. Ecological ditches (eco-ditch), as specific wetland systems, can be used to manage agricultural NPS water and achieve both ecological and environmental benefits. In order to understand which type of eco-ditch systems (Es, soil eco-ditch; Ec, concrete eco-ditch; Eh, concrete eco-ditch with holes on double-sided wall) is more suitable for plant nutrient balance meanwhile reducing NPS water (total nitrogen [TN], about 10 mg/L; total phosphorus [TP], about 1 mg/L), it is essential to evaluate the plant (Vallisneria natans) stoichiometry response to water in different types of eco-ditches under static experiment. The results indicated that there were no significant differences in TP removal efficiency among three eco-ditches, yet Eh systems had the best TN removal efficiency during the earlier experimental time. Addition of agricultural NPS water had varying effects on plants living in different types of eco-ditch systems. Plant organ stoichiometry of V. natans varied in relation to eco-ditch types. Plant stoichiometry (C:N, C:P, and N:P) of V. natans in Eh systems could maintain the homeostasis of nutrients and was not greatly affected by external changing environment. V. natans in Es systems can more easily modify the nutrient contents of organs with regard to nutrient availability in the environment. Our findings provide useful plant stoichiometry information for ecologists studying other specific ecosystems.

This study aims to evaluate the practical potential of using constructed wetlands (CWs) for treating saline wastewater containing various heavy metals. The results demonstrated that CWs growing Canna indica with porous slag as substrate could efficiently remove heavy metals (Cu, Zn, Cd, and Pb) from saline wastewater at an electrical conductivity (EC) of 7 mS/cm, especially under low influent load. Salts with salinity level (characterized as EC) of 30 mS/cm suppressed the removal of some heavy metals, dependent on heavy metal species and their influent concentrations. The presence of salts in CWs can improve the accumulation of Cu, Zn, and Pb in plant tissues as compared to control treatment, irrespective of metal concentrations in solution. The influence of salts on Cd accumulation depended on both salinity levels and Cd concentrations in solution. Although more heavy metals were accumulated in roots than in shoots, the harvesting of aboveground plant materials is still efficient addition for heavy metal removal due to the greater biomass and growth rate of aboveground plant material. Furthermore, replacing all plants instead of preserving roots from harvested plants in CWs over a period of time is essential for heavy metal removal, because the continued accumulation by roots can be inhibited by the increasing accumulated heavy metals from saline wastewater.

This study aimed to estimate in laboratory the temporal production of methylmercury during the filling of reservoirs of hydropower plants and to correlate it to the ecosystem of different locations in northern Brazil: Jirau hydropower plant in the Madeira River in the state of Rondônia (white waters—under construction), Cana Brava hydropower plant in the Tocantins River in the state of Goiás (clear waters—completed), and the Negro River in the Amazon (black waters—comparative). After collecting water, soil, and sediment samples in the regions mentioned, a microcosm was created to reproduce the conditions close to those found in nature. Water/soil/Hg⁰/Hg²⁺ and water/sediment/Hg⁰/Hg²⁺ were added to glass recipients. Next, methylmercury concentration was monitored by atomic fluorescence spectrometry, total organic carbon by TOC 5000A, and physical and chemical parameters such as pH, redox potential, and dissolved oxygen, for 25 days. The results obtained allow concluding that organic matter plays an important role, providing excess methyl groups to react with inorganic Hg and form organic Hg. The Negro River, which has higher contents of organic matter in its soil, water, and sediment, presented higher potential of mercury methylation in both experiments performed, followed by rivers Madeira and Tocantins.

The presence of polycyclic aromatic hydrocarbons, such as anthracene (AN) and benzo[a]pyrene (BaP), in water has become a problem of great concern due to the detrimental health effects caused to humans and living beings. In this work, the efficiency of the UV/H₂O₂ system for degrading the target compounds at ultra-trace levels in surface water has been evaluated. For this purpose, a previous optimization step using a face-centered central composite experimental design has been conducted, considering the effect of the UV-C irradiance and the initial concentration of H₂O₂. It was evidenced that under optimal operating conditions (11 mg L⁻¹ H₂O₂ and 0.63 mW cm⁻² irradiance), AN and BaP removal percentages were higher than 99.8%. Additionally, 69.3% of the organic matter, in terms of total organic carbon, was mineralized without the production of transformation by-products more harmful than the parent compounds. These findings demonstrate the oxidation capacity of the examined system in a natural matrix for degrading micropollutants that cannot be converted through conventional treatment processes. Consequently, new horizons are opened for the effective use of the UV/H₂O₂ system for drinking water production, providing the accomplishment of other regulated parameters related to water quality.

This study applies asymmetric causality to renewable energy (REC), carbon dioxide emissions (CE), and real GDP using non-linear broadcasting between these variables through the non-linear autoregressive distributed lag model (NARDL) to examine the short- and long-run asymmetries in the inconsistency of greenhouse gas emissions among the variables and to unpack the asymmetric causality of selective variables through positive and negative shocks for time series data from the Kingdom of Saudi Arabia between 1990 and 2014. The bounds cointegration test shows the existence of long-term dealings among all considered variables in the presence of asymmetry. The non-linear asymmetric causality test shows that negative shocks in carbon dioxide emissions had only positive impacts on real GDP in the long-term but are unobservable in the short-term. Additionally, the short- and the long-term incidences of positive shocks on real GDP are not similar to the negative shock to REC, implying the existence of asymmetric impacts on REC in both short- and long-term forms. Finally, the asymmetric causal relationship from carbon dioxide emissions to REC is neutral in the long-term. Both positive and negative shocks to REC consistently had an adverse effect on CE in the long-term. The presence of asymmetry between economic growth, CE, and REC could be of major substantial for more helpful policymakers and the action plan of sustainable development goals (SDGs) in Saudi Arabia.

Previous studies have shown that plant diversity can enhance methane (CH₄) emission and nitrogen purification efficiency in constructed wetlands (CWs), but effect of plant diversity on carbon dioxide (CO₂) flux and carbon removal efficiency in CWs is unknown. Therefore, we established four plant diversity levels (each level containing 4, 3, 2, and 1 species, respectively) in laboratory-scale wetland microcosms fed with simulated wastewater. Results showed that plant species richness enhanced CO₂ emissions (84.7–124.7 mg CO₂ m⁻² h⁻¹, P < 0.01), carbon fixation rate (P < 0.05), and microbial biomass carbon (P < 0.001), but did not improve carbon removal (P > 0.05). The presence of Pontederia cordata increased CO₂ emissions, carbon fixation rate of belowground, and microbial biomass carbon (P < 0.05), whereas the presence of Phragmites australis only enhanced CO₂ emission (P < 0.05). However, the presence of Typha orientalis or Lythrum salicaria did not show an influence on CO₂ emissions and carbon removal (P > 0.05). Hence, our study highlights the importance of plant diversity in mediating CO₂ emission intensity and carbon processes but not carbon removal in CWs.

Heterogeneous photocatalytic oxidation (PCO) is a widely studied alternative for the elimination of volatile organic compounds (VOC) in air. In this context, research on novel photoreactor arrangements to enhance PCO rates is desired. Annular fluidized bed photoreactors (AFBPR) have yielded prominent results when compared to conventional thin film reactors. However, very few works aimed at optimizing AFBPR operation. In this study, TiO₂ photocalytic agglomerates were synthesized and segregated in specific size distributions to behave as Geldart groups A, B, C, and D fluidization. The TiO₂ agglomerates were characterized by XRD, FTIR spectra, and N₂ adsorption. Photocatalyst performances were compared in a 10-mm gapped AFBPR for degrading the model pollutant methyl-ethyl-ketone (MEK), using a 254-nm radiation source. Geldart group C showed to be inadequate for AFBPR operation due to the short operation range between fluidization and elutriation. In all the cases, photocatalytic reaction rates were superior to sole UV photolysis. Group A and group B demonstrated the highest reaction rates. Considerations based on mass transfer suggested that the reasons were enhanced UV distribution within the bed at lower flow rates and superior catalyst surface area at higher flow rates. Results also revealed that groups A, B, and D perform equally per catalyst area within an AFBPR if the fluidization numbers (FN) are high enough.

In this work, an ultrathin polyamide (PA) membrane was fabricated via in situ removing polysulfone (PSF) substrate from the PSF-PA forward osmosis membrane for the first time. The physicochemical properties of the PA membranes were confirmed by means of surface morphology, chemistry analysis, and surface charge characterization. The performance of PA, PSF-PA, and physically combined PSF+PA membrane was compared in terms of water flux, reverse salt flux, and selectivity. The flux performance of these three membranes followed the order of PA>PSF-PA>PSF+PA membranes, and the possible mechanism for their performance was proposed. Compared with home-made PSF-PA and PSF+PA membranes, the ultrathin PA membrane had high water flux (i.e., 80.54 LMH) due to its low membrane resistance and minimized internal concentration polarization under same operation conditions (i.e., DI water feed solution, 1.0 M NaCl draw solution, and AL-FS orientation). This study would provide insights on the preparation and application of ultrathin PA membranes with high permeability in the context of global water/energy-related crisis.