Graphene Oxide/Polyethyleneimine-Modified Cation Exchange Membrane for Efficient Selective Recovery of Ammonia Nitrogen from Wastewater.

Competition for the migration of interfering cations limits the scale-up and implementation of the Donnan dialysis process for the recovery of ammonia nitrogen (NH4+-N) from wastewater in practice. Highly efficient selective permeation of NH4+ through a cation exchange membrane (CEM) is expected to be modulated via tuning the surface charge and structure of CEM. In this work, a novel CEM was designed to form a graphene oxide (GO)-polyethyleneimine (PEI) cross-linked layer by introducing self-assembling layers of GO and PEI on the surface of a commercial CEM, which rationally regulates the surface charge and structure of the membrane. The resulting positively charged membrane surface exhibits stronger repulsion for divalent cations compared to monovalent cations according to Coulomb's law, while, simultaneously, GO forms π-metal cation conjugates between metal cations (e.g., Mg2+ and Ca2+), thus limiting metal cation transport across the membrane. During the DD process, higher NH4+ concentrations resulted in a longer time to reach Donnan equilibrium and higher NH4+ flux, while increased Mg2+ concentrations resulted in lower NH4+ flux (from 0.414 to 0.213 mol·m-2·h-1). Using the synergistic effect of electrostatic interaction and non-covalent cross-linking, the designed membrane, referred to as GO-PEI (20) and prepared by a 20 min impregnation in the GO-PEI mixture, exhibited an NH4+ transport rate of 0.429 mol·m-2·h-1 and a Mg2+ transport rate of 0.003 mol·m-2·h-1 in single-salt solution tests and an NH4+/Mg2+ selectivity of 15.46, outperforming those of the unmodified and PEI membranes (1.30 and 5.74, respectively). In mixed salt solution tests, the GO-PEI (20) membrane showed a selectivity of 15.46 (~1.36, the unmodified membrane) for NH4+/Mg2+ and a good structural stability after 72 h of continuous operation. Therefore, this facile surface charge modulation approach provides a promising avenue for achieving efficient NH4+-selective separation by modified CEMs.

Effects of benthic bioturbation on anammox in nitrogen removal at the sediment-water interface in eutrophic surface waters.

Anaerobic ammonium oxidation (anammox) significantly contributes to nitrogen loss in freshwater ecosystems. The sediment-water interface (SWI), known as a "hot spot" for anammox, also harbors numerous macroinvertebrates. However, the impact of their bioturbation on anammox has generally been overlooked. This study compared the effects of three representative macroinvertebrates (i.e., Propsilocerus akamusi, Branchiura sowerbyi and Radix swinhoei) with different bioturbation modes on anammox and the N-removal processes at the SWI by using a microcosmic system. The results demonstrated that all three benthic macroinvertebrates promoted anammox in addition to denitrification processes. The highest N-removal was achieved in the presence of P. akamusi considered as a gallery-diffuser, where the relative abundance of Planctomycetes (to which the anammox bacteria belong) increased by 70%. P. akamusi increased the abundance of anammox hzsB gene by 2.58-fold and promoted potential anammox rate by 12.79 nmol N g-1 h-1, which in turn facilitated total N-removal mass increased by 2.42-fold. In the presence of B. sowerbyi and R. swinhoei, the potential anammox rates increased by 4.81 and 5.57 nmol N g-1 h-1, respectively. These results underscore the substantial impact of macroinvertebrates on anammox and N-removal processes, highlighting their crucial role in N pollution control, and sustaining the overall health and stability of eutrophic water bodies.

Hybrid Donnan dialysis-electrodialysis for efficient ammonia recovery from anaerobic digester effluent.

Ammonia recovery from wastewater is crucial, yet technology of low carbon emission and high ammonia perm-selectivity against complex stream compositions is urgently needed. Herein, a membrane-based hybrid process of the Donnan dialysis-electrodialysis process (DD-ED) was proposed for sustainable and efficient ammonia recovery. In principle, DD removes the majority of ammonia in wastewater by exploring the concentration gradient of NH4 + and driven cation (Na+) across the cation exchange membrane, given industrial sodium salt as a driving chemical. An additional ED stage driven by solar energy realizes a further removal of ammonia, recovery of driven cation, and replenishment of OH- toward ammonia stripping. Our results demonstrated that the hybrid DD-ED process achieved ammonia removal efficiency >95%, driving cation (Na+) recovery efficiency >87.1% for synthetic streams, and reduced the OH- loss by up to 78% compared to a standalone DD case. Ammonia fluxes of 98.2 gN m-2 d-1 with the real anaerobic digestion effluent were observed using only solar energy input at 3.8 kWh kgN -1. With verified mass transfer modeling, reasonably controlled operation, and beneficial recovery performance, the hybrid process can be a promising candidate for future nutrient recovery from wastewater in a rural, remote area.

Nitrogen removal enhanced by benthic bioturbation coupled with biofilm formation: A new strategy to alleviate freshwater eutrophication.

Excessive nitrogen input into the water caused eutrophication thereby reducing biodiversity and degrades freshwater function. Nitrogen pollution in sediments is one key reason that makes eutrophication difficult to control. The physicochemical technologies such as dredging and coverage for sediment pollution easily destroyed and homogenized aquatic habitats. To alleviate freshwater eutrophication in ecological way, this work combined the functions of bioturbation and biofilm to test their effect on the removal of nitrogen from sediment and water. The total nitrogen removal by employing the coupled function (bioturbation + biofilm, SCB) was greater than that of the single function (bioturbation or biofilm). The mean efficiency of total nitrogen removal in SCB treatment was 3.19 times that of the control without chironomids nor biofilm medium. Chironomid bioturbation promoted nitrogen release from sediments to the overlying water. Biofilm enhanced the conversion and removal of nitrogen stirred up by chironomids, resulting the lowest concentration of total nitrogen in overlying water of SCB treatment. The enhancement of nitrogen removal may be due to the coupled function increased the abundance of denitrifying and anammox functional bacteria in sediment and biofilm. Therefore, the method of combining benthic animals with biofilm medium is not only a viable solution for reducing sedimentary nitrogen loading in freshwater ecosystems, but also a solution to mitigate eutrophication in the overlying water. The restoration and management for aquatic ecosystems should consider protecting habitat for benthic organisms while maintaining heterogeneity for biofilm.

Donnan Dialysis-Osmotic Distillation (DD-OD) Hybrid Process for Selective Ammonium Recovery Driven by Waste Alkali.

This work proposed an innovative and energy-efficient Donnan Dialysis (DD) and Osmotic Distillation (OD) hybrid process for alkali-driven ammonium recovery from wastewater. The efficiency and feasibility of ammonium removal and recovery from synthetic and real wastewater using NaOH and waste alkali were investigated. Ammonium in the feed first transported across the cation exchange membrane and accumulated in the receiver chamber. It is then deprotonated as ammonia, passing through the gas permeable membrane and finally is fixed as ammonium salt in the acid chamber. Our results indicated that employing waste alkali (red mud leachate) as driving solution led to excellent ammonium recovery performances (recovery efficiency of >80%), comparable to those of NaOH solution. When the initial ammonium concentration was 5 and 50 mM, the waste alkali driven DD-OD process achieved acceptable NH4+-N flux density of 16.8 and 169 g N m-2 d-1, at energy cost as low as 8.38 and 2.06 kWh kg-1 N, respectively. Since this alkali driven DD-OD hybrid process is based on solute concentration (or partial pressure) gradient, it could be an energy-effective technology capable of treating wastewaters containing ammonium using waste alkali to realize nutrients recovery in a sustainable manner.

Effect of microplastics on ecosystem functioning: Microbial nitrogen removal mediated by benthic invertebrates.

While ecotoxicological impacts of microplastics on aquatic organisms have started to be investigated recently, impacts on ecosystem functions mediated by benthic biota remain largely unknown. We investigated the effect of microplastics on nitrogen removal in freshwater sediments where microorganisms and benthic invertebrates (i.e., chironomid larvae) co-existed. Using microcosm experiments, sediments with and without invertebrate chironomid larvae were exposed to microplastics (polyethylene) at concentrations of 0, 0.1, and 1 wt%. After 28 days of exposure, the addition of microplastics or chironomid larvae promoted the growth of denitrifying and anammox bacteria, leading to increased total nitrogen removal, in both cases. However, in microcosms with chironomid larvae and microplastics co-existing, nitrogen removal was less than the sum of their individual effects, especially at microplastics concentration of 1 wt%, indicating an adverse effect on microbial nitrogen removal mediated by macroinvertebrates. This study reveals that the increasing concentration of microplastics entangled the nitrogen cycling mediated by benthic invertebrates in freshwater ecosystems. These findings highlight the pursuit of a comprehensive understanding of the impacts of microplastics on the functioning in freshwater ecosystems.

Meiofauna promotes litter decomposition in stream ecosystems depending on leaf species.

Litter decomposition, a fundamental process of nutrient cycling and energy flow in freshwater ecosystems, is driven by a diverse array of decomposers. As an important component of the heterotrophic food web, meiofauna can provide a trophic link between leaf-associated microbes (i.e., bacteria and fungi)/plant detritus and macroinvertebrates, though their contribution to litter decomposition is not well understood. To investigate the role of different decomposer communities in litter decomposition, especially meiofauna, we compared the litter decomposition of three leaf species with different lignin to nitrogen ratios in litter bags with different mesh sizes (0.05, 0.25, and 2 mm) in a forested stream, in China for 78 days. The meiofauna significantly enhanced the decomposition of leaves of high-and medium- quality, while decreasing (negative effect) or increasing (positive effect) the fungal biomass and diversity. Macrofauna and meiofauna together contributed to the decomposition of low-quality leaf species. The presence of meiofauna and macrofauna triggered different aspects of the microbial community, with their effects on litter decomposition varying as a function of leaf quality. This study reveals that the meiofauna increased the trophic complexity and modulated their interactions with microbes, highlighting the important yet underestimated role of meiofauna in detritus-based ecosystems.

Beneficial CNT Intermediate Layer for Membrane Fluorination toward Robust Superhydrophobicity and Wetting Resistance in Membrane Distillation.

Robust membrane hydrophobicity is crucial in membrane distillation (MD) to produce clean water, yet challenged by wetting phenomenon. We herein proposed a robust superhydrophobization process, by making use of a carbon nanotube (CNT) intermediate layer over commercial hydrophobic membrane, indirectly grafting the low-surface-energy material 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FAS), with the achieved membrane denoted as PVDF-CNT-FAS, in systematic comparison with direct grafting FAS on alkalinized PVDF denoted as PVDF-OH-FAS. Superhydrophobicity with water contact angle of 180° was easily achieved from initial hydrophilic interface for both two resultant membranes. Interestingly, the existence of a CNT intermediate layer significantly maintained the stable hydrophobicity in various harsh conditions and improved mechanical properties, at an expense of ca. 20% smaller pore size and extended membrane thickness than PVDF-OH-FAS. In the MD experiment, the PVDF-CNT-FAS exhibited no vapor flux sacrifice, giving constant flux with the control and doubled that for PVDF-OH-FAS. A mass-heat transfer modeling suggested no significant heat loss but facilitated vapor flux with the CNT layer, unlike the impeded transfer for the counterpart membrane. A superior wetting resistance against 0.4 mM SDS further confirmed the benefit of constructing the CNT intermediate layer, presumably because of its excellent slippery property. This study demonstrates the important role of the CNT intermediate layer toward robust superhydrophobic membrane, suggesting the interest of applying the functional nanomaterial for controllable interface design.

Dam operation altered profiles of per- and polyfluoroalkyl substances in reservoir.

Information on the impact of dam operation on per- and polyfluoroalkyl substances (PFASs) distribution in reservoirs is very limited. In the present study, water, riparian soils and floating wastes samples were collected from the Three Gorges Reservoir, China during the storage and the discharge periods to characterize the PFASs distribution. The total PFASs concentrations of water samples in the storage period (50.4-146 ng/L) were 4.7 times higher than those in the discharge period (1.40-38.6 ng/L). The main types of PFASs in water samples changed from PFOA in the discharge period to short-chain species in the storage period. The main analogues in riparian soils and floating wastes were PFOA and PFOS. Wastes contributed little to PFASs mass in the reservoir, while PFASs accumulated in soils accounted for 49.7 % of the total mass when the riparian zone was submerged during the storage period. Changes in profiles of PFASs caused by dam operation suggested that the potential water safety and the shift of riparian soils between source and sink of PFASs may vary with the annual operation cycle of dam. The water resources protection in reservoirs needs strategies that consider the variation of dam operation cycle.

Combined effects of elevated carbon dioxide and temperature on phytoplankton-zooplankton link: A multi-influence of climate change on freshwater planktonic communities.

It is essential to understand the combined effects of elevated CO2 and temperature on phytoplankton-zooplankton link when attempting to predict climate change responses of freshwater ecosystems. Phytoplankton species differ in stoichiometric and fatty acids composition, and this may result in phytoplankton-mediated effect on zooplankton at elevated CO2 and temperature. Beyond the isolated analysis of CO2 or temperature effect, few studies have assessed zooplankton growth under the phytoplankton-mediated effects of elevated CO2 and temperature. In this study, three algal species (green alga, diatom, cyanobacteria) were fed on zooplankton Daphnia magna, under the conditions of CO2 concentrations of ambient (390 ppm) and elevated (1000 ppm) levels and temperatures at 20, 25 and 30 °C. Elevated CO2 increased the algal biomass, while it reduced the phosphorus (P) and ω3 polyunsaturated fatty acids (ω3 PUFAs) to carbon (C) ratios. Elevated temperature decreased the P/C ratios in all algal cultures and ω3 PUFAs/C ratios in the diatom and the cyanobacteria cultures. Phytoplankton-mediated effect of elevated CO2 reduced the growth of zooplankton fed on the green and the mixed three algae culture. The stimulation of zooplankton fed on the diatom and the cyanobacteria by elevated temperature can be offset by decreasing food P and ω3 PUFAs contents. The combined effects of elevated CO2 and temperature on the growth of daphnids were mainly mediated by ω3 PUFAs/C ratios in the phytoplankton. Rising temperature as a combined direct and indirectly phytoplankton-mediated effect on zooplankton may be able to ameliorate the negative effects of elevated CO2. The results indicated that the combined effects of increased CO2 and temperature increased the fatty acid content of the green alga but not the other algae. This study highlighted that climate change with simultaneously increasing temperature and CO2 may entangle the carbon transfer in freshwater planktonic communities.