Salinity Gradient Research Articles

Phytoplankton retention mechanisms in estuaries: a case study of the Elbe estuary

Abstract. Due to their role as primary producers, phytoplankton are essential to the productivity of estuarine ecosystems. However, it is important to understand how these nearly passive organisms are able to persist within estuaries when river inflow results in a net outflow to the ocean. Estuaries also represent challenging habitats due to a strong salinity gradient. Little is known about how phytoplankton are able to be retained within estuaries. We present a new individual-based Lagrangian model of the Elbe estuary which examines possible retention mechanisms for phytoplankton. Specifically, we investigated how reproduction, sinking and rising, and diel vertical migration may allow populations to persist within the estuary. We find that vertical migration, especially rising, favors retention, while fast sinking does not. We further provide first estimates of outwashing losses. Our simulations illustrate that riverbanks and tidal flats are essential for the long-term survival of phytoplankton populations, as they provide refuges from strong downstream currents. These results contribute to the understanding needed to advance the ecosystem-based management of estuaries.

Dietary reconstruction and influencing factors of oysters cultured in a typical estuarine bay of South China

Blue carbon can be transferred from primary producers to primary consumers through oyster filter feeding. Understanding the carbon (food) sources is crucial for understanding coastal food webs and management of oyster aquaculture ecosystems. In the present study, Bayesian stable isotope mixing models (MixSIAR) were used to identify and quantify food composition within a cultured oyster ecosystem at six sites along a salinity gradient in the estuarine bay of Zhenhai (ZHB). The carbon stable isotope (δ13C) signatures of oyster Crassostrea hongkongensis ranged from −22.06 to −27.27‰, with significant spatial differences (p < 0.001), indicating variations in food resources along the salinity gradient. The primary food source for cultured oysters was suspended particle organic matter across ZHB (55.7–90.6%), with sedimentary organic matter contributing more in the mid-estuary (28.9–44.3%). Partial least squares regression (PLSR) was used to explore the effects of habitat environment and human disturbance on spatial variation in oyster diets. The results indicated that dissolved oxygen, sediment particle size, elevation, and mangrove-related landscape indices were the main factors influencing oyster diet in ZHB. The findings provide insights into the dietary composition of cultured oysters and the underlying factors in a typical shallow bay in southern China, with particular emphasis on the role of terrestrial landscape features. In addition, the findings validate PLSR as a reasonable tool for predicting the food composition of consumers in estuarine bays.

Sustainable energy recovery from salt-lake brines through a novel selective reverse electrodialysis process

The salinity gradient energy (SGE) in salt-lake brines can be exploited as a sustainable energy source through reverse electrodialysis (RED). However, the composition of brines is notably intricate. In order to avoid the adverse effects of multivalent cations on RED in brines, we introduced a novel selective reverse electrodialysis (SRED) here. The results demonstrate that when increasing the salinity ratio between high and low concentration feeds (HCFs, LCFs) from 20 to 60, the SRED maximal maximum power density (Pd,max) improves from 0.069 to 0.109 W/m2. The feed salinity has less obvious effect on SRED. Moreover, the role of Mg2+ on the adverse effects is regulated. The maximal Pd,max of SRED is 1.13 ∼ 1.23 times higher than that of the conventional RED at the Mg2+ concentrations of 4861.00 ∼ 14583.00 ppm in HCFs and 40.51 ∼ 121.53 ppm in LCFs. This suggests that SRED is a feasible and efficient strategy for capturing SGE in salt-lake brines.

Augmentation of heat storage capacity of compact salinity gradient solar pond using dragon fruit extract coated magnesium powder and paraffin wax

Compact salinity gradient solar ponds are gaining recognition as a source of heat storage medium in densely populated areas and small residences around the globe. However, their efficiency is limited because of the small capacity of saline water and lower potential. In this study, a compact salinity gradient solar pond was developed to enhance heat storage efficiency. This experimental study investigates the effect of incorporating a novel phase change medium composed of paraffin wax infused with dragon fruit extract-coated magnesium oxide powder to enhance the heat storage capability of the compact salinity gradient solar pond. The dragon fruit extract is utilised as a natural additive to enhance the thermal properties of the magnesium oxide powder. This improves the heat storage capability of the phase change medium and provides eco-friendliness to the system. The experiment allowed heat addition and rejection from the saline water through natural convection mode. After coating the MgO nanopowders, the peak temperature in the lower convective zone of the compact salinity gradient solar pond was 51.2 ℃ on the seventh day. The efficiency of the solar pond increased when the previous day exhibited a drop in solar intensity. It showed a maximum efficiency of 35 %. An optimisation study revealed that the solar pond efficiency can be 28.16%. It is concluded that the heat stored in the compact salinity gradient solar pond increases because of the periodic heating of the saline water and the presence of the heat storage medium.

Light-responsive and ultrapermeable two-dimensional metal-organic framework membrane for efficient ionic energy harvesting

Nanofluidic membranes offer exceptional promise for osmotic energy conversion, but the challenge of balancing ionic selectivity and permeability persists. Here, we present a bionic nanofluidic system based on two-dimensional (2D) copper tetra-(4-carboxyphenyl) porphyrin framework (Cu-TCPP). The inherent nanoporous structure and horizontal interlayer channels endow the Cu-TCPP membrane with ultrahigh ion permeability and allow for a power density of 16.64 W m−2, surpassing state of-the-art nanochannel membranes. Moreover, leveraging the photo-thermal property of Cu-TCPP, light-controlled ion active transport is realized even under natural sunlight. By combining solar energy with salinity gradient, the driving force for ion transport is reinforced, leading to further improvements in energy conversion performance. Notably, light could even eliminate the need for salinity gradient, achieving a power density of 0.82 W m−2 in a symmetric solution system. Our work introduces a new perspective on developing advanced membranes for solar/ionic energy conversion and extends the concept of salinity energy to a notion of ionic energy.

PET-hydrogel heterogeneous membranes that eliminate concentration polarization for salinity gradient power generation

Salinity gradient (blue) energy has been identified as a promising power source and many technologies have been proposed for its recovery, one of which is nanopore power generators. The single-pore/channel membranes selectively transport ions to achieve efficient salinity gradient power generation. However, porous membranes fail due to concentration polarization. Here, we constructed a heterogeneous membrane by covering ion-selective hydrogel on a polyethylene terephthalate (PET) cylindrical nanochannel porous membrane for the first time to eliminate concentration polarization. The gel layer enhances the ion selectivity of cylindrical nanochannels, making heterogeneous membrane present the ion current rectification effect. The gel layer also avoids the negative impact of concentration polarization on the power generation of porous membranes. The results show that PET porous membrane can hardly generate electricity, but the power density of PET-hydrogel heterogeneous membrane can reach 1.92 W/m2 under 50 times salinity gradient, which is superior to commercial ion exchange membranes. Our work provides a strategy (ion-selective hydrogel covering) to eliminate concentration polarization of porous membranes, and shows the prospect of cylindrical nanochannels for salinity gradient power generation.

Ultra-Thin Ion Exchange Membranes by Low Ionomer Blending for Energy Harvesting.

Exploring the utilization of ion exchange membranes (IEMs) in salinity gradient energy harvesting, a technique that capitalizes on the salinity difference between seawater and freshwater to generate electricity, this study focuses on optimizing PVDF to Nafion ratios to create ultra-thin membranes. Specifically, our investigation aligns with applications such as reverse electrodialysis (RED), where IEMs facilitate selective ion transport across salinity gradients. We demonstrate that membranes with reduced Nafion content, particularly the 50:50 PVDF:Nafion blend, retain high permselectivity comparable to those with higher Nafion content. This challenges traditional understandings of membrane design, highlighting a balance between thinness and durability for energy efficiency. Voltage-current analyses reveal that, despite lower conductivity, the 50:50 blend shows superior short-circuit current density under salinity gradient conditions. This is attributed to effective ion diffusion facilitated by the blend's unique microstructure. These findings suggest that blended membranes are not only cost-effective but also exhibit enhanced performance for energy harvesting, making them promising candidates for sustainable energy solutions. Furthermore, these findings will pave the way for advances in membrane technology, offering new insights into the design and application of ion exchange membranes in renewable energy.

Spatial and seasonal variability of dissolved metals in a monsoonal estuarine environment

Distribution of metals in estuarine surface water reflects their respective sources and abundances in the fresh- and marine- water end-members, mixing processes and ability to take part in various geochemical reactions prevailing in the transitional zone. However, these elements tend to show complex geochemical behaviour, which varies both within and among different estuaries. This makes it difficult to delineate a universal distribution pattern. In this study, the behaviour of multiple dissolved metals (Sr, Ba, Fe, Mn, Mo, V, Co, Cr, Cu, Ni, Zn, U and Pb) have been evaluated in a tropical monsoonal estuary located on the east coast of India during summer (May, 2018), monsoon (September, 2018) and winter (February, 2019) season. Concentration of Sr, Mo, V, Cu and U increases towards the mouth of the estuary, whereas, Ba, Fe, Mn, Pb and Cr concentration shows a declining trend along increasing salinity gradient. All the elements show non-conservative behaviour in the estuary during at least one season within the studied period. Rapid removal of Fe and Mn takes place in the low salinity region, whereas, Mo, V, Pb and U get removed in the low- to mid- salinity zone of the estuary as a result of salinity-induced flocculation. A pronounced mid-estuarine maxima is exhibited by Ni and Zn. Barium shows peak concentrations in the low salinity region. Spatial variation in the occurrence of Ba maxima along the salinity gradient exists between the low and high discharge periods. Seasonally, highest concentration of Sr, Mo, V and U are found in the summer, followed by winter and monsoon. On the other hand, Ba, Fe, Mn, Co, Cr, Cu, Ni and Zn exhibit the highest concentrations during the monsoon period as a result of increased inputs from fluvial sources. The intrusion of seawater during the dry seasons and dilution during monsoon, which shows the highest freshwater discharge, are responsible for the seasonal variations. Thus, this study highlights the variability in distribution of elements as well as the differences in the behaviour of individual elements under contrasting environmental conditions within an estuary.

Ternary donor/acceptor system for simple single-emitting-layer white organic light emitting diodes based pure exciplexes emission

Herein, we designed pure exciplex emission, white organic light emitting diodes (WOLED) with simple single-emitting-layer (S-EML) structure through a ternary donor/acceptor system. Three kinds of donor/acceptor materials in one emitting layer of mCP:PO-T2T(1:1): x% m-MTDATA or TAPC:TPBi (1:1): x% PO-T2T could produce the white emission with high energy blue and low energy orange exciplex emission. As a result, the pure exciplex WOLED based on S-EML structure were achieved successfully with the optimized maximum luminance of 4047 cd m−2, current efficiency of 10.26 cd A−1, power efficiency of 10.57 lm W−1 and EQE of 3.4% with the two complementary color exciplex emissions. To our knowledge, this is the first time to achieve a S-EML structure pure exciplex emission WOLED. The efficient energy transfer from high energy exciplex to low energy exciplex could be also observed in this ternary system.

Surfactant-polymer flood with seawater for a high temperature carbonate reservoir

Developing ultralow tension surfactant formulations for carbonate reservoirs at high temperatures (> 100 °C) in the presence of hard brine is challenging. The target carbonate reservoir had been flooded with seawater and changing of the injection brine (softening or low salinity) for enhanced oil recovery was considered expensive. Seawater was presumed to be the injection brine for this study. The goal of this research work was to develop an ultralow tension surfactant-polymer (SP) formulation with seawater for this carbonate reservoir at 115 °C and to evaluate core floods with and without salinity gradient. Phase behavior was studied for many surfactants and ultralow IFT formulations were identified with the optimal salinity near the seawater salinity. SP core floods were conducted in reservoir sand packs at 115 °C; a novel 2-oven technique was used to collect the effluent with no wax formation and minimal hydrocarbon vaporization. The residual oil saturation was reduced to less than 1% in floods with salinity gradient and about 5% in constant salinity floods (at seawater salinity). The novelty of the work lies in developing SP formulations with seawater at a high temperature and studying the effect of salinity gradient. This process can be considered for many off-shore fields which have been waterflooded with seawater.

Tidal Intrusion Fronts, Surface Convergence, and Mixing in an Estuary with Complex Topography

Abstract Observations from a tidal estuary show that tidal intrusion fronts occur regularly during flood tides near topographic features including constrictions and bends. A realistic model is used to study the generation of these fronts and their influence on stratification and mixing in the estuary. At the constriction, flow separation occurs on both sides of the jet flow downstream of the narrow opening, leading to sharp lateral salinity gradients and baroclinic secondary circulation. A tidal intrusion front, with a V-shaped convergence zone on the surface, is generated by the interaction between secondary circulation and the jet flow. Stratification is created at the front due to the straining of lateral salinity gradients by secondary circulation. Though stratification is expected to suppress turbulence, strong turbulent mixing is found near the surface front. The intense mixing is attributed to enhanced vertical shear due to both frontal baroclinicity and the twisting of lateral shear by secondary circulation. In the bend, flow separation occurs along the inner bank, resulting in lateral salinity gradients, secondary circulation, frontogenesis, and enhanced mixing near the front. In contrast to the V-shaped front at the constriction, an oblique linear surface convergence front occurs in the bend, which resembles a one-sided tidal intrusion front. Moreover, in addition to baroclinicity, channel curvature also affects secondary circulation, frontogenesis, and mixing in the bend. Overall in the estuary, the near-surface mixing associated with tidal intrusion fronts during flood tides is similar in magnitude to bottom boundary layer mixing that occurs primarily during ebbs.

Coupled Stable Isotope Tracing and Sulfamic Acid Reduction (SIT-SAR) Method to Determine the Ammonia and Nitrite Oxidation Rates in Water and Sediments.

Herein, a method was developed to measure the ammonia oxidation rate (Ra) and the nitrite oxidation rate (Rn) of water and sediment samples using a coupled stable isotope tracing and sulfamic acid reduction (SIT-SAR) method. 15NH4+ was used as a tracer to determine the ammonia oxidation rates (Ra) by calculating the concentrations of produced 15NO2- and 15NO3- during incubation, while 15NO2- was used as a tracer to determine the nitrite oxidation rates (Rn) by calculating the increase of 15NO3- during incubation. 15NO2- was chemically reduced to 29N2 with 15 mmol·L-1 sulfamic acid (SA). 15NO3- was first reduced to 15NO2- with a zinc-cadmium reducing agent, and then 15NO2- was subsequently reduced to 29N2 with SA. The produced 29N2 was measured by a membrane inlet mass spectrometer (MIMS). Under optimized experimental conditions, this method provides a sensitive (detection limit: 0.5 μmol·L-1) and precise (relative standard deviation: 4.80% for 15NO2-, 3.82% for 15NO3-) approach to quantify the concentrations of 15NO2- (0.5-150 μmol·L-1) and 15NO3- (0.5-120 μmol·L-1) in water and sediment samples over a wide range of salinities (0-30‰) with excellent calibration curves (R2 ≥ 0.999). This method was a successful application to estuarine water and sediments along the salinity gradient. Overall, the SIT-SAR method provided a rapid, accurate, and cost-effective means to determine Ra and Rn simultaneously.

Evaluating top-down, bottom-up, and environmental drivers of pelagic food web dynamics along an estuarine gradient.

Identification of the key biotic and abiotic drivers within food webs is important for understanding species abundance changes in ecosystems, particularly across ecotones where there may be strong variation in interaction strengths. Using structural equation models (SEMs) and four decades of integrated data from the San Francisco Estuary, we investigated the relative effects of top-down, bottom-up, and environmental drivers on multiple trophic levels of the pelagic food web along an estuarine salinity gradient and at both annual and monthly temporal resolutions. We found that interactions varied across the estuarine gradient and that the detectability of different interactions depended on timescale. For example, for zooplankton and estuarine fishes, bottom-up effects appeared to be stronger in the freshwater upstream regions, while top-down effects were stronger in the brackish downstream regions. Some relationships (e.g., bottom-up effects of phytoplankton on zooplankton) were seen primarily at annual timescales, whereas others (e.g., temperature effects) were only observed at monthly timescales. We also found that the net effect of environmental drivers was similar to or greater than bottom-up and top-down effects for all food web components. These findings can help identify which trophic levels or environmental factors could be targeted by management actions to have the greatest impact on estuarine forage fishes and the spatial and temporal scale at which responses might be observed. More broadly, this study highlights how environmental gradients can structure community interactions and how long-term data sets can be leveraged to generate insights across multiple scales.

Plasma‐oxidized 2D MXenes subnanochannel membrane for high‐performance osmotic energy conversion

AbstractNanofluidic channels inspired by electric eels open a new era of efficient harvesting of clean blue osmotic energy from salinity gradients. Limited by less charge and weak ion selectivity of the raw material itself, energy conversion through nanofluidic channels is still facing considerable challenges. Here, a facile and efficient strategy to enhance osmotic energy harvesting based on drastically increasing surface charge density of MXenes subnanochannels via oxygen plasma is proposed. This plasma could break Ti–C bonds in the MXenes subnanochannels and effectively facilitate the formation of more Ti–O, C═O, O–OH, and rutile with a stronger negative charge and work function, which leads the surface potential of MXenes membrane to increase from 205 to 430 mV. This significant rise of surface charge endows the MXenes membrane with high cation selectivity, which could make the output power density of the MXenes membrane increase by 248.2%, reaching a high value of 5.92 W m−2 in the artificial sea‐river water system. Furthermore, with the assistance of low‐quality heat at 50°C, the osmotic power is enhanced to an ultrahigh value of 9.68 W m−2, which outperforms those of the state‐of‐the‐art two‐dimensional (2D) nanochannel membranes. This exciting breakthrough demonstrates the enormous potential of the facile plasma‐treated 2D membranes for osmotic energy harvesting.

A Review of Polymer-Based Environment-Induced Nanogenerators: Power Generation Performance and Polymer Material Manipulations.

Natural environment hosts a considerable amount of accessible energy, comprising mechanical, thermal, and chemical potentials. Environment-induced nanogenerators are nanomaterial-based electronic chips that capture environmental energy and convert it into electricity in an environmentally friendly way. Polymers, characterized by their superior flexibility, lightweight, and ease of processing, are considered viable materials. In this paper, a thorough review and comparison of various polymer-based nanogenerators were provided, focusing on their power generation principles, key materials, power density and stability, and performance modulation methods. The latest developed nanogenerators mainly include triboelectric nanogenerators (TriboENG), piezoelectric nanogenerators (PENG), thermoelectric nanogenerators (ThermoENG), osmotic power nanogenerator (OPNG), and moist-electric generators (MENG). Potential practical applications of polymer-based nanogenerator were also summarized. The review found that polymer nanogenerators can harness a variety of energy sources, with the basic power generation mechanism centered on displacement/conduction currents induced by dipole/ion polarization, due to the non-uniform distribution of physical fields within the polymers. The performance enhancement should mainly start from strengthening the ion mobility and positive/negative ion separation in polymer materials. The development of ionic hydrogel and hydrogel matrix composites is promising for future nanogenerators and can also enable multi-energy collaborative power generation. In addition, enhancing the uneven distribution of temperature, concentration, and pressure induced by surrounding environment within polymer materials can also effectively improve output performance. Finally, the challenges faced by polymer-based nanogenerators and directions for future development were prospected.

Unraveling Flow Effect on Capacitive Energy Extraction from Salinity Gradients.

The harvesting of salinity gradient energy through a capacitive double-layer expansion (CDLE) technique is directly associated with ion adsorption and desorption in electrodes. Herein, we show that energy extraction can be modulated by regulating ion adsorption/desorption through water flow. The flow effects on the output energy, capacitance, and energy density under practical conditions are systematically investigated from a theoretical perspective, upon which the optimal operating condition is identified for energy extraction. We demonstrate that the net charge accumulation displays a negative correlation with the water flow velocity and so does the surface charge density, and this causes a nontrivial variation in the magnitude of output energy when water flows are introduced. When high water flows are introduced in both the charging and discharging processes, the energy extraction can be significantly reduced by 47.69-49.32%. However, when a high flow is solely exerted in the discharging process, the energy extraction can be enhanced by 12.94-14.49% even at low operation voltages. This study not only offers a comprehensive understanding of the microscopic mechanisms of surface-engineered energy extraction with water flows but also provides a novel direction for energy extraction enhancement.

Comparative root transcriptome analysis of Kandelia candel Druce and Rhizophora mucronata Lam. germinating propagules under salinity gradients reveal their tolerance mechanisms and ecological adaptations

Comparative root transcriptome analysis of Kandelia candel Druce and Rhizophora mucronata Lam. germinating propagules under salinity gradients reveal their tolerance mechanisms and ecological adaptations

Fluid Oscillation‐Driven Bi‐Directional Air Turbine Triboelectric Nanogenerator for Ocean Wave Energy Harvesting

AbstractOcean wave energy is one of the important renewable energy sources. However, random, low‐frequency, and micro‐amplitude characteristics of ocean waves make it difficult for traditional electromagnetic generators to collect wave energy efficiently. The emergence of triboelectric nanogenerator (TENG) provides an extremely effective technical means for the collection of low‐frequency and micro‐amplitude wave energy. In this work, a non‐contact turntable‐structured oscillating water column TENG (OWC‐TENG) is designed by combining an OWC wave energy conversion mechanism with a TENG for the first time. The OWC is optimized through simulation experiments according to the principles of ship hydrostatics and wave theory. The output performances of the OWC‐TENGs with different structural parameters under different water wave excitation conditions are then tested. The TENG delivers a maximum output current of 55.45 µA and an output power of 5.28 mW, which corresponds to a power density of 114.8 W m−3, enabling a stable power supply for small sensors at sea. This work provides a new solution to the efficient collection of low‐frequency micro‐amplitude ocean wave energy for powering various offshore equipments, presenting broad application prospects in ocean blue energy development and offshore Internet of Things.

Efficient self-powered cathodic corrosion protection system based on multi-layer grid synergistic triboelectric nanogenerator and power management circuits

The last few years have seen a boom in the use of triboelectric nanogenerators (TENGs) for capturing wave energy in the ocean. This work reports a multi-layer grid synergistic triboelectric nanogenerator (MLGS-TENG). In this work, the independent layer triboelectric nanogenerator in the grid format is skillfully employed, and the design of the multi-layer parallel structure can effectively increase the capture rate of blue energy in the ocean. The shell of the triboelectric nanogenerator adopts a gyroscopic structure, which, when combined with its internal architecture, operates effectively on the surface of ocean waves. Moreover, it can be very effective in harvesting low-frequency wave energy in the ocean and converting it into mechanical energy, and then converting it into electrical energy through the friction of the internal self-assembly layer. In low-frequency mechanical tests, when the frequency is about 1 Hz and the load impedance is 400 MΩ, the peak power output of the generator in a single layer can reach 77 mW/m2. Given the excellent energy harvesting performance of MLGS-TENG, it is applied to build a high-efficiency cathodic corrosion protection system in conjunction with a buck rectifier circuit. This work is of great significance for the wide application of triboelectric nanogenerators in the marine environment and provides a feasible way to ensure the long-term stability of offshore platforms.

Two-dimensional metal–organic framework nanocomposite membranes with shortened ion pathways for enhanced salinity gradient power harvesting

Harnessing energy through the reverse electrodialysis-based salinity gradient power conversion is an exciting and promising technological endeavor. The key to its success lies in developing the advanced ion-selective membrane capable of enhancing ion flux and accelerating ion transport at confined nanospaces. Traditional two-dimensional (2D) membranes typically employ a layer-by-layer arrangement of nanosheets with an extremely narrow layer separation, limiting ion flux and creating elongated 2D ion pathways. In response to this limitation, our study introduces an innovative design of the 2D metal–organic framework (MOF)-based nanocomposite membrane (named as Cu-TCPP@SNF), which is composed of the 2D Cu-TCPP and natural-based silk nanofibers (SNFs). We show that the introduction of the space-charged SNFs can significantly enhance the stability of the 2D MOF nanocomposite membrane in electrolyte solutions, and this unique framework facilitates a multitude of abundant one-dimensional ion pathways across the membrane while maintaining the advantage of minimal layer separation. Our results demonstrate that the Cu-TCPP@SNF exhibits low resistance, thus facilitating ion transport. Furthermore, numerical simulations elucidate the impacts of the SNF content and membrane thickness on power performance. Remarkably, our membrane achieves a notable power density of ∼6.48 W/m2 by mixing artificial seawater and river water. Impressively, an ultrahigh performance of ∼29.5 W/m2 can be achieved under a 5 M/0.01 M NaCl gradient, surpassing the existing advanced membranes. These findings hold significant promise for the future of composite 2D membrane development, paving the way for scalable applications in salinity-based energy harvesting.