Ions In Seawater Research Articles

Long‐term Durability of Seawater Electrolysis for Hydrogen: From Catalysts to Systems

AbstractDirect electrochemical seawater splitting is a renewable, scalable, and potentially economic approach for green hydrogen production in environments where ultra‐pure water is not readily available. However, issues related to low durability caused by complex ions in seawater pose great challenges for its industrialization. In this review, a mechanistic analysis of durability issues of electrolytic seawater splitting is discussed. We critically analyze the development of seawater electrolysis and identify the durability challenges at both the anode and cathode. Particular emphasis is given to elucidating rational strategies for designing electrocatalysts/electrodes/interfaces with long lifetimes in realistic seawater including inducing passivating anion layers, preferential OH−adsorption, employing anti‐corrosion materials, fabricating protective layers, immobilizing Cl− on the surface of electrocatalysts, tailoring Cl− adsorption sites, inhibition of OH− binding to Mg2+ and Ca2+, inhibition of Mg and Ca hydroxide precipitation adherence, and co‐electrosynthesis of nano‐sized Mg hydroxides. Synthesis methods of electrocatalysts/electrodes and innovations in electrolyzer are also discussed. Furthermore, the prospects for developing seawater splitting technologies for clean hydrogen generation are summarized. We found that researchers have rethought the role of Cl− ions, as well as more attention to cathodic reaction and electrolyzers, which is conducive to accelerate the commercialization of seawater electrolysis.

Pyrite Depression by Sodium Metabisulfite in Freshwater and Seawater with Copper Activation

Pyrite has natural floatability and thus readily enters valuable mineral flotation concentrates, diluting their quality and decreasing their economic value. Its separation is challenging, depending on process conditions, the presence of activating ions, and water quality, particularly in regions where seawater is used. This study examines the effect of various doses of sodium metabisulfite (SMBS) on pyrite depression in freshwater and seawater under weakly alkaline conditions and with different copper ion concentrations. Without the addition of activators or depressants, pyrite recovery is 40% in freshwater and 60% in seawater, whereas with 10 ppm of SMBS, recoveries drop to 28% and 38%, respectively. The addition of 10 ppm Cu2+ increases recovery by 10% in freshwater and by 20% in seawater. In the presence of 75 ppm of PAX, maximum recovery reached 50% in freshwater and 80% in seawater. These results suggest that cationic bridges formed by seawater ions, combined with CaOH+ activation, play a critical role in pyrite activation, even in the presence of depressants.

The Influence of Seawater on Fe(II)-Catalyzed Ferrihydrite Transformation and Its Subsequent Consequences for C Dynamics.

Short-range-ordered minerals like ferrihydrite often bind substantial organic carbon (OC), which can be altered if the minerals transform. Such mineral transformations can be catalyzed by aqueous Fe(II) (Fe(II)aq) in redox-dynamic environments like coastal wetlands, which are inundated with seawater during storm surges or tidal events associated with sea-level rise. Yet, it is unknown how seawater salinity will impact Fe(II)-catalyzed ferrihydrite transformation or the fate of bound OC. We reacted ferrihydrite with Fe(II)aq under anoxic conditions in the absence and presence of dissolved organic matter (DOM). We compared treatments with no salts (DI water), NaCl-KCl salts, and artificial seawater mixes (containing Ca and Mg ions) with or without SO42-/HCO3-. Both XRD and Mössbauer showed that NaCl-KCl favored lepidocrocite formation, whereas Ca2+/Mg2+/SO42-/HCO3- ions in seawater overrode the effects of NaCl-KCl and facilitated goethite formation. We found that the highly unsaturated and phenolic compounds (HuPh) of DOM selectively bound to Fe minerals, promoting nanogoethite formation in seawater treatments. Regardless of salt presence, only 5-9% of Fe-bound OC was released during ferrihydrite transformation, enriching HuPh relative to aliphatics in solution. This study offers new insights into the occurrence of (nano)goethite and the role of Fe minerals in OC protection in coastal wetlands.

Protection of Fe Single‐Atoms by Fe Clusters for Chlorine‐Resistant Oxygen Reduction Reaction

AbstractDirect seawater zinc‐air batteries (S‐ZABs), with their inherent properties of high energy density, intrinsic safety, and low cost, present a compelling avenue for the development of energy storage technology. However, the presence of chloride ions in seawater poses challenges to their air electrode, resulting in sluggish reaction kinetics and poor stability for the oxygen reduction reaction (ORR). Herein, Fe atomic clusters (ACs) decorated Fe single‐metal atoms (SAs) catalyst (FeSA‐FeAC/NC) is prepared using a plasma treatment strategy. The aberration‐corrected transmission electron microscope images confirm the successful construction of Fe SAs surrounding Fe ACs, which delivers robust ORR activity with a half‐wave potential of 0.886 V in alkaline seawater electrolyte. When utilized as the cathode catalyst in assembled S‐ZABs, the battery demonstrates excellent discharge performance, achieving a peak power density of 222 mW cm−2, which is 2.3 times higher than Pt/C based S‐ZABs. Moreover, density functional theory calculations unveil that the synergy effect between the Fe ACs and SAs not only reduces the spin‐down d‐band center in the Fe SAs, but also efficiently suppresses Cl−1 adsorption on Fe SAs due to their strong Cl−1 absorption ability of the Fe ACs, thereby enhancing both the activity and stability of the catalyst.

Controlled synthesis of the M doped (M = Fe, Cu, Zn, Mn)-Co4S3/Ni3S2 catalyst with high electrocatalytic performance for hydrogen evolution reaction in seawater and urea

Electrolysis of seawater is a promising method for producing hydrogen, but sluggish reaction kinetics and electrolyte containing corrosive ions limit its development. In this work, a series of M doped (M = Fe, Cu, Zn, Mn)-Co4S3/Ni3S2 catalyst has been prepared on Ni foam by hydrothermal method with excellent seawater and urea splitting performance. The unique 3D porous structure provides a large number of active sites for the catalyst, which has strong catalytic activity. Fe-Co4S3/Ni3S2 electrode showed excellent hydrogen evolution reaction (HER) catalytic activity in 1 M KOH + seawater and 1 M KOH + 0.5 M urea, driving a current density of 10 mA cm−2 at a low overpotential of 81 and 66 mV. In addition, Fe-Co4S3/Ni3S2 electrode also shows strong stability in 15 h stability test. The results show that although the impurity ions in seawater affect the activity of catalyst, the corrosion resistance of catalyst also improves the catalytic activity and stability. Density functional theory calculations show that this Fe-Co4S material exhibits the optimal Gibbs free energy of hydrogen, the introduction of this Ni3S2 material enhances the electrical conductivity of the material, and the synergistic catalysis of the two materials promotes the optimal hydrogen production performance of the Fe-Co4S3/Ni3S2 material. This work provides a novel understanding for the research of catalysts used in the electrolysis of seawater and urea.

Mechanical and electrochemical characterization of CuAlNi alloys

The effect of copper composition on the structure and mechanical properties of CuxAlNi alloys was investigated using MD simulation and characterization methods. It was found that the structure of CuxAlNi alloys markedly resembles Cu composition, which alterations from the initial single (FCC) to (BCC) structure and then to a duplex BCC structure as the Cu content is raised. Nanoindentation measurements show that the hardness of CuAlNi alloys increases with Cu content. When there are more Al elements, the surface of the material is first combined with ions in seawater so that the corrosion potential is significantly reduced. This research seeks to identify CuAlNi alloys with improved properties through molecular dynamics simulations and experimental analyses, highlighting the connections between microstructure and mechanical behavior.

NiP/Fe3P/MoP@NF nanoblocks as a bifunctional electrocatalyst for water splitting in alkaline seawater and urea solution

Compared with conventional electrolytic fresh water for hydrogen generation, electrolytic seawater for hydrogen production is more cost-effective, because seawater resources are rich, and fresh water resources are short. However, there are many impurity ions in seawater, especially chloride ions, which have great corrosion on the catalyst, so it is a major factor restricting the development of electrolytic seawater. In this study, a novel NiP/Fe3P/MoP@NF catalyst was made via conventional hydrothermal and phosphating method. Experimental results show that the overpotential of NiP/Fe3P/MoP@NF catalyst in the current density of 10 mA cm−2 is just 40 mV for hydrogen evolution reaction (HER) and the overpotential of NiP/Fe3P/MoP@NF catalyst in the same condition is 135 mV for oxygen evolution reaction (OER) in 1 M KOH and seawater solution. In electrolyte solution of 0.5 M urea and 1 M KOH, the overpotential of NiP/Fe3P/MoP@NF catalyst in the same current density of 10 mA cm−2 is 51 mV for HER. Furthermore, the whole alkaline seawater splitting requires an applied voltage of 1.66 V at current density of 10 mA cm−2. Experimental results show that the increased catalytic activity is assigned to the rapid electron transport rate, the exposure of more activity sites, and their synergistic catalysis. The density functional theory (DFT) can show that MoP has the best Gibbs free energy for hydrogen and electrical conductivity compared with NiP and Fe3P, thus enhancing the catalytic performance of NiP/Fe3P/MoP@NF. The paper proposes novel ideas concerning the exploration of catalysts in alkaline seawater and urea solution.

Interface mechanical properties of CFRP-seawater sea sand concrete under simulated seawater immersion

In this study, the interfacial mechanical properties of carbon fiber-reinforced polymer (CFRP) sheet reinforced seawater sea sand concrete (SSSC) in simulated seawater immersion environment were studied. The results present that the interface bearing capacity of CFRP-SSSC decreases significantly after seawater immersion. The decreasing rate increases greatly with the extending of immersion periods. Seawater immersion also affects the failure mode of interface between CFRP and SSSC, which leads to the damaged surface expanding from the base layer of concrete to the junction section of resin and aggregate. The distribution of strain and stress along the bonding direction presents the load transfer from the loading end to the free end. The shear stress peak locates near the loading end and decreased about 15–20 % due to seawater immersion as a result of the deterioration of the interfacial property. The analysis on the microstructure of CFRP-SSSC interface proves that the chemical ions in seawater causes the hydrolyzation of epoxy resin adhesive layer. The CFRP-SSSC interface presents a lower interfacial bonding properties than CFRP reinforced normal concrete (NC). The improvement of strength grade of concrete from C30 to C50 increases the bonding properties of the CFRP about 9–12 % and mitigates the influence of seawater immersion. The obtained bond-slip curve indicates that the seawater immersion decreases the interface ductility. A modified Popovics model is applied to predict the bond-slip curve and presents a high fitting degree.

Three-Dimensional Double-Layer Multi-Stage Thermal Management Fabric for Solar Desalination.

Water scarcity is a serious threat to the survival and development of mankind. Interfacial solar steam generation (ISSG) can alleviate the global freshwater shortage by converting sustainable solar power into thermal energy for desalination. ISSG possesses many advantages such as high photothermal efficiency, robust durability, and environmental friendliness. However, conventional evaporators suffered from huge heat losses in the evaporation process due to the lack of efficient thermal management. Herein, hydrophilic Tencel yarn is applied to fabricate a three-dimensional double-layer fabric evaporator (DLE) with efficient multi-stage thermal management. DLE enables multiple solar absorptions, promotes cold evaporation, and optimizes thermal management. The airflow was utilized after structure engineering for enhanced energy evaporation efficiency. The evaporation rate can reach 2.86 kg·m-2·h-1 under 1 sun (1 kW·m-2), and 6.26 kg·m-2·h-1 at a wind speed of 3 m·s-1. After a long duration of outdoor operation, the average daily evaporation rate remains stable at over 8.9 kg·m-2, and the removal rate of metal ions in seawater reaches 99%. Overall, DLE with efficient and durable three-dimensional multi-stage thermal management exhibits excellent practicality for solar desalination.

Multi-objective optimization of seawater mixing in cement-based materials with supplementary cementitious materials using response surface methodology

Seawater instead of freshwater to produce concrete is an effective measure to save freshwater resources. However, the presence of harmful ions in seawater restricts its broader application. The incorporation of supplementary cementitious materials (SCMs) has the potential to significantly mitigate these adverse effects. Consequently, there is a growing interest in understanding the mechanisms by which SCMs function in seawater-mixed cementitious systems. In this study, a quadratic polynomial prediction model with the mechanical properties and dry shrinkage of the composite matrix as the response target was analyzed by the response surface method (RSM). Tests and analytical techniques elucidated the effects of metakaolin (MK) and ground granulated blast-furnace slag (GGBS) on the hydration process of the pastes after seawater mixing. The results revealed that the compressive strength and dry shrinkage were influenced by single-factor or multi-factor interactions. The optimal mix design (MK=20.2 % and GGBS=10.0 %) was experimentally validated, showing an absolute error of 1.5 %. The microstructural analysis indicated that MK and GGBS expedited essential pozzolanic reactions. Enhanced strength in mixtures before 3 d was attributed to cation exchange and initial flocculation. In the 28 d specimens, the increase in strength was related to the growth of Friedel’s salt crystals and C-(A)-S-H gels. Due to the presence of the C-(A)-S-H gels and AFm phases, chloride ions were efficiently immobilized, and the shrinkage of the matrix was reduced.

Molecular insight into the dual effect of salts: Promoting or inhibiting the nucleation and growth of carbon dioxide clathrate hydrates

Facing the huge gap between the growing CO2 emissions and net-zero emissions target to mitigate global warming, the long-term storage of excess CO2 becomes an intractable issue. Sequestering CO2 in the oceans as clathrate hydrates is a promising solution. Many unanswered questions still exist at the molecular level regarding how ions in seawater kinetically affect the formation of CO2 hydrates, particularly the dual effect: as the concentration rises, salt ions shift from being a promoter of hydrate formation to becoming an inhibitor. Herein, we report the homogeneous nucleation process of CO2 hydrates in saline solutions via molecular dynamics simulations. Consistent with experimental findings, our results show that nucleation is promoted within certain salt concentration ranges; however, the evidence suggests that ions do not act as nucleation sites. Multiple ion-induced variations in the aqueous phase are analyzed, including changes in the mobility, structure, and CO2 solubility. We find that the presence of ions strengthens the hydrophobic interactions among the dissolved CO2 molecules, which are associated with the salting-out phenomenon, accounting for the promoting effect of salts at low concentrations. However, as the salt concentration increases, this advantage is offset, leading to the inhibiting effect at high salt concentrations. We demonstrate that anions are inevitably involved in the construction of the solid hydrate phase. Their presence disrupts the native tetrahedral connectivity of water molecules and imparts negative charges to the solid phase, thereby attracting cations. In addition to advancing our understanding of the intricate interactions among water, guest, and additive molecules to germinate ordered solid structures, these findings can accelerate the development and utilization of sustainable hydrate-based applications.

Novel insights on the setting process, hardened properties, and durability of sustainable ultra-high-performance seawater sea sand concrete

Using seawater and sea sand to prepare concrete could effectively alleviate the shortage of freshwater and river sand resources and promote the sustainable development of concrete. However, the current research on the setting process, hardened properties, and sustainability of ultra-high-performance seawater sea sand concrete (UHPSSC) remain considerably limited. In this paper, the UHPSSC was designed to mitigate the autogenous shrinkage and enhance the sustainability, and the effect of seawater, untreated and desalinated sea sand on the setting process, mechanical properties, durability, and sustainability was explored by various tests. Results show that the abundant ions (e.g., Cl- and SO42-) in seawater and untreated sea sand promote the dissolution and nucleation of cement, accelerating the hydration process, and seawater provides a high pH hydration environment, which enhances the pozzolanic reaction activity of silica fume and fly ash. This leads to the enhanced flexural and compressive strengths at early age, reduced flowability, and increased autogenous shrinkage. Although the seawater and untreated sea sand considerably alleviate the natural raw materials consumption, the mechanical performance at 28 d and durability are reduced, thereby decreasing the sustainability index by 7.8 %. The desalination treatment of sea sand decelerates the hydration process of UHPSSC with untreated sea sand due to the lower ion content in desalinated sea sand. The application of desalinated sea sand mitigates the adverse impact of untreated sea sand on the workability, mechanical properties, and durability. However, this desalination process requires more energy and freshwater consumption, thereby decreasing sustainability index. The fly ash incorporation could improve the workability of UHPSSC, and decrease the autogenous shrinkage value by 17.9 %. Moreover, it mitigates the environmental effect without compromising the mechanical properties and durability, leading to an impressive 42.8 % improvement in sustainability. Durability assessment reveals that the prepared UHPSSC specimen maintains outstanding mechanical properties even after 360 d of exposure to severe marine environment, indicating its excellent durability. This paper provides novel insights for achieving a cleaner and more sustainable concrete production.

Long-term Durability of Seawater Electrolysis for Hydrogen: From Catalysts to Systems.

Direct electrochemical seawater splitting is a renewable, scalable, and potentially economic approach for green hydrogen production. However, issues related to low durability caused by complex ions in seawater pose great challenges for its industrialization.In this review, a mechanistic analysis of durability issues of electrolytic seawater splitting is discussed.We critically analyze the development of seawater electrolysis and identify the durability challenges at both the anode and cathode. Particular emphasis is given to elucidating rational strategies for designing electrocatalysts/electrodes/interfaces with long lifetimes in realistic seawater including inducing passivating anion layers, preferential OH-adsorption, employing anti-corrosion materials, fabricating protective layers, immobilizing Cl- on the surface of electrocatalysts, tailoring Cl- adsorption sites, inhibiting binding of OH- to Mg2+ and Ca2+, inhibiting adherence of Mg and Ca hydroxide precipitation, and co-electrosynthesis of nano-sized Mg hydroxides. Synthesis methods of electrocatalysts/electrodes and innovations in electrolyzer are also discussed. Furthermore, the prospects for developing seawater splitting technologies for clean hydrogen generation are summarized. We found that researchers have changed the attitude towards Cl- ions from "hate" to accept to utilize, as well as more attention to cathodic reaction and electrolyzers, which is conducive to accelerate the commercialization of seawater electrolysis.

The influence of divalent ions on the osmotic energy conversion performance of 2D cation exchange membrane in reverse electrodialysis process

As reverse electrodialysis (RED) technology evolves, achieving high-performance osmotic energy conversion (OEC) necessitates porous membranes with exceptional ion selectivity and permeability. Two-dimensional membranes crafted from graphene oxide (GO) or MXene stand out for their immensely high surface charges, which facilitate superb ion selectivity during transport. Their unique layered stacking structure further yields ultra-small nanochannel radii, bolstering ion selectivity while maintaining high permeability. This makes them prime candidates for osmotic energy conversion. However, the presence of divalent ions in natural seawater readily alters the surface charge density and interlayer spacing of these 2D membranes, significantly impacting their OEC performance, resulting in a power density reduction of about 6 % -10 %. Prior research focused on the ionic properties of divalent ions but overlooked their impact on the nanochannel structure, including changes in interlayer spacing and surface charge density. This study thus delves into the OEC performance variations of GO and MXene 2D layered membranes under varying divalent ion concentrations and salt ratios. Starting from the ion properties of divalent ions and their impact on the nanochannel structure, combined with simulation research, explain their mechanism of action. This research offers theoretical foundations for the future design and application of 2D layered membranes, providing novel solutions and insights to tackle critical challenges like enhancing power density and adapting to real seawater conditions.

Enhancing the Flotation Separation of Galena and Pyrite in the Presence of Seawater.

The effect of seawater on the flotation of sulfide minerals is a hot research topic at the present. Using seawater instead of fresh water to separate lead-sulfur can save fresh water resources and reduce lime consumption, which has broad prospects in terms of economic and environmental protection. Many studies have shown that divalent calcium and magnesium ions in seawater are hydrolyzed to form hydroxyl complexes under alkaline pH values, which affects the flotation separation of sulfide ore. In this paper, the flotation separation of galena and pyrite in seawater was tested, and the mechanism was analyzed. Compared with that in deionized water, the lead grade of concentrate in seawater flotation increased from 57.40 to 60.20%. Meanwhile, the recovery of galena declined from 68.66 to 61.17%, and the SI value increased from 2.020 to 2.072. With the addition of SHMP, SS, and EDTA, the SI value increased from 2.072 to 2.609, 2.525, and 2.287, respectively. Noteworthily, when EDTA was excessive, the grade of Pb was 69.60%, and the SI value was 3.050. Wettability analysis, zeta potential analysis, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, density functional theory calculation, and E-DLVO calculation verified the microflotation results and revealed the mechanism of action.

Advances in using seawater in slag-containing cement systems

The use of seawater as mixing water in concrete is motivated by an increasing danger of freshwater shortages. The factors restricting its use in concrete include a high risk of concrete structure degradation as well as steel reinforcement corrosion initiated by the chlorides and sulphates in seawater. The objective of this study is to evaluate the potential of ternary cementitious systems containing the substitution of a clinker constituent in cements with ground granulated blast furnace slag (GGBFS) at rate of 45 wt% and 85 wt% and zeolite (10 wt%) mixed with seawater. Moreover, the effects of alkaline activators were evaluated as a potential method to compensate the low strength of high volume slag-zeolite-cement systems containing seawater. The hydration process was evaluated by means of calorimetry, X-ray diffraction (XRD) and scanning electron microscopy (SEM), and was supplemented by the compressive strength development (2, 7, 28 days) evaluation. It was found that presence of GGBFS in seawater-mixed systems, promotes an increase in the volume of low-calcium silicate hydrate gel-like phases, which (in addition to the layered double hydroxide phases) bind the Cl-- and SO42--ions of seawater. However, at high volume replacement level (85 wt%) the accelerating effect of seawater is limited resulting in slow rate of concrete strength gain. The proposed alkaline activation, allowed the strength enhancement, but also satisfied the conditions for the complete binding of Cl-- and SO42− -ions, due to the formation of zeolite-like alkaline aluminosilicate hydrates in the resultant cement matrix. It was found that the addition of natural zeolite accelerates hydration reactions and crystallization processes in cement systems. As an outcome, due to synergistic effect of seawater and alkali-activation, the proposed ternary blend containing 5 wt% of Portland cement exhibited only 24 % lower 28 days compressive strength when compared to freshwater-mixed Portland cement (90 wt%) – zeolite (10 wt%) reference mix.

Investigation on methane hydrate formation behaviors in deep-sea methane seepage environments

Deep-sea cold seep is a natural phenomenon where methane leaks from the sea floor. Methane can be partially conversed through physical, chemical, and biological processes, but the excess methane that remains uncaptured escapes into the atmosphere and contributes to the greenhouse effect. Physical carbon sequestration via hydrate formation is a fast and high-energy density type of carbon sequestration; however, it has been largely overlooked. This study investigated hydrate formation in authigenic carbonate rocks through deep-sea geological surveys in cold seeps in the South China Sea. Investigations have shown that authigenic carbonate rocks can sequester carbon by blocking the methane plume from the seepage vents. This perspective provides a new physical carbon sequestration mode of carbonate rocks by observing the artificial collection of methane plumes in deep-sea seepage areas. The mode was simplified into two steps including the formation of a thin layer of planar hydrate film on carbonate rocks and a thick layer of hydrate through the stacking of bubbles under the thin film. Salinity, a key distinction between seawater and freshwater, was further investigated for its impact on the carbon sequestration mechanism. Results showed that saline ions in seawater had little impact on the thickening rate of the planar hydrate film but reduced the lateral growth rate of hydrates on the bubble by 60%. This reduction in the lateral growth rate greatly facilitated the three-dimensional growth, increased the hydrate thickness on the bubble, and improved the carbon sequestration efficiency. Hydrate formation on the exposed carbonate rock can effectively impede methane plume migration into upper water and atmosphere, mitigate the greenhouse effect, and present a promising strategy for carbon sequestration in deep-sea methane seepage areas.

N,S co-doped SnO2 catalysts in gas-liquid interface dielectric barrier discharge for formate formation via CO2 reduction

N,S co-doped SnO2 were deposited on MWNTs/sponge to prepare a series of related N-S-SnO2/MWNTs/sponges for the reduction of CO2 by dielectric barrier discharge (DBD). With optimizing the composition, the N-S-SnO2/MWNTs/sponge (N:S:Sn = 1:0.2:1) achieves higher yields of formate (4791.40 μmol/L) and formaldehyde (66.11 μmol/L) after 60 min. The structural properties and DFT calculations reveal that, the S doped increased oxygen-containing functional groups on catalysts surface, thus the nonmetallic N and S modified Sn based catalysts affected the adsorption sites and reduced the reaction energy barrier of CO2 on the catalyst surface in order to promote energy utilization of DBD system for CO2 catalytic reduction. In addition, different types and concentrations of seawater ions such as K+, Na+, Ca2+, Mg2+, Cl- and SO42- affected on the formation of formic acid and formaldehyde during discharge, which showed that DBD catalysis technology was advanced used for CO2 capture and storage in the ocean.

Synthesis of eco-sustainable seawater sea-sand geopolymer mortars from ternary solid waste: Influence of microstructure evolution on mechanical performance

To fully utilize abundant marine resources and reduce the carbon footprint in the construction sector, this study developed a seawater sea-sand geopolymer mortar (SSGM) using seawater, sea-sand, and ternary solid waste. Three different alkaline content levels (4%, 5%, and 7%) were tailored for the SSGM, in addition to freshwater river sand mixed geopolymer mortars (FRGM), seawater sea-sand ordinary Portland cement mortar (SSCM), and SSGM without additional fibres (SSGM-0). The workability, setting time, mechanical performance, and drying shrinkage of all samples were studied. The microstructural characteristics of each mortar were meticulously scrutinized through techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and mercury intrusion porosimetry (MIP). The results showed that as alkaline content increased, SSGM formed more hybrid sodium, calcium aluminosilicate hydrate (N, C-A-S-H) gels, resulting in a denser matrix. Under the influence of magnesium ions and sulfate ions in seawater, the formation of magnesium aluminosilicate hydrates, magnesium silicate hydrates, and silica gels filled the pores, making SSGM has more mesopores and gel pores compared to FRGM, thus exhibiting superior mechanical properties. However, the Ca/Si ratio of the primary hydration products of SSGM was lower than that of SSCM and FRGM, indicating a more disordered microstructure of SSGM, leading to greater shrinkage. Despite moisture migration reaching a stable state, SSGM exhibited persistent shrinkage, revealing their inherent time-dependent (creep) response to drying conditions, indicative of typical viscoelastic/visco-plastic matrix behavior.

Engineering shrinkage resistance of nano-structured hydrogels in seawater for fast uranium capture

Synthetic hydrogels, which are 3D networks composed of crosslinked hydrophilic polymer chains, have been created using various crosslinked ways for uranium capture due to full exposure of chelate functional groups. However, the conventional honeycomb-like hydrogels exhibit non-connected micropore structure, which locks abundant water molecules and hence is unfavorable for mass transfer. Herein, we demonstrate a simple synthesis of the nano-structured hydrogel with activated nanogels as nano-crosslinkers (denoted as PGP hydrogel), and probe into the relationship between hydrogel structural parameters and uranium adsorption kinetics. It is found that the PGP hydrogel with a low crosslinking density of ∼0.02 mol/m3 possesses interconnected micropores, thereby facilitating diffusion of uranyl ions (UO22+) in 3D polymeric networks. Furthermore, a freeze-assisted soaking strategy is proposed to endow PGP hydrogel (denoted as target hydrogel) with strong shrinkage resistance in natural seawater, resulting in a ∼40 % increase in water permeance to reach 369 Lm−2h−1MPa−1. Correspondingly, the adsorption equilibrium time is shortened to ∼11 days. Although the uranium uptake capacity of PGP hydrogels with and without freeze-assisted soaking treatment remain basically unchanged (∼9.73 mg-U/g-Ads), a high uranium adsorption rate of 0.88 mg/g/d for the target hydrogel surpasses most currently available polymer adsorbents. The presented strategy is generalizable to a broader range of applications involving the adsorption of economically valuable ions in seawater or brine lake.