Boya Qiu,Patricia Gorgojo,Xiaolei Fan

1 Department of Chemical Engineering and Analytical Science,School of Engineering,The University of Manchester,Oxford Road,Manchester M13 9PL,United Kingdom

2 Nanoscience and Materials Institute of Aragón (INMA) CSIC-Universidad de Zaragoza,Mariano Esquillor,50018 Zaragoza,Spain

3 Chemical and Environmental Engineering Department,Universidad de Zaragoza,Pedro Cerbuna 12,50009 Zaragoza,Spain

Keywords:Adsorption desalination Adsorbents Porous materials Operation modes

ABSTRACT With the continuous growth of the world population,the demand for fresh water is ever increasing.Water desalination is a means of producing fresh water from saline water,and one of the proposed solutions in the scientific community for solving the current global freshwater shortage.Adsorption is foreseen as a promising technology for desalination due to its relatively low energy requirements,low environmental impact,low cost and high salt removal efficiency.More importantly,chemicals are not required in adsorption processes.Active carbons,zeolites,carbon nanostructures,graphene and coordination framework materials are amongst the most investigated adsorbents for adsorption desalination,which show different performances regarding adsorption rate,adsorption capacity,stability and recyclability.In this review,the latest adsorbent materials with their features are assessed (using metrics) and commented critically,and the current trend for their development is discussed.The adsorption mode is also reviewed,which can provide guidance for the design of adsorbents from the engineering application point of view.

1.Introduction

Water scarcity is one of the most serious global challenges of our time.Now,over one-third of the world’s population lives in water-stressed countries,and,by 2025,this is predicted to rise to nearly two-thirds[1,2].The challenges of providing ample and safe drinking water are further complicated by population growth,industrialization,contamination of available freshwater resources and climate change.Accordingly,several measures to alleviate the stresses on water supply should be implemented,including water conservation,improved infrastructure resilience and improved catchment/distribution systems.However,it is worth noting that these measures can only improve the use of existing water resources,rather than exploiting additional resources.An increase in water supply beyond what is available from the hydrological cycle can be achieved by desalination and water reuse [3].In general,several source waters can be utilized including seawater,brackish ground water,brine water,surface water,shale gas produced water andet al.The key contaminants in these source water include minerals,metals cations and anions,and they can be expressed by the concentration of total dissolved solids (TDS),which is in the range of 500-50000 mg·L-1.To obtain drinking water,the TDS should be reduced to ＜200 mg·L-1.In addition,some organic matters can also appear in the source water,which need to be removed.Desalination can effectively reduce the contaminants in the source water and provide the steady and ample supply of high-quality water without impairing natural freshwater ecosystems.Hence,the number of installed water desalination facilities has increased significantly in water-stressed countries in the past decade [1,4-6].To date,there are more than 20,000 plants constructed around the world,and an expected 10 billion USD investment in the next five years would add 5.7 million cubic meter per day of new production capacity.This capacity is even expected to double by 2030.

Fig.1.Overview of the current desalination processes.

As illustrated in Fig.1,early large-scale desalination plants,mostly in the arid Gulf countries,were based on thermallyactivated desalination processes including multi-effect distillation(MED) and multi-stage flash (MSF),in which seawater is heated,evaporated and condensed to produce fresh water [7],which are energy intensive and associated with large carbon footprints [5].Although thermal process based on solar energy(i.e.,solar distillation)can be sustainable,its intermittency can be the issue for sustaining fresh water supply [8,9].Currently,desalination plants worldwide are mainly based on membrane technologies,including reverse osmosis (RO),forward osmosis (FO),nanofiltration (NF)and membrane distillation (MD),which account for 2.2 million m3·d-1of annual contracted capacity in 2017 and present over 59% of total desalination capacity (in comparison,thermal processes account for just 0.1 million m3·d-1).Membrane-based processes are pressure-activated systems which require high pressures to operate,resulting in energy penalty.Also,costs incurred by feed pre-treatment and membrane regeneration (to deal with membrane fouling) may make membrane-based desalination relatively less attractive.In addition to the thermally-/pressureactivated systems,chemical-activated desalination processes such as ion-exchange and precipitation are also proposed [10,11].However,such chemical processes are not very ideal due to the use of chemical additives.Comparatively,adsorption processes using porous adsorbents have relatively low energy consumption (＜1.5 kW·m-3) with environmental-friendly desalting process to regenerate the adsorbent at moderate temperatures,thus being attractive for designing novel desalination processes[12].

In 2005,adsorption desalination was proposed for the first time,and since then various porous materials have been investigated and different adsorption modes have been developed and applied[13].Adsorption desalination is a phase transfer process that occurs at the interface between a solid and an aqueous phase where salt ions (the adsorbate) adsorb on the surface of porous solids (the adsorbent)viaphysical interactions (such as van der Waals interaction and hydrogen bonding) and/or chemical bonding.Adsorption can be reversed,i.e.,desorption,especially physical adsorption,to regenerate the adsorbent,which is key for the development of practical processes.Porous materials possess a unique set of properties,such as high specific area and pore volume,and fluid permeability,and hence provide high adsorption capacity,fast adsorption kinetic and good selectivity[14].In general,a high surface area is often seen as an important characteristic to improve the adsorption capacity,since adsorption is related to the interaction between the guest molecules and adsorbent surface.However,the surface,especially the internal surface,of porous materials needs to be accessible by the guest molecules.The surface area of porous materials depends on pore sizes,i.e.,the surface area increase as the averaged pore size decreases.Again,accessibility of the porous network is vital for the applications.To enable fast adsorption kinetics,the averaged pore size of a material needs to be bigger than the kinetic diameter of the guest molecules.If secondary pores are present in the material,then the pore network needs to be hierarchical to allow the improved accessibility of the guest to the host surface.Although several reviews on current desalination technologies are available in the literature,including brief overviews of materials in adsorption desalination,systematic discussion on the state-of-the-art porous adsorbent materials for desalination is lacking [15-17].Since it is a promising alternative to address water scarcity,here,this review provides a critical snapshot of the state-of-the-art of adsorption desalination covering the latest porous candidate materials as adsorbents for adsorption desalination,the current trend for developing new adsorbents and different adsorption modes employed by desalination.This review can be used for guiding the further research in developing adsorption desalination technologies with high efficiencies for practical settings.It should be noted that this review focuses on the adsorption removal of the contaminants in aqueous solution for desalination,rather than water adsorption,therefore,thermal adsorption desalination,in which water is collected by evaporation-triggered-adsorption and desorption-activated-con densation processes [18,19],is not included.

2.Overview of State-of-the-art Adsorbent Materials

2.1.Activated carbon (AC)

AC is a class of versatile carbonaceous porous materials,which can be derived from various economical raw materials (or precursors) such as wood straw,nut shells,coal,waste plastics,papermaking and municipal wastes [15,20].Depending on the precursors and methods of activation,porous properties of AC can be tuned.AC with hierarchical porous structures can be very beneficial to adsorption desalination due to their high porosity,large surface area,versatile surface functionality,etc.AC has been extensively researched and applied for adsorption of various organic/inorganic contaminants in aqueous systems including salts,metal ions,heavy metals,etc.,which are common in seawater and brackish water [21,22].

To enhance the adsorption capacity of AC,the microstructure and chemical properties of AC need to be engineered,which can be achieved by various post-synthetic modification methods such as oxidation which can be attained by chemical/air/electrochemical oxidation and plasma/ozone treatments [23-39].Acid treatment using nitric and sulfuric acid is the most common method for AC oxidation [40-42],which can endow the modified AC with surface oxygenated acid groups.These groups are proton donors and favorable to interact with metal ions to form metal complexes,and hence to improve the adsorption capacity of AC for metal ions such as Cr3+,Zn2+,Hg2+,Pb2+,Mn2+,Co2+,Ni2+and Cu2+[43-45].The affinity of metal ions to the acid-treated AC were found to coincide with the stability of metal complexes formed,i.e.,Mn2+＜Co2+＜Ni2+＜Cu2+[46,47].Grafted AC with basic nitrogen(N) functionalities,such as amides,amine and pyridines,showed relatively high adsorption ability for negatively charged species[48,49] due to dipole-dipole interactions,hydrogen-bonding and covalent bonding.Base-treated AC can be beneficial to the removal of organic species form water,such as 4-chlorophenol and 2,4-dichlorophenol (2,4-DCP) [50,51].For example,the adsorption of 2,4-DCP on ammonia treated AC was improved by 22.86% [51].Both acid and base treatments tend to reduce the AC’s specific surface area and pore volumes,which could bring negative effect on adsorption.For example,the adsorption capacity of AC for Cr(VI)was not enhanced despite a higher number of oxygen groups on the NaOH treated AC [52].This could be mitigated by tuning the treatment conditions,for example,the porosity and pore structure of AC after thermal ammonia treatment was reported to maintain which contributed to the increased adsorption capacity for perchlorate[53].Microwave irradiation also found their usages in treating AC,which can provide a selective yet rapid heating to reduce the treatment time and energy consumption[54-57].Additionally,other treatments employing ozone and plasma were reported as well [42,58-65].However,it should be noted that AC adsorption was commonly used for removal of heavy metals and organic contaminants,the assessment of AC’s adsorption capacity for ionic salts(e.g.,Na+,Cl-)was rather limited,and AC was mainly used in capacitive deionization (CDI) technologies as electrodes to deionize water sources [66-68].

2.2.Zeolites

Natural zeolites can also be used as adsorbents for desalination[69-72].The general chemical formula for zeolites is Mx/n[AlxSiyO2(x+y)]·pH2O where M is(Na,K,Li)and/or(Ca,Mg,Ba,Sr),nis cation charge,y/x=1-6 andp/x=1-4,and zeolites have the well-defined 3-dimensional (3D) frameworks with cages and channels inside[69,73].Zeolites are intrinsically negatively charged due to the valences of silicon and aluminum;therefore,they have the ion exchange ability for cations,as well as a variety of positively charged organic molecules.In addition,the high adsorption kinetic can be enabled due to the high porosity of zeolite (typically 30%-40% with pore sizes of 0.3-0.8 nm).The zeolite water can also accelerate the adsorption kinetic since it can be taken as bridge for ion-exchange between ionic salts and exchangeable cations.Zeolites have a wide range of sources in nature,including clinoptilolite,mordenite,phillipsite,chabazite,stilbite,analcime and laumontite.The adsorption property of zeolites varies depending on types,and the adsorption capacity and kinetics are closely related to the crystalline structure and composition of zeolites.For example,mordenite type zeolite,which is an abundant natural zeolite,can effectively reduce the concentration of Na+in aqueous solutions by about 73%,as well as has the increased adsorption capacity of Ca2+and Mg2+due to ion exchange between Na+and the two cations.However,its adsorption ability for anions such as Cl-andis relatively low,only about 20%and 30%,respectively,due to the negative charge of the zeolite [70].Natural zeolite bigadic clinoptilolite is also able to absorb Ca2+from water,with the exchange capacity of 11 mg·g-1[74].

Natural zeolites also found applications in for seawater desalination [73].For example,Wajimaet al.[70] used a modified mordenite-type zeolite from Fukushima Japan for removing NaCl from seawater.The zeolites were heated at 60°C for 12 h and further treated with silver nitrate(AgNO3) prior to adsorption,which led to 81.73%NaCl reduction under batch condition.Wibowoet al.[75] reduced seawater salinity using the modified natural zeolites from Indonesia(clinoptilolite).The sorption capacity of clinoptilolite was enhanced (up to 51.43 mg·g-1under batch conditions)through thermal activation at 225 °C for 3 h without the use of any chemical reagents.The adsorption capacity of zeolites are generally better than that of AC,which is 2-20 mg·g-1[69].Hence,natural zeolites can be cost-effective sorbent materials for seawater desalination.In addition to ionic salts and metal ions,zeolites can also be used in wastewater treatment for the removal of heavy metal ions (e.g.,Pb2+,Fe3+,Cu2+,Zn2+,Cd2+,Co2+,Ni2+,Mn2+,Cr3+),inorganic anions (e.g.,CN-) [76-79],and organic molecules including dyes (e.g.,methylene blue,rhodamine B,malachite green,reactive black 5,reactive red 239,reactive yellow 176,basic red 46 [80-83]).

Synthetic zeolites,such as LTA,Zeolite Y and Zeolite P.which are synthesizedviahydrothermal methods,can also be used as adsorbents.By varying synthesis conditions,chemical and physical properties of synthetic zeolites can be tuned to improve adsorption capacity.For example,to reduce diffusion resistance,mesoporous LTA (with intracrystalline mesopores ofca.3 nm) was developed and showed the enhanced adsorption rate of Mg2+due to the improved diffusion rate of ions in its meso-micro-porous framework,which was 17.5 times higher than that in the pristine microporous LTA[71].

2.3.Carbon nanotubes (CNTs)

Since the discovery of carbon nanotubes (CNTs) by Iijima [84],they have become one of the most investigated materials for a wide range of applications including desalination.CNTs are 1D cylinders composed by graphene sheets with micro-or nanoscale diameter which can be effectively used for water purification and desalination mainly owing to their large surface areas,ease of functionalization,high porosity and hollow/layered structures.The structure of CNTs also allows them to interact with some solutes through π-π electronic and hydrophobic interactions,showing very high adsorption capacity(up to 4,000 mg·g-1)and adsorption rate [85-89].

For desalination,the adsorption of metal ions on CNTs can be attributed to electrostatic attraction,sorption-precipitation and chemical interactions between metal ions and surface functional groups on CNTs.Surface chemical nature of CNTs is key to regulate their adsorption property [90,91].Oxidation is one of the most widely used methods for modifying the surface of CNTs,which was achieved by various strategies such as ozone,plasma and thermal treatment[92].Oxidation processes can open the tips of CNTs and introduce defects at the pentagons on graphene,then a large amount of oxygen-containing functional groups can be functionalized on the defects easily.The presence of oxygen-containing functional groups can hinder the aggregation of CNTs,thereby improving the dispersion of CNTs in aqueous media during water treatment.The single pair of electrons on oxygen atoms in the functional group can be donated to metal ions,which improves cation exchange capacity of CNTs[93,94].For example,the adsorption capacity of Ca2+on CNTs can be improved from 1.1 mg·g-1to 2.6,5.1 and 11.0 mg·g-1,respectively,after the oxidation of CNTs by H2O2,HNO3and KMnO4[92,95].Other surface functional groups such as arboxyls,lactones and phenols can also be introduced to CNTs to improve the ion exchange (adsorption) capacity [96].However,diffusion resistance may be higher in the functionalized CNTs,in which the possible occupation of the interior space of CNTs (even they are uncapped) may reduce the effective surface area of the modified CNTs and the accessibility of the ions to the inner surface of CNTs [91,97].

Although CNTs are effective in desalination,concerns of health risks and environmental impact (such as physicochemical stability and apparent biopersistence in the lung [98]) hinder their practical applications as in their bulk forms.The high aspect ratio(＞100) along with poor solubility of CNTs also raises safety concerns in scientific communities,which are likely analogous to hazardous fibers such as asbestos [99].One solution for industrial adoption of CNTs for adsorption is engineering special devices to host CNTs.CNTs can be integrated with membranes to remove saltviacapacitive electrostatic interactions.For example,CNTs composite films were fabricated by chemical vapor deposition and used for Na+adsorption.However,the adsorption capacity of the development was rather limited (i.e.,22.09 mg·g-1) due to the low loading of CNTs on the substrate [100].Another way is to form CNT sheets,that is,vertically aligned CNTs with functionalized open ends which allows water to pass through[101,102].The water permeance can be high due to the reduced friction on CNT walls.More importantly,ionic salts are adsorbed rather than rejected,and hence the mass transfer resistance can be low,suggesting low applied pressure during operation and low energy consumption.Salts removal from aqueous media by CNT sheets was demonstrated by Yanget al.[66].They showed that plasma treatment of CNTs resulted in exceptionally high salt adsorption capacity exceeding 400% by mass (i.e.,4,000 mg·g-1for NaCl solution desalination),which is two magnitudes higher than that of AC.It is worth noting that fabrication of CNT sheets with large surface areas is very challenging,which is also key to advance the technology towards commercialization [103].The schematic of strategies to improve the adsorption capacity of CNTs are concluded in Fig.2.

2.4.Graphenes

Graphene and its derivatives are one of the most popular nanomaterials for adsorption and desalination.Graphene is a 2D nanomaterial composed by carbon atoms with a sp2-bonded aromatic structure [104,105].The distinctive structure of graphene endows it high surface area (theoretically 2,630 m2·g-1) since both side of the single layer of the graphene can be exposed to the aqueous solution during adsorption.Graphene can also be prepared easily with good yields (compared to other 2D materials such as 2D metal organic frameworks,MOFs) and potential for scaling up.Graphene oxide (GO),as the major derivative of graphene,is also promising for adsorption applications due to the presence of abundant surface oxygen-containing groups to interact with adsorbents.Graphene and its derivatives show good adsorption performance for a variety of contaminants such as metal ions,and organic pollutants due to multiple interactions in the systems such as electrostatic interaction,π-π stacking,hydrophobic interaction,and complexation.Specifically,cationic and anionic (e.g.,Cl-,,and F-)species are adsorbed mainly based on electrostatic interaction and anion-π interactions [106-109];π-π stacking and hydrophobic interaction are responsible for adsorbing organic contaminants;complexation is the mechanism for adsorption of heavy metal ions on GO [110].Additionally,the combination of very high specific surface area and conductivity also makes graphene-based electrodes very attractive for CDI application for seawater desalination.For example,Mishra and Ramaprabhu [109] developed a GO based supercapacitor for arsenic removal and seawater desalination,and the maximum adsorption capacity for Na+was found to be～122 mg·g-1.GO can be further functionalized with additional functional groups,such as ethylenediamine triacetic acid (EDTA) and chitosan which can be grafted on GO [111-113],to improve its dispersity and adsorption capacity.Functionalization of GO with EDTA was found to improve surface area and adsorption capacity due to the metal chelating ability of EDTA [112].Polyacrylamide-grafted GO showed the enhanced adsorption capacity of Pd2+(by about 100%),since large numbers of acetylamine groups can interact with metal ions by chemical or physical adsorption [111].

Functionalization of GO can improve recovery/regeneration of materials as well,as shown in Fig.3,which can be beneficial to practical application.For example,incorporation of perylene bisimides-containing poly(N-isopropylacrylamide) on GO (vianon-covalent bonding) can take advantage of the thermoresponsive property of the functional group,and hence tune dispersity by varying the system temperature.In detail,the functionalized GO could rapidly aggregate and sediment atT＞36°C,which can facilitate materials recoveryviafiltration.Conversely,atT＜34 °C,it could be well dispersed in aqueous media [116].

Fig.2.Schematic of strategies to improve the adsorption capacity of CNTs((a)general strategy for adsorption desalination with CNTs;(b)Surface functionalization of CNTs via different routes).

Composites based on graphene and GO were also proposed to integrate their virtues for improving adsorption applications[118].Magnetic nanoparticles such as iron and iron oxides can be immobilized on graphene to develop composites for metal ions removal from aqueous solutions [114,118-123].Although,as adsorbents,magnetic nanoparticles are good candidates,however,they tend to aggregate to form larger particles,therefore reduce the surface area and active sites accessible for adsorption.As an alternative,graphene can stabilize the magnetic nanoparticles to prevent the aggregation of nanoparticles.As a result,the nanocomposites showed the improved adsorption capacity due to the synergistic effect.In addition,the magnetic nanoparticles can also facilitate the recovery of the composites which may improve the feasibility of the nanocomposties to be used in practical settings.Nanocomposite materials consisting of graphene were also developed,such as graphene-containing hydrogels or aerogels with the crisscross structure and showed the improved adsorption capacity [115,124,125].Composites based on graphene and GO for NaCl removal from aqueous solutions are listed in Table 1,in which their performance in CDI,regarding specific capacitance,salt removal efficiency and adsorption capacity,was compared.In addition,recently,graphene was also functionalized by β-cyclodextrin to prepare membranes for continuous adsorption with high removal efficiency of bisphenol A and high water flux,which demonstrated the potential of graphene-based membrane adsorber.

Table 1Comparison of desalination performance of different graphene/GO-based materials for CDI

Although graphene is one of the most promising materials for desalination,the economic aspect of the materials needs to be assessed carefully.The cost of graphene is relatively high,which was estimated to be as high as 150 USD to 250 USD per gram compared to 0.002 USD for the raw graphite when traded commercially [132].This could be due to the complicated pathways for preparing graphene and its derivatives/composites,especially at large scale.However,continuous improvement in synthesis strategies may help to drive down the cost for these materials.For example,a new method has been developed to prepare GO using eucalyptus extracts,which can reduce the cost compared to the conventional reduction methods [133].It should be noted the adsorption of a variety of pollutants including dyes and natural organic matter on GO is mostly irreversible,which imposes the recovery/regeneration issues as compared with conventional adsorbents such as ACs.Therefore,the development of regenerative GO-based adsorbents can be one of the directions for future research [134,135].

2.5.Metal-organic frameworks (MOFs)

MOFs are framework materials composed of organic ligands and metal clusters by coordination bonding and are widely known for their high specific surface areas (ranging from 1,000 to 10,000 m2·g-1),high porosity,uniform pore size,tunable physiochemical properties and ability to be functionalized [136-139].The advantageous characteristics make MOFs highly promising next generation adsorbents with remarkably high adsorption efficiencies [138,140-148].MOFs with cation charge ability such as SLUG-21,SLUG-22,SLUG-35 and 1-ClO4,can effectively adsorb anionic substrates (e.g.,[149-152]).A number of Zn-based MOFs (e.g.,ZIF-8,FIR-53,and FIR-54) were applied to adsorption of,and some Cr-based MOFs(e.g.,UiO-66,UiO-66-NH2,and NU-1000)show high adsorption capacity for some anionic adsorbates such as F-,,[136,153-157].3D cationic MOFs can also be used for the ion exchange adsorption of anion ions [158-161].However,the stability of MOFs in aqueous media is problematic for their application in desalination.As illustrated in Fig.4,a large number of MOFs with relatively good water stability was synthesized during these decades,which showed high adsorption property as well.For example,water stable PCN-100 was prepared to adsorb heavy metal ions such as Cd2+and Hg2+,and the framework is stabilized through hydrogen bonding [173].

MOFs can be engineered rationally to improve their adsorption performance,and the strategies can be divided into:(i) synthesis MOFs with long organic linkers to enlarge the pore size of MOFs and facilitate the diffusion of substances in MOFs [155,162];(ii)design defective MOFs to enlarge the pore size and/or introduce additional adsorption sites into MOFs [165,174-177];and(iii) functionalization of MOFs.Especially,functionalization of MOFs can improve their ion adsorption/exchange and regeneration properties.Two strategies have been proposed for functionalizing MOFs:(i) grafting the metal sites of MOFs with charge-balancing anionic species (such as polyelectrolyte),which can be achieved by introducing charged secondary building units during synthesis and post-synthesis anion stripping modifications [149,156,178];(ii) grafting counterions to the ligands of MOFs (viaself-assembly or post synthesis ligand exchange modifications),in which the additional,pendant functional groups (e.g.,—NH2,—OH,and —SH) can offer additional adsorption sites and also improve the selectivity of pristine MOFs [166,179-184].

Fig.3.Strategies to improve reusability of graphene or GO-based materials for adsorption desalination((a)Thermo-responsive GO;(b)magnetite-reduced GO composite;(c)holey graphene hydrogel with in-plain pores;(d).GO-based adsorption membrane).Adapted from Ref.,[114,115].Copyright American Chemical Society,and[116,117]with permission.

Another strategy for boosting the adsorption performance of MOFs is incorporation of guests in MOFs,as shown in Fig.4,for example,introducing guest ions in pores of MOFs by impregnation or one-pot synthesis [185,167].The regular pore structure and rigid framework of MOFs can also enable the confinement of ionexchange polymers in their cavities.Compared to other porous materials,regular and controllable pores of MOFs are beneficial to disperse ion-exchange polymers and prevent entanglement,as well as ensuring the accessibility of adsorbed ions to the inner pore space [168,186].Consequently,the polymers-MOFs composites showed higher adsorption property than either bulk polymers or MOFs [187-193].Anion-exchangeable polymers were incorporated into ZIF-8 for effective Au(CN)2-extraction,and after adsorption the composite could be regenerated easily by washing with ethanolic NaOH solution [168].Non-crosslinked polymer chains can also be sufficiently interlocked in MOFs’framework structures,without further need for polymerization.Such strategy can preserve the active site in non-crosslinked polymers for ion exchange compared to the bulk crosslinked counterparts,thus leading to fast ion exchange rates [169].For instance,as illustrated in Fig.4,polyvinyl benzyl trimethylammonium hydroxide (PVBTAH) is synthesizedin-situin the pores of MOFs without crosslinking,and the active sites can be fully exposed [168].It has been proved that the polymer chains can be immobilized in MOF with high stability.The polymer chains are segregated by the ordered MOF structure without entanglement.In addition,the large volume of the pores in MOFs can also guarantee the accessibility of the active site on the polymer in MOFs.

Fig.4.Various MOFs and methods for preparing MOFs-based materials for adsorption (water stable MOFs:[155,162,163,164];ligand/defects engineering:[156,165,166];guests incorporation:[167,168,169];MOF-derived adsorbents:[170-193].)

MOFs have been applied to CDI [194,195],however,due to the low electric conductivity and chemical stability,MOFs cannot be used as electrode materials directly.Accordingly,carbon materials were developed using MOFs (as the templates) to enable conductivity with the controlled porosity derived from the framework of the parent MOFs[196-198]for CDI applications.MOFs can be converted to carbon materials by direct thermal pyrolysis[198-200].Panet al.compared the desalination performance of three carbon materials derived from Zn-MOFs (of ZIF-8-C,RT-MOF-5 and ZnFumarate) [200].After carbonization at 1,000 °C,microporous carbon was obtained from ZIF-8-C,whilst carbons drived from RT-MOF-5 and Zn-Fumarate possessed meso-micro-porous structures,especially the latter exhibited the largest specific surface area due to the well-developed mesopores,which showed comparatively high desalination capacity at 13.1 mg·g-1(applied voltage=1.2 V,CNaCl=5 mmol·L-1) and stable performance with 60 adsorption-desorption cycles.This work showed the possibility of tuning the porous structure of MOF-derived carbons to regulate CDI performance.Surfactant templates can also be used to design MOF-derived carbons with the well-defined hierarchical pore networks.For example,when cetyltrimethylammonium bromide(CTAB) was used as the template during the thermal pyrolysis of ZIF-8,worm-shaped interconnected pore structure with hierarchical pores was achieved in the resulting carbon,instead of microporous structures.The obtained material shows a higher salt adsorption capacity (20.05 mg·g-1) than the normal ZIF-8-derived carbon (13.01 mg·g-1) (at 1.4 V,CNaCl=500 mg·L-1),and the obtained electrode presents a rapid salt removal rate and excellent cycling stability [171].By thermal pyrolysis,bimetallic MOFs and hollow MOFs can also be used for preparing porous carbons with increased deionization capacity [158,170,172,201,202].

Regeneration of MOF-based materials after ion exchange adsorption is challenging,which may require strong acids and/or bases,and hence damage the structure of MOFs,as well as associated with environmental impact [203].Stimuli-responsive MOFs,with the ability to be responsive to physical stimuli (e.g.,light and temperature,exemplified in Fig.5),can be the solutions to address the issue.Stimuli-responsive MOFs were developed by functionalizing the MOFs’ ligands and applied in adsorption applications in aqueous media [204-210].For instance,a temperatureresponsive MOF,PDMVBA-MIL-121 was synthesized by introducing a tertiary amine monomer (DMVBA) into the pores of MIL-121 andin-situpolymerization of impregnated monomers.The obtained structure showed very stable performance in cyclic adsorption/desorption of salts for desalination.The NaCl adsorption capacity(53.76 mg·g-1)of PDMVBA-MIL-121 decreased gradually by～10% in the first three cycles,then became steady afterwards[203].Such novel materials hold the potential for developing environmental-friendly processes for desalination and water purification though thermal energy is required to enable the temperature change above 80 °C [203,211,212].In comparison,sunlight is a renewable source of energy and is one of the most abundant sustainable resources on earth[213].Therefore,integration of light-responsive materials with MOFs can be another strategy to improve the adsorption capacity and regeneratively of MOFs-based materials.Wanget al[214]confined poly(spiropyran acrylate (SP)),which is a light-responsive polymer,in MIL-53,and developed a sunlight-responsive composite structure.This composite adsorbent,i.e.,PSP-MIL-53,quickly adsorbed monovalent and divalent ions from salt water under dark conditions,up to 168.30 mg·g-1of NaCl and underwent fast regeneration under sunlight irradiation.The confinement effect of MIL-53 could also keep merocyanine (MC,zwitterionic state of SP) units isolated,which largely suppressed photodegradation and ensured fast and efficient reversible isomerization.Due to the high adsorption capacity and fast adsorption/regeneration of PSP-MIL-53,drinking water could be purified in less than 30 min,whilst PDMVB-MIL-121 needed 300 min to achieve the adsorption equilibrium,and commercial adsorbent Sirotherm TR-20 needed 120 min for desalination[215].The comparative performance of common adsorbents for desalination is illustrated in Table 2,which shows that the developed sunlight-responsive PSP-MIL-53 composite can serve as an efficient solution for the sustainable desalination with the utilization of solar energy.

Fig.5.Improved reusability in adsorption of thermal responsive MOFs.Adapted from Ref.[203] with permission.

Table 2The performance of common adsorbents for desalination

2.6.Covalent organic frameworks (COFs)

COFs are crystalline framework materials constructed by covalent bonding of light elements such as carbon,nitrogen,hydrogen,oxygen and boron[219].They are proposed as promising materials for adsorption desalination in aqueous solutions due to their intrinsic properties including excellent chemo stability,high surface area,abundant functional sites,and uniform adjustable aperture sizes.COFs have been applied to the adsorption removal of organic dyes [220,221],heavy metals [222-231],as well as salts[232,233].Dye molecule adsorption on COFs is mainly based on electrostatic interaction,hydrogen-bonding,Lewis acid-base interactions and π-π stacking.For example,highly water stable polycationic COFs could remove several anionic organic dye pollutants(e.g.,methyl orange,acid green 25,direct fast brown M,indigo carmine,and acid red 27) at very low concentrations of 32 × 10-6mol·L-1,mainly due to electrostatic attraction between anionic groups of dyes and the bipyridinium cations of the COF [220].The combination of MOF-5 and melamine-terephthaldehydebased intergrade 2D π-conjugated COFs also showed high adsorption capacity for auramine O and rhodamine B at 17.95 and 16.18 mg·g-1,respectively [221].COFs also showed the capability of adsorbing heavy metal ions such as U6+,Cr4+,Cr3+,Ag+,Pd2+,Fe3+,Cr3+,Cu2+,and Ni2+[222,223,225,227,228,234-236].

Water stability of COFs is the prerequisite for any potential applications,and many COFs have the limited stability in aqueous solutions,especially the ones constructed by boroxines or boronate esters which are easily attacked by nucleophiles [237].In order to improve the water stability of COFs,linkers such as imine,hydrazine,triazine and azine,which are relatively stable in water,can be used to prepare COFs[238-243].Strategy of introducing intramolecular hydrogen bonds can also be employed to improve the stability of COFs under harsh conditions with acidic/alkaline solutions[244,245].Combining COFs with other water stable materials is also a promising option for improving their water stability.For example,GO was integrated with COF which is composed by 1,3,5-triformylphloroglucinol (TFP) and 2,6-diaminoanthraquinone(DAAQ),the resulting graphene-synergized COF (GS-COF) showed the improved uranium and plutonium adsorption capacities compared to GO and COF,e.g.,for uranium 220.1 mg·g-1(GS-COF)vs.92.5 mg·g-1(GO)vs.105.0 mg·g-1(COF),The acid stability of the resultant composite in aqueous media was also improved due to the interaction of π systems between the participating materials and different kinds of layers,as well as chemical bonding generated during the complex process[234].

Beyond that,functionalization modification of COFs also endows COFs with new or modified properties.For example,1,3,5-tris(4-aminophenyl)benzene(TAPB)-2,5-bis(methylthio)terephthaladehycle(BMTTPA-COF) was fabricatedviaa bottom-up method with sulfide functional monomers.Due to the introduced short sulfide functional chains and large pore size,high adsorption capacity of 734 mg·g-1can be provided,with fast kinetics (about 99% of mercury ions are removed within 5 min) [225].Sulfur derivatives including thiol and triazole functional groups were grafted to imine-based COFs by ‘Click Chemistry’ to improve the adsorption capacity of mercury ions to 4,395 mg·g-1,with fast adsorption kinetics of 99.98%removal within 2 min[227].In addition,other modification methods including functionalization with hydroxyl,amide and EDTA were also to aid adsorption on COFs[228].

Application COFs to salt removal from aqueous solutions such as brackish water has been currently explored [232].For example,a redox-active COF (DAAQ-TFP-COF) with a salt adsorption capacity of 22.8 mg·g-1has been introduced as the cathode material,combined with a nitrogen-doped porous carbon anode,and applied to a hybrid CDI [232].Although the adsorption capacity of COFs is still limited compared to other materials,however,due to the high flexibility of COFs,they still hold potential for further exploration and can provide new insight into the material development for adsorption desalination [219,233].

2.7.Other adsorbents

Fig.6.Metrics for comparison of different materials for adsorption desalination.

In addition to the adsorbents discussed above,other materials including industrial by-products and wastes,such as fly ash,waste slurry,blast furnace slag,were reported as well for adsorption desalination [246].For example,fly ash from power station showed a slightly better Na+removal efficiency (23%) than zeolite Y (21%) [247],and is much more economical.Agricultural byproducts and wastes(such as shells and/or stones of fruits either in their natural forms or after physical/chemical treatments) were also investigated as the low-cost bio-adsorbent for water/wastewater treatment [246].The use of waste materials for desalination presents an attractive feature regarding costs.However,their performance needs to be improved before it can reach commercialization.

Some other materials,such as 2D inorganic MXenes,were also recently considered as adsorbents for salts removal applications.For example,MXene Ti3C2Txnanosheets were fabricated to have high electrical conductivities and high specific surface areas,which led to a removal capacity of 26.8 mg·g-1in capacitive desalination[248].The findings suggest the potential of MXenes for desalination,which deserves further development and investigation.

2.8.Comparison of adsorbents

Metrics for comparing different materials for adsorption desalination is shown in Fig.6.Based on five aspects under consideration,ACs and zeolites are two relatively cheap adsorption materials for desalination with good stability,whilst they have relatively low adsorption capacities (＜52 mg·g-1).AC is the most environmentally friendly and safe material.Both graphene and carbon nanotube have relatively high adsorption capacity and various functional groups with high versatility,but they tend to impose environmental impact and risks to human health.Therefore,appropriate shaping strategies to immobilize them need to be developed for potential practical applications.Shaped aligned carbon nanotube shows the highest adsorption capacity(4,000 mg·g-1)to date[103].In comparison,MOFs have shown relatively high adsorption capacities (168.3 mg·g-1for PSP-MIL-53)[214],also,the high versatility due to the flexibility and tunability of MOFs’ structures and compositions endows MOFs great potential to be applied in adsorption desalination.It should also be noted that MOF-based composite PSP-MIL-53 shows the fastest adsorption/regeneration in less than 30 min.However,the cost and stability are the two aspects which need to considerate in order to advance the application of MOFs.The stability of MOFs should also be improved to avoid environmental impacts such as leaching of heavy metals due to decomposition.Being similarly to MOFs,COFs also have the great potential to be applied in adsorption desalination.However,investigation of COFs in this area is still in its infant stage,and at present,the adsorption capacity,stability,and cost of COFs are not comparable with other candidate adsorbent materials for adsorption desalination.

3.Adsorption Modes for Desalination

3.1.Batch and fixed-bed configurations

Adsorption desalination can be achieved in different modes under different conditions which depends on the type and size of adsorbents.For example,for the powdered-form adsorbent,batchoperation is appropriate to avoid separation and pressure drop issues,whilst regarding the granular forms (or pellets),fixed-bed configurations can be applied.Comparison between batch and fixed-bed operations is presented in Table 3.Under batch conditions,the use of the powdered-form adsorbents leads to fast adsorption kinetics and can reach the adsorption equilibrium quickly due to the reduced mass transfer resistance between the powder adsorbents and adsorbate molecules.However,the recovery and regeneration of powder adsorbents can be difficult and uneconomic,and hence,in most cases,they are disposed or incinerated,which suggests the possible environmental impact.On the contrary,granular/pelletized adsorbents can be prepared as fixed(or backed) beds,in which water can be pumped through the bed with comparatively low pressure drop.Therefore,adsorption is enabled under continuous-flow conditions and additional separation process is not needed.However,due to the large particle size,high mass transfer resistance and short residence time,the adsorption kinetic is relatively slow compared to the powdered forms under batch conditions.

Table 3Comparison between batch and fixed-bed configurations for adsorption desalination

3.2.Capacitive deionization (CDI)

CDI is an emerging desalination technology,in which electrical potential difference is applied over two electrodes to enable eletro-sorption deionization of water.The electrodes are commonly based on porous carbon materials.During CDI,the feed solution flows through the space between the two electrodes or cross them,and the ions in the solution with positive or negative charge move to the electrodes under the electric field force and are adsorbed onto the polarized electrodes [232].Once the electrodes are saturated,regeneration of electrodes is neededviadesorption,in which reverse(or zero)potential was applied to the electrodes,as shown in Fig.7,to enable desorption of the adsorbed ions to the effluent to form concentrate for discharge.CDI systems have advantages such as simple design,low energy consumption and cost-effectiveness,and has been applied to the desalination of seawater/brackish water and wastewater remediation[251-253].It is worth noting that the performance of CDI system depends largely on the porous materials for making the electrodes,and stable and conductive materials with high specific areas and pore volumes and suitable pore size distribution are required to ensure a high salt adsorption capacity.

Carbon-based materials are common for preparing the electrodes in CDI due to their high conductivity and good stability.ACs are the most popular materials among the carbon-based materials since they are economical with high porosity.Other carbon materials were also proposed to design new CDI electrodes to improve the salt adsorption capacity,such as activated carbon fibers,carbon nanofibers,activated/ordered mesoporous carbons,carbon aerogels,carbon cloths,carbide derived carbons,carbon nanotubes and GO [127,254-264].In addition,materials based on redox-chemistry such as metal oxides (e.g.,sodium manganese oxide),MOFs (e.g.,MOF-5,ZIF-8),COFs (e.g.,2,6-diaminoanthraqui none-modified β-keto-enamine-linked COFs) and Prussian blue analogues (PBAs,e.g.,CuFe),were also explored in recent years[233,265-268].

Fig.7.Schematic diagram of adsorption (a) and regeneration (b) process in CDI and the experimental set-up (c).Adapted from Ref.[249,250] with permission.

To tune the selectivity of CDI systems,two aspects can be considered,that is,electrode engineering and operation optimization.Materials selection (according to a CDI process) and optimization(viamodification of the physical/chemical properties of electrode materials) are the initial steps to enhance the adsorption capacity and selectivity of the CDI electrodes.For example,pore size distribution can be adjusted to allow preferential ions to transport selectively across the pore network and gain access to the adsorption site,i.e.,size selective[269].The chemical property of the selected electrode can be further engineeredviaa variety of strategies to improve the selectivity and/or capacity.For example,grafting functional groups,which have the selectivity to the target adsorbate,on the surface of the electrode to enhance the selective interaction between the electrode and adsorbate during CDI [270-274].

CDI was conventionally designed with the water flowing through or between the fixed/moving electrodes,as shown in Fig.8(a)-8(b),which generally requires the separate regeneration step to recover the saturated electrodes after the adsorption cycle,which is not very convenient.To improve CDI,new designs for CDI configuration/operation were proposed and investigated during the past decade,such as flow-electrode CDI (FCDI),membrane CDI (MCDI),rocking-chair CDI (RCDI),and hybrid CDI (HCDI),exemplified by Fig.8(c)-8(e).Specifically,FCDI uses flowable carbon suspensions as electrode rather than static electrodes which enables a continuous desalination process.FCDI process can deliver an unlimited desalination capacity through a consistent supply of flow electrodes that are regenerated in separate units [275].Although being promising,FCDI experiences issues of relatively high energy consumption (which is associated with pumping flow electrode such carbon slurry),possible clogging of the pump and system by electrode suspensions(especially in long-term continuous operation),and low conductivity (compared to conventional fixed CDI) [232,276].MCDI utilizes ion exchange membranes on the separator-side of each electrode.The ion exchange membranes can be either freestanding or coated(directly onto the porous electrode).The main advantage of MCDI is that the membrane can be tailored to have selectivity to different ions [252,277-280].For example,a monovalent membrane can be used to prepare MCDI,which can exclude the permeation of multi-valent ions and allow high permeance for monovalent ions,therefore enhance the CDI selectivity to monovalent ions [280,281].On the contrary,membranes (e.g.,amine functionalized poly(vinyl alcohol) (QPVA) and Ca2+ion exchange resin) which allow selective transportation of bivalent ions (e.g.,Ca2+,) over monovalent ions (Na+and Cl-) can be integrated with a CDI to change the selectivity[282,283].

Fig.8.Schematic of different designs for operating CDI ((a) flow-electrode architectures including feed flow between electrodes,feed flow across electrodes,and CDI with flow electrode;(b) Membrane CDI;(c) rocking-chair CDI).

Although CDI processes are efficient and promising,limitations exist.Degradation of CDI electrodes is one of the primary limitations which reduced the CDI cell life.Oxidation of the positively charged electrode [284,285] is known to affect CDI performance adversely during desalination.Oxidation results in an increase in oxygen-containing surface functional groups on the micropore surface of the carbon anode,which increases the resistivity of the positive electrodes and thus the decreased salt adsorption capacity[286].Thus,further investigation on improving long-term performance of CDI electrodesviamaterials innovation and/or new engineering designs are highly encouraged.In addition,regarding FCDI,the electrode materials are still key to the system which directly determine the desalination capacity.Efforts need to be geared towards the research of carbon materials with high specific surface area and pore volume and formulation of high-performance of electrode suspensions to improve ion adsorption.

3.3.Adsorption membrane filtration

Membrane-based processes such as reverse osmosis and nanofiltration are also effective for water treatment and desalination applications.Membrane adsorption refers to the processes combining membrane separation with adsorption under continuous adsorption conditions,which is also called ‘a(chǎn)dsorption followed filtration’.In membrane adsorption,the adsorbent is suspended in the feed solution,and after adsorption,the membrane can retain the adsorbent in the system and allow the treated aqueous solution to transport across the membrane for discharge.Due to the relatively high flux,microfiltration or nanofiltration membranes,which can reject the adsorbents,are generally used in such process.The main advantage of the membrane adsorption process (compared to the traditional fixed-bed column) lies in the use of adsorbents with smaller sizes (e.g.,1-500 μm),which enhance the accessibility of active sites/pore network of the adsorbents and mass transfer in the system.Therefore,the amount of adsorbents required can be reduced,and thus the operation cost[287].Although,it is possible that cake layer can be formed on the membrane,which can lead to high pressure.Cake formation on the membrane can be avoided to a large extent if the permeate does not exceed the critical value of the flux.In this case,the adsorbent will not settle on the membrane surface,and instead they can be concentrated at the membrane surface in forms of suspension which can flow tangentially to the membrane surface.Therefore,the pressure drop would be low since it only depends on membrane resistance,rather than on particle size.Besides,thin cakes or membrane fouling could also be removed by several effective methods,such as pulsatile flow and turbulence promoters,backpulsing,and backflushing,skimming and gas sparking [288-291].The integrated process of adsorption membrane filtration has been applied for boron adsorption[291].The total cost of the adsorption membrane filtration was evaluated and compared with conventional fixed-bed column adsorbers,with the size of adsorbent being 1 μm and 1 mm,respectively.The findings showed that the operation cost can be significantly reduced from 10.01 Euro·d-1for a fixed-bed column adsorber to 0.07 Euro·d-1for the adsorption membrane filtration [292].

Fig.9.Strategies of fabricating membrane adsorbers ((a) Phase inversion;(b) TIPS-HoP;(c) in-situ immobilization by flow-synthesis).Adapted from Ref.[293].Copyright American Chemical Society,and [294,295] with permission.

Although this method can help to reduce the cost of separation,the process for adsorbent separation still leads to unnecessary energy consumption and cost because addition membrane filtration is needed for recovering adsorbents which are still suspended in the solution after the adsorption process.In addition,if the size of adsorbents is reduced (to ＜ 100 nm) due to attrition,nanofiltration/ultrafiltration is necessary which are associated with high energy penalty as well.Alternatively,structuring the adsorbents in or/on the membrane,with the specially designed water transportation channels,can be the solution to address recovery issues for the suspended fine adsorbent particles.In addition,adsorbent nanoparticles with small sizes (e.g.,＜100 nm) can be anchored within the membrane structure for continuous adsorption.In this case,the surface area and adsorption capacity of the hybrid system (or membrane adsorber) can be tuned by varying the loading of adsorbent nanoparticles in the membrane.

Various approaches to prepare membrane adsorbers have been proposed as illustrated in Fig.9.For example,poly(sulfobetaine methacrylate) (PSBMA)-functionalized MOF UiO-66-PSBMA has been incorporated in polysulfone(PSf)casting solution to fabricate novel hybrid ultrafiltration membranesviathe phase-inversion method [293].However,it was found that high MOF loadings led to phase segregation with the increased fragility and brought the difficulty in producing large-area membranes without cracks.Conversely,low loadings (lower than 1% (mass) of UiO-66-PSBMA in PSf) resulted in low specific adsorption sites and consequently caused low adsorption.Continuous research leads to the advance in the field with new strategies developed,which enabled high loading of adsorbents with highly exposed adsorption sites.For example,Wanget al.[294]developed a variety of membrane absorbers with different MOFs including NH2-UiO-66,MIL-100(Cr),Zn-BLD,NH2-UiO-66-MIL-100(Cr).They achieved high loading of MOFs(～86% (mass)) based on thermally induced phase separation-hot pressing method(TIPS-HoP),and used polymers(i.e.,polyethylene)with higher-molecular weight(＞1,500,000)to prevent the cracking of the membrane with high MOFs loading.Qiuet al.[295],fabricated ZIF-8 and ZIF-67 membrane adsorber by a flow-synthesis method,in which the metal ion solution and the organic ligand solution flow across the polyethersulfone (PES) porous membrane alternatively,leading toin-situcrystallization and stable immobilization of MOFs on the inner surface of the porous membrane.The membrane adsorber showed the improved adsorb capacity compared to bulk ZIF-8 and ZIF-67 due to highly exposed surface area and reduced size of absorbance nanoparticles.The adsorption kinetics was enhanced as well due to the enhanced mass transfer in the confined space in membrane pores.Other materials such as graphene and carbon nanotubes were also explored for constructing membrane adsorbers[117,296].It should be noted that membrane fouling is one of the critical issues in applications related to membrane filtration,therefore,pretreatment of the feed is necessary to remove contaminants as much as possible.In addition,additional energy consumption is also needed if high pressures are necessary to drive membrane filtration processes.

3.4.Comparison of adsorption modes

Fig.10.Metrics to compare different operation modes for adsorption desalination.

Different adsorption modes were compared using the metrics against five aspects of material reusability,long-term stability,energy consumption,adsorption kinetics,and operation feasibility(Fig.10).Specifically,the batch/fixed-bed configuration shows feasible operation and low energy cost,but low material reusability,long-term stability,and adsorption kinetics due to the difficulty of powder recovery and poor mass transfer,or the attrition and high mass transfer resistance of granulated adsorbents.Capacitive deionization shows high material reusability since adsorbents are fixed on probes,but long-term stability and energy cost need to be addressed properly.Adsorption membrane filtration has high material reusability and long-term stability due to the immobilization of adsorbents in the membrane.Rapid mass transfer in the membrane adsorber during filtration also contributes to high adsorption kinetics.However,pretreatment of the feed may be necessary,which reduces the operation feasibility of the membrane-based processes.Also,the high pressure required by membrane filtration leads to additional energy consumption.

4.Summary and Outlooks

Adsorption is a promising technology for desalination which can potentially replace the current thermally-activated and membrane-based desalination processes.Porous materials lie in the heart of adsorption desalination processes.Various porous materials including active carbons,zeolites,carbon nanotubes,graphene,metal organic frameworks (MOFs) and covalent-organic frameworks (COFs) have been investigated as the adsorbent for adsorption desalination,and the metrics show that:

(1) AC and zeolites are cheaper and safe materials with good stability,although they have relatively low adsorption capacity;

(2)graphene and carbon nanotubes have relatively high adsorption capacity as well as high versatility,but their safety risks need to be addressed properly;

(3)MOFs have high adsorption capacity and great potential due to their high versatility in composition and structure,but the cost and stability are keys to advance the application of MOFs in industry;

(4) COFs also have great potential to be applied in adsorption desalination due to its high versatility,although,investigation of COFs at present are still in its infant stage.

In addition,different adsorption modes of adsorption desalination including batch adsorption,fixed-bed adsorption,capacitive deionization and adsorption membrane filtration were also discussed critically,which show that capacitive deionization and adsorption membrane filtration can be promising for developing advanced,efficient and practical adsorption desalination processes.

Based on the critical review presented here,we proposed the following directions of research for consideration by the relevant research community to further progress desalination towards practical applications:

(1) Effective and efficient desorption of saturated adsorbents are critical to practical desalination which can reduce operational cost/time significantly,as well as energy consumption (and hence lower carbon footprints).Therefore,the regeneration properties of adsorbent materials should be thoroughly evaluated and assessed,and feasible strategies for adsorbents regeneration under mild conditions should be developed.

(2) Powdered form adsorbents are advantageous in many aspects in comparison with the shaped ones.However,most of the current research was focused on the adsorption performance of the powdered forms,and there is still gap to translate the relevant materials developed to practical adsorption desalination at large scales.Accordingly,feasible strategies to utilize powdered form adsorbents effectively at scales should be investigated and developed.

(3) Most of the studies were performed in laboratory/pilot scales with the simulated brackish water and/or synthetic seawater rather than real ones.The complexity of real water samples such as different pH values and existence of multiple compounds can have significant effects on adsorption behaviors(such as kinetics,capacity,and stability) of different porous adsorbents.Therefore,relevant research of developing and assessing practical porous adsorbents against adsorption desalination of real seawater,brackish water and/or wastewater should be considered.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Patricia Gorgojo is grateful to the Spanish Ministerio de Economía y Competitividad and the European Social Fund for her Ramon y Cajal Fellowship (RYC2019-027060-I/AEI/10.13039/5011 00011033).B.Q.thanks the China Scholarship Council(202006240076)-University of Manchester joint studentship for supporting her PhD research.