Organic polymer materials are widely used as support materials because of their adjustable pore size, abundant functional groups and good stability. Organic polymer carriers mainly include synthetic polymers such as ion exchange resins and natural polymers (Table 1). Porous organic polymers can effectively control the size of metal nanoparticles through the spatial confinement of cooperative pores, immobilize metal species through the coordination or electrostatic interaction between organic functional groups and metal ions, and improve the pollutant purification efficiency through the interaction between carriers and target pollutants.

(1) macroporous ion exchange resin

Ion exchange resin is a kind of polymer material with crosslinked network structure, which is usually modified by suspension copolymerization of styrene and divinylbenzene, followed by sulfonation, chloromethylation, amination and other chemical reactions. Macroporous ion exchange resins with good macroporous network structure, high mechanical strength and certain specific surface area can be obtained by adding pore-forming agents in the copolymerization reaction, which have been widely used in many industrial fields. Ion exchange resin is an ideal carrier for the preparation of nanostructured composites because it can immobilize nanoparticles through the mesh confinement effect and has a pre-concentration effect on charged pollutants through the Donnan membrane effect.

The milli-nanostructured composite materials with macroporous resin as the carrier are usually prepared by the "dipping-in situ growth method" (also known as "precursor introduction-nano-mesh confined nucleation"), in which the precursor of nanoparticles (such as metal ions or metal complex molecules) is introduced into the resin by ion exchange, evaporation and concentration, etc.Then, the precursor is promoted to grow into the target nanoparticle material in the nano-mesh by heat treatment, alkali treatment, redox reaction, etc., to prepare the macroporous resin-based milli-nano structure composite material (Figure 1b). Pan Bingcai et al. Used this method to load nano-hydrated zirconia (HZO), hydrated lanthanum oxide (HLO), hydrated iron oxide (HFO) and nano-zero-valent iron (nZVI) on macroporous strongly basic anion exchange resin D201.A series of milli-nano structured composite materials are prepared, which show fast adsorption kinetics and high adsorption selectivity for inorganic pollutants in water, such As phosphate, heavy metal complexes, As (V), etc., and can be efficiently desorbed and regenerated^{[1][2,3][4,5][1][6]}. Caspian University and Bangladesh University of Engineering and Science have developed a commercial adsorbent ArsenX^np for arsenic removal, which is composed of macroporous anion exchange resin loaded with HFO nanoparticles. The apparent diameter of the adsorbent is 0. 3 ~ 1. 2 mm, and the loading of HFO on the resin is 150 mg Fe/G. The material has high mechanical strength and good stability, and is suitable for fixed-bed reactors^[7,8].

In addition, milli-nanostructured composites suitable for macroporous ion exchange resin supports can also be prepared by the "ion exchange-in situ deposition" method (Figure 1 C). The method utilizes quaternary ammonium groups uniformly distributed on a polystyrene skeleton to introduce a precursor into resin pore channels through ion exchange, and then uniformly loaded nanoparticles are prepared through an in-situ deposition method^[9]. For example, Pan et al. Used \mathrm{F} \mathrm{e} \mathrm{C} {{\mathrm{l}}_4}^{-} as HFO precursor to prepare macroporous anion exchange resin-based nano-HFO composite (HFO-201) by ion exchange-in situ deposition method, which has good adsorption and removal capacity for phosphate^[10].

However, the pore structure of macroporous resin support is often disordered and the pore size distribution is wide (10 ~ 200 nm), which leads to the size of nanoparticles growing in the pores varies from several to tens of nanometers, and it is difficult to achieve size uniformity^[4,11]. The formed nanoparticles are easy to block the pore channels of the carrier and reduce the availability of the active sites. For example, the in situ growth method encapsulating nano-iron oxide in porous polystyrene and mesoporous silicate caused severe pore blockage, which inhibited the diffusion of pollutants to the surface of Nanoparticles (NPs) and led to a 35% reduction in the density of active sites^{[12⇓~14][15]}. Another preparation method, in which the polymer precursor is mixed with NPs followed by a phase change, results in a composite with a lower porosity^[16]. The phase separation of the polymer matrix from NPs at the solid-liquid interface leads to severe agglomeration of NPs, such that the number of active sites accessible to contaminants is reduced^[17,18]. In addition, the content of nanoparticles loaded in the carrier also has a great influence on the adsorption performance of the composite. Some scholars have found that when HFO is encapsulated in the carrier, the increase of Fe content leads to pore blockage, which leads to the decrease of adsorption site density and adsorption capacity^[19,20]. In one study, α-Fe₂O₃ and Fe₃O₄ nanoparticles were encapsulated in macroporous polymer gel, and the adsorption capacity for As (Ⅲ) was found to be reduced by 30%^[21]. At present, it is still a challenge to prepare resin materials with uniform pore size distribution and control the size and uniformity of confined nanoparticles.

(2) gel-type resin

Because the preparation of the gel type resin does not need to add a pore-forming agent, the production cost of the gel type resin is obviously reduced compared with that of the macroporous resin. However, the chemical structure of gel-type resin is not permanent, and only after full swelling in water, it produces a pore structure with a diameter of 0. 5 ~ 5.0 nm, and the overall porosity is small. The method for preparing the milli-nano structure composite material by using the characteristic that the non-porous gel swells to form pores when meeting water is mainly a "swelling-in-situ deposition" method (fig. 1e), that is, the non-porous gel and a nanomaterial precursor solution are stirred to be uniformly distributed in the pores of the gel,The nano-particles are deposited in situ in the gel pores with a spatial grid structure by adding an alkali solution and the like, and after vacuum drying and dehydration treatment, the pores generated by swelling disappear, and the nano-particles in the pores are extruded by a crosslinking skeleton to realize a confined structure, so that the milli-nano structure composite material is obtained. For example, Liu Yan et al. Used the commercial gel-type strong base anion exchange resin 201 × 4 as the matrix to prepare the composite material loaded with nano-HFO by the "swelling-in situ deposition" method, and found that the HFO particles played a supporting role in the gel resin skeleton, which made it have obvious physical pore structure and significantly improved the mechanical strength^[22]. The HFO nanoparticles are mainly needle-like and have a large specific surface area, and the available active site density is increased by about 67% compared with the macroporous composite material. Due to the lack of rigid pores, the swelling rate of gel resin in water is large, the mechanical strength is low, and it is easy to break in the process of repeated regeneration and swelling, which affects its long-term application performance.

(3) Homopolymeric hypercrosslinked resin

At present, the commonly used polystyrene resin is mainly prepared by suspension copolymerization, and then the pore structure is regulated by controlling the degree of post-crosslinking reaction. Compared with macroporous resin, hypercrosslinked resin has a certain uniformity of pore size distribution, and has the characteristics of high pore volume and strong mechanical strength. Li et al. Prepared milli-nanostructured composite HFO-NS by "ion exchange-in situ deposition" method in three cross-linked anion exchange resins (NS) with different pore size distributions to study the effects of confined space size on HFO crystal form, particle size, surface chemical properties and As (V) adsorption kinetics^[12]. The results showed that the average particle size of confined HFO NPs decreased from 31. 4 nm to 11. 6 nm with the decrease of the average pore size of the anion exchange resin from 38. 7 nm to 9. 2 nm, and the density of surface active sites increased due to the size effect, and the adsorption capacity of As (V) increased significantly.

(4) Other synthetic polymers

Polystyrene beads and sponges are the main synthetic polymers used as supports for milli-nano structured composites. The porous structure can be used to infiltrate the precursor solution of the nanomaterial into the pores of the porous material by means of vacuumizing or soaking, and then the milli-nano structure composite material can be prepared by drying, baking and other processes.

Zhang et al. Developed a sub-10nm iron oxide composite preparation method based on the "flash freezing-in situ growth" technique (Fig. 1 d)^[13]. In this method, the microphase separation between polymer molecules and solvent molecules is initiated by rapid cooling, and the pore-forming effect is produced by the micro-nano crystallization of solvent molecules to form a rich and ordered mesoporous structure, and then the sub-10 nm iron oxide is formed in the nanopore by the "in-situ growth method". The prepared nanostructured nanocomposites have a uniform size of about 1 mm, and the iron oxide nanoparticles in the nanostructured nanocomposites have a narrow particle size distribution (2. 0-7. 3 nm), which is adjustable and well dispersed, and can avoid the blockage of the pore structure of the carrier while obtaining a higher loading, thus ensuring the high availability of the active sites on the surface of the nanoparticles. Compared with the rod-like iron oxide (18 nm × 60 nm), the as-prepared nanostructured composites have a 10 ~ 15 times higher standardized treatment capacity for arsenic, showing significant size effect and surface effect.

Sponge is a kind of porous polymer with high specific surface area, such as polyurethane sponge, which is widely used. It has chemical inertness and good mechanical properties, and its pore structure composed of organic framework can load functional nanomaterials. In previous studies, nano-functional materials were loaded on the polyurethane skeleton by hydrothermal growth method, that is, polyurethane sponge was placed in metal ion solution, and nanoparticles were attached to the surface of the skeleton under high temperature and high pressure. In order to prevent the polyurethane sponge from dissolving, the water temperature used should be lower than 180 ℃. For example, Nguyen et al. Used the hydrothermal growth method to load iron oxide particles into polyurethane sponge to obtain polyurethane sponge composite, which was applied to the adsorption of As (Ⅲ) and As (V) in water. For the solution containing As (V) with an initial concentration of 1000 μg/L, the adsorption column after 28 days of continuous operation can achieve a stable removal of the effluent concentration of less than 50 μg/L^[23].

The synthetic polymer materials mentioned above (such as polystyrene resin and polyurethane sponge) have rich pore structure and high specific surface area, and can be reused many times after elution, but most of them are not easy to biodegrade, so secondary treatment after waste should be considered. For example, the polystyrene matrix is easy to embrittle, the low temperature impact resistance is poor, and the heat deformation temperature is relatively low, so there may be a potential risk of microplastics in the long-term use process. Therefore, the structural stability of organic polymer-based nanocomposites still needs to be further improved.

(5) Natural polymer materials

Synthetic polymer materials are relatively expensive, while natural polymer materials, such as chitosan and cellulose, have the advantages of wide source, low cost, environmental friendliness and good biodegradability. Among them, cellulose is a macromolecular polysaccharide composed of glucose units, which mainly exists in plant cell walls and is the most widely available in natural polymer materials. At present, researchers use cellulose loaded functional nanomaterials to achieve selective removal of pollutants in water. Among them, the representative metal oxides used for nanocellulose-based composite adsorption materials are iron oxide (β-FeOOH), zinc oxide (ZnO) and titanium dioxide (TiO₂), etc^[24][25][26]. Metal oxide nanoparticles and cellulose are adsorbed on the surface of nanocellulose through van der Waals force and electrostatic attraction to form a composite material, which is directly used for pollutant removal. For example, Alipour et al. Modified Fe₃O₄ and ZnO magnetic nanoparticles by thiourea and coated them on the surface of cellulose to obtain a composite material, which had a maximum adsorption capacity of 554.4 mg/G for Pb^2+[25].

(1) granular activated carbon

Activated carbon is a kind of carbon-based material obtained by carbonization and activation of carbon-rich materials, which has many advantages, such as large specific surface area, rich surface chemical groups, developed pore structure system and so on, and has been widely used in practical water treatment projects. Activated carbon adsorption is one of the important process units in water treatment plants. The preparation of nanocomposites by loading nanoparticles on granular activated carbon carriers with abundant pores can fully combine the high selectivity and adsorption activity of nanomaterials with the operational flexibility of the carriers themselves. The preparation method of granular activated carbon matrix composite is mainly impregnation-calcination method, that is, the precursor solution of nanomaterials is infiltrated into the pores of porous materials by means of vacuumizing or soaking, and the composite is prepared by drying and calcination after impregnation and equilibration (Fig. 1f). For example, Mullick and Neogi prepared activated carbon-zirconium dioxide nanocomposite by loading zirconium dioxide on activated carbon, and the study showed that the specific surface area of activated carbon-zirconium dioxide was 1103 m²/g.Zirconium dioxide particles were uniform in size (about 52. 3 nm) and stably and effectively dispersed and immobilized on activated carbon, showing good fluoride removal adsorption performance and less interference from coexisting anions in water^[27]. However, the encapsulation of nanoparticles in activated carbon may lead to a decrease in the surface area of nanoparticles and may also block the pore size of activated carbon^[28]. For example, in one study, even if the proportion of Fe in the HFO-loaded composite was increased to 79%, the adsorption capacity of arsenic was only 0.024 mg/G^[29]. In addition, the equilibrium time for the adsorption of arsenic in water by the composite adsorption material with activated carbon carrier is longer, and the adsorption performance is greatly affected by pH. In addition, the mechanical strength of activated carbon materials under high fluid resistance is also facing a certain test, and the crushing and loss of carrier materials inevitably lead to the loss of active nanoparticles.

(2) Biochar

Biochar is often obtained by pyrolysis and carbonization of straw, sawdust, fruit peel, fruit shell, sludge and other biomass under anaerobic or anoxic conditions, which is extremely rich in raw materials, with high specific surface area, rich pore structure and stable chemical properties, and has significant economic and environmental benefits. The preparation of milli-nano structure composite material with biochar as carrier is mainly to mix and impregnate biomass or biochar with metal salt precursor or nanoparticle solution, and then generate biochar through different thermochemical processes (such as slow pyrolysis and hydrothermal carbonization) to obtain functional composite material. For example, Sun et al. used a carbonization-activation two-step method (Fig. 1G) to grow FeNi nanoparticles in corncob-derived carbon (CCAC) in a confined manner. The porous structure and high specific surface area of CCAC effectively dispersed the FeNi nanoparticles, which significantly enhanced the photo-Fenton catalytic performance of the composite, and its catalytic degradation efficiency for rhodamine was 2.3 times that of the unconfined FeNi nanoparticles^[30]. Wang et al. Loaded nanoscale zero-valent iron (nZVI) on the surface of konjac glucomannan derived carbon by impregnation-pyrolysis reduction method, and the nZVI was uniformly coated in the carbon carrier, which effectively slowed down the oxidation and agglomeration of nZVI^[31]. The composite material showed a uranium enrichment rate of up to 90.1% within 60 min, and the enrichment capacity was 720.8 mg/G. However, most of the current studies on biochar adsorption of pollutants are laboratory studies, lacking long-term pilot studies and large-scale application reports, and there are few studies on the regeneration and reuse of materials, so far there is a lack of common standards for biochar preparation methods and application processes. At the same time, large-scale production is also a key factor limiting the large-scale commercial application of biochar^[32,33].

Natural mineral materials are a class of environmentally friendly materials in nature, which are abundant and have no potential secondary pollution, and have become one of the key directions of functional materials for water treatment. The main minerals used to prepare functional materials for water treatment are zeolite, diatomite, bentonite, etc. (Table 2). Due to the stable physical pore structure of these natural mineral materials, the preparation method of milli-nanostructured composites based on these materials is mainly the "impregnation-calcination" method (Fig. 1 f).

(1) Zeolite

Natural zeolite is a kind of porous hydrous aluminosilicate with wide distribution, low price and skeleton structure, which has the ability of ion exchange and adsorption. Natural zeolites have been used in engineering applications such as ammonia nitrogen adsorption, heavy metal and radioactive element fixation. In recent years, researchers have used natural zeolite as a carrier to improve its adsorption capacity and selectivity by loading functional nanoparticles (such as Fe, Mg, La oxides). For example, Zeng Qi et al. First loaded silica on natural zeolite, and then loaded lanthanum hydroxide nanoparticles with high adsorption activity for F^-, which significantly increased the specific surface area of natural zeolite and significantly improved its adsorption performance for F^-, with the maximum adsorption capacity of 14. 2 mg/G^[34]. Jiang Yinshan et al. Used natural zeolite as carrier and TiCl₄ as titanium source to load anatase TiO₂ on zeolite without high temperature treatment in aqueous system, which combined firmly and had excellent and stable catalytic performance.The decomposition and decolorization rate of methyl orange and methylene blue under sunlight is nearly 100%, and the catalyst can be reused, which not only improves the adsorption capacity of pollutants on the catalyst, but also solves the problem that the TiO₂ powder is not easy to recycle^[35].

(2) Diatomite

Diatomite is a biochemical sedimentary stone formed by the siliceous wall of diatoms. Its main chemical composition is silicon dioxide. It has high porosity, uniform pore size distribution and high chemical stability. The nano-sized TiO₂ composite photocatalyst prepared with diatomite as carrier can efficiently treat rhodamine B dye wastewater with a decolorization rate of more than 90%^[36]. In addition, the diatomite supported BiOCl/TiO₂ composite prepared by precipitation calcination method can be used to degrade formaldehyde under visible light by using the unique photoelectric properties of BiOCl with layered crystal structure^[37]. When the mass ratio of TiO₂ to BiOCl is 55 ∶ 45, the sintering temperature is 600 ℃, and the pH value of the solution is 6, the degradation rate of formaldehyde by BiOCl/TiO₂ composite is as high as 84. 1%. However, granular diatomaceous earth is fragile when disturbed, which will lead to an increase in water turbidity.

Natural minerals have excellent chemical stability, high specific surface area, porous structure and a variety of functional groups, which are pollution-free when used in ecological restoration and conform to the concept of green ecology. However, the active sites of natural minerals are limited, and the specific adsorption capacity of pollutants is low. The adsorption capacity of natural minerals can be improved by loading functional nanomaterials. At the same time, the natural mineral carrier can fix nanoparticles, combined with the adsorption and ion exchange characteristics of mineral pores, to further enhance the decontamination efficiency of natural mineral-based composites. However, in the past decade, there have been few studies on milli-nano structured composites supported by natural minerals, and the existing literature mainly focuses on the fields of fluoride removal and photocatalysis^[38]. In addition, when natural minerals are used as functional materials for water treatment, it is often necessary to pretreat the organic impurities existing in the natural minerals themselves. Bentonite is sticky and easy to expand when it meets water, so it is necessary to heat treat the raw material of bentonite. If it is directly used in the dynamic water treatment system, it is easy to cause pipeline blockage. At the same time, there are few studies on the recycling of functional materials with natural minerals as carriers, most of which are disposable, resulting in high cost. China is rich in non-metallic mineral resources, including diatomite, zeolite, bentonite and other porous minerals, and the research on making full use of natural minerals to develop high-performance functional materials for water treatment needs to be further strengthened.

UNK 1 porous pellet

As a high-performance catalyst support material, alumina has excellent characteristics such as high specific surface area, pore volume and thermal stability, and is often used as a support to prepare Mn, Ti and Mg composite catalytic ozonation catalysts (Table 3). The particle size of the active γ-Al₂O₃ pellet is 3 – 5 mm, and it has excellent mechanical strength, so it is a carrier material suitable for practical application, and its service life can reach 3 – 5 years in a fixed-bed reactor, so it has broad prospects for industrialization^[39]. Wang et al. Used Fe(NO₃)₃·9H₂O as the metal precursor and citric acid-modified Al₂O₃ beads as the porous support to prepare a composite with highly uniformly dispersed Fe species by the "impregnation-calcination" method, in which the Fe species existed in the structure of Al-O-Fe^III[40]. Compared with the Fe₂O₃/Al₂O₃ catalyst, the prepared material has higher specific surface area, the Lewis acid sites on the surface of the catalyst are significantly increased, and the prepared material shows higher catalytic ozonation activity and organic matter degradation performance. Qiao et al. Prepared metal composite catalyst Mn-Ti-Mg/Al₂O₃ by impregnation method and applied it to the advanced treatment of coal chemical wastewater by catalytic ozonation^[41]. The results showed that the organic pollutants in the wastewater were well degraded by the Mn-Ti-Mg/Al₂O₃ catalytic ozonation process.

(2) ceramic membrane

Ceramic membrane is a kind of inorganic membrane separation material, which is mainly made of γ-Al₂O₃, ZrO₂ and TiO₂. Compared with polymer membrane, ceramic membrane has the advantages of high temperature resistance, chemical corrosion resistance, high mechanical strength and long service life. However, the traditional ceramic membrane separation process only has a single physical separation function, and there is a problem of membrane fouling in operation, which makes it difficult to intercept dissolved organic matter and refractory organic matter in water. The coupling of ceramic membrane and advanced oxidation technology has the functions of separation and catalytic degradation, which can not only improve the effluent quality by catalytic degradation of pollutants, but also effectively alleviate membrane fouling, and overcome the shortcomings of difficult catalyst recovery in advanced oxidation technology alone, so it has attracted wide attention in water treatment^[42]. The ceramic membrane has a porous structure and is an ideal carrier for loading nanoparticles (Table 3). Therefore, the preparation of catalytic ceramic membranes loaded with metal oxides or multimetallic compounds catalysts has become an important basis for promoting the popularization and application of membrane filtration and catalytic coupling technology. Researchers have prepared catalytic separation ceramic membranes by loading catalyst nanoparticles in ceramic membranes, and developed a series of coupling technologies of membrane separation and advanced oxidation, including photocatalysis, persulfate (PMS) activation, catalytic ozonation, heterogeneous Fenton, etc. Zhang et al. Prepared a tubular ceramic membrane loaded with Mn-TiO₂ by impregnation-calcination method, and designed a pilot-scale study of catalytic ozonation coupled membrane separation process for dye wastewater treatment; the water treatment capacity of the process was 10 t/d at an average permeate flux of 110 L/(m²·h), and the effluent COD_Cr concentration was always less than 10 mg/L^[43]. Bao et al. Prepared a porous functional ceramic membrane (CoCM) loaded with Co₃O₄ for activated PMS degradation of sulfamethoxazole (SMX), and the removal rate of SMX increased by 34% compared with the original ceramic membrane^[44]. In addition, the contact angle of CoCM is smaller, the permeability of pure water is higher, and the formation of Co — O — Al bond effectively inhibits the leaching of cobalt during membrane filtration. Chen et al. Supported TiO₂ and Ti-Mn oxide successively in a tubular ceramic membrane, and Ti-Mn oxide particles with a size of about 20 nm were uniformly distributed on the surface and in the internal pores of the TiO₂ ceramic membrane^[45]; Furthermore, a pilot plant of membrane separation coupled with catalytic ozonation process was designed to study the treatment of aquaculture wastewater.Compared with the TiO₂ ceramic membrane, the Ti-Mn-TiO₂ ceramic membrane showed higher catalytic ozonation ability (the removal rate of organic matter was up to 52. 1%) and membrane anti-fouling ability. Chen et al. Prepared a flat ceramic membrane loaded with Mn oxide, and carried out a pilot study on the advanced treatment of secondary effluent from municipal wastewater treatment plant by coupling with in-situ ozone oxidation process^[46]. The results showed that the ceramic membrane loaded with Mn oxide had stronger ozone catalytic activity than the traditional alumina ceramic membrane, and the system operated stably for 48 days at a higher membrane water flux (Dissolved organic carbon)) and a lower ozone dose (5 mg/L), and the effluent Dissolved organic carbon (DOC) concentration was always lower than 5 mg/L. Lee et al. Prepared the Ce-TiO_x modified catalytic ceramic membrane with hierarchical porous structure by sol-dip-calcination method, and the Ce nanoparticles with the size of about 8.3 nm were uniformly distributed on the TiO_x layer^[47]. In the process of DEET degradation by membrane filtration coupled with catalytic ozonation, the mineralization rate of pollutants is as high as 35%.

From the above study, it can be seen that the confinement space of the millimeter-scale carrier can significantly improve the dispersion and stability of nanoparticles, and has a significant regulatory effect on the particle size of nanoparticles. At the same time, the confined space can also control the crystal form and crystal orientation of nanoparticles, thereby affecting their reactivity. The confinement space of milli-nanostructured composites not only affects the structure of the prepared materials, but also affects the reaction process, decontamination mechanism, water phase characteristics and so on. These are attributed to the confinement effect of milli-nanostructured composites, which will be described in detail in the next section.

Previous studies on confined crystallization and other related directions have shown that nanoconfinement can affect the nucleation and growth of nanoparticles from multiple levels, including crystal stability, growth orientation and crystal face exposure. First of all, the spatial confinement may change the growth of nanoparticles from thermodynamically dominated to kinetically dominated, which affects the stability of the crystal form, that is, it is conducive to the formation and stable existence of metastable nanocrystals. Kinetically dominated metastable structures are first formed in the nucleation and growth of crystals. Due to the limitation of space size, the metastable structures in the nanoconfined system are difficult to effectively distort or undergo Ostwald ripening, so that the metastable structures can exist and be maintained^[48]. For example, zirconium phosphate crystals mostly exist in the thermodynamically stable α phase in the open system, while a mixture of α phase and metastable γ phase is formed in the confined space of 7. 9 nm. Compared with α-zirconium phosphate nanoparticles, the P — O bond length of confined zirconium phosphate is shorter, the bond energy is larger, the O 1s electron binding energy is increased by about 1. 0 eV, and the adsorption performance for target metal ions is improved by 10 ~ 90 times. The study shows that there is a significant difference in the adsorption mechanism of zirconium phosphate on heavy metals between the confined system and the open system. The former is mainly the internal sphere coordination with strong selectivity, while the latter is the non-specific electrostatic interaction (Fig. 2)^[49].

Secondly, the type of confined space also affects the orientation of crystal growth and the exposure of dominant crystal planes. With the decrease of the confinement size, the influence of the carrier on the nanoparticles increases, and the shape, crystal growth orientation and dominant crystal plane of the nanoparticles may be significantly different. For example, the growth orientation of hydroxyapatite in the columnar pores of etched membranes and anodic aluminum oxide (AAO) membranes is quite different from that in the open system. In the confined system, hydroxyapatite grows mainly along the [001] crystal axis, forming a dominant orientation along the long axis of the columnar nanopores^[50]. At the same time, the physical and chemical properties of the support can also affect the nucleation and growth of nanoparticles. Due to the influence of the hydrophilic and hydrophobic properties of the support and the functional groups, the nanopore wall can exert obvious interfacial tension and electrostatic interaction on the crystal nucleus, thus changing the volume free energy and surface free energy of crystal nucleus growth, and further affecting the critical size of crystal nucleation, the shape of crystal nucleus and the growth orientation. For example, Bi nanorods grow along the (202) crystal plane parallel to the film plane in the pores of the AAO film etched by phosphoric acid, while there are two main crystal planes of (012) or (202) in the AAO film etched by sulfuric acid^[51]. Completely different from the Fe₃O₄ nanoparticles with the main exposed crystal face (111) in the open system, the Fe₃O₄ nanoparticles grown in the 3D graphene nanoconfined space preferentially expose the (400) crystal face, and the adsorption capacity of roxarsone is as high as 454. 48 mg/G, which is nearly 2. 8 times higher than that of the former^[52]. Similarly, the confined growth of ZnFe₂O₄ nanocrystals in the graphene lattice exhibited a high proportion of exposure of (311) facets, which led to a substantial enhancement in the photocatalytic degradation rate of methylene blue, about 20 times higher than that of the previously reported ZnFe₂O₄ spinel materials^[53]. From the above study, it can be seen that nanoconfinement can be used as a means of crystal plane control to control nanoparticles with highly active crystal plane exposure, thereby improving the decontamination performance of materials. However, there is a lack of in-depth research on the confined growth characteristics of nanoparticles in milli-nano structured materials for water treatment, and it is urgent to systematically explore the influence of the size, shape and surface chemical properties of the confined space on the crystal structure of nanoparticles and the means of regulation.

The confined space can not only change the crystal growth behavior of the immobilized nanoparticles, but also change the adsorption performance of nanomaterials to pollutants by changing the solution properties of the confined phase and the acid-base properties of the interface, regulating the internalization process of pollutants, and improving the anti-pollution performance.

The surface physicochemical properties of confined nanoparticles are often significantly different from those of bulk materials. For example, the point of zero charge of hydrated manganese oxide in nanoporous polystyrene confined system increases from about 6.2 in open system to about 10.5 in confined system, and the surface charge reverses from negative charge to positive charge under nearly neutral conditions, resulting in a nearly 10-fold increase in phosphate adsorption capacity^[54]. In the open system, the point of zero charge of nano-goethite is about 9. 2, which rises to about 10. 5 in the nanopore of 7. 9 ~ 26.2 nm, and the saturation adsorption capacity of As (V) is increased by 7 ~ 10 times^[13,55]. With the decrease of the average pore size from 38. 7 nm to 9. 2 nm, the surface net charge of ferrihydrite loaded in the pore increases gradually, and the adsorption performance of As (III/V) increases significantly^[12]. By fitting the surface complexation model, it was found that the adsorption affinity of confined ferrihydrite for H⁺ was weaker than that of the open system, and accordingly, the adsorption affinity of confined ferrihydrite for metal cations such as Cu²⁺ was weakened, while the adsorption affinity for anions such as \mathrm{A} \mathrm{s} {\mathrm{O}}_4^{3-} was significantly enhanced^[15].

In addition, the properties of the aqueous phase in the nanoconfined system also change significantly. Water phase is the working environment for the purification of pollutants by nanomaterials, and the properties of water phase have an important influence on the interfacial process and mechanism of pollutant purification. For example, the dielectric coefficient of water in low-dimensional nanopores decreases sharply, which makes it easy for pollutants to form electrically neutral ion pairs with ions of different charges through association, thus changing the Coulomb interaction between nanomaterials and pollutants^[56]. In addition, the decrease of the dielectric constant of confined water reduces the hydration number of ions, which in turn affects the interaction between ions and adsorption sites and the adsorption mechanism^[57]. For example, zeolites and other materials have nanopore structures, and ions larger than the size of the pore structure need to be partially dehydrated before entering the pore channel. The adsorption mechanism of the dehydrated ions in the nanopore changes from surface ion exchange to inner sphere complexation, thus increasing the adsorption capacity and selectivity^[58]. At the same time, many studies have observed that the pH of the solution in the nanoconfined space is significantly lower than that in the open system, and the adsorption process of heavy metals, arsenic, phosphate and other pollutants on iron oxides is very sensitive to pH, which can be dominated by monodentate mononuclear coordination, bidentate binuclear coordination, outer ring coordination or surface precipitation at different pH^{[59,60][61,62]}.

In addition to changing the adsorption mode of phosphorus, arsenic, fluorine and other pollutants on nanoparticles, confinement can also promote the reversible internalization of target pollutants in nano-metal oxides under certain conditions. The so-called internalization refers to the process in which pollutants are adsorbed by solid phase materials and then enter their crystal lattices to form new crystal structures. For example, the main mechanism of phosphate removal by hydrous lanthanum oxide (HLO) is inner sphere complexation, whereas the HLO nanoparticles confined in the crosslinked polystyrene support immobilize phosphate through the formation of LaPO₄·xH₂O nanocrystals, and the regeneration efficiency can be increased to about 95%^[4]. For another example, nanoconfinement changes the adsorption of phosphate on La(OH)₃ from surface coordination to lattice internalization, which can be transformed into LaPO₄ crystals in only about 1 day (Figure 3A). This may be due to the fact that the nanoconfinement makes the La(OH)₃ in a metastable state, and phosphate can quickly enter the lattice to achieve internalization^[63]. At the same time, the LaPO₄ formed by internalization is also in a thermodynamic metastable state, and the OH^- can quickly enter the crystal lattice during regeneration to promote the transformation of the LaPO₄ crystal into a La(OH)₃,O as to realize the reversible interconversion between the crystal forms of the LaPO₄ and the La(OH)₃ and successfully break through the bottleneck that the La(OH)₃ is difficult to regenerate and recycle after adsorbing and dephosphorizing^[4].

In addition, the carrier space structure of the milli-nano structure composite material can improve the enrichment, adsorption selectivity and anti-coexisting matrix interference performance of the material to the target pollutant through the Donnan film effect. For example, when ion exchange resin is used as a millimeter-scale carrier, its positively charged quaternary ammonium functional groups exclude cations (such as Ca²⁺, Mg²⁺, and K⁺) in water through the Donnan membrane effect, while enhancing mass transfer and pre-enrichment of inorganic anionic pollutants (such as arsenic, fluorine, and phosphate), which can be removed by adsorption of immobilized nanoparticles (such as HFO and HZO) (Figure 3B)^[2,11,12,64].

Nano-confinement can also significantly improve the anti-pollution ability of nanoparticles. Ca²⁺, Mg²⁺ and silicate, which coexist widely in water, are easily adsorbed by nano-hydrated iron oxide (HFO) and polymerized on the surface to form scale, resulting in a significant decline in the deep decontamination performance of the material. Although some studies have been carried out to alleviate the adverse effects of coexisting substances on advanced water treatment by adding scale inhibitors, coagulation and membrane separation, it is difficult to fundamentally overcome the problems that the scale content is much higher than the target pollutant and the nano-HFO is seriously polluted by scale. Zhang et al. Found that the scaling behavior of Ca²⁺ and Mg²⁺ plasma in confined nano-iron oxide composites was quite different from that in open systems, which changed from mainly scaling on the surface of nano-HFO to zonal independent crystallization.The coexisting ions such as Ca²⁺ and Mg²⁺ not only do not affect the adsorption activity of confined HFO, but also provide additional adsorption sites through the formation of phosphate precipitates, which greatly improves the adsorption performance of pollutants such as phosphate (Figure 3C)^[65].

Nano-confined catalysts have been widely used in the study of catalytic oxidation of organic pollutants in water, and the confinement effect has led to significant enhancement of the activity and stability of many catalysts. Among them, the development of some nano-confined catalysts is mainly used to explore the performance and mechanism of confinement effect, such as carbon nanotubes, graphene, metal organic frameworks and other materials as carriers, which are still easy to agglomerate and difficult to recycle, and are difficult to be applied to large-scale water treatment. The following will focus on the catalytic oxidation characteristics of confined catalysts with milli-nano structure, such as composite materials with alumina templates, alumina spheres and ceramic membranes as carriers.

Firstly, the nanoconfined space can effectively shorten the mass transfer distance from free radicals to pollutants, and solve the problem of low utilization efficiency of hydroxyl radicals (HO ·) caused by mass transfer limitation in heterogeneous Fenton reaction, thus rapidly degrading pollutants. For example, when the Fe₃O₄ nanoparticles grown in the nanopores of the alumina template are used as Fenton-like catalysts, the efficient degradation of methylene blue can be achieved in a very short residence time (14 s), which is far better than the catalytic reaction of unconfined Fe₃O₄ nanoparticles under the same conditions (≥ 3 H)^[66]. The study suggests that within the critical confinement scale of 20 nm, the catalytic reaction can achieve high-intensity and intensive space utilization of HO · (Figure 4A).

Secondly, nanoconfinement can improve the activity of the catalyst by regulating the solution properties in the microenvironment. The active sites on the surface of iron oxides are easily competed by hydroxyl ions, which leads to the sensitivity of catalytic Fenton-like reaction activity to the pH of the solution. The unique hydrogen bond network structure of water molecules in the nanoconfined space, the interaction of the "pore wall-nanoparticle" surface electric double layer, and the functional groups of the carrier can all lead to the change of the pH of the solution in the microdomain^[67][68,69]. For example, when the pore size of AAO is below 58 nm, the pH of the solution in the confined space is lower than that in the open system, and decreases with the decrease of the pore size of the membrane, which leads to the degradation rate of phenols, aniline and other pollutants by Mn₃O₄ formed in AAO nanopores (3 ~ 5 nm) is three orders of magnitude higher than that in the open system^[70,71][72]. Similarly, the pore size of mesoporous silicon decreases from 1000 nm to 1 nm, and the difference between the pH of the solution in the pore and the bulk increases from nearly 0 to nearly 2 units^[73]. At the same time, some studies have proposed that nanoconfinement can adjust the microenvironment of catalytic sites, thereby changing the Fenton-like reaction path and kinetic process^[74]. However, at present, there is a lack of characterization means and computational methods to deeply analyze the micro-mechanism and micro-environment.

In addition, nanoconfinement can also provide new ideas for improving the oxidation selectivity of advanced oxidation reactions. For example, the active species produced by Fenton reaction is generally HO ·, which is easily quenched by dissolved organic matter widely existing in water, and the oxidation selectivity of target pollutants is insufficient. Zhang et al. Used FeOCl nanocatalyst grown in situ in the pores of ultrafiltration ceramic membrane to carry out confined catalytic Fenton-like reaction (Fig. 4B), and found that nanoconfinement could inhibit organic matter with larger molecular size from entering the pores through steric hindrance effect.At the same time, the high-intensity limited oxidation of small molecular organic matter is carried out in the nano-pores, so as to realize the selective oxidation of small molecular organic pollutants such as p-chlorobenzoic acid and other new pollutants^[75].

In recent years, the research of catalytic separation membrane is increasing. The functional coupling of advanced oxidation and membrane separation not only promotes the large-scale application of advanced oxidation technology, but also realizes seamless docking with existing common processes. Membrane pore filtration can solve the problem of mass transfer limitation relying on diffusion in traditional sequencing batch reactor, and enhance the mass transfer effect. At the same time, the nano-membrane pores are equivalent to numerous nano-reactors, and the confined space can not only improve the free radical generation efficiency of advanced oxidation and the degradation efficiency of pollutants, but also inhibit membrane fouling and achieve the self-cleaning effect of the separation membrane. For example, Chen et al. Prepared a catalytic ceramic membrane doped with manganese oxide in the membrane pores, and coupled it with in-situ ozone and biological activated carbon, and applied it to the pilot study of reclaimed water treatment and reuse, revealing the synergistic effect of the coupling process^[46]. Among them, the manganese oxide ceramic membrane can significantly catalyze ozone oxidation, greatly improve the generation rate of hydroxyl free radicals, effectively remove dissolved organic matter, polysaccharides and proteins in reclaimed water, improve the removal rate of biopolymers and humic-acid-like substances, and greatly reduce the resistance of filter cake layer and gel layer, effectively alleviate the pollution of ceramic membrane.

The milli-nano structural composite material is convenient to use and operate, and can realize continuous flow, large-scale production and engineering application. In the practice of adsorption decontamination engineering, adsorption separation technology is mainly applied through fixed bed/adsorption column. The milli-nano structural composite material is filled into a fixed bed, and the treatment effect of the milli-nano structural composite material on actual water body under the condition of continuous long-term operation is tested, so as to verify the stability of large-scale production and the practical application potential. At present, there are few pilot studies on the application of composite nanomaterials in the field of adsorption and decontamination, and there are even fewer commercial and engineering applications. Relevant research teams at home and abroad, such as Pan Bingcai of Nanjing University, Lee of Seoul University in Korea and SenGupta of Caspian University in the United States, have carried out continuous exploration and research in this direction, and some milli-nano structural composites have achieved large-scale production and engineering application. Based on the types of target pollutants (phosphorus, fluorine, arsenic, heavy metals, etc.), the practical application of milli-nano structure composite materials in the field of adsorption and separation will be introduced below.

Advanced oxidation technologies (such as ozonation, Fenton/Fenton-like, electrocatalysis, photocatalysis) can effectively degrade new pollutants, such as pharmaceuticals and personal care products (PPCPs) and endocrine disrupting chemicals (EDCs), by producing reactive species with strong oxidizing properties. However, a large number of advanced oxidation technology research is still in the laboratory stage, and the practical application is affected by many factors, such as the suitability of catalyst and process equipment, operation cost and so on. The development of milli-nano composite catalyst provides a good platform for the practical application and promotion of advanced oxidation technology.It not only overcomes the problems of easy agglomeration of catalyst particles, difficult separation and recovery, and difficult realization of continuous flow in the traditional sequencing batch advanced oxidation process, but also improves the removal efficiency of pollutants and alleviates pore blockage and pollution by using the confinement effect of nanopores. However, there are few reports on other practical applications of AOPs processes in academic research, such as Fenton, photocatalysis, electrocatalysis, etc., except for traditional Fenton technology^[90]. The reason is that Fenton or Fenton-like reaction processes such as H₂O₂ or persulfate often require a large amount of oxidant, and the transportation and storage of oxidant is a challenge for large-scale wastewater treatment. Electrochemical oxidation technology is difficult to be applied to large-scale treatment projects because of its limited treatment scale and high electrode cost. Photocatalytic oxidation requires high uniformity of light, and the suspended solids and high chroma in wastewater are not conducive to the transmission of light, which limits the efficiency of photocatalytic technology and the feasibility of engineering. Relatively speaking, the preparation method and generator equipment of ozone are relatively mature, which can provide stable oxidant supply for large-scale wastewater treatment. The addition of ozone is similar to air aeration, which can scour the membrane surface and catalyst, and enhance the mass transfer effect. Therefore, the practical application of AOPs process based on catalytic ozonation is relatively more, and the milli-nano structural composite materials used mainly include catalytic separation membranes and functional alumina pellets.

At present, Quan Xie of Dalian University of Technology, Zhang Xihui of Tsinghua University, Masten of Michigan State University, Wang Jianbing of China University of Mining and Technology (Beijing) and Zhang Guoquan of Dalian University of Technology are mainly committed to the development of heterogeneous catalytic ozonation technology and its application in the pilot test and engineering application of actual water bodies at home and abroad. The following is a brief introduction to the characteristics and practical applications of the milli-nano structural composites of these research groups.

1) The mechanism of carrier-nanoparticle interaction is unknown. Nanoconfinement can significantly change the structure of nanomaterials, but there are few reports on the regulation and mechanism of carrier-nanoparticle interaction on the structure and activity of nanoparticles, which is still in the stage of passive discovery and appearance observation. From the point of view of development, the growth mechanism of metal oxide crystals in confined space can be combined with classical nucleation theory.The effects of confinement size and surface properties on the surface energy, critical size of nucleation and supersaturation during the crystal growth of nanoparticles were systematically analyzed, so as to master the control methods of directional exposure of crystal faces of metal oxides and realize the preparation of high-performance composites. In addition, the mechanism of the effect of confined space on the regeneration process of nanomaterials is not clear and needs to be further explored.

2) The understanding of carrier-nanoparticle coupling/synergy is not deep. Although the improvement mechanism of the decontamination performance of nano-functional materials has been recognized by the limited carrier at present, it is still not deep and comprehensive enough, and it stays on the superficial analysis of the treatment effect. For example, the synergistic and coupling effects of carriers and nanoparticles in the decontamination process are seriously neglected, and the adsorption and transformation characteristics of target pollutants on the surface and interface of composite nanomaterials and their process mechanisms are in the black box or gray box stage, which urgently need to be discussed in depth from multiple dimensions by combining theoretical calculations and characterization techniques with high spatial and temporal resolution.

3) Lack of in-situ characterization of decontamination characteristics and processes. At present, there is a lack of research on the pattern recognition of milli-nanostructured composite decontamination, the form and transformation process of pollutants in confined environment, and the reaction process and characterization of nanoparticles, and the information revealed by the existing ex-situ characterization methods such as electron spectroscopy is insufficient, so it is urgent to build an in-situ, high spatio-temporal resolution characterization method. For example, in the process of phosphorus removal, the formation of phosphorus-containing nanocrystals is the key way to improve the capacity and selectivity of phosphorus removal, but the mechanism of in-situ formation and reverse transformation (regeneration) of phosphorus-containing nanocrystals in the confined structure is not clear, which restricts the regulation and enhancement of the structure and process of phosphorous-containing nanocrystals.

1) It is necessary to construct a systematic method for the preparation and structural regulation of milli-nano structural composites. The size of nanoparticles in the existing milli-nano structure composite materials is generally too large, and the nano-effect has not been fully utilized. The results show that when the size of nanoparticles is reduced to less than 10 nm, the proportion of highly reactive edge atoms on the surface of materials increases sharply, resulting in significant size and surface effects. In addition, the adsorption reaction usually occurs only on the surface of the material, while the ions in the solution may produce lattice substitution and solid diffusion with the internal crystal structure of the sub-10 nm particles, thus changing the nature of the adsorption process. Therefore, the development of millimeter-sized composites with sub-10 nm structure is the key to improve the deep removal performance of pollutants. At present, there are two main methods for the preparation of milli-nanostructured composites: confined growth method and embedding assembly method, but they face the problems of uneven distribution, agglomeration and overgrowth of nanoparticles caused by the difficulty of low-dimensional pore diffusion and the high surface energy of small nanoparticles. These problems have brought great challenges to the preparation and mass production of milli-nano structural composites with high stability and high activity. It is still a challenge to realize the macro-scale preparation of milli-nanostructured composites with confined sub-10 nm particles. The innovation of preparation methods depends on further revealing the interaction mechanism between carriers and nanoparticles and clarifying the influence of carriers on the stability and activity of nanoparticles.

2) It is necessary to consider the application and adaptability in real scenarios. It has always been a good vision for researchers to effectively connect scientific discoveries and technological innovations at the molecular or micro level to provide scientific basis and technical support for the synergy of pollution reduction and carbon reduction. Despite the excellent pollutant removal efficiency of milli-nano structured composites with nano-effects, most of the current research work is in the laboratory, and the efficiency test tends to be carried out with simulated sewage and wastewater. Especially for the advanced oxidation technology using nano-confined catalysts, the related research focuses on the research and development of new materials, but few studies focus on its practical application value. It is worth noting that in real scenarios, temperature, dissolved oxygen and coexisting substrates will synergistically affect the purification effect, material stability and regeneration process. For example, shedding, leaching, and regeneration of nanomaterials are issues of high concern for practical applications. In addition, for the development of composite nanomaterial treatment unit process, it is necessary to consider the organic connection and efficient integration with the existing water treatment process. Furthermore, selective adsorption and catalytic oxidation are expected to achieve high-purity enrichment and directional transformation of high-value pollutants in sewage and wastewater, thus building resource recovery strategies and approaches. However, there is an obvious lack of relevant research in this direction, and relevant technological innovation and application are urgently needed.

3) The economy and safety of new nanomaterials need to be systematically evaluated. At present, the preparation steps of composite nanomaterials are generally complicated and the material and energy consumption are high, so it is particularly important to develop an efficient, low-cost, green and mass production process of nanomaterials for practical application. In recent years, scholars have begun to consider the safety of nanomaterials from the perspective of the whole life cycle, taking into account the environmental impact of solvents, energy consumption and waste used in the preparation process.It also includes by-products, microplastics and metal ion dissolution produced in the process of removing pollutants in the application stage of nanomaterials, as well as reagent consumption and energy consumption in the process of regeneration or cleaning, and disposal of waste materials. At present, the safety assessment of nanomaterials mostly focuses on the chemical safety risk of the material itself, ignoring the overall environmental impact caused by the complex interaction with other coexisting substrates, natural environment and equipment used in the long-term operation of actual water treatment.

To sum up, this paper reviews the research status of milli-nanostructured composites in the field of water treatment from the aspects of preparation methods, structural properties, confined growth characteristics of nanoparticles, adsorption characteristics, catalytic oxidation characteristics and practical applications.The research deficiencies and application challenges of related directions are put forward, in order to provide theoretical basis and technical reference for promoting the practical application of nanomaterials.