Recent advances in the detection and removal of heavy metal ions using functionalized layered double hydroxides: a review

Heavy metal pollution is one of the most severe environmental problems, possessing high ecotoxicity and health risk. Therefore, it is important to develop effective methods and corresponding materials for the detection and removal of heavy metals. Recent studies reveal the great potential of layered double hydroxides (LDHs) in detecting and removing heavy metals owing to their designable structure and tunable surface composition. In this review, we majorly discuss the recently adopted detection and removal of heavy metal ions based on LDHs. This review starts with an introduction of the structural characteristics and functionalization of LDHs. Then, the sensing tactics and mechanisms are introduced regarding LDH-based heavy metal ion detection. Based on the type of interaction, the removal of heavy metal ions with LDHs is summarized into two categories: reversible adsorption and irreversible mineralization. This review ends with a discussion on the challenges and future trends of LDH-based detectors and adsorbents for heavy metal ions.


Introduction
As one of the most severe environmental problems, heavy metal pollution causes health risks and is difficult to treat. 1 Even at the ppb level, some heavy metals are still harmful due to their strong binding affinity toward N, O, and S atoms of biomolecules, which inhibits biological functions and interrupts physiological processes. 2For example, the catalytic activity of some enzymes is dramatically inhibited after Hg 2+ or Cu 2+ binding, which is known as heavy metal poisoning.Excess intake of heavy metals also leads to a variety of diseases, such as Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. 3,4In addition, heavy metals show high stability in many environments in comparison to organic compounds. 5As a consequence, the detection and removal of environmental heavy metals is significant to ecological security.
][8][9][10] For instance, rhodamine and coumarin derivatives have been widely explored as fluorescent probes for various heavy metal ions. 11Using hyperbranched polyethyleneimine-capped silver nanocluster as fluorescence probe, Yuan et al. achieved sensitive Cu 2+ detection with a limit of detection (LOD) of 10 nM. 128][19][20][21] Their easy preparation, low-cost, and exchangeable interlayer anions make LDHs attractive in removing heavy metal ions in comparison to other materials. 22,23For example, Li et al. reported the selective and electrochemical detection of Pb 2+ with Fe/Mg/Ni ternary LDHs. 24Using CaAl-LDH as an adsorbent, Kong et al. explored efficient Cd 2+ removal in aqueous media with a high adsorption capacity of 592 mg g −1 . 25In consideration of the adjustable layer composition and interlayer anions, appropriately functionalized LDHs can be applied for productive heavy metal ion detection and removal by integrating proper monitoring.We have witnessed recent advances in LDH-based probes and adsorbents in rapid and efficient heavy metal ion detection and removal due to their improved sensitivity, selectivity, and stability.
Despite the report of few excellent review articles on LDHbased adsorbent exploration in the past few years, [26][27][28][29] summarizing some of the most recent developments in both heavy metal ion detection and removal will help junior researchers to understand the fundamental principles and to select proper strategies for constructing expected LDH probes and adsorbents.In this review, the structure and functionalization of LDHs are first introduced.Then, recent advances in LDH-based analysis systems for heavy metal ions (e.g., Cd 2+ , Pb 2+ , and Cu 2+ ) are summarized in four aspects: colorimetric assays, fluorimetric assays, electrochemical assays, and atomic absorption spectrometry (National standard method).Meanwhile, the sensing mechanisms or approaches of analyte-induced signal changes are discussed.Recent studies showed that ion exchange-induced mineralization is more effective to realize ultrastable adsorption of heavy metal ions.Thus, regarding the LDHbased nanosorbents, the removal of heavy metal ions is classified into reversible adsorption and irreversible mineralization for the first time.In consideration of the page limit, only few examples are selected in each described section.Finally, this review ends with the discussion of current challenges and future prospects of LDHs for sensing and mineralization applications.

Structure and functionalization of LDHs
LDHs are a large class of two-dimensional materials with positively charged brucite-like metal hydroxide layers and charge-balancing intercalated anions, as shown in Fig. 1a. 30he metal ions of outer layers possess unsaturated coordination, which requires auxiliary chelation with hydroxyl groups.The abundance of surface hydroxyl groups usually makes LDH suspensions alkaline.The structural formula of the LDH can be described as [M 2+ 1−x -N 3+ x (OH This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

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to form MN-LDHs rather than M(OH) 2 and N(OH) 3 .So far, many synthetic procedures have been explored for the preparation of LDHs, including coprecipitation, hydrothermal synthesis, ion exchange, structure reconstruction, and so on. 31imilar to other two-dimensional materials, LDHs also show large specific surface areas and high adsorption capability toward ions and molecules. 32,33And the adsorption performances of LDHs are related to the chemical composition, structure, shape and size of inorganic metal hydroxide layers and also the interlayer gallery anions.In addition, the chemical reactivity of LDHs connects with the thickness of the cationic layer, which is controlled by the chemical identity of the metal ions and the size of intercalated anions.Interestingly, the chemical composition of metal hydroxide layers, as well as the interlayer anions can be precisely controlled. 34espite the easy preparation of LDHs with low costs, the finite functional groups and simple structural components limit the applications of original LDHs in sensing and adsorption.Therefore, it is important to enhance the performances or create new functions of LDHs by importing functional groups or structural components.To increase the functional groups, two pathways are considered.One is to introduce functional molecules or compounds; the other is to break the integrated structure of LDHs.The former fixes functional molecules on the LDH surface or into the interlayer, while the latter creates defects on LDHs.Functionalization of LDHs can be achieved simply by surface modification with organic molecules containing functional groups.The increased functional groups promote the binding affinity for various heavy metal ions.By changing organic molecules into nanomaterials, the resulting nanomaterial-LDH hybrids contribute combined functions toward different applications, such as adsorption, magnetic separation, catalysis, etc.In addition, the change of interlayer anions into inorganic and organic anions, coordination compounds, polyoxometalates, and biomolecules is also an effective way to construct function-designable LDHs. 35Moreover, substituting metal cations of hydroxide layers with various divalent and trivalent metal ions also benefits the binding of various small molecules or polymers. 36Meanwhile, the substitution might produce defects into the LDH layer, which is regulated by changing the dosage of the metal source. 30,37s referred in Chen's report, oxygen defects into the LDH layer enhance the ion exchange between the LDH layer and heavy metal ions. 38Besides metal substitution, plasma treatment is also an effective means to generate defects into the LDH layer.In a word, the functionalization of LDHs could be realized by surface modification, nanocomposites, intercalation science and defect engineering (Fig. 1b).

Detection of heavy metal ions
As mentioned above, heavy metal pollution causes severe environmental problems, and the development of sensitive and selective detection methods is attracting growing attention in recent years.The rich hydroxyl group on the surface, as well as the various interlayer anions, makes LDHs a good candidate to construct efficient heavy metal ion sensors by integrating proper techniques.Accordingly, for the detection of heavy metal ions, LDH-based sensing systems have been proposed according to four categories: colorimetric assays, fluorimetric assays, electrochemical assays and atomic absorption spectrometry.

Colorimetric assays
][41][42][43] For example, cysteamine functionalized gold nanoparticles have been explored for colorimetric detection of Cu 2+ through forming aggregates. 12his is because the aggregation of gold nanoparticles alters surface plasmonic resonance and leads to color changes of the colloid solution from red to blue.However, neither dispersive nor aggregated LDHs show a visible colour as in gold nanoparticle systems.As a result, chromophores are introduced into LDH-based sensing systems to produce a colour change.
The metal hexacyanoferrates show unique colours, which has been applied to metal ion identification.On the basis of this character, Wang et al. developed a colorimetric Fe 3+ and Cu 2+ sensing system using Fe(CN) 6  4− anion interlayered natural colour of filter paper.Interestingly, a dramatic colour change was observed within 30 s upon adding Fe 3+ or Cu 2+ .Blue colour appeared after the addition of Fe 3+ , which is due to the formation of Prussian blue, while the introduction of Cu 2+ led to the formation of copper hexacyanoferrate precipitates, resulting in the appearance of reddish brown colour.With naked-eye observation, 50 μM Fe 3+ or 10 μM Cu 2+ were detectable.
In addition to forming colourful precipitates, few transition heavy metal ions possess strong catalytic activity toward H 2 O 2 , called Fenton or Fenton-like reaction, which generates reactive oxygen species and leads to the oxidation of chromophore precursors.Based on this property, Tang et al. reported a colorimetric system toward Cu 2+ and Fe 3+ . 45In their work, ethylenediaminetetraacetic acid (EDTA)-nickel ion (Ni 2+ ) chelates were first intercalated into LDHs by co-precipitation reaction.The chelated Ni 2+ could be replaced by Cu 2+ due to the high complex formation constant of EDTA-Cu 2+ .The Ni 2+ -Cu 2+ exchange reaction was effective under pH 6.0, which extracts soluble Cu 2+ to the LDH surface.As is known, the acidic effect coefficient of EDTA is oppositely proportional to the solution pH.That is, the lower the pH, the stronger the acidic effect coefficient.Large acidic effect coefficient results in low complex formation constant and the release of chelated metal ions.When the solution pH was adjusted to 5, the chelated Cu 2+ was released from the LDH surface and catalysed the oxidation of 3,3,5,5-tetramethylbenzidine (TMB) in the presence of H 2 O 2 through Fenton-like reaction, which produces a chromogenic compound with high absorption coefficients (Fig. 2b).The blue colour of oxidized TMB allowed for the colorimetric detection of Cu 2+ in the concentration range from 0.05 to 100 μM, with a LOD of 10 nM.
They also found that aniline catalytic polymerization was realized in the presence of Cu 2+ or Fe 3+ . 46With the combination of metal ion exchange and aniline catalytic polymerization, sensitive detection of Cu 2+ or Fe 3+ was achieved using LDHs as the support.The enrichment of Cu 2+ and Fe 3+ was conducted at pH 6.5 and 4.5, respectively.Through the pH adjustment, free Cu 2+ or Fe 3+ was released from the complex and catalysed the polymerization of aniline.The strong hydrolysis of Fe 3+ required the enrichment with high acidity.However, in a weakly acidic or neutral environment, Fe(OH) 3 was easily formed.In this case, the EDTA-Fe 3+ complex formation constant at pH 6.5 was smaller than that of pH 4.5.In contrast, EDTA-Cu 2+ complex formation constant increased with the increasing pH from 4.5 to 6.5.Taken together, the enrichment pHs for Cu 2+ and Fe 3+ were 6.5 and 4.5, while the release pHs were 4.5 and 6.5, respectively.According to the absorption spectra of polymeric aniline, the detection of Cu 2+ and Fe 3+ was performed at 0.1 and 1 nM, respectively.

Fluorimetric assays
Fluorimetry shows the advantages of low background, high sensitivity and strong interference rejection, and has attracted wide research interest in the past decades. 47,48Ligand functionalized nanomaterials (e.g., gold nanoclusters and semiconducting quantum dots) have been applied in the detection of heavy metal ions, including Pb 2+ , Hg 2+ , Cu 2+ , and so on. 49,50Since the autofluorescence of LDHs is generally ignorable, fluorescent materials are usually introduced to construct LDH-based fluorimetric heavy metal ion detection systems.Through the fluorescence variation, these systems can be classified into two categories: fluorescence turn-off system and fluorescence turn-on system. 51,52or many heavy metals, the paramagnetic atomic orbital easily causes the fluorescence quenching of adjacent fluorophores, making the fluorescence quenching probes commonly used in the detection of heavy metal ions.To light up LDHs, fluorescent organic or inorganic materials are used. 52In general, ultrathin films with bright emission were fabricated through layer-by-layer assembly of LDHs and fluorophores. 53For example, Shi et al. proposed a Cd 2+ probe based on ZnAl-LDH nanosheets and 2,20-azino-bis(3ethylbenzothiazoline-6-sulfonate) (ABTS). 54As shown in Fig. 3a, fluorescent ultrathin films through layer-by-layer deposition were first constructed.The addition of Cd 2+ caused the distinct fluorescence quenching of ABTS, and the intensity at 435 nm can be used for the monitoring of Cd 2+ concentration, with a LOD of 9.5 nM.Such a Cd 2+ -induced fluorescence inhibition can be recovered by adding EDTA.This is because the complex formation constant of EDTA-Cd 2+ is larger than that of ABTS-Cd 2+ .This character allowed for the cyclic Cd 2+ detection.By replacing ABTS with pyrenetetrasulfonate (PTS), they established a Cu 2+ detection platform with a similar tactic. 55The strong PTS-Cu 2+ binding affinity ruled out the interference from other metal ions, such as Pb 2+ , Co 2+ and Ni 2+ .In addition to organic fluorophores, fluorescent inorganic nanomaterials were also applied for the fabrication of fluorimetric sensors.For instance, Mn-doped ZnS (Mn-ZnS) semiconducting quantum dots and MgAl-LDHs were used to establish a fluorimetric sensing platform toward Pb 2+ , Cr 3+ and Hg 2+ . 56The glutathione stabilized Mn-ZnS quantum dots adsorbed onto the MgAl-LDH surface via electrostatic attraction, yielding fluorescent nanocomposites.Under an optimal quantum dot/LDH ratio, the fluorescence was greatly enhanced, partially due to the inhibited aggregation of Mn-ZnS quantum dots by MgAl-LDH supports.The fluorescence quenching of Mn-ZnS quantum dots was assigned to the surface cation exchange reaction, which breaks the energy gap.Interestingly, the fluorescence intensities decreased with similar trends upon adding Pb 2+ , Cr 3+ and Hg 2+ .The comparable quenching efficiencies were attributed to the similar K sp values of PbS (10 -27.1 ), HgS (10 -24.5 ) and Cr 2 S 3 (insoluble in water) and relatively high K sp value of MnS (10 -12.6 ).The low K sp drove the rapid surface cation exchange reaction and caused dramatic fluorescence suppression.
Fluorescent Eu-MOF has also been explored as the reporter; Yang et al. proposed a Fe 3+ turn-off sensor by integrating Eu-MOF and NiAl-LDHs. 57The outer layer of Eu-MOF acted as the fluorescence reporter, while the inner NiAl-LDHs functioned as the Fe 3+ captor.The Fe 3+ -induced fluorescence quenching was divided into two pathways: inner filter effect and electron transition.A moderate overlap between the absorption spectrum of Fe 3+ solution and the excitation spectrum of Eu-MOF was observed, resulting in a visible inner filter effect.In addition, the Fe 3+ possesses a 3d 5 outer electronic structure with five half-filled orbitals, which facilitates the electron transfer from the conduction band of the Eu-MOF to the 3d orbital of Fe 3+ and quenches the fluorescence.This system enabled sensitive Fe 3+ detection with a LOD of 0.1 μM.
Apart from fluorescence quenching, fluorescence turn-on systems have also been reported.As indicated above, direct modification with fluorophores usually results in quenching behavior after addition of heavy metal ions.It is reported that LDHs show visible peroxidase-like activity, and this character has been widely applied in the development of sensitive chemiluminescence sensing systems.To avoid fluorescence quenching, a non-fluorescent precursor was first introduced; then, the production of a fluorescent product by catalytic reaction was applied.Amini et al. presented a fluorescence turnon system toward Cr(VI) based on Fe(CN) 6  4− intercalated NiAl-LDHs and terephthalic acid (TA). 58The Fe(CN) Note that electron transition energy in certain metal atoms is characteristic, and the corresponding atomic emission can be used to identify metal species.With the combination of the X-ray fluorescence technique, fluorimetric heavy metal detection can be realized without the involvement of fluorescent reporters.For example, based on this mechanism, Zawisza et al. proposed a turn-on gold detection method using MgAl-LDH/graphene oxide nanocomposites. 51High sorption efficiencies toward both gold nanoparticles and Au 3+ were realized by the nanocomposites.Through X-ray fluorescence determination, sensitive gold identification was achieved with a LOD of 0.06 ng mL −1 .

Electrochemical assays
For most colorimetric and fluorimetric assays, only one heavy metal target can be detected in a single test to prevent the interference.However, multiple heavy metal ions usually exist if pollution occurs.Therefore, the development of simple and effective approaches for achieving simultaneous detection of multiple heavy metal ions is appealing.As is well known, different heavy metals show diverse redox potentials, which provides the possibility to discriminate multiple heavy metal ions by electrochemical assays. 59To realize potential-resolved heavy metal ion detection, electrodes with high electrochemical performances have been explored.For example, graphene-based electrodes with high electron transfer capability have been reported for the simultaneous detection of Cd 2+ , Pb 2+ , Cu 2+ , and Hg 2+ . 60,61ince LDHs show high affinity toward metal ions due to the surface hydroxyl groups and various interlayer anions, LDH-based electrochemical approaches were established for the detection of multiple heavy metal ions. 62However, sole LDHs usually have poor conductivity and selectivity.To overcome this drawback, the combination of LDHs and other materials with high conductivity is reasonable to facilitate the electrochemical detection. 63For instance, Ma et al. reported a sensitive electrochemical sensor based on a MgFe-LDH modified graphene electrode (Fig. 4a). 64Through a onestep hydrothermal method, MgFe-LDHs were immobilized onto the surface of graphene nanosheets.The resulting hybrid materials displayed high specific surface areas and good electrical conductivity, which enhances the heavy metal ion binding and subsequent electron transfer.Based on the proposed functional electrode, simultaneous detection of Pb 2+ and Cd 2+ in aqueous media was achieved with good interference rejection.Using peak current as the signal, linear responses of Pb 2+ (−0.55 V) and Cd 2+ (−0.8 V) were found in the concentration range of 0.05-1.0μM and 0.1-1.0μM, respectively.The LODs towards Pb 2+ and Cd 2+ were 2.7 nM and 5.9 nM, respectively.The good average recoveries (96.25 to 107.5%) and small relative standard deviations (<4%) support the potential of practical heavy metal ion analysis in real water samples.
Besides graphene, ionic liquids with high conductivity have also been applied for developing effective LDH-based electrochemical sensors.Using an N,N-dimethyl-N-2-propenyl-2-propen-1-aminium chloride homopolymer ionic liquid doped MgAl-LDH modified glassy carbon electrode, Zhou et al. proposed a high-throughput electrochemical sensor for simultaneous detection of Cd 2+ , Cu 2+ , Hg 2+ and Pb 2+ based on anodic stripping voltammetry. 65The peak potentials of Cd 2+ , Cu 2+ , Hg 2+ and Pb 2+ were −768, +42, +302 and −541 mV with an Ag/AgCl reference electrode (Fig. 4b).The introduction of an ionic liquid not only provides a mesoporous architecture, but also facilitates the electron transfer.As a result, sensitive detection of Cd 2+ , Cu 2+ , Hg 2+ and Pb 2+ was realized with LODs of 250, 25, 250 and 16 ng L −1 , respectively.The proposed approach has been applied for heavy metal analysis in spiked black tea extracts and dried tangerine peel.

Atomic absorption spectrometry
As the National Standard protocol (GB/T 17141-1997), atomic absorption spectrometry shows high sensitivity and accuracy toward heavy metal ions.Similar to X-ray fluorescence, atomic absorption spectrometry utilizes intraatomic electron transition, which provides characteristic spectral lines for certain heavy metals.And such an electron transition is not affected by the valence state.Despite the requirement of a special ionic source, the high sensitivity and intrinsic multiple target sensing character make atomic absorption spectrometry suitable for the detection of heavy metal ions with low concentrations.
To achieve sensitive detection with LDHs, a simple route is to enrich heavy metal ions by surface modification.In this case, organic molecules and inorganic nanoparticles have been investigated.In addition, in order to simplify the separation, Fe 3 O 4 magnetic nanoparticles were accompanied to produce LDH adsorbents.For example, through the coprecipitation method, Fe 3 O 4 nanoparticle decorated NiAl-LDHs were prepared for Cd 2+ and Pb 2+ analysis with the assistance of micro solid phase extraction by Arghavani-Beydokhti et al. 66 The large specific surface area of the asprepared Fe 3 O 4 -NiAl-LDH hybrid materials allowed for effective capture of Cd 2+ and Pb 2+ .After extraction and magnetic separation, the heavy metal contents were determined with flame atomic absorption spectrometry by acidic dissolution.Under the optimal conditions, sensitive Cd 2+ and Pb 2+ analysis with good linearity in the concentration range of 0.75-35 ng mL −1 and 4.0-370 ng mL −1 was realized.The LODs toward Cd 2+ and Pb 2+ were found to be 0.25 and 1.0 ng mL −1 , respectively.
Despite the magnetic separation character, the enrichment capability of Fe 3 O 4 -LDH hybrid materials is still poor.To further enhance the capture and detection of heavy metal ions, organo-functionalization of LDHs was applied.With the co-functionalization of a surfactant and polymer, Rajabi et al. developed two kinds of Fe 3 O 4 -LDH nanosorbents for the detection of heavy metal ions. 67,68Note that the chelation between organic molecules and heavy metal ions promotes their pre-concentration and detection; polymers with strong coordinating atoms (e.g., N and S) were thus chosen.To prevent the isolation of the polymer or surfactant, the surfactant-embedded polymer was generally prepared first.Using sodium dodecyl benzene sulfonate (DBSNa) embedded polyaniline (PA) as the co-sorbent, they proposed a PA-DBSNa-Fe 3 O 4 -ZnAl-LDH nanosorbent for the preconcentration and detection of Ni 2+ , Pb 2+ , Co 2+ , and Cd 2+ . 67uch a high throughput analysis capability is partially due to the wide chelation between N and metal ions.After optimization, linear detection of Ni 2+ , Pb 2+ , Co 2+ and Cd 2+ was achieved in the concentration range of 5-500, 7-750, 5-500 and 3-100 ng mL −1 , respectively.With the integration of a CO 2 -effervescence assisted dispersive micro solid-phase extraction procedure, sensitive detection of Ni 2+ , Pb 2+ , Co 2+ and Cd 2+ was realized with LODs of 1.4, 2.1, 1.5 and 0.9 ng mL −1 , respectively.When changing PA into polythiophene (PTH, S as the coordinating atom), a selective PTH-DBSNa-Fe 3 O 4 -ZnAl-LDH nanosorbent for Ni 2+ and Cd 2+ was proposed. 68With the assistance of ultrasound-assisted dispersive micro solid phase extraction and flame atomic absorption spectrometry, they achieved sensitive Ni 2+ and Cd 2+ detection in the concentration ranges of 5-300 and 2.5-100 ng mL −1 with good linearity.The LODs towards Ni 2+ and Cd 2+ were 1.3 and 0.7 ng mL −1 , respectively.

Removal of heavy metal ions
The existence of toxic heavy metal ions in polluted environments hinders the utilization of resources, including rivers, farmlands and woods.Therefore, their removal is essential to environmental treatment and recycling of resources.To meet this demand, it is urgent to explore rapid and efficient sorbents for their adsorption, and considerable effort has been devoted to the development of hybrid LDHbased nanosorbents because of the high capture capacity.However, the adsorption mechanisms of heavy metal ions using different LDH-based nanosorbents are not exactly the same.In this section, we will introduce LDH-based removal of heavy metal ions in two categories: reversible adsorption and irreversible mineralization.

Reversible adsorption
The positive charge and partially empty d orbital make heavy metal ions attracted toward negatively charged and electrondonative species.In consideration of the positively charged LDH layer, intercalated and surface anions are beneficial to interact with heavy metal ions through electrostatic attraction and chelation.Such interactions generally allow for the reversible adsorption of heavy metal ions.In this part, removal of heavy metal ions by reversible adsorption is described based on electrostatic attraction or chelation.
By intercalating carboxyl compounds, LDH-based nanosorbents with improved adsorption capacity toward heavy metal ions are prepared.Small molecules including EDTA and mercaptocarboxylic acid (MPA) have been reported for preparing intercalated LDH nanosorbents toward heavy metal ion removal. 69,70The introduction of carboxyl groups not only increases the negative charge of LDHs, but also provides more binding sites to heavy metal ions by chelation.As shown in Fig. 5a, using EDTA as the intercalating anion, Liu et al. explored MgAl-LDH nanosorbents for the removal of Ni 2+ with an adsorption capacity of 108.2 mg g −1 , which is much higher than that of the bare MgAl-LDHs (23.47 mg g −1 ). 69Interestingly, the introduction of the sulfurcontaining MPA intercalation anion endowed the nanosorbents with high adsorption capacity for multiple heavy metal ions, including Hg 2+ , Pb 2+ , and Cu 2+ . 70Under pH 5 working condition, the maximum adsorption capacities for Hg 2+ , Pb 2+ , and Cu 2+ were determined to be 250.30,122.10, and 105.33 mg g −1 , respectively.
In addition to small molecules, carboxylic polymers are also adaptable for LDH modification.For instance, Chen et al. reported sodium polyacrylate (PAAS) intercalated MgAl-LDHs for the effective removal of Pb 2+ and Hg 2+ from waste water (Fig. 5b). 71The PAAS intercalation changes the surface charge and hydrophilicity of LDHs, which benefits the adsorption of Pb 2+ and Hg 2+ .The maximum adsorption capacities were determined to be 345.4 and 142.7 mg g −1 , respectively.Peptides with carboxyl residues can also act as effective intercalation anions.Using polyaspartic acid intercalated MgAl-LDHs, Yin et al. constructed eco-friendly nanosorbents for the removal of heavy metal ions from aqueous solutions. 72The orderly carboxylic groups in polyaspartic acid facilitate the inner-sphere complexation, which promotes the treatment of heavy metal ions.The maximum adsorption capacities toward Pb 2+ and Hg 2+ were 229.2 and 208.6 mg g −1 , respectively.
Beyond carboxyl compounds, highly charged inorganic anions have also been utilized for intercalating LDHs. 73Zhou et al. found that the intercalation of sulfide anions (S 2− ) enhanced the adsorption of Co 2+ and Ni 2+ with increased electrostatic attraction. 74The maximum adsorption capacities toward Co 2+ and Ni 2+ were 88.6 and 76.2 mg g −1 , respectively.Interestingly, this nanosorbent allowed for the adsorption of an anionic heavy metal (CrO 4 2− ).The unusual adsorption of CrO 4 2− was attributed to the anionic exchange between S 2− and CrO 4 2− .By using MoS 4 2− as the intercalation anion, Rathee et al. proposed a NiFeTi-LDH nanosorbent for effective removal of heave metal ions within 10 min. 75In their work, the responses toward various metal ions were listed as the following order: Ni 2+ < Cu 2+ < Zn 2+ < Fe 3+ < Pb 2+ < Ag + .The maximum adsorption capacities toward Pb 2+ and Ag + were 856 and 653 mg g −1 , respectively.The nanomaterial-LDH hybrids have also been applied for the exploration of efficient nanosorbents. 38,76,77By grafting thiol-or amino-functionalized grapheme oxide onto MgAl-LDHs, Liao et al. reported two kinds of nanosorbents for the adsorption of Cu 2+ and Cd 2+ . 76The adsorption isotherms could be well fitted by both Langmuir and Freundlich models.The maximum adsorption capability toward Cu 2+ didn't show distinct variations by thiol (234.80 mg g −1 ) or amino (204.80 mg g −1 ) functionalization.In contrast, the maximum adsorption capability toward Cd 2+ with thiol functionalization (102.77mg g −1 ) was about triple that of amino (37.99 mg g −1 ) functionalization.The enhanced Cd 2+ capture might be attributed to the synergistic effect of LDH and thiolated grapheme oxide.With the use of lignin derived carbon and CaFeAl-LDHs, Chen et al. explored a nanosorbent for the removal of high valence metals (U(VI) and Cr(VI)). 38hey found that the plasma-regulated defect in carbon was critical to the adsorption performance due to the changed complexation between metal ions and carbon.Under the optimal condition, nanosorbents were acquired with a maximum adsorption capability of 267.65 mg g −1 toward U(VI).Similarly, with the use of glycerol-modified CaAl-LDHs, Zou et al. proposed a U(VI) nanosorbent with a maximum adsorption capability of 266.5 mg g −1 . 78Changing CaAl-LDHs into NiAl-LDHs, the maximum adsorption capability decreased to 142.3 mg g −1 because the Ca-O group shows high-activity toward U(VI).

Irreversible mineralization
As mentioned above, heavy metal ions have a devastating effect on ecological systems because of their nonbiodegradability, bioaccumulation, and high toxicity.For example, Cd 2+ contaminated agriculture soil causes severe health risk to humans through the food chain, and it becomes a global issue because of the anthropogenic and geographic activity.Despite the development of various LDHbased nanosorbents, many of them only capture heavy metal ions by reversible adsorption, which is difficult to deal with heavy metal ions in complicated environments, such as soil.Therefore, the effective removal or stabilization of heavy metal ions is critical to reduce the ecological risk.
A general strategy for permanent fixation is changing heavy metal ions into a part of materials by chemical reaction, which avoids the involvement of reversible attraction or chelation. 28,79To meet this demand, considerable effort has been dedicated to the exploration of chemical reduction and ion exchange based approaches.For instance, using Mo 3 S 13 2− anion intercalated MgAl-LDHs (Mo 3 S 13 -LDH), Yang et al. developed effective nanosorbents for permanent Ag + fixation with a maximum adsorption capability of 1073 mg g −1 . 80The as-prepared Mo 3 S 13 -LDH captured Ag + through two mechanisms: 1. formation of Ag 2 S due to Ag-S complexation and precipitation; 2. reduction of Ag + to metallic silver (Ag 0 ), as shown in Fig. 6a.The Mo 3 S 13 -LDH also showed excellent capture capability for Hg 2+ (594 mg g −1 ), with high selectivity response toward heavy metal ions in the following order: Ag + (K d 5.4 × 10 7 mL g −1 ) > Hg 2+ (3.6 × 10 5 mL g −1 ) > Cu 2+ (7.0 × 10 4 mL g −1 ) > Pb 2+ (1.4 × 10 3 mL g −1 ) ≥ Co 2+ (71 mL g −1 ), Ni 2+ (81 mL g −1 ), Zn 2+ (86 mL g −1 ), Cd 2+ (84 mL g −1 ).It was noticed that the Ag + removal rates remained >99.9% even with 500-fold Cu 2+ existence, suggesting the practical potential of selective Ag + extraction in Cu-rich environments.Another effective strategy is to fix heavy metal ions by ion exchange-based replacement.The cationic exchange processes have been widely applied in divalent heavy metal ion detection using ZnS or MnZnS quantum dots. 49For LDHbased mineralization cases, the divalent metal in the layer is replaced by the toxic heavy metal ions, which results in the formation of a new LDH layer (Fig. 6b). 87In contrast, the trivalent metal in the layer of LDHs is difficult to replace because of the high coordination number and large steric hindrance.Such a replacement reaction is generally related to several parameters, including ionic size and coordination number of metal ions, K sp values of metal hydroxides and corresponding LDHs, and defect of LDHs.In general, the replacement is only triggered by heavy metal ions with similar ionic sizes and coordination number.In addition, the replacement is promoted with the increased K sp difference between metal hydroxides and corresponding LDHs.In addition, replacement reaction rate is partially enhanced by the increased defect of LDHs.On the basis of this fixation mechanism, intercalation with anions, small molecules or polymers is not necessary.
For example, Kong et al. reported a rapid and efficient in situ Cd 2+ mineralization approach using CaAl-LDHs as the stabilizer. 25As shown in Fig. 6c, the immobilization of Cd 2+ was realized by two steps: a. fast trapping of Cd 2+ onto the CaAl-LDH surface by adsorption; b. in situ ion exchange-induced reconstruction of CdAl-LDHs.The adsorption of Cd 2+ finished within 5 min, which is demonstrated by inductively coupled plasma opticalemission spectrometry (ICP-OES).According to the transmission electron microscope (TEM) and X-ray diffraction (XRD) results, the intermediate underwent metal exchange and complete formation of CdAl-LDHs with a 30 min reaction window.With the integration of the cation exchange mechanism, the CaAl-LDHs showed super-stable mineralization of Cd 2+ in solution with a 592 mg g −1 adsorption capacity.Using CaAl-LDHs as the stabilizer, 96.9% Cd 2+ immobilization efficiency in soil remediation was achieved.Importantly, the scaling-up production of CaAl-LDHs reached 3000 tons per year, indicating the potential application of efficient remediation of Cd contaminated soil with CaAl-LDHs.By employing CaFe-LDHs as the stabilizer, Chi et al. realized efficient mineralization of Ni 2+ with a similar mechanism. 88They also found that the MgFe-LDHs displayed lower removal capacity (184 mg g −1 ) than CaFe-LDHs (321 mg g −1 ).As is known, Ca 2+ possesses a larger radius compared with Mg 2+ and CaFe-LDHs show a higher K sp value than MgFe-LDHs.Consequently, the CaFe-LDHs easily convert into NiAl-LDHs in comparison to MgFe-LDHs.
Except Ca 2+ and Ni 2+ , other divalent heavy metal ions (e.g., Cu 2+ and Pb 2+ ) can also be trapped and immobilized by Cacontaining or Mg-containing LDHs. 79,81,83As indicated in Zhang's work, the CaAl-LDHs also showed a high maximum adsorption capability of 381.9 mg g −1 toward Cu 2+ through the Langmuir model. 82However, it was still much lower than that of Cd 2+ (1035.4mg g −1 ).In spite of the similar adsorption kinetics, the different surface compositions may lead to diverse cation exchange efficiencies.For Cd 2+ assays, no CdCO 3 or Cd(OH) 2 was observed with XRD.In contrast, the Cu(OH) 2 signal was easily observed after adsorption in Cu 2+ assays.The formed Cu(OH) 2 may hinder the rapid cation exchange and subsequent formation of CuAl-LDHs.Therefore, CaAl-LDHs showed high adsorption capability toward Cd 2+ .With the intercalation of thiolates, an enhanced adsorption capacity was observed due to the assistance of precipitation of metal sulfide and surface complexation (Fig. 6d). 85To facilitate the comparison of different LDH nanosorbents, the adsorption performances are summarized in Table 1.The adsorption capacity (q e ) could be calculated with the following equation: q e = [(c 0 −c e )V]/m, where c 0 (mg L −1 ) and c e (mg L −1 ) are the concentrations of heavy metal ions before and after adsorption, V (mL) is the volume of heavy metal ion solution, and m (g) is the dosage of LDH nanosorbents.
The mineralization reaction not only achieves the fixation of heavy metal ions, but also produces new LDHs, sometimes with unexpected chemical reactivity.It is reported that transition metal-based LDHs present efficient photocatalytic activities due to the strong metal-to-metal chargetransfer. 89,90Interestingly, the heavy metal pollutants (e.g., Ni, Pb, Co and Cu) contribute high metal-to-metal chargetransfer efficiency in LDHs.After mineralization, the formed ternary LDHs might be good catalysts for realizing photocatalytic application.In Chi's work, the formed ternary NiCaFe-LDH intermediates showed high photocatalytic activity toward CO 2 reduction. 88Under light irradiation, CO 2 was converted into CO and CH 4 .Similarly, after the adsorption of Hg 2+ , Pb 2+ , and Cu 2+ onto MPA-LDHs, photocatalytic degradation of organic dyes with high efficiency (>95%) was achieved based on the formed LDH intermediates. 70Therefore, the LDH-based nanosorbents not only remove heavy metal pollutants, but also provide feasibility to yield effective photocatalysts.

Conclusion and perspectives
In this review, we have provided an overview on the recent advances in the detection and removal of heavy metal ions based on LDHs.Based on the enrichment effect, most LDH-based nanosensors show enhanced sensitivity toward toxic heavy metal ions by integrating proper detection techniques.The anion intercalation endows LDH-based nanosorbents good adsorption capabilities through electrostatic attraction and chelation.With further cation exchange amplification, some LDH-based stabilizers allow the permanent immobilization of heavy metal ions in aqueous media and soil remediation.
Despite the successful exploration of effective LDH-based nanosensors and nanosorbents for the detection and removal of heavy metal ions, some challenges regarding this research area still exist.For LDH-based assays, several limitations still exist and need to be addressed in future.Here, we'd like to share our viewpoints of challenges focusing on sensing and mineralization performances of LDHs.
1) Selective pre-concentration of specific heavy metal ions.Although numbers of LDH-based nanosensors with high enrichment capabilities have been reported, most of them can't selectively capture one metal ion.For example, intercalated LDHs usually adsorb two or more heavy metal ions with comparable adsorption capability.In this case, the co-concentrated metal ion may affect the detection of target ions.On the other hand, the occupied binding site may have the side effect of weakening the sensitivity.The modified LDHs capture metal ions through electrostatic attraction and chelation, and these two pathways have non-unified contribution.The exploration of LDH nanosensors with designed pre-concentration capability toward expected heavy metal ions might be an interesting research direction in future.
2) Unclear mechanism of mineralization diversity.It is reported that Ca-containing or Mg-containing LDHs show non-uniform removal efficiencies toward different heavy metal ions.And in general, the capture capability toward Cd 2+ is much higher than that for other divalent metal ions with the same LDHs.Meanwhile, different LDHs also display different capture capabilities toward the same metal ions.That is, the cation exchange behaviour is dependent on the atomic structure of metal ions.However, the relationship between cation exchange efficiency and atomic structure is missing, which limits the development of universal LDHbased nanosorbents and stabilizers for immobilizing heavy metal ions.
3) Direct visualization of the mineralization process.As referred in the last section, the permanent immobilization of heavy metal ions is promoted by cation exchange reaction, and the time-dependent ICP-OES, XRD and TEM results have been provided.These results, however, cannot represent the adsorption and in situ exchange process.And the sample preparation process may change the structure of LDHs, which brings in false results.Therefore, direction visualization of adsorption and cation exchange processes is essential to illustrate the mineralization mechanism.In addition, the kinetics behaviour might benefit the deep understanding of mineralization diversity toward different heavy metal ions.

) 2 ]
x+ (A − ) x/n •mH 2 O, where M 2+ and N 3+ stand for divalent and trivalent metal cations, and A − represents interlayer anions.In many cases, the K sp values of LDHs are much smaller than that of the corresponding M(OH) 2 and N(OH) 3 due to the ultrastable steric configuration of the layer structure and high coordination number of metal cations.As a result, the co-existence of M 2+ , N 3+ and OH − has a tendency Zhiqin Yuan Zhiqin Yuan is currently an Associate Professor at the College of Chemistry, Beijing University of Chemical Technology.He obtained his PhD from the College of Chemistry and Chemical Engineering, Hunan University, in 2013 with Dr Yan He and Dr Edward S. Yeung.His research interests are focused on synthesis and analytical application of metal nanoparticles and nanoclusters, carbon quantum dots, and layered nanomaterials.Chao Lu Chao Lu received his Ph.D. degree in Materials Science from Chinese Academy of Sciences in 2004.He is currently a full professor of Green Catalysis Center, College of Chemistry, Zhengzhou University.In 2011, he was selected to participate in the 'New Century Outstanding Talent' scheme of the Ministry of Education.His research interests are focused on the synthesis and characterization of nanostructured materials, nanosensors, and chemiluminescence.He is responsible for national and international research projects and has published more than 100 papers.Industrial Chemistry & Materials Review Open Access Article.Published on 17 November 2022.Downloaded on 11/25/2023 12:24:32 AM.

Fig. 2
Fig. 2 (a) Illustration of the concept for the formation of Fe(CN) 6 4− anion-interlayered MgAl-LDH-modified test strips and the detection of heavy 6 4− intercalated NiAl-LDHs exhibited peroxidase-like activity towards the TA-H 2 O 2 system, which produces a highly fluorescent product (Fig. 3b).Such a peroxidase-like activity was dramatically promoted by the introduction of Cr(VI), because of the oxidation of Fe(CN) 6 4− and formation of Fe(CN) 6 3− .The Fe(CN) 6 4− and Fe(CN) 6 3− mixture undergoes conversion of Fe 2+ /Fe 3+ , which accelerates the electron transfer and enhances the catalytic reaction.As a result, the fluorescence intensity at 422 nm was proportional to the Cr(VI) concentration, and showed good linearity in the concentration range from 0.067 to 10 μM.With this proposed system, sensitive Cr(VI) detection was achieved with a LOD of 0.039 μM.The Cr(VI)-caused Fe(CN) 6 4− oxidation and subsequent activity promotion was selective; other heavy metal ions didn't show a similar response.