A novel and facile green synthesis method to prepare LDH/MOF nanocomposite for removal of Cd(II) and Pb(II)

To date, many nanoadsorbents have been developed and used to eliminate heavy metal contamination, however, one of the challenges ahead is the preparation of adsorbents from processes in which toxic organic solvents are used in the least possible amount. Herein, we have developed a new carboxylic acid-functionalized layered double hydroxide/metal–organic framework nanocomposite (LDH/MOF NC) using a simple, effective, and green in situ method. UiO-66-(Zr)-(COOH)2 MOF nanocrystals were grown uniformly over the whole surface of COOH-functionalized Ni50Co50-LDH ultrathin nanosheets in a green water system under a normal solvothermal condition at 100 °C. The synthesized LDH/MOF NC was used as a potential adsorbent for removal of toxic Cd(II) and Pb(II) from water and the influence of important factors on the adsorption process was monitored. Various non-linear isotherm and kinetic models were used to find plausible mechanisms involved in the adsorption, and it was found that the Langmuir and pseudo-first-order models show the best agreement with isotherm and kinetic data, respectively. The calculated maximum adsorption capacities of Cd(II) and Pb(II) by the LDH/MOF NC were found to be 415.3 and 301.4 mg g−1, respectively, based on the Langmuir model (pH = 5.0, adsorbent dose = 0.02 g, solution volume = 20 mL, contact time = 120 min, temperature = 25 ℃, shaking speed 200 rpm).

To date, many nanoadsorbents have been developed and used to eliminate heavy metal contamination, however, one of the challenges ahead is the preparation of adsorbents from processes in which toxic organic solvents are used in the least possible amount. Herein, we have developed a new carboxylic acid-functionalized layered double hydroxide/metal-organic framework nanocomposite (LDH/MOF NC) using a simple, effective, and green in situ method. UiO-66-(Zr)-(COOH) 2 MOF nanocrystals were grown uniformly over the whole surface of COOH-functionalized Ni 50 Co 50 -LDH ultrathin nanosheets in a green water system under a normal solvothermal condition at 100 °C. The synthesized LDH/MOF NC was used as a potential adsorbent for removal of toxic Cd(II) and Pb(II) from water and the influence of important factors on the adsorption process was monitored. Various non-linear isotherm and kinetic models were used to find plausible mechanisms involved in the adsorption, and it was found that the Langmuir and pseudo-first-order models show the best agreement with isotherm and kinetic data, respectively. The calculated maximum adsorption capacities of Cd(II) and Pb(II) by the LDH/MOF NC were found to be 415. 3  The escalating level of heavy metal released in the aquatic environment is a cause of widespread public health problems associated with environmental pollution, especially water contamination. Heavy metals released through various anthropogenic activities such as leather, cosmetics, electronics, and battery manufacturing industries are one of the primary sources of this worsening environmental pollution 1,2 . In order to decrease the negative impact of heavy metals on the environment, the level of heavy metal pollution in the environment must be minimized. The challenge is to develop appropriate removal techniques for the remediation of heavy metals 3 . Adsorption technique is considered as an energetically efficient, simple, and economical strategy for heavy metals removal [4][5][6][7][8][9] . For this adsorption-based process to be efficient, it is essential to have porous adsorbents at the nano-/microscale with acceptable adsorption performance [10][11][12][13][14] . Over the past decade, metal-organic frameworks (MOFs), an interesting class of hybrid crystalline nanoporous substances, have quickly developed into one of the most exciting fields of research in material science, physics, chemistry, and interdisciplinary fields 15,16 . The physicochemical properties of MOFs permit their structural features to be finely tuned-based on reticular synthesis-over an extremely broad range. Indeed, several recent works claimed that these materials could be utilized as an ideal platform for the adsorption of pollutants like heavy metals [17][18][19][20] .
To date, heavy metal adsorption performance of various kinds of MOFs has been evaluated, primarily on the criteria of (1) a large heavy metal adsorption capacity though this is not necessarily of primary importance to effectively remove heavy metals under practical conditions, and (2)  show a similar pattern to the same samples synthesized in previous works which express the successful synthesis of these samples 18,20,21,[23][24][25] . The PXRD pattern of the composite shows that peaks representing both the LDH phase and MOF phase are still present in the composite, indicating the simultaneous presence of both phases in the composite structure and, in other words, the growth of MOF crystals on the LDH sheets. Compared to pure MOF, the crystalline peaks in the composite are less intense due to the simultaneous presence of two phases of LDH (with low crystallinity) and MOF (with high crystallinity) in the composite structure.
FT-IR measurement. FT-IR spectra of the samples are given in Fig. 1. The characteristic absorption bands related to the functional groups and chemical structure of the pure LDH and MOF were observed and are in agreement with the literature 18,20,21,25 . After surface functionalization of LDH, the presence of new absorption bands at 1572 cm −1 and 1491 cm −1 was observed which are attributed to the aromatic structure of the pyromellitic acid molecules. Also, a strong broad absorption band in the range of 3000-3600 cm −1 region is due to the hydrogen bonding between carboxyl groups. These observations are indicative of successful surface modification of LDH with carboxylic acids groups (LDH-COOH). In the FT-IR spectrum of the composite sample, characteristic absorption bands of both LDH and MOF are observed, indicating the simultaneous presence of both phases in the composite structure. Absorption bands and corresponding functional groups for each sample are listed in Table 1. Figure 2 shows the FESEM figures of the samples. Figure 2 (first row) clearly reveals the micrographs of UiO-66(Zr)-(COOH) 2 nanoparticle. Also, as shown in Fig. 2 Fig. 3. The images reveal that LDH/MOF NC possesses a uniform surface structure with a homogeneous distribution of elements, implying that MOF nanocrystals are uniformly distributed on the LDH ultrathin sheets. Also, the EDX peaks corresponding to the structural elements of composite (Co, Ni, O, N, C, Si, and Zr) are depicted in EDX spectra (Fig. 3). The presence of nitrogen atom in the structure of the composite is due to the presence of   (2) interlayer nitrate anions (gallery anion) in LDH material (Scheme 1). Also, the presence of Si atom is due to the presence of the silane coupling agent grafted on the surface of LDH sheets (Scheme 1). N 2 adsorption-desorption isotherms. The adsorption-desorption isotherms of the samples were measured with N 2 gas at 77 K to study their porosity and texture properties. The obtained results are tabulated in Table 2 and corresponding adsorption-desorption isotherms are shown in Fig. 4. The UiO-66(Zr)-(COOH) 2     www.nature.com/scientificreports/ exhibits a combination of Type 1(b) and Type IV(a), which is characteristic of microporous and mesoporous material, respectively. Type 1(b) isotherm is the result of a microporous solid having pore size distribution in the micro-meso range, including wide micropores (pore size: 1-2 nm) and possibly narrow mesopores (pore size: < 2.5 nm) 26 . The Ni 50 Co 50 -LDH shows a Type III isotherm with a Type H1 hysteresis loop which is representative of the presence of mesopores and macropores 27 . Adsorption-desorption isotherms of N 2 for LDH/MOF NC has a similar pattern to UiO-66(Zr)-(COOH) 2 , except that the LDH/MOF NC has a wider hysteresis loop than UiO-66(Zr)-(COOH) 2 , which can be attributed to the presence of larger mesopores in the LDH/MOF NC structure.

FESEM images and EDX mapping analyses.
The presence of such a bi-/trimodal pore system in a porous solid material can facilitate the mass transfer, which in turn can significantly improve the transfer rate of species in the system and, consequently, increase adsorption capacity and reduce equilibrium adsorption time 7,8,17,20,28 . BJH pore size distribution of the samples (Fig. 5) revealed that LDH/MOF NC has a wider pore size distribution than pure UiO-66(Zr)-(COOH) 2 , which is in accordance with the findings from the hysteresis loops.
Adsorption studies. Characterization of the LDH/MOF NC structure showed that this hybrid material can be used as an effective adsorbent for the removal of heavy metal species due to its abundant adsorption sites within its porous structure. For this purpose, the adsorption behavior and the removal performance of the synthesized adsorbent for the uptake of Cd(II) and Pb(II) cations were studied. A batch adsorption system for removal of Cd(II) and Pb(II) was used to investigate the adsorption behavior of Cd(II) and Pb(II) on the LDH/ MOF NC as an adsorbent. For this purpose, the influence of important adsorption parameters affecting the adsorption performance of the adsorbent was studied at room temperature ( T = 25 ℃), including the effect of pH, adsorbent dose, initial metal concentration, and contact time. The heavy metal adsorption capacity on the adsorbent at equilibrium and any time t, as well as removal percentage, can be calculated using the following equations 29 : where Q e and Q t represent the uptake capacity of heavy metals at equilibrium (mg g −1 ) and any time t (mg g −1 ), respectively. C i , C e , and C t are heavy metal concentration (mg g −1 ) at the initial stage (before adsorption), at equilibrium, and at any time t , respectively. V and W parameters are the volume of heavy metal solution (L) and mass of the adsorbent (g), respectively. In the adsorption process, pH and adsorbent dose factors have a direct effect on the amount of adsorption. It has been reported that solution pH has the most significant effect on the physicochemical properties of the adsorbent surface as well as the solution chemistry 30,31 . Therefore, the simultaneous effects of pH and adsorbent dose on the removal of Cd(II) cations by the LDH/MOF NC were monitored and the results are shown in Fig. 6.
For both heavy metals, as the pH increases from 2.0 to 5.0, the removal percentage increases steadily to reach its maximum level (pH 5.0) and then decreases slightly until the pH value of 7.0 above which begins to decrease significantly. This adsorption trend can be seen in all three adsorbent doses. Soltani et al. 12 have reported that this trend of increasing-maximum-decreasing (IMD) pattern is observed in many cases for the adsorption of cationic heavy metals on adsorbents with active functional groups such as -OH, -SH, -NH, -NH 2 , and -C=O.
In acidic pHs (pH = 2.0-4.0), metal adsorption is low due to the competition between proton (H 3 O + ) and metal cations for interaction with functional groups (carboxylate groups) 32 . Moreover, according to the previous studies 33 , in low pH environments, low adsorption is observed due to the repulsive interaction between cationic  www.nature.com/scientificreports/ adsorbate species and the positive surface charge of the MOF. Gradually, with increasing pH of the solution and decreasing competition, the adsorption increases to reach its maximum at a certain point, then with further increase in the pH and augmenting the concentration of hydroxide anions, metal cations begin to interact with them, which reduces the adsorption on the adsorbent surface. Zhao et al. 18 reported that with increasing pH of the solution, the carboxylic form of linkers (-COOH) is converted to carboxylate (-COO − ), which leads to a stronger interaction between functional groups and heavy metals cations, resulting in increased adsorption. Also, for both Cd(II) and Pb(II), with an increase in the amount of adsorbent from 1.0 to 2.0 mg, the removal percentage increased considerably, but with an increase to 5.0 mg, there was no significant increase in adsorption as shown in Fig. 6. It is reported that because of the stronger electron-accepting affinity of heavy metals like Cd(II) and Pb(II) cations than that of H 3 O + cations, acidic adsorption sites like carboxylic groups can be effective for the capture of metal cations 18 . Here, in acidic pH 5.0 interaction between heavy metals and surface -COOH groups of adsorbent is stronger. Accordingly, pH 5.0 and an adsorbent dose of 2.0 mg were selected as optimum conditions for further adsorption studies. The effects of heavy metal concentration and contact time on the adsorption process were studied and are depicted in Fig. 7a,c. As shown in Fig. 7a, the changes in the adsorption capacity with equilibrium heavy metal concentration, with increasing heavy metal concentration the adsorption capacity increases dramatically until it is almost fixed at a point (experimental maximum adsorption capacity, Q m,exp. (mg g −1 )) and the adsorption reaches equilibrium. Also, an almost similar adsorption pattern was observed for adsorption capacity changes with increasing contact time (Fig. 7c).
In order to investigate the adsorption behavior of Cd(II) and Pb(II) cations on the synthesized LDH/MOF NC and to study the possible adsorption mechanism/s in the process different isotherm and kinetic models were fitted to experimental data. The non-linear equations of these isotherms (Langmuir, Freundlich, and Redlich-Peterson (RP)) and kinetics (pseudo-first-order (PFO), pseudo-second-order (PSO), and Elovich) are given as follows 34 :  where, in the Langmuir equation, Q m,cal. and K L represent the calculated maximum adsorption capacity of the adsorbent at equilibrium (mg g −1 ) and Langmuir isotherm constant (L g −1 ), respectively. K L and n are, Freundlich isotherm constant (mg g −1 ) (mg L −1 ) −1/n and a parameter representative of the adsorption intensity (dimensionless) in the Freundlich isotherm, respectively. K R−P (L g −1 ), α R−P (mg L −1 ) −g , and g (dimensionless) are R-P isotherm constants, where 0 ≤ g ≤ 1 . In the aforementioned kinetic equations, Q e,cal. , k 1 , k 2 , α , and β are the calculated uptake capacity at equilibrium time (mg g −1 ), the rate constant in the PFO model (min −1 ), the rate constant in the PSO model (g mg −1 min −1 ), the initial adsorption rate (mg g −1 min −1 ) in the Elovich model, and the adsorption constant (g mg −1 ) in the Elovich model, respectively. The values of isotherm and kinetic parameters as well as the R 2 values obtained from the non-linear fitting method are given in Table 3. Compared to the Freundlich model, the Langmuir model has a higher R 2 value for adsorption of both Cd(II) and Pb(II) on the adsorbent. Also, the amounts of Q m,cal. (for Cd(II) = 415.3 mg g −1 and for Pb(II) = 301.4 mg g −1 ) in the Langmuir model was close to the experimental maximum adsorption capacities ( Q m,exp. = 420.5 and 300.3 for Cd(II) and Pb(II), respectively), so the Langmuir model is in better agreement with (5) Freundlich :  www.nature.com/scientificreports/ the experimental data and can provide an appropriate isotherm approximation. Based on the Langmuir isotherm, the maximum uptake capacity is related to the complete monolayer coverage on the adsorbent surface 35 . The R-P isotherm can be applied to determine whether the adsorption behavior follows the Freundlich or the Langmuir. In the R-P model, when g = 0 and g = 1 R-P equation becomes the Freundlich equation and the Langmuir equation, respectively 5,8,20 . For adsorption of both Cd(II) and Pb(II), the value of this parameter was very close to unity, demonstrating that the equilibrium data for adsorption of these heavy metals on the LDH/MOF NC fit much better with the Langmuir model than the Freundlich model. The Langmuir model assumes that the adsorption phenomenon takes place on the surface of the LDH/MOF HNC adsorbent with a limited number of identical localized sites via a monolayer coverage. According to Hall et al. 36 an essential feature of the Langmuir model could be demonstrated in terms of separation factor ( R L ) which is a dimensionless equilibrium parameter and suggests the adsorption nature and possibility of the adsorption process: R L = 0 , irreversible;0 < R L < 1 , favorable;R L = 1 , linear; R L > 1 , unfavorable. As shown in Fig. 7b, for the adsorption of Cd(II) and Pb(II) cations on the synthesized LDH/MOF NC adsorbent, the values obtained are between zero and one, implying a favorable adsorption process.
By evaluating the theoretical values of the kinetic parameters obtained after the non-linear fit of the experimental equilibrium data for the adsorption of both Cd(II) and Pb(II), it was found that the PFO and PSO kinetic models have higher values of R 2 than the Elovich models. Also, compared to the PSO model, the PFO model has a Q e,cal. value closer to the Q e,exp. value ( Q e,exp. : experimental adsorption capacity at equilibrium) as shown in Table 3. Consequently, it can be concluded that the adsorption mechanism of Cd(II) and Pb(II) cations on the LDF/MOF NC in an aqueous media is a combination of the PFO and PSO kinetic models with more characteristics of the PFO model, suggesting that the adsorption process takes place initially via the fast response (PFO model). The PFO model points that the rate of adsorption site occupation on the adsorbent is proportional to the number of unoccupied adsorption sites. The PSO kinetic model assumes the rate-limiting step as the formation of a chemisorptive type bond involving sharing or exchange of electrons between surface functional groups of adsorbent and adsorbate.
As a result and based on the data obtained from isotherm and kinetic models, it can be suggested that the adsorption mechanism of Cd(II) and Pb(II) on the synthesized LDF/MOF NC is monolayer adsorption on a homogeneous surface with an initial fast adsorption response involving a combination of cation-π interactions (between heavy metal cations and π electron cloud of the aromatic system in MOF structure) and chemisorption involving valency forces through sharing or exchange of electrons (between carboxylate functional groups as complexing functionality and heavy metal cations as well as direct bonding between metal cations with the free hydroxyl and amine groups on the surface of Ni 50 Co 50 -LDH-COOH), as depicted in Scheme 2. The most important characteristics of an adsorbent material that directly affect the rate of adsorption and equilibrium adsorption capacity are (1) the degree of porosity and specific surface area, (2) surface functionality and the physicochemical nature of the adsorbent surface, (3) the availability of that surface and its active adsorption sites to adsorbate species, and (4) the morphology and physical size of the adsorbent particles. The Ni 50 Co 50 -LDH-COOH/UiO-66(Zr)-(COOH) 2 NC synthesized in this study, having all these properties, can play the role of an effective adsorbent for the removal of toxic heavy metal cations from aqueous solution.
Comparison study. The synthesized LDH/MOF NC possesses a considerably enhanced adsorption capacity for both Cd(II) and Pb(II) cations compared with most LDH-based and MOF-based adsorbents as well as the other nanoporous adsorbents (Table 4). Furthermore, in comparison to other MOF-based and LDH-based adsorbents, as well as the other nanoporous adsorbents, that have been synthesized using toxic organic solvents, the LDH/MOF NC adsorbent was prepared using environmentally friendly solvents such as water and ethylene glycol.

Conclusions
Here, for the first time, we have developed a new LDH/MOF NC using a simple, effective, and green in situ approach in a round bottom flask under ambient pressure conditions. MOF nanocrystals (UiO-66-(Zr)-(COOH) 2 ) were grafted and grown uniformly over the whole surface of micrometer-sized ultrathin nanosheets of COOH-functionalized LDH (Ni 50 Co 50 -LDH-COOH) in a typical solvothermal condition at 100 °C in a water system. XRD, FT-IR, FESEM-EDX mapping, TEM, and N 2 adsorption-desorption analyses were applied to characterize and investigate the physicochemical properties of the synthesized samples. The prepared LDH/MOF NC was used as a potential adsorbent for the uptake of Cd(II) and Pb(II) heavy metals cations from aqueous solution and the influence of pH, adsorbent dose, initial metal concentration, and contact time on the adsorption process were investigated. Several non-linear isotherm and kinetic models were applied to find plausible mechanisms involved in the Cd(II) and Pb(II) adsorption, and it was found that the Langmuir and pseudo-first-order models have the best agreement with isotherm and kinetic data, respectively. The calculated maximum adsorption

Methods
Materials. All chemicals were used without further purification and are listed in Table 5.

Synthesis of Ni 50 Co 50 -LDH-COOH (LDH-COOH).
Carboxylic acid-functionalized LDH (LDH-COOH) was synthesized according to the following method: 1.0 g LDH and a certain amount of APTES was introduced into a round bottom flask containing 60 mL ethanol and ultrasonicated for 15 min. Afterward, the mixture was refluxed (24 h), cooled to room temperature, filtered off, washed with ethanol and water, and ovendried for 24 h. The resulting fine powder was then added to a round bottom flask containing 0.4 g pyromellitic acid and 60 mL ethanol and ultrasonicated for 15 min. The mixture was then refluxed at 160 ℃ for 12 h. Finally, the reaction mixture was cooled, Buchner-filtered, repeatedly washed with water and ethanol, and the resulting precipitate was dried (at 40 °C for 12 h and 100 °C for 12 h).