PEI@Mg₂SiO₄: an efficient carbon dioxide and nitrophenol compounds adsorbing material

Abstract

The PEI@Mg₂SiO₄ hybrid composite is readily synthesized for adsorbing carbon dioxide (CO₂) and nitrophenol compounds (NACs), in which more than 11% PEI is embedded. By the removal rate of 4-nitrophenol (10 mg L⁻¹) and the capture capacity of CO₂, the Mg²⁺ : PEI : SiO₃²⁻ optimal mole ratio of the PEI@Mg₂SiO₄ is 1 : 0.6 : 1.5. The composite shows the highest adsorption amounts of 0.94 mmol g⁻¹ CO₂ at 50 °C. The heat treatment regeneration exhibits a good stability, e.g. the CO₂ capture capacity decreases by only 20% after four cycles, and the desorption rate of CO₂ remains more than 91%. It also exhibites a fast adsorption and high capacity for 4-nitrophenol (NP), 2,4-dinitrophenol (DNP) and 2,4,6-trinitrophenyl group (TNP). The adsorption capacity of NP, DNP and TNP is 2.40 mmol g⁻¹, 2.66 mmol g⁻¹ and 4.85 mmol g⁻¹ respectively, which is more than that of the conventional sorbents. This work presents the higher capacity and eco–friendly sorbent for CO₂ and nitrophenol compounds.

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Inorganic–organic (IO) materials with hierarchical structures and complex morphologies assembled through “host–guest” chemistry have attracted much attention for a fundamental interest in environmental science and their potential applications in water treatment and gas remediation.¹ It is necessary to find special properties by intercalating appointed organic material into a stable inorganic matrix. Some entity frameworks in layered architectures have been utilized as ideal “hosts” that aid in the construction of multifunctional adsorbents such as layered double hydroxides,² layered transition metal oxides,³ graphite, and other lamellar materials.^4,5 In particular, layered magnesium silicate has been applied in absorbing to pollutants⁶ because of its high surface area,^7,8 properties of ion exchange, hydrophobic character and molecular sieves.⁹

As a guest, surfactants and block-copolymers often are used extensively in the shape-controlled synthesis of various functional materials, in which they play the role of a soft template or stabilizer.^10,11 Among the potential “guest” molecules, diverse amine species have prospective absorption of applications in heavy metal,¹² organic contamination,¹³ gaseous pollutants¹⁴ and other fields. Polyethyleneimine (PEI) can effectively adsorb pollutants due to the high amine density, accessible primary amine sites on chain ends and good water-solubility.¹⁵ However, before being used in the absorption for pollutants, the water-soluble PEI must be processed into a solid form. Fortunately, the combination of PEI with inorganic matrix may achieve this goal.¹⁶ For example, Deng¹⁷ and Ting reported that the fungal biomass with PEI-grafting showed high sorption capacity for anionic Cr(VI) and Cr(III). Tailor¹⁸ and co-workers found that the MCM-41-supported modified PEI markedly increased the SO₂ capture capacity than that of traditional adsorbents. Gao et al.¹⁹ introduced an interesting study for adsorbing phenol perfectly with modified diatomite by PEI impregnating.

Owing to the burning of fossil fuels and various chemical processes, there is a large amount of emission of CO₂, which is a greenhouse gas, into the atmosphere.²⁰ More and more people believe that the anthropogenic emissions of greenhouse gases contribute to global warming and climate change.^21,22 Moreover, the widespread of NACs in water is a main concern that impels researchers to look for remedies.^23–26 These compounds, in particular nitrobenzene, and their transformation products in the environment occur as contaminants worldwide and are known to be hazardous to ecology and human health.^27–29 Amine-layered IO materials have been shown to be successful for the removal of CO₂ and NACs, such as amine-layered LDHs,³⁰ amine-functionalized mesoporous silica³¹ and milli-sized calcium.³² Currently, there are two main classes of such amine-based IO materials: (a) amines covalently bound to the support via the use of chemical grafting^33–35 and (b) physically impregnated into the support based on a “wet impregnation” method.^31,36–39 The sorbents mentioned above have some obvious disadvantages, involving lack of stability, relatively low efficiency, less content of functional compound, and poor regeneration stability.⁴⁰ Our previous studies indicate that the layered IO material by coprecipitation technique has a great effect on overcoming these disadvantages.^7,35,43–45 Therefore, there is a significant and untapped opportunity to improve the adsorption characteristics by developing the PEI-layered magnesium silicate material by coprecipitation technique. The objective of this study is (1) to form a hybrid material (PEI@Mg₂SiO₄) by coprecipitation technique, (2) to utilize the high amine density and positive charge of the PEI@Mg₂SiO₄ to adsorb NACs and CO₂ and (3) to investigate their resultant adsorptions. The results indicate that the PEI@Mg₂SiO₄ is promising to capture CO₂ and adsorb NACs from wastewater decently.

Experimental section

Apparatus and materials

The infrared spectra were obtained with a Fourier transform infrared spectrometer (Model NICOLET 5700, Thermo Electron Co., USA) to indicate PEI embedded into the hybrid materials and NAC adsorbed into the sorbent. The heat weight change of the materials was performed with a thermogravimetric analyser (Model NETZSCH TG 209 F1, NETZSCH-Geratebau GmbH, Germany). A scanning electron microscope (SEM) (Model Quanta 200 FEG, FEI Co., USA) was used to measure the size and shape of the hybrid sorbents. A transmission electron microscope (TEM) (Model TECNAI G2, S-TWIN, FEI Co., USA) was used to characterize the morphology of the PEI@Mg₂SiO₄ and Mg₂SiO₄. A ζ-potential instrument (Zetasizer Nano Z, Malvern, UK) was used to determine the surface potential of the sorbents. The surface area of the materials was measured with a surface area and porosimetry analyser (Model ASAP2020, Micromeritics Co., USA). The small-angle X-ray diffraction (SAXRD) (Model D/Max-2550 PC, Japan) was recorded using CuKa radiation at a voltage of 30 kV and current of 50 mA, and the wide-angle X-ray diffraction (WAXRD) (Model Bruker D8 Advance, Germany) in the 2θ range of ∼10° to 90° at 40 kV and 40 mA. The elemental analysis device (Model Vario EL III, Germany) used to determine C, N and H content of hybrid composite. A photodiode array spectrometer (Model S4100, Scinco, Korea) with the Labpro plus software (Firmware Version 060105) was used to determine the concentration of various NAC solutions.

Sodium metasilicate (Aladdin Agents, China), magnesium nitrate (Aladdin Agents, China), sodium hydroxide (Aladdin Agents, China), PEI (M.W. 600, Aladdin Agents, China), sodium chloride (Aladdin Agents, China), and hydrochloric acid (Sinopharm Chemical Reagent Co., Ltd, China) were used. The adsorption capacities of five phenolic compounds (Aladdin Agents, China) on the PEI@Mg₂SiO₄ were investigated, which was NP, DNP, TNP, p–cresol and p–chlorophenol respectively.

Synthesis of the PEI@Mg₂SiO₄ hybrid composite and Mg₂SiO₄

2.0 g of magnesium chloride hexahydrate (AR, Aladdin Reagents Co., China) was dispersed into 100 mL of deionized water and mixed with 2.0 g ethylene imine polymer (M.W. 600, 99%, Aladdin Reagents Co., China) for 10 min with an ultrasonic fragmentation device (Model JY92-II, Xinzhi Biotechnol. Co, Ltd, Ningbo, China). 100 mL of 4.3% (w/v) sodium silicate was added rapidly with stirring. After aging for 24 h, the hybrid composite was washed repeatedly with deionized water to remove the excess PEI. The PEI@Mg₂SiO₄ suspension liquid was used to dry by freeze-drying. As reference, magnesium silicate was prepared according to the same procedure. PEI@Mg₂SiO₄ and Mg₂SiO₄ powder were used to adsorb CO₂, whereas the PEI@Mg₂SiO₄ suspension liquid was used to adsorb NACs.

Adsorption of CO₂ to the PEI@Mg₂SiO₄ and Mg₂SiO₄ material

The amount of adopted CO₂ was determined by using a thermogravimetric analyzer (Model NETZSCH TG 209 F1, NETZSCH-Geratebau GmbH, Germany). In a typical measurement procedure, the sample was degassed at 150 °C for 30 min in N₂ to remove any physically adsorbed molecules. After the temperature was decreased to 25 °C, the sample was allowed to adsorb CO₂ by passing CO₂ at a flow rate of 30 mL min⁻¹, and this process was continued for 40 min.

The influences of time and temperature on the adsorption of CO₂ were tested by using a thermogravimetric analyzer (Model NETZSCH TG 209 F1, NETZSCH-Geratebau GmbH, Germany). The sample was degassed at 150 °C for 30 min in N₂ to remove any physically adsorbed molecules, after the temperature was decreased to 50 °C, the sample was allowed to adsorbed CO₂ by passing CO₂ at a flow rate of 30 mL min⁻¹, the process was continued for 40 min. The temperature of sample was adjusted from 25 °C to 100 °C. The adsorption–desorption cycling was also measured. The sample after adsorption of CO₂ was heated at 150 °C with a ramp rate of 5 °C min⁻¹ in a nitrogen atmosphere, and then the sample was maintained at this temperature for 100 min to remove the adsorbed CO₂. After the temperature was decreased to 50 °C, the sample was allowed to adsorb CO₂ again by re-passing CO₂ at a flow rate of 30 mL min⁻¹. The process was continued for another 80 min, and the process was repeated for four times.

Adsorption of NACs and phenols

The adsorption capacities of the PEI@Mg₂SiO₄ to three kinds of NACs, and two kinds of phenols were determined. The 0.02% (w/v) of the PEI@Mg₂SiO₄ was added into the NACs and solutions of phenols, e.g. DNP from 15 to 100 mg L⁻¹, TNP from 20 to 150 mg L⁻¹, p-cresol from 20 to 300 mg L⁻¹, NP from 50 to 420 mg L⁻¹ and p-chlorophenol from 60 to 600 mg L⁻¹. The PEI@Mg₂SiO₄ pollutants were mixed for 10 min by ultrasonication. After the mixtures were centrifuged, the concentrations of pollutants in the supernatants were determined by spectrophotometry.

In addition, the influences of pH, ionic strength and time on the adsorption of NP (53 mg L⁻¹), DNP (19 mg L⁻¹) and TNP (20 mg L⁻¹) on the PEI@Mg₂SiO₄ were investigated, respectively. NaCl (1 M) was used to adjust ionic strength of the liquids from 0 to 0.2 M. The sorption time was varied from 0 to 30 min, and the pH was varied from 4 to 10. In all the experiments, the pollutants in the supernatants were determined by spectrophotometry.

Results and discussion

Interaction of PEI with magnesium silicate

Our previous study indicates that the initial optimal mole ratio of Mg²⁺ : SiO₃²⁻ is 1 : 1.5 for preparing the organics-layered magnesium silicate.⁶ From the interaction of PEI with in situ formed magnesium silicate, an embedded amount of PEI increases with the increase of SiO₃²⁻, and then the embedded amount of PEI approaches equilibrium.⁶ The hybridization obeys the Langmuir isotherm model (Fig. 1A and B), e.g. 1/q_e = 1/(K_ac_eq_∞) + 1/q_∞,^41–44 where c_e is the equilibrium molarity of PEI in g L⁻¹, q_e is the amount of PEI binding to magnesium silicate in mg g⁻¹, q_∞ is the saturation amount of PEI in mg g⁻¹ and K_a is the binding constant. The q_∞ of PEI is calculated as 0.41 g per gram of magnesium silicate, i.e. the content maximum of embedded PEI in the PEI@Mg₂SiO₄ approaches 29.4%, which is more than other results of previous similar studies.^19,45,46 During the growing of Mg–O–Si particles, the positively charged amino group of PEI can bind to the negatively charged Mg–O–Si ones via the electrical attraction. The ζ-potential of the magnesium silicate-only is determined to be −45.5 mV in aqueous media, which may be attributed to the fact that lots of OH⁻ in basic media are adsorbed via the interaction with Mg²⁺ and O–Si.⁴⁷

Fig. 1 Effect of PEI on the hybridization. (A) Plots q_e versus c₀, (B) plots c_e/q_e versus c_e. (C) The removal rate of NP with the different content of embedded PEI in the sorbent. (D) The adsorption of CO₂ with the different content of embedded PEI in the sorbent.

However, the results of previous studies show that there is a substantial decrease in the adsorption capacity of IO-sorbent with increase in the content of embedded PEI due to partial blockage of pores by the grafted amine molecules.^46,48,49 In order to optimize the ratio of PEI in the PEI@Mg₂SiO₄, CO₂ and NP (10 mg L⁻¹) are selected as representatives to measure the optimal pollutant adsorption capacity of the different content of embedded PEI in the PEI@Mg₂SiO₄, e.g. 94 mg g⁻¹, 130 mg g⁻¹, 175 mg g⁻¹, 219 mg g⁻¹, 260 mg g⁻¹ and 295 mg g⁻¹. As a result, the removal rates of NP are 2.50%, 2.63%, 2.07%, 2.01%, 1.98% and 0.16%, which are obtained by using the above corresponding PEI@Mg₂SiO₄ (Fig. 1C). The adsorptions of CO₂ are 0.81 mmol g⁻¹, 0.66 mmol g⁻¹, 0.35 mmol g⁻¹, 0.06 mmol, 0 mmol and 0 mmol g⁻¹, respectively (Fig. 1D). Thus, the adsorption of NP and CO₂ are maximum in the adsorbents with the Mg²⁺ : PEI : SiO₃²⁻ ratio of 1 : 0.6 : 1.5. The PEI@Mg₂SiO₄ is formed and used. The molar ratio of Mg to PEI and Si in the PEI@Mg₂SiO₄ is calculated as 1 : 0.13 : 1.3, and it was determined by element analysis and ICP.

Characterization of the PEI@Mg₂SiO₄

An initial estimate of PEI bilayer content in the PEI@Mg₂SiO₄ is obtained from infrared (IR) spectra. As shown in Fig. 2A, the Si–O absorption peak is at 1059 cm⁻¹. The absorption peaks at 2935 (C–H stretching), 2819 (C–H stretching), (1756 –NH₂ and –NH– deformation), 1472 (–NH₂ and –NH– deformation) and 1290 cm⁻¹ (–NH₂ and –NH– deformation) indicate that PEI is embedded into the PEI@Mg₂SiO₄. From the elemental analysis of the PEI@Mg₂SiO₄, both 7.1% C and 3.8% N indicate that 11.4% PEI is embedded into the PEI@Mg₂SiO₄ (Table S1, ESI†). From the small-angle XRD (SAXRD) (curve 2 in Fig. 2B), the PEI@Mg₂SiO₄ composite possesses the layered structure. During the synthesis, the positively charged amino group of PEI (structured in Fig. 2B) can bind to the negatively charged Mg–O–Si particles via electrical attraction. The ζ-potential of Mg₂SiO₄-only is determined to be −45.5 mV in aqueous media, which may be attributed to the fact that lots of OH⁻ in basic media are adsorbed via the interaction with Mg²⁺ and O–Si.⁴⁷ After the intercalation of PEI, the ζ-potential of the PEI@Mg₂SiO₄ changes to +15.9 mV. The electrostatic interaction occurs between PEI⁺ and magnesium silicate. In addition, the alkyl chain (–[CH₂–CH₂–NH]_n–) of PEI in length of 3.4 nm may be embedded into two Mg₂SiO₄ sheets. PEI is fixed between Mg₂SiO₄ particles in the interval of 3.9 nm (Fig. 2B).

Fig. 2 IR spectra (A) of Mg₂SiO₄ (1), the PEI@Mg₂SiO₄ (2) and PEI (3) and SAXRD and (B) of Mg₂SiO₄ (1) and the PEI@Mg₂SiO₄ (2).

As shown in Fig. 3, the PEI@Mg₂SiO₄ shows the sheet morphology with an average diameter of 40–60 nm (Fig. 3B), which is similar to that of Mg₂SiO₄ (Fig. 3D). However, the PEI@Mg₂SiO₄ also shows the layer structure (Fig. 3A) because two sheets of Mg₂SiO₄ are aged into the layer-by-layer material by electric attraction. Moreover, the width of silt is about 4 nm (Fig. 3A and C), which is consistent with the SAXRD. As shown in the SEM images (Fig. 3E and F), the PEI@Mg₂SiO₄ has flaky structure with an average diameter of 60 to 100 nm, while Mg₂SiO₄ has the granule structure with the average diameter of 60–80 nm (Fig. 3G). The presence of PEI affects the stacking of lamellar Mg₂SiO₄, which is only stringed by the intermolecular action with PEI.⁶ The PEI interior of the hybrid is also confirmed by EDX (Fig. S1, ESI†).

Fig. 3 TEM images of the PEI@Mg₂SiO₄ (A–C) and Mg₂SiO₄ (D). SEM of the PEI@Mg₂SiO₄ (E and F) and Mg₂SiO₄ (G).

Fig. 4 illustrates the N₂ sorption isotherm and the pore-size distribution with Barrett–Joyner–Halenda method of Mg₂SiO₄ and the PEI@Mg₂SiO₄. All of the isotherms are classical type IV, which refers to the mesoporous materials. Furthermore, the isotherms have two hysteresis loops, indicating a bimodal pore size distribution in the mesoporous regions. The shapes of the two hysteresis loops are different from each other.⁵⁰ At the pressure between 0.8 and 0.9, the hysteresis loops are type H1, suggesting the presence of cylindrical mesopores.⁵¹ The above bimodal pore-size distribution is further confirmed by the pore-size distribution in Fig. 4a and b. From the pore plots, no peak appeared in the pore, indicating the presence of irregular mesopores. The mesopores may be formed by the stacking of lamellar Mg₂SiO₄. The BET analysis of the materials indicates that the specific surface areas of Mg₂SiO₄ and PEI@Mg₂SiO₄ were determined to be 51.61 and 89.51 m² g⁻¹, respectively. The increased area should be attributed to the partial impregnation of PEI into Mg₂SiO₄.

Fig. 4 N₂ absorption–desorption isotherm and pore-size distribution (inset) for Mg₂SiO₄ (A(a)) and the PEI@Mg₂SiO₄ (B(b)).

As seen in the thermogravimetric analysis (TGA) and differential thermal gravity of the PEI@Mg₂SiO₄ (Fig. S2, ESI†), the first step in the range from 40 to 200 °C is attributed to the removal of the physisorbed water and interlayer water, the second step in the range from 200 to 400 °C is due to the removal of the silicate layer, and the third step in the range from 400 to 600 °C is due to the removal of interlayer PEI. At 700 °C, the weight loss of the PEI@Mg₂SiO₄ is 33.2%, where the physisorbed water, silicate layer and interlayer PEI distribution was 15.0%, 7.9% and 10.3%, respectively, approaching that obtained from the element analysis.

Prediction of CO₂ adsorption uptakes in the PEI@Mg₂SiO₄

The CO₂ capture capacity of Mg₂SiO₄ and the PEI@Mg₂SiO₄ materials is measured at 25 °C by using a thermogravimetric analyzer. The chemisorption of CO₂ may occur through the formation of carbamate by the reaction of the secondary amine group (R₂NH) and primary amine (RNH₃) of PEI with CO₂, as shown in eqn (1) and (2) below.

This suggests that CO₂ would form a weak chemical bond with the secondary amine group, and subsequently lead to the formation of carbamate zwitterions.⁵² The CO₂ capture capacity on Mg₂SiO₄ is around 0.69 mmol g⁻¹, which is higher than that of the results reported.^30,53,54 The interlayer anions in Mg₂SiO₄ (ref. 47) is favourable to CO₂ capture.^31,55 The CO₂ capture capacity increases up to 0.75 mmol g⁻¹ with the PEI@Mg₂SiO₄. These competing kinetic (diffusion) and thermodynamic (adsorption) effects has been observed and reported previously.^45,53 The nanopores of the PEI@Mg₂SiO₄ provide CO₂ with a diffusion path, where PEI located on the nanopore wall offers the active adsorption sites.

For CO₂ capture, any adsorbent should have not only a high sorption capacity but also fast adsorption to be energy-efficient.⁵⁶ A recent study demonstrated the high loading of amine may hamper the adsorption kinetics but increase the capacity.⁵⁷ A double-exponential model (DEM) is applied to describe the CO₂ adsorption kinetics on the PEI@Mg₂SiO₄, expressed by the relation:

q_t = q_e − D₁ exp(−K₁t) − D₂ exp(−K₂t) (3)

where D₁ (mg g⁻¹) and K₁ (min⁻¹) are the adsorption rate and diffusion parameter for the fast step, and D₂ and K₂ are those for the slow step. The model is suitable for an adsorbent with two different types of adsorption sites, and the adsorption contains fast and slow adsorption steps.^56,58 The experimental data are fitted by the DEM model at 25, 50, 75 and 100 °C (Fig. 5A). Both D₁ and D₂ first increase, and then decrease with the increase of temperature from 25 to 100 °C, and the highest value appeared at 50 °C (Table S1, ESI†). At 100 and 75 °C, the CO₂ adsorption equilibrium reached within 5 min. With decrease in temperature, the adsorption of CO₂ goes slow on the PEI@Mg₂SiO₄. The slow adsorption is much more obvious at 25 and 50 °C, and the equilibrium has not been reached at 15 min. This demonstrates a strong diffusion of CO₂ is hindered by PEI phase at low temperature,⁵⁷ proven by the diffusion parameter (K₁, K₂) (Table S2, ESI†). Evidently, the PEI@Mg₂SiO₄ exhibits the highest adsorption for CO₂ at 50 °C. The kinetic results are also consistent with previous reports.⁵⁶ This may provide a valuable application to collect CO₂ from flue gas. In addition, the CO₂ adsorption equilibrium on the PEI@Mg₂SiO₄ is completed in 18 min, while that on the Mg₂SiO₄ is completed in 35 min (Fig. S3, ESI†). The adsorption time is close to other types of adsorbents.^10,56,59,60

Fig. 5 (A) The CO₂ adsorption kinetics of the PEI@Mg₂SiO₄ at 25 °C (1), 50 °C (2), 75 °C (3) and 100 °C (4) fitting by DEM. (B) Cycles of CO₂ adsorption–desorption for Mg₂SiO₄ (1) and the PEI@Mg₂SiO₄ at 50 °C.

The successive CO₂ adsorption–desorption cycle may provide an assessment of the regeneration stability of the materials. The trend of cycle curves is consistent with that of other studies^53,61,62 (Fig. 5B). During four cycles, the CO₂ capture amounts decrease from 0.94 to 0.75 mmol g⁻¹ with PEI@Mg₂SiO₄; however, with Mg₂SiO₄, it decreases, from 0.74 to 0.46 mmol g⁻¹. The CO₂ adsorption capacity of the PEI@Mg₂SiO₄ regenerated decreases by only 15–20%. In addition, from curve 1 more than 18% CO₂ is always left on the Mg₂SiO₄ every cycle. On the contrary, from curve 2 the desorption rate of CO₂ remains more than 91% with the PEI@Mg₂SiO₄.

Prediction of NACs adsorption uptakes in the PEI@Mg₂SiO₄

As described above, the PEI@Mg₂SiO₄ carries a great deal of positive charges. It may adsorb strongly NACs, such as NP, DNP and TNP, via electric attraction. Moreover, there are many electron-withdrawing groups (nitro group) in NACs, which are in favour of being adsorbed by the PEI@Mg₂SiO₄. The NP, DNP and TNP are tried to be adsorbed by the PEI@Mg₂SiO₄ hybrid composite, and the adsorption obeys the Langmuir isotherm model (Fig. 6B), 1/q_e = 1/(K_ac_eq_∞) + 1/q_∞. From Table 1, q_∞ value is positively related to lg K_o/w value, e.g. 2.40 mmol g⁻¹ for NP (lg K_o/w = 1.91), 2.66 mmol g⁻¹ for DNP (lg K_o/w = 1.54), 4.85 mmol g⁻¹ for TNP (lg K_o/w = 2.03), which is superior to the other traditional absorbents.^63–67 The ζ-potential of the hybrid composite approaches +7.29 mV, +6.43 mV and −0.39 mV after adsorption of NP, DNP and TNP, respectively. The ± charge attraction plays a primary role in the adsorption of NACs,⁶ i.e. PEI embedding into the hybrid composite captured NACs from aqueous solution.

Fig. 6 IR spectra (A) of the PEI@Mg₂SiO₄–NP (1), the PEI@Mg₂SiO₄–DNP (2), the PEI@Mg₂SiO₄–TNP (3), the PEI@Mg₂SiO₄ (4), Mg₂SiO₄ (5) and adsorption of NP (B1), DNP (B2), TNP (B3), p-cresol (B4) and p-chlorophenol (B5).

Table 1 The adsorption capacity of the PEI@Mg₂SiO₄ to NACs with K_a and comparison with the other sorbents reported

Pollutants	Group category	Nitro groups number, n	q∞		lg Ka	R^2	q∞ comparison
			mmol g^−1	mg g^−1			Sorbent	q∞/mg g^−1
TNP	Electron-withdrawing groups	3	4.85	1111	3.05	0.9962	Hydrotalcite	504 (ref. 63)
DNP		2	2.66	455	2.66	0.9884	activated carbon fibers	417 (ref. 64)
NP		1	2.40	333	2.52	0.9763	Granular activated carbon	206 (ref. 65)
p-Chlorophenol		—	2.23	286	2.47	0.9767	—	—
p-Cresol	Electron-donating groups	—	1.68	182	2.26	0.9398	—	—

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Nitro groups can increase the electronegativity of the phenolic hydroxy groups such that the amino groups of PEI@Mg₂SiO₄ can combine with the phenolic hydroxy groups of NACs more efficiently.⁶⁸ Such NACs with more nitro groups often exhibit the higher adsorption capacity. In addition, the electron-withdrawing effect of chlorine group is less than that of nitro groups. Methyl groups can reduce the electronegativity of the phenolic hydroxy groups, which weighs against the combination between the amino groups of the PEI@Mg₂SiO₄ and the phenolic hydroxy groups of p-cresol. Therefore, the absorption ability of pollutants from high to low is: TNP > DNP > NP > p-chlorophenol > p-cresol. The lg(K_a) values of NP, DNP and TNP are calculated to be 2.52, 2.66 and 3.05, respectively, which are superior to their lg K_o/w. There are many long hydrophobic chains, i.e. –[CH₂–CH₂–N⁺]_n– in PEI, which indicates that PEI@Mg₂SiO₄ carries decent hydrophobicity.⁶⁹ Therefore, the hydrophobic interaction occurs between the hydrophobic groups of NACs and the long alkyl chains of embedded PEI in the PEI@Mg₂SiO₄, in addition to the electrical charge interaction.

On the basis of the above experiments, we further carried out in situ IR experiments to clarify the NACs kinetics on the adsorbents. In situ IR difference spectra upon NACs adsorption, the absorption peaks of nitro groups at 1385–1565 cm⁻¹ indicate that NACs are embedded into the hybrid materials (Fig. 6A). The pH of solution affected obviously the adsorption of NACs. With increasing pH from 4 to 6, the q_e of NP and DNP increases by 50% and 215%, respectively, and remains almost constant between pH 7 to 10. The q_e of TNP increases by 20% between pH 4 to 6 and decreases by 37% at pH 10 (Fig. S4, ESI†). The adsorption of NACs increased slightly with increase of ionic strength. This is attributed to the fact that the protonation of PEI is triggered in strong acid.⁷⁰ The q_e of NP, DNP and TNP increases by 16%, 4% and 13%, respectively, in 0.25 M NaCl (Fig. S5, ESI†). This is attributed to the fact that ionic strength can strengthen the hydrophobic interaction between NACs and PEI. The adsorption equilibrium is completed in 8 min (Fig. S6, ESI†), which is much faster than that of activated carbon.⁷¹ Therefore, the hybrid sorbent prepared is favorable for treatment of the highly salt and slightly alkaline wastewater.

Conclusions

This work provides a facile layered sorbent (PEI@Mg₂SiO₄) for adsorbing CO₂ and NACs by PEI hybridizing into Mg₂SiO₄. More than 11% PEI is embedded such that the sorbent contains a large numbers of reactive sites to cause an exceptional CO₂ capture capacity. The secondary amine group of PEI with CO₂ plays a key role, and the experimental data fits the DEM. The sorbent exhibits the highest adsorption capacity for CO₂ at 50 °C, e.g. approaching 0.94 mmol g⁻¹. The sorbent regenerated by heat treatment exhibits a good stability with the adsorption–desorption cycling of CO₂, e.g. decreasing by 20% for four cycles. In addition, the desorption rate of CO₂ remains more than 91% with the PEI@Mg₂SiO₄. Moreover, such a material appears simultaneously multifunctional, e.g. adsorption, ionic exchange and flocculation. From the adsorptions of three NACs, the PEI@Mg₂SiO₄ exhibits a high adsorption capacity to NACs, where the electrostatic interaction plays a dominant role. The effect between the electron-donating groups and electron-withdrawing groups plays a supporting role. Then, it adsorbs NP, DNP, and TNP according to the Langmuir isotherm model with a high q_e at 2.40 mmol g⁻¹, 2.66 mmol g⁻¹ and 4.85 mmol g⁻¹, respectively. This work may suggest a new strategy for development of CO₂ capture materials and a functionalized material for treatment of NACs wastewater.

Acknowledgements

The authors acknowledge the financial support from the Foundation of State Key Laboratory of Pollution Control and Resource Reuse (Tongji University), China (PCRRK11003).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra04928d

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