Organically-modified magnesium silicate nanocomposites for high-performance heavy metal removal

Abstract

A disulfide-grafted polyethyleneimine (PES)@Mg₂SiO₄ composite was synthesized, characterized, and used successfully to remove heavy metals from wastewater. It was grafted with a huge number of dithiocarbamate moieties as sites for specific adsorption. By comparative studies, it was found that the morphology, structure and properties were influenced by the trapped organic components in the composites. PES@Mg₂SiO₄ exhibits a larger specific surface area and better adsorption activity compared with Mg₂SiO₄, polyethyleneimine (PEI)@Mg₂SiO₄ and PEI@Mg₂SiO₄–CS₂. The adsorption behaviors of the obtained composites were investigated for the recovery of Hg²⁺, Pb²⁺ and Cd²⁺. PES@Mg₂SiO₄ shows the highest adsorption capacities of 714 mg g⁻¹ for Hg²⁺, 400 mg g⁻¹ for Pb²⁺ and 323 mg g⁻¹ for Cd²⁺, which fits the Langmuir isotherm model. Besides this, it also exhibits excellent adsorption capacity for Cu²⁺, Bi³⁺ and metal complexes. Its application in petrochemical wastewater demonstrates its enormous potential for heavy metal wastewater treatment.

---

1. Introduction

In recent years, heavy metal pollution has become a difficult problem with the rapid increase of global industrial activities, such as metal finishing, the manufacture of pigments and fertilizers, electroplating, mining and so on.^1–4 Environmental and occupational exposure to heavy metals such as mercury, lead and chromium results in severe health impacts including prenatal and developmental defects.^5–9 For example, Hg²⁺ has a very high tendency for binding proteins, mainly causing damage to the renal and nervous systems;^10,11 Pb²⁺ can cause mental retardation, kidney disease, anemia, and so forth;¹² Cd²⁺ is a toxic metal ion and a potent inhibitor of neuromuscular enzymes.^7,13–15 In addition to the ionic state, heavy metals mainly exist in complexed states in wastewater.^16,17 In recent years, common techniques that have been applied to the elimination of heavy metals include ion exchange,^11,18,19 chemical precipitation,^20,21 adsorption,^22–25 membrane processes,^26–29 oxidation processes³⁰ and so on. Of these, the adsorption technique exhibits its own advantages over the others because of its simple design and the small investment in terms of both initial cost and land required.³¹ However, for the removal of metal complexes, the adsorption technique suffers from inadequate removal capacity, due to a lack of adsorbents with high binding capacities, which can adsorb metals from complexes by competing with the ligands. Therefore, prior to the adsorption treatment, other pre-treatments have to be performed to dissociate metals from their complexes. For instance, photocatalytic oxidation has been evaluated as a promising technique for the dissociation of complexed metals by degrading organic ligands.^30,32–35 Undoubtedly, additional treatments add to the cost and reduce the efficiency. Therefore, to maintain the technical advantages of adsorption, it becomes necessary to exploit novel versatile adsorbents with increased binding capacity, which not only can adsorb metal ions, but also can competitively adsorb metal atoms from complexes.

Inorganic–organic (IO) composites, showing many advantages in water treatment, are considered to be a promising new class of adsorbent of heavy metals.^36–41 This is because the adsorption abilities of the IO material can be achieved by profiting from the natures of both the inorganic material and the embedded organic components.^42–46 Therefore, the proper fabrication of IO composites, of both the organic and inorganic components, provides the possibility of delivering adsorbents for the simultaneous removal of ionic and complexed metals. Organic polyethyleneimine (PEI) can effectively adsorb heavy metals due to its high amine density, accessible primary and secondary amine sites, and good water-solubility.^47,48 It has been trapped on different host materials to generate hybrid adsorbents for the removal of heavy metals. For example, Chen and co-workers reported that a novel hybrid adsorbent D001-PEI was fabricated by immobilizing PEI within a macroporous cation exchange resin D001. D001-PEI as an adsorbent exhibited enhanced efficiency for the removal of heavy metals from waste effluent.⁴⁹ Gao and co-workers investigated the chelating adsorption properties of PEI/SiO₂ for Pb²⁺.³⁹ Meanwhile, the primary and secondary amine groups in PEI can readily react with carbon disulfide (CS₂) to form dithiocarbamate moieties, which are strong chelating agents for metal ions, resulting in disulfide-grafted PEI (PES). Even compared to EDTA, dithiocarbamates not only can promptly bind many heavy metal ions,^50,51 but also can competitively bind metals from EDTA–metal complexes. However, the sorption sites may be limited to the surface of PES aggregates as a result of resistance to intraparticle diffusion during coagulation–flocculation. The immobilization of PES in a suitable nano-structured inorganic matrix may address this issue, and may also benefit the recovery of spent adsorbents at the end of the process.⁵² Magnesium silicate is an excellent layered inorganic matrix, which has been applied as an inorganic adsorbent in the adsorption of heavy metal ions³⁶ because of its high surface area,^42,45 its capacity for ion exchange and the reactivity of its OH groups towards metal ions.^49,53 For instance, Wang and co-workers found that as-synthesized magnesium silicate hollow spheres showed an ability for the efficient removal of heavy metal ions from wastewater.⁵⁴ Recently, our group has utilized magnesium silicate as a host material to trap PEI to generate IO materials for the adsorption of carbon dioxide and nitrophenols.⁵⁵ Therefore, embedding the tailored PES into the layered magnesium silicate scaffold has become a very promising approach for the achievement of nano-structured adsorbents with increased binding capacity towards heavy metals.

In this work, we firstly grafted PEI with CS₂ to obtain PES. The conventional co-precipitation approach was employed to produce the titular nano-structured IO hybrid PES@Mg₂SiO₄. For comparison, Mg₂SiO₄ and PEI@Mg₂SiO₄ were prepared with further post-modification processing of PEI@Mg₂SiO₄ to give PEI@Mg₂SiO₄–CS₂. We systemically studied their adsorption behaviors towards mercury, cadmium, and lead ions and their relevant complexes in aqueous solutions. The primary results indicate that PES@Mg₂SiO₄ is promising for the removal of heavy metals from wastewater and more efficient than Mg₂SiO₄, PEI@Mg₂SiO₄ and PEI@Mg₂SiO₄–CS₂. The results of petrochemical wastewater treatment testing demonstrate that PES@Mg₂SiO₄ has potential for practical application.

2. Experimental

2.1 Materials

Sodium metasilicate, magnesium nitrate, sodium hydroxide, PEI (MW 600), sodium chloride, CS₂, EDTA, triethylamine, 1000 ppm lead standard, mercury standard and cadmium standard and 100 ppm (As, Bi, Cd, Cu, Cr, Fe, Hg, Mn, Ni, Se, Pb, Zn) multi-element mixed standard solution were purchased from Aladdin Agents, China. Hydrochloric acid was purchased from Sinopharm Chemical Reagent.

2.2 Synthesis of the PEI@Mg₂SiO₄, PEI@Mg₂SiO₄–CS₂, PES@Mg₂SiO₄ composite and Mg₂SiO₄

20 mL of CS₂, 4 g PEI and 7 mL of triethylamine were dispersed into 100 mL of absolute ethanol under stirring. After stationary aging for 1 h, the resulted precipitant (PES) was washed repeatedly with absolute ethanol to remove the excess CS₂. The obtained PES was then dried by freeze drying. 2.0 g magnesium chloride hexahydrate was dissolved into 100 mL of deionized water and mixed with 1.8 g of dry PES for 60 min with an ultrasonic fragmentation device. 100 mL of 4.3% (w/v) sodium silicate was added under stirring. After stationary aging for 24 h, the hybrid composite was washed repeatedly with deionized water to remove the excess PES. The hybrid composite was denoted as PES@Mg₂SiO₄. The optimal dosage of PES has been investigated (Fig. S1 ESI†). The PES@Mg₂SiO₄ suspension liquid was dried by freeze drying. As controls, Mg₂SiO₄, PEI@Mg₂SiO₄ and PEI@Mg₂SiO₄–CS₂ were synthesized. The synthetic procedures were as follows:

100 mL of deionized water containing 2.0 g MgCl₂·H₂O was mixed with 10.0 g PEI by ultrasound for 10 min. 100 mL of deionized water containing 4.3 g Na₂SiO₃·9H₂O was added to the above mixture under stirring. After stationary aging for 24 h, the hybrid composite was washed several times to remove the excess PEI. The hybrid composite was denoted as PEI@Mg₂SiO₄. As a control, Mg₂SiO₄ was prepared the same way without PEI.

0.7 g dry PEI@Mg₂SiO₄ was dispersed into 20 mL of CS₂ and mixed with 7 mL of triethylamine under stirring. After stationary aging for 24 h, the hybrid composite was washed several times to remove the excess CS₂. The hybrid composite was denoted as PEI@Mg₂SiO₄–CS₂.

2.3 Adsorption of dissociated heavy metals on the Mg₂SiO₄, PEI@Mg₂SiO₄, PEI@Mg₂SiO₄–CS₂ and PES@Mg₂SiO₄ composites

The adsorption capacities of the sorbents to three kinds of dissociated heavy metals were determined. 0.01% (w/v) of the Mg₂SiO₄, PEI@Mg₂SiO₄, PEI@Mg₂SiO₄–CS₂ and PES@Mg₂SiO₄ were added to solutions of the heavy metals: Hg²⁺ from 10 to 200 mg L⁻¹, Pb²⁺ from 20 to 200 mg L⁻¹ and Cd²⁺ from 20 to 200 mg L⁻¹. The influences of adsorption time (0–180 min) and pH (3.3–7.0) on the adsorption of Hg²⁺ (14.4 mg L⁻¹), Pb²⁺ (51.5 mg L⁻¹) and Cd²⁺ (44.8 mg L⁻¹) on PES@Mg₂SiO₄ were investigated. The optimal dosage was investigated too. 0.01%, 0.02%, 0.03%, 0.04% and 0.05% PES@Mg₂SiO₄ (w/v) were added into the (As, Bi, Cd, Cu, Cr, Fe, Hg, Mn, Ni, Se, Pb, Zn) multi-element mixed standard solution (1 mg L⁻¹). The procedure was as follows: the sorbent–pollutant liquids were mixed for 120 min using a magnetic stirrer. After the mixtures were centrifuged, the concentrations of the pollutants in the supernatants were determined using an inductively coupled plasma optical emission spectrometer (ICP), Agilent 720ES, USA.

2.4 Adsorption of complexed heavy metals on the PES@Mg₂SiO₄ composite and practical application

A certain amount of EDTA was added into the (As, Bi, Cd, Cu, Cr, Fe, Hg, Mn, Ni, Se, Pb, Zn) multi-element mixed standard solution (1 mg L⁻¹), and the concentration ratios were 0, 2 and 20. 0.04% (w/v) of the PES@Mg₂SiO₄ was added into these mixed solutions at pH 5 under stirring for 120 min. After the mixtures were centrifuged, the concentrations of the pollutants in the supernatants were determined by ICP. 0.04% (w/v) of the PES@Mg₂SiO₄ was added into solutions of petrochemical wastewater. The solutions were mixed for 120 min using a magnetic stirrer. After the mixtures were centrifuged and digested, the concentrations of the pollutants in the supernatants were determined by ICP.

3. Results and discussion

3.1 Experimental principle and possible structures

It is widely recognized that CS₂ can readily react with the primary or secondary amines in a PEI scaffold to form dithiocarbamate moieties, resulting in CS₂-grafted PEI (PES) (Fig. 1A), which exhibits a strong chelating ability to heavy metal ions and complexes (Fig. 1B)^56,57 and is thus extensively used for the treatment of heavy metal wastewater.^58–61 Herein, the obtained PES was further applied to prepare the hybrid PES@Mg₂SiO₄ composite. For comparison, Mg₂SiO₄, PEI@Mg₂SiO₄, and PEI@Mg₂SiO₄–CS₂ were also prepared (Fig. 1C).

Fig. 1 Schematic synthesis of PES (A), the binding reaction between dithiocarbamate in PES and metal ions, and competitive binding with metal complexes (B), and illustrated structures of Mg₂SiO₄, PEI@Mg₂SiO₄, PEI@Mg₂SiO₄–CS₂ and PES@Mg₂SiO₄ (C).

3.2 Characterization of the sorbents

The ζ-potential of Mg₂SiO₄ is −6.5 mV at pH 8 because of the adsorbed OH⁻ in basic media.⁶² After reaction with organics, the ζ-potentials of the PES@Mg₂SiO₄, PEI@Mg₂SiO₄–CS₂ and PEI@Mg₂SiO₄ change to be +5.1, +8.5 and +15.9 mV respectively. In order to reasonably evaluate the heavy metal adsorption capacity among PES@Mg₂SiO₄, PEI@Mg₂SiO₄–CS₂ and PEI@Mg₂SiO₄, the content of PEI in each of those sorbents is set to an approximate value (Table 1). Elemental analysis indicates that the CS₂ content in PES@Mg₂SiO₄ is greater than in PEI@Mg₂SiO₄–CS₂. The coprecipitation technique is more efficient than the physical dipping method, which can improve the content of functional compounds.^36,63,64

Table 1 Change of N, S, PEI and CS₂ content in the sorbents

	N%	S%	PEI%	CS2%
PES@Mg2SiO4	4.194	4.628	12.76	5.51
PEI@Mg2SiO4–CS2	3.471	1.151	12.92	1.37
PEI@Mg2SiO4	4.610	0	12.92	0
PES	19.880	25.33	69.85	30.15

---

The Fourier transform infrared (FTIR) spectra of these samples were obtained and are presented in Fig. 2A. The absorption peaks at 1059 cm⁻¹ represent the Si–O bending vibrations. The absorption peaks near 2940 and 2820 cm⁻¹ represent C–H stretching, and the adsorption peaks at 1690 and 1470 cm⁻¹ represent –NH₂ and –NH– deformation, respectively. These results indicate that PEI is successfully doped into the PES@Mg₂SiO₄, PEI@Mg₂SiO₄–CS₂ and PEI@Mg₂SiO₄. The PES exhibited spectral features corresponding to the monodentate binding mode of N–C=S at 1110 cm⁻¹, proving that there are dithiocarbamate groups in the product.⁶⁵ There are also peaks near 970 and 663 cm⁻¹, which may be related to C=S and C=C.^58,66 However, the absorption peaks for N–C=S and C=S are not observed in the FTIR spectra of the PES@Mg₂SiO₄ and PEI@Mg₂SiO₄–CS₂. This is because the Si–O absorption peak covers up the absorption peaks of N–C=S and C=S. However, the results of the elemental analysis (Table 1) show that there is some elemental sulphur in the PES@Mg₂SiO₄ and PEI@Mg₂SiO₄–CS₂, which proves that CS₂ has reacted with PEI in those sorbents. As shown in Fig. 2B, all the X-ray diffraction peaks show the characteristics of Mg₃Si₄O₁₀(OH)₂ (reported data: JCPDS no. 03-0174), which presents a talc structure.⁵⁴ The peak intensity for PES@Mg₂SiO₄ is weaker than that for PEI@Mg₂SiO₄, suggesting that the alkyl chain in PEI grows in length after the chelation to CS₂. It should be noted that the peak for d₂₀₀ in the PES@Mg₂SiO₄ shifts from 37.93 Å to 35.17 Å which implies some lattice deformation to expand the layer spacing.¹

Fig. 2 (A): IR spectra of the PES (1), PES@Mg₂SiO₄ (2), PEI@Mg₂SiO₄–CS₂ (3), PEI@Mg₂SiO₄ (4) and Mg₂SiO₄ (5), (B): XRD of PES@Mg₂SiO₄ (1), PEI@Mg₂SiO₄–CS₂ (2), PEI@Mg₂SiO₄ (3) and Mg₂SiO₄ (4).

The N₂ sorption isotherms and the pore-size distributions of PES@Mg₂SiO₄, PEI@Mg₂SiO₄–CS₂, PEI@Mg₂SiO₄ and Mg₂SiO₄ are shown in Fig. 3. All of the isotherms present the characteristics of a mesoporous material, which is of classical type IV. Furthermore, the isotherms have hysteresis loops at relative pressures between 0.9 and 1.0; the hysteresis loops are an obvious H3, which suggests the presence of slit-like mesopores.⁶⁷ The pore size distributions of these adsorbents are characterized, which shows that there are some stacking holes in the PES@Mg₂SiO₄, PEI@Mg₂SiO₄ and Mg₂SiO₄. The BET analysis of the materials indicates that the specific surface areas of the PES@Mg₂SiO₄, PEI@Mg₂SiO₄–CS₂, PEI@Mg₂SiO₄ and Mg₂SiO₄ are 112.40, 58.12, 70.19 and 51.61 m² g⁻¹, respectively. It’s worth noting that the PEI@Mg₂SiO₄–CS₂ has a smaller specific surface area than PES@Mg₂SiO₄ and PEI@Mg₂SiO₄. This result is expected, because during the impregnation of PEI@Mg₂SiO₄ with CS₂, some amount of sulfurizing agent is probably trapped in the pores of the adsorbent. Besides this, PES@Mg₂SiO₄ has a higher specific surface area than PEI@Mg₂SiO₄–CS₂, PEI@Mg₂SiO₄ and Mg₂SiO₄, which is conducive to the adsorption of heavy metal ions from wastewater.

Fig. 3 N₂ absorption–desorption isotherm and pore-size distribution (inset) for the PES@Mg₂SiO₄ (A, a), PEI@Mg₂SiO₄–CS₂ (B, b), PEI@Mg₂SiO₄ (C, c) and Mg₂SiO₄ (D, d).

The SEM images illustrate that the PEI@Mg₂SiO₄–CS₂ and PEI@Mg₂SiO₄ have flaky structures, unlike the granular PES@Mg₂SiO₄ and Mg₂SiO₄ (Fig. 4). The reaction between Mg₂SiO₄ and PEI makes PEI@Mg₂SiO₄–CS₂ form a structure with lamellar stacking.³⁶ This flaky structure is affected by the intervention of CS₂, which makes PES@Mg₂SiO₄ tend to form a grainy structure. The results of energy dispersive X-ray analysis can also be used to demonstrate the presence of PEI and CS₂ in these composites. The TEM images further prove the presence of the layered structure features. Moreover, after adsorbing heavy metals, the structure of the PES@Mg₂SiO₄ changes from loose (Fig. 5E) to compact (Fig. 5F). This is because two dithiocarbamate molecules are used to chelate each metal atom (Fig. 1C and D).

Fig. 4 SEM images of the PES@Mg₂SiO₄ (A, a), PEI@Mg₂SiO₄–CS₂ (B, b), PEI@Mg₂SiO₄ (C, c) and Mg₂SiO₄ (D).

Fig. 5 TEM images of the PES@Mg₂SiO₄ (A and E), PEI@Mg₂SiO₄–CS₂ (B), PEI@Mg₂SiO₄ (C), Mg₂SiO₄ (D) and PES@Mg₂SiO₄–metal (F).

As shown in the thermal gravimetric analysis (TGA) with differential thermal gravity (Fig. S2, ESI†), three steps are involved in the TGA of the PES@Mg₂SiO₄ (Fig. S2A†), PEI@Mg₂SiO₄–CS₂ (Fig. S2B†), and PEI@Mg₂SiO₄ (Fig. S2C†). Firstly, the physisorbed water and interlayer water are removed between 50 to 200 °C, then layer silicate is removed in the range of 200 to 400 °C, finally in the range of 400 to 750 °C the doped organics are completely oxidized. At 750 °C, the weight loss of the PES@Mg₂SiO₄ is about 40%, which is composed of 13% physisorbed water, 16% layer silicate and 11% interlayer PES; the weight loss of the PEI@Mg₂SiO₄–CS₂ is about 33%, which is composed of 11% physisorbed water, 11% layer silicate and 11% interlayer PEI–CS₂; the weight loss of the PEI@Mg₂SiO₄ is about 39%, which is composed of 12% physisorbed water, 17% layer silicate and 10% interlayer PEI–CS₂.

The values of the point of zero charge (pH_pzc) obtained are about 8.1, 7.9, and 9.3 for PES@Mg₂SiO₄, PEI@Mg₂SiO₄–CS₂ and PEI@Mg₂SiO₄, respectively (Fig. S3, ESI†). The sulfurization treatments make the pH_pzc of PES@Mg₂SiO₄ and PEI@Mg₂SiO₄–CS₂ lower than that of PEI@Mg₂SiO₄. This sulfurization treatment facilitates the electrostatic interaction between the sorbents and metal cations, which is conductive to the improvement of their adsorption capabilities.⁶⁰ Some studies find that it benefits the adsorption of metal cations when the initial pH of the solution is higher than the pH_pzc of the adsorbents.^68,69 Thus, it is expected that the sulfurization treatment could broaden the optimum pH range for metal adsorption.

3.3 Evaluation of dissociated heavy metal adsorption by sorbents

As described in the above, the PES@Mg₂SiO₄, PEI@Mg₂SiO₄–CS₂ and PEI@Mg₂SiO₄ carry many dithiocarbamate or amino groups. They may strongly adsorb heavy metal ions, e.g. Hg²⁺, Pb²⁺ and Cd²⁺ via chelation. Pertinent metal uptake data for Mg₂SiO₄ are also included for comparison. As shown in Table 2, the R² values for the Langmuir isotherm model are greater than those for the Freundlich isotherm model. These indicate that those adsorptions are more consistent with the Langmuir isotherm model, 1/q_e = 1/(K_aC_eq_∞) + 1/q_∞,^70–73 where C_e is the equilibrium molarity in mg L⁻¹, q_e is the adsorbance of the heavy metal in mg g⁻¹, q_∞ is the saturation adsorbance of the heavy metal in mg g⁻¹ and K_a is the binding constant. The q_∞ order for the different sorbents was as follows: PES@Mg₂SiO₄ > PEI@Mg₂SiO₄–CS₂ > PEI@Mg₂SiO₄ > Mg₂SiO₄. This indicates that the main influence factor on the adsorption capacity is the content of dithiocarbamate or amino groups, but not the specific surface area. The q_∞ order for PES@Mg₂SiO₄ for Hg²⁺, Pb²⁺ and Cd²⁺ was as follows: Hg²⁺ > Pb²⁺ > Cd²⁺. This can be explained by hard and soft acid–base theory. The central theme of this theory is that a soft acid tends to react with a soft base, while a hard acid tends to react with a hard base. Thus, the soft acid Hg²⁺ has a stronger chelating ability with the soft base dithiocarbamate. Besides this, the selectivity coefficient for Hg²⁺ is much higher than that for Pb²⁺ and Cd²⁺ (Table S1, ESI†). This indicates that PES@Mg₂SiO₄ has the strongest adsorption capacity for Hg²⁺, and preferably adsorbs Hg²⁺. Compared with other silicon-based materials, PES@Mg₂SiO₄ has a much better adsorption capacity for Hg²⁺, Pb²⁺ and Cd²⁺.

Table 2 Kinetic parameters for Hg²⁺, Pb²⁺ and Cd²⁺ ion adsorption by the sorbents

Metal ions	Sorbents	Freundlich parameters			Langmuir parameters			q∞ comparison
		n	Kf	R^2	q∞ (mg g^−1)	Ka	R^2	Sorbent	q∞ (mg g^−1)
Hg^2+	PES@Mg2SiO4	0.656	1.656	0.959	714	0.065	0.954	Porous clay heterostructures^1	600
	PEI@Mg2SiO4–CS2	0.573	1.667	0.871	500	0.069	0.951
	PEI@Mg2SiO4	0.572	1.457	0.884	370	0.055	0.979
	Mg2SiO4	0.373	1.461	0.939	78	0.551	0.962
Pb^2+	PES@Mg2SiO4	0.361	1.814	0.928	323	0.188	0.998	Hollow Mg2SiO4 (ref. 54)	300
	PEI@Mg2SiO4–CS2	0.443	1.299	0.903	270	0.028	0.964
	PEI@Mg2SiO4	0.783	0.369	0.950	169	0.011	0.996	Palygorskite clay^49	104
	Mg2SiO4	0.560	0.501	0.909	88	0.022	0.989
Cd^2+	PES@Mg2SiO4	0.697	1.258	0.969	556	0.022	0.995	Poly(amidoxime)/SiO2 (ref. 22)	45
	PEI@Mg2SiO4–CS2	0.671	0.858	0.965	256	0.017	0.998
	PEI@Mg2SiO4	0.714	0.459	0.830	200	0.008	0.98
	Mg2SiO4	0.623	0.569	0.808	71	0.035	0.966

---

As indicated by Fig. 6A, the adsorption equilibrium time is about 20 min for all three metals, which is less than that of activated carbon.³¹ pH has almost the same influence on the adsorption of all three heavy metal ions (Fig. 6B). The adsorption quantity increased from pH 3.3 to 5.3 and decreased from pH 5.3 to 7. This is attributed to the chelate interaction between the lone pair electrons of the sulphur atom and the heavy metal ion. When pH < 5.5, H⁺ competed with the heavy metal ions for chelation sites, the concentration of H⁺ and competitive force decreased with the increase in pH, and the adsorption quantity of heavy metals increased.⁷⁴ When pH > 5.3, heavy metal ions tended to form hydroxides covering the surface of the composites, and the adsorption ability would be relatively reduced.⁷⁵ The effect of sorbent dosage on the adsorption activity was studied (Fig. 6C). The removal rate increased with the increase in dosage from 0.01% to 0.04%. Then there is no significant increase in the removal rate with the increase in dosage from 0.04% to 0.05%. When the dosage is 0.04% (w/v) PES@Mg₂SiO₄, the removal rates for Hg²⁺, Pb²⁺, Cd²⁺, Cu²⁺, Fe³⁺, Bi³⁺, and Se⁴⁺ are all larger than 90%. So the optimal dosage is determined to be 0.04% (w/v) for practical applications.

Fig. 6 The adsorption curves of heavy metal ions (1 – Hg²⁺, 2 – Pb²⁺ and 3 – Cd²⁺) for the PES@Mg₂SiO₄ composite at different times (A) and pH (B), and the removal rate for the multi-metal mixed standard solution with the PES@Mg₂SiO₄, with concentrations of 0.01% (1), 0.02% (2), 0.03% (3), 0.04% (4) and 0.05% (5) (w/v) (C).

3.4 Evaluation of complexed heavy metal adsorption by sorbents and practical application

As shown in Fig. 7A, the increase in EDTA concentration has some effect on the removal rate. When the concentration of EDTA increased from 0 to 20 mg L⁻¹, the removal rates for Hg, Cu, Bi and Se decreased a little but were still more than 90%, and those for Cd and Bi were both more than 80%. Besides this, the removal rates for Ni, Cr and Mn even increased with the increase in EDTA concentration. These indicate that PES@Mg₂SiO₄ was an excellent sorbent for metal complexes. Then the composite was used to treat wastewater from petrochemical works in Huai’an, China. The result is presented in Fig. 7B. It is obvious that Hg, Pb, Cd, Se and Cu were totally removed. Over 90% of Al and Zn were removed, and the removal rates for Fe and Ni were close to 50%. So the composite has great potential in applications for industrial wastewater treatment.

Fig. 7 The adsorption of the multi-metal mixed standard solution with different concentrations of EDTA: 0 mg L⁻¹ (1), 2 mg L⁻¹ (2) and 20 mg L⁻¹ (3) (A); treatment of petrochemical wastewater with the PES@Mg₂SiO₄ (B).

4. Conclusion

In this work, PES was firstly tailored to deliver organic components with increased binding capacity even towards metal complexes. Nano-hybrid PES@Mg₂SiO₄ was then fabricated to remove dissociated and complexed heavy metals from aqueous solution. The results show that PES@Mg₂SiO₄ is an effective sorbent for the removal of Hg²⁺, Pb²⁺ and Cd²⁺ and their complexes. Equilibrium isotherms were more in accordance with the Langmuir equation than the Freundlich equation, giving maximum adsorption capacities of 714 mg g⁻¹ for Hg²⁺, 400 mg g⁻¹ for Pb²⁺ and 323 mg g⁻¹ for Cd²⁺, which are better than those of PEI@Mg₂SiO₄–CS₂, PEI@Mg₂SiO₄ and Mg₂SiO₄. Also, the composite has the best performance in terms of the preferential adsorption of Hg²⁺. The dosing quantity optimization experiment indicated that the optimal dosage was 0.04% (w/v), and the composite also can efficiently remove Cu²⁺, Bi³⁺ and metal complexes from aqueous solution. Besides this, its application on petrochemical wastewater showed positive results, which demonstrated its enormous potential in heavy metal wastewater treatment.

Acknowledgements

The work was financially supported by the National Natural Science Foundation of China (11375111) and the Research Fund for the Doctoral Program of Higher Education of China (20133108110021).

References

1. A. l. J. Tchinda, E. Ngameni, I. T. Kenfack and A. Walcarius, Chem. Mater., 2009, 21, 4111–4121.
2. J. J. Fu, A. Q. Zhang, T. Wang, G. B. Qu, J. J. Shao, B. Yuan, Y. W. Wang and G. B. Jiang, Environ. Sci. Technol., 2013, 47, 7437–7445.
3. A. J. Verschoor, J. P. M. Vink, G. R. de Snoo and M. G. Vijvert, Environ. Sci. Technol., 2011, 45, 6049–6056.
4. B. Y. Song, Y. Eom and T. G. Lee, Appl. Surf. Sci., 2011, 257, 4754–4759.
5. M. Monier and D. A. Abdel-Latif, J. Hazard. Mater., 2013, 250–251, 122–130.
6. B. Wang, L. Axe, Z. H. Michalopoulou and L. P. Wei, Environ. Sci. Technol., 2012, 46, 517–524.
7. E. K. Yetimoğlu, M. Fırlak, M. V. Kahraman and S. Deniz, Polym. Adv. Technol., 2011, 22, 612–619.
8. A. Tal, Science, 2006, 313, 1081–1084.
9. W. Chouyyok, Y. Shin, J. Davidson, W. D. Samuels, N. H. Lafemina, R. D. Rutledge, G. E. Fryxell, T. Sangvanich and W. Yantasee, Environ. Sci. Technol., 2010, 44, 6390–6395.
10. Y. Zhao, Y. Chen, M. Li, S. Zhou, A. Xue and W. Xing, J. Hazard. Mater., 2009, 171, 640–646.
11. A. E. Gash, A. L. Spain, L. M. Dysleski, C. J. Flaschenriem, A. Kalaveshi, P. K. Dorhout and S. H. Strauss, Environ. Sci. Technol., 1998, 32, 1007–1012.
12. Y. Yang, Y. Xie, L. Pang, M. Li, X. Song, J. Wen and H. Zhao, Langmuir, 2013, 29, 10727–10736.
13. M. C. Aragoni, M. Arca, F. Demartin, F. A. Devillanova, F. Isaia, A. Garau, V. Lippolis, F. Jalali, U. Papke, M. Shamsipur, L. Tei, A. Yari and G. Verani, Inorg. Chem., 2002, 41, 6623–6632.
14. K. A. Hill, L. W. Lion and B. A. Ahner, Environ. Sci. Technol., 2002, 36, 5363–5368.
15. S. M. Lee, Lalhmunsiama and D. Tiwari, Environ. Sci. Pollut. Res., 2014, 21, 3686–3696.
16. M. S. Vohra and A. P. Davis, J. Colloid Interface Sci., 1998, 198, 18–26.
17. F. L. Fu and Q. Wang, J. Environ. Manage., 2011, 92, 407–418.
18. J. H. Zhu, S. Y. Wei, H. B. Gu, S. B. Rapole, Q. Wang, Z. P. Luo, N. Haldolaarachchige, D. P. Young and Z. H. Guo, Environ. Sci. Technol., 2012, 46, 977–985.
19. Y. L. Chen, B. C. Pan, H. Y. Li, W. M. Zhang, L. Lv and J. Wu, Environ. Sci. Technol., 2010, 44, 3508–3513.
20. W. Ciesielski, C. Y. Lii, M. T. Yen and P. Tomasik, Carbohydr. Polym., 2003, 51, 47–56.
21. L. Charerntanyarak, Water Sci. Technol., 1999, 39, 135–138.
22. B. Gao, Y. Gao and Y. Li, Chem. Eng. J., 2010, 158, 542–549.
23. C. Xiong, Q. Jia, X. Chen, G. Wang and C. Yao, Ind. Eng. Chem. Res., 2013, 52, 4978–4986.
24. N. Wu, H. H. Wei and L. Z. Zhang, Environ. Sci. Technol., 2012, 46, 419–425.
25. R. Ahmad and S. Haseeb, Desalin. Water Treat., 2015, 57, 17826–17835.
26. A. Oehmen, D. Vergel, J. Fradinho, M. A. M. Reis, J. G. Crespo and S. Velizarov, J. Hazard. Mater., 2014, 264, 65–70.
27. M. Hua, Y. N. Jiang, B. Wu, B. C. Pan, X. Zhao and Q. X. Zhang, ACS Appl. Mater. Interfaces, 2013, 5, 12135–12142.
28. M. L. Chen, C. B. Gu, T. Yang, Y. Sun and J. H. Wang, Talanta, 2013, 116, 688–694.
29. K. K. Bhatluri, M. S. Manna, P. Saha and A. K. Ghoshal, J. Membr. Sci., 2014, 459, 256–263.
30. X. Zhao, L. B. Guo, B. F. Zhang, H. J. Liu and J. H. Qu, Environ. Sci. Technol., 2013, 47, 4480–4488.
31. K. Kadirvelu, C. Faur-Brasquet and P. Le Cloirec, Langmuir, 2000, 16, 8404–8409.
32. T. H. Madden, A. K. Datye, M. Fulton, M. R. Prairie, S. A. Majumdar and B. M. Stange, Environ. Sci. Technol., 1997, 31, 3475–3481.
33. A. P. Davis and D. L. Green, Environ. Sci. Technol., 1999, 33, 609–617.
34. P. Kocot, A. Karocki and Z. Stasicka, J. Photochem. Photobiol., A, 2006, 179, 176–183.
35. K. R. Lee, S. J. Kim, J. S. Song, J. H. Lee, Y. J. Chung and S. Park, J. Am. Ceram. Soc., 2002, 85, 341–345.
36. Y.-P. Wei and H.-W. Gao, J. Mater. Chem., 2012, 22, 5715.
37. F. Ciesielczyk, A. Krysztafkiewicz and T. Jesionowski, J. Mater. Sci., 2007, 42, 3831–3840.
38. L. Zhang, X. Hu, C. Yu, R. Crawford and A. Yu, Int. J. Environ. Anal. Chem., 2013, 93, 1274–1285.
39. F. An and B. Gao, J. Hazard. Mater., 2007, 145, 495–500.
40. A. Mittal, R. Ahmad and I. Hasan, Desalin. Water Treat., 2015, 57, 17790–17807.
41. R. Ahmad and A. Mirza, Groundwater for Sustainable Development, 2015, 1, 50–58.
42. G. Decher, Science, 1997, 277, 1232–1237.
43. D. Y. Zhong, J. H. Franke, S. K. Podiyanachari, T. Blomker, H. M. Zhang, G. Kehr, G. Erker, H. Fuchs and L. F. Chi, Science, 2011, 334, 213–216.
44. M. C. B. Sheng Dai, Y. H. Ju and H. J. Gao, J. Am. Chem. Soc., 2000, 122, 992–993.
45. M. Kimura, R. Sakai, S. Sato, T. Fukawa, T. Ikehara, R. Maeda and T. Mihara, Adv. Funct. Mater., 2012, 22, 469–476.
46. A. B. Thompson, S. J. Cope, T. D. Swift and J. M. Notestein, Langmuir, 2011, 27, 11990–11998.
47. J. Yu, Y. Le and B. Cheng, RSC Adv., 2012, 2, 6784.
48. M. A. Abu-Saied, E. S. Abdel-Halim, M. M. G. Fouda and S. S. Al-Deyab, Int. J. Electrochem. Sci., 2013, 8, 5121–5135.
49. H. Chen and A. Q. Wang, J. Colloid Interface Sci., 2007, 307, 309–316.
50. C. Yuan, B. Liu, F. Liu, M.-Y. Han and Z. Zhang, Anal. Chem., 2014, 86, 1123–1130.
51. S. Sung, H. Holmes, L. Wainwright, A. Toscani, G. J. Stasiuk, A. J. P. White, J. D. Bell and J. D. E. T. Wilton-Ely, Inorg. Chem., 2014, 53, 1989–2005.
52. C. Bertagnolli, A. Grishin, T. Vincent and E. Guibal, Ind. Eng. Chem. Res., 2016, 55, 2461–2470.
53. F. Ciesielczyk, A. Krysztafkiewicz and T. Jesionowski, J. Mater. Sci., 2007, 42, 3831–3840.
54. Y. Wang, G. Wang, H. Wang, C. Liang, W. Cai and L. Zhang, Chem.–Eur. J., 2010, 16, 3497–3503.
55. Z. Y. Chen, H. W. Gao and J. X. Yang, RSC Adv., 2014, 4, 33866–33873.
56. F. Dubois, B. Mahler, B. Dubertret, E. Doris and C. Mioskowski, J. Am. Chem. Soc., 2007, 129, 482–483.
57. Y. Zhao, W. Perez-Segarra, Q. C. Shi and A. Wei, J. Am. Chem. Soc., 2005, 127, 7328–7329.
58. H.-b. Zhen, Q. Xu, Y.-y. Hu and J.-h. Cheng, Chem. Eng. J., 2012, 209, 547–557.
59. L. H. Liu, J. Wu, Y. L. Ling, X. Li and R. J. Zeng, J. Appl. Polym. Sci., 2013, 127, 2082–2094.
60. N. Asasian and T. Kaghazchi, Ind. Eng. Chem. Res., 2012, 51, 12046–12057.
61. P. Stathi, K. Litina, D. Gournis, T. S. Giannopoulos and Y. Deligiannakis, J. Colloid Interface Sci., 2007, 316, 298–309.
62. S. L. Burkett, A. Press and S. Mann, Chem. Mater., 1997, 9, 1071–1073.
63. D. H. Zhao, Y. L. Shen, Y. L. Zhang, D. Q. Wei, N. Y. Gao and H. W. Gao, J. Mater. Chem., 2010, 20, 3098–3106.
64. Z. Y. Chen, H. W. Gao and Y. Y. He, RSC Adv., 2013, 3, 5815–5818.
65. A. Warshawsky, I. Rogachev, Y. Patil, A. Baszkin, L. Weiner and J. Gressel, Langmuir, 2001, 17, 5621–5635.
66. N. Singh, B. P. R. Metla and A. J. Elias, J. Organomet. Chem., 2012, 717, 99–107.
67. S. B. Yang, L. Zhan, X. Y. Xu, Y. L. Wang, L. C. Ling and X. L. Feng, Adv. Mater., 2013, 25, 2130–2134.
68. U. Farooq, J. A. Kozinski, M. A. Khan and M. Athar, Bioresour. Technol., 2010, 101, 5043–5053.
69. M. Iqbal, A. Saeed and S. I. Zafar, J. Hazard. Mater., 2009, 164, 161–171.
70. S. K. Parida, S. Dash, S. Patel and B. K. Mishra, Adv. Colloid Interface Sci., 2006, 121, 77–110.
71. T. Kumeria, A. Santos and D. Losic, ACS Appl. Mater. Interfaces, 2013, 5, 11783–11790.
72. C. S. Cheng, J. Deng, B. Lei, A. He, X. Zhang, L. Ma, S. Li and C. Zhao, J. Hazard. Mater., 2013, 263, 467–478.
73. D. Suteu, A. C. Blaga, M. Diaconu and T. Malutan, Cent. Eur. J. Chem., 2013, 11, 2048–2057.
74. O. Ercan and A. Aydin, J. Braz. Chem. Soc., 2013, 24, 865–872.
75. K. Saeed, S. Haider, T. J. Oh and S. Y. Park, J. Membr. Sci., 2008, 322, 400–405.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20181d

---