J. Cent. South Univ. (2018) 25: 709-718

DOI: https://doi.org/10.1007/s11771-018-3775-y

Absorption performance of DMSA modified Fe₃O₄@SiO₂core/shell magnetic nanocomposite for Pb²⁺ removal

TIAN Qing-hua(田庆华), WANG Xiao-yang(王晓阳), MAO Fang-fang(毛芳芳), GUO Xue-yi(郭学益)

School of Metallurgy and Environment, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract: The purpose of this study is to explore the adsorption performance of meso-2, 3-dimercaptosuccinic acid (DMSA) modified Fe₃O₄@SiO₂ magnetic nanocomposite (Fe₃O₄@SiO₂@DMSA) for Pb²⁺ ions removal from aqueous solutions. The effects of solution pH, initial concentration of Pb²⁺ions, contact time, and temperature on the amount of Pb²⁺ adsorbed were investigated. Adsorption isotherms, adsorption kinetics, and thermodynamic analysis were also studied. The results showed that the maximum adsorption capacity of the Fe₃O₄@SiO₂@DMSA composite is 50.5 mg/g at 298 K, which is higher than that of Fe₃O₄ and Fe₃O₄@SiO₂ magnetic nanoparticles. The adsorption process agreed well with Langmuir adsorption isotherm models and pseudo second-order kinetics. The thermodynamic analysis revealed that the adsorption was spontaneous, endothermic and energetically driven in nature.

Key words: lead removal; adsorption; Fe₃O₄@SiO₂ core/shell structure; DMSA modification; magnetic nanocomposite

Cite this article as: TIAN Qing-hua, WANG Xiao-yang, MAO Fang-fang, GUO Xue-yi. Absorption performance of DMSA modified Fe₃O₄@SiO₂ core/shell magnetic nanocomposite for Pb²⁺ removal [J]. Journal of Central South University, 2018, 25(4): 709–718. DOI: https://doi.org/10.1007/s11771-018-3775-y.

1 Introduction

Heavy metals are generally considered to be a threat toward humans and ecosystems owing to their high potential toxicity [1]. Lead is a strong poison and toxic to vital target organs and body systems, which could cause a wide range of health problems [2, 3]. A number of techniques have been developed to remove the metal ions from wastewater such as chemical precipitation and ion exchange process [4], biotechnology [5], membrane and reverse osmosis processes [6], electrochemical methods [7], and adsorption [8–10]. Among them, adsorption is the best available method due to the convenient operation process, wide applicability, high efficiency, low energy requirement and cost effectiveness [11, 12]. Recently, many research groups have explored several nanoparticles for heavy metal removal, such as alumina [13], carbon [14], silica [15] nanomaterials, because of the ease of their surface functionality modification and their high surface area-to-volume ratio for increased adsorption capacity and efficiency [16].

In the last decade, iron oxide magnetic nanoparticles (MNPs) have been regarded as attractive adsorbents due to their high surface area and the unique advantage of easy separation under external magnetic fields [17]. However, MNPs are unstable and usually tend to aggregate and, therefore, greatly weaken their efficiency for environmental application [18]. It is shown that coating of MNPs with functional groups can prevent the magnetite core from particle aggregation and improve the dispersion stability in suspension medium [19]. Many materials such as chitosan [20], carbon [21], polyrhodanine [22], are considered to be good alternatives for the surface modification of iron oxide MNPs. Among these, SiO₂ is deemed to an ideal shell composite because it is stable under acidic conditions and inert to redox reactions [23]. Additionally, the availability of —OH moieties can provide the binding sites for other organic molecules which may further extend heavy metal removal applications [24].

Nowadays, in order to achieve the most effectiveness in adsorption systems, Fe₃O₄@SiO₂ nanoparticles have been functionalized with different materials such as amino, porphyrin, and acetoacetanilide for heavy metal removal from water [25–27]. The dimercaptosuccinic acid (DMSA, C₄S₂O₄H₆) has attracted considerable attention as a new class of highly dispersible biocompatible material for Fe₃O₄ nanoparticles functionalization recently [28, 29]. Besides, the DMSA has an abundance of surface sulfhydryl groups, which can enhance the adsorption capability of magnetic nanoparticles.

Considering the excellent dispersed property and adsorption ability of DMSA, here we explored the DMSA modified Fe₃O₄@SiO₂ core/shell nanoparticles for Pb²⁺ removal from water by a simple and effective synthesis method. In addition, we investigated the adsorption capacity of Fe₃O₄@SiO₂@DMSA composites for Pb²⁺ removal with different solution pH, contact time and metal ion concentration. We also studied the adsorption isotherms, kinetics and thermodynamics to understand the mechanism of the lead ions adsorption of as-prepared MNPs adsorbent and explored the effect of MNPs reuse. The comparative analysis of adsorption capacity of naked Fe₃O₄ MNPs, SiO₂ coated Fe₃O₄ MNPs (Fe₃O₄@SiO₂), and Fe₃O₄@SiO₂@DMSA MNPs were also carried out in the study.

2 Experimental

2.1 Materials

Dimethyl sulfoxide (DMSO), aqueous ammonia (25%–28%), tetraethyl orthosilicate (TEOS), meso-2, 3-dimercaptosuccinic acid (DMSA), ethanol, lead nitrate, sodium hydroxide, 5, 5-Dithiobis-(2-nitrobenzoic acid) (DTNB) were purchased from Aladdin Industrial Corporation (Shanghai, China). Toluene was purchased from Aladdin Industrial Corporation (Shanghai, China). Nitric acid was provided by Xingkong Industrial Corporation (Zhuzhou, China) and hydrochloric acid was provided by Kaixin Industrial Corporation (Hengyang, China). Water used in all experiments was purified using a Milli-Q Plus 185 water purification system (Millipore, Bedford, MA). All chemicals were of analytical grade.

2.2 Synthesis of Fe₃O₄@SiO₂@DMSA core/shell nanoparticles

The Fe₃O₄ magnetic nanoparticles were supplied by Research Institute for Resource Recycling, (Changsha, China). The detailed description of the synthetic methods of Fe₃O₄@SiO₂ and DMSA modified Fe₃O₄@SiO₂ MNPs were presented in the previous research [30]. To synthesize the Fe₃O₄@SiO₂ nanocomposite,100 mg as-prepared hydrophilic Fe₃O₄ MNPs were suspended in ethanol (150 mL) under sonication for 40 min. Aqueous ammonia (3 mL) and deionized water (47 mL) were added in sequence to the suspension, and the mixture (with the volume ratio of ethanol-water-ammonia 75.0%-23.5%-1.5%) was sonicated for 40 min. Then, 0.6 mL TEOS was added slowly under mild continuous stirring at room temperature for another 8 h. The silica-coated nanoparticles were isolated by magnetic separation and were washed with ethanol several times. The final product was collected and dried at 60 °C under vacuum.

The DMSA modified Fe₃O₄@SiO₂ nanocomposites were obtained by the following process. 100 mg Fe₃O₄@SiO₂ nanoparticles were dispersed in 10 mL toluene under sonication for 30 min, and then 10 mL DMSO and 250 mg DMSA were added in the solution. The mixture was stirred for 12 h. The final products were collected by magnetic separation, washed with ethanol and deionized water several times, and dried at 60 °C under vacuum.

2.3 Adsorption experiments

Adsorption of Pb²⁺ ions from aqueous solution was determined in batch experiments. We investigated the effect of pH (2.0–6.0), kinetics time (10–130 min), adsorption isotherms (initial concentration 0–500 mg/L) of Pb²⁺ ions, and thermodynamics (298–323 K). Analyzing adsorption behavior of the MNPs involved adding 20 mg (Fe₃O₄, Fe₃O₄@SiO₂, and Fe₃O₄@SiO₂@ DMSA) to 20 mL of solution of the Pb²⁺ ions concentrations at 100 mg/L at 298 K. The pH was maintained at a constant value during adsorption. The contact time was 4 h. The adsorbents were separated by powerful magnets and centrifuging at 5000 r/min for 10 min. The Pb²⁺ ion concentration adsorbed per gram of adsorbent q_t at time t was calculated as follows [31]:

                         (1)

where C₀ and C_t are the concentrations of the Pb²⁺ in the solution before and after adsorption, respectively (mg/L); V is the volume of the solution (L), and m is the amount of dry MNPs used (g).

2.4 Desorption experiments

To evaluate the ability to reuse the Fe₃O₄@ SiO₂@DMSA nanoparticles, we investigated the effects of the concentrations of hydrochloric acid (0.05–1.60 mol/L) on the stability of the adsorbents and the amount of desorbed Pb²⁺ ions. The desorption experiments involved adding 40 mg Pb²⁺ ion-loaded adsorbents to 20 mL aqueous solution with 0.10 mol/L hydrochloric acid. The mixture was shaken for 3 h to reach desorption equilibrium. Adsorption-desorption cycles were repeated five times by using the same MNPs.

3 Results and discussion

3.1 Characterization of prepared MNPs

In the previous studies, the detailed characterizations of the morphology, structure, functional groups, surface charge, magnetic susceptibility of the Fe₃O₄, Fe₃O₄@SiO₂, and Fe₃O₄@SiO₂@DMSA MNPs were carried out by transmission electron microscopy (TECNAI G2 60-300, FEI), X-ray diffraction (S-3000N, Hitachi), Fourier transform infrared spectrometer(6700, NICOLET), X-ray photoelectron spectroscopy (ESCALAB 250Xi, Thermo Fisher), zeta potential analysis (Nano-2S&MPT-2, Malvern), dynamic laser scattering (Nano-2S&MPT-2, Malvern) and vibrating sample magnetometer (HH-15) [30]. The results revealed that: 1) The size of the DMSA modified Fe₃O₄@SiO₂ MNPs could be tuned with core diameter of 150–200 nm and layer thickness of 15–19 nm. 2) The FTIR and XPS results showed that DMSA molecules were bound to the particle surface via COO— groups. 3) The content of —SH on the surface of the Fe₃O₄@SiO₂@DMSA nanoparticles was calculated to be 0.089 mmol/g. 4) The Fe₃O₄@SiO₂@DMSA nanoparticles exhibited fine colloidal stability, enhanced dispersibility, and optimal magnetization after DMSA modification, which is confirmed by ZETA, DLS, and VSM measurements. The Pb (II) ions were supposed to interact with the adsorbents through the active sites on the surface of MNPs (Scheme 1).

3.2 Results of batch experiments

3.2.1 Effect of pH

The initial pH is an important variable for adsorption of the metal ions [19]. The effect of pH on the adsorption for Pb²⁺ has been studied in the pH range 2.0–6.0 and is illustrated in Figure 1. The removal efficiency increased with pH 2.0–4.5, which might be ascribed to the competition of Pb²⁺and Pb(OH)⁺ ions with H⁺ ions for the active sites on the MNPs surface [31]. As the alkalinity of solution increases, the —SH and —OH turn into —S— and —O— anions and the adsorption increases gradually until pH>pH_zpc (pH at zero point charge). Thereafter, the —SH and —OH completely turn into —S— and —O— anions, with almost no change in adsorption [32]. Considering that Pb²⁺ will precipitate as hydroxide when the pH up to a certain value, we chose pH=5.5 for further experiments [27].

Scheme 1 Presentation of possible mechanism of prepared MNPs for Pb (II) ions adsorption

Figure 1 Effect of pH on adsorption of Pb²⁺ ions; initial Pb²⁺concentration: 100 mg/L, adsorbent dose: 1 mg/mL, reaction temperature: 298 K, contact time: 4 h

3.2.2 Adsorption isotherm

Figure 2 shows the adsorption isotherms of Pb²⁺ on Fe₃O₄, Fe₃O₄@SiO₂, and Fe₃O₄@SiO₂@ DMSA MNPs at 298 K. The equilibrium capacities of the Pb²⁺ ions are increased with increasing concentrations until reaching equilibrium. Besides, the maximum capacity of Pb²⁺ on the MNPs decreases in the order Fe₃O₄@SiO₂@DMSA> Fe₃O₄@SiO₂>Fe₃O₄, indicating the adsorption capacity is improved after DMSA modification. The observed differences in the maximum capacities of Pb²⁺ ions are most probably due to the different affinity to interact with —SH and —OH groups of the MNPs [33].

Equilibrium data can be analyzed by widely used adsorption isotherms, Langmuir isotherm and Freundlich isotherm, which provide the basis for the design of adsorption systems. The former is based on the monolayer and homogenized coverage of the adsorbent surface, where all sorption sites are identical and energetically equivalent [34] (Eq. (2)). The Freundlich model assumes multilayer, reversible and non-ideal adsorption at the active sites with exponential distribution of energies, which endorses the heterogeneity of the surface [35] (Eq.(3)).

Figure 2 Effect of initial Pb²⁺concentration on adsorption of Pb²⁺ ions (pH=5.5, adsorbent dose:1 mg/mL, reaction temperature: 298 K, contact time: 4 h)

                         (2)

                         (3)

where C_e and q_e are the concentration and adsorption capacity at equilibrium, and q_m and K_L are Langmuir constants, representing the maximum adsorption capacity and energy of adsorption (L/mg), respectively. K_F and n are Freundlich constants related to adsorption capacity and intensity, respectively.

For each isotherm, the values of q_m and K_L were calculated from experimental data through Figure 2. The values of K_F and n are obtained similarly. The results are presented in Table 1 with the correlation coefficients (R²). It can be seen from Table 1 that the Langmuir model yields a better fit than the Freundlich model by comparing the results of correlation coefficient values. Further, it shows a clear consistency between the calculated (q_m) and experimental values (q_m^a) of the adsorption capacity at equilibrium. Therefore, it can be considered that monolayer adsorption takes place at specific sites present on the surface of MNPs, which are energetically identical. It further confirms that chemisorption occurs due to the complexation between Pb²⁺ ions and the organic groups present on the surface of the nanoadsorbent [36, 37].

Table 1 Parameters of Langmuir and Freundlich adsorption isotherm models for Pb²⁺ adsorption

Nowadays, because Fe₃O₄@SiO₂ nanoparticles have been functionalized with different materials for Pb²⁺ removal, a comparison of the adsorption performance of functionalized Fe₃O₄@SiO₂ nanoparticles are presented in Table 2 [25, 38–41]. Although the comparison is not precise due to the different experimental conditions and the uncompleted data, the conclusion can be acquired that the DMSA modification is basically achieved and has enhanced efficiency for Pb²⁺ ions adsorption.

Table 2 Applications of functionalized Fe₃O₄@SiO₂ MNPs in Pb²⁺ removal

3.2.3 Adsorption kinetics

Figure 3 shows the effects of contact time on the adsorption of the Pb²⁺ ions. It is found that the Pb²⁺ adsorption of Fe₃O₄@SiO₂@DMSA MNPs is very fast, and the process almost completely reaches equilibrium within 30 min, which is shorter than that of Fe₃O₄@SiO₂, and Fe₃O₄. The high adsorption rate can be attributed to the presence of sufficient adsorption sites and ease of their accessibility as well as strong complexation ability towards Pb²⁺ ions [38].

Figure 3 Effect of time on adsorption of Pb²⁺ ions (pH=5.5, initial Pb²⁺concentration: 100 mg/L, adsorbent dose: 1 mg/mL, reaction temperature: 298 K)

The adsorption kinetics of metal ions with MNPs was investigated by Lagergren pseudo-first- order (Eq.(4)) and pseudo-second-order (Eq.(5)) models [42]:

                      (4)

                           (5)

where k₁ (min^–1) and k₂ (g mg^–1 min^–1) are the kinetic rate constants for the pseudo-first-order and the pseudo-second-order models. Then q_e and q_t are the amounts of Pb²⁺ adsorbed at equilibrium and at time t, respectively.

The kinetic adsorption data are fitted to Eqs.(3) and (4), and the calculated results are depicted in Table 3. As it is clear from Table 3, the correction coefficients (R²) for the second order kinetic model are greater than 0.99, suggesting the applicability of this kinetic equation and confirms the second order nature of the adsorption phenomenon of Pb²⁺ ions on the MNPs. The second-order nature of the adsorption processes confirms that the Pb²⁺ ions have been adsorbed via chemical reactions [43]. This observation is consistent with this fact that functional groups on the surface of the MNPs can adsorb the Pb²⁺ ions via complexation reaction.

Table 3 Parameters of applied kinetic models for Pb²⁺ adsorption

3.2.4 Adsorption thermodynamics

Entropy and energy factors should be considered in order to determine what processes will occur spontaneously in engineering practice. Figure 4 shows the effects of temperature on the adsorption of the Pb²⁺ ions by MNPs. The improved sorption performance of MNPs with elevated temperature elucidates that the adsorption process is thermodynamically driven.

Figure 4 Effect of temperature on adsorption of Pb²⁺ ions (Initial Pb²⁺concentration:100 mg/L, pH=5.5, adsorbent dose: 1 mg/mL, contact time: 4 h)

The Gibbs free energy indicates the degree of spontaneity of the adsorption process and higher negative value reflects a more energetically favorable adsorption. The Gibbs free energy change of adsorption is defined as

                           (6)

where R is the universal gas constant (8.314 J/(mol·K)) and T is the absolute temperature in Kelvin [44]. The equilibrium constant may be expressed in terms of standard enthalpy change of adsorption (△H) and entropy change of adsorption (△S) as a function of temperature. The relationship between the K_L and temperature is given by the Vant Hoff equation:

                        (7)

△H and △S can be obtained from the slope and intercept of the plot of ln K_c versus 1/T [45] (Figure 5). Thermodynamic parameters quantified with the measured data under a preset range of temperatures are listed in Table 4. △G at all temperatures is a negative value, confirming the spontaneous nature and the thermodynamically favorable adsorption. The positive value of △H implies that the interaction between Pb²⁺ ions and MNPs is endothermic in nature, while the positive value of △S may be attributed to the increase in randomness at the solid-liquid interface [46]. This increase results from the extra translational entropy gained by the water molecules previously adsorbed onto MNPs but displaced by Pb²⁺ions. It is noteworthy that adsorption process with △G values between –80 and –400 kJ/mol corresponds to chemisorptions [47, 48]. From the △G values obtained in this study, it can be deduced that the adsorption mechanism is dominated by physisorption [47]. Thus, the adsorption process cannot be simply attributed to a single chemisorption or physisorption. The physisorption may be contributed to the MNPs’ high surface area-to-volume ratio for Pb²⁺ adsorption.

Figure 5 lnK_c vs 1/T plots

Table 4 Estimated values of thermodynamic parameters at different temperatures

3.2.5 Desorption and repeated use

The different concentrations of H⁺ in solution to determine the best conditions for reusing MNPs were investigated (Figure 6). The adsorbents are corroded by the H⁺, which can be seen from the contents of the released Fe³⁺ ions. After SiO₂ coating, the contents of the released Fe³⁺ ions are reduced dramatically, indicating that the SiO₂ shell can prevent the MNPs corrosion by H⁺ efficiently. Considering the desorption efficiencies which are more than 90% with 0.1 mol/L H⁺ and the corrosion effect of the MNPs, the concentration of 0.1 mol/L H⁺ was selected in the desorption process. The Pb²⁺ ions adsorption capacities of MNPs decrease at first recycle but change less after second cycle, which indicates the reusability of the MNPs adsorbents (Figure 7). Our recyclability studies suggest that nano-adsorbent can be repeatedly used as an efficient adsorbent in water treatment.

Figure 6 Contents of released Fe³⁺ ions at different H⁺ concentrations of desorption

4 Conclusions

The DMSA modified Fe₃O₄@SiO₂ nanocomposites were developed for the removal of Pb²⁺from aqueous solutions in this study. This results show that DMSA modified Fe₃O₄@SiO₂ nanocomposites could be used for the removal of Pb²⁺ ions from water in a very short time with high removal efficiency.

Figure 7 Effect of recycle numbers on adsorption of Pb²⁺ ions (pH=5.5, adsorbent dose: 1 mg/mL,initial Pb²⁺ concentration: 100mg/L, reaction temperature: 298 K, contact time: 4 h)

1) The maximum enrichment capacity of Fe₃O₄@SiO₂@DMSA nanoparticles was 50.5 mg/g at 298 K, pH=5.5, which is higher than that of Fe₃O₄ and Fe₃O₄@SiO₂ magnetic nanoparticles.

2) Adsorption was very rapid and equilibrium was achieved in less than 30 min, and the correction coefficients (R²) for the pseudo-second-order kinetic model was greater than 0.99.

3) The Langmuir and Freundlich adsorption models were used to express the sorption phenomenon, with a better fitting to the Langmuir model than the Freundlich model.

4) The thermodynamics of Pb²⁺ adsorption onto the MNPs confirmed the endothermic nature of the adsorption process, the spontaneity and energetically driven in nature.

5) Desorption and regeneration studies indicated that the SiO₂ shell could prevent the MNPs corrosion by H⁺ efficiently, and nanoadsorbents could be used repeatedly.

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(Edited by HE Yun-bin)

中文导读

DMSA改性Fe₃O₄@SiO₂核壳结构磁性纳米复合材料对Pb²⁺的吸附行为

摘要：本研究旨在探究2,3-二巯基丁二酸（DMSA）改性的Fe₃O₄@SiO₂核壳结构纳米复合材料（Fe₃O₄@SiO₂@DMSA）对水溶液中Pb²⁺的去除效果。考察了溶液pH、初始Pb²⁺浓度、吸附时间、温度对Pb²⁺吸附量的影响。结果表明，在298 K的条件下Fe₃O₄@SiO₂@DMSA复合材料对Pb²⁺的饱和吸附量为50.5 mg/g，明显高于Fe₃O₄和Fe₃O₄@SiO₂两种磁性纳米材料。 Fe₃O₄@SiO₂@DMSA对Pb²⁺的吸附过程符合Langmuir等温吸附模型，为准二级动力学吸附。热力学分析显示Fe₃O₄@SiO₂@DMSA对Pb²⁺的吸附为自发的、吸热的过程。

关键词：除铅；吸附；Fe₃O₄@SiO₂核壳结构；DMSA改性；磁性纳米复合材料

Foundation item: Project(2013DFA51290) supported by International S&T Cooperation Program of China

Received date: 2016-07-27; Accepted date: 2016-12-20

Corresponding author: GUO Xue-yi, PhD, Professor; Tel: +86–731–88877863; E-mail: xyguo@csu.edu.cn