Highly Efficient Lead(II) Sequestration Using Size-Controllable

Apr 2, 2017 - In this work, we successfully prepared the mussel-inspired polydopamine microspheres (PDA-Ms) with controllable sizes, through a facile ...
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Research Article pubs.acs.org/journal/ascecg

Highly Efficient Lead(II) Sequestration Using Size-Controllable Polydopamine Microspheres with Superior Application Capability and Rapid Capture Qingrui Zhang,†,‡ Qinggang Yang,‡,§ Pikky Phanlavong,‡,§ Yixiang Li,‡ Zikang Wang,‡ Tifeng Jiao,*,†,‡ and Qiuming Peng*,† †

State Key Laboratory of Metastable Materials Science and Technology and ‡School of Environmental and Chemical Engineering, Yanshan University, No. 438 hebei avenue, Qinhuangdao 066004, P.R. China S Supporting Information *

ABSTRACT: In this work, we successfully prepared the musselinspired polydopamine microspheres (PDA-Ms) with controllable sizes, through a facile self-oxidative polymerization method. The prepared PDA-M biomaterial with environmentally benign properties exhibits efficient lead(II) sequestration against high salts of competitive Ca(II), Mg(II), or Na(I) ions. It reveals 30 times greater than the commercial ion-exchanger 001x7 by selectivity evaluation. Kinetic results show that an exceedingly rapid lead(II) uptake can be achieved below 1 min. More attractively, the prepared PDA-Ms further exhibit the distinguished application ability with superior treated capacity of ∼42000 kg contaminated water/kg sorbent, and the effluents can be reduced from 1000 μg/L to below 10 μg/L, reaching the drinking water standard (WHO), which is equal to 200 times greater than commercial ion exchanger resin (∼210 kg) and granular activated carbon (∼120 kg). In addition, the exhaust PDAM material can be well regenerated and repeated use using binary 1% HCl + 5% Ca(NO3)2 solution. X-ray photoelectron spectroscopy (XPS), zeta potential, and FT-IR analysis prove that such satisfactory performances can be ascribed to the following aspects (1) the well-dispersed nanoscale morphology and highly charged property will achieve the rapid adsorption and sufficient sorbent utilization. That is, the negatively-charged PDA sphere can exert the famous Donnan membrane effects for target lead(II) enrichment and diffusion enhancement; (2) the strong amine and carbonyl/hydroxyl group within the matrix can offer sorption selectivity for powerful lead(II) capture. Effective performances as well as environmentally friendly features suggest PDA-M material is a promising lead(II)-removing candidate for water remediation. KEYWORDS: Polydopamine, Heavy metal, Adsorption, Application

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INTRODUCTION In recent years, heavy metal pollution has gotten worse and has caused an increasing number of pollution incidents, which always bring about environmental degradation and harm human life.1 Lead(II) ion is a representative toxic metal, which is ubiquitous in aquatic environments. The serious exposure of lead(II) can lead to potential damage to kidneys and the alimentary and reproductive systems. Serious cancer can also be further induced, even at trace levels of lead ingestion.2 In China and India, more and more lead pollution incidents have erupted in the past few years and the maximum allowable wastewater discharge concentration is 0.1 mg/L (1 mg/L before 2008) for wastewater and 10 μg/L for drinking water in China, while EPA and WHO have also enforced strict regulations of 15 and 10 μg/L in drinking water, respectively.3,4 Up to now, various technologies, such as chemical precipitation,5 adsorption,6,7 membrane filtration,8 and biological treatment,9 have been applied for lead-contaminated waters. © 2017 American Chemical Society

Among the available methods, adsorption can provide an important platform and option for enhanced heavy metal sequestration, due to its simplicity and high efficiency as well as low secondary pollution.10 Now, a great many adsorbents have been explored to sequestrate toxic metals in aqueous environments, such as activated carbon, graphene oxide, fly ash, chitosan, metal oxide, zeolite, montmorillonite, kaolinite, and ion-exchange resin,11−13 etc. Particularly the ion-exchange resin has been greatly developed for use with commercial products, due to their easier preparation, facilitation of modification, and high capacity properties.14 However, heavy metal uptake onto ion-exchange resin is still far from being ready for real application due to nonspecific electrostatic properties and lack of selectivity.14,15 In recent decades, the Received: January 13, 2017 Revised: March 6, 2017 Published: April 2, 2017 4161

DOI: 10.1021/acssuschemeng.7b00129 ACS Sustainable Chem. Eng. 2017, 5, 4161−4170

Research Article

ACS Sustainable Chemistry & Engineering

surface areas as well as the well-dispersed spherical features can further achieve the target lead(II) enrichment and rapid purification, according to the famous Donnan membrane effects.42 It will favor trace level heavy metal capture. Inspiringly, by contrast to the commercial ion exchanger resin 001x7, PDA-Ms is a nontoxic, widespread, biocompatible, and environmentally benign material,43 which can realize surface functionalization by one-step devices at room temperature. Thus, it probably replaces the conventional ion-exchangers for actual application. Therefore, the evaluation of heavy metal capture onto PDA-Ms, particularly in application view, will be outstandingly important and urgent. In this work, we have successfully prepared the PDA microspheres with different sizes by a modified facile selfoxidative polymerization route. Lead(II), the representative toxic metal, listed of priority pollutants by EPA, is selected as model pollutants for adsorption evaluation. More attractively, we perform it in actual application view. The solution pH effects, temperature interference, kinetics tests, and sorption capacity assessment as well as the actual continual flow purification for lead(II) removal were systematically examined. The commercial ion exchanger 001x7 and granular activated carbon (GAC) were also involved as references. Considering the characteristics of PDA microspheres, some distinguished advantages beyond conventional adsorbents are expected. Significantly, the obtained PDA-Ms exhibits superior application performances with the treated capacity of 42 000 kg wastewater/kg sorbent, which is equal to 200 times greater than the commercial ion exchanger 001x7 and GAC; an extremely rapid lead(II) uptake can be finished within below 1 min or 30 s and greater Kd values for selectivity were also realized by comparing with commercial 001x7.

chelating group modification was also conducted to improve the sorption affinity of ion-exchangers toward toxic metal capture, e.g., the imine group,16,17 thiol group,18 carboxyl group, etc.19 Nevertheless, the nonstabilized ligand groups are readily exfoliated from the matrix and cause the potential secondary contamination. In addition, the preparation and graft process will also gradually release a variety of toxic substances, such aniline, chloromethylmethyl ether, etc.20 Mitch and coworkers21 also demonstrated that high cancerogenic nitrosamine, dimethylnitramine, and chloropicrin can be generated and released immediately from quaternary ammonium groups, which is unfavorable for water treatment evidently, especially for drinking water purification. Therefore, the development of a new adsorbent with high selectivity and environmentally friendly characteristics will be an important challenge and urgent task, to replace the traditional organic ion-exchange resin. Recently, Mezzenga and Bolisetty developed an efficient composited membrane based on β-lactoglobulin amyloid fibrils and activated carbon. It reveals an exceedingly large applicability toward various heavy metals with serval orders decreases.22 This work opens a new insight into efficient adsorbent exploration using protein. Polydopamine (PDA), inspired from the mussel adhesive protein, is an eco-friendly biopolymer, which was first discovered and synthesized by Lee et al. through the facile self-polymerization process under spontaneous air oxidation of dopamine at pH = 8.5 with the buffer solution of Tris.23 Furthermore, PDA can be also readily coated on nearly all the surface of substances by self-oxidation reaction,24 such as metals, silica, wood, oxides, and even polymer. These unique features further expand its application capability in chemical, biological, physical, and medical science fields.25−28 As an efficient coating medium, the deposition of PDA can achieve the bridge connection and compatibility between organic and inorganic substrates.29−31 More importantly, the sufficient catechol and amine groups within the PDA matrix can strongly bind to toxic metals for effective removal.32−35 Therefore, PDA as an efficient adsorbent is further explored in the environmental remediation realm. Up to now, series works have been undertaken through coating technology for new adsorbent development. For instance, the PDA coated silica for various metal retention, 32 the sequestrating Pb(II)/Cd(II) using PDA-CaCO3 composites,36 and the composited GO-PDA for enhanced lead removal with the capacity of approximately 300 mg/g.37 Therefore, PDA surface coating is effective for improving metal-removal of raw material. However, the deposited PDA film with dense nanostructure of 40 nm and low surface areas will inhibit its efficient application in environmental remediation. More recently, polydopamine microspheres (PDA-Ms), a kind of new biological polymeric constituent, have attracted widespread attention.38,39 They can be easier to synthesize by dopamine monomer self-assembled polymerization. The size of microspheres can be well controlled by adjusting the alkaline concentration. Just recently, the efficient adsorption behaviors for organic dyes were well demonstrated through powerful π−π interactions, further widening their usage in environmental domain.40 Completely different from the organic dye removal, PDA-Ms will possibly reveal significant applicability for trace metal sequestration. Particularly, the nanosized structures with plenty of catechol or amine groups can endow strong sorption affinity toward heavy metals.41 More attractively, the highly negatively charged property of the PDA matrix and large

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MATERIALS AND METHODS

Materials. All the chemical agents are analytical grade without further purification and the dopamine hydrochloride for material synthesis was purchased from Sigma-Aldrich. A stock solution of Pb(II) (1000 mg/L) was prepared by dissolving desired amount of Pb(NO3)2 into deionized water. The commercial strongly acidic cationic exchange resin 001x7 was also selected for a reference with exchanged capacity of 4.2 mequiv/g, which was kindly provided by Zhejiang Zhengguang resin Co., Ltd. Note that, prior to use, the referred 001x7 beads were subjected to washing by 1% HCl and NaOH solution for five cycles to remove the possible residues. 2.2. Preparation of PDA Microspheres. PDA microspheres were prepared by controlled ammonium adjustment methods. Briefly, a selfassembled polymerization can be achieved by facile dopamine (2 mg/ L) air oxidation in water−ethanol at room temperature for 24 h reaction. The ammonium solutions with different concentrations were used to obtain the desired size of PDA-Ms. Afterward, the resultant PDA microspheres were centrifuged and washed using deionized water completely, until pH < 7.0. Finally, the obtained PDA microspheres with different sizes were subjected to a freeze-drying treatment. Batch Sorption Tests for Lead(II) Removal. Batch adsorption tests were conducted to evaluate the lead(II) sequestration. The detailed experimental procedures were well described as follows: Effects of solution pH: briefly, quantitative PDA-Ms were added into the flasks containing 50 mL of 0.25 mM lead(II) solutions. The initial solution pH was adjusted using 1% HCl solution with the range of approximately pHs = 0.5−6.5. Afterward, the above flasks were transferred into an incubator shaker at desired temperature for 24 h reaction, in order to approaching the sorption equilibrium. Finally, the resultant solutions were certificated and the remainder concentrations and solution pH at equilibrium was determined. Competing experiments were conducted using similar methods, the common competitive ions, including Ca(II)/Mg(II)/Na(I) with 4162

DOI: 10.1021/acssuschemeng.7b00129 ACS Sustainable Chem. Eng. 2017, 5, 4161−4170

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Characterization of PDA microspheres: (a) SEM image of PDA microspheres; (b) TEM of PDA microspheres; (c) higher magnification of part b; (d) SEM-EDS analysis of PDA microspheres. different concentrations, were involved respectively and the refereed ion exchanger resin 001x7 was also employed for comparisons. Next, similar sorption equilibrium tests were performed, and the Pb(II) concentrations in effluents were assayed. Kinetic experiments were carried out in a 1000 mL solution with 40 mg/L of lead ions and 100 mg PDA-M additions. Here, 1 mL solutions were taken at various time intervals. The desired kinetic curves were obtained by determining the sampled concentrations versus the corresponding time records. Sorption isotherms were realized by obtaining the maximum lead(II) uptake capacity with different lead ion content at various solution temperatures (293, 313, and 333 K). Continuous Adsorption Experiments for Application. A 0.1 g portion of the desired PDA-Ms was added into a 2000 mL container containing 1 mg/L lead(II) ions with comparative Ca(II), Mg(II), and Na(I) ion at high concentrations as feeding solutions. A mechanical agitator was used to ensure the reaction completely. A Lange-580 pump (China) was employed to attach a constant flow rate and an automatic fraction collector was used to collect the effluent samples at various time intervals. Finally, the effluent history of treated capacity versus lead(II) contents after purification was obtained. Note that, before sampling, 2 min for static sediment is necessary to avoid the possible loss of PDA microspheres. The flow velocity is approximately 20 mL lead(II)-contaminated water per hour. Analysis and Characterization. The content of lead(II) was determined by a Shimadzu AA-6800 atomic absorption/emission spectrometer equipped with a graphite furnace atomizer and deuterium background correction. The size and morphology of PDA-Ms were investigated with a scanning electron microscope (SEM, Hitachi S-4800) with the accelerating voltage of 5−15 kV, and TEM images were obtained using a high-resolution transmission electron microscopy (HRTEM, JEM2010), equipped with a Gatan CCD camera under the accelerating voltage of 200 kV. The XRD patterns of

PDA-Ms were well described on an XTRA X-ray diffractometer (Switzerland) and Cu Kα radiation (λ = 1.5418 Å) with a scan rate of 2° s−1. FTIR spectra were recorded on a Fourier infrared spectroscopy (Thermo Nicolet Corporation) with PDA-Ms by the conventional KBr disk tablet method. The zeta potentials were conducted by a Zetaplus 4 instrument (Brookhaven Instrument) with a dynamical light scatter (DLS) for size survey. XPS analysis of prepared PDA-Ms was conducted with a spectrometer (UlVAC-PHI model 5000 Versa probe), and the results were corrected and fitted using the software of XPS-peak4.1.

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RESULTS AND DISCUSSION Characterization. The systematical structural characterization was performed by SEM, TEM, XRD, and FT-IR investigation. It can be observed that the obtained PDA-Ms exhibit the well-dispersed spherical particles with size of approximately 400 nm by SEM images (Figure 1a). TEM analysis further provides similar evidence for size distributions. The low magnification image (Figure 1b) reveals the uniform particle distribution in our preparation,44 and the high magnification images further demonstrate the perfect spherical shape with accurate size of 420 nm (Figure 1c). Additionally, such shapes and sizes of PDA microspheres can be well controlled through adjusting the concentrations of ammonium solutions additions. It can be seen that the high amounts of ammonium solutions will be favorable for reducing the particle size. For instance, the 200 nm size of PDA-Ms can be readily formed using 0.5 mol/L NH3·H2O, while 0.25, 0.125, and 0.06 mol/L NH3·H2O additions will obtain 400, 550, and 900 nm PDA microspheres, respectively (Figure S1a−d). Afterward, the lead(II) uptake capacities with different size of PDA-Ms were 4163

DOI: 10.1021/acssuschemeng.7b00129 ACS Sustainable Chem. Eng. 2017, 5, 4161−4170

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ACS Sustainable Chemistry & Engineering

components can also further improve the lead(II) capture by formation of strong ligand complexation. The detailed mechanism is elucidated by XPS investigation below. Sorption Isotherms and Kinetics. The sorption isotherms were also conducted at various temperatures to evaluate the maximum sorption capacity (Figure 3c). Interestingly, the adsorption capacity exhibits outstanding variations from ∼86.8 to ∼165 mg/g at different solution temperatures, and higher temperatures will favor for the lead(II) sequestration. In general, the adsorption process will accompany the heat release/obtained inevitably. Specifically, as for the physical adsorption, the adsorption process always exhibits the exothermic reaction with the weak π−π or H-bond interaction and the lower temperatures will favor pollutant adsorption. On the contrary, in this work, the strong chemical adsorption will endow an endothermic process and the high adsorption capacity in higher temperature suggests the strong affinity between toxic Pb and PDA-Ms. Similar phenomenon was also observed in heavy metal uptake by metal oxide with strong inner sphere complexation.48 In addition, the lead(II) sorption results are described by the classic Langmuir and Freundlich models with the equations below: Langmuir model

further evaluated with the results of Figure S1e. It can be seen that the smaller size of 200 nm can reveal better lead(II) adsorption. But as for the 200 nm PDA-Ms in this study, it is very difficult to separate from liquid in application, due to its small size and suspension property. Therefore, considering the application or industrial view, the 400 nm samples with good solid−liquid separation properties are selected. Frankly, 400 nm exhibits a similar adsorption performance with the following 550 and 900 nm. Furthermore, the XRD pattern suggests the resultant PDAMs are amorphous phase without distinct diffraction peaks, which coincides with the previous study45 (Figure S2). SEMEDS investigation further proves the C, N, and O components of PDA-Ms (Figure 1e). The present functional groups of the PDA microspheres are examined by FT-IR measurement (Figure 2). The adsorption band at ∼3381 cm−1 is assigned

Qe =

Q max KLCe 1 + KLCe

(1)

Freundlich model lg Q e = lg KF +

Figure 2. FT-IR investigation of the prepared PDA microspheres.

1 lg Ce n

(2)

Where Ce represents the concentration of lead(II) in equilibrium and Qe is the corresponding adsorption capacity, Qmax is assigned to the maximum lead uptake in calculation, KL (L/g) is a binding constant, KF represents the Freundlich coefficient, n is a constant, and the detailed parameters of fitting are illustrated in Table 1. Evidently, the high correlation coefficient (R2> 0.98) suggests the lead(II) sorption behaviors can be well described by the Langmuir sorption model with a maximum sorption capacity of 165.8 mg/g, which can be roughly compared with the lead(II) adsorbents in the literature (Table 2) . Sorption kinetics was also conducted to evaluate the sorption rate for lead(II) removal with the results of Figure 3d. Amazingly, an exceedingly rapid lead(II) adsorption was achieved in approximately 1 min for approaching equilibrium. Such satisfactory kinetic behavior suggests the great possibility for actual application. It is mainly ascribed to the unique feature of PDA-Ms: (1) the highly negative charge surrounding (>40 mV) PDA spheres can accelerate the target lead(II) diffusion and enrichment, prior to sequestration, which is similar to the so-called Donnan membrane effect;49 (2) the ultrafine uniformed microsphere features with abundance of catechol and amine groups can provide large specific areas and numerous of active sites. It further improves the sorption capability and kinetic rates. It is noteworthy that it is difficult to describe the extremely rapid sorption behavior by the classic kinetic models. A similar phenomenon was also observed in lead(II) uptake by ZrP nanoparticles.50 Competing Sorption of Common Anions. Cases are known, the representative cations, including Ca(II), Mg(II), and Na(I), are always coexisting in toxic metal contaminated water/wastewaters with high concentrations. Therefore, the

to the stretching vibration of catechol O−H and N−H,46 and the peak located at ∼1616 cm−1 is attributed to the stretching vibration of aromatic ring and bending ring vibration of N− H;38 the adsorption peaks at ∼1292 and ∼1390 cm−1 can be ascribed to the C−OH stretching and bending vibration respectively,40,47 while the peak at ∼1516 cm−1 is assigned to the shearing vibration of the amide group. These characteristic peaks suggest that PDA microspheres have been successfully prepared by the oxidation polymerization of dopamine. Solution pH Effects and Sorption Isotherm. The influence of solution pH on toxic lead(II) removal was evaluated in Figure 3a. It can be observed that lead(II) uptake onto PDA-Ms is strongly associated with the solution pH surroundings with optimal pH = 4.0−6.5. Specifically, the negligible adsorption can be attached under solution pH < ∼3.2 and outstanding lead(II) sequestration improving is realized at pH > ∼3.5 with the maximum sorption condition of pH = 6.5. Note that further increase solution pH will inevitably lead to the precipitation and lead species variations (Figure S3). That is, at pH > 6, the bivalent Pb2+ will be gradually transformed to the Pb(OH)+ species; thus, the high adsorption at neutral surroundings is ascribed to the multicomponent uptake of Pb2+ and Pb(OH)+. Additionally, such pH-dependent adsorption can be well interpreted by the surface charges of PDA-Ms (Figure 3b). At low pH conditions (pH < ∼3.2.), the positively charged PDA-Ms species will generate a strong rejection toward target lead(II) by the protonated NH3+ or OH2+ formation; thus the slight adsorption (near to zero) is attached. Relatively, when solution pH > ∼3.2, the negatively charged property of PDA-Ms will exhibit a preferable adsorption toward lead(II) removal. Moreover, the present active amine or catechol 4164

DOI: 10.1021/acssuschemeng.7b00129 ACS Sustainable Chem. Eng. 2017, 5, 4161−4170

Research Article

ACS Sustainable Chemistry & Engineering

Figure 3. (a) Solution pH effects on lead(II) adsorption (conditions: dose 0.5 g/L sorbent, initial lead(II) concentration is 0.25 mM at 298 K). (b) Zeta potential analysis at different solution pH onto PDA-Ms. (c) Lead(II) sorption isotherms at 293, 313, and 333 K, respectively (condition: dose 0.5 g/L sorbent, pH = 5.8−6.5 at 298 K). (d) Sorption kinetic curves of PDA-Ms for lead(II) sequestration (condition: dose 0.1 g/L sorbent, initial lead(II) content is 40 mg/L at 298 K with solution pH = 6.2−6.6) .

sorption preferential will be an important index to verify the applicability of the given adsorbent. The commercial cation exchange resin 001x7 was also involved for a reference. As shown in Figure 4a−c, the bivalent Ca(II)/Mg(II) except for Na(I) addition can bring about a dramatic lead(II) sorption decrease onto 001x7, and the final lead(II)-removing is near to zero, suggesting the present nonspecific affinity and lack of sorption selectivity. Considering the chemical components of 001x7, it is believed that the binding −SO3H groups within the matrix will exert a broad-spectrum adsorption toward various cationic ions by weak electrostatic attraction; thus, the added high concentrations of Ca/Mg will significantly occupy the available sorption sites and inhibit the target lead(II) sequestration greatly.60 Comparatively, the resultant PDA-Ms adsorbents exhibit outstanding lead(II) sorption enhancement, regardless of Ca(II)/ Mg(II) or Na(I) additions with a sorption efficiency of approximately 50% above. Attractively, the high level of competing ions can generate an observed lead(II) uptake improving, and similar phenomenon were also observed in metal determination by PEI and metal oxide materials, suggesting the possible presences of strong ligand complexation.61,62 Such higher ionic strength with favorable adsorption possibly could be attributed to the change in conformation of chains onto polydopamine. Specifically, the strong Ca(II) or Mg(II) ionic strength addition might also bring about the possible transformation of conformation of PDA from a tightly collapsed form to loosely extended state, thus more active sites are released to elevate the target Pb(II) adsorption. As expected, the PDA-Ms are rich in catechol or amine groups,

Table 1. Parameters of Langmuir and Freundlich Models for the Adsorption of Lead(II) onto PDA-Ms at Different Temperatures Langmuir model

Freundlich model

temperature (K)

Qmax (mg/g)

KL (L/mg)

R2

KF (mg/g)

1/n

R2

293 313 333

165.8 125.0 86.8

0.0483 0.0223 0.0114

0.994 0.983 0.996

13.6 10.9 21.8

0.281 0.425 0.356

0.864 0.974 0.985

Table 2. Comparison of Lead Ion Sorption Capacities in the Literature adsorbent hydroxyapatite/chitosan composite sugar cane bagasse/ multiwalled carbon nanotube composite silica modified calcium magnesium silicate hollow spheres g-Fe2O3@MgSNTs flowerlike zinc silicates functionalized mesoporous silica flowerlike Cu3(PO4)2 MoS2-carbon dotsdots (CDs) PDA microsphere

Qmax (mg/g)

kinetics (min)

temperature

12.04

∼30 min

298 K

51

58.6

∼120 min

310 K

52

18.9 65

∼120 min ∼15 min

298 K 298 K

53 54

430 80 184

∼60 min NA ∼55 min

298 K 298 K NA

55 56 57

225 588.2

∼30 min ∼180 min

298 K 298 K

58 59

165.87