Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 35785-35793
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Biogenic Calcium Carbonate with Hierarchical Organic−Inorganic Composite Structure Enhancing the Removal of Pb(II) from Wastewater Xueli Zhou,†,‡ Weizhen Liu,*,†,‡ Jian Zhang,† Can Wu,†,‡ Xinwen Ou,† Chen Tian,†,‡ Zhang Lin,*,†,‡ and Zhi Dang†,‡ †
School of Environment and Energy, The Key Laboratory of Pollution Control and Ecosystem Restoration in Industry Clusters (Ministry of Education), South China University of Technology, Guangzhou, Guangdong 510006, China ‡ Guangdong Engineering and Technology Research Center for Environmental Nanomaterials, South China University of Technology, Guangzhou, Guangdong 510006, China S Supporting Information *
ABSTRACT: Calcium carbonate from geological sources (geo-CaCO3, e.g., calcite, aragonite) is used extensively in removing heavy metals from wastewater through replacement reaction. However, geo-CaCO3 has an intrinsically compact crystalline structure that results in low efficiency in pollutant removal and thus its use may produce enormous sludge. In this work, biogenic calcium carbonate (bio-CaCO3) derived from oyster shells was used to remove Pb(II) from wastewater and found to significantly outperform geo-CaCO3 (calcite). The thermodynamics study revealed that the maximum adsorption capacity of bio-CaCO3 for Pb(II) was three times that of geoCaCO3, reaching up to 1667 mg/g. The kinetics study disclosed that the dissolution kinetics and the rate of intraparticle diffusion of bio-CaCO3 were faster than those of geo-CaCO3. Extensive mechanism research through X-ray powder diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption/desorption test and mercury intrusion porosimetry showed that the hierarchical porous organic−inorganic hybrid structure of bio-CaCO3 expedited the dissolution of CaCO3 to provide abundant CO32− active sites and facilitated the permeation and diffusion of Pb(II) into the bulk solid phases. In addition, Fourier transform infrared spectroscopy (FTIR) study, X-ray photoelectron spectroscopy (XPS) analysis, and the examination of Pb(II) removal ability of bio-CaCO3 after calcination indicated that the organic functional groups of bio-CaCO3 also facilitated the immobilization of Pb(II) into CaCO3 particles, although the major contribution was from the hierarchical porous structure of bio-CaCO3. KEYWORDS: CaCO3, lead, porous structure, microstructure, functional groups, wastewater treatment
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INTRODUCTION Calcium carbonate (CaCO3) is a widely occurring mineral. It is typically obtained from geological and biogenic sources1 and is used extensively in water treatment and environmental remediation because it is environmentally friendly and inexpensive.2−4 Contemporary industries mainly use CaCO3 obtained from geological sources, including marble, limestone, calcite, aragonite, etc. However, geological calcium carbonate (geo-CaCO3) normally has a dense bulk structure and low specific surface area, referring to an inferior ability for water treatment. As a result, using geo-CaCO3 to remove heavy metals from wastewater may hardly satisfy the water discharge standards while avoiding enormous output sludge is compulsory.5 In contrast, because of its unique hierarchical porous microstructure and organic functional groups, bio-CaCO3 is expected to outperform geo-CaCO3 in heavy metal removal. This is appealing in particular since seashells, oyster shells, and other materials containing bio-CaCO3 are usually discarded on © 2017 American Chemical Society
the coast, which may litter the environment and create foul odor.6 Making use of bio-CaCO3 to substitute geo-CaCO3 appears to be an excellent approach to remove heavy metals in the contaminated water. It has been reported that the bio-CaCO3 derived from marine shells, eggshells, os sepia, etc., shows high adsorption capacity toward Pb, Zn, Cd, Cu, and Ni.7−11 However, the efficiency of bio-CaCO3 and geo-CaCO3 in heavy metal removal has been rarely differentiated. It was reported that bio-CaCO3 outperformed geo-CaCO3 in immobilizing heavy metals, but the mechanism was mainly attributed to the organic functional groups,12 which is not very convincing since the content of the organic matrix in bio-CaCO3 is only about 3%− 5% (wt %). Instead, it is more likely that the higher adsorption Received: June 28, 2017 Accepted: September 26, 2017 Published: September 26, 2017 35785
DOI: 10.1021/acsami.7b09304 ACS Appl. Mater. Interfaces 2017, 9, 35785−35793
Research Article
ACS Applied Materials & Interfaces
Preparation of Pb(II) Solution. All the chemicals used in the experiments were analytical reagents purchased from Aladdin (China). The Pb(II) solution was prepared by dissolving a certain amount of Pb(NO3)2 in deionized water. Adsorption Kinetic Experiments. The adsorption equilibrium time was measured by adding bio-CaCO3 or geo-CaCO3 (0.1 g/L) into the Pb(II) solution (200 mg/L, 300 mL) in a 500 mL Erlenmeyer flask. The flask was sealed with a stopper and shaken at 200 rpm in an incubator maintained at 25 °C. Aliquots (∼5 mL) were taken at 5 min, 10 min, 15 min, 30 min, 60 min, 2 h, 4 h, 8 h, 12 h, 24 h, and 48 h. The aliquots were passed through a 0.22 μm filter, and the concentrations of Pb(II) and Ca(II) were measured by inductively coupled plasma optical emission spectrometry (ICP-OES). After 48 h of agitation, the solution remaining in the flask was centrifuged at 5000 rpm to collect the precipitates. Finally, the collected solids were dried in a vacuum oven at 90 °C for 12 h and used for further studies. The adsorption capacity (q) for Pb(II) at different time was calculated according to eq 1
efficiency of bio-CaCO3 is not only related to the organic functional groups but also associated with the microstructure of bio-CaCO3 itself. Lead (Pb) is one of the most hazardous heavy metals.13 Traditional wastewater treatment with geo-CaCO3 can reduce residual Pb(II) down to 1 mg/L, but it still has problems in meeting new standards that require the discharge to contain no more than 0.5 mg/L Pb(II).14 As a result, it is worth studying whether bio-CaCO3 can be used to substitute geo-CaCO3 for wastewater treatment, especially in improving Pb(II) removal efficiency and in decreasing the amount of sludge. Sequestration of Pb(II) by geo-CaCO3 is a well-studied process.4,15−17 The immobilization of Pb(II) is generally considered to proceed through layer-by-layer epitaxial crystal growth along the solid−liquid interface. However, Yuan18 used an innovative full-field transmission X-ray microscopy (TXM) to examine the process and showed that dissolution-permeable recrystallization was responsible for the incorporation of Pb(II) by geo-CaCO3. They argued that the formation of nanopores played a critical role in the penetration and diffusion of Pb(II) into CaCO3 particles. In addition, Ma19 prepared CaCO3 from calcium chloride and ammonium with organic maltose as an additive. The synthetic hierarchical vaterite CaCO3 was found to have a high adsorption capacity for Pb(II) up to 3242 mg/g. In view of these results, the microstructure of CaCO3 may have a major influence on the performance of heavy metal removal. However, the underlying mechanism of Pb(II) immobilization by bio-CaCO3 has been rarely reported. In fact, most biogenic shells are organic−inorganic composite materials, with polygonal lamellar stacks of CaCO3 crystals being separated by layers of organic polymer matrix.20,21 Hence, bio-CaCO3 is expected to have a similar hierarchical structure to the abovementioned synthetic CaCO3 and probably also to possess superior ability for Pb(II) removal. In this work, we explored the removal efficiency of Pb(II) with bio-CaCO3 and geo-CaCO3 of similar grain size. We quantified the rate of Pb(II) removal and the adsorption capacity of the two materials through a series of batch experiments and kinetics study, and systematically investigated the underlying mechanism. Through XRD, SEM, energy dispersive spectroscopy (EDS), N2 adsorption/desorption test, and mercury intrusion porosimetry, we studied how the microstructure of bio-CaCO3 influenced its interfacial interaction process with Pb(II). Furthermore, we analyzed in bioCaCO3 the complexation between the organic functional groups and Pb(II) with FTIR and XPS. This study provides a deep investigation on the mechanism of Pb(II) removal by bioCaCO3 and thus lays an important theoretical basis for the utilization of bio-CaCO3 in the treatment of wastewater contaminated with heavy metals.
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q=
C0 − Ce V m
(1)
where q (mg/g) is the adsorption capacity of the adsorbents at time t, m is the weight of the adsorbents (g), V is the volume of solution (L), and C0 and Ce (mg/L) are the initial and equilibrium concentrations of the adsorbates in the solution, respectively. The data were fitted by using the pseudo-first-order eq 2, the pseudo-second-order eq 3, and the intraparticle diffusion eq 4
log(qe − qt ) = log qe −
k1t 2.303
(2)
t 1 t = + 2 qt q k 2qe e
(3)
qt = kdi t + Ci
(4)
where qe (mg/g) is the adsorption capacity at equilibrium, qt (mg/g) is the adsorption capacity at time t, and k1 (min−1) and k2 (g/mg·min) are the rate constants of pseudo-first-order and pseudo-second-order kinetics, respectively. The rate constants k1 and k2 were determined by plotting log(qe − qt) versus t and t/qt versus t, respectively. The rate constant of intraparticle diffusion kdi (mg/g h1/2) was calculated by plotting qt versus t1/2 to find the slope, and the intercept (Ci) was used to evaluate the boundary layer thickness. Adsorption Isotherm Models. All the experiments were performed in triplicate. The relationship between the concentration of Pb(II) in the solution and the amount of Pb(II) adsorbed on the solid phase at equilibrium was measured as follows. Bio-CaCO3 or geo-CaCO3 of 0.1 g/L was added to 60 mL Pb(II) solution of varying concentrations (100, 200, 300, 400, and 500 mg/L). Then, the mixture was stirred mechanically at 200 rpm for 48 h. After the above process, 5 mL of solution was taken and passed through a 0.22 μm filter, and the concentration of Pb(II) was measured by ICP-OES. The adsorption data were fitted to the Langmuir equation and the Freundlich equation. The Langmuir adsorption model has the form
qe =
MATERIALS AND METHODS
Preparation of Calcium Carbonate. Waste oyster shells were collected from a seafood market in Guangzhou, China. The shells were brushed, washed with deionized water, and then dried at 80 °C in a vacuum oven for 24 h. The dried shells were pulverized in agate mortar to fine granules and sifted through nylon sieves (200 mesh) to give the bio-CaCO3 ready for use. Moreover, some of the dried shells were calcined in a programmable furnace at 600 °C for 2 h to remove organic matter. The calcined shells were pulverized, and sifted in a similar way as bio-CaCO3 and then stored until use. Ground calcite was purchased from Guangzhou Yifeng Mining Technologies Co., Ltd. It was cleaned, dried, pulverized, and sifted in the similar way as bio-CaCO3 to give geo-CaCO3.
kLqmCe 1 + kLCe
(5)
where Ce (mg/L) and qe (mg/g) are the solute concentration and adsorption capacity at equilibrium, respectively, and qm (mg/g) and kL (L/mg) are the maximum monolayer adsorption capacity and the binding energy of adsorption, respectively. The Freundlich adsorption model has the form qe = k f Ce1/ n
(6)
where kf and n are the Freundlich constants measuring the adsorption capacity and the adsorption intensity, respectively. Characterization. XRD Analysis. XRD patterns were collected on a Bruker X-ray powder diffractometer (advance D8) with Cu−Kα 35786
DOI: 10.1021/acsami.7b09304 ACS Appl. Mater. Interfaces 2017, 9, 35785−35793
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) XRD patterns of bio-CaCO3 and geo-CaCO3. (b) SEM image of the top view of bio-CaCO3. (c) SEM image of the side view of bioCaCO3 viewed from the direction of the white arrow in panel b. (d) SEM image of geo-CaCO3.
Figure 2. (a) Residual Pb(II) concentration in aqueous solution after treatment by bio-CaCO3 and geo-CaCO3 with different dosages. Inset is the enlarged curve in the range of 1.0 and 1.2 mg/L. (b, c) Langmuir and Freundlich isotherms for Pb(II) adsorption by bio-CaCO3 and geo-CaCO3. (d) Adsorption kinetics of bio-CaCO3 and geo-CaCO3 for Pb(II). (e) Pseudo-second-order model fitting for Pb(II) adsorption. (f) Intraparticle diffusion model fitting for Pb(II) adsorption. In the vertical axis of panels b−f, q denotes adsorption capacity. XPS Analysis. The XPS spectra were recorded using an Axis Ultra DLD instrument (Kratos Analytical, U.K.) with an A1 Kα X-ray source, at a pass energy of 160 eV for survey scans and 40 eV for highresolution scans.
radiation. The tube voltage was 40 kV and the tube current was 40 mA. Diffraction patterns were collected over 2θ = 10°−80° at 1°/min. The step size of the scan was 0.02°. Porous Structure Analysis. The N2 adsorption−desorption isotherms of the samples were recorded at −196 °C with a Micromeritics ASAP 2020 after degassing in vacuum at 200 °C for 4 h. The Brunauer−Emmett−Teller (BET) surface areas were calculated according to a multipoint BET method using adsorption data in the relative pressure (P/P0) range of 0.05−0.25. Moreover, the pore size distributions were determined from desorption branch of nitrogen isotherms by the Barret−Joyner−Halenda (BJH) method. The macropores of all the samples were investigated by mercury intrusion porosimetry using a Micrometrics Autopore IV 9510. FTIR Analysis. The FTIR spectra of the samples were recorded on an RX1 PerkinElmer FTIR spectrometer using KBr as a diluter. SEM and EDS Analysis. The morphology of the samples were observed using a JSM-7100F SEM with an Oxford INCA EDS.
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RESULTS AND DISCUSSION Characterization of bio-CaCO3 and geo-CaCO3. Figure 1 shows the phase composition and microstructure of bioCaCO3 and geo-CaCO3. The XRD patterns of both bio-CaCO3 and geo-CaCO3 (Figure 1a) can be well indexed as the calcite phase (COD9007689), which is the most stable phase of CaCO3.22 Compared with geo-CaCO3, bio-CaCO3 had weaker peak intensity, which was indicative of lower crystallinity. Moreover, its local crystallite size was also smaller, as was indicated by the full width at half-maximum (fwhm) of diffraction peak from the (104) reflection of bio-CaCO3 and 35787
DOI: 10.1021/acsami.7b09304 ACS Appl. Mater. Interfaces 2017, 9, 35785−35793
Research Article
ACS Applied Materials & Interfaces
mg for bio-CaCO3 and geo-CaCO3, respectively. Hence, compared with geo-CaCO3, bio-CaCO3 had much higher adsorption affinity for Pb(II). Adsorption Kinetics. Figure 2d shows that with an initial Pb(II) concentration of 200 mg/L, the adsorption capacity of both bio-CaCO3 and geo-CaCO3 increased with increasing time and reached equilibrium within 12 h. The adsorption kinetics data were fitted to both pseudo-first-order model (Figure S3) and pseudo-second-order model (Figure 2e), and the related model parameters are listed in Table 2. Figure 2e shows that the adsorption kinetics of the two adsorbents conformed to the pseudo-second-order model, which was indicative of chemisorption.10 To further explore the adsorption mechanism, the adsorption kinetics data were fitted to the intraparticle diffusion kinetic model. Figure 2f presents a linear fit of the intraparticle diffusion model for Pb(II) adsorption by bio-CaCO3 and geoCaCO3, and the corresponding model parameters are listed in Table 3. According to the fitting results, three distinct steps took place in the adsorption process. The first step was the stage of external surface adsorption, during which high initial Pb(II) concentration served as the driving force of diffusion. As a result, the instantaneous adsorption of Pb(II) was completed at a rapid rate within 30 min. In comparison, the adsorption in the second step was much more gradual. It could be speculated that the precipitation transformation between CaCO3 and Pb(II) took place during this stage. This step was the rate limiting step for both bio-CaCO3 and geo-CaCO3. Note that in Figure 2f, the curve slope remained relatively steep for bioCaCO3 in this step, indicating that bio-CaCO3 experienced much smaller resistance in intrapore diffusion than geo-CaCO3 because of its interior hierarchical porous structure. The final step was the equilibrium stage. At this point, the adsorption slowed down because the concentration of residual Pb(II) had become low. The adsorption capacity of bio-CaCO3 and geoCaCO3 reached 1605.45 and 520.93 mg/g, respectively, as the adsorption process gradually approached equilibrium. Mechanism of Pb(II) Removal by bio-CaCO3 and geoCaCO3. Contribution of the Microstructures. Figure 3a shows that the amount of Ca(II) released from solids was negatively correlated to the amount of Pb(II) in the solution. Therefore, the dissolution of CaCO3 particles must be a key factor in Pb(II) removal. It can be clearly seen that bio-CaCO3 released more Ca(II) than geo-CaCO3 and was thus more efficient in removing Pb(II) from the solution. However, when the adsorption of Pb(II) reached equilibrium, the molar ratio of ΔPb(II)/ΔCa(II) was 0.931 and 0.606 for bio-CaCO3 and geoCaCO3, respectively. That is to say, compared with bio-CaCO3, geo-CaCO3 caused less Pb(II) adsorbed for each unit amount of Ca(II) released. Stoichiometry dictates that each unit amount of Ca(II) released in the solution should correspond to one unit amount of CO32− active sites in the solid which consequently combines with one unit amount of Pb(II). However, for geo-CaCO3, less Pb(II) combined for each unit amount of CO32−, indicating that the freed CO32− active sites
geo-CaCO3 (Table S1). The XRD analysis was further corroborated by the SEM observations. Specifically, bioCaCO3 had an ordered multilayer structure of crystalline CaCO3 platelets of approximately 30−50 μm in length with tiny, fragmented rough surfaces (Figure 1b). And from the side view of the sample, bio-CaCO3 mainly exhibited hierarchical and open network-like structures with slit-like pores (Figure 1c), whereas geo-CaCO3 had a typical dense structure of calcite with smooth surfaces (Figure 1d). In addition, N2 adsorption/ desorption test combined with mercury intrusion porosimetry was carried out to study the pore structure of bio-CaCO3 and geo-CaCO3 (Figures S1 and S2). It was found that bio-CaCO3 had a wide pore size distribution ranging from a few nanometers to many micrometers while geo-CaCO3 had few pores, being consistent with the SEM observations (Figure 1). The specific surface areas of bio-CaCO3 and geo-CaCO3 were calculated to be 7.358 and 0.692 m2/g, respectively (Table S2). The above results collectively demonstrated that bio-CaCO3 and geo-CaCO3 had the same primary constituent but were different in morphology and microstructure. Pb(II) Removal by bio-CaCO3 and geo-CaCO3. Residual Pb(II) Concentration after Treatment. Figure 2a shows that with an initial Pb(II) concentration of 500 mg/L, both bioCaCO3 and geo-CaCO3 could decrease the residual Pb(II) concentration in the solution with increasing adsorbent dosage from 0.2 to 1.2 g/L, while bio-CaCO3 exhibited much better performance. Specifically, with the adsorbent dosage of 0.4 g/L, the residual Pb(II) concentration was still 297.22 mg/L for geoCaCO3 but dropped to 0.134 mg/L when bio-CaCO3 was used. Clearly, bio-CaCO3 was much superior than geo-CaCO3 in removing Pb(II) from wastewater to meet the discharge standard (
