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Abundance, chemical composition and lead adsorption properties of sedimentary colloids in a eutrophic shallow lake
Chemosphere 218 (2019) 534e539
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Abundance, chemical composition and lead adsorption properties of sedimentary colloids in a eutrophic shallow lake Huacheng Xu a, *, 1, Li Ji b, 1, Ming Kong c, Mengwen Xu a, d, Xizhi Lv e, ** a
State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China b College of Harbour, Coastal and Offshore Engineering, Houhai University, Nanjing 210098, China c Nanjing Institute of Environmental Sciences, Ministry of Environmental Protection, Nanjing 210042, China d College of Biology and the Environment, Nanjing Forestry University, Nanjing 210037, China e Yellow River Institute of Hydraulic Research, Key Laboratory of the Loess Plateau Soil Erosion and Water Loss Process and Control of Ministry of Water Resources, Zhengzhou 450003, China
h i g h l i g h t s Abundance, composition and adsorption properties of sedimentary colloids were studied. The ultrasonic-induced extraction efficiency enhancement was related to colloidal composition and sediment type. Sedimentary colloids had Fe-, Al- and Si-containing mineralogical structures with various conformations. Total colloids exhibited higher adsorption capacity for Pb(II) than the inorganic colloids. Langmuir isotherm, pseudo-second-order kinetics, and chemical mechanism accounted for the adsorption process.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2018 Received in revised form 31 October 2018 Accepted 22 November 2018 Available online 23 November 2018
Colloidal particles are omnipresent in lake sediments and substantially influence the retention, transportation, and fate of contaminants in lake ecosystems. In this study, the abundance, chemical composition and adsorption behavior of sedimentary colloids (including total and inorganic colloids) from different ecological regions, were for the first time investigated via ultrasonic extraction, spectral analysis and batch absorption experiments. Results showed that the extraction efficiencies of sedimentary colloids showed an ultrasonic energy-dependent enhancement, and the algae-dominated area contained comparable colloidal abundance with the macrophyte-dominated area (i.e., 198.5 vs. 183.3 mg/g). Despite the different ecosystems, these sedimentary colloids usually had a wide size distribution of 30e200 nm, and were characterized with montmorillonite-, kaolin-, volkonskoite-, and quartz-rich chemical compositions. Batch experiment showed that the total pristine colloids exhibited higher adsorption capacity for Pb(II) than the inorganic colloids both for the macrophyte- and algae-dominated sediments, and the adsorption process followed pseudo-second-order kinetics and Langmuir isotherm, irrespective of different colloidal types. Thus, sedimentary colloids can immobilize the heavy metals in sediment and decrease their release into the water column, which can be considered as a sink for contaminants. This study highlighted the significance of sedimentary colloids in determining the physicochemical properties of lake sediments and in evaluating the environmental behavior and fate of contaminants in lake ecosystems. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Eutrophic shallow lakes Lake sediments Colloids Extraction Adsorption properties
1. Introduction
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Xu), [email protected] (X. Lv). 1 Huacheng Xu and Li Ji contributed equally to this work. https://doi.org/10.1016/j.chemosphere.2018.11.147 0045-6535/© 2018 Elsevier Ltd. All rights reserved.
Colloidal particles, with the size ranging from 1 nm to 1 mm, are omnipresent in natural environment, including sediments, soils, and waters, etc. (Kaegi et al., 2008; Tang et al., 2009; Yu et al., 2017). Presence of colloidal particles definitely poses important impacts
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on environmental and ecological systems. For instance, due to the large specific surface areas, the colloidal particles usually exhibited high adsorption capacities for the contaminants and nutrients, which can significantly affect their behavior, fate, and bioavailability (Gibson et al., 2007). Moreover, large amounts of colloidal particles can also induce obvious toxicity to microorganism metabolism (Hou et al., 2018) and ecosystem balance (Colman et al., 2014). Due to the important roles in determining the environmental behavior and bio-availability of contaminants as well as ecosystem health, environmental colloidal particles are receiving more and more attention in recent years. Lake sediments can be considered as the sources or sinks of nutrients and contaminants, which exhibit contrasting influences on the quantity of overlying water (Xu et al., 2013; Ding et al., 2015). Colloidal particles in lake sediment consisted of various mineralogical/chemical compositions, including silicates, carbonates, metals (Fe, Al, Mn, Si) and the corresponding hydroxides, etc (Chanudet and Filella, 2009). The sedimentary colloids, especially for those in the shallow lakes, are easily re-suspended under the windy conditions, which contribute highly the turbidity of the surface waters (Xing and Kong, 2007). Previous studies showed that the process of sediment re-suspension in eutrophic shallow lake under high environmental wind speed (e.g., 8 m/s) can contribute up about 55% of total colloidal abundance (mass based) in the water column (Xu et al., 2018a). Owing to the small-size properties, these aquatic colloids are prone to stabilize in surface waters for a long time and thus significantly impair the transparency of the water column (Zheng et al., 2015). In addition, some contaminants (e.g., heavy metals) adsorbed on the surface of sedimentary colloids can also change their bio-availability and toxicity to the ecological systems. Despite the important roles in influencing the contaminant transportation and water quality, detailed information on the physicochemical properties of sedimentary colloids and their adsorption properties for heavy metals is still lacking. The objectives of this study were thus to 1): determine the abundance of sedimentary colloids, including total and inorganic colloids, from different ecological regions as a function of ultrasonic energies; 2) characterize the shapes, sizes and mineralogical compositions of these sedimentary colloids; 3) investigate the adsorption properties of the sedimentary colloids at environmentally relevant conditions. To accomplish these goals, two lake sediments, namely macrophyte- and algae-dominated sediments from Lake Taihu, were sampled for the extraction of sedimentary colloids. The spectral and microscopic techniques, including high resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) were applied to characterize the colloids extracted. The representative heavy metals, Pb(II), was applied, due to its relatively high concentration in polluted ecosystems and potential toxicity to microorganisms, and the adsorption properties and mechanism were explored based on the batch experiments and spectral techniques. Results obtained here would be beneficial for a better insight into the physicochemical properties of sedimentary colloids and their contributions in contaminant transportation. 2. Materials and methods 2.1. Collection of lake sediment samples Lake Taihu, with a surface water area of 2338 km2, is one of the largest freshwater lakes in China. It is also a eutrophic and shallow lake, with the maximum and mean depths of 3.4 m and 1.9 m, respectively (Qin et al., 2007). Moreover, two different ecological areas, namely algae- and macrophyte-dominated areas, coexisted within the lake Taihu. The macrophyte-dominated area exhibits
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plenty of submersed vegetation, immobilized sediments, and clear water. In comparison, the algae-dominated area is characterized by frequent algal blooms, sedimentary re-suspension, and turbid water (Qin et al., 2007). Surface sediments samples, which were located within 0e10 cm below the sediment-water interface, were sampled from the macrophyte-dominated area (i.e., East Taihu Bay) and algaedominated area (i.e., Meiliang Bay), respectively (Xu et al., 2013). The samples were collected in September, 2017. After collection, the sedimentary samples were transported to laboratory on ice as soon as possible, followed by freeze-dried. The dried samples, with the organic matter contents of about 2e5%, were then ground and sieved through a 2 mm mesh to remove the large-size particles. 2.2. Extraction of sedimentary colloidal particles The sedimentary colloids were extracted by an Ultrasonic Processor (XO-1000D, Nanjing, China) with a 40% amplitude of stable power output. The probe tip, with a diameter of 13 mm, was inserted into 2 cm below the suspension surface, which were obtained by mixing 15 g sediments with 100 ml carbonate sodium solutions (0.025 M). The carbonate sodium solutions were applied herein to avoid the aggregation of colloidal particles (Li et al., 2012). To avoid the temperature effects, the suspensions were placed in a cooler during the whole ultrasonic treatment. To explore the ultrasonic-dependent extraction efficiencies, eight levels of ultrasonic energy, i.e., 12, 24, 48, 96, 192, 288, and 480 KJ were used to extract the sedimentary colloids. After extraction, the solutions suffering with different ultrasonic energies were settled in a cylinder. The colloidal particles were separated from the whole solutions by siphon based on the different sedimentation velocities (i.e., <0.3 cm/h) derived from the Stokes, law. An amicon stirred ultrafiltration cell coupled with a 1 kDa ultrafilter membrane was applied to concentrate the colloidal particles, with a concentration factor of about 25 (Xu and Guo, 2017; Xu et al., 2018a). After ultrafiltration, the retentate was carefully collected, which can be denoted as the total colloidal particles (TCPs). Afterwards, 30% of hydrogen peroxide was used to an aliquot of the retentate to remove the organic layers that adsorbed on the colloidal surfaces to obtain the inorganic colloidal particles (ICPs) (Dumat et al., 1997; Xu et al., 2018a). After centrifugation at 15000g for 10 min, the obtained TCPs and ICPs were both freeze-dried and stored in the desiccator for the following chemical composition and adsorption analysis. 2.3. Adsorption experiments The sorption processes were all performed in duplicate at room temperature. For the studies of equilibrium sorption isotherms, 50 ml solutions containing different Pb(II) concentrations [Pb(NO3)2, 2e75 mg L1] were added to 100 ml vials which contained 40 mg TCPs or ICPs. The vials were shaken in an incubator shaker (ZWY2102C, ZHCHENG, Shanghai) at 180 rpm for 24 h. The pH was determined by a pH meter (PB-10, Sartorius, Germany), with the initial pH of 6.4 ± 0.1. During the adsorption process, the pH values of solutions were adjusted at 6.0 at intervals of 8 h via the addition of NaOH or HCl solution. For the kinetic experiments, 40 mg of colloidal samples (e.g., TCPs or ICPs) were mixed with 50 ml of Pb(II) solutions (40 mg L1, pH 6.0), with the adsorption capacities determining at the interval times (10 min, 20 min, 30 min, 1 h, 2 h, 4 h, 6 h, 12 h, and 24 h, respectively). For the studies of environmental factor-dependent adsorption heterogeneities, various pHs (4e7) and ionic strengths (0.001e0.1 M) were applied, with an initial Pb2þ concentration and contact time of 40 mg L1 and 24 h, respectively. After adsorption, the mixed solutions were centrifuged at 15000 g
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for 20 min, followed by microfiltration (0.22 mm), with Pb(II) concentration measuring via the inductively coupled plasma-atomic emission spectrometry (ICP-AES, Prodigy, USA). Difference of Pb(II) between the total and residual concentrations were determined to calculate the adsorption efficiency.
dominated area during the ultrasonic extraction process. The results showed that both the abundances and compositions of sedimentary colloids were highly related to ultrasonic energies.
2.4. Characterization of the colloidal particles and statistic analysis
Fig. 2 shows the TEM micrographs of the extracted TCPs and ICPs. It was shown that the morphologies of sedimentary TCPs did not exhibit differences between the macrophyte- and algaedominated areas that they were both characterized with the mixed shapes of spherical, elliptical and polygonal. As for the colloidal sizes, they were characterized with a broad size range from 30 nm to 200 nm, showing an uneven distribution patterns. Further analysis showed that obvious organic layers can be found on the surfaces of TCPs, which would be attributed to the chemicophysical adsorption of humic substances, biopolymers, and other macromolecules in natural waters (Sch€ afer et al., 2007; Xu et al., 2018a). Compared with the sedimentary TCPs, the ICPs from the two areas exhibited different morphologies. Specifically, the sedimentary ICPs from macrophyte-dominated area were mostly characterized with polygonal and prismatic shapes, whereas those from algae-dominated area were mainly spherical. In addition, the sizes of sedimentary ICPs from the two areas ranged mainly from 20 to 40 nm, which were significantly smaller than those of the TCPs.
The sizes and morphology of sedimentary colloids were measured via the TEM observation (JEM JEOL-200CX). The crystal phases of sedimentary colloids were recorded by X-ray diffraction analysis (Siemens D5000) using Cu-Ka radiation. The operational tube for XRD analysis was characterized with a current and voltage of 30 mA and 40 kV, respectively. The spectra of ATR-FTIR were determined via a Thermo Nicolet iS50 spectrometer at room temperature with the spectral scan of 600e4000 cm1 and a resolution of 1 cm1. As for the statistic analysis, significance of data differences in colloidal abundances of lake sediments between two ecological types was examined via the independent-sample T-test model. 3. Results and discussion 3.1. Abundances of extractable sedimentary colloids as affected by ultrasonic energies Ultrasonic treatment can be used to disperse the lake sediments as well as to extract the small-size colloidal particles, with increasing extraction efficiency at higher energies. Taken the TCPs as example, the extraction efficiencies exhibited an initially rapid increase followed by a gradual enhancement both for the two sedimentary colloids (Fig. 1). With the highest ultrasonic energy, the abundances of sedimentary TCPs from algae- and macrophytedominated areas were 198.5 ± 7.5 and 183.3 ± 8.2 mg/g, respectively. The abundances of colloids measured in lake sediments were comparable with those reported in soils (68e453 mg/g, 12e72 KJ) (Li et al., 2012). In addition, compared with the sedimentary TCPs, the extraction efficiencies of ICPs showed different variation trends between the two lake sediments. Although the extraction efficiencies of sedimentary ICPs both increased sharply during the initial ultrasonic treatment, there existed a slight increase for further treatment for the algae-dominated area but no further enhancement in extraction efficiency can be observed for the macrophyte-dominated area. The final abundances of sedimentary ICPs from the macrophyte-dominated area were thus 62.0 ± 8.2 mg/g, which was significantly lower than those from the algaedominate area (111.2 ± 6.2 mg/g) (p < 0.01). In addition to the extraction efficiency, the percentages of inorganic fraction also showed an ultrasonic energy-related variation. The initial ratios of ICPs to TCPs for the two sediments were comparable (42.7%~44.6%), while it decreased to 33.9% for the macrophyte-dominated area but increased to 55.8% for the algae-
Fig. 1. The extraction efficiencies of sedimentary colloids under various ultrasonic intensities.
3.2. Morphologies and sizes of the sedimentary colloids
3.3. The mineralogical composition of sedimentary colloids XRD can be used to characterize the mineral composition of sedimentary colloids, with the results shown in Fig. 3a. No obvious heterogeneities can be observed in XRD patterns in terms of peak intensities and positions among the four samples, indicating the similar mineral patterns between different sediments and that the treatment of the removal of organic matters (i.e., the usage of hydrogen peroxide) did not influence the mineral compositions of sedimentary colloids. Comparisons in the XRD patterns of sedimentary colloids with those of the standard minerals showed that the main mineral structure for sedimentary colloids were montmorillonite, nontronite, volkonskoite, and quartz. These results indicated that ferric, aluminum, and silicon were the main element composition within the sedimentary colloids in the eutrophic shallow lakes.
Fig. 2. TEM micrograph of the sedimentary TCPs (aeb) and ICPs (ced) from macrophyte- and algae-dominated areas, respectively.
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Fig. 3. XRD (a) and FTIR (b) patterns of sedimentary TCPs and ICPs from different ecological areas.
To further explore and characterize the chemical compositions of the sedimentary colloids, the infrared techniques were applied herein, with the ATR-FTIR spectra shown in Fig. 3b. Some obvious absorption peaks, e.g., 3700, 3620, 911, 798 and 754 cm1, can be observed for the two kinds of sedimentary colloids. We assigned these peaks as follows: peaks 3700 and 3620 cm1 were attributed to the internal OH groups and were characteristic of kaolin material; peak 911 cm1 can be ascribed to the OH deformation of smectite, and the presence of peaks 798 and 754 cm1 originated from montmorillonite (Li et al., 2012). The result of ATR-FTIR also demonstrated the main mineral structures of kaolin, smectite and montmorillonite for the sedimentary colloids. However, further analysis showed that the sedimentary TCPs from the two areas were characterized with obvious amide I peaks at 1650 cm1 (Xu and Jiang, 2015), indicating the presence of organic layers (e.g., protein-like substances) on the surface of sedimentary TCPs. However, compared with the TCPs, the intensities of peak 1650 cm1 in sedimentary ICPs was much lower, demonstrating the efficient removal of organic matters from the TCPs. 3.4. The adsorption potential of sedimentary colloids for Pb(II) The adsorption potential of colloidal particles can be explored via the application of adsorption isotherms. Two isotherms models, i.e., Freundlich and Langmuir models, were usually applied to fit the experimental data and to characterize the adsorption process (Liang et al., 2011). The Freundlich and Langmuir isotherm models are expressed as follows:
qe ¼
1
=
q ¼ KF Ce
n
q m K a Ce 1 þ Ka Ce
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Fig. 4aeb shows the Freundlich isotherms for Pb(II) adsorption on sedimentary TCPs and ICPs. Large variations among the experiment data were observed both for the two kinds of sedimentary colloids, suggesting that the experimental data did not fit well in this model. In addition, the low R2 value (<0.87) indicated the unsuitability of Freundlich model to predicate the adsorption properties of Pb(II) on sedimentary colloids. The Langmuir equation was thus applied to describe the adsorption behavior. It was shown that the Langmuir model predicted much better than the Freundlich model for the adsorption of Pb(II) on both TCPs and ICPs, with a high R2 value of 0.99 (Fig. 4ced). Further analysis showed that the adsorption capacity (qm) of sedimentary ICPs were 42.0 ± 1.4 mg/g for macrophytedominated area and 44.8 ± 2.4 mg/g for algae-dominated area, while those of sedimentary TCPs were 58.1 ± 1.0 mg/g for macrophyte-dominated area and 65.3 ± 0.6 mg/g for algaedominate area, respectively. The adsorption capacity of sedimentary colloids calculated herein was comparable with those of carbon nanotubes (30e40 mg/g) (Li et al., 2005) and g-Fe2O3 nanoparticles (<90 mg/g) (Brianna et al., 2010) but significantly lower than that of ZnO nanoparticles (556 mg/g) (Xu et al., 2016). In addition, compared with the ICPs, the higher adsorption capacity of TCPs would be ascribed to the organic layers of colloidal surfaces. Previous studies had shown that the organic layers on particle surfaces can interact with metals to form metal-metal complexes via hydrogen binding and/or ligand complexation (Xu et al., 2013), which resultantly enhanced the capacity of Pb(II) adsorption on sedimentary TCPs.
3.5. Adsorption kinetics for Pb(II) Due to the large specific surface areas and high surface activity, it is believable that sedimentary colloids can exhibit a fast adsorption potential for Pb(II) (Xu et al., 2016). Analysis on the adsorption capacities as a function of time showed that the process of Pb(II) adsorption on sedimentary TCPs and ICPs were both composed of a rapid initial adsorption and a slow equilibrium phase. For example, about 90% of total Pb(II) concentrations can be adsorbed in the first 2 h, whereas the following adsorption process contributed only 10% of adsorption capacity. To explore the detailed adsorption kinetics, two kinetic models, namely pseudo-first-order and pseudo-second-order equations,
(1) (2)
where Ce represents the Pb(II) concentration (mg L1) in the solutions, KF is the Freundlich constant. In addition, q and qm are the amount of Pb(II) adsorbed under each condition and the maximum adsorption amount for a monolayer, respectively. During the calculation of adsorption potential, both the two isotherm models were usually rearranged to a liner form:
log q ¼
1 logCe þ logKF n
Ce 1 Ce ¼ þ qe qm Ka qm
(3)
(4)
For the Freundlich model, the constants ‘KF’ and ‘1/n’ can be obtained based on the linear plots of ‘log q’ vs. ‘log Ce’, while for the Langmuir model, the constants ‘qm’ and ‘Ka’ can be calculated via the linear plots of ‘Ce/qe’ vs. ‘Ce’.
Fig. 4. The adsorption isotherms of marcolyte-(a, c) and algae-(b, d) dominated sedimentary colloids for Freundlich and Langmuir model, respectively.
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were applied in this study and expressed as follows:
logðqe qt Þ ¼ logqe
k1 t 2:303
t 1 t ¼ þ qt k2 ðqe Þ2 qe
(5)
(6)
where qt and qe represent the adsorbed amounts of heavy metal Pb(II) at time t and equilibrium, respectively; k1 and k2 are the rate constants of pseudo-first-order and pseudo-second-order adsorption processes, respectively. Studies on the pseudo-first-order rate equation showed a significant deviation between the experimental and the estimated ‘qe’ (Fig. 5), indicating that this mode was not suitable for kinetic analysis. By comparison, application of pseudo-second-order kinetic equation can successfully estimate the adsorption kinetics, with a high R2 of 1.00 both for the two kinds of sedimentary colloids. The results of adsorption kinetics were in agreement with those of some other colloidal and/or nanoparticles, such as nanoparticles ZnO, carbon nanotubes and graphene (Xu et al., 2016, 2018b).
3.6. Effects of environmental factors on Pb(II) adsorption Adsorption of heavy metals was highly related with environmental factors, which can significantly influence on surface properties of adsorbents (Xu et al., 2016). In this study, the efficiencies of Pb(II) adsorption increased with enhancing pHs both for the two kinds of sedimentary colloids (Fig. 6). Take ICPs from macrophytedominated area as example, the adsorption capacity was 12.5 ± 0.5 mg/g at pH 4, while it reached to 45.1 ± 1.8 mg/g when the solution pH increased to 7. Based on the pH-dependent species of lead, Pb2þ was the predominant species when the solution pH was below 7. The coexistence of Hþ and Pb2þ can compete each other for the active sites on colloidal surfaces, which accounted for the low adsorption efficiency under acidic pH conditions. The pHdependent enhancement of adsorption capacity of Pb(II) can also be found for other pure and/or polymer-modified nanoparticles (Ge et al., 2012; Xu et al., 2016).
Fig. 5. The adsorption kinetics of marcolyte-(a, c) and algae-(b, d) dominant sedimentary colloids for the pseudo-first-order and pseudo-second-order kinetics, respectively.
Fig. 6. Effects of pH and ionic strengths on Pb(II) adsorption for sedimentary colloids.
Compared with the pH-dependent enhancement of adsorption capacity, the ionic strengths did not exhibit obvious influence on Pb(II) adsorption both for the two kinds of sedimentary colloids. Presence of the electrolyte ions can significantly influence the thickness and interface potential of double layer, which determine the adsorption efficiency for the outer-sphere surface complexes (Li et al., 2013). Thus, results on the non-sensitivity to ionic strength but sensitivity to pH observed herein indicated that inner-sphere complexation would be the main mechanism responsible for Pb(II) adsorption on sedimentary colloids. The results were consistent with those reported for other adsorbents, such as carbon nanotubes (Sheng et al., 2010), magnetite nanoparticles (Wang et al., 2010), and zinc oxide nanoparticles (Xu et al., 2016).
3.7. Environmental significance of this study Sedimentary colloids can pose obvious environmental and ecological effects to the aquatic ecosystems. Firstly, the abundance, composition and distribution of colloidal particles can influence the physicochemical properties of lake sediments. Secondly, these sedimentary colloids in lakes, especially in the eutrophic shallow lakes, are easily to re-suspend into the overlying water, which can substantially impair the transparence of water column and light penetration (Xu et al., 2018a). Moreover, these suspended colloidal particles can also exhibit potential bio-toxicity to microorganisms and the whole ecosystems (Colman et al., 2014; Hou et al., 2018). The first and utmost importance is to characterize the sedimentary colloids, including the extractable amounts, sizes, morphologies, and mineral compositions, etc. This is the first study investigating the abundance and chemical compositions of sedimentary colloids in the eutrophic shallow lake. Due to the small particle sizes and large specific surface areas, the sedimentary colloids usually exhibit substantial impacts on the transportation of contaminates (Gibson et al., 2007). However, there exists a debate in recent years that the lake sediments should be considered as the source or sink of contaminants. The large specific surface area endow the sedimentary colloids a high adsorption potential, which indicated a sink of contaminants for the lake sediment; whereas the properties of small sizes endow sedimentary colloids a high mobility and can thus be considered as the source of contaminates. In this study, we found that the abundance of colloidal particles exhibited somewhat heterogeneities between the two lake sediments. Thus, the debate on the source or sink of contaminants for lake sediments should be highly dependent on the ecological types and sediment properties. In addition, this study found a high adsorption capacity and the chemical adsorption mechanism of sedimentary colloids for heavy metals (e.g., Pb2þ), suggesting the high immobility of heavy metals in lake sediments. Therefore, sediments in the eutrophic shallow lakes can be considered as the sink for heavy metals, especially for the macrophyte-dominated areas.
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4. Conclusions Colloids can be extracted effectively from lake sediments, with the extraction efficiencies highly correlating to colloidal compositions and sediment types. These extracted colloids were characterized with montmorillonite-, kaolin-, volkonskoite-, and quartzrich chemical compositions, and a size distribution of 30e200 nm. As for the adsorption potential, the sedimentary colloids from different ecological regions both exhibited considerable potential for Pb(II) adsorption under environmentally relevant conditions. However, the sedimentary TCPs exhibited higher adsorption capacity than the ICPs, regardless of different ecological regions. Langmuir isotherm, pseudo-second-order kinetics, and chemical complexation can successfully characterize the adsorption process. This is the first systematic study probing into the abundance, chemical composition and adsorption behaviors of sedimentary colloids. Our results highlighted the importance of sedimentary colloids in analyzing the physicochemical properties of lake sediments and in evaluating the environmental behavior and fate of contaminants in aquatic ecosystems. Acknowledgments We gratefully thank the National Natural Science Foundation of China (51479187, 41601538), Youth Innovation Promotion Association CAS (2016286), Open Research Foundation of Key Laboratory of the Pearl River Estuarine Dynamics and Associated Process Regulation, Ministry of Water Resources (2018KJ02), and the State Key Laboratory of Lake Science and Environment, Chinese Academy of Sciences (2018SKL010) for their financial support on this study. References Brianna, R.W., Brandon, T.S., James, A.H., 2010. Magnetic g-Fe2O3 nanoparticles coated with poly-l-cysteine for chelation of As(III), Cu(II), Cd(II), Ni(II), Pb(II) and Zn(II). J. Hazard Mater. 161, 848e853. Chanudet, V., Filella, M., 2009. Size and composition of inorganic colloids in a perialpine, glacial flour-rich lake. Geochem. Cosmochim. Acta 72, 1466e1479. Colman, B.P., Espinasse, B., Richardson, C.J., Matson, C.W., Lowry, G.V., Hunt, D.E., Wiesner, M.R., Bernhardt, E.S., 2014. Emerging contaminant or an old toxin in disguise? Silver nanoparticle impacts on ecosystems. Environ. Sci. Technol. 48, 5229e5236. Ding, S.M., Han, C., Wang, Y.P., Yao, L., Wang, Y., Xu, D., Sun, Q., Williams, P.N., Zhang, C.S., 2015. In situ, high-resolution imaging of labile phosphorus in sediments of a large eutrophic lake. Water Res. 74, 100e109. Dumat, C., Cheshire, M.V., Fraser, A.R., Shand, C.A., Staunton, S., 1997. The effect of removal of soil organic matter and iron on the adsorption of radiocaesium. Eur. J. Soil Sci. 48, 675e683. Ge, F., Li, M.M., Ye, H., Zhao, B.X., 2012. Effective removal of heavy metal ions Cd2þ, Zn2þ, Pb2þ, Cu2þ from aqueous solution by polymer-modified magnetic
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nanoparticles. J. Hazard Mater. 211e212, 366e372. Gibson, C.T., Turner, I.J., Roberts, C.J., Lead, J.R., 2007. Quantifying the dimensions of nanoscale organic surface layers in natural waters. Environ. Sci. Technol. 41, 1339e1344. Hou, J., Wu, Y.Z., Li, X., Wei, B.B., Li, S.G., Wang, X.K., 2018. Toxic effects of different types of zinc oxide nanoparticles on algae, plants, invertebrates, vertebrates and microorganisms. Chemosphere 193, 852e860. Kaegi, R., Wagner, T., Hetzer, B., Sinnet, B., Tzvetkov, G., Boller, M., 2008. Size, number and chemical composition of nanosized particles in drinking water determined by analytical microscopy and LIBD. Water Res. 42, 2778e2786. Liang, L., Luo, L., Zhang, S.Z., 2011. Adsorption and desorption of humic and fulvic acids on SiO2 particles at nano- and micro-scales. Colloid. Surface. A Physicochem. Eng. Aspect. 384, 126e130. Li, W., He, Y., Wu, J., Xu, J., 2012. Extraction and characterization of natural soil nanoparticles from Chinese soils. Eur. J. Soil Sci. 63, 754e761. Li, W., Feng, X.H., Yan, Y.P., Sparks, D.L., Phillips, B.L., 2013. Solid-state NMR spectroscopic study of phosphate sorption mechanisms on aluminum (Hydr)oxides. Environ. Sci. Technol. 47, 8308e8315. Li, Y.H., Di, D.C., Ding, J., Wu, D.H., Luan, Z.K., Zhu, Y.Q., 2005. Adsorption thermodynamic, kinetic and desorption studies of Pb2þ on carbon nanotubes. Water Res. 39, 605e609. Qin, B.Q., Xu, P.Z., Wu, Q.L., Luo, L.C., Zhang, Y.L., 2007. Environmental issues of lake Taihu, China. Hydrobiologia 581, 3e14. €fer, T., Chanudet, V., Claret, F., Filella, M., 2007. Spectromicroscopy mapping of Scha colloidal/particulate organic matter in Lake Brienz, Switzerland. Environ. Sci. Technol. 41, 7864e7869. Sheng, G.D., Li, J.X., Shao, D.D., Hu, J., Chen, C.L., Chen, Y.X., Wang, X.K., 2010. Adsorption of copper(II) on multiwalled carbon nanotubes in the absence and presence of humic or fulvic acids. J. Hazard Mater. 178, 333e340. Tang, Z.Y., Wu, L.H., Luo, Y.M., Christie, P., 2009. Size fractionation and characterization of nanocolloidal particles in soils. Environ. Geochem. Health 31, 1e10. Wang, X.S., Lu, H.J., Zhu, L., Liu, F., Ren, J.J., 2010. Adsorption of lead(II) ions onto magnetite nanoparticles. Adsorpt. Sci. Technol. 28, 407e417. Xing, P., Kong, F.X., 2007. Intra-habitat heterogeneity of environmental factors regulating bacterioplankton community composition in Lake Taihu, China. Aquat. Microb. Ecol. 48, 113e122. Xu, H.C., Yu, G.H., Yang, L.Y., Jiang, H.L., 2013. Combination of two-dimensional correlation spectroscopy and parallel factor analysis to characterize the binding of heavy metals with DOM in lake sediments. J. Hazard Mater. 263, 412e421. Xu, H.C., Jiang, H.L., 2015. Effects of cyanobacterial extracellular polymeric substances on the stability of ZnO nanoparticles in eutrophic shallow lakes. Environ. Pollut. 197, 231e239. Xu, H.C., Li, L.N., Lv, H., Liu, X., Jiang, H.L., 2016. pH-dependent phosphatization of ZnO nanoparticles and its influence on subsequent lead sorption. Environ. Pollut. 208, 723e731. Xu, H.C., Guo, L.D., 2017. Molecular size-dependent abundance and composition of dissolved organic matter in river, lake and sea waters. Water Res. 117, 115e126. Xu, H.C., Xu, M.W., Li, Y.N., Liu, X., Guo, L.D., Jiang, H.L., 2018a. Characterization, origin and aggregation behavior of colloids in eutrophic shallow lake. Water Res. 142, 176e186. Xu, J., Cao, Z., Zhang, Y.L., Yuan, Z.L., Lou, Z.M., Xu, X.H., Wang, X.K., 2018b. A review of functionalized carbon nanotubes and graphene for heavy metal adsorption from water: preparation, application, and mechanism. Chemosphere 195, 351e364. Yu, G.H., Xiao, J., Hu, S.J., Polizzotto, M.L., Zhao, F.J., McGrath, S.P., Li, H., Ran, W., Shen, Q.R., 2017. Mineral availability as a key regulator of soil carbon storage. Environ. Sci. Technol. 51, 4960e4969. Zheng, S.S., Wang, P.F., Wang, C., Hou, J., 2015. Sediment resuspension under action of wind in Taihu Lake, China. Int. J. Sediment Res. 30, 48e62.
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Abundance, chemical composition and lead adsorption properties of sedimentary colloids in a eutrophic shallow lake Huacheng Xu a, *, 1, Li Ji b, 1, Ming Kong c, Mengwen Xu a, d, Xizhi Lv e, ** a
State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China b College of Harbour, Coastal and Offshore Engineering, Houhai University, Nanjing 210098, China c Nanjing Institute of Environmental Sciences, Ministry of Environmental Protection, Nanjing 210042, China d College of Biology and the Environment, Nanjing Forestry University, Nanjing 210037, China e Yellow River Institute of Hydraulic Research, Key Laboratory of the Loess Plateau Soil Erosion and Water Loss Process and Control of Ministry of Water Resources, Zhengzhou 450003, China
h i g h l i g h t s Abundance, composition and adsorption properties of sedimentary colloids were studied. The ultrasonic-induced extraction efficiency enhancement was related to colloidal composition and sediment type. Sedimentary colloids had Fe-, Al- and Si-containing mineralogical structures with various conformations. Total colloids exhibited higher adsorption capacity for Pb(II) than the inorganic colloids. Langmuir isotherm, pseudo-second-order kinetics, and chemical mechanism accounted for the adsorption process.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2018 Received in revised form 31 October 2018 Accepted 22 November 2018 Available online 23 November 2018
Colloidal particles are omnipresent in lake sediments and substantially influence the retention, transportation, and fate of contaminants in lake ecosystems. In this study, the abundance, chemical composition and adsorption behavior of sedimentary colloids (including total and inorganic colloids) from different ecological regions, were for the first time investigated via ultrasonic extraction, spectral analysis and batch absorption experiments. Results showed that the extraction efficiencies of sedimentary colloids showed an ultrasonic energy-dependent enhancement, and the algae-dominated area contained comparable colloidal abundance with the macrophyte-dominated area (i.e., 198.5 vs. 183.3 mg/g). Despite the different ecosystems, these sedimentary colloids usually had a wide size distribution of 30e200 nm, and were characterized with montmorillonite-, kaolin-, volkonskoite-, and quartz-rich chemical compositions. Batch experiment showed that the total pristine colloids exhibited higher adsorption capacity for Pb(II) than the inorganic colloids both for the macrophyte- and algae-dominated sediments, and the adsorption process followed pseudo-second-order kinetics and Langmuir isotherm, irrespective of different colloidal types. Thus, sedimentary colloids can immobilize the heavy metals in sediment and decrease their release into the water column, which can be considered as a sink for contaminants. This study highlighted the significance of sedimentary colloids in determining the physicochemical properties of lake sediments and in evaluating the environmental behavior and fate of contaminants in lake ecosystems. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Eutrophic shallow lakes Lake sediments Colloids Extraction Adsorption properties
1. Introduction
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Xu), [email protected] (X. Lv). 1 Huacheng Xu and Li Ji contributed equally to this work. https://doi.org/10.1016/j.chemosphere.2018.11.147 0045-6535/© 2018 Elsevier Ltd. All rights reserved.
Colloidal particles, with the size ranging from 1 nm to 1 mm, are omnipresent in natural environment, including sediments, soils, and waters, etc. (Kaegi et al., 2008; Tang et al., 2009; Yu et al., 2017). Presence of colloidal particles definitely poses important impacts
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on environmental and ecological systems. For instance, due to the large specific surface areas, the colloidal particles usually exhibited high adsorption capacities for the contaminants and nutrients, which can significantly affect their behavior, fate, and bioavailability (Gibson et al., 2007). Moreover, large amounts of colloidal particles can also induce obvious toxicity to microorganism metabolism (Hou et al., 2018) and ecosystem balance (Colman et al., 2014). Due to the important roles in determining the environmental behavior and bio-availability of contaminants as well as ecosystem health, environmental colloidal particles are receiving more and more attention in recent years. Lake sediments can be considered as the sources or sinks of nutrients and contaminants, which exhibit contrasting influences on the quantity of overlying water (Xu et al., 2013; Ding et al., 2015). Colloidal particles in lake sediment consisted of various mineralogical/chemical compositions, including silicates, carbonates, metals (Fe, Al, Mn, Si) and the corresponding hydroxides, etc (Chanudet and Filella, 2009). The sedimentary colloids, especially for those in the shallow lakes, are easily re-suspended under the windy conditions, which contribute highly the turbidity of the surface waters (Xing and Kong, 2007). Previous studies showed that the process of sediment re-suspension in eutrophic shallow lake under high environmental wind speed (e.g., 8 m/s) can contribute up about 55% of total colloidal abundance (mass based) in the water column (Xu et al., 2018a). Owing to the small-size properties, these aquatic colloids are prone to stabilize in surface waters for a long time and thus significantly impair the transparency of the water column (Zheng et al., 2015). In addition, some contaminants (e.g., heavy metals) adsorbed on the surface of sedimentary colloids can also change their bio-availability and toxicity to the ecological systems. Despite the important roles in influencing the contaminant transportation and water quality, detailed information on the physicochemical properties of sedimentary colloids and their adsorption properties for heavy metals is still lacking. The objectives of this study were thus to 1): determine the abundance of sedimentary colloids, including total and inorganic colloids, from different ecological regions as a function of ultrasonic energies; 2) characterize the shapes, sizes and mineralogical compositions of these sedimentary colloids; 3) investigate the adsorption properties of the sedimentary colloids at environmentally relevant conditions. To accomplish these goals, two lake sediments, namely macrophyte- and algae-dominated sediments from Lake Taihu, were sampled for the extraction of sedimentary colloids. The spectral and microscopic techniques, including high resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) were applied to characterize the colloids extracted. The representative heavy metals, Pb(II), was applied, due to its relatively high concentration in polluted ecosystems and potential toxicity to microorganisms, and the adsorption properties and mechanism were explored based on the batch experiments and spectral techniques. Results obtained here would be beneficial for a better insight into the physicochemical properties of sedimentary colloids and their contributions in contaminant transportation. 2. Materials and methods 2.1. Collection of lake sediment samples Lake Taihu, with a surface water area of 2338 km2, is one of the largest freshwater lakes in China. It is also a eutrophic and shallow lake, with the maximum and mean depths of 3.4 m and 1.9 m, respectively (Qin et al., 2007). Moreover, two different ecological areas, namely algae- and macrophyte-dominated areas, coexisted within the lake Taihu. The macrophyte-dominated area exhibits
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plenty of submersed vegetation, immobilized sediments, and clear water. In comparison, the algae-dominated area is characterized by frequent algal blooms, sedimentary re-suspension, and turbid water (Qin et al., 2007). Surface sediments samples, which were located within 0e10 cm below the sediment-water interface, were sampled from the macrophyte-dominated area (i.e., East Taihu Bay) and algaedominated area (i.e., Meiliang Bay), respectively (Xu et al., 2013). The samples were collected in September, 2017. After collection, the sedimentary samples were transported to laboratory on ice as soon as possible, followed by freeze-dried. The dried samples, with the organic matter contents of about 2e5%, were then ground and sieved through a 2 mm mesh to remove the large-size particles. 2.2. Extraction of sedimentary colloidal particles The sedimentary colloids were extracted by an Ultrasonic Processor (XO-1000D, Nanjing, China) with a 40% amplitude of stable power output. The probe tip, with a diameter of 13 mm, was inserted into 2 cm below the suspension surface, which were obtained by mixing 15 g sediments with 100 ml carbonate sodium solutions (0.025 M). The carbonate sodium solutions were applied herein to avoid the aggregation of colloidal particles (Li et al., 2012). To avoid the temperature effects, the suspensions were placed in a cooler during the whole ultrasonic treatment. To explore the ultrasonic-dependent extraction efficiencies, eight levels of ultrasonic energy, i.e., 12, 24, 48, 96, 192, 288, and 480 KJ were used to extract the sedimentary colloids. After extraction, the solutions suffering with different ultrasonic energies were settled in a cylinder. The colloidal particles were separated from the whole solutions by siphon based on the different sedimentation velocities (i.e., <0.3 cm/h) derived from the Stokes, law. An amicon stirred ultrafiltration cell coupled with a 1 kDa ultrafilter membrane was applied to concentrate the colloidal particles, with a concentration factor of about 25 (Xu and Guo, 2017; Xu et al., 2018a). After ultrafiltration, the retentate was carefully collected, which can be denoted as the total colloidal particles (TCPs). Afterwards, 30% of hydrogen peroxide was used to an aliquot of the retentate to remove the organic layers that adsorbed on the colloidal surfaces to obtain the inorganic colloidal particles (ICPs) (Dumat et al., 1997; Xu et al., 2018a). After centrifugation at 15000g for 10 min, the obtained TCPs and ICPs were both freeze-dried and stored in the desiccator for the following chemical composition and adsorption analysis. 2.3. Adsorption experiments The sorption processes were all performed in duplicate at room temperature. For the studies of equilibrium sorption isotherms, 50 ml solutions containing different Pb(II) concentrations [Pb(NO3)2, 2e75 mg L1] were added to 100 ml vials which contained 40 mg TCPs or ICPs. The vials were shaken in an incubator shaker (ZWY2102C, ZHCHENG, Shanghai) at 180 rpm for 24 h. The pH was determined by a pH meter (PB-10, Sartorius, Germany), with the initial pH of 6.4 ± 0.1. During the adsorption process, the pH values of solutions were adjusted at 6.0 at intervals of 8 h via the addition of NaOH or HCl solution. For the kinetic experiments, 40 mg of colloidal samples (e.g., TCPs or ICPs) were mixed with 50 ml of Pb(II) solutions (40 mg L1, pH 6.0), with the adsorption capacities determining at the interval times (10 min, 20 min, 30 min, 1 h, 2 h, 4 h, 6 h, 12 h, and 24 h, respectively). For the studies of environmental factor-dependent adsorption heterogeneities, various pHs (4e7) and ionic strengths (0.001e0.1 M) were applied, with an initial Pb2þ concentration and contact time of 40 mg L1 and 24 h, respectively. After adsorption, the mixed solutions were centrifuged at 15000 g
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for 20 min, followed by microfiltration (0.22 mm), with Pb(II) concentration measuring via the inductively coupled plasma-atomic emission spectrometry (ICP-AES, Prodigy, USA). Difference of Pb(II) between the total and residual concentrations were determined to calculate the adsorption efficiency.
dominated area during the ultrasonic extraction process. The results showed that both the abundances and compositions of sedimentary colloids were highly related to ultrasonic energies.
2.4. Characterization of the colloidal particles and statistic analysis
Fig. 2 shows the TEM micrographs of the extracted TCPs and ICPs. It was shown that the morphologies of sedimentary TCPs did not exhibit differences between the macrophyte- and algaedominated areas that they were both characterized with the mixed shapes of spherical, elliptical and polygonal. As for the colloidal sizes, they were characterized with a broad size range from 30 nm to 200 nm, showing an uneven distribution patterns. Further analysis showed that obvious organic layers can be found on the surfaces of TCPs, which would be attributed to the chemicophysical adsorption of humic substances, biopolymers, and other macromolecules in natural waters (Sch€ afer et al., 2007; Xu et al., 2018a). Compared with the sedimentary TCPs, the ICPs from the two areas exhibited different morphologies. Specifically, the sedimentary ICPs from macrophyte-dominated area were mostly characterized with polygonal and prismatic shapes, whereas those from algae-dominated area were mainly spherical. In addition, the sizes of sedimentary ICPs from the two areas ranged mainly from 20 to 40 nm, which were significantly smaller than those of the TCPs.
The sizes and morphology of sedimentary colloids were measured via the TEM observation (JEM JEOL-200CX). The crystal phases of sedimentary colloids were recorded by X-ray diffraction analysis (Siemens D5000) using Cu-Ka radiation. The operational tube for XRD analysis was characterized with a current and voltage of 30 mA and 40 kV, respectively. The spectra of ATR-FTIR were determined via a Thermo Nicolet iS50 spectrometer at room temperature with the spectral scan of 600e4000 cm1 and a resolution of 1 cm1. As for the statistic analysis, significance of data differences in colloidal abundances of lake sediments between two ecological types was examined via the independent-sample T-test model. 3. Results and discussion 3.1. Abundances of extractable sedimentary colloids as affected by ultrasonic energies Ultrasonic treatment can be used to disperse the lake sediments as well as to extract the small-size colloidal particles, with increasing extraction efficiency at higher energies. Taken the TCPs as example, the extraction efficiencies exhibited an initially rapid increase followed by a gradual enhancement both for the two sedimentary colloids (Fig. 1). With the highest ultrasonic energy, the abundances of sedimentary TCPs from algae- and macrophytedominated areas were 198.5 ± 7.5 and 183.3 ± 8.2 mg/g, respectively. The abundances of colloids measured in lake sediments were comparable with those reported in soils (68e453 mg/g, 12e72 KJ) (Li et al., 2012). In addition, compared with the sedimentary TCPs, the extraction efficiencies of ICPs showed different variation trends between the two lake sediments. Although the extraction efficiencies of sedimentary ICPs both increased sharply during the initial ultrasonic treatment, there existed a slight increase for further treatment for the algae-dominated area but no further enhancement in extraction efficiency can be observed for the macrophyte-dominated area. The final abundances of sedimentary ICPs from the macrophyte-dominated area were thus 62.0 ± 8.2 mg/g, which was significantly lower than those from the algaedominate area (111.2 ± 6.2 mg/g) (p < 0.01). In addition to the extraction efficiency, the percentages of inorganic fraction also showed an ultrasonic energy-related variation. The initial ratios of ICPs to TCPs for the two sediments were comparable (42.7%~44.6%), while it decreased to 33.9% for the macrophyte-dominated area but increased to 55.8% for the algae-
Fig. 1. The extraction efficiencies of sedimentary colloids under various ultrasonic intensities.
3.2. Morphologies and sizes of the sedimentary colloids
3.3. The mineralogical composition of sedimentary colloids XRD can be used to characterize the mineral composition of sedimentary colloids, with the results shown in Fig. 3a. No obvious heterogeneities can be observed in XRD patterns in terms of peak intensities and positions among the four samples, indicating the similar mineral patterns between different sediments and that the treatment of the removal of organic matters (i.e., the usage of hydrogen peroxide) did not influence the mineral compositions of sedimentary colloids. Comparisons in the XRD patterns of sedimentary colloids with those of the standard minerals showed that the main mineral structure for sedimentary colloids were montmorillonite, nontronite, volkonskoite, and quartz. These results indicated that ferric, aluminum, and silicon were the main element composition within the sedimentary colloids in the eutrophic shallow lakes.
Fig. 2. TEM micrograph of the sedimentary TCPs (aeb) and ICPs (ced) from macrophyte- and algae-dominated areas, respectively.
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Fig. 3. XRD (a) and FTIR (b) patterns of sedimentary TCPs and ICPs from different ecological areas.
To further explore and characterize the chemical compositions of the sedimentary colloids, the infrared techniques were applied herein, with the ATR-FTIR spectra shown in Fig. 3b. Some obvious absorption peaks, e.g., 3700, 3620, 911, 798 and 754 cm1, can be observed for the two kinds of sedimentary colloids. We assigned these peaks as follows: peaks 3700 and 3620 cm1 were attributed to the internal OH groups and were characteristic of kaolin material; peak 911 cm1 can be ascribed to the OH deformation of smectite, and the presence of peaks 798 and 754 cm1 originated from montmorillonite (Li et al., 2012). The result of ATR-FTIR also demonstrated the main mineral structures of kaolin, smectite and montmorillonite for the sedimentary colloids. However, further analysis showed that the sedimentary TCPs from the two areas were characterized with obvious amide I peaks at 1650 cm1 (Xu and Jiang, 2015), indicating the presence of organic layers (e.g., protein-like substances) on the surface of sedimentary TCPs. However, compared with the TCPs, the intensities of peak 1650 cm1 in sedimentary ICPs was much lower, demonstrating the efficient removal of organic matters from the TCPs. 3.4. The adsorption potential of sedimentary colloids for Pb(II) The adsorption potential of colloidal particles can be explored via the application of adsorption isotherms. Two isotherms models, i.e., Freundlich and Langmuir models, were usually applied to fit the experimental data and to characterize the adsorption process (Liang et al., 2011). The Freundlich and Langmuir isotherm models are expressed as follows:
qe ¼
1
=
q ¼ KF Ce
n
q m K a Ce 1 þ Ka Ce
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Fig. 4aeb shows the Freundlich isotherms for Pb(II) adsorption on sedimentary TCPs and ICPs. Large variations among the experiment data were observed both for the two kinds of sedimentary colloids, suggesting that the experimental data did not fit well in this model. In addition, the low R2 value (<0.87) indicated the unsuitability of Freundlich model to predicate the adsorption properties of Pb(II) on sedimentary colloids. The Langmuir equation was thus applied to describe the adsorption behavior. It was shown that the Langmuir model predicted much better than the Freundlich model for the adsorption of Pb(II) on both TCPs and ICPs, with a high R2 value of 0.99 (Fig. 4ced). Further analysis showed that the adsorption capacity (qm) of sedimentary ICPs were 42.0 ± 1.4 mg/g for macrophytedominated area and 44.8 ± 2.4 mg/g for algae-dominated area, while those of sedimentary TCPs were 58.1 ± 1.0 mg/g for macrophyte-dominated area and 65.3 ± 0.6 mg/g for algaedominate area, respectively. The adsorption capacity of sedimentary colloids calculated herein was comparable with those of carbon nanotubes (30e40 mg/g) (Li et al., 2005) and g-Fe2O3 nanoparticles (<90 mg/g) (Brianna et al., 2010) but significantly lower than that of ZnO nanoparticles (556 mg/g) (Xu et al., 2016). In addition, compared with the ICPs, the higher adsorption capacity of TCPs would be ascribed to the organic layers of colloidal surfaces. Previous studies had shown that the organic layers on particle surfaces can interact with metals to form metal-metal complexes via hydrogen binding and/or ligand complexation (Xu et al., 2013), which resultantly enhanced the capacity of Pb(II) adsorption on sedimentary TCPs.
3.5. Adsorption kinetics for Pb(II) Due to the large specific surface areas and high surface activity, it is believable that sedimentary colloids can exhibit a fast adsorption potential for Pb(II) (Xu et al., 2016). Analysis on the adsorption capacities as a function of time showed that the process of Pb(II) adsorption on sedimentary TCPs and ICPs were both composed of a rapid initial adsorption and a slow equilibrium phase. For example, about 90% of total Pb(II) concentrations can be adsorbed in the first 2 h, whereas the following adsorption process contributed only 10% of adsorption capacity. To explore the detailed adsorption kinetics, two kinetic models, namely pseudo-first-order and pseudo-second-order equations,
(1) (2)
where Ce represents the Pb(II) concentration (mg L1) in the solutions, KF is the Freundlich constant. In addition, q and qm are the amount of Pb(II) adsorbed under each condition and the maximum adsorption amount for a monolayer, respectively. During the calculation of adsorption potential, both the two isotherm models were usually rearranged to a liner form:
log q ¼
1 logCe þ logKF n
Ce 1 Ce ¼ þ qe qm Ka qm
(3)
(4)
For the Freundlich model, the constants ‘KF’ and ‘1/n’ can be obtained based on the linear plots of ‘log q’ vs. ‘log Ce’, while for the Langmuir model, the constants ‘qm’ and ‘Ka’ can be calculated via the linear plots of ‘Ce/qe’ vs. ‘Ce’.
Fig. 4. The adsorption isotherms of marcolyte-(a, c) and algae-(b, d) dominated sedimentary colloids for Freundlich and Langmuir model, respectively.
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were applied in this study and expressed as follows:
logðqe qt Þ ¼ logqe
k1 t 2:303
t 1 t ¼ þ qt k2 ðqe Þ2 qe
(5)
(6)
where qt and qe represent the adsorbed amounts of heavy metal Pb(II) at time t and equilibrium, respectively; k1 and k2 are the rate constants of pseudo-first-order and pseudo-second-order adsorption processes, respectively. Studies on the pseudo-first-order rate equation showed a significant deviation between the experimental and the estimated ‘qe’ (Fig. 5), indicating that this mode was not suitable for kinetic analysis. By comparison, application of pseudo-second-order kinetic equation can successfully estimate the adsorption kinetics, with a high R2 of 1.00 both for the two kinds of sedimentary colloids. The results of adsorption kinetics were in agreement with those of some other colloidal and/or nanoparticles, such as nanoparticles ZnO, carbon nanotubes and graphene (Xu et al., 2016, 2018b).
3.6. Effects of environmental factors on Pb(II) adsorption Adsorption of heavy metals was highly related with environmental factors, which can significantly influence on surface properties of adsorbents (Xu et al., 2016). In this study, the efficiencies of Pb(II) adsorption increased with enhancing pHs both for the two kinds of sedimentary colloids (Fig. 6). Take ICPs from macrophytedominated area as example, the adsorption capacity was 12.5 ± 0.5 mg/g at pH 4, while it reached to 45.1 ± 1.8 mg/g when the solution pH increased to 7. Based on the pH-dependent species of lead, Pb2þ was the predominant species when the solution pH was below 7. The coexistence of Hþ and Pb2þ can compete each other for the active sites on colloidal surfaces, which accounted for the low adsorption efficiency under acidic pH conditions. The pHdependent enhancement of adsorption capacity of Pb(II) can also be found for other pure and/or polymer-modified nanoparticles (Ge et al., 2012; Xu et al., 2016).
Fig. 5. The adsorption kinetics of marcolyte-(a, c) and algae-(b, d) dominant sedimentary colloids for the pseudo-first-order and pseudo-second-order kinetics, respectively.
Fig. 6. Effects of pH and ionic strengths on Pb(II) adsorption for sedimentary colloids.
Compared with the pH-dependent enhancement of adsorption capacity, the ionic strengths did not exhibit obvious influence on Pb(II) adsorption both for the two kinds of sedimentary colloids. Presence of the electrolyte ions can significantly influence the thickness and interface potential of double layer, which determine the adsorption efficiency for the outer-sphere surface complexes (Li et al., 2013). Thus, results on the non-sensitivity to ionic strength but sensitivity to pH observed herein indicated that inner-sphere complexation would be the main mechanism responsible for Pb(II) adsorption on sedimentary colloids. The results were consistent with those reported for other adsorbents, such as carbon nanotubes (Sheng et al., 2010), magnetite nanoparticles (Wang et al., 2010), and zinc oxide nanoparticles (Xu et al., 2016).
3.7. Environmental significance of this study Sedimentary colloids can pose obvious environmental and ecological effects to the aquatic ecosystems. Firstly, the abundance, composition and distribution of colloidal particles can influence the physicochemical properties of lake sediments. Secondly, these sedimentary colloids in lakes, especially in the eutrophic shallow lakes, are easily to re-suspend into the overlying water, which can substantially impair the transparence of water column and light penetration (Xu et al., 2018a). Moreover, these suspended colloidal particles can also exhibit potential bio-toxicity to microorganisms and the whole ecosystems (Colman et al., 2014; Hou et al., 2018). The first and utmost importance is to characterize the sedimentary colloids, including the extractable amounts, sizes, morphologies, and mineral compositions, etc. This is the first study investigating the abundance and chemical compositions of sedimentary colloids in the eutrophic shallow lake. Due to the small particle sizes and large specific surface areas, the sedimentary colloids usually exhibit substantial impacts on the transportation of contaminates (Gibson et al., 2007). However, there exists a debate in recent years that the lake sediments should be considered as the source or sink of contaminants. The large specific surface area endow the sedimentary colloids a high adsorption potential, which indicated a sink of contaminants for the lake sediment; whereas the properties of small sizes endow sedimentary colloids a high mobility and can thus be considered as the source of contaminates. In this study, we found that the abundance of colloidal particles exhibited somewhat heterogeneities between the two lake sediments. Thus, the debate on the source or sink of contaminants for lake sediments should be highly dependent on the ecological types and sediment properties. In addition, this study found a high adsorption capacity and the chemical adsorption mechanism of sedimentary colloids for heavy metals (e.g., Pb2þ), suggesting the high immobility of heavy metals in lake sediments. Therefore, sediments in the eutrophic shallow lakes can be considered as the sink for heavy metals, especially for the macrophyte-dominated areas.
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4. Conclusions Colloids can be extracted effectively from lake sediments, with the extraction efficiencies highly correlating to colloidal compositions and sediment types. These extracted colloids were characterized with montmorillonite-, kaolin-, volkonskoite-, and quartzrich chemical compositions, and a size distribution of 30e200 nm. As for the adsorption potential, the sedimentary colloids from different ecological regions both exhibited considerable potential for Pb(II) adsorption under environmentally relevant conditions. However, the sedimentary TCPs exhibited higher adsorption capacity than the ICPs, regardless of different ecological regions. Langmuir isotherm, pseudo-second-order kinetics, and chemical complexation can successfully characterize the adsorption process. This is the first systematic study probing into the abundance, chemical composition and adsorption behaviors of sedimentary colloids. Our results highlighted the importance of sedimentary colloids in analyzing the physicochemical properties of lake sediments and in evaluating the environmental behavior and fate of contaminants in aquatic ecosystems. Acknowledgments We gratefully thank the National Natural Science Foundation of China (51479187, 41601538), Youth Innovation Promotion Association CAS (2016286), Open Research Foundation of Key Laboratory of the Pearl River Estuarine Dynamics and Associated Process Regulation, Ministry of Water Resources (2018KJ02), and the State Key Laboratory of Lake Science and Environment, Chinese Academy of Sciences (2018SKL010) for their financial support on this study. References Brianna, R.W., Brandon, T.S., James, A.H., 2010. Magnetic g-Fe2O3 nanoparticles coated with poly-l-cysteine for chelation of As(III), Cu(II), Cd(II), Ni(II), Pb(II) and Zn(II). J. Hazard Mater. 161, 848e853. Chanudet, V., Filella, M., 2009. Size and composition of inorganic colloids in a perialpine, glacial flour-rich lake. Geochem. Cosmochim. Acta 72, 1466e1479. Colman, B.P., Espinasse, B., Richardson, C.J., Matson, C.W., Lowry, G.V., Hunt, D.E., Wiesner, M.R., Bernhardt, E.S., 2014. Emerging contaminant or an old toxin in disguise? Silver nanoparticle impacts on ecosystems. Environ. Sci. Technol. 48, 5229e5236. Ding, S.M., Han, C., Wang, Y.P., Yao, L., Wang, Y., Xu, D., Sun, Q., Williams, P.N., Zhang, C.S., 2015. In situ, high-resolution imaging of labile phosphorus in sediments of a large eutrophic lake. Water Res. 74, 100e109. Dumat, C., Cheshire, M.V., Fraser, A.R., Shand, C.A., Staunton, S., 1997. The effect of removal of soil organic matter and iron on the adsorption of radiocaesium. Eur. J. Soil Sci. 48, 675e683. Ge, F., Li, M.M., Ye, H., Zhao, B.X., 2012. Effective removal of heavy metal ions Cd2þ, Zn2þ, Pb2þ, Cu2þ from aqueous solution by polymer-modified magnetic
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