Published online January 25, 2018
Soil Chemistry
Pb(II) and Cu(II) Adsorption and Desorption Kinetics on Ferrihydrite with Different Morphologies Lei Tian† Yuzhen Liang† Yang Lu Lanfang Peng Pingxiao Wu Zhenqing Shi*
School of Environment and Energy South China Univ. of Technology Guangzhou Guangdong 510006 People’s Republic of China The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters Ministry of Education South China Univ. of Technology Guangzhou Guangdong 510006 People’s Republic of China Guangdong Engineering and Technology Research Center for Environ. Nanomaterials South China Univ. of Technology Guangzhou Guangdong 510006 People’s Republic of China
Understanding the effect of ferrihydrite morphology on the kinetic reactions of metals with ferrihydrite is essential for predicting the dynamic behavior of metals in soil. In this study, kinetics of Pb(II) and Cu(II) adsorption and desorption on two types of ferrihydrite, the freshly precipitated gellike ferrihydrite and freeze-dried dense ferrihydrite representing two typical morphologies under extreme soil conditions, were studied. The high-resolution transmission electron microscopy (TEM) images revealed that the gellike ferrihydrite loosely aggregated with open structure, while the dense ferrihydrite compactly aggregated with more consolidated and thicker structure. The energy dispersive spectroscopy (EDS), at the nanometer scale, showed that Pb(II) and Cu(II) distributed evenly on gellike ferrihydrite but localized on dense ferrihydrite after adsorption. The stirred-flow kinetic experiments showed that higher pH and higher influent metal concentrations increased metal adsorption for both types of ferrihydrite. The dense ferrihydrite adsorbed slower and much less metals than gellike ferrihydrite because of less reactive sites. The desorption of heavy metals from ferrihydrite was affected by combined factors of the ferrihydrite morphology, adsorbed metal concentrations, pH, and metal re-adsorption rates. The mechanic kinetics model based on the CD-MUSIC model successfully described the adsorption and desorption kinetics of Pb(II) and Cu(II) on both gellike and dense ferrihydrite under various chemistry conditions. Our model provided quantitative tools for considering the effects of ferrihydrite morphology and heterogeneous binding sites under varying solution chemistry conditions when predicting the dynamic behavior of Pb(II) and Cu(II) in soil environment. Abbreviations: BF, bright field; CD-MUSIC, Charge Distribution and Multisite Surface Complexation Model; DEPP, N,N-diethyl piperazine; EDS, energy dispersive spectroscopy; HAADF, high-angle annular dark field; HR-TEM, high resolution transmission electron microscopy; MES, 2-(N-morpholino)ethanesulfonic acid; SCM, surface complexation model; TEM, transmission electron microscopy; STEM, scanning transmission electron microscopy; XRD, X-ray powder diffractometer.
T Core Ideas • Gellike and dense ferrihydrite had different structures and Pb/Cu distributions. • Ferrihydrite morphology and solution chemistry affected sorption and desorption kinetics. • The kinetics model successfully described the stirred-flow kinetic data of Pb and Cu. • Different binding sites controlled adsorption and desorption kinetics differently. Soil Science Society of America Journal
he ferrihydrite, an amorphous and poorly crystalline Fe oxyhydroxide, is one of the most naturally ubiquitous and reactive iron oxides (Sparks, 2003; Wang et al., 2013; Wang et al., 2015a). Because of its abundance, large surface area, and highly reactive surface sites, ferrihydrite is an important adsorbent in natural soil and plays a key role in controlling the fate and transport of heavy metals (Brown et al., 1999; Gustafsson et al., 2011; Jain et al., 1999; Lion et al., 1982; Martinez and McBride, 1999). Typical heavy metals, such as Pb(II) and Cu(II), may form strong complexes with ferrihydrite (Gustafsson et al., 2011; Tiberg et al., 2013; Trivedi et al., 2003). Therefore, understanding the adsorption and desorption properties of heavy metals on ferrihydrite is essential to understand the fate, bioavailability, and toxicity of heavy metals in soil. In soil environments under different climatic conditions, ferrihydrite may exhibit various aggregation morphologies and thus aggregation structures. However,
† L. Tian and Y. Liang contributed equally. Soil Sci. Soc. Am. J. doi:10.2136/sssaj2017.08.0279 Received 16 Aug. 2017. Accepted 13 Nov. 2017. *Corresponding author (
[email protected]). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA. All Rights reserved.
the impact of ferrihydrite morphologies on the reactivity of ferrihydrite with heavy metals and underlying mechanisms remain unclear. The changing of aggregation state and morphology of ferrihydrite was found to have large effects on metal uptake, retention, and speciation (Dale et al., 2015; Gilbert et al., 2009; Stegemeier et al., 2015; Vindedahl et al., 2016). The gellike ferrihydrite aggregate forming through fast precipitation represents the newly formed ferrihydrite during Fe cycling in soil, while the denser ferrihydrite aggregate occurs through slow redox precipitation or through seasonal drying of ferrihydrite gel in redoxomorphic soils, both of which had significant impact on metal adsorption behavior (Scheinost et al., 2001). Previous studies found that increasing ferrihydrite aggregation led to the decreasing percentage of metal adsorption (Dale et al., 2015), and the transition from low to high aggregation states reduced pore space between aggregated particles and resulted in the more condensed aggregate and less reactive surface area (Dale et al., 2015; Gilbert et al., 2009; Hofmann et al., 2004; Scheinost et al., 2001; Stegemeier et al., 2015). Therefore, the changes of morphology and reactive surface areas may have significant implications on the fate of heavy metals in soil environments. Numerous surface complexation models (SCMs) have been developed to describe the adsorption equilibrium (Dzombak and Morel, 1990; Goldberg, 1992; Gustafsson et al., 2011; Hiemstra and Van Riemsdijk, 1996; Lofts and Tipping, 1998; Wang et al., 2015b; Westall and Hohl, 1980). The Charge Distribution and Multisite Surface Complexation (CD-MUSIC) model (Hiemstra and Van Riemsdijk, 1996), is able to account for the heterogeneous surface binding sites of Fe minerals and varying charge distribution at the interface, and has shown success for predicting metal sorption on Fe minerals (Gustafsson et al., 2011; Tiberg and Gustafsson, 2016; Tiberg et al., 2013). Equilibrium and structural investigation has provided some fundamental information on metal adsorption on ferrihydrite with different morphologies under various reaction conditions. However, kinetics of heavy metal adsorption and desorption on ferrihydrite are less systematically understood (Subramaniam et al., 2003; Subramaniam and Yiacoumi, 2001; Yiacoumi and Tien, 1995a,1995b), and seldom quantitatively studied for ferrihydrite with different morphologies (Scheinost et al., 2001). The typical biphasic adsorption kinetics, with rapid initial uptake and slow continued reactions, have been observed for heavy metals (Farrell and Chaudhary, 2013; Scheinost et al., 2001; Swedlund et al., 2014; Xu et al., 2006). In the study by Tian et al. ( 2017), metal adsorption and desorption kinetics on freshly synthesized gellike ferrihydrite was systematically studied and a unified kinetics model was developed. It was found that the majority of the gellike ferrihydrite binding sites were accessible for metals within a short time with little diffusion limitation. However, it has also been reported that the slow sorption reactions in the freeze-dried dense ferrihydrite aggregates were pronounced, probably because of the reduced porosity, smaller diameters of pores and loss of reactive sites (Hofmann et al., 2004; Scheinost et al., 2001). So far, how the ferrihydrite morphology, in combination with the variations ∆
of solution chemistry and heterogeneity of ferrihydrite binding sites, affects the heavy metal adsorption and desorption kinetics is still poorly understood. This significantly limits our ability to predict heavy metal behavior in soil, and further studies on how ferrihydrite morphology affects metal kinetic reactions, at both quantitative and mechanistic levels, are desired. In this kinetic study, two types of ferrihydrite with different morphologies were used, a freshly precipitated ferrihydrite suspension (denoted as gellike ferrihydrite) and another freezedried ferrihydrite (denoted as dense ferrihydrite), which represent ferrihydrite morphologies under extreme soil conditions. The objective of this study is to elucidate the difference of both Pb(II) and Cu(II) kinetic reactions with the gellike and the dense ferrihydrite at both mechanistic and quantitative levels. The original and metal-adsorbed ferrihydrite aggregates were investigated using the high-resolution transmission electron microscopy (HR TEM) coupled with EDS at the nanoscale. The adsorption and desorption kinetics of heavy metals on both gellike and dense ferrihydrite were studied under varying solution chemistry conditions using a stirred-flow reactor. The mechanistic kinetics model developed in the previous study (Tian et al., 2017) was used to describe the heavy metal adsorption and desorption kinetics for both types of ferrihydrite. We expect that the results can further enhance our understanding on the kinetic behavior of heavy metals in a soil environment, and thus help environmental risk assessment and soil remediation work.
Materials and methods Reagents
The background electrolyte solutions were 10 mM Ca(NO3)2 and 10 mM NaNO3 for dense and gellike ferrihydrite experiments, respectively, which were buffered with either N,N-diethyl piperazine (DEPP, pH 5) or 2-(N-morpholino) ethanesulfonic acid (MES, pH 5.5 and 6) to maintain a desired pH. Lead(II) and Cu(II) stock solutions were made with Pb(NO3)2 and Cu(NO3)2 salts, respectively. The heavy metal stock solution was added to the buffered background electrolyte to prepare different concentrations of heavy metal solutions for the kinetic experiments.
Synthesis of Ferrihydrite Two-line ferrihydrite was synthesized using the standard method (Cornell and Schwertmann, 2003) by dissolving Fe(NO3)3·9H2O into deionized water and then adding 1 M NaOH with stirring to bring the pH between 7 and 8. The ferrihydrite suspension was then shaken for 24 h and the resulted suspension was the gellike ferrihydrite. A portion of the gellike ferrihydrite suspension was kept in the refrigerator at 4°C and used within 2 d. Another portion of the gellike ferrihydrite suspension was washed and centrifuged, which was repeated for a few times. The collected ferrihydrite particles were then freezedried to obtain the dense ferrihydrite aggregates. The dense ferrihydrite aggregates were kept in the refrigerator at 4°C.
Soil Science Society of America Journal
Stirred-Flow Adsorption–Desorption Kinetic Experiment The stirred-flow experiments were conducted to study the adsorption and desorption kinetics of Pb(II) and Cu(II) on dense and gellike ferrihydrite. In the stirred-flow reactor, the experimental variables were well-controlled (Shi et al., 2005, 2008, 2013; Tian et al., 2017), which provides good conditions to study the kinetic reactions of metals with ferrihydrite particles– aggregates (Tian et al., 2017). Since both Pb(II) and Cu(II) adsorption and desorption kinetics on the gellike ferrihydrite were examined extensively in the previous study (Tian et al., 2017), the kinetic experiments focused on the dense ferrihydrite for both Pb(II) and Cu(II). We also conducted a few additional experiments using the gellike ferrihydrite for comparison and testing the applicability of the kinetics model. To start the experiments, a certain amount of ferrihydrite samples were added in the stirred-flow reactor (7.5 mL), which was then filled with the background electrolyte solution, with a Teflon-coated magnetic stir bar placed in it. This gave a final dense ferrihydrite concentration of 13.3 g L-1 or gellike ferrihydrite concentration of 1.3 or 0.33 g L-1 in the stirred-flow reactor. The reactor was sealed with a membrane filter (25-mm diam. and 0.22-mm pore size) to retain the ferrihydrite particles in the reactor, in which the release of ferrihydrite particles was negligible based on the total Fe concentrations in the effluent samples. After completely mixing the ferrihydrite with the background electrolyte in the reactor using the stir bar, the background electrolyte solution was continuously pumped through the reactor for 30 min. Then, the Pb(II) or Cu(II) solution was pumped through the reactor for 4 h at a constant flow rate of 1 mL min-1. The desorption process was followed by switching the influent solution to the background electrolyte solution for another 4 h. During the adsorption and desorption processes, the effluent solution was collected every 5 min. The effluent solution was then acidified using concentrated HNO3 and the metal concentration was determined using an atomic absorption spectrometer (AA-6880, Shimadzu Corp., Japan) or an inductively coupled plasma mass spectrometer (Agilent 7900, Agilent Technologies, USA). All the experiments were conducted at 25°C. Duplicate experiments of adsorption and desorption kinetics were conducted to ensure the reproducibility of the experiments.
Characterization of Ferrihydrite The composition of the ferrihydrite was identified using a Bruker D8 Advance X-ray powder diffractometer (Bruker, Germany). The fine structure and elemental distribution of the fresh and metal-adsorbed ferrihydrite particle aggregates were examined with a JEM-2100F TEM ( JEOL Ltd., Japan) and a FEI Titan Themis 200 TEM (FEI, USA) that was fitted with the high-angle annular dark field (HAADF), bright field (BF), and EDS (Bruker super-X EDS, Bruker, Germany). To prepare the metal-adsorbed ferrihydrite samples for TEM analysis, the heavy metal stock solution was added dropwise to the ferrihydrite suspension with stirring, and then the www.soils.org/publications/sssaj
mixture was continuously stirred for 4 h at pH 6.0. After 4-h adsorption, the metal-adsorbed ferrihydrite suspension was sampled for TEM analysis. The samples for TEM analysis first were prepared by ultra-sonication for 20 min, where the vessel with the ferrihydrite suspension dissolved in ethyl alcohol was placed in cold water to prevent ferrihydrite transformation. Then 10 mL of the suspension was pipetted onto a 200-molybdenum-mesh grid coated with a 3- to 5-nm carbon-sputtered Formvar support film (Gilder Grids Ltd., UK) and dried with an infrared lamp for 3 to 5 min. The nanoscale morphology and HR TEM images of the pure and metal-adsorbed ferrihydrite aggregates were conducted with the JEM-2100F TEM working in the TEM mode. Elemental distribution of the ferrihydrite samples were mapped with the FEI Titan Themis 200 in the STEM mode using STEMEDS spectrometer as well as HAADF and BF detectors.
Kinetics Model The details of the kinetics model for ion adsorption–desorption on ferrihydrite can be found in the previous work (Tian et al., 2017). Briefly, to describe the adsorption and desorption kinetics of Pb(II) or Cu(II) on a specific ferrihydrite binding site and variations of metal concentrations in solution in the stirredflow reactor, Eq. [1] and [2] are used:
d C pi dt
= − kdiC pi + kaiC ion [1]
Q ( C ion −C ion,0 ) [2] d C ion = ∑ kdi mC pi −∑ kai mC ion − dt V where, for a specific ferrihydrite binding site i, kai (L g-1 min-1) is the adsorption rate coefficient, kdi (min-1) is the desorption rate coefficient, Cpi (mg g-1) is the adsorbed heavy metal concentration, Cion (mg L-1) is the heavy metal concentration in the solution, m (g L-1) is the ferrihydrite concentration, Q (L min-1) is the flow rate, V (L) is the volume of the reactor, and Cion,0 (mg L-1) with subscript 0 is the influent heavy metal concentration. We integrated the equilibrium model CD-MUSIC into the kinetics model to account for the variations of solution chemistry, heterogeneity of the binding sites, and the nonlinear binding of Pb(II) and Cu(II) to ferrihydrite. At equilibrium, for specific binding site i, there are below relationships among Kpi (equilibrium partition coefficient), kai, kdi, Cpi, and Cion:
kaiC ion = kdiC pi K pi =
C pi C ion
[3] [4]
Combining Eq. [3] and [4] yields
= kai
kdiC pi = kdi K pi C ion
[5]
The above equation relates the kinetic parameters, kai and kdi, with the equilibrium parameter Kpi that is a function of the ∆
solution chemistry (e.g., Cpi and pH) and can be calculated by CD-MUSIC at specific Cpi and pH values. Note that the kinetics model assumes that kdi is independent of solution chemistry and the variations of solution chemistry is accounted by kai via Eq. [5]. The CD-MUSIC model built in the Visual MINTEQ software (Gustafsson, 2015) was used for Kpi calculations, which adopted the surface charging parameters from Tiberg et al. (Tiberg and Gustafsson, 2016; Tiberg et al., 2013) based on the recent advanced spectroscopic structural analysis of ferrihydrite and equilibrium studies (Gustafsson et al., 2011; Hiemstra, 2013; Hiemstra and Van Riemsdijk, 2009; Tiberg and Gustafsson, 2016; Tiberg et al., 2013). Generally, the Kpi values decreased with the increase of Cpi and the decrease of pH and showed a highly nonlinear adsorption behavior for both Pb(II) and Cu(II) (Fig. 1), which was specifically considered in the kinetics model. For the Pb(II) binding sites with stronger binding, Kpi values were larger. The major surface complexation reactions used in CD-MUSIC are summarized in Table 1. Three types of bidentate complexes form between Pb and ferrihydrite with weak, medium, and strong strength, denoted as Fh-bi-weak, Fh-bi-medium, and Fh-bi-strong, respectively. The three types of sites consist of 99, 0.9, and 0.1% of total binding sites, with varying metal binding constants (Table 1). Cu(II) only forms the same type of bidentate complexes with ferrihydrite, denoted as Fh-bi-weak (Table 1). An implicit finite difference numerical method was used to solve Eq. [1] and [2]. The Kpi value at each time step was calculated by CD-MUSIC Model with the corresponding Cpi and pH. For the ferrihydrite concentration m, the exact experimental ferrihydrite concentration was used for the gellike ferrihydrite in the model calculations since most of the binding sites were accessible, and we used the kinetic model parameters published in the previous study (Tian et al., 2017) to conduct the model calculations. For the dense ferrihydrite, based on our preliminary modeling tests, only a portion of the experimental ferrihydrite concentrations [6 and 3% for Pb(II) and Cu(II), respectively] was used since most of the binding sites of the dense ferrihydrite were not accessible under our experimental conditions and short time scales. The kd values of the Fh-bi-weak complex for both Pb(II) and Cu(II) were selected as the model fitting parameters, and kd for the other complexes of Pb(II) were calculated based on the relationship between metal binding constants KM and Table 1. Parameters used in CD-MUSIC for surface complexation reactions for ferrihydrite (Gustafsson, 2015). Log KM 8.1 −0.6 −0.6 3.17 0.97† 9.69 (99%); 12.35 (0.9%); 14.24 (0.1%)‡ † KM of Cu complexed with ferrihydrite bidentate weak site. ‡ KM of Pb complexed with ferrihydrite bidentate weak, medium, and strong sites, respectively. Values in the parenthesis denote the percentages of each binding site. Equation FeOH1/2- + H+ = FeOH21/2+ FeOH1/2- + Na+ = FeOHNa1/2+ Fe3O1/2- + Na+ = Fe3ONa1/2+ FeOH1/2- + Ca2+ = FeOHCa3/2+ 2FeOH1/2- + Cu2+ + H2O = (FeOH)2CuOH + H+ 2FeOH1/2- + Pb2+ = (FeOH)2Pb+
∆
desorption rate coefficients kd (Tian et al., 2017). The Solver program was used to minimize the total squared error (the sum of the squared differences between model calculated and experimental Cion), and the optimized kd was obtained.
Results and discussion
Characterization of Gellike and Dense Ferrihydrite Aggregates X-ray diffraction analysis confirmed that the aggregates were 2-line ferrihydrite (data not shown). The HR TEM images revealed two different types of morphology (Fig. 2). The gellike ferrihydrite had open and loosely aggregated structure while the dense ferrihydrite was more consolidated and thick, which is consistent with the previous observations (Scheinost et al., 2001; Tian et al., 2017). The compact structure of the dense ferrihydrite was maintained even after rehydration in the kinetic experiments since the drying process irreversibly resulted in collapsing of the open structure. The STEM-EDS mappings of the metal-adsorbed gellike ferrihydrite (Fig. 3a) showed that the distribution of Pb and Cu elements correlated well with both the mappings of Fe and O elements and the fine structure of the ferrihydrite aggregates, indicating an even distribution of Pb and Cu on gellike ferrihydrite after 4-h adsorption as observed previously (Tian et al., 2017). However, localized distribution was observed for Pb and Cu on dense ferrihydrite (Fig. 3b). The elemental distribution of Pb and Cu did not correlate well with the structure of dense ferrihydrite aggregates and elemental distribution of Fe and O. Large voids in the STEM-EDS mappings of Pb and Cu on dense ferrihydrite were observed, where the STEMEDS mappings of Fe and O had significant signals. The different heavy metal distributions revealed the distinct ferrihydrite structures and sorption properties between gellike and dense ferrihydrite. The even adsorption of heavy metals on gellike ferrihydrite may be because of the open and loosely aggregated structure, which provided larger surface area, more accessible binding sites, and higher sticking efficiency (Wang et al., 2013, 2016, Wang et al., 2015a). However, freeze-drying resulted in a much denser structure with reduced surface area and less accessible metal binding sites (Dale et al., 2015; Scheinost et al., 2001; Stegemeier et al., 2015). As a result, metal could only adsorb on certain accessible binding sites but not a great number of other hidden binding sites of dense ferrihydrite in the short term. Those easily accessible binding sites may not distribute homogeneously and the hidden binding sites may control metal adsorption at different time scales, which, in combination, may account for the uneven distribution of metals on dense ferrihydrite.
Pb(II) and Cu(II) Adsorption and Desorption Kinetics by Gellike and Dense Ferrihyrite Aggregates The adsorption and desorption kinetics of Pb(II) on both gellike and dense ferrihydrite with varying influent Pb(II) concentrations, pH, and ferrihydrite concentrations are shown in Fig. 4. Generally, as the influent Pb(II) concentrations increased, the amount of Pb(II) adsorbed on both ferrihydrite increased. Soil Science Society of America Journal
Fig. 1. The variations of equilibrium distribution coefficients (Kpi) of Pb(II) and Cu(II) for the binding sites of dense and gellike ferrihydrite predicted by Charge Distribution and Multisite Surface Complexation Model (CD-MUSIC Model) with varying pH and heavy metal concentrations in a specific ferrihydrite bind site (Cpi). The ranges of Cpi values of each binding site are obtained from the kinetics model results under various experimental conditions.
In the desorption stage, the effluent concentrations decreased in the order of the influent Pb(II) concentrations, as expected, but the differences were not dramatic. As pH increased, the Pb(II) adsorption increased, which was because of less competition from H+ at higher pH. pH had relatively small effect on the de-
sorption kinetics since the overall desorption was controlled by both metal desorption and re-adsorption reactions. Overall, ferrihydrite morphology had significant impact on the adsorption kinetics of gellike and dense ferrihydrite. The Pb(II) adsorption on gellike ferrihydrite was faster than that on
Fig. 2. High resolution transmission electron microscopy (TEM) images of (a, b) gellike ferrihydrite aggregates and (c, d) dense ferrihydrite aggregates. www.soils.org/publications/sssaj
∆
Fig. 3. High-angle annular dark field (HAADF) images and scanning transmission electron microscopy–energy dispersive spectrocopy (STEM-EDS) mappings of elemental distribution on the (a) gellike and (b) dense ferrihydrite aggregates after 4-h adsorption.
dense ferrihydrite and, to adsorb equivalent amount of Pb(II), much higher concentrations of dense ferrihydrite were needed under similar experimental conditions. The differences were because the gellike ferrihydrite had larger amount of easily accessible sites than the dense ferrihydrite. Relatively slower Pb(II) desorption was observed for gellike ferrihydrite than dense ferrihydrite under similar solution conditions, which may be because of the stronger re-adsorption reaction of gellike ferrihydrite. Similar to Pb(II), higher influent Cu(II) concentrations and higher pH values increased the adsorption of Cu(II) for both dense and gellike ferrihydrite (Fig. 5). The above-mentioned major differences of Pb(II) adsorption properties between the gellike and dense ferrihydrite applied to Cu(II) as well. For the kinetic experiments with the dense ferrihydrite, the effluent Cu(II) concentrations increased quickly during the adsorption process and decreased quickly during the desorption process (Fig. 5), indicating a much lower adsorption ability of ∆
the dense ferrihydrite. In the desorption process, pH had little effect on the Cu(II) desorption by dense ferrihydrite and the effluent concentrations dropped and reached zero concentration rapidly. The desorption of Cu(II) from the gellike ferrihydrite, however, behaved differently under varying solution conditions as shown in Fig. 5, mainly because of the different re-adsorption of Cu ions at various conditions. Most kinetic studies on metal adsorption on ferrihydrite with a batch method also showed rapid Cu(II) and Pb(II) adsorption kinetics (Liu and Huang, 2003; Mikutta et al., 2012; Rout et al., 2012; Xu et al., 2006) and distinct difference between the gellike and dense ferrihydrite (Scheinost et al., 2001). Similarly, the reaction rates were found to be significantly affected by solution chemistry conditions, which is consistent with what we observed in this study. In most of the batch kinetic experiments, the diffusion-limitation may be significant because of the rapid decrease of solution metal concentrations, which is minimized in Soil Science Society of America Journal
Fig. 4. Lead(II) adsorption and desorption kinetics for the (a) gellike and (b, c) dense ferrihydrite aggregates with varying initial Pb(II) concentrations and pH. The symbols represent the experimental data, and the solid lines are model calculations.
Fig. 5. Copper(II) adsorption and desorption kinetics for the (a) gellike and (b, c) dense ferrihydrite aggregates with varying initial Cu(II) concentrations and pH. The symbols represent the experimental data, and the solid lines are model calculations.
our stirred-flow experiments by continuously injecting the metal stock solutions (Tian et al., 2017). Simple kinetic models, such as a second-order equation, have been proposed to describe the batch reaction kinetics, which generally resulted in different rate constants under various reaction conditions. Note that, during the adsorption kinetic process in the batch reactor, the overall reaction rates are controlled by both the adsorption rates and desorption rates, which usually is not well described by simple kinetic equations. Our stirred-flow kinetic experiments, coupled with the mechanistic-based kinetics model, provide a consistent way to describe metal adsorption–desorption kinetics under various reaction conditions, as explained in the following section.
weak ferrihydrite sites bound the most Pb(II) and the contributions of both medium and strong sites were relatively small when Pb(II) concentrations were high, because of the low abundance of both sites (Gustafsson et al., 2011). When the influent Pb(II) concentrations and pH decreased, the contribution of medium and strong sites to Pb(II) adsorption–desorption kinetics may be significant (Tian et al., 2017). We further compared the difference of adsorption–desorption kinetics of Pb(II) and Cu(II) on both ferrihydrite aggregates using model calculations with the parameters obtained in this study (Fig. 6). Overall, the adsorption rates of both Pb(II) and Cu(II) on gellike ferrihydrite were significantly larger than the dense ferrihydrite, as shown by the effluent kinetic curves (Fig. 6a). As a result, the amount of Pb(II) and Cu(II) adsorbed on the gellike ferrihydrite was much larger than that adsorbed on the dense ferrihydrite (Fig. 6b). Under the same experimental conditions, ferrihydrite adsorbed more Pb(II) than Cu(II), similar to what was observed previously (Scheinost et al., 2001; Tian et al., 2017). This can be explained by stronger binding of Pb(II) on ferrihydrite than Cu(II) as indicated by the much larger log K value of Pb(II) than that of Cu(II) (Table 1) (Tiberg et al., 2013). The variations of adsorption rate coefficients (kai) and desorption rate coefficients (kdi) of ferrihydrite binding sites were shown in Fig. 7 and Table 2, respectively. As pH increased and
Modeling Pb(II) and Cu(II) Adsorption and Desorption Kinetics The kinetics model based on the CD-MUSIC model described the experimental data reasonably well for both Pb(II) and Cu(II) at different reaction conditions (Fig. 4 and 5). The root mean square errors (RMSEs) for the model fits in Fig. 4 and 5 were usually
