Effects of the dissolved organic matter on Cs transport in the weathered granite soil

Journal of Environmental Management 254 (2020) 109785

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Research article

Effects of the dissolved organic matter on Cs transport in the weathered granite soil Takahiro Tatsuno *, Shoichiro Hamamoto, Naoto Nihei, Taku Nishimura Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-8657, Japan

A R T I C L E I N F O

A B S T R A C T

Keywords: Dissolved organic matter Cesium Soil Laboratory column Experiment Fukushima

It is important to understand the migration of Cesium (Cs) in soils, particularly after the nuclear power plant accident at Fukushima Dai-ichi, Japan. Dissolved organic matter (DOM) is one of factors affecting the migration of Cs in soils under flowing water conditions. We investigated the effect of DOM on the migration of Cs adsorbed to the clay planar site via laboratory column experiments. The sequence of DOM application had a significant influence on Cs transport in the soil. When DOM was applied concurrently with or prior to Cs application, the DOM adsorbed on to the clay planar site adsorbed onto the soil solid surface and enhanced Cs adsorption; consequently, it slowed Cs migration in the soil. In particular, in the case of DOM loaded prior to the application of Cs solution, a noticeable delay in Cs migration was observed. On the other hand, when DOM was applied to the soil where the Cs solution had been previously applied, the DOM desorbed Cs from the soil. DOM in liquid phase enhanced the migration of Cs through the formation of binding to organic matter. Majority of Cs affected by DOM was the exchangeable fraction that adsorbed to the clay planar site. In other words, DOM attached to the soil would adsorb Cs as a easily exchangeable form and depress migration of Cs. On the other hand, DOM in the soil solution may up take adsorbed Cs from the soil and enhanced the transport in the form of Cs bound to DOM.

1. Introduction Radionuclides were released to the atmosphere by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident on March 11, 2011, Japan (Chino et al., 2011). Vast areas of Fukushima prefecture, Japan, were contaminated with high concentrations of radionuclides. In particular, Cesium-137 (137Cs) has a long half-life of about 30.1 years and a sustained influence on humans and surrounding environments. It is therefore important to understand Cs dynamics in the environment. Rosen et al. (1999) investigated changes in the vertical distribution of Cs in several soil profiles in Sweden from 1987 to 1995 after the Chernobyl nuclear accident, and found that the annual downward migration differed between the surveyed sites. Similarly, in Fukushima prefecture after the FDNPP accident, mobility of Cs varied depending on the survey site, soil density, clay content and adsorption of organic matter on clay (Koarashi et al., 2012). Takahashi et al. (2015) showed that the distribution of Cs concentration in the surface soil of forests, grass-land, and farm-land from 0 to 10 cm varied for two years after initial contamination.

Cs strongly adsorbs onto soils with 2:1 type clay mineral such as illite and vermiculite (Lee et al., 2017; Zachara et al., 2002). Frayed Edge Site (FES), siloxane ditrigonal cavity, and clay planar site are known as the Cs adsorption sites of clay minerals. Cs electrically adsorbed weakly to the clay planar site and is easily exchanged with other cations (Comans et al., 1991; Comans and Hockley, 1992; Ohkubo et al., 2018). On the other hand, cations with low hydration energy, such as Kþ, NH4þ, and Csþ, cause interlayer dehydration of clay minerals and are fixed in in­ terlayers (Sawhney, 1977). Comans and Hockley, 1992 reported that for a K-saturated illite, clay planar sites with low adsorption selectivity adsorbed 24.8% of the total Cs. Adsorption of Cs onto illite clay took approximately 100 days for Cs to be fixed to FES, while adsorption to the clay planar site is in equilibrium with the solution in several hours (Comans and Hockley, 1992). Nakanishi et al. (2014) investigated changes in Cs concentration and DOC concentration in soil solution during rainfall events by collecting forest soil solutions using lysimeter at Fukushima prefecture, Japan. They showed that Cs was moving in the soil accompanying with water flow during the rainfall event. Consid­ ering the reaction time required to reach equilibrium for Cs adsorption

* Corresponding author. E-mail addresses: [email protected] (T. Tatsuno), [email protected] (S. Hamamoto), [email protected] (N. Nihei), takun@ soil.en.a.u-tokyo.ac.jp (T. Nishimura). https://doi.org/10.1016/j.jenvman.2019.109785 Received 31 July 2019; Received in revised form 21 October 2019; Accepted 25 October 2019 Available online 13 November 2019 0301-4797/© 2019 Elsevier Ltd. All rights reserved.

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Journal of Environmental Management 254 (2020) 109785

at clay planar site and FES, Cs adsorbed to the clay planar site is not negligible under flowing water condition due to relatively shorter Cs-soil contact time. Organic matter is one of the factors affecting Cs migration in soils under flowing water conditions. Most organic matters in organic rich soil are humic substances that are hardly decomposable polymer com­ pounds (Fang et al., 2016). Humic substances have acidic functional groups such as carboxyl groups and phenolic hydroxyl groups. Humic substances are negatively charged at pH 3–11 owing to the dissociation of the hydrogen ion from the functional group, enabling them to adsorb metal cations such as Cs (Celebi et al., 2009). Dissolved organic matter (DOM) comprises of organic matter passed through 0.45 μm opening filter (Tan, 2017). DOM is the most mobile form of soil organic matter (Jansen et al., 2014). Previous field obser­ vation indicated that Cs migration in soil was correlated with the €rr, 1996; Nakanishi et al., 2014). There migration of DOM (Tegen and Do are two possible processes in regard to DOM migration-induced Cs transport. The first is the possibility of the DOM acting as a transfer medium for Cs as in the case of colloid-facilitate transport of contami­ nants (Fig. 1-a) (Zhuang et al., 2003; Flury and Qiu, 2008). In the case of alkaline metals such as Liþ, Naþ, Kþ and Csþ, binding between organic matter and the cations is owing to electrically binding rather than complexation with functional groups of the organic matter (Bonn and Fish, 1993; Lofts et al., 2002; Celebi et al., 2009). Lofts et al. (2002) conducted Cs adsorption experiment with humic acid (HA) and found that the distribution coefficient of Cs with HA was 100–1000 L kg 1 under acidic conditions (pH 3 to 5). In comparison, the distribution coefficient of Cs with the clay planar site of illite, one of the major minerals in the Hamadori area near the FDNPP (Kamei et al., 2003), was around 900 L kg 1 (Dumat and Staunton, 1999). On the other hand, the distribution coefficient of Cs with FES of illite was estimated greater than 105 L kg 1 (Dumat and Staunton, 1999). As mentioned above, while the distribution coefficient of Cs adsorption to FES is significantly different from that to organic matter, the distribution coefficient of Cs adsorption to the clay planar site is relatively close to that to organic matter. Fang et al. (2016) showed that colloids could work as a transfer medium for metal ions when there was no significant difference between the distribution coefficient of metal ions with colloids and that of metal ions with soil. Therefore, in the case of considering Cs adsorption to the clay planar site, it is considered that DOM may act as a transfer medium for Cs in soils when Cs moves under flowing water condition. The second possibility in regard to DOM migration-induced Cs transport is desorp­ tion of Cs adsorbed on the clay planar site by DOM in soil solution. Weng et al. (2002) showed that when HA solution was passed through metal-contaminated soil, HA detached metal cations such as Al3þ and Cu2þ from the soil and took them to a deeper layer. Unlike Cs fixed to

FES, Cs adsorbed to the clay planar site is easily exchanged with other cations (Sawhney, 1977; Fan et al., 2014). These facts suggest that DOM detaches Cs adsorbed on the clay planar site and enhance the migration of Cs in soil (Fig. 1-b). Although the aforementioned literature suggested that DOM in soils enhances migration of Cs adsorbed to the clay planar site, there is an alternative possibility that DOM delays Cs migration due to adsorption of metal-DOM complexes to the soils (Fig. 1-c) (Flury and Qiu, 2008). Furthermore, the effect of DOM adsorbed on soil is also considered. DOM adsorbs to the soil by ligand exchange between DOM and hydroxyl groups on the soil solid surface and cation bridging which multivalent cations act as the bridge between DOM and soil solid surface (Shen, 1999; Bryan et al., 2012). In addition, DOM deposits in soil due to a decrease in negative charge of the particle or aggregation of DOM itself (Yamashita et al., 2013). Soil organic matter has a large cation exchange capacity (CEC) depending on pH and can adsorb cations (Liang et al., 2006). Fan et al. (2012) showed that samples of Beishan soil that had adsorbed either HA or fulvic acid (FA) adsorbed more Cs than soil without humic substances (Fig. 1-d). The effect of the DOM on Cs migration is as follows. a. Cs-DOM form can promote Cs migration as a transport medium. b. DOM can desorb Cs from the soil solid phase and promote migration in the process of DOM movement in soil. c. Cs-DOM form adsorbed on soils. As a result, the Cs-DOM form will slow Cs migration. d. DOM on soil will adsorb Cs and depress migration. As mentioned above, DOM may have some mechanism to alter migration of Cs adsorbed to the clay planar site by adsorption to the soil solid surface or moving with Cs in soil solution. Previous field obser­ vation showed that Cs transport was correlated to the migration and €rr, 1996; Nakanishi et al., adsorption of DOM in soil (Tegen and Do 2014), however, it was not clear which mechanism were actually effective under flowing water condition (Fig. 1). In the field observation, it is difficult to understand only the effect of DOM since various factors €rr, 1996; other than DOM also affect Cs migration (Tegen and Do Nakanishi et al., 2014; Takahashi et al., 2015). Therefore, we tried to evaluate the effect of DOM on Cs migration using laboratory column experiment under condition which DOM application was controlled. Refaey et al. (2017) showed the effect of DOM on migration of metal cations such as Cu2þ, Ni2þ and Zn2þ in soils was depended on whether DOM was attached on solid phase or floated in liquid phase (e.g. adsorbed form onto the solid phase or mobile form in liquid phase as DOM or DOM bonded with metal cations) by using column transport experiments with different sequences of DOM application. Considering the above, there is a possibility to clarify the effect of DOM on migration of Cs adsorbed to the clay planar site by using the column experiments with different sequence of DOM application as follows; 1) the column experiment applied Cs and DOM mixed solution correspond to evalu­ ating the movement of Cs binding to DOM in soil (as shown Fig. 1-a and 1-c), 2) the column experiment applied DOM prior Cs application correspond to evaluating the effect of DOM adsorbed on the soil solid phase (as shown in Fig. 1-b), and 3) the column experiment to which DOM is applied after Cs application is corresponded to evaluating the effect of DOM desorbing Cs adsorbed on clay planar site. Furthermore, the concentrations of dissolved organic carbon (DOC) in the DOM solution applied in previous laboratory experiments were higher than 100 mg-C L 1 (Weng et al., 2002; Fan et al., 2012), while Nakanishi et al. (2014) showed that the DOC concentration of soil so­ lution collected from a Fukushima prefecture forest ranged from 5 to 30 mg L 1. To evaluate the effect of DOM in the actual soil, it is neces­ sary to conduct experiments applying a DOM solution in which the DOC concentration is similar to that of an actual soil solution. The purpose of this study is to evaluate the effects of DOM on the migration of Cs adsorbed to the clay planar site in Fukushima soil using

Fig. 1. Conceptual model of Cs transport by DOM. 2

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Journal of Environmental Management 254 (2020) 109785

laboratory column experiments. The column transport experiments were conducted under different sequences of DOM application; DOM was applied concurrently with Cs loading in order to clarify the migra­ tion of Cs bounded to DOM (as shown Fig. 1-a and 1-c), after Cs loading in order to evaluate desorption of Cs adsorbed on the clay planar site by DOM in liquid phase (as shown in Fig. 1-b) or before Cs loading in order to evaluate the effect of DOM attached to the soil on Cs migration (as shown in Fig. 1-d).

Table 2 Characteristics of dissolved organic matter solution. Al (mg L-1) 0.07

2. Materials and methods

0.62

Cu (mg L-1)

Fe (mg L-1)

0.00

K (mg L-1)

0.00

0.35

Mg (mg L-1)

Mn (mg L-1)

0.00

0.00

Na (mg L-1)

133

Cs (mg L-1)

NO3 (mg L1 )

Cl (mg L-1)

SO4 (mg L1 )

pH

EC (μS cm-1)

DOC (mg-C L1 )

20.94

0.05

0.20

37.22

0.65

6.00

140

20.00

EC - Electrical conductivity at 25 � C; DOC – Concentration of dissolved organic carbon. Each element was measured using ICP mass spectrometer (NexION 350S, Perkin Elmer), ICP Optical Emission spectrometer (Optima 7300DV, Perkin Elmer) or Ion chromatography (LC-20AD, Shimadzu, Japan).

2.1. Materials 2.1.1. Soil Weathered granite called masa-soil in the local dialect, was collected from subsoil at an abandoned forest in Iitate village, Fukushima pre­ fecture, Japan. The soil sample was air-dried and sieved (<2 mm) prior to use. Soil properties are shown in Table 1. The soil texture was a sandy - clay. Soil pHH2O was measured using 1:2.5 ratio. The CEC was measured following Hendershot and Duquette (1986). The distribution coefficient of Cs on the soil was measured by the following adsorption experiment. First, 1 g of dry soil and 50 mL of Cs solution was mixed in a centrifuge tube and shaken for 1 h. The concentration in the applied Cs solution was 0.15 mmol L 1, which was prepared by diluting a stable CsCl (Kanto Chemical Co., Inc., Japan). Then, the ionic strength and pH of the solution were adjusted to 1 mM and 6.0, respectively, using NaBr and NaOH. After the shaking, the solid and liquid phases were separated by centrifugation at 10,000 rpm for 30 min. Temperature was held constant at 25 � C. Adsorption experiment was conducted three repli­ cates. The distribution coefficient, Kd, was defined as follows (Eq. (1)),

2.1.3. Cs solutions for column transport experiments A Cs solution with 0.15 mmol L 1 was prepared using a stable CsCl (Kanto Chemical Co., Inc., Japan). Cs concentration of the solution was equivalent to 6.4 � 1010 Bq L 1 when converted to the radioactive concentration of 137Cs, which was much higher than that collected from €rr, 1996; Nakanishi et al., 2014). Fan et al. actual field. (Tegen and Do (2014) showed that the greater Cs concentration in the soil solution, the greater the contribution of Cs adsorption to the clay planar site in total Cs adsorbed to the soil. Furthermore, in previous study focused on Cs adsorption to the clay planar site, Cs solution with relatively high con­ centration which was from 1 � 10 6 mol L 1 to 1 � 10 3 mol L 1 was used (Dumat and Staunton, 1999; Fan et al., 2014). Therefore, in order to evaluate Cs adsorption to the clay planar site, we used a Cs solution with relatively high concentration. The Cs-DOM mixed solution (hereinafter referred to as Cs and DOM mixture) was prepared by mixing Cs and the DOM solution for 72 h at a constant temperature (25 � C), resulting in Cs with 0.15 mmol L 1 and DOC with 20 mg-C L 1. The ionic strength and pH of the Cs and DOM mixture was adjusted to 1 mM and 6.0 using NaBr and NaOH, respectively.

(1)

Kd ¼ S=C

Ca (mg L-1)

where S is the amount of Cs adsorbed on unit mass of the solid phase (mol kg 1) and C (mol L 1) is the Cs concentration in the liquid phase. The Kd was 92.35, which was not significantly different in the distri­ bution coefficient of Cs adsorbed on the clay planar site of illite (Dumat and Staunton, 1999). The soil organic matter content was measured using a C/N analyzer (Elementar Vario EL, Elementar, Germany).

2.2. Methods 2.2.1. Zeta potential of soil sample As the DOC concentration in the DOM solution in this study was much lower than those used for adsorption experiments in previous studies (Weng et al., 2002; Fan et al., 2012), it is difficult to evaluate the change in cation adsorption capacity of the soil before and after DOM adsorption. Therefore, effects of DOM adsorption on surface potential of the soil were studied using a streaming potential method. Zeta potential is a potential near the solid surface and is an indicator of electrical interaction (Shiratori et al., 2008; Pham et al., 2013). Zeta potential of the soil with and without DOM adsorption treatment were measured using the streaming potential method. Zeta potential was calculated using Helmholtz-Smoluchowski’s equation (Eq. (2)) (Delgado et al., 2007):

2.1.2. Dissolved organic matter (DOM) solution DOM solution was extracted from a litter collected from the forest floor at university forest of the University of Tokyo in Chichibu city, Saitama prefecture, Japan, in 2008. First, the litter sample was mixed with pure water for 24 h with a soil to water mass ratio of 1:4. After shaking, the supernatant solution was collected by centrifugation at 4000 rpm for 30 min (50A-IVD, Sakuma., Corp., Japan). The superna­ tant solution was filtered using a 0.45 μm membrane filter (A045A142C, ADVANTEC) and the filtrate solution was centrifuged at 6000 rpm for 30 min（Microsep Advance Centrifugal Device 1 K MWCO，Pall Co.）. The remaining solution in the reservoir was used as the DOM stock solution. After measuring the concentration of dissolved organic carbon (DOC) using a TOC analyzer (TOC-V CPH/CPN, Shimadzu, Japan), the stock solution was diluted to prepare the DOM solution with a DOC concentration of 20 mg-C L 1. The ionic strength and pH of the solution were adjusted to 1 mM, and 6.0 using NaCl and NaOH, respectively. Table 2 shows selected composition of DOM applied to the experiments.

ζ¼

Ψ str ηKL ΔP εε0

(2)

where ζ is the zeta potential (mV), Ψ str is the different potential (mV), ΔP is the pressure difference (mbar), η is the solution viscosity (mPa⋅s), KL is the solution conductivity (mS/cm), ε is the relative dielectric constant of

Table 1 Selected properties of the weathered granite soil used in this study. Particle site distribution (%) Clay (<2 μm)

Silt (2–20 μm)

Sand (20–200 μm)

26.39

20.72

52.89

Soil texture

pH

CEC (cmol kg 1)

Sandy-clay

5.0

11.08

CEC - Cation exchange capacity; Kd - Distribution coefficient; SOM - Soil organic matter. 3

Zeta potential (mV) 8.63

Kd (L kg 1)

SOM (%)

92.35

0.92

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the liquid and ε0 is the electric permittivity of a vacuum (8.854 � 10 12 F/m). Zeta CAD (CAD Instruments) was used to measure zeta potential of the soil sample. A 5 cm long glass column with inner diameter of 1.5 cm was repacked by the soil sample at the dry bulk density of 1.30 g cm 3. After packing the soil sample, the column was saturated with 1 mM NaCl solution (pH6.0 adjusted by NaOH solution). NaCl solution was applied at constant flux of 0.15 cm min 1 for 200 pore volumes (PVs). One pore volume was calculated by subtracting the volume of the solid phase from the column volume, which was 1.22 cm3 in this column. Then zeta potential of the soil samples was obtained by measuring electrical potential difference (Ψ str) in the column as well as other parameters shown in Eq. (1) under variable pressure differences (Table 1). After the measurements, DOM solution with a DOC concen­ tration of 20 mg-C L 1 or 50 mg-C L 1 was applied for 350 PVs and NaCl solution was applied for 200 PVs. After applying those solutions, the zeta potentials of the samples adsorbed with DOM were measured again. Measuring the zeta potential was conducted three replicates at constant temperature (25 � C).

applied again for 30 PVs to flush excess DOM solution from the column. During the Cs loading phase, Cs solution was applied for 350 PVs. Then, 350 PVs of 1 mM NaCl solution were applied during the Cs flushing phase. 0.57 mmol Cs was applied to each column experiment, and 75.60 mg-C DOM was applied to each column experiment except for the column without DOM. During the column experiments, effluents from the bottom of each column were collected at 10 PVs intervals using a fraction collector (CHF121SA, ADVANTEC). DOC concentrations of effluents were measured using a TOC analyzer (TOC-V CPH/CPN, Shimadzu). The Cs concentration was measured with either atomic absorbance spectros­ copy (AAS6200, Shimadzu) or ICP-MS (NexION 350, PerkinElmer) depending on the Cs concentration of the samples. In addition to the measurement of Cs concentration of the effluents, the water-soluble Cs and the Cs bound to DOM (herein after referred to as Cs-DOM form) were fractionated using a centrifuge ultrafiltration device (Microsep Advance Centrifugal Device 1 K MWCO，Pall Co.) (Kautenburger and Beck, 2007). The existence of Cs-DOM form in the effluents would suggest Cs transport by DOM carriers. The target effluent samples used for ultrafiltration were sampled during the Cs loading phase (0–350 PVs) and the Cs flushing phase (350–700 PVs) was as follows; 180, 350, 360, 530, and 700 PVs for the column with the Cs and DOM mixture, 360, 530, and 700 PVs for the column with DaCs, and 180, 350, 360, 530, and 700 PVs for the column with DpCs. First, the target solution samples were placed in the device reservoir. After ultrafiltration of the sample at 6000 rpm for 30 min, water-soluble Cs was filtered and set into the receiver. The amount of Cs-DOM form was measured by the difference between the amount of Cs initially charged in the reservoir and the amount of water-soluble Cs that was filtered and entered into the reservoir.

2.2.2. Column transport experiment Air dried soil was packed uniformly into an acrylic plastic cylinder, 5 cm long with an inner diameter of 3 cm, to form a soil column 3 cm high with dry bulk density of 1.3 g cm 3. After packing the soil sample, the column was saturated from the bottom by 1 mM NaCl solution, adjusted to pH6.0 by NaOH solution, and the saturated condition was maintained overnight. For the breakthrough experiments, the flow direction was changed downward by maintaining a constant ponding depth of 1 cm on the top and constant flux was adjusted to 0.15 cm min 1. Four types of experiments were conducted with different sequences of DOM addition (Table 3). Each column experiment was conducted three replicates at constant temperature (25 � C). The first was a column experiment applied without DOM, denoted as the column without DOM. The second column experiment, hereinafter called the column with Cs and DOM mixture, applied the Cs and DOM mixture. The third experi­ ment applied DOM solution after the Cs solution, and is indicated as the column with DOM after Cs (DaCs). The fourth experiment applied DOM prior to Cs solution, and is referred to as the column with DOM prior to Cs (DpCs). Applied solutions were successively changed over three phases. First, in the pre-loading phase, the equilibrium condition was prepared prior to the breakthrough experiment. Next, in the Cs loading phase, Cs was applied to the column soil. Finally, in the Cs flushing phase, adsorbed Cs was desorbed from the column soil. During the preloading phase, 1 mM NaCl solution with pH6.0 was applied for 30 PVs to each column. In the column transport experiments, one PV was 10.79 cm3. For the column without DOM and the column with Cs and DOM mixture, 350 PVs of Cs solution or Cs and DOM mixture were applied during the Cs loading phase, then 1 mM NaCl was applied during the Cs flushing phase. The column with DaCs applied the Cs solution during the Cs loading phase and the DOM solution for the Cs flushing phase. For the column with DpCs, 350 PVs of the DOM solution were applied after the pre-loading phase. Then, 1 mM NaCl solution was

2.2.3. Sequential extraction After the column experiment, soil in the column was sliced into 1 cm sections and absorbed Cs in each section was analyzed using the sequential extraction procedure described by Fan et al. (2014), which is shown in Table 4. Here, five Cs fractions were extracted in the following order: easily exchangeable fraction (F0), less exchangeable fraction (F1), carbonate fraction (F2), easily reducible fraction (F3), and organic fraction (F4). 2.2.4. Statistical treatment of results In our study, triplicate experiments were conducted for all condi­ tions. Statistical treatment based on t-test was performed to determine the difference between the averages of the results obtained under different experimental conditions. The probability value (p-value) of less than 0.05 was considered statistically significant. Statistical treatments Table 4 Summary of sequential extraction for Cs adsorbed on soil samples (Fan et al., 2014).

Table 3 Applied solution in the column experiments. columns

I. Pre-loading phase

II. Cs loading phase

III. Cs flushing phase

without DOM

NaCl (30 PVs)

Cs (350 PVs)

with the Cs and DOM mixture with DaCs

NaCl (30 PVs)

Cs-DOM (350 PVs) Cs (350 PVs)

with DpCs

NaCl (30 PVs) →DOM (350 PVs) →NaCl (30 PVs)

NaCl (350 PVs) NaCl (350 PVs) DOM (350 PVs) NaCl (350 PVs)

NaCl (30 PVs)

Cs (350 PVs)

Fraction

Extractant

Treatment

Time (h)

Temperature (oC)

F0

1 mol/L NaCl, pH7.0

24

25 � 2

F1

1 mol/L NH4Cl pH 7.0

24

25 � 2

F2

0.11 mol/L CH3COOH

24

25 � 2

F3

0.5 mol/L NH2OH⋅HCl (0.05 mol/L HNO3) (a) 8.8 mol/L H2O2 (pH 2.0 with HNO3) (b) 1.0 mol/L CH3COONH4 (pH 2.0 with HNO3) Residual phase

end-over-end shaking end-over-end shaking end-over-end shaking end-over-end shaking digestion

24

25 � 2

8

85 � 2

24

25 � 2

F4

F5

The pore volumes in parentheses indicates the amount applied to the soil.

end-over-end shaking

The weight ratio of dry soil to extractant solution was 1:40. 4

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were conducted as follows; comparing 1) DOM discharge from the col­ umns during application of solution including DOM, 2) Cs discharge from the column without DOM with that from the columns which DOM was applied at different sequences, 3) in the case of the column with Cs and DOM mixture, the ratio of Cs-DOM form to the total Cs in the applied solution with that in effluent, and 4) Cs adsorption for each fraction between the column without DOM and the columns which DOM was applied at different sequences.

3.3. Changes in Cs concentration of effluents during Cs loading and flushing Fig. 3 shows changes in Cs concentration of the effluents during the Cs loading phase (0–350 PVs) and the Cs flushing phase (350–700 PVs). During the Cs loading phase, the maximum Cs concentrations in effluent from the column with the Cs and DOM mixture and DpCs were lower than that of the other two conditions. Cs concentration in the effluent from the column with DpCs increased at 190 PVs, while the column without DOM and with the Cs and DOM mixture showed increased Cs concentration at 80 PVs. During the Cs flushing phase, there was no significant difference in Cs concentration among the effluents from the column without DOM, with the Cs and DOM mixture and DpCs. Though, the Cs concentration in effluents of the column with DaCs was higher than that of the other three experiments during the early in the Cs flushing phase, 350–560 PVs. There was no significant difference among Cs concentrations of the ef­ fluents from of all the column experiments after 560 PVs. The Cs loading phase was sampled between 0 and 350 PVs. The Cs flushing phase was sampled between 350 and 700 PVs. Table 6 shows the ratio of Cs-DOM forms to the effluents collected in each column experiment. Notably, the Cs-DOM form comprised 42.8% of total Cs in the applied Cs and DOM mixture. In the column experiment with Cs and DOM mixture, the ratio of the Cs-DOM forms to the total Cs concentration of the effluent solution was 34.8–44.3%. During the Cs flushing phase of the column with DaCs, the ratio of the Cs-DOM forms was 16.3–26.5%. The Cs-DOM form was not detected in the effluents of the column with DpCs. Fig. 4 shows Cs concentration of the Cs-DOM form and water-soluble Cs in applied solution and effluents collected from the column without DOM, the column with the Cs and DOM mixture, and the column with DaCs. In the column with the Cs and DOM mixture, Cs-DOM form was detected in the applied solution and all effluents. In the column with DaCs, Cs concentrations in effluents collected at 360 PVs and 530 PVs were higher than those from the column without DOM. However, there were no significant differences in the concentrations of water-soluble Cs in each effluent among two conditions. The closed objects indicate Cs concentration of water-soluble Cs. The hatched objects indicate Cs concentration of Cs-DOM form. The vertical error bar indicates the standard error of Cs concentra­ tion of Cs-DOM form or water-soluble Cs.

3. Results 3.1. DOMs adsorption and discharge Amounts of discharged and adsorbed DOM after the column exper­ iments are shown in Table 5. Total amount of DOM applied to the col­ umn was 75.60 mg-C for the column with Cs and DOM mixture, DpCs and DaCs. When either the Cs and DOM mixture or the DOM solution was applied, DOM discharged from the column of the Cs and DOM mixture, DaCs, and DpCs were 40.03, 50.99 and 48.28 mg-C, respec­ tively. Adsorbed DOM in the column with applied Cs and DOM mixture was 35.57 mg-C, greater than that of the column with DaCs (24.61 mgC) and DpCs (27.32 mg-C). Adsorption of DOM to an unit mass of soil was calculated by dividing the adsorption of DOM by the mass of dry soil in the column (27.55 g), which was 1.29 mg-C g 1, 0.89 mg-C g 1, and 0.99 mg-C g 1 in the column with Cs and DOM mixture, column with DaCs, and column with DpCs, respectively. In the column with DpCs, 36.14% of the applied DOM was adsorbed before the Cs loading phase. However, during the Cs loading phase, discharge of DOC from the column with DpCs was 4.25 mg-C, which was approximately equal to the discharge of DOC from the column without DOM (4.27 mg-C). It was therefore considered difficult to desorb DOM adsorbed to the soil with subsequent water flow. 3.2. Zeta potential of the soil with and without DOM treatment Fig. 2 shows the zeta potential of soil with and without DOM treat­ ment. When DOM solution with a DOC concentration of 20 mg-C L 1 and 50 mg-C L 1 was applied, the amounts of DOM adsorption were 1.12 mg-C g-soil 1 and 1.50 mg-C g-soil 1, respectively. Zeta potential was 9.68 mV when the DOM adsorption was 1.12 mg-C g-soil 1, while it was 10.02 mV when adsorption was 1.50 mg-C g-soil 1. As DOM adsorption to soil increased, the magnitude of zeta potential increased. The horizontal error bar indicates the standard error of the adsorp­ tion of DOM per unit mass of soil, and the vertical error bar indicates the standard error of Zeta Potential. Measuring zeta potential was con­ ducted three replicates.

3.4. Ratio of discharged and adsorbed Cs to applied Cs Table 7 shows the fractions of different forms of adsorbed Cs after the column experiments with and without DOM. In Table 7, “Total”

Table 5 DOM discharge from columns and adsorption on the soil. DOM discharge (mg-C)

Column

without DOM With Cs and DOM mixture with DaCs with DpCs

DOM adsorption on soil

Phase

preloading

Cs loading

Cs flushing

Total

Averaged (error) e average (error) average (error) average (error)

0.33 (�0.02) 0.32 (�0.01) 0.42 (�0.09) 48.28 (�0.88)

4.27 (�0.42) 40.03 (�1.92) 4.65 (�0.92) 4.25 (�0.53)

3.01 (�0.33) 7.63 (�0.70) 50.99 (�2.95) 3.59 (�1.20)

7.61 (�0.71) 47.99 (�1.24) 56.06 (�3.23) 54.39 (�0.86)

adsorption of DOMa (mg-C)

adsorption rateb (%)

adsorption of DOM to an unit mass of soilc (mg-C g 1)

35.57 (�1.92) 24.61 (�2.95) 27.32 (�0.88)

47.05 (�2.54) 32.55 (�3.90) 36.14 (�1.59)

1.29 (�0.07) 0.89 (�0.11) 0.99 (�0.03)

a The adsorption of DOM was the amount of adsorption while DOM solution or Cs and DOM mixture was applied, which was calculated by subtracting the discharge of DOM during the phase with DOM application from the application of total DOM (75.60 mg-C). b The adsorption rate was calculated by dividing the adsorption of DOM by application of DOM. c The adsorption of DOM to an unit mass of soil was calculated by dividing the adsorption of DOM by the mass of dry soil in the column (27.55 g). d “average” indicates the average values calculated from experiments conducted three replicates. e “error” indicates the standard error calculated from experiments conducted three replicates.

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Journal of Environmental Management 254 (2020) 109785

Fig. 2. Zeta potentials of the soil sample before and after DOM adsorption.

Fig. 3. Breakthrough curves of Cs concentration during Cs loading and flushing phases. Cs ：Cs concentration of effluents (mmol/L). Table 6 Ratios of Cs-DOM form to total Cs of effluents in columns with the Cs and DOM mixture, DaCs, and DpCs. Ratio of Cs-DOM form to the total Cs of effluents (%) Cs loading phase Column

With Cs and DOM mixture with DaCs with DpCs

a b

Averagea Errorb average error average error

Cs flushing phase

180 PVs

350 PVs

360 PVs

530 PVs

700 PVs

41.91 (�1.19)

44.32 (�4.67)

0.00 (�0.00)

0.00 (�0.00)

36.27 (�9.70) 26.51 (�3.06) 0.00 (�0.00)

34.75 (�2.29) 25.80 (�7.28) 0.00 (�0.00)

40.44 (�7.30) 16.33 (�8.70) 0.00 (�0.00)

“average” indicates the average ratio of Cs-DOM form to the total Cs of effluent calculated from experiments conducted three replicates. “error” indicates the standard error calculated from experiments conducted three replicates.

indicates the sum of adsorption and “Unknown” indicates the missing fraction, which represents the difference between the sum of applied Cs and the sum of discharged and adsorbed Cs. During the Cs loading and flushing phase, Cs discharge from the columns without DOM, with the Cs and DOM mixture, DaCs and DpCs were 0.29, 0.26, 0.34 and 0.24 mmol, respectively. The proportions of Cs discharge from the columns without

DOM, with the Cs and DOM mixture, and with DaCs and DpCs were 50.9, 45.6, 59.6 and 42.1%, respectively. Most of the adsorbed Cs was an easily exchangeable fraction (F0). The sum of discharged and adsorbed Cs was 0.50 mg or more, and 87.7–92.9% of applied Cs was recovered as a discharged or adsorbed fraction. 6

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Journal of Environmental Management 254 (2020) 109785

Fig. 4. Concentration of Cs-DOM form and water-soluble Cs in (a) applied solution and. (b) effluents from column without DOM, with Cs and DOM mixture, and DaCs. Table 7 Discharge and adsorption of Cs during column experiments with and without DOM. Cs discharge (mmol)

Column

Without DOM with Cs and DOM mixture With DaCs With DpCs

a b

Averagea (error) b average (error) average (error) average (error)

Cs adsorption on soil (mmol)

Loading phase

Flushing phase

total

F0

F1

F2

F3

F4

total

0.14 (�0.01) 0.11 (�0.01) 0.14 (�0.02) 0.09 (�0.01)

0.16 (�0.00) 0.16 (�0.00) 0.20 (�0.02) 0.15 (�0.01)

0.29 (�0.02) 0.26 (�0.02) 0.34 (�0.02) 0.24 (�0.01)

0.20 (�0.00) 0.23 (�0.02) 0.14 (�0.00) 0.23 (�0.03)

0.03 (�0.00) 0.03 (�0.00) 0.02 (�0.00) 0.03 (�0.00)

0.01 (�0.00) 0.01 (�0.00) 0.01 (�0.00) 0.00 (�0.00)

0.00 (�0.00) 0.00 (�0.00) 0.00 (�0.00) 0.00 (�0.00)

0.00 (�0.00) 0.00 (�0.00) 0.00 (�0.00) 0.00 (�0.00)

0.23 (�0.01) 0.27 (�0.03) 0.17 (�0.00) 0.26 (�0.03)

Unknown (mmol) 0.04 (�0.01) 0.04 (�0.01) 0.06 (�0.01) 0.07 (�0.02)

“average” indicates the average amount of Cs discharge or adsorption calculated from experiments conducted three replicates. “error” indicates the standard error calculated from experiments conducted three replicates.

Cs discharge indicates the amount of Cs discharge from the column during Cs loading and flushing phase. Cs adsorption on soil indicates the Cs amount extracted from the column soil after Cs flushing phase. Each extracted fraction was as fol­ lows; F0- easily exchangeable fraction; F1- less exchangeable fraction; F2carbonate fraction; F3- easily reducible fraction; F4- organic fraction; total - total Cs adsorption of F0, F1, F2, F3 and F4; Unknown - missing fraction, which represents the difference between the sum of applied Cs and the sum of discharged and adsorbed Cs.

a decrease in negative charges of DOM or aggregation of itself (Shen, 1999; Bryan et al., 2012; Yamashita et al., 2013). The DOM solution contained almost no multivalent ions except Ca, which is only 0.62 mg L 1 (Table 2). The cations contained in other solutions such as Cs solution and Na solution were only monovalent. Therefore, there was less possibility that DOM adsorbed to the soil due to cation bridging with multivalent cations. When DOM was applied to the soil, the solution applied to the col­ umn with Cs and DOM mixture contained Cs, unlike the DOM solution applied to the columns with DaCs and DpCs. Although it was considered that Cs in the mixture might enhance DOM adsorption or deposition in the soil, the mechanism of DOM staying in the soil could not be clarified in this study. In order to understand mechanism which DOM remained in the soil, it is necessary to research in more detail, for example, to analyze DOM discharge using a model based on the deposition kinetics (Yamashita et al., 2013) and to conduct column experience under con­ ditions applying solutions with different pH and ionic strength (Shen, 1999).

4. Discussion 4.1. Mechanism of DOM staying in the soil In this study, three types of column experiments with different sequence of DOM application were conducted. There were no significant differences in DOM discharge during application of DOM solution be­ tween the column with DaCs and the column with DpCs (p < 0.05), on the other hand, DOM discharge from the column with Cs and DOM mixture was less than that from other two conditions during application of DOM solution (p > 0.05) (Table 5). These results suggested that the mechanism of DOM staying in the soil was different between the column with Cs and DOM mixture and the other two columns with DOM application. It was considered that DOM was remained in the soil by adsorption by ligand exchange or cation bridging, and deposition due to

4.2. Migration of the Cs-DOM form in the soil (Fig. 1-a and 1-c) Table 6 shows that Cs-DOM forms were part of the Cs discharged from the column with the Cs and DOM mixture. During the Cs loading phase, there were no significant difference in the ratios of the concen­ tration of Cs-DOM forms to the total Cs concentration among effluent 7

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Journal of Environmental Management 254 (2020) 109785

collected at 180 PVs, that at 350 PVs and applied Cs and DOM mixture (p > 0.05)(Table 6). This indicated that neither promotion nor inhibition of Cs migration occurred as a result of forming Cs-DOM forms when the Cs and DOM mixture was applied to the soil (Fig. 1-a and 1-c). During the Cs loading phase, discharge of Cs from the column with the Cs and DOM mixture was less than that from the column without DOM (Table 7). This result suggests that DOM in the Cs and DOM mixture depressed Cs migration. DOM adsorbed to the soil increased the adsorption of metal cations to soil because of dissociation of functional groups such as phenol hydroxyl and carboxyl groups (Tan et al., 2008; Fan et al., 2012; Refaey et al., 2014). In the column with Cs and DOM mixture, half of the applied DOM was adsorbed by the soil (Table 5) at the end of the Cs loading phase. It was considered that as the Cs and DOM mixture was applied to the column soil, the adsorption of DOM to the soil gradually increased; consequently, DOM on the soil would enhance Cs adsorption and depress Cs migration. On the other hand, Cs adsorption as an easily exchangeable fraction (F0) of the soil in the column with the Cs and DOM mixture was greater than that in the column without DOM (Table 7). The difference in the F0 fraction between the two columns was 0.3 mmol, which was almost similar to the difference in Cs discharge between the two columns. These results suggest that there was a possibility that DOMs adsorbed to the soil increased easily exchangeable Cs. Consequently, DOM adsorbed to the soil may depress Cs migration.

was similar to that from the column without DOM, while Cs concen­ tration of the effluent from the column with DpCs was lower than that from the other conditions (Fig. 3). At the end of pre-loading phase, adsorption of DOM in the column with DpCs was 27.32 mg-C (Table 5), while DOM was not applied in the column with the Cs and DOM mixture. Soon after switching from the pre-loading phase to the Cs loading phase, adsorption of DOM to soil in the column with DpCs was greater than that of the column with the Cs and DOM mixture. Fig. 2 shows that the greater the adsorption of DOMs, the more negative the zeta potential of the soil. Because the DOM-adsorbed soil had more negative charge than the soil without DOM, the DOM-adsorbed to the soil could enhance the adsorption of the monovalent Cs cation (Csþ). These results suggested that, compared to the column with the Cs and DOM mixture, Cs migration retarded in the case of the column with DpCs since the soil had more DOM before Cs application. 5. Conclusions This study showed that DOM influenced the migration of Cs adsorbed to the clay planar site in weathered granite soil. When DOM was applied to the soil concurrently with Cs application, Cs-DOM forms in the so­ lution didn’t enhance nor depress Cs migration, neither. Early in applying DOM, there was no significant difference in Cs migration compared to the case of Cs application without DOM. However, as DOM applied to the soil and the amount of the DOM adsorption increased, DOM attached to the soil enhanced Cs adsorption and depressed the Cs transport. When DOM was applied to the soil prior to Cs application, as with the case of DOM application concurrently with Cs solution, DOM adsorbed on the soil trapped Cs and depressed the Cs transport. Furthermore, in that case, Cs concentrations of the effluent took more time to rise than those DOM was applied concurrently with Cs. This was because more DOM was adsorbed to the column soil before Cs appli­ cation than when the column soil applied DOM simultaneously with Cs. However, when DOM was applied to the soil where Cs had been adsorbed, DOM in the soil solution took up the adsorbed Cs and enhanced the Cs transport in the form of the Cs bound to DOM. Whether DOM is on the soil or in the soil solution, majority of the Cs affected by DOM was the easily exchangeable fraction which was corresponded to Cs adsorbed to the clay planar site (Comans et al., 1991; Fan et al., 2014). This suggests that DOM attached to the soil would enhance the adsorption of Cs as easily exchangeable form and depress Cs migration. On the contrary, DOM in the soil solution would up take Cs weakly adsorbed to the clay planar site and enhance its transport in the form of Cs-DOM forms.

4.3. Cause of desorption of adsorbed Cs by DOM (Fig. 1-b) Samples collected at 360 PVs and 530 PVs showed higher Cs con­ centrations of the effluent from the column with DaCs than from the column without DOM (Fig. 4). On the other hand, there were no sig­ nificant differences in the concentrations of water-soluble Cs in effluents collected at 360 PVs and 530 PVs between the two column conditions (p > 0.05) (Fig. 4). These results indicate that DOM in the applied so­ lution may enhance Cs migration by taking up Cs adsorbed on the soil (Fig. 1-b). Table 7 showed that Cs discharge from the column with DaCs during the Cs flushing phase was greater than that from the column without DOM. On the other hand, adsorption of the easily exchangeable fraction (F0) in the column without DOM was greater than in the column with DaCs, and there was no significant different in adsorption of the other Cs fractions (F1, F2, F3 and F4) (p > 0.05). These results indicated that DOM in the applied solution could act as a flushing agent for easily exchangeable Cs adsorbed to the soil. 4.4. Delay of Cs migration by DOM adsorbed to the soil (Fig. 1-d)

Acknowledgments

Table 7 shows that discharge of Cs from the column with DpCs was less than it was from the column without DOM during the Cs loading phase, and the difference in discharge of Cs between them was 0.05 mmol. DOM adsorbed to the soil could retard Cs migration. On the other hand, the adsorption of easily exchangeable fraction (F0) of the column with DpCs was 1.2 times greater than that of the column without DOM (Table 7). The difference in adsorption of easily exchangeable fraction between them was 0.03 mmol, that was approximately 60% of the difference in discharged Cs between the two columns during the Cs loading phase. However, there was no significant difference in the adsorption of the other Cs fractions (F1, F2, F3, and F4 in Table 7) be­ tween the two columns. These results suggest that, in the column with DpCs, DOM on the soil enhanced Cs adsorption in the form of easily exchangeable fraction. Consequently, DOM on the soil depressed Cs migration. The effect of DOM adsorbed on the soil was the same as the column with the Cs and DOM mixture. On the other hand, there was the difference in Cs concentrations of the effluent between the column with DpCs and the column with the Cs and DOM mixture. Early in the Cs loading phase, 0 to 240 PVs, Cs con­ centration of the effluent from the column with Cs and DOM mixture

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