# Adsorption and fixation of radiocesium compared to properties, landuse and sampling location of contrasting soils from Taiwan

Staunton Siobhan, Chih-Yu Chiu

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Staunton Siobhan, Chih-Yu Chiu. Adsorption and fixation of radiocesium compared to properties, landuse and sampling location of contrasting soils from Taiwan. Journal of Environmental Radioactivity, 2025, 287, pp.107727. 10.1016/j.jenvrad.2025.107727. hal-05199610

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# Adsorption and fixation of radiocesium compared to properties, landuse and sampling location of contrasting soils from Taiwan

Staunton Siobhan<sup>a, *, ID</sup>, Chih-Yu Chiu<sup>b</sup>

## ABSTRACT

Adsorption and desorption of radiocesium has been measured on 57 soils from different climates, landuses and altitudes in Taiwan, and compared to soil properties. A large range in radiocesium adsorption (assessed by the distribution coefficient, Kd) was found. Largest values of Kd were observed for arable soils. These were sampled near sea-level, tended to have neutral to alkaline pH, and to be depleted in water-extractable potassium and organic matter. Kd values of soil from undisturbed and secondary forests fell in a smaller range, and were on average smaller than those of arable soils, and decreased with increasing altitude. Kd was not significantly related to clay content. A fraction of adsorbed radiocesium was not readily desorbed in a solution containing stable cesium. This fixed fraction, denoted $f_{fixed}$, tended to increase with increasing Kd, and was greater in arable soils. The $f_{fixed}$ tended to decrease with increasing altitude for soils from undisturbed forests. Fixation increased with pH and was inversely related to carbon, extractable potassium and clay contents. We conclude that content of clay-sized fraction is a poor predictor of radiocesium dynamics, because small amounts of mineral clay are sufficient to adsorb trace amounts of radiocesium. Mineral weathering, controlled by landuse and climate, determines both affinity and fixation of radiocesium.

## 1. Introduction

Radiocesium, and in particular $^{137}$Cs, with a half-life of 30.2 years, is one of the most widely studied environmentally relevant radioelements. Fairly uniform atmospheric fallout in the Northern hemisphere from weapons testing in the middle of the 20th century has led to $^{137}$Cs being used as a tracer of soil erosion (Zhuang et al., 2015). It was a major component of the fallout from the notorious accidents at Chernobyl in 1986 and Fukushima in 2011 (Igarashi et al., 2019; Konoplev, 2022; Onda et al., 2020).

Cesium is an alkali metal with no known beneficial biological role. The large concentrations at which the stable element may become toxic are usually not environmentally relevant. However, its similarity to the macronutrient potassium is key to its dynamics in the biosphere. Extensive studies of the dynamics of $^{137}$Cs have shown that its fate depends to a large extent on its adsorption on soil organo-mineral surfaces, and Cs-K discrimination at mineral and biological interfaces (Massas et al., 2010; Nakao et al., 2014; Ohnuki and Kozai, 2013; Park et al., 2019; Staunton and Roubaud, 1997; White and Broadley, 2008; Zhu and Smolders, 2000). Adsorption of radiocesium in soil tends to be strong and quasi-irreversible (Fuller et al., 2015). The most important soil parameters that determine radiocesium dynamics are reported to be soil clay content and mineralogy, soil potassium status and soil organic

matter (Ramirez-Guinart et al., 2020). There is some selectivity for cesium over other cations on regular exchange sites, such as the planar sites of phyllosilicates. The greatest selectivity, leading to strong adsorption, is associated with edge sites associated with the weathering of micas to form illites and vermiculites. These sites, known as frayed edge sites are reported (FES) to account for a small proportion of cation exchange capacity of these minerals: for example Cremers reported two classes of FES in illite accounting for 0.5 % and 2–5 % of cation exchange capacity (Cremers et al., 1988). Adsorption of cesium on organic matter and metal (hydr)oxides is not thought to be selective, but may contribute to Cs adsorption (Semenkova et al., 2023). Adsorption of trace amounts of radiocesium is dominated by highly selective sites, and less selective sites usually contribute little to adsorption until FES are saturated. Following adsorption, these sites collapse, with loss of the hydration water, which causes the irreversibility of adsorption, also known as fixation. Some studies indicate that fixation increases with time, a process known as aging. This may be due to diffusion of adsorbed Cs within mineral interlayers. Organic matter may impede the collapse of FES and thus decrease adsorption and fixation (Ramirez-Guinart et al., 2020; Rigol et al., 2002; Staunton, 2024; Staunton et al., 2002). Experimental observations of this effect (Cheshire et al., 2000; Dumat et al., 1997, 2000; Dumat and Staunton, 1999) have been confirmed by extended x-ray fine structure spectroscopic (EXAFS) measurements (Fan et al.,

<footnote><sup>a</sup> INRAE, Eco&Sols, INRAE-IRD-Cirad-InstAgroMontpellier-University of Montpellier, Place Viala, 34060, Montpellier, France</footnote>

<footnote><sup>b</sup> Biodiversity Research Center, Academia Sinica, Taipei, 115, Taiwan, Republic of China</footnote>

<footnote>* Corresponding author.
E-mail addresses: siobhan.staunton@inrae.fr (S. Siobhan), bochiu@gate.sinica.edu.tw (C.-Y. Chiu).</footnote>
<figure><img src="image_4.png" /><figcaption>Fig. 1. Location of soil sampling points on a topographical map of Taiwan.</figcaption></figure>

<table><caption>**Table 1**
Some soil properties according to landuse: minimum-maximum range, average and median.</caption><thead><tr><th></th><th>Undisturbed</th><th>Secondary forest</th><th>Arable land</th></tr></thead><tbody><tr><td>pH</td><td>3.2-4.89; 4.0; 4.08</td><td>3.62-6.12; 4.60; 4.58</td><td>4.15-8.55; 6.21; 6.18</td></tr><tr><td>SOC/%</td><td>1.25-72.4; 17.4; 12.3</td><td>2.78-17.9; 9.35; 8.54</td><td>0.41-10.3; 2.11; 1.09</td></tr><tr><td>CEC mmol<sub>c</sub> kg<sup>-1</sup></td><td>11.5-135.9; 42.0; 36.1</td><td>19.1-90.3; 34.8; 34.9</td><td>19-84; 42.35; 37.0</td></tr><tr><td>clay content (&lt;2 µm)/%</td><td>22-41; 29.3; 27.0</td><td>12-47; 31.0; 32.5</td><td>4.0-39.0; 17.2; 17.0</td></tr><tr><td>Clay/SOC</td><td>2.32-17.7; 11.3; 12.1</td><td>1.3-6.67; 3.57; 3.18</td><td>3.4-33.1; 14.5; 15.1</td></tr><tr><td>[K]<sub>aq</sub>/mM</td><td>0.12-1.2; 0.34; 0.27</td><td>0.10-1.02; 0.30; 0.27</td><td>0.05-0.90; 0.20; 0.12</td></tr></tbody></table>

2014a, 2014b). Since the accident at the nuclear power plant at Fukushima more data have been obtained for volcanic soils and soils from tropical and subtropical climatic regions. Comparison of this more recent data set with data accumulated from previous decades of research indicates that radiocesium is less strongly fixed on Japanese soils than would be expected on European soils. It has been suggested that this may be due to a smaller proportion of weathered mica, and hence FES (Almahayni et al., 2019; Uematsu et al., 2016).

The objective of this study was to elucidate the radiocesium adsorption properties of soils from tropical and sub-tropical climates. A large number of soils from Taiwan, including volcanic soils and soils from wet climates and high altitudes were surveyed. One of the originalities of this study is the emphasis on desorbability, not simply

<figure><img src="image_5.png" /><figcaption>Fig. 2. Kd as a function of a) altitude and b) rainfall for all soils for each of the three landuse classes (only undisturbed and secondary forest soils for altitude since all arable soils were sampled at about sea level). Open symbols are used to distinguish volcanic soils.</figcaption></figure>

adsorption. Both adsorption and desorbability were compared to soil properties. We focused on the roles of soil potassium, clay and organic carbon contents. Soils were sampled from under undisturbed forest, and secondary forest, and arable land. The standard conditions of adsorption were a background solution of dilute (10 mM) CaCl₂ solution. Adsorption was also measured in solutions containing potassium or stable Cs to assess the selectivity of adsorption. Desorption in dilute Cs solution was measured and used to obtain a measurement of the fraction of adsorbed radiocesium that was fixed.

## 2. Materials and methods

Fifty-seven soils from different landuses in Taiwan were selected for this study. The landuses were grouped as undisturbed (largely) forest, secondary forest and arable (rice and horticulture). Fig. 1 shows the sampling points on a topographical map of Taiwan. Arable land was always at about sea level whereas the altitude of secondary forest sites ranged from 725 to 2200 m and that of undisturbed sites ranged from 50 to 3200 m. Annual precipitation was large: about 2000–4000 mm for undisturbed and secondary forest soils, and a similar average, but larger range, for the arable soils, 1200–6400 mm. Some soil properties were measured using routine methods: pH measured in 1:2 water suspension; cation exchange capacity (CEC) by the ammonium acetate method; soil organic carbon content (SOC) by CHN elemental analysis (soils were not carbonaceous and so no pretreatment or correction was required for non organic C); soil solution potassium concentration by Atomic Absorption spectroscopy of supernatant solution of soil suspended in 0.01 M CaCl₂ solution with a soil:solution ratio of 1:10 g/ml); and clay sized fraction (<2 µm) was obtained after oxidation of organic matter, dispersion and sedimentation (denoted clay content). Table 1 shows the minimum -
<figure><img src="image_6.png" /><figcaption>Fig. 3. Kd as a function of various soil properties for each of the three landuse classes; a) pH, b) soil organic carbon (SOC), c) cation exchange capacity (CEC), d) clay content (<2 µm), e) clay/C ratio, f) [K]. Open symbols are used to distinguish volcanic soils. A semi logarithmic scale is used because of the large range of values of Kd.</figcaption></figure>

maximum range, mean and median values of some soil properties, classified by landuse. A complete table of values is given as Supplementary Information. Undisturbed and secondary forest soils were acid whereas arable soils tended towards neutral, with some alkaline soils. The SOC contents were greater in undisturbed forest soils, with some decrease in secondary forests, and much smaller C contents in arable soils. The range of clay content varied little with landuse, but mean and median values were smaller for arable soils than for other soils. Clay content cannot be reliably measured for soils with very large organic matter content. Arable soils were depleted in water-soluble potassium, in comparison to undisturbed and secondary forest soils.

The soils were air-dried, sieved to 200 µm and stored until required. The distribution coefficients (Kd, defined as the ratio of adsorbed to aqueous Cs) of ¹³⁷Cs were measured in suspension (1g/10 ml) with a background electrolyte of 0.01 M CaCl₂ and ¹³⁷Cs (5–50 kBq ml⁻¹, depending on the Kd determined in preliminary experiments). Suspensions were shaken end-over-end for 24 h, then phases separated by centrifugation. Suspensions were centrifuged at 19000 g for 15 min, and aliquots of the apparently clear supernatant solution removed and centrifuged again for 15 min in 2-ml Eppendorf tubes. Triplicate aliquots of 0.5 ml were taken for quantification of ¹³⁷Cs. Activity was measured

in a 3-inch gamma spectrometer (Wallac Wizard, 1480, PerkinElmer, Waltham, MA, USA). The Kd value was calculated by difference between initial (L₀) and final (Lₜ) concentrations of ¹³⁷Cs in solution, and the ratio of solution volume (Vₜ) to soil mass (M₅).

$$Kd = x_{ads} / L_{t} = (V_{L} / (M_{s}) * (L_{0} - L_{t}) / L_{t}) \quad 1$$

Distribution coefficients were also measured in solution of 0.01 M CaCl₂ also containing 1 mM either KCl or CsCl, denoted KdK and KdCs. The desorption of previously adsorbed ¹³⁷Cs was measured on lyophilized soils by resuspending soil in 0.01 M CaCl₂ +1 mM CsCl solution, and following the same procedure as for adsorption. Soil was lyophilized to avoid artefacts arising from drying soil and poor resuspension of dried soil plugs. The desorption Kd, Kddes, was calculated from the amount of previously adsorbed ¹³⁷Cs, Ms*xads, and the activity in solution after desorption, Ldes.

$$Kd_{des} = (M_{s} * x_{ads} - V_{L} L_{des}) / M_{s} * L_{des} \quad 2$$

The fraction fixed, namely not desorbed in this step, ffixed, was calculated as follows,

$$f_{fixed} = (Kd_{des} - Kd_{Cs}) L_{des} / x_{ads} \quad 3$$
<figure><img src="image_7.png" /><figcaption>Fig. 4. Kd as a function of other adsorption or desorption parameters: a) Kd measured in 10 mM CaCl₂ + 1 mM KCl; b) Kd measured in 10 mM CaCl₂ + 1 mM CsCl; c) desorption Kd, Kd<sub>des</sub>; d) fraction of adsorbed <sup>137</sup>Cs calculated to be fixed, f<sub>fixed</sub>. Logarithmic scales are used for Kd and Kd<sub>K</sub> because of the large ranges of values. Linear regression data are indicated for the regression between Kd and Kd<sub>K</sub>.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 5. Solution phase concentration of potassium in 10 mM CaCl₂ without and with 1 mM KCl. Linear regression data are indicated on the figure.</figcaption></figure>

This equation is derived assuming that Kd<sub>Cs</sub> gives the distribution of <sup>137</sup>Cs expected in the absence of fixation in CaCl₂+CsCl solution; the excess amount remaining adsorbed after the desorption step is defined as fixed. The potassium concentration of supernatant solutions of suspensions (with background solutions of CaCl₂, with or without KCl) was measured in suspensions without <sup>137</sup>Cs, but otherwise identical to Kd measurements.

### 3. Results and discussion

#### 3.1. <sup>137</sup>Cs adsorption

The largest range of Kd was observed for arable soils (300–22300 dm³ kg⁻¹), while undisturbed and secondary forest soils had a narrower range of Kd (8–3240 dm³ kg⁻¹). Fig. 2 shows the relationship between Kd and a), land altitude for undisturbed and secondary forest soils and b) with mean annual rainfall for all the soils. A logarithm scale is used because of the large range of Kd values observed. Experimental variability in the values of Kd was less than 5 %. Arable soils were all sampled at about sea level and so are not shown on Fig. 2a. Cs adsorption affinity decreased markedly with increasing altitude, and for a given altitude, the Kd of secondary forests was larger than that of undisturbed forest soils. This trend may be linked to soil properties, as affected by climate and thus clay mineral weathering. Volcanic soils, distinguished as open symbols, do not stand out from other soils in each landuse group. Kd tended to decrease with increasing rainfall for the entire data set, but no significant relation was observed for each land-use group analysed separately. These observations strongly indicate that apparent effects of rainfall or altitude on Cs affinity are due to soil properties. It should be remembered that rainfall was much larger than in temperate climates, and not closely related to altitude or soil properties. Fig. 3 shows the relation between Kd values and various soil properties, and as before, volcanic soils are distinguished using open symbols.

Kd tended to increase with increasing pH at acid pH, whereas Kd decreased with increasing pH at neutral to alkaline pH for some of the arable soils. A direct effect of pH on Cs dynamics is not expected, but it has been suggested that intensive weathering in Japanese soils may explain the paucity of FES (Nakao et al., 2008). The inverse relations between Kd and pH at acid and at neutral to alkaline pH must reflect different weathering mechanisms. In neutral to alkaline arable soils, potassium was also depleted, promoting weathering and creation of FES. The driving force for the observed trend is thus more likely to be mineral
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Fraction of adsorbed ¹³⁷Cs calculated to be fixed, $f_{fixed}$ as a function of a) altitude and b) rainfall for each of the three landuse classes (only undisturbed and secondary forest soils for altitude, since all arable soils were sampled at about sea level). Open symbols are used to distinguish volcanic soils.</figcaption></figure>

weathering due to depletion in K, rather than the pH per se.

The effect of SOC content was complex. There was a nonlinear inverse relation between Kd and SOC content. Adsorption was greatest in C-depleted arable soils, and these soils also tended to be depleted in water-extractable potassium and less acid. Organic matter is not expected to make an important contribution to cesium adsorption, as the adsorption sites are not highly selective (Chiu et al., 2002, 2008; Rigol et al., 2002). However it has been hypothesized that organic matter prevents the collapse of FES, thereby reducing their selectivity (Cheshire et al., 2000; Staunton, 2024; Staunton et al., 2002), and this has been confirmed by spectroscopic studies (Fan et al., 2014a, 2014b). The accumulation of organic matter is also an indicator of pedo-climatic conditions that also determine mineral weathering. The trend observed for CEC was similar to that of SOC, resulting from the strong correlation between CEC and SOC. There was an inverse relation between clay content and Kd for undisturbed and secondary forest soils, but no coherent relation for arable soils. It should be noted that clay content could not be determined for all undisturbed and secondary forest soils because of the large SOC content. The absence of a relation between clay content and Kd may be surprising as clay content is often considered to be a predictor of Kd in semi-empirical models (Almahayni et al., 2019; Ramirez-Guinart et al., 2020). We propose that a tiny amount of minerals containing highly selective adsorption sites is sufficient to dominate the adsorption of environmentally relevant trace amounts of radiocesium. The often-reported correlation between Cs affinity and clay content may not be causative. Furthermore clay mineralogy of soils may be more important than simply the amount of clay

present.

There was no coherent relationship between Kd and the ratio of clay and SOC contents. This ratio has been proposed to be an indicator of free organic matter (Dexter et al., 2008). Estimates for soils of temperate climate suggest that soil minerals have the capacity to adsorb 0.1 g of organic carbon, and this represents a threshold for free organic matter (Dexter et al., 2008). However equivalent data are not available for more weathered soils of tropical climates. Free organic matter is not expected to play a large role in the adsorption of Cs. The tendency for Kd to increase with increasing clay:SOC ratio for most of the soils is coherent with organic matter blockage of specific adsorption sites on minerals. For the arable soils with very large Kd, both organic matter and potassium are depleted, and thus the apparent decrease in Kd with increasing clay:SOC ratio may not be causative.

There was a strong inverse nonlinear relationship between Kd and water-extractable potassium concentration. Arable soils had much lower K concentrations and a large range of Kd. This is in accordance with the mineralogical changes induced by potassium depletion in the rhizosphere (Hinsinger and Jaillard, 1993), and the observation that root action enhances Cs adsorption on soil and reduces Cs uptake by roots (Guivarch et al., 1999; Gommers et al., 2005).

## 3.2. ¹³⁷Cs adsorption as a function of solution composition

There was a positive relation between Kd values obtained in indif- ferent salt solution, CaCl₂, and in the presence of 1 mM KCl, KdK (Fig. 4a). KdK was about 15 % lower for undisturbed and secondary forest soils, and 60 % lower for arable soils. This observation reflects the competition between Cs and K. The relation between KdK and the water- extractable K concentration (data not shown) was similar to that be- tween Kd and [K] (Fig. 3f), with a smaller number of outlying points at large Kd and low Kaq. The lower KdK values reflects the competition between Cs and K at organo-mineral surfaces. The KdCs values are on average 3 % than Kd, (Fig. 4b). This is an illustration of the well-known concentration dependence of Cs adsorption, that is often shown to follow a Freundlich isotherm (Comans et al., 1991; Staunton and Rou- baud, 1997). These values were required to calculate the fraction fixed, but are of limited interest to understand the dynamics of radiocesium. They give some indication of the adsorption on regular exchange sites that are not highly selective for Cs. There was no correlation between Kd values obtained in CaCl₂ and CaCl₂+CsCl (Fig. 4b), but the contrast was markedly greater for some of the arable soils, reflecting the greater proportion of selective sites in these soils.

## 3.3. ¹³⁷Cs desorption

The desorption Kd values, Kddes, obtained by resuspending soil in CaCl₂+CsCl solution after ¹³⁷Cs adsorption were more than an order of magnitude lower than the adsorption Kd values. There was no signifi- cant correlation between Kd and Kddes (Fig. 4c). Importantly, Kddes was larger than Kdcs, the adsorption Kd measured in solution with the same composition, This demonstrates that a proportion of adsorbed radio- cesium was not readily exchanged. The desorption data were used to calculate the fraction of adsorbed Cs that was not readily desorbed, defined as the fraction fixed, $f_{fixed}$. The fixed fraction increased with increasing affinity, with markedly larger $f_{fixed}$ observed for arable soils (Fig. 4d). For undisturbed and secondary forest soil a range of $f_{fixed}$ between 0.1 and 0.8 was calculated, whereas most values of $f_{fixed}$ for arable soils were in the range 0.8–0.9.

## 3.4. Potassium distribution

Although the focus of this study is the distribution of radiocesium, not potassium, some data on the potassium distribution are given because of the importance of K in Cs dynamics. The average potassium concentration in suspensions with added K were similar at 0.8 mM for
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Fraction of adsorbed <sup>137</sup>Cs calculated to be fixed, f<sub>fixed</sub>, for each of the three landuse classes as a function of some soil properties; a) pH, b) SOC, c) CEC, d) clay content (<2 µm), e) clay/C ratio, f) [K]. Open symbols are used to distinguish volcanic soils.</figcaption></figure>

each landuse, and a linear correlation was observed between concentrations with and without added K, again with no landuse effect (Fig. 5). The ratio of change in exchangeable K (ΔKex) to change in water-extracted K (ΔKaq), which approximates to the tangent of the adsorption isotherm of K for each soil, varied by an order of magnitude, between 2 and 28 dm<sup>3</sup> kg<sup>-1</sup>, with average values in the order arable < undisturbed forest < secondary forest. There was no significant correlation between the ratio ΔKex:ΔKaq and Kaq (data not shown) which illustrates that the soil matrix differs between soils, and was not simply depleted in K. The potassium concentrations in solution containing Cs were slightly lower than in CaCl<sub>2</sub> (10 %), with no distinction between landuse categories (data not shown).

### 3.5. Fixation of adsorbed <sup>137</sup>Cs

Fig. 6a shows the altitude effect on f<sub>fixed</sub> for undisturbed and secondary forest soils. The f<sub>fixed</sub> for secondary forest soils were very constant, at about 0.7, with no relation with altitude. However the f<sub>fixed</sub> decreased with increasing altitude, apart from three outlying points at high attitude. Although they appeared as outlying points with respect to altitude, they were not outlyers when affinity or fixation are compared to soil properties. The fraction fixed decreased with increasing rainfall

with data strongly grouped by landuse (Fig. 6b).

Fig. 7 shows how the values of f<sub>fixed</sub> varied with soil properties. The relationships were markedly different for each of the three landuse categories. Average f<sub>fixed</sub> increased from undisturbed forest < secondary forest < arable (0.55 ± 0.18, 0.67 ± 0.05 and 0.77 ± 0.15 respectively). The fixed fraction increased with increasing pH, and decreased with increasing SOC content, CEC and clay content. Although there was no net effect of the ratio clay/SOC content on f<sub>fixed</sub> for arable and secondary forest soils, there was a positive relation for undisturbed forest soils. There was a strong inverse relation between the fraction fixed and water-extractable potassium concentration, and marked landuse differences linked to soil solution potassium levels is evident. Kd values integrates the selectivities of all adsorption sites, whereas fixation more strongly reflects the content of highly selective sites. The marked landuse effect must arise from the long term weathering effect of climate. Acid soils with high organic matter content, often from high altitude, appear to have fewer highly selective adsorption sites for Cs. In contrast, Cs adsorption was dominated by highly selective FES in soils from arable land, depleted in exchangeable K and organic matter. The driving force for the weathering causing the formation of FES would include K depletion. Soil mineralogical differences between well-drained hill top and marshy lowland soils in Taiwan have been attributed to the blocking
of weathering under water-saturated conditions (Pai et al., 2007). However in the present study lowland soils are not waterlogged as they have been used for arable culture, and so mineralization was not impeded. Potassium depletion in arable soils is likely to play a significant role in weathering and creation of selective adsorption sites (Nakao et al., 2014). The inverse correlation between fixation and clay content may be surprising. This illustrates the importance of distinguishing between adsorption and fixation. This distinction is generally overlooked by models that only consider adsorption. The amount of weathered non swelling mineral, containing highly selective FES, required to dominate radiocesium adsorption would be tiny, probably too small to be quantified, and possibly undetectable. The total clay content does not thus determine either adsorption or fixation.

Many studies attempt to quantify the amount of FES in soils using a simple experimental approach. The original method involved using a bulky thiourea complex as a masking agent for regular exchanges sites ensuring that adsorption takes place only on FES (Wauters et al., 1994). It is more common to measure Kd in a dialysis bag to obtain a known solution composition, with potassium concentration of 0.5 mM (Nakao et al., 2008; Uematsu et al., 2016). The calculation assumes a universal Cs-K selectivity coefficient of 1000. Although we did not attempt to calculate the FES contents or the Radiocesium Interception Potential (RIP) of these soils using the dialysis bag method, it is nevertheless interesting to point out that calculation of RIP from data from the suspensions in both CaCl2 and CaCl2+KCl, using the measured concentrations of K, yielded marked different estimations of RIP. The RIP appeared to be more than three times greater with added K than without, despite the measured K concentration being similar to that recommended for the dialysis bag technique. We did not attempt to measure ammonium concentration nor to remove this cation. Nevertheless, this observation seems to highlight a weakness in the assumptions regarding RIP quantification.

## 4. Conclusions

A large range of Cs Kd values is reported, and this range is similar to data reported for temperate climate soils. We report for the first time relationships between altitude and annual rainfall, and both affinity (adsorption assessed by Kd) and fixation (irreversibility of adsorption). Adsorption properties also varied with landuse. We conclude that climate and landuse effects on Cs adsorption properties are not directly causal, and reflect soil properties. Water-extractable K was strongly related to Cs dynamics. This is explained by both the absence of K to compete with Cs for adsorption and, in the longer term, mineral weathering and creation of FES caused by depletion of K. Upland Taiwanese soils contain enough clay mineral to immobilize radiocesium, in contrast to upland soils from temperate climates that often have high organic matter and little clay mineral.

### CRediT authorship contribution statement

**Staunton Siobhan:** Writing – review & editing, Writing – original draft, Validation, Methodology, Investigation, Formal analysis, Conceptualization.
**Chih-Yu Chiu:** Writing – review & editing, Validation, Investigation, Data curation.

### Declaration of competing interest

The author declares that she has no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jenvrad.2025.107727.

## Data availability

Data will be made available on request.

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