Decontamination of Cs from Japanese cedar (Cryptomeria japonica) via kraft cooking

Abstract

To investigate the possibility of decontaminating ¹³⁷Cs-contaminated Cryptomeria japonica wood, kraft pulping was conducted and the Cs behavior in the reaction process was examined. ¹³³Cs-treated or ¹³⁷Cs-contaminated bark, sapwood, and heartwood chips of Cryptomeria japonica were digested using an aqueous solution of NaOH and Na₂S. The pulp was washed with ultrapure water and filtered, after which the filtrate (black liquor) was collected. The black liquor was acidified to separate the supernatant and precipitation. The Cs (¹³³Cs and ¹³⁷Cs) concentrations in the chip and reaction products were measured. As for wood samples, the majority of Cs was present in black liquor, while only a minor amount of Cs was retained in the pulp (<1%). In the case of bark, although the majority of Cs was present in the black liquor, the proportion of Cs in the pulp was much higher than that in the wood pulp. In addition, the Cs in the precipitation of the bark was higher than that in the wood, possibly because the Cs in the bark was combined with some components, which is insoluble in alkaline solution. Our results suggest that ¹³⁷Cs-contaminated Cryptomeria japonica wood can be used in the pulp and paper industries.

Introduction

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in March 2011 resulted in a release of large amounts of radionuclides into the Fukushima Prefecture [1]. Because forests cover approximately 70% of the entire prefectural area and are highly efficient at intercepting gaseous and particulate contaminants [2, 3], the radioactive contamination of forests and forest products is a serious problem in the Fukushima Prefecture.

Japanese cedar (Cryptomeria japonica D. Don) is one of the most abundant forest resources in Japan, comprising 43% of the country’s artificial forests [4], and it is one of the most valuable plantations for the timber industry [5]. At present, in Japan, because there is no regulation in place for the radiation levels of building materials using wood from conifer forests [6], the risk of using ¹³⁷Cs-contaminated wood is difficult to evaluate and the use of C. japonica as timber is restricted without sufficient decontamination. To maintain the public function of the C. japonica forest and to revive the forest industry in the Fukushima Prefecture, it is important to explore uses of C. japonica other than as timber. After being deposited onto forests, ¹³⁷Cs can be absorbed and translocated to wood through three main pathways: surface absorption through leaves [7, 8] and bark [9, 10], and root uptake from the soil. In the case of C. japonica, we confirmed that ¹³³Cs ions applied to bark were absorbed into the heartwood through the bark [10]. Therefore, a part of the ¹³⁷Cs in the wood of C. japonica is likely derived from bark absorption in an ionic form. In tree stem wood, ¹³⁷Cs may form ionic bonds with carboxylic groups, in cell walls or in the cytoplasm of living cells, and is very mobile across tree rings [11, 12], which indicates that ¹³⁷Cs could be removed from contaminated wood via chemical treatments. Therefore, one promising application of ¹³⁷Cs-contaminated C. japonica is to chemically convert ¹³⁷Cs-contaminated wood into pulp, which is then used to produce paper and related products. Kraft pulping is the most frequently used chemical method to transform wood into pulp [13]. A conventional kraft pulping process usually involves digesting the wood chips at elevated temperature and pressure with an aqueous solution of sodium hydroxide and sodium sulfide. Most of the lignin is dissolved and separated from the pulp via a washing process and filtration. The pulp is further processed until it is suitable for the manufacture of paper and other related products [14]. At present, because the behavior of ¹³⁷Cs during the kraft pulping of C. japonica wood is not well understood, the use of ¹³⁷Cs-contaminated C. japonica wood in pulp and paper industries is still in suspense.

Cesium is a metal element of group 1. ¹³³Cs is the only natural stable isotope, and ¹³⁷Cs is radioactive and has a half-life of ca. 30 years. It is extremely difficult to find the difficulties in chemical behavior between the two isotopes. In this study, laboratory kraft cooking of ¹³³Cs-treated and ¹³⁷Cs-contaminated wood is performed. The purposes of the study are to elucidate the behavior of Cs during the kraft pulping process and to evaluate whether ¹³⁷Cs-contaminated C. japonica wood can be used in pulp and paper industries. In addition, even though bark is usually removed from wood and is not used to produce pulp, the kraft cooking of bark is also conducted to study whether the behavior of Cs differs between the bark and wood during the kraft cooking process.

Materials and methods

Sampling and sample preparation

The ¹³³Cs-treated wood was prepared by applying a ¹³³CsCl solution onto the bark of C. japonica at Nagoya University (Fig. 1, NU). On May 15, 2015, a 40-ml aliquot of 0.01 M ¹³³CsCl solution was spread uniformly on a paper towel. The towel was then attached to the bark at a height of 1.2 m above the ground. To prevent the towel from drying, it was covered with plastic wrap and fixed using plastic wires. Further, to prevent soil contamination from ¹³³CsCl caused by stemflow, a 3-cm-thick urethane sheet was wrapped around the trunk below the paper towel, which was then fixed using plastic wires that also functioned as stemflow gutters. A hose was connected to the urethane sheet to direct the stemflow to a bucket. The juncture between the hose, bark, and urethane was filled with silicon resin to prevent leakage (Fig. 2). On June 4, 2015, the treated C. japonica was cut down, and 10-cm-thick disks were collected at heights of 1.2 m. As a control, untreated C. japonica was also cut and disks were collected at a height of 1.2 m. The surface soil (0–5 cm) was randomly collected at three points around each tree and assessed for whether the soil was contaminated with ¹³³Cs.

Fig. 1

A map of the sampling site; YF indicates the Yamakiya forest and NU indicates Nagoya University. FDNPP indicates the Fukushima Daiichi Nuclear Power Plant

Fig. 2

Application of ¹³³Cs to Japanese cedar bark. A paper towel with 40 ml of a 0.01 M ¹³³CsCl solution was wrapped around the bark at 1.2 m. A urethane sheet and hose were used to direct the stemflow to the guttering

The ¹³⁷Cs-contaminated C. japonica disk was sampled from a C. japonica forest, located in Setohachiyama, Yamakiya district, Fukushima Prefecture, approximately 36 km northwest of the FDNPP (Fig. 1, YF). The stand age was 60 years, the air dose rates (μSv h⁻¹) was 1.5 μSv/h on September 24, 2015, which were measured with a scintillation survey meter (TCS-161, Aloka), about 1 m above ground level and were presented as an average from 10 different points. On that day, 10-cm-thick disks were collected at a height of 0.4 m for this study. The collected disks were divided into bark, sapwood, and heartwood. Wood and bark chips were prepared from each portion, and the average size of the wood and bark chips was 11 mm × 10 mm × 5 mm and 10 mm length × 10 mm width, respectively.

Kraft cooking

Wood and bark chips (1.5 g) were digested in a 10-ml high-pressure stainless steel tube, using “white liquor,” which is an aqueous solution of 7.5 ml NaOH and Na₂S·9H₂O. The active alkalinity and sulfidity of the alkaline solution was 20 and 28%, respectively. Kraft cooking was performed in an oven, the cooking temperature was gradually increased from 20 to 170 °C within 1.5 h, and this temperature was maintained for 2 h. After cooling, the kraft pulp was washed with 15 ml of Mill-Q water (with resistivity more than 18 MΩ) and filtered with 5-μm filter paper. The filtrate was collected and is referred as the 1st black liquor, which consisted primarily of water, dissolved wood components, and the spent digesting agents. The kraft pulp was further washed with 15 ml of ultrapure water and filtered an additional two times to collect the 2nd and 3rd black liquors. Then, the pulp was continuously washed until the filtrate became nearly colorless; the obtained pulp is referred to as the washed pulp. A portion of the black liquor (3 ml) was taken and acidified with 2 ml of 10% H₂SO₄ to adjust the pH to 2–3, resulting in the formation of precipitation, the main component of which was lignin. The precipitation and supernatant were then separated with a 0.45-μm membrane filter. The wood, bark chips not used for kraft cooking, washed pulp, and the precipitation were oven-dried at 80 °C for more than 48 h to a constant weight. A flow chart of the kraft cooking process is illustrated in Fig. 3.

Fig. 3

Flow-chart of the kraft pulping process

¹³³Cs and ¹³⁷Cs determination

¹³³Cs determination: The chips, washed pulp, and precipitation were digested with HNO₃ using a graphite block acid digestion system (EcoPre; ODLAB). The concentrations of ¹³³Cs in the abovementioned components, as well as in the supernatant, were determined using an inductively coupled plasma mass spectrometer (ICP-MS) (iCAPQc, Thermo Scientific) with indium (In) as the internal standard. The ion-exchangeable ¹³³Cs in the soil samples (2 g) were extracted using 1 M NH₄AC (40 ml, pH 7) and its concentration was determined with ICP-MS.

¹³⁷Cs determination: The chip, pulp powder, black liquor, and supernatant were placed in plastic containers (Ø 56 mm × H 35 mm; PP-U9, Sekiya Rika), and the ¹³⁷Cs activities were measured using a high-purity germanium (HPGe) detector (GC3520, Canberra; GEM-30195-P, Seiko EG&G Ortec). Standard ¹³⁷CsCl solutions (Japan Radioisotope Association) diluted with ¹³³CsCl were used as standard sources for the HPGe detector. Measurements were continued for 80,000 s or until a counting error of less than 5% of the ¹³⁷Cs peak was achieved. The radioactive concentration of ¹³⁷Cs (Bq/kg dry weight) was calculated based on the gamma ray spectrum using the Gamma Explorer software (Canberra Japan KK, Tokyo). The activities were corrected for decay back to the sampling date. The ¹³⁷Cs activity for the precipitation was calculated from the difference between the ¹³⁷Cs activities in the corresponding black liquor and supernatant.

Statistical analysis

The Shapiro–Wilk test was employed to check the normality of the data set, and the Mann–Whitney U test (significance level of 5%) was used to compare the ¹³³Cs concentration between the treated samples and the control samples.

Results

¹³³Cs and ¹³⁷Cs concentrations in the bark and wood of C. japonica

The ¹³³Cs and ¹³⁷Cs concentrations in the bark, sapwood, and heartwood are shown in Fig. 4a for ¹³³Cs-treated trees and in Fig. 4b for the trees from Fukushima. The ¹³³Cs concentrations in the sapwood and heartwood of the ¹³³Cs-treated trees were considerably higher than those of the control trees, even though there were no significant differences between them. Because no significant differences were observed between the average ¹³³Cs concentrations in the soils under the treated trees (0.097 ± 0.021 μg g⁻¹) and those under the control trees (0.075 ± 0.029 μg g⁻¹), this indicates that the influence of root-uptake caused by the leakage of the ¹³³CsCl solution was negligible, and that the ¹³³Cs entered the wood through the bark and was translocated into the heartwood. For the ¹³⁷Cs-contaminated trees from Fukushima, the ¹³⁷Cs concentration was highest in the bark, and the concentration in the heartwood was higher than that in the sapwood (Fig. 4b).

Fig. 4

a ¹³³Cs and b ¹³⁷Cs concentrations in the bark, sapwood, and heartwood. The Y-axis of a is in logarithmic units. The error bars indicate the standard deviations of three replicates

Pulp yields of the bark and wood

The pulp yields of the bark, sapwood, and heartwood of ¹³³Cs-treated and ¹³⁷Cs-contamianed C. japonica are given in Table 1. The pulp yields of the sapwood and heartwood were nearly the same and the pulp yield of the bark was the lowest of the three parts of the wood for both the ¹³³Cs-treated and ¹³⁷Cs-contamianed C. japonica. The lower pulp yields in the bark compared to the wood may occur because the amounts of polyphenols with high molecular weight, extractives, and ash in bark are higher than those in wood, which will lead to a lower pulp-yield in bark than in wood.

Table 1 Pulp yields (%) of the ¹³³Cs-treated and ¹³⁷Cs contaminated bark and wood (n = 3)

Proportion of Cs in each component after kraft cooking

To investigate the behavior of Cs during the kraft cooking process, we calculated the amount of ¹³³Cs (μg) and the activity of ¹³⁷Cs (Bq) in the chips and each component after kraft cooking; the results are shown in Tables 2 and 3, respectively. Then, we calculated the proportion of Cs in each component based on these results. The proportion was calculated to be the percent ratio of ¹³³Cs or ¹³⁷Cs in each component to the total amount of ¹³³Cs or ¹³⁷Cs in all the components after kraft cooking. The results are shown in Fig. 5. For sapwood, approximately 99% of the ¹³³Cs was present in the supernatant, while that in the pulp (0.59%) and precipitation (0.1%) was less than 1%. The ¹³⁷Cs in the sapwood was detected in the 1st and 2nd black liquors, which include the supernatant and precipitation, and the ¹³⁷Cs proportion of the 1st black liquor (68%) was greater than that of the 2nd black liquor (32%). The proportion of Cs in the heartwood was similar to that in the sapwood in which the majority of the ¹³³Cs (approximately 99%) was retained in the supernatant while that in the pulp (0.84%) and precipitation (0.31%) were less than 1%, and the distribution of ¹³⁷Cs was greater in the 1st black liquor (56%) than in the 2nd (27%) and 3rd (17%) black liquors. In the case of bark, even though the majority of the ¹³³Cs (95%) and ¹³⁷Cs (86%) was present in the supernatant, the proportions of the ¹³³Cs (4.2%) and ¹³⁷Cs (5.9%) in the pulp were higher than those in the sapwood and heartwood. In addition, the proportions of ¹³³Cs and ¹³⁷Cs in the precipitation of the bark were also higher than those in the sapwood and heartwood.

Table 2 The amount of ¹³³Cs (μg) in each component after kraft cooking 1.5 g of chips

Table 3 The activity of ¹³⁷Cs (Bq) in each component after kraft cooking 1.5 g of chips

Fig. 5

Proportion of ¹³³Cs and ¹³⁷Cs in each component after kraft cooking: a, b bark; c, d sapwood; and e, f heartwood. The proportion was calculated as the percent ratio of the ¹³³Cs or ¹³⁷Cs in each component to the total amount of ¹³³Cs or ¹³⁷Cs in all the components (chips were not included). The ¹³⁷Cs radioactivity in the sapwood and heartwood was expressed in the form of black liquor because the ¹³⁷Cs radioactivity was not detected in the supernatant and precipitation. n.d. indicates not detected. The Y-axis of a is in logarithmic units

Discussion

Use of ¹³⁷Cs contaminated C. japonica wood in pulp mills

We explored the possibility of using radionuclide-contaminated woods as resources for pulp in this study. To use the contaminated wood, at least the following two requirements should be fulfilled: (1) the ¹³⁷Cs activity in the pulp should be under the detection limit or below the safety level and (2) the ¹³⁷Cs in the black liquor should be properly processed.

In this study, because ¹³⁷Cs was not detected (Fig. 5d, f) and only a minor amount of ¹³³Cs was present (Fig. 5c, e) in the wood pulp, this suggests that the ¹³⁷Cs-contaminated C. japonica wood could potentially be used to produce paper and related products via the kraft pulping process. The existence of minor amounts of ¹³³Cs in the wood pulp may be due to the insufficient washing process because only 45 ml of ultrapure water in total was used. In industrial pulping processes, the pulp obtained after kraft cooking will undergo several further processes, such as bleaching and washing, before it can be used to produce paper. It is expected that the amount of Cs retained in the wood-pulp will be further reduced after such processes.

The majority of the ¹³⁷Cs was present in the black liquor (Fig. 5d, f), which consists primarily of dissolved wood components, spent digesting agents, and other impurities. In a pulp mill, to improve the economic benefits and to minimize waste, the digesting agents are recycled. Because most of the ¹³⁷Cs was concentrated and retained in the black liquor (Fig. 5d, f) and the recovery process for the digesting agents would considerably enhance the average concentration of radionuclides compared to those in the wood [15, 16], developing methods to remove ¹³⁷Cs from black liquor is necessary to reduce not only the exposure to workers in pulp plants but also the discharge of ¹³⁷Cs into aquatic ecosystems. The application of a Cs-selective absorbent used to remove ¹³⁷Cs from radioactive water appears to be a promising choice [17,18,19,20].

Our results show that only a minor amount of Cs was present in the precipitation (Fig. 5c, e). The main component of the precipitation is lignin, and this means that the lignin could be further converted to added-value products, such as biosorbents of trace metals from wastewater [21, 22], precursors for carbon fibers [23], or amphiphilic derivatives with high surface activity [24], as well as several other applications [25, 26].

Implications for ¹³⁷Cs speciation in bark

Understanding the chemical speciation of ¹³⁷Cs is essential to evaluate its formation process and behaviors in the environment [27]. After its release into the atmosphere, ¹³⁷Cs was transported to the northwestern region of FDNPP [28] in the form of aerosols [29], including small particles such as Cs balls [30], or associated with sulfate salts and organic matter [27, 28]. Radionuclides can be trapped by tree bark directly via atmospheric deposition, stemflow, and resuspended soil particles resulting from wind erosion [31]. The chemical speciation of ¹³⁷Cs in bark plays a significant role in its behavior, such as its penetration into the wood through the bark [10] and its degradation.

In this study, we found that the proportions of ¹³³Cs and ¹³⁷Cs in the bark pulp were higher than those in the wood pulp (Fig. 5), suggesting that part of the ¹³³Cs and ¹³⁷Cs in the bark were not dissolved in the kraft pulping process. The insolubility of ¹³³Cs and ¹³⁷Cs may have two causes: (1) the ¹³³Cs and ¹³⁷Cs absorbed by the bark combined with unknown components have low solubility or cannot dissolve in the kraft pulping process and/or (2) the ¹³⁷Cs was deposited in an insoluble form such as a Cs-ball [30]. In addition, we found that the proportions of ¹³³Cs and ¹³⁷Cs in the precipitation of the bark (Fig. 5) were higher than those in the wood (Fig. 5); this can be explained in two ways: (1) ¹³³Cs and ¹³⁷Cs dissolved during the kraft pulping process but co-precipitated together with specific components in the bark via acidification and/or (2) the pore sizes of the filter paper (5 μm) used to separate the pulp and the black liquor and the membrane filter (0.45 μm) used to separate the supernatant and the precipitation were different. Therefore, it is likely that Cs-bearing particles with diameters smaller than 5 μm but larger than 0.45 μm passed through the filter paper but were retained on the membrane filter. One possible Cs-bearing particle is a ¹³⁷Cs-ball, the diameter of which has been reported to be approximately 2 μm [30]. Previous studies have also reported that ¹³⁷Cs absorbed on bark likely exists in a stable chemical form and is not easily washed off the bark by weak acid solutions [32]. Further investigations are required to fully understand the speciation of ¹³⁷Cs in bark.

Conclusions

In this study, laboratory kraft cooking of ¹³³Cs-treated and ¹³⁷Cs-contaminated C. japonica wood and bark was conducted to investigate the behavior of Cs during the kraft cooking process and to evaluate whether the ¹³⁷Cs-contaminated C. japonica could be used in pulp and paper industries. Most of the Cs was transferred into the black liquor, while only a minor amount of the Cs was retained in the pulp. A minor amount of the Cs was also contained in the precipitation yielded by the acidification of the black liquor. From these results, it is suggested that the pulp from ¹³⁷Cs-contaminated woods be used if this minor amount of Cs retained in the pulp can be removed. The digested agents in black liquor can be recovered if the ¹³⁷Cs can be removed by a ¹³⁷Cs-special absorbent. In addition, the lignin could be used for further applications. The different behavior of Cs in the kraft cooking of bark and wood suggests that ¹³⁷Cs is bonded with unknown components in the bark that are difficult to dissolve in an alkaline solution. Further study at the pulp mill-scale would be interesting and is required for the forest industry in the Fukushima Prefecture to recover.