Evaluation of Ion-Exchange Characteristics of Cesium in Natural Japanese Rocks

technologies

Article Evaluation of Ion-Exchange Characteristics of Cesium in Natural Japanese Rocks

Takuya Miura 1,*, Atsushi Sasaki 2 and Masatoshi Endo 1

1 Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan; [email protected] 2 Faculty of Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-238-26-3142

Received: 20 June 2018; Accepted: 10 August 2018; Published: 17 August 2018

Abstract: A large amount of soil was contaminated by radioactive cesium due to the accident at the Fukushima Daiichi Nuclear Power Plant in Japan in 2011. The adsorption behavior of cesium ions (Cs+) is strongly influenced by numerous factors, including the components, structure and weathering conditions of natural soil. The adsorption and ion exchange characteristics of Cs+ ions onto and from natural Japanese rocks with well-known components were studied. Cs+ adsorption onto volcanic rocks (0.9–5.3 mg/g) occurred more easily than that onto plutonic rocks (0.7–0.8 mg/g) due to differences in crystallinity. In addition, the adsorption quantity of cesium increased with increasing lattice water content and content of ion-exchangeable cations in the rock samples. The cesium adsorption ability of rock was inhibited by seawater and coexisting ions in the solution. Cesium adsorption quantities onto andosol, containing the corrosion products, increased approximately 2.7-fold with increasing pH from neutral to basic. Cesium desorption differed depending on the type of salt used, and the desorption rates were highest with ammonium salts. Cs+ desorption from regions such as the soil interlayer and the pores were inhibited by melting of the rock surface.

Keywords: cesium; natural Japanese rocks; adsorption/desorption; heat treatment

1. Introduction The accident at the Fukushima Daiichi Nuclear Power Plant, which occurred as a consequence of the Great East Japan Earthquake on March 11, 2011, resulted in widespread contamination with radioactive materials [1–3]. A large amount of soil was contaminated by this accident. However, efficient and practical remediation of contamination caused by radioactive substances is difficult. Several reports have examined cesium adsorption/desorption onto zeolite, bentonite, and mica [4–12]. The adsorption processes of exchangeable cations on the adsorption site of their minerals are different. For example, cesium ions (Cs+) are adsorbed in the interlayer of lamellar minerals and in the pores of zeolite [8,11,12]. However, the mechanism by which Cs+ adsorb onto and desorb from all minerals on the earth has not been fully elucidated. Natural soil is mixed with various kinds of rock [1,4,13]. Additionally, weathering processes, such as erosion, cause structural and chemical changes in natural minerals. The radioactive materials from the Fukushima Daiichi Nuclear Power Plant have been incorporated into clay minerals—such as mica—in the natural soil in the surrounding environment. Cs+ ions are known to adsorb strongly onto the natural soil, such as biotite [7,14,15]. In particular, the half-time of Cs-137 is about 30 years [7,10]. Therefore, quick response to soil contamination is necessary. Since the adsorption and desorption of Cs+ ions onto and from natural soil are strongly inﬂuenced by several factors, including the components, structure, and weathering conditions of the soil, the behavior on Cs in an actual environment has not yet been determined. The clear elucidation of the

Technologies 2018, 6, 78; doi:10.3390/technologies6030078 www.mdpi.com/journal/technologies Technologies 2018, 6, x FOR PEER REVIEW 2 of 12

+ TechnologiesSince2018 the, 6 , 78adsorption and desorption of Cs ions onto and from natural soil are strongly2 of 12 influenced by several factors, including the components, structure, and weathering conditions of the soil, the behavior on Cs in an actual environment has not yet been determined. The clear elucidation cesiumof the adsorptioncesium adsorption and desorption and desorption mechanism mechanis is necessary.m is necessary. Therefore, Therefore, in this study, in this the study, adsorption the + andadsorption ion exchange and ion characteristics exchange characteristics of Cs ions ontoof Cs and+ ions from onto natural and from Japanese natural rocks Japanese with well-knownrocks with componentswell-known werecomponents studied. were studied.

2.2. MaterialsMaterials andand MethodsMethods 2.1. Natural Japanese Rocks 2.1. Natural Japanese Rocks NaturalNatural JapaneseJapanese rocksrocks were obtained from the Ge Geologicalological Survey Survey of of Japa Japann in in National National Institute Institute ofof AdvancedAdvanced IndustrialIndustrial Science and Technology (A (AIST).IST). These These natural natural Japanese Japanese rocks rocks were were ground ground intointo powder powder inin aa mortarmortar andand sievedsieved with 0.149 mm mm sieves. sieves. Basic Basic data, data, such such as as the the sample sample components components andand collection collection locations, locations, were were previously previously reported reported [16 [16].]. Figure Figure1 shows 1 shows that that the referencethe reference samples samples were classiﬁedwere classified as igneous as igneous rock, sedimentary rock, sedimentary rock, metamorphic rock, metamorphic rock, and rock, sediments. and sediments. In this study,In this andesitestudy, (JA),andesite basalt (JA), (JB), basalt granodiorite (JB), granodiorite (JG), gabbro (JG), (JGb),gabbro chert (JGb), (JCh), chert slate (JCh), (JSl), slate andosol (JSl), andosol (JSO), and(JSO), stream and sedimentstream sediment (JSd) were (JSd) selected. were selected. “J” in these “J” in abbreviations these abbreviations denotes denotes that these that samples these samples were collected were + incollected Japan. Thein Japan. silica oxideThe silica (SiO 2oxide) content (SiO and2) content crystallinity and crystallinity of rock strongly of rock inﬂuence strongly Cs influenceadsorption Cs+ behavior.adsorption Igneous behavior. rock Igneous and the rock other and rock the groups othe werer rock further groups divided were intofurther two divided categories: into beforetwo weatheringcategories: before and after weathering weathering. and after weathering.

Classification Acid rock Intermediate Basic rock Ultrabasic rock rock SiO2 More than 66% 65～53% 52～45% Less than 44% Granite Serpentinite Syenite JG Diorite Gabbro Peridotite Granodiorite Quartz-diorite Pyroxenite rock JGb Monzonite Amphibolit e Plutonic Plutonic

Granite-porphyry Holocrystalline or Quartz-porphyry Porphyrite Diabase Igneous rock Dyke rock Dyke rock Hypabyssal JA JB Rhyolite Andesit e Basalt Liparite Dacite Hypocrystalline Obsidian ･ Pitchstone

Volcanic rock Pumice Scoria Welded tuff

Rocks Glassy

Sedimentary rock Chert ，Sandstone，Mudstone，Dolomite，・・・ JCh Metamorphic ，，，・・・ rock Slate Quartz-schist Biotite-schist JSl The others Andosol ， Stream sediment，Marine sediment，・・・ JSO JSd

JA, JB, JG, JGb, JCh, JSl, JSO and JSd were shown as andesite, basalt, granodiorite, gabbro, chert, slate, andosol and stream sediment.

FigureFigure 1.1. ClassiﬁcationClassification of rocks used in this study: igneous igneous rock rock (JA, (JA, JB, JB, JG JG,, and and JGb), JGb), sedimentary sedimentary rockrock (JCh),(JCh), metamorphicmetamorphic rockrock (JSl),(JSl), andand other rocks (JSO and JSd).

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2.2. Cesium Adsorption Experiments Cs+ aqueous solutions of 200 or 2000 mg/L were prepared from dissolving cesium chloride powder (Wako Pure Chemical Industries, Ltd., Osaka, Japan, Assay min. 99.0%) into deionized water. The particle diameter of all the mineral samples was less than 0.149 mm. The Cs+ adsorption saturation time of the minerals in this study was 5 min for zeolites and 30 min for the other minerals. A Cs+ solution of 50 mL and a mineral sample of 0.5 g were placed in a 200 mL polystyrene container and stirred for 2 h. Each sample solution was filtered through a 0.20-µm syringe filter, and the saturation quantities of adsorbed Cs+ were determined on the basis of the Cs+ concentration in the filtrates, as measured by inductively coupled plasma-mass spectrometry (ICP-MS, ELAN-DRCII, Perkin Elmer, Yokohama, Japan). The adsorption tests with seawater were carried out using the commercial product Tetra Marin® Salt Pro sea salt (Spectrum Brands Japan Co., Ltd., Yokohama, Japan). Cs+ was added at a concentration of 200 ppm to seawater adjusted to a predetermined concentration (specific gravity 1.020–1.023, 1 L water, and 33 g sea salt). Cs+ adsorption experiments on 0.5 g of the rocks were conducted with seawater using the procedure described in the cesium adsorption test using deionized water. Solutions with coexisting ions, including potassium ions (K+), magnesium ions (Mg2+), calcium ions (Ca2+), and barium ions (Ba2+), with concentrations of 100 or 200 ppm, were prepared from potassium chloride, magnesium nitrate, calcium carbonate, and barium nitrate, respectively. The initial concentration of Cs+ used in these adsorption tests was 200 ppm. Cs+ adsorption experiments on 0.5 g of JB-1a and JSO-1 using solutions with coexisting ions were conducted using the procedure described in the cesium adsorption test, using deionized water. The pH of the Cs+ solution (200 ppm) was adjusted to 2, 4, 6, 8, 10, and 12 with hydrochloric acid or sodium hydroxide solution. Cesium adsorption experiments were conducted on JB-1a and JSO-1 (0.5 g) under various pH conditions using the procedure described in the cesium adsorption test using deionized water.

2.3. Cesium Desorption Experiments

Hydrochloric acid (HCl), diammonium hydrogen phosphate ((NH4)2HPO4), ammonium chloride (NH4Cl), ammonium fluoride (NH4F), ammonium oxalate ((NH4)2C2O4), and ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA-2Na·2H2O) were used in the desorption tests. First, 50 mL of 0.1 mol/L solution of each reagent was added to a JB-1a sample with Cs+ adsorbed at 2.01–2.94 mg/0.5 g, and the resultant solution was stirred for 2 h. Similar desorption tests were carried out after the adsorption tests. Solutions with coexisting ions K+, Mg2+, Ca2+ and Ba2+ were prepared from potassium chloride, magnesium nitrate, calcium carbonate, and barium nitrate, respectively. Next, 50 mL of a solution with 200 ppm of each ion was added to JB-1a and JSO-1 samples with Cs+ adsorbed at 1.9–2.5 mg/0.5 g and 2.5–2.7 mg/0.5 g, respectively. The resultant mixtures were stirred for 2 h. Cs+ desorption experiments on 0.5 g of JB-1a and JSO-1 in the presence of the coexisting ions were conducted using the procedure described in cesium desorption test with ammonium salts.

2.4. Heat Treatment Experiments The rock samples were calcined at 400, 600, 800, and 1000 ◦C, and Cs+ adsorption experiments were conducted with the calcined samples. In addition, a desorption experiment using 0.1 mol/L hydrochloric acid on the calcined rock samples was carried out. The rock and solution ratios and the procedure after stirring were similar to those used in the adsorption experiments. Calcined rocks were analyzed by X-ray diffraction (XRD, Ultima-IV, Rigaku, akishima, Japan), thermogravimetric and differential analysis (TG-DTA, 2000 SA, Bruker-AXS, Yokohama, Japan), and scanning electron microscopy (SEM, SU 8000, Hitachi, Minato, Japan). TechnologiesTechnologies2018 2018, 6, ,6 78, x FOR PEER REVIEW 44 ofof 1212

3. Results and Discussion 3. Results and Discussion 3.1. Adsorption of Cs+ onto Natural Japanese Rocks 3.1. Adsorption of Cs+ onto Natural Japanese Rocks The components and crystallinity of natural minerals differ based on the formation process of the mineralThe components with ground and heat crystallinity and pressure of natural [16]. minerals The results differ of basedthe Cs on+ adsorption the formation tests process indicate of that the + + mineralCs+ was with adsorbed ground onto heat the and JA pressure and JB [ 16group]. The rocks results (volcanic of the Cs rock:adsorption merocrystalline). tests indicate In thatcontrast, Cs was Cs+ + adsorbedwas hardly onto adsorbed the JA andonto JB JG group and JGb rocks (plutonic (volcanic rock rock:s: holocrystalline). merocrystalline). The In adsorption contrast, Cs quantitieswas hardly for + adsorbedCs+ onto ontoplutonic JG and rocks, JGb (plutonicwhich are rocks: highly holocrystalline). crystalline, were Theadsorption substantially quantities lower forthan Cs theonto adsorption plutonic rocks,quantities which onto are highly the JA crystalline, and JB were groups, substantially which lowerare poorly than the crystalline adsorption (Table quantities 1 and onto theFigure JA and 2). JBAdditionally, groups, which the are high poorly cesium crystalline adsorption (Table ability1 and Figure of JSO2). was Additionally, due to the the corrosion high cesium product adsorption on the abilitysurface of of JSO JSO was [1]. due Cesium to the was corrosion adsorbed product onto on the the adsorption surface of JSOsites [1 on]. Cesium JSO and was the adsorbed surface hydroxyl onto the adsorptiongroups of the sites corrosion on JSO and product. the surface hydroxyl groups of the corrosion product.

TableTable 1.1.The Themajor majorconstituent constituentof ofnatural natural JapaneseJapanese rocks.rocks.

ConstituentConstituent JA-1JA-1 JA-3JA-3 JB-1JB-1 JB-1aJB-1a JB-2JB-2 JB-3 JG-1 JG-1 JGb-1 JGb-1 JSl-1 JSl-1 JCh-1 JCh-1 JSd-2 JSd-2 JSO-1 JSO-1 SiO2 (%) 64.0 62.3 52.4 52.4 53.3 51.0 72.3 43.7 59.5 97.8 60.8 38.4 SiO2 (%) 64.0 62.3 52.4 52.4 53.3 51.0 72.3 43.7 59.5 97.8 60.8 38.4 Al2O3 (%) 15.2 15.6 14.5 14.5 14.6 17.2 14.2 17.5 17.6 0.734 12.3 18.1 Al2O3 (%) 15.2 15.6 14.5 14.5 14.6 17.2 14.2 17.5 17.6 0.734 12.3 18.1 Fe2O3 (%) 2.59 1.15 2.33 2.55 3.33 3.20 0.380 4.79 1.88 0.272 4.55 8.58 Fe2O3 (%) 2.59 1.15 2.33 2.55 3.33 3.20 0.380 4.79 1.88 0.272 4.55 8.58 FeOFeO (%) (%) 3.98 3.98 4.834.83 5.995.99 5.785.78 9.989.98 7.85 1.61 1.61 9.43 9.43 4.52 4.52 0.087 0.087 5.96 5.96 2.52 2.52 MgOMgO (%) (%) 1.57 1.57 3.723.72 7.717.71 7.837.83 4.624.62 5.19 0.740 0.740 7.85 7.85 2.41 2.41 0.075 0.075 2.73 2.73 2.11 2.11 CaOCaO (%) (%) 5.70 5.70 6.246.24 9.259.25 9.319.31 9.829.82 9.79 2.20 2.20 11.9 11.9 1.48 1.48 0.045 0.045 3.66 3.66 2.55 2.55 NaNa2O2O (%) (%) 3.84 3.84 3.193.19 2.772.77 2.732.73 2.042.04 2.73 3.38 3.38 1.20 1.20 2.18 2.18 0.031 0.031 2.44 2.44 0.670 0.670 K2KO2O (%) (%) 14.2 14.2 1.411.41 1.431.43 1.401.40 0.4200.420 0.780 3.98 3.98 0.240 0.240 2.85 2.85 0.221 0.221 1.15 1.15 0.340 0.340 + H2HO2O(%)+ (%) 0.720 0.720 0.2000.200 1.02 1.02 0.920 0.920 0.2500.250 0.1800.180 0.540 0.540 1.28 1.28 3.92 3.92 0.360 0.360 2.55 2.55 7.88 7.88 − H2HO2O(%)− (%) 0.300 0.300 0.1100.110 0.9500.950 0.9200.920 0.1300.130 0.0700.070 0.070 0.070 0.130 0.130 0.654 0.654 0.152 0.152 0.451 0.451 - -

3000 pure water seawater 2500 (µg)

+ 2000

1500

1000

Adsorption of Cs 500

0 JA-1 JA-3 JB-1 JB-1a JB-2 JB-3 JG-1 JGb-1 JSl-1 JCh-1 JSd-1 JSO-1

FigureFigure 2.2. AdsorptionAdsorption quantityquantity ofof CsCs++ onto the natural Japanese rocks (0.5 g)g) withwith purepure waterwater oror seawater.seawater. InitialInitial CsCs++ concentration:concentration: 200 mg/L.

Water components in natural minerals are divided into H2O++ (water in the crystal lattice) and Water components in natural minerals are divided into H2O (water in the crystal lattice) and H2O−− (moisture content) [17]. The adsorption quantity of Cs++ onto JA and JB groups differed with the H2O (moisture content) [17]. The adsorption quantity of Cs onto JA and JB groups differed with the collection location and the H2O++ content [16]. The evaporation temperature of H2O− − and H2O++ range collection location and the H2O content [16]. The evaporation temperature of H2O and H2O range fromfrom roomroom temperaturetemperature to to 105 105◦ °CC andand 105105 toto 950950◦ °C,C, respectivelyrespectively [[17].17]. ExchangeableExchangeable cationscations areare heldheld between H2O++ and H2O+ in the soil layers. Therefore, the number of exchangeable cations increases between H2O and H2O in the soil layers. Therefore, the number of exchangeable cations increases with increasing H2O++ content in the rocks. with increasing H2O content in the rocks. Figure 3 shows the variation in the ratio between the adsorbed quantity of Cs+ + and the H2O++ Figure3 shows the variation in the ratio between the adsorbed quantity of Cs and the H2O content in the rock samples. The H2O++ content in the rock samples was proportional to the adsorbed content in the rock samples. The H2O content in the rock samples was proportional to the adsorbed + quantityquantity ofof CsCs+ ions.ions. However, thethe slopesslopes ofof thethe twotwo straightstraight approximationapproximation lineslines differeddiffered accordingaccording to the weathering condition of the rocks. This relationship is influenced by the leaching of cations as

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rocks weather in the natural environment [18]. In addition, Table 1 shows that the cation content toratios the weatheringof Mg2+, Ca condition2+, Na+, and of K the+ in rocks. the igneous This relationship rocks wereis relative inﬂuencedly lower by thethan leaching those in of the cations other asrock rocks groups, weather such in as the JSl, natural JCh, JSd, environment and JSO. Those [18]. In cations addition, elute Table from1 showsrock as that weathering the cation progresses. content ratiosTherefore, of Mg cesium2+, Ca2+ adsorption, Na+, and Kquantity+ in the for igneous weathe rocksred wererocks relatively were higher lower than than for those the unweathered in the other rockrocks. groups, such as JSl, JCh, JSd, and JSO. Those cations elute from rock as weathering progresses. Therefore, cesium adsorption quantity for weathered rocks were higher than for the unweathered rocks.

30 Weathered rocks y = 24.218x - 0.1627 (%)

+ R² = 0.9565 20

Unweathered rocks 10 y = 2.2172x + 0.9725 R² = 0.9811 Adsorption of Cs

0 0.00 2.00 4.00 6.00 8.00 10.00 + Contents of H2O in rocks (%) FigureFigure 3. 3.The The correlation correlation between between the the adsorption adsorption quantity quantity for for Cs Cs+ +onto onto the the natural natural Japanese Japanese rocks rocks and and ++ thethe H H2O2O ofof the samples. The solid and broken lines revealreveal thethe resultsresults forfor unweatheredunweathered (igneous(igneous rocks) rocks) andand weathered weathered (others) (others) rocks, rocks, respectively. respectively.

3.2.3.2. Adsorption Adsorption Behavior Behavior of of Cs Cs++ withwith SeawaterSeawater FigureFigure2 shows2 shows that that the Csthe+ adsorptionCs+ adsorption quantities quantities on allrock on all samples rock weresamples lower were when lower the adsorption when the wasadsorption carried outwas in carried seawater out thanin seawater when the than adsorption when the was adsorption carried outwas in carried pure water. out in Thepure decrease water. The in adsorptiondecrease in quantity adsorption for Cs +quantityonto JB-1, for JB-1a, Cs+ andonto JSO-1, JB-1, whichJB-1a, haveand largeJSO-1, saturated which adsorptionhave large quantitiessaturated andadsorption high exchangeable quantities cationand high content, exchangeable exceeded approximately cation content, 50%. exceeded The influence approximately of seawater 50%. on theThe + Csinfluence+ adsorption of seawater quantities on varied the Cs with adsorption the type quantities of rock, and varied the decrease with the in type the Cs of+ rock,adsorption and the quantities decrease + rangedin the fromCs adsorption 12% (minimum: quantities JGb-1) ranged to 90% from (maximum: 12% (minimum: JA-1). Seawater JGb-1) contains to 90% high (maximum: concentrations JA-1). + + 2+ 2+ ofSeawater cations, suchcontains as Na high+,K concentrations+, Ca2+, and Mg of2+ cations,[19]. The such adsorption as Na , K of, Ca Cs+ ,was and inhibited Mg [19]. by The these adsorption cations. + Basedof Cs on was these inhibited results, by cesium these adsorptioncations. Based in seawater on these was results, more cesium difficult adsorption than in pure in seawater water. was more difficult than in pure water. 3.3. Adsorption of Cs+ with Coexisting Ions 3.3. Adsorption of Cs+ with Coexisting Ions The adsorption quantity of Cs+ onto JB-1a was low when divalent ions coexisted. In addition, the CsThe+ adsorption adsorption quantity quantity decreasedof Cs+ ontowith JB-1a increasing was low when concentration divalent ions of divalentcoexisted. ions In addition, (Figure4 the). TheCs+ surfacesadsorption ofrocks quantity are negativelydecreased chargedwith increasing by isomorphous concentration replacement of divalent [20– 22ions]. The (Figure adsorption 4). The afﬁnitysurfaces associated of rocks are with negatively the adsorption charged of by divalent isomorphous ions onto replacement rock is higher [20–22]. than The that adsorption associated affinity with theassociated adsorption with of the monovalent adsorption ions. of divalent Therefore, ions the onto observed rock is inhibition higher than of Cs that+ adsorption associated onto with rock the increasedadsorption in theof monovalent sequence K+ ions.< Ba2+ Theref≈ Mgore,2+ < the Ca2+ observed. inhibition of Cs+ adsorption onto rock increasedIn contrast, in the Cs sequence+ adsorption K+ < Ba onto2+ ≈ JSO-1Mg2+ < was Ca2+ more. affected by the concentration of coexisting ions than byIn theircontrast, valence. Cs+ adsorption Cs+ adsorption onto characteristics JSO-1 was more of JB-1aaffected and by JSO-1 the concentration differed according of coexisting to the cation ions typethan and by concentration.their valence. CsCs++ adsorptionadsorption in characteristics both JB-1a and of JSO-1 JB-1a was and inhibited JSO-1 differed most effectively according by to Ca 2+the. cation type and concentration. Cs+ adsorption in both JB-1a and JSO-1 was inhibited most effectively by Ca2+.

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3000

2500 (µg) (µg) + + 2000

1500

1000 Adsorption of Cs Adsorption of Cs 500

0 Cs K K Mg Mg Ca Ca Ba Ba Cs K K Mg Mg Ca Ca Ba Ba only 100 200 100 200 100 200 100 200 only 100 200 100 200 100 200 100 200 JB-1a JSO-1

FigureFigure 4.4. AdsorptionAdsorption quantityquantity for Cs+ inin solutions solutions with with coexisting coexisting ions: ions: K K+,+ Mg, Mg2+,2+ Ca, Ca2+, 2+and, and Ba2+ Ba. The2+. Theinitial initial concentration concentration of each of each coexisting coexisting ion ionwas was 100100 or 200 or200 mg/L. mg/L.

3.4.3.4. AdsorptionAdsorption QuantityQuantity ofof CsCs++ forfor pHpH ChangeChange TheThe adsorptionadsorption quantityquantity of Cs + ontoonto JB-1a JB-1a and and JSO-1 JSO-1 increased increased with with increasing increasing pH pH (Figure (Figure 5).5 ).In Inparticular, particular, the the adsorption adsorption of of Cs Cs+ +ontoonto JSO-1 JSO-1 increased increased approximately approximately 2.7-fold as the conditionsconditions werewere adjustedadjusted fromfrom neutralneutral toto basic. basic. TheThe adsorptionadsorption quantityquantity ofof Cs Cs++ ontoonto JSO-1JSO-1 ﬂuctuatedfluctuated dramaticallydramatically whenwhen large large quantities quantities of of corrosion corrosion products products were were present. present. The The corrosioncorrosion products products include includesoil soilorganic organic materialsmaterials that that exhibit exhibit a a negativenegative surfacesurface chargecharge underunder basicbasic pHpH conditionsconditions inin aqueousaqueous solutionsolution [[23,24].23,24]. Therefore,Therefore, the the quantity quantity of of adsorbed adsorbed Cs Cs++ onon JSOJSO increasedincreasedwith withincreasing increasingpH. pH.

6000 JB-1a

(µg) JSO-1 (µg) JSO-1 + + 4000

2000 Adsorption of Cs Adsorption of Cs

0 2 4 6 8 10 12 14 pH pH Figure 5. Effect of pH on the adsorption quantity for Cs+ onto JB-1a and JSO-1 (0.5 g). Initial Cs+ FigureFigure 5.5. EffectEffect ofof pHpHon onthe the adsorption adsorption quantity quantity for for Cs Cs+ onto ontoJB-1a JB-1aand and JSO-1 JSO-1 (0.5 (0.5 g). g).Initial InitialCs Cs+ concentration: 200 mg/L. concentration:concentration: 200200 mg/L.mg/L.

+ 3.5.3.5. DesorptionDesorption ofof CsCs+ fromfrom NaturalNatural JapaneseJapanese RocksRocks + FigureFigure6 6shows shows that that desorption desorption of of Cs Cs+ variedvaried withwith thethe typetype ofof saltsalt andandthat thatthe thedesorption desorptionratio ratio waswas highesthighest inin thethe case of ammonium ammonium salts. salts. The The ioni ionicc radius radius of ofexchangeable exchangeable cations cations increases increases in the in 2+ + + + + order Ca2+ < 2+Na+ < K+ < NH+4+ < Cs+ [25,26].+ + The experimental results are likely attributable to a strong the order Ca < Na < K < NH4 < Cs [25,26]. The experimental results are likely attributable + + chemical attraction among NH4+ ions, whose+ ionic radii are essentially equal to those of Cs+ ions. In to a strong chemical attraction among NH4 ions, whose ionic radii are essentially equal to those + ofaddition, Cs+ ions. the In desorption addition, theratio desorption of Cs in ratiothe presence of Cs+ in of the EDTA presence was ofapproximately EDTA was approximately 80%, which is + 80%,attributed which to is the attributed ion-coordinating to the ion-coordinating ability of EDTA ability and ofthe EDTA ion-exchange and the ability ion-exchange of Na . abilityStructure of components, such as Al3+ in JB-1a, can be dissolved by hydrochloric acid [10]. In this study, Cs+ was easily desorbed from rock samples in H2O (desorption ratio 30%) and in hydrochloric acid

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Na+. Structure components, such as Al3+ in JB-1a, can be dissolved by hydrochloric acid [10]. In this Technologies+ 2018, 6, x FOR PEER REVIEW 7 of 12 study, Cs was easily desorbed from rock samples in H2O (desorption ratio 30%) and in hydrochloric Technologies 2018, 6, x FOR PEER REVIEW + 7 of 12 acid (desorption(desorption ratio ratio 80%), 80%), due due to tothe the intentionally intentionally high high concentrations concentrations of Cs+ ofadsorbed Cs adsorbed onto the ontorock the rock samples. samples. + (desorption ratio 80%),+ due to the intentionally high concentrations of Cs adsorbed onto the rock The desorption of Cs ions+ was reported to be difﬁcult under conditions where the adsorbed samples.The desorption of Cs ions was reported to be difficult under conditions where the adsorbed + + quantityquantity forThe Cs fordesorption Csonto onto the ofthe substrate Cs substrate+ ions was was was reported low, low, as as is toistrue truebe difficult for natural under soil soil conditions [20,21]. [20,21 ].In Inwherethis this work, the work, however,adsorbed however, + + + + we observedwequantity observed thatfor Cs that high+ onto high desorption the desorption substrate ratios ratios was of low, of Cs Cs aswere wereis true duedue for to natural the large large soil adsorbed [20,21]. adsorbed In quantity this quantity work, of Cs ofhowever, Cs. . we observed that high desorption ratios of Cs+ were due to the large adsorbed quantity of Cs+. 100 100 80 (%) + 80 (%)

+ 60 60 40 40 Desorption of Desorption Cs 20 Desorption of Desorption Cs 20 0 H₂aO b HCl c d(NH ₄ )₂ HPO ₄e fNH ₄ Cl g NH₄F(NH₄)₂C₂O₄ EDTA・2Na・2H₂O 0 H₂aO b HCl c d(NH ₄ )₂ HPO Desorption₄e fNH ₄ Cl agents g NH₄F(NH₄)₂C₂O₄ EDTA・2Na・2H₂O

Desorption agents FigureFigure 6. Desorption 6. Desorption ratio ratio (%) (%) of of Cs Cs+ +from from JB-1aJB-1a (0.5 (0.5 g) g) using using 0.1 0.1 mol/L mol/L solutions solutions of different of different salts. a: salts. H2O, b: HCl, c: (NH4)2HPO, d: NH4Cl, e: NH4F, f: (NH4)2C2O4, and g: Ethylenediaminetetraacetic acid a: H2O,Figure b: HCl, 6. Desorption c: (NH4)2 ratioHPO, (%) d: of NH Cs4+Cl, from e: NHJB-1a4F, (0.5 f: (NHg) using4)2C 0.12O mol/L4, and solutions g: Ethylenediaminetetraacetic of different salts. a: disodium salt, 2-hydrate. The desorption ratios were calculated based on the Cs+ adsorption +quantities acid disodiumH2O, b: HCl, salt, c: (NH 2-hydrate.4)2HPO, d: The NH desorption4Cl, e: NH4F, ratiosf: (NH4 were)2C2O4 calculated, and g: Ethylenediaminetetraacetic based on the Cs adsorption acid in Figure 2. quantitiesdisodium in Figure salt, 2-hydrate.2. The desorption ratios were calculated based on the Cs+ adsorption quantities in Figure 2. 3.6. Desorption of Cs+ with Coexisting Ions 3.6. Desorption of Cs+ with Coexisting Ions 3.6. DesorptionFigure 7 shows of Cs+ withthat Coexistingthe Cs+ desorption Ions ratio in the presence of other coexisting ions differed + Figure7 shows that the Cs desorption ratio in the presence of+ other coexisting ions differed dependingFigure on7 shows the type that of therock. Cs We+ desorption confirmed ratio that in desorption the presence of Cs of otherfrom JB-1acoexisting using ions monovalent differed depending on the type+ of rock. We confirmed that desorption of Cs+ from JB-1a using monovalent dependingcations such on as the K wastype easier of rock. than We the confirmed desorption that using desorption divalent of cations. Cs+ from Desorption JB-1a using from monovalent JSO-1 was + 2+ 2+ cationsmorecations such strongly such as K as wasinfluenced K+ was easier easier by than Bathan the due the desorption desorptionto the ionicusing usingradius divalent of Ba beingcations. cations. similar Desorption Desorption to that from offrom cesium JSO-1 JSO-1 [27].was was + 2+ + 2+ moremoreWe strongly confirmed strongly influenced influencedthat K most by by Ba strongly Badue2+ due promoted to to the the ionic ionic Cs radiusradius desorption of Ba 2+from beingbeing JSO-1 similar similar and toJB-1a, tothat that respectively,of cesium of cesium [27]. in[ 27]. + + We confirmedWethe presenceconfirmed that of that coexisting K Kmost+ most stronglycations. strongly promotedpromoted Cs Cs+ desorptiondesorption from from JSO-1 JSO-1 and andJB-1a, JB-1a, respectively, respectively, in in thethe presence presence of of coexisting coexisting cations.cations. 40 40 30 (%) + 30 (%) + 20 20 10

Desorption of Cs 10 0 Desorption of Cs 0 KMgCaBa KMgCaBa KMgCaBaJB-1a KMgCaBaJSO-1

JB-1a JSO-1 Figure 7. Desorption ratio (%) of Cs+ from JB-1a and JSO-1 (0.5 g) in solutions with coexisting ions + (initial concentration: 200 mg/L); the+ desorption ratios were calculated based on the Cs adsorption FigureFigure 7. Desorption 7. Desorption ratio ratio (%) (%) of of Cs Cs+ from from JB-1a JB-1a andand JSO-1JSO-1 (0.5 (0.5 g) g) in in solutions solutions with with coexisting coexisting ions ions quantities in the absence of the coexisting ions (Figure 2). + (initial(initial concentration: concentration: 200 200 mg/L); mg/L); the the desorption desorption ratiosratios werewere calculated based based on on the the Cs Cs adsorption+ adsorption quantities in the absence of the coexisting ions (Figure 2). quantities in the absence of the coexisting ions (Figure2).

Technologies 2018, 6, x FOR PEER REVIEW 8 of 12

Technologies 2018, 6, 78 8 of 12 3.7. Effect of the Heat Treatment of Rocks

Table 2 shows the influence of calcination temperature on the adsorption of Cs+ onto the rock 3.7. Effect of the Heat Treatment of Rocks samples. The adsorption quantity for Cs+ onto all rocks decreased with increasing calcination temperature.Table2 shows The theadsorption influence quantity of calcination of Cs temperature+ onto rocks on calcined the adsorption at 1000 of °C Cs was+ onto limited the rock to approximatelysamples. The adsorption 20% or less quantity of the for saturated Cs+ onto alladsorbent rocks decreased quantities with in increasing the absence calcination of calcination. temperature. The Theadsorption adsorption quantities quantity of of Cs Cs+ +ontoonto all rocks rocks calcined calcined at 1000 at 1100◦Cwas °C were limited below to approximately the lower detection 20% or less limit of ofthe the saturated ICP-MS adsorbent instrumentation. quantities in the absence of calcination. The adsorption quantities of Cs+ onto all rocks calcined at 1100 ◦C were below the lower detection limit of the ICP-MS instrumentation. Table 2. Adsorption ratio (%) for Cs+ onto rocks (0.5 g) calcined at various temperatures, as calculated Tablebased 2.onAdsorption the Cs+ adsorption ratio (%) quantities for Cs+ onto with rocksout calcination (0.5 g) calcined (Figure at various 2). temperatures, as calculated based on the Cs+ adsorption quantities without calcination (Figure2). 400 °C 600 °C 800 °C 1000 °C 1100 °C JB 40068.1◦C 60058.3◦C 28.6 800 ◦C 19.8 1000 ◦C 0.01100 ◦C JBJSO 68.198.5 76.7 58.3 45.1 28.6 17.5 19.8 0.0 0.0 JSOJA 98.535.8 24.5 76.7 26.4 45.1 14.2 17.5 0.0 0.0 JAJSd 35.8100 30.0 24.5 13.6 26.4 0.0 14.2 0.0 0.0 JSdJSl 10090.2 83.4 30.0 48.0 13.6 21.2 0.0 0.0 0.0 JSl 90.2 83.4 48.0 21.2 0.0 Dehydration, oxidation decomposition, oxidation, and melting events for each rock sample were detectedDehydration, by TG-DTA oxidation (Figure decomposition, 8). These results oxidation, indicate andthat meltingstructural events changes, for each such rock as samplemelting were and decomposition,detected by TG-DTA occur (Figureduring8 ).heating, These leading results indicateto a reduction that structural in the soil changes, interlayer such distance as melting by dehydration.and decomposition, occur during heating, leading to a reduction in the soil interlayer distance by dehydration.

33.4 0

2 TG 33.3 DTA -10 33.2 3 -20 33.1 TG (mg) 33 DTA (µV) -30

32.9 1 -40 32.8

4 32.7 -50 0 200 400 600 800

Temperature (°C) Figure 8. Thermogravimetric and differential analysis (TG- (TG-DTA)DTA) patterns for JB-1a: 1. dehydration, dehydration, 2. 2. oxidation decomposition, 3. oxidation, and 4. melting.

In addition, Figure 9 shows the XRD patterns for rock samples calcined at different In addition, Figure9 shows the XRD patterns for rock samples calcined at different temperatures. temperatures. The diffraction peaks of the rock samples decreased in intensity with increasing The diffraction peaks of the rock samples decreased in intensity with increasing calcination temperature. calcination temperature. These results indicate that the skeleton structure of the rocks was destroyed These results indicate that the skeleton structure of the rocks was destroyed by heat. by heat.

TechnologiesTechnologies2018 ,20186, 78, 6, x FOR PEER REVIEW 9 of 129 of 12

Technologies 2018, 6, x FOR PEER REVIEW 9 of 12

1100 ᵒC

1100 ᵒC 1000 ᵒC

1000 ᵒC 800 ᵒC

800 ᵒC ᵒ

Intensity/a.u. 600 C

ᵒ

Intensity/a.u. 600 C 400 ᵒC

400 ᵒC Without calcination Without calcination 0 20406080 2θ /deg CuKα (°) 0 20406080 Figure 9. X-ray diffraction (XRD) patterns of2θ JB- /deg1a calcined CuKα at(°) various temperatures (●: albite calcian; Figure 9. X-ray diffraction (XRD) patterns of JB-1a calcined at various temperatures ( : albite calcian; 4: augite;△: augite; CuK CuKα;α scan; scan rate: rate: 10 10°/min;◦/min; and and step step scan scan size: size: 0.02°). 0.02 ◦). Figure 9. X-ray diffraction (XRD) patterns of JB-1a calcined at various temperatures (●: albite calcian; △The: augite; effects CuK of αmelting; scan rate: of the10°/min; surfaces and stepof the scan rock size: samples 0.02°). were observed by SEM (Figure 10). The surfacesThe effects of rocks of melting not subjected of the to surfaces calcination of thewere rock uneven, samples as shown were in observed Figure 10a,c. by SEM The (Figuresurface 10). The surfacesstructureThe effects ofof rocksthe rocksof melting not calcined subjected of the at surfaces1000 to calcination °C becameof the rock weresmooth. samples uneven, All were rock as observedsamples shown inwere by Figure SEM changing (Figure 10a,c. state 10). The dueThe surface structuresurfacesto heat. of Therefore, theof rocks rocks not calcinedthe subjectedadsorption at 1000 to of calcination ◦CsC+ became onto the were smooth. rocks uneven, was All inhibited as rock shown samples by in a decreaseFigure were 10a,c. changing in the The number statesurface of due to structure of the rocks calcined at 1000 °C became smooth. All rock samples were changing state due heat.adsorption Therefore, sites the upon adsorption heat treatment. of Cs+ onto the rocks was inhibited by a decrease in the number of to heat. Therefore, the adsorption of Cs+ onto the rocks was inhibited by a decrease in the number of adsorption sites upon heat treatment. adsorption sites upon heat treatment.

(a) (b)

Figure 10. Scanning electron(c )micro scopy (SEM) images of (a) JB-1a and( d(c)) JSO-1 without calcination and (b) JB-1a and (d) JSO-1 after calcination at 1000 °C; magnification: 30,000×. Figure 10. Scanning electron microscopy (SEM) images of (a) JB-1a and (c) JSO-1 without calcination Figure 10. Scanning electron microscopy (SEM) images of (a) JB-1a and (c) JSO-1 without calcination and (b) JB-1a and (d) JSO-1 after calcination at 1000 °C; magnification: 30,000×. and (b) JB-1a and (d) JSO-1 after calcination at 1000 ◦C; magniﬁcation: 30,000×.

Technologies 2018, 6, 78 10 of 12

Table3 shows that the desorption quantity of Cs + from rocks tended to decrease with increasing calcination temperature. This result is related to changes in the skeletal structure and the Cs+ adsorption behavior. The desorption of Cs+ from, for example, soil interlayers and pores, would be inhibited by the melting rock surface. In particular, the inhibitory effect of Cs+ desorption was high in cases where the rock was calcined at temperatures greater than at 1000 ◦C. Nevertheless, Cs+ desorption from rock samples calcined at 400 and 600 ◦C occurred easily due to incomplete structural changes and because the cations were not trapped inside the rock. Consequently, heat treatments, similar to the calcination at 1000 ◦C described above, are expected to be used as the ﬁnal disposal method for contaminated soil.

Table 3. Desorption ratio (wt %) of Cs+ from the rocks (0.5 g) calcined at various temperatures, as calculated based on the Cs+ adsorption quantities t without calcination (Figure2).

Water HCl (pH 1) (◦C) Non calcination 400 600 800 1000 1100 400 600 800 1000 1100 JB 31.0 3.4 2.2 0.6 0.2 0.2 87.1 47.1 4.8 1.4 0.2 JSO 39.2 11.2 13.0 5.9 0.5 0.8 88.6 50.2 23.8 2.6 1.5 JA 31.0 10.2 8.8 4.2 2.3 0.9 58.4 31.1 16.6 3.8 0.3 JSd 90.1 31.6 15.1 9.9 3.0 1.3 95.9 52.0 16.1 4.3 0.8 JSl 65.5 29.3 16.7 15.5 1.1 0.5 100 100 35.2 2.2 0.4

4. Conclusions We confirmed that Cs+ adsorption onto volcanic rocks (0.9–5.3 mg/g) occurred more easily than that onto plutonic rocks (0.7–0.8 mg/g). Adsorption quantity of Cs+ onto rock samples increased with increasing exchangeable cation quantity in the solution and decreasing rock crystallinity. Cesium adsorption quantities for the before weathering rocks were higher than for the after weathering rocks. The adsorption quantity for Cs+ onto all the rock samples decreased when seawater was used. The adsorption of Cs+ was inhibited under adsorption experiments conducted with Ca2+ ions coexisting in the solution. In addition, the adsorption quantity of Cs+ onto JSO increased approximately 2.7-fold when the solution pH was changed from acidic to basic. We determined that the adsorption behavior in the case of JSO-1 was influenced by the pH-dependency of the corrosion product. + + The desorption ratio of Cs in the case of ion exchange with NH4 salts was relativity high. 2+ + + + + The ionic radius of exchangeable cations occurs in the increasing order: Ca < Na < K < NH4 < Cs . Cesium desorption behavior with ammonium salts was attributable to a strong chemical attraction + + among NH4 , whose ionic radius is substantially equal to that of a Cs ion. Desorption behavior with coexisting ions was related to the valence and radius of the ions. The quantity of Cs+ adsorbed onto and desorbed from all the rock samples decreased with increasing rock calcination temperature. The results were influenced by the melting of the rock surface, which decreased the number of available adsorption sites. In this study, we confirmed that the cesium adsorption and desorption characteristics were changed by the major constituent elements in the natural Japanese rocks and the condition of the reaction solution (seawater, coexisting ions, and pH). These characteristics of rock in natural environments are influenced by many factors, such as structure, weathering condition, exchangeable cations, and pH. This research result, using natural Japanese rocks with well-known components, is expected to help elucidate and simplify the cesium adsorption/desorption characteristics for contaminated soil. Additionally, we were able to control the cesium adsorption and desorption with the calcination of rocks. Technologies 2018, 6, 78 11 of 12

Author Contributions: Conceptualization, T.M.; M.E.; Data Curation, T.M.; A.S.; M.E.; Formal Analysis, T.M.; A.S.; M.E.; Investigation, T.M.; Methodology, T.M.; A.S.; M.E.; Project Administration, T.M.; M.E; Supervision, M.E.; Validation, A.S.; M.E.; Writing Original Draft Preparation, T.M.; Writing Review and Editing, A.S.; M.E; All authors have read and approved the final manuscript. Atsushi Sasaki is my adviser for the experiment and writing article. Masatoshi Endo is my PhD supervisor. Funding: This research received the funding from Japan Society of Ion Exchange. Conflicts of Interest: The authors declare no conflicts of interest.

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