Isotopic analysis of nickel, copper, and zinc in various freshwater samples for source identification
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Geochemical Journal, Vol. 55, pp. 171 to 183, 2021 doi:10.2343/geochemj.2.0627
Isotopic analysis of nickel, copper, and zinc in various freshwater samples
for source identification
SHOTARO T AKANO,1* MAO TSUCHIYA,1 SHOJI IMAI,2 YUHEI YAMAMOTO ,2 YUSUKE FUKAMI,3
KATSUHIKO SUZUKI3 and YOSHIKI S OHRIN1
1
Institute for Chemical Research, Kyoto University, Japan
2
Graduate School of Technology, Industrial, and Social Sciences, Tokushima University, Japan
3
Japan Agency for Marine-Earth Science and Technology, Japan
(Received January 21, 2021; Accepted April 13, 2021)
Nickel (Ni), copper (Cu), and zinc (Zn) are commonly used in human activities and pollute aquatic environments
including rivers and oceans. Recently, Ni, Cu, and Zn isotope ratios have been measured to identify their sources and
cycles in environments. We precisely determined the Ni, Cu, and Zn isotope ratios in rain, snow, and rime collected from
Uji City and Mt. Kajigamori in Japan, and investigated the potential of isotopic ratios as tracers of anthropogenic materi-
als. The isotope and elemental ratios suggested that road dust is the main source of Cu in most rain, snow, and rime
samples and that some of the Cu may originate from fossil fuel combustion. Zinc in the rain, snow, and rime samples may
be partially attributed to Zn in road dust. Zinc isotope ratios in the Uji rain samples are lower than those in the road dust,
which would be emitted via high temperature processes. Nickel isotope ratios are correlated with V/Ni ratios in the rain,
snow, and rime samples, suggesting that their main source is heavy oil combustion. Furthermore, we analyzed water
samples from the Uji and Tawara Rivers and the Kakita River spring in Japan. Nickel and Cu isotope ratios in the river
water samples were significantly heavier than those in rain, snow, and rime samples, while Zn isotope ratios were similar.
This is attributed to isotopic fractionation of Cu and Ni between particulate-dissolved phases in river water or soil.
Keywords: spring water, rain, snow, rime, river
used as powerful tracers of the sources of trace metals in
INTRODUCTION
various fields including the ocean (Cameron and Vance,
Heavy metals, such as Fe, Mn, Ni, Cu, Zn, and Pb are 2014; Conway and John, 2014; Homoky et al., 2013;
commonly used in human activities and pollute aquatic Vance et al., 2008), river (Chen et al., 2008), soil (Bigalke
environments including rivers and oceans. The enrich- et al., 2010; Ratié et al., 2016), and atmosphere (Dong et
ment of aquatic environments with heavy metals affects al., 2013, 2017; Ochoa-Gonzalez et al., 2016). In par-
aquatic organisms including phytoplankton (Morel and ticular, isotope ratios of multi-elements powerfully con-
Price, 2003; Stefania et al., 2017). Anthropogenic metals strain sources and processes associated with their elements
emitted from industrial and residential wastewater flow (Guinoiseau et al., 2018; John and Conway, 2014; Souto-
into the aquatic environment. In addition, anthropogenic Oliveira et al., 2019; Takano et al., 2020). For example,
metal emissions in the atmosphere are transferred to the Souto-Oliveira et al. (2018, 2019) have determined Cu,
aquatic environment via dry and wet deposition. It is dif- Zn, and Pb isotope ratios in atmospheric particles col-
ficult to evaluate the amount of metals in the aquatic en- lected in Sao Paulo, and determined the sources of these
vironment that is supplied by human activities, because metals, such as vehicular traffic, construction activities,
of the natural sources of metals that include rock weath- and metallurgical industries.
ering, decomposition of organic matter, and input of min- The multi-elemental isotopic study of heavy metals
eral dust (Chester and Jickells, 2012). for atmospheric wet deposition has not been conducted
The development of multi-collector inductively cou- previously. Wet deposition is a dominant mechanism for
pled plasma mass spectrometry (MC-ICP-MS) has al- transporting atmospheric particles to the Earth’s surface
lowed for the quick and precise measurements of isotope in East Asia. In southeast China, wet deposition fluxes of
ratios for heavy metals. Their isotope ratios are widely Cu and Zn are 1.5–6 times more than the dry depositions
(Ye et al., 2018). In this study, we present an effective
and precise method for determining Ni, Cu, and Zn iso-
*Corresponding author (e-mail: takano.shotaro.3r@kyoto-u.ac.jp)
tope ratios in freshwater samples. This method can si-
Copyright © 2021 by The Geochemical Society of Japan. multaneously separate Ni, Cu, and Zn from freshwater
171
Fig. 1. Sampling sites. River water was collected from the Uji and Tawara Rivers. Rainwater was collected at the Institute for
Chemical Research and Mt. Kajigamori. Spring water was collected from Observatory-1 and Observatory-2 in the Kakita River
spring. This figure is based on the Digital Basic Map published by the Geospatial Information Authority of Japan.
samples only using two-step separation, and successfully lab wares, such as pipette tips and centrifuge tubes, were
determine their isotope ratios using a MC-ICP-MS. Then, soaked in warm 3 M HNO3 and rinsed with ultrapure
we analyze the Ni, Cu, and Zn isotope ratios in rainwa- water.
ter, snow, rime, spring, and river water collected in Japan
to evaluate the potential of the isotope ratios of metals as Samples
tracers of anthropogenic materials in aquatic environ- Sampling sites are shown in Fig. 1. Rainwater sam-
ments. ples were collected from the rooftop of the Institute for
Chemical Research, Kyoto University, Uji, Kyoto
(34.910°N, 135.802°E). Uji is a suburban city with
METHOD
180,000 habitants. Rainwater, snow, and rime samples
Reagents and materials were collected from the top of Mt. Kajigamori, Otoyo,
Ultrapure water was supplied by Milli-Q integral MT Kochi (33.759°N, 133.752°E) in the Shikoku Mountains.
or Milli-Q IQ 7005 water purification system (Millipore). Spring water samples that originate from Mt. Fuji were
High purity reagents of HCl, HNO 3, HF (Ultrapur-100, collected at observatory-1 (35.108°N, 138.900°E) and
Kanto Chemical), H2O2 (Tamapure-AA10, Tama Chemi- observatory-2 (35.107°N, 138.901°E) at Kakita River
cals) and CH3COOH (Optima, Fisher Scientific) were Park, Shimizu, Shizuoka. River water samples were col-
used for sample preparation. Standard solutions for el- lected from the Uji River, Uji, Kyoto (34.917°N,
emental concentration measurements were prepared by 135.790°E) and the Tawara River, Ujitawara, Kyoto
mixing commercial standard solutions (Na, Mg, Al, Ca, (34.846°N, 135.899°E), which is a tributary flowing into
Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Ga, Mo, Cd, Ba, and Pb; the Uji River. Rainwater samples were collected in low-
Wako Pure Chemical Industries). For isotopic measure- density polyethylene (LDPE) bottles attached to a
ments, secondary standard materials (NiWako, CuWako, and polyethylene (PE) funnel (Yamamoto et al., 2019). Snow
ZnAA-ETH) certified by primary reference materials (NiNIST samples were collected in plastic containers and trans-
SRM986 , CuNIST SRM976 , and ZnJMC-Lyon) were used (Archer ferred to LDPE bottles. Rime samples were collected from
et al., 2017; Takano et al., 2017). those grown on a PE net. River water samples were col-
Polytetrafluroacetate (PFA) vials (Savillex) were lected using a PE bucket and a PE rope. Spring water
cleaned by soaking in diluted alkaline detergent for 12 h, samples were collected using a polytetrafluoroethylene
rinsed with ultrapure water, and then boiled in 3 M HNO3 (PTFE) beaker or PTFE Bailer sampler with a Kevlar rope.
for 2 h before rinsing with ultrapure water. Disposable Samples were filtered using 0.45 µm membrane filters
172 S. Takano et al.
(Millex, Millipore) and the filtrates were acidified to 0.02 Ti and Mn, 1 mL of 12 M CH 3COOH-1.7 M HCl and 0.1
M HNO3. In addition, riverine standard material SLRS-5 mL of 10 M HCl were passed, respectively. Copper was
(National Research Council of Canada) was used to vali- eluted with 2.5 mL of 4 M HCl. After the elution of Fe
date our analysis. and Mo with 1 mL of 1 M HCl, Zn was eluted with 1 mL
To assess the contribution of mineral dust to fresh of 1 M HNO3. The eluates of Ni, Cu, and Zn were evapo-
water, a leaching experiment of sand was carried out: rated, and the organic residue was digested by reflux in 1
metals were leached from 238 mg of a sand sample col- mL of 69% HNO3 and 0.1 mL of 35% H2O2 for 12 h at
lected from Luntai in the northern Taklamakan Desert (site 160°C on a hotplate. After evaporation of the acid, the Ni
79, Chang et al., 2000) with 500 g of ultrapure water for fraction was dissolved in 2% HNO3 and the Cu and Zn
90 h at room temperature, and the obtained supernatant fractions in 0.3% HNO3.
was acidified using 0.02 M HNO 3.
Measurements of isotope ratios
Solid-phase chelate extraction Nickel, Cu, and Zn isotope measurements were per-
Nickel, Cu, and Zn were preconcentrated from sam- formed on a Neptune Plus MC-ICP-MS (Thermo Fisher
ples by solid-phase chelate extraction. The chelate ex- Scientific) at the Research Institute for Humanity and
traction process was based on previous studies (Takano Nature and on a different Neptune Plus at the Japan
et al., 2017, 2020) and was performed inside a clean booth. Agency for Marine-Earth Science and Technology. The
A NOBIAS Chelate PA-1W column (Hitachi high tech- cup configurations are shown in Supplementary Table S3.
nologies) was used for chelate extraction. The column Sample and standard solutions were introduced to the MC-
made of polypropylene contained 600 mg of NOBIAS ICP-MS via a PFA nebulizer (50 µL/min flow rate) and a
Chelate PA-1 resin. Before preconcentration, a sample glass spray chamber for Cu isotope measurements, and
with a CH3COOH-CH 3COONH4 buffer (final concentra- via a PFA nebulizer and an Aridus II desolvater (Cetac)
tion 0.03 M) was added to adjust the pH to 4.6–5.0. All for Ni and Zn isotope measurements. With respect to Ni
solutions except for the eluent were passed through the measurements, N2 gas was added to sweep gas of Aridus
column by a peristaltic pump. A sample solution was II at a flow rate of 5 mL/min to decrease interferences of
passed through the column at a flow rate of 10 mL/min oxides, such as 40Ar18O+ and 42Ca16O+. Copper was meas-
(Supplementary Table S1) after cleaning and condition- ured at a low-resolution mode, and Ni and Zn were meas-
ing the inside of the column. Subsequently, ultrapure water ured at a high-resolution mode. Data collection consisted
was passed at 60 mL/min to remove alkali metals and of 4.2 s × 20–30 cycles.
earth metals. Then, 1.5 M NH4F (pH 3.8–4.0) solution Interferences of isobar ions or double charged ions
was passed at 1 mL/min to elute Al, Ti, Fe, V, and Mn were corrected using Eq. (S1) in Supplementary mate-
from the resin after which, ultrapure water was passed at rial. The Fe interference on Ni was corrected by measur-
60 mL/min. Finally, 8 mL of 1 M HNO 3 was passed ing 58Fe+, the Ba2+ interferences on Cu, Ga, and Zn were
through the column by gravity to elute metals absorbed corrected by measuring 137Ba2+, and the Ni interference
on the resin. The eluate was completely evaporated on a on Zn was corrected by measuring 62Ni+.
hot plate (180°C) and the residue was dissolved in 0.4 Mass biases in Ni and Zn isotope measurements were
mL of 15 M CH3COOH-1.7 M HCl. corrected using the double-spike technique. The detailed
calculation and procedure were described in previous
Anion exchange separation studies (Siebert et al., 2001; Takano et al., 2017). Double
Samples preconcentrated by chelate extraction were spikes of 61Ni-62Ni and 64Zn- 67Zn were added to samples
processed by anion exchange to separate Ni, Cu, and Zn more than 12 h before chemical separation. δ 60Ni and
from matrices. Supplementary Table S2 shows the proce- δ66Zn were calculated relative to secondary standards (i.e.,
dure of anion exchange. The anion exchange column was NiWako and ZnAA-ETH). For Cu isotope measurements, mass
made of PFA tubing and a polyethylene frit. The dimen- biases were corrected by standard-bracketing in combi-
sions were 1 cm bed height by a 3 mm inner diameter. nation with the external correction method using Ga iso-
AG MP-1M anion exchange resin (100–200 mesh, Bio- topes. After chemical separation, a Ga standard solution
Rad) was packed into the column. Samples and eluents (GaWako, Wako Pure Chemicals) was added to be 1000
were added to the column using a pipette and passed by ppb in sample solution. A standard solution containing
gravity. Before loading the samples, the resin was cleaned 400 ppb of CuWako and 1000 ppb of GaWako was measured
with 1.5 mL of 1 M HNO3 and conditioned with 0.4 mL every 2–4 samples. δ 65Cu was calculated relative to
of 15 M CH3COOH-1.7 M HCl. Then, a sample dissolved Cu Wako via the method described in previous studies
in 0.4 mL of 15 M CH3COOH-1.7 M HCl, and 0.3 mL of (Maréchal et al., 1999; Takano et al., 2013). Measured δ
15 M CH3COOH-1.7 M HCl was subsequently passed values of Ni, Cu, and Zn relative to NiWako, Cu Wako, and
through the anion exchange column to collect Ni. To elute ZnAA-ETH were expressed as δ 60NiWako, δ 65CuWako, and
Isotopic analysis of Ni, Cu, and Zn in freshwater for source identification 173
Fig. 2. Elution curves of anion exchange for metal ions using 15 M CH 3COOH (a), 15 M CH3COOH-1.7 M HCl (b), or 12 M
CH 3COOH-1.7 M HCl (c).
δ 66ZnAA-ETH, and those relative to primary standards as 4.6–4.9. Furthermore, 1 M HNO3 was passed for elution.
δ60NiNIST986, δ65CuNIST976, and δ66ZnJMC-Lyon, respectively. Metal recoveries were determined via the measurement
δ 60NiNIST986, δ65CuNIST976, and δ66ZnJMC-Lyon were calcu- of the eluate (Supplementary Fig. S1). The NH4F solu-
lated by subtracting the δ value of the secondary stand- tion with a pH of 3.8–4.0 was more effective in removing
ard (Archer et al., 2017; Takano et al., 2017). Al, Mn, V, Fe, Ga, and Mo. The recoveries of Ni, Cu, and
Zn through the optimal procedure of chelate extraction
Measurements of elemental concentrations (Table S1) are 96.2 ± 1.9%, 95.9 ± 1.3%, and 96.6 ± 1.2%
Concentrations of Ni and Zn were determined using (mean ± 1 standard deviation, SD), respectively.
the isotope dilution method at the same time as the iso- Generally, Cu is isolated by anion exchange using HCl
topic measurement. The concentration of Cu was meas- media (Borrok et al., 2007; Maréchal et al., 1999;
ured by comparison of the 63Cu signal intensity with the Yamakawa et al., 2009). However, because the distribu-
bracketing standards in the isotopic measurement. Con- tion coefficient of Cu on the anion exchange resin is low,
centrations of the other elements in rain, rime, river, and an anion exchange column with a long bed height or mul-
spring water samples were determined by the calibration tiple cycles of ion exchange is required to separate Cu
curve method using an Element 2 HR-ICP-MS (Thermo from the other metals, such as Ti and Na. Recently, it has
Fisher Scientific) at the Institute for Chemical Research, been reported that Cu is successfully separated via an
Kyoto University. anion exchange column with a short bed height using a
CH3COOH-HCl media (Yang et al., 2019). We optimized
CH3COOH and HCl concentrations in the media used for
RESULTS AND DISCUSSION
anion exchange. First, 15 M CH3COOH (0.85 mL) con-
Recoveries and procedural blanks in chemical separation taining 250 ng each of Fe, Ni, Cu, and Zn was passed
Some freshwater samples contain large amounts of Al, through an anion exchange resin column of 5 cm bed
Mn, V, and Fe, all of which cause potential interferences height. However, all the metals were not adsorbed on the
during isotope measurements of Ni, Cu, and Zn. anion exchange resin in 15 M CH3COOH (Fig. 2a). It is
Aluminum, Mn, and Fe are effectively removed by pass- suggested that metal complexes in pure CH3COOH are
ing an NH 4F solution through the NOBIAS Chelate-PA1 cations or neutral species. According to the results of
column after sample loading (Takano et al., 2020). We molecular structure analysis using the electron spin reso-
evaluated the removal efficiency of interfering elements nance spectrum (Sharrock and Melník, 1985), Cu is
in chelating extraction using solutions of different pH of present as a neutral complex
NH 4F. After conditioning, 1 L ultrapure water with 2 µg [Cu(CH3COO) 2·(H2O)·CH3COOH] in CH3COOH solu-
of Na, Mg, Al, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, tion.
Mo, Cd, and Ba was passed through the NOBIAS Chelate- Next, 15 M CH 3 COOH-1.7 M HCl or 12 M
PA1 column, and subsequently an NH4F solution with a CH3COOH-1.7 M HCl were used as media in anion ex-
pH ranging from 3.8–4.0 or that with a pH ranging from change. The CH3COOH-HCl solution (0.85 mL) contain-
174 S. Takano et al.Fig. 3. Isotope ratios plotted against the reciprocal concentrations for Ni (a, d), Cu (b, e), and Zn (c, f) in Uji rainwater (a, b, c)
and the Tawara River water (d, e, f) with/without doping the secondary standards of Ni, Cu, and Zn. Error bars indicate 2-
standard error of δ values in MC-ICP-MS measurements. The linear regression curves are shown as solid lines. The 95% predic-
tion bands are shown as dashed lines.
ing 2900 ng Na, 710 ng Mg, 1100 ng Al, 700 ng Ca, 760 respectively (Table S2).
ng Ti, 730 ng V, 720 ng Cr, 770 ng Fe, 710 ng Ni, 790 ng The recoveries of 15 elements through anion exchange
Cu, 690 ng Zn, 90 ng Ga, and 1400 ng Mo was passed using columns of 1 cm bed height are shown in Supple-
through an anion exchange column of 1 cm bed height. mentary Table S4. Nickel is eluted concurrently with al-
After that, the same CH 3COOH-HCl solution without kali metals and alkali earth metals. However, most of these
metals was passed to elute metals from the anion exchange metals are removed by chelate extraction prior to anion
column (Figs. 2b and 2c). In CH 3COOH-HCl solutions, exchange. Although 60% of the Mn is eluted together with
Cu was adsorbed on the anion exchange resin, suggest- Cu in anion exchange, more than 99.9% of the Mn is also
ing that Cu was present as an anion complex removed in chelate extraction. In the previous study
[Cu(CH 3COO) 2 ·Cl·CH 3 COOH] – by replacing a water (Takano et al., 2017), the anion exchange process using a
molecule in [Cu(CH3COO)2·(H2O)·CH3COOH] with a Cl– long column of 7 cm took 10–11 h. In this study, the short
ion. In 15 M CH3COOH-1.7 M HCl, the anion exchange bed height of the column reduced the processing time to
resin column retained ~100% of Ti, Fe, Cu, Zn, Ga, Mo, 5–6 h.
and 71% of V after passing 1 mL of eluent. In 12 M We evaluated procedure blanks through this analyti-
CH3COOH-1.7 M HCl, Ti, V, and Cr were more rapidly cal method, by processing 1 L of ultrapure water as a sam-
eluted. Therefore, we decided to use 15 M CH3COOH- ple. The procedure blanks of Ni, Cu, and Zn were 0.007
1.7 M HCl for elution of Ni, and 12 M CH 3COOH-1.7 M ± 0.002 nmol, 0.007 ± 0.004 nmol, and 0.02 ± 0.01 nmol
HCl for elution of Ti, V, and Cr. For the elution of Fe, Cu, (n = 8), respectively. The minimum amounts of Ni, Cu,
and Zn, we used 1 M HCl, 4 M HCl, and 1 M HNO3, and Zn contained in our natural water samples were 0.245
Isotopic analysis of Ni, Cu, and Zn in freshwater for source identification 175Table 1. Isotope ratios and concentrations of Ni, Cu, and Zn in a certified material of river water SLRS-5
δ6 0NiNIST986 δ6 5CuNIST976 δ6 6ZnJMC-Lyon Concentration nmol/kg
a a a
Mean 2SE Mean 2SE Mean 2SE Ni Cu Zn
SLRS-5 #1 0.69 0.05 0.50 0.02 0.38 0.02 7.92 289 13.2
#2 0.75 0.05 0.49 0.02 0.39 0.03 7.95 288 13.2
#3 0.72 0.05 0.50 0.02 0.36 0.02 8.08 295 13.2
Mean 0.72 0.50 0.38 7.98 291 13.2
2SDb 0.05 0.02 0.03 0.17 7 0.02
Certified value 8.11 ± 1.09 274 ± 20 12.9 ± 1.5
a
2-standard error in MC-ICP-MS measurements.
b
2-standard deviation in replicate analyses.
nmol, 0.259 nmol, and 2.00 nmol, respectively. The con- by Fe. The interference was sufficiently corrected using
tributions of the procedure blanks were less than 3% for the intensity of 57Fe (Eq. S1). Interferences of Ca and Ti
Ni, 3% for Cu, and 1% for Zn. were not observed below 29 mol/mol of Ca/Ni and 0.45
mol/mol of Ti/Ni, respectively. For measurements of
External correction using Ga in the isotope measurement of δ65Cu, the shifts exceeding 0.05‰ were caused by inter-
Cu ferences of Ti and Mn at >0.04 mol/mol Ti/Cu and >0.5
The instrumental mass bias during Cu isotope meas- mol/mol Mn/Ga, respectively. The interference of Cr was
urements is often corrected by external correction using not observed below 0.05 mol/mol of Cr/Cu, and that of
Zn isotopes. This correction is sensitive to the Zn/Cu ra- Ba was successfully corrected by 137Ba2+ intensity. For
tio in measured solutions. Different Zn/Cu ratios between measurements of Zn isotopes, detectable interferences
the sample and the standard solution cause an analytical were not found in our experiment. Supplementary Figure
artifact (Archer and Vance, 2004; Zhu et al., 2015). There- S4 shows ratios of potentially interfering elements to Ni,
fore, Zn must be doped to achieve a constant Zn/Cu ratio Cu, or Zn in natural water samples (i.e., snow, rime, river
among the samples and the standard solution for accurate water, and rain) after chemical separation. Threshold lines
isotope measurement. Instead of Zn, Ga is used for the in Fig. S4 indicate maximum elemental ratios below which
external correction of the Cu isotope measurement in this undetectable interference is observed. For most of the
study. Figure S2 shows δ65CuWako values measured in 100 samples, the elemental ratios are below these lines, thus
ppb Cu Wako solution doped with different amounts of indicating the effectiveness of our method in removing
Ga Wako or ZnAA-ETH. Only when the Zn/Cu ratio is identi- interfering elements for isotopic measurements of Ni, Cu,
cal to the standard solution (Zn/Cu = 3), δ65CuWako is ~0. and Zn. The samples containing interfering elements
δ 65Cu values shift to the positive direction for solutions above the threshold lines are excluded from the follow-
containing higher Zn/Cu ratios, and vice versa. The shift ing sections.
in δ 65Cu is attributable to the interference of 64Zn1H on
65
Cu. In the case of Ga doping, δ65Cu Wako is ~0 at any Accuracy and precision
Ga/Cu ratios, and matching Ga/Cu ratios between the sam- The accuracy and precision of the isotope analyses of
ple and standard solutions are not required. Therefore, Ni, Cu, and Zn were evaluated by repeat analysis of river
the use of Ga for external correction allows for more ac- water collected from the Tawara River on October 10,
curate and simpler measurements of Cu isotopes. 2019 and rainwater collected from Uji City from July 23
to 27, 2019. The river water was divided into nine
Interferences during isotope measurements of Ni, Cu, and subsamples of 500 mL each. Three of them were doped
Zn with 1.5–1.6 nmol of NiWako, 2.5 nmol of CuWako, and 3.6–
Various ions cause interferences during the isotope 3.7 nmol of ZnAA-ETH. The rainwater was divided into 16
measurements of Ni, Cu, and Zn. In Supplementary Fig. subsamples of 500 mL each. Ten of them were doped with
S3 and Supplementary Table S5, we evaluated the inter- 1.9–7.3 nmol of NiWako, 4.8–19.3 nmol of CuWako, and
ferences of Na, Mg, Ti, Cr, Mn, and Ba in measurements 53–210 nmol of ZnAA-ETH. Analyzed isotope ratios of Ni,
for Cu isotopes; Ca, Ti, and Fe in measurements for Ni Cu, and Zn in all the subsamples are shown in Supple-
isotopes, and Mg, Al, Ca, Ti, V, Cr, and Ba in measure- mentary Table S6. Recoveries of Ni, Cu, and Zn calcu-
ments for Zn isotopes (Table S5). Isotope ratios of NiWako, lated using Eq. (S2) are close to 100%; 108 ± 5% for Ni,
Cu Wako, and ZnAA-ETH were measured after doping with 99 ± 4% for Cu, and 92 ± 5% for Zn (mean ± SD, n = 3)
these interfering elements. In measurements of Ni iso- for the Tawara River water and 99 ± 6% for Ni, 99 ± 4%
topes, a detectable interference (>0.05‰) was not caused for Cu, and 100 ± 11% for Zn (n = 8) for the rainwater.
176 S. Takano et al.The 2SDs for the δ values in undoped samples are
Hu and Balasubramanian (2003)Fig. 4. Enrichment factors relative to the continental crust for
Ni, Cu, and Zn in rainwater, snow, rime, and water-soluble frac-
tion of the Taklamakan sand. Solid lines represent the mean
values.
Fig. 5. Isotope ratios of Ni, Cu, and Zn in freshwater samples
The EFs for Ni, Cu, and Zn in rain, snow, and rime sam- and aerosols. Solid lines indicate the mean value. Yellow bands
ples are 101–105 (Fig. 4). One possibility to explain such represent the mean ± 1 standard deviation in terrigenous mate-
high EFs is the different solubilities of the metals in min- rials (Cameron et al., 2009; Little et al., 2014). *literature data
eral dust originated from the continental crust. However, for aerosols (Dong et al., 2017; Ochoa-Gonzalez et al., 2016;
the EFs in a water-soluble fraction of the Taklamakan sand Souto-Oliveira et al., 2018) and rivers (Cameron and Vance,
is as low as 2–5, which indicates that preferential disso- 2014; Little et al., 2014; Vance et al., 2008).
lution of Ni, Cu, and Zn from the mineral dust is not
enough to explain the high EFs of the metals in rain, snow,
and rime. It is to be noted that the leaching condition of tion coefficients for V, Ni, Cu, Zn, Ba, Cd, and Pb largely
mineral dust in our experiment is different from that in vary according to location and sample types (i.e., rain,
actual rainwater. Actual rainwater is acidic more than snow, and rime). These metals have high EFs (>100), and
ultrapure water, and mineral dust in the atmosphere is finer their anthropogenic input is significant. The high vari-
than bulk sand. Therefore, solubilities of metals in min- ability of the correlation coefficients suggests that these
eral dust would be higher in actual rainwater, which may metals are from multiple sources and that the contribu-
increase the EFs of Ni, Cu, and Zn. Another possibility tion of the sources varies according to location and sam-
explaining high EFs is the input of anthropogenic Ni, Cu, ple types.
and Zn, as these metals are common in human activities. Isotope ratios of Ni, Cu, and Zn in rain, snow, rime,
Correlation coefficients among metal concentrations in river water, and spring water samples are shown in Fig.
rain, snow, and rime are shown in Supplementary Tables 5, together with literature data. The isotope ratios of Cu
S9–S12. Strong correlations are found among Al, Ti, Cr, and Zn in rainwater, snow, and rime samples are within
Mn, and Fe in rain, snow, and rime samples. EFs of these the ranges of those of aerosols from an island in the South
elements are less than 100. Therefore, the dominant source China Sea (Takano et al., 2020), and from cities in Eu-
of these elements would be mineral dust. Strong correla- rope (Dong et al., 2017; Ochoa-Gonzalez et al., 2016)
tions are also observed between Na and Mg, suggesting and South America (Souto-Oliveira et al., 2018, 2019).
that both these elements originate from sea salt. Correla- Nickel isotope ratios in aerosols have been reported only
178 S. Takano et al.Fig. 6. Isotope ratios of Ni, Cu, and Zn in our freshwater samples plotted against enrichment factors (EF). Error bars indicate 2-
standard errors of δ values in MC-ICP-MS measurements. Stars represent the mean for terrigenous materials (Cameron et al.,
2009; Little et al., 2014)
Fig. 7. δ65Cu NIST976 values plotted against Sb/Cu ratios in rain, snow, and rime (a). Error bars indicate 2-standard errors of δ
values in MC-ICP-MS measurements. δ66ZnJMC-Lyon values plotted against Cu/Zn ratios in rainwater, snow, and rime (b). Solid
boxes represent the range of road dust (Dong et al., 2017).
for those collected from an island in the South China Sea. lower δ66ZnJMC-Lyon (–0.8 to –0.1‰). Rainwater samples
Variations of δ60Ni NIST986 in rainwater, snow, and rime collected in summer from Mt. Kajigamori exhibit high
samples are larger than those in aerosols in the South EFs of Zn (>105).
China Sea. For rain, snow, and rime samples, Ni, Cu, and Dong et al. (2017) and Ochoa-Gonzalez et al. (2016)
Zn isotope ratios are plotted against their EFs in Fig. 6. investigated Cu and Zn in atmospheric particles and traf-
For Ni, isotope ratios are higher at lower EFs. For Cu, fic emission sources, and suggested traffic emission as
isotope ratios tend to be higher at higher EFs in rainwa- one of the main sources of Cu and Zn in the atmospheric
ter and rime. For Zn, most of the data are plotted within particles. Dong et al. (2017) used Sb as an anthropogenic
the range of 103 to 10 4 of EFs and –0.1 to +0.3‰ of tracer in combination with δ 65CuNIST976 to distinguish the
δ66ZnJMC-Lyon. Several rainwater samples from Uji exhibit sources of Cu in atmospheric particles. In Fig. 7a,
Isotopic analysis of Ni, Cu, and Zn in freshwater for source identification 179Fig. 8. Vanadium concentrations plotted against Ni concentrations in rainwater, snow, and rime (a). Error bars indicate 2- standard errors of δ values in MC-ICP-MS measurements. The solid line represents the linear regression curve. The dashed line represents the V/Ni ratio in crude oil (Barwise, 1990). δ60NiNIST986 plotted against V/Ni ratios in rainwater, snow, and rime (b). The solid line represents the linear regression curve. δ65CuNIST976 values are plotted against Sb/Cu ratios in our mote mountain area with a high-altitude of 1400 m, sam- rain, snow, and rime samples. Most of them are plotted in ples collected there are strongly affected by atmospheric the range of road dust as reported in the previous study particles over long-range transported from the Asian con- (Dong et al., 2017). Therefore, road dust is a dominant tinent (Imai et al., 2017). source of Cu in rain, snow, and rime collected in Nickel isotopes in aerosols have been reported only Kajigamori and Uji. The high δ65Cu and low Sb/Cu in for those collected from an island in the South China Sea some samples may be attributed to emission from fossil where the source of Ni is expected to be crude oil com- fuel combustion (Dong et al., 2017; Ochoa-Gonzalez et bustion, based on the correlation between Ni and V con- al., 2016). The high Sb/Cu ratio (>0.4‰) in some sam- centrations (Takano et al., 2020). Similar correlation is ples from Kajigamori is possibly because of the higher observed in the rain, snow, and rime samples (Fig. 8a), solubility of Sb than Cu in fine atmospheric particles (Sarti with a similar slope as that observed in crude oil (Barwise, et al., 2015). In Fig. 7b, δ66ZnJMC-Lyon values are plotted 1990). Therefore, the dominant source of Ni in these sam- against Cu/Zn ratios in the rain, snow, and rime samples. ples is emission from crude oil combustion. δ60NiNIST986 Some of the rain samples obtained from Uji are placed in and V/Ni are found to be correlated (Fig. 8b), which sug- the range of road dust (Dong et al., 2017). Low gests two significant sources of Ni. δ 60NiNIST986 in crude δ66ZnJMC-Lyon (
ably supplied from the basaltic lava via water-rock inter- than those in rain, snow and rime samples, while Zn iso-
action. However, this cannot be firmly concluded due to tope ratios are similar. This is attributed to isotopic
the small number of samples. fractionation of Ni and Cu between particulate-dissolved
Isotope ratios of Ni, Cu, and Zn in our river water phases in river water or soil.
samples are within the range of those reported for rivers
worldwide (Fig. 5). δ60Ni NIST986 and δ65CuNIST976 in the Acknowledgments—We are grateful to Assoc. Prof. Rumi
river water are 0.72–1.24‰ and 0.50–0.87‰, respec- Sohrin for her aid in sample collection from the Kakita River
tively. These are significantly heavier than δ values in spring. We would also like to thank Prof. Yoshio Takahashi for
water sources, such as rain and spring water (Fig. 5). providing the Taklamakan sand. This study was supported by
the Joint Research Grant for Environmental Isotope Study from
Heavy isotope ratios of dissolved Cu in river water are
the Research Institute for Humanity and Nature, the Mitsumasa
attributable to the removal of light Cu by adsorption on
Ito Memorial Research Grant from the Research Institute for
riverine particles (Vance et al., 2008) or oxic soil (Vance Oceanochemistry Foundation (R2-R2), and JSPS KAKENHI
et al., 2016). For Ni, similar processes would explain Grants (18K14250, 19H01148 and 20K19957).
heavy isotope ratios in the river water, although isotope
ratios of particulate Ni have not yet been determined.
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