Applications of Selective Ion Exchange for Perchlorate Removal, Recovery and Environmental Forensics

Chapter 22

Isotopic Tracing of Perchlorate in the Environment

Neil C. Sturchio, John Karl Bohlke, Baohua Gu, Paul B. Hatzinger, and W. Andrew Jackson

Abstract Isotopic measurements can be used for

tracing the sources and behavior of environmental

contaminants. Perchlorate (ClO4�) has been detected

widely in groundwater, soils, fertilizers, plants, milk,

and human urine since 1997, when improved analyti-

cal methods for analyzing ClO4� concentration

became available for routine use. Perchlorate inges-

tion poses a risk to human health because of its

interference with thyroidal hormone production. Con-

sequently, methods for isotopic analysis of ClO4�

have been developed and applied to assist evaluation

of the origin and migration of this common contami-

nant. Isotopic data are now available for stable iso-

topes of oxygen and chlorine, as well as 36Cl isotopic

abundances, in ClO4� samples from a variety of natu-

ral and synthetic sources. These isotopic data provide

a basis for distinguishing sources of ClO4� found in

the environment, and for understanding the origin of

natural ClO4�. In addition, the isotope effects of

microbial ClO4� reduction have been measured in

laboratory and field experiments, providing a tool for

assessing ClO4� attenuation in the environment. Iso-

topic data have been used successfully in some areas

for identifying major sources of ClO4� contamination

in drinking water supplies. Questions about the origin

and global biogeochemical cycle of natural ClO4�

remain to be addressed; such work would benefit

from the development of methods for preparation

and isotopic analysis of ClO4� in samples with low

concentrations and complex matrices.

22.1 Introduction

Perchlorate (ClO4�) is a stable oxyanion consisting of

four O2� ions bonded in tetrahedral coordination with

a central Cl7+ ion. It is ubiquitous in the environment at

trace concentrations, and has natural and anthropogenic

sources. Natural ClO4� is present in precipitation (gen-

erally <0.1 mg L�1), in soil (generally <10 mg kg�1),

and in groundwater at concentrations ranging from

background levels of about 0.01 to >100 mg L�1 in

some arid regions where perchlorate has been concen-

trated by evaporation (Dasgupta et al. 2005; Jackson

et al. 2005a; Rao et al. 2007; Rajagopalan et al. 2006,

2009; Parker et al. 2008).

A major production mechanism for natural perchlo-

rate apparently involves reactions of atmospheric Cl

species with ozone (O3) (Simonaitis and Heicklen

1975; Jaegle et al. 1996; Bao and Gu 2004; Dasgupta

et al. 2005; Kang et al. 2008; Catling et al. 2010; Rao

et al. 2010). Anthropogenic ClO4� salts are synthesized

by electrolysis of NaCl brines (Schumacher 1960) in

quantities on the order of 107 kg year�1 (DasGupta

et al. 2006) for military, aerospace, and other industrial

applications, e.g., solid rocket fuel, explosives, fire-

works, road flares, electroplating solutions.

Perchlorate salts (NH4ClO4, KClO4, NaClO4) are

soluble in water and some organic solvents. The

N.C. Sturchio (*)

University of Illinois at Chicago, Chicago, IL 60607, USA

e-mail: [email protected]

J.K. Bohlke

U.S. Geological Survey, Reston, VA 20192, USA

B. Gu

Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

P.B. Hatzinger

Shaw Environmental, Inc., Lawrenceville, NJ 08648, USA

W.A. Jackson

Texas Tech University, Lubbock, TX 79409, USA

M. Baskaran (ed.), Handbook of Environmental Isotope Geochemistry, Advances in Isotope Geochemistry,

DOI 10.1007/978-3-642-10637-8_22, # Springer-Verlag Berlin Heidelberg 2011

437

ClO4� ion in aqueous solution is non-complexing and

unreactive at low temperature, resistant to O exchange

with H2O (Hoering et al. 1958), and adsorbs weakly to

solids present in soils and aquifers. It is not readily

removed by conventional water treatment processes,

but special ion exchange resins have been developed

to remove ClO4� from drinking water (Gu et al. 2007,

2011). Perchlorate can be reduced by microbes under

anoxic conditions after available nitrate (NO3�) is

depleted (Coates and Achenbach 2004), and this bio-

degradation process has been utilized for both in situ

and ex situ treatment of water (Hatzinger 2005). Its

chemical properties give ClO4� high mobility and

general persistence in surface waters, vadose zone

environments, and shallow oxic aquifers.

The widespread occurrence of ClO4� in drinking

water and food supplies (Gullick et al. 2001; Kirk et al.

2003; Jackson et al. 2005b; Sanchez et al. 2005, 2006)

has attracted attention because of the potential delete-

rious health effects of ClO4� ingestion caused by

interference with human thyroidal hormone produc-

tion (NRC 2005). Pathways of natural and synthetic

perchlorate through the environment, and potential

natural attenuation processes, are thus of great interest

with respect to human health and the protection of

environmental quality.

Stable isotope ratio analyses of oxygen (O) and

chlorine (Cl), along with 36Cl abundance measure-

ments, have shown that different ClO4� sources have

distinct isotopic compositions that provide multiple

isotopic tracers of the sources and behavior of ClO4�

in the environment (Bao and Gu 2004; Bohlke et al.

2005; Sturchio et al. 2006, 2009; Jackson et al. 2010).

In this chapter we review the development and current

status of sampling and isotopic analytical methods

for ClO4�, along with a survey of published data.

Remaining questions about the origin and behavior

of natural ClO4� are outlined.

22.1.1 Isotope Abundances and Notation

There are three stable isotopes of O (16O, 17O, and18O) having natural abundances of approximately

99.76, 0.04, and 0.2%, respectively (Coplen et al.

2002; Hoefs 2009). Variations in O isotope ratios are

reported as d18O and d17O, defined as the difference

between the 18O/16O or 17O/16O atom ratio, respec-

tively, of a sample and that of Vienna Standard Mean

Ocean Water (VSMOW):

d18O = 18O=16O� �

sample= 18O=16O� �

VSMOW� 1

� �

(22.1)

d17O = 17O=16O� �

sample= 17O=16O� �

VSMOW� 1

� �

(22.2)

These d values are normally reported in parts per

thousand (‰) following multiplication of both sides

of (22.1) and (22.2) by 1,000. In systems where isoto-

pic fractionation is strictly mass-dependent, d17Offi 0.52 � d18O and is not normally reported. However,

natural ClO4� commonly has 17O in excess of this

relationship (Bao and Gu 2004; Bohlke et al. 2005)

and the 17O anomaly commonly is reported as a

deviation from the abundance expected for mass-

dependent fractionation, according to the approxima-

tion (Thiemens 2006):

D17O = d17O� 0.52 � d18O (22.3)

Alternatively (Miller 2002), and in this paper:

D17O = 1þ d17O� �

= 1þ d18O� �0:525h i

� 1 (22.4)

The D17O value is normally reported in parts per

thousand (‰) following multiplication of both sides of

(22.3) and (22.4) by 1,000.

There are two stable isotopes of Cl (35Cl and 37Cl)

having natural abundances of approximately 75.76

and 24.24%, respectively (Coplen et al. 2002; Hoefs

2009). Chlorine stable isotope ratios are reported as

d37Cl, defined as the difference between the 37Cl/35Cl

atom ratio of a sample and that of Standard Mean

Ocean Chloride (SMOC) (Long et al. 1993; Godon

et al. 2004):

d37Cl = 37Cl=35Cl� �

sample= 37Cl=35Cl� �

SMOC� 1

� �

(22.5)

Values of d37Cl are normally reported in parts per

thousand (‰) following multiplication of both sides of

(22.5) by 1,000.

438 N.C. Sturchio et al.

Chlorine-36 is a long-lived radioactive isotope

(half-life ¼ 301,000 years) produced largely by cosmic-

ray interactions with atmospheric Ar as well as

by thermal neutron capture on 35Cl in the terrestrial

subsurface environment. Chorine-36 abundances are

measured by accelerator mass spectrometry (AMS)

and are expressed as the atom ratio of 36Cl/Cltotal.

This ratio ranges from about 10�15 to 10�11 in terres-

trial materials (Phillips 2000).

22.2 Methods

All high-precision stable isotope data (d37Cl, d18O,D17O) reported for ClO4

� to date have been measured

by isotope-ratio mass spectrometry (IRMS). Methods

published by Ader et al. (2001) and Sturchio et al.

(2003) are suitable only for determination of Cl iso-

tope ratios in ClO4�, and these have been replaced in

routine analysis by the methods developed and applied

to isotopic analysis of both O and Cl by Bohlke et al.

(2005); Sturchio et al. (2006); Hatzinger et al. (2009),

and Jackson et al. (2010), which are described

below in Sect. 22.2.3. Such measurements begin with

a pure alkali-ClO4 salt, preferably KClO4, RbClO4, or

CsClO4. A typical isotopic analysis of ClO4� by IRMS

requires at least 0.2 mg of ClO4�. Although synthetic

ClO4� reagents are readily available in large amounts,

the concentration of ClO4� in most environmental

materials is low (mg/kg) and requires elaborate methods

of preconcentration, extraction, and purification prior

to isotopic analysis (Gu et al. 2011).

22.2.1 Sampling

The most successful and widely used method for isoto-

pic sampling of ClO4� from environmental materials

has involved the use of a highly ClO4-selective bifunc-

tional ion-exchange resin for preconcentration (Gu

et al. 2001, 2007, 2011). This resin was developed

initially for large-scale water treatment and is com-

mercially available as Purolite® A530E. Its use for

preconcentration requires that the ClO4� be contained

in an aqueous solution. Thus A530E is suitable for

preconcentration of ClO4� directly from natural

waters or from aqueous solutions obtained by leaching

ClO4-bearing solid materials such as soil, caliche,

plants, or industrial products.

In practice, the A530E resin (20–30 mesh size) is

normally packed into 100-mL size columns fabricated

from 38 mm (1.5 in.) diameter clear polyvinyl chloride

(PVC) tubing. Water is passed through the column at

an optimal rate of 1–2 L per minute to preconcentrate

ClO4� on the resin, with the objective of obtaining at

least 3 mg for isotopic analysis. A prefilter (5–10 mm)

may be used to remove particles from the water before

passing through the column. Common anions such as

Cl�, NO3�, and SO4

2�, along with humic material, if

present in sufficient quantities, tend to interfere with

uptake of ClO4� on the A530E resin. This interference

increases with salinity and results in lower efficiency

of ClO4� adsorption and early breakthrough, thus it is

advisable to pass a larger volume of water through the

column than that required by the assumption of 100%

efficiency. Further details about the performance of

A530E resin in ClO4� removal are given by Gu et al.

(2001, 2007, 2011).

22.2.2 Extraction and Purification

When ClO4� has been concentrated on the A530E

resin column, there are normally substantial quantities

of other common anions present. The first step in

recovering ClO4� is to flush the column with four to

five bed volumes of 4 M HCl. This displaces a large

portion of the adsorbed NO3�, SO4

2�, carbonates, andorganic anions, but adds substantial Cl� to the resin.

The ClO4� is then eluted using a solution of 1 M FeCl3

in 4 M HCl, and the ClO4� is displaced from the

A530E resin by the FeCl4� anion (Gu et al. 2001).

After ClO4� has been eluted from the column, there

are multiple options for Fe3+ and Cl� removal and

further purification (Gu et al. 2011). Depending on

the bulk sample composition, various issues may

arise in ClO4� purification, and certain samples are

more difficult to purify than others. Generally, lower-

concentration samples (ClO4� <~1 mg/L) are more

challenging. Ultimately, the purified ClO4� is nor-

mally precipitated as CsClO4, the purity of which is

verified by ion chromatography and/or Raman spec-

troscopy, for isotopic analysis.

22 Isotopic Tracing of Perchlorate in the Environment 439

22.2.3 Isotopic Analysis

22.2.3.1 Oxygen Isotopes

Oxygen isotope ratios in ClO4� may be analyzed by

two IRMS methods. First, d18O values may be deter-

mined by reaction with glassy carbon at 1,325�C (or

higher) to produce CO, which is transferred in a He

carrier through a molecular-sieve gas chromatograph

to an isotope-ratio mass spectrometer and analyzed in

continuous-flow (CF) mode by monitoring peaks at m/

z 28 and 30 (this method is referred to as CO-CFIRMS,

for CO continuous-flow isotope-ratio mass spectrom-

etry). Yields of O (as CO) typically are 100 � 2% for

pure ClO4� reagents and samples. The 1s analytical

precision of d18O values ranges from �0.1 to 0.3‰,

based on replicate analyses of samples and isotopic

reference materials.

For the second IRMS method, both d18O and d17Ovalues are measured on O2 produced by in vacuo

decomposition of the CsClO4 at 600–650�C, accord-ing to the reaction:

CsClO4 ! CsCl þ 2O2 (22.6)

Decomposition can be done in sealed quartz or

Pyrex glass tubes (Bohlke et al. 2005; Sturchio et al.

2007) or in a vacuum system equipped with an O2 trap

(Bao and Gu 2004). The O2 gas is admitted to an

isotope-ratio mass spectrometer and analyzed in

dual-inlet (DI) mode by measurements at m/z 32, 33,

and 34 (this method is referred to as O2-DIIRMS, for

O2 dual-inlet isotope-ratio mass spectrometry). Yields

of O (as O2) by the sealed-tube method are typically

within �5% of those expected from processed sample

amounts for pure ClO4� reagents and samples and

measured aliquots of tank O2. Partial exchange of O

between O2 and glass may occur during this decompo-

sition reaction, as a function of sample size, tempera-

ture, and time.

Oxygen isotope analyses of ClO4� are calibrated

using a pair of KClO4 isotopic reference materials

(USGS37, USGS38) with isotopic compositions that

differ by more than the samples being analyzed. To

monitor drift, analyses include an internal laboratory

reference gas (either CO or O2, see below), against

which all samples and isotopic reference materials are

analyzed in the mass spectrometer during a single

batch of analyses in which all samples and reference

materials are the same size (in terms of O) and are

treated identically. The isotopic reference materials

consist of reagent-grade KClO4 salts (USGS37,

USGS38) that were prepared specifically for calibra-

tion of ClO4� isotopic analyses. The d18O scale is

based on CO-CFIRMS analyses of the ClO4� isotopic

reference materials against international water, nitrate,

and sulfate isotopic reference materials as described

by Bohlke et al. (2003) and all data are referenced to

the conventional VSMOW-SLAP (Standard Light

Antarctic Precipitation) scale. The D17O scale for

ClO4� is based provisionally on the assumption that

the normal reagent KClO4 reference material

(USGS37) has 18O:17O:16O ratios that are related to

those of VSMOW by normal mass dependent relations

(D17O ¼ 0.0‰) with exponent l ¼ 0.525 (see (22.4)

above) (Miller 2002; Bohlke et al. 2005). Typical 1sreproducibility of the D17O measurements is approxi-

mately �0.1‰ after normalization, based on replicate

analyses of samples and isotopic reference materials.

22.2.3.2 Chlorine Isotopes

Stable isotope ratios of Cl in purified ClO4� samples

are analyzed using IRMS techniques, whereas 36Cl/Cl

ratios are determined using AMS (Sturchio et al.

2007, 2009). For d37Cl determinations, ClO4� salts

are first decomposed at 600–650�C in evacuated

glass tubes to produce alkali chloride salts, which

are then analyzed according to well established

methods (Sturchio et al. 2007). The alkali chloride

salts produced by ClO4� decomposition are dis-

solved in warm 18.2 MO deionized water, and Cl�

is precipitated as AgCl by addition of AgNO3. The

resulting AgCl is recovered by centrifugation,

washed in dilute HNO3, dried, and reacted in a sealed

glass tube with excess CH3I at 300�C for 2 h to

produce CH3Cl. The resulting CH3Cl is purified

using gas chromatography, cryo-concentrated, and

then admitted to an isotope-ratio mass spectrometer

and analyzed in either continuous-flow or dual-inlet

mode (depending on sample size) by measurements at

m/z 50 and 52.

Chlorine isotope ratio analyses are normalized

using the same ClO4� isotopic reference materials

described for O analysis in Sect. 22.2.3.1 (USGS37,

USGS38). To monitor drift, analyses include an


internal laboratory reference gas (CH3Cl), against

which all samples and reference materials are ana-

lyzed in the mass spectrometer during a single batch

of analyses. The d37Cl scale is based on isotopic ana-

lyses of the USGS ClO4� isotopic reference materials

against SMOC. After normalization, the 1s analytical

precision of d37Cl values is approximately �0.2‰,

based on replicate analyses of samples and isotopic

reference materials.

Analysis of 36Cl in ClO4� is performed by AMS

(Sturchio et al. 2009). Prior to AMS, the CsClO4 is

decomposed in a sealed tube as described above, then

the chloride is recovered as CsCl and precipitated as

AgCl by addition of excess AgNO3 solution. The

AgCl is dissolved in dilute NH4OH solution and the

Cl� is purified by anion chromatography on analytical

grade 1-X8 resin (using an unpublished protocol

provided by the PRIME Lab of Purdue University)

to ensure removal of trace amounts of S that might

cause isobaric interference (by 36S) at mass 36. An

alternative method for purification of Cl� involves

cation-exchange chromatography (Jiang et al. 2004).

Purified Cl� is then precipitated as AgCl for AMS

measurement. Analysis of seawater Cl� provides

a reference datum of 36Cl/Cl ¼ 0.5 � 10�15 (Argento

et al. 2010).

22.3 Results and Discussion

22.3.1 Stable Isotopic Compositionof Synthetic Perchlorate

Stable isotope data for samples of synthetic ClO4�

reagents and other commercial ClO4�-bearing pro-

ducts have been published by Ader et al. (2001), Bao

and Gu (2004), Bohlke et al. (2005), and Sturchio et al.

(2006). Data for samples of synthetic ClO4� in which

both d18O and d37Cl isotope ratios were measured are

shown in Fig. 22.1. Published d37Cl values for syn-

thetic ClO4� range from �3.1 to þ2.3‰ and have a

mean value near þ0.6‰. This mean value is within

the range of common industrial NaCl sources, such as

halite from Phanerozoic bedded evaporites having

d37Cl values in the range of 0.0 � 0.9‰ (Eastoe

et al. 2007), and indicates that relatively little (<1‰)

isotopic fractionation of Cl accompanies ClO4� syn-

thesis. Published d18O values of synthetic ClO4� range

from�24.8 to�12.5‰, presumably reflecting a range

in composition of the local water sources used in

production and O isotopic fractionation that occurs

during ClO4� synthesis (Sturchio et al. 2006). In con-

trast, D17O values of synthetic ClO4� are consistently

0.0 � 0.1‰, indicating little or no mass-independent

isotopic fractionation in the ClO4� synthesis process.

All stable isotopic data generated to date for primary

synthetic ClO4� (as of September, 2010) are within

the ranges of d18O and d37Cl shown in Fig. 22.1.

Preliminary measurements on ClO4� produced by

disproportionation reactions in commercial NaOCl

(bleach) solutions, however, have indicated some

anomalously low values of d18O and anomalously

high values of d37Cl (e.g., d37Cl ¼ þ14.0‰; (Sturchio

et al. 2009)), yielding a potential means by which to

distinguish this source of ClO4� from that produced by

electrochemical synthesis.

22.3.1.1 Isotopic Fractionation of Perchlorate

by Microbial Reduction

Kinetic isotopic fractionations are caused by mass

dependent differences in reaction rates of different iso-

topic species during fast, incomplete, or unidirectional

reactions such as diffusion, evaporation, and microbial

respiration, where reaction products are generally

Fig. 22.1 d37Cl (‰) versus d18O (‰) for representative sam-

ples of synthetic ClO4� grouped by source (see legend). Analyt-

ical errors shown at �0.3‰. Dashed line is d37Cl referencevalue of 0.0‰ for Standard Mean Ocean Chloride. Data from

Sturchio et al. (2006)


enriched in the lighter isotopes (Criss 1999). The isoto-

pic fractionation factor, a, is defined as

a ¼ RA=RB (22.7)

where R is an isotope ratio, and A and B are two

substances (in the present case, product and reactant,

respectively). For O and Cl isotope ratios, R represents18O/16O and 37Cl/35Cl, respectively. Values of a can

be obtained from the experimental results by assuming

the exponential function

R=R0 ¼ f a�1 (22.8)

where R and R0 are the O or Cl isotope ratios (18O/16O

or 37Cl/35Cl) of the residual ClO4� and the initial,

unreacted ClO4�, respectively, and f is the fraction of

ClO4� remaining. In terms of the d values of the

ClO4�, as defined in (22.1), (22.2), and (22.5),

dþ 1ð Þ= d0 þ 1ð Þ ¼ f a�1 (22.9)

where d is the isotopic composition of the ClO4� at

any value f, and d0 is the isotopic composition at f ¼ 1.

The value of a can be obtained by fitting data with the

natural log of (22.8):

a� 1 ¼ ln R=Roð Þ= ln f (22.10)

This describes the mass-dependent Rayleigh-type

isotopic fractionation that accompanies a variety of

natural processes (Broecker and Oversby 1971). Iso-

topic fractionation factors are commonly expressed in

terms of e, where

e ¼ a� 1 (22.11)

with e normally expressed in parts per thousand (‰).

Isotopic fractionation of Cl and O accompanying

microbial reduction of ClO4� have been investigated

in several laboratory experiments and in a field study

involving an in situ aquifer push-pull test. Two inde-

pendent liquid culture experiments published in 2003

used the bacterial species Azospira suillum with ace-

tate as the electron donor and ClO4� as the sole elec-

tron acceptor to investigate the Cl kinetic isotope

effect (Coleman et al. 2003; Sturchio et al. 2003).

Coleman et al. (2003) maintained the temperature of

their cultures at 37�C, whereas Sturchio et al. (2003)

maintained the temperature of their cultures at 22�C,during incubation. Coleman et al. (2003) reported

e37Cl values of �15.8 � 0.4 and �14.8 � 1.3‰obtained from two separate cultures, both exhibiting

nearly complete ClO4� reduction in about 90 min.

Sturchio et al. (2003) reported comparable e37Clvalues of �16.6 and �12.9‰ obtained from two sep-

arate cultures with complete ClO4� reduction times of

18 days and 5.5 h, respectively, noting that less frac-

tionation was observed in the experiment in which

ClO4� was consumed more rapidly. A subsequent

presentation and re-evaluation of the Coleman et al.

(2003) data concluded a single and more precise e37Clvalue of �14.98 � 0.15‰, following a statistical

refinement of the complete experimental data set

(Ader et al. 2008).

Further work by Sturchio et al. (2007) produced

data on both Cl and O isotope effects of microbial

ClO4� reduction, this time using two different bacte-

rial genera (A. suillum JPLRND and Decholospirillum

sp. FBR2) in liquid cultures at temperatures of 22 and

10�C, and including an experiment using 18O-enriched

water to test for O exchange between ClO4� and H2O.

This set of experiments resulted in composite e37Cland e18O values (regressed from the combined data of

the five separate experiments, with all points weighted

equally) of �13.2 � 0.5 and �33.1 � 1.2‰, respec-

tively. There was no evidence (within analytical error)

for dependence on either bacterial strain or tempe-

rature, and no evidence for O exchange between

ClO4� and H2O, during ClO4

� reduction. A key result

of this work was the observation that, despite a

range in apparent e values of the individual experi-

ments, the ratio e18O/e37Cl showed a constant value of2.50 � 0.04 for the combined experimental results

over a range of f from 1.00 to 0.01 (Fig. 22.2).

The applicability of laboratory isotopic fraction-

ation factors in the field was explored by Hatzinger

et al. (2009), who performed a push-pull test in 2006 at

a site where a soybean oil emulsion had been injected

three years earlier in a demonstration of its potential to

stimulate ClO4� bioremediation (Borden 2007). In the

single injection push-pull test, 405 L of contaminated

upgradient groundwater (containing 5.5 mg L�1

ClO4� and 7.7 mg L�1 NO3

�) was amended with

NaBr (as a tracer for groundwater mixing) and injected

at a rate of 30 L min�1 into the zone containing

residual emulsified oil (the monitoring well was


screened at depths of 1.8–4.9 m, where total well depth

was 5 m below ground surface). Nine water samples of

1–30-L volume (increasing with time) were removed

periodically over a period of 30 h for chemical and

isotopic analysis. Apparent in situ isotopic fraction-

ation factors for both O and Cl in ClO4� were only

about 0.3–0.4 times the values reported for pure cul-

ture studies (Sturchio et al. 2007; Ader et al. 2008);

similarly, apparent fractionation factors for N and O in

NO3� were only 0.2–0.6 times those reported in labo-

ratory culture studies. The relatively small apparent

isotopic fractionation factors observed in the push-pull

test may be attributed to physical and chemical hetero-

geneity in the aquifer, as similar effects have been

observed and modeled for NO3� reduction in ground-

water and surface water (Mariotti et al. 1988; Lehman

et al. 2003; Green et al. 2010). Despite the relatively

small apparent in situ values of e18O and e37Clmeasured for ClO4

� in the push-pull test, the e ratio

e18O/e37Cl was 2.63, in excellent agreement with the

laboratory-determined value of 2.50 � 0.04 (Sturchio

et al. 2007). This indicates that the fundamental pro-

cess of ClO4� reduction may have been the same in

laboratory and field settings, but the heterogeneity of

the field setting was such that it caused substantial

underestimation of the extent of ClO4� reduction

when using an isotopic approach based on laboratory

fractionation factors.

22.3.2 Stable Isotopic Compositionof Natural Perchlorate

22.3.2.1 Atacama Desert, Chile

The most well-known occurrence of naturally occur-

ring ClO4� is in the extensive NO3

�-rich “caliche”-

type salt deposits of the hyperarid Atacama Desert of

northern Chile, which contain up to 0.6 wt.% ClO4�

(Ericksen 1981). The O isotopic composition of Ata-

cama ClO4� was first measured by Bao and Gu (2004)

who reported d18O values ranging from �24.8 to

�4.5‰ and D17O values ranging from þ4.2 to

þ9.6‰. The elevated 17O abundance was interpreted

by Bao and Gu (2004) as a reflection of the atmo-

spheric origin of ClO4� through photochemical reac-

tions of atmospheric Cl species with O3. Additional

isotopic data for Atacama ClO4� were reported by

Bohlke et al. (2005), Sturchio et al. (2006), and Jack-

son et al. (2010), extending the O isotopic results of

Bao and Gu (2004) and, in addition, documenting

extreme 37Cl-depletion with d37Cl values ranging

from �14.5 to �11.8‰. An atmospheric origin for

ClO4� is consistent with N and O isotopic evidence

for the atmospheric origin of NO3� in the Atacama salt

deposits and soils (Bohlke et al. 1997; Michalski et al.

2004; Ewing et al. 2007), where both compounds may

have accumulated for millions of years (Ericksen

1981; Rech et al. 2010; Ewing et al. 2006). There is

growing interest in the origin and isotopic composition

of Atacama ClO4� for several reasons: (1) billions of

kg of Atacama NO3� deposits have been imported to

the U.S. for use in agricultural fertilizer (DasGupta

et al. 2006), and the distinctive isotopic composition

of Atacama ClO4� has been found in aquifers

across the U.S., indicating past use of Atacama nitrate

fertilizer is a source of ClO4� in U.S. groundwater

(Bohlke et al. 2005, 2009; Sturchio et al. 2011);

and (2) the recent discovery of ClO4� in Martian

and Antarctic soils has led to renewed efforts to under-

stand the mechanism of atmospheric ClO4� formation

and the planetary geochemical cycles for ClO4� on

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0 0.01 0.02 0.03 0.04 0.05

ln R/Ro 37Cl

ln R

/Ro

18O

JPL 22C HW

JPL 22C

FBR2 22C

FBR2 10C

JPL 10C

e18O/e 37Cl = 2.5

Fig. 22.2 Relation of e18O and e37Cl in combined results of

five liquid culture experiments of microbial ClO4� reduction

(Sturchio et al. 2007). A constant slope of 2.50 � 0.04 was

observed, independent of bacterial strain and temperature. The

experiment labeled “HW” was done in 18O-enriched water

(d18O ¼ þ198‰) and indicates no measurable O exchange

between ClO4� and H2O during microbial reduction under

these conditions


both Earth and Mars (Hecht et al. 2009; Ming

et al. 2010; Catling et al. 2010; Kounaves et al. 2010).

22.3.2.2 Southwestern United States

The occurrence of indigenous natural ClO4� (unre-

lated to imported Atacama ClO4�) throughout the

United States, especially in the arid southwestern

regions, has become increasingly apparent through

surveys of ClO4� concentrations in precipitation,

soil, and groundwater (Plummer et al. 2006; Jackson

et al. 2005a; Rajagopalan et al. 2006, 2009; Rao et al.

2007; Parker et al. 2008). The stable isotopic compo-

sition of indigenous natural ClO4� analyzed to date

(hereafter referred to as SW ClO4�) is distinct from

both Atacama ClO4� and synthetic ClO4

� (Jackson

et al. 2010) (Fig. 22.3).

The three types of ClO4� shown in Fig. 22.3 can be

distinguished completely by their d18O values. Two

types of ClO4� have overlapping d37Cl values: syn-

thetic ClO4� has a range from about �3 to þ2‰,

whereas SW ClO4� has a range from about �3 to

+6%. In contrast, Atacama ClO4� has a uniquely low

range in d37Cl from about�15 to�9%. The reason for

such a wide range in d37Cl values for natural ClO4� is

not yet understood, but may be related to regional or

hemispheric differences in the source(s) of atmo-

spheric Cl reactants or the mechanism(s) involved in

ClO4� production.

The D17O values of synthetic ClO4� are uniformly

0.0 � 0.2%. Natural ClO4� shows a significant 17O

excess, with Atacama ClO4� showing a relatively

narrow range in D17O values from þ8.1 to þ10.5‰,

whereas SW ClO4� shows a much wider range from

aboutþ0.2‰ in groundwaters from the Southern High

Plains to as high as þ18.4‰ for an unsaturated zone

salt sample from the Zabriskie area (Ericksen et al.

1988) near Death Valley, CA. This mass-independent

isotopic fractionation effect (Thiemens 2006) is diag-

nostic of all natural ClO4� analyzed to date. The D17O

values of the SW ClO4� summarized in Fig. 22.3

display a bimodal distribution, with one group (labeled

SHP, for Southern High Plains) having a range from

Fig. 22.3 d37Cl (per mil) versus d18O (per mil) values

(upper diagram) and D17O (per mil) versus d18O (per mil)

values (lower diagram) for samples of synthetic ClO4�,

Atacama ClO4�, and SW ClO4

�. Southwest ClO4� is

subdivided into Southern High Plains (SHP) and Death

Valley (DV) as described in text. Data from Bohlke et al.

(2005); Sturchio et al. (2006); Jackson et al. (2010)


about þ0.2 to þ2.6‰, and a second group (labeled

DV, for Death Valley) having a range from þ8.6 to

þ18.4‰. This distribution may be related in part with

local climate and the mode of ClO4� occurrence, as

the lower range of values represents ClO4� extracted

from a number of groundwater samples and an unsat-

urated zone soil leachate from the Southern High

Plains and Middle Rio Grande Basin (Texas and

New Mexico), whereas the higher values are from

unsaturated zone salts from the vicinity of Death

Valley, California. The ranges of O isotopic composi-

tions could imply either (1) there is a primary, high-

D17O atmospheric ClO4� signature that is retained for

thousands of years in the most arid unsaturated zone

profiles (and for millions of years in the hyperarid

Atacama Desert) but diminished by slow isotopic

exchange with O in slightly wetter environments

with time, or (2) there is another production mecha-

nism (e.g., involving UV but not O3) that results in

lower D17O values for ClO4� (Jackson et al. 2010).

22.3.3 Chlorine-36 Abundancein Perchlorate

The radioactive isotope 36Cl (half-life ¼ 301,000 years)

provides an additional tracer for ClO4�. Chlorine-36

is produced in the atmosphere largely by cosmic-

ray interactions with Ar, and also in the subsurface by

thermal neutron capture on 35Cl (Lehmann et al. 1993).

The range in measured 36Cl/Cl ratios of Cl� in pre-

anthropogenic groundwater across the continental U.

S. is from ~10 � 10�15 near the coasts to as high

as 1,670 � 10�15 in the central Rocky Mountains

(Davis et al. 2003). The lower ratios near the coasts

reflect dilution by marine aerosols in which 36Cl/Cl ¼0.5 � 10�15 (Argento et al. 2010). Testing of thermo-

nuclear bombs in the Pacific Ocean during 1952–1958

injected a large amount of 36Cl, produced by neutron

irradiation of seawater Cl�, into the stratosphere,

resulting in worldwide 36Cl fallout for several years

thereafter (Phillips 2000). The presence of this bomb-

pulse Cl� may be identified from its anomalously high36Cl/Cl ratio and its association with relatively

high tritium activity. The highest measured 36Cl/Cl

ratio reported for bomb-affected groundwater Cl� is

12,800 � 10�15 (Davis et al. 2003). The 36Cl/Cl ratio

of Cl� in Long Island (NY) rainwater sampled in 1957

(during the peak of the nuclear bomb testing era in the

Pacific) was as high as 127,000�10�15 (Schaeffer et al.

1960). Arctic deposition in 1957 also had anomalously

high 36Cl/Cl (28,600 � 10�15) as observed in the

Dye-3 ice core (Synal et al. 1990).

A reconnaissance survey of 36Cl in ClO4�, includ-

ing a variety of synthetic and natural ClO4� samples,

showed a range of 36Cl/Cl ratios exceeding four orders

of magnitude (Sturchio et al. 2009) (Fig. 22.4). Syn-

thetic ClO4� samples had relatively low 36Cl/Cl ratios

from �2.5 � 10�15 to 40 � 10�15, reflecting those

expected from ancient, marine-derived NaCl sources

0.1

1

10

100

1000

10000

100000

–20 –15 –10 –5 0 5 10

36C

l/Cl (

10–1

5 )

δ37Cl (‰)

U.S.A.groundwater Cl–

Atacama ClO4–

SW ClO4–

synthetic ClO4–Atacama Cl–

Fig. 22.4 36Cl/Cl (atom ratio) versus

d37Cl (‰) in representative samples

of synthetic ClO4�, Atacama ClO4

�

and associated Cl�, and SW ClO4�

(adapted from Sturchio et al. 2009)


used in the electrochemical synthesis, such as bedded

evaporites and salt domes. Southwest ClO4� samples

had extremely high 36Cl/Cl ratios in the range

3,130 � 10�15 to 28,800 � 10�15. Although the pres-

ence of bomb 36Cl could not be ruled out for some of

these samples, 36Cl/Cl ratios of 3,130 � 10�15 to

12,300 � 10�15 were measured in ClO4� extracted

from groundwaters having no detectable tritium, indi-

cating groundwater recharge prior to bomb testing and

implying that the high 36Cl/Cl ratios in these ClO4�

samples are unrelated to the bomb tests. The fact that

these ratios are much higher than any others measured

for Cl� from pre-anthropogenic groundwater implies a

largely stratospheric source for the ClO4�, as any

near-surface ClO4� production mechanism should

yield 36Cl/Cl ratios in the range of near-surface Cl�

(i.e. <2,000 � 10�15).

Atacama ClO4� had an intermediate range of 36Cl/

Cl ratios from 22 � 10�15 to 590 � 10�15, and Cl�

from two samples of the Atacama caliche and one

sample of an Atacama commercial NaNO3 product

had relatively low 36Cl/Cl ratios identical within

error to those of the co-occurring ClO4�. The Atacama

data could be consistent with initially high 36Cl/Cl

ratios (similar to those of SW ClO4� samples) that

have decreased by radioactive decay. In those samples

where ClO4� and Cl� have identical 36Cl/Cl ratios,

ClO4� and Cl� have apparently approached secular

equilibrium with the local radiation environment (i.e.,

a condition in which the rate of radioactive decay of36Cl is equal to its rate of production within the sample

environment). There has been adequate time for secu-

lar equilibrium to be attained, as many of the Atacama

caliche deposits began forming at least 9.4 million

Fig. 22.5 d37Cl (per mil) versus d18O (per mil)

values (upper diagram) and D17O (per mil) versus

d18O (per mil) values (lower diagram) showingstable isotope data for ClO4

� in groundwater samples

from the Chino Basin, California (red symbols)in comparison to the principal known ClO4

�

source types in the region as identified in Fig. 22.3.

A dominantly Atacama source is inferred for the

Chino Basin samples (Sturchio et al. 2008)


years ago (Rech et al. 2006), whereas it takes only

about two million years for 36Cl to reach a condition of

>99% secular equilibrium.

22.3.4 Tracing Perchloratein Contaminated Aquifers

The isotopic data accumulated for ClO4� during the

past decade have created opportunities for tracing

ClO4� in contaminated aquifers with relatively robust

results. The three principal sources of ClO4� found in

contaminated groundwaters of the US (i.e., synthetic,

Atacama, and SW ClO4�) are isotopically distinct

with respect to stable isotopes of O and Cl (Bao and

Gu 2004; Bohlke et al. 2005; Sturchio et al. 2006)

(Jackson et al. 2010) as well as 36Cl (Sturchio et al.

2009). Furthermore, the low rate of O exchange

between ClO4� and H2O (Hoering et al. 1958) could

maintain the integrity of ClO4� source O isotopic

ratios over at least several decades under normal

groundwater conditions (Bohlke et al. 2009). Biodeg-

radation of ClO4� produces systematic stable isotope

enrichments that do not significantly affect D17O

values or 36Cl/Cl ratios, thus potentially allowing

detection of the biodegradation without masking

the initial source(s) of ClO4� (Sturchio et al. 2007;

Hatzinger et al. 2009).

Several studies involving the application of stable

isotope measurements for tracing ClO4� sources in

contaminated aquifers have been published with sup-

porting information from other chemical and isotopic

tracers. Increasing concentrations of ClO4� detected

in municipal water supplies of Chino, Ontario, and

Pomona, California during 2004 prompted an isotopic

investigation to determine the source of this increase.

The data showed a dominantly agricultural source


values (upper diagram) and D17O (per mil) versus

d18O (per mil) values (lower diagram) showingstable isotope data for ClO4

� in groundwater

samples (labeled red symbols) from Long Island,

New York (Bohlke et al. 2009), in comparison to

the principal known ClO4� source types in the

region as identified in Fig. 22.3


with isotopic compositions resembling those of Ata-

cama ClO4�, with apparent minor contributions of

synthetic and SW ClO4� (Fig. 22.5). The northern

portion of the Chino Basin was formerly used for

citrus cultivation and there is anecdotal evidence for

extensive use of Chilean NO3� fertilizer (Sturchio

et al. 2008).

Bohlke et al. (2009) demonstrated that ClO4� ori-

ginating from known synthetic and agricultural

sources in Long Island, New York could be traced

from stable isotopic compositions (Fig. 22.6). A his-

tory of groundwater contamination by Atacama ClO4�

in an agricultural area in eastern Long Island was

inferred from depth profiles of environmental tracer

ages along with concentration and isotopic data for

ClO4� and NO3

� (Bohlke et al. 2009). The possible

sources of ClO4� in the groundwater of Pasadena,

California also were investigated using multiple chem-

ical and isotopic tracers, and evidence was found for

at least three distinct sources of ClO4�, of which

at least two were synthetic (Slaten et al. 2010).

The most intensive local isotopic investigation of

ClO4� sources in groundwater to date was performed

in the southeastern San Bernardino Basin, California.

Discrete, intersecting ClO4� plumes were found to

have distinct sources (Sturchio et al. 2011). One

plume containing synthetic ClO4� emanated from a

former rocket-testing site that had been used subse-

quently as an artificial recharge basin (labeled “site” at

the right side of Fig. 22.7), and another plume contain-

ing Atacama-type ClO4� emanated from a predomi-

nantly agricultural area. Where these plumes intersect,

some groundwater wells contain mixtures of synthetic

and Atacama ClO4� (Figs. 22.7 and 22.8).

Fig. 22.7 Map showing outlines of discrete groundwater

ClO4� plumes in Bunker Hill Basin southwest of San Bernar-

dino, California in relation to groundwater potentiometric

surface. Map legend identifies symbols indicating predominant

ClO4� source type. Adapted from Sturchio et al. (2011)


22.4 Summary and ResearchOpportunities

Isotopic data can be used for tracing the origin and

behavior of ClO4� in the environment. Four indepen-

dently varying parameters have been measured on indi-

vidual ClO4� samples for this purpose: d37Cl, d18O,

D17O, and 36Cl/Cl. At least three distinct types of

ClO4� have been identified isotopically (i.e., synthetic,

Atacama, and SW ClO4�), and these distinctions have

proven to be useful in forensic applications. Additional

data for indigenous, natural ClO4� are needed, how-

ever, to obtain a global picture of its isotopic variations.

Improved methods for sample preparation and isotopic

analysis with much better sensitivity would be helpful

for measuring ClO4� isotopic variations in some sample

types such as aerosols and precipitation as well as

foodstuffs and body fluids, which have been precluded

by the impracticality of obtaining the currently required

milligram amounts of ClO4�. Further experimental and

theoretical investigations of atmospheric ClO4� pro-

duction mechanisms may lead to improved explana-

tions of observed isotopic variations in natural samples.

Acknowledgements Much of the work reviewed in this chapter

was supported by contracts from the Strategic Environmental

Research and Development Program and the Environmental

Security Technology Certification Program of the U.S. Depart-

ment of Defense, and by the National Research Program of the

U.S. Geological Survey. Use of product or trade names in this

paper is for identification purposes only and does not constitute

endorsement by the U.S. government. We thank Hans Eggen-

kamp, Stephanie Ewing, Doug Kent, and an anonymous referee

for constructive reviews of the manuscript.


(upper diagram) and D17O (per mil) versus d18O(per mil) (lower diagram) for samples of

groundwater ClO4� plumes in the southeastern

San Bernardino Basin, California (red symbols)in comparison to the three principal known ClO4

�

source types in the region as identified in Fig. 22.3.

One plume has a synthetic source, and the other

has a predominantly agricultural (i.e. Atacama-type)

source, with some apparent mixing between sources

(data from Sturchio et al. 2011)


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Applications of Selective Ion Exchange for Perchlorate Removal, Recovery and Environmental Forensics