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NOVO
Environmental
r-.
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.r".i."t"gy
and Chemi,'try, V"L 2\, N". 5,1'1'.980-983,2002
" 2002 SETAC
I'rinted in the USA
11730-72"'\102 $9.1)4) .t .1111
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OXlDATlON CHEMISTRY OF ACID-YOLATlLE SULFIDE DURING ANALYSIS
ADRIAN M. GONZALEZ*
Amtcch Scrvi"cs In<:.,Dn lIannah Avcrmc, Knoxvillc, "Tcnncs.'<Cc
:\7'>21,USA
(R.xâved
21 M{~y 2001; Accel'ted
21 Oct<)ner 2001)
Ab5tract.--The susccptibility of some componcnts of sediment add-volatile sul/ide (AVS) to chemical oxidation is a critical factor
impacting accurate measurcment of AVS in se<!iment samples. This well-documeutcd susccptibility tooxidatiau 100to Ih<:n:quircment
for oxygen-free cou(ütions in lhe analytical method devcloped for AVS. lu light ofthis aL'"Utcpott.'t1tial tooxidi7.e, lhe sercnilipitous
finding that Rir can bc use<! in thc analysis of sOOiment AVS is countt.'Iintuitive and unexpccted. To dcmonstrate and investigate
lhis interesting observation, extraction cxperiments were per{ormed using aqueous and solid-pbase sullide spcci<."S.Expcrimenls
using Rir as lhe earrier gas showcd a mean percentage recovery of sul/ide matching that af traditional (nitrogen gíts) analysis (i.e.,
>91"1..) and a time to completion ofless lhan 30 mio for aqueous sulfidc and less than (,() mio for scilimcnt samples. These results
are consistenl with those of sul/ide oxidation studles rcported in lhe líterature. Usi~ mr asthe anaIytical carrier gas can provido:
an interesting alternative for devclopiog aR analytical method to determine AVS paramcters in lhe field.
Keywords.--Acid-volalile
sulfide
Metal toxidty
&-diment
INTROD1JCTION
Certain componenls of sediment acid-volatile sulfide (AVS;
í.e., labile metal sulfides [primarily amorphous iron (11) and
manganese (fI) sulfideJ, FeS, and/or MnS)arequíte susceptíble
to chemical oxidalÍon [1-3). Detennining concentrations 01'
simultaneously extmcled metaIs (SEM) and AVS is useful in
screening sediments for potential toxicity doe to elevated metal
concentrations. The accuracy of lhe SEM and AVS data, however, is limiled by lhe slability of AVS in lhe sediment sample.
Changes in AVS concentration, primarily through oxidation
during sampling, shipment, and manipulation, can lead to inaccurate SEM:AVS ratios and to inaccurate toxico}ogica} classification of sedimcnts [4,5]. The current, /aboratory-based
melhod foI determining SEM and se.iiment AVS concentratíons [6J altempts to do this by using ínert gas (e.g., nitrogen)
10transfer hydrogen sulfide (B}S) released during sample acidífication fiom tJlC reaction vessel to a suUide-stabilizing mediurno The method BIso specífies using flasks, water, acid rcagent, and su/fide-stabilizíng solution purged of oxygen (i.e.,
deaerated).
The goal of tbis papel" was to document lhe successful use
of air as a carricr-gas matrí.1\ for AVS ana/ysis. AJthough inilially counterintuitive, this phenornenon is repcatable and reproducibJe and can be cxplained in light of sulfide oxidation
Iheory as reflected in lhe current lileralure. Thís I)henomenon
míght Jind some usefulncss ín lhe devclopmclll of analytical
mclhods tor dctemlining AVS paranlelers in lhe field.
MATERIALS
Oxidation
for sulfidc content. Surface-rinsed sodium sulfide crystals
(Na1S'9H2O) were dissolved in deaerated, àistilled walcr to
make stoek solutions lhat were then standarrli1',ed iorlomeuícal/y. Oue samp/e of a freshwater sediment was collected ftom
a suburban pond (Rivcrdale, NY, USA) using a Petite-Ponar
dredge (Wildlife Suprir, Buffalo, NY, USA) a1ld passed
through a I.OO-mm sieve to remove Jarge gravei and debris.
PolentiaJ oxidation caused by sieving this se.iiment sample
was ilTelevant to this study, because the goal was to compare
rclatíve AV3 concc1luations in spli\, hornogenized suhsamples
by two methods (i.e., aír.and nitrogen). Thc LFS batches wcrc
made by mixing equal volume\: 01' a natural \:ubmergcd clay
(Oak Ridge, TN, USA; sieved to a particle size of < 1,000
I-Lm) anã sand (from a local vendar, KnoxviJIe, TN, USA;
sieved to a particle si2'.c of <500 ILm). ~nthetic AVS for-
"
mu/ations [7J were made by comhiningstoichiometricam~ijJ1ISr
of iron (ll) sulfate (FeSO..7Hp) and sodium sulfid€' (both ,
dissolved in deaemted, distilled walcr) directly within lhe LFS
matrix
to forro
the
iron
(lI)
sult,de
precipitate.
Su1f\de
ana1)!s)s
t
\
in lhe ínitial Jnvestigations followed lhe mcthod described by
Allen ct ai. [6}. wilh the following minor modifications.
The nitrogen-bascd sulfide analytical method (Nz-AVS)
used laboratory-grade nitrogen anã Ihrec 125-ml glass Erlenmeye~ flash (Belko Glass, Vineland, NJ, USA) for lhe reaction vessels (i.e., the reaction vessel, pU 4 buffer trap, and
sulftde stabili",ation
trap). A cartridge
01' oxygen-stripping
resin ' " j "
(OxiClear; Diamond Tool anã Die, CIe~'elalld, 011, USA) W3S(
installed 'o lhe nitrogen supply line, replacing \he vanadium I
ch/oride scrubbing solution described by Allen et aI. [6]. The
ANO METIIOOS
To demollstrale. lhe negligihle effe.ct ~~fair on sulfode recovery during AVS analysis, extraction cxpcrirnc1Its were pcrformcd on sulllde species ín severa I malríces. Prcpared solulions ofsodiurn sulfidc, sampled aliquots oinatural {resh~ater
sedimenl, and prepared samples of a laboratory-iormulaled
sedimellt (LrS) amcndcd wíth synthetic AVS were analyzed
Analysis
reaction vesscl in eacb analytical traio was charged wilh SO\
r
i \\
ml 01' 1 M hydrochloric acid (instead of water) that was de- " 'oxygenated wítb nitrogen for more than 30 min before sample
addition. '-oss of sulfidc by adding samples díreclly 10 lhe acid
was negligible, because the rcaclion nask was sealed ímlllediately
«
1 s) al\cr
addi\ion
01' \he
sample.
Two
sult.de
rc-
covery chccks iu this syslem (using lhe 1>1andardizedsodíurn
sulfide solutioll) resultcd in 96.6% recovcry.
The air-ha"oo ..ult;.u, analytical m..thod (air-AV~) u"..d
uu-
* Tu whom cmrc~pm\<km:c may bc a_"lres","
\accn 1117(l.1)hotmait.<:om.\
')80
981
Ellvimlt. Tt,x;.:..,t. Ch<,m. 21, 2002
Oxidation chemistry 01' AYS during analysis
..
Table I. Acid-volatilc sulfide conccntrations dctcnnined in lhe laboratory using nitrogen and air CarriL'T-gaS
metbods
Acid-volatile
sulfide (,L111Ol/gdry sediment)
Air mcthod
Nitrogen mcthod
8a111l'le
identilication'
Mean
SOl'
n
CY'
Mcan
SI>
n
CY
NS-I
LFS-I
LFS-2
LFS-3
LFS-4
LFS-5
LFS-6
LFS-6b'
LrS- 7
125.tr'
<0.14/
0.16
2.&&.
6.87/
8.82/
\ 1.6/
13.9/
17.4/
NAd
0.00
0.16
0.10
0.58
0.21
0.84
0.28
0.11
I
3
3
2
3
3
3
3
3
NA
NA
1.00
o.m
0.08
0.02
0.07
0.02
0.01
126.0 /
<0.14,
0.16.
2.69
5.83/
8.47/
11.9/
13.2/
NA
0.00
0.16
0.43
0.37
0.S9
0.08
0.84
0.32
I
3
3
3
3
3
3
3
3
NA
NA
1.00
tU6
(1.(16
0.07
0.0\
0.(16
0.02
17.1 /
U,I"
DiffL'Tene<."
tU
0.0
0.0
6.6
. IS.I
-4.0
2.6
-- 5.0
-- 1.7
SigniflCant
at fi
O.OS'!
(p value)
NA'
NA
No (0.9809)
.
No (tH'Io.120)/
No (J.OS84)/
No (0.3887)
No (05:W.2) ,
No (0.2563)
No (0.2007)
/
.NS = natural (tield-collected) sediment; LFS
~
laboratory-fonnulated
sedimcnt containing synthetic acid-volatile sullidc.
SD = standard dcviation.
'CV = coefficient 01' variation (standard dcviation .:. mean).
d % Difference
= IO{) X (air - nitrogen)/nitrogen.
'NA = not applicable.
I LFS-6a and LFS-6b wcre repeated L"Xperinu..-nts
using LFS-6 sedirncnt.
h
fillered air oblaincd from a laboratory air tine. an aquarium
air pump. or a peristaltic pwnp for ana1yses in positive-pressure mode and from a portable vacuum pump for analyses in
ncgative-prcssurc (i.c.. vacuum) mode.
Side-by-side air-AVS and N,-AVS analyses (conducted in
125-ml glass Erlenmcycr flasks in triplicatc. exccpt for tbe
analysis of Ibe natural sediment sample) were perfonned to
compare suJfide recovery by tbe two melhods. A total ofnine
scparate experiments were conducted witb scdiment samples
(one natural sed imen I ~ple
and seve" LFS samples; one
LFS samplc was used twiee) conlaining mcan AVS eoneentTalions tbal ranged from <0.14 to 126.0 jLmol/g dry sediment.
These eorresponded to inilial sulfide eoncentrations ranging
fTom <0.005 to 8.8 mM (based on.81l..mLofJ.M.J:!Ç!). The
AVS concenlrations in the seven LFS samples were < 18 !Lmol/
g dry sediment (i.e.. <0.9 mM). Single detcnninations of a
natural sediment sample containing extrernely bigb AVS concentrations (previously measured aI 126.0 jLmol/g dry sediment) by tbe air-AVS and N,-AVS rnetbods were BIso perforrned. Samples in replícated experimenls were added to alternat1ng treatmenl tures (nitrogen, a1T.nittogcn. air, cte.) to
mjnjmize introduction of syslernalic bias caused by sulfide
oxidation in samples awaiti~g distribuliol1. Samples were aeidified. and lhe gas-pbase B,S transfer was contil1ued unlil lhe
silver suICIde precipitale bad coalesced.
The maximllm recovery attainab1e and the tota1 time required for maxímum sultide recovery in lhe small-scale airAVS cquipmenl were detennined in tWQtirne-course experimenls. Equal amollnts of sulfide stock solulion (42.4 p.mol
sulfide) were added to a series of small-scaJe analytkal trains.
and tbe aeid transfer/reaclion was iniliated. AI 5 to tO min
ínlervals. lhe sulfidc-trap tllbe fcom ooe analylical traio was
removed. and the contents were filtered and dTied to determine
lhe mass 01' I.he predpitale. A secolld experimenl was needed
to oblaín recovery data aI times of less tban 10 mino Recovery
was calculaled tTom Ibe ralio of measured to total sulfide added.
The precision and accuracy of 111eIwo rnelhods were evalualed hy comparing percenlage diftcrenccs in mcan AVS conccntralions among mctho\1$ and by comparing standani deviatíons and coenieienls of varialíon (standard deviation .;mean) of AYS eonccnlrations measured by botb melbods. Dít:.
ferences in sulfide rccovery data florn the two rnetbods wcre
ana1Y7..ed statistiea11y
for signiflCance
at lhe a
=
0.05
leve1
using Lhegeneral linear model and Tukey's Studentized Range
test inctudcd iR lhe SAS~ statistical IOOftware{Si.
RESULTS
Bomogeni7..ed sediment samplcs were analyzed side-bysitie using botb lhe N,-A VS and tlle air-A VS melhods to compare lhe recovery of sulfide (Table \). The absolute p<..-rccntage
differences in sediment AVS coneenlTations belween lhe Iwo
metbods were less than 6.6'%, witb one exception (LFS-4.
-15.1%). Six oftbe seven LFS samples bad AVS eoneenlralions grealer tban the detection limit (-O. t 4 fLIDol/gdry sediment for 3-4-g samp1es). Analysis of 000 samp1e (LFS-6) was
perfOlmed twiCd. Mean concentrations weriJ lower as determined by lhe air-i\VS mcthod retative tQ lhe N,-i\YS method
(i.e.. pereentage differences were negative) in tive experíments. were bigbcr in two (lhe natural sedimenl, NS-l. and
tbe first experiment using LFS-6), and were approxirnatcly
cqual in two (LFS-I and LFS-2). The range of stanciard deviations ofthe AVS concentrations in LFS-3. -4, -5. -6a. -6b.
aRQ -7 usjng bolh ana)yJieaJ metborls was O.JO 10 0.84 and
0.16 10 0.84 p.mol/g dry sedirnent for lhe N,-A VS and ai rAVS melhods. respeetivcly. Tbcse values were consistent
across a range ofmean eoncentrations spanning two orders of
magnitude (0.16-11.4 fLIDol/g dry sedimenl). Variabi1ity in
AVS concentralíons exprcssed as coefficienls of variatíon
showed the two methods to be similar: ,!te average cocffident
of variation for lhe N[AVS rnctbod was 4.0 :t 2.9"/0, and tbal
for lhe aÍf-AVS mcthod was 6.4 ..!.:4.9%. Based on Ibese resutis, tbe nittogen metbod appears 10 have becn more precise
Ihan lhe air-A VS metllod in detennining sample AVS eoncentralÍons.
The potential for systematic oxidalion 01'sedirnent samples
during. experiment set-up was addressed by altemating tbe addition of sampte belwecn tlte two mclbod trealrnents. Tbe success 01'this procedure was evaluated by ptotting measured AVS
e<mcentralions from eacb experirnent as a funelion of sample
order. The resu/ts (oot shown) di<! nol suggest thal AVS conccntralions were "ystematkaUy Iowcr in sam\,lcs l\feparc<.-\lalcr
(e.g.. samples numbercd 4, 5. and 6) relalive to samples prepared earlier (c.g.. samplcs nurnbcrcd I, 2. and 3). The only
982
Envimn.
AM. GonzaIez
Tox;co[. Ch.'m. 21.2002
sample to which Ihis mar have occurrcd was sarople LFS-4.
tbe sample with lhe bigbest percentage difference (- 15. I %)
and lhe Iowesl p value (0.0584) (Table I). Total preparalion
time between tbe flrst and last sample was nev~ nmre than
15 mino Bascd on Ihese results, il was assumcd Ihal any 01>scrved differences in mcan AVS concentrations bctween the
two analytical methods were due primarily 10 difference.<; in
lhe efTccl of lhe carrier-gas composition.
Differcnces in AVS collcelltrations bctwcen me two anaIytical melhods in seven oflhe nine experiments were analyzed
for significance. StatisticaJ comparisons were not applicable
to experiments NS-I (analy"..cd in singlet) or LFS-I (concentration Icss than lhe detection levei). Allhough sulfide concentrations in five of lhe analyses were )ower wbcn deterrnincd
by lhe air-A VS melhod Ihan when determined by lhe N,-A VS
method, no diff~ences in AVS concentrations among lhe saropie pairs were significantly differcnt at o: = 0.05 (Table I).
The percentage recovery of aqucous sulfide in lhe smallscale apparatus using negative-pressure (i.e., vacuum) airflow
was 92. I 1: 2.9%(n
= 4)
afier 10 mino This result isconsistent
with sulfide rccovery (91.7 1: 7.0'Yo.n = 20) ill side-by-side
experiments perforrned under positive-pressure mode in this
study, wilh aqueous su)fide spike recoveries (>90%) reportcd
by Allcn et aJo [6). ano with aqueous sulfide spike recoveries
(93.81: 6.7%) reported for a diffusion method ofdeterrnining
AVS (9). Completion of tbe H,S transfer, as indicated by coalescence of lhe silver sullide precipitale in lhe sullide traI',
occuned within 20 min of acidification of &:\ucous sulflde
spikes ano from 30 to 60 min for scdiment samples. Longer
times to prccipitate coalescence likely indicate longer sulfide
transfer times caused by inefficient or incomplete scdimelltl
acid reagent mixing. Sulfide'recovery efficiellcy experiments
wcre nol pcrforrncd wilh sedimcnt samples.
DlSCUSSION
The sulfide oxjdation literature was reviewcd for Ihree sullide oxidation processes: Gascous H1S. aqucous sulfide. ano
solid-phasc FeS. The results of Ihis review show Ihat. allhough
these Ihree processes are bigbly complex ano dependent on
experimental condltions, lhe oxidation rale under lhe experimental condirions of AVS analysis (í.e.,low pH) is extremely
slow. The difference be\ween II,S oxidation ra1es ano extraction and sequestration rales is wbat makes using air as the
carrier gas successful in this particular case. The relative potential impact of each oxidation OIcchanism on deterroining
accurate AVS conccntrations using air as lhe c3JTÍer gas is
discusscd in light of Ihis literature review.
The oxidation of gaseous 1-12Sin Rir is slow ano strongly
dependent on tbe atmospberic' bydroxyl radical (011') concentration [10). This mar partiaJly explain ils 311llOying(and
I'otenlially toxic) pcrsistencc in lhe atmo\:\,here when relea~d.
The residence time ofH1S in lhe Rir spacc wilhin the analytical
traiu (-60 ml) at the Rir flow rale uscd (200 ml/min) is approximately
60 ml + 200 rol/mia
=
0.3 miR
=
18 s. This is
much shorter than lhe residence lime of H2S in a doscd system
( 102--IO' sI, wbcrc lhe only sulfide sink is almospheric oxidalion [ I IJ. Oxjd111ionby gas-pbase reaclion wiili almospheric
oxygcn wilhin lhe AVS system should not oceur to any appreciable degrce within lhe analytieal systcm anã likely is not
a sígnificant mcchanism for sulfidc loss.
A number of investígators bave studicd lhe oxidatíon 01'
sulfide ill marine or freshwaler aqueous matriccs {12 181-The
bel<texample oflhcse I<ludicl<was one rerform,e,d by Chen 3nd
Morris [12]. They investigatcd oxidation kinetics of aqucous
sulftde (as Na2S.Q\l20) in (nonsaline) water under a wide range
of 1'11conditions (6.00-- I 1.75), inilial sulfade .:oocentralions
(0.5 X 10'4..2.0 X 10.4 M), and initial OKygC4lo
~cntrations
(1.(, X \0-4 .8.0 x 10 . M). They consistenlly observcd an
induction períod, ranging fiom 0.2 to 6.0 h, during which
reactant concentrations remaincd uuchanged. In addition, lhe
speciflC oxidation rale (k; observcd afier lhe induction period)
was a cmnplex, nonlinear funclion of pH ano had maxrmum
values aI pll 8.3 ano p/J J 1.2. Oelween pll .8.3 anel 6.0 (the
lowest pH uscd), lhe spccifie rcachon rale dropped from approximately 23 M-O9 h-I to ncar ".cm. Because at pU < 6 any
sulfide prescnt is predominantly in lhe f0l11l of H2S, Ihese
autbors spcculated tbat the spccific rale of oxidalion of su)f.dc
in this 1'11range would be extremely slow.
In ali cases, it was ei~
shown, ar assumed through evideare from other studies, thal lhe primary reduced sulfur species being oxidizcd was lhe bisulfide ion (HS--), not dissolved
1-/2S.This has dircct implicatíons on lhe oxidation linetics of
aqueous-phase ~ulfide wilhin lhe AVS anaJysis system. The
convcrsion of sulfidc to H2S by I M HCI will be exuernely
rapid, limitcd onJy by lhe samplelacid reagent mixing efficiency [12). Sulfide spcciation at lhe ,,1-1ofa I M HCI 8Olution
(pH
= O) l4)
will strongly
favor lhe 1-12Sfonu. Underextremely
seidic pH conditions, lhe concentration ofthe HS- spccies is
negligib'e; Iherefore, lhe sulflde oxidation rale is expeclcd to
bc extremely slow. Even if lhe sborlest induclÍOII period is
assumcd to apply under lheGe analytical condilions (0.2 b, or
12 ruiu), significant sulfide oxidation should not occur betwecn
lhe time lhe sample is acidificd ano lhe time the relcascd H,S
is stabilizcd.
The oxidation of solid-pbase FeS within sel/eral difTcrenl
lypcs of matrices ha.",been invcstigaled 1I,2,7}. Expcrrmenls
wilh varying concentrations of synlhelic iron (11) sulfide in
aeralcd waler [I) showed that sulfide concentrations'reacbcd
.
nondetectcd levels in approximalely I to 2 h, regardless of
initial sulfide concentrations ranging bclween roughly 3 and
17 ruM. Similar results were ohtaincd by Di Toro et aI. [2j
during oxidalion studies usíng natural scdiment samples (containing AVS) suspended in oxygen-saturated watcr and by
Gonzalez {7) during oxidaliOll cxperimcnts usíng synlhetic FeS
iu a LFS matrix suspended in oKygen-salurated water. However, sulfide samples in solid-phase forros (e.g.. FeS) would
similarly be conl/ertcd to lhe 112Sforro at lhe experimental
pl I, with its complete conversion bcing limitcd only hy lhe
mixing ef11ciency of lhe scdimentlacid reagcnt matrix.
Oftbe tbrce possible mechanisms for suiCIdeoxidation (i.e.,
gasoous, aqucous. and 8Olic:1phase), S<llid-phase sulfidc llXilUltion Iike1y would impacl AVS concentra\io"s in samplcd
sediment most readily. None oflhethrcc mcchanisms ofsulfidc
oxidalioll, however, is expccted to iml'3CI thc rcsults 01' Ihc
AVS analysis ollce lhe 1'11ofthe saml'lc matri is 10werC4.!hy
acidiflCalion.
CONCUJSIONS
h lIas been shown lhal scdiment AVS can bc eJ<tracted using
air (as tlle carrier gas inlheacid extraction) willlout significanl
impacts on its quantitative (CCOl/cryand measuremcnt otlce
the samplc has becn acidiflCd. IR light of the acutc susceptihility ofreduccd sulfur sl'ccics (e.g.. scdiment AVS) to chclllical oxidation. this fmding is rdtbcr intercsting. Sullide rccovcry cxpcrimcllts using Rir werc Rol statisticall) difTcrcnt ti'oll1
I<ulfldecanccntratiolli: obtaincd usi,,!!. laboratory-gradc "i\(o-
Oxidation ChL'l1listryof A VS during anaJysis
Envirml. ToxicoJ. Ch<m. 21, 2002
%3
;,
gell. The fiean recovery efficiency for aqlleolls sodillffi slIlfide
spikes, rallgíng from 10.2 to 42.4 v.mol, was 91.7 -'- 7J)% (n
= 20), which ís colIsistelll wíth the resu/Cs of olher íllvesligations l4,9}. The successlul use 01' air as a carricr gas for
AVS analysis works ill I1lis paI1icuJar applicalioll, howcver,
bolh because the dominant form of sulfide present during analysis is not the rapidly oxidized HS- but lhe slowly oxidized
H2S and because lhe rale of extraction/transfer of dissolved
H2S is much grcaler than the oxidation rale at acidie pll. Thc
release, transler, and stabilízation 01'sulfide occur betore SIIbstantialloss via oxÍllation is realized. This obscrvation might
prove 10 he ali illtcresting alternative for developing an allllIytical method 01' detennining AVS pararnclers in the field.
Acknow/nigement...This
study was pcrformed in part under Suocontract 18X-ST297C with CKY at Oak Ridb~ National Laboratory, managed by Lockheed Martin Encrgy Rescarch for the U.S. DcpartrnL"'!1t
af Energy undi..-rcontract DE-ACO5-960R22464. Tbe l'Upt'artofl.ynn
Kszos and the assistance of Clint Rash are VL'TYmuch apprcciated.
Critica! review of the manuscript by several anonY111ousrevicwcrs is
also apprecialed.
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