Post on 26-Oct-2019
Arsenic speciation in sulfidic waters:Arsenic speciation in sulfidic waters:
Reconciling contradictory spectroscopic andg y
chromatographic evidence
h d XAS thi itor: why does XAS see thioarsenites
and IC-ICP-MS thioarsenates?
Prof. Dr. Britta Planer-Friedrich
Group Seminar, 12.11.2010
Thioarsenites
OH SH SH SHOH OH SH SH
As
OH
As
OH
As
OH
As
SHOH OHOH SH
Mono~ Di~ Tri~ Tetra~
OHOH OH SH SH
Mono Di Tri Tetra
O
OH – As = O
O
OH – As = O
O
SH – As = O
S
SH – As = O SH – As = S
SHOH SH SH SH
Thioarsenates
The early papers: Thioarsenates
B B & T í k F U b di Ei i k S h f l t ff f
y p p
Brauner, B. & Tomícek, F., Ueber die Einwirkung von Schwefelwasserstoff auf Arsensäure. Fresen. J. Anal. Chem. 27 (1), 508‐513 (1888).
Precipitation of As2S5 (low pH high H2S low temp ) vs As2S3 Precipitation of As2S5 (low pH, high H2S, low temp.) vs. As2S3
McCay, L.W., Die Einwirkung von Schwefelwasserstoff auf Arsensäure Fresen. J. Anal. Chem. 27 (1), 632‐634 (1888).
McCay, L.W., Die Einwirkung von Schwefelwasserstoff auf Arsensäure. Zeitschrift für anorganische Chemie 29, 36‐50 (1901).
Isolation of monothioarsenate and dithioarsenate
McCay, L.W. Z. Anorg. Chem. 29, 36‐50 (1902).
McCay, L.W. & Foster, W., Über die Trisulfoxyarsensäure. Zeitschrift für Anorganische Chemie 41, 452‐473 (1904). Alkalinizing As S yields trithioarsenate at low OH concentrations; formation Alkalinizing As2S5 yields trithioarsenate at low OH concentrations; formationof mono‐, di‐ and tetrathioarsenate at higher OH concentrations
1970: Thioarsenites
at pH ca. 6AsS4
3- + 1.5 H3O+ 1/n AsS33- + 0.5 SH- + 0.5 H2S + 1.5 H2O
Tetrathioarsenate Trithioarsenite
at pH ca 3at pH ca. 32 AsS3
3- + 2 H3O+ 1/x As2S5 + H2S + 2 H2OTrithioarsenite precipitation as As-S-mineral
Upon acidification, thioarsenates are stable as long as AsOH-groups canform; once the addition of further protons forces the formation of AsSHform; once the addition of further protons forces the formation of AsSH-groups, the ions become instable
hydrolysis to trithioarsenate can only be a minor reaction as tetrathioarsenate becomes immediately instable
CE, IC: Thioarsenates
• First determination of thioarsenates by IC‐ECD and CE
• Synthesis of mono‐, di‐ and tetrathioarsenate
tetrathioarsenate
thioarsenate stability over pH:
• monothioarsenate entire pH range ! compare Thilo paperp g
• dithioarsenate max. pH 3, 5‐6, 10
• tetrathioarsenate only pH 10‐13
trithioarsenate
tetrathioarsenate only pH 10 13
IC‐ICP‐MS: Thioarsenites
‐‐> compareto Schwedt
o thioarsenic species can be determined by IC (20-100 mM NaOH) - ICP-MS (std.
mode) at low concentrations!
o based on their occurrence under reducing conditions (arsenite + sulfide) they are
assigned as thioarsenites
a problem occurs with the assignment of peak No. 4 proposed AsS4Hxx-3, with no
b di l t inon-bonding electron pair
o thermodynamic constants were derived CAUTION! Those are WRONG!
IC‐ICP‐MS: Thioarsenates
d th d Wilki t l 2003 (IC ICP MS) it lfid tdo used same method as Wilkin et al. 2003 (IC-ICP-MS) on arsenite-sulfide std.
solutions and natural samples
b d i ith th i d td d ld lit t i id tifi tio based on comparison with synthesized stds. and old literature reviews identification
as thioarsenates (not thioarsenites as Wilkin et al. 2003)
o occurrence of thioarsenic species (mainly tri~) in groundwater of a former paper millo occurrence of thioarsenic species (mainly tri~) in groundwater of a former paper mill
(sulfur acid production pyrite-burning) environmental relevance!
o proposed mechanism: disproportionation As(III) + S(-II) As(V)-S + As(0)o proposed mechanism: disproportionation As(III) + S(-II) As(V)-S + As(0)
problem: we see no „missing“ As(0); disproportionation questionable
IC‐ICP‐MS: Thioarsenates
o S/As ratios determined in synthetic samples
trithio-arsenate
as 1:1 to 4:1
o ES/MS characterisation of IC-fractions dithio-arsenate
arsenate
as thioarsenates
What we measure with IChi
are definitely thioarsenates!monothio-arsenate
problem: are the original mixtures of
arsenite + sulfide really thioarsenates 800 1000 1
tetratharsena
arsenite + sulfide really thioarsenates
or only oxidized by IC-ICP-MS and ES-MS?
800 1000 1time [s]
XAS: Thioarsenites
All XAS paper report thioarsenites (ONLY!) no thioarsenatesAll XAS paper report thioarsenites (ONLY!), no thioarsenates
XAS: Thioarsenitesupon destruction of
ThioarsenatesAs(V)As(III)
• first spectra of synthesizedthioarsenates
„Tetrathioarsenate“ acidified
Thioarsenates
4
Tetrathioarsenat • first detection ofthioarsenites and -nates in the same solution
pH 2.8pH 6.3
pH 5.8
3
led
CN As2S3
• formation of thioarsenites upon acidification oftetrathioarsenate
1
2
mod
el
Monothioarsenat
Dithioarsenat
0
1
11867 11868 11869 11870 11871 11872
Na-Arsenat
Monothioarsenat
Na-Arsenit
11867 11868 11869 11870 11871 11872
E0 [eV]
80
100
cent
ratio
n
A: EXAFS Speciation
20
40
60
ulat
ed IT
T co
nc
XAS: Trithioarseniteforms from Arsenite + Sulfide
4
0 C
alc
B: EXAFS Coordination Numbers
Component 1 (Arsenite) Component 2 (Trithioarsenite)
1
2
3
CN
0
C: IC-ICP-MS Speciation CNAs-S CNAs-O
100s)
40
60
80
cont
ent (
% T
As
IC‐ICP‐MS: Dithioarsenateforms from Arsenite + Sulfide
l ti f thi it
0
20
0.1 1 2 3 4 10
Spec
ies
AsIII S:As S:As S:As S:As S:As S:As AsS3
• co-elution of thioarsenites and thioarsenates?
• oxidation of thioarsenites? Loss = TAs - Sum species Tetrathioarsenate (HxAsVS4
3-x) Trithioarsenate (HxAsVOS33-x)
Dithioarsenate (HxAsVO2S23-x) Monothioarsenate (HxAsVO3S3-x)
Arsenate (HxAsVO43-x) Arsenite (HxAsIIIO3
3-x)
• hydrolysis of thioarsenites?
Thioarsenates in chromatography from arsenite‐lfid l ti id ti t f t!sulfide solutions are an oxidation artefact!
5.0x108 Arsenite 10 mM Arsenite 10 mM + Sulfide 100 mM anaerobic Arsenite 10 mM + Sulfide 100 mM aerobic
2 „arsenite“ peaks, when
• S:As ↑
6.0x107
8.0x107
er se
cond
• Total As ↑
• NaOH ↓
lf i ith f th 2 k
7
4.0x107
Cou
nts p
e no sulfur in either of the 2 peaks
150 200 800 900 1000 1100 1200 13000.0
2.0x107
Time (s)
60
70
80A
ol/L
)
cap-closed IC vials
30
40
50
60
entra
tion
(µm
o p
Oxidation kinetics of a solution containing0
10
20
30Sp
ecie
s con
ce
solution containing0.1 mM arsenite+ 1 mM sulfide
0 1000 2000 3000 4000 5000 6000
70
80
Time (min)B
) IC i l 1 mM sulfide
40
50
60
70
tion
(µm
ol/L
) open IC vials
10
20
30
40
cies
con
cent
ra
0 1000 2000 3000 4000 5000 60000
Loss = TAs sum species
Spec
Time (min)Loss = TAs - sum species Tetrathioarsenate Trithioarsenate Dithioarsenate Monothioarsenate Arsenate Arsenite
Why arsenite instead of thioarsenites?
80
100A
s) Loss = TAs - Sum species Tetrathioarsenate (HxAsVS4
3-x)
60
80
ent (
% T
A Trithioarsenate (HxAsVOS33-x)
Dithioarsenate (HxAsVO2S23-x)
Monothioarsenate (H AsVO S3-x)
40
ecie
s con
te Monothioarsenate (HxAs O3S )
Arsenate (HxAsVO43-x)
Arsenite (HxAsIIIO33-x)
0
20Spe
Analysis underidi i diti
SH:OH 5.0 2.5 1.3 1.0 0.2
As(mM) 10 5 1 0.5 0.1pH 12.3 12.3 11.9 11.7 11.8
oxidizing conditions:thioarsenates = thioarsenites
• Dilution (even when preserving pH) leads to a decrease in the SH:OH ratio with an excess of OH thioarsenites are instable and transform to arsenite (hydroloysis, „competitive dissociation“)• Without initial formation of thioarsenites, no thioarsenates form!
Effect of pH80
100
TAs)
A
40
60s c
onte
nt (%
T
immediately after preparation
2.8 5.0 6.0 7.0 9.2 11.1 12.1 13.00
20
Spec
ie
pHSH OH 2 108 1 106 1 105 1 104 63 0 8 0 1 0 01
immediately after preparationin the glovebox
SH:OH 2.108 1.106 1.105 1.104 63 0.8 0.1 0.01
80
100B
As)
40
60
80
cont
ent (
% T
A
2.7 4.2 6.1 6.8 8.7 11.1 11.9 13.00
20
pH
Spec
ies after one week in the glovebox
SH:OH 2.108 6.106 8.104 2.104 200 1.0 0.1 0.01
Loss = TAs - Sum species Tetrathioarsenate (HxAsVS4
3-x) Trithioarsenate (HxAsVOS33-x)
Rapid decomposition of thioarsenites to arsenite but relatively slow formation from
Dithioarsenate (HxAsVO2S23-x) Monothioarsenate (HxAsVO3S3-x)
Arsenate (HxAsVO43-x) Arsenite (HxAsIIIO3
3-x)arsenite electrostatic repulsion of negatively charged arsenite and sulfide?
Summary of what happens duringion chromatography
Summary interpretation of previous observationsSummary interpretation of previous observations
thi it d t i d i XAS b it d• thioarsenites are determined in XAS because arsenite and thus sulfide concentrations (10-100 mM) are high compared to OH-concentrations (pH 11-12 = 1-10 mM)OH concentrations (pH 11 12 1 10 mM)
• dilution necessary for IC (nM - µM range) and analysis at highly alkaline pH leads to a decrease in the SH/OH ratio, thioarsenites decompose to arsenite (anaerobic cond.) or
idi t thi t ( bi d )oxidize to thioarsenates (aerobic cond.)
• preparation of arsenite and sulfide at high pH preventspreparation of arsenite and sulfide at high pH prevents formation of thioarsenites, no thioarsenates form, only arsenite is detected under aerobic cond. thioarsenites are necessary intermediate key species for thioarsenate formation
ImplicationsImplications
d t li itl d t i thi it• we need to explicitly determine thioarsenites analytical challenge ion pair chromatography? synthesis of trithioarsenite?synthesis of trithioarsenite? what is the “loss”? prove pathway of formation (“oxidative thio-lation”)? role of polysulfides?
h t th fi ld t di th ti thi t• what are the field studies worth reporting thioarsenate occurrence with IC-ICP-MS?
improve sampling methods (O2-exclusion!)improve sampling methods (O2 exclusion!) look for thioarsenites under anaerobic conditions role of polysulfides?