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?