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Arsenopyrite Weathering and Leachingof Arsenic in an Austrian SoilDejene A. Tessema a , Aberra Mogessie a & Walter Kosmus aa Karl-Franzens University of Graz, Graz, AustriaPublished online: 14 Jul 2011.

To cite this article: Dejene A. Tessema , Aberra Mogessie & Walter Kosmus (2011) ArsenopyriteWeathering and Leaching of Arsenic in an Austrian Soil, Soil and Sediment Contamination: AnInternational Journal, 20:5, 550-563, DOI: 10.1080/15320383.2011.587044

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Soil and Sediment Contamination, 20:550–563, 2011Copyright © Taylor & Francis Group, LLCISSN: 1532-0383 print / 1549-7887 onlineDOI: 10.1080/15320383.2011.587044

Arsenopyrite Weathering and Leaching of Arsenicin an Austrian Soil

DEJENE A. TESSEMA, ABERRA MOGESSIE,AND WALTER KOSMUS

Karl-Franzens University of Graz, Graz, Austria

The extent of arsenopyrite weathering in relation to co-existing minerals in an Aus-trian soil and the leaching of arsenic from the soil has been investigated. Soil andunderlying bedrock samples were collected and characterized by chemical and min-eralogical analyses. The solubility of the soil arsenic under anaerobic conditions wasstudied by incubating the soil sample in distilled water for different periods of timeusing a customized lycimeter. The solubility of arsenic from pure arsenopyrite mineraland mixtures of arsenopyrite with chalcopyrite or pyrite was studied by incubatingthe pulverized minerals. Speciation of arsenic in the incubated and non-incubated soilsamples was carried out by sequential leaching, solvent-extraction, and ion exchangechromatographic techniques.

Results of SEM analysis indicated that arsenopyite (FeAsS), the most commonmineral in the area, occurs in paragenesis with pyrite (FeS2) and chalcopyrite (CuFeS2).The existence of these minerals with arsenopyrite was found to enhance its solubilization.From the speciation study it was found that nearly all (92%) of the arsenic in the soilexists in the inorganic form. Out of the total inorganic arsenic, the trivalent inorganicspecies accounted for only 3% and the remaining 89% was found to be the pentavalentform. The low solubility of As in the Graz soil is attributed to the prevalence of thispentavalent inorganic species.

Keywords Mobility, speciation, extraction, incubation, pyrite, chalcopyrite

1. Introduction

Arsenic is an element of great concern in terrestrial as well as aquatic environments becauseof the high toxicity of certain species of arsenic to living organisms. Arsenic release intothe environment can occur as a result of rock weathering, geothermal activity, and fromseveral anthropogenic sources such as mining tailings, coal ash, and arsenical pesticides.Depending on environmental conditions, As in soils can be mobilized into the ground andsurface water, where living organisms are readily exposed to it, and it may accumulate inthe trophic chain (Mello et al., 2006). In soils, although the inorganic species As(III) andAs(V) have been preferentially considered, organisms are able to metabolize these speciesinto organo-arsenic compounds (Smedley and Kinniburgh, 2002). The relative distributionof As(III) and As(V) in the soil is influenced by the redox potential (Eh) of the soil.

Address correspondence to Dejene A. Tessema, Institute of Chemistry, Analytical Chemistry,Karl-Franzens University of Graz, Universitaetsplatz 1, A-8010 Graz, Austria. E-mail: dayeletese@gmail.com

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Arsenopyrite Weathering and Leaching of Arsenic 551

Under oxidizing conditions, As(V) predominates, and in reducing media As(III) is favoredthermodynamically. In the soil As(III) is more mobile than As(V) and this is believed tobe the result of its high pKa, because it should be less adsorbed than the As(V) oxyanions(Deschamps et al., 2003; Foster et al., 1997).

The arsenic found in Austrian soils is mainly of geogenic origin (Kock and Pichler-Semmelrock, 1993; Thalmann et al., 1989). Typical amounts of arsenic in natural, uncon-taminated soils range between 5 and 6 mg kg−1 (Bhumbla and Keefer, 1994). Arsenic isfrequently present above the permissible level of 20 ppm in Austrian soils. It is also reportedthat about 1.3% of the soils consist of arsenic concentration higher than 80 mg/kg (Thal-mann et al., 1989). Its distribution mostly follows the distribution of iron and associatedelements, which is characteristic of the magmatic processes of an early Paleozoic islandarc. The ore mineralizations largely coincide with areas of high arsenic level in the soil(Thalmann et al., 1989).

In the soils of most parts of Styria (SE Austria), the level of arsenic is much higherthan the average concentration found in uncontaminated soils. Analysis of soil samplescollected from 519 sampling sites showed that about 30% of the samples contained abovethe permissible level (Sommerhuber, 2000). For example, the soil in Strassegg (Gasen),a village in Styria located 50 km NE of Graz along the Strassegg electrum-arsenopyritevein-type mineralization zone, contains an unusually high level of arsenic, ∼3000 mg kg−1

(Tessema and Walter, 2001). Due to this, arsenic is considered to be one of the principalsoil contaminants in Styria (Kock and Pichler-Semmelrock, 1993).

Soil arsenic concentrations in excess of 20 to 50 mg/kg are reported to be injurious toplant growth and development (Alloway, 1995). Moreover, 40 mg kg−1 is established as thetoxicity threshold for crop plants (Sheppard, 1992). In Gasen, although the level of arsenicin the soil is much higher than these values, plants grown in the area and animals grazingon the pasture do not show any symptom of toxicity. Although the soil arsenic levels insoils in the region are high, the average level of arsenic determined from the soil solutionscollected at various seasons of the year was very low compared to the concentration in thesoil. Concentrations determined in soil water collected during spring were relatively higherthan in those collected in the other seasons (Sommerhuber, 2000).

Although the existence of high levels of arsenic in the soil may be a threat to environ-mental quality due to its toxic properties, the danger it poses mainly depends on the extentof mobility and availability of the arsenic. The total content in the soil simply represents asource term for that unrealistic environmental scenario in which the entire mineral struc-ture of the solid is digested, and application to other environmental scenarios may leadto over-estimation of the leachability of constituents held in geologically stable mineralphases (Bentley and Chasteen, 2002; Garrabrants, and Kosson, 2000; Jaime et al., 2006).

Many environmental factors, such as pH, redox potential, presence of other elements,organic matter content, texture, mineralogy, fungal or bacterial activity and fauna, areknown to influence the relative abundance of different arsenic forms in soils and thus theirmobility and bioavailability. When environmental conditions such as climate, organic andinorganic components of the soil, and redox potential change, the speciation and mobilityof arsenic may also change (Tessema et al., 2002; Geiszinger et al., 2002). Therefore, thescientific description of the danger that a certain level of arsenic in the soil may pose requiresknowledge of the main species of arsenic in the area and the biotic and abiotic componentsfound in the soil that influence the mobility of the arsenic. Without such knowledge ofthe mobility of the arsenic in the soil and its plant availability, it would be impossibleto estimate the danger it poses to human health, ground water contamination, the healthygrowth of plants cultivated in the area, or the livestock grazing on the pastures.

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To date, there are no reported studies on the extent of mobility of arsenic in the Gasensoil and that describe the physico-chemical or pedogenic factors which may contributeto the mobilization or retention of the arsenic in the soil based on empirical evidence.Therefore, this study has been carried out to understand the physico-chemical factors thatcontributed to the high arsenic level in the Gasen soil, to study the speciation and mobilityof arsenic under conditions that are considered to impact arsenic mobility in the soil. Thestudy was specifically focused on: i) finding out the minerals that co-exist with arsenopyritein the underlying bedrock and studying their influence on the weathering of arsenopyrite;and ii) investigating the influence of change in Eh on the speciation and mobilization ofarsenic in the soil.

2. Materials and Methods

2.1 Soil Physico-Chemical Properties and Arsenic Analysis in Soil and Minerals

Surface (0–20 cm) samples from the soil profile of a sloppy grazing land were collectedfrom the Gasen area. Randomly collected grab samples, along the mineralization vein,were combined in a trough and large particles were crushed using a mortar. The sampleswere thoroughly mixed using a trowel to form a homogenized composite sample. Thehomogenized sample was then air-dried, allowed to pass through a 2 mm aperture sieve,stored in polyethylene sacks, and used for all experiments related to the soil. Underlyingrock samples were also collected using a stainless-steel hammer after shovelling awaythe relatively thin (<1.5 m thick) soil layer. Soil physical and chemical properties weremeasured using standard methods. Soil pH was measured in a 1:2 aqueous suspension usingan ORION model 920-A meter equipped with a glass combination pH electrode (McLean,1982). Soil redox potential was measured using a 1:1 soil/water mass ratio with a platinumelectrode in combination with a calomel reference electrode and an Orion Model SA720 pH meter. The dry ashing method (Karam, 1993) was employed for estimating the soilorganic matter content. Texture was determined following the pipette method (Sheldrickand Wang, 1993). All glassware used in all the experiments were washed with distilledwater and rinsed with Milli Q+ deionized water before use.

Pyrite, chalcopyrite, and arsenopyrite minerals were pulverized using a ball mill, and0.5 g mass of each mineral was accurately weighed and transferred into a high pressuretetrafluormethaxil (TFM) microwave digestion vessel. The same amount of soil samplewas also accurately weighed and transferred into TFM digestion vessels. A mixture of4.0 mL of sub-boiling distilled HNO3, 1.0 mL 30% (w/w) H2O2, and 2.0 mL 40% (w/w)HF was added into each of the samples. In case the reaction was too vigorous, the mixturewas swirled intermittently for about one minute under a fume-hood before microwavedigestion. Then the samples were mineralized by applying the microwave heating programgiven in Table 1. After digestion, the samples were quantitatively transferred to 100 mLvolumetric flasks and diluted to the mark with MilliQ+ water. Then 5 mL aliquot of theclear solution was transferred to a test tube and diluted to 10 mL with water. Total elementanalysis was carried out using a VG Plasma Quad Turbo Plus ICP-MS (VG Elemental,Winsford, UK) equipped with a Meinhard concentric glass nebulizer and a double-passScott-type spray chamber water cooled to 0◦C. A Gilson Miniplus-3 peristaltic pump wasused to supply standard and sample solutions. For maximum intensity the instrument wasoptimized at m/z 115 (115In). For the molecular interference on arsenic analysis from40Ar35Cl+ at m/z 75, an interference correction for chloride was employed.

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Arsenopyrite Weathering and Leaching of Arsenic 553

Table 1Microwave-heated, closed-vessel heating program used for the mineralization of soil

and mineral samples

Step 1 2 3 4 5 6 7 8 9Power (Watt) 250 0 250 0 450 0 600 500 ventTime (min) 2 0.5 10 0.5 5 0.5 7 7 2

Quality control measures included analytical sample duplicates and comparison of themean arsenic concentrations in the NIST Montana Soil Reference soils: SRM 2709, SRM2710, SRM 2711, with their suggested values. Average results of triplicate measurementsof SRM 2709, SRM 2710, and SRM 2711, respectively, agreed within ±9%, ±1.4%, and±7.7% of the certified values of As, ±8.5%, ±4.5%, and ±9.2% of that of Al, ±6.6%,±2.2%, and ±8.3% of that of Fe, and ±2.2%, ±2%, and ±6.5% of that of Mn. It was alsoconfirmed that all errors are not significant (P = 0.05).

2.2 Scanning Electron Microprobe Analysis of Minerals

For the scanning electron microprobe analysis of minerals, polished sections were madefrom the mineralized rock samples. The minerals were identified using a combination ofreflected light microscopy and scanning electron microscopy (SEM) analysis. The analysiswas carried out on a JEOL - 6310 SEM with an attached link energy dispersive system (EDX)and a MICROSCPEC wavelength dispersive system (WDS) using 20 kV acceleratingvoltage and counting time of 100 sec calibrated on cobalt. Standards of natural sulfideminerals as well as pure metals were used for the calibration and analysis of the varioussulfides (Table 2). Matrix effects were corrected according to ZAF for the EDX and WDSanalyses.

The practical detection limits in these analyses vary from 0.05 to 0.1% (w/w) for theMICROSPEC wavelength dispersive system, and 0.1 to 0.5-wt% for the LINKISIS energydispersive system.

Table 2Standards used for the analysis of some elements in the sulfide

minerals

Element Standard

S Chalcopyrite Western mines 4/28/00Fe Chalcopyrite Western mines 4/28/00Cu Chalcopyrite Western mines 4/28/00Zn Zn 60.3Fe 6.43 S 33.27 4/28/00As Cabri-526 (PtAs2) 4/28/00Ag Silver metal 1/28/00Sb Dutrizac - 1584 4/28/00Te Cabri -Calav (AuAgTe) 4/28/00Pb Dutrizac PbTe Cabri 4/28/00

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554 D. A. Tessema et al.

2.3 Anaerobic Incubation of Soil and Arsenic Speciation

To estimate the potential for in situ reductive dissolution of As, about 0.5 kg of the room-temperature, dried and homogenized soil sample was saturated with deionized water andincubated anaerobically in a small, rectangular (20 cm × 10 cm × 10 cm), home-madelysimeter for two months. Such a small lysimeter has been successfully utilized by Kalbitzand Wennrich (1998) to study the mobilization of heavy metals in the wetland soil of theMulde River in the industrial district of Bitterfeld-Wolfen (Germany). The temperature ofthe medium was regulated at 2 ± 1◦C. The soil received no treatment to activate the possiblereduction of arsenic and therefore any reductive dissolution is attributed to indigenousorganisms and substrates in the soils. Beginning with the first day of incubation (day 0)and at weekly intervals after that, a portion of the soil solution was collected through adrain fitted to the bottom of the lysimeter after monitoring the pH and Eh of the suspension.Each collected solution was filtered through a 0.2 µm Millipore filter and the clear solutionwas used for solvent extraction speciation of total inorganic arsenic and trivalent arsenic(Chappell et al., 1995) and speciation of As(III), As(V), and MMA by ion-exchangechromatography (Aggett and Kadwani, 1983). The precision and accuracy of the solventextraction and ion-exchange chromatographic methods were verified using analytical spikesamples. Recoveries of all species (total and inorganic arsenic, As(III), As(V) and MMA)by both methods were found to be ≥92%. Total arsenic recovery from the solution by thesolvent extraction technique was 95%, inorganic arsenic 96%, and trivalent arsenic 93%.The total As, As(III), As(V), and MMA recoveries of the ion-exchange chromatographymethod, respectively, were 94%, 92%, 94%, and 92%. As a result, the methods were found tobe accurate, precise, and suitable for our speciation studies. Paired t-test statistical analysisof the trivalent and total arsenic concentrations obtained by the ion-exchange and solventextraction speciation techniques confirmed that the two methods do not give significantlydifferent results (P = 0.05).

At the end of the incubation period, a portion of the incubated soil was transferredinto a centrifuge tube under O2-free N2 and centrifuged for 5 min at 3000 rpm to partiallyremove bulk water. Then, about 5.0 g of this soil sample and the same quantity of the“field received” (non-incubated) soil sample were transferred into each of 50 mL screw-cap centrifuge tubes and sequential extraction of arsenic bound to the various soil phaseswas carried out following the scheme developed by Tessier et al. (1979). Finally, arsenicconcentration was determined by ICP-MS.

2.4 Leaching Experiment of Pulverized Minerals

A leaching experiment was carried out on the pure and mixed minerals to investigate theinfluence of the co-existence of pyrite (PY) and chalcopyrite (CP) with arsenopyrite (AP)on arsenopyrite weathering. Five sets of replicate samples of the pulverized minerals wereprepared in 50 mL centrifuge tubes. Three of these sets consisted of 1 g of each of the pureAP, PY, or CP minerals and each of the remaining two sets consisted of a mixture of 1 gAP + 1 g PY or 1 g AP + 1 g CP. Another four sets, in which two of them consisted of2 g of each of the pure PY and CP minerals and the remaining two consisted of 1 g AP+ 2 g PY or 1 g AP + 2 g CP were prepared. Ten mL of Milli Q+ water was added toeach of the tubes and allowed to stand for two weeks at room temperature with intermittentshaking. Finally, all samples were centrifuged on a Jouan B3.11 centrifuge at 3000 rpm,filtered through a 0.2 µm Millipore filter, and the concentration of As in the leachates wasdetermined by ICP-MS.

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Arsenopyrite Weathering and Leaching of Arsenic 555

Figure 1. SEM image of arsenopyrite showing its relationship with chalcopyrite. Arsenopyrite (AP),Chalcopyrite (CP), Electrum (El).

3. Results and Discussion

3.1 SEM Analysis and Leachable As in Incubated Mineral Mixtures

Results of the representative electron microprobe analyses of phases in the mineralizedzone (Table 3) indicated that arsenopyrite, pyrite, and chalcopyrite are the most commonminerals underlying the soil. Arsenopyrite, however, is the most abundant mineral andit occurs as prismatic crystals that are up to a few centimeters in length. It often occursin paragenesis with pyrite and chalcopyrite. The association of these minerals with oneanother is shown in Figures 1 and 2.

Results of the leach experiment of the mixed minerals (Figure 3) show that the con-centrations of arsenic in the leachate of 1.0 g of pure AP were found to be less than theconcentration in the leachates of the mixture with PY or CP. The amount of arsenic deter-mined from 1.0 g or 2.0 g of pure PY or CP leachates was less than 0.5 mg kg−1. However, theamount of arsenic released from 1.0 g of AP increased from 6.75 mg kg−1 to 15.42 mg kg−1

when 1.0 g of PY was added to it and incubated. This quantity also increased to 19.8 mg kg−1

when the mass of PY in the mixture was doubled. A similar increase was observed in the

Figure 2. SEM image of arsenopyrite showing its association with pyrite and chalcopyrite. Pyrite(PY), Chalcopyrite (CP), Arsenopyrite (AP).

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Tabl

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posi

tions

ofre

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anal

yses

ofph

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tras

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Pyri

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)1(F

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)1(C

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S 2)1

Ele

men

tE

lem

ent%

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%E

lem

ent%

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mic

%E

lem

ent%

Ato

mic

%

Fe48

.71

±0.

2135

.00

±0.

1535

.05

±0.

1633

.49

±0.

1531

.35

±0.

1525

.51

±0.

12A

s—

—45

.42

±0.

5832

.35

±0.

41—

—S

51.7

4±

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64.7

6±

0.15

20.3

5±

0.14

33.8

7±

0.23

35.2

1±

0.12

49.8

9±

0.17

Au

——

1.07

±0.

260.

29±

0.07

——

Cu

0.38

±0.

080.

24±

0.05

——

34.4

1±

0.24

24.6

0±

0.17

Zn

——

——

——

1 Cal

cula

ted

form

ula.

556

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Arsenopyrite Weathering and Leaching of Arsenic 557

Figure 3. Concentration of arsenic in the leachates of pure AP, PY, CP minerals and mixtures of PYor CP with AP.

AP-CP mixture leachate. The observed difference between the amount of arsenic leachedfrom pure AP and that of arsenic leached from AP-PY or AP-CP mixtures is quite largeand cannot be considered to have come from the dissolution of PY or CP that is mixedwith AP. This indicates that nearly all the arsenic is derived from arsenopyrite dissolution.From the observed increase in the amount of arsenic in the leachates of AP-PY or AP-CPmixtures from that of pure AP leachate, the dissolution of AP appears to be enhanced bythe co-existence of PY and/or CP with AP. Based on these results, we can suggest that theco-existence of the two minerals with AP might have contributed to the rapid weatheringof arsenopyrite and the accumulation of such a high level of arsenic in the soil.

It has been shown that when sulfide minerals are in contact, one will have an acceler-ating or a retarding effect on the chemical changes of the other (Ahlberg and Asbjornsson,1993). The observed increase in AP dissolution in the presence of PY and CP could bean electrochemical corrosion process in which dissolved species such as Fe3+ and Cu2+

undergo reduction at the arsenopyrite surface, behaving as an anode while arsenopyrite actsas the cathode. Once released from arsenopyrite, the arsenic can be transported by variousphysical processes resulting in elevated levels of arsenic in the soils.

3.2 Leachable As and its Speciation in the Soil

The amount of mobile arsenic (water soluble plus exchangeable) in the incubated soilsample was found to increase successively relative to that of the “field-received” sample(Figure 4). Mobile arsenic concentration accounted for 0.14% of the total extracted arsenicin the “field received” soil, whereas it was 0.72% of total arsenic in the two-month incubatedsoil. However, in both samples more than 99% of the arsenic was found to be bound tothe recalcitrant soil phases and remained immobile. This could be a good indication forthe geogenic origin of the arsenic in the Gasen soil. Metals deposited from anthropogenic

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558 D. A. Tessema et al.

Figure 4. Arsenic concentrations in the various phases of the “field received” (FRec) and eightweeks incubated (Incub) soil samples.

sources are relatively more mobile than those of geogenic origin, and become immobilizedwith time because of slow diffusion into minerals under natural conditions (Hirner, 1992).

The concentration of arsenic in each of the recalcitrant fractions (amorphous iron oxidebound to residual) of the incubated soil sample is less than its corresponding concentrationin the “field received” soil extracts. Although the excess quantity of arsenic in the mobilephase of the incubated soil from that of the “field received” soil is expected to have beenreleased from the carbonate-bound soil phase, since this is the phase in which arsenic isbound loosely next to the water soluble and exchangeable phases, the amounts of arsenicextracted from this soil phase in the two soil samples were nearly the same. This may beattributed to the fact that the observed increase in pH and the corresponding lowering of theEh (Table 4) might have created a thermodynamically favorable condition for the resorptionof part of the arsenic released from other fractions by the carbonate-bound fraction.

Table 4Eh and pH values of the soil solution of eight successive weeks of incubation and corre-

sponding rH values

Week Eh (mV) pH (H2O) rH

0 313 5.01 20.811 120 5.88 15.902 96 6.01 15.333 55 6.15 14.204 45 6.2 13.955 24 6.25 13.336 13 6.37 13.197 −2 6.4 12.738 −14 6.45 12.42

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Arsenopyrite Weathering and Leaching of Arsenic 559

Figure 5. Concentrations of total As, inorganic As, and As(III) in the lysimeter leachates of variousweeks as speciated by solvent extraction.

Results of the solvent extraction speciation revealed that nearly all (92%) of the arsenicin the “field received” soil existed in the inorganic form; the pentavalent inorganic arsenicspecies accounted for about 97% of this inorganic arsenic (Figure 5). Trivalent arsenic wasnot detected in the water extract of the “field received” soil. However, in the soil solutioncollected after one-week incubation, a relatively higher level of trivalent arsenic (4.5%)has been determined. The proportion of inorganic arsenic was observed to decrease withincubation time, which indicates an increase in the organic arsenic proportion. The higherrate of increase of organic arsenic to that of the trivalent form in the first two weeks,followed by a remarkable decrease in the inorganic arsenic, may indicate that methylationof arsenic by microorganisms is a step that follows its reduction from the higher pentavalentto the trivalent form.

The ion-exchange chromatography speciation results, depicted in Figure 6, agreed withthose of solvent extraction results. The ion-exchange chromatography speciation resultsshow that pentavalent arsenic is the major species in the soil solution. As in the case ofthe solvent extraction speciation, the trivalent species was not detected at the beginning ofthe incubation (day 0). However, in the soil solution collected at the end of the first weekof incubation 0.03 mg L−1 of this species was detected, and accounts for 9.4% of the totalarsenic determined in the leachate. Monomethylarsonate (MMA) was detected beginningthe second week of incubation and increased successively at an average rate of 0.4 mg L−1

per week. The pentavalent and trivalent species were also found to increase with incubationtime. However, the rate of increase of trivalent arsenic was fairly uniform (0.3 mg L−1 perweek), while that of the pentavalent species decreased successively. In addition to this, theratio of pentavalent arsenic to those of the trivalent and MMA species in the soil water wasobserved to decrease with an increase in the duration of incubation.

The relative proportion of trivalent and MMA to that of total arsenic in the leachatesof day 0 and week eight also increased from 9 to 20%, and 4 to 13%, respectively. Theamount of pentavalent arsenic, however, declined from 92 to 58%. In general, results of theincubation study revealed that mobile arsenic species in the soil increases with an increasein incubation period.

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560 D. A. Tessema et al.

Figure 6. Concentrations of total As, As(V), As(III), and MMA in the lysimeter percolates of variousweeks as speciated by ion-exchange chromatography.

Results of the physico-chemical analysis of the soil indicated that the soil at Gasenis slightly acidic (pH 5.01) and fairly oxidizing (Eh 313 mV). With an increase in theanaerobic incubation time, an increase in soil pH and a decrease in Eh were observed. Theredox level of the soil is characterized by combining the two parameters and calculatingthe rH (reduction intensity) value from the equation

rH = Eh (mV)/29 + 2pH

A reduction intensity value of zero is defined as prevalence of a completely reducedcondition while a value of 42.2 as prevalence of an entirely oxidized condition (Munch,and Ottow, 1983). The reduction intensity value of the “field received” (day 0) soil solution(Table 4) indicates that the soil at Gasen has a typical redox environment. However, withan increase in incubation period, the soil environment became more and more reducing andtotal soluble arsenic went on increasing.

Previous studies (Salzsauler et al., 2004; Forstner, 1991; Masscheleyn, et al., 1991)have also revealed that at higher reduction intensity values As(V) is the predominant arsenicspecies and arsenic solubility is observed to be low. Further, reduction of As(V) to As(III)was found to enhance mobilization of arsenic. On the other hand, arsenic is known to beeffectively removed from acidic solutions by adsorption into soil minerals, indicating thatarsenic oxyanions were naturally retained in the presence of oxides and clay minerals andstrongly adsorbed by iron oxides, aluminum oxides, and manganese oxides. Consequently,the presence of oxides of Al, Fe, and Mn in soils will govern the mobility of arsenic inthe soil system. Furthermore, AlAsO4 and FeAsO4 are known to be the prevalent arsenatecompounds in soils at lower pH and oxidizing mediums (Deschamps et al., 2002). Therelatively high concentrations of these elements in the Gasen soil (Table 5) may indicatethat the soil arsenic exists mainly as the insoluble arsenates of these elements.

Arsenic combined with Fe and Al oxides is also known to be liberated upon hydrolysiswith the reduction of soil potential (Mello et al., 2006). In our experiment, the observedincrease in mobile arsenic with incubation time can be attributed to the reduction of arsenic

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Table 5Concentrations of As, Al, Fe, and Mn in the Gasen soil, Montana soil SRMs 2709–2711

and mineral samples

Concentration (mg kg−1)

Sample As Al Fe Mn

Gasen soil 2980 ± 17 7782 ± 14 2350 ± 20 800 ± 12SRM 2709 16 ± 0.75 6.86 ± 0.26 3.27 ± 0.09 526 ± 7SRM 2710 617 ± 3.46 6.15 ± 0.15 3.31 ± 0.03 9901 ± 96SRM 2711 97 ± 3.12 5.93 ± 0.25 2.65 ± 0.1 596 ± 24Pyrite 270 ± 5 488 ± 7 391000 ± 322 ∗

Chalcopyrite 265 ± 3 648 ± 0.6 248700 ± 76 ∗

Arsenopyrite 370900 ± 265 472 ± 3 276700 ± 141 15 ± 0.4

∗ = <1 mg/kg.

from its higher oxidation state to the more labile lower oxidation states by indigenousmicroorganisms and substrates in the soil as well as the reduced potential of the soil.

4. Conclusion

Results of our study have shown that arsenopyrite is the most common mineral in theunderlying bedrock and the mineral co-exists with chalcopyrite and pyrite. Pedo-chemicalinteraction of these co-existing sulfide minerals with arsenopyrite, presumably an electro-chemical corrosion process in which dissolved species such as Fe3+ and Cu2+ undergoreduction at the arsenopyrite surface behaving as an anode and arsenopyrite as the cathode,seems to be the predominant factor for the extremely high level of arsenic in the soil.

Speciation of nearly all of the arsenic (>95%) is the pentavalent inorganic form. Thepredominant pentavalent inorganic arsenic species is strongly bound to the recalcitrantsoil fractions. Arsenic existing under this condition is known to be hardly mobile andphytoavailable. The observed low mobility of arsenic in the Gasen soil could be attributedto its pentavalent-inorganic speciation and strong binding to the soil recalcitrant fractions.However, the increase in the mobility of arsenic with a decrease in the soil Eh and the factthat spatial and seasonal variations of soil Eh and pH change the dynamics of metals in thesoil solution could be a good indication that a further study on the seasonal variation of thelevel of arsenic in the ground water has to be conducted.

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