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Arsenic removal from groundwater of the Chaco-Pampean Plain (Argentina) using natural geologicalmaterials as adsorbentsJochen Bundschuh a b c , Prosun Bhattacharya a , Ondra Sracek d e , M. Fernanda Mellano f

, Antonio E. Ramírez f , Angel del R. Storniolo f , Raúl A. Martín f , Julia Cortés f , Marta I.Litter g h i & Jiin-Shuh Jean ca KTH-International Groundwater Arsenic Research Group, Department of Land and WaterResources Engineering, Royal Institute of Technology (KTH), Stockholm, Swedenb Institute for Applied Research, Karlsruhe University of Applied Sciences, Karlsruhe,Germanyc Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwand Department of Geology, Faculty of Science, Palacký University, Olomouc, Czech Republice OPV s.r.o. (Protection of Groundwater Ltd), Praha, Czech Republicf Departamento de Geología y Geociencias, Facultad de Ciencias Exactas y Tecnologias,Universidad Nacional de Santiago del Estero (UNSE), Santiago del Estero, Argentinag Gerencia Química, Comisión Nacional de Energía Atómica, San Martín, Prov. de BuenosAires, Argentinah Instituto de Ingeniería e Investigación Ambiental, Universidad Nacional de Gral. SanMartín, San Martín, Prov. de Buenos Aires, Argentinai Consejo Nacional de Investigaciones Científicas y Técnicas, Buenos Aires, Argentina

Available online: 31 Aug 2011

To cite this article: Jochen Bundschuh, Prosun Bhattacharya, Ondra Sracek, M. Fernanda Mellano, Antonio E. Ramírez, Angeldel R. Storniolo, Raúl A. Martín, Julia Cortés, Marta I. Litter & Jiin-Shuh Jean (2011): Arsenic removal from groundwater ofthe Chaco-Pampean Plain (Argentina) using natural geological materials as adsorbents, Journal of Environmental Science andHealth, Part A, 46:11, 1297-1310

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Journal of Environmental Science and Health, Part A (2011) 46, 1297–1310Copyright C© Taylor & Francis Group, LLCISSN: 1093-4529 (Print); 1532-4117 (Online)DOI: 10.1080/10934529.2011.598838

Arsenic removal from groundwater of the Chaco-PampeanPlain (Argentina) using natural geological materials asadsorbents

JOCHEN BUNDSCHUH1,2,3, PROSUN BHATTACHARYA1, ONDRA SRACEK4,5,M. FERNANDA MELLANO6, ANTONIO E. RAMIREZ6, ANGEL DEL R. STORNIOLO6,RAUL A. MARTIN6, JULIA CORTES6, MARTA I. LITTER7,8,9 and JIIN-SHUH JEAN3

1KTH-International Groundwater Arsenic Research Group, Department of Land and Water Resources Engineering, Royal Instituteof Technology (KTH), Stockholm, Sweden2Institute for Applied Research, Karlsruhe University of Applied Sciences, Karlsruhe, Germany3Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwan4Department of Geology, Faculty of Science, Palacky University, Olomouc, Czech Republic5OPV s.r.o. (Protection of Groundwater Ltd), Praha, Czech Republic6Departamento de Geologıa y Geociencias, Facultad de Ciencias Exactas y Tecnologias, Universidad Nacional de Santiago delEstero (UNSE), Santiago del Estero, Argentina7Gerencia Quımica, Comision Nacional de Energıa Atomica, San Martın,Prov. de Buenos Aires, Argentina8Instituto de Ingenierıa e Investigacion Ambiental, Universidad Nacional de Gral. San Martın, San Martın,Prov. de Buenos Aires, Argentina9Consejo Nacional de Investigaciones Cientıficas y Tecnicas, Buenos Aires, Argentina

Use of natural geological materials for arsenic (As) removal is an emerging solution at a household level for poor people in remoterural settlements, especially when the materials are locally available and can be collected by the local population. Their low or zero costmakes these materials very attractive compared with synthetic or commercial materials. Sometimes, this may be the only option toprovide safe water to very poor settlements. Their suitability for As removal from water is mainly due to adsorption, co-precipitationand ion exchange processes involving Fe- and Al-rich minerals and clay minerals present in the soils or sediments. In the presentstudy, various clay-rich soils from the Santiago del Estero province (SDE, NW Argentina) and, for comparison, a laterite from theMisiones province have been tested as adsorbents for As in shallow naturally contaminated groundwaters of the Rıo Dulce alluvialaquifer in SDE. Batch adsorption experiments showed higher As(V) removal for the Misiones laterite sample (99 %) as comparedwith the soils from SDE (40–53 %), which can be related to lower contents of water-soluble and oxalate extractable Al and Fe inthe last samples. These results suggest the application of the Misiones laterite soil as an alternative for As removal. However, hightransportation costs from Misiones to SDE can be an economical restriction for the low-income population of SDE.

Keywords: Groundwater arsenic removal, geogenic arsenic sorbents, sequential extraction.

Introduction

Metals and metalloids, including arsenic (As), can bepresent in mineral phases within their crystal lattice orcan be attached onto their surface as inner- or outer-sphere complexes. The respective processes between min-

Address correspondence to Jochen Bundschuh and Prosun Bhat-tacharya, KTH-International Groundwater Arsenic ResearchGroup, Department of Land and Water Resources Engineering,Royal Institute of Technology (KTH), Teknikringen 76, SE-10044Stockholm, Sweden; E-mail: jochenbundschuh@yahoo.com (J.Bundschuh) and prosun@kth.se (P. Bhattacharya).

eral phases and water (precipitation/dissolution, sorp-tion/desorption, ion exchange, formation of colloidal com-plexes), determine the release of As into the aqueous phaseand the sequestration of As from the water. The processesdepend on the mineral phases present in the system andon geochemical conditions such as pH, Eh, ionic strength,ionic composition of the fluid phase, concentration andspeciation of metals dissolved in the water phase, etc.

The genesis of As-rich waters might be dependent uponthe specific geochemical conditions, favorable for its releasefrom the solid phase to the aqueous phase. Similarly, underspecific geochemical conditions As in the aqueous phasecould be sorbed onto a mineral phase (or a mixture of

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1298 Bundschuh et al.

mineral phases in a sediment or soil) and thus could beused for removal of As from drinking water.

Groundwater is the main source of potable water forthe population in a major part of the globe. However,in many parts of the world, including Latin American(LA) countries (Mexico, Argentina, Chile, Peru, Bolivia,Nicaragua, Uruguay, Venezuela, Ecuador, etc.), As ispresent in groundwater and a population of 14 million isestimated to be at the potential risk of As exposure (>10µg/L).[1]

Conventional technologies (coagulation–coprecipit-ation, adsorption, reverse osmosis, etc.) can be applied atmedium or large scale and also at household level. How-ever, for domestic or community purposes as those fre-quently found in rural or urban isolated population inLA, low-cost technologies using non-expensive materials,sunlight or biological methods should be developed[2–3].Experiences on emerging technologies obtained especiallyduring the last two decades in Latin America com-prise: (i) coagulation/filtration with iron and aluminumsalts, scaled-down for small community- and household-scale-applications;[4–7] (ii) adsorption techniques using low-cost arsenic sorbents, such as geological materials (clays,laterites, soils, limestones),[8–10] natural and modifiedorganic-based sorbents,[11–14] and synthetic materials.[15]

TiO2-heterogeneous photocatalysis and zerovalent iron, es-pecially using nanoscale particles, appear to be promis-ing emergent technologies.[16–17] However, further researchis needed to find additional methods and to assessthe effectiveness of the proposed technologies. Socioeco-nomic features should be carefully taken into accountto select the technology. Although various studies havebeen undertaken by numerous local researchers, andproven treatment methods for the specific water condi-tions have been encountered, technologies have been not yetcommercialized.

Natural As is frequently reported in groundwater fromseveral parts of Argentina where most groundwater fordrinking water supply for the rural population are ab-stracted from the shallow aquifers generally at a few metersto tens of meters in depth.[18–24] In poor rural areas of theChaco-Pampean Plain of Argentina, which extends overmore than 1,000,000 km2, at least 4.5 millions of people[25]

depend on drinking water with high As concentrationsat toxic levels (more than 10 µg/L); often values exceed1,000 µg/L).[18–24] In the province of Santiago del Estero(SDE, NW Argentina; Fig. 1), which belongs to the semi-arid Chaco plain, two large areas with groundwater Ascontamination are found.

The first area is situated along the northern border of theSDE province and extends into the provinces of Salta andChaco. The second area is the Rıo Dulce Alluvial Conelocated to the East of SDE city in the counties of Bandaand Robles. In the first area, the shallow aquifer (depthmostly <15 m) and the unsaturated zone are character-ized by a discontinuous accumulation of loessic sediments

(up to few meters thick), which are deposited over fluvialand aeolian fine sand to silty sediments of Quaternary age.The mechanism of As mobilization is thought to be dueto alkaline desorption from secondary As-bearing mineralphases such as Al, Fe and Mn (oxy)hydroxides; volcanicash layers and volcanic glass dispersed in aquifers and theunsaturated zone are considered as the principal primaryAs source.[26–33]

The present study focuses on As removal by adsorptionon natural materials followed by sedimentation to separatethe water from the As adsorbing media. Natural materi-als containing metal-oxides and -hydroxides could have Asadsorption features. In many soil types, these oxides andhydroxides occur as coatings on mineral grains. Arsenate isgenerally more easily removed since it occurs as negativelycharged anionic species that adsorb on the surface of thepositively charged media. Under certain conditions, clayscan adsorb As because of the oxide-like structures at theiredges.

In previous works, different clay-rich soils and sedimentshave been tested as natural and locally available adsorbentsfor As removal from groundwater from SDE. Claesson andFagerberg[31] tested at a household scale various soil sam-ples taken within 100 km from the capital city of SDE andthree soil samples collected in Misiones, a province 1100km east of SDE (NE Argentina; Fig. 1), where the soil isknown to be lateritic. The highest As adsorption rates werefound using an acidic Fe-rich laterite from Misiones whosemajor components were hydrous oxides of Fe and Al, witha pHzpc of 7–9.

Experiments were performed with SDE groundwaterscontaining initially 500 µg/L As and using a 1:10 lat-erite:water mass ratio; the initial As concentration was de-creased to 10–30 µg/L within 40 min, corresponding to aremoval efficiency of 94–98 %. The high removal capacitywas explained by the higher Fe content of the material andthe low pH values of the groundwater, which favored Asadsorption and made it the most suitable material to re-move As. The other soils were unfavorable for As removal.The cost for transporting the laterite from Misiones to SDEwas considered as the principal economic limitation for ap-plication of this method, as Misiones is 1100 km far fromSDE; these authors estimated the annual cost to be US$43(year 2003) for a household of 10 persons (average dailyconsumption: 4 L water/person).

Later, Lindback and Sjolin[32] studied the effectivenessof five sediment and soil samples from different Argen-tine provinces (SDE, Tucuman, Salta and Misiones). Theyfound that in the explored regions in SDE (Robles County),there were no remediation soils with a low pH and a highcontent of Fe encountered, suitable for remediation pur-poses. However, the laterite from the Misiones Provincehad good properties for As remediation and further stud-ies should focus on this adsorbent.

The present study evaluates in which form the metals Fe,Mn, Al and the metalloid As are present in clay-rich soils

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As removal from Chaco-Pampean Plain, Argentina 1299

Fig. 1. Northern Argentina: Soil and volcanic ash sampling sites in the provinces of Misiones and SDE.

and volcanic ash from SDE aquifers and compares themwith a lateritic soil from a tropical area (Misiones province),their release into the water phase and the suitability of thesesoils to remove As from drinking water. Arsenic removalwith this material may be a simple and low-cost method,which uses a locally available material for poor rural areasof Argentina. Geochemical and chemical conditions whichfavor release and sequestration of As from/by the solidphases are investigated and discussed.

Materials and methods

Reagents and equipment

All reagents were of analytical quality (Merck). Water wasdeionized with a Milli-Q Water Purification System. Aque-ous solutions or suspensions were shaken with a Fanemhorizontal shaker, model 2540, unless indicated.

Soil sampling

Samples were taken from two areas: (i) SDE (NWArgentina, Fig. 1), an area with semiarid climate, (ii)the Misiones province (NE Argentina, Fig. 1), which ischaracterized by a tropical climate and related lateriticweathering. Samples of five clay-rich soils (S1–S5) werecollected from different sites of the SDE area. The sampleswere selected based on macroscopic information aboutthe content of iron i.e., on red color of samples. SampleS5 is from the area of the Rıo Dulce alluvial cone, whose

shallow groundwater is highly contaminated by As (As:<10–14900 µg/L).[27] In this area, additionally, foursamples of volcanic ash were taken from two sites (V1a andb; V2a and b). The sample from the Misiones Province wasa laterite sample (M1) and was included for comparison(Fig. 1). At all sites, samples of ca. 1 kg were taken fromthe surface to a depth of about 10 cm.

Clay mineralogy

The mineralogical composition of the clay fraction of thesamples M1, S1–S5 and V1 was determined by X-raydiffraction using a Philips PW2510 X-Ray Diffractome-ter at the Department of Stratigraphy and Sedimentology,Miguel Lillo Institute, National University of Tucuman.

pH, electrical conductivity and cation exchange capacity ofsoil saturation extract

The pH, electrical conductivity and cation exchange ca-pacity of soil saturation extracts were determined for allsamples with the exception of V1b and V2.

pH. For determination of the pH of the soil saturationextract (soil/water ratio: 1:2 V/V), 25 mL of distilled waterwere added to 10 g of sieved soil, shaken for 5min andallowed to settle for 30 min before measuring the pH in thesupernatant.

Electrical conductivity (EC). First, 100 mL of distilled wa-ter were added to 10 g of sieved soil and agitated for 30 min

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1300 Bundschuh et al.

before measuring the electrical conductivity of the super-natant.

Cation exchange capacity (CEC). 5 g of soil were filledinto a centrifuge tube and 33 mL of 0.1 M ammoniumacetate were added; the mixture was stirred for 5 min. Af-ter centrifugation for 5 min at 1,000 rpm, the supernatantwas decanted into a 250 mL flask. This procedure was re-peated two more times, and the volume of the flask wasfilled up with ammonium acetate. This solution containedexchangeable cations of soil, replaced by ammonia.

Then, 33 mL of ethanol were added to the tube, thetube was sealed, shaken for 5 min and centrifuged. Theclear supernatant liquid was decanted and discarded. Thisprocedure was repeated 3 times.

Now, 33 mL of ammonium acetate were added to thetube and the tube was covered, stirred for 5 min and cen-trifuged. The clear supernatant was decanted and filledinto 250 mL flasks. This procedure was repeated 3 times.Finally, the flasks were filled up to 20 mL with deionizedwater and the concentrations of exchangeable cations weredetermined by acid-base titration with 0.1 M HCl.

Sequential extraction

A sequential extraction was applied to all samples (M1,S1–S5, V1a,b, V2a,b). Extractants used were in order oftheir application: deionized water (DIW), sodium bicar-bonate (NaHCO3), sodium acetate (NaOAc), ammoniumoxalate ((NH4)2C2O4) and, finally, 7 M HNO3.

Deionized water (DIW) extraction. The samples wereleached with DIW buffered to pH 6.95 for the quantifi-cation of the water-soluble fraction metals. 4 g of samplewere mixed in 250 mL conical flasks with 100 mL DIW. Af-ter shaking on a horizontal shaker for 2 h (GFL OverheadRotator 3040 horizontal orbital shaker) the solutions werefiltered through Munktell R© 00K quantitative acid washedfilter (ash content 0,007 %), acidified with 0.5 mL concen-trated HNO3 and stored in 50 mL plastic bottles. If precip-itation occurred in the filtrate, the solutions were re-filteredusing a 0.2 µm membrane filter (Whatman) and stored in22 mL vials.

Bicarbonate (NaHCO3) extraction. The residual fractionsof the DIW extractions were allowed to decantate and theliquid fraction was removed. These liquids were extractedwith 100 mL of 0.01 M NaHCO3; after shaking (2 h), andsettling (12 h) to allow sedimentation of the solid particles,the procedure of the previous extraction step was repeated.Solutions were filtered (Munktell R© 00K –filters), acidified,filtered again through 0.2 µm membrane filters and storedin 22 mL vials. The bicarbonate extraction, which is appliedto samples with pH<7, was only applied to samples M1 andS1, since all other samples had high pH values (7–10).

Acetate (NaOAc) extraction. The contents remainingfrom the previous extraction step were allowed to decantateand the liquid fraction was removed. Then 200 mL of 1.0M sodium acetate (NaOAc) was added to each flask. Theflasks were left for 2 h on a horizontal shaker. Then the so-lutions were filtered through Munktell R© 00K filters, acid-ified, and stored in 50 mL bottles. Since no precipitationoccurred, it was not necessary to filter the samples again.

Oxalate ((NH4)2C2O4) extraction. Oxalate extractionprimarily quantifies Fe, Al, and Mn bound to amorphousoxides and hydroxides. The remaining contents of theconical flask from the previous extraction step were left fordecantation and the liquid fraction was removed. Then,100 mL of 0.2 M buffered (NH4)2C2O4 (pH 3.5) wereadded in the dark and left for 4 h on the horizontal shaker.The contents of the conical flask from these phases wereleft for decantation and the majority of the liquid fractionwas removed. Samples were centrifuged for 15 min at4000 g. The supernatants were filtered through 0.45 µmSartorius filters and diluted 5 times prior to analysis.

Residual fraction (7 M HNO3) extraction. The remainingsediment in the conical flasks was dried overnight at 30 ◦C.Then, 2 g of sediment samples were mixed in a conical flaskwith 15 mL of 7 M HNO3 and thereafter boiled for 2.5 hon a sand bed with small glass balls for good circulation.The samples were digested at 70 ◦C for 30 min, and then at100 ◦C for 2 h. Then, the solutions were cooled down,filtered through OOK filters, and diluted to 50 mL withDIW.

Chemical analysis of solutions from sequential extractions.All liquid fractions from the different sediment extractionswere analyzed at Stockholm University (Department ofGeology and Geochemistry) for Fe, Mn, Al and As usingICP-OES (model Varian: Vista Pro Ax).

Adsorption experiments

Adsorption experiments were carried out with soil samplesfrom SDE (S1 and S5) and sample M1 from Misiones inpolypropylene bottles. A batch mode was applied using 10 gof sorbent and arsenate solutions prepared by dissolutionof K2H2AsO4 with initial concentrations of 0.5, 1 and 2mg/L of As(V). The soil:solution ratio was 1:10. For eachexperiment, three As solutions with pH values of 4.5, 5.9and 8.6 were prepared. The ionic strength was maintainedconstant by addition of 2 mL 0.01 M NaCl. The flaskscontaining the soil arsenic solution mixture were closedand put for 10 min on a horizontal shaker. The pH valuewas not adjusted during the progress of the experiment.

The initial pH was measured and the equilibration timewas determined for As(V) adsorption. Filtered solutions(0.45 µm Whatman membrane filters) were subsequentlykept in the refrigerator (4◦C) before being analyzed for

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As removal from Chaco-Pampean Plain, Argentina 1301

As(V) after contact times between the As-spiked water andthe solid sample of 0.5, 1.5, 4, 24 and 48 h and the pH ofthe solution was measured periodically. After 30 min, mostsamples were clear and transparent, but in some cases, sed-imentation was slower and 30 min was not enough forgravity settling. In such cases, the flask was left for fur-ther sedimentation or filtered through 0.45 µm Whatmanmembrane filters. Blanks were prepared following the sameprocedure without the addition of the absorbent for thedetermination of initial As concentrations.

The As concentration was measured by ICP-OES (Var-ian, model: Vista Pro Ax). The adsorbed amount of As(V)was calculated as a difference between the initial As con-centration in solution at the beginning of the experimentand the As concentration in the supernatant for each con-tact time between fluid and water (0.5, 1.5, 4, 24 and 48 h)and initial pH (4.5, 5.9 and 8.6), resulting in 15 data pointsfor each solid sample.

Results

Sediment characteristics

Mineralogical characteristics. The soil samples from Mi-siones and SDE revealed different clay mineralogies, prin-cipally caused by the different climate in these both areas.The sample from Misiones (M1) showed a dominance ofkaolinite (70 %) followed by beidellite (28 %), which is atypical composition for laterite (Table 1). The SDE soilsamples (S1–S5) showed a significant variability in the claymineralogical composition. Montmorillonite (up to 99 %),illite (up to 71 %) and beidellite (up to 61 %) reached thehighest contents, whereas chlorite (up to 6 %) showed onlylower shares (Table 1). The volcanic ash sample indicateddominance of illite (71 %).

Soil saturation extract. The pH of the saturation extracts ofthe clay sediment samples from SDE (S1–S5) were slightlyacidic to alkaline (pH 6.5–9.9; median: 7.4), whereas thesample from Misiones (M1) was acidic (pH 4.5), indicatinglow buffer capacity (Table 2) as reflected by the low EC of72 µS/cm. In comparison, the saturation extracts of SDEsamples (S1–S5) indicated much higher EC values rangingbetween 360–5700 µS/cm (median: 2928 µS/cm) (Table2). The soil saturation extract of the volcanic ash sample(V1a) was alkaline (pH 9.9) and high EC (5100 µS/cm).

Cation exchange capacity. The average cation exchange ca-pacity (CEC) of the SDE samples (S1–S5 and V1: 25–40meq/100 g; median: 37.9 meq/100 g) is about doubleof that of the Misiones sample (M1: 18.3 meq/100 g)(Table 2).

Sequential extraction of metals

The results of the individual sequential extraction steps arepresented in Figure 2 and Table 3.

Deionized water (DIW) extraction. The extraction stepwith deionized water quantifies water-soluble fraction ofAs and other trace elements in the sediments; it was able toremove significant amounts of metals from the solid phase.The highest removal was found for the soils from SDE(S1–S5), followed by the volcanic ash samples (V1, V2); thelowest values corresponded to the sample from Misiones(M1).

In the soil samples from SDE (S1–S5), the highest mo-bilization was for Al (AlDIW: 27.5–173.2 mg/kg, median:137.4 mg/kg), whereas those for extractable Fe (FeDIW2.3–27.8 mg/kg; median: 13.5 mg/kg), and Mn (MnDIW2.9–34.7 mg/kg; median: 10.7mg/kg) were much lower.The extractable As content was 0.47–5.74 mg/kg (median:1.29 mg/kg).

The volcanic ash samples (V1a,b, V2a,b) also indicatedthe highest range of concentrations for AlDIW (24.8–107.0mg/kg, median: 81.8 mg/kg) > FeDIW (8.2–22.6 mg/kg,median: 12.5 mg/kg) > MnDIW (1.4–7.0 mg/kg, me-dian: 5.7 mg/kg). The contents of extractable AsDIW were0.12–2.55 mg/kg (median: 2.5 mg/kg).

The M1 sample showed the lowest extractable amounts.In contrast to previous sample groups, the highest ex-tractable amount was found for Mn (47.6 mg/kg) followedby Al (38.2 mg/kg) and Fe (5.9 mg/kg).

Bicarbonate (NaHCO3) extraction. Bicarbonate extrac-tion quantifies the release of As and other traceelements from the sediments under high pH. Thebicarbonate extraction step was only applied to samplesM1 and S1, since all other sediment samples had pH val-ues >7. The extractable amounts were low compared withthe previous extraction step: AlHCO3 (0.31–0.62 mg/kg),FeHCO3 (0.02–0.43 mg/kg), MnHCO3 (0.18–0.53 mg/kg),and AsHCO3 (0.12–0.82 mg/kg). Only the AsHCO3 value of0.82 mg/kg, corresponding to sample S1, was considerable.

Acetate (NaOAc) extraction. Acetate (NaOAc) extrac-tion quantifies the elements mainly bound to carbonateand phosphate phases in the sediments. Extraction ofsamples M1 and S1–S5 with NaOAc at pH 5.4 revealedlow Al amounts; the median was 0.44 mg/kg, (range:AlOAc (0.15–1.93 mg/kg, except for sample S3 with 27.4mg/kg); the volcanic ash samples showed even lower val-ues (0.03–0.14 mg/kg, median: 9.04 mg/kg). In all sam-ples, the FeOAc amounts in the NaOAc extracts were verylow (0.02 mg/kg). The MnOAc amounts in the soil sampleswere generally low, median: 0.84 mg/kg; with exception ofS5 (4.17 mg/kg), all samples were in the range of 0.03–1.58mg/kg. The extractions of the volcanic ash samples (V1a,b,V2a,b) indicated low amounts: MnOAc (0.43–0.94 mg/kg,median: 0.52 mg/kg) > AlOAc (0.03–0.14 mg/kg, median:0.04 mg/kg) � FeOAc, (0.02 mg/kg). The contents ofextractable As were of similar low order for all samples(0.24–0.82 mg/kg, median: 0.33 mg/kg).

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Tab

le1.

Min

eral

com

posi

tion

ofso

ilsus

edfo

rad

sorp

tion

expe

rim

ents

.

Min

eral

com

posi

ton

(%)

Sam

ple

H-m

ontm

orill

onit

eL

-mon

tmor

illon

ite

kaol

init

eL

-bei

delli

teill

ite

smec

tite

chlo

rite

quar

tzm

usco

vite

heul

andi

tegy

psum

Cry

stal

linit

y

M1

——

7028

—<

0.5

—<

1.5

——

—Po

orS1

——

—61

39—

——

——

—Po

orS2

38—

——

58—

6—

——

—G

ood

S399

——

——

——

1–

–M

oder

ate-

poor

S425

——

—69

∗—

6—

—69

∗—

—M

oder

ate

S5—

60—

—39

—1

——

—Po

orV

117

——

—71

—6

—6

Poor

L:L

ow;H

.Hig

h;∗

illit

eas

soci

ated

wit

hm

usco

vite

69%

.

1302

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As removal from Chaco-Pampean Plain, Argentina 1303

Fig. 2. Distribution of Al, Fe, Mn and As released during the sequential extraction of the soil and volcanic ash samples from Misionesand SDE.

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1304 Bundschuh et al.

Table 2. Chemical characterization of soil samples used for ad-sorption experiments.

Sample pH EC (µS/cm) CEC (meq/100g)

M1 4.46 72.2 18.3S1 6.49 2910 39S2 7.45 2945 36.7S3 7.14 360 40S4 7.98 5700 25S5 7.36 1250 40V1a 9.88 5100 34.4V1b — — —V2b — — —

Oxalate ((NH4)2C2O4) extraction. This extraction stepquantifies Fe, Mn, and Al bound to oxides and hydroxidesand quantifies As in these phases. Oxalate extractionof the laterite sample (M1) showed high extractablemetal amounts: Feox (52.6 mg/kg) > Mnox (48.8 mg/kg)� Alox (13.4 mg/kg). Oxalate-extractable As was 0.12mg/kg. The soils from SDE showed comparable highamounts of extractable metals, but in different order: Alox,(41.9–102.2 mg/kg, median: 72.2 mg/kg) > Feox (32.5-39.1 mg/kg, median: 34.6 mg/kg) � Mnox (10.7–21.8mg/kg, median: 18.9 mg/kg) � Asox (0.13-1.64 mg/kg,median: 0.21 mg/kg). The oxalate extractable amountswere generally lower in the volcanic ash samples: Alox,(19.4–90.4 mg/kg, median: 25.6 mg/kg) > Feox (9.8–17.0mg/kg, median: 14.4 mg/kg) � Mnox (1.86–6.09 mg/kg,median: 1.6 mg/kg) � Asox (0.11–2.8 mg/kg, median:0.12 mg/kg).

Residual fraction (7 M HNO3) extraction. The extractionof the residual fraction in 7 M HNO3 during the last stepof the sequential extraction procedure quantify the con-tents of Al, Fe, Mn and As in the residue, which wereimmobile from the sediments during the preceding steps ofsequential extraction. The procedure showed high contentsof Feres (52600 mg/kg) and Alres (13370 mg/kg) in the lat-erite sample (M1). In comparison, the soil samples (S1–S5)had much lower values: Feres (930–7150 mg/kg, median:5490 mg/kg) and Alres (1570–4850 mg/kg, median: 2760mg/kg). The volcanic ash samples (V1a,b, V2a,b) fromSDE revealed even lower amounts of extractable metals(Alres 1660–1840 mg/kg, median: 1715 mg/kg; and Feres:2230–2540 mg/kg, median: 2420 mg/kg). Amounts ofMnres (41–170 mg/kg, median: 42.5 mg/kg) were relativelylow in all samples, while the median Asres contents rangedfrom 0.29–2.78 mg/kg, with an exception of sample M2(Asres: 20.7 mg/kg, the median is 0.50 mg/kg).

Adsorption experiments

Soil samples from SDE (S1 and S5) were used for adsorp-tion experiments because they have high contents of metals

available as As adsorption sites and can be easily minedand transported; additionally, sample M1 from Misioneswas tested for comparison. During the progress of the ex-periment, pH was not adjusted to make the experimentalconditions close to the conditions of the application site,where the clays shall be used on a household scale in smallfilters.

It is important to note that a limitation of this adsorptionsystem is that it does not consider other ions in solutioncompeting with As(V) for adsorption sites on the clays. Theexperiments aimed at determining the amount of adsorbedAs as a function of the contact time ranging from 30 minto 48 h. At the same time, arsenate elimination by each clayas a function of pH was determined.

Effects of reaction time on the pH of the solid-fluid mixture

The purpose of the experiment was to identify the reac-tion time required for adsorption to reach equilibrium, animportant performance factor for the applicability of an ad-sorbent for As removal from drinking water on household-scale. Figure 3 shows variations of pH as a function of time.It can be observed in Figure 4 that most of the arsenateadsorption on clays took place within the first 4 h of con-tact time. Depending whether the initial solute pH (pH0)or the soil saturation extract pH (dashed line) was higher,the fluid phase pH of the mixture decreased or increasedsharply within the first 0.5 h of contact time. Afterwards,the pH values fluctuated only within small pH ranges. After0.5 h of contact time, the pH values were 4.2–6.0 for theMisiones sample and 6.5–8.0 for the SDE samples.

In the case of the Misiones clay, a sudden drop frompH0 8.6 to 6.0 within the first 0.5 h was observed, andfinally pH 5.0 was reached after 48 h of reaction time. Inthe case of pH 4.5 and 6.9, the change in pH was withina narrower range. However, it was observed that the batchexperiments at the three pH reached a final pH ca. 5.0 after48 h of equilibration.

Role of initial pH and As concentration in arsenic removalefficiency

The effect of initial pH on adsorption was studied by de-termining the amount of adsorbed As at pH 4.5, 5.9 and8.63. Figure 4 shows the adsorption of As from As(V) so-lutions in the samples S1, S5 and M1 for different initial Asconcentrations in the solutions and for different initial pHvalues of the solution (pH0) for solid-fluid contact timesof 0–48 h. A comparison of the adsorption curves a-c, d-fand g-i for the 3 samples shows that the change in the equi-librium pH during the progress of the experiment does notsignificantly affect the final As removal.

The soil samples from SDE showed a low As removalcapacity. Most of the As adsorption on clays took placewithin the first 4 h of contact time, but fluctuations also oc-curred at smaller reaction times. After that period, removal

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Tab

le3.

Res

ults

ofse

quen

tial

extr

acio

nof

soils

used

for

adso

rpti

onex

peri

men

ts(m

g/kg

).S

ampl

eA

l DIW

Al H

CO

3A

l OA

cA

l ox

Al re

s∑

Al

FeD

IWFe

HC

O3

FeO

Ac

Feox

Fere

s∑

FeM

n DIW

Mn H

CO

3M

n OA

cM

n ox

Mn r

es∑

Mn

As D

IWA

s HC

O3

As O

Ac

As o

xA

s res

∑A

s

Mis

ione

sM

138

.20.

620.

1550

.913

370

1345

95.

90.

430.

0282

.652

600

5268

947

.60.

180.

0948

.817

026

70.

120.

120.

270.

120.

551.

18Sa

ntia

goso

ilS1

173.

20.

310.

3041

.927

6029

7527

.80.

430.

0232

.526

9027

518.

590.

530.

0310

.744

645.

740.

820.

821.

6420

.729

.72

S213

7.4

—0.

4480

.715

7017

8919

.1—

0.02

39.1

6360

6418

34.7

—1.

5820

.179

135

1.29

0.41

0.21

1.08

2.99

S327

.5—

17.4

102.

229

6031

072.

3—

0.02

4.4

930

937

2.91

—0.

841.

2313

180.

47—

0.30

0.19

0.33

1.29

S410

0.9

—0.

2170

.626

8028

5213

.5—

0.02

34.6

5490

5538

20.0

—0.

8421

.874

117

0.79

0.41

0.13

0.84

2.17

S51.

93—

1.93

72.3

4850

5069

7.4

—0.

0237

.071

5071

9410

.7—

4.17

18.9

106

140

3.85

—0.

540.

272.

787.

44m

edia

n13

7.4

—0.

4472

.327

6029

7613

.5—

0.02

34.6

5490

5538

10.7

—0.

8418

.974

117

1.29

—0.

410.

211.

082.

99V

olca

nic

ash

V1a

24.8

—0.

0324

.117

7018

1915

.1—

0.02

17.0

2490

2522

1.38

—0.

481.

8641

450.

12—

0.38

0.12

0.33

0.95

V1b

87.6

—0.

1490

.416

6018

389.

9—

0.02

9.8

2230

2250

6.12

0.94

6.09

4154

2.51

0.24

2.80

0.32

5.87

V2a

75.9

—0.

0327

.116

5017

538.

2—

0.02

17.5

2350

2376

5.31

—0.

561.

3338

452.

480.

390.

110.

293.

27V

2b10

7.0

—0.

0419

.418

4019

6622

.6—

0.02

11.7

2540

2574

7.00

—0.

431.

3241

50—

0.28

0.11

0.44

3.38

med

ian

81.8

—0.

0425

.617

1518

2912

.5—

0.02

14.4

2420

2449

5.72

—0.

521.

6041

472.

50—

0.33

0.12

0.33

3.33

1305

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1306 Bundschuh et al.

0

pH v

alue

(-)

a

Time (h)0

2

4

6

8

10

0.5 1.5 4 24 480b

Time (h)0 0.5 1.5 4 24 48

0c

Time (h)0 0.5 1.5 4 24 48

pH v

alue

(-)

2

4

6

8

10

pH v

alue

(-)

2

4

6

8

10

pH0= 4.5pH0= 5.9pH0= 8.6

S5S1M1

Fig. 3. Effect of the variation of pH as a function of contact time on the As(V) adsorption on the different studied media: a) M1laterite from Misiones; b) S1 from SDE; and c) S5 from SDE. The dashed line represents the pH value of the respective soil solutionextract.

Fig. 4. Sorption of arsenate on clay samples from SDE (S1 and S5) and Misiones (M1), with varying contact time at three differentinitial pH values (pH0) and initial As concentrations.

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As removal from Chaco-Pampean Plain, Argentina 1307

using sample S1 fluctuated between 20 and 60 % (Figure4a–c), while S5 had a removal capacity of 40–60 % (Figure4d–f). No clear correlation between As removal percent-age and initial As concentration in the fluid was found forcontact times higher than 4 h; at lower contact times, theremoval rate was generally lower for the highest initial Asconcentrations. In respect to the lateritic soil (M1) fromMisiones, complete removal of arsenate was found for alltested initial As concentrations after a very short contacttime (0.5 h, Figure 4g-i). The removal efficiency was almostcomplete at all the pH ranges from 4.5 to 8.6.

Discussion

Based on the results of the sediment geochemistry, twoclays from SDE (S1 and S5) and, for comparison, one fromMisiones (M1) were selected to test their efficiency as ad-sorbents for As removal. Samples S1 and S5 from SDEwere selected because of their higher contents of Fe, Al andMn compared to the other SDE samples. Fe, Al and Mnare present in the chosen materials mainly as oxides andhydroxides, and this makes them suitable for As adsorp-tion; in addition, these samples can be easily mined andtransported.

Salient geochemical characteristics of the sediments fromSDE were evaluated for their suitability as adsorbents forAs removal from groundwater. The main constituents ofthe samples were identified by XRD studies as quartz alu-minum silicates (predominantly kaolinite) and iron oxides.

Mobilization of arsenic from solid to water phase

Sequential extraction procedure was employed to investi-gate sediment composition and quantify the amounts oftotal Al, Fe, Mn, Al and As bound to or present as spe-cific mineral phases such as water-soluble fraction (DIWextractable), phases extractable at higher pH (bicarbon-ate extractable), carbonate and phosphate phases (acetateextractable), amorphous oxides and hydroxides (oxalate ex-tractable), and other phases (HNO3 extraction for residualphases), which is important to understand the possible ad-sorption sites in the materials and hence to evaluate thepossibility of their application for remediation of ground-waters of SDE containing high As concentrations. In thisway, it is possible to investigate in which form and to whichsite metals such as Fe, Al and Mn and As are bound on thesolid phases, as each step suggests the amount of differentcomponents that can be mobilized from the samples.

Not considering the residual fraction step, which repre-sents under most of the natural conditions the insolublepart, the DIW and oxalate extractions were those whichextracted the highest amounts of Al, Fe and Mn (generallyAl > Fe � Mn). This indicates that a large fraction of Al,Mn and Fe is only weakly attached to the soil particles(DIW extractable) and another large part corresponds toamorphous Fe-, Al- and Mn-oxides and hydroxides that are

readily dissolved in oxalate. Oxalate extractions indicatedthat Al and Fe are preferentially present as amorphousand poorly crystalline oxides and hydroxides and in sig-nificantly higher amounts as compared to Mn oxides andoxyhydroxides. This indicates that these oxides and hydrox-ides of Al and Fe can be well suited for removal of As fromcontaminated groundwater.

Sequestration of arsenic by soils for drinking waterproduction

The dependency of reaction time and the speciation ofAs in adsorption processes have been studied in a num-ber of previous studies.[25–37] The promising experiments inSouth Asia showed that for laterite most of the adsorptiontakes place within 5–10 min. [38] Because the dependency oftime on the adsorption characteristics is governed by thegeochemical characteristics of the experimental conditions,this study aimed at measuring the As concentration in thewater first after a time interval of 30 min after mixing thesoil into the water. Approximately 90 min, 4 h, 24 h and48 h later tests were made to assure further reactions.

The laterite soil (M1) from the province Misiones showedthe highest and almost complete As retention (99 % ofdissolved As was adsorbed) at all the pH values from 4.5to 8.6. This is consistent with results with iron-rich tropicalsoils from other parts of the world such as Sri Lanka.[39]

In contrast, removal efficiency was much lower for clays S5and S1 from SDE, with As adsorption efficiencies of only53 and 40 %, respectively.

Previous analysis by Mellano and Ramirez[33] identifiedthe following clay characteristics that make the materi-als good adsorbents for As: a) high iron and aluminumcontent, b) high specific surface area, c) low pH, d) highgoethite content. Fe and Al oxides are effective anion adsor-bents because of surface charge, surface area, and favorablegeometry of terminal OH groups for specific adsorption. [40]

The highest As adsorption was correlated with the highestcontent of Al- and Fe-oxihydroxides that are characterizedby large specific surface area. [35,41] Goethite (γ -FeOOH),the second most common naturally occurring iron oxidemineral after hematite (Fe2O3), has also been reported tobe effective for As adsorption. [42-43]

In addition, As removal strongly depends on the pHof the soil, which affects the pH of the slurry. Maximumadsorption of As(V) occurred in the pH range 3-6.[44] Thismakes the Misiones clay the best adsorbent comparedwith those of SDE and agrees with previous studies. It isimportant to remember that iron oxides are adsorbentsfor other anions that form surface complexes and competewith As(V) and As(III) species for adsorption sites. Assaid before, this is a limitation of this adsorption system,and can lead to decreased adsorption ability. Principaloxyanionic species competing with As(V) and As(III) foradsorption site at the Chaco-Pampean Plain are phosphateand vanadate.[23–24,27,30]

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Based on previous studies, suggested mechanism ofadsorption for arsenate is a formation of inner spherecomplexes on the surface of Fe(III) oxyhydroxides.[39,45]

Rajapaksha et al.[39] suggested formation of a bidentatebinuclear-bridging complex, but monodentate complex wassuggested in the presence of phosphate.

At alkaline pH (pH > 8.6), the clays were probablyaffected by precipitation of Fe- and/Al-hydroxides (asindicated by changes in the color of the solution). Thisobservation was consistent with the removal of As(V)from the suspensions. At elevated pH (> 8.6) adsorptionof As(V) is limited and hence As removal from theaqueous phase decreased with an increase in pH. Theincrease of pH in the solutions was not proportional toAs concentrations, and for As concentrations higher than2 mg/L, pH reached a plateau and remained constant.Several factors may explain the increased pH[27,30] butthe most important is the uptake of protons by mineralweathering and ion exchange reactions, combined with theeffect of evaporation in arid and semiarid regions. This pHincrease is commonly associated with the development ofsalinity and the salinization of soils.

The lateritic clay from Misiones Province, which is foundto be promising for As remediation (about 99 % of dissolvedarsenic adsorption), further demands cost-benefit analysisas costs of transportation (more than 1000 km) from oneprovince to the other have to be taken into account. How-ever, the reduction of As levels from groundwater by claysfrom SDE was found to range only between 40 and 53 %.This is a rather low adsorption efficiency and the As-richwater needs to pass several times the clay filter. Most ad-sorption occurs during the first 90 min of contact time andseveral loops of that period are recommended. However,further studies are recommended to optimize the time formaximum adsorption. In addition, studies with real watersamples are imperative due to the effects of other competingoxyanionic species during the adorption processes.

Conclusions

The use of natural geological materials (soils and sedi-ments) for As removal at a household level is an emergingsolution for the economically disadvantaged populationsin remote rural settlements, especially when the materialsare locally available and can be collected by local popula-tion. Their low cost makes these materials a very attractiveoption for As removal as compared to synthetic or com-mercial materials. Sometimes, this may be the only optionto provide safe water to very poor settlements. Their suit-ability for As removal from water is mainly due to processessuch as adsorption, co-precipitation and ion exchange in-volving Fe- and Al-rich minerals and clay minerals presentin the soils.

In the present study, different clay-rich soils with highcontents of amorphous iron and aluminum hydroxide have

been tested and compared as adsorbents to remove As fromthe groundwater of the Rıo Dulce alluvial aquifer (SDE)at a laboratory scale. Soils from SDE and from Misioneswere tested.

Batch adsorption experiments (solid/liquid ratio of 1:10wt/wt) using laboratory water at different initial pH values(4.5, 5.6, 8.6) and different initial As(V) concentrations (0.5,1.0 and 2.0 mg/L) showed much higher As(V) removalwhen using the Misiones tropical soil laterite sample (99%) as compared with the soils from SDE (40–53 %). Thiscan be related to lower contents of oxalate extractable Aland Fe in the SDE soils formed under subtropical climateconditions.

These results confirm the high Fe content, low pH andgood remediation capacity of the Misiones province lat-eritic soils. However, high costs of transport may be a lim-iting factor for the application of this technique to remotesites, as there is no laterite soil in the area near SDE dueto the geomorphological setting. As a recommendation forthe future, studies on As-removal techniques using naturalclays from SDE as adsorbents, which may provide a cost ef-fective solution in the region, should be further expanded.The lateritic clay from Misiones Province, which is foundto be promising for As remediation, further demands cost-benefit analysis. Studies with real waters are necessary toaccount for competitive adsorption with other oxyanionicspecies such as phosphate and vanadate.

Acknowledgments

This study was carried out as a part of the Linnaeus-PalmeAcademic Exchange Programme between the UniversidadNacional de Santiago del Estero (UNSE) and the Depart-ment of Land and Water Resources Engineering, Royal In-stitute of Technology (KTH) through the grant 466-2003from the International Programs Office (IPK), Stockholm,Sweden. MFM and AEF thank the IPK (Sweden) and theInternational Office of the KTH for the fellowship as PalmeExchange students in Sweden. PB and JB/MIL greatlyappreciate the financial support provided by the Foun-dation for Strategic Environmental Research (MISTRA)through the grant 2005-035-137) and by the CYTED Net-work IBEROARSEN (grant 406RT0282) for finalizing themanuscript. JB and JSJ thank the National Science Councilof Taiwan for the financial support. MIL thanks CNEA,CONICET and ANPCyT agencies in Argentina for sup-port during various phases of the study. We also thankthe laboratory help provided by Carl-Magnus Morth atthe Department of Geology and Geochemistry, StockholmUniversity for the chemical analyses.

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