21
This article was downloaded by: [Colorado College] On: 07 November 2014, At: 13:47 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Soil and Sediment Contamination: An International Journal Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/bssc20 Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake Indranil Das a , Koushik Ghosh a , D. K. Das a & S. K. Sanyal a a Department of Agricultural Chemistry and Soil Science , Bidhan Chandra Krishni Viswavidyalaya , Nadia, West Bengal, India Published online: 28 Nov 2011. To cite this article: Indranil Das , Koushik Ghosh , D. K. Das & S. K. Sanyal (2011) Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake, Soil and Sediment Contamination: An International Journal, 20:7, 790-809, DOI: 10.1080/15320383.2011.609199 To link to this article: http://dx.doi.org/10.1080/15320383.2011.609199 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms- and-conditions

Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

  • Upload
    s-k

  • View
    212

  • Download
    0

Embed Size (px)

Citation preview

Page 1: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

This article was downloaded by: [Colorado College]On: 07 November 2014, At: 13:47Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Soil and Sediment Contamination: AnInternational JournalPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/bssc20

Studies on Fractionation of Arsenic inSoil in Relation to Crop UptakeIndranil Das a , Koushik Ghosh a , D. K. Das a & S. K. Sanyal aa Department of Agricultural Chemistry and Soil Science , BidhanChandra Krishni Viswavidyalaya , Nadia, West Bengal, IndiaPublished online: 28 Nov 2011.

To cite this article: Indranil Das , Koushik Ghosh , D. K. Das & S. K. Sanyal (2011) Studies onFractionation of Arsenic in Soil in Relation to Crop Uptake, Soil and Sediment Contamination: AnInternational Journal, 20:7, 790-809, DOI: 10.1080/15320383.2011.609199

To link to this article: http://dx.doi.org/10.1080/15320383.2011.609199

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the“Content”) contained in the publications on our platform. However, Taylor & Francis,our agents, and our licensors make no representations or warranties whatsoever as tothe accuracy, completeness, or suitability for any purpose of the Content. Any opinionsand views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Contentshould not be relied upon and should be independently verified with primary sourcesof information. Taylor and Francis shall not be liable for any losses, actions, claims,proceedings, demands, costs, expenses, damages, and other liabilities whatsoever orhowsoever caused arising directly or indirectly in connection with, in relation to or arisingout of the use of the Content.

This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Page 2: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

Soil and Sediment Contamination, 20:790–809, 2011Copyright © Taylor & Francis Group, LLCISSN: 1532-0383 print / 1549-7887 onlineDOI: 10.1080/15320383.2011.609199

Studies on Fractionation of Arsenic in Soilin Relation to Crop Uptake

INDRANIL DAS, KOUSHIK GHOSH, D. K. DAS,AND S. K. SANYAL

Department of Agricultural Chemistry and Soil Science, Bidhan Chandra KrishniViswavidyalaya, Nadia, West Bengal, India

Widespread arsenic (As) contamination in West Bengal and Bangladesh is of greatconcern as it affects millions of people due to its toxicity. Groundwater, when used forirrigation, helps entry of arsenic into the food chain via a soil-plant-animal continuum.In this study the extent of geo accumulation is measured in order to assess the degreeof As contamination in soil. A sequential fractionation study of As revealed the con-centration of different arsenic fractions in the order: As held at the internal surfacesof soil aggregates (20.7%) > freely exchangeable As (20.3%) > calcium associatedAs (18.7%) > chemisorbed As (17%) > residual As (15.7%) > labile As (3.29%). Thevariation in fractions may be attributed to the mineralogical make-up of soils along withsome physicochemical factors. Statistical correlations and path analyses revealed thattotal and Olsen extractable arsenic (plant available arsenic) are dependent upon the Asheld at the internal surfaces of soil aggregates and chemisorbed arsenic fraction, whichare directly influenced by the mineralogy of these experimental soils. The crop uptakeby Kharif rice and mustard grown in these areas also corroborates the above fact.The poor reflection of exchangeable forms of soil arsenic in crop availability revealedthat arsenic has undergone transformation via minerals through the continuous use ofarsenic-laden water for irrigation.

Keywords Soil accumulation, forms of arsenic, mineralogy, soil physico-chemicalproperties, correlation, crop uptake

Introduction

The arsenic (As) calamity in the state of West Bengal in India is now one of the largest knownmass poisoning incidents in human history with an estimated 7-8 million people beingexposed to As-contaminated drinking water. As is present in soils or sediments in variousforms with varying degree of bioavailability, toxicity, and mobility. Thus, the assessmentof As enrichment of soil in farming areas, notably the ones affected by groundwatercontamination, is of great importance. The assessment of soil enrichment with the elementcan be carried out in many ways; the most common are the index of geo accumulationand enrichment factors (Loska et al., 2003). Determination of total concentration of theseelements in solid materials is therefore considered to be of limited use in assessing potential

Address correspondence to Mr. Indranil Das, Assistant Agricultural Chemist, Office of theAgricultural Chemist, Fertilizer Control Laboratory, Pulses and Oilseeds Research Station, P.O.-Berhampore, Dist. Murshidabad, Pin 742101, West Bengal, India. E-mail: [email protected]

790

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 3: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

Fractionation of Arsenic in Soil 791

environmental impacts. In order to assess As toxicity and impact, a good understandingof the chemical forms of the element is required (Shiowatana et al., 2001). The use ofsequential extraction technique for fractionation of metals in solid materials and evaluationof their potential effects has been widely used and well recognized (Tessier et al., 1979).Fractionation of soil arsenic is like phosphorus fractionation as arsenic chemistry in soilclosely resembles phosphorus chemistry. In the 1970s, researchers used the adaptation ofthe Chang and Jackson’s (1957) soil phosphorus fractionation procedure to explore arsenicchemistry in contaminated soils (Jacobs et al., 1970; Woolson et al., 1971; Johnston andBarnard, 1979). However, the limitations of Chang and Jackson’s procedure in relationto the problems of interpretation for soil phosphorus and hence of soil arsenic have beenpointed out by many researchers (Tiessen and Moir, 1993; McLaren et al., 1998). McLarenet al. (1998) used the alternative phosphorus fractionation scheme of Hedley et al. (1982)for fractionation of arsenic in soils from contaminated (with sodium arsenite oxidized toarsenate) cattle dip sites. As a result of the use of these sequential extraction methods,the occurrence of arsenic in the solid phase of contaminated soils as specific minerals(iron arsenate, arsenopyrite) and the adsorption mechanism of arsenic on clay minerals,organic matter, and Fe/Mn/Al-oxides were revealed. Under these circumstances, the presentstudy was undertaken to study the different fractions of arsenic in soil in relation to cropuptake.

Materials and Methods

Four surface (0–0.15m) soil samples (replicated thrice) were collected from the areas,namely Nonaghata-Uttarpara (S1 & S2) and Nonaghata-Dakhinpara (S3) (Haringhata Block,District-Nadia, West Bengal, India) as arsenic-affected and Kalyani (K) (Chakdah Block,District-Nadia, West Bengal, India) as arsenic-unaffected soil. The physico-chemical prop-erties of these soils were determined according to the standard methodology specified byJackson (1973). The clay fractions of the selected soil samples of the study areas were sep-arated by the method described by Jackson (1973). For determination of total arsenic, thesoil samples were digested on a sand bath with triacid mixture (HNO3 : H2SO4 : HClO4::10 : 4 : 1 by volume) to obtain a clear digest. A portion of the soil samples was also treatedwith Olsen extractant (0.5 M NaHCO3, pH 8.5) to extract the labile arsenic pool of the soil(representing the plant available pool of arsenic). The arsenic content of soil digest andalso the soil extract were measured with the use of an Atomic Absorption Spectrophotome-ter (AAS) (GBC – 932B) coupled with a hydride generator unit (HG 3000). For routineinterpretation of the minerals present, the clay samples were Ca-saturated, Ca-glycolated,K-saturated, and heated to 300◦C and 550◦C. For the semi-quantitative estimation of theclay minerals, the methodology proposed by Gjems (1967) was adopted. It involved ap-plication of a proper background curve, which was based on a hyperbola with formulax, y = a in the X-ray diffractograms and delineation of each diagnostic. From the X-raydiffractograms of soil clay samples (Ca-saturated, Ca-glycolated, K-saturated, and heatedto 300◦C and 550◦C) the 7 nm, 10 nm, and 17 nm peaks were taken for kaolinite, illite, andmontmorillonite, respectively. The area lying between the 10 nm and 14 nm peaks for theMg-saturated clays was assumed to represent mixed layer minerals in the diffractogramsof glycolated samples. The diffractograms for each sample are presented in groups of four,the uppermost for Ca – saturated samples and then in sequence below, the curves for theCa-glycolated and K-saturated samples heated to 300◦C and 550◦C, respectively.

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 4: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

792 I. Das et al.

Assessment of Soil Arsenic Enrichment

Assessment of soil arsenic enrichment was done employing an index of geoaccumulationand enrichment factors.

Index of Geoaccumulation (Igeo). The index of geoaccumulation (Igeo) was used as ameasure of bottom sediment contamination by many researchers (Muller, 1969; Mikoet al., 2000; Kwapuliniski et al., 1996). It determines contamination by comparing currentmetal contents with pre-industrial levels (Loska et al., 2003; Muller, 1981). The metalcontent accepted as background was multiplied each time by the constant 1.5 in order totake into account natural fluctuations of a given substance in the environment as well asvery small anthropogenic influence. The value of the geoaccumulation index is describedby the following equation:

Igeo = log2[Cn/(1.5 ∗ Bn)]

Where Cn - arsenic content in tested soil, Bn - background content and the average arseniccontent in the earth’s crust is 1.5 mg/kg (Taylor and McLennan, 1995). The interpretation ofthe obtained was done according to Muller (1969) as: Igeo ≤ 0 -practically uncontaminated,0 < Igeo < 1 - uncontaminated to moderately contaminated, 1 < Igeo <2 - moderatelycontaminated, 2 < Igeo <3 - moderately to heavily contaminated, 3 < Igeo <4 - heavilycontaminated, 4 < Igeo < 5 - heavily to very heavily contaminated and Igeo ≥ 5 - veryheavily contaminated.

Enrichment Factor (EF). The use of the Enrichment Factor (EF) for assessment of bottomsediment contamination with metals was done using the method followed by Buat-Menard(1979) and Loska et al. (2003):

EF = (Cn/Cref)/(Bn/Bref)

where:Cn - content of the examined element in the examined environment; Cref - content of

the examined element in the reference environment; Bn - content of the reference elementin the examined environment and Bref - content of the reference element in the referenceenvironment. An element is regarded as a reference element if it is present in the environmentin trace amounts (Loska et al., 2003). It is also possible to apply an element of geochemicalnature, substantial amounts of which occur in the environment but have no characteristiceffects, i.e. synergism or antagonism towards an examined element. The most commonlyused reference elements are Sc, Mn, Al, and Fe (Loska et al., 1997).

Five contamination categories are recognized on the basis of Enrichment Factor assuggested by Sutherland (2000): EF < 2 – depletion to minimal enrichment, EF = 2–5 –moderate enrichment, EF = 5–20 – significant enrichment, EF = 20–40 – very highenrichment, EF > 40 – extremely high enrichment.

Despite certain shortcomings (Reimann and Caritat, 2000), the enrichment factor, dueto its universal formula, is a relatively simple and easy tool for assessing the degree ofenrichment and comparing the contamination of different environmental media (Loskaet al., 2003).

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 5: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

Fractionation of Arsenic in Soil 793

Batch Sequential Extraction Procedure for Different Fractions of As

Solutions used to extract As sequentially, described by McLaren et al. (1998) with somemodifications, were 0.1 M HCl with anion exchange membrane, 0.5 M NaHCO3 (pH 8.5),0.1 M NaOH, 0.1 M NaOH solution with sonicator, 1 M HCl, and finally digestion withtriacid mixture (HNO3:H2SO4:HClO4:: 10:4:1, by volume).

Step 1 (Water Soluble/Extractable Arsenic): To fractionate the inorganic As, a 2 gportion of each soil sample collected from the areas was taken in a polypropylene centrifugetube in a suspension with 30 mL of deionized water. Five grams of anion exchange resinwere taken in a dialysis paper bag, which was immersed in the suspension. The latter wasshaken for 24 hours, centrifuged, and then filtered with Whatman No.1 filter paper. Theanion exchange resin, with its adsorbed (exchanged) arsenic loading, was extracted with30 mL of 0.1 M HCl.

Step 2 (Sodium Bicarbonate Extractable Arsenic): The soil residue from Step 1 waswashed with 30 mL of water and centrifuged, and the washing was discarded by decantation.The soil residue was resuspended in 30 mL of NaHCO3 (0.5 M, pH 8.5) and shaken for16 hours. The soil suspension was then centrifuged as in Step 1 and the supernatant solutionwas filtered through Whatman No.1 filter paper, collected, and kept for analysis.

Step 3 (Sodium Hydroxide Extractable Arsenic): The soil residue from Step 2 waswashed with 30 mL of water, centrifuged, and the washing was discarded by decantation.The residual soil left was again resuspended in 30 mL of 0.1 M NaOH and shaken for16 hours. The suspension was then centrifuged as in Step 1 and the supernatant solutionfiltered through Whatman No.1 filter paper was collected for analysis.

Step 4 (Sonicated Sodium Hydroxide Extractable Arsenic): The soil residue from Step3 was washed with 30 mL of water and centrifuged. The washing was discarded bydecantation. Further, the soil residue was sonicated using an ultrasonic probe (Bransonsonifier operated at 20 KHZ) for 3 minutes in 0.1 M NaOH solution. The system wascentrifuged and the supernatant solution was filtered through Whatman No. 42 filter paper.

Step 5 (Hydrochloric Acid Extractable Arsenic): The soil residue from Step 4 waswashed with 30 mL of water, centrifuged, and the washing was discarded by decantation.The soil residue was then treated with 30 mL of 1 M HCl solution, shaken for 16 hours,centrifuged as in Step 1, and the supernatant solution filtered through Whatman No.1 filterpaper was collected for analysis. The soil residue was then dried at 60◦C.

Step 6 (Residual Arsenic): The dried soil residue from Step 5 was finally digested withthe triacid mixture. The supernatant and the centrifuged solution from each correspondingstep were collected separately and the arsenic content of each such solution was measuredusing the AAS coupled with a hydride generator unit. The sequential extraction withthe extractants mentioned above would identify freely exchangeable arsenic form (A),nonexchangeable but readily available or labile As associated with soil mineral surfaces(B), arsenic held by strongly chemisorbed Fe and Al components form (C), arsenic held atinternal surfaces of soil aggregates (D), calcium associated As (E), and residual recalcitrantAs forms in the experimental soils (F).

Measurement of Crop Uptake of Arsenic

For measuring crop uptake, the plant samples, namely kharif rice and mustard taken fromthe arsenic affected soil (S1, S2 and S3) as well as unaffected soil (K), were cleaned properly,processed, and further dried in an air-oven at 105◦C for 24 hrs. The dried plant sampleswere ground. Then a portion of the given plant and soil samples were digested in a sand bath

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 6: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

794 I. Das et al.

with triacid mixture (HNO3 : H2SO4 : HClO4 :: 10 : 4 : 1 by volume) to obtain clear digest.A portion of the soil samples was also treated with Olsen extractant (0.5 M NaHCO3, pH8.5) to extract the labile arsenic pool of the soil. The arsenic content of soil and plant digestsand also the soil extracts were measured by AAS (GBC – 932B) coupled with a hydridegenerator unit (HG 3000).

Statistical Analyses

Finally, the statistical analysis was performed by the standard statistical package employingMS EXCEL software. Path analysis was done using “INDOSTAT” software. Path analysis isa statistical technique which provides a measure of relative importance of each independentvariable for prediction of changes in the dependent one (Basalma, 2008). In other words,one uses this analysis to test assumptions of direct and indirect effects of variables (Loehlin,1987). Conceptually, a path analysis is presented as a path diagram with the assumed causalorder represented by the direction of arrows. This technique is applied basically in socialsciences. However, in soil science experiments such analysis was also applied to describethe cause-effect relationship (Feiziene and Feiza, 2003; Zhang et al., 2005).

Results and Discussion

The important physicochemical properties as well as the taxonomic classification of thesoils studied are given in Table 1. These soils belonged to the order Inceptisols, signify-ing that they are of recent origin in the pedogenic development process. Regarding thephysicochemical properties of the three surface soils under study, the soil reaction (pH)was found to be varied from slightly acidic to neutral, ranging from 6.33 to 7.54, whilethe organic carbon content varied from 4.18 to 7.70 g/kg. The cation exchange capacityof these soils ranged from 16.3 to 27.6 cmol(p+)kg−1. Interestingly, three types of texturalvariations (silty clay loam, silty clay, and clay loam) were noticed in these soils where theclay content varied from 28.5 to 45.3%. The exchangeable sodium plus potassium contentwas found to be the highest in Nonaghata Uttarpara (S2) soil and the lowest in Kalyani (K)soil, whereas the exchangeable calcium plus magnesium content was maximum in Kalyani(K) soil and lowest in Nonaghata Uttarpara-I (S1) soil. Such variations were also reflectedin soil pH. This was again supported by the presence of amorphous iron and aluminumoxides in soils in the following order: Nonaghata Dakshinpara (S3) (0.40% + 0.39%) >

(S2) (0.37% + 0.24%) > S1 (0.31% + 0.21%) > K (0.19% + 0.17. The total arseniccontent (mg/kg) varied from 17.3 to 20.0 in the arsenic affected soils while in the controlsite (Kalyani, K) the value was rather low (3.33).The Olsen extractable arsenic content wasfound to be relatively higher in the soil S3 in comparison to the soils of S1 and K. Suchvariations in the arsenic content may be attributed to clay content, amorphous iron andaluminum oxides, calcium and magnesium content of these soils.

The clay fraction of the experimental soils were further analyzed to assess the semi-quantitative composition of different predominant minerals in these soils (Table 1). From theanalysis it was revealed that the clay of the experimental soils was mostly illite dominatedwith appreciable amounts of kaolinite and chlorite. The illite content of these soils rangedfrom 38.0 to 48.8%, which were almost half of the total minerals present in these soils.However, the Kalyani soil (K) not affected with arsenic contained more vermiculite (17.0%)and montmorillonite minerals (28.0%) compared to the arsenic affected soils, namely S2

(6.25% and 13.2%, respectively) and S3 (3.55% and 8.71%, respectively). These twosoils (i.e., S2 and S3) also showed the presence of appreciable amounts of vermiculite and

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 7: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

Fractionation of Arsenic in Soil 795

Table 1Physicochemical, mineralogical, and arsenic accumulation parameters of the selected field

soils under study

Typic HaplusteptsSoilParameters S1

∗ S2∗ S3

∗ K∗

Soil Taxonomic ClassClay (%) 28.5 36.2 45.3 38.7Silt (%) 52.4 48.2 46.5 19.6Sand (%) 19.1 15.6 9.16 41.7pH (1:2:5) 6.33 6.74 6.82 7.54Organic C (g/kg) 4.79 4.20 7.70 4.18Surface area (m2/g) 106 115 111 81Amorphous Fe (%) 0.31 0.37 0.40 0.19Total Fe (%) 1.81 1.24 1.38 1.12Amorphous Al (%) 0.21 0.24 0.39 0.17CEC [cmol (p+)/ kg] 24.7 20.6 27.6 16.3

Exchangeable cations [cmol (p+)/kg]Na 0.09 0.16 0.10 0.09K 0.27 0.43 0.36 0.24Ca + Mg 7.34 8.80 11.9 12.3Total arsenic (mg/kg) 20.0 17.3 19.7 3.33Olsen extractable As (mg /kg) 4.49 3.22 4.98 0.34Olsen extractable P (mg /kg) 51.4 54.3 52.3 57.6Total N (%) 0.091 0.042 0.098 0.064

Clay MinerologyMontmorillonite 13.2 13.2 8.71 28.0Illite 44.5 44.5 48.8 38.0Chlorite 13.3 13.3 10.3 6.0Vermicullite 6.25 6.25 3.55 17.0Kaolinite 16.5 16.5 18.3 11.0Mixed- layer 4.75 4.75 10.3 0.0Index of geoaccumulation (Igeo) 3.15 2.94 3.13 0.57Enrichment Factor (EF) 41.5 52.4 53.6 11.2

∗Soil samples, denoted by S1, S2 and S3, were collected from an arsenic-affected area (namelyNonaghata Uttarpara (I & II) and Dakshinpara, Haringhata Block of Nadia District) while the othersoil sample, K, was collected from an arsenic- unaffected area (namely Kalyani, Chakdaha Block ofNadia District).

mixed-layered minerals, while soil K did not contain any mixed layer mineral. The kaolinitecontent of these soils followed the order S3 > S2 & S1 > K.

The extent of arsenic accumulation was evaluated using the index of geoaccumulation(Igeo) and Enrichment Factor (EF). The index of geoaccumulation varied from 0.57 to 3.15(Table 1). Applying the classification of Muller (1969), the experimental soils, namelyNonaghata Uttarpara (S1 and S2) and Nonaghata Dakshinpara (S3), may be described asmoderately arsenic contaminated to heavily arsenic contaminated soils whereas Kalyani soilwas the uncontaminated soil. When calculating the Enrichment Factor, iron (total content)

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 8: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

796 I. Das et al.

Table 2Sequential extraction of arsenic (mg/kg) by extractants to identify different arsenic fractions

in soils under study

Soil sampleExtractants Solvent/extractable arsenic (mg/kg)fractions from the experimental soils S1 S2 S3 K

Resin extractable As or freely exchangeablearsenic (A)

1.75 4.60 5.22 0.98

0.5 M NaHCO3 (pH 8.5) extractable As OrNonexchangeable but readily available/labileAs associated with soil mineral surfaces (B)

2.07 1.33 0.90 0.25

0.1 M NaOH extractable As or Arsenic held bystrongly chemisorbed Fe and Al components(C)

3.74 2.48 3.59 0.72

Sonicated 0.1 M NaOH extractable As or Arsenicheld by internal surfaces of soil aggregates (D)

4.19 3.41 4.05 0.50

1 M HCl extractable As or Calcium associated As(E)

4.17 3.49 2.90 0.57

Residual recalcitrant As (F) 4.10 2.03 3.05 0.51

was taken as a reference element as described by Loska et al. (1997). Arsenic content of1.5 mg/kg in the earth’s crust (Taylor and McLennan, 1995) was accepted as a referencevalue in the tested environment. The mean content of iron in the earth’s crust of 5.63 ×104 mg/kg was accepted as reference value (Draggan, 2007). The iron has been taken asreference element in this study as the crop fields in these areas are irrigated with iron-richwater (>1 mgL−1), which led to some anthropogenic addition in these soils. In addition, theiron content in these soils is also high geochemically. The values of the Enrichment Factorrevealed that the soils of Nonaghata Uttarpara (S1 and S2) and Nonaghata Dakshinpara (S3)are extremely enriched with arsenic while significant enrichment was found for Kalyani(K) soil. Unlike the interpretation of Loska et al. (1997), both the indices reflected a similartrend as found for the total or real arsenic content of these soils. Both the indices indicatedthe arsenic contamination in the experimental soils in the order: S1>S3>S2>K.

Different fractions of arsenic in experimental soils as obtained from the sequentialextraction by different extractants are presented in Table 2. These reflect the amount ofarsenic held in the soil colloidal surfaces and subsequently their capacity to hold arsenicin their matrices. The resin extractable As form varied from 0.98 to 5.22 mg As/kg whereS3 soil showed the highest extractability. This is probably because S3 soil is endowedwith high Olsen extractable arsenic as well as high organic carbon, the latter causing ahigher degree of complexation of arsenic leading to its higher resin extractability (Table 2and Figure 1). With the exception of S1 and K soils, the other two soils showed higherextractability of resin extractable arsenic fraction, which might be attributed to the lesserextent of preferential binding of arsenate against the phosphate for the common adsorptionsites (i.e. soil surface). The 0.5 M NaHCO3 (pH 8.5) extractable arsenic content variedfrom 0.25 mg As/kg to 2.07 mg As/kg whereas 0.1M NaOH extractable As varied from0.72 to 3.74 mg As /kg. Further, the extractability was improved in all four soils whentreated with sonicated 0.1M NaOH. Similar results were obtained by McLaren et al. (1998)

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 9: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

Fig

ure

1.Pe

rcen

tage

Dis

trib

utio

nof

diff

eren

tfra

ctio

nsof

arse

nic

inth

eex

peri

men

tals

oils

unde

rst

udy.

797

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 10: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

798 I. Das et al.

and Ghosh et al. (2002). This was probably due to dispersion and dissolution of boundarsenic fraction in the soils. The extractable arsenic by the latter extractant varied from0.50 to 4.19 mg As/kg. Also, a considerable extractability was found in the case of 1MHCl extraction where the values ranged from 0.57 to 4.17 mg As/kg. The dissolution ofCa and Mg bound arsenic in these soils might have resulted in such an increase in arsenicextractability. However, S1 soil showed higher content of all the arsenic fractions in all theabove-mentioned extractants. This was probably because S1 soil is endowed with highertotal and labile forms of arsenic reflected in the release by the extractants. S2 and S3 soilshave higher content of clay, amorphous iron and aluminum oxides, calcium and magnesiumcontent, thereby releasing the fractions in considerable amounts. In K soils, higher sandand lower arsenic content resulted in a lower magnitude of release by these extractants.A considerable amount of inert arsenic was found in all the soils that were unable to beextracted by these extractants (Table 2 and Fig. 1).

Regarding different fractions of As, the resin extractable forms correspond to the freelyexchangeable form of arsenic. In all these soils this form accounted for 9 to 29% of the totalarsenic loading. Excepting S1 soil, the higher content of freely exchangeable form maycontribute considerably to plant availability. The lower content (0.25 to 2.07 mg As/kg) ofarsenic (as a fraction of total loading) in the nonexchangeable but readily available arsenic[extracted with 0.5M NaHCO3 (pH 8.5) solution] or arsenic held at mineral surfaces, espe-cially by illite, which is the dominant clay mineral (Table 2) in these soils, might be tracedto the fact that As in ionic form is negatively charged, hence non-specific (electrostatic)adsorption by the mineral surface would be lower. Further, arsenic may protonate the sur-face via ligand exchange but that is possible in very high pH, which is not possible in theexisting soil pH range of 6.33 to 7.54. The nonexchangeable but readily available arsenicform is found least among all the forms in all the soils and it accounts for only 5 to 10%of the total arsenic loading. Chemisorbed arsenic by amorphous iron and aluminum oxideshowed increased level of arsenic extractability (0.52 to 3.74 mg As/kg) as the surfacesof iron and aluminum are positive and arsenic being the anion, held electrostatically bythese surfaces of the soils and thereby released accordingly when treated with specificextractants. Unlike the extent of freely exchangeable form, this form contributes a consid-erable portion (14–19%) of the total arsenic content of these soils. Arsenic held by internalsurface of soil aggregates was found to be higher (varied from 0.50 to 4.50 mg As/kg),which may possibly be attributed to the higher clay content of these soils. The proportion ofcontribution of the above-mentioned form to the total arsenic content is also considerablyhigh as it accounts for 15–21% in all these soils (Fig. 1). These soils being in the almostneutral to slightly alkaline pH range, the Ca-associated As fractions (ranged from 0.52 to4.17 mg As/kg) in these soils were also significantly high (15–21% of the total arseniccontent of these soils) as reflected in the release by a specific extractant. The much higheranion exchange membrane extractable As pool in the soil sample, S3, might have arisenfrom a persistent anoxic condition prevailing in this rice soil. The anoxic condition helps inmobilizing arsenic into the corresponding soil solution. Similar reports of arsenic increasein flooded soil were reported by McGeehan and Naylor (1994) and Reynolds et al. (1999).The K (control) soil, showing much less total arsenic loading, also exhibited smaller valuesof all the arsenic pools as compared to the remaining (arsenic-affected) soils (Table 2). Aconsiderable amount of arsenic found in different fractions in S1, S2 and S3 soils which,in general, correspond to the differential nature of soils having variations in texture, amor-phous iron and aluminum oxides, calcium, magnesium and organic carbon content of thesesoils. Such findings are in agreement with those of McLaren et al. (1998) and Ghosh et al.(2002).

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 11: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

Fractionation of Arsenic in Soil 799

Table 3Correlation among different fractions, soil physicochemical, mineralogical properties of

arsenic

(A) (B) (C) (D) (E) (F)

(A) 1 .076 .515 .605 .426 .264(B) .076 1 .782 .798 .934 .872(C) .515 .782 1 .982(∗) .893 .962(∗)(D) .605 .798 .982(∗) 1 .940 .919(E) .426 .934 .893 .940 1 .886(F) .264 .872 .962(∗) .919 .886 1Olsen Ext. As .623 .699 .990(∗) .979(∗) .856 .915Total As .633 0.788 0.973(∗) 0.999(∗∗) 0.941 0.900Clay content .516 −.785 −.256 −.255 −.529 −.471pH −.315 −.964(∗) −.912 −.930 −.987(∗) −.939Organic Carbon .620 −.061 .568 .492 .166 .418Surface area .836 .595 .883 .941 .838 .734Amorphous Fe .997(∗∗) .004 .450 .542 .360 .191Amorphous Al .801 −.102 .535 .509 .194 .330CEC .589 .536 .942 .892 .696 .863Ca + Mg content .062 −.939 −.539 −.596 −.830 −.652Clay minerology

Montmorrilonite −.784 −.623 −.934 −.966(∗) −.845 −.806Illite .824 .465 .894 .904 .716 .740Chlorite .440 .902 .782 .866 .975(∗) .762Vermicullite −.772 −.649 −.937 −.972(∗) −.864 −.813Kaolinite .788 .612 .933 .963(∗) .836 .802Mixed layers .827 .300 .819 .809 .567 .649Igeo 0.664 0.781 0.952(∗) 0.992(∗∗) 0.945 0.869EF 0.862 0.570 0.836 0.912 0.825 0.676

∗Correlation is significant at the 0.05 level (2 tailed).∗∗Correlation is significant at the 0.01 level (2 tailed).

The correlation coefficient (r) values of different arsenic fractions among themselvesand with important physicochemical and mineralogical properties of the given soils arepresented in Table 3. The correlation coefficients among different forms revealed that thechemisorbed arsenic form showed significant positive correlations with internally boundarsenic (r = 0.982∗) and Ca associated arsenic fractions (r = 0.962∗). This indicates thevital role of Fe, Al, and Ca in modifying arsenic transformation in these soils. Similarresults were obtained by McLaren et al. (1998), Shiowatana et al. (2001), and Ghoshet al. (2002). Total arsenic was positively and significantly correlated with chemisorbedarsenic (r = 0.973∗) and internally bound arsenic (r = 0.999∗∗). Olsen extractable arsenicalso showed a significant positive relationship with these arsenic fractions in soil. Theserelationships indicate that all the fractions influence mobility and transformation of arsenicin these soils. The clay content showed negative correlation with all the forms, which is dueto the electrostatic repulsion between negatively charged clay surfaces and arsenic anions.Significant negative correlations were observed between pH and nonexchangeable butreadily available arsenic (r = −0.964∗) and the calcium bound arsenic form (r = −0.987∗).

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 12: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

800 I. Das et al.

Negative charge in the mineral surfaces (especially illite and montmorillonite) increaseswith the rise in pH of soils which debar arsenic from adsorption, whereas the protonationof the arsenic ions by ligand exchange mechanism is not possible as the pH of the soil isnear neutral. However, for calcium, nonspecific (electrostatic) attraction may be responsiblefor arsenic bonding. The pH-dependent arsenic reactivity in soil was well documented byWelch et al. (1988), Sanyal (2005), and Masue et al. (2007). Whereas at low pH the Feand Al sites have positive surfaces which bind arsenic electrostatically, arsenic in thesesoils was weakly bonded and could be released easily by the extractant as the pH of thesesoils was near neutral. Lower adsorption of arsenic at about 6.5 pH was also reported byMasue et al. (2007). The reduction of As adsorption with increasing pH was also reported byMcLaren et al. (1998) and Reynolds et al. (1999). The CEC and surface area were positivelycorrelated with the freely exchangeable form. Significant positive correlation was foundfor freely exchangeable arsenic with amorphous iron (r = 0.997∗∗) while only positivecorrelation was observed for the same form with amorphous aluminum (r = 0.801). Thepositive surfaces of Fe and Al hold exchangeable arsenic electrostatically and are releasedeasily whenever extracted with specific extractants. Negatively significant correlation wasobserved for the nonexchangeable but readily available arsenic pool with pH (r =−0.964∗∗).Nonsignificant positive correlation was observed for iron/aluminum bound arsenic with pH,Ca +Mg content, and CEC of the soils. This was duly reflected by the increased reactivesurface area of iron and aluminum (Table 1), hence the CEC, to hold more arsenic inthese surfaces. The internally held arsenic positively correlated with surface area, CEC,and Olsen extractable arsenic content whereas it showed negative relation with pH. Thecalcium bound arsenic showed negative correlation with pH and calcium plus magnesiumions as these conditions govern the arsenic dissolution by solubility product principlesimilar to phosphate dissolution. The inert arsenic content showed negative correlationwith pH, which indicates the possibility of the decrease of inert arsenic content with therise in pH. Significant negative correlation for internally held arsenic was observed formontmorillonite (r = −0.966∗) and vermiculite (r = −0.972∗), whereas the same formof arsenic significantly and positively correlated with kaolinite (r = 0.963∗). This may bedue to the fact that montmorillonite and vermiculite both have a larger surface area foradsorption. Similar results were also observed by Ghosh et al. (2002). As, an anion arsenic,forms monodentate and bidentate ligands over these internal surfaces of minerals. Arsenicmay also form binuclear and trinuclear complexes with different iron oxides by which theyget strong retention over the surfaces (Violante and Pigna, 2002). These ligands bind arsenicirreversibly, thereby restricting release of arsenic. However, monmorillonite possesses lessinternal surface area but the surface charge is developed from broken bonds. Here, arsenicis held (via ligand exchange) only loosely onto the outer surfaces of this mineral, therebybeing easily released by specific extractants. The significant positive correlation betweenCa bound arsenic and chlorite (r = 0.975∗) may be due to the magnesium content of thismineral, which competes with calcium in binding arsenic electrostatically. In addition, illiticminerals showed a more or less positive relationship with all these forms. This is becauseillitic minerals are a less expanding type, so internal surfaces are less exposed for arsenicto form ligand. Only the outer surfaces of the broken bond form ligand, which are weakerbonds where arsenic may be released upon extraction. Significant positive correlationbetween the internally held arsenic form and Ca associated arsenic form revealed the factthat arsenic accumulation has been facilitated by these forms, which is in agreement withthe earlier findings of McLaren et al. (1998) and Ghosh et al. (2002).

Path analysis differentiates between correlation and causation by partitioning simplecorrelation coefficients between independent variables (soil properties, including physico-chemical and mineralogical properties) and dependent variables (total As and Olsen As)

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 13: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

Fractionation of Arsenic in Soil 801

Figure 2. Path analysis model for total and Olsen extractable arsenic with different parameters ofthe soils under study.

into direct and indirect effects (Table 4 and Figure 2). This type of statistical analysis wasperformed earlier by Afifi and Clark (1984), Basta et al. (1993), and Zhang et al. (2005).Path analysis provides a numerical value for both direct and indirect effects, thus indicatingthe relative strength of causal relationships (Loehlin, 1987). Direct effects are referred toas path coefficients and are standardized partial regression coefficients (Basta et al., 1993).The significant relationship of parameters with the total as well as Olsen extractable arsenic

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 14: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

Tabl

e4

Path

anal

ysis

for

tota

land

Ols

enex

trac

tabl

ear

seni

cw

ithdi

ffer

entf

ract

ions

,soi

lphy

sico

chem

ical

,min

eral

ogic

alpr

oper

ties

ofar

seni

c

Cor

rela

tion

Dir

ect

Form

sPa

ram

eter

sco

effic

ient

(r)

Eff

ect

Posi

tive

Indi

rect

Eff

ectw

ithot

her

para

met

ers

Tota

lArs

enic

(C)

0.97

3∗0.

563

D(0

.553

),F(

0.54

2),O

ls.E

xt.A

s(0

.557

),M

ont.

(0.5

03),

Kao

(0.5

25).

(D)

0.99

9∗−0

.656

C(0

.644

),E

(0.6

16),

F(0.

602)

,Ols

.Ext

.As

(0.6

42),

pH(0

.610

),SA

(0.6

17),

Mon

t.(0.

633)

,Ver

mi.(

0.63

7)(E

)0.

941

−0.3

64B

(0.3

40),

D(0

.342

),pH

(0.3

59),

Cho

r.(0.

355)

Ols

enE

xtra

ctab

lear

seni

c

0.97

4∗−0

.004

D(0

.004

),C

EC

(0.0

02),

Kao

.,(0

.004

),M

ont.,

Ver

mi.(

0.00

4)

pH−0

.923

0.01

4B

,D,F

(0.0

13),

E(0

.014

),C

hl.(

0.01

3)Su

rfac

ear

ea0.

953∗

1.18

∗∗D

(1.0

5∗∗),

Ols

.Ext

.As(

1.08

∗∗),

Mon

t,V

erm

i(1.

16∗∗

),M

ontm

orill

onite

−0.9

72∗

1.63

∗∗C

(1.5

2∗∗),

D(1

.57∗∗

),Il

l.(1.

59∗∗

),V

erm

.(1.

63∗∗

),K

ao(1

.63∗∗

)Il

lite

0.90

82.

49∗∗

D(2

.25∗∗

),O

ls.E

xt.A

s.(2

.36∗∗

),SA

(2.3

8∗∗),

Mon

t.(2

.43∗∗

),K

ao(2

.44∗∗

),M

ix.L

(2.4

4∗∗)

Ver

mic

ullit

e−0

.979

∗0.

287

D(0

.279

),O

ls.E

xt.A

s.(0

.278

),SA

(0.2

85),

Mon

t(0.

287)

,Ill.

(0.2

78),

Kao

(0.2

87)

Kao

linite

0.96

9∗−0

.776

Mon

t.(0.

776)

Ver

mi.(

0.77

5)O

lsen

extr

acta

ble

(C)

0.99

0∗∗−3

.00∗∗

arse

nic

(D)

0.97

9∗3.

08∗∗

C(3

.02∗∗

),E

(2.9

0∗∗),

Tota

lAs.

(3.0

8∗∗),

SA(2

.90∗∗

),K

ao(2

.97∗∗

)To

tala

rsen

ic0.

974∗

5.31

∗∗C

(5.1

6∗∗),

D(5

.30∗∗

),E

(5.0

0∗∗),

SA(5

.06∗∗

),K

ao(5

.14∗∗

)Su

rfac

ear

ea0.

925

−4.3

5∗∗M

ont.(

4.31

∗∗∗ )

,Ver

mi(

4.31

∗∗),

CE

C0.

964∗

−1.1

7∗∗

Mon

tmor

illon

ite−0

.968

∗−7

.88∗∗

D(7

.61∗∗

),To

talA

s(7.

66∗∗

),SA

(7.8

0∗∗),

Illit

.(7.

69∗∗

)K

ao(7

.88∗∗

)Il

lite

0.94

8−1

4.3∗∗

Mon

t.(14

.0∗∗

),V

erm

i(13

.8∗∗

),V

erm

icul

lite

−0.9

67∗

−1.6

2∗∗D

(1.5

7∗∗),

Tota

lAs.

(1.5

8∗∗),

SA(1

.60∗∗

∗ ),I

llit.(

1.56

∗∗),

Kao

.(1.

61∗∗

)K

aolin

ite0.

968∗

7.27

∗∗D

(7.0

0∗∗),

Tota

lAs.

(7.0

4∗∗),

SA(7

.19∗∗

),Il

lit.(

7.12

∗∗),

Ver

mi.(

1.61

∗∗)

Unc

orre

late

dR

esid

ualV

alue

is0.

011

and

Coe

ffici

ento

fD

eter

min

atio

nis

0.99

5

∗ Cor

rela

tion

issi

gnifi

cant

atth

e0.

05le

vel(

2ta

iled)

∗∗C

orre

latio

nis

sign

ifica

ntat

the

0.01

leve

l(2

taile

d).

802

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 15: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

Fractionation of Arsenic in Soil 803

was chosen for path analysis. The noncorrelated residue value (R), which indicates thenumber of free parameters in the analysis model, was low (0.011) and the coefficient ofdetermination measuring the fraction of variability between arsenic forms and the effectof other parameters was high (0.995), indicating that the path analysis model explainedthe majority of variations in the total as well as Olsen extractable arsenic forms. For totalAs, path analysis showed a significant (p < 0.01) direct relationship with illite (2.49∗),montmorillonite (1.63∗) and surface area (1.18∗), a nonsignificant relationship (p > 0.05)with the chemisorbed arsenic form (C) (0.563), and a negative nonsignificant relationshipor no direct effect was observed for total As with kaolinite (−0.776) and internally heldarsenic by soil aggregates (D) (−0.656). The minerals, namely illite and montmorillonite,are responsible for influencing total arsenic reserve in the soil. Negative or no correlationfor kaolinite may be due to lesser chances of strong bonding for arsenic as a result of lowsurface area and specific sites for adsorption, while for the case of the internally boundform, arsenic was bound irreversibly by ligand formation, which cannot be released easily,and the bound arsenic may restrict further adsorption of arsenic by these sites. Regardingthe indirect effects, a positive significant (p < 0.01) effect was observed for surface areawith As held by internal surfaces of soil aggregates (D) (1.05∗), Olsen extractable As(1.08∗), montmorillonite (1.16∗) and vermicullite (1.16∗), montmorillonite with As heldby strongly chemisorbed Fe and Al components (C)(1.52∗), As held by internal surfacesof soil aggregates (D) (1.57∗), illite (1.59∗), vermicullite (1.63∗), and Olsen extractablearsenic (1.59∗) and internally held arsenic (1.57∗). Similar results were also observed forillite with these parameters, except montmorillonite. Though a nonsignificant relationship(p > 0.05) existed between the direct effect for total arsenic with the chemisorbed arsenicform (C), yet the As held by internal surfaces of soil aggregates (D), residual recalcitrant As(F) form, Olsen extactable As, montmorillonite and kaolinite seemed to be the importantfactors influencing the chemisorbed arsenic for a direct relationship with total As at p >

0.05. No correlation or negative nonsignificant correlation with the direct effect of D formfor total As was also influenced by a nonsignificant (p > 0.05) indirect effect of C form, Eform, F form, Olsen extractable form, pH, surface area, montmorillonite and vermiculite,while no correlation in the direct effect of E form for total As was influenced by B form,D form, pH and chlorite. In the case of the Olsen extractable arsenic form, path analysispartitioning (Table 4) showed a significant direct relationship (p < 0.01) with internallyheld arsenic (3.08∗), total arsenic (5.31∗), and kaolinite (7.88∗), whereas a negative or nocorrelation was observed with chemisorbed arsenic fraction (C) (−3.00∗), montmorillonite(−7.88∗), illite (−14.3∗), vermiculite (−1.62∗). The significant direct effects of D form forthe Olsen extractable arsenic was further significantly and indirectly influenced (p < 0.01)by C form (3.02∗), E form (2.90∗), total arsenic (3.08∗), surface area (2.90∗), and kaolinite(2.97∗). The direct effects of total arsenic were significantly and indirectly influenced byC, D, E forms, surface area and kaolinite, whereas the direct similar effects of kaolinitewas influenced indirectly by D form, total As, surface area, illite, and vermiculite. Thoughnegative significant or no correlation was observed in the direct effect of Olsen extractableAs for C form, illite, montmorillonite and vermiculite, these attributes (except C form) wereindirectly and significantly influenced by mineralogical forms, total As, D forms, and sur-face area. From these results it may be inferred that there remains a contrasting relationshipin the direct and indirect effect of different attributes for the total As and Olsen extractableAs form. As held at internal surfaces of soil aggregates was low due to the dominance ofillitic minerals, which can bind As loosely through the broken bonds developed in theirmineral matrices and hence release As under specific soil conditions, which is reflected asthe Olsen extractable As (crop available form). Further, the kaolinite mineral, which has a

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 16: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

804 I. Das et al.

Table 5Total arsenic uptake (mg kg −1) by different crops in the experimental area

CropsSoils Kharif Rice Mustard

Nonaghata- Uttarpara (S1) 36.0 25.3Nonaghata- Uttarpara (S2) 28.1 19.5Nonaghata-Dakhinpara (S3) 43.4 28.0Kalyani (K) 1.55 2.83

very limited adsorption site to bind As, can release As easily and hence a direct, positive,significant relationship exists. However, for total As the minerals, namely montmorilloniteand vermiculite, had the larger outer surface area to bind arsenic specifically (via ligandexchange) in the mineralogical matrices and form inner sphere complexes. The inner spherecomplexes of these minerals for arsenic were also reported by Goldberg (2002). The min-eralogical make-up of soils is also reflected in the significant positive relationship of thesurface area for total arsenic. The C form of As fraction representing Fe and Al chemisorbedarsenic form held arsenic nonspecifically (electrostatically) restricts release of As by theirstronger bonding (Jacobs et al., 1970). These direct effects were indirectly modified by theforms B and E, pH, surface area, and mineralogical properties of these experimental soils.

Regarding the arsenic uptake by crops, namely kharif rice and mustard, it was observedthat kharif rice accumulated much higher arsenic compared to mustard (Table 5). This ismainly due to the fact that kharif rice is grown during the rainy season, where plentyof water in the soil creates partial dissolution of arsenic bound by minerals, Fe, Al andCa under the anoxic conditions (McGeehan and Naylor, 1994; Reynolds et al., 1999).The dissolution of As from iron oxides under reducing condition was also reported byMahimairaja et al. (2005); thereby arsenic uptake was facilitated more in the kharif season.Rainwater dilutes arsenic concentration but during the year and season of the experimentabnormally low rainfall caused less removal of arsenic in soil solution, thereby causinggreater accumulation of arsenic in soil.

However, after the rainy season, when the evaporative demand in the soil is high andthe arsenic content in the soil increases, there is greater uptake in crops like mustard.The uptake pattern of these crops in these soils followed the order S3>S1>S2>K. Thisuptake was directly related with the arsenic content and accumulation parameters of theexperimental soils. It may be concluded that soil acts as a sink (as it is contaminated throughthe irrigation water source) for arsenic, which travels its way from soil to crop directly withthe facilities of the soil physicochemical and mineralogical make-up (Sanyal, 2005).

A simple correlation coefficient was derived for crop uptake and different arsenicfractions, physicochemical properties, and mineralogical properties of the experimentalsoils (Table 6 and Figure 3). Significant positive correlation was observed for kharif riceuptake with internally held arsenic (r = 0.976∗), Olsen extractable arsenic (r = 0.965∗), andtotal arsenic (r = 0.983∗), while for mustard such fractions also showed a similar relationship(r = 0.982∗, 0.989∗∗ and 0.956∗), respectively. Significant positive correlation was observedfor As uptake of kharif rice and mustard with surface area (r = 0.991∗∗ and 0.987∗), kaolinite(r = 0.996∗∗ and 0.987∗) and accumulation parameters Igeo and EF, whereas a significantnegative correlation was observed for montmorillonite and vermiculite. The negative impactof these two minerals in the Olsen extractable form was also reflected in the crop uptake

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 17: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

Fractionation of Arsenic in Soil 805

Table 6Simple correlation coefficients R© and Path analysis for uptake by Kharif rice and mustardwith different fractions of arsenic, physicochemical and mineralogical properties of soil

Correlation Direct Positive Indirect Effect withParameters coefficient R© Effect other parameters

Kharif RiceTotal As 0.983∗ −0.087 Mont(0.085), Vermi (0.085),(D) 0.976∗ −0.568 Mont (0.549), Vermi (0.552)Olsen Ext. As 0.965∗ −0.563 Mont (0.545), Vermi(0.544)Surface area 0.991∗∗ 0.221 Total As (0.211), D(0.208), ILL

(0.211), Kao(0.219)Montmorillonite −0.998∗∗ −0.400 Total As (0.389), D(0.387), Ols. Ext.

As(0.388), SA(0.397), ILL(0.391),Kao(0.400)

Illite 0.959∗ −0.121 Vermi(0.117)Vermicullite −0.998∗∗ −1.21∗∗ Total As(1.19∗∗), D(1.18∗∗), Ols. Ext.

As(1.17∗∗),SA(1.20∗∗), ILL(1.17∗∗),Kao(1.21∗∗)

Kaolinite 0.996∗∗ 0.652 Total As(0.631), D(0.628), Ols. Ext.As(0.631), SA(0.644), ILL(0.639),

Igeo 0.994∗∗ — —EF 0.910 — —

MustardTotal As 0.956∗ −0.084 Mont(0.082), Vermi (0.082)(D) 0.982∗ −1.46∗∗ Mont (1.41∗∗), Vermi (1.42∗∗)Olsen Ex. As 0.989∗∗ −1.42∗∗ Mont (1.37∗∗), Vermi(1.38∗∗)Surface area 0.987∗ −0.203 Mont(0.201), Vermi (0.201),Montmorillonite −0.989∗∗ −3.25∗∗ Total As (3.16∗∗), D(3.14∗∗), Ols. Ext.

As(3.15∗∗), SA(3.22∗∗), ILL(3.17∗∗),Kao(3.25∗∗)

Vermicullite −0.994∗∗ −3.69∗∗ Total As (3.62∗∗), D(3.60∗∗), Ols. Ext.As(3.58∗∗), SA(3.66∗∗), ILL(3.58∗∗),Kao(3.69∗∗)

Kaolinite 0.987∗ 0.010 Total As(0.01), D(0.01), Ols. Ext.As(0.01), SA(0.01), ILL(0.01)

Igeo 0.996∗∗ — —EF 0.902 — —

Uncorrelated Residual Value is SQRT(1-1.000) and Coefficient of Determination is 1.0

∗Correlation is significant at the 0.05 level (2 tailed) ∗∗Correlation is significant at the 0.01 level(2 tailed).

of As. After the determination of a simple correlation coefficient, path analysis was alsocarried out between crop uptake and different fractions of soil arsenic, physicochemicaland mineralogical properties of the experimental soils (Table 6). In the case of arsenicuptake by kharif rice, the nonsignificant direct (p > 0.05) effects (as derived by the pathcoefficients) were observed for surface area (0.221) and kaolinite (0.652). Negative or no

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 18: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

806 I. Das et al.

Figure 3. Path Analysis Model with Kharif rice and mustard crop uptake and different fractions ofArsenic in soil.

correlation for kharif rice uptake was observed with vermiculite, D form, Olsen extractableAs form and montmorillonite. The higher direct value for kaolinite (its weaker adsorptionon the low surface area makes arsenic easily available to plants) indicates greater cropuptake of arsenic while vermiculite and montmorillonite do not favor such uptake as theybind arsenic probably through a ligand exchange mechanism. This is comparable with theearlier interpretation made through path analysis of the total and Olsen extractable form.The direct effect of kharif rice uptake for surface area and kaolinite was strongly influencedby the indirect effect of total As, D forms, Olsen extractable As, and illite. In spite ofthe negative relationship of vermicullite with kharif rice uptake, the indirect effects by theattributes (total As, D, Olsen extractable As, surface area, illite, and kaolinite) significantly(p < 0.01) influence the uptake. Montmorillonite and vermicullite also influenced indirectlythe direct effects of D form, total As, and Olsen extractable As for kharif rice uptake.

In the case of arsenic uptake by mustard, the negative significant or no correlation in thedirect effects was observed for internally held arsenic (D)(−1.46∗∗), Olsen extractable form(−1.42∗∗), montmorillonite (−3.25∗∗) and vermicullite (−3.69∗∗), while a nonsignificant(p > 0.05) direct effect was observed only with kaolinite (0.010).

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 19: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

Fractionation of Arsenic in Soil 807

Though montmorillonite and vermiculite showed no correlation with arsenic uptakeby mustard, the indirect effects controlling these minerals showed a significant positiverelationship (p < 0.01) with total As (3.16∗∗), D form (3.14∗∗), Olsen extractable As(3.15∗∗), surface area (3.22∗∗∗), illite (3.17∗∗) and kaolinite (3.25∗∗). The negative impactsof vermiculite and montmorillonite for arsenic uptake by both the crops may be due to theirirreversible bonding with As in their mineral matrices. As mustard is grown under irrigatedor controlled limited water conditions, the release of arsenic facilitated by the kaolinitemineral is also restricted, and hence a nonsignificant relationship exists.

Conclusion

This study revealed the fact that the experimental soils were highly contaminated byarsenic, which is reflected by the accumulation indices. The As fractionation study ingeneral revealed that the As affected soils were endowed with internally held arsenicfollowed by Fe and Al chemisorbed arsenic and Ca associated arsenic. However, thefreely exchangeable form was also found considerably in two of the arsenic affected soils,but this form doesn’t correlate with the physicochemical and mineralogical properties ofthe experimental soils. The fractions of arsenic varied mainly due to the mineralogicalmake-up of the soils, surface area, pH, total and Olsen extractable As, amorphous ironand, to a smaller extent, calcium and magnesium content of these soils. Such findingswere also established by the simple correlation coefficient of the parameters. However,path analysis for total as well as Olsen extractable As revealed that out of these sixfractions of soil arsenic only two fractions, namely chemisorbed As (C) and internallyheld arsenic (D) forms, directly governed the total and Olsen extractable As form, whileCa associated As (E) indirectly modifies the direct effect of C and D forms. The total Asform was directly governed by the mineralogical minerological make-up of soils, namelymontmorillonite, illite (the dominant fraction in these affected soils) and vermiculite, whilethe Olsen extractable form was directly influenced by kaolinite. Crop uptake of As in soilwas directly facilitated by As held at the internal surfaces of soil aggregates form (D),kaolinite minerals, and indirectly governed by chemisorbed As (C) form, surface area,montmorillonite, vermiculite, and illite minerals. The crop uptake data revealed greateruptake by kharif rice due to dissolution of As during the rainy season as compared tomustard grown in the rabi season under irrigated or controlled water conditions. Thefactors responsible for influencing the Olsen extractable form of As are also responsiblefor crop uptake. These interpretations are also supported by the Path analysis data for cropuptake of kharif rice and mustard. However, from the study it may be concluded that Asavailability is facilitated by the As held at the internal surface of soil aggregates form andthe availability is modified by the mineralogical make-up, especially montmorillonite, illite,vermiculite, and kaolinite minerals of the soil clay fraction, which ultimately contributesto the transformation of As in soil. Adequate management practices (under continuedirrigation through As laden groundwater) are needed in order to restrict the release from Asheld at internal surface of soil aggregates form and also the dissolution of As from mineralsso that much less As is left in the soil solution for crop uptake and thus only a little Asenters into the food chain, causing harm to human and animal health.

Acknowledgement

The corresponding author is thankful to Dr. N.C. Sahu, Programme Coordinator (Reader),Dakshin Dinajpur Krishi Vigyan Kendra, Uttar Banga Krishi Viswavidyalaya, Majhian,

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 20: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

808 I. Das et al.

P.O. Patiram, District Dakshin Dinajpur, Pin. 733133, West Bengal, India, for editing andmodifying the entire manuscript.

References

Afifi, A. A., and Clark, V. 1984. Path analysis. Computer-aided Multivariate Analysis, LifetimeLearning Publ., Belmont, Conn., pp. 235–237.

Basalma, D. 2008. The correlation and path analysis of yield and yield components of different winterrapeseed (Brassica napus ssp. oleifera L.) cultivars. Res. J. Agricul. and Biol. Sci. 4(2), 120–125.

Basta, N. T., Pantone, D. J., and Tabatabai, M. A.1993. Path analysis of heavy metal adsorption bysoil. Agron. J. 85, 1054–1057.

Buat-Menard, P. 1979. Influence de la Retombee Atmospherique sur la Chimie des Metaux en Tracedans la Maticre en Suspension de l’Atlantique Nord. Thesis, Univ. Paris VI, Paris, France, p.434.

Chang, S. C. and Jackson, M. L. 1957. Fractionation of soil phosphorus. Soil Science. 84, 133–144.Draggan, S. (ed.). 2007. Iron. In: Encyclopedia of Earth (Cleveland,Cutler J., ed.), Environmental

Information Coalition, National Council for Science and the Environment, Washington, D.C.Available at: http://www.eoearth.org/article/Iron.

Feiziene, D. and Feiza, V. 2003. Changes in soil-available P and K on eroded slopes of West Lithuania.Zemes ukio Mokslai. 3, 8–18.

Ghosh, A. K., Sarkar, D., Sanyal, S. K. and Nayak, D. C. 2002. Status and distribution of arsenic inalluvium derived soils of West Bengal and their interrelationship with some soil properties. J.Ind. Soc.of Soil Sci. 50, 51–56.

Gjems, O. 1967. Meddelelser fra Det Norske Skogforsokvesen. 81(21), 303.Goldberg, S. 2002. Competitive adsorption of arsenate and arsenite on oxides and clay minerals. Soil

Sci. Soc. Am. J. 66, 413–421.Hedley, M. J., Stewart, J. W.B., and Chauhan, B. S. 1982. Change in inorganic and organic soil

phosphorus fractions induced by cultivation practices and by laboratory incubations. Soil Sci.Soc. Am. J. 46, 970–976.

Jackson, M. L. 1973. Soil Chemical Analysis, Prentice-Hall of India, New Delhi, p. 326.Jacobs, L. W., Syers, J. K., and Keeney, D. R. 1970. Arsenic sorption by soils. Soil Sci. Soc. Am.

Proc. 34, 750–754.Johnston, S. E. and Barnard, W. M. 1979. Comparative effectiveness of fourteen solutions for ex-

tracting arsenic from Western New York soils. Soil Sci. Soc. Am., J. 43, 304–308.Kwapuliniski, J., Miroslawaski, J., Wiechula, D., Rochel, R., Burczyk, J., Sowada, B., and Iwanek,

K. 1996. The use of ecotoxicological parameters for estimation of the quality of medicinal plantyielding areas. Bromat. Chem. Toksykol. 29 (3), 243 (in Polish).

Loehlin, J. C .1987. Path models in factor, path, and structural analysis. In: Latent Variable Models:An Introduction to Factor, Path, and Structural Analysis, E. Erlbaum Assoc., Hillsdale, NJ, pp.1–37.

Loska, K., Cebula, J., Pelczar, J., Wiechula, D., Kwapuliniski, J. 1997. Use of enrichment andcontamination factors together with geoaccumulation indexes to evaluate the content of Cd, Cu,and Ni in the Rybnik Water Reservoir in Poland. Water, Air, Soil Pollut. 93, 347.

Loska, K., Wiechuła, D., Barska, B., Cebula, E., and Chojnecka, A. 2003. Assessment of arsenicenrichment of cultivated soils in southern Poland. Polish J. Environ. Studies. 12(2), 187–192.

Mahimairaja, S., Bolan, N. S., Adriano, D. C.C. and Robinson, B. 2005. Arsenic contamination andits risk management in complex environmental settings. Adv. in Agron, 86, 1–82.

Masue,Y R., Loeppert, H., and Kramer, T. A. 2007. Arsenate and arsenite adsorption and desorptionbehavior on coprecipitated aluminum:iron hydroxides. Environ. Sci. Techn. 41(3), 837–842.

McGeehan, S. L. and Naylor, D. V. 1994. Sorption and redox transformation of arsenite and arsenatein two flooded soils. Soil Sci. Soc. Am. J. 58, 337–342.

McLaren, R. G., Naidu, R., Smith, J., Tiller, K. G. 1998. Fractionation and distribution of arsenic insoils contaminated by cattle dip. J. Environ. Qual. 27, 348–354.

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14

Page 21: Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

Fractionation of Arsenic in Soil 809

Miko, S., Peh, Z., Bukovec, D., Prohic, E., and Kast-Muller, Z. 2000. Geochemical baseline mappingand Pb pollution assessment of soils in the karst in Western Croatia. Nature Croatia. 9 (1), 41.

Muller, G. 1969. Index of geoaccumulation in sediments of the Rhine River. Geojournal. 2, 108.Muller G. 1981. Die schwermetallbelastung der sedimenten des neckars und seiner nebenflusse.

Chemiker-Zeitung 6, 157.Reimann, C., and Caritat, P. 2000. Intrinsic flaws of element enrichment factors (EFs) in environmental

geochemistry. Environ. Sci. Techn. 34, 5084.Reynolds, J. G., Naylor, D. V. and Fendorf, S. E. 1999. Arsenic sorption in phosphate-amended soils

during flooding and subsequent aeration. Soil Sci. Soc. Am. J. 63, 1149–1156.Sanyal, S. K. 2005. Arsenic contamination in agriculture: A threat to water-soil-crop-animal-human

continuum. Presidential Address, Section of Agriculture & Forestry Sciences, 92nd Session ofthe Indian Science Congress Association (ISCA), Ahmedabad, January 3–7, 2005; The IndianScience Congress Association, Kolkata.

Shiowatana, J., McLaren, R. G., Chanmekha, N., and Samphao, A. 2001. Fractionation of arsenic insoil by a continuous-flow sequential extraction method. J. Environ. Qual. 30, 1940–1949.

Sutherland, R. A. 2000. Bed sediment-associated trace metals in an urban stream, Oahu, Hawaii.Environ. Geol. 39 (6), 611.

Taylor, S. R., and McLennan, S. M. 1995. The geochemical evolution of the continental crust. Reviewsin Geophysics. 33, 241.

Tessier, A., Campbell, P. G.C. and Bisson, M. 1979. Sequential extraction procedure for the speciationof particulate trace metals. Anal. Chem. 51, 844–851.

Tiessen, H., and Moir, J. O.1993. Characterisation of available phosphorus by sequestial extraction.In: Soil Sampling and Methods of Analysis (Carter, M. R., ed.), Lewis Publishers, Boca Raton,Florida, pp. 75–86.

Violante, A. and Pigna, M. 2002. Competitive sorption of arsenate and phosphate on different clayminerals and soils. Soil Sci. Soc. Am. J. 66, 1788–1796.

Welch, A. H., Lico, M. S. and Hughes, J. L. 1988. Arsenic in groundwater of the Western UnitedStates. Ground Water. 26, 333–347.

Woolson, E. A., Axley, J. H., and Kearney, P. C. 1971. Correlation between available soil arsenic,estimated by six methods, and response of corn (Zea mays L). Soil Sci. Soc. Am. Proc. 35(1),101–105.

Zhang, H., Schroder, J. L., Fuhrman, J. K., Basta, N. T., Storm, D. E., and Payton, M. E. 2005.Path and multiple regression analyses of phosphorus sorption capacity. Soil Sci. Soc Am J. 69,96–106.

Dow

nloa

ded

by [

Col

orad

o C

olle

ge]

at 1

3:47

07

Nov

embe

r 20

14