IASWS 2014: THE INTERACTIONS BETWEEN SEDIMENTS AND WATER

Speciation of selected tracemetals (Fe,Mn, Cu and Zn) with depthin the sediments of Sundarban mangroves: India and Bangladesh

Alok Kumar1,2 & AL Ramanathan1

Received: 17 February 2015 /Accepted: 4 September 2015# Springer-Verlag Berlin Heidelberg 2015

AbstractPurpose The study was conducted to highlight the distribu-tion of trace metals in the world’s largest mangrove ecosys-tem, Sundarban, in India and Bangladesh. A speciation tech-nique was used to document the bioavailability and environ-mental hazards associated with operationally defined chemi-cal forms of trace metals.Materials and methods Seven sediment cores were selected tostudy the speciation of selected trace metals (Fe, Mn, Cu andZn) in the sedimentary mangrove environment. A modifiedEuropean Community Bureau of Reference (BCR) three-stepsequential extraction procedure was used to target chemicalforms.Results and discussion Texture analysis showed that the hy-drodynamic conditions prevailing in the region are high inenergy that leads to the deposition of sediment fractions inthe following order: silt > clay > sand. The fine-grained sed-iments have the potential for binding trace metals. The tracemetal content was higher in the fine-grained fraction that wasdominant in the Indian Sundarban (IS) than BangladeshSundarban (BS). The ranges of concentrations of four tracemetals were as follows: 38,760–52,829 μg g−1 for Fe, 424–

770 μg g−1 for Mn, 36–82 μg g−1 for Cu and 55–83 μg g−1 forZn in IS, while in BS, the ranges were 29,081–45,025 μg g−1

for Fe, 342–792 μg g−1 for Mn, 12–45 μg g−1 for Cu and 29–75 μg g−1 for Zn. Speciation suggested the dominance of Mnin the acid-soluble fraction (>68 % IS; >52 % BS) while Fe(>83 % IS; >80 % BS), Cu (>56 % IS; >60 % BS) and Zn(>77 % IS; >80 % BS) dominated in the residual fraction. Thespeciation pattern showed that due to low organic carbon(OC) content (<0.99 % IS; <1.52 % BS), the Fe–Mnoxyhydroxide fraction (fraction 2) acts as a scavenging phasefor Fe, Cu and Zn. The Risk Assessment Code (RAC) showedmoderate to very high riskwith respect to Cu andMn, whereasFe and Zn were in the low-risk category suggesting an anthro-pogenic influence with respect to Cu and Mn.Conclusions The dominance of Cu and Mn in the non-residual fraction suggests anthropogenic sources, such asport-related activities and sewage runoff. The study high-lights the dominance of anthropogenic activity and lack offreshwater flow in IS compared to BS, which affects thespeciation profile of trace metals in the mangrovesediments.

Keywords Mangroves . Risk assessment . Sediments .

Speciation . Tracemetals

1 Introduction

Sundarbanmangrove complex is the largest conglomerationof mangroves in the world situated on the fringe of deltaicfloodplains of the Ganga–Meghna–Brahmaputra rivers.Sundarban mangroves suffer from environmental degrada-tion due to rapidly expanding human settlements, tourismactivity, deforestation, intense fishing, and agricultural, in-dustrial and aquacultural activities (Saha 2006; FAO 2007;

Responsible editor: Carolyn Oldham

Electronic supplementary material The online version of this article(doi:10.1007/s11368-015-1257-5) contains supplementary material,which is available to authorized users.

* AL Ramanathanalrjnu@gmail.com; alr0400@mail.jnu.ac.in

1 School of Environmental Sciences, Jawaharlal Nehru University,New Delhi 110067, India

2 Department of Environmental Science, Central University ofRajasthan, Kishangarh, Ajmer, Rajasthan 305817, India

J Soils SedimentsDOI 10.1007/s11368-015-1257-5

Fattorini 2013). Mangroves are potentially large deposition-al stores of sediment-associated trace metals due to their lowredox potential and significant amount of H2S (Huerta-Diazand Morse 1992; Jones and Turki 1997; Silva Filho et al.2010) making it important to assess the mobility of tracemetals in such environments (Andrade et al. 2012). Thetotal metal content of the sediments may not be able toprovide a clear picture of the metal loadings and their be-haviour (such as mobility, toxicity and bioavailability) withchanging environmental conditions. The chemical state ofelements in the sediments (e.g. easily exchangeable ions,metal carbonates, oxides, sulphides, organometallic com-pounds, ions in crystal lattices of minerals) depends on pre-vailing environmental conditions (Usero et al. 1998; Weiszet al. 2000; Yu et al. 2001; Pérez-López et al. 2008; Delgadoet al. 2011). Anthropogenic influences alter the prevailingenvironmental conditions (e.g. pH, redox potential, organicmatter) which in turn remobilize the sediment-bound metalions and make them bioavailable to the environment. Tracemetals thus released from the sediment into the environmentmay have adverse impacts on living organisms (e.g. Li et al.

2001; Morillo et al. 2002, 2007; Peng et al. 2009; Delgadoet al. 2011).

Speciation not only helps in identifying the degree ofassociation of metals in different phases but also distin-guishes the origin (lithogenic or anthropogenic) of tracemetals in the environment. Several authors (e.g. Izquierdoet al. 1997; Sundaray et al. 2011) have demonstrated thatmetals associated with the exchangeable and carbonatefraction have an anthropogenic origin, while the residualfraction corresponds to metals with a lithogenic origin.Over the years, numerous sequential extraction methodshave been developed which differ according to the type ofreagent being used as an extractant, the experimental con-ditions applied and the number of steps involved toachieve different chemically bound fractions (e.g. Tessieret al. 1979; Gibson and Farmer 1986; Ure et al. 1993;Quevauviller et al. 1997; Rauret et al. 1999; Dold 2003).In the present study, the three-step modified EuropeanCommunity Bureau of Reference (BCR) sequential ex-traction procedure (Rauret et al. 1999) was used. TheBCR, now Standard Measurements and Testing (SM&T),

Fig. 1 Study area indicating sediment core locations in the Indian Sundarban (IS) and Bangladesh Sundarban (BS)

J Soils Sediments

proposed the original sequential extraction procedure inorder to standardize various existing procedures,obtaining acid-soluble, acid-reducible and acid-oxidizable fractions (Ure et al. 1993); the scheme wassubsequently improved by various authors (e.g. Rauretet al. 1999; Sahuquillo et al. 1999; Arain et al. 2008).The digestion method applied (USEPA 3051 1997) pro-vide information on total or pseudo-total content of tracemetals in sediments without giving a direct evaluation ofanthropogenic contributions and the consequent pollutionlevel of the sites (Pérez-Santana et al. 2007). Existingstudies of trace metal pollution in the Sundarban man-groves (Chatterjee et al. 2007, 2009; Mukherjee et al.2009; Silva Filho et al. 2010; Banerjee et al. 2012) givean account of trace metals in the Sundarban but focuson the Indian sector of the Sundarban mangroves only,and there is a lack of baseline data for sediment tracemetal pollution in the Bangladesh sector. This study wasconducted to understand the speciation and mobility ofselected trace metals (i.e. Fe, Mn, Cu and Zn) in thesediments, thus providing information on the bioavail-ability and the risk they pose in the Sundarban man-grove ecosystem.

2 Material and methods

Seven sediment core profiles were collected: three from IndianSundarban (IS) and four from Bangladesh Sundarban (BS)(Fig. 1) mangrove settings using 1.5-m-long plexiglass auger(inner diameter, 7 cm) in February 2012. The samples weretransported to the field laboratory where they were sectionedat a 5-cm interval. The maximum length of the core collectedwas 45 cm. The core sections were collected and packed in aprecleaned polythene bags and stored at 4 °C until analysis.The samples were freeze-dried, finely powdered and homog-enized for metal extraction.

2.1 Grain size analysis

Grain size fractions were measured using laser particlesize analyser (Microtrac, S3500, USA). Before analysis,the sediments were pretreated using 30 % H2O2 andboiled with 10 % HCl and tetrasodium pyrophosphate(decahydrate) was added (Konert and Vandenberghe1997). Samples were run in triplicates, and the averagevalue was considered.

Fig. 2 Ternary diagram showingdeposition of sand, silt and clayunder varying hydrodynamicenergy conditions after Pejrup(1988) for Indian and BangladeshSundarban sediment core profiles

Table 1 BCR three-step sequential extraction scheme

Fraction Extraction reagent Extraction method

F1—exchangeable, water and acid soluble 40 ml 0.11 M CH3COOH Shaking at 22±5 °C for 16 h

F2—reducible 40 ml 0.5 M NH2OH·HCl(adjusted to pH=1.5 with HNO3)

Shaking at 22±5 °C for 16 h

F3—oxidizable 10 ml 8.8 M H2O2/another 10 ml 8.8M H2O2/50 ml 1.0 M NH4OAc(adjusted to pH=2.0 with HNO3)

Digesting at room temperature for 1 h with occasionalmanual shaking, then cover the tube and heat at 85±2°C for 1 h, then remove the cover and reduce the volumeto a few millilitres/cover the tube and heat at 85±2 °C for1 h, and then remove the cover and reduce the volume tonear dryness/shaking at 22±5 °C for 16 h

F4—residual Aqua regia Oven digestion

Source: Rauret et al. (1999)

J Soils Sediments

2.2 Chemicals and solutions

All the solutions prepared were in deionized water (Milli-QPlus system; Millipore) with 18.2 mΩ cm−1 resistivity. All thestandard reagents were standardized and kept at 4 °C in dark-ness until use. Dilutions of these standard solutions were pre-pared daily for the analyses. Concentrated nitric acid (Merck)and concentrated hydrochloric acid (Merck) were used for theaqua regia digestion method. The reagents used—30 % H2O2

(Merck), acetic acid (Merck), hydroxylammonium chloride(Merck), ammonium acetate (Merck) and tetrasodium pyro-phosphate (decahydrate) (Merck)—were analytical reagent

grade. The standard NIST SRM 1646a (Estuarine sediment)was used to validate our results. The precision and biasassessed by the reagent blanks and replicate samples were<5 % of the mean analytical concentrations for all the ele-ments. The recovery rates in the sediment samples for all ofthe measured elements were ~88–113 %.

2.3 Sediment digestion and speciation

The USEPA 3051 method was used for sediment digestion(USEPA 1997). Sediments were digested using a microwavedigestion system (MARS 5; CEM Corp, Matthews, NC,

Fig. 3 Depth profile of Fe (in %), Mn, Cu and Zn (in μg g−1) in different sediment core profiles of Indian and Bangladesh Sundarban

J Soils Sediments

USA). The freeze-dried and finely powdered sediment sam-ples (0.2–1.0 g) were weighed into a dry, clean Teflon diges-tion vessel, and 5 ml of aqua regia was added. The digestionsolution was transferred to a 50-ml volumetric flask, dilutedwith Milli-Q water, filtered through a Millipore membrane(0.45 μm) and kept in a plastic container for analysis (Rahmanet al. 2009). Inductively coupled plasma mass spectrometry(ICP-MS; Agilent 7500c Agilent Technologies, Tokyo, Japan)was used to determine the total metal concentrations. Organiccarbon analysis was determined using rapid titration method(Walkley and Black 1934).

Inductively coupled plasma atomic emission spectroscopy(ICP-AES, Jobin Yvon Ultima 2, Paris, France) was used todetermine speciation pattern in the sediment core profiles. Themodified SM&T (formerly BCR) three-step sequential extrac-tion protocol (Rauret et al. 1999) was used to determine dif-ferent operationally defined species of selected trace metals(i.e. Fe, Mn, Cu and Zn) in the sediment core profiles. Thefour fractions obtained, and extractants that were used, are asfollows:

F1: acid-soluble fraction, which comprises exchangeableand carbonate-bound fraction (0.11 M, acetic acid; pH,7);F2: bound to Fe/Mn oxides (0.5 M, hydroxylammoniumhydrochloride; pH, 1.5–2);F3: bound to organic material and sulphides (8.8 M, hy-drogen peroxide; pH 2–3, 1 M, ammonium acetate; pH2); andF4: residual (digestion with aqua regia) (Table 1).

3 Results and discussion

3.1 Sediment texture and hydrodynamics

Sediment texture analysis is an efficient tool to differentiatethe various depositional environments of a region (Mason andFolk 1958; Nordstrom 1977; Perjup 1988; Flemming 2000).The grain size composition of Sundarbanmangrove sediments

Fig. 4 Percentage of Fe extracted in each step of the sequential extraction procedure for sediment core profiles of IS and BS

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followed the order silt > clay > sand (Electronic Supplemen-tary Material, Fig. S1a, b). The % sand content in IS rangedfrom 2.46 to 7.62 % and in BS ranged from 3.93 to 11.51 %.Similarly, there was a dominance of silt fraction in BS than ISwhich ranged from 67.20 to 80.63 % and 64.14 to 73.92 %,respectively, indicating dominance of higher energy condi-tions (Massolo et al. 2012) in BS than IS. The % clay contentwas higher in IS (19.60 to 31.22 %) compared to BS (12.11 to27.74 %) suggesting the prevalence of low-energy areas ofsuspension deposits in IS than BS.

The sorting of sediments under the influence of vary-ing degrees of hydrodynamic energy conditions is givenin Fig. 2 as a ternary diagram (Pejrup 1988). The tex-tural characteristics provide a measure of energy condi-tions during deposition. The channel morphology,source, weathering, abrasion, corrosion, sorting and de-position of the sediment influence the textural pattern.Previously, various authors (Trefethen 1950; Shepard1954; Folk 1954 and 1968; Pejrup 1988; Flemming2000) have used modifications of ternary diagrams forthe classification of sediments and sedimentary rocks,

distinguishing different depositional facies and hydrody-namic conditions of both recent and paleoenvironments.As shown in Fig. 2, the samples from IS fall under thegroup D-III while all BS sediment cores fall in groupD-III and D-IV. Thus, BS sediments are sorted underhigher and variable energy conditions than are the ISsediments that exhibit calm conditions. There is a dom-inance of fine-grained sediments in IS than BS, whichmay also suggest higher binding of trace metals in theIS sediments than BS sediments. Fine-grained sedimentsare an important sink for a wide range of contaminants,particularly trace metals (Cundy and Croudace 1995;Spencer et al. 2003). Due to this, mangrove environ-ments may continue to release trace metals into estua-rine waters even after the cessation of inputs from ef-fluent sources. As a result of physical, chemical andbiological processes in the sediments and throughgroundwater discharge, continuous mixing and remobili-zation of trace metals into the water column are likelyto take place (Deng et al. 2010; Silva Filho et al. 2010;Sanders et al. 2012).

Fig. 5 Percentage of Mn extracted in each step of the sequential extraction procedure for sediment core profiles of IS and BS

J Soils Sediments

3.2 Organic carbon

The range for total organic carbon (TOC) in the different coresvaried as follows: cores I-1 (0.608–0.972 %), I-2 (0.609–0.831 %), I-3 (0.586–0.995 %), B-1 (0.683–1.524 %), B-2(0.568–0.860 %), B-3 (0.614–0.915 %) and B-4 (0.507–0.929 %). This indicates moderately low TOC contents inthe mangrove environment, which can be attributed to rapidtidal export of locally produced organic matter to the coastalzone (Rogers et al. 2013).

3.3 Vertical sediment core profile speciation for Fe, Mn,Cu and Zn

Figure 3 shows variations of total elemental concentrationswith depth in seven sediment cores. The concentration ofpseudo-total metals for selected trace metals in the IS rangedfrom 38,760 to 52,829 μg g−1 for Fe, 424 to 770 μg g−1 forMn, 36 to 82μg g−1 for Cu and 55 to 83μg g−1 for Zn, and forBS values ranged from 29,081 to 45,025 μg g−1 for Fe, 342 to

792 μg g−1 for Mn, 12 to 45μg g−1 for Cu and 29 to 75 μg g−1

for Zn. Iron, Cu and Zn showed a consistently higher concen-tration in IS compared to BS except for Mn, which showed acomparable concentration in both mangrove settings. The dis-tribution pattern of speciation of Fe, Mn, Cu and Zn wasdetermined by the modified BCR three-step sequential extrac-tion protocol (Figs. 4, 5, 6 and 7). The speciation pattern doesnot follow any specific pattern when comparing samples spa-tially between IS and BS.

Iron and Zn in the sediment profiles of cores I-1, I-2, I-3,B-1, B-2, B-3 and B-4 (with an exception of only Fe for B-4)were found to be dominant in the residual fraction followed byreducible f ract ion (F2) , associa ted with Fe–Mnoxyhydroxides and oxidizable fraction (F3), bound to the sul-phides and minimum concentration in the acid-soluble frac-tion (F1), associated with exchangeable and carbonate bound.Thus, the association of Fe and Zn with the residual fractionsuggests its non-availability under prevailing natural condi-tions. The association with the reducible fraction (F2) as thesecond most dominant fraction suggests that it plays an

Fig. 6 Percentage of Cu extracted in each step of the sequential extraction procedure for sediment core profiles of IS and BS

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important role in the leaching of metals into the environment.Low abundance of Fe and Mn in the F3 and F1 fractionssuggests low release and availability under suboxic and natu-ral conditions. For metals Fe and Mn in core B-4, the residualfraction is dominant followed by the reducible fraction (F2).While the available fraction (F1) is more in abundance thanthe oxidizable fraction (F3), their abundance in these fractionsdoes not pose a threat to the sediment quality and biota. Thedominance of Fe in the residual fraction indicates that it existsas crystalline iron peroxides (e.g. goethite, limonite, magne-tite, haematite) (Massolo et al. 2012). The second most dom-inant fraction for Zn was the Fe–Mn oxyhydroxide fraction(F2) which may be due to the high stability constants of Znoxides and may be due to the ability of Fe oxides to adsorbconsiderable quantities of Zn; these oxides may also occludeZn in the lattice structures (Banerjee 2003). For Cu in thesediment core profiles, a major fraction was bound with theresidual, thus not easily available under natural conditions(Davidson et al. 2006; Delgado et al. 2011). The higher

proportion of Cu in the residual fraction may be due to sorp-tion to clay minerals (Pickering 1986; Massolo et al. 2012).The secondmost abundant fraction for Cuwas associatedwiththe reducible fraction (F2), suggesting adsorption of thesemetals by the Fe–Mn colloids which act as important scaven-gers of these metals in sediments, thus indicating the role ofFe–Mn colloids in controlling the mobility of Cu in the envi-ronment (Burt et al. 2003; Kaasalainen and Yli-Halla 2003;

Fig. 7 Percentage of Zn extracted in each step of the sequential extraction procedure for sediment core profiles of IS and BS

Table 2 RiskAssessment Code (RAC) RAC Criteria (%)

No risk <1

Low risk 1–10

Medium risk 11–30

High risk 31–50

Very high risk >50

Source: Perin et al. (1985)

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Álvarez et al. 2006; Davidson et al. 2006; Delgado et al.2011). A small portion of Cu is also bound to the availablefraction (F1) suggesting its bioavailability while the oxidiz-able fraction bound to sulphur, and organics is the least abun-dant fraction and does not act as a significant scavenger forSundarban sediments. The largest fraction ofMnwas found inthe available fraction (F1) for all the sediment core profiles,except B-1 and B-2, where slightly less abundance was ob-served than the residual fraction. In the available fraction,metals are weakly sorbed and are retained on the sedimentsurface by weak electrostatic interaction and as a result canbe released by ion exchange processes and precipitated or co-precipitated with the carbonates present in many sediments(Marín et al. 1997; Filgueiras et al. 2004; Delgado et al.2011). The metals in the available fraction pose a significantenvironmental risk for aquatic ecosystems (Morillo et al.2007; Kumar et al. 2012), as they may easily leach in neutralor slightly acidic waters. Thus, they are prone to assimilationby organisms and could, therefore, be used to assess potentialecological risks (Kabata-Pendias 1993; Álvarez-Valero et al.2009). The abundance of Mn is least in the oxidizable fraction(F3), indicating its absence as a scavenger for Mn in the sed-iments probably due to the occurrence of low TOC as reportedin other studies (e.g. Banerjee et al. 2012). The Fe–Mnoxyhydroxide fraction (F2) is the more significant phase as ascavenger of Mn than the oxidizable fraction (F3).

3.4 Internal check of recovery

An internal check was performed by comparing the totalamount of metal extracted by different reagents during thesequential extraction procedure with the results of the totaldigestion. The recovery of the sequential extraction methodwas calculated as follows:

Recovery ¼ CFraction 1 þ CFraction 2 þ CFraction 3 þ CResidual

CTotal digestion

� �� 100

The results indicated that the sums of the four fractionswere in good agreement with the total digestion results, withsatisfactory recoveries (88–113 %).

3.5 Risk assessment and bioavailability

The Risk Assessment Code (RAC) was used to assess thepotential mobility and environmental risk of trace metals inthe sediments using sequential extractions as a characteri-zation method (Perin et al. 1985; Jain 2004; Singh et al.2005; Liu et al. 2009; Rodríguez et al. 2009; Kumar et al.2012). The percentage of trace metals bound to the ex-changeable and carbonate fractions (in this study, the avail-able fraction, F1) was used to assess the availability fromthe sediments. Table 2 and Fig. 8 show the classificationindex for RAC. Iron showed low to moderate risk for coresB-1, B-3, B-4, I-2 and I-3 as they fall near the low-riskcategory (1 %). For cores B-2 and I-1, there was a higheranthropogenic influence as they were near human settle-ments; their RAC values were >2 %, suggesting a moderaterisk that may be due to anthropogenic influence. The riskassociated with Zn was minimal for all the sediment coreprofiles, as RAC falls near and under the low-risk category.Copper showed moderate risk for all the sediment core pro-files except B-3 and B-4 which fall under the low-risk cat-egory. The RAC values for Mn fall under the high-risk to thevery high-risk category for all the sediment core profiles.Thus, the excess concentrations of Cu and Mn, though re-quired as a nutrient, in the sediments may not have an ap-parent effect on mangrove plants but may be harmful tosediment-dwelling organisms and human health.

Fig. 8 Risk assessment code (RAC) of trace metals Fe, Mn, Cu and Zn for sediment core profiles of IS and BS

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4 Conclusions

The distribution of the trace metals Fe, Mn, Cu and Zn invarious operationally defined fractions was investigated. Itwas found that the sediment texture in the Sundarban man-groves was dominated by silt followed by clay and sand, in-ferring the prevalence of high-energy conditions in the studyarea. Speciation analysis of trace metals suggests that Fe, Cuand Zn are dominant in the residual fraction (>60 %) whileMn was dominant in the acid-soluble fraction indicating itsaffinity for carbonates. The supply of fresh organic matterthrough the mangroves and variable hydrodynamic conditionsmay determine its affinity to the acid-soluble fraction. Copper,Zn and Fe were associated with the colloids of Fe–Mn oxidesas the second most dominating phase, suggesting the Fe–Mnoxyhydroxide phase as a significant scavenger for these tracemetals. The RAC suggests low toxicity for Fe and Zn and hightoxicity for Cu and Mn, which may be harmful to sediment-dwelling organisms. There is a need to understand the role ofvegetation in trace metal partitioning in sediments for the ef-fective management of the Sundarban mangrove ecosystem.

Acknowledgments The Asia-Pacific Network for Global Change Re-search, Japan, (ARCP2012-07CMY-Ramanathan) funded the project.The authors thank Dr. M.B.K. Prasad (University of Maryland, USA),Prof. Dilip Datta, Dr. Mahmood Hossain,Mr.MohammadRaqibul HasanSiddique andMr. Kushal Roy fromKhulna University for their assistancein sample collection and field analysis. The first author would also like tothank Dr. Jayant Kumar Tripathi and Dr. Sandeep Gautam for helpingwith the ICP-AES analysis at the National Facility for Geochemical Re-search, School of Environmental Sciences, Jawaharlal Nehru University,New Delhi. The authors are also thankful to Dr. Mohammad MahmudurRahman, University of South Australia, for helping with ICP-MSanalysis.

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