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Marine Pollution Bulletin 79 (2014) 342–347

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Marine Pollution Bulletin

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Baseline

Impact of total organic carbon (in sediments) and dissolved organiccarbon (in overlying water column) on Hg sequestration by coastalsediments from the central east coast of India

0025-326X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.marpolbul.2013.11.028

⇑ Corresponding author. Tel.: +91 8322450 495; fax: +91 8322450 602.E-mail address: [email protected] (P. Chakraborty).

Parthasarathi Chakraborty a,⇑, Brijmohan Sharma a,b, P.V. Raghunath Babu a, Koffi Marcellin Yao a,c,Saranya Jaychandran a,d

a Geological Oceanography Division, National Institute of Oceanography, Dona Paula, Goa 403004, Indiab TERI University, 10, Institutional Area, Vasant Kunj, New Delhi, Indiac Centre de Recherches Oceanologiques, BP V18 Abidjan, Cote d’Ivoired Cochin University of Science and Technology, Cochin 682022, Kerala, India

a r t i c l e i n f o a b s t r a c t

Keywords:Hg-sediment interactionHg speciationHg–TOC complexesHumic acid–Hg complexes

Total organic carbon (TOC) (in sediment) and dissolved organic matter (DOM) (in water column) playimportant roles in controlling the mercury sequestration process by the sediments from the central eastcoast of India. This toxic metal prefers to associate with finer size particles (silt and clay) of sediments.Increasing concentrations of DOM in overlying water column may increase complexation/reduction pro-cesses of Hg2+ within the water column and decrease the process of Hg sequestration by sediments. How-ever, high concentrations of DOM in water column may increase Hg sequestration process by sediments.

� 2013 Elsevier Ltd. All rights reserved.

Mercury (Hg) has received a worldwide attention due to its sig-nificant global adverse impact on both environment and humanhealth (Ratcliffe et al., 1996; Boening, 2000; Wolfe et al., 2009).Due to its high toxicity, distributions of Hg compounds in coastal,estuarine and marine environments have been the subject of manyresearchers (Valette-Silver, 1993; Daskalakis and O’Connor,1995;Long et al., 1995; Gagnon et al., 1997; Kannan and Falandysz,1998; Kannan et al., 1998; Horvat et al., 1999; Borja et al., 2000;Hines et al., 2000). It is well known that sediment, an integraland inseparable part of aquatic system, plays an important rolein controlling Hg pollution in aquatic systems (Farmer, 1991; Tackand Verloo, 1995; Bubb and Lester, 1996; Liu et al., 2006). Hg accu-mulates in sediments globally from many physical, chemical, bio-logical, geological and anthropogenic environmental processes.Thus, sediment can be a good indicator of water quality of a partic-ular area (Dai et al., 2007; Yu et al., 2008; Bilotta and Brazier,2008).

The toxicity and bioavailability of Hg is dependent on its chem-ical speciation rather than its total concentrations in sediments. Insediments, Hg can be associated with different binding phases.These forms, or species, are of key importance in risk assessmentsof Hg in sediments (as well as in the overlying water column).However, determination of Hg associated with different bindingphases in sediments is a very difficult task due to intrinsic

complexity of binding sites in sediment (Chakraborty, 2010,2012, 2012a, 2013).

In addition to that there are several factors (dissolved organicmatter in water column, adsorption capacity of sediments,different type of binding sites, pH of the water column, chlorideion concentrations, ionic strength, cation exchange capacity, oxida-tion–reduction potential) may also influence the distribution andspeciation of metals in sediments (Chakraborty and Chakrabarti,2006, 2008; Gopalapillai et al., 2008; Chakraborty, 2010; Chakr-aborty et al., 2010, 2012b,c). Sequential extraction procedures(SEP) are designed to determine metal contents associated to dif-ferent solid phases present in sediments. These methods lead toobtain operationally defined fractions related to the mobility andpotential toxicity of metals and rarely with specific individualspecies. Several operationally-defined sequential extraction proce-dures have been widely used to assess the bioavailable Hg fractionand their mobility in sediment. In this work a modified BCR(sequential extraction) method was used for the evaluation ofthe geochemistry of Hg and determines non-residual (bioavailable)Hg species and phases in the coastal sediment from the central eastcoast of India. The aims of this study were also to identify thefactors which control Hg-sediment interaction and elucidate theimpacts of DOM (in this case humic acid in the overlying watercolumn) on the sequestration processes of Hg by the coastal sedi-ments from the central east coast of India.

Sediment samples were collected from the four different envi-ronmentally significant sites of the central east coast of India as

Fig. 2. The schematic diagram of the modified BCR protocol for Hg fractionationstudy in coastal sediments.

Table 1Percentages of silt, clay, sand, TOC, total Hg, in the coastal sediments.

Station Sand(%)

Silt(%)

Clay(%)

TOC(%)

[Hg]T (lg/kg)

Bhimili 67.2 15.8 20.4 0.06 16.6Visakhapatnam 58.2 39.3 2.5 0.12 28.9Gangavaram 14.2 50.4 35.4 0.42 59.4Kakinada 6.6 88 5.4 0.24 88.4

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shown in the Fig. 1. The coastal sediments were collected from (1)Bhimili, (2) Visakhapatnam, (3) Gangavaram, and (4) Kakinada. Thegeneral description, geographic location of the sampling sites, thedistance from the shore, and the depth from where the sedimentsamples were collected are given as supporting document(Table 1SD).

A Van Veen stainless steel grab sampler with an area of 0.02 m2

was used to collect the sediments; without emptying the grab, asample was taken from the centre with a polyethylene spoon toavoid contamination by metallic parts of the dredge. The sampleswere stored at �20 �C for 15 days, and then dried at room temper-ature (25–30 �C) by keeping the sediment samples on Petri dishes(covered with parafilm). However, the sediment samples (of largequantity) were also dried at 60 �C in a forced air oven (Kadavil Elec-tro Mechanical Industry Pvt. Ltd. India, Model No. KOMS.6FD).There was no loss of Hg found from the oven dried sediment com-pared to the sediments dried at room temperature.

A series of batch extractions were performed on the coastal sed-iments, following BCR protocol (Quevauviller et al., 1993; Ure et al.,1993). This sequential extraction protocol allows us to determinethe sum of ion-exchangeable, water soluble and carbonate/bicar-bonate form of Hg (Fr. 1): Hg bound to Fe–Mn oxides i.e., reduciblefraction of Hg (Fr. 2); fraction of total Hg bound to organic matter(Fr. 3) and residual Hg fraction (Fr. 4) in sediment. The protocol ispresented in Fig. 2.

Total Hg was analyzed by direct mercury Analyzer (Tri cellDMA-80) from Milestone, Italy. Operational conditions were estab-lished according to EPA method 7473 (EPA, 2007). The sensitivityand the accuracy of the DMA was monitored in each experimentby analyzing certified reference material (Reference marine sedi-ments (MESS-2, PACS-1) from Natural Resources of Canada)followed by a reagent blank between every 10 samples. total or-ganic carbon (TOC) in the studied sediments was determined byfollowing the Walkley–Black method (Schumacher, 2002).

The general description and texture analysis of the studied sed-iments are presented in Table 1. The total Hg content in the coastalsediments was found to vary from 16.6 to 88.4 lg kg�1. The max-imum concentration of Hg was found in the sediment collectedat Kakinada (88.4 lg kg�1) followed by Gangavaram (59.4 lg kg�1),Visakhapatnam (28. 9 lg kg�1) and Bhimli (16. 6 lg kg�1). It isinteresting to note that the total Hg content in the coastal sedi-ments was found to increase from north to south of the central east

Fig. 1. Different environmentally significant sam

coast of India. The concentrations of the finer size (sum of silt andclay contents) particles was found to influence the accumulation ofHg in the studied sediments. A positive correlation (R2 = 0.96) wasfound between the total Hg content and the finer particles

pling sites off the central east coast of India.

Fig. 3. Distribution of Hg in different binding phases of the coastal sediments (byfollowing BCR protocol).

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fractions in the coastal sediments. Yoshida et al. (2006) have alsoreported that finer particles in sediments strongly influence thedistribution of Hg between sediment and the overlying watercolumn.

TOC content in the studied sediment was found to increase withthe increasing concentrations of the finer particles and decreasewith the increasing coarser fraction (sand) in the studied sedi-ments. A strong correlation (R2 = 0.83) was found between theTOC concentrations with the fraction of finer particles in the coast-al sediments. Percival et al. (2000) has reported that finer particlesof sediment control organic carbon level in sediments. A strongcorrelation (R2 = 0.92) between the total concentrations of Hg withthe TOC content in the sediments (as shown in Table 2) indicatesthat TOC in the coastal sediments probably plays an important rolein controlling Hg sequestration process by the sediment from thecentral east coast of India.

It has been reported that TOC in sediment controls the specia-tion, transformation, and fate of Hg in the sediment (Stein et al.,1996; Wallschläger et al., 1998; Ravichandran, 2004; Lambertssonand Nilsson, 2006). TOC (in sediment) has been reported toenhance Hg sorption on sediments by providing additionalsorption sites (Melamed et al., 1997; Bengtsson and Picado,2008). However, TOC may also cause reduction of Hg2+ to Hg0 byintramolecular electron transfer (Barkay et al., 1997; Ravichan-dran, 2004). The strong correlation between total Hg and TOC inthe studied sediment indicates that Hg had strong affinity forTOC associated with the finer particles of the coastal sediments(under the environmental conditions, pH, Salinity, etc.). Furtherinvestigation was carried out to understand the association of Hgwith the different binding phases in the coastal sediments byapplying a modified sequential extraction protocol proposed byCommunity Bureau of Reference (BCR) (Quevauviller et al., 1993;Ure et al., 1993).

Fig. 3 shows the distributions of Hg associated with differentbinding phases in the coastal sediments. The concentrations ofFr. 1 of Hg (which include water soluble Hg complexes + Hg inexchangeable form + carbonate/bicarbonate complexes of Hg)were found to vary from �2% to 23% of the total Hg content ofthe sediments.

The concentrations of Hg associated with Fe and Mn-oxides (Fr.2) were found to vary along with the coastal sediments. The high-est concentrations of Hg was associated with Fe and Mn-oxidephases in the sediments collected at Bhimli (up to 23% of the totalHg) followed by Visakhapatnam (13.0%), Gangavaram (11.9%) andKakinada (6%).

A major fraction of Hg was associated with organic phases (Fr 3)in the studied sediments. Fig. 3 shows that the concentrations ofHg associated with organic fraction gradually increased with theincreasing TOC content in the studied sediments. The highest per-centage of Hg was found to associate with organic carbon in sedi-ments at Gangavaram (52.3% of the total Hg concentration)followed by Bhimli (44.1%) Visakhapatnam (29.4%), and Kakinada(21.7%). A positive correlation coefficient (R2) value of 0.98 wasfound between the concentration of Hg associated with organics

Table 2Pearson correlations matrix between percentages of silt, clay, sand, TOC, and total Hgin the sediments.

Sand(%)

Silt(%)

Clay(%)

Fine TOC(%)

[Hg]T (lg/kg)

Sand (%) 1.00Silt (%) �0.88 1.00Clay (%) �0.20 �0.29 1.00Fine �1.00 0.87 0.22 1.00TOC(%) �0.83 0.51 0.60 0.83 1.00[Hg]T (lg/kg) �0.96 0.97 �0.06 0.96 0.65 1.00

and the TOC content of the sediments. This observation suggeststhat TOC content in the sediments play a key role in controllingthe distribution and speciation of Hg in the studied sediments.The residual Hg concentration (probably Hg complexes with sulfurcontaining group in the sediments) was found to vary from –20% to70% of the total concentrations of Hg in the studied sediments. Thedata are presented as supporting document (Table 2SD).

Several equilibrium adsorption isotherms were studied tounderstand the Hg sorption capacity of the studied sediments. Ki-netic uptake study of Hg (from water column) by the coastal sed-iments was performed to determine the time required for Hg (inthe water column) and the coastal sediments to reach equilibrium.Fig. 4a shows that the dissolved concentrations of Hg (from theoverlying water column) gradually decreased in the presence ofthe coastal sediments. This finding indicates that Hg interactedquite fast with the coastal sediments and the Hg-sediments systemreached equilibrium within 2 h. An equilibrium time of 2 h wasused though out this study to understand Hg adsorption isothermby the coastal sediments.

The adsorption isotherms of Hg were constructed to compareHg adsorption capacity among the four coastal sediments samplescollected from the four environmentally significant sites. Theadsorption of Hg by the sediment was found to decrease in the fol-lowing order: Visakhapatnam > Kakinada > Bhimili > Gangavaram.Hg was found to adsorb completely by the studied sediments atlow Hg loading. The adsorbed concentrations of Hg (by the sedi-ments) were found to increase with the increasing Hg concentra-tions in the overlying water column. However, the adsorbedconcentration of Hg in the sediments reached a plateau with thehigher concentrations of Hg in the solution. This is because of thesaturation of the adsorbing sites for Hg on the sediment. The iso-therms of Hg adsorption on the sediments are presented inFig. 4b. These isotherms are described by the simple Langmuirand Freundlich isotherms.

The Freundlich parameters, KD and bF, were used to measuresthe adsorption affinity and the intensity of adsorption of the sedi-ments (Table 3SD). The value of bF, between 0 and 1; confirm theheterogeneity of the adsorbent. The value of bF for sediments col-lected at Bhimili was found to be 0.85, Visakhapatnam (0.95),Gangavaram (0.7) and Kakinada (0.85). The KD values of Bhimili(0.11), Visakhapatnam (0.18), Gangavaram (0.33), and Kakinada(0.59), suggests that the order of adsorptive capacity of sedimentscollected from the different stations for Hg were Kakinada > Gang-avaram > Visakhapatnam > Bhimili. It is important to note that theTOC content in the studied sediments collected from different sta-tions were Kakinada > Gangavaram > Visakhapatnam > Bhimili.This finding again indicates that the adsorption affinity of Hg insediments was influenced by TOC content of the sediments.

The monolayer maximum adsorption (Qmax) values (from theLangmuir isotherm) were 0.22 mg g�1 for Bhimili, 0.40 mg g�1 for

Fig. 4. (a) Adsorption of mercury (from solution) by sediment as a function of time, (b) Isotherms of Hg adsorption in the four coastal sediments collected from the centraleast coast of India. (4) Kakinada;(.) Gangavaram; (o) Visakhapatnam, (d) Bhimili.

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Visakhapatnam, 0.11 mg g�1 for Gangavaram, and 0.26 mg g�1 forKakinada.

The ‘‘b’’ is a constant related to the binding energy and partlyreflect adsorption energy level. Positive value of b indicates thatadsorption reaction could spontaneously proceed at ordinary tem-perature. The larger the value of b is, the stronger the degree ofspontaneous reaction. The ‘‘b’’ value was 0.05 for Bhimili, 0.04 forVisakhapatnam, 0.5 for Gangavaram, and 0.29 for Kakinada. Thisagain indicates that TOC of sediment played a crucial role in con-trolling Hg distribution, speciation in the coastal sediment. How-ever, the impact of DOM (humic acids) in water column incontrolling sequestration of Hg by the studied sediments was fur-ther investigated.

Fig. 5 shows the adsorption isotherms of the costal sedimentsfor Hg in the presence of varying concentrations of humic acid(HA) (as a representative of DOM) in the overlying water columns.The data are presented as supporting documents (Table 4SD). Theassociation of Hg was found to be highest in absence of HA(0.0 ppm), followed by 0.1 mg dm�3 1.0 mg dm�3 and10.0 mg dm�3.

The maximum monolayer adsorption (Qmax) of the coastal sed-iment collected from Bhimli was found to decrease from0.22 mg g�1 (0.0 mg dm�3 of HA) to 0.11 mg g�1 (0.1 mg dm�3 ofHA) 0.02 mg g�1 (1.0 mg dm�3 of HA) to 0.02 mg g�1

(10.0 mg dm�3 of HA) (Fig. 5a).The Qmax of the coastal sediment collected from Visakhapatnam

decreased from 0.4 mg g�1 (0.0 mg dm�3 of HA) to 0.35 mg g�1

(0.1 mg dm�3 of HA) to 0.25 mg g�1 (1.0 mg dm�3 of HA) to0.12 mg g�1 (10 mg dm�3 of HA) (Fig. 5b). The Qmax of the sedimentcollected from Gangavaram was found to decrease from0.11 mg g�1 (0.0 mg dm�3 of HA) to 0.08 mg g�1 (0.1 mg dm�3 ofHA) to 0.06 mg g�1 (1 mg dm�3 of HA) to 0.05 mg g�1 (10 mg dm�3

of HA) (Fig. 5c). The Qmax of the sediment collected from Kakinadadecrease from 0.26 mg g�1 (0.0 mg dm�3 of HA) to 0.19 mg g�1

(0.1 mg dm�3 of HA) to 0.09 mg g�1 (1.0 mg dm�3 of HA) to0.08 mg g�1 (10.0 mg dm�3 of HA) (Fig. 5d). The maximum adsorp-tion capacity (Qmax) of all the coastal sediments suggests that accu-mulation of Hg in the sediments gradually decreased with theincreasing DOM concentrations in the water column.

Comparison of Qmax values revealed that the reactivity of Hgwith the sediments decreased with the increasing HA concentra-tions in the overlying water column for all the coastal sediments(Fig. 6a). The maximum amount of Hg was absorbed by the coastalsediments collected from Visakhapatnam followed by Kakinada,Bhimli and Gangavaram. The decrease in Qmax was prominentwhen the concentrations of HA (in water column) varied from0.0 to 1.0 mg dm�3. The change in Qmax was found to be insignifi-cant with increasing concentration of HA from 1.0 to10.0 mg.dm�3. This observations probably suggest that the com-plexing sites of Hg in dissolved HA in (1.0 mg dm�3) the water col-umn was sufficient enough to bind all available Hg2+ ion in thesolution and thus, the change in Qmax was found to be insignificant.

A steady decrease in adsorption affinity (KD) of the coastal sed-iments for Hg was observed with the increasing HA concentrationsin the overlying water column (Fig. 6b). The decrease in KD valueswas significant for all the studied sediments with changing HAconcentration from 0 to 0.1 to 1.0 mg dm�3. Interestingly, furtherincrease in HA concentrations (to 10.0 mg dm�3) increased theadsorption affinity of the studied sediments for Hg. This is probablybecause of the adsorption of HA on the surface of the sedimentsand by increasing the available binding sites for Hg on the costalsediments.

The intensity of adsorption (bF) was also found to decrease withthe increasing HA concentrations in the water column. The bF ofthe coastal sediment collected from Bhimli was found to decreasefrom 0.85 (0.0 mg dm�3 of HA) to 0.80 (0.1 mg dm�3 of HA) to 0.76(1.0 mg dm�3 of HA) to 0.66 (10.0 mg dm�3 of HA). The bF of thesediment collected from Visakhapatnam decrease from 0.95(0.0 mg dm�3 of HA) to 0.90 (0.1 mg dm�3 of HA) to 0.80(1.0 mg dm�3 of HA) to 0.70 (10.0 mg dm�3 of HA). The bF of thesediment collected from Gangavaram was found to decrease from0.7 (0.0 mg dm�3 of HA) to 0.8 (0.1 mg dm�3 of HA) to 0.6(1.0 mg dm�3 of HA) to 0.5 (10.0 mg dm�3 of HA).

This study suggests that DOM (in overlying water column) andTOC (within the sediments) can alter the speciation of Hg in a nat-ural system. DOM in overlying water column decrease Hg seques-tration process by sediment. However, high concentrations of DOMin overlying water column may also increase adsorption of DOM on

Fig. 5. Adsorption isotherms of Hg in the coastal sediments as a function of different concentrations of humic acid in the water column.

Fig. 6. Variation of (a) maximum monolayer adsorption (Qmax) and (b) adsorption affinity of the coastal sediments as a function of different concentrations of humic acid inthe overlying water column.

346 P. Chakraborty et al. / Marine Pollution Bulletin 79 (2014) 342–347

sediments and this may lead to increase Hg sequestration process.It is important to note that adsorption of DOM from water columnto sediment is dependent on several factors (such as, texture of

sediment, nature of DOC, pH, ionic strength etc). However, furtherdetail investigation is required to understand the interactionbetween Hg2+, DOM (in water column) and TOC of sediments.

P. Chakraborty et al. / Marine Pollution Bulletin 79 (2014) 342–347 347

Acknowledgments

Authors are thankful to the Director, NIO, Goa for his encour-agement and support. This work is a part of the Council of Scientificand Industrial Research (CSIR) supported PSC0106. S. Jayachandranand Brijmohan Sharma acknowledge the Summer Research Fellow-ship provided by the Indian Academy of Science. Kofi Marcellin Yaoacknowledges Department of Science and Technology, India forproviding CV Raman Postdoctoral Fellowship. This article bearsNIO Contribution Number 5507.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.marpolbul.2013.11.028.

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