Mobile and bound forms of trace metals in sediments of the lower ganges

Wat. Res. Vol. 26, No. 11, pp. 1541-1548,1992 Printed in Great Britain. All rights reserved

0043-1354/92$5.00+0.00 Copyright © 1992Pe~non Press Ltd

MOBILE AND BOUND FORMS OF TRACE METALS IN SEDIMENTS OF THE LOWER GANGES D. P. MovAt*, K. P. SINOH, H. C'nANt)P.Aand P. K. RAY Industrial Toxicology Research Centre, P.O. Box 80, M.G. Marg, Lucknow-226 001, India

(First received April 1990; accepted in revised form February 1992) Mmtract--Mobile and bound trace metals associated with sediment components (viz. exchangeable, carbonate, organic, Fe/Mn oxide and residual fractions) were determined at five locations on the River Ganges in the lower reaches. In the exchangeable phase, 5-22% of Pb, 5-14.4% of Cr, 3-16.4% of CA, 3-16% of Zn and 1-13.5% of Cu were found, and in the carbonate phase 73-87% of Zn, 38--41% of CA, 13-27% of Ni and 3-10.1% of Pb were found. The Fe/Mn oxide phase retained about 79-83% of Mn, 31)--40% of Cr and Fe, 22-25% of Cu, 14-16% of Ni and 9-11% of Pb. In the orsanic phase about 36-47% of CA, 22-28% of Cu and 10-15% of Pb were found. The order of release of metals was Cd > Cr > Pb > Cu > Zn > Ni > Mn > Fe, and the order of adsorption characteristics of most of the mobile metal fractions was Fe/Mn oxide > organic > clay. Correlations of the physico-chemical parameters with adsorption characteristics were also determined and a good correlation (r = 0.7) of cation exchange capacity with the clay fraction was found. I~o (geoaccumulation indices) of metals in the sediments were also evaluated. Results showed a considerable enrichment of trace metals in the sediment phase at almost all the sites.

Key words--trace metals, mobile metals, bound metals, Ganges sediment, geoaccumulation indices, sequential extraction, cation exchange capacity

INTRODUCTION

organic carbon, sand, silt and clay fractions, mixed oxide and inorganic carbon have been examined. Geoaccumulation indices for trace metals at five different locations have also been evaluated.

The presence of high levels of mobile trace metals such as Cd, Cu, Ni, Cr and Pb, and low levels of Fe and Mn (Modak eta/., 1989a), with correspondingly high uptake values of trace metals (at a low concentration), by humic and fulvic acids in the River Ganges water (Modal< et al., 1989b) encouraged the authors to investigate the mobile fraction of metals in the sediment, and also the bound fraction of metals held within the mineral matrix of the sediment. A large number of papers relating to chemical associ-

MAT~L,~LS AND METHODS

Sampling locations

ation of trace metals in sediments have appeared in the past (Sa]omons and De Groot, 1978; Tessier and Campbell, 1979; Salomons and Forstner, 1980; Forstner and Patchineelam, 1980), but data for mobile and bound metals in sediments of the Ganges have not yet been reported. This study on the sediments of the Ganges is of considerable interest because of the increased risk factor associated with the exposure of elevated levels of mobile metals to the aquatic biota (Ortner et al., 1983; Gibson and Farmer, 1986). This paper reports the results of eight trace metals (mobile and bound) associated with five sediment fractions (exchangeable, carbonate, organic, Fe/Mn oxide and residual fraction) at five locations on the River Ganges in the lower reaches. Correlation of mobile and bound metals with some physico-chemical properties of sediments such as cation exchange capacity, *Address all correspondence to: Dr D. P. Modak, Bose Institute, 9311 Acharya Pratulla Chandra Road, Calcutta-700 009, India.

The major industrial activities on the River Ganges are located in West Bengal from Kalyani to Uluberia, where the possibility exists of trace metal contamination in the Ganges water. Hence, four sampling stations (Kalyani, Palta, Dakshineswar and Uluberia) (Fig. 1) were selected in this industrially-rich belt. Baharampur, a point 140kin upstream of Kalyani and an industry-free zone, has also been included in this study for comparison.

Sediment sample Ten sediment samples were collected at points 5 m from both banks of the river and 1.5 m below the water surface at each site. Surface sediment was collected and transported to the Industrial Toxicology R_o~_rch Centre (ITRC), Lucknow, for analysis. The sediment utmples have been analysed for pH, texture, organic carbon, water holding capacity, mixed oxide, inorganic carbon and cation exchange capacity, pH was determined by equilibrating one part of sediment with two parts of water for 2 h. Scdin~nt texture (sand, silt and clay) was determined by the pipette method (Allan, 19$1) ming Stock's law. Organic carbon was determined by the dichromate oxidation method (Black, 1965). The water holding capacity was determined by moistening the sediment sample for 24 h in a circular Imm box with a Whatman filter paper in the bottom. The amount of water thus absorbed by the sediment wag determined and expressed as a percentage of dry sediment OPiper, 1950). For mixed oxide, samples were digested with nitric acid-parchloric acid (5:1) until a clear solution was obtained and

1541

1542

D.P. MODAKet al.

..-1 Baharampur

== Palashi

NO

NabadwlpI ( Gangs0basin 800 I I t t I

~ Bandel Triveni

o Ranaghat ! ~-2Kalyant

Samnagar Ichapur tO N

Srirampur ~,~3 Palta

H~,~

Calcutta "*'4Dakshineswar"~ BudgBudg S Uluberia

Dimond harbour Fig. 1. Map showing the sampling sites on the River Ganges. mixed oxide was precipitated from the filtrate by ammonium hydroxide at pH 7, udng methyl red as indicator. The dried reddue was e x p ~ as a percentage of dry sediment (Vogel, 1961). Inorganic carbon was determined by the method reported by Smith (1984). Index of geoaccumalation The index of geoaccumulation (Ipo) is calculated by the method of Muller (1979) using the following mathematical relation: l°g21-~D.-'-n Ipo where C, is the measured total concentration of the element n in <63 #m fraction of sediment. B, is the average (crystal) concentration of element n in shale (background) and 1.5 is the factor compemating the background data (correction factor) due to lithogenic effects (Tayler, 1964). The IFo can be dauified into seven grades (0-6). The highest grade, 6, reflects a 100-fold enrichment and 0 reflects the background concentration. Estimation o f mobUe and bound metals 10 g of air-dried sediments were processed for sequential trace metal extraction by batch operation as shown in Table I. At the end of each extraction step, samples were centrifuged for 25 rain at 4000Z and the supernatant was collected and acidified with HNO3 (pH < 2) prior to analyds. The pellets formed were further extracted for trace metais after being washed with distilled water, and no loss of metals was noted. Metal analysis Acidified extracts were analysed for CA, Cr, Cu, Fe, Mn, Pb, Ni and Zn by air--acetylene flame atomic absorption spectre*copy (AAS) (Rand and Greenberg, 1985). Total metal content was determined by nitric acid--perchloric acid digestion (5:1). Metal levels at each site showed a variation within +2%.

Quality assurance Precision and accuracy of the data were assured through repeated analysis (n = 11)of National Bureau of Standards No. 70G for Cd and 42G for the rest of the metals and the results were found to be within +2% of certified values. Recoveries of metals from the sediment samples were also studied for evaluating the matrix effect by the standard addition technique and found to be between 94 and 101%. The minimum detection limit for Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn was found to be 2, 20, 10, 20, 10, 20, 50 and 5 #g/l, respectively.

RESULTS AND DISCUSSION The IFo values for sediment from the River Ganges at five locations are shown in Fig. 2. Cd showed I~o class 4 (i.e. 16-fold enrichment compared to natural background value) at most of the sites (Baharampur, Palm, Kalyani and Uluberia) and increased to Ipo class 5 at Dakshineswar. High levels of total Cd in the sediment could not be due to industrial activities because similar levels were observed in the industryfree zone (Baharampur). This possibly reflects the weathering of pyrites upstream (Roy, 1973). Ipo values for Fe and Mn remained in class 0 (background concentration) and Pb I~o was class 1 (i.e. 2-fold enrichment above the natural background value) at all the sampling sites. Cr was found in I~o class 1 at Kalyani and Palta, and increased to class 2 at Dakshineswar and onwards. Discharge of chromium plating and tannery wastes in this region may possibly increase the Cr level in these sediments (Modak et al., 1989a). Ni showed 0 class at Baharampur and Kalyani, then increased at Palta

Metals speciationin the Ganges sediments

1543

Table i. Scheme of sequential extraction of mettls from Ganges sediment Sediment air-dried I

I

I M NH(OAC, pH 7, solid/solution ratio I: 15, shaking time 6 h and centrifuged at 4000E for 25 rain (Tessier and Campbell, 1979)

I I

Supcmatant (exchangeable fraction) I

1 M Acetate buffer, pH 5, solid/solution ratio 1:15, shaking time 6h and centrifuged (Tessier and Campbell, 1979)

I Supernatant (carbonate fraction)

I

1M NH2OH, 0.02 M HCI, shaking time 24h and centrifuged (Chao and Zhou, 1983)

I Supernatant I (Fe/Mn oxide fraction) I 30% H202, 0.01 M HNO3, 95°C, extracted by ! M NH40AC, shaking time 24 h and centrifuged (Gupta and Chen, 1975)

I I

HNO3+ HCIO4,digested, 120°C

I

Supernatant (organic fraction) ~ Residual fraction [ 1

1

to class 1 and reached class 2 at Dakshineswar and onwards. Zn was found to be in class 2 at l~harampur and Palta, and decreased to class 1 at the other three sites. Cu was found to be 0 class up to Palta and reached class 1 at Dakshineswar and Uluberia. Thus, it is evident that considerable enrichment of trace metals in the sediment occurred

6

Od .

5 c~ .

4

at most of the sites.

~,

The physico-chemical characteristics of the sediments are presented in Table 2. Silt was found to be dominant in all the samples (67.62-73,21%), followed by clay (15.84-24.3%) and sand (2,8-15.64%). High percentages of silt and clay fractions found in the sediment may be due to the continuous deposition of alluvium on the river bed from the upper Ganges. A high percentage of silt in the sediment forms a very loose fabric with a very high porosity and high permeability (data not shown), resulting in easy transportation of the sediment downstream. A linear

~

~l •

c.~ •

.

a ~ 2

?.n Zn M Ni,Or Pb ~2.n,Fo Ca,Ni,FeOaJ_n,~ Ou2.n 1 . • . . . 0 - Cr~.u Cu,~J~n C~Fe Or,Fee FeJ~n.

-

FeJ~ln,Ni

NI

Mn

Mn

I

I

I

I

Pb

I

s-1

S-2

s~

s-4

s-s

Sites Fig. 2. Geoaccumulation indices for metals in sediment of the River Ganges at five different locations.

1544

D.P. MODAl[et ai.

g Is 14

o

..: . ~ 444444+144

~

~3

• r~11 to

12odooo +t+1+1+1+t

11

g~--;~oN

c o 0

10

12

14

16

18

20

22

24

"~

Clay (%) Fig. 3. C o r r e l a t i o n b e t w e e n t h e c l a y f r a c t i o n a n d c a t i o n

°-H ~-H ~-H ~-H ~+1 ~

,~: o~ .z . e~_: "~oo~ -.

~ , ~ ~ ~ ~ ~o

'~ ° o ~ °4444+14-I-I-I ~ __~ ~ ~. ~

~,[ ii

- ~ ~, o = =4q ~4-I ~44 -44 ~44 ~ ~ ~ ~g ~ ~ oa ~

exchange capacity of the sediment of the lower Ganges. relationship of the clay fraction with the cation exchange capacity was found (Fig. 3). The X-ray analysis of the sediments collected in the delta of the River Ganges showed that ilfite was the dominant mineral followed by smectite, vermiculite, chlorite and kaolinite (Sahoo et al., 1988). Both the water holding capacity and mixed oxide (AI~O3+ Fe203) content were found to be high in all the samples, indicating the presence of appreciable amounts of hydrous oxide in the sediment. Inorganic carbon was also found at an elevated level (1.22-1.9%). The denudation of carbonate minerals (limestone, calcite etc.) from the upper Ganges catchment area may be one of the reasons for this. Organic carbon in the sediment was traced at between 0.91 and 1.9%. The sources of organic carbon possibly originated from the discharge of sewage and plant materials from surrounding catchment areas. Chemical association of eight metals in the sediment of the Ganges at five locations is illustrated in Figs 4-6. The concentrations of trace metals in different fractions were found to vary with sites. Varying contents of clay, organic matter, inorganic carbon, Fe/Mn oxyhydroxide (Table 2) and changing influx pattern of metals from the industrial belt possibly cause the varying level of metals in the sediment. Different competitive affinity of metals with sediment components may also be one of the reasons for variation in concentration of metals in different extractant-determined components. Pb and Cd have special affinity for clay mineral structure due to their ionic radii which are very similar to those of potassium metal primarily associated with clay minerals (Soong, 1974). Similar situations were also found in our Observations, in the exchangeable phase about 5-22 '/o of Pb, 5-14.4 Voof Cr, 3-16.4% of Cd, 3-16% of Zn and 1-13.5% of Cu were found. A high percentage of clay and silt present in the sediment phase may possibly act as an adsorbent, retaining metals through ion exchange and other processes

-¢ ~ o = ~ ~ ~ ~ ,~ "~ "~ ~ +~"~ ~~~~~

~ g ~ ~ "8 ~ ~ ~ ~ .~'~

'~ ~-~ ~ .~ ~.

o ~ ~,,q .~ ~ .~"~' .~' ~ ~ ~, :$ ~. -~ ~ ~

~~~~~ "~-~~+i '~ -~~-~-~~ ~~~~ -

~~~~ =l ~ ~-~~'~~a~ I "od ~ ~ o~ ~ ~ ~ ~ e~ I ~ oaoa ~.~

Metals speciation in the Ganges sediments 34%

42%

1545

Residual

43%

45% ~

! 7% 8%

4o~

14%

~'--.-.-~,,%

9%

I

Dakshineswar

3s~'..~ "~'~4%

~

Uluberia

39%

37% Cadmium

Chromium

Copper

Fig. 4. Capt/on on p. !547.

(Riemer and Toth, 1979; Nriagu, 1979; Gupta and Hai'rison, 1981) with a net result of increased levels of trace metals in the exchangeable phase. This fraction is generally considered to be one which constitutes the immediate nutrient reservoir for aquatic organisms. Interestingly, the sites at Uluberia and Dakshineswar are situated on the tidal plane, and there is a possibility of leaching of metals from the exchangeable fraction by saline water during high tide. A high level of trace metals found near the mouth of the Bay of Bengal (data not shown) supports the above facts, The presence of an appreciable amount of inorganic carbon in the sediment phase caused increased occurrence of some metals as carbonate bound. About 73-87% of Zn, 38-.41% of CA, 13-27% of Ni and 3-10% of Pb were found in this phase. This is because Zn, CA, Ni and Pb have a special affinity

with carbonate and may coprecipitate with carbonate minerals at high pH (Forstner and Wittmann, 1983). The trace metal content in the Fe/Mn oxide phase, which has been proved to be sensitive to anthropogenic inputs (Salomons and Fontner, 1980), retained about 79-83% of Mn, followed by 30-40% of Cr and Fe, 22-25% of Cu, 14-16% of Ni and 9-11% of Pb. At high pH, hydrous oxides are known to be negatively charged [R(H20), + n O H - - , R ( O H ~ - + n H=O], creating greater al~nity for cations and h i g h / ~ values (ratio of the amounts of metals in solid and liquid phases at equilibrium) of Fe, Mn, Cr and Cu to hydrous oxide (Ghosh, 1982), which may be the reason for the increased level of these metals. Significant amounts of metals such as CA (36--47%), Cu (22-28%) and Pb (10-15%) were found in the organic phase. The

1546

D.P. MODAl[et al.

40%

16% 3%

6%

~ i ~

79%

~

~

1 ~ ~ 6 %

lO~ ~

4

60%

~ 8 7 %

ls%

lo~

~

%

9

l Kalyani

Ferno~de

[ ~ O r g anle

S2% 15%

30%

43%~

~

13%

14%

[~]

ReldduaJ

.,,., e%~i:::~. 23% Dakshineswar

Iron

Manganese

Lead

Fig. 5. Caption opposite.

presence of organic carbon may possibly have as Cd > Cr > Pb > Cu > Zn > Ni > Mn > Fe increased the levels of these metals in the sediment (Figs 4--6). phase (Christensen, 1989). Recently the authors found a preference of Cd at low concentration with CONCLUSIONS oqpmic matter (Modak et a/., 1989b) in Ganges water. It is also known that organic Cu (mainly Chemical association of metals with the sed_i_m__ent Cu-humate) is soluble to a certain extent in water component is a competitive adsorption phenomenon (Salomons and Forstner, 1984). The presence of a and sequential extraction of metals with appropriate constant amount of C'u in the water phase (Modak chemical agents may yield consistent information on eta/., 19891))may be one of the reasons for the above the binding strength of the metals with the sediment observation. Most of the metals (Cr, Cu, Fe, Ni and component. The order of adsorption characteristics Pb) were found in the residual fraction, which is of most of the mobile metal fraction is Fe/Mn assumed to be held within the mineral matrix. Trace oxide > organic > clay and the order of release of elements in this form are not soluble under exper- metals is CA > Cr > Pb > Cu > Zn > Ni > Mn > Fe. imental conditions and may, therefore, be considered The geoaccumulation factor (Ipo) of most of the as bound or stationary metals. Experiments con- metals shows a considerable enrichment of trace ducted on the sequential extraction of metals from metals in sediments at almost all sites. The geoaccuthe sediment indicate the order of release of metals mulation indices obtained under this study for the (ratio of bound and mobile fraction) or mobility various metals in the lower Ganges sediments indicate

Metals speciation in the Ganges sediments

1547

t0% Baharampur

IBm"

m

01"70

10% Kalyani

15%

1m

FelMn oxide

[~

Organic

I

Carbonete

9%

Palta

14%

Dakshineswar

16%

10%

Ulubeda

Nickel

Zinc Fig. 6

Figs 4-6. The mobile and bound metal fractions in sediment collected from five locations in the lower Ganges.

a considerable input from various anthropogenic sources, Acknowledgements--We are grateful to the Ganga Project Directorate, Government of India, for financial support, and to Dr B. M. Gupta, FNA, for his valuable guidance and

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1548

D. P. MOD~d¢et al.

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