Indian Journal of Marine Sciences Vol. 28., December 1999, pp. 365-369

A comparative study of atomic absorption spectrophotometry and anodic stripping voltammetry for the determination of trace metals Zn, Cd, Pb and Cu in coastal

waters of Visakhapatnam, east coast of India

P V S .Prabhakara Murty· & D Satyanarayana

School of Chemistry, Andhra University, Visakhapatnam 530003, AP, India

Received 19 JUli e 1998; revised 12 August 1999

Concentration levels of some of the dissolved trace metals like Zn, .Cd, Pb and Cu have been determined in the coastal waters along three transects (Gangavaram, Harbour and Rushi hill) near Visakhapatnam. Estimations. made hy anodic stripping voltammetry in the present study are in good agreement with the reported values by atomic ahsorption spectrophotometry. Vertical distribution of dissolved Zn , Cd and Cu showed an increasing trend from the surface waters to bottom. Surface depletion and bottom enrichment of these elements resembling those of the di stribution pattern of nutrients are attributed to their involvement in bio-geochemical cycles. Dissolved Pb showed enrichment in surface compared to bottom waters indicating anthropogenic input. Dissolved Zn, Cd and Cu showed signiticant positive correlations among themselves and with nutrients indicating their common association and source of occurrence. The atomic ratios of Zn 10 Si and Cd to N were found to be higher while, that of Cd to P was lower when compared with those of the o ffshore waters of the Bay of Bengal. This was attributed to the differences in the biological uptake of these metals.

Determination of trace metals in marine environment has received considerable attention in recent years. During the last two decades, sensitive and accurate methods such as atomic absorption spectrophotometry (AAS), anodic stripping voltammetry (ASV) and neutron activation analysis (NAA) have been developed for their determination in a variety of matrices. Among them, AAS and ASV methods are most commonly employed for the estimation of trace metals in seawater l

-13

_ However, sufficient information is not available on the intercomparison of these two methods. Further, the distribution of trace metals in coastal waters and the processes controlling their distribution are not well­studied. In the present work a detailed study of the distribution of elements like Zn, Cd, Pb and Cu and their speciation have been undertaken using ASV technique and the results were compared with the AAS data reported earlier l 3

. The possible interrelations of these elements with nutrients (nitrate, phosphate and silicate) have also been discussed.

Materials and Methods Seawater samples were collected with Niskin water

*Occupational Health Ser~iees & Research Center, Steel Plant, Visakhapatnam 530 031 , India ,

samplers at monthly intervals over a period of one year (September 1986 to August 1987) along three transects at Gangavaram (G), Harbour (H) and Ru shi hill (R) situated on Visakhapatnam coast (Fig. I). The horizontal distance of each station from the coast along the transects G(G I, G2 and G3), H(H I, H2 and H3) and R(R I, R2 and R3) is shown in Table I. Seawater samples were filtered through a pre­weighed acid cleaned membrane (0.45 11m pore size) to separate di ssolved and particulate fractions. Nutrients (nitrate, phosphate and si licate) were determined photometrically using standard methods 14.

An aliquot (800 ml) of seawater sample was acidified to pH 3-4, and the trace metal s (Zn, Cd , Pb and Cu) were determined by anodic stripping voltammetry in differential pulse mode. EG & G, PAR model 264A, in combination with a cell having three electrode assembly (model 303A), consisting of a hangi ng mercury drop as working electrode, Ag / AgCI as reference electrode and a Pt wire as counter electrode.

X-Y recorder RE 0089 was employed for obtaining current-voltage curves. Acetate buffer (0. 1 M, pH 4.5) was used as a supporting electrolyte. About 10 ml of seawater sample (PH 3-4) mixed with 10 ml buffer in the cell, was purged with pure nitrogen gas to expel the dissolved oxygen completely , The electrolytic deposition of metals was then carried out

366 INDIAN I. MAR. SCI., VOL., 28. DECEMBER 1999

0'

• Fig. I-Transects locat~ along Visakhapatnam coast

for a fixed tiiTIeof 6 min by holding the potential at -1.20V while stirring the solution throughout. The solution was allowed to attain equilibrium for 1 min 'while discontInuing stirring. Stripping of metals was ·then noted bY' anodic scan at 5 mV/s with a pulse height of 50m\l..l'J:1~ resulting current - voltage plot was recorded at a' current sensiti vity of 10 JlA. An

. <;tliquot ,of the solution containing a known concentration of mixture Zn, Cd, Pb and Cu was added to the' seawater samples using a micro syringe. The electrolysis and stripping were repeated under identical conditions of instrumental setting in all the cases. The peak heights corresponding to the addition were computed at the peak potentials - 0.98, - 0.63, - 0.42 and - 0.19 V corresponding to the oxidation of Zn, Cd, Pb and Cu respectively. Concentrations of these trace metals in the seawater were computed using the equation :

Cm = i, v Cs / i2 V + (h - i, ) V

where Cm = concentration of metal IS seawater, i, = original peak height, i2 = spiked peak height,

Table I-Annual average concentration levels of dissolved trace metals (lig. 1-') and nutrients (limo!. 1-') in coastal waters of

Visakhapatnam

Trace metal/Vertical

Nutrient

Zn

Cd

Pb

Cu

NO,-N

POcP

distance

(m)

o 10

20

30

50

o 10

20

30

50

o 10

20

30

50

o 10

20

30

50

o JO

20

30

50

o 10

20

30

50

o JO

20

30

50

G

( 0 - 8)

7.90

8.57

10.45

12.80

12.70

0.67

0.87

1.10

1.33

1.40

8.30

6.63

5.95

5.00

3.80

3.10

3.87

3.85

4 .35

4.30

6.70

7.43

7.60

8.55

1).40

0.68

0.86

0.91

1.16

1.50

11.60

14.00

16.50

18.60

22.1 0

Transects"

H

(0- 9)

9 .70

11.50

11.90

13.60

14.50

0.73

0.92

I. 03

1.10

1.25

7.77

6.60

6.25

5.45

5.00

3.73

4.13

4.45

4.75

5.00

8.03

8.33

8.65

9.40

10. 10

1.02

1.09

1.24

1.47

1.70

11.57

11.73

12.45

13.80

15.70

R (0 - II)

8.80

9.77

11.30

11.90

13.00

0.61

0.75

0.96

1.08

1.15

5.93

5.77

4.85

4.40

4.00

2.43

2.83

2.65

3. 15

3.00

4.27

4.97

5.60

6.90

7.90

0.36

0.52

0.56

0.74

0 .97

11.97

11.90

11.95

13.80

14.90

*Values given in parentheses are horizontal distance (km) from the coast.

v = volume of standard solution added, V = volume of seawater taken, and Cs = concentration of standard solution used for spiking. The procedure was repeated thrice for its reproducibility III each case and

)

-«

MURTY & SAY ANARA Y ANA: AAS & ASV FOR DETERMINATION OF TRACE METALS 367

calculated the percentage of deviation as ± 8, ± 6, ± 12 and ± lOin case of Zn, Cd, Pb and Cu, respectively.

Results and Discussion The annual averages of dissolved trace metals (Zn,

Cd, Pb and Cu) and nutrients (N03 - N, P04 - P and Si04 - Si) present in the seawaters of the three transects of Visakhapatnam coast are depicted in Table 1. Typical vertical profiles of these constituents at 50 m depth are shown in Fig. 2. Nutrients and trace metals (Pb and Cu) at surface waters revealed in general, a decreasing trend from coast to offshore. This can be attributed to the impact of land run off and its gradual dilution and dispersion towards offshore.

TRACE METAL CONCENTRATION ().1g.(1 )

Zn Cd Pb Cu

Zinc-The vertical distribution pattern , in general, resembles those of nutrients (N03 - N, P04 - P and Si04 - Si). It revealed an increasing trend from surface to bottom (Fig. 2), which can be attributed to its regeneration in the water column. Thi s is also evident from the significant positive correlations observed between Zn and nutrients (nitrate, phosphate and silicate) in the present study (Table 2). Surface depletion and bottom enrichment of Zn can be attributed to its uptake by organisms at surface and subsequent regeneration of it from sinking biological matter at bottom waters. Since phytoplankton is known to concentrate large amounts of Zn, it is possible that diatoms may play an important role in bio-geochemical cycling. Similar vertical di stribution of dissolved Zn was reported earlier in coastal waters

NUTRIENTS ( }lmol'.. [-I)

S i04- Si 5 10 15 0 1 2 3 6 9 0 3 6 o .......... >-r-...., r----1;)r---, ......,---,...-j::t., r--CiM-----'

o 2 10 15 20 . ' 25

1 0

2' 0

30

~O

5 0

Vl 0 llJ n::

10 I-LU

( H ) ~

20

z 30

:r: 40 I-a.. LU 50 Cl

0

1 0 ( R)

20

3 0 -

4 0

5 0

Fig. 2-Vertical profiles of trace metals and nutrients at the three transects G. H and R

. , .

368 INDIAN 1. MAR. SCI., VOL., 28, DECEMBER 1999

of Visakhapatnam7, and in the coastal and offshore

waters of Bay of Bengal" )) which was attributed to its regeneration from organisms. The atomic ratio of Zn to Si (5.16 x 10-') derived from the regression equation for the coastal waters of Visakhapatnam was found to be higher than reported value (l.40 x 10-') for the offshore waters of Bay of Bengal)) .

The concentration levels (range and average) of Zn (Table 3) are in good agreement with the earlier values obtained by AAS. Both of them indicated a relatively higher concentration of Zn at Harbour than that at Gangavaram and Rushi hill transects. This may be attributed to anthropogenic input of Zn in the Visakhapatnam harbour and its subsequent dispersion into the coastal regions particularly along Harbour transect.

Cadmium - Vertical distribution of dissolved Cd resembles that of Zn (Fig. 2), which is also evident from significant positive correlation (r = 0.84, P = 99%) obtained between them (Table 2). Vertical profiles of dissolved Cd showed relatively higher concentration in bottom when compared to surface waters (Fig. 2) similar to those of nutrients. This is also evident from the significant positive correlations observed between Cd and nutrients. This can be explained by the removal of Cd in the surface waters

on to the particulate orgal1lc tissues and its subsequent regeneration via oxidative processes. Since Cd has no physiological role, it is probably adsorbed on the surface of plankton debris or faecal pellets during its transport to bottom waters '5. Based on the regression equation, the atomic ratio of Cd to N (4.73xlO-4) obtained is higher than the reported value for offshore waters of Bay of Bengal" (2.8xlO-4). However, the ratio of Cd to P (2.73xI0-' ) is iower than the reported value (6.20x 10-3

) " . Thi s may probably be due to differences in its biological uptake and other physico-chemical processes. The Cd levels found along the three transects of Visakhapatnam coast are in good agreement with the AAS data reported earlier" (Table 3). Further, both the data indicated high concentrations at Gangavaram than other two transects.

Lead - Unlike Zn and Cd, the concentration levels of dissolved Pb showed relatively high at surface as compared to bottom waters (Table I). Concentration depth profiles (Fig. 2) also indicated similar trend suggesting its distribution controlled through atmospheric input. However, it wa~; attributed earlier7

,1J to its anthropogenic input. Sanzgiri & Braganca4 observed higher concentration of Pb at surface waters when compared to bottom waters of

Table 2-Correlation matrix among dissolved trace metals and between nutrients and dissolved trace metab in coastal waters of Visakhapatnam

Cd(nM) Pb(nM) Cu(nM) N03-N(IlM) P04- P(IlM) SiOrSi(IlM)

Zn(nM)

Cd(nM)

Pb(nM)

Cu(nM)

0.84 - 0.61 0.36- 0.45 0.55 0.43*

-0.56 0.3 1-- 0.39- 0.46 06~

0.87

Number of samples (n) analysed in each case is 33; the level of significance (P) is >99%; - P.> 95% ; -- P> 90 % ; ----- not significant.

0.90

- 0.51

0.42'

Table 3-Comparison of range and average concentration levels of dissolved trace metals (Ilg. 1- 1) with the reported data (ref. 13) in the coastal waters of V isakhapatnam

Trace Transect Remarks

metal G H R

Zn 3.50 - 18.00(9.90) 5.50 - 19.50(11.70) 4.00 - 18.00( 10.50) Present study

ND - 22.80(12.70) ND-21.90(13 .20) ND - 21.20(11.70) Reported I .'

Cd 0.25 -2.00(0.99) 0.34 -1 .95(0.95) 0.20 -1.80(0.84) Present study

0.11 -2.11(1.03) 0.11 -2.11 (0.99) 0.11 -2.32(1.01 ) Reported I,

Pb 4.00 -11.00(6.40) 5.00 -12.50(6.50) 3.80 -10.00(5.2 1) Present study

0.16 -11.20(2.97) 0.31 - 11 .20(3.00) 0.31 -7.90(2.24) Reported I )

Cu 2.00 -6.00(3.78) 2.50 -7.50(3.44) 2:10 -4.50(2.76) Present study

0.09 -6.59( 1.80) ND -8.88( 1.82) 0.18 -6.59(1.57) Reported I.'

ND = Not detected ; Values in parentheses are annual averages

)

....

MURTY & SAY ANARA Y ANA: AAS & ASV FOR DETERMINA nON OF TRACE METALS 369

Andaman Sea. Dissolved Pb showed significant negative correlations with nutrients unlike other trace metals (Table 2) indicating its non-association with them. The present data has comparatively higher values than the reported by AAS (Table 3). However, both of them showed, a similar distribution of Pb with relatively high concentration at Harbour compared to Gangavaram and Rushi hill transects, which can be attributed to its anthropogenic input.

Copper - Vertical profiles of dissolved Cu at all the three stations revealed an increasing trend with depth indicating its depletion at the surface and enrichment at the bottom waters. Its distribution in coastal waters was same as those of Zn and Cd and nutrients. This is evident from the significant positive correlations (Table 2) which further indicates their common association and source of occurrence in seawater. This can be attributed to its (i) remobilisation from shelf sediments and subsequent diffusion into the overlying waters, and (ii) release from organisms during degradation and decay. Studies3 on the distribution of trace metals in coastal and offshore waters of Bay of Bengal revealed enrichment of dissolved Cu in the bottom water layers coinciding with the minimum levels of dissolved oxygen. The enrichment of Cu in both dissolved and particulate forms at the bottom layers of water near Harbour and coastal areas of Visakhapatnam7 was attributed to its release during the decomposition of the organisms. Comparison of the concentration levels of Cu obtained in the present study broadly agrees with those values reported by AAS (Table 3). However, the average concentration of Cu obtained by the ASV method is relatively high in Gangavaram while it is almost same in Gangavaram and Harbour waters as obtained by AAS technique.

In conclusion, values of trace metal concentrations reported in the present study using ASV technique

broadly agree with those reported by AAS technique (Table 3). However, ASV technique has several advantages over AAS for trace metal analysis particularly in seawater. These include (i) simple and reliable, (ii) multi element technique, (iii) no need of pre-concentration prior to analysis and (iv) relatively free from interference.

Acknowledgement Authors wish to express their gratitude to the

Defence Research and Development Organisation, Govt. of India for sponsoring this work.

References I Sen Gupta R, Singbal S Y S & Sanzgiri S, Illdiall J Mar Sci,

7 (1978) 295.

2 Sanzgiri S & Moraes C, indiall J Mar Sci , g (1979) 254.

3 Braganca A & Sanzgiri S, indian J Mar Sci, 9 (1980) 283.

4 Sanzgiri S & Braganca A, indiall J Mar Sci, 10 ( 1981 ) 23g. 5 Rajendran A, De Sousa S N & Reddy C V G, Illdiall .I Mar

Sci, II (1982) 43 .

6 George M D, Sawkar K & Reddy C V G, Illdiall J Mar Sci. \3 (1984) 64.

7 Satyanarayana D, Rao I M & Reddy B P R, Illdiall J Mar Sci, 14 (1985) 139.

8 George M D, Indiall J Mar Sci , 15 (1986) 271.

9 Satyanarayana D, Prasada Reddy B, Dileep Kumar M & Ramesh A, in Contributions in marine sciellces, edited by T S S Rao, R Natarajan, B N Desai , C Narayanaswami & S R Bhat, (Dr S Z Qasim Sastyabdapurti Felicitations Committee, NIO, Goa) 1987,329.

10 George M D, indian J Mar Sci, 17 (1988) I I I. II Satyanarayana D, Prabhakara Murty P V S & Sarma V V,

Indian J Mar Sci, 19 (1990) 206.

12 Satyanarayana D, Prabhakara Murty P V S & Sanna V V, Mahasagar, 23 (1990) 103 .

13 Pr"bhakara Murty P V S & Satyanarayana D. Illdiall JEll\! Protec, 10 (1990) 919.

14 Grashoff K, Methods of seawater analysis, (Verlag Chemie, Weinheim) 1976, pp 307.

15 Kremling K, Pinze I, Brockel V K & Wong C S, Mar Bioi, 48 (1978) I.