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Inter-relationship between dissolved lead and sediment lead in the backdrop of climate change induced acidification

Authors:
  • Techno India University, West Bengal

Abstract and Figures

The surface water pH, dissolved lead and biologically available lead in the surface sediment were monitored for 30 years in three different stations of coastal West Bengal namely Shankarpur, Kakdwip and Ajmalmari. The gradual decrease of surface water pH played a significant role in the process of compartmentation of heavy metals in the coastal ecosystem. It is observed that lowering of pH triggered the process of transference of heavy metals from the sediment to the overlying aqueous system. The correlation coefficient values (for Shankarpur, dissolved Pb × sediment Pb =-0.888, p < 0.01; for Kakdwip, dissolved Pb × sediment Pb =-0.817, p < 0.01 and for Ajmalmari, dissolved Pb × sediment Pb =-0.8810, p < 0.01, respectively), support our findings. The role of climate change induced acidification is confirmed through this study.
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CHAPTER 14
Inter-relationship between dissolved lead and sediment lead in the backdrop of climate
change induced acidification
Arpita Saha1 and Abhijit Mitra2
1 Department of Oceanography, Techno India University, West Bengal, Salt Lake Sector V,
Kolkata-700091, India
2Department of Marine Science, University of Calcutta, 35, Ballygunge Circular Road, Kolkata
700 019, India
Abstract
The surface water pH, dissolved lead and biologically available lead in the surface sediment
were monitored for 30 years in three different stations of coastal West Bengal namely
Shankarpur, Kakdwip and Ajmalmari. The gradual decrease of surface water pH played a
significant role in the process of compartmentation of heavy metals in the coastal ecosystem. It is
observed that lowering of pH triggered the process of transference of heavy metals from the
sediment to the overlying aqueous system. The correlation coefficient values (for Shankarpur,
dissolved Pb × sediment Pb = -0.888, p < 0.01; for Kakdwip, dissolved Pb × sediment Pb = -
0.817, p < 0.01 and for Ajmalmari, dissolved Pb × sediment Pb = -0.8810, p < 0.01,
respectively), support our findings. The role of climate change induced acidification is confirmed
through this study.
Keywords: Acidification, water pH, dissolved heavy metals, biologically available heavy metals
1. Introduction
Ocean acidification has become an important topic of research in recent times. The change in pH
of the aquatic phase is a reflection of the rate of acidification. The pH of sea water, which is
about 8.20, is largely a function of the dissociation of the dissolved inorganic carbon (DIC),
whose relative proportions by mass are ~0.5% aqueous carbon dioxide (CO2), ~89% bicarbonate
(HCO3-) and ~11% carbonate (CO3- 2). In the ocean water, these components together form
~2200µmol/kg. Acidification of seawater is caused when the rate of increase in carbon dioxide
exceeds the rate at which weathering, transport and mixing processes can deliver HCO3- and
CO3-2 to buffer against the increased carbonic acid (H2CO3). The process of acidification largely
regulates the concentration of conservative pollutants (primarily heavy metals) in the water and
sediments. It has been documented by several researchers that aquatic pH controls the process of
dissolution / precipitation and there by regulates the level of heavy metals in the aquatic phase
and the underlying sediment compartments (Lakshmanan and Nambisan 1983; Ramamoorthy
1988; Chakraborty et al. 2009; Mitra et al. 2011; Mitra et al. 2012). The present paper is an
attempt to find the role of acidification in three stations along the coastal regions of Indian
Sundarbans on the concentration in relation to lead (Pb) in the ambient media. We have taken up
this study in the backdrop of gradual increase of carbon dioxide in the Indian atmosphere due to
global warming and subsequent decrease of pH in the coastal regions of the Bay of Bengal
coastline.
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2. Materials and Methods
2.1. Study Area
The sampling stations of Shankarpur (21.6491˚ N, 87.5699 ̊ E), Kakdwip (21.8760˚ N, 88.1852 ̊
E) and Ajmalmari (21°49'42.9''N, 88°37'13.7''E) were selected for the present study.
2.2. Measurement of aquatic pH
pH of the surface water in the selected sampling stations was measured during high tide
condition with a portable pH meter (sensitivity = ±0.02).
2.3. Analysis of dissolved Pb
Surface water samples were collected using 10-l Teflon-lined Go-Flo bottles, fitted with Teflon
taps and deployed on a rosette or on Kevlar line, with additional surface sampling carried out by
hand. Shortly after collection, samples were filtered through Nuclepore filters (0.4 µm pore
diameter) and aliquots of the filters were acidified with sub-boiling distilled nitric acid to a pH of
about 2 and stored in cleaned low-density polyethylene bottles. Dissolved heavy metals were
separated and pre-concentrated from the seawater using dithiocarbamate complexation and
subsequent extraction into Freon TF, followed by back extraction into HNO3as per the procedure
of Danielsson et al (1978). Extracts were analyzed for Pb by Atomic Absorption
Spectrophotometer (Perkin Elmer: Model 3030). The accuracy of the dissolved heavy metal
determination is indicated by good agreement between our values and that reported for certified
reference seawater materials (CASS 2) (Table 1).
Table 1: Analysis of reference material for near shore seawater (CASS 2)
Element
Certified value (µg l-1)
Laboratory results (µg l-1)
Pb
0.019 ± 0.006
0.029 ± 0.009
2.4. Analysis of biologically available Pb
Sediment samples from surface (1 cm depth) were collected by scrapping using a pre-cleaned
and acid washed plastic scale and immediately kept in clean polythene bags, which were sealed.
The samples were washed with metal free double distilled water and dried in an oven at 105oC
for 5 6 hours, freed from visible shells or shell fragments, ground to powder in a mortar and
stored in acid washed polythene bags. Analyses of biologically available metals were done after
re-drying the samples, from which 1 gm was taken and digested with 0.5 (N) HClas per the
standard procedure outlined by Malo (1977). The resulting solutions were then stored in
polythene containers for analysis. The solutions were finally aspirated in the flame Atomic
Absorption Spectrophotometer (Perkin Elmer: Model 3030) for the determination of metal
concentrations. No detectable trace metals were found in the reagent blank. Analysis of the NIES
Sargasso sample was carried out to assure the quality of the data (Table 2).
Table 2: Analysis of reference material (NIES Sargasso sample) for sediments obtained
from the National Institute of Environmental Studies, Japan
Element
Certified value (µg g-1)
Laboratory results (µg g-1)
Pb
2.4
2.9
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2.5. Statistical Analysis
Inter-relationships between aquatic pH, selected dissolved heavy metal and biologically available
heavy metal in sediment were determined through correlation coefficient values, for all possible
combinations. All statistical calculations were performed with SPSS 9.0 for Windows.
3. Results
3.1. Surface water pH
The surface water pH exhibited variation within a small range. At Shankarpur highest value was
recorded during 2009 (8.34) and lowest during 2019 (8.29). Kakdwip recorded highest pH value
in 1991 (8.30) and lowest in 2019 (8.08) and for Ajmalmari highest was in 2001(8.32) and
lowest in 2019 (8.25) (Fig 1). The gradual lowering of pH (1.3×10-3/yr in Shankarpur; 7.0×10-
3/yr in Kakdwip; 2.0×10-3/yr in Ajmalmari) clearly confirms the phenomenon of acidification of
coastal water in the study area.
3.2. Dissolved metal
The order of dissolved lead depicted a gradual increase in all the sampling sites. For Shankarpur,
highest value of 117.55 ppb was found in 2019, followed by Kakdwip (48.37ppb) and Ajmalmari
(45.15). During the initial study period in 1989, the value of dissolved Pb ranged from 8.59 ppb
(in Ajmalmari) to 10.69 ppb in (Kakdwip). The lowest value for Shankarpur was 17.85 ppb
(1990).
3.3. Sediment metal
In sediment compartment, the temporal variation of biologically available lead exhibited a
decreasing trend. Both Kakdwip and Ajmalmari recorded lowest values of 4.88 ppm during 2019
and highest values of 32.49 ppm during 1990. The lowest value for Shankarpur was 5.75 ppm
during 2019. It is also noted that the order of biologically available lead in sediment is
Shankarpur> Kakdwip > Ajmalmari (Table 3).
Table 3: Inter-relationships between pH, dissolved heavy metals and biologically available
heavy metals in sediment
Combination
p value
Shankarpur
Kakdwip
Ajmalmari
pH × sediment Pb
0.61344
0.89555
0.64383
<0.01
pH × dissolved Pb
-0.72473
-0.89162
-0.77475
<0.01
4. Discussion
The increase of atmospheric carbon dioxide in West Bengal, a maritime state in northeast coast
of India has touched almost 51% since 1980 (Ghoshal and Bhattacharyya 2008). The gradual
increase of carbon dioxide coupled with unplanned expansion of shrimp culture, urbanization
and industrialization has lowered the aquatic pH considerably (Chakraborty et al. 2013). The
process has significantly increased the rate of dissolution of precipitated heavy metals from the
sediment compartment to the water column and altered the speciation of heavy metals including
lead, in the study areas. The phenomenon of chemical speciation is governed by a number of
factors like distribution, mobility, biological availability of chemical elements (it’s chemical or
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physical association), pH, redox potential and availability of reactive species such as complexing
ligands (organic and inorganic), particle surface for adsorption and colloidal matter. In the
present study, such in-depth effort has not been attempted, but significant negative correlations
between aquatic pH and dissolved heavy metals and significant positive correlations between
aquatic pH and biologically available heavy metals in sediment (Table 3) confirm the role of pH
as one of the key factors influencing chemical speciation of the heavy metals in the present
geographical locale. The correlation values explain the significant negative and positive
relationships of aquatic pH with dissolved and biologically available heavy metals in surface
sediments respectively. The present study is in accordance with the earlier works in this area
(Chakraborty et al. 2009), which was conducted in short term temporal scale. A consequence of
ocean acidification is a decreased concentration of OH- and CO3-2. These anions form strong
complexes in ocean water with divalent and trivalent metals (Millero et al. 2009) and such
reduction is expected to change the speciation of numerous metal ions in seawater (Byrne 2002).
To our knowledge, this is the first study to evaluate the impact of acidification on the heavy
metal level in the ambient media of Bay of Bengal coastal water, which confirms the regulatory
role of gradual acidification of Pb concentrations in the aquatic phase and underlying sediment
compartment of the present study area. The present study suggests the inclusion of acidification
phenomenon of coastal water while formulating management action plan to reduce heavy metal
pollution in the north east coast of West Bengal, India.
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