DETERMINATION
OF Pb IN WATER AND SEDIMENT OF TWO
SALTY PONDS AND ITS RELATION WITH pH, SALINITY, AND SULPHATE
Coribell Nava1 ,
Elizabeth Gutiérrez2, Hilda Ledo Medina2
1coribell3@hotmail.com -
coribell@latinmail.com
2Departament of Chemistry,
Laboratory of Environmental Chemistry, Experimental Faculty of Science, Zulia University,Venezuela. elizabet@mail.ciens.luz.ve
ABSTRACT
The presence of contaminants, as lead, in the
environment has increased the interest for monitoring industrial influenced
areas.Water and sediment samples from two salty ponds located at the northern
of Maracaibo (Venezuela), specifically at Las Peonías site were used and
manually collected during 7 months in 1995 and 1996. The sediments samples were
taken under the salty cover. Pb concentrations in the lyophilized sediment and
water samples were determined by FAAS.Likewise, the relationship between these
concentrations and pH, salinity and sulphate concentrations was established.
Sulphate was determined by the Turbidimetric Standard Method. Mean values are:
Pond 1: Pb Concentration in sediment (mg/Kg) 46,9; Pb concentration in water (mg/L) 3,49; pH 7,41; SO4-2 (mg/L) 10.095,2; Salinity (g/L) 58,5.
Pond 2: Pb Concentration in sediment (mg/Kg) 49,5; Pb concentration in water
(mg/L) 3,79; pH 7,31; SO4-2 (mg/L) 8.220,9; Salinity
(g/L) 169,06. Significant diference was not found between both ponds, showing
that the metal comes from the same source. Sediment of Pond 1 has correlation
with sulphate concentration and negative with pH.On the other hand, Pb
concentration had negative correlation with salinity in water. In pond 2, both
water and sediment showed positive correlation with sulphate concentration,
while the sediment had negative correlation with pH. In Pond 1 the negative correlation
with salinity could sugest that Pb does not
react directly with chloride, instead induces changes originating its
precipitation as agregates with sediment.This can be due to high salinity
levels which produce enormous amounts of
suspended solids which can cause metal trapping.
INTRODUCTION
Salty system are aquatic environments widely spread
in all continents and in terms of the total biosphere water they are an only a
little less important than fresh water
systems. It is know that inland bodies of water represent 0.008% of the total earth surface (aprox. 104 Km2) (Vallentyne, 1972). There
are many inland bodies of water in South America (Wiliams,1981) being by
definition those with a concentration of total dissolved solids higher than 3%.
This type of ecosystems exist in several counties in Zulia State. Among them is
Las Peonías Lagoon (Maracaibo County) which have been exploited
rudimentarily with comercial purposes (animal and human use).
DESCRIPCION OF
THE AREA
The Lagoon is fed with water from the Maracaibo Lake through Araguato
Creek. The petrochemical plant El Tablazo is located at aprox. 26 Km. from the
Lagoon. High lead concentrations have been found in water and sediment of the
Maracaibo Lake in that area. South of Las Peonías Lagoon are the salines of
comercial use which share freatic floor with the Lagoon. Due to the extraction
of salt for animal and human consumption in the region it is important to
determine the quality of the product in terms of heavy metal concentrations.
METHODOLOGY
Collection of
samples
Sediment and water samples were taken by hand
(manually) in triplicate. Sediment sample were extracted from the superficial
cover (aprox. 5 cm of thickness) from
both ponds. This cover is in contact with the salt and represent the
bioavailable area of the pond. Samples
were stored in 250 ml plastic bottles and kept at -5 ° C until analysed.
Samples were collect from December 1995 to June 1996.
Determination
of pH, sulphate and salinity
pH values were measured just after collection
of samples with a field pH-meter.
Sulphate concentration was determined by means of the Standard Turbidimetric
Method (Standard Methos, 1989).
Determination
of Pb in sediment and water
Samples of sediment were lyophillized for 6 hours
and 0.1 gr. were digested in a high
pressure reactor (Parr type) with 3 mL of HNO3 concentrate Analytical Grade
and 5 mL of deionized water. The vessel was heated at 110 ° C in a oven for 6
hours and when cooled, the sample was diluted with 25 ml of deionized water,
filtered and analyzed by FAAS using air/acetilene flame by means of a Perkin
Elmer Model 3000 Atomic Absorption Spectrometer. Samples the water were
analyzed directly, without pretreatment, by FAAS.
RESULT AND
DISCUSION
We found that the lead concentrations in water are
higher than the reported values for sea water of coastal superficial zones
(Förstner and Witman, 1981). Likewise, both ponds presented lead concentration
higher in sediments than reported values for salty systems (Lyons et. Al., 1990). It has also being reported lead
concentration in water of 1 x 10-4 ppm of Pb for comercial salines.
Higher concentrations belong to saturated salines during the comercial process
of exploitation (Javor,1989). Lead concentration in water samples from both
ponds is 300 times higher than the reported value. This could be atributed to
evaporation which causes concentration of Pb coming from the Lake. There have
been reported values as high as 93.10 ppm in the narrower area
(Estrecho) of the Lake (Barco,1989). There is also an additional contribution
to the system caused by rain. Atmosferic lead concentration around the
northeast region of Maracaibo has a reported mean value of 0.247 mg/m3 which could mean antropogenic
enrichment. Sulphate concentration is 9.785 mg /m3 wich is
higer as compared with the estimated marine aerosol (Barrios,1993). Therefore,
atmosferic particulate could be an important
factor causing the presence of Pb and sulfate since high temperatures
cause that emissions from the earth crust and superficial recirculation be
higher due to dryness of the soil and currents of air. A Variance Analysis of
the data does not indicate significant diferences (P< 0.05) for lead
concentration in sediment samples for both ponds. Table 2 shows correlation
between lead concentration in sediment and sulphate in water while there was no
correlation with salinity. This last factor could suggest that lead does not
react directly with chloride.
Tabla 1
|
Pond 1 |
||||||
|
Indep. Var. |
R |
R2 |
F-calc. |
Probability |
Estimated Std. Err. |
Model |
|
Water |
N.S |
- |
- |
- |
- |
- |
|
Salinity |
N.S |
- |
- |
- |
- |
- |
|
SO4 |
0.49 |
24.24 |
6.07 |
> 0.05 |
0.69 |
Multiplicative |
|
PH |
-0.58 |
34.24 |
9.89 |
> 0.01 |
0.11 |
Lineal |
This relation could be associated with two
tendencies a) increasing salinity may increase also suspended solids which trap
a big part of the metal and b) increasing salinity causes an increase the ratio
of other ions, as sulphate. Pond 1 showed a sulphate concentration of 10,095.2
mg/L ( 104.5 mM) while in Pond 2
8,220.92 (85.09mM) was found. These values are higher that the reported
values for salty comercial system (Barrios, 1993). Correlation between lead
concentration in sediment and pH (7.41) was found. It is known that in alkaline
conditions and in presence of sulfate, lead forms metallic salt which can float
as small particles and get attached to the sediment (Becker,1983) as hydroxides
with more alcaline pH (Föstner and Prosi,1979). Sulphate concentration showed
correlation in water and sediment in Pond 2 as ilustred in Table 3.
Tabla 2
|
Pond 2 |
||||||
|
Indep.Var. |
R |
R2 |
F-calc. |
Probability |
Estimated Std. Err. |
Model |
|
Water |
-0.47 |
22.68 |
5.57 |
> 0.05 |
0.13 |
Lineal |
|
Salinity |
N.S |
- |
- |
- |
- |
- |
|
SO4 |
0.46 |
21.53 |
5.21 |
> 0.05 |
0.62 |
Exponential |
|
pH |
-0.45 |
20.93 |
5.02 |
> 0.05 |
0.13 |
Lineal |
The data could indicate that lead precipitates as
sulphate in the sediment or possibly as sulphur.This can indicate that
increasing their concentration would lead to a higher formation of lead sulphate or sulphur. Other authors have found
the same behavior (Lyons et.al., 1990). There was no correlation between lead
concentration in water and sediment with salinity in Pond 2. This can be due to
the difference in salinity in both ponds (58.5 and 169.048). Lead concentration
in sediment of Pond 2 showed correlation with pH. Opposite than Pond 1 it was
found correlation for water and sediment in Pond 2 with lead concentration
possibly because its higher salinity causes trapping of the metal by the
floccules formed by suspended solids.
Since lead concentrations in both ponds are similar it is suggested that the
physical-chemical relations are determined by differences in salinity and
sulphate concentrations. These two
parameters possibly play an important role in the metal distribution in the
sediment-water phase and apparently have relationship with suspended solids.
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