(119e) Arsenic Mobilization In Alluvial Soils of Punjab, North-West India Under Flooded Conditions

Elevated arsenic contaminant in
drinking water above the WHO (1996) safe limits of 10 μgL^-1   was reported at many locations in
underground water of Punjab, North-West India. In rural areas, flooded irrigations and maintenance of standing water in
paddy fields likely to create anoxic conditions in the soils over-lying above
the shallow aquifers. The objective of the present investigation was to study
the mechanism of arsenic mobilization in surface alluvial soils. Fifty gram of
each of 18 soil samples were transferred to 200 ml polythene bottles, and 100
ml of deionized water was added to maintained flooded conditions. These
soil-water systems were kept in BOD incubator at 25⁰C for
equilibration under anaerobic conditions of 24 and 240 hours period,
respectively. After equilibration periods, pH, EC, soluble HCO₃^‑
and Cl^- were determined for each soil-water equilibrated systems.
Supernatant aliquots were withdrawn using a syringe, centrifuged, filtered and
acidified with a drop of nitric acid prior to analysis for Fe, Mn, SO₄^2-
and PO₄^3- on ICAP-AES. Arsenic in the filtrates solution
was determined using hydride generation assembly on ICAP-AES.

Total
dissolved As in 24 hour soil-water equilibrated systems varied from 3 to 16
μgL^-1 with mean value of 9 μgL^-1 (Figure 1),
whereas its concentration range in different soil-water suspensions after 240
hour of equilibration period enhanced to 33 to 1761 μgL^-1 with
mean value of 392 μgL^-1. The median, 75th and 90th percentiles
concentrations of As were 8, 12 and 14 μgL^-1 after 24 hour and
eventually elevated to 190, 490 and 1013 μgL^-1, respectively
after 240 hours of anaerobic conditions. Arsenic concentration was enhanced
with decreasing redox potential. The results showed antagonistic relationship
of arsenic concentration with the magnitude of redox potential status of
alluvial soil-water suspensions (Figure 2a). Dissolved Fe concentration versus
redox status of soil-water systems determined after two equilibration periods
are illustrated in Figure 2b. As the pe decreased, the Fe concentration
declined in different alluvial soil water systems.

The
effect of redox potential on soluble Mn concentrations determined at different
flooding periods in alluvial soil-water suspensions is shown in Figure 2c. As
the pe decreased, the soluble Mn concentration enhanced in soil water systems.
The soluble concentrations of Mn was ranged between 0.034 to 0.407 mgL^-1
with a mean value of 0.132 mgL^-1 after 24 hour equilibration period,
but were considerably enhanced to 0.449-44.44 with mean value of 22.398 mgL^-1
after 240 hour of equilibration periods. The behavior of SO₄^2-
in different alluvial soil-water systems under different redox status is shown
in Figure 2d. With decrease in pe, SO₄^2- concentration
declined in all soil water systems. Total dissolved concentration of SO₄^2-
anion in soil-water suspensions also considerably lowered to range of
0.003-25.30 mgL^-1 with mean value of 1.95 mgL^-1 after 240
hour of equilibration periods from 4.56 to 81.6 mgL^-1 with a mean
value of 25 mgL^-1 after 24 hours of equilibration periods. Under
reducing conditions, sulfate concentrations decreased significantly.

Pourbaix
(pe-pH) diagrams for arsenic and ferromagnesian oxides (Figure 3 and 4) were
constructed by to identify the chemical species for each of these elements in
different soil-water suspensions. Aqueous arsenic species, HAsO₄^2-
and H₃AsO₃⁰ along with the possible solid
phase, orpiment (amorphous As₂S₃) as were identified in
soil-water systems (Figure 3). The mono valent arsenate oxy-anion (HAsO₄^‑)
specie was predominant in all the soil-water suspensions except soils 2, 4 and
8, where arsenite (H₃AsO₃⁰) specie was
identified after 24 hour of anaerobic condition. The predominance of neutral
arsenite (H₃AsO₃⁰) specie was observed after
240 hours of reduced incubated period in all soil water systems except soils
12, 15 and 16 in which orpiment (As₂S₃) as solid phase
was identified. The neutral arsenite (H₃AsO₃⁰)
and arsenate oxy-anions (H₂AsO₄^-) has been
found to be predominantly occurred in the alluvial aquifers of Punjab between
the pH of 7.05 to 8.25 and under reduced pe value from 0.77 to -4.49,
respectively.  Thus, it is conclusively
evident that two chemical aqueous species controlling As chemistry and its
toxicity in soils are arsenate and arsenite. Arsenate is the thermodynamically
stable form under aerobic conditions. Arsenite is the predominant species under
anaerobic conditions and it is a neutral species at natural pH and is more
soluble, mobile and phototoxic than arsenate. The orpiment (As₂S₃)
solid phase domain predominates near the bottom of pe-pH diagram and lying over
the H₂O / H₂g stability line in the Pourbaix diagram
(Figure 3). After 240 hours of equilibrium period, arsenite (H₃AsO₃⁰)
specie was predominant in all alluvial soil-water suspensions. The increase in
arsenic concentration was also primarily due to an increase in As (III) species
in all the soil-water suspensions under more reduced conditions.  In some soil water systems (soil number
12, 15 and 16) apparent precipitation of orpiment was identified on pe-pH
diagram. It is evident that total arsenic concentration continuously enhanced in
these soil-water systems with decreased magnitude of redox potential. The
simultaneously drastic decline in concentration of both Fe and SO₄^2-
in contrast to accumulation As in reduced soil-water suspensions of alluvial
Holocene validated the formation of iron sulfide mineral in reduced alluvial
soil-water systems of Punjab. Because of the differences in solubility of iron
(DG= -117.11 Kcal mole^-1) versus arsenic (DG = -382.64
Kcal mole^-1) sulfides, precipitation of iron sulfide may remove
sulfide from solution but not arsenic since the precipitation rates of iron
sulfide was very rapid compared to amorphous arsenic sulfide.

The
aqueous species of Fe²⁺, Mn²⁺ and MnHPO₄ and
solid phase of hematite (Fe₂O₃) were predominant in all
the alluvial soil-water equilibrated solutions after 24 hours within the pe and
pH ranges of -1.75 to 0.77 and 7.25-8.25, respectively (Figure 4). After 240
hours of incubation period, reduction potential
status range in soil-water suspensions declined between -4.49 to -2.74 within
pH range of 7.05 to 7.94. Aqueous species of Fe²⁺, Mn²⁺
and MnHPO₄ and solid phase of hematite [Fe₂O₃]
were predominant in soil-water suspensions of soils 1, 5, 6, 11 and 14.
Pourbaix diagram (Figure 4) for iron
illustrate that declined in iron was due to the predominantly precipitation of
iron sulfide [FeS₂] and or as vivianite [Fe₃ (PO₄)₂
8H₂O] minerals. The reduced solid phase
vivianite [Fe₃ (PO₄)₂ 8H₂O] was
found in soil 2, 3, 7 and 13, respectively. Pyrite [FeS₂] was found to
be predominant in soil 8, 9, 10, 12, 16, 17 and 18, respectively. All
the water composition falls in the stability field for aqueous Mn²⁺
ion (Figure 4) and its concentrations varied from 0.45 to 44.44 mgL^-1.
Stability diagram for manganese showed that Mn²⁺ ion is
thermodynamically more stable and predominant across a wide pe-pH range than
for Fe²⁺ ion. Consequently, the Mn²⁺ ion remain in
solution in aqueous environment in which Fe²⁺ ions is oxidized to Fe³⁺
and precipitated as Fe (OH)₂.7Cl_0.30 below pH 6.8 at pe
> -1.8 and Fe₂O₃ above pH 6.8 at pe < -1.8 and thus
eventually contributed to higher dissolved Mn²⁺ concentration under
reduced environments in soils. Iron and manganese concentration in the anoxic
soil-water systems were as high as 1.634 and 44.44 mgL^-1,
respectively. The largest contributor of ferrous to the equilibrated soil-water
interaction appears to be an iron oxide phase (probably maghemite, Gamma-Fe₃O₃,
an alteration of magnetite). These results showed that dissolve Fe
concentration were probably controlled by hematite (Fe₂O₃)
under alkaline oxic condition and by iron sulfide/pyrite (FeS) or in some soils
by vivianite [Fe₃(PO₄)₂8H₂O] under
reduced environment, where soluble phosphate concentration was high. Under
reduced condition (developed after 240 hours of flooding), soil solution Fe and
SO₄^2- are governed by newly formed precipitates of pyrite
(FeS₂).