Journal of Oceanography, Vol. 61, pp. 591 to 601, 2005
Influence of Salinity on pH and Aluminum Concentration on the Interaction of Acidic Red Soil with Seawater
M OHAMED M. K OMBO, SAID A. VUAI, MAKI ISHIKI and AKIRA TOKUYAMA *
Graduate School of Engineering and Science, University of the Ryukyus,
Senbaru, Nishihara-cho, Okinawa 903-0213, Japan
(Received 3 October 2003; in revised form 4 October 2004; accepted 4 October 2004)
Contamination of acidic red soil in the coastal areas of Okinawa Islands is a serious
environmental problem. This study was conducted to examine the effects of the salinity on pH and aluminum concentration when the acidic red soil interacts with seawater.
Acidic red soil from Gushikawa recreation center was fractionated into bulk soil,
coarse sand and silt + clay. Different weights of each fraction were equilibrated with
seawater solutions. The pH and concentrations of Al 3+, Na +, K +, Ca 2+ and Mg2+ were
then analyzed in the extracts. The results showed a decreasing trend of pH with increasing soil to solution ratio while the extracted Al3+ revealed an increasing trend.
The lowest pH values were 3.85, 4.06, 4.41, 4.66 and their corresponding highest Al3+
concentrations were 2.50, 1.01, 0.062 and 0.036 mmolL–1 in the seawater extracts,
one-tenth seawater extracts, one-hundredth seawater extracts and one-thousandth
seawater solution extracts, respectively. Mostly, the concentrations of Na+, Ca2+, Mg2+
and especially K+ decreased with increasing soil weight in the high salinities but showed
the opposite trend in the low salinity samples. Potassium concentration decreased by
39%, 53% and 40% in the seawater extracts, one-tenth and one-hundredth seawater
extracts but increased by 200% in one-thousandth seawater extracts. The coincidence
of the increase in Al3+ and H+ concentrations, and the decrease of Na+, K +, Ca 2+ and
Mg 2+ concentrations in the solutions suggests ion exchange/adsorption, while the increased patterns, particularly at low salinity could be attributed to the dissolution of
the species from the soils.
Keywords:
⋅ Aluminum,
⋅ cation exchange,
⋅ pH,
⋅ red soil,
⋅ salinity,
⋅ soil acidity.
which tends toward soil inflow to aquatic systems (Omija,
2000).
The major mineral compositions of the red soils observed on a subtropical Island at Okinawa are mainly
vermiculite, intergraded minerals, vermiculite/illite mixed
layer, illite, kaolinite, and halloysite (Nurcholis et al.,
1998). The dominant mineral compositions demonstrated
in the Gushikawa recreation center (GRC) red soil are
vermiculite-chlorite and kaolinite in the clay, quartz and
muscovite in the silt and quartz in the fine sand (Vuai et
al., 2003). Other minor mineral compositions in the soil
include vermiculite/illite, goethite, feldspar, vermiculite
and kaolinite. Tokashiki (1993) found that red soil, which
is locally called Kunigami Mahji, covers about 55% of
total area of Okinawa Islands (2250 km2) and is characterized by low percentage base saturation, dominated by
exchangeable aluminum. The red soil has low effective
cation exchange capacity (<3 cmolkg–1) where Al is dominant (>50%) (Vuai, 2001). Using X-ray fluorescence,
Yonaha (2002) examined the chemical compositions of
1. Introduction
Red soil, which causes what is now termed “red soil
pollution”, is a predominant feature of Okinawa Islands
(Onaga, 1992). The soil contamination in coastal areas
of the islands is a serious environmental problem that has
attracted the attention of many researchers. The red soil
erosion, which has accompanied development projects in
Okinawa has polluted waterways and beautiful coral waters, impacting on the ocean ecology and is an adverse
reflection on ways of interaction with the natural environment (Ota, 1994). A survey on accumulation of reddish soil in estuaries at the northern part of Okinawa Island revealed very noticeable reddish soil pollution
(Nishihira, 1987). The reddish soil pollution is often triggered by land development activities and other artificial
factors, combined with the natural condition of Okinawa,
* Corresponding author. E-mail: [email protected]
Copyright © The Oceanographic Society of Japan.
591
our soil sample from Gushikawa recreation center (GRC)
and found that the bulk soil is composed of 75.9%, 10.7%,
5.56% and 1.24% for the SiO2, Al 2O3, Fe2O 3 and base
cations, respectively. For the other fractions he found SiO2
(77.1, 64.9%), Al2O3 (10.1, 15.3%), Fe2O 3 (5.74, 8.17%)
and base cations (1.20, 1.83%) for coarse sand and silt +
clay, respectively.
Aqueous-phase, adsorbed and solid-phase forms of
Al(III) are of critical importance in acidic soils (Sposito,
1989). Aluminum is a major problem in many acidic soils
and the toxicity of Al3+ has been noted in several regions
of the eastern United States, Canada and in the tropics
where acidic soils are found (Stevenson and Cole, 1999).
Chemically active or labile soil Al can have a variety of
forms, controlled primarily by the pH and mineralogical
composition of the system (Bertsch and Bloom, 1996).
These authors further pointed out that Al could bind to a
negatively charged clay surface by electrostatic forces,
and is freely exchanged by other cations such as Ca 2+,
Mg2+ or K +.
When the acidic red soil encounters seawater solution the cation exchange reactions are favored by the high
ionic strength of seawater. Consequently, cations held by
negatively charged sites on colloidal particles of both clay
minerals (inorganic surface) and organic surface could
be displaced by the cations in the seawater solutions. In
acid soils with appreciable amounts of exchangeable
monomeric Al3+, the displacement of the ion and its hydrolysis are probably responsible for pH lowering
(Thomas and Hargrove, 1984). The pH is an important
variable influencing the ion exchange and/or adsorption
upon the interaction of the soil and solution. Through the
surface electrochemical properties, the soil minerals control the adsorption and release behavior of chemical constituents to water or soil solution (Evangelou, 1998). The
release of Al is greatly influenced by the technique, type
and concentration of the extracting agent and pH (Jardine
and Zelazny, 1996). This study was conducted in the presence of multiple chemical species that are available in
seawater at different ionic strengths to reflect the environment in the estuaries, whereas the rivers discharge
most of the terrigenous suspended matter ultimately to
the marine ecosystems. The purpose of the study was to
investigate factors, processes and mechanisms control-
ling the release of Al as well as to quantify the magnitude
of decrease in pH accompanied with the Al3+ when the
red soil interacts with seawater at different salinities.
2. Materials and Methods
2.1 Study site
The study was conducted in Okinawa Island (Fig.
1), which has an area of 1250 km2. The Island is located
in the southwest part of Japan with its central point at
26°N and 128°E. The Island has a sub-tropical climate
and is inhabited by 1.35 million people (Okinawa prefecture, 2004). The annual average temperature and annual
average rainfall are 22.3°C and 2155 mm, respectively.
Surface soil (0–15 cm depth) was sampled from
Gushikawa recreation center (GRC) in the central part of
Okinawa Island. The seawater sample was collected at
the Zampa cape located on the western side of the Island.
2.2 Sample treatments and analysis
The soil sample was air dried at room temperature.
It was then lightly crushed and sieved through a 2-mm
sieve and fractionated into bulk soil (<2 mm), coarse sand
128 00
The Motobu
Peninsula
Nago city
26 30
The Zampa Cape
GRC
Naha city
e
0
10
20km
n
ya
eK
p
Ca
Th
Bulk soil
Coarse sand
Silt + clay
592
M. M. Kombo et al.
pH
4.65
4.74
4.61
Kunigami mahji
(Red soil)
Chuseki so
Shimajiri mahji
Jahgalu
Fig. 1. Map of Okinawa Island showing the sampling points.
Table 1. Soil pH, pH and salinity (S) of seawater samples.
Soil fractions
The Hedo
Cape
Seawater solutions
Seawater (SW)
10 times diluted SW
100 times diluted SW
1000 times diluted SW
pH
S (g kg – 1 )
8.21
7.33
6.46
5.92
34.4
3.47
0.347
0.035
(2–0.2 mm) and silt + clay (<63 µm). The soil pH was
measured in a suspension of the soil in distilled water at
a ratio of 1:2.5 (w/v) using TOA glass electrode pH meter model HM-21P. Salinity of the seawater was determined immediately after sampling using an inductively
coupled salinometer, model 601 MK III. The seawater
sample was diluted 10, 100, and 1000 times using milliQ water. The different weights of the soil (1, 3, 5, 10, 15,
20, 30 and 40 g) were mixed with 100 mL of seawater,
one-tenth seawater, one-hundredth seawater and one-thousandth seawater solutions. The mixtures were shaken for
4 hours using an end-over-end Miyamoto shaker at a constant speed of 163 rpm and a temperature of 25°C. After
shaking, the samples were centrifuged for 20 minutes.
Immediately after centrifuging, the pH of the supernatant
was measured for each sample using a glass electrode pH
meter. The supernatants were then filtered through a 0.45
µm membrane filter. The solutions were analyzed for dissolved monomeric Al3+, Na+, K+, Mg2+ and Ca2+. A JarrellAsh flame emission spectrophotometer model AA-782
was used to measure the concentrations of Na+ and K+.
The concentrations of Ca2+ and Mg2+ were determined
using a Jarrell-Ash atomic absorption spectrophotometer.
The total dissolved Al3+ was analyzed by the Tiron method
(Yotsuyanagi et al., 1967) using a Hitachi model U-1500
spectrophotometer. During the analysis of aluminum, a
known concentration of synthetic seawater was added in
the standards to maintain similar matrix effect in the samples and the standards.
seawater to 4.06 in the soil extracts. Initially, the pH in
the one-tenth seawater solution was 7.33 but dropped
abruptly to about 5 in all fractions after mixing with a
gram of the soil (Figs. 2(a)–(c)). It further decreased
gradually with increasing soil to solution ratio to the mini-
3. Results
3.1 Soil pH, salinity and pH of the seawater solutions
The results obtained after analyzing the soil pH, the
salinity and the pH of the seawater solutions are shown
in Table 1. The soil pH revealed a trend of silt + clay <
bulk soil <coarse sand. The bulk soil pH 4.65 indicated
that the soil sample was acidic. The pH and salinity of
the seawater were 8.21 and 34.4 g kg–1, respectively.
3.2 Influence of salinity on pH of the soil-seawater solution extracts
At low soil to solution ratios (<5 g/100 mL), the pH
values of seawater-soil extracts were the highest of all
other diluted solution-soil extracts (Table 2). However,
with increasing soil weight the minimum pH values were
found in the seawater-soil extracts compared to the lower
salinities (Figs. 2(a)–(c)). The pH in the seawater extracts
ranged from 8.21 to 3.85 in 40 g/100 mL solution in silt +
clay (Fig. 2(c)). The minimum pH values in other soil
fractions were 3.87 and 3.89 in bulk soil and coarse sand,
respectively (Figs. 2(a) and (b)). Kombo et al. (2003)
demonstrated similar results whereas the pH of red soil
from Okinawa royal golf club decreased from 8.13 in
Fig. 2. Changes of pH with soil weight at different salinities of
seawater solutions in bulk soil (a), coarse sand (b) and silt
+ clay (c).
Influence of Salinity on pH and Aluminum
593
Table 2. pH values and concentrations of cations in seawater and diluted the seawater extracts (meqL–1).
Soil weight
(g/100 mL)
Seawater
+
+
One-tenth diluted seawater
2+
pH
Na
K+
Mg 2+
Ca2+
Al3+
*ND
0.544
0.629
0.812
2.14
2.77
3.85
5.12
6.60
7.33
4.99
4.53
4.46
4.34
4.28
4.25
4.16
4.14
46.0
45.4
45.6
44.2
44.0
44.2
44.1
43.2
42.7
0.975
0.886
0.856
0.805
0.710
0.629
0.605
0.507
0.459
10.9
10.6
10.5
10.3
10.1
10.0
10.0
9.74
9.54
1.94
2.03
2.06
2.04
2.05
2.18
2.25
2.28
2.31
*ND
0.506
0.623
0.855
0.920
1.18
1.27
1.54
1.74
17.0
16.6
15.8
16.0
16.2
15.8
15.9
15.4
0.56
0.59
0.60
1.91
3.08
3.98
5.61
6.59
4.86
4.45
4.35
4.21
4.17
4.15
4.11
4.06
44.8
45.0
44.5
44.3
44.2
44.0
43.5
42.7
0.928
0.844
0.826
0.710
0.650
0.600
0.516
0.488
9.95
10.1
9.98
9.76
9.59
9.54
9.33
9.08
1.78
1.81
1.81
1.92
1.96
2.13
2.23
2.34
0.556
0.715
0.821
1.01
1.13
1.20
1.30
1.45
16.1
16.3
16.5
16.7
17.5
16.7
15.0
16.1
1.11
1.12
1.34
1.74
3.15
4.70
5.22
7.51
4.88
4.45
4.32
4.24
4.21
4.14
4.13
4.10
45.1
44.7
43.6
43.8
43.7
43.6
43.4
42.5
0.848
0.813
0.759
0.679
0.622
0.566
0.503
0.459
10.4
10.5
10.2
9.98
9.90
9.36
9.24
8.87
1.77
1.79
1.87
1.89
1.95
2.08
2.17
2.26
0.718
1.05
1.23
1.65
1.99
2.24
2.60
2.84
Ca
2+
pH
Na
K
Mg
Bulk soil
0
1
3
5
10
15
20
30
40
8.21
6.86
5.88
4.74
4.28
4.11
4.05
3.93
3.87
460
463
470
460
459
457
457
456
455
9.79
10.1
10.4
10.1
8.63
8.35
7.82
7.52
6.88
109
108
106
106
106
108
104
103
102
19.5
17.5
16.9
18.7
18.0
18.9
17.6
17.8
17.5
Coarse sand
1
3
5
10
15
20
30
40
6.87
5.94
4.83
4.29
4.14
4.07
3.96
3.89
466
467
472
460
458
458
457
455
10.1
9.17
8.87
8.81
7.97
7.56
7.65
7.14
105
104
104
102
101
101
103
99.5
Silt + clay
1
3
5
10
15
20
30
40
7.37
6.25
4.94
4.30
4.11
4.03
3.92
3.85
456
456
453
452
450
450
453
451
10.1
9.71
9.03
8.80
8.40
7.73
5.94
6.00
102
103
102
102
104
103
98.3
97.0
Al
mum values of 4.14, 4.06 and 4.10 for bulk soil, coarse
sand and silt + clay, respectively (Table 2). In the onehundredth seawater solution extracts, the pH in the sample without soil was 6.46, dropping to ~5 at a ratio of 1 g
of soil per 100 mL solution in almost all the soil fractions
(Figs. 2(a)–(c)). It gradually decreased further as the soil
weight increased, reaching to minimum values of 4.42,
4.51 and 4.41 for bulk soil, coarse sand and silt + clay,
respectively (Table 2). The pH in the one-thousandth
seawater extracts ranged from 5.92 in the sample without
soil to 4.71, 4.73 and 4.66 in bulk soil, coarse sand and
silt + clay, respectively (Figs. 2(a)–(c)). The trend of the
pH in all the extracts was mostly silt + clay < bulk soil <
coarse sand, reflecting the original soil pH.
3.3 Influence of salinity on aluminum in the soil-seawater
solution extracts
The Al concentration was below detection level in
the seawater sample at the pH value of 8.21, but Boyd
(2000) reported 0.01 mgL–1 as an average concentration
594
M. M. Kombo et al.
3+
+
of Al in seawater. The concentrations of aluminum in the
extracts were significantly different among the salinities,
increasing with increasing salinity, soil to solution ratio
and decreasing the pH (Table 2). In the seawater-soil extracts the highest Al3+ concentration was 2.50 mmolL–1
in 40 g of soil/100 mL of the silt + clay (Fig. 3(c)). For
the other fractions the maximum concentrations were 2.28
and 2.20 mmolL–1 for bulk soil and coarse sand, respectively (Figs. 3(a) and (b)). Kombo et al. (2003) reported
similar observation, whereas Al3+ concentration increased
from undetected level in the seawater to 1.94 mmolL–1 at
ratio of 1:2.5 soil to seawater in red soil from Okinawa
royal golf club. In the one-tenth seawater-soil extracts,
the maximum concentration of Al3+ was 0.946 mmolL–1
in silt + clay (Fig. 3(c)). Likewise, Al3+ concentrations
were 0.580 and 0.483 mmolL–1, in bulk soil and in coarse
sand, respectively (Figs. 3(a) and (b)). Aluminum concentrations in the one-hundredth seawater solution increased with soil weight to maximal values of 0.0453,
0.0347 and 0.062 mmolL–1 for bulk soil, coarse sand and
Table 2. (continued).
Soil weight
(g/100 mL)
One-hundredth diluted seawater
+
+
2+
One-thousandth diluted seawater
pH
Na+
K+
Mg 2+
Ca2+
Al3+
*ND
0.075
0.087
0.094
0.096
0.119
0.121
0.130
0.136
5.92
4.90
4.83
4.79
4.77
4.76
4.75
4.73
4.71
0.465
0.474
0.481
0.483
0.490
0.522
0.532
0.567
0.584
0.010
0.010
0.010
0.010
0.011
0.012
0.013
0.014
0.014
0.110
0.085
0.081
0.078
0.078
0.084
0.085
0.092
0.094
0.020
0.028
0.049
0.050
0.059
0.080
0.068
0.085
0.089
*ND
0.050
0.052
0.052
0.054
0.057
0.058
0.059
0.084
0.184
0.226
0.245
0.280
0.306
0.347
0.391
0.419
0.061
0.073
0.077
0.082
0.083
0.085
0.101
0.104
5.15
4.97
4.86
4.81
4.79
4.77
4.75
4.73
0.463
0.465
0.463
0.473
0.481
0.498
0.514
0.523
0.010
0.010
0.009
0.009
0.010
0.010
0.011
0.011
0.090
0.090
0.093
0.105
0.109
0.117
0.120
0.134
0.045
0.049
0.059
0.048
0.049
0.053
0.062
0.061
0.033
0.042
0.052
0.062
0.063
0.064
0.066
0.068
0.195
0.248
0.275
0.321
0.351
0.366
0.388
0.388
0.146
0.149
0.156
0.156
0.164
0.172
0.176
0.186
4.95
4.84
4.82
4.76
4.73
4.70
4.67
4.66
0.465
0.467
0.510
0.522
0.547
0.562
0.583
0.617
0.011
0.012
0.013
0.014
0.015
0.016
0.018
0.019
0.086
0.082
0.083
0.086
0.090
0.095
0.106
0.110
0.034
0.047
0.047
0.055
0.060
0.063
0.081
0.081
0.059
0.087
0.088
0.096
0.096
0.095
0.098
0.108
Ca
2+
pH
Na
K
Mg
Bulk soil
0
1
3
5
10
15
20
30
40
6.46
4.70
4.55
4.53
4.48
4.46
4.45
4.40
4.42
4.64
4.52
4.46
4.44
4.45
4.44
4.47
4.46
4.38
0.101
0.0811
0.0815
0.0747
0.0656
0.0595
0.0564
0.0554
0.0547
1.10
0.977
0.921
0.883
0.821
0.761
0.764
0.751
0.692
0.190
0.220
0.227
0.238
0.259
0.280
0.278
0.280
0.294
Coarse sand
1
3
5
10
15
20
30
40
4.73
4.58
4.57
4.55
4.54
4.53
4.52
4.51
4.50
4.46
4.54
4.58
4.54
4.49
4.54
4.54
0.0810
0.0758
0.0736
0.0701
0.0613
0.0595
0.0572
0.0564
1.07
1.07
1.02
0.994
0.970
0.864
0.869
0.856
Silt + clay
1
3
5
10
15
20
30
40
4.67
4.57
4.55
4.53
4.49
4.48
4.44
4.41
4.56
4.49
4.47
4.46
4.45
4.45
4.43
4.31
0.0816
0.0726
0.0682
0.0592
0.0583
0.0574
0.0563
0.0563
0.952
0.982
0.964
0.825
0.776
0.752
0.728
0.703
Al
3+
*ND: Not detected .
silt + clay, respectively (Figs. 3(a)–(c)). In comparison
to the other extracts the Al3+ concentrations were lowest
in the one-thousandth seawater extracts at a maximum of
0.036 mmolL–1 (Fig. 3(c)). In the other fractions the Al 3+
concentrations were maximum at 0.0278 and 0.0227
mmolL–1 for bulk and coarse sand, respectively. The Al 3+
concentrations showed a trend of silt + clay > bulk soil >
coarse sand at almost all salinities, similar to the H+ concentrations.
3.4 Influence of salinity on behavior of the base cations
Decreases in concentrations of cations, Na+, Ca2+ and
2+
Mg and particularly K+ were commonly observed with
increases of soil to solution ratio (Table 2). Sodium concentration in seawater was 460 mmolL–1 but fluctuated
with increasing soil to solution ratio. It decreased by
2.17%, 7.65%, and 7.11% for the seawater extracts, onetenth, and one-hundredth seawater extracts, respectively.
The Na+ concentration showed certain increased patterns
in bulk soil and coarse sand in the seawater-soil extracts
to a maximum value of 472 mmolL–1 at 5 g/100 mL of
coarse sand (Table 2). However, both one-tenth and onehundredth seawater extracts showed only decreasing patterns, ranged from 46.0 to 42.5 mmolL–1 and 4.64 to 4.31
mmolL–1, respectively (Table 2). The increased pattern
for Na+ concentration was remarkable in one-thousandth
seawater extracts, which increased from 0.465 mmolL–1
to a maximum value of 0.617 mmolL–1 in the silt + clay
(Table 2).
The percentage decrease of the K+ exceeded other
base cations that showed the trend of silt + clay > bulk
soil > coarse sand at the higher salinities. Although there
were few exceptions of increasing patterns in seawater
and one-thousandth seawater extracts, the concentration
of K+ generally decreased with soil weight (Table 2). The
concentration of K+ was 9.79 mmolL–1 in the seawater
Influence of Salinity on pH and Aluminum
595
2.5
Seawater (SW)
10 times diluted SW
100 times diluted SW
1000 diluted SW
3+
-1
Al (mmolL )
2.0
1.5
1.0
0.5
0
0
(a)
20
30
40
50
Weight of soil (g/100mL)
2.5
Seawater (SW)
10 times diluted SW
100 times diluted SW
1000 times diluted SW
2.0
3+
-1
Al (mmolL )
10
1.5
1.0
0.5
0
0
(b)
10
20
30
40
50
Weight of soil (g/100 mL)
3.0
Seawater (SW)
10 times diluted SW
100 times diluted SW
1000 times diluted SW
2.0
1.5
3+
-1
Al (mmolL )
2.5
1.0
0.5
0
0
(c)
10
20
30
40
50
Weight of soil (g/100 mL)
Fig. 3. Changes of Al 3+ concentration with soil weight at different salinities of seawater solutions in bulk soil (a), coarse
sand (b) and silt + clay (c).
sample. In the seawater extracts it increased to a maximum of 10.4 at 5 g/100 mL of the bulk soil, decreasing to
the lowest value of 5.94 mmolL –1 in the silt + clay. In
both one-tenth and one-hundredth extracts, only decreas-
596
M. M. Kombo et al.
ing trends were found for all the soil fractions. The concentrations of K+ ranged from 0.975 to 0.459 mmolL–1
and from 0.101 to 0.0547 mmolL–1 for the 10 and 100
times diluted seawater extracts, respectively. The reverse
trend was noted in one-thousandth seawater solution extracts where the K+ concentration increased from 0.010
mmolL–1 to the highest value of 0.019 mmolL–1.
Calcium concentration in the seawater sample was
9.76 mmolL–1 (Table 2). It generally decreased with increasing soil weight in the seawater-soil extracts. The
minimum Ca2+ concentration was 7.51 mmolL–1 in 30 g/
100 mL for silt + clay. The concentration of Ca2+ in the
one-tenth seawater solution was 0.970 mmolL–1 and did
not show a definite pattern throughout the extracts. The
highest concentration (1.17 mmolL–1) was found in 40 g/
100 mL solution of coarse sand, while the minimum concentration was 0.885 mmolL–1 in silt + clay. The Ca2+
concentration in one-hundredth seawater-soil extracts was
dominated by increasing patterns. It increased from 0.095
mmolL–1 to a maximum of 0.210 mmolL–1 in 40 g/100
mL solution of coarse sand. In one-thousandth seawater
extracts the concentration of the Ca2+ in the sample without soil was 0.010 mmolL–1 and only increasing patterns
were observed to the highest concentration of 0.045
mmolL–1 in 40 g/100 mL for bulk soil.
Magnesium concentration revealed decreasing patterns in all four extracts examined (Table 2). The Mg2+
concentration in the seawater sample was 54.8 mmolL–1.
The concentration generally decreased with increasing
soil to solution ratio to the minimum concentration of 48.5
mmolL–1 in 40 g/100 mL of silt + clay. The initial concentration of Mg2+ in one-tenth seawater solution was 5.45
mmolL–1 but decreased to 4.44 mmolL–1 in 40 g/100 mL
of the silt + clay. Likewise, the concentration of Mg2+ in
the one-hundredth seawater solution extracts decreased
from 0.548 mmolL –1 to the lowest value of 0.346
mmolL–1 in 40 g/100 mL solution of bulk soil. In onethousandth seawater solution extracts, the Mg2+ concentration diminished from 0.055 mmolL–1 in the solution
without soil to the lowest value of 0.039 mmolL–1 in the
bulk soil.
4. Discussion
4.1 Soil pH
The pH value (4.65) of the bulk soil observed in this
study shows that the sample was strongly acidic soil. The
soil pH is consistent with that reported by Nurcholis et
al. (1997), who stated that the red soil pH, at six locations in the northern part of Okinawa, ranged from 4.20
to 5.10 in a soil to solution ratio of 1:2.5 in distilled water. Xu et al. (2003) also found pH values of 4.85 and
4.60 for the red soil in the subtropical region in southern
China obtained at 0–5 cm and 20–25 cm depth, respec-
tively. Soils with pH values ranging from 4.5 to 5.0 are
classified as strongly acid in terms of soil reaction (Brady
and Weil, 2002). At pH values of four to five, the presence of exchangeable trivalent Al is encountered in mineral soils and even in certain organic soils (Thomas, 1996).
The lowest pH in the silt + clay reflects a large amount of
clay mineral, probably associated with more adsorbed H+
and Al in the soil exchangeable pool. The soil pH is due
to release of H+ from the soil and Al hydrolysis. However, the total soil acidity is largely determined by the
soil composition, ion exchange, and hydrolysis reactions
associated with the silicate layers, the oxide minerals, the
soluble acids and the soil organic matter. According to
Thomas and Hargrove (1984) the H+ ions responsible for
the lower soil pH may come from organic matter, hydrolysis of Al or Fe in the soil mineral interlayer, or on the
surface edge of the layer silicates.
4.2 pH of the soil-seawater solution extracts
When the red soil interacted with seawater solutions
the pH fell with increasing soil weight and salinity (Figs.
2(a)–(c)). From above 10 g of soil per 100 mL of seawater,
the pH values in the seawater extracts were lower than
actual soil pH determined in distilled water (Table 2). This
indicates an additional increase of H+ and Al 3+ from the
soil exchangeable pool compared to the actual soil pH
observed in distilled water. This might be attributed to
the ion exchange effect in the soil-aqueous system. The
coincidence of the maximum release of Al3+ with the lowest pH values in almost all the extracts suggests a contribution of the Al3+ to the acidity of the solution. Hydrolysis is an important process that lowers the soil pH in a
soil solution containing Al 3+ (Thomas and Hargrove,
1984). Aluminum in soil contributes acidity as follows:
Micelle Al 3+(adsorbed Al) ↔ Al3+(solution)
(1)
and the Al3+ in the solution is then hydrolyzed as:
Al3+ + 2H2O → Al(OH)2+ + 2H+
in the mixture. In relative terms, soil pH dropped much
more in the high salinity than in the lower salinity because of the great displacement of H+ and Al 3+ from the
exchange sites under the higher salt concentrations. According to Donald et al. (2003) the increase of pH in soils
with dilution and decreased pH with salt addition suggest that the negatively charged clays dominate the soil
exchange properties. A net negative charge on the soil
mineral surface is generated when the coordinating cations (Si4+ and Al 3+ in the aluminosilicates) are replaced
by cations of lower valence, such as aluminum for silicon or Fe2+ and Mg 2+ for aluminum, resulting in a deficit
of internal positive charge in the soil (Evangelou, 1998).
The pH in seawater extracts decreased gradually after mixing the solution with soil, unlike the three lower
dilution seawater extracts (Table 2). This indicated that
the quantity of the acid released from the seawater-soil
extracts did not completely neutralize alkali at low soil
to solution ratio (<5g/100 mL). At least 10 g of soil/100
mL of seawater could release acidity that completely neutralized the alkalinity in the seawater-soil extracts. The
abrupt drop in the pH of the diluted solution extracts after mixing with only one gram of soil per 100 mL of the
solutions could be attributed to the full neutralization of
the alkalinity by the released H+ in to the extract solutions. Kombo et al. (2003) found that the concentration
of HCO 3 – fell from 138 mgL –1 in seawater to 1.58
mgL–1 at 1:10 red soil to seawater ratio and alkalinity
was not detected at a soil to seawater ratio of 1:7.
The quantity of chemical constituents, quantity and
type of mineral, as well as particle size of a given sample
were key intrinsic factors that governed the differences
in pH among the soil fractions. The consequence of exchange of H+ and Al 3+ from the soils for K+, Ca2+, Mg2+
and Na+ from the solution led to a lowering of the pH of
the solution extracts. Thorjørn et al. (1999) also revealed
the decrease of K+, Na+, Mg2+ and Ca2+ from soil solution in Chinese acid soils, which accounted for a large
release of Al 3+ and H+ and attributed to the ion exchange.
(2)
where the released H + lowers the pH of the solution
(Brady, 1990).
The remarkable influence of the salinity on the pH
of the solutions may be explained by the lowest pH values observed in the soil-seawater extracts. With the exception of few cases at higher soil to solution ratios (>10
g 100 mL), the pH trend was seawater extracts < onetenth seawater extracts < one-hundredth seawater extracts
< one-thousandth seawater extracts (Figs. 2(a)–(c)). Two
important experimental variables that affected soil pH
were the soil/solution ratio and the salt concentrations of
the solution. The effect of the soil/solution ratio tended
to lower pH values with larger concentrations of the soil
4.3 Aluminum in the soil-seawater solution extracts
The influence of the salinity on the extracted Al3+ is
well defined in Figs. 3(a)–(c), which reveal that the maximum Al3+ concentration was observed in the seawatersoil extracts (2.50 mmolL–1) while the lowest value was
found in the one thousand times diluted seawater-soil
extracts (0.036 mmolL–1). The effect was also observed
at intermediate salinities, where the maximum Al3+ concentrations were only 38% and 2.48% of the seawater
extracts in the 10 times and the 100 times dilutions for
the silt + clay. The relatively high concentrations of Al3+
in the higher salinity-extracts were mainly a result of
cation exchange, while relatively low Al3+ concentrations
in the two lower salinities reflect dissolution process.
Influence of Salinity on pH and Aluminum
597
Increasing the concentrations of K+, Na + and Ca2+ in certain extracts could explain the dissolution process from
the soil minerals. According to Sposito (1989) the cations that can become soluble through the weathering of
clay minerals may be Na+, K+, Mg2+ and Ca2+. Our results agree with the finding of Liao et al. (1997) that the
aluminum concentrations in leachates from soil that was
exposed to different solutions depended mainly on the
ionic strength. With increasing ionic strength, the activity coefficients of the ions involved decrease to a different degree, favoring an increase in the concentration of
trivalent cations to soil solution (Matschonat and Vogt,
1998).
Aluminum concentrations in the seawater extracts
and one-tenth seawater extracts increased gradually at pH
values above 5 and then increased sharply with decreasing pH values below 5 (Table 2). Nurcholis et al. (1997)
found similar results, that exchangeable Al of most
Okinawa and Java (Indonesia) red soil samples sharply
decreased with rising pH. According to Ajwa and
Tabatabai (1995) polymerization of the hydrolysis products and precipitation of Al(OH)3 may control the activity of Al 3+ at higher pH values (>5). The sharp increase
of aluminum concentration, particularly in the seawater
extracts, was imposed by the large cation exchange in
comparison to other lower salinities. According to Brady
(1990), the exchangeable Al 3+ and H+ present in acidic
soils could be released into the solution by salts such as
KCl as follows:
Micelle Al 3+, H+(soil solid) + 4KCl(solution)
↔ Micelle 4K+(soil solid) + AlCl 3 + HCl(solution). (3)
The logarithmic concentrations (M) of Al and pH are
plotted in Figs. 4(a)–(c). A well-defined relation between
the log [Al3+] and pH is found in all the soil fractions at
all the salinities examined. The control of soluble Al by
exchange reactions with soil organic matter is explained
by a linear relationship between –logAl3+ and pH in solutions (Gerard, 2003). The pH changes were observed over
a range of 8.21 to 3.85, 7.33 to 4.06, 6.46 to 4.41 and
5.92 to 4.66 from the highest to the lowest salinities in
the four extracts (Table 2). These magnitudes of released
H+ are sufficient to effect major changes in the dissolved
aluminum. Liao et al. (1998) found that soil with pH of
3.5 resulted in higher Al3+ concentrations than soil with
pH of 4.3, implying that aluminum release from soil is
highly pH-dependent. Boyd (2000) reported that the concentration of total aluminum in natural water with pH
values above 5 is usually very low and water with pH
values below 4 may have a high aluminum concentration.
The quantity of the soil, quantity and type of minerals, their chemistry and their particle sizes also imposed
598
M. M. Kombo et al.
Fig. 4. Relationship between log [Al3+] and pH at different
salinities of seawater solutions in bulk soil (a), coarse sand
(b) and silt + clay (c).
significant influences on the extracted Al 3+. In all the soil
fractions, the quantity of the dissolved aluminum released
to solution increased continuously with increasing soil
weight. Nurcholis et al. (1998) also revealed that the exchangeable Al increased with increasing quantity of kao-
cal composition of the soil. The chemical analysis of the
soil fractions showed that the quantities of Al and Fe were
relatively higher in the silt + clay than in the bulk soil
and coarse sand (Yonaha, 2002). This reflects the large
amount of clay minerals and the large surface area for
ion exchange. The smaller size particles in a given mass
of soil have a greater surface area, which is exposed for
adsorption and other surface phenomena (Brady and Weil,
2002). The analysis of the soil minerals showed that
vermiculite/chlorite and kaolinite dominated in the clay
fraction, while quartz and muscovite dominated in the silt
fraction and only quartz dominated in the sand fraction
(Vuai et al., 2003). These observations suggest a great
deal of exchange in the silt + clay fraction relative to
coarse sand, which is dominated by quartz (SiO2). The
coarse soil particles are to a large extent quartz and feldspar, which contain most of the silicon in the mineral soil
(Clemens et al., 1998). Such quartz and feldspar are responsible for low Al contents (Yeong-gill et al., 1999). It
is widely recognized that the major controlling processes
in acidic soils are reversible formation of hydroxyl-Al
compounds and adsorption of soluble Al in soil organic
matter (Breggren and Mulder, 1995). In fact, these two
Al-controlling processes depend on pH and the organic
matter content of the soil.
4.4 Influence of salinity on behavior of the base cations
The relationships between the total adsorbed base
cations and pH are shown in Figs. 5(a)–(c). A well-defined inverse relationship between the adsorbed cations
and pH was prevalent at all the salinities investigated.
Similar trends were also revealed for the Al3+ concentrations. The data strongly suggest that at the high salinities,
the H+ and Al 3+ were actively exchanged with K+, Na+,
Ca2+ and Mg2+ in the solution. The salt effect increased
Al3+ concentration through exchange with the base cations and consequently decreases the pH of the extracts.
According to Critter and Airoldi (2003), the equilibrium
with the attached acidic functional groups in soil can be
represented by a general equation:
SHn + Mn+ ↔ SM + nH+
Fig. 5. Relationship between sum of adsorbed base cations and
pH at different salinities of seawater solutions in bulk soil
(a), coarse sand (b) and silt + clay (c).
lin minerals, illite and vermiculite/illite mixed layer minerals in the red soils from Okinawa and Java (Indonesia).
The smallest particles, silt + clay in almost all cases, released relatively large amounts of Al3+ than the bulk soil
and coarse sand (Figs. 3(a)–(c)), as observed in the chemi-
(4)
where SH is soil surface containing H+ ions available for
exchanging, Mn+ is the exchanged cation and SM is the
soil surface that adsorbed the cation. Liao et al. (1998)
noted that the released Al was higher in soil with lower
pH and higher concentrations of sulfate and base cations
than in soil with higher pH and negligible base cation
contents, suggests the importance of ion exchange between base cations and aluminum ions on the soil exchange complex.
In addition to the exchange between the acidic cations and basic cations, exchange among the base cations
Influence of Salinity on pH and Aluminum
599
in the soil-solution interface may also occur. This might
be explained by the increasing patterns of certain base
cations. The most important readily exchangeable metal
cation in acidic soils is Al 3+, followed by Ca2+ (Sposito,
1989). On the other hand, the dissolution of the soil minerals such as feldspar, geothite can also release Na+, K+,
Mg2+ or Ca 2+. The solubility of the species from the soil
is clearly observed at the lowest salinity extracts (Table
2). Matschonat and Vogt (1998) pointed out that the total
cation concentration in the system is a key factor for cation
exchange during soil-solution interactions. Our results
also suggest that the ion exchange coincides with the adsorption of certain base cations. Libes (1992) reported
that, due to the strong attractive force, relatively small
cation, K+ is adsorbed but not readily exchanged in soil
minerals such as illites. There is a consensus that ions
adsorbed specifically like K+ are not considered readily
exchangeable (Sposito, 1989).
The percentage of adsorbed K+ concentration was
relatively higher than other cations, probably due to the
smallest hydrated radius (i.e. K + = 5.32, Na = 7.90,
Ca2+ = 9.60 and Mg2+ = 10.8 Å). The adsorption selectivity rule for the same valence cations favors the ion with
the smallest hydrated radius (Evangelou, 1998). Further,
Jardine and Zelazny (1996) argued that the potassium
adsorption selectivity by clays among larger hydrated ions
such as Ca2+ or Mg2+, is due to the space limitations for
diffusion of the latter in the wedge zone, and they showed
that amount of exchangeable aluminum in soil was influenced by the cation in the order of NH4+ > K+ > Ca2+ >
Na+. The concentrations of Ca2+ and K+ generally showed
similar patterns to Na + and Mg2+ but much adsorption
was revealed for Ca2+, which followed K+. Thabet and
Selim (1996) also reported the general preferential adsorption of Ca2+ with respect to Mg2+ for Sharkey and
Mahan soils, almost similar to our observations although
in different background extracting solutions. This study
revealed that the percentage adsorption trend for the base
cations was K + > Ca2+ > Mg 2+ > Na+.
Our results demonstrate that red soil carried from
the terrestrial area to the aquatic systems might acidify
the fresh water and influence the estuarine pH. Ion exchange between the adsorbed acid cations in the soil and
base cations in the river and the estuarine waters might
occur in the areas which are significantly affected by red
soil erosion in Okinawa Islands. Vuai et al. (2003) pointed
out that the red soils contain appreciable amounts of soluble H+ which can lower the pH of fresh water when they
come in to contact. According to them, the percentage of
H+ which may associate with SO42– and likely form H2SO4
are in the range of 26–52% for the red soil from
Gushikawa recreation center (Okinawa). The authors reported that some acidic waters found in Okinawa red soildominated areas have pH ranging from 4.95–5.81. Their
600
M. M. Kombo et al.
base cation concentrations were also lower than those of
neutral river water with trends of Na+ > K+ and Mg2+ >
Ca2+. The low concentrations of these chemical species
might be due to cation exchange between them and Al/
H+ from red soil. On the other hand, Upadhyay and Gupta
(1995) observed that the sorption/exchange processes
dominated the geochemical interactions of Al in waters
at the Mandovi estuary, whereas Al from riverine was
actively exchanged between solution and solid surfaces.
The phenomenon might possibly occur under prevailing
condition of estuaries in the central to the northern part
of Okinawa Island and ultimately may affect the marine
ecological systems.
5. Conclusion
An experiment on the effect of salinity upon red soil
and seawater interaction has been conducted. Processes
such as ion exchange, adsorption, dissolution and hydrolysis occurred. The cation exchange took place when H+
and Al3+ were displaced from the soil exchangeable pool
while Na+, Ca2+, Mg 2+ and K+ were adsorbed/exchanged
or dissolved depending on the salinity of the solutions.
As the salinity and soil weight increased, the ions exchange and/or adsorption also increased and hence the
concentrations of the base cations, particularly K+, decreased from the solutions, favoring the release of H+ and
Al3+ to the solution. The dissolution process that was
mostly intensified in the lower salinity was also increased
with increasing soil to solution ratio. The H+ released from
the soil exchangeable pool and the hydrolysis of Al3+ were
responsible for lowering the pH in the solution extracts,
showing a relatively higher magnitude in the silt + clay
than in bulk soil and coarse sand. With increasing soil to
solution ratio the lowest pH values were found in the
seawater-soil extracts and the highest values were observed in the one-thousandth seawater solution-soil extracts. Although the magnitude of the impacts observed
in this study might differ from the field environment, the
study succeeded in demonstrating the factors, the mechanisms, and types of processes that occur when the acidic
red soil is discharged to the coastal area such as estuaries.
Acknowledgements
We wish to express our sincere gratitude to Prof.
Oomori T. and Ass. Prof. Ohde S. for their constructive
comments, which improved this work. We would also like
to thank all colleagues in our environmental chemistry
laboratory for their assistance in sampling and analysis
during this investigation. Special thanks are due to Mr.
John M. N. for critical reading of an earlier draft of this
manuscript. Our thanks are extended to two anonymous
reviewers for their insightful comments. Last but not least,
we would also like to acknowledge the ministry of Edu-
cation, Science, Sports and Culture of Japan
(MONBUKAGAKUSHO) for the financial support.
References
Ajwa, H. A. and M. A. Tabatabai (1995): Metal-induced sulfate
adsorption by soils: I. Effect of pH and ionic strength. Soil
Science, 159, 32–42.
Bertsch, P. M. and P. R. Bloom (1996): Aluminum. p. 517–550.
In Methods of Soil Analysis Part 3 Chemical Methods, ed.
by D. L. Sparks, Soil Science of America, Inc., U.S.A.
Boyd, C. E. (2000): Water Quality: An Introduction. Kluwer
Academic Publ., 330 pp.
Brady, N. C.(1990): The Nature and Properties of Soils. 10th
ed., The Macmillan Company, New York, 621 pp.
Brady, N. C. and R. R. Weil (2002): The Nature and Properties
of Soils. 13th ed., Pearson Education, Inc., 960 pp.
Breggren, D. and J. Mulder (1995): The role of organic matter
in controlling aluminum solubility in acidic mineral soil
horizons. Geochem. Cosmochim. Acta, 59, 4167–4180.
Clemens, R., P. de Caritat, H. Niskavaara, T. E. Finne, G.
Kashulina and V. A. Pavlov (1998): Comparison of elemental contents in O- and C-horizon from the surroundings of
Nikel, Kola Peninsula, using different grain size fractions
and extractions. Geoderma, 84, 65–87.
Critter, S. A. M. and C. Airoldi (2003): Adsorption-desorption
processes of calcium on Brazilian soils. Geoderma, 111, 57–
74.
Donald, G. McG., R. J. Southard and R. J. Zasoski (2003):
Mineralogical comparison of agriculturally acidified and
natural acidic soils. Geoderma, 114, 355–368.
Evangelou, V. P. (1998): Environmental Soil and Water Chemistry: Principles and Applications. John Wiley & Sons, Inc.,
564 pp.
Gerard, F., J. Ranger, C. Menetrier and P. Bonnoud (2003): Silicate weathering mechanisms determined using soil solutions
held at high matric potential. Chem. Geol., 202, 443–460.
Jardine, P. M. and L. W. Zelazny (1996): Surface reactions of
aqueous Aluminum species. p. 221–270. In The Environmental Chemistry of Aluminum, ed. by G. Sposito, Lewis
Publ.
Kombo, M. M., S. A. Vuai and A. Tokuyama (2003): Impact of
acidic red soil on pH and aluminum under the soil-seawater
interacting environment. Bull. Fac. Sci., Univ. Ryukyus, 75,
75–87.
Liao, B., H. M. Seip and T. Larssen (1997): Response of two
Chinese forest soils to acidic inputs: Leaching experiment.
Geoderma, 75, 53–73.
Liao, B., L. Thorjørn and M. S. Hans (1998): Response of five
Chinese forest soils to acidic inputs: Batch experiment.
Geoderma, 86, 295–316.
Libes, S. M. (1992): An Introduction to Marine Biogeochemistry. John Wiley & Sons, Inc., New York, 734 pp.
Matschonat, G. and R. Vogt (1998): Significance of the total
cation concentration in acid forest soils for the solution
composition and the saturation of exchange sites. Geoderma,
84, 289–307.
Nishihira, M. (1987): Natural and human interference with coral
reef and coastal environment in Okinawa. Galaxea, 6, 311–
321.
Nurcholis, M., Y. Tokashiki, K. Oya, M. Shimo and T. Shinjo
(1997): Alterations of pH and exchangeable Al by liming
in the acidic soils of Okinawa and Java islands. Pedologist,
41, 89–98.
Nurcholis, M., Y. Tokashiki, K. Oya, M. Shimo and N. Miyauchi
(1998): Relationship between clay mineralogy and exchangeable Al in red and yellow soils from the islands of
Okinawa and Java. Aust. J. Soil Res., 36, 411–421.
Omija, T. (2000): Soil run-off and coral reef preservation. Soil
Run-off Section, 13, Okinawa Prefectural Institute of Health
and Environment, Okinawa, Japan.
Onaga, K. (1992): The present state of soil conservation in Japan. Australian Journal of Soil and Water Conservation, 5,
141–146.
Ota, M. (1994): The Okinawa Prefecture Red Soil Erosion Prevention Ordinance. No. 36, 20 pp.
Sposito, G. (1989): The Chemistry of Soils. Oxford University
Press, Inc., 277 pp.
Stevenson, F. J. and M. A. Cole (1999): Cycles of Soil Carbon,
Nitrogen, Phosphorous, Sulfur, Micronutrients. 2nd ed.,
John Wiley & Sons, Inc., 427 pp.
Thabet, A. G. and H. M. Selim (1996): Determination of cation
selectivity coefficients of soils: A thin disk flow method.
Soil Science, 161, 405–410.
Thomas, G. W. (1996): Soil pH and soil acidity. p. 475–491. In
Methods of Soil Analysis Part 3 Chemical Methods, ed. by
D. L. Sparks, Soil Science of America, Inc., U.S.A.
Thomas, G. W. and W. L. Hargrove (1984): The chemistry of
soil acidity. p. 3–56. In Soil Acidity and Liming, 2nd ed.,
ed. by F. Adams, Madison, Wisconsin, U.S.A.
Thorjørn, L., R. D. Vogt, H. M. Seip, G. Furuberg, B. Liao, J.
Xiao and J. Xiong (1999): Mechanisms of aluminum releases in Chinese acid forest soils. Geoderma, 91, 65–86.
Tokashiki, Y. (1993): Soil survey; Jahgaru, Shimajiri Mahji,
Feichisya, Kunigami Mahji. p. 63–88. In Soil and Nature
in Okinawa Island, ed. by R. Hirayama and I. Yamada, Society (Meeting) of the Pedologist (in Japanese).
Upadhyay, S. and R. S. Gupta (1995): The behavior of Al in
waters of the Mandovi estuary, west coast of India. Mar.
Chem., 51, 261–276.
Vuai, S. A. (2001): Effects of red soil on chemical composition
of natural waters. M.Sc. Thesis, University of the Ryukyus,
Japan, 107 pp.
Vuai, S. A., M. Ishiki and A. Tokuyama (2003): Acidification
of fresh waters by red soil in a subtropical silicate rock area,
Okinawa, Japan. Limnology, 4, 63–71.
Xu, R., A. Zhao, Q. Li, X. Kong and G. Ji (2003): Acidity regime of the Red Soils in a subtropical region of southern
China under field conditions. Geoderma, 115, 75–84.
Yeong-gill, C., C. B. Lee and M. S. Choi (1999): Geochemistry
of surface sediments of the Southern and Western coasts of
Korea. Mar. Geol., 159, 111–129.
Yonaha, T. (2002): Interaction between sodium chloride solution and red soil. Graduate Thesis, Faculty of Science, University of the Ryukyus, Japan, 32 pp. (in Japanese).
Yotsuyanagi, T., K. Goto and M. Nagayama (1967):
Absorptimetry of Aluminum with Tiron: Application for
dissolved state of aluminum complex. Nihon Kagaku Zasshi,
12, 1282–1287 (in Japanese).
Influence of Salinity on pH and Aluminum
601