Ammonium Adsorption by Surface Sediments in the Loughor Estuary, UK

Abstract

Ammonium is a form of nitrogen that can be present in natural water systems due to various sources, including agricultural runoff, wastewater discharge, and decomposition of organic matter. High concentrations of ammonium in seawater can have several significant consequences for marine ecosystems such as harmful algal blooms, oxygen depletion, acidification, and changes in nutrient ratios. Therefore, monitoring and regulating nutrient inputs are essential for protecting marine ecosystems and maintaining the health and productivity of coastal and open ocean environments. In this study, adsorption isotherm experiments were used to study ammonium adsorption by surface bed sediments in the Loughor Estuary, South Wales, UK. The findings indicated that the adsorption isotherm was linear and fitted the Freundlich adsorption isotherm. The adsorption coefficient of ammonium in the study area ranged from 9.3 to 18 ml/g and the dimensionless ammonium adsorption coefficient was found to be ranged between 23.0 and 36.5. These values correlated well with the organic carbon content, of the sediments and can be considered as the main factors controlling ammonium sorption. The results also showed that salinity affected the adsorption of ammonium and the distribution of ammonium between the sediments and the water column. The amount of ammonium adsorption on the sediments was found to decrease gradually with the increment of the salinity levels.

Full text

Journal – The Institution of Engineers, Malaysia (Vol. 84, No. 1, June 2023)
22
FATHIA ABDULGAWAD, FANG YENN TEO, EBRAHIM AL-QADAMI,
BETTINA BOCKELMANN-EVANS, ROGER A. FALCONER
ABSTRACT
Ammonium is a form of nitrogen that can be present in natural water systems due to various sources, including agricultural
runoff, wastewater discharge, and decomposition of organic matter. High concentrations of ammonium in seawater can have
several signicant consequences for marine ecosystems such as harmful algal blooms, oxygen depletion, acidication, and
changes in nutrient ratios. Therefore, monitoring and regulating nutrient inputs are essential for protecting marine ecosystems
and maintaining the health and productivity of coastal and open ocean environments. In this study, adsorption isotherm
experiments were used to study ammonium adsorption by surface bed sediments in the Loughor Estuary, South Wales, UK.
The ndings indicated that the adsorption isotherm was linear and tted the Freundlich adsorption isotherm. The adsorption
coefcient of ammonium in the study area ranged from 9.3 to 18 ml/g and the dimensionless ammonium adsorption coefcient
was found to be ranged between 23.0 and 36.5. These values correlated well with the organic carbon content, of the sediments
and can be considered as the main factors controlling ammonium sorption. The results also showed that salinity affected the
adsorption of ammonium and the distribution of ammonium between the sediments and the water column. The amount of
ammonium adsorption on the sediments was found to decrease gradually with the increment of the salinity levels.
Keywords: Adsorption Coefficients, Ammonium, Estuaries, Salinity, Surface Sediments
1.0 INTRODUCTION
Over the centuries the sub-aerial processes of erosion and
deposition have led to the formation of river valleys and
estuaries [1]. An estuary conveys marine conditions into a river
valley potentially up to the tidal limit to form a semi-enclosed
coastal body of water connected to the open sea at one end and
to an inux of fresh river water at the other [1], [2]. When the
sea level rise exceeds the peak lling level, then the estuary is
regarded as well-developed and persists. Marine environments
and estuaries have acted as lters for a range of constituents,
with large quantities of materials such as fertilisers and organic
materials from the land being deposited in the receiving
estuarine/marine basin [3]. This organic matter is decomposed
by various heterotrophic organisms and produces large amounts
of ammonium ions [4], [5].
Ammonium ions can accumulate in pore water and can
be re-incorporated into organisms, adsorbed onto sediment
particles, or diffused out of the sediments and into the overlying
water column [6], [7]. The ammonium ions can be adsorbed onto
the sediments due to adsorption at the cation exchange sites [8],
[9]. These sites are present on the surface of clay minerals and
organic matter [10], [11]. Organic matter controls the behaviour
of ammonium sorption on sediments with low clay content [12],
[13]. The amount of ammonium adsorption depends on the ion
exchange capacity of the sediment which is usually related to
organic matter and clay content of the sediments [14]. Therefore,
the adsorption of ammonium by the sediments has an important
inuence on nitrogen cycling [15], which affects not only the
diffusive ux of ammonium into the overlying water column
but also the coupled nitrication/ denitrication occurring in the
sediments.
Fluctuating salinity in estuarine sediments plays a major role
in controlling the ammonium ion adsorption of the sediments
[16]. It was reported by Seitzinger et al. (1991) [17] that the
amount of adsorbed ammonium was lower in estuarine sediments
as compared to freshwater sediments. Salinity can inuence
the ammonium ion adsorption rate when freshwater mixes with
seawater in estuaries. Sea water cations (e.g. Mg++, Na+, K+)
compete with the NH4+ ions on the surface of clay particles and
the adsorption rate decreases with increasing salinity [17], [18],
[19]. Increasing the ammonium ion desorption rate, as well as
increasing the salinity in an estuarine system, can potentially
affect the nitrication rate due to the decreasing residence time
of ammonium on the surface of the clay [20], [21].
AMMONIUM ADSORPTION BY SURFACE SEDIMENTS
IN THE LOUGHOR ESTUARY, UK
(Date received: 05.09.2023/Date accepted: 02.11.2023)
Fathia Abdulgawad1*, Fang Yenn Teo2, Ebrahim Al-Qadami3,
Bettina Bockelmann-Evans4, Roger A. Falconer5
1,4,5Hydro-environmental Research Centre, School of Engineering, Cardiff University, Cardiff CF24 3AA, UK
2Faculty of Science and Engineering, University of Nottingham Malaysia, 43500 Semenyih, Selangor, Malaysia
3Eco Hydrology Technology Research Center (Eco-Hytech), Faculty of Civil Engineering & Built Environment,
Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja, Batu Pahat, Johor, Malaysia
*Corresponding author: fathia00@hotmail.com

Journal – The Institution of Engineers, Malaysia (Vol. 84, No. 1, June 2023) 23
AMMONIUM ADSORPTION BY SURFACE SEDIMENTS
IN THE LOUGHOR ESTUARY, UK
The Loughor Estuary, located in the Bristol Channel in the UK,
has a land-sea transitional area which is a typical environmentally
vulnerable zone. The changes in the environmental factors,
particularly salinity, are very intense and can inuence the
ammonium adsorption rate as a result of the interaction between
the fresh and seawater. The authors have measured the ammonium
and nitrate concentrations in the Loughor Estuary waters and found
that concentrations were between 0.08 to 4.12 mg/l for ammonia
and 2.65 to 8.31 mg/l for nitrate. The ammonium adsorption rate
by the sediments from the Loughor Estuary has not previously
been reported in the literature. The objectives of the present study
have therefore been to determine the adsorption coefcients for
ammonium at the Loughor Estuary sediments and to establish the
effect of salinity on this ammonium adsorption coefcient.
2.0 MATERIALS AND METHODS
2.1 Study Area and Sample Collection
Samples were obtained from two sites in the Loughor Estuary as
shown in Figure 1. The Loughor Estuary is located in Southwest
Wales and is one of the main tributaries discharging into
Carmarthen Bay and the Bristol Channel. The Bristol Channel
is located on the west coast of the UK, is a funnel-shaped
estuary, and has the second-highest tidal range in the world (up
to 14.5m). As for most macro tidal estuaries, the tides in the
Bristol Channel and Carmarthen Bay play a major role in mixing
fresh and saltwater and in re-suspending sediments from the bed
and transporting the suspended sediments landward or seaward.
During spring tides, the suspended sediments in the Loughor
Estuary are transported into the outer bay and deposited in the
near shore region just beyond the river mouth. Human activity,
including agricultural practices, sewage treatment works, and
disused mine discharges all arising along the Loughor River
basin, generally have a negative inuence on the receiving basin
water quality.
The Loughor Estuary is of particular interest in studying the
transport pathways and the adsorption and desorption behaviour
of nutrients associated with the sediments; eutrophication
problems are not uncommon, and this has economic implications
for the coastal waters in West Wales, particularly in connection
with the shing and tourism industries. A study by Abdulgawad
et al. (2008) [22] indicated that high concentrations of all
nutrients were found to occur in the Loughor Estuary and both
the Loughor Estuary and the receiving coastal water body were
affected regularly by algal blooms.
Bed sediment samples were taken close to the water’s
edge (at depths of 0-3 cm along the Loughor Estuary. These
samples were taken as part of a project that was sponsored by the
Environment Agency (EA) Wales and the samples were used for
laboratory analysis. Three sediment samples were taken at each
site and at hourly intervals. Samples 1b1, 1b2, 1b3, and 21, 22, 23
were collected at times of 1 pm, 2 pm, and 3 pm, respectively.
The tidal predictions for 14th December show that the tide was
above neap tide with tides ranging between 7.9 m at 8:48 am and
2.1 at 3:12 pm. Plastic bags were used for collecting the surface
sediments, with the samples being stored in a refrigerator at 4 ºC
for the next day (or night) for a series of adsorption experiments.
2.2 Determination of Sediment Physio-Chemical
Parameters
The sediment-water content was measured by determining the
weight loss of a known amount of wet sediment, dried at 105 ºC
for 48 hr. The sediment particle density was measured as a mass
of a known volume of the solid sediment. The sediment porosity
was determined by use of the following formula [23]:
( ){ }
WWW
s
+−=
ρφ
/100/
(1)
Where;
φ
= sediment porosity; W (%) = sediment water content
and
s
ρ
(g /cm3) = sediment particle density.
The organic carbon (OC) content was determined after
acidication with phosphoric acid and was obtained using
a SHIMADZU analyser. The particle size distribution of
the sediments was determined by using a laser particle
size instrument, namely a Malvern Master sizer. Dry
samples were measured and dispersed in distilled water
and ultrasound was used to prevent occulation. The
corresponding results are given in table (1).
2.3 Fixed Ammonium on the
Sediment
Measurement of the level of xed ammonium on the
sediments was obtained by KCl extraction. In this procedure,
the slurry was made by adding 50 ml 2 M KCl solution to the
centrifuge tubes containing a portion of the wet sediments.
The slurries were shaken for 2 hr, centrifuged at 3,000 rpm
for 20 min, and the supernatant was removed for analysis.
The ammonium concentration in the supernatant was
determined using a spectrophotometer, together with a
HACH reagent. The spectrophotometer and the HACH
(b): Location of the Two Sampling
Sites where Sediments were
Collected
Figure 1: Sewage Treatment Station Outfall (SWO)
(c): Cross-Section for Site 1b, with
Grey Rectangles Indicating Sample
Locations at Spring and Neap Tides
Respectively
(a): Location of the Loughor Estuary,
South Wales, UK

Journal – The Institution of Engineers, Malaysia (Vol. 84, No. 1, June 2023)
24
FATHIA ABDULGAWAD, FANG YENN TEO, EBRAHIM AL-QADAMI,
BETTINA BOCKELMANN-EVANS, ROGER A. FALCONER
meter (i.e., a colour meter) employed a similar concept of
measurement which depends on the light wavelength absorbed
by the sample. The wavelength of ammonium is 640 nm. The
reagents used in both methods were identied as being roughly
similar, which is used in the Phenate method [24]. All the
measurements were made on wet sediments within 24 hours of
collection.
2.4 Ammonium Adsorption Isotherm
Ammonium adsorption studies were undertaken using sediments
again taken from the surface and between depths of 0 to 3 cm.
Wet homogenised sediments were taken at each site, weighed,
and placed in a centrifuge tube containing 40ml of NH4+Cl, at
varying concentrations of NH4+. These sediment samples gave
concentrations of 2, 6, and 10 mg/l NH4+. The centrifuge tube was
placed on a shaker with continuous agitation for 24 hours, at a
temperature of 20 oC +/- 2 oC. The samples were then centrifuged
at 3000 rpm for 15 min. The supernatant was collected, ltered
through a 0.45 μm cellulose lter paper, and then analysed for
NH4+. The determination of ammonium concentration in the
supernatant was undertaken using a Spectrophotometer Lambda
EZ150, set at 640 nm [[24]. The ammonium concentration in the
supernatant was considered to be the equilibrium concentration
for the water. The adsorption isotherm experiments were followed
by an adsorption procedure, in order to remove ammonium ions
from the solids. 40ml of 2 M KCl solution was added to the
sediments remaining in the centrifuge tubes and these sediments
were then used to replace those on the shaker. After 2 hours
of agitation, the solutions were centrifuged at 3000 rpm for
15 min. The supernatant was removed and analysed using the
spectrophotometer. This procedure was repeated until all of the
NH4+ was removed from the sediments.
2.5 Salinity Effect on Ammonium Adsorption
To study the effects of the salinity on ammonium adsorption,
four different concentrations of articial seawater were prepared
including 1, 2, 3, and 4 parts per thousand (ppt). Articial
seawater was prepared according to the Scottish Association
for Marine Science procedure [25](MASM, 2007), with the
pH value of the articial seawater being set to 8. The same
methodology was used as that outlined for the above-mentioned
isotherm adsorption experiments.
3.0 RESULTS
3.1 Sediment Characteristics
The sediments in the Loughor Estuary at the two sampling sites
constituted ne sand with mean particle sizes (D50) of 83 to 130
µm and 130 to 170 µm respectively (see Figure 2), samples
were collected when the tidal range was greater than neap tide
on 14th December. Sample 1b1 had the largest mean particle
size of 130µm at site 1b. The mean particle diameter for site 1b
samples (i.e., 1b1, 1b2, 1b3) decreased, declining gradually in
cross-sectional area and with diameters of the order of 130, 100,
and 83 µm, respectively. In addition, the mean grain diameter
decreased during the outgoing tide. Sample 22 had the largest
particle size among the samples for both sites (170 µm) and was
collected in the middle of the cross-sectional area. At 23, i.e., the
position furthest going down along the cross-section, sediment
samples had the lowest mean particle size at 130 µm among the
samples of site 2. Site 1b was located closer to the sea along
the Loughor estuary, and was typically the area most affected by
the interaction of seawater and freshwater along the estuary and
further from the sea, as shown in Figure 1. The channel width at
site 2 was larger than at site 1b, thus typically the water velocity
at site 1b was generally higher than that at site 2. Samples
collected from site 2 had a measured particle size higher than site
1b due to the water velocity being higher at site 2. The sediment
porosity is ranged between 0.43 and 0.58. Sample 1b1 had the
highest porosity value of 58% attributed to the amount of organic
carbon contained in the sample. Sediments from site 1b were
high in carbon content, constituting typically 1.53% – 3.74 %
when compared to the sediment samples taken from site 2, which
ranged from 1.29% to 4.81%. Sample 21 had the highest amount
of organic carbon (4.81%), among the samples for both sites 1b
and 2. This sample was collected at high tide. In contrast, sample
23 had the lowest amount of organic carbon at 1.29% which was
collected at low tide compared to other samples from sites 1b and
2. The particle and the bulk density typically ranged from 1.0 to
1.6 mg/m3 and 2.60 to 2.70 mg/m3 respectively. The particle and
bulk density are inuenced by the mineralogy of the sediments
and the organic matter content. The density of organic matter
is much lower than the mineral solids density. Sediments high
in organic matter and also some clay minerals have low bulk
density. The particle density for sediments from both sites 1b and
2 ranged between, 2.63 to 2.69 mg/m3, see Table 1. The Particle
density for a quartz-dominated sediment is normally expected to
Table 1: Sediment Characteristics for Sites 1b and 2
of the Loughor Estuary
Sample Sample
Number
Porosity Median
Grain Size
(µm)
Organic
Carbon
(%)
Particle
Density
(g cm-3)
1b11 0.58 130 3.74 2.65
1b22 0.51 100 2.82 2.67
1b33 0.52 83 1.53 2.69
211 0.43 160 4.81 2.65
222 0.51 170 1.39 2.68
233 0.54 130 1.29 2.63
Figure 2: Particle Size Distribution of the Loughor
Estuary Sediment Samples at Sites 1b and 2
(The subscript numbers refer to the Sampling Time)

Journal – The Institution of Engineers, Malaysia (Vol. 84, No. 1, June 2023) 25
AMMONIUM ADSORPTION BY SURFACE SEDIMENTS
IN THE LOUGHOR ESTUARY, UK
be close to 2.65 mg/m3 the slight variation of the particle density
between the samples was believed to be due to the organic carbon
and clay minerals content for both sites ranging from 2.63 to 2.69
g cm-3. The sediment characteristics are summarised in Table (1).
The results of X-ray diffraction (XRD) for the sediment samples
collected at sites 1b and 2 indicated that the sediments from both
sites mostly comprise quartz (66.1 to 88.3%), followed by calcite
(9.9 to 17.5%), with minor amount of halloysite (0.1 to 8.4%),
and Kaolinite (0.7 to 7.9%).
3.2 Adsorption Isotherm of Ammonium on
Sediments
During out-going tides over a period of 3 hours at sites 1b and
2 samples were collected at regular intervals (i.e., samples 1b1,
1b2, 1b3, and 21, 22, 23), as shown in Figure (1). Figure (3) shows
comparisons between the experimental or eld data and the
theoretical results, using Langmuir and Freundlich adsorption
isotherm equations. In general, the results show good agreement
between the experimental or eld data and theoretical
results. The Langmuir model ts the data particularly well for
all the samples tested, according to the error analysis that was
performed for all samples. The Langmuir adsorption equations
have the following forms:
where,
QCbQqC e+= 1
is Langmuir constants related to adsorption
capacity, b is a constant related to; the energy of adsorption and
the Langmuir adsorption coefcient (KL), qe is the ion adsorption
amount (µg/g dry wt), Ce is the equilibrium concentration of the
solute remaining in solute [26].
Freundlich equation has the following form [26], [27]:
qe = x/m = K* C 1/n (4)
Where qe is the ion adsorption amount (µg/g dry wt), x
is the amount of the adsorbate, m is the mass of the adsorbent,
C is the ion equilibrium concentration in water (mg/l), K*
and n are constants and n is almost equivalent to 1, i.e., the ion
adsorption is linear [28]. In these experiments n 1. The equation
is more useful in its logarithmic form, as expressed by Martin et
al., 1979.
Table 2 shows the error analysis for the Langmuir and
Freundlich adsorption isotherms. The Chi-squared test (X2) and
nonlinear regression (R2) were used to evaluate the t of the
theoretical data with the experimental data. The error analysis
was applied to all of the samples using deionised water. The
Chi-squared test for the Langmuir isotherm has lower values
compared to the Freundlich isotherm for all sampling sites,
showing that the Langmuir isotherm better ts the sample data.
Sample 1b1 had the highest value of X2 for all of the samples,
with a value of 31.10. In contrast, sample 1b2 had the lowest
X2 of 8.93, in comparison with the other samples at both sites.
The R2 test also resulted in higher values for Langmuir isotherm
data for all samples at both sites (1b and 2) in comparison with
the Freundlich isotherm data. These results also illustrate that
the Langumuir isotherm better ts the data of the ammonium
adsorption for the Loughor Estuary samples.
The adsorption coefcient for ammonium is an important
factor in calculating the concentration of nitrogen in the
sediments. The formula of the ammonium adsorption coefcient
of the sediment for the Loughor Estuary has the following form:
qCKQ +∗= *
(5)
Where, Q (µg/g dry wt) is the amount of ammonium adsorbed
on the sediment of the Loughor Estuary, K* is the slope of the
regression line (adsorption coefcient), C (mg/l) is the ammonium
ion equilibrium concentration in water, q is the xed ammonium
content in the sediments being zero.
The ammonium adsorption coefcient of the sediments for
the Loughor Estuary was calculated according to the following
formula.
K = *
1
k∗∗
−
ρ
φ
φ
(6)
Where K is the dimensionless ammonium adsorption
coefcient; Ф is the porosity of the sediments; ρ is the density of
the sediment (g cm -3); and K* is the slope of the regression line
or the Adsorption coefcient (ml g -1).
Table 3 summarises the relative parameters of the ammonium
adsorption coefcient, such as the porosity, particle density total
organic carbon, and the adsorption coefcient of ammonium.
These parameters are important to calculate the dimensionless
adsorption coefcient of ammonium (K) for the sediments of
the Loughor Estuary. The table shows that the values of the
dimensionless adsorption coefcient for ammonium ranged
from 23.0 to 36.5. The highest dimensionless adsorption
coefcient was found in the sediment containing the highest
amount of total organic carbon, which was in sample 21 and with
Table 2: Error Analysis for Langmuir and Freundlich
Adsorption Isotherms using Chi-squared Test (X2) and
Nonlinear Regression (R2)
Samples Langmuir Isotherm Freundlich Isotherm
X2 R2X2R2
1b19.30 0.94 31.10 0.75
1b22.87 0.97 8.93 0.88
1b34.99 0.94 13.17 0.81
217.86 0.95 15.65 0.80
226.30 0.95 14.15 0.86
237.80 0.94 15.65 0.84
Table 3: Ammonium Adsorption Coefficients of the Loughor Estuary,
its Relative Parameters, and Total Organic Carbon Content (TOC)
Parameters Sampling Sites
1b11b21b3212223
K34.5 26.2 23.0 36.5 25.7 23.0
Ф0.58 0.51 0.52 0.43 0.51 0.54
K*(ml/ g) 18 10.20 9.3 10.4 10 10.3
ρs (g /cm3)2.65 2.67 2.69 2.65 2.68 2.63
TOC (%) 3.74 2.82 1.53 4.81 1.39 1.29

Journal – The Institution of Engineers, Malaysia (Vol. 84, No. 1, June 2023)
26
FATHIA ABDULGAWAD, FANG YENN TEO, EBRAHIM AL-QADAMI,
BETTINA BOCKELMANN-EVANS, ROGER A. FALCONER
values of 36.5 and 4.81(%) respectively. In contrast, the lowest
dimensionless adsorption coefcient was found in the sediment
samples 1b3 and 23 which contained the smallest amounts of total
organic carbon. The corresponding adsorption coefcient and
TOC values were 23, 23, and 1.53 (%). 1.29 (%) respectively.
These results indicate that total organic carbon is an important
factor affecting the adsorption of ammonium onto the sediments
in the Loughor Estuary. However, the results did not show any
inuence for other parameters namely Ф and ρs.
K is dimensionless adsorption; Ф is the sediment porosity;
coefcient K*(ml g-1) is the slope of the regression line
(Adsorption coefcient); ρs (g cm-3) is sediment density; TOC
(%) is organic carbon.
3.3 Salinity Effects on Ammonium Adsorption
Site 1b was located in the mouth of the Loughor Estuary and
in the region most affected by the interaction of seawater with
riverine freshwater. Site 2 is located more towards the middle
of the estuary. The salinity in the water column between low
and high tides ranged from
0.02 to 26.90 ppt at site 1b
and from 0.32 ppt to 17.21
ppt at site 2 (low to high tides)
respectively. Sites 1b and 2
were therefore regarded as
being ideal example sites for
studying the salinity effects on
the adsorption of ammonium
by sediments for the Loughor
Estuary. The results are
shown in Figure 4, which
indicates the salinity effect
on the adsorption coefcient
(K*) and the dimensionless
adsorption coefcient for
samples from sites 1b and 2.
The results shown in the gure
indicate that the adsorption
coefcients (K* and K) for
ammonium at both sites 1b and
2 were highest for zero salinity
conditions and continuously
dropped with increasing
salinity up to 25 ppt. This result
was as expected, caused by the
sediment cation exchange sites
being increasingly occupied
by seawater cations and
decreasingly by ammonium
ions as salinity rose. The
adsorption coefcients for
ammonium were found to
be higher for samples from
site 1b1 compared to samples
from site 21 for all salinity
conditions. This was thought
to be done in sample 1b1
containing a higher amount
of organic matter than sample
21, which resulted in more
exchange sites for ammonium being present. Thus, there was
found to be a correlation between the level of organic matter in
the bed sediments and the amount of ammonium adsorbed onto
the sediments.
Figure 3: Experimental Results and Theoretical (Langmuir and Freundlich) Adsorption Isotherms of
Ammonium for Samples in Distilled Water. b is the Langmuir Constant and Adsorption Coefficient,
K is the Freundlich Adsorption Coefficient, and n is the Freundlich Constant
Figure 4: Ammonium Adsorption for Samples from
Sites 1b1 and 2 under Varying Salinity Concentrations

Journal – The Institution of Engineers, Malaysia (Vol. 84, No. 1, June 2023) 27
AMMONIUM ADSORPTION BY SURFACE SEDIMENTS
IN THE LOUGHOR ESTUARY, UK
Figures 5 and 6 show the difference in the ammonium
adsorption under different salinity conditions and based on the
same initial ammonium concentration (of 6 mg/l) in the water
column for samples from sites 1b and 2. Equation 9 relates to site
1b for sample 1b1 and Equation 10 refers to site 2 for sample 21.
The ammonium adsorption value in the sediments for samples
1b1 and 21 gradually decreased with increasing salinity levels,
varying from 86.5 mg/g at 0 ppt to 64 mg/g at 25 ppt and 45.5
mg/g at 0 ppt to 28.5 mg/g at 25 ppt respectively, and was found
to be best represented by the following equations:
Equation for sample 1b1:
Q = 0.0098 S2 - 1.0756 S + 85.477 (7)
The equation for sample 21:
Q = 0.0393 S2 - 1.8321 S + 50.916 (8)
Where, Q is the ion adsorption quantity (µg/ g dry wt); and
S is salinity in the water column (ppt).
3.4 Organic Carbon and Grain Size Effect on
Ammonium Adsorption
Sediments at Site 1b were found to be higher in carbon content
when compared to the sediment samples taken from Site 2. It
was found that the organic carbon content had a signicant
effect on the ammonium adsorption and this result agreed well
with previous studies, such as those of de Lange, (1992); and
Raaphorst and Malschaert, (1996) [29], [30]. The samples from
Site 1b were found to have higher ammonium adsorption values
than those at Site 2. It was also found that the samples taken from
Site 1b, with a higher content of organic carbon, had a higher
ammonium adsorption than those taken from the same site but
with a lower organic carbon content. This nding was attributed
to the fact that Site 1b was rich in organic carbon in comparison
with Site 2. Studies published by other authors have indicated
that sediments with a high ammonium adsorption rate also have
a relatively large amount of organic matter, with this result being
compatible with the studies of [12], [18]. The corresponding
results are shown in Figure 7.
4.0 DISCUSSION
The ammonium ion adsorption level in the sediments was found
to obey the Langmuir isotherm equation, in particular, the study
showed that the rate of adsorption by the sediments was found
to be linear and tted the Langmuir adsorption equation within
the controlled range of ammonium concentrations in the water
column. Figure 3 indicated that the experimental data tted well
with the theoretical data of adsorption (Langmuir isotherm)
than (Freundlich isotherm) and there was found to be good
agreement between the values for ammonium adsorption using
both methods. The error analysis (Chi-squared and nonlinear
regression) conrmed that ammonium adsorption data ts the
Langmuir isotherm well. There was found to be good agreement
between the values for ammonium adsorption using both error
analysis methods.
The ammonium adsorption coefcient, dened as the ratio
of the concentration of adsorbed ammonium and the equilibrium
ammonium, was found to be important in highlighting the
characteristics of ammonium adsorption on the sediments. K is
the dimensionless equivalent of K*, with K being larger than
K*, which is due to the sediment physio-chemical parameters.
The sediment physio-chemical parameters are known to have a
signicant inuence on the behaviour of ammonium adsorption
[31]. The particle size of the sediments was also found to have
a signicant inuence on the amount of ammonium adsorption.
Samples with a small mean particle size have a larger overall
surface area per unit diameter and therefore will have a high
ammonium adsorption coefcient. The present study shows that
samples with a 65 µm grain size, had the highest ammonium
adsorption coefcient when compared to the other samples with
a larger mean grain size; this result agrees with the ndings of
Raaphorst and Malschaert (1996) [29].
The present study showed that the ammonium adsorption
coefcient did not appear to have any signicant correlation with
the sediment porosity. However, the adsorption coefcient was
found to be highest in the sediments with comparatively higher
Figure 5: Salinity-Dependent Ammonium Adsorption in the
Sediments for Sample 1b1, Q (µg/ g dry wt); S (ppt)
Figure 6: Salinity-Dependent Ammonium Adsorption in the
Sediments for Sample 21, Q (µg/ G Dry Wt); S (Ppt)
Figure 7: Correlation of Ammonium Adsorption with
Organic Carbon Content for the Loughor Estuary Sediment
Samples at Sites 1b and 2

Journal – The Institution of Engineers, Malaysia (Vol. 84, No. 1, June 2023)
28
FATHIA ABDULGAWAD, FANG YENN TEO, EBRAHIM AL-QADAMI,
BETTINA BOCKELMANN-EVANS, ROGER A. FALCONER
amounts of organic carbon. Availability of organic matter was
therefore considered to be one of the important factors controlling
the degree of ammonium adsorption in the Loughor Estuary (see
Figure 7); as can be seen, the adsorption coefcients were higher
at Site1b when compared to the corresponding values measured
at Site 2. This nding was attributed to the fact that Site 1b was
rich in organic carbon in comparison with it 2. Studies published
by other authors have indicated that sediments with a high
ammonium adsorption rate also have a relatively large amount
of organic matter, with this result being comparable with the
studies of Hou et al (2003); Boatman and Murray (1982); and
Mackin and Aller (1984) [12], [18], [28].
The difference in the adsorption isotherm of ammonium for
the Loughor Estuary sediments, particularly at Sites 1b and 2,
for different salinity concentrations showed that salinity affected
the distribution of ammonium between the sediments and the
water column. The ammonium adsorption coefcient was found
to decrease with increasing salinity concentrations; meaning
that lower salinity levels were found to be more favourable
to ammonium adsorption by the sediments. The ammonium
adsorption levels detected in the Loughor Estuary sediments,
as taken from Site 1b and 2 when linked to the salinity levels
were tted to Equations 9 and 10, with the rst derivative of this
equation being given as:
dQ/dS = 0.0196 S – 1.0756 (9)
dQ/dS = 0.0786 S – 1.8321 (10)
Where, dQ/dS is the rate of change of the quantity of
ammonium (µg g -1 ppt); and S - the salinity -ranged from 0
to 25 ppt. From the above rst-order derivative equations, it is
possible to calculate the rate of change of ammonium adsorption
in the sediments with respect to increasing salinity. The rate
of change is higher within the lower range of salinity, thereby
reecting the fact that a small variation in the salinity at the
start of the incoming tide has a relatively large effect on the
rate of ammonium adsorption. One of the obvious challenges
in analysing such processes in estuarine and coastal waters is
the huge variation in the salinity levels during the mixing region
between the freshwater and seawater ows.
Past studies by other authors of ammonium adsorption for
Montmorillonite, Kaolinite, and ne and coarse sand, for different
salinity concentrations, have indicated that the adsorption
coefcient of ammonium was higher for distilled water and lower
for articial seawater conditions for Montmorilonite, Kaolinite
and ne and coarse sand, respectively. For the current samples,
Montmorilonite was found to have the highest adsorption
coefcient, both for distilled water and articial seawater [22].
This result led to further studies being undertaken to compare
further the results between the eld data and the modelled
results, with both clay and sand being used. Figure 8 shows the
adsorption coefcient for the eld data, taken at both Sites 1b
and 2 for clean clays (i.e., Montmorillonite and Kaolinite) and
sand both (ne and coarse) under different salinity conditions.
For all samples, it can be seen that the adsorption coefcients
decreased gradually with increasing salinity concentrations.
Also, the results indicated that Montmorillonite had much higher
adsorption coefcients for all salinities than the eld samples
(1b1 and 21), the clays, and the sand samples. The adsorption
coefcients for Montmorillonite, at 0 and 25 ppt salinity ranged
from 126 ml/g to 34 ml/g respectively. Kaolinite had the second
highest adsorption coefcient ranging from 19.3 ml/g at 0 ppt
salinity, to 8.5 ml/g at 25 ppt. Sample 1b1 at 25 ppt had a higher
adsorption coefcient than Kaolinite with a value of 13 ml/g
compared to 8.5 ml/g. Furthermore, the adsorption coefcients
for sample 1b1 were higher than for ne sand, coarse sand, and
sample 21 for all saline concentrations.
The xed ammonium levels in the sediments for the Loughor
Estuary were zero, as indicated earlier. This was attributed to
the constant exchange of ammonium being adsorbed by the
sediments, with large quantities of seawater cations such as Na +,
K+, and Ca 2+ from the area controlled by seawater (Sites 1b and
2). The salinity was found to have a signicant inuence on
the xed ammonium levels in the sediments. In addition, the
ammonium levels were also thought to have been inuenced
by the nitrication process, which was relatively high in the
estuary. This high of ammonium could be due to the agricultural
activities along this section of the river (next to the farms) or
due to a SWO outfall future upstream or have come with the
river. This nitrication process is due to the level of inorganic
carbon and dissolved oxygen, both of which are readily available
as a result of the ammonium produced in the sediments and was
rapidly converted to nitrate.
5.0 CONCLUSIONS
Ammonium adsorption isotherms for the sediments have been
analysed for the Loughor Estuary, in the U.K. These analyses
showed that the adsorption of ammonium by the sediments was
almost linear and tted to the Langmuir adsorption isotherm
equation. The ammonium adsorption coefcient K* for the
Loughor Estuary sediments ranged from 9.3 to 18 ml/g and the
dimensionless ammonium adsorption coefcient, K, ranged
from 23.0 to 36.5. These results showed a close afnity with
the total organic carbon content and the types of sediments
analysed indicated that the organic matter and mineralogy of the
sediments were the main factors controlling the adsorption of
ammonium. The main parameter found to affect the adsorption
of ammonium by the sediments was salinity. Low salinity levels
resulted in more ammonium adsorption on the sediments, in
comparison with the levels of adsorption measured for higher
levels. The xed amount of ammonium measured on the
Loughor sediments was zero, which was thought to be due to
the competition of seawater cations to replace the ammonium
on the surface of the sediment particles. Also, nitrication was
Figure 8: Comparison of Adsorption Coefficient of Field Samples
1b1 & 21 and Clean Clays (Montmorillonite and Kaolinite)
and Sand (Fine and Coarse)

Journal – The Institution of Engineers, Malaysia (Vol. 84, No. 1, June 2023) 29
AMMONIUM ADSORPTION BY SURFACE SEDIMENTS
IN THE LOUGHOR ESTUARY, UK
anticipated to be higher in the estuary sediments as a result of
the ammonium produced within the sediments being rapidly
converted to nitrate. However, the adsorption of ammonium by
surface bed sediments in aquatic environments is complex and
inuenced by several factors. Understanding these factors is
essential for managing water quality and the fate of ammonium
in natural systems. Future studies should in-depth investigate the
effects of sediment composition, pH of the water, ionic strength,
and sediment depth on the ammonium adsorption rate by the
surface sediments.
6.0 ACKNOWLEDGMENTS
The authors would like to thank the Environment Agency Wales
for providing the necessary resources for the samples acquired
for this project.
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BETTINA BOCKELMANN-EVANS, ROGER A. FALCONER
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PROFILES
FATHIA ABDULGAWAD is a water resources and water quality expert with the international management, research and teaching experience.
She teaches material relating to her research expertise in water resource management at British Water Engineer College. She delivers training in
water resources management in UK wide. She obtained her PhD from the Hydro-environmental Research Centre, in the School of Engineering, at
Cardiff University, UK and her MSc in Environmental sciences from Wageningen University Netherlands. She has worked as a water engineer at
consultancy rm RPS Group and as adviser to UN on water quality.
Email address: fathia00@hotmail.com
EBRAHIM AL-QADAMI is a postdoctoral research fellow in the Faculty of Civil Engineering and Built Environment, Universiti Tun Hussein
Onn Malaysia (UTHM). Dr. Al-Qadami obtained his Ph.D. from Universiti Teknologi PETRONAS (UTP) in 2022 and his master's degree from
Universiti Putra Malaysia (UPM) in 2017. Dr. Al-Qadami does research on Coastal Engineering, Flood Risk Modelling, Urban Stormwater
Management, and Water Resources Management.
Email address: ebrahim@uthm.edu.my
FANG YENN TEO is a Professor of Water Engineering at the University of Nottingham Malaysia, and Chair of the International Association
for Hydro-Environment Engineering and Research (IAHR), Malaysia Chapter. He obtained a PhD in Civil Engineering from Cardiff University,
UK. He is the Professional Engineer (P.Eng.) with Practising Certicate; Professional Technologist (P.Tech.); International Professional Engineer
Register (Int.PE); ASEAN Chartered Professional Engineer (ACPE); Fellow of the Institution of Engineers, Malaysia (FIEM); Fellow of Higher
Education Academy (FHEA, UK); Fellow of ASEAN Academy of Engineering and Technology (FAAET); and Member of the Institution of
Engineering and Technology (MIET).
Email address: fangyenn.teo@nottingham.edu.my
BETTINA BOCKELMANN-EVANS is Senior Lecturer at the Hydro-environmental Research Centre, School of Engineering, Cardiff University
and also Senior Bid Developer in Net Zero, Research and innovation services at Cardiff University.
Email address: bockelmannevans@cardiff.ac.uk
ROGER A. FALCONER is an Emeritus Professor of Water Engineering (since 2018) and Professor and Founding Director (1997-18) of the
Hydro-environmental Research Centre, in the School of Engineering, at Cardiff University, UK. He is also Chair Professor at Hohai University
and the Yangtze Institute for Conservation and Development (since 2019), China. He was previously Professor of Water Engineering (1987-97) at
the University of Bradford. He is a Fellow of the: UK Royal Academy of Engineering, Chinese Academy of Engineering (Foreign member), and
European Academy of Sciences. He was President of the International Association for Hydro-Environment Engineering and Research (2011-15)
and Honorary Member (since 2017) and is Vice President of the International Association for Coastal Reservoir Research (since 2017).
Email address: falconerra@cardiff.ac.uk
[30] De Lange, G. J. (1992). Distribution of Exchangeable, Fixed,
Organic and Total Nitrogen in Interbedded Turbiditic/Pelagic
Sediments of the Madeira Abyssal Plain, Eastern North Atlantic,
Mar. Geol., vol. 109, no. 1–2, pp. 95–114, 1992.
[31] Morse, J. W., and Morin, J. (2005). Ammonium Interaction with
Coastal Marine Sediments: Inuence of Redox Conditions on K.
Mar. Chem., vol. 95, no. 1–2, pp. 107–112, 2005.