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The small sized powdered ferric oxy-hydroxide, termed Dust Ferric Hydroxide (DFH), was applied in batch adsorption experiments to remove arsenic species from water. The DFH was characterized in terms of zero point charge, zeta potential, surface charge density, particle size and moisture content. Batch adsorption isotherm experiments indicated that the Freundlich model described the isothermal adsorption behavior of arsenic species notably well. The results indicated that the adsorption capacity of DFH in deionized ultrapure water, applying a residual equilibrium concentration of 10 µg/L at the equilibrium pH value of 7.9 ± 0.1, with a contact time of 24 h (i.e., Q10), was 6.9 and 3.5 µg/mg for As(V) and As(III), respectively, whereas the measured adsorption capacity of the conventionally used Granular Ferric Hydroxide (GFH), under similar conditions, was found to be 2.1 and 1.4 µg/mg for As(V) and As(III), respectively. Furthermore, the adsorption of arsenic species onto DFH in a Hamburg tap water matrix, as well as in an NSF challenge water matrix, was found to be significantly lower. The lowest recorded adsorption capacity at the same equilibrium concentration was 3.2 µg As(V)/mg and 1.1 µg As(III)/mg for the NSF water. Batch adsorption kinetics experiments were also conducted to study the impact of a water matrix on the behavior of removal kinetics for As(V) and As(III) species by DFH, and the respective data were best fitted to the second order kinetic model. The outcomes of this study confirm that the small sized iron oxide-based material, being a by-product of the production process of GFH adsorbent, has significant potential to be used for the adsorptive removal of arsenic species from water, especially when this material can be combined with the subsequent application of low-pressure membrane filtration/separation in a hybrid water treatment process.
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Article
Performance Evaluation of Small Sized Powdered
Ferric Hydroxide as Arsenic Adsorbent
Muhammad Usman 1, *ID , Ioannis Katsoyiannis 2, Manassis Mitrakas 3ID ,
Anastasios Zouboulis 2ID and Mathias Ernst 1, *ID
1Institute for Water Supply and Water Resources, Hamburg University of Technology, Am Schwarzenberg
Campus 3, 21073 Hamburg, Germany
2Laboratory of Chemical and Environmental Technology, Department of Chemistry, Aristotle University of
Thessaloniki, 54124 Thessaloniki, Greece; katsogia@chem.auth.gr (I.K.); zoubouli@chem.auth.gr (A.Z.)
3
Analytical Chemistry Laboratory, Department of Chemical Engineering, Aristotle University of Thessaloniki,
54124 Thessaloniki, Greece; manasis@eng.auth.gr
*Correspondence: muhammad.usman@tuhh.de (M.U.); mathias.ernst@tuhh.de (M.E.);
Tel.: +49-40-42878-3177 (M.U.); +49-40-42878-3453 (M.E.)
Received: 3 July 2018; Accepted: 18 July 2018; Published: 20 July 2018


Abstract:
The small sized powdered ferric oxy-hydroxide, termed Dust Ferric Hydroxide (DFH),
was applied in batch adsorption experiments to remove arsenic species from water. The DFH was
characterized in terms of zero point charge, zeta potential, surface charge density, particle size
and moisture content. Batch adsorption isotherm experiments indicated that the Freundlich model
described the isothermal adsorption behavior of arsenic species notably well. The results indicated
that the adsorption capacity of DFH in deionized ultrapure water, applying a residual equilibrium
concentration of 10
µ
g/L at the equilibrium pH value of 7.9
±
0.1, with a contact time of 24 h (i.e., Q
10
),
was 6.9 and 3.5
µ
g/mg for As(V) and As(III), respectively, whereas the measured adsorption capacity
of the conventionally used Granular Ferric Hydroxide (GFH), under similar conditions, was found
to be 2.1 and 1.4
µ
g/mg for As(V) and As(III), respectively. Furthermore, the adsorption of arsenic
species onto DFH in a Hamburg tap water matrix, as well as in an NSF challenge water matrix,
was found to be significantly lower. The lowest recorded adsorption capacity at the same equilibrium
concentration was 3.2
µ
g As(V)/mg and 1.1
µ
g As(III)/mg for the NSF water. Batch adsorption
kinetics experiments were also conducted to study the impact of a water matrix on the behavior of
removal kinetics for As(V) and As(III) species by DFH, and the respective data were best fitted to the
second order kinetic model. The outcomes of this study confirm that the small sized iron oxide-based
material, being a by-product of the production process of GFH adsorbent, has significant potential to
be used for the adsorptive removal of arsenic species from water, especially when this material can
be combined with the subsequent application of low-pressure membrane filtration/separation in a
hybrid water treatment process.
Keywords:
arsenic adsorption; small sized powdered ferric hydroxide; granular ferric hydroxide;
water matrix; adsorption kinetics; drinking water
1. Introduction
Arsenic is globally considered as one of the major pollutants in drinking water sources and
a worldwide concern because of its toxicity and carcinogenicity [
1
]. The presence of arsenic at
elevated concentrations in natural environments can be attributed to both natural and anthropogenic
inputs [
2
]. Arsenic pollution is primarily caused by natural processes, such as the weathering of
rocks and minerals, followed by leaching and industrial activities that lead to the pollution of soil and
Water 2018,10, 957; doi:10.3390/w10070957 www.mdpi.com/journal/water
Water 2018,10, 957 2 of 15
groundwater [
3
]. The discharge of arsenic polluted waters from mining or mining-related activities,
the pharmaceutical industry and agricultural activities plays an important role in anthropogenic
arsenic pollution in Asia [
4
]. However, the introduction of arsenic into groundwaters is expected to
occur mainly as a result of its natural geological presence in rocks [5].
Arsenite As(III) and arsenate As(V) are considered as the main oxidation states of inorganic arsenic
found in natural waters. As(V) anions are predominant and stable in oxygen-rich environments,
whereas the As(III) anions prevail in moderately reduced environments (i.e., anaerobic or anoxic
groundwaters). Therefore, arsenic speciation mostly depends on pH and redox potential (Eh)
conditions. Under oxidizing conditions and at pH values relevant to drinking water treatment,
H
3
AsO
4
is present as an oxyanion in the forms of H
2
AsO
4
and/or HAsO
42
, whereas at low Eh
values, arsenic becomes dominant as H
3
AsO
3
. Up to pH 9, H
3
AsO
3
does not dissociate and, therefore,
is present in most natural waters as the uncharged arsenious acid [
6
]. Therefore, As(III) species are
considered as much more mobile in aquifers and cannot be easily adsorbed (and removed) onto the
usually co-existing mineral surfaces, such as those of iron oxides. Moreover, As(III) is more toxic for
the biological systems, as compared to As(V) [3,7].
The pollution of drinkable water sources by arsenic has been reported in more than 70 countries,
where more than 150 million inhabitants are under high health risk [
8
]. Due to its high toxicity
to humans, the World Health Organization [
9
] lowered the guideline value for arsenic in drinking
water from 50 to 10
µ
g/L in 2004, aiming to minimize the health-related problems associated with
arsenic pollution. The same standards also apply in the European Commission, as well as the US
Environmental Protection Agency. Among other countries, the arsenic pollution of groundwater is
considered as a particularly serious health-related problem in Pakistan, as a recent survey reveals [
10
].
Approximately 50 to 60 million people relying on groundwater as a source of drinking water in the
Indus Valley are at high health risk [
11
]. In Punjab, more than 3% of the inhabitants are exposed to
arsenic concentrations higher than 50
µ
g/L in drinking water, and 20% of the population is exposed to
concentrations higher than 10
µ
g/L, while 16% and 36% of inhabitants in Sindh areas are exposed to
arsenic pollution of concentrations higher than 50 µg/L and 10 µg/L, respectively [10].
Several treatment technologies to remove arsenic from drinking water have been applied
worldwide [
12
], including adsorption using activated alumina [
13
] or iron oxide-based adsorbents,
such as tetravalent manganese feroxyhyte [
14
], bayoxide [
15
17
], granular ferric hydroxide (GFH) [
18
],
etc. Other treatment methods include the application of oxidation and arsenic removal using
zero-valent iron (especially in Bangladeh) [
19
], coprecipitation of arsenic with iron or aluminum
salts [
2
], preliminary arsenic oxidation by ozonation or biological oxidation [
19
], ion exchange [
20
],
high pressure membrane separation [
21
,
22
] and electrocoagulation [
23
]. According to Tresintsi [
14
],
chemical precipitation by ferric coagulation has significantly higher arsenic removal efficiencies in
comparison to adsorption by iron oxy-hydroxides, and a drinking water regulation limit can be
achieved at an affordable price, with operational costs estimated between 0.09 and 0.16
/m
3
for
initial arsenic concentrations, ranging between 19 and 208
µ
g/L. However, the major part (>90%) of
treatment costs was attributed to the management of produced sludge, since the coagulant costs are
estimated to be
0.01
[
15
]. Previous studies have identified high pressure membrane processes as
an emerging technology, due to their high removal efficiencies and easy operation features [
21
,
22
],
but these high pressure membrane processes are rather energy (and cost) intensive, and subjected to
the fouling of membranes. Moreover, the disposal of produced brine (high salt concentrations) is a
considerable challenge.
On the other hand, for the treatment of waters with moderate arsenic concentrations, i.e.,
slightly higher than the regulation limits, adsorption onto iron oxide-based adsorbents has been
proved to be the most economically efficient procedure [
15
]. The two mostly commercially applied
adsorbents are the Granular Ferric oxy-Hydroxides (GFH) and the Bayoxide E33 (GFO), which are
favorable in terms of cost, removal efficiency, simplicity of design, operation, maintenance and
minimizing the (secondary) waste production [
24
]. The GFH has been tested to remove arsenic from
Water 2018,10, 957 3 of 15
drinking water sources under both laboratory-scale and full-scale water treatment plants [
15
,
18
,
25
,
26
].
Arsenic adsorption onto GFH is usually performed in a column filtration mode, which is a rather
simple process and can be continuously operated, but the production of this material is relatively
cost intensive. The cost (on dry basis) for GFH and Bayoxide materials was estimated to be 9
/kg
and 12.5
/kg, respectively [
15
]. Currently, the small sized fraction (dust ferric hydroxide, DFH)
generated during the industrial production of GFH cannot be employed in the common column
filtration mode, since the small sized adsorbents can rapidly clog the fixed beds in filter columns,
causing an increased pressure head, thereby increasing energy costs and maintenance and, hence,
reducing the system performance.
Adsorption combined with the application of low-pressure membrane filtration is considered
as a newly developed hybrid water treatment process. Low-pressure membrane processes, such as
microfiltration or even ultrafiltration, have a reasonable energy demand and produce superior quality
treated water with a rather controllable fouling of membranes and incurring quite low capital
and operational costs [
27
]. The low-pressure membrane processes are not able to remove mono-
and polyvalent ions, i.e., arsenic species, from water sources, although they can efficiently remove
suspended solids, colloids, bacteria, viruses and micro-particles [
28
]. If the cost-effective small sized
GFH adsorbent (having a substantially lower commercial price of only 1.6
/kg on a dry basis) has
the potential to remove arsenic species from drinking water sources, it might then be employed in
the adsorption-microfiltration (MF) hybrid treatment scheme to economically and efficiently remove
arsenic. The idea of a submerged membrane filtration adsorption hybrid system could be exploited in
this regard, which allows the pollutant to be in contact with adsorbents for longer time.
The objectives of the study were: (i) To assess the adsorption potential/performance of the smaller
fraction of GFH material with a particle size of <0.250 mm, which is abbreviated as DFH henceforth,
for removing As(V) and As(III) species from different water matrixes; (ii) to determine the kinetics
of arsenic adsorption on the studied material; (iii) to examine the effect of a water matrix on arsenic
removal; and (iv) to compare the efficiencies of both major inorganic arsenic species, As(V) and
As(III), with the established, conventionally applied adsorbents, such as GFH. Badruzzaman [
29
]
studied the use of small sized GFH in packed bed columns, but investigated the adsorption potential
of this material only in the case of As(V) and ultra-pure water and has found promising results.
However, to the best of our knowledge, no comprehensive study concerning the application of
DFH material, systematically studying the arsenic adsorption efficiency of both arsenic species and
different water matrices, such as the tap water of Hamburg (HH tap water) and the NSF (National
Sanitation Foundation) challenge water, used to simulate typical arsenic-containing groundwater,
has yet been performed.
2. Materials and Experimental Methods
2.1. Reagents
For the preparation of As(III) or As(V) 100 mg/L stock solutions, the standard solution of As(III),
as As
2
O
3
in 2% HNO
3
, and As(V), as H
3
AsO
4
in HNO
3
, with a concentration of 1 g/L, were used,
obtained from Carl Roth GmbH + Co. KG (Karlsruhe, Germany) and Merck Chemicals GmbH
(Darmstadt, Germany), respectively. The pH buffer solution, N,N-Bis-(2-hydroxyethyl)-2-aminoethane
sulphonic acid (BES), used in the experiments focusing on arsenic removal from deionized (DI) water,
was obtained from Carl Roth GmbH + Co. KG.
2.2. Material Characterization
The DFH material, with a particle size of <0.250 mm, was supplied by GEH–Wasserchemie
GmbH & Co. KG, Osnabrück, Germany. The material is predominantly akaganeite, a specific form
of an iron oxy-hydroxide [
16
]. DFH is mainly characterized by a relatively large specific surface area
(252 m2/g) [29] and surface charge density (Table 1).
Water 2018,10, 957 4 of 15
Table 1. Main properties of used DFH material.
Properties Value Literature Value
Chemical composition β-FeOOH and Fe(OH)
Dry solids content (%) ~50 a~50 b
Moisture content (%) ~50 a~50 b
Particle size (µm), dp7.4–250 a1.8–250 b
Mean particle size (µm) 78.40
Point of zero charge (PZC) 5.3 ±0.2 ~5.5 c
Isoelectric point (IEP) 7.8 ±0.2 ~7.8 c
Surface charge density 0.9 mmol [OH]/g
Note: aAverage values from triplicate analysis, bData by Ref. [29], cData by Ref. [30].
Particle size distribution was determined by EyeTech
TM
instrument (combi, AmbiValue, Nijerdal,
The Netherlands), ranging between 7.4 and 250
µ
m. The liquid flow cell of EyeTech was filled with
1 L of deionized water, and approximately 100 mg of material was added. Mechanical shaking was
provided in the liquid flow cell, which keeps the material particles in suspension. Then, suspension
was supplied to the optical cell and circulated through it for 5 min at a pump speed of 0.674 L/min.
Three cycles of the suspension were performed to determine the particle size distributions.
To determine the surface charge of DFH in the suspension, the Isoelectric Point (IEP) and the
Point-of-Zero Charge (PZC) were quantified. IEP was determined by a zeta-potential curve at
20
±
1
C of adsorbent dispersion in 0.01 M NaNO
3
, with the respective pH of solution, using a
Micro-electrophoresis Apparatus (Mk II device, Rank Brothers Ltd, Cambridge, England), while PZC
and the surface charge density were defined by the application of acid/base potentiometric mass
titration in suspensions of the adsorbent and for various ionic strengths [31].
2.3. Water Matrix
The test solution was initially prepared using deionized water (DI), spiked with either
As(III) or As(V) species, at an initial concentration of 190
µ
g/L. 2 mM of N,N-Bis(2-
hydroxyethyl)-2-aminoethanesulfonic acid (BES) was added to the test solution, made of DI water,
for pH control at pH 7.9. In addition to DI water, As(V) and As(III) test solutions were prepared in HH
tap water and NSF water with the same initial arsenic concentration, as used in the case of DI water,
in order to study the effect of different water matrixes on the arsenic adsorption capacity. The major
physicochemical parameters of the HH tap water and of NSF challenge water are listed in Table 2.
Table 2.
Water quality parameters of Hamburg tap water (* data obtained from Hamburgwasser) and
NSF challenge water.
Parameter. (mg/L)
Water Matrixs.
HH Tap Water * NSF Challenge Water
Na+14 73.7
Ca2+ 42 40.1
Mg2+ 4 12.6
HCO3150–300 183.0
Cl19 71.0
SO4223 50.0
NO30.62 2.0
F0.13 1.0
PO430.05–0.15 0.123
SiO216.6–18.5 20
DOC 0.8 ±0.2
Water 2018,10, 957 5 of 15
The NSF challenge water was prepared according to the National Sanitation Foundation (NSF)
international and contains the following: 252 mg NaHCO
3
, 12.14 mg NaNO
3
, 0.178 mg NaH
2
PO
4·
H
2
O,
2.21 mg NaF, 70.6 mg NaSiO
3·
5H
2
O, 147 mg CaCl
2·
2H
2
O and 128.3 mg MgSO
4·
7H
2
O in 1 L of DI
water. Prior to adsorption experiments, the pH was adjusted to 7.9 by adding either NaOH or HCl
standard solutions (0.1 N) [32].
2.4. Batch Adsorption Procedure
Batch equilibrium and kinetic adsorption tests were performed to study the adsorption potential of
DFH for removing arsenic species from the different examined test solutions/water matrixes. To derive
the adsorption isotherms, the method of adding various quantities of adsorbent to a constant solution
volume (500 mL), having the same initial concentration of As(V) or As(III) species, was adopted.
Additionally, As(III) test solutions were preliminary bubbled for 30 min with pure N
2
gas at 0.1 bar
(flowrate 11.25 mL/min) to minimize the influence of dissolved oxygen on As(III) potential oxidation
and adsorption, and the flasks were immediately closed and placed on the platform shaker in darkness
in the thermostate cabinet (20
±
0.5
C) to insure the stability of As(III) species during and after
adsorption onto the examined iron oxide-based adsorbents.
The evaluation of the examined adsorbent material focused on its ability to decrease the residual
arsenic concentration below the drinking water regulation limit of 10
µ
g/L (termed Q
10
hereafter),
rather than studying the (more convenient) maximum capacity (Q
max
) at higher residual arsenic
concentrations. If efficiency of the adsorbent was evaluated through Q
max
, which usually points to
high residual concentrations and, indeed, brings high adsorption capacities, but provides marginal
information on its ability to reach low concentrations, such as the regulation limits [33].
Different adsorbent dosages were placed in flasks for the three different water matrixes, while only
adsorbent dosages, ranging between 5–40 mg/L, 6–50 mg/L and 10–80 mg/L, provided equilibrium
As(V) concentrations between 1 and 120
µ
g/L in DI water, HH Tap water and NSF water, respectively.
Adsorbent dosages of 10–60 mg/L, 15–100 mg/L and 40–200 mg/L were found to provide the same
range of equilibrium concentrations in the experiments focusing on the removal of As(III) in DI water,
HH Tap water and NSF water, respectively. For comparison, batch adsorption isotherm studies were
also conducted with the GFH material, using DI water to compare the adsorption characteristics of
GFH with those obtained when using DFH, i.e., to examine the efficiency of both particle size fractions
of this adsorbent. GFH dosages, ranging between 10–80 and 20–120 mg/L, were carefully placed in
flasks for the removal of As(V) or As(III) species, respectively. For each experimental test focusing
on adsorption isotherm, a reference blank sample (i.e., without the presence of an adsorbent) was
filled. The flasks were stirred using a platform shaker for 24 h at 20
±
0.5
C. The equilibration
time was determined for the corresponding kinetic experiments. At the end of the equilibration
time, the suspensions were immediately filtered through a 0.45
µ
m membrane syringe filter (PVDF,
Carl Roth GmbH + Co. KG), and the filtrates were collected and stored for the subsequent analytical
determination of residual (still dissolved and removed) arsenic.
In the kinetic studies, the initial arsenic concentration and adsorbent quantity was kept at 190
µ
g/L
and 50 mg/L, respectively. The initial concentrations of either only As(V) or only As(III) species were
the same as in the respective isotherm studies. Batch adsorption kinetics tests were conducted at the
initial pH value of 7.9. Unlike the isotherm studies, a magnetic stirrer (100 rpm) was used in the kinetic
studies experiments. The samples were collected at regular time intervals and the residual arsenic
in the solution was analyzed. Each set of adsorption batch isotherm and kinetics experiments was
replicated at least twice, and the average values are reported.
2.5. Chemical Analytics
Initial and residual arsenic concentrations were determined by Graphite Furnace Atomic
Absorption Spectrophotometry (Perkin-Elmer 4100ZL, Baesweiler, Germany), using a Perkin-Elmer
4100ZL instrument [
34
]. The limit of detection was 0.5
µ
g/L. Prior to analysis, As(III) water samples
Water 2018,10, 957 6 of 15
from the isotherm experiments were acidified (2 < pH < 4) and passed through a 30 mL column (with
ID = 2 cm), containing an anion exchange resin (Dowex®1×8–100, mesh size 50–100, Sigma-aldrich
Chemie GmbH, Taufkirchen, Germany), which retained As(V), whereas the total arsenic concentration
of water samples from the adsorption kinetics experiments were analyzed. This method of arsenic
speciation needs approx. 50 mL of water sample, noting that, in the kinetics experiments, only small
volumes (~7 mL) of water samples were collected at regular intervals; accordingly, arsenic speciation
using this method was not possible. Therefore, only the total arsenic concentration of the water
samples from adsorption kinetics was analyzed, presenting the concentration of individual arsenic
species in the water samples. The initial concentration of phosphate in HH tap water was measured
using ICP-MS (NexION 300D, PerkinElmer, Baesweiler, Germany).
3. Results and Discussion
3.1. Particle Size Distribution
The particle size has a strong effect on the removal kinetics of arsenic. Banerjee [
35
] observed that
the removal of As(III) by the pulverized/powdered GFH (with d
p
< 63
µ
m) was faster than that of
as-received GFH (0.320 mm < d
p
< 2 mm) at same experimental conditions. A similar trend was also
recorded by Tresintsi [
36
] during the adsorption of arsenic species onto an iron oxide-based adsorbent.
The length-based and volume-based particle size distributions of DFH are shown in Figure 1a,b.
The major fraction of this material has a length-based particle size ranging from 7 to less than 65
µ
m,
while the volume-based particle size has two peaks ranging between (i) 65 and 100
µ
m, and (ii) 200
and 250
µ
m. As DFH has a constant density, the volume-based distribution gives an indication of mass
distribution. The average length-based particle size of DFH particles, as determined by the EyeTech
instrument, is 78.4 µm.
Water 2017, 9, x FOR PEER REVIEW 6 of 14
concentration of individual arsenic species in the water samples. The initial concentration of
phosphate in HH tap water was measured using ICP-MS (NexION 300D, PerkinElmer, Baesweiler,
Germany).
3. Results and Discussion
3.1. Particle Size Distribution
The particle size has a strong effect on the removal kinetics of arsenic. Banerjee [35] observed
that the removal of As(III) by the pulverized/powdered GFH (with dp < 63 µm) was faster than that
of as-received GFH (0.320 mm < dp < 2 mm) at same experimental conditions. A similar trend was
also recorded by Tresintsi [36] during the adsorption of arsenic species onto an iron oxide-based
adsorbent. The length-based and volume-based particle size distributions of DFH are shown in
Figure 1a,b. The major fraction of this material has a length-based particle size ranging from 7 to less
than 65 µm, while the volume-based particle size has two peaks ranging between (i) 65 and 100 µm,
and (ii) 200 and 250 µm. As DFH has a constant density, the volume-based distribution gives an
indication of mass distribution. The average length-based particle size of DFH particles, as
determined by the EyeTech instrument, is 78.4 µm.
(a) (b)
Figure 1. (a) Length-based particle size distribution, and (b) volume-based particle size distribution
of DFH particles, as measured by the EyeTech instrument.
Conventional GFH, which is applied in fixed bed adsorption columns (commercially available),
has particle size ranging from 0.32 to 2 mm, while the particle size of tetravalent manganese
feroxyhyte media, produced by Tresintsi [14] in a kilogram-scale continuous process, is of non-
uniform size. A fine fraction of adsorbent media (with a particle size of <250 µm) is also generated
during its production at the kilogram-scale in a laboratory two-stage continuous flow reactor.
3.2. Batch Isotherm Studies
The batch equilibrium adsorption experiments are conducted to evaluate the adsorption
potential of arsenic species onto small sized powdered ferric hydroxide (DFH). The amount of arsenic
adsorbed at the equilibrium stage is calculated using the mass balance of an adsorption system:
Q=
, (1)
0
10
20
30
40
50
Length (%)
Particle size (µm)
0
5
10
15
20
25
30
35
40
Volume (%)
Particle size (µm)
Figure 1.
(
a
) Length-based particle size distribution, and (
b
) volume-based particle size distribution of
DFH particles, as measured by the EyeTech instrument.
Conventional GFH, which is applied in fixed bed adsorption columns (commercially available),
has particle size ranging from 0.32 to 2 mm, while the particle size of tetravalent manganese feroxyhyte
media, produced by Tresintsi [
14
] in a kilogram-scale continuous process, is of non-uniform size. A fine
fraction of adsorbent media (with a particle size of <250
µ
m) is also generated during its production at
the kilogram-scale in a laboratory two-stage continuous flow reactor.
Water 2018,10, 957 7 of 15
3.2. Batch Isotherm Studies
The batch equilibrium adsorption experiments are conducted to evaluate the adsorption potential
of arsenic species onto small sized powdered ferric hydroxide (DFH). The amount of arsenic adsorbed
at the equilibrium stage is calculated using the mass balance of an adsorption system:
Qe=(CoCe)V
m, (1)
where Q
e
is the amount of arsenic adsorbed at the equilibrium stage per mass of adsorbent, C
o
and
C
e
are the initial and equilibrium concentrations of arsenic in the test solution, respectively; m is the
quantity (mass) of the adsorbent used and V is the volume of test solution.
The non-linear form of the Freundlich and Langmuir isotherm model is used to describe the
adsorption behavior:
Qe=KFC1/n
e, (2)
Qe=Qm
KLCe
1+KLCe, (3)
where K
F
and n are constants, explaining the adsorption capacity and the intensity, respectively;
K
L
and Q
m
are the Langmuir adsorption constant and maximum adsorption capacity per unit mass
of adsorbent, respectively. To identify the fitting of the isotherm model to the experimental data,
a chi-squared value (
χ2
, Equation (4)) was also calculated, in addition to the calculation of correlation
coefficients, for the non-linear form of the isotherm model. According to Tran [
37
], this indicates the
bias in the experimental and model results. Its value is close to zero, if the data obtained using a model
are similar to the experimental data, whereas its high value indicates the high biasness between the
experimental data and the model estimations.
χ2=Qe,exp Qe,cal2
Qe,cal
, (4)
where
Qe,exp
is the amount of arsenic adsorbed at equilibrium, and
Qe,cal
is the amount of arsenic
adsorbed as calculated from the isotherm model.
3.2.1. As(V) Adsorption
The major parameters of the Freundlich isotherm in the case of As(V), along with the correlation
coefficients and the respective chi-squared values, are presented in Table 3, while the Langmuir
isotherm parameters are shown in Table S15 (supplementary information). The correlation coefficients
and the chi-squared values indicated that the Freundlich model described the isothermal adsorption
behavior of arsenic species notably well. The R
2
of the Freundlich model was greater than 0.91 and
χ2
was less than 1 for all of the (3) water matrixes. Badruzzaman [
29
] also reported the fitting of the
Freundlich model for small fractions of GFH with an R2of greater than 0.92.
Table 3.
Parameters of the Freundlich isotherm for As(V), along with the correlation coefficients and
the respective chi-squared values.
Water Matrix Adsorbent n () KF* Q10 (µg/mg) R2χ2
DI water GFH 2.03 0.68 2.1 0.960 0.136
DI water DFH 3.10 3.25 6.9 0.991 0.117
HH tap water DFH 3.41 3.20 6.3 0.941 0.551
NSF water DFH 2.39 1.22 3.2 0.918 0.367
Note: * (µg/mg)/(µg/L)1/n .
Water 2018,10, 957 8 of 15
The As(V) adsorption isotherms for DFH and GFH in DI water at 20
C, with an equilibrium pH
value of 7.9
±
0.1 after a contact time of 24 h, are shown in Figure 2a. The adsorption characteristics of
the DFH and GFH in DI water were analyzed and evaluated using the Freundlich isotherm equation.
The K
F
value for the case of DI water was 3.25 (
µ
g As(V)/mg DFH)/(
µ
g/L)
1/n
and the n value was
3.10, while the corresponding K
F
and n values, in the case of As(V) adsorption onto GFH in DI water,
were found to be 0.68 (
µ
g As(V)/mg GFH)/(
µ
g/L)
1/n
and 2.03, respectively. The adsorption capacity
at an equilibrium liquid phase As(V) concentration of 10
µ
g/L (Q
10
) and an equilibrium pH value
of 7.9
±
0.1, produced by setting the isotherm parameters in the Freundlich model, was found to be
6.9 As(V)/mg DFH and 2.1 mg As(V)/mg GFH. At the equilibrium As(V) concentration of 10
µ
g/L,
the adsorption capacity (Q
10
) of DFH for As(V) was almost triple that of GFH, which is also shown
in Figure 2a. However, the DFH/GFH ratio of adsorption capacity diminishes from 3.2 times to 2.4
and 2.2 times as the equilibrium As(V) concentration was increased to 50
µ
g/L and to 100
µ
g/L,
respectively. Banerjee [
35
] also reported a higher adsorption capacity of the pulverized/powdered
(particle size <63
µ
m) GFH during the adsorption of As(V) in comparison to the as-received GFH
material (with a particle size of 0.32–2.0 mm) after a contact time of 24 h and at the equilibrium pH
value of 7.0–7.5. The Q
10
value reported by Banerjee [
35
] for As(V) is approximately 4 times higher for
pulverized GFH compared to the as-received GFH, whereas the Q10 value of DFH is 3.2 times higher
than the as-received GFH in the current study. These results can be considered to be in agreement,
since the observed small differences are considered negligible and could be attributed to the respective
difference in the initial material used, since the pulverized GFH used by Banerjee [
35
] presented a
particle size smaller than the examined DFH in this study.
Water 2017, 9, x FOR PEER REVIEW 8 of 14
to 2.4 and 2.2 times as the equilibrium As(V) concentration was increased to 50 µg/L and to 100 µg/L,
respectively. Banerjee [35] also reported a higher adsorption capacity of the pulverized/powdered
(particle size <63 µm) GFH during the adsorption of As(V) in comparison to the as-received GFH
material (with a particle size of 0.32–2.0 mm) after a contact time of 24 h and at the equilibrium pH
value of 7.0–7.5. The Q10 value reported by Banerjee [35] for As(V) is approximately 4 times higher
for pulverized GFH compared to the as-received GFH, whereas the Q10 value of DFH is 3.2 times
higher than the as-received GFH in the current study. These results can be considered to be in
agreement, since the observed small differences are considered negligible and could be attributed to
the respective difference in the initial material used, since the pulverized GFH used by Banerjee [35]
presented a particle size smaller than the examined DFH in this study.
In another study, Badruzzaman [29] obtained KF (4.45 (µg As(V)/mg DFH)/(µg/L)1/n) and n (3.57)
values of a similar magnitude using the same fraction of the same adsorbent at the equilibrium pH
value of 7 and 24 ± 0.5 °C after 18 days of contact time. The calculated Q10 value in this case was 8.5
µg As(V)/mg, which is higher than the recorded value in the current study. The divergence in Q10
value between the current study and Badruzzaman [29] could be ascribed to the differences of
experimental conditions (equilibrium pH value, temperature, water matrix and longer contact time).
At pH 7, As(V) is present as an oxyanion in the form of H2AsO4, while it transforms into HAsO42 at
pH 8. The latter requires two active adsorption sites to be adsorbed on the absorbent surface. In
addition, Badruzzaman [29] used bicarbonate as a pH buffer. In the current study, BES was used as
a pH buffer to facilitate the required constant pH condition, since no influence on arsenic adsorption
was observed, which is in agreement with the results reported by Banerjee [35].
(a) (b)
Figure 2. (a) As(V) and (b) As(III) adsorption isotherms for DFH and GFH materials in DI water. Solid
lines represent the Freundlich model using non-linear fitting. Experimental conditions: Initial (As(V))
190 µg/L, initial (As(III)) 190 µg/L, equilibrium pH value 7.9 ± 0.1 and temperature 20 °C.
In the case of granular GFH with particle sizes ranging from 0.32 to 2 mm, Banerjee [38] obtained
a KF value of 3.13 (µg As(V)/mg GFH)/(µg/L)1/n and an n value of 0.23 at the equilibrium pH of 6.5
and at 20 °C. An adsorption capacity of 5.3 µg As(V)/mg GFH is calculated by the Freundlich model,
setting KF and n values at the equilibrium liquid phase, with an As(V) concentration of 10 µg/L.
3.2.2. As(III) Adsorption
The adsorption capacity of DFH in the case of As(III), which is also higher than that obtained by
the commercially-used GFH material, is shown in Figure 2b. The calculated Q10 value in DI water,
under experimental conditions similar to As(V) adsorption isotherm experiments, was 3.5 µg
As(III)/mg DFH (Table 4). Consequently, the Q10 value for As(III) was almost half of the
corresponding value for As(V). The difference in the adsorption efficiencies between As(V) and
As(III) could be attributed to the different behavior of arsenic species at the equilibrium pH value,
because at pH 8, As(V) species predominantly exist as HAsO42 in aqueous solutions, while As(III)
species predominantly exists as undissociated H3AsO3 (pKa = 9.2). As(V) adsorption onto GFH takes
place via electrostatic attraction and Lewis acid–base interactions (ligand exchange reactions) [38,39].
Figure 2.
(
a
) As(V) and (
b
) As(III) adsorption isotherms for DFH and GFH materials in DI water. Solid
lines represent the Freundlich model using non-linear fitting. Experimental conditions: Initial (As(V))
190 µg/L, initial (As(III)) 190 µg/L, equilibrium pH value 7.9 ±0.1 and temperature 20 C.
In another study, Badruzzaman [
29
] obtained K
F
(4.45 (
µ
g As(V)/mg DFH)/(
µ
g/L)
1/n
) and n
(3.57) values of a similar magnitude using the same fraction of the same adsorbent at the equilibrium
pH value of 7 and 24
±
0.5
C after 18 days of contact time. The calculated Q
10
value in this case
was 8.5
µ
g As(V)/mg, which is higher than the recorded value in the current study. The divergence
in Q
10
value between the current study and Badruzzaman [
29
] could be ascribed to the differences
of experimental conditions (equilibrium pH value, temperature, water matrix and longer contact
time). At pH 7, As(V) is present as an oxyanion in the form of H
2
AsO
4
, while it transforms into
HAsO
42
at pH 8. The latter requires two active adsorption sites to be adsorbed on the absorbent
surface. In addition, Badruzzaman [
29
] used bicarbonate as a pH buffer. In the current study, BES was
used as a pH buffer to facilitate the required constant pH condition, since no influence on arsenic
adsorption was observed, which is in agreement with the results reported by Banerjee [35].
In the case of granular GFH with particle sizes ranging from 0.32 to 2 mm, Banerjee [
38
] obtained
a K
F
value of 3.13 (
µ
g As(V)/mg GFH)/(
µ
g/L)1/n and an n value of 0.23 at the equilibrium pH of 6.5
Water 2018,10, 957 9 of 15
and at 20
C. An adsorption capacity of 5.3
µ
g As(V)/mg GFH is calculated by the Freundlich model,
setting KFand n values at the equilibrium liquid phase, with an As(V) concentration of 10 µg/L.
3.2.2. As(III) Adsorption
The adsorption capacity of DFH in the case of As(III), which is also higher than that obtained by
the commercially-used GFH material, is shown in Figure 2b. The calculated Q
10
value in DI water,
under experimental conditions similar to As(V) adsorption isotherm experiments, was 3.5
µ
g As(III)/
mg DFH (Table 4). Consequently, the Q
10
value for As(III) was almost half of the corresponding value
for As(V). The difference in the adsorption efficiencies between As(V) and As(III) could be attributed
to the different behavior of arsenic species at the equilibrium pH value, because at pH 8, As(V) species
predominantly exist as HAsO
42
in aqueous solutions, while As(III) species predominantly exists as
undissociated H
3
AsO
3
(pKa = 9.2). As(V) adsorption onto GFH takes place via electrostatic attraction
and Lewis acid–base interactions (ligand exchange reactions) [38,39]. The higher adsorption capacity
of As(V) is possibly due to As(V) presenting a greater electrostatic attraction to the charged DFH
particles, as compared to the electrically neutral As(III) at circumneutral pH values. Therefore, As(V)
adsorbs better onto DFH than As(III).
Table 4.
Parameters of the Freundlich isotherm in the case of As(III), along with the correlation
coefficients and the chi-squared values.
Water Matrix Adsorbent n () KF* Q10 (µg/mg) R2χ2
DI water GFH 2.66 0.58 1.4 0.985 0.01
DI water DFH 3.96 1.96 3.5 0.987 0.023
HH tap water DFH 4.34 1.64 2.8 0.972 0.036
NSF water DFH 4.39 0.64 1.1 0.970 0.005
Note: * (µg/mg)/(µg/L)1/n.
As shown in Figure 2b, DFH has a higher As(III) adsorption capacity than GFH within the
investigated concentration range. In particular, the obtained Q
10
value of DFH was 2.5 times higher,
than the respective values obtained by GFH. This difference was, however, reduced to 2.1 and 1.9
times at the equilibrium As(III) concentrations of 50
µ
g/L and of 100
µ
g/L, respectively. The Q
10
value of pulverized GFH for As(III), reported by Banerjee [
35
], is approximately 1.8 times higher than
that obtained by the granular GFH experiments. The divergence in Q
10
value might be attributed to
difference in the initial concentration, particle size and equilibrium pH.
The results of this study can be compared with similar studies using very advanced nanomaterials
to achieve efficient arsenic adsorption. The study of Bolisetty [
40
] shows that amyloid–carbon hybrid
membranes containing 10% (by weight) amyloid fibrils indeed diminished the arsenic concentration in
ultrapure water within the drinking water regulation limit, but the adsorption capacity is lower than
0.3 and 1.2
µ
g/mg for As(V) and As(III), respectively, and the adsorption efficiency of amyloid–carbon
hybrid membranes for As(V) is almost 25 times lower than that of DFH (recorded from adsorption
isotherms) and 3 times lower in case of As(III). DFH media is a by-product, otherwise useless in the
water industry, and can find real scale applications in a short period of time with a rather higher
adsorption capacity and lower operational costs.
3.3. Effect of Water Matrix on Arsenic Adsorption
DFH batch adsorption isotherms studies were also conducted with HH tap water and NSF water
to assess the real and practical adsorption potential for removing As(III) and As(V) from drinking
water. The adsorption isotherms for As(V) and As(III) onto DFH at 20
C and at the equilibrium pH
value of 7.9
±
0.1 in three different water matrixes after a contact time of 24 h are shown in Figure 3a,b.
Water 2018,10, 957 10 of 15
Water 2017, 9, x FOR PEER REVIEW 9 of 14
The higher adsorption capacity of As(V) is possibly due to As(V) presenting a greater electrostatic
attraction to the charged DFH particles, as compared to the electrically neutral As(III) at
circumneutral pH values. Therefore, As(V) adsorbs better onto DFH than As(III).
Table 4. Parameters of the Freundlich isotherm in the case of As(III), along with the correlation
coefficients and the chi-squared values.
Water Matrix Adsorbent n () K
F
* Q
10
(μg/mg) R
2
DI water GFH 2.66 0.58 1.4 0.985 0.01
DI water DFH 3.96 1.96 3.5 0.987 0.023
HH tap water DFH 4.34 1.64 2.8 0.972 0.036
NSF water DFH 4.39 0.64 1.1 0.970 0.005
Note: * (µg/mg)/(µg/L)
1/n.
As shown in Figure 2b, DFH has a higher As(III) adsorption capacity than GFH within the
investigated concentration range. In particular, the obtained Q
10
value of DFH was 2.5 times higher,
than the respective values obtained by GFH. This difference was, however, reduced to 2.1 and 1.9
times at the equilibrium As(III) concentrations of 50 µg/L and of 100 µg/L, respectively. The Q
10
value
of pulverized GFH for As(III), reported by Banerjee [35], is approximately 1.8 times higher than that
obtained by the granular GFH experiments. The divergence in Q
10
value might be attributed to
difference in the initial concentration, particle size and equilibrium pH.
The results of this study can be compared with similar studies using very advanced
nanomaterials to achieve efficient arsenic adsorption. The study of Bolisetty [40] shows that amyloid–
carbon hybrid membranes containing 10% (by weight) amyloid fibrils indeed diminished the arsenic
concentration in ultrapure water within the drinking water regulation limit, but the adsorption
capacity is lower than 0.3 and 1.2 µg/mg for As(V) and As(III), respectively, and the adsorption
efficiency of amyloid–carbon hybrid membranes for As(V) is almost 25 times lower than that of DFH
(recorded from adsorption isotherms) and 3 times lower in case of As(III). DFH media is a by-product,
otherwise useless in the water industry, and can find real scale applications in a short period of time
with a rather higher adsorption capacity and lower operational costs.
3.3. Effect of Water Matrix on Arsenic Adsorption
DFH batch adsorption isotherms studies were also conducted with HH tap water and NSF water
to assess the real and practical adsorption potential for removing As(III) and As(V) from drinking
water. The adsorption isotherms for As(V) and As(III) onto DFH at 20 °C and at the equilibrium pH
value of 7.9 ± 0.1 in three different water matrixes after a contact time of 24 h are shown in Figure
3a,b.
(a) (b)
Figure 3. (a) As(V) and (b) As(III) adsorption isotherms for DFH using three different water matrixes.
Solid lines represent the Freundlich model by using non-linear fitting. Experimental conditions: Initial
(As(V)) 190 µg/L, initial (As(III)) 190 µg/L, equilibrium pH value 7.9 ± 0.1 and temperature 20 °C.
Figure 3.
(
a
) As(V) and (
b
) As(III) adsorption isotherms for DFH using three different water matrixes.
Solid lines represent the Freundlich model by using non-linear fitting. Experimental conditions: Initial
(As(V)) 190 µg/L, initial (As(III)) 190 µg/L, equilibrium pH value 7.9 ±0.1 and temperature 20 C.
The adsorption capacity of DFH in HH tap water and in NSF water decreased, as compared to
DI water, over the entire range of equilibrium As(V) concentrations is shown in Figure 3a. However,
the decrease of the adsorption capacity was found to be more significant in the case of NSF water.
For example, the Q
10
value for As(V) in HH tap water and in NSF at the equilibrium pH of 7.9
±
0.1
was 6.3
µ
g As(V)/mg DFH and 3.2
µ
g As(V)/mg DFH, respectively. Due to the presence of different
competing interfering ions, such as phosphate and silica, the reduction of 8.2% and 53.4% in Q
10
values
for As(V) was observed regarding the cases of HH tap water and of NSF water, respectively.
In the case of As(III), the Q
10
value of 3.5
µ
g As(III)/mg observed in DI water at the equilibrium pH
of 7.9
±
0.1 was reduced to 2.8 and 1.1
µ
g As(III)/mg in HH tap water and in NSF water, respectively.
The observed reduction of Q
10
values in the cases of HH tap water and of NSF water, can be attributed
to As(III) speciation, because As(III) is electrically neutral at the set pH value of 7.9, resulting in
nearly negligible electrostatic attraction, while the co-presence of their anions may have significant
electrostatic attraction with the charged surface of DFH. Therefore, reductions of 20.1% and 69.0%
were recorded in the Q
10
values for As(III) in HH tap water and in NSF water, respectively, indicating
also that in NSF water, the concentration of competing and interfering ions is higher, as compared with
HH tap water.
Especially, the presence of phosphates and silicates showed the most adverse effect on the arsenic
adsorption capacity of iron-based adsorbents [
41
,
42
]. At pH 8.2, GFH has a strong affinity with
phosphate, existing as HPO
42
[
43
], and strongly competes with arsenic species for similar adsorption
sites. Amy [
16
] reported a reduction of 3% in the As(V) adsorption capacity of GFH in the presence
of only 125
µ
g/L phosphate at pH 8 during batch tests. However, the reduction of the Q
10
value
increased to 36% when the phosphate concentration was increased to 250
µ
g/L. In the case of silica
competition under the same experimental conditions, Amy [
16
] reported a reduction of 25% and 60%
in the Q
10
values for As(V) when silica was present with 13.5 and 22 mg/L concentrations, respectively.
During the adsorption of As(V) onto tetravalent manganese feroxyhyte (TMF) in batch adsorption tests
at the equilibrium pH of 8, the measured Q
10
values of 10.3
µ
g As(V)/mg and 10.9
µ
g As(III)/mg in DI
water were reduced to 5.4
µ
g As(V)/mg and 4.6
µ
g As(III)/mg in the case of NSF water, resulting in
reductions of 47.6% and 57.8% for As(V) and As(III), respectively [
14
]. In the case of arsenic adsorption
onto Bayoxide in NSF water, Amy [
16
] reported reductions of 54.6% and 96.9% at the equilibrium pH
of 7.5 for As(V) and As(III), respectively. Silicate also presents strong competition with arsenic species
for similar adsorption sites because it exists as H
3
SiO
4
at pH 8, which requires one active site for
adsorption [44].
Water 2018,10, 957 11 of 15
3.4. Arsenic Removal Kinetics
The rate adsorptions of As(V) and As(III) onto DFH in three different water matrixes, at 20
C and
an initial pH of 7.9, respectively, are shown in Figure 4a,b. First-order and second-order kinetic models
were considered to analyze the removal rates of As(V) and As(III) from these aqueous solutions.
Banerjee [
38
] used the first-order kinetic equation to study the adsorption of arsenic species onto
GFH, while Eljamal [
45
] and Saldaña-Robles [
46
] employed the second-order kinetic equation for the
adsorption of As(V). The simple forms of the first- and second-order kinetic models can be expressed
as [38,46]:
lnAst
Aso=k1t, (5)
1
Ast
1
Aso
=k2t, (6)
SE =v
u
u
t"CeCp2
n2#(7)
where
As0
is the initial concentration of arsenic species (either As(V) or As(III)),
Ast
is the liquid
phase arsenic concentration remaining in the solution at time t, and
k1
and
k2
are the first- and
second-order rate constants, respectively.
Ce
and
Cp
are the experimental and the predicted solid
phase arsenic concentrations, and n is the total number of data points. The fitting of the kinetic model
was determined by R2and by the standard error of estimation (SE, Equation (7)).
Water 2017, 9, x FOR PEER REVIEW 12 of 14
surface sites, as well as by the relative surface charge, adsorbed species, and complexation rate of
dissolved species with the surface sites.
(a) (b)
Figure 4. Rate of adsorption of (a) As(V) and (b) As(III) in different water matrixes at an initial pH of
7.9 and temperature of 20 °C, using an adsorbent dosage of 50 mg/L and an initial (As(V)) 190 µg/
L
a
nd initial (As(III)) 190 µg/L.
4. Conclusions
In this study, the potential of a cost-effective DFH adsorbent, considered as a by-product of GFH
production, for As(V) and As(III) was investigated in a systemic and detailed batch-scale study. The
results show that the adsorption isotherm data obtained for As(V) and As(III) at an initial pH of 7.9
were well described by the Freundlich isotherm equation. The calculated adsorption capacity in
deionized ultrapure water at the equilibrium liquid phase concentration of 10 µg/L was 6.9 and 3.5
µg/mg for As(V) and As(III), respectively. The calculated adsorption capacity of GFH at the same pH
value and equilibrium liquid phase concentration, as determined in the present study, was lower
than that of DFH under the applied experimental conditions. At the equilibrium pH value of 7.9 ± 1,
DFH has a considerably higher adsorbent capacity and, therefore, can bind more arsenic within the
given contact time in a technical installation. However, the presence of different competing
interfering ions reduces the adsorption capacity significantly, and the lowest adsorption capacity was
measured in the case of NSF water that has rather elevated levels of silicate and phosphate anions.
The adsorption kinetic data for both As(V) and As(III) fitted well to a second-order kinetic model.
The different interfering ions of HH tap water and NSF artificial water matrixes strongly decrease
the rate of uptake of As(V) and As(III), and the latter is even more greatly affected by the water matrix.
To conclude, this study suggests that DFH might be successfully employed for arsenic removal from
groundwaters, for example, in an adsorption-low pressure membrane hybrid system, which will be
investigated in ongoing research.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Tables S7–S14: Used
adsorbent dosages for adsorption isotherm experiments. Tables S15 and S16: Langmuir isotherms parameters
for arsenic adsorption.
Author Contributions: This article was written by M.U. within his PhD project. M.E. supervised the overall
activities of the research project. These authors designed the research project and conducted the laboratory tests
and analyses. I.K. participated in editing the paper. M.M. and A.Z. participated in the final write up.
Acknowledgments: The authors are grateful to the Higher Education Commission (HEC) of Pakistan, German
Academic Exchange Service (DAAD), Greece State Scholarships Foundation, the German Society for Academic
Exchanges (IKYDA), and the Technische Universität Hamburg.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Smith, A.H.; Hopenhayn-Rich, C.; Bates, M.N.; Goeden, H.M.; Hertz-Picciotto, I.; Duggan, H.M.; Wood, R.;
Kosnett, M.J.; Smith, M.T. Cancer risks from arsenic in drinking water. Environ. Health Perspect. 1992, 97, 259.
Figure 4.
Rate of adsorption of (
a
) As(V) and (
b
) As(III) in different water matrixes at an initial pH of
7.9 and temperature of 20
C, using an adsorbent dosage of 50 mg/L and an initial (As(V)) 190
µ
g/L
and initial (As(III)) 190 µg/L.
R
2
and SE coefficients for both kinetic models, for As(V) and As(III) species, in all (3) examined
water matrixes are shown in Table 5. The calculated values of SE are lower, whereas the R
2
values are
higher for the second-order equation, providing a closer fit. Accordingly, the second-order kinetic
model was used to further analyze the kinetics data of all water matrixes. The second-order rate
constants (k
2
) for As(V) and As(III) in DI water, in HH tap water and in NSF water, are presented in
Table 6. The R
2
ranged from 0.992 to 0.998 and from 0.980 to 0.996 for As(V) and As(III), respectively,
whereas the values of k
2
are in the range of 1.82
×
10
3
to 7.51
×
10
3
h
1
and of 0.34
×
10
3
to
1.34 ×103h1,
as calculated in the case of As(V) and As(III), respectively. The interfering ions present
in the HH tap water and in the NSF water results in significantly lower k
2
values, while the lowest k
2
value was calculated in the case of NSF water.
Water 2018,10, 957 12 of 15
Table 5.
Correlation coefficient (R
2
) for the first- and second-order kinetic models, along with the
standard error of estimation (SE).
Water Matrix
As(V) As(III)
First Order Second Order First Order Second Order
R2SE R2SE R2SE R2SE
DI water 0.529 59.99 0.998 4.93 0.802 35.31 0.996 14.52
HH tap water 0.579 57.62 0.994 14.31 0.769 21.59 0.969 6.72
NSF water 0.714 23.76 0.992 10.68 0.905 14.45 0.980 8.12
Table 6. Second-order rate constant (k2) for the three examined different water matrixes.
Water Matrix
As(V) As(III)
k2(h1) k2(h1)
DI water 7.51 ×1031.34 ×103
HH tap water 4.23 ×1030.85 ×103
NSF water 1.82 ×1030.34 ×103
The results from batch adsorption kinetics reveal that the rate of the adsorption of As(V) onto
DFH is initially fast, followed by a slower rate of adsorption, which eventually approaches an
equilibrium plateau. A similar trend was observed during the adsorption of As(III) for the same
experimental conditions and initial concentration of adsorbate. However, the observed uptake rate
of As(III) onto DFH is slower than that of As(V), possibly because of the insignificant presence of
electrostatic attraction (Coulombic interaction) in the case of As(III) adsorption [
38
,
39
]. For example,
55% and 33% adsorption of As(V) and As(III) occurred within the first 1 h of contact time, respectively,
which increased up to 90% and 59% after at the end of 6 h of contact time. The slower adsorption rate
of both arsenic species after 6 h of contact time can be attributed to the majority of adsorption sites,
already occupied by the adsorbate species, leaving a relatively small number of adsorption sites still
available for adsorption.
The results reveal that the competitive interfering ions present in HH tap water and in NSF water
(in comparison with the respective experiments of DI water) can substantially reduce the removal
rate of both As(V) and As(III). More specifically, the presence of interfering ions has a significant
influence on the behavior of adsorption kinetics. In the case of HH tap water and NSF water, as shown
in Figure 4, the strong interference of competing ions resulted in a lower adsorption rate of As(V) onto
DFH, as concluded from the significant decrease of k
2
values, which decreases from
7.51 ×103h1
(for DI water) to 4.23
×
10
3
h
1
and 1.82
×
10
3
h
1
in the cases of HH tap water and of NSF
water, respectively (Table 6). A similar negative influence, regarding the presence of phosphate and
silicate ions on the As(V) adsorption rate by GFH, was also observed by Xie [
41
] and Nguyen [
42
].
Furthermore, during the adsorption of lead by iron-based adsorbents, Smith [
47
] also reported that the
behavior of adsorption kinetics is influenced by the accessibility and availability of adsorbent surface
sites, as well as by the relative surface charge, adsorbed species, and complexation rate of dissolved
species with the surface sites.
4. Conclusions
In this study, the potential of a cost-effective DFH adsorbent, considered as a by-product of
GFH production, for As(V) and As(III) was investigated in a systemic and detailed batch-scale study.
The results show that the adsorption isotherm data obtained for As(V) and As(III) at an initial pH
of 7.9 were well described by the Freundlich isotherm equation. The calculated adsorption capacity
in deionized ultrapure water at the equilibrium liquid phase concentration of 10
µ
g/L was 6.9 and
3.5
µ
g/mg for As(V) and As(III), respectively. The calculated adsorption capacity of GFH at the same
Water 2018,10, 957 13 of 15
pH value and equilibrium liquid phase concentration, as determined in the present study, was lower
than that of DFH under the applied experimental conditions. At the equilibrium pH value of 7.9
±
1,
DFH has a considerably higher adsorbent capacity and, therefore, can bind more arsenic within the
given contact time in a technical installation. However, the presence of different competing interfering
ions reduces the adsorption capacity significantly, and the lowest adsorption capacity was measured in
the case of NSF water that has rather elevated levels of silicate and phosphate anions. The adsorption
kinetic data for both As(V) and As(III) fitted well to a second-order kinetic model. The different
interfering ions of HH tap water and NSF artificial water matrixes strongly decrease the rate of uptake
of As(V) and As(III), and the latter is even more greatly affected by the water matrix. To conclude,
this study suggests that DFH might be successfully employed for arsenic removal from groundwaters,
for example, in an adsorption-low pressure membrane hybrid system, which will be investigated in
ongoing research.
Supplementary Materials:
The following are available online at http://www.mdpi.com/2073-4441/10/7/957/s1,
Tables S7–S14: Used adsorbent dosages for adsorption isotherm experiments. Tables S15 and S16: Langmuir
isotherms parameters for arsenic adsorption.
Author Contributions:
This article was written by M.U. within his PhD project. M.E. supervised the overall
activities of the research project. These authors designed the research project and conducted the laboratory tests
and analyses. I.K. participated in editing the paper. M.M. and A.Z. participated in the final write up.
Acknowledgments:
The authors are grateful to the Higher Education Commission (HEC) of Pakistan, German
Academic Exchange Service (DAAD), Greece State Scholarships Foundation and the German Society for Academic
Exchanges (IKYDA), and the Technische Universität Hamburg.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Smith, A.H.; Hopenhayn-Rich, C.; Bates, M.N.; Goeden, H.M.; Hertz-Picciotto, I.; Duggan, H.M.; Wood, R.;
Kosnett, M.J.; Smith, M.T. Cancer risks from arsenic in drinking water. Environ. Health Perspect.
1992
,97, 259.
[CrossRef] [PubMed]
2.
Violante, A.; Ricciardella, M.; Del Gaudio, S.; Pigna, M. Coprecipitation of arsenate with metal oxides:
Nature, mineralogy, and reactivity of aluminum precipitates. Environ. Sci. Technol.
2006
,40, 4961–4967.
[CrossRef] [PubMed]
3.
Tantry, B.A.; Shrivastava, D.; Taher, I.; Nabi Tantry, M. Arsenic exposure: Mechanisms of action and related
health effects. J. Environ. Anal. Toxicol. 2015,5, 1. [CrossRef]
4.
Mukherjee, A.; Sengupta, M.K.; Hossain, M.A.; Ahamed, S.; Das, B.; Nayak, B.; Lodh, D.; Rahman, M.M.;
Chakraborti, D. Arsenic contamination in groundwater: A global perspective with emphasis on the Asian
scenario. J. Health Popul. Nutr. 2006, 142–163.
5.
Garelick, H.; Jones, H.; Dybowska, A.; Valsami-Jones, E. Arsenic pollution sources. In Reviews of Environmental
Contamination; Springer: New York, NY, USA, 2009; Volume 197, pp. 17–60.
6.
Ware, G.W.; Albert, L.A.; Crosby, D.G.; Voogt, d.P.; Hutzinger, O.; Knaak, J.B.; Mayer, F.L.; Morgan, D.P.;
Park, D.L.; Tjeerdema, R.S.; et al. Reviews of Environmental Contamination and Toxicology; Springer: New York,
NY, USA, 2005.
7.
Mandal, S.; Sahu, M.K.; Patel, R.K. Adsorption studies of arsenic(III) removal from water by zirconium
polyacrylamide hybrid material (ZrPACM-43). Water Resour. Ind. 2013,4, 51–67. [CrossRef]
8.
Abejón, R.; Garea, A. A bibliometric analysis of research on arsenic in drinking water during the 1992–2012
period: An outlook to treatment alternatives for arsenic removal. J. Water Process Eng.
2015
,6, 105–119.
[CrossRef]
9.
Organization, W.H. Guidelines for Drinking-Water Quality; World Health Organization: Geneva, Switzerland,
2004.
10.
Sanjrani, M.A.; Mek, T.; Sanjrani, N.D.; Leghari, S.J.; Moryani, H.T.; Shabnam, A.B. Current situation of
aqueous arsenic contamination in Pakistan, focused on Sindh and Punjab Province, Pakistan: A review.
J. Pollut. Eff. Cont. 2017,5, 207.
Water 2018,10, 957 14 of 15
11.
Podgorski, J.E.; Eqani, S.A.M.A.S.; Khanam, T.; Ullah, R.; Shen, H.; Berg, M. Extensive arsenic contamination
in high-pH unconfined aquifers in the Indus Valley. Sci. Adv. 2017,3, e1700935. [CrossRef] [PubMed]
12. Hering, J.G.; Katsoyiannis, I.A.; Theoduloz, G.A.; Berg, M.; Hug, S.J. Arsenic removal from drinking water:
Experiences with technologies and constraints in practice. J. Environ. Eng. 2017,143, 3117002. [CrossRef]
13.
Wang, L.; Chen, A.S.C.; Sorg, T.J.; Fields, K.A. Field evaluation of as removal by IX and AA. J. Am. Water
Work. Assoc. 2002,94, 161–173. [CrossRef]
14.
Tresintsi, S.; Simeonidis, K.; Estradé, S.; Martinez-Boubeta, C.; Vourlias, G.; Pinakidou, F.; Katsikini, M.;
Paloura, E.C.; Stavropoulos, G.; Mitrakas, M. Tetravalent manganese feroxyhyte: A novel nanoadsorbent
equally selective for As(III) and As(V) removal from drinking water. Environ. Sci. Technol.
2013
,47, 9699–9705.
[CrossRef] [PubMed]
15.
Tresintsi, S.; Simeonidis, K.; Zouboulis, A.; Mitrakas, M. Comparative study of As(V) removal by ferric
coagulation and oxy-hydroxides adsorption: Laboratory and full-scale case studies. Desalt. Water Treat.
2013
,
51, 2872–2880. [CrossRef]
16.
Amy, G.L.; Chen, H.-W.; Dinzo, A.; Brandhuber, P. Adsorbent Treatment Technologies for Arsenic Removal;
American Water Works Association: Denver, CO, USA, 2005.
17.
´
Curko, J.; Matoši´c, M.; Crnek, V.; Stuli ´c, V.; Mijatovi´c, I. Adsorption characteristics of different adsorbents
and iron(III) salt for removing As(V) from water. Food Technol. Biotechnol.
2016
,54, 250–255. [CrossRef]
[PubMed]
18.
Thirunavukkarasu, O.S.; Viraraghavan, T.; Subramanian, K.S. Arsenic removal from drinking water using
granular ferric hydroxide. Water Sa 2003,29, 161–170. [CrossRef]
19.
Katsoyiannis, I.A.; Mitrakas, M.; Zouboulis, A.I. Arsenic occurrence in Europe: Emphasis in Greece and
description of the applied full-scale treatment plants. Desalt. Water Treat. 2015,54, 2100–2107. [CrossRef]
20.
An, B.; Steinwinder, T.R.; Zhao, D. Selective removal of arsenate from drinking water using a polymeric
ligand exchanger. Water Res. 2005,39, 4993–5004. [CrossRef] [PubMed]
21.
Abejón, A.; Garea, A.; Irabien, A. Arsenic removal from drinking water by reverse osmosis: Minimization of
costs and energy consumption. Sep. Purif. Technol. 2015,144, 46–53. [CrossRef]
22.
Víctor-Ortega, M.D.; Ratnaweera, H.C. Double filtration as an effective system for removal of arsenate
and arsenite from drinking water through reverse osmosis. Process Saf. Environ. Prot.
2017
,111, 399–408.
[CrossRef]
23.
Nidheesh, P.V.; Singh, T.S.A. Arsenic removal by electrocoagulation process: Recent trends and removal
mechanism. Chemosphere 2017,181, 418–432. [CrossRef] [PubMed]
24.
Lata, S.; Samadder, S.R. Removal of arsenic from water using nano adsorbents and challenges: A review.
J. Environ. Manag. 2016,166, 387–406. [CrossRef] [PubMed]
25.
Driehaus, W.; Jekel, M.; Hildebrandt, U. Granular ferric hydroxide—A new adsorbent for the removal of
arsenic from natural water. J. Water Supply Res. Technol. AQUA 1998,47, 30–35. [CrossRef]
26.
Pal, B.N. Granular ferric hydroxide for elimination of arsenic from drinking water. Technol. Arsen. Remov.
Drink. Water 2001, 59–68.
27.
Katsoyiannis, I.A.; Zouboulis, A.I.; Mitrakas, M.; Althoff, H.W.; Bartel, H. A hybrid system incorporating
a pipe reactor and microfiltration for biological iron, manganese and arsenic removal from anaerobic
groundwater. Fresenius Environ. Bull. 2013,22, 3848–3853.
28.
Kalaruban, M.; Loganathan, P.; Shim, W.; Kandasamy, J.; Vigneswaran, S. Mathematical modelling of nitrate
removal from water using a submerged membrane adsorption hybrid system with four adsorbents. Appl. Sci.
2018,8, 194. [CrossRef]
29.
Badruzzaman, M.; Westerhoff, P.; Knappe, D.R.U. Intraparticle diffusion and adsorption of arsenate onto
granular ferric hydroxide (GFH). Water Res. 2004,38, 4002–4012. [CrossRef] [PubMed]
30.
Kersten, M.; Karabacheva, S.; Vlasova, N.; Branscheid, R.; Schurk, K.; Stanjek, H. Surface complexation
modeling of arsenate adsorption by akagenéite (
β
-FeOOH)-dominant granular ferric hydroxide. Colloids Surf.
A Physicochem. Eng. Asp. 2014,448, 73–80. [CrossRef]
31. Kosmulski, M. Surface Charging and Points of Zero Charge; CRC Press: Boca Raton, FL, USA, 2009.
32.
Simeonidis, K.; Papadopoulou, V.; Tresintsi, S.; Kokkinos, E.; Katsoyiannis, I.; Zouboulis, A.; Mitrakas, M.
Efficiency of iron-based oxy-hydroxides in removing antimony from groundwater to levels below the
drinking water regulation limits. Sustainability 2017,9, 238. [CrossRef]
Water 2018,10, 957 15 of 15
33.
Simeonidis, K.; Mourdikoudis, S.; Kaprara, E.; Mitrakas, M.; Polavarapu, L. Inorganic engineered
nanoparticles in drinking water treatment: A critical review. Environ. Sci. Water Res. Technol.
2016
,2,
43–70. [CrossRef]
34. Skoog, D.A.; Leary, J.J. Principles of instrumental analysis. Clin. Chem.-Ref. Ed. 1994,40, 1612.
35.
Banerjee, K.; Nour, S.; Selbie, M.; Prevost, M.; Blumenschein, C.D.; Chen, H.W.; Amy, G.L. Optimization
of process parameters for arsenic treatment with granular ferric hydroxide. In Proceedings of the AWWA
Annual Conference, Anaheim, CA, USA, 15–19 June 2003.
36.
Tresintsi, S.; Mitrakas, M.; Simeonidis, K.; Kostoglou, M. Kinetic modeling of AS(III) and AS(V) adsorption
by a novel tetravalent manganese feroxyhyte. J. Colloid Interface Sci. 2015,460, 1–7. [CrossRef] [PubMed]
37.
Tran, H.N.; You, S.-J.; Hosseini-Bandegharaei, A.; Chao, H.-P. Mistakes and inconsistencies regarding
adsorption of contaminants from aqueous solutions: A critical review. Water Res.
2017
,120, 88–116.
[CrossRef] [PubMed]
38.
Banerjee, K.; Amy, G.L.; Prevost, M.; Nour, S.; Jekel, M.; Gallagher, P.M.; Blumenschein, C.D. Kinetic and
thermodynamic aspects of adsorption of arsenic onto granular ferric hydroxide (GFH). Water Res.
2008
,42,
3371–3378. [CrossRef] [PubMed]
39.
Manning, B.A.; Fendorf, S.E.; Goldberg, S. Surface structures and stability of arsenic(III) on goethite:
Spectroscopic evidence for inner-sphere complexes. Environ. Sci. Technol. 1998,32, 2383–2388. [CrossRef]
40.
Bolisetty, S.; Reinhold, N.; Zeder, C.; Orozco, M.N.; Mezzenga, R. Efficient purification of
arsenic-contaminated water using amyloid-carbon hybrid membranes. Chem. Commun.
2017
,53, 5714–5717.
[CrossRef] [PubMed]
41.
Xie, B.; Fan, M.; Banerjee, K. Modeling of arsenic (V) adsorption onto granular ferric hydroxide. J. Am. Water
Works Assoc. 2007,99, 92–102. [CrossRef]
42.
Nguyen, V.L.; Chen, W.-H.; Young, T.; Darby, J. Effect of interferences on the breakthrough of arsenic: Rapid
small scale column tests. Water Res. 2011,45, 4069–4080. [CrossRef] [PubMed]
43.
Genz, A.; Kornmüller, A.; Jekel, M. Advanced phosphorus removal from membrane filtrates by adsorption
on activated aluminium oxide and granulated ferric hydroxide. Water Res.
2004
,38, 3523–3530. [CrossRef]
[PubMed]
44.
Tresintsi, S.; Simeonidis, K.; Vourlias, G.; Stavropoulos, G.; Mitrakas, M. Kilogram-scale synthesis of iron
oxy-hydroxides with improved arsenic removal capacity: study of Fe(II) oxidation—Precipitation parameters.
Water Res. 2012,46, 5255–5267. [CrossRef] [PubMed]
45.
Eljamal, O.; Sasaki, K.; Tsuruyama, S.; Hirajima, T. Kinetic model of arsenic sorption onto zero-valent iron
(ZVI). Water Qual. Expo. Health 2011,2, 125–132. [CrossRef]
46.
Saldaña-Robles, A.; Saldaña-Robles, N.; Saldaña-Robles, A.L.; Damian-Ascencio, C.; Rangel-Hernández, V.H.;
Guerra-Sanchez, R. Arsenic removal from aqueous solutions and the impact of humic and fulvic acids.
J. Clean. Prod. 2017,159, 425–431. [CrossRef]
47.
Smith, E.H. Surface complexation modeling of metal removal by recycled iron sorbent. J. Environ. Eng.
1998
,
124, 913–920. [CrossRef]
©
2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).
... To improve the arsenic adsorption, some references have shown that micro-sized iron hydroxide particles can increase the adsorption capacity [19,22,23] and that processes operated using these small particles in stirred systems increase the arsenic-adsorbent masstransfer [17,19,22] when compared to the GFH in columns. In the case of iron-based NMs, several reports have shown that the maximum adsorption capacity is clearly increased in stirred batch systems by reducing the particle size within the nanometer range [10,13,14]. ...
... To improve the arsenic adsorption, some references have shown that micro-sized iron hydroxide particles can increase the adsorption capacity [19,22,23] and that processes operated using these small particles in stirred systems increase the arsenic-adsorbent masstransfer [17,19,22] when compared to the GFH in columns. In the case of iron-based NMs, several reports have shown that the maximum adsorption capacity is clearly increased in stirred batch systems by reducing the particle size within the nanometer range [10,13,14]. ...
... When the samples reached equilibrium, q e and C e were obtained with Equation (1). The Freundlich (Equation (4)) and Langmuir (Equation (5)) isotherms were used to fit the experimental equilibrium data of the arsenic concentration [2,22,37]. ...
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The Most Detailed Resource Available on Points of Zero Charge With their work growing in complexity, chemists involved with surface phenomena-related projects have outgrown the common resources available to them on points of zero charge (PZC) of oxides. Reporting on a limited number of materials in a limited number of scenarios, these resources often leave scientists wondering if the variances reported in the results they depend upon are due to actual differences in properties among particular samples or due to differences between isoelectric points (IEP) and points of zero charges obtained by titration. Taking on the monumental task of building a complete reference, Marek Kosmulski, a leading authority in the field of surface chemistry (Hirsch index of 22), takes a new approach to provide chemists with the most detailed resource on the points of zero charge of oxides available to date. Surface Charging and Points of Zero Charge presents PZC data on well-defined specimens of materials sorted by trademark, manufacturer (commercial materials), location (natural materials), and specific recipe (synthetic materials). The text emphasizes the comparison between particular results obtained for different portions of the same or very similar material. Synthesizing information published in research reports over the past few decades, this invaluable reference: • Characterizes materials in terms of thermochemical data, chemical composition (level of impurities), crystallographic structure, specific surface area (various methods), particular size, and morphology • Provides additional references to more detailed sample characterization (SEM and TEM images, XRD patterns, and particle size distributions) • Reviews the PZC and IEP--with all possible details regarding the method, type of instrument, and experimental conditions • Pays special attention to correlations of the PZC and IEP with other physical quantities and properties, surface charging in mixed and nonaqueous solvents, surface charging at high ionic strengths, and ion-specificity in 1-1 electrolytes All available sources were used to obtain the data in this reference making it the definitive resource on PZC/IEP. Destined to become a classic, Surface Charging and Points of Zero Charge points the way for further research with tried and true methods that help researchers avoid the doubt that can lead to countless hours of unnecessary research.
Reviews of Environmental Contamination and Toxicology attempts to provide concise, critical reviews of timely advances, philosophy, and significant areas of accomplished or needed endeavor in the total field of xenobiotics, in any segment of the environment, as well as toxicological implications.
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