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Arsenic removal from drinking water using by-products from conventional iron oxyhydroxides production as adsorbents coupled with submerged microfiltration unit

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Arsenic is among the major drinking water contaminants affecting populations in many countries because it causes serious health problems on long-term exposure. Two low-cost micro-sized iron oxyhydroxide-based adsorbents (which are by-products of the industrial production process of granular adsorbents), namely, micro granular ferric hydroxide (μGFH) and micro tetravalent manganese feroxyhyte (μTMF), were applied in batch adsorption kinetic tests and submerged microfiltration membrane adsorption hybrid system (SMAHS) to remove pentavalent arsenic (As(V)) from modeled drinking water. The adsorbents media were characterized in terms of iron content, BET surface area, pore volume, and particle size. The results of adsorption kinetics show that initial adsorption rate of As(V) by μTMF is faster than μGFH. The SMAHS results revealed that hydraulic residence time of As(V) in the slurry reactor plays a critical role. At longer residence time, the achieved adsorption capacities at As(V) permeate concentration of 10 μg/L (WHO guideline value) are 0.95 and 1.04 μg/mg for μGFH and μTMF, respectively. At shorter residence time of ~ 3 h, μTMF was able to treat 1.4 times more volumes of arsenic-polluted water than μGFH under the optimized experimental conditions due to its fast kinetic behavior. The outcomes of this study confirm that micro-sized iron oyxhydroxides, by-products of conventional adsorbent production processes, can successfully be employed in the proposed hybrid water treatment system to achieve drinking water guideline value for arsenic, without considerable fouling of the porous membrane. Graphical abstract
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RECENT DEVELOPMENTS AND INNOVATIVE STRATEGIES IN ENVIRONMENTAL SCIENCES IN EUROPE
Arsenate removal from drinking water using by-products
from conventional iron oxyhydroxides production as adsorbents
coupled with submerged microfiltration unit
Muhammad Usman
1
&Ioannis Katsoyiannis
2
&Josma Henna Rodrigues
1
&Mathias Ernst
1
Received: 6 January 2020 /Accepted: 4 March 2020
#The Author(s) 2020
Abstract
Arsenic is among the major drinking water contaminants affecting populations in many countries because it causes serious health
problems on long-term exposure. Two low-cost micro-sized iron oxyhydroxide-based adsorbents (which are by-products of the
industrial production process of granular adsorbents), namely, micro granular ferric hydroxide (μGFH) and micro tetravalent
manganese feroxyhyte (μTMF), were applied in batch adsorption kinetic tests and submerged microfiltration membrane adsorp-
tion hybrid system (SMAHS) to remove pentavalent arsenic (As(V)) from modeled drinking water. The adsorbents media were
characterized in terms of iron content, BET surface area, pore volume, and particle size. The results of adsorption kinetics show
that initial adsorption rate of As(V) by μTMF is faster than μGFH. The SMAHS results revealed that hydraulic residence time of
As(V) in the slurry reactor plays a critical role. At longer residence time, the achieved adsorption capacities at As(V) permeate
concentration of 10 μg/L (WHO guideline value) are 0.95 and 1.04 μg/mg for μGFH and μTMF, respectively. At shorter
residence time of ~ 3 h, μTMF was able to treat 1.4 times more volumes of arsenic-polluted water than μGFH under the
optimized experimental conditions due to its fast kinetic behavior. The outcomes of this study confirm that micro-sized iron
oyxhydroxides, by-products of conventional adsorbent production processes, can successfully be employed in the proposed
hybrid water treatment system to achieve drinking water guideline value for arsenic, without considerable fouling of the porous
membrane.
Keywords Arsenic removal .Granular ferric hydroxide .Micro-sized iron oxyhydroxides .Waste utilization .Adsorption
kinetics .Submerged membrane adsorption hybrid system .Drinking water production
Introduction
Groundwater is globally the foremost source of drinking water
for human consumption. Arsenic contamination of drinking
waters across the world is one of the most serious water related
problems, because it affects big parts of the global population
and it is very harmful to human health. It is well-known that
inorganic forms of arsenic are a strong human carcinogen. The
World Health Organization (WHO) has set a guideline value
of 10 μg/L in drinking water. Both natural and anthropogenic
sources contribute to elevated concentrations of arsenic in
natural environments (Violante et al. 2006). In groundwaters,
arsenic is present mainly with its inorganic forms, arsenite,
which is the trivalent form of arsenic [As (III)], and arsenate,
which is the pentavalent form of arsenic [As(V)]. More than
150 million inhabitants are under high health risk in more than
70 countries due to pollution of drinking water by arsenic
(Abejón and Garea 2015). Therefore, many different attempts
such as the use of zero valent iron with main applications in
South east Asian countries (Katsoyiannis et al. 2015b) and
adsorption of arsenic onto iron oxides and iron oxide-coated
Responsible Editor: Bingcai Pan
*Muhammad Usman
muhammad.usman@tuhh.de
*Mathias Ernst
mathias.ernst@tuhh.de
1
Institute for Water Resources and Water Supply, Hamburg
University of Technology, Am Schwarzenberg-Campus 3,
20173 Hamburg, Germany
2
Department of Chemistry, Laboratory of Chemical and
Environmental Technology, Aristotle University of Thessaloniki,
54124 Thessaloniki, Greece
https://doi.org/10.1007/s11356-020-08327-w
/ Published online: 10 April 2020
Environmental Science and Pollution Research (2021) 28:59063–59075
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
sand in fixed-bed adsorption filters (Katsoyiannis et al. 2015a;
Callegari et al. 2018) are being made to remove it so that
people have safe drinking water supplies.
The use of fixed-bed adsorption media filters has gained
considerable attention, especially for the treatment of waters
with relatively low initial arsenic concentrations (i.e., in the
range of 20 to 50 μg/L) due to the simplicity of operation and
the efficiency of arsenic removal. These filters usually use the
granular size of the adsorbents, i.e., higher than 250 μm
(Thirunavukkarasu et al. 2003). However, micro-sized frac-
tions (particle sizes of 1 to 250 μm) of granular ferric hydrox-
ide that are very effective for adsorption of trivalent arsenic
(As (III)) and pentavalent arsenic (As(V)) as reported by
Usman et al. (2018) cannot be used in fixed-bed filters be-
cause of high clogging potential in fixed-bed adsorption fil-
ters, rapidly causing an increased pressure head, and thereby
increasing energy costs and maintenance (Kalaruban et al.
2018; Vieira et al. 2017). However, considerable amounts of
micro-sized fraction of granular ferric hydroxide (GFH),
termed μGFH and tetravalent manganese feroxyhyte (TMF),
termed μTMF are generated. μGFH is a by-product from in-
dustrial production of GFH, while μTMF produced during
kilogram-scale production at the laboratory scale. These by-
products might be applied in drinking water production not
only to reduce the cost of water treatment, but also supply
methods for by-product utilization. If fine fractions of adsor-
bent media will be added to a fluidized bed reactor, which
treats water containing arsenic, adsorption of arsenic will take
place. The treated water will subsequently be separated by
low-pressure membrane submerged in a reactor. Recently,
Hilbrandt et al. (2018) combined embedment of μGFH in a
up-flow rapid filter with subsequent particle separator. The
objective of particle separator is to retain the particles that
might be washed out during the filtration process. However,
they have tested the aforementioned process using cylindrical
glass columns (2.4 × 100 cm) and its scaling up has not yet
been performed, since rapid filter closely resemble the fixed-
bed adsorption filter and fixed-bed filters packed with fine
fractions of adsorbents are known to be prone to clogging
during the filtration process in waterworks.
In the present study, the use of micro-sized fractions of the
proposed adsorbents is investigated and its application is pro-
posed in a combined unit with low-pressure membrane filtra-
tion to create an innovative hybrid treatment process, which
could reduce the overall costs of treatment by using low-cost
adsorbents and create a treatment process easily applicable from
the household to the community based level. Low-pressure
membrane processes, such as microfiltration (MF) or even ul-
trafiltration (UF), have a reasonable energy consumption and in
general produce excellent hygienic quality treated water with a
rather controllable membrane fouling at moderate capital costs
(Katsoyiannis et al. 2013). A study of Drouiche et al. (2001)on
economic performance of ultrafiltration membrane process
indicated that a drinking water system (480 m
3
/day) treating
surface water in the Kabylia region of Algeria incurred a total
cost of 0.235$/m
3
including capital, energy (0.010$/m
3
), mem-
brane cleaning, and replacement costs.
Recent studies show that the removal of organic and inor-
ganic pollutants from drinking water by a submerged mem-
brane adsorption hybrid system (SMAHS) using micro-sized
adsorbents is a promising technology (Vigneswaran et al.
2003; Kalaruban et al. 2018; Hilbrandt et al. 2019). The per-
formance of a SMAHS depends on the adsorption capacity of
the applied adsorbent media to remove specific pollutants,
mode of adsorbent dosage (initially or continuously dosed to
adsorption reactor), adsorption reactor configuration, and op-
erating conditions including water matrix, hydrodynamic con-
ditions such as air bubbling rate, water flux, feed water pH,
temperature, etc. (Vigneswaran et al. 2003; Campos et al.
2000; Jia et al. 2009). High membrane water fluxes reduce
costs due to large amounts of water being treated by small
footprint installations. However, if due to increasing mem-
brane water fluxes the hydraulic retention time in the adsorp-
tion reactor is rather short, the pollutant removal efficiency
may decrease. Also, high fluxes may increase the rate of foul-
ing on the membrane (critical flux phenomena). Nevertheless,
applying aeration to the adsorbent suspension keeps the ad-
sorbent particles completely dispersed in the reactor and helps
to reduce the solid deposition on the membrane surface by the
air scouring effect (Kalaruban et al. 2018; Stylianou et al.
2018;Choietal.2009). For example, in powdered activated
carbon (PAC) adsorption-membrane filtration systems, PAC
might be initially or continuously dosed into the adsorption
reactor. For the PAC initially dosedmode, the required
PAC is added into the reactor at the start of each filtration
cycle. For the PAC continuously dosedmode, the PAC is
continuously dosed into the adsorption reactor during the fil-
tration cycle. Mathematical modeling using different adsorp-
tion kinetic models has shown that higher removal efficiency
canbeacquiredwiththeadsorbent initially dosed modedue
to higher adsorbent use efficiency with this approach (Campos
et al. 2000; Chang et al. 2003).
The SMAHS and fixed-bed adsorption filters are dynamic
and continuous flow treatment systems, which are more rele-
vant to real-water treatment process than the static batch sys-
tem of water treatment. The advantage of a SMAHS over
conventional-bed filters is that micro-sized adsorbents
exhibiting high surface area can be used in this system
(Kalaruban et al. 2018). Moreover, the micro-sized iron
oxyhydroxides are cheaper than the granular fractions.
Currently, the costs (on dry basis) for GFH and μGFH mate-
rials was estimated to 9 /kg vs. 1.6 /kg, respectively (Usman
et al. 2018). Another synergistic advantage of a SMAHS over
fixed-bed absorbers is that low-pressure membranes achieve
simultaneous removal of colloids, microorganisms, and
suspended solids (Lebeau et al. 1998).
59064 Environ Sci Pollut Res (2021) 28:59063–59075
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
The primary objective of the study is to exploit the
performance of micro-sized porous iron oxyhydroxides
and also to identify the low-cost materials suitable for
application in SMAHS for As(V) removal from a model
drinking water source at varying operating conditions. The
adsorbent efficiency was assessed in terms of volume of
treated water until 10 μg As(V)/L (EU drinking water
directives, US environmental protection agency (EPA)
and World Health Organization (WHO) guideline value)
was reached as well as the amount of arsenic adsorbed
per unit mass of adsorbent. Moreover, the applied adsor-
bent media was characterized to better understand adsorp-
tion behavior. To the best knowledge of the authors, the
performance of an adsorption-submerged membrane hybrid
system using micro-sized fractions of conventional adsor-
bents to remove arsenic from drinking water under contin-
uous flow operation is presented for first time in this
study.
Material and methods
μGFH was obtained from GEH Wasserchemie GmbH &
Co, Osnabrück, Germany, and TMF was kindly supplied
by colleague Manassis Mitrakas from Aristotle University
of Thessaloniki (Tresintsi et al. 2013a). μGFH is a by-
product generated during the industrial production of
GFH, which is produced from a ferric chloride solution
by neutralization and precipitation with sodium hydroxide.
The ferric hydroxide precipitate was centrifuged and gran-
ulated by a high-pressure process (Thirunavukkarasu et al.
2003), while preparation of TMF involves the
coprecipitation of FeSO
4
and KMnO
4
in a kilogram-scale
continuous process. The kilogram-scale production in a lab-
oratory two-stage continuous flow reactor includes the
coprecipitation into the water of the iron source (FeSO
4
·
H
2
O) at pH 4 and the manganese source (KMnO
4
), which
is an oxidant for the process and also used to adjust the
reactions redox to 850 mV (Tresintsi et al. 2013a). The
generated fraction of μTMF during a laboratory-scale pro-
duction of TMF was ca. 10%. Therefore, granular TMF
(0.32 mm) was grounded to achieve abundant quantity
of μTMF to apply in continuous flow experiments. All
results were presented on a dry mass basis of both iron
oxyhydroxides after drying at 105 °C for 24 h and subse-
quent cooling in a desiccator.
Adsorbent characterization
Particle size distribution was determined by EyeTechan-
alyzer (Combi, AmbiValue, the Netherlands). The iron con-
tent of the adsorptive media was determined by acid diges-
tion. Briefly, 1 g of media (on dry basis) was added to
50 mL of 10% HNO
3
in a glass beaker and the suspension
was heated using a hot plate to boiling point. After 2 h, the
iron oxide in the suspension was completely dissolved and
the acid solution turned yellow (AWWARF 1993)). At this
point, heating was ceased, the suspension after cooling was
made up to 1 L with distilled deionized (DI) water, filtered
through 0.45-μm filter, and the iron content determined by
DIN 38406 method using a photometer (model UV-1700,
Shimadzu, Germany). The surface area of the media was
determined by nitrogen gas adsorption at liquid N
2
temper-
ature (77 K) using a surface area analyzer (Nova 4200,
Quantachrome Instruments, USA) according to Brunauere
Emmette Teller (BET) model. Six-point surface area mea-
surements were employed to determine the surface area of
the samples.
Batch adsorption kinetic procedure
The kinetics of adsorption was conducted in model ground
water (prepared according to National Sanitation
Foundation (NSF) standards, termed NSF water hereafter)
at pH 8 ± 0.1 with an adsorbent dose of 100 mg/L and a
As(V) concentration of 190 μg/L by shaking Schott flask
(2 L) containing test solution (1 L) at 150 rpm using a
platform shaker. NSF water has the following composition:
252 mg NaHCO
3
,12.14mgNaNO
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
(Simeonidis et al. 2017). The standard solution of As(V)
was H
3
AsO
4
in HNO
3
(0.05 mol/L) with a concentration of
1 g/L. It was obtained from Merck chemical GmbH
(Darmstadt, Germany). The experiments for each adsorbent
were performed in duplicate.
Samples were taken at specific time intervals for a period
up to 360 min. Upon sampling, samples were filtered with
0.45-μm (PESmembrane) syringe filters and the filtrates were
stored and analyzed at pH 2 using hydrochloric acid for total
arsenic. A Perkin-Elmer atomic absorption spectrometer with
a Perkin-Elmer Graphite Furnace Tube atomizer was used to
measure the arsenic concentrations (Bower 1992). Argon gas
was used to atomize the samples. The instrument setup param-
eters were 380 mA lamp current, detection at a wavelength of
193.7 nm, 0.7 nm silt width, and peak area as measurement
mode.
Submerged membrane adsorption hybrid system
A self-assembled MF membrane module was made with
hollow fiber outside-in PVDF-type membrane (Microza
microfilter, Pall membrane) with specifications of 0.1 μm
nominal pore size and 0.018 m
2
was used in a SMAHS to
separate the loaded adsorbent particles (Fig. 1). The inner
and outer diameter of hollow MF fiber is 0.7 and 1.3 mm,
59065Environ Sci Pollut Res (2021) 28:59063–59075
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
respectively. The feed solution was prepared using NSF
water spiked with As(V) to adjust a concentration of either
190 μg/L or 380 μg/L, and the pH of the solution was
maintained at 8 ± 0.1.
The SMAHS experiments were carried out in a continuous
flow operation. The adsorbents (1 g and 5 g each) were ini-
tially added to As(V) contaminated NSF water (1 L) in the
slurry reactor and the MF membrane was submerged into the
reactor. A peristaltic pump was used to feed the influent water
to the slurry reactor and permeate was drawn from the reactor
through a second peristaltic pump. The flow rates of both feed
and permeate pumps were maintained identical and in order to
keep the water level in the reactor constant throughout the
experiments. Air was entered continuously from the bottom
to keep the adsorbent particles in suspension and to generate
scoring on membrane surface. In the absence of air, adsorbent
particles could settle at the bottom of the slurry reactor and
thus circumventing close contact between adsorbent and
As(V) species. The transmembrane pressure was measured
by a signal conditioned precision vacuum pressure transducer
(423SC15D-PCB, Sensortechnics). The data were collected
automatically by a data logger.
In most of studies (Adham et al. 1993; Hashlamon et al.
2017;Quetal.2018), the adsorption process is chosen as a pre-
treatment. However, in this system, the iron oxyhydroxides
were added directly to the slurry reactor. Consequently,
As(V) adsorption and separation of arsenic-loaded adsorbent
particles by MF membrane take place simultaneously in a sin-
gle reactor.
Results and discussion
Characterization of adsorbents
Table 1summarizes the physicochemical data derived by own
analyses for both applied adsorbents.
As it can be seen in Table 1, both materials present a quite
high Fe content and surface area. The results are similar to
results of other studies. The determined BET surface area and
Fe content of TMF were 187 m
2
/g and 38.1 wt%, respectively
(Tresintsi et al. 2013a), while the BET surface area of μGFH
reported by Hilbrandt et al. (2018)is305m
2
/g. Both ad-
sorbents have quite high Fe content, which is important re-
garding their adsorptive capacity, since the adsorption of ar-
senic takes place mainly because of the iron-based adsorption
sites. In particular the adsorption of As(V) is believed to be
dominated by (monodentate and bidentate) inner-sphere com-
plexation between media surface groups and adsorbing
moelcules. These types of surface complexes are restricted
Air diffuse r
Pressurized air
Air bubbles
Adsorbent in
suspension
Hollow fiber MF
membrane
Feed water
tank Permeate
water tank
Peristalc
pump
Peristalc
pump
Overflow pipe
Slurry react or
Air flow me ter
Vacuum press ure
transducer
Fig. 1 Schematic diagram of the
submerged membrane adsorption
hybrid system (SMAHS)
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Content courtesy of Springer Nature, terms of use apply. Rights reserved.
to ions such asarsenicthat have a high affinity for surface sites
and can bind to the media surface through covalent bonding
(Essington 2015). The possible ligand exchange reaction for
adsorption of As(V) by iron oxide-based adsorbent may in-
clude (Banerjee et al. 2008):
H2AsO4
aqðÞ
þFeOOH sðÞ
FeH2AsO4sðÞþOHˉð1Þ
Regarding the surface area of the adsorbents, μGFH has
considerably higher BET surface area than μTMF. This might
play an important role on the adsorption efficiency of arsenic,
acting synergistically to the very high Fe content. Both adsor-
bents have an isoelectric point at values above 7, in particular
at 7.2 for μTMF and 7.8 for μGFH. The IEP value of the solid
adsorbents plays a critical role as at solution pH values lower
than the IEP, the overall surface charge of the adsorbent is
positive. Given the fact that As(V) species are anionic over a
broad pH range (i.e., from 3 up to9) (H
2
AsO
4
and HAsO
42
),
their adsorption onto porous (oxy) hydroxides (μGFH and
μTMF) takes place mainly via electrostatic (Coulombic inter-
action) forces as well as ligand exchange reactions (Lewis
acid-base interactions) to form monodentate and bidentate
inner-sphere complexes (Banerjee et al. 2008). In the present
study, the feed solution pH was much higher than the isoelec-
tric point (IEP), which indicates that Coulombic interaction
was not expected to be the main mechanism for As(V) adsorp-
tion and it is adsorbed onto iron oxyhydroxides by the forma-
tion of monodentate and bidentate inner-sphere complexes.
Generally, it is presumably believed that porous nature of iron
(oxy) hydroxides leads to As(V) adsorption at internal iron
complexation sites (Sinha et al. 2002; Badruzzaman et al.
2004).
Effect of contact time
Batch adsorption kinetics experiments were conducted to
study the effect of contact time on the rate of adsorption of
As(V) onto adsorbent media (Fig. 2). The adsorption kinetic
plots exhibit that initial rate of As(V) adsorption onto both
adsorbents is rapid and removal rate increases with increasing
contact time. At the end of the experiment (contact time of
6 h), about 95% of the As(V) was adsorbed onto μTMF rel-
ative to μGFH (93%). Due to the large concentration gradient
between the bulk solution and media surface, the rapid initial
removal rate of As(V) has been observed. This can be seen in
Fig. 2that about 7080% of As(V) was adsorbed within the
first 1 h of contact and only 2030% additional As(V) adsorp-
tion has occurred in the following 5 h. Banerjee et al. (2008)
reported that similar pattern during adsorption of As(V) onto
GFH (with particle sizes between 0.32 and 2 mm) in ultrapure
deionized water, but at a much higher initial arsenic to media
ratio of 0.4 μg As(V)/mg GFH. In the present study, initial
arsenic to media ratio is 1.9 μg As(V)/mg. It can beconcluded
that small size of μGFH favors faster removal rate of As(V)
compared to GFH.
First- and second-order adsorption kinetic models were
considered to analyze the removal rates of As(V) from aque-
ous solution. The simple forms of the first- and second-order
kinetic models can be expressed as follows:
The first-order kinetic model:
ln Ast
Aso

¼k1t;ð2Þ
The second-order kinetic model:
1
Ast
1
Aso
¼k2t;ð3Þ
Fig. 2 Effect of contact time onAs(V) adsorption rate onto adsorbents in
NSF water (n= 2). Solid lines represent the fitting using second-order
adsorption kinetic model. Experimental conditions: Adsorbent dosage =
100 mg/L, initial As(V) = 190 μg/L, pH = 8 ± 0.1 and T = 20 °C, residual
As(V) concentration for μTMF and μGFH is 9.5 and 13.8 μg/L,
respectively
Table 1 Main physicochemical
characteristics of used adsorbent
media
Media Moisture
content
(%)
Fe
content
(wt%)
BET
surface
area (m
2
/g)
Pore
volume
(mL/g)
Mean
particle
size (μm)
Surface charge
density (mmol
OH¯/g)
Isoelectric
point (IEP)
μGFH ~ 50 ± 2 59.8 283 ± 3 0.28 78.4 0.9
a
7.8 ± 0.2
a
μTMF ~ 5 44.5 178 ± 8 0.35 40.0 2.7
b
7.2 ± 0.1
c
a
Usman et al. (2018),
b
Tresintsi et al. (2014b),
c
Tresintsi et al. (2013a)
59067Environ Sci Pollut Res (2021) 28:59063–59075
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
where As
0
is the initial concentration of As(V), As
t
is the
liquid phase As(V) concentration remaining in the solution
at time t, and k
1
and k
2
are rate constants of first- and
second-order kinetic models, respectively. The second-order
kinetic model shows better fit to the kinetic data of As(V)
adsorption onto iron oxyhydroxides as indicated by the higher
correlation coefficient (R
2
) values in Table 2.
The calculated k
2
values for the initial contact time of 3h
for μTMF and μGFH are 0.02 and 0.01 L/(mg*h), respective-
ly (Table 2). The higher value of k
2
for μTMF indicates the
faster adsorption rate. This shows that As(V) removal oc-
curred more rapidly with the μTMF which has a smaller par-
ticle size (Table 1). Though higher As(V) adsorption rate onto
μTMF after a contact time of 3 h, μTMF removes 22% more
arsenic within this time compared to μGFH.
As(V) removal using submerged membrane
adsorption hybrid system
The influence of various operating conditions on slurry reactor
combining adsorption onto adsorbent media and a submerged
MF membrane has been studied. In this unit, the added adsor-
bent media is used to remove pollutants, e.g., As(V) which is
present in the source water, and at a second step, the sub-
merged membrane functions as a complete barrier to arsenic
loaded media particles. In the following section, the influence
of several operational parameters have been studied, in order
to define the optimum conditions for efficient operation of the
hybrid treatment system.
Hydrodynamic conditions/influence of air bubbling
rate
The influence of bubbling on the adsorption process has been
studied at bubbling rates 1.25, 2, and 3 L
air
/(min. L
slurry
). Air
was transported from an air cylinder by PVC tubing to a
sintered glass diffuser to generate fine air bubbles.
The adsorption process normally follows four consecutive
steps (Jia et al. 2009): (1) external diffusion from bulk solution
to liquid film, (2) diffusion in the liquid film surrounding the
particle surface, (3) surface diffusion along the adsorbent in-
ner surface, (4) adsorption of pollutant onto the active sites in
the micropores. Among these four steps, the air bubbling rate
will have an effect on the first two steps. Jia et al. (2009)
reported during adsorption of Atrazine on PAC that mass
transfer in the liquid film surrounding the adsorbent particle
is very sensitive to air bubbling rate, and therefore, optimum
air bubbling rate should be achieved for better removal of
pollutant via adsorption in SMAHS.
Figure 3shows that at all three air bubbling rates, As(V)
removal efficiency of about 80% was achieved after approx-
imately 5 min. At air bubbling rates of 2 and 3 L/min, an
increase in the As(V) concentration with time was slow and
As(V) removal efficiency of over 70% was achieved in a 6 h
long continuous flow experiment. However, in case of 1.25 L/
min, the increase in As(V) concentration with time was com-
paratively faster than that of 2 and 3 L/min, and at the end of
6 h, the As(V) permeate concentration was approximately
70 μg/L (just over 60% removal efficiency) at air bubbling
rate of 1.25 L/min, while at 2 L/min, the As(V) permeate
concentration was around 1.5 times lower than that of at
1.25 L/min air bubbling rate. It is concluded that air bubbling
rate affects the arsenic concentration profile over time and
reveals positive effect on adsorption process with increasing
air bubbling rate from 1.25 to 2 L/min. Further increase in
bubbling to 3 L/min did not noticeably enhance As(V) adsorp-
tion rate. According to Jia et al. (2005), there is a limit to
Table 2 The first- and second-
order rate constants (k
1
&k
2
)for
the two adsorbents with different
contact times
Media First-order kinetic equation for Second-order kinetic equation for
Contact time (3 h) Contact time (6 h) Contact time (3 h) Contact time (6h)
k
1
(L/mg*h) R
2
k
1
(L/mg*h) R
2
k
2
(L/mg*h) R
2
k
2
(L/mg*h) R
2
μGFH 1.06 0.68 0.53 0.86 0.010 0.95 0.014 0.99
μTMF 0.74 0.95 0.65 0.50 0.020 0.99 0.018 0.99
Fig. 3 As(V) concentration in the permeate water over time in a SMAHS
with μGFH for varying air bubbling rates with As(V) = 190 μg/L,
adsorbent dosage = 1 g/L, membrane flux = 20 L/(m
2
.h), feed solution
pH = 8.0 and permeate pH = 8.08.2
59068 Environ Sci Pollut Res (2021) 28:59063–59075
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
bubble-induced mass transfer. This indicates that hydrody-
namic conditions at 2L
air
/min for As(V) mass transfer are
optimized and a further increase in air bubbling rate will not
promote the adsorption rate. It can also be concluded that air
bubbles produced by means of sintered glass diffuser keep the
adsorbent in suspension, therefore promotes the contact be-
tween adsorbent and adsorbate.
Influence of adsorbent dosage
Once the effect of the air inflow rate was quantified, the next
important parameter in continuous operation units is the ad-
sorbent dosage. In this section, the evaluation of the adsorp-
tion media in SMAHS was studied in terms of its ability to
decrease the As(V) permeate concentration below the drink-
ing water guideline value of 10 μg/L (termed Q
10,SMAHS
here-
after), rather than to enhance the maximum capacity (Q
max
)
and/ or higher removal efficiencies, which provides marginal
information on ability of a specific adsorbent to reach guide-
line value set by EU drinking water directives, US environ-
mental protection agency (EPA), and the World Health
Organization (WHO). At an adsorbent dosage of 1 g/L, over
80% As(V) removal efficiency was obtained but As(V) con-
centration in the permeate exceeded the desire 10 μg/L WHO
guideline value. Therefore, the amount of adsorbent initially
dosed into the slurry reactor was increased to 5 g/L from 1 g/L
to guarantee the As(V) permeate concentration below the
WHO guideline value of 10 μg/L (Fig. 4).
It is pertinent to mention that by increasing the adsorbent
dosage a slight increase to the pH value of the permeate (rang-
ing between 8.0 and 8.3) was observed which can be attribut-
ed to the release of hydroxyl ion during adsorption of As(V)
onto hydrous iron oxyhydroxide-based adsorbent (Eq. 1).
While at the dosage of 1 g/L of adsorbent initially dosed to
the reactor, a slight difference in the As(V) adsorption effi-
ciency between the two adsorbents was observed, at the dos-
age of 5 g/L, both adsorbent removed almost completely ar-
senic, and final concentrations were very low even up to the
end of the experiment, i.e., after 6 h. Furthermore, at the dos-
age of 1 g/L, there is a continuous increase in the As(V) per-
meate concentration with time which starts to be evident even
from the first hour of the experiment, most likely because of
the gradual exhaustion of the adsorbent sites. In the case of
5 g/L, only after 4 h of the experiment, a slight increase in the
permeate concentrations starts to be detected, but in all mea-
surements, the As(V) permeate concentration was below the
10 μg/L.
However, it was found in this work that at an adsorbent
dosage of 1 g/L, both adsorbents failed to meet the guideline
value of 10 μg/L for arsenic in drinking water, as indicated at
the EU Directive 98/83/EC. In the first case, the dosed adsor-
bent corresponds to 0.19 mg As(V)/g of adsorbent while in the
second case, the ratio is 5 times lower, thus equals to 3.8 ×
10
2
mg As(V)/g Adsorbent, under optimized conditions of
air bubbling. During removal of nitrate in a submerged mem-
brane adsorption system using ion exchange resins at water
flux of 15 LMH, Kalaruban et al. (2018) used a ratio of 8 mg
NO
3
/g to lower the adsorbate concentration from 20 mg/L to
less than 11.3 mg/L (roughly 7 mg/L) in the reactor or perme-
ate concentration, which is the guideline value for nitrate.
Under these conditions, both adsorbent failed to maintain the
nitration permeate concentration below 11.3 mg/L after 34h
at a hydraulic retention time of 2.7 h in the reactor.
Influence of hydraulic residence time
The residence time is a limiting factor in the slurry reactor as the
adsorption kinetic plot (Fig. 2) shows that As(V) removal rate
Fig. 5 As(V) concentration in permeate vs. time for both media in a
SMAHS at two different hydraulic residence times for adsorbent
dosages of 5 g/L with initial As(V) concentration of 190 μg/L, air
bubbling rate = 2 L/min and permeate pH = 8.08.3. The dashed line
indicates the WHO guideline value for arsenic in drinking water
Fig. 4 As(V) concentration in permeate over time in a SMAHS with
μGFH and μTMF for adsorbent dosages of 1 and 5 g/L with initial
As(V) concentration of 190 μg/L, air bubbling rate = 2 L/min and
permeate pH = 8.08.3
59069Environ Sci Pollut Res (2021) 28:59063–59075
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
increased with increasing contact time. Therefore, the influence
of hydraulic residence time on the performance of SMAHS has
been studied at water fluxes of 10 L/(m
2
h)and20L/(m
2
h).
Because the hydraulic retention times of As(V) in the slurry
reactor were 2.8 h and 5.6 h at 20 L/(m
2
h)and10L/(m
2
h),
respectively, accordingly, the initial 3 h and 6 h contact times of
As(V) with media in adsorption kinetics were considered.
Figure 5shows the As(V) permeate concentration profiles de-
rived by monitoring the SMAHS experiments using micro-
sized iron oxyhydroxides. The results show that the addition
of adsorbent results in a sudden decrease in As(V) concentra-
tion from 190 μg/L to a minimum value in the slurry reactor
and after a day the As(V) concentration starts to increase with
time. However, for the 10 LMH, As(V) concentration in the
reactor stayed at minimum value for3days,andafterwhich,the
As(V) concentration in the permeate starts to rise but at a slower
rate than 20 LMH.
It can be seen in Fig. 5that As(V) permeate concentration
profiles over time for both media in hybrid system are effected
by the membrane water fluxes. However, the increase in
As(V) permeate concentration over time for μGFH was rapid
at both water fluxes. This shows that the μTMF is more effec-
tive than μGFH in adsorbing As(V) in the presence of com-
peting ions.
The performance of media has been assessed in terms of
the amount of As(V) adsorbed per unit mass of adsorbent and
volume of treated water up to a guideline value of 10 μg/L.
Q
10, SMAHS
was calculated by dividing the area above the
As(V) concentration curves by the initially added dry mass
of adsorbent.
It can be seen from Fig. 6that higher residence time (or
lower flux of 10 LMH) increases the adsorption efficiency
of media in removing As(V) from modeled groundwater,
and thus is favorable, although it produces less treated wa-
ter per unit time. The amount of As(V) adsorbed per unit
mass of media has been decreased at 20 LMH. At 20
LMH, the recorded adsorption capacity of μGFH is
36.2% less than that of μTMF. The difference decreases
to 9.9% at 10 LMH. This difference in Q
10,SMAHS
at lower
contact times can be explained by lower k
2
values of
μGFH (Table 2). These results showed that although the
kinetics of As(V) adsorption was much faster for μTMF,
the amount of arsenic adsorbed was finally similar for both
adsorbent, and therefore, when the flux was decreased, the
difference in arsenic capacity of both adsorbentsbefore
As(V) permeate concentration reaches the 10 μg/Lhas
been reduced. This difference in Q
10,SMAHS
value might
also be due to greater surface charge density of μTMF
(2.7 mmol OHˉ/g) than μGFH (0.89 mmol OHˉ/g). It is
concluded that either particle size or surface charge charac-
teristics or both have significant influence on practical ad-
sorption capacity (Q
10,SMAHS
) for drinking water production
observed in SMAHS, even though μGFH has higher BET
surface area and IEP than μTMF.
The volume of water treated by unit mass of adsorbent to
reach As(V) permeate concentration of 10 μg/L was calculat-
ed by defining as specific system productivity (SSP):
SSP ¼QT
10
VM
ad
ð4Þ
Where Q is the volumetric flow rate at corresponding mem-
brane water flux, T
10
is time taken to reach 10 μgAs(V)/L
concentration in permeate, V is the liquid volume in the reac-
tor, and M
ad
is the mass of adsorbent initially dosed into the
reactor. The results revealed (Fig. 6b) that the system produc-
tivity is higher at lower membrane water flux and vice versa.
Furthermore, the recorded system productivity using μTMF is
higher, compared with μGFH at both water fluxes. Figure 7
shows transmembrane pressure profile for constant flux
Fig. 6 aAdsorption efficiency of
both media in a SMAHS. b
Specific system productivity at
initial As(V) = 190 μg/L and
permeate pH = 8.08.3
59070 Environ Sci Pollut Res (2021) 28:59063–59075
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
filtration of 10 and 20 LMH. On the whole, it can be seen that
the no fouling occurred during operation for both applied
fluxes. It is recommended that membrane fouling behavior
must be considered for long-term operation.
In summary, the As(V) adsorption capacity in SMAHS at
contact time 6husingμTMF is higher compared to μGFH
at two different water fluxes. In a former study, we determined
adsorption capacity (Q
10,batch
) through batch adsorption iso-
therm studies of 3.2 μg As(V)/mg and 3.3 μgAs(V)/mgin
NSF water for μGFH and at an equilibrium liquid phase con-
centration of 10 μg/L and at same pH with much longer con-
tact time until adsorption equlibrium was reached (Usman
et al. 2018). In batch adsorption tests, the recorded adsorption
capacity of μTMF was 3.1% higher than μGFH. However, the
obtained Q
10
value for μTMF in SMAHS is 9.5% higher than
the respective Q
10
value for μGFH; this might be attributed to
smaller particle size of μTMF, due to which adsorption rate is
rapid as indicated by the adsorption kinetic plot of μTMF.
Influence of initial As(V) concentration
The volume of water treated and amount of As(V) adsorbed
per unit mass of adsorbent in the SMAHS using both adsor-
bents at varying As(V) feed concentrations and at identical
flux is listed in Table 3.
As anticipated, specific system productivity using the
micro-sized ion oxyhydroxides has been declined, when the
As(V) feed concentration was increased from 190 to
380 μg/L. A reduction of 6% and 10% are recorded for
μGFH and μTMF, respectively. However, Q
10,SMAHS
value
has been increased at the same water flux for both iron
oxyhydroxides since the amount of arsenic entering the slurry
reactor per unit time has beenincreased,and subsequently, the
concentration gradient between the adsorbate in solution and
the media solid surface has been increased and lead to higher
Q
10
value of both adsorbents. The time taken to reach the
As(V) permeate concentration of 10 μg/L has been decreased
in case of higher initial As(V) concentration of 380 μg/L.
Hilbrandt et al. (2019) reported that during adsorption of phos-
phate onto μGFH in an adsorption-membrane hybrid system
that a sharp slope of the breakthrough curve is favorable as it
indicates less influence of film diffusion on adsorption. Due to
which, a sharp increase in As(V) permeate concentration was
observed and target contaminant concentration of 10 μg/L in
the treated permeate with As(V) feed concentration of
380 μg/L was accomplished earlier than with 190 μgAs(V)/
L.
Comparison of As(V) removal efficiency using SMAHS
and fixed-bed filtration filter
The performance of the SMAHS using micro-sized iron hy-
droxides can be compared with laboratory- and full-scale
fixed-bed adsorbers used for As(V) removal from the water
via adsorption onto granular fractions of iron oxyhydroxide-
based adsorbents as well as iron oxide-coated sand (Table 4).
Tresintsi et al. (2013b) obtained higher As(V) adsorption ca-
pacity (i.e., Q
10
value) of GFH, with particle size ranging
between 250 and 500 μm, in rapid small-scale column test
(RSSCT) than that of SMAHS using μGFH (1.7 vs.
0.95 μg/mg at 10 LMH or 0.61 μg/mg at 20 LMH). This
difference in adsorption capacity might be explained because
the two studies were conducted under different experimental
setups and conditions, since the actual adsorption capacity of
an adsorbent for a specific pollutant depends on experimental
setups, water matrix, and solution pH.
Table 3 Volume of water treated
and Q
10,SMAHS
value for As(V)
concentration < 10 μg/L for two
adsorbent media at varying As(V)
feed concentrations with
adsorbent dosage = 5 g/L, water
flux = 20 LMH, pH = 8 ± 0.1 and
air bubbling rate = 2 L/min. L
slurry
Media Influent
As(V) conc.
(μg/L)
Total
operation
time (h)
Total
volume
filtrated (L)
T
10
(h)
Amount of
As(V) adsorbed
(mg)
a
SSP
(L/g)
a
Q
10,SMAHS
(μg/ mg)
μGFH 190 69 24.9 45.8 3.0 3.3 0.61
380 68 27.5 43.0 5.8 3.1 1.15
μTMF 190 103 37.1 64.1 4.1 4.6 0.82
380 102 36.6 57.0 7.7 4.1 1.54
a
When As(V) concentration in permeate reached the WHO guideline value of 10 μg/L
Fig. 7 Transmembrane pressure profile during constant water flux in a
submerged membrane adsorption hybrid system
59071Environ Sci Pollut Res (2021) 28:59063–59075
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
Better performance of RSSCT packed with granular iron-
oxyhydroxides for As(V) is associated to a larger concentra-
tion gradient between the adsorbate in solution and the media
solid surface. The adsorbent in RSSCT is always in contact
with the influent arsenic concentration, which results in a
higher driving force over the whole adsorption process.
However, in a SMAHS, the influent arsenic concentration is
in contact with slurry, which has very low liquid phase arsenic
concentration especially at the start of adsorption process
when all adsorption sites are empty and arsenic removal oc-
curred very rapidly, due to which the mass transfer driving
force (concentration gradient) is very low compared to
RSSCT. This is becoming more obvious, when the initial
arsenic concentration increased to 380 μg/L. This provided
more contact of arsenic species with the slurry of iron oxides
andcausedanincreaseinQ
10,SMAHS
from 0.82 to 1.54 for
μTMF and from 0.61 to 1.15 for μGFH respectively.
Regarding the experimental conditions, As(V) removal
from the water via adsorption onto iron oxyhydroxides is
known to be impacted by solution pH and presence of
competing ions in the drinking water matrix. Westerhoff
et al. (2005) investigated the arsenic adsorption in RSSCT
packed with GFH using a different drinking water matrix with
an As(V) influent concentration of 14 μg/Lwhere the con-
centration driving force might be in the same range as in
SHAHSeven though the obtained Q
10
value in RSSCT
packed with GFH is lower than that of SMAHS using
μGFH. This is possible because the both studies were con-
ducted in a different drinking water matrix. A study by Amy
et al. (2005) on effect of water matrix on arsenic adsorption
reported a reduction of 70% in As(V) adsorption capacity onto
GFH in the presence of 13.5 mg/L silica (SiO
2
)atpH8in
batch adsorption tests. Similarly, 75% reduction in As(V) ad-
sorption capacity was recorded in presence of 250 μg/L PO
43
under similar experimental conditions.
Concerning the pH value, most commonly used adsorbents
adsorb arsenic more effectively at pH values below IEP and
their adsorptive capacities increase with decreasing pH
(Tresintsi et al. 2012). During adsorption of As(V) onto iron
oxyhydroxides, synthesized in laboratory at kilogram scale, in
RSSCT, the Q
10
value was increased from 2.8 to 6.8 to
10.7 μg/mg at 8, 7, and 6, respectively. This study demon-
strated that by decreasing the solution pH by one unit from 8
to 7, the adsorption efficiency of adsorbent increased by 2.4
times. Similar results were obtained by Katsoyiannis and
Zouboulis (2002), where As(V) adsorption was studied in
fixed-bed columns using amorphous iron oxides as adsorbent;
the bed volumes treated before arsenic concentration reached
the 10 μg/L were increased as the pH decreased from 9 to 7.
This is because as the pH decreases, the surface charge of the
adsorbent becomes more positive and favors the adsorption of
oxyanion species on their surface.
In summary, this difference in adsorption capacity between
two studies could be attributed to water matrix as well as
solution pH in addition to As(V) influent concentration, and
therefore, it is relevant to mention that these factors play piv-
otal role while comparing the removal efficiencies of two
water treatments systems for As(V) at roughly the same con-
centration driving force. From the above discussion, it is con-
cluded that the removal efficiencies of both treatment systems
are comparable for As(V) especially when the concentration
driving force is higher in the slurry reactor and also taking into
account the effect of the water matrix and solution pH. In one
system, adsorbent media is fixed, while in hybrid system,
adsorbent media is in suspension. Takin into account the ex-
perimental setup difference, it may be concluded that the use
of this adsorption onto micro-sized iron oxyhydroxides
followed by membrane separation might be an efficient solu-
tion for treatment of high arsenic content waters, as found in
many countries including India, Bangladesh, Pakistan, Nepal,
Table 4 As(V) adsorption capacity of different iron oxyhydroxide-based adsorbents in a fixed-bed adsorption filter and SMAHS
Type of system Test solution pH Phosphate
conc. (mg/L)
Silica conc.
(mg/L)
Influent
As(V) conc.
Q
10
(μg/mg)
Reference
GFH packed fixed-bed adsorber Mitrousi (Greece) tap water 7.3 0.6 14 19 1.6 (Tresintsi
et al. 2013b)GFH packed RSSCT (lab-scale) Thessaloniki (Greece) tap
water
7.9 0.3 20 100 1.7
GFH packed RSSCT (lab-scale) Arizona (USA) groundwater 7.6 34 14 0.75 Westerhoff
et al. (2005)GFH packed fixed-bed adsorber Arizona (USA) groundwater 7.8 39 34 1.6
SMAHS using μGFH at 20 LMH Artificial groundwater 8 0.12 20 190 0.61 This work
8 0.12 20 380 1.15
SMAHS using μTMF at 20 LMH Artificial groundwater 8 0.12 20 190 0.82
8 0.12 20 380 1.54
Iron oxide-coated sand
packed fixed-bed adsorber
Groundwater 7.6 ––1000 0.002 Callegari
et al. (2018)
Amyloid fibril carbon hybrid mem-
branes
Ultrapure water 7 –– 239 0.27 Bolisetty et al.
(2017)
59072 Environ Sci Pollut Res (2021) 28:59063–59075
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
and China. This is because, at higher concentration driving
force, the achieved Q
10
values of the both applied adsorbents
have been increased significantly.
Other types of fixed-bed filter that used low-cost sand coat-
ed with iron oxide to remove arsenic from groundwater,
(Callegari et al. 2018), reported As(V) removal efficiency up
to 99%. The iron oxide-coated sand fixed-bed filter could
safely treat about 22.25 L water/L filter volume until
10 μg/L in the effluent was reached. With this volume of
product water treated, the corresponding Q
10
value of the iron
oxide-coated sand is 0.002 μg/mg, which is at least three
orders of magnitudes lower than the SMAHS using micro-
sized iron oxyhydroxides in a complex water matrix.
Additionally, the fixed-bed filter packed with iron oxide-
coated sand needs around 2.5 h to reach stable arsenic effluent
concentration (below 10 μg/L), from a 1000-μg As(V)/L ini-
tial concentration. However, in a SMAHS using low-cost mi-
cro-sized iron oxyhydroxides, the arsenic permeate concentra-
tion immediately reached arsenic concentrations below the
drinking water guideline value starting from a 380-μg
As(V)/L initial concentration. Moreover, the fined-sized ad-
sorbent can be used in a SMAHS in addition to simultaneous
removal of colloids, microorganisms, and suspended solids.
Furthermore, the examined hybrid system is a relatively sim-
ple, effective option to treat arsenic-contaminated water and
can find its application in decentralized water treatment sys-
tem. Structural costs of the hybrid system are quite low, and
the energy demand of pumps is relatively low that could be
provided by solar photovoltaic panels.
The SMAHS performance can also be compared with sim-
ilar studies using very advanced nanomaterials to achieve
As(V) removal. The study of Bolisetty et al. (2017)shows
that amyloidcarbon hybrid membranes containing 10% (by
weight) amyloid fibrils indeed diminished the As(V) concen-
tration in ultrapure water within the drinking water guideline
value, but the adsorption capacity is only 0.27 μg/mg for
As(V), and thus is almost 3 times lower than that of μGFH
and μTMF in SMAHS, even without the presence of compet-
ing ions, showing that in the hybrid treatment system pro-
posed in this study, the critical factor for optimized perfor-
mance is the iron-based materials.
Considering environmental sustainability of process, spent
adsorbent might be regenerated using an integrated procedure
proposed by Tresintsi et al. (2014a). They have employed low-
cost MgO to regenerate the used iron oxyhydroxides during
As(V) removal in a fixed-bed adsorption filter. This approach
combines the As(V) leaching (leaching step) and adsorption of
leached As(V) onto MgO (adsorption step) under strong alka-
line environment (0.05 N NaOH) in a continuous recirculation
configuration. The MgO (below 3 wt%) is generally used in
clinker which is a major constituent of cement. Arsenic-
contained MgO can be successfully incorporated in commer-
cially concrete products without any secondary pollution.
Therefore, it can be concluded that the examined hybrid treat-
ment system can be used without potential health risk in the
process of waste handling.
Conclusions
Two low-cost micro-sized iron-based oxyhydroxides were
proved to be efficient in removing As(V) from artificial
groundwater. The second-order adsorption kinetic described
the adsorption kinetic data of both media well and confirms
that As(V) adsorption kinetic was faster with μTMF than with
μGFH under the same experimental conditions.
The SMAHS tests showed that both adsorbents can be
applied by this approach without considerable fouling of MF
membrane. Like batch adsorption tests, the adsorption kinetics
of As(V) onto μTMF was faster than that of μGFH in hybrid
system. Within the given SMAHS setup air bubbling rate of
2L
air
/(min. L
slurry)
) was necessary to reach optimal condi-
tions for the required mass transfer of As(V). Based on the
continuous flow tests, the hybrid system benefits from a
higher adsorbent dosage. The higher residence time of
5.6 h in the slurry reactor was favored for As(V) removal
from contaminated water. As(V) adsorbent capacity in the
hybrid system for As(V) increased and got almost double
when the As(V) feed concentration increased to 380 μg/L
from 190 μg/L at smaller hydraulic retention time. The
Q
10
value of hybrid system and fixed-bed adsorption fil-
ters were in similar ranges, taken into consideration the
complete difference in the two compared units.
The media cost was estimated to be as low as 0.30 /L
of treated water and media cost can be decreased signifi-
cantly if pH of the raw water is lower than in the present
study, i.e., between 6.5 and 7.5. Additionally, the media
cost can be reduced remarkably by reusing spent iron
oxyhydroxides, which can contribute to the low treatment
cost of this hybrid system. In future research, the objective
is the development of mathematical models to predict the
As(V) permeate concentration profiles in a continuous
flow hybrid system.
Acknowledgments Open Access funding provided by Projekt DEAL.
The authors are grateful to the Higher Education Commission (HEC) of
Pakistan, German Academic Exchange Service (DAAD), and the
Technische Universität Hamburg. The authors gratefully acknowledge
Manassis Mitrakas, Department of Chemical Engineering, Aristotle
University of Thessaloniki, Greece, for delivering tetravalent manganese
feroxyhyte media. We acknowledge support for the Open Access fees by
Hamburg University of Technology (TUHH) in the funding programme
Open Access Publishing.
Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of
interest.
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Attribution 4.0 International License, which permits use, sharing, adap-
tation, distribution and reproduction in any medium or format, as long as
you give appropriate credit to the original author(s) and the source, pro-
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... However, the convective flow, which drives the feed through the packed bed, occurs around the microporous beads bypassing the active pores within them. In order to optimize the process via a diffusive transport mechanism from the exterior surface of the bead to the active sites within the beads, long residence times to ensure that the active sites saturate with the feed are necessary and realized thanks to the increased size of the equipment, which in turn increases capital and operating costs [110]. Conventional adsorptive membranes ( Figure 5A) are usually prepared by surface modification of porous UF/MF polymeric membranes, e.g., pore diameters, d p ≈ 10−1000 nm (see Figure 1), with specific functional groups or the addition of nanoparticles onto the membrane surface, which can bind the metal ions through ion exchange or surface complexation, even though the pore sizes of these membranes (MF/UF) are larger than the size of metal ions. ...
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... This depends mainly on the separation process of van der waals and the electrostatic attraction forces among adsorbed molecules [4] . It should be noted that the efficiency of this technique is affected by exposure period, acting pH, the presence of other chemical species, adsorbent dose, initial arsenic concentration, and temperature [5,6]. ...
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Arsenic contamination has been widely recognized as one of the most consequential environmental pollutants due to its anthropogenic activities.
Chapter
Arsenic (As) contamination of soil and water bodies due to various natural and anthropogenic activities has become a primary environmental concern worldwide. The As level is increasing due to contaminated irrigation water and the use of As‐based agrochemicals, which is leading to the deterioration of soil fertility. In developing countries, As concentration in drinking water has far exceeded the permissible limit stated by the World Health Organization (WHO). It is highly toxic to plants, animals, and humans with severe consequences to present and coming generations. As is a nondegradable element, and thus removed from the contaminated site, or water bodies are the only solution. Ion exchange and membrane technologies are one of the aesthetical approaches for the removal of As from wastewater. Various environmental factors such as temperature, pH, and state of reduction/oxidation influence the rate and type of ionic conversion reactions. This book chapter provides an insight into the different membrane and ion exchange technologies to assess the status of As removal from different water bodies. Attempts have also been made to summarize the role of different types of membrane technologies like microfiltration, nanofiltration, reverse osmosis, and ultrafiltration processes in As removal. Furthermore, this chapter will also compile the flaws of both the technologies and challenges which need to be investigated for developing sustainable approaches.
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A lot of anthropogenic activities can discharge arsenic into the ecosystem such as industrial wastes, incineration of municipal, pesticide production and wood preserving. In addition, most arsenic soluble species can enter surface waters via runoff and leach into the groundwater. Around forty million people from all over the world are affected by arsenic through drinking water above the maximum contaminant level of 0.01 mg/L. The affected by inorganic arsenic through drinking water can cause a lot of diseases especially a unique peripheral vascular disease and blackfoot disease. These diseases usually cause gangrene and end with amputation of the legs and can also cause severe systemic atherosclerosis. In addition, the wastewater treatment techniques can be divided into two groups, adsorbents and membrane separations such as electrodialysis, nanofiltration and reverse osmosis. Furthermore, most of these techniques do not function at a low level of concentration, so that moderate to high levels of concentration are required. However, the use of some of these arsenic removal approaches is costly because they require a lot of energy and reagents. Moreover, this review discusses readily adsorption technologies that have been applied to remove arsenic from wastewater along with an analysis of arsenic chemistry and contamination. This review is also focused on the removal of arsenic from wastewater using different adsorbents such as iron, aluminium, natural and biological adsorbents. Its goal is to increase our fundamental understanding of this developing research subject and to identify future research and development strategies for sustainable and cost-effective arsenic adsorption technology.
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The fine fraction of granular ferric hydroxide (µGFH, <0.3 mm) is a promising adsorbent for the removal of heavy metals and phosphate, but properties of µGFH were hitherto not known. The present study aimed at characterizing µGFH regarding its physical and chemical properties and at evaluating methods for the conditioning of fixed-bed filters in order to develop a process that combines filtration and adsorption. Conditioning was done at different pH levels and for different particle sizes. Anthracite, coke, pumice and sand were studied as potential carrier materials. A method for the evaluation of the homogeneity of the iron hydroxide particle distribution on pumice filter grains using picture analysis was developed. Pre-washed pumice (pH 8.5) proved to lead to high embedment and a homogeneous distribution of µGFH. Filter runs with phosphate (2 mg/L P) showed similar breakthrough curves for the embedded fine fraction adsorbent and for conventional GFH.
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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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Arsenic is among the most dangerous contaminants which can limit groundwater use for drinking water consumption. Among the most diffused As-removal technologies around the world, adsorptive media systems are usually favored for relatively low cost and simplicity of operation. This study examines the performance of a laboratory-scale iron oxide-coated sand (IOCS) column filter, to remove arsenic (arsenate (As[V]) and arsenite (As[III])) from groundwater. This technology could be adopted in small communities, as it showed consistent removal rates of 99% with an easy-to-operate process. Some considerations about the possible introduction of such technology in developing countries are provided, highlighting the general impacts to human health related to high arsenic concentrations in groundwater. This, among other adsorption processes, could be recommended as a sustainable mean of ensuring good drinking water quality in developing regions, reducing human health impacts.
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Chromium (Cr(VI)) is a very toxic and carcinogenic element, which is widely present in groundwaters, mainly due to geogenic conditions. The limit of Cr(VI) in drinking water is expected to be reduced to 10 μg/L in both the USA and the European Union. Recent literature findings indicated that the most efficient process in reducing Cr(VI) levels to below 10 μg/L proved to be Cr(VI) reduction by Fe(II), by applying a molar ratio Fe(II)/Cr(VI) of around 9. In the present work, we investigated the reduction of Cr(VI) by Fe(II) in pipe flocculation reactors followed by filtration of insoluble products by microfiltration. The proposed technology involves re-circulation of a part of the sludge in the pipe reactors, in order to improve kinetics and efficiency of the process. The obtained results showed that with a Fe(II) dose of around 1 mg/L, Cr(VI) was reduced to below 10 μg/L, by even an initial concentration as high as 300 μg/L of Cr(VI), corresponding to a molar ratio Fe(II)/Cr(VI) of around 3, thus reducing the overall quantity of reductive reagents and of the produced sludge. This ratio was also confirmed by the XPS analysis, which also showed that Cr(VI) was reduced to Cr(III) and then precipitated either as Cr(OH)3 or associated with the produced iron oxides.
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Nitrate contamination of ground and surface waters causes environmental pollution and human health problems in many parts of the world. This study tests the nitrate removal efficiencies of two ion exchange resins (Dowex 21K XLT and iron-modified Dowex 21K XLT (Dowex-Fe)) and two chemically modified bio-adsorbents (amine-grafted corn cob (AG corn cob) and amine-grafted coconut copra (AG coconut copra)) using a dynamic adsorption treatment system. A submerged membrane (microfiltration) adsorption hybrid system (SMAHS) was used for the continuous removal of nitrate with a minimal amount of adsorbents. The efficiency of membrane filtration flux and replacement rate of adsorbent were studied to determine suitable operating conditions to maintain the effluent nitrate concentration below the WHO drinking standard limit of 11.3 mg N/L. The volume of water treated and the amount of nitrate adsorbed per gramme of adsorbent for all four flux tested were in the order Dowex-Fe > Dowex > AG coconut copra > AG corn cob. The volumes of water treated (L/g adsorbent) were 0.91 and 1.85, and the amount of nitrate removed (mg N/g adsorbent) were 9.8 and 22.2 for AG corn cob and Dowex-Fe, respectively, at a flux of 15 L/(m(2)/h).
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This study evaluates the efficiency of iron-based oxy-hydroxides to remove antimony from groundwater to meet the requirements of drinking water regulations. Results obtained by batch adsorption experiments indicated that the qualified iron oxy-hydroxide (FeOOH), synthesized at pH 4 for maintaining a high positive charge density (2.5 mmol OH−/g) achieved a residual concentration of Sb(III) below the EU drinking water regulation limit of 5 μg/L by providing an adsorption capacity of 3.1 mg/g. This is more than twice greater compared either to similar commercial FeOOHs (GFH, Bayoxide) or to tetravalent manganese feroxyhyte (Fe-MnOOH) adsorbents. In contrast, all tested adsorbents failed to achieve a residual concentration below 5 μg/L for Sb(V). The higher efficiency of the qualified FeOOH was confirmed by rapid small-scale column tests, since an adsorption capacity of 3 mg Sb(III)/g was determined at a breakthrough concentration of 5 μg/L. However, it completely failed to achieve Sb(V) concentrations below 5 μg/L even at the beginning of the column experiments. The results of leaching tests classified the spent qualified FeOOH to inert wastes. Considering the rapid kinetics of this process (i.e., 85% of total removal was performed within 10 min), the developed qualified adsorbent may be promoted as a prospective material for point-of-use Sb(III) removal from water in vulnerable communities, since the adsorbent’s cost was estimated to be close to 30 ± 3.4 €/103 m3 for every 10 μg Sb(III)/L removed.
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The use of micro-sized iron hydroxide adsorbents in mixed reactors is a promising technique for the removal of inorganic contaminants from wastewater within minutes of contact time. This study focusses on phosphate adsorption onto fine fraction granular ferric hydroxide (μGFH) and iron oxy(hydr)oxide agglomerates (IOAs) in a reactor with submerged ultrafiltration (UF) membrane. The performance of the hybrid adsorption/UF membrane system was evaluated for various adsorbents and phosphate concentrations, residence times and concentrations of co-existing ions. The membrane was not fouled at the experimental conditions used (up to 6.3 g/L adsorbent). Phosphate loadings of 20 and 60 mg P/g Fe (36.1 and 108.3 mol P/mol Fe) were reached for μGFH and IOAs, respectively (C0(P)=4.5 mg/L, deionized water at pH 8, C(Fe)=0.6 g/L). A shortened residence time of 15 min in the reactor led to a decrease in final loading of 6 mg/g compared to 30 min residence time (54 mg/g compared to 60 mg/g). An extension to 60 min did not result in higher loadings. An increase in adsorbent (IOA) concentration from 0.1 to 0.3 mg/L resulted in an increase of phosphate removal (27 to 35%). Simultaneously, loadings decreased from 50 to 35 mg/g. The application of the developed process for the treatment of artificial secondary effluent resulted in an increase of 87 and 60% in treated volumes until breakthrough (50%) for μGFH and IOAs, respectively, compared to deionized water. Thus, the combined process of adsorption and particle separation using a submerged membrane can be well adjusted according to water composition, initial pollutant concentrations and desired removals.
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For decentralized drinking water treatment in rural areas, a hybrid process of biological activated carbon (BAC) filter and ultrafiltration (UF) was applied to obtain potable water. A pilot-scale study was performed by comparing a BAC-UF and a UF, and bench scale experiments were also conducted to investigate the effect of BAC pretreatment on membrane fouling in UF. Performances of pilot scale BAC-UF and UF were continuously monitored for130 days. BAC-UF performed better than UF in terms of dissolved organic matters (DOM) and ammonia removals. BAC pretreatment significantly alleviated membrane fouling in the subsequent UF. Besides reducing the organic loading entered into the UF, BAC pretreatment also changed the quality of the DOM. BAC pretreatment shifted the major fluorescence foulants from fulvic and humic acid-like substances in raw water to protein-like substances in BAC effluent, and increased apparent molecular weight of dissolved organics. Bench-scale UF tests showed that the BAC pretreatment reduced both reversible and irreversible fouling. Fouling mechanism analysis indicated that cake layer formation instead of pore blocking dominated fouling in UF with BAC pretreatment, which resulted in the alleviated flux decline and higher stabilized flux. This results indicated that the changes in both quantity and quality of DOM enabled by BAC pretreatment contributed to retarding flux decline and increasing stabilized flux in BAF-UF.
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We show that amyloid fibrils-based membranes purify water from arsenic, adsorbing both the arsenate(+5) and arsenite(+3) oxidation forms at efficiencies of ~99%. Binding isotherms indicate that amyloid fibrils possess multiple binding residues capable of strongly adsorbing arsenic ions via metal-ligand interactions, delaying the saturation of the membrane. We also show that these membranes can be reused for several cycles without any efficiency drop, and validate our technology in purifying real contaminated ground water by removing arsenic with an efficiency as high as 99.6%. These result make this technology promising for inexpensive, efficient and low-energy removal of arsenic from contaminated water.