Citation: Steuer, A.; Dippon-Deißler,
U.; Mahringer, D.; Ruhl, A.S. Can
Selenium Be Removed in a Pilot Plant
for Biological Iron and Manganese
Removal? Water 2023,15, 3147.
https://doi.org/10.3390/w15173147
Academic Editor: Dimitrios
E. Alexakis
Received: 24 July 2023
Revised: 21 August 2023
Accepted: 31 August 2023
Published: 2 September 2023
Copyright: © 2023 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 (https://
creativecommons.org/licenses/by/
4.0/).
water
Article
Can Selenium Be Removed in a Pilot Plant for Biological Iron
and Manganese Removal?
Andrea Steuer 1,2,* , Urs Dippon-Deißler 3, Daniel Mahringer 1and Aki S. Ruhl 1,2
1Section II 3.3, German Environment Agency (UBA), Schichauweg 58, 12307 Berlin, Germany
2Water Treatment, Technische Universität Berlin, KF 4, Straße des 17. Juni 135, 10623 Berlin, Germany
3Section II 2.6, German Environment Agency (UBA), Wörlitzer Platz 1, 06844 Dessau-Roßlau, Germany
*Correspondence: andr[email protected]
Abstract:
Selenium (Se) is essential to human health, yet harmful in high doses. Of the water-soluble
Se redox species, Se(IV) readily adsorbs onto iron and aluminium oxides. Se(VI), the dominant form
in oxygenated waters, is more mobile and less readily adsorbed. In this study, the removal of Se(VI)
by reduction with Fe(II) to Se(IV) and subsequent adsorption onto iron hydroxides is investigated in a
pilot plant for biological iron and manganese removal from groundwater to investigate an economical
approach for Se removal during drinking water production. While Se(IV) is removed by up to 90%,
Se(VI) shows no removal over 48 h. In batch-shaking tests, the adsorption of Se(IV) and Se(VI) onto
iron hydroxides with and without addition of Fe(II) or dithionite as reducing agents was studied.
Se(IV) was removed to a greater extent by adsorption than Se(VI) (7% and 2.6%, respectively, at a
starting concentration of 0.1 mg/L) and the addition of reducing agents resulted in no significantly
higher removal of Se(VI). Reducing Se(VI) with Fe(II) or dithionite and consequent adsorption onto
iron hydroxides can therefore be excluded as viable removal mechanism for Se(VI).
Keywords: selenium removal; groundwater; iron hydroxide; pilot scale; drinking water
1. Introduction
Selenium (Se) concentrations in soils and natural water strongly correlate with Se
contents in the parent rock [
1
,
2
] but can also be influenced by anthropogenic activity
such as agricultural run-off [
3
], through which Se can be transported into surface water
and groundwater [
4
]. Groundwater concentrations typically range from 0.1–400
µ
g/L [
3
].
Especially high concentrations of Se were found in Punjab, India (324
µ
g/L) [
1
], and in the
US states Colorado, New Mexico, and Utah (up to 4700 µg/L) [5].
While low concentrations are necessary for human health, Se is highly toxic in elevated
amounts [
1
–
3
]. Therefore, the WHO suggests a limit of 40
µ
g/L [
6
], the European drinking
water limit is 20 µg/L [7], and the German limit value is 10 µg/L [8].
Se naturally occurs in four redox states: selenate (Se(VI)), selenite (Se(IV)), elemental
selenium (Se(0)), and selenide (Se(-II)) [
9
,
10
]. The speciation of Se is dependent on redox
conditions and pH [
3
]. Reducing and acidic conditions favour the formation of Se(0) and
Se(-II) [
3
,
4
], many of which are water insoluble. Oxidizing and alkaline conditions favour
water-soluble Se(VI) and Se(IV) [3,4].
A large variety of methods for Se removal from water have been researched over the
years. For Se(IV), various adsorption strategies have been proposed, including aluminium-
based water treatment residuals [
11
], manganese-aluminium hydroxides [
12
], iron oxy-
hydroxides [
13
], and carbon-substituted hydroxylapatite [
14
]. In most cases, Se(VI) is not as
readily adsorbable as Se(IV). Therefore, alternative approaches such as co-precipitation with
barite [
15
], removal with ion exchange resins [
16
], and reduction to Se(0) via zero-valent
iron [
17
,
18
] and ferrous hydroxide [
19
] have been studied for Se(VI) removal. Most of the
studies to date focus on laboratory set-ups. In pilot-scale and full-scale applications, Se(VI)
Water 2023,15, 3147. https://doi.org/10.3390/w15173147 https://www.mdpi.com/journal/water
Water 2023,15, 3147 2 of 12
removal is mainly investigated for the treatment of mining waste water. Biological treatment
in plug flow biotreatment reactors [
20
], ion exchange resin [
21
], and nanofiltration [
21
] have
been successfully implemented.
This study investigates the combination of chemical reduction, coagulation, and
biological filtration as an economic, low-waste approach to Se(VI) removal from ground-
water in a pilot plant for biological iron and manganese removal under Fe(II) dosage.
Mahringer et al. [22]
have shown that this process is an effective way to remove Cr(VI)
from drinking water. As Fe(II) has the potential to reduce Se(VI), it is tested whether Se(VI)
can be reduced to Se(IV) and subsequently adsorbed to in situ-produced iron hydroxides
in a technical set-up. Therefore, the pilot plant was fed with iron- and manganese-free
drinking water, and spiked with Se(VI). Fe(II) was added to act as a reducing agent and,
after itself being oxidized to Fe(III) with dosed oxygen, to provide adsorption sites in the
subsequent filter bed.
The reduction of Se(VI) and adsorption of Se(IV) were further studied in batch tests
using Fe(II) and dithionite as reducing agents at different concentrations.
2. Materials and Methods
2.1. Modelling
Thermodynamic Se speciation was modelled using PHREEQC (2019) [
23
] with Phree-
Plot (2022) [
24
] for batch processing and plotting of a Pourbaix diagram with the Lawrence
Livermore National Laboratory (LLNL) dataset from 2017. Boundary conditions for the
model were set to be 10
◦
C, carbonate available as CO
2
at atmospheric pressure, a Se con-
centration of 1
µ
mol/L, and a sodium chloride background of 1 mmol/L in the pH range
from 2 to 12 and Eh range from
−
0.75 V to 1.25 V. Only soluble species were considered for
the model.
2.2. Chemicals and Stock Solutions
Se(IV) stock solution of 1 g/L was prepared from Na
2
SeO
3
(Sigma Aldrich, St Louis,
MO, USA) in pure water, which was produced on-site by reverse osmosis and ion exchange.
Se(VI) stock solution of 1 g/L was prepared from Na
2
SeO
4
(Carl Roth, Karlsruhe, Germany)
in pure water, which was purged with nitrogen to remove oxygen.
Stock solutions (50 mg/L) for reducing agents were prepared using oxygen-free pure
water and adding either FeSO
4·
7 H
2
O (Th. Geyer, Renningen, Germany) or Na
2
S
2
O
4
(Merck KGaA, Darmstadt, Germany).
Fe(II) solution for the pilot-scale experiments was prepared from FeSO
4·
7 H
2
O, which
was acidified to a pH below three to enhance the stability of Fe(II).
2.3. Pilot Plant Description
The experiments were conducted at a fully automated two-stage pilot plant for biolog-
ical iron and manganese removal, which is described in detail elsewhere [
22
] and depicted
in Figure 1. In short, the pilot plant consists of two filter columns (in row; 148 mm inner
diameter, 1550 mm height, 20.64 L empty bed volume, 1200 mm filter bed height) filled
with expanded polystyrene filter material and operated in up-flow. Due to the extended
run time of the filters, these have been naturally converted into biofilters. The influent is
aerated with technical oxygen before entering the first column and again before the second
column. The oxygen flow is adjusted to remove iron mainly within the first column and
manganese in the second column.
Oxygen dosage was set at 3 NmL/min (ca. 1.0 mg/L) resulting in a redox potential of
153 mV after the first column. During normal operation, the pilot plant treats groundwater
at 250 L/h, which correspond to a filtration velocity of 15 m/h.
Water 2023,15, 3147 3 of 12
Water 2023, 15, x FOR PEER REVIEW 3 of 14
Figure 1. Simplified scheme of the pilot plant with dosing points for Se(VI) or Se(IV) and optional
Fe(II), which was only dosed when Se(VI) removal was examined. Numbers depict the sampling
ports.
2.4. Pilot-Scale Experiments
Experiments were conducted within one filtration cycle until backwashing (50 h).
Two experiments were conducted using iron- and manganese-free drinking water as raw
water. Se(VI) was dosed into the pilot plant influent to achieve an influent concentration
of 1.4 mg/L, a realistic concentration in highly contaminated areas [5]. To investigate
adsorption of Se(VI), the filters were pre-loaded with precipitated iron for 4 h by treating
iron- and manganese-containing groundwater before switching to the iron-free drinking
water in the first experiment. Samples were obtained at different sampling points along
the pilot plant to examine Se reduction during groundwater treatment. All samples were
analysed for Fe(II) and total selenium (Se
tot
). Sampling was conducted daily.
To test potential reduction of Se(VI) to Se(IV) by Fe(II) and subsequent adsorption,
Fe(II) was dosed at three different concentrations (1.0, 2.0, and 4.5 mg/L) to provide a
reducing agent during the second experiment. Sampling was conducted in the same
manner as without Fe(II) dosing. The filters were not pre-loaded for this experiment.
To verify and quantify Se(IV) adsorption within the pilot plant under natural
conditions, a third experiment was run with natural groundwater as the raw water (ca. 2.5
mg/L Fe(II)) and Se(IV) was dosed into the pilot plant influent to achieve a concentration
of 0.5 mg/L. The dosing tank of Se(IV) was purged with nitrogen to avoid oxidation.
Sampling was conducted on the first and third day of the experiment.
All samples were filtered over 0.45 µm PET filters (Chromafil, Macherey-Nagel,
Düren, Germany). Fe(II) samples were analysed immediately, and Se samples (50 mL)
were acidified with 300 µL HNO
3
(65%, Merck KGaA, Darmstadt, Germany) for
conservation.
2.5. Lab-Scale Batch Experiments
To further elucidate the reduction of Se(VI) and the adsorption behaviour of Se(IV)
and Se(VI), batch-shaking tests were conducted in drinking water.
For reduction and adsorption tests, Se(VI) solutions were prepared at 1.0 mg/L and
0.1 mg/L. Dithionite was added as a reducing agent at concentrations of 5 mg/L and 25
mg/L. Fe(II) was also tested as a reducing agent at 5 mg/L. As an adsorbent, iron
Figure 1.
Simplified scheme of the pilot plant with dosing points for Se(VI) or Se(IV) and op-
tional Fe(II), which was only dosed when Se(VI) removal was examined. Numbers depict the
sampling ports.
2.4. Pilot-Scale Experiments
Experiments were conducted within one filtration cycle until backwashing (50 h).
Two experiments were conducted using iron- and manganese-free drinking water as raw
water. Se(VI) was dosed into the pilot plant influent to achieve an influent concentration
of 1.4 mg/L, a realistic concentration in highly contaminated areas [
5
]. To investigate
adsorption of Se(VI), the filters were pre-loaded with precipitated iron for 4 h by treating
iron- and manganese-containing groundwater before switching to the iron-free drinking
water in the first experiment. Samples were obtained at different sampling points along
the pilot plant to examine Se reduction during groundwater treatment. All samples were
analysed for Fe(II) and total selenium (Setot). Sampling was conducted daily.
To test potential reduction of Se(VI) to Se(IV) by Fe(II) and subsequent adsorption,
Fe(II) was dosed at three different concentrations (1.0, 2.0, and 4.5 mg/L) to provide a
reducing agent during the second experiment. Sampling was conducted in the same
manner as without Fe(II) dosing. The filters were not pre-loaded for this experiment.
To verify and quantify Se(IV) adsorption within the pilot plant under natural condi-
tions, a third experiment was run with natural groundwater as the raw water (ca. 2.5 mg/L
Fe(II)) and Se(IV) was dosed into the pilot plant influent to achieve a concentration of
0.5 mg/L. The dosing tank of Se(IV) was purged with nitrogen to avoid oxidation. Sam-
pling was conducted on the first and third day of the experiment.
All samples were filtered over 0.45
µ
m PET filters (Chromafil, Macherey-Nagel, Düren,
Germany). Fe(II) samples were analysed immediately, and Se samples (50 mL) were
acidified with 300 µL HNO3(65%, Merck KGaA, Darmstadt, Germany) for conservation.
2.5. Lab-Scale Batch Experiments
To further elucidate the reduction of Se(VI) and the adsorption behaviour of Se(IV)
and Se(VI), batch-shaking tests were conducted in drinking water.
For reduction and adsorption tests, Se(VI) solutions were prepared at 1.0 mg/L and
0.1 mg/L. Dithionite was added as a reducing agent at concentrations of 5 mg/L and
25 mg/L. Fe(II) was also tested as a reducing agent at 5 mg/L. As an adsorbent, iron
hydroxides were added in the form of backwash sludge from the first column of the pilot
Water 2023,15, 3147 4 of 12
plant in regular operation mode with unspiked reduced groundwater. The sludge was
added as a suspension at a total suspended solid concentration of 12 mg/L in the batch.
To separate adsorption from reduction, in a separate trial, Se(IV) solutions of 1.0 mg/L
and 0.1 mg/L were prepared from the stock solution. The adsorbent was added in the
same manner to both Se(VI) and Se(IV) solutions at both concentrations.
All samples (50 mL) were shaken at 120 rpm for 30 min, filtered over 0.45
µ
m PET
filters, and acidified with 300 µL HNO3(65%) for conservation.
2.6. Analyses
Se(IV) and Se(VI) were measured as Se
tot
using inductively coupled plasma combined
with mass spectrometry (ICP-MS; model NexION 300 D, Perkin Elmer, Waltham, MA, USA)
according to ISO 17294-2.
Fe(II) samples were collected into 10 mL glass vials and analysed immediately. The
photometric determination of Fe was carried out using the Spectroquant
®
Iron Test Kit
(Merck KGaA, Darmstadt, Germany) with a UV/VIS spectrometer (Lambda 35, Perkin
Elmer, Waltham, MA, USA). Measurements were carried out in a 10 mm quartz glass
cuvette at a wavelength of 562 nm in a calibration range from 50 to 6000
µ
g/L with a
regression factor of 0.99.
3. Results
3.1. Modelling
The speciation of Se in dependence of pH and Eh was modelled with PHREEQC
(2019) using the Lawrence Livermore National Laboratory (llnl.dat) 2017 database and is
depicted in Figure 2. At a high oxidation redox potential (ORP), the very mobile [
1
,
3
,
9
,
25
]
Se(VI) is the most dominant species over the entire pH range, present as SeO
42−
. At
moderate ORP, Se(IV) is the dominant speciation, changing from H
2
SeO
3
to HSeO
3−
to SeO
32−
with increasing pH. Se(IV) is considered to be more toxic than Se(VI) due to
greater bioavailability [
25
–
27
], but is less mobile due to higher adsorption towards iron
and aluminium oxides [
3
]. At low ORP, Se is present as Se(
−
II), in the form of H
2
Se at low
pH and HSe
−
at moderate-to-high pH. Under strongly acidic and reducing conditions, Se
is present as insoluble Se(0) [3] (not shown in Figure 2).
Water 2023, 15, x FOR PEER REVIEW 4 of 14
hydroxides were added in the form of backwash sludge from the first column of the pilot
plant in regular operation mode with unspiked reduced groundwater. The sludge was
added as a suspension at a total suspended solid concentration of 12 mg/L in the batch.
To separate adsorption from reduction, in a separate trial, Se(IV) solutions of 1.0
mg/L and 0.1 mg/L were prepared from the stock solution. The adsorbent was added in
the same manner to both Se(VI) and Se(IV) solutions at both concentrations.
All samples (50 mL) were shaken at 120 rpm for 30 min, filtered over 0.45 µm PET
filters, and acidified with 300 µL HNO
3
(65%) for conservation.
2.6. Analyses
Se(IV) and Se(VI) were measured as Se
tot
using inductively coupled plasma combined
with mass spectrometry (ICP-MS; model NexION 300 D, Perkin Elmer, Waltham, MA,
USA) according to ISO 17294-2.
Fe(II) samples were collected into 10 mL glass vials and analysed immediately. The
photometric determination of Fe was carried out using the Spectroquant
®
Iron Test Kit
(Merck KGaA, Darmstadt, Germany) with a UV/VIS spectrometer (Lambda 35, Perkin
Elmer, Waltham, MA, USA). Measurements were carried out in a 10 mm quartz glass cu-
vette at a wavelength of 562 nm in a calibration range from 50 to 6000 µg/L with a regres-
sion factor of 0.99.
3. Results
3.1. Modelling
The speciation of Se in dependence of pH and Eh was modelled with PHREEQC
(2019) using the Lawrence Livermore National Laboratory (llnl.dat) 2017 database and is
depicted in Figure 2. At a high oxidation redox potential (ORP), the very mobile [1,3,9,25]
Se(VI) is the most dominant species over the entire pH range, present as SeO
42−
. At mod-
erate ORP, Se(IV) is the dominant speciation, changing from H
2
SeO
3
to HSeO
3−
to SeO
32−
with increasing pH. Se(IV) is considered to be more toxic than Se(VI) due to greater bioa-
vailability [25–27], but is less mobile due to higher adsorption towards iron and alumin-
ium oxides [3]. At low ORP, Se is present as Se(−II), in the form of H
2
Se at low pH and
HSe
−
at moderate-to-high pH. Under strongly acidic and reducing conditions, Se is pre-
sent as insoluble Se(0) [3] (not shown in Figure 2).
Figure 2.
Pourbaix diagram of soluble Se species in an open system, computed with Lawrence
Livermore National Laboratory dataset (llnl.dat). Dark grey field is Se(VI), light grey field is Se(IV),
and white field is Se(-II).
Water 2023,15, 3147 5 of 12
The presented modelling results are consistent with literature investigating Se spe-
ciation under different conditions. Kumar et al. [
25
] investigated Se in groundwater in
Chennai (India) and identified both Se(VI) and Se(IV) under slightly oxidizing to oxidizing
conditions, with Se(VI) making up the majority of detected Se. Kuisi et al. [
28
] analysed
total Se in eight different aquifers in Jordan, ranging from slightly oxidizing to oxidizing
conditions depending on the aquifer depth. The speciation was not directly measured
but assumed to be both Se(VI) and Se(IV) due to the prevailing conditions. It was further
proposed that Se(VI) is more dominant due to its greater mobility [
28
]. In Berlin groundwa-
ter, Se levels tend to be below the limit of quantification (0.9
µ
g/L), but there have been
reports of up to 4.5
µ
g/L [
29
]. Based on mean values for pH and ORP [
29
,
30
], Se(IV) would
be expected.
Based on the thermodynamic calculation, it is hypothesized that Fe(II) can be used to
reduce Se(VI) to Se(IV) at moderate-to-high ORP as would be expected for Se(VI)-containing
groundwater.
3.2. Removal in the Pilot Plant
The adsorption of Se(VI) without Fe(II) dosing onto iron hydroxides and manganese
oxides showed no significant removal over the duration of the filter cycle (Figure 3). While
there is a small decrease in the Se(VI) concentration between sampling points 1 (influent)
and 10 (effluent of the first filter) at the beginning of the experiment, adsorption stalled
after 24 h, and after 48 h, the effluent concentration even exceeded influent concentration.
As Fe(II) is completely removed by the filter, it can be assumed that ca. 1200 mg freshly
precipitated Fe(III) was present in the filter at the start of Se(VI) dosing. This would
indicate a Se(VI) loading of 114
µ
g Se(VI) per mg Fe at the first sampling event after 2 h
of Se(VI) dosing. This loading is about three orders of magnitude higher than loadings
found by Kalaitzidou et al. [
13
] onto FeOOH in rapid small-scale column tests (RSSCT)
with synthesized FeOOH (10
µ
g Se(VI)/g FeOOH). Due to the high Se concentration of 1.4
(
±
0.1) mg/L, it is plausible that the initial adsorption capacity was quickly exhausted. As
no further iron was introduced during the experiment, no further adsorption sites were
created and were available, and Se(VI) was not removed.
Water 2023, 15, x FOR PEER REVIEW 6 of 14
Figure 3. Se(VI) concentrations in the pilot plant after dosing of Se(VI). Error bars indicate minimum
and maximum concentrations from duplicate samplings per sampling point.
During continuous Fe(II) dosing, Fe(II) was removed to a great extent in the first col-
umn and further to values below the detection limit (50 µg/L) in the second column inde-
pendent of the feed concentration (Figure 4). Se(VI), on the other hand, was not signifi-
cantly removed in either column of the pilot plant. As Se(VI) is poorly removable via ad-
sorption to iron hydroxides [13], these results indicate that the reduction potential of Fe(II)
is insufficient to reduce Se(VI) or reaction time between aeration and filter bed entry (ap-
prox. 13 min) is too short to achieve reduction. This confirms lab-scale results of Yoon at
al. [18], who found Fe(II) insufficient to reduce Se(VI). Furthermore, no initial adsorption
can be observed. The ratio of spiked Se to Fe(II) was lower after pre-loading (235 mg Se(VI)
per g Fe(II) at the beginning of the experiment) than without pre-loading (ratio of Se to Fe
was 300–1300 mg Se(VI) per g Fe(II) throughout the experiment), meaning the ratio of
freshly produced adsorption sites to Se was higher, leading to the observed difference in
Se removal during the first sampling of both experiments. Due to the high Se concentra-
tions, the adsorption capacity was likely quickly exhausted, and the amount of Fe(II)
dosed was not sufficient to remove Se consistently over the course of the experiment.
Figure 3.
Se(VI) concentrations in the pilot plant after dosing of Se(VI). Error bars indicate minimum
and maximum concentrations from duplicate samplings per sampling point.
Water 2023,15, 3147 6 of 12
During continuous Fe(II) dosing, Fe(II) was removed to a great extent in the first
column and further to values below the detection limit (50
µ
g/L) in the second column
independent of the feed concentration (Figure 4). Se(VI), on the other hand, was not
significantly removed in either column of the pilot plant. As Se(VI) is poorly removable
via adsorption to iron hydroxides [
13
], these results indicate that the reduction potential
of Fe(II) is insufficient to reduce Se(VI) or reaction time between aeration and filter bed
entry (approx. 13 min) is too short to achieve reduction. This confirms lab-scale results
of
Yoon at al. [18]
, who found Fe(II) insufficient to reduce Se(VI). Furthermore, no initial
adsorption can be observed. The ratio of spiked Se to Fe(II) was lower after pre-loading
(235 mg Se(VI) per g Fe(II) at the beginning of the experiment) than without pre-loading
(ratio of Se to Fe was 300–1300 mg Se(VI) per g Fe(II) throughout the experiment), meaning
the ratio of freshly produced adsorption sites to Se was higher, leading to the observed
difference in Se removal during the first sampling of both experiments. Due to the high Se
concentrations, the adsorption capacity was likely quickly exhausted, and the amount of
Fe(II) dosed was not sufficient to remove Se consistently over the course of the experiment.
Water 2023, 15, x FOR PEER REVIEW 7 of 14
Figure 4. Se (top) and Fe (bottom) concentrations in the pilot plant after dosing of Se(VI) and Fe(II).
Error bars indicate minimum and maximum concentrations from duplicate samplings per sampling
point.
The high Se concentrations of 1.5 (±0.2) mg/L are realistic in certain areas [5] and were
therefore chosen to evaluate the efficiency of the treatment process, yet the dosed Fe(II)
concentrations are insufficient to remove such high concentrations. Previous studies
dosed Se at an Se/adsorbent ratio of 0.1 to 5 mg/g [31,32], which is significantly lower than
the ratio used in this study of 300 to 1300 mg Se per g Fe(II). Based on the mass balance of
Se and Fe removed, an adsorbent loading of up to 113.6 mg Se per g Fe(II) (without Fe(II)
dosing) and 22.35 mg/g (with continuous Fe dosing) was achieved. This is up to 100 times
higher than adsorbent loadings found for hydroxide Fe3O4 (1.4 mg Se(VI)/g) [31] and iron
oxides (1.9 mg Se(VI)/g) [32]. For a better understanding of the removal capacity of the
tested system, lower Se/Fe(II) ratios should be tested by either lowering the spiked Se con-
centration or further increasing the Fe(II) dosage.
As total Se was measured, the results can only indicate whether Se(VI) was reduced
to Se(IV) based on the fact that Se(IV) is generally better adsorbable and would be ex-
pected to be removed in the pilot plant. To investigate this hypothesis, Se(IV) was dosed
at 0.43 (±0.03) mg/L in a third experiment using natural groundwater as influent.
Fe(II) was removed to concentrations below the limit of quantification within the first
filter column (Figure 5). Se(IV) was removed by 85% within the first filter column and
Figure 4.
Se (
top
) and Fe (
bottom
) concentrations in the pilot plant after dosing of Se(VI) and
Fe(II). Error bars indicate minimum and maximum concentrations from duplicate samplings per
sampling point.
The high Se concentrations of 1.5 (
±
0.2) mg/L are realistic in certain areas [
5
] and
were therefore chosen to evaluate the efficiency of the treatment process, yet the dosed
Water 2023,15, 3147 7 of 12
Fe(II) concentrations are insufficient to remove such high concentrations. Previous studies
dosed Se at an Se/adsorbent ratio of 0.1 to 5 mg/g [
31
,
32
], which is significantly lower than
the ratio used in this study of 300 to 1300 mg Se per g Fe(II). Based on the mass balance
of Se and Fe removed, an adsorbent loading of up to 113.6 mg Se per g Fe(II) (without
Fe(II) dosing) and 22.35 mg/g (with continuous Fe dosing) was achieved. This is up to 100
times higher than adsorbent loadings found for hydroxide Fe
3
O
4
(1.4 mg Se(VI)/g) [
31
]
and iron oxides (1.9 mg Se(VI)/g) [
32
]. For a better understanding of the removal capacity
of the tested system, lower Se/Fe(II) ratios should be tested by either lowering the spiked
Se concentration or further increasing the Fe(II) dosage.
As total Se was measured, the results can only indicate whether Se(VI) was reduced to
Se(IV) based on the fact that Se(IV) is generally better adsorbable and would be expected
to be removed in the pilot plant. To investigate this hypothesis, Se(IV) was dosed at 0.43
(±0.03) mg/L in a third experiment using natural groundwater as influent.
Fe(II) was removed to concentrations below the limit of quantification within the first
filter column (Figure 5). Se(IV) was removed by 85% within the first filter column and
above 90% over the entire pilot plant shortly after the start of the experiment. Removals
decreased to 43% in the first column and 57% overall by the end of the filtration cycle.
These results show that Se(IV) can be effectively removed during the iron removal step of
the pilot plant. This is in accordance with Kalaitzidou et al. [
33
], who demonstrated good
removal of Se(IV) with FeOOH in RSSCT. At a dosing concentration of 100
µ
g/L Se(IV), the
adsorption capacity (defined as loading at residual concentrations of 10
µ
g/L) was found to
be 4.3
µ
g Se(IV) per mg FeOOH [
33
]. In this study, the amount of dosed Fe(II) was 5.8 times
the amount of Se(IV), which resulted in a loading of 170
µ
g Se(IV) per mg Fe(III) after the
first two hours and 98
µ
g Se(IV) per mg Fe(III) after 48 h of dosing. The decrease in the
loading over the experimental period could indicate the exhaustion of the adsorptive sites.
This seems unlikely despite the high Se dosage, as adsorption sites on iron hydroxides and
manganese oxides are continuously produced during the filtration cycle, yet it is possible
that iron hydroxides remaining in the filter after backwash could have enhanced Se(IV)
adsorption at the beginning of the experiment. Potentially, Se(IV) might have been partially
oxidized to Se(VI) in the dosage tank (despite purging with nitrogen), which might have
led to decreased adsorption. As total Se was measured and no distinction between the
species could be made, this cannot be ruled out entirely.
The results cannot unrefutably demonstrate to what extent Se(VI) can be removed and
which underlying mechanism is responsible for the removal. The good removal of Se(IV)
indicates that rapid reduction of Se(VI) before entering the filter column could present a
solution for enhanced removal of Se(VI). Further investigations into Se(VI) reduction and
adsorption are necessary to assess the feasibility of removal during groundwater treatment
The Se removal was investigated within one filter cycle. During continuous oper-
ation, groundwater treatment filters are periodically backwashed to remove the solids
that accumulate in the filter bed. Adsorbed Se can therefore be expected to occur in the
backwash sludge. There is some research on the reuse of Se from solids, for example,
as a fertilizer in Se-poor soils [
34
,
35
], in the production of semi-conductors [
36
], or the
adsorption of mercury [
37
]. Se might be recovered from solids using NaOH, which has
been demonstrated to work well in extracting arsenic from hydroxyl-enriched CeO2[38].
If the selenium cannot be extracted and sludge cannot be reused, safe disposal to avoid
secondary contamination is necessary. Staicu et al. [
36
] reported that Se-containing iron
sludge has a low TCLP (toxicity characteristic leaching procedure) value and can be treated
as non-dangerous waste. Se-containing sludge from groundwater treatment would need to
be similarly tested before disposal options are discussed.
Water 2023,15, 3147 8 of 12
Water 2023, 15, x FOR PEER REVIEW 9 of 14
Figure 5. Se (top) and Fe (bottom) concentrations in the pilot plant after dosing of Se(VI). Fe(II) was
provided from anoxic groundwater.
The results cannot unrefutably demonstrate to what extent Se(VI) can be removed
and which underlying mechanism is responsible for the removal. The good removal of
Se(IV) indicates that rapid reduction of Se(VI) before entering the filter column could pre-
sent a solution for enhanced removal of Se(VI). Further investigations into Se(VI) reduc-
tion and adsorption are necessary to assess the feasibility of removal during groundwater
treatment
The Se removal was investigated within one filter cycle. During continuous opera-
tion, groundwater treatment filters are periodically backwashed to remove the solids that
accumulate in the filter bed. Adsorbed Se can therefore be expected to occur in the back-
wash sludge. There is some research on the reuse of Se from solids, for example, as a fer-
tilizer in Se-poor soils [34,35], in the production of semi-conductors [36], or the adsorption
Figure 5.
Se (
top
) and Fe (
bottom
) concentrations in the pilot plant after dosing of Se(VI). Fe(II) was
provided from anoxic groundwater.
3.3. Reduction and Adsorption Tests
To examine the reducing potential of Fe(II) and dithionite, batch experiments were
carried out with two initial Se concentrations and backwash sludge from the pilot plant as
the adsorbent (Figure 6). At the initial concentration of 0.1 mg/L, Se(IV) was removed by
7% by the adsorbent alone, and Se(VI) was removed by 2.6%. The addition of dithionite
resulted in slightly higher removals of 3% and 4% Se(VI) for 5 mg/L and 25 mg/L dithionite,
respectively. Fe(II) did not result in significantly higher removals (0.4%). These removal
rates translate to a loading of 0.007
µ
g Se(IV) per mg sludge and 0.002
µ
g Se(VI) per mg
sludge only via adsorption, which is significantly lower than loadings achieved during
pilot-scale removal. The addition of the reducing agents of Fe(II) at 5 mg/L and dithionite
at 5 and 25 mg/L resulted in no significant increase in the Se(VI) loading (0.0004, 0.002, and
0.003 µg Se(VI) per mg sludge, respectively).
Water 2023,15, 3147 9 of 12
Water 2023, 15, x FOR PEER REVIEW 10 of 14
of mercury [37]. Se might be recovered from solids using NaOH, which has been demon-
strated to work well in extracting arsenic from hydroxyl-enriched CeO2 [38].
If the selenium cannot be extracted and sludge cannot be reused, safe disposal to
avoid secondary contamination is necessary. Staicu et al. [36] reported that Se-containing
iron sludge has a low TCLP (toxicity characteristic leaching procedure) value and can be
treated as non-dangerous waste. Se-containing sludge from groundwater treatment
would need to be similarly tested before disposal options are discussed.
3.3. Reduction and Adsorption Tests
To examine the reducing potential of Fe(II) and dithionite, batch experiments were
carried out with two initial Se concentrations and backwash sludge from the pilot plant
as the adsorbent (Figure 6). At the initial concentration of 0.1 mg/L, Se(IV) was removed
by 7% by the adsorbent alone, and Se(VI) was removed by 2.6%. The addition of dithionite
resulted in slightly higher removals of 3% and 4% Se(VI) for 5 mg/L and 25 mg/L dithio-
nite, respectively. Fe(II) did not result in significantly higher removals (0.4%). These re-
moval rates translate to a loading of 0.007 µg Se(IV) per mg sludge and 0.002 µg Se(VI)
per mg sludge only via adsorption, which is significantly lower than loadings achieved
during pilot-scale removal. The addition of the reducing agents of Fe(II) at 5 mg/L and
dithionite at 5 and 25 mg/L resulted in no significant increase in the Se(VI) loading (0.0004,
0.002, and 0.003 µg Se(VI) per mg sludge, respectively).
At an initial Se concentration of 1 mg/L, removals were overall higher but showed
larger variation. Se(VI) and Se(IV) were removed to a similar degree via adsorption. The
addition of reducing agents resulted in no further Se(VI) removal, and on the contrary,
the removals were lower. The addition of 5 mg/L Fe(II) and 25 mg/L dithionite resulted in
a similar removal of approximately 13%, and the addition of 5 mg/L dithionite resulted in
an even smaller Se(VI) removal. Calculated loadings due to adsorption are 0.008–0.015 µg
Se(IV) per mg sludge and 0.012–0.014 µg Se(VI) per mg sludge. These are slightly higher
than at lower initial Se concentrations. The impact of the reducing agents was negligible,
indicating the ratio between Se(VI) and the reducing agents to be too low to achieve re-
duction.
Figure 6. Se adsorption onto iron hydroxides in the presence of reducing agents for Se(IV) (blue)
and Se(VI) (green) removals at Se starting concentrations of 0.1 mg/L (left) and 1.0 mg/L (right).
Error bars indicate minimum and maximum values of two replicates.
Fe(II) has been shown to effectively reduce Cr(VI) to Cr(III), resulting in removal of
Cr(III) via co-precipitation [22]. Although Se(VI) reduction by Fe(II) is thermodynamically
Figure 6.
Se adsorption onto iron hydroxides in the presence of reducing agents for Se(IV) (blue) and
Se(VI) (green) removals at Se starting concentrations of 0.1 mg/L (
left
) and 1.0 mg/L (
right
). Error
bars indicate minimum and maximum values of two replicates.
At an initial Se concentration of 1 mg/L, removals were overall higher but showed
larger variation. Se(VI) and Se(IV) were removed to a similar degree via adsorption. The
addition of reducing agents resulted in no further Se(VI) removal, and on the contrary, the
removals were lower. The addition of 5 mg/L Fe(II) and 25 mg/L dithionite resulted in a
similar removal of approximately 13%, and the addition of 5 mg/L dithionite resulted in
an even smaller Se(VI) removal. Calculated loadings due to adsorption are
0.008–0.015 µg
Se(IV) per mg sludge and 0.012–0.014
µ
g Se(VI) per mg sludge. These are slightly higher
than at lower initial Se concentrations. The impact of the reducing agents was negligible,
indicating the ratio between Se(VI) and the reducing agents to be too low to achieve
reduction.
Fe(II) has been shown to effectively reduce Cr(VI) to Cr(III), resulting in removal of
Cr(III) via co-precipitation [
22
]. Although Se(VI) reduction by Fe(II) is thermodynamically
possible [
39
], Fe(II) did not reduce Se(VI) to Se(IV) to a significant extent over time in this
experiment. The increased contact time between Fe(II) and Se(VI) in the batch experiment
in comparison to the pilot-scale experiment (30 min and 13 min, respectively) did not aid
Se(VI) reduction. A previous study on the kinetics of Se(VI) reduction in the presence
of iron oxides has shown that the reduction does not occur over the course of multiple
hours [
39
], indicating an extension of the reaction time would not increase Se(VI) reduction
in this set-up.
Dithionite also did not improve Se(VI) removal. It has been shown that dithionite can
rapidly reduce selenious acid from weakly acidic sulphate solutions [
40
]. At neutral pH,
dithionite is able to reduce Se(IV) and Se(VI) to Se(0) under UV radiation [
41
]. Therefore,
the boundary conditions in this study might not have been optimal to utilize the reducing
potential of dithionite for Se(VI) removal.
Further research is needed to identify adequate redox partners to reduce Se(VI) for
subsequent Se(IV) adsorption. Other studies on the reduction of Se(VI) focus on the reduc-
tion to Se(0), either biologically [
42
–
44
] or using zero-valent iron [
18
], titanium dioxide [
45
],
or granulated iron and organic carbon [
16
]. While employing these reducing agents and
relying on co-precipitation of Se(0) can present an alternative to reduction to Se(IV) and
consequent adsorption, they require additional chemicals or long operational times to
achieve sufficient removal.
Unexpectedly, the removal of Se(VI) and Se(IV) were very low in this experiment. Other
studies have also looked into Se adsorption onto iron-containing adsorbents.
Jadhav et al. [46]
Water 2023,15, 3147 10 of 12
did a thorough investigation of Al-Fe mixed oxides and found an almost complete removal
of both Se(IV) and Se(VI) at neutral pH values and a ratio 4
µ
g Se per mg adsorbent. It was
also shown that sulphate can significantly decrease Se adsorption [
46
]. In this study, the
effect of competing anions in the water matrix was not studied and could be a factor in the
observed poor removal.
Another factor impacting Se removal is the Se to adsorbent ratio. Qureshi et al. [
47
]
demonstrated that ratios below and above the optimal ratio of 20
µ
g Se(IV) per mg adsor-
bent resulted in decreased Se removal with Fe-Mn-based adsorbents. In this batch test,
two ratios of 8.3
µ
g Se per mg adsorbent and 83
µ
g Se per mg adsorbent were tested. Both
resulted in relatively low removals for Se(VI) and Se(IV) via adsorption alone, indicating
that the dose was not optimal. In contrast to the pilot-scale experiments, the higher Se to
adsorbent ratio resulted in better Se removal, but the achieved loading of the adsorbent was
significantly lower than in the filter experiment. This effect highlights the importance of
conducting pilot-scale experiments alongside batch experiments when looking for feasible
treatment technologies.
4. Conclusions
The removal of the Se species, Se(IV) and Se(VI), during groundwater treatment in a
pilot plant for biological iron and manganese removal was investigated. It was shown that
up to 90% of Se(IV) was removed, while Se(VI) was not removed in significant amounts.
This indicates that Fe(II) was not able to reduce Se(VI) within the given set-up. The removal
of Se(IV) declined over the duration of the filtration cycle. Further investigations are needed
to determine whether this is due to insufficient adsorption sites or an oxidation of Se(IV) to
Se(VI) before entering the filtration column.
Batch experiments over 30 min confirmed the better adsorption of Se(IV) over Se(VI)
onto backwash sludge. At a starting concentration of 0.1 mg/L, Se(IV) was removed by 7%,
while Se(VI) was removed by less than 3%. The reducing agents dithionite and Fe(II) were
not able to reduce Se(VI) over the duration of the experiment. Further research is needed to
identify appropriate reducing agents and conditions to remove Se(VI) from water.
Author Contributions:
Conceptualization, D.M.; methodology, D.M.; formal analysis, A.S. and U.D.-D.;
investigation, A.S.; data curation, A.S.; writing—original draft preparation, A.S.; writing—review and
editing, A.S., D.M. and A.S.R.; visualization, A.S.; supervision, A.S.R.; project administration, D.M.;
funding acquisition, A.S.R. and D.M. All authors have read and agreed to the published version of the
manuscript.
Funding:
This research was funded by the Federal Ministry of Education and Research under grant
number 02WV1565E. The responsibility for the content of this publication lies with the authors.
Data Availability Statement:
The original contributions presented in the study are included in the
article. Further inquiries can be directed to the corresponding author.
Acknowledgments:
We would like to acknowledge Alexander Kämpfe, Christian Höra, and Christoph
Merdan of the Umweltbundesamt for Selenium measurement and quality control. A sincere thank
you goes out to C. Czekalla of Hamburg Wasser for the provision of selenium substances and helpful
discussions on the topic of selenium removal. Lastly, we would like to thank Dominic Braun of the
Umweltbundesamt for sample collection.
Conflicts of Interest: The authors declare no conflict of interest.
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