water
Article
Conditioning Fixed-Bed Filters with Fine Fractions of
Granulated Iron Hydroxide (µGFH)
Inga Hilbrandt *, Aki S. Ruhl and Martin Jekel
Water Quality Control, Technical University Berlin, Str. des 17 Juni 135, 10623 Berlin, Germany;
*Correspondence: [email protected]; Tel.: +49-303-142-6429
Received: 25 July 2018; Accepted: 4 September 2018; Published: 25 September 2018
Abstract:
The fine fraction of granular ferric hydroxide (
µ
GFH, <0.3 mm) is a promising adsorbent
for the removal of heavy metals and phosphate, but properties of
µ
GFH were hitherto not known.
The present study aimed at characterizing
µ
GFH 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
µ
GFH. Filter runs with phosphate (2 mg/L P) showed
similar breakthrough curves for the embedded fine fraction adsorbent and for conventional GFH.
Keywords:
granular ferric hydroxide;
µ
GFH; fixed-bed adsorber; distribution analysis;
filter conditioning
1. Introduction
Granular ferric hydroxide (GFH) is an established adsorbent in water purification for the removal
of arsenic and phosphate [
1
,
2
]. It consists predominantly of akaganeite (
β
-FeOOH) with small
contributions of ferrihydrite and other iron oxides [
3
]. During the production process of GFH, the
fine fraction with particle sizes below 0.3 mm (
µ
GFH) is separated out and disposed of unused since
µ
GFH causes pressure losses in conventional GFH fixed-bed filters. Adsorption onto conventional
GFH (0.3–2.0 mm) is limited by internal diffusion, which leads to nonideal breakthrough curves [
3
]. As
adsorption kinetics onto small particles typically provides reduced intra-particle diffusion resistance
resulting in higher k values for pseudo first- and second-order kinetics [
4
] a more ideal breakthrough
behavior is expected for
µ
GFH [
5
]. For the predicted advantages of the ferric hydroxide fine fraction
and to avoid disposal of valuable resources, an easy-to-use and efficient application for
µ
GFH is
sought after.
The integration of particular adsorbents into deep-fed filtration has been studied for powdered
activated carbon (PAC) and proved to be an energy- and space-saving option for the removal of organic
micropollutants (OMP) [
6
]. Altmann et al. [
7
] reported that the conditioning step of the fixed-bed
filter determines the distribution of PAC in the carrier material and that conditioning in turbulent
upflow led to improved efficiency due to the more homogenous distribution of the adsorbent. Also,
the PAC distribution in a filter bed of foamed polystyrene beads was decisive for OMP removal [
8
].
The combination of PAC and deep-bed filtration led to increased OMP removal which could be further
improved by recirculation of PAC in the backwash water [9].
Iron-coated sand combines the high adsorbent capacities of iron oxides with the stability and
filter efficiency of sand, but its overall adsorbent loadings with phosphate and arsenate are low
Water 2018,10, 1324; doi:10.3390/w10101324 www.mdpi.com/journal/water
Water 2018,10, 1324 2 of 12
(0.7 mg/g P) compared with GFH (18 mg/g P) [
2
,
10
]. To our knowledge, no attempts to combine
particulate iron hydroxide adsorbents with a carrier material for application in fixed-bed filters have
been reported before now. Target pollutants for
µ
GFH are arsenate, phosphate, bromate, heavy metals
and silicate [
3
,
11
–
14
]. The proposed method integrates an adsorption step into an existing filtration
process and, thus, less space and investments are required [6].
Separation efficiency and the corresponding embedment of particles in a filter bed are a function
of transport efficiency and adhesion probability [
15
]. The transport efficiency describes what ratio of
the material has contact with the filter grain surface. For particles greater than 1
µ
m, interception and
sedimentation are the dominating retention mechanisms. Also, the geometrical surface structure can
have an influence. The adhesion probability is mainly controlled by the strength of the double layer
or van der Waals forces [
15
]. By shear forces, the attached particles can be separated from the carrier
material surface. Thus, a conditioning process with optimal embedment depends highly on the surface
charge of the adsorbent and the carrier material and on the flow rate of the conditioning.
The present study aimed at evaluating the integration into fixed-bed filtration of adsorption
onto
µ
GFH. Hence, anthracite, coke, pumice and sand were studied as carrier materials. Different
particle size fractions (0–63, 0–125, 0–300
µ
m) were used for conditioning. Conditioning in up- and
downstream mode was investigated to obtain the optimal distribution and to maximize the embedded
adsorbent mass. Also, the effect of the pH of the carrier material and the adsorbent was studied. The
point of zero charge (pH
PZC
) of conventional GFH is 7.5 [
16
], which results in a neutral to slightly
positive surface charge at pH 7. Around pH 7.0–7.5, agglomeration of
µ
GFH can be observed. A shift
to higher pH values leads to repulsion between
µ
GHF particles and the carrier material, preventing
agglomeration and leading to deeper infiltration of the carrier material.
2. Materials and Methods
2.1. Characterization of µGFH
GEH Wasserchemie (Osnabrück, Germany) provided
µ
GFH with a water content of ca. 53%.
Concentrations of approximately 50 mg/L
µ
GFH were suspended in ultra-pure water and shaken
thoroughly to prevent sedimentation during triplicate measurements of the particle size distribution
using a particle analyzer (PAMAS SVSS) with a laser diode sensor. Particle volumes were calculated
assuming spherical shapes with the particle size as diameter. The specific BET surface areas were
determined for selected particle size fractions using nitrogen as described by Gregg and Sing [
17
]
with an AutoSorb-1-MP system (Quantachrome, Boynton Beach, FL, USA). The samples were dried at
room temperature (not in an oven) to prevent thermal transformations. The mineral composition of
µ
GFH was determined using X-ray powder diffraction (D2 Phaser, Bruker, Billerica, MA, USA) with a
copper anode. Furthermore, the adsorbency was characterized by scanning electron microscopy with
energy-dispersive X-ray spectroscopy.
2.2. Fixed Bed Conditioning
Fixation of
µ
GFH was investigated in small glass columns (24
×
200 mm). The properties of
the tested materials (anthracite, coke, pumice and sand) as determined following the respective
standards [
18
] are listed in Table 1. Constant filter bed volumes of 180 cm
3
were used for the materials
which vary widely in density, grain size and pore volume.
Table 1. Physical characteristics of the filter materials.
Material Product (Supplier) Main
Component
Particle Size
(mm)
Bulk Density
(kg/m3)
Grain Density
(g/cm3)
Anthracite Hydroanthrasit-P (Rheinkalk Akdolit)
Carbon (92%) 1.4–2.5 718 1250
Coke Filter coke H Type II (Evers GmbH) Carbon (88%) 1.4–2.5 523 1667
Pumice EVERZIT BI (Evers GmbH) SiO2(55%) 0.8–1.5 375 1250
Sand Quartz sand (Sand-Schulz) SiO2(98%) 1.0–2.0 1485 2625
Water 2018,10, 1324 3 of 12
The materials were submerged in deionized water and degassed in vacuum, then rinsed in the
columns with deionized water for 24 h to wash out the fine fraction. The pH was adjusted to 8.5
to cause negative surface charges to repulse the negatively charged
µ
GFH particles during filter
conditioning. Due to electrostatic repulsion, the particles could penetrate deeply into the fixed bed
and the building of a cake layer was prevented.
Conditioning with a highly concentrated
µ
GFH suspension in up- and downstream mode was
examined.
µ
GFH was air-dried and suspended in 100 mL deionized water at pH 8.5 (set with NaOH).
A pH value above the pH
PZC
of GFH (7.5) was set to assure negatively charged surface groups and to
suppress agglomeration that occurred at pH 7.5.
Conditioning in upstream mode was performed at a velocity of 30 m/h, which was accompanied
by a filter bed expansion of approximately 50%. After embedment, mobile
µ
GFH was washed out with
500 mL of deionized water at 5 m/h. To determine the mass of embedded
µ
GFH, the carrier material
was removed from the column and washed with deionized water until the supernatant was clear. The
µ
GFH in 20 mL (V
suspension
) of the collected supernatants (V
wash-off
) was dissolved by adding defined
volumes of 32% HCl (V
HCL
) and 64% HNO
3
(V
HNO3
) and stirring at 80
◦
C for one hour. Dissolved
iron (cFE) was then quantified with atomic absorption spectroscopy. The mass of
µ
GFH was then
calculated with the total volume of the suspension, the dilution from the acids for the disintegration
and the iron content of µGFH following Equation (1):
mµGFH =cFe· VSuspension +VHCl +VHNO3
VSuspension !·1
wFe
·Vwash−off (1)
For image analyses, the filters were conditioned in downstream mode and the effluent was
recirculated four times: three times with 5 m/h and one time with a decreased rate of 2.5 m/h.
2.3. Image Analysis
Conditioned columns were frozen at
−
18
◦
C for 24 h to analyze the
µ
GFH distribution within the
filter bed. The frozen cores were removed from the columns and cut in the frozen state with a circular
saw into an upper and lower half (each 5 cm) and subsequently vertically into two parts each (H1 and
H2). All conditioning modes were done twofold.
Digital images were taken of each cross-sectional area individually with a Canon EOS 700D
camera at constant distance, illumination and angle.
ImageJ [
19
] was used for picture analyses following the scheme displayed in Figure 1. To exclude
wall effects, equally sized rectangles in the middle of the pictures were selected (Step II). The selected
sections were divided into twelve subsections to evaluate the homogeneity of the core (Step III).
Color thresholds were applied to differentiate between pumice (light color),
µ
GFH (dark brown) and
water-filled pore volume and to convert the image into a binary picture (Step IV). The area covered by
µGFH was then calculated. Further explanation can be found in the supplementary information.
Water2018,10,xFORPEERREVIEW 4of13
Figure1.PictureanalyseswithImageJwith(I)photographofthefrozenandcutcolumn,(II)core
excludingwalleffects,(III)subsection(1of12)and(IV)convertedbinaryimage.
2.4.FixedBedAdsorptionTests
Fixedbedadsorptionwasstudiedincylindricalfiltercolumns(acrylicglass,2.4×100cm)filled
withdegassedandextensivelyflushedpumicetoabedheightof40cm(ca.180mLfilterbed).Grain
sizesofthecarriermaterialandadsorbentparticleswerechosensoastonotincreasetheheadlossof
thefilter.Conditioningwasperformedindownstreammode(2.5m/h)witha30g/LμGFH
suspension(100mL,pH8.5).Amodelsolutionofphosphate(2mg/LP),0.6g/LNaClandorganic
buffer(N,N‐Bis(2‐hydroxyethyl)‐2‐aminoethanesulfonicacid,pK
a
=7.1,0.4mg/L)settopH7.0with
NaOHindeionizedwaterwasappliedforadsorptiontestswithaflowrateof2.2m/h(1L/h)
followingexperimentsbySperlichetal.[2].Samplesweretakenautomaticallyevery3hatthefilter
outlet.Orthophosphatewasmeasuredviaflowinjectionanalysisaccordingtothestandardmethod
[20].
BreakthroughdataforconventionalGFHwastakenfromPatel[21]forcomparison.Dueto
differencesinfiltervelocity(3m/hforconventionalGFHand2.2m/hforμGFH),modelleddatais
included.InputparametersformodelingarelistedinTableS1.ThesoftwareFAST(fixedbed
simulationadsorptiontool)hasbeenproventopredictthebreakthroughbehaviorofGFHin
numerousstudies[2,14,22].Itisbasedonthehomogenoussurfacediffusionmodel(HSDM)and
combinesthemassbalanceequationwithintraparticletransportaccordingtoFick’ssecondlaw.The
modelisdescribedindetailinSperlichetal.[2].
3.Results
3.1.PhysicalCharacterization
Particlesizeanalysesvialaserlightextinctionofpreviouslywet‐sievedμGFH(tobelow300μm)
revealedthatapproximately30%byvolume(ormass,assumingahomogeneousdensitydistribution)
iscontributedbyeachoftheparticlesizeranges0–63μm,63–125μmand125–250μm(Figure2).The
fractionsoflargerparticles(250–300μm)contributeapproximately15%byweightbutconsistofonly
comparativelyfewparticles.Hence,thesmallparticleshaveagreatinfluenceontheadsorption
kinetics.
Figure 1.
Picture analyses with ImageJ with (
I
) photograph of the frozen and cut column, (
II
) core
excluding wall effects, (III) subsection (1 of 12) and (IV) converted binary image.
Water 2018,10, 1324 4 of 12
2.4. Fixed Bed Adsorption Tests
Fixed bed adsorption was studied in cylindrical filter columns (acrylic glass, 2.4
×
100 cm) filled
with degassed and extensively flushed pumice to a bed height of 40 cm (ca. 180 mL filter bed). Grain
sizes of the carrier material and adsorbent particles were chosen so as to not increase the head loss of the
filter. Conditioning was performed in downstream mode (2.5 m/h) with a 30 g/L
µ
GFH suspension
(100 mL, pH 8.5). A model solution of phosphate (2 mg/L P), 0.6 g/L NaCl and organic buffer
(N,N-Bis(2-hydroxyethyl)-2-aminoethanesulfonic acid, pK
a
= 7.1, 0.4 mg/L) set to pH 7.0 with NaOH
in deionized water was applied for adsorption tests with a flow rate of 2.2 m/h (1 L/h) following
experiments by Sperlich et al. [
2
]. Samples were taken automatically every 3 h at the filter outlet.
Orthophosphate was measured via flow injection analysis according to the standard method [20].
Breakthrough data for conventional GFH was taken from Patel [
21
] for comparison. Due to
differences in filter velocity (3 m/h for conventional GFH and 2.2 m/h for
µ
GFH), modelled data
is included. Input parameters for modeling are listed in Table S1. The software FAST (fixed bed
simulation adsorption tool) has been proven to predict the breakthrough behavior of GFH in numerous
studies [
2
,
14
,
22
]. It is based on the homogenous surface diffusion model (HSDM) and combines
the mass balance equation with intraparticle transport according to Fick’s second law. The model is
described in detail in Sperlich et al. [2].
3. Results
3.1. Physical Characterization
Particle size analyses via laser light extinction of previously wet-sieved
µ
GFH (to below 300
µ
m)
revealed that approximately 30% by volume (or mass, assuming a homogeneous density distribution)
is contributed by each of the particle size ranges 0–63
µ
m, 63–125
µ
m and 125–250
µ
m (Figure 2).
The fractions of larger particles (250–300
µ
m) contribute approximately 15% by weight but consist
of only comparatively few particles. Hence, the small particles have a great influence on the
adsorption kinetics.
Water2018,10,xFORPEERREVIEW 5of13
Figure1.ParticlesizedistributionoffinefractionGFH(μGFH)byvolumeestimatedfromparticle
sizesdeterminedusingaparticleanalyzer;numbersindicatedetectedparticlenumbers(in%ofthe
totalnumber)ofthefraction.
Almostidenticalspecificsurfaceareasintherange299–309m2/gweredeterminedforthe
fractions<63μm,63–125μm,125–250μmand>250μm,indicatinganegligibleimpactoftheouter
particlesurface.Badruzzamanetal.[11]reportedloweraveragedvaluesof236±9m2/gfor
conventionalGFHwhileSperlichetal.[22]measuredvaluesbetween229and257m2/g.They[22]
showedthatsurfaceareacouldnotbelinkedtograinsizeandstatedthatthevariationislargerthan
themeasurementuncertaintyoftheBETmethod,whichwasexplainedbytheheterogeneityofthe
adsorbentmedia.Otherstudiesreportedspecificsurfaceareasofupto300m2/g[14].Theresults
obtainedinthisstudyareintheupperrangeofthatmagnitude.Therefore,nosignificantrisein
equilibriumadsorptioncapacityofthefinefractionadsorbentincomparisontoconventionalGFH
canbeexpected.
MineralogicalanalyseswithpowderX‐raydiffractionconfirmedβ‐FeOOHasthemain
constituentofallparticles(FigureS1inthesupportinginformation).Additionally,particleslarger
than63μmshowedreflexeswhichcorrespondtothechemicalstructureofFeO(OH,Cl),whereas
thespectrumofparticlessmallerthan63μmrevealedthepresenceofFe2O3.Storedparticlessmaller
than63μm(fromanotherbatch)consistedmainlyofhematite.Thisleadstotheassumptionthat
smallerparticlesaremorelikelytotransformtohematiteduetothelackofstabilizingchlorineatoms
[14].
ThescanningelectronmicroscopicimagesofμGFHunderdifferentmagnificationsinFigure3
indicateirregularshapesandthewiderangeofparticlesizes.However,nomacro‐porescouldbe
observed.Smallerparticlesattachtobiggerparticles(Figure3b).Energy‐dispersiveX‐ray
spectroscopy(FigureS2)confirmed,asexpected,highcontentsofironandoxygenwithtracesof
phosphorusandsulfur.TheparticlesarestableinthepHrange3–14andcanonlybedissolvedusing
highlyconcentratedacidsandelevatedtemperatureswhichmakesthemusableforavarietyof
applications.
Figure 2.
Particle size distribution of fine fraction GFH (
µ
GFH) by volume estimated from particle
sizes determined using a particle analyzer; numbers indicate detected particle numbers (in % of the
total number) of the fraction.
Almost identical specific surface areas in the range 299–309 m
2
/g were determined for the fractions
<63
µ
m, 63–125
µ
m, 125–250
µ
m and >250
µ
m, indicating a negligible impact of the outer particle
surface. Badruzzaman et al. [
11
] reported lower averaged values of 236
±
9 m
2
/g for conventional GFH
Water 2018,10, 1324 5 of 12
while Sperlich et al. [
22
] measured values between 229 and 257 m
2
/g. They [
22
] showed that surface
area could not be linked to grain size and stated that the variation is larger than the measurement
uncertainty of the BET method, which was explained by the heterogeneity of the adsorbent media.
Other studies reported specific surface areas of up to 300 m
2
/g [
14
]. The results obtained in this study
are in the upper range of that magnitude. Therefore, no significant rise in equilibrium adsorption
capacity of the fine fraction adsorbent in comparison to conventional GFH can be expected.
Mineralogical analyses with powder X-ray diffraction confirmed
β
-FeOOH as the main constituent
of all particles (Figure S1 in the supporting information). Additionally, particles larger than 63
µ
m
showed reflexes which correspond to the chemical structure of FeO (OH, Cl), whereas the spectrum of
particles smaller than 63
µ
m revealed the presence of Fe
2
O
3
. Stored particles smaller than 63
µ
m (from
another batch) consisted mainly of hematite. This leads to the assumption that smaller particles are
more likely to transform to hematite due to the lack of stabilizing chlorine atoms [14].
The scanning electron microscopic images of
µ
GFH under different magnifications in Figure 3
indicate irregular shapes and the wide range of particle sizes. However, no macro-pores could be
observed. Smaller particles attach to bigger particles (Figure 3b). Energy-dispersive X-ray spectroscopy
(Figure S2) confirmed, as expected, high contents of iron and oxygen with traces of phosphorus
and sulfur. The particles are stable in the pH range 3–14 and can only be dissolved using highly
concentrated acids and elevated temperatures which makes them usable for a variety of applications.
Water2018,10,xFORPEERREVIEW 6of13
Figure3.Scanningelectronmicroscopicimagesof(a)unfractionatedμGFHbulkmaterial,(b)an
individualparticlewithattachedfinesand(c)asmallparticledepositedonacellulosenitrate
membrane.
3.2.CarrierMaterialSelection
Allfourtestedgranularcarriermaterials—pumice,coke,anthraciteandsand—showedagood
fixationoftheimbeddedμGFH,asaftertheadditionoftheμGFHconditioningsuspension
accompaniedwithaninitialreleaseofparticles,onlylowμGFHconcentrationscouldbefoundinthe
columneffluent(FigureS3).Sand,however,showedthebuildupofafiltercakeandnopenetration
oftheμGFHparticlesintolowerfilterlayers(FigureS4).Theintergranularporesizesofthesand
werethusnotsufficientforasuccessfuldistributionindifferentfilterstrata.
TheuseofcokeresultedinariseinpHofthefiltratetoabove12evenaftermultiplewashing
cycles.Thiscausesanegativeeffectonadsorptionasmosttargetpollutantslikephosphateand
arsenateshowhigheradsorptionatpHvaluesbelow7[2,23].
AnthraciteshowedaslightlylowerfixationofμGFHthanpumice(95%comparedto97%)but
wouldbeequallysuitableasacarriermaterial.
Pumicewaschosenasmostpromisingcarriermaterialfortheembedmentofthefinefraction
ironhydroxideasitsusedidnotresultintheformationofafiltercakeorachangeinpH.
Furthermore,itslightcolormakesiteasilydistinguishablefromtheironhydroxideparticles.
ConditioningintheupstreammodeledtoahomogenousdistributionofμGFHinallcarrier
materialsinsmalltestcolumns(bedheight10cm).Thiscouldnotbereproducedinlargercolumns
(bedheight40cm),asagglomerationofμGFHparticlesoccurredinpartsofthecolumnswhich
resultedinpreferentialflowpathsoftheadsorbatesolution.Thus,subsequentconditioningwasdone
indownstreammode.
3.3.OptimizationofPumiceLoading
Toestimatetheefficiencyofembedment,conditioningsuspensionswithrisingμGFH
concentrations(3–50g/L)wereaddedtopumice‐filledcolumns.Almostcompleteretention(98%)
andloadingsofapproximately8and20mg/gwerereachedwithsuspensions(50mL)of3and6g/L
μGFH.Theretentionwasreducedto67%and56%whenhighersuspensionconcentrationsof24and
50g/L,repectively,werefiltered.However,theresultingfilterloadingsweremuchhigherat48and
83mg/g.
HeatmapsweredrawntoshowthefractionofembeddedμGFHindifferentlyconditioned
columns(Figure4).DarkcolorsindicateahighmassofembeddedμGFHwhereaslightcolors
indicatealmostnoembedment.Differentinformationcanbetakenfromthemaps:howthe
embeddedμGFHisdistributedoverthecore,whereitislocatedandanapproximationofhowmuch
μGFH(in%volume)canbefixedonthecolumn.ThefractionofareafilledwithμGFHvariesbetween
0%and45%forμGFHparticlesizes0–125μm(Figure4).Higherloadingsarefoundmainlyinthe
middleareaoftheprofile,whileloadingsarelowerneartheouterareasofthecore.Thismayindicate
walleffectsordifferentflowconditionsduringcolumnconditioning,whichleadtoareducedfixation
oftheparticlesintheouterareas.Asaresult,aninhomogeneousdistributionofμGFHoverthe
Figure 3.
Scanning electron microscopic images of (
a
) unfractionated
µ
GFH bulk material, (
b
)
an individual particle with attached fines and (
c
) a small particle deposited on a cellulose
nitrate membrane.
3.2. Carrier Material Selection
All four tested granular carrier materials—pumice, coke, anthracite and sand—showed a
good fixation of the imbedded
µ
GFH, as after the addition of the
µ
GFH conditioning suspension
accompanied with an initial release of particles, only low
µ
GFH concentrations could be found in the
column effluent (Figure S3). Sand, however, showed the buildup of a filter cake and no penetration of
the
µ
GFH particles into lower filter layers (Figure S4). The intergranular pore sizes of the sand were
thus not sufficient for a successful distribution in different filter strata.
The use of coke resulted in a rise in pH of the filtrate to above 12 even after multiple washing
cycles. This causes a negative effect on adsorption as most target pollutants like phosphate and
arsenate show higher adsorption at pH values below 7 [2,23].
Anthracite showed a slightly lower fixation of
µ
GFH than pumice (95% compared to 97%) but
would be equally suitable as a carrier material.
Pumice was chosen as most promising carrier material for the embedment of the fine fraction iron
hydroxide as its use did not result in the formation of a filter cake or a change in pH. Furthermore, its
light color makes it easily distinguishable from the iron hydroxide particles.
Conditioning in the upstream mode led to a homogenous distribution of
µ
GFH in all carrier
materials in small test columns (bed height 10 cm). This could not be reproduced in larger columns
Water 2018,10, 1324 6 of 12
(bed height 40 cm), as agglomeration of
µ
GFH particles occurred in parts of the columns which
resulted in preferential flow paths of the adsorbate solution. Thus, subsequent conditioning was done
in downstream mode.
3.3. Optimization of Pumice Loading
To estimate the efficiency of embedment, conditioning suspensions with rising
µ
GFH
concentrations (3–50 g/L) were added to pumice-filled columns. Almost complete retention (98%)
and loadings of approximately 8 and 20 mg/g were reached with suspensions (50 mL) of 3 and 6 g/L
µGFH. The retention was reduced to 67% and 56% when higher suspension concentrations of 24 and
50 g/L, repectively, were filtered. However, the resulting filter loadings were much higher at 48 and 83
mg/g.
Heat maps were drawn to show the fraction of embedded
µ
GFH in differently conditioned
columns (Figure 4). Dark colors indicate a high mass of embedded
µ
GFH whereas light colors indicate
almost no embedment. Different information can be taken from the maps: how the embedded
µ
GFH is
distributed over the core, where it is located and an approximation of how much
µ
GFH (in % volume)
can be fixed on the column. The fraction of area filled with
µ
GFH varies between 0% and 45% for
µ
GFH particle sizes 0–125
µ
m (Figure 4). Higher loadings are found mainly in the middle area of the
profile, while loadings are lower near the outer areas of the core. This may indicate wall effects or
different flow conditions during column conditioning, which lead to a reduced fixation of the particles
in the outer areas. As a result, an inhomogeneous distribution of
µ
GFH over the transverse profile can
be seen. In the later adsorption phase, this could lead to preferential flow paths along the areas with
lower embedded masses and, therefore, an incomplete exploitation of the adsorbent. It can also be
seen that significant differences occur between H1 and H2, which are different layers of the same core.
Cross-sectional images and heat maps of all conditioned columns can be found in Figures S5–S13.
Water2018,10,xFORPEERREVIEW 7of13
transverseprofilecanbeseen.Inthelateradsorptionphase,thiscouldleadtopreferentialflowpaths
alongtheareaswithlowerembeddedmassesand,therefore,anincompleteexploitationofthe
adsorbent.ItcanalsobeseenthatsignificantdifferencesoccurbetweenH1andH2,whichare
differentlayersofthesamecore.Cross‐sectionalimagesandheatmapsofallconditionedcolumns
canbefoundinFiguresS5–S13.
Figure4.Verticalcrosssectionsoftheupperandlowerpartsofafrozencore(left)andresultingheat
mapswithregardtotheaccumulationofμGFHparticles(0–125μm)intheporesofpumice.
AcomparativeevaluationofallconditionedpumicecolumnsisgiveninFigure5.Threedifferent
particlesizefractionsandtwoadditionalpreparatorystepsweretestedregardingtheireffectson
μGFHdistributioninthefixedbed.Thesamecolorsareusedforduplicatecolumns.
ImpactofpHonμGFHdistribution:
EquallyhomogenousdistributionsarereachedwithandwithoutsettingthepHofthe
conditioningsuspensionto8.5(Figure5).Thus,aneutralpHdoesnotleadtoagglomerationof
particleswhichaffectstheembedmentoftheparticles.AsmallincreaseinembeddedμGFHof
approximately5%isreachedwiththeelevatedpH.Whenwashingthecarriermaterialwith
deionizedwaterwithoutsettingthepHbeforehand(pHapproximately7),equallyhighloadingsare
reachedbuttheduplicatecolumnsdiffersignificantly.Also,theareacoveredbyμGFHvarieswidely
withloadingsofapproximately40%intheupperpartsofonecolumnandonly10%inthesamepart
oftheduplicatecolumn(seeFiguresS10andS11).WithoutsettinganelevatedpH,pumiceparticles
areunequallycharged,whichresultsinnonreproducibleloadingswiththeadsorbent.Thus,the
settingofanelevatedpHbeforehandisessentialforhomogenousandreproducibleloadings.
Figure 4.
Vertical cross sections of the upper and lower parts of a frozen core (left) and resulting heat
maps with regard to the accumulation of µGFH particles (0–125 µm) in the pores of pumice.
Water 2018,10, 1324 7 of 12
A comparative evaluation of all conditioned pumice columns is given in Figure 5. Three different
particle size fractions and two additional preparatory steps were tested regarding their effects on
µGFH distribution in the fixed bed. The same colors are used for duplicate columns.
•Impact of pH on µGFH distribution:
Equally homogenous distributions are reached with and without setting the pH of the
conditioning suspension to 8.5 (Figure 5). Thus, a neutral pH does not lead to agglomeration of
particles which affects the embedment of the particles. A small increase in embedded
µ
GFH of
approximately 5% is reached with the elevated pH. When washing the carrier material with deionized
water without setting the pH beforehand (pH approximately 7), equally high loadings are reached
but the duplicate columns differ significantly. Also, the area covered by
µ
GFH varies widely with
loadings of approximately 40% in the upper parts of one column and only 10% in the same part of
the duplicate column (see Figures S10 and S11). Without setting an elevated pH, pumice particles are
unequally charged, which results in nonreproducible loadings with the adsorbent. Thus, the setting of
an elevated pH beforehand is essential for homogenous and reproducible loadings.
Water2018,10,xFORPEERREVIEW 8of13
Figure5.InfluenceofpreparatorystepsonareacoveredbyμGFH;samecolorsindicateduplicate
columns.
Impactofparticlesizes:
Avariationinparticlesizeleadstobigdifferencesintheamountofembeddedadsorbent.While
thefraction0–125μmcoversapproximately25%ofthearea,thefraction0–63μmcoverslessthan
10%(Figure6).Duetotheverysmallparticlesizes,interceptionandsedimentationislimitedandthe
transportefficiencyisreduced.Thisleadstoanincreaseddischargeoftheparticlessmallerthan63
μm.Eventhoughthetotalembeddedmassoftheseparticlesissmaller,amorehomogenous
distributionoverthefilterprofile(xcoordinateinFigure6)isreached.Thebiggestfraction0–300μm
showsslightlylesscross‐sectionalareacoveredbyμGFHcomparedwiththefraction0–125μm(18%
incomparisonto25%)andamoreinhomogeneousdistribution(Figure7).Thiscorrespondswellto
experimentswithlow‐concentrationconditioningsuspensions(1g/LμGEH),inwhich50timesmore
particlesintherange0–63μmand5timesmoreparticlesintherange125–300μmincomparisonto
particlesintherange63–125μmwerefoundincolumneffluent.Thehigherconcentrationofbigger
particlesincomparisontomedium‐sizedparticlesintheeffluentcanbeexplainedbythetarget
surfacetheyoffertoshearforces.Therefore,theyaremoreeasilydetachedfromthecarriermaterial
surface.
Verticaldistribution:
TheupperpartofthecoreshowsmoredivergentcoveragewithμGFH(Figure7).Thisleadsto
theconclusionthatthefirstfewcentimetersofthecolumnsdifferfromthedeeperlayersbecause
eithertheapproachingflowisnotlaminarorthealreadyembeddedparticlesinfluencetheadhesion
offurtherparticlesgreatly.Forparticlessmallerthan63μm,adecreaseinembedmentcanbeseen
overfilterdepth(Figure6).Fortheothersizefractions,nocleartrendintheamountofembedded
μGFHcanbeobserved.
Figure 5.
Influence of preparatory steps on area covered by
µ
GFH; same colors indicate
duplicate columns.
•Impact of particle sizes:
A variation in particle size leads to big differences in the amount of embedded adsorbent. While
the fraction 0–125
µ
m covers approximately 25% of the area, the fraction 0–63
µ
m covers less than
10% (Figure 6). Due to the very small particle sizes, interception and sedimentation is limited and the
transport efficiency is reduced. This leads to an increased discharge of the particles smaller than 63
µ
m.
Even though the total embedded mass of these particles is smaller, a more homogenous distribution
over the filter profile (xcoordinate in Figure 6) is reached. The biggest fraction 0–300
µ
m shows slightly
Water 2018,10, 1324 8 of 12
less cross-sectional area covered by
µ
GFH compared with the fraction 0–125
µ
m (18% in comparison
to 25%) and a more inhomogeneous distribution (Figure 7). This corresponds well to experiments
with low-concentration conditioning suspensions (1 g/L
µ
GEH), in which 50 times more particles in
the range 0–63
µ
m and 5 times more particles in the range 125–300
µ
m in comparison to particles in
the range 63–125
µ
m were found in column effluent. The higher concentration of bigger particles in
comparison to medium-sized particles in the effluent can be explained by the target surface they offer
to shear forces. Therefore, they are more easily detached from the carrier material surface.
•Vertical distribution:
The upper part of the core shows more divergent coverage with
µ
GFH (Figure 7). This leads
to the conclusion that the first few centimeters of the columns differ from the deeper layers because
either the approaching flow is not laminar or the already embedded particles influence the adhesion of
further particles greatly. For particles smaller than 63
µ
m, a decrease in embedment can be seen over
filter depth (Figure 6). For the other size fractions, no clear trend in the amount of embedded
µ
GFH
can be observed.
Water2018,10,xFORPEERREVIEW 9of13
Figure6.VerticalandhorizontaldistributionsofμGFHincrosssectionsdependingonparticlesize.
Figure7.Cross‐sectionalareafractionscoveredbyμGFHinverticaldistribution;samecolorsindicate
duplicatecolumns.
Imageanalysisisausefultooltoevaluatetheembedmentoffineparticulateadsorbentsina
carriermaterialinwater‐filledcolumns.InthecaseofμGFHembedmentinpumice,greatdifferences
werefoundfordifferentpreparatorystepsandparticlesizes.Also,dataanalysisrevealedabroad
distributionofGFHembedmentperpumiceareaindifferentcrosssectionswhichcomplicates
interpretation.Asaconsequence,abigdatapoolisnecessarytoobtainstatisticallyreliableresults.
However,thedataobtainedinthisstudysuggesttheuseofastocksuspensioncontainingthe
particlesizefraction0–125(pH8.5)withpumice,pre‐washedat8.5,asafavoredconditioningmethod
Figure 6. Vertical and horizontal distributions of µGFH in cross sections depending on particle size.
Water 2018,10, 1324 9 of 12
Water2018,10,xFORPEERREVIEW 9of13
Figure6.VerticalandhorizontaldistributionsofμGFHincrosssectionsdependingonparticlesize.
Figure7.Cross‐sectionalareafractionscoveredbyμGFHinverticaldistribution;samecolorsindicate
duplicatecolumns.
Imageanalysisisausefultooltoevaluatetheembedmentoffineparticulateadsorbentsina
carriermaterialinwater‐filledcolumns.InthecaseofμGFHembedmentinpumice,greatdifferences
werefoundfordifferentpreparatorystepsandparticlesizes.Also,dataanalysisrevealedabroad
distributionofGFHembedmentperpumiceareaindifferentcrosssectionswhichcomplicates
interpretation.Asaconsequence,abigdatapoolisnecessarytoobtainstatisticallyreliableresults.
However,thedataobtainedinthisstudysuggesttheuseofastocksuspensioncontainingthe
particlesizefraction0–125(pH8.5)withpumice,pre‐washedat8.5,asafavoredconditioningmethod
Figure 7.
Cross-sectional area fractions covered by
µ
GFH in vertical distribution; same colors indicate
duplicate columns.
Image analysis is a useful tool to evaluate the embedment of fine particulate adsorbents in a
carrier material in water-filled columns. In the case of
µ
GFH embedment in pumice, great differences
were found for different preparatory steps and particle sizes. Also, data analysis revealed a broad
distribution of GFH embedment per pumice area in different cross sections which complicates
interpretation. As a consequence, a big data pool is necessary to obtain statistically reliable results.
However, the data obtained in this study suggest the use of a stock suspension containing the
particle size fraction 0–125 (pH 8.5) with pumice, pre-washed at 8.5, as a favored conditioning method
for
µ
GFH on pumice, as it led to the most homogeneous distribution with high embedded
µ
GFH
masses (approximately 25%).
3.4. Fixed Bed Adsorption
A column conditioned with
µ
GFH according to the method described above leads to comparable
adsorption breakthrough curves for phosphate compared with conventional GFH under similar
adsorption parameters (pH, flow rate) (Figure 8). Conventional GFH is used in packed fixed-bed
filters while in this study
µ
GFH was used as a fine fraction adsorbent embedded in a carrier material,
which results in a need for a substitute for bed volume as the reference value. For better comparability,
the increase of phosphate in column effluent is plotted against the specific throughput (L/g) with
regard to the applied adsorbent. As no data with identical flow rates was available for conventional
GFH, a modelled curve was added for comparability. Probably due to retarded phosphate diffusion
to particles not exposed to the flow, all displayed breakthrough curves vary significantly from the
ideal s-shaped breakthrough curve. This known effect of conventional GFH could not be improved
by the use of fine particulate adsorbent and is explained by a limitation of adsorption through slow
surface diffusion.
By using embedded
µ
GFH, no complete holdback of phosphate could be achieved since from
the start of the experiment small concentrations of approximately 0.1 mg/L were measured in the
filter effluent. The slope of the breakthrough curve shows a slower increase in concentration in
comparison to conventional GFH. As shown above, a homogeneous distribution of
µ
GFH on the
carrier material is hard to achieve. As a consequence, preferential flow paths are established and the
adsorbent is unequally loaded with the target compounds. In areas with high amounts of embedded
µ
GFH, the outer areas are likely to adsorb while the inner grains do not come into contact with the
target pollutants. At 80% breakthrough, loadings of 24 mg/g P are reached for the fine fraction, while
conventional GFH reaches 24 mg/g P at 100% breakthrough [
21
]. The embedment of the adsorbent
did not lead to an increase of head loss in the filter.
Water 2018,10, 1324 10 of 12
Water2018,10,xFORPEERREVIEW 10of13
forμGFHonpumice,asitledtothemosthomogeneousdistributionwithhighembeddedμGFH
masses(approximately25%).
3.4.FixedBedAdsorption
AcolumnconditionedwithμGFHaccordingtothemethoddescribedaboveleadstocomparable
adsorptionbreakthroughcurvesforphosphatecomparedwithconventionalGFHundersimilar
adsorptionparameters(pH,flowrate)(Figure8).ConventionalGFHisusedinpackedfixed‐bed
filterswhileinthisstudyμGFHwasusedasafinefractionadsorbentembeddedinacarriermaterial,
whichresultsinaneedforasubstituteforbedvolumeasthereferencevalue.Forbetter
comparability,theincreaseofphosphateincolumneffluentisplottedagainstthespecificthroughput
(L/g)withregardtotheappliedadsorbent.Asnodatawithidenticalflowrateswasavailablefor
conventionalGFH,amodelledcurvewasaddedforcomparability.Probablyduetoretarded
phosphatediffusiontoparticlesnotexposedtotheflow,alldisplayedbreakthroughcurvesvary
significantlyfromtheideals‐shapedbreakthroughcurve.ThisknowneffectofconventionalGFH
couldnotbeimprovedbytheuseoffineparticulateadsorbentandisexplainedbyalimitationof
adsorptionthroughslowsurfacediffusion.
ByusingembeddedμGFH,nocompleteholdbackofphosphatecouldbeachievedsincefrom
thestartoftheexperimentsmallconcentrationsofapproximately0.1mg/Lweremeasuredinthe
filtereffluent.Theslopeofthebreakthroughcurveshowsaslowerincreaseinconcentrationin
comparisontoconventionalGFH.Asshownabove,ahomogeneousdistributionofμGFHonthe
carriermaterialishardtoachieve.Asaconsequence,preferentialflowpathsareestablishedandthe
adsorbentisunequallyloadedwiththetargetcompounds.Inareaswithhighamountsofembedded
μGFH,theouterareasarelikelytoadsorbwhiletheinnergrainsdonotcomeintocontactwiththe
targetpollutants.At80%breakthrough,loadingsof24mg/gParereachedforthefinefraction,while
conventionalGFHreaches24mg/gPat100%breakthrough[21].Theembedmentoftheadsorbent
didnotleadtoanincreaseofheadlossinthefilter.
Figure8.PhosphatebreakthroughcurveinembeddedμGFH(flow2.2m/h)comparedto
conventionalGFH(3m/h),modelleddata(2.2m/h)andtheidealbreakthroughcurvewithoutsurface
diffusionlimitation(pH7,buffered,c0(P)=2mg/L).
RemovalofexploitedμGFHbybackwashandsuccessfulreplacementwithfreshμGFHisa
requirementforsuccessfulutilizationofμGFHinfixed‐bedfilters.Furthermore,itispreferableto
Figure 8.
Phosphate breakthrough curve in embedded
µ
GFH (flow 2.2 m/h) compared to conventional
GFH (3 m/h), modelled data (2.2 m/h) and the ideal breakthrough curve without surface diffusion
limitation (pH 7, buffered, c0(P) = 2 mg/L).
Removal of exploited
µ
GFH by backwash and successful replacement with fresh
µ
GFH is a
requirement for successful utilization of
µ
GFH in fixed-bed filters. Furthermore, it is preferable to
remove the loaded adsorbent as completely as possible to raise the embedding capacity for fresh
adsorbent. Tests revealed that multiple conditioning of pumice with
µ
GFH leads to comparable
loadings of the carrier material (60 ±4 mg µGFH/g pumice) and similar phosphate adsorption.
Compared with the use of conventional GFH, the described process is space and cost efficient, as
existing rapid filters can be upgraded for heavy metal or phosphate removal. As the adsorbent is a
by-product in the production process of conventional GFH and is currently disposed, its use reduces
waste, is resource-efficient and is sustainable. Possible applications of this process are highly dependent
on the target pollutant but include water restoration and drinking and waste water treatment.
4. Conclusions
This study characterized an iron hydroxide fine fraction adsorbent, named
µ
GFH and introduced
a method using image analyses for the evaluation of embedment of fine fraction particles on carrier
materials in water-saturated filter columns. The main findings are as follows:
•
BET analyses quantified surface areas of 300 m
2
/g for the fine fraction, equivalent to those of
conventional GFH. Therefore, similar equilibrium adsorption capacities can be expected. The
material consists of 30% (by mass) particles below 63 µm and very few particles above 120 µm.
•
For the utilization of
µ
GFH in fixed-bed filtration, pumice proved to be a good carrier material.
High embedment of the adsorbent (approximately 60 mg/g) and a homogeneous distribution
could be observed when the pumice was pre-washed with deionized water (pH set to 8.5). The
use of a concentrated stock suspension containing particle sizes 0–125
µ
m with pH set to 8.5 led
to a homogeneous distribution on the carrier material.
•
A method of image analysis of cross sections of frozen filter bed cores proved applicable
for distribution analysis of fine fraction particle embedment in filter columns filled with a
carrier material.
Water 2018,10, 1324 11 of 12
•
A laboratory-scale fixed-bed filter with embedded
µ
GFH showed breakthrough curves for
phosphate similar to those of conventional GFH fixed-bed adsorbers. The use of
µ
GFH did
not lead to accelerated intra-particle diffusion. Loadings of 24 mg/g P were reached.
Supplementary Materials:
The following are available online at http://www.mdpi.com/2073-4441/10/10/1324/
s1, Figure S1: XRD diffractograms of (a) 63–300
µ
m and (b) 0–63
µ
m
µ
GFH particle sizes with vertical lines
indicating reflex positions, Figure S2: Energy dispersive X-ray spectrum of
µ
GFH with major peaks for iron (Fe),
oxygen (O), phosphor (P), sulfur (S), silicon (Si) and carbon (C, used for coating to enhance electric conductivity),
Figure S3: Cumulative masses of
µ
GFH in column effluent over treated volume for different carrier materials with
downstream conditioning, Figure S4: Conditioned columns with (a) anthrazite, (b) pumice, (c) filter coke and (d)
sand, Figure S5: Photographs of vertical cross-sections of frozen filter beds (pH 8.5 of the conditioning suspension
and during pre-wash) loaded with
µ
GFH particles in the size range 0–63
µ
m (left) and resulting/corresponding
distribution patterns obtained by image analyses as described in section S1, Core A, Figure S6: Photographs of
vertical cross-sections of frozen filter beds (pH 8.5 of the conditioning suspension and during pre-wash) loaded
with
µ
GFH particles in the size range 0–63
µ
m (left) and resulting/corresponding distribution patterns obtained
by image analyses as described in section S1, Core B, Figure S7: Photographs of vertical cross-sections of frozen
filter beds (pH 8.5 of the conditioning suspension and during pre-wash) loaded with
µ
GFH particles in the
size range 0–125
µ
m (left) and resulting/corresponding distribution patterns obtained by image analyses as
described in section S1, Core A, Figure S8: Photographs of vertical cross-sections of frozen filter beds (pH 8.5 of
the conditioning suspension and during pre-wash) loaded with
µ
GFH particles in the size range 0–300
µ
m (left)
and resulting/corresponding distribution patterns obtained by image analyses as described in section S1, Core
A, Figure S9: Photographs of vertical cross-sections of frozen filter beds (pH 8.5 of the conditioning suspension
and during pre-wash) loaded with
µ
GFH particles in the size range 0–300
µ
m (left) and resulting/corresponding
distribution patterns obtained by image analyses as described in section S1, Core B, Figure S10: Photographs of
vertical cross-sections of frozen filter beds (pH 6 of the conditioning suspension and pH 8.5 during pre-wash)
loaded with
µ
GFH particles in the size range 0–125
µ
m (left) and resulting/corresponding distribution patterns
obtained by image analyses as described in section S1, Core A, Figure S11: Photographs of vertical cross-sections
of frozen filter beds (pH 6 of the conditioning suspension and pH 8.5 during pre-wash) loaded with
µ
GFH
particles in the size range 0–125
µ
m (left) and resulting/corresponding distribution patterns obtained by image
analyses as described in section S1, Core B, Figure S12: Photographs of vertical cross-sections of frozen filter
beds (pH 8.5 of the conditioning suspension and pH 7 during pre-wash) loaded with
µ
GFH particles in the
size range 0–125
µ
m (left) and resulting/corresponding distribution patterns obtained by image analyses as
described in section S1, Core A, Figure S13: Photographs of vertical cross-sections of frozen filter beds (pH 8.5 of
the conditioning suspension and pH 7 during pre-wash) loaded with
µ
GFH particles in the size range 0–125
µ
m
(left) and resulting/corresponding distribution patterns obtained by image analyses as described in section S1,
Core B, Table S1: Input parameters for Fixed bed adsorption simulation tool.
Author Contributions:
The article was written by I.H. within her Ph.D. project, with supervision by A.S.R.
and M.J.
Funding:
The current research as part of the German–Israeli project AdsFilt (02WIL1389) is funded by the
German Ministry of Education and Research (BMBF) within the framework of the German–Israeli water
technology cooperation.
Acknowledgments:
We acknowledge support by the German Research Foundation and the Open Access
Publication Funds of TU Berlin.
Conflicts of Interest: The authors declare no conflict of interest.
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2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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