School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China
a b s t r a c ta r t i c l e i n f o
Article history:
Received 12 May 2011
Received in revised form 27 June 2011
Accepted 13 July 2011
Available online 11 August 2011
Keywords:
Adsorption
Column systemFerric sludge
Phosphate
The binding ef ficiency of phosphate onto the ferric sludge was investigated by batch and column experimentsto control the deterioration of water quality caused by eutrophication. Several adsorption isotherm models
and kinetics models including pseudo-first order, pseudo-second order, Elovich mass transfer model and
intra-particle diffusion model for the batch system and Thomas equation for thefi
xed column bed systemwere used to evaluate the equilibrium and kinetics data, respectively. Results showed that M-Langmuir modeland pseudo-second order model were recommended to describe the adsorption equilibrium and kinetics
characteristics. Moreover, the column experimental data fitted well with Thomas model with obtaining themaximum adsorption capacity of about 30 mg/g at pH 5.5, which was in conformity with aresult presented in
batch experiment. The insignificant effect of SO42− and Cl− on phosphate adsorption indicated that binding
phosphate was through a kind of inner-sphere complexation. During the adsorption process, the pH changedsignificantly, indicating an anion/OH− exchange reaction. Further analysis of the adsorption kinetics indicated
that the intra-particle diffusion was the rate limiting process. The above study implies that the ferric sludgehas excellent potential in binding phosphate during wastewater treatment process.
Phosphorus is an indispensable element for the growth of animals
and plants. However, several million tons of phosphorus containingwastewater is straightly discharged into the watercourses with therapid developmentof industry andagriculture annually.It is wellknownthatthe increased phosphate concentrations andalterednutritionratios
will inevitably cause severe environment pollution, such as theeutrophication of the waterways which frequently cause fish kills,phytoplankton blooms, and the deterioration of water quality. The costof eutrophication to the UK water industry is estimated at N£15 M per
annum [1], while it is reported that about 30% of the Irish river channellength is polluted mainly from eutrophication [2]. Currently, an issueabout the increased trend of phosphate concentration in many coastalwaterways of China has been reported. The major source is industrial
waste ef fluence. In order to respond to the demand for controlling andremedying the eutrophication problems and improve the ecologicalenvironment, several methods have been applied for phosphateremoval, such as adsorption, chemical treatment, ion exchange,
membrane separation, bioreactor and biodegradation [3–6]. Recently,the development of adsorption research on removing phosphate fromaqueous solutions has attracted much attention. The key problem is to
seek out an extensively ef ficient adsorbent. Several low-cost or easilyavailable materials and by-products such as zeolite [7], fly ash [8], blast
furnace slag [9], steel furnace slag [10], Al–bentonite, Fe–bentonite and
Al–
Fe–
bentonite [11], aluminum and iron oxides [12], have all beenwidely and systematically investigated recently. Zhao group [13,14] and
Motula and Gagnon [15] presented phosphate removal from drinkingwater by alum residuals and the results indicated good effectiveness of alum sludge for phosphorus immobilization.
In contrast to the abundant research conducted to the ef ficiency of
alum sludge from drinking water treatment plants, there are fewstudies having been reported on the use of ferric sludge to adsorbphosphate, although some researchers have reported some wastesolids containing iron oxide such as tailing material, red mud and
blending adsorbents with ferrous and ferric material [16–20].Compared with the aluminum salt, the ferric salt has the same good
flocculation effect, more important, the residue concentration of ferricsalt in aqueous solution has no health threats.
Therefore, in this study, an attempt was made to explore the ef ficiencyof using ferric sludge from drinking waterworks for phosphate bindingfrom aqueous solutions. Farther, the characteristics of equilibrium andkinetics behavior in batch and fixed bed system were investigated.
2. Materials and methods
2.1. Ferric sludge and reagents
Dewatered ferric sludge cakes were collected from an industrial
filter press of the dewatering unit of the Shenyang Water Treatment
Plant, China. The chemical compositions of the dry sludge were Fe2O3:~60%, CaO: ~30%, SiO2: 5–7%, Al2O3: b2%. The sludge samples were
heated at 103 °C±2 and then cooled at room temperature in a sealedcontainerfor 3–4 h. The sludge was then ground andsievedto providethe adsorbent with diameterb0.098 mm. Then the sludge wascalcined at 723 K for 4 h.
Allreagents usedin experimentswere analyticalgradecommerciallyavailable, andan appropriatevolumeof 0.1 mol/L HClor NaOHsolutionswas used to adjust thepH of the solution. Thephosphate solutions were
prepared by diluting the stock solution of 100 mg/L (calculated as P,
prepared by dissolving KH2PO4 in deionized water) to desiredconcentrations.The X-ray diffraction (XRD) was measured using a Philips semi-
automatic X-ray diffractometer with Co K α radiation generated at30 kV, 15 mA. The microstructure and surface morphology of thesludge sample were observed by scanning electron microscopy (SEM,
JEOL JSM-5600LV).
2.2. Adsorption studies
2.2.1. Batch adsorption experiments
For the adsorption of phosphate, the batch adsorption experiments
were conducted with a given dose of sludge in 250 mL Erlenmeyer
flasks mixed with 50 mL phosphate solutions in a thermostat shakerat 120 rpm and 298 K for 24 h. Liquid samples were collected atvarious time intervals and separated by centrifugation (2500 rpm,10 min). Then the supernatant solutions were used to determine the
phosphate concentrations by the molybdate blue spectrophotometricmethod at λmax of 700 nm with the blank sample containing onlydeionized water and corresponding ferric sludge as a reference
solution.The effect of pH in the range of 3 to 9 adjusted by 0.1 mol/L HCl or
NaOH was investigated with a sludge dose of 0.05 g and the initialphosphate concentration of 27.43 mg/L. The initial phosphate concen-tration ranging from 5 to 50 mg/L was studied to evaluate the
adsorption capacity with a sludge dose of 0.05 g at pH 5.5 and 298 K.The sludge dose effect was examined with pH 5.5 and the initialphosphate concentration of 27.43 mg/L at 298 K. In order to investigatethe selective adsorption of Cl−, SO4
2− and PO43− by the ferric sludge,
binary-solute systems of P/Cl−, P/SO42− were investigated followed bythe introduction of Cl− and SO4
2− at concentrations ranging from 0 to200 mg/L with the initial phosphate concentration of 27.43 mg/L and
adsorbent dose 0.15 g. Based on the above experimental data,adsorption isotherms and kinetics were investigated. The adsorptionisotherms were carriedout with sludge dose 0.15 g and pH 5.5 at 298 K,and the contact time effect was conducted with the initial phosphate
concentration of 27.43 mg/L and sludge dose 0.15 g in the initial pH 5.5at 298 K. Each adsorption experiment was duplicated under identicalconditions to get the average value to evaluate the adsorptionperformance.
The following equation was used to compute the equilibriumuptake capacity of phosphate qe (mg/g):
qe = C i−
C eð Þ × V 1000w
ð1Þ
where V is the volume of phosphate solution in mL, w is the mass of
adsorbent in g, C i and C e are the initial and equilibrium phosphateconcentrations in mg/L, respectively.
2.2.2. Column adsorption experiments
The column experiments were carried out at room temperature incylindrical columns with 28 mm internal diameter, where mixedsludge and sand were packed with a weight ratio of 1:1 in order toimprove the penetrate performance. For the column experiments,
4.0 g, 6.0 g, 8.0 g sludge sample was packed in column witha height of 20 mm, 30 mm and 40 mm under the same packing density,
respectively. The medium in the fixed bed column was prewashed
by deionized water for hours, and the inlet flow rate was adjusted to
10 mL/min controlled by a flow controller. Synthetic sample solutionat a series of initial phosphate concentrations (10―30 mg/L) wascirculated with a pump and fedinto the fixed bed column. The columnwas operated using downward flow at 298 K in the air conditionedlaboratory.
The adsorbent packing heights of 20 mm, 30 mm and 40 mm wereused to study the effect of packing height on adsorption performancewith the phosphate concentration of 30 mg/L and pH 5.5. The ef fluentwas then collected physically at given time intervals and the
phosphate concentration was analyzed. Effects of initial phosphateconcentrations of 10 mg/L, 20 mg/L, 30 mg/L were evaluated byThomas model with a packing height of 40 mm. Furthermore, theintermittent ef fluent experiments were conducted at 13 h intermit-
tent operationfor deeply understanding the effect of particle diffusionwith the initial phosphate concentration of 30 mg/L packed heights of 20 mm, 40 mm, respectively.
3. Results and discussion
3.1. XRD and SEM analysis
Fig. 1 illustrates the X-ray diffraction patterns of the ferric sludgebefore and after adsorption. It can be seen clearly that the sludge
exhibits quite similar XRD characteristics before and after adsorption.The XRD output signals for the ferric sludge are strong with distinctpeaks, implying a higher crystallinity degree of ferric oxide.
Fig. 1. The XRD with Co K α radiations generated at 30 kV, 15 mA of ferric sludge from
drinking water works.
Fig. 2. The SEM of ferric sludge.
385 X. Song et al. / Desalination 280 (2011) 384– 390
The scanning electron microscope (SEM) observation of the sludgeadsorbent is shown in Fig. 2, which indicates that anomalistic
floccules exist in the porous surface.
3.2. Effect of initial pH on phosphate adsorption
Theamountof phosphateadsorbedonto ferric sludge at various pHvalues was illustrated in Fig. 3. As presented in Fig. 3, phosphate
adsorption by ferric sludge was profoundly affected by pH. Themaximum adsorption amount of phosphate appeared at pH 5.5 anddecreased with either decreasing or increasing pH. The material hasgood adsorption capacity (above 10 mg/g) at pH 4–6, at which point
the dominant phosphate form is the monovalent H2PO4−. The low pH
values areadvantageous to the phosphateadsorption because of anionadsorptioncombined withthe release of hydroxyl anions [21].Itiswell
known that certain anions (phosphate, nitrate, arsenate, etc.) areadsorbed onto adsorbent via electrostatic attraction and ligand
exchange [22–24]. The process of ligand exchange can change thepH value of the adsorption system [25]. In the study, the pH increased
during the adsorption process (Fig. 3), indicating the phosphateadsorption may be accompanied with a ligand exchange process.Moreover, the phosphate adsorption must occur at the active sites onthe surface of the ferric sludge. At low pH, the surface hydroxyl groups
are protonized and –OH2+ is easier to replace at the binding sites
compared with the hydroxyl groups [26]. As showed in Fig. 3, a biggerchange occurred betweenthe initial pH and equilibrium pH at low pH,
which can be explained that ligand exchange process is promoted tosome extent. The decrease of phosphate adsorption above pH 5.5 maybe due to a change in surface charge caused by the ferric sludge
becoming more negative at higherpH values [11]. Itis wellknown thatthe k2 of the phosphoric acid is equal to the concentration of HPO 4
2−
approximately. This process would then strengthen the electrostaticrepulsionbetween theexchange site andthe incoming phosphateions
(PO43−, HPO4
2−) with the increase of pH [22,27].
3.3. Effect of initial phosphate concentration
To study the effect of initial concentration on phosphateadsorption capacity by the ferric sludge, some experiments wereconducted accordingly and the results were shown in Fig. 4. As seenfrom Fig. 4, the equilibrium adsorption capacity increased with the
increase of the initial concentration, indicating that phosphate
adsorption onto the sludge has not reached a saturation due to thelow contact opportunity in low phosphate concentration.
3.4. Effect of sludge dose
It is well known that the amount of ferric sludge added into the
solution determines the number of binding sites available foradsorption. Results from this study indicate that the absorbent doseseems to have a great influence on phosphate adsorption. As showed
Fig. 3. Effectof initial pH onthe P (KH2PO4) adsorption capacity by ferric sludge and the
relationship between the initial pH and equilibrium pH.
Fig. 4. Effect of initial concentration on the P (KH2PO4) adsorption capacity.
Fig. 5. Effect of the adsorbent dose on the P (KH2PO4) adsorption capacity and removal
ef ficiency.
Fig. 6. Effect of the co-existing anions on the phosphate adsorption capacity.
386 X. Song et al. / Desalination 280 (2011) 384– 390
in Fig. 5, we can gain the information that the value of qe decreased,but the removal ef ficiency of phosphate increased from 68% to 99%with an increase in amount of adsorbent up to 3 g/L. It is explainedthat more active sites are available for the phosphate binding with the
addition of more adsorbent. However, the phosphate removalpercentage was fairly constant when the adsorbent dose was greaterthan 3 g/L with a continuous decrease in phosphate adsorptioncapacity. Therefore, it is suggested that the effectiveness decreasedwith an increase of adsorbent dose beyond 3 g/L. Thus the adsorbent
dose was limited to 3 g/L in subsequent experiments.
3.5. Effect of co-existing anions on phosphate adsorption
The wastewater contains many ions such as sulfate, chloride,which may compete for the availablesites. The effects of Cl− and SO4
2−
on the phosphate adsorption capacity of the sludge were shown inFig. 6. As presented in Fig. 6, it almost tended to show no evidentchange of the phosphate adsorption capacity despite the concentra-tions of Cl− and SO4
2− varying from 0 to 200 mg/L, indicating the
existence of co-anions nearly has no significant influence in thephosphate adsorption onto the sludge.
3.6. Adsorption isotherms study
The Langmuir, Freundlich equations and Multi-Langmuirexpressed in Eqs. (2), (3) and (4) were used for evaluating theseadsorption isotherm data. The adsorption isotherms of phosphate on
the ferric sludge were investigated with the optimized conditions:adsorbent dosage 3 g/L, initial pH 5.5, contact time 24 h. The plots
were presented in Fig. 7 and the relative parameters were listed in
Table 1.Langmuir isotherm has been widely used to quantify the perfor-
mance of different adsorbents by non-line expression as Eq. (2) [9]:
qe
¼qmbC e
1þbC eð2Þ
where qm is the maximum adsorption amount of sludge (mg/g), b isthesorption equilibrium constant (L/mg) and C e, qe are the equilibriumconcentration of phosphate in aqueous phase (mg/L) and solid phase
(mg/g), respectively.The Freundlich isotherm equation is also often used to describe
adsorption in the solution due to its assuming multilayer coverage of sorption in heterogeneous system and is given by [5]
qe = K cC 1 =ne ð3Þ
where K c is a constant for relative adsorptive capacity (qe= K c, when
C e=1 mg/g) and n indicates to some extent how dramatically the
binding strength changes as the adsorption density changes.Moreover, multi-Langmuir (M-Langmuir) model [28] which
considers the effect of pH, if pH changes a l ittle, the term
kh⋅10−ΔpH≈kh, can be described as Eq. (4). kh is constant with pHvalues.
qe¼qmbC 1
=ne
1þkhþbC 1=ne
ð4Þ
It can be seen that all the isotherms showed a similar shape and
were nonlinear over a wide range of aqueous equilibrium
Fig. 7. The adsorption isotherms of the P (KH2PO4) by ferric sludge.
Table 1The equilibrium constants and isotherm parameters obtained from experimental datanonlinear fitting.
Langmuir isotherm qm (mg/g) 26.801
b 2.790
R2 0.982
ε 2 0.001
Freundlich isotherm K c 18.032
n 2.955
R2 0.974
ε 2 0.002M-Langmuir qm (mg/g) 36.670
n 1.530
kh 2.303
b 0.884
R2 0.994
ε 2 0.001
Table 2
Comparison of phosphate adsorption maxima (qm) of tested materials with some
concentrations. The results showed that M-Langmuirfitting plots withdetermination coef ficient R2 0.994 and average square error ε 2 0.001better fitted the experimental data than the Langmuir and Freundlichisotherms. The Langmuir model and the Freundlich model are often
used to evaluate the adsorption process by monolayer and multilayeradsorbents regardless of the pH effect.
The qm values obtained in this study could be compared with thoseother adsorbents reported in the literature (Table 2). It can be seen
that the phosphate adsorption capacity of ferric sludge was very highamong the adsorbents. Therefore, ferric sludge has the potentialcapability for binding phosphate from aqueous solution.
3.7. Effect of sorption time on the adsorption and the kinetic performance
With regard to further adsorption characteristics, the effect of contact time at initial phosphate concentration of 27.43 mg/L waspresented in Fig. 8. It can be seen that the plot rapidly increased in the
beginning of the adsorption process. However, it almost tended toshow no further increase of adsorption capacity after 1000 min withthe maximum uptake amount of phosphate (25.5 mg/g) achieved. Itimplies that the phosphate adsorption on ferric sludge reaches
equilibrium in a short time.
3.8. Adsorption kinetics performance study
In order to deeply understand the kinetics characteristics, severalkineticsmodels, suchas pseudo-first order model, pseudo-second order
model, Elovich mass transfer model and intra-particle diffusion model
wereemployed to evaluate phosphate adsorptionkinetics performance.The following Eqs. (5) to (8) were applied for describing the fourkinetics models, respectively [29–36]:
1
qt
=k1
qm
×1
t +
1
qm
ð5Þ
Fig. 9. The sorption kinetics of phosphate.
Table 3
The parameters of kinetic model.
Pseudo-first-order Pseudo-second-order The Evolich model Intra-particle diffusion model
where qt (mg/g) is the experimental uptake capacity of phosphate attime t , qm (mg/g) is the maximum adsorption capacity, k1 (min−1) is
the rate constant of the pseudo-first order model, k2 (g/(mg·min)) isthe rate constant of the pseudo-second order model. The initialadsorption rate is k2qm
2 (mg/(g·min)). α is the initial adsorption rateconstant (mg/(g·min)) and the parameter β (g/mg) is related to the
extent of surface coverage and activation energy for chemi-sorption.kp (mg/(g·min0.5)) is the rate constant of the intra-particle diffusionmodel and C is obtained from the intercept.
The fitting of the experimental data to the linear forms of the four
adsorption kinetics models were shown in Fig. 9a–d, respectively.Additionally, the relative rate constants and parameters were shownin Table 3. It can be seen that the experimental data fit better with thepseudo-second order model with determination coef ficient (R2) of
0.997 compared with the first order model with R2 of 0.817. The highdetermination coef ficient implies the good applicability of the secondorder kinetics equation for phosphate adsorption using ferric sludgeunder the experimental condition. A similar result was reported by
Liangguo Yan et al. [11].On the other hand, the initial adsorption rate α and coef ficient β of
activated energy of the Elovich model were shown in Table 3, whichimplies that the Elovich model is not suitable to describe the
adsorption process compared with the other models mentionedabove.
In terms of the intra-particle diffusion model, it can be seen thatthe R2 was 0.775 for the whole adsorption process while 0.995 for the
initial time of the adsorption process, which indicates that the intra-particle diffusion is the limiting step at the initial time of theadsorption process [34–37]. A similar result was reported by Cheung
et al. [38].
3.9. Fixed column bed performance study
Thomas model is one of the extensively used models in fixed bedperformance theory which can predict the amount of adsorption byper mass adsorbent. The expression by linear Thomas equation for anadsorption column is given as follows [37–39]:
lnc 0c t−1
=
kThqmm
v−kThc 0t ð9Þ
where c t represents the ef fluent phosphate ion concentration (mg/L), c 0is the initial concentration (mg/L), kTh is the Thomas rate constant(L/(mg·min)) and qm is the maximum phosphate adsorption capacity
(mg/g), ν is thevolumetricflowrate (L/min) andt is theoperation time of ferric sludgefixed bed (min).The values of kThand qm canbe determinedfrom a plot of ln(c 0/c t−1) against t at a given flow rate using linearregression analysis when the values of c t/c 0 is within 0.05–0.95.
The breakthrough curves were presented in Fig. 10 under differentconditions which were presented in Table 4. It revealed that thecolumn service time decreased with the decrease of bed packing massand the increase of initial concentration.
The data obtained from the column study were used to calculateboth the maximum concentration of phosphate adsorbed onto ferricsludge and the adsorption rate constant in Thomas model. The modelplots were presented in Fig. 11 and the parameters of Thomas modelwere also summarized in Table 4. It can be seen that the phosphate
adsorption capacity strongly depends on the inlet phosphateconcentration and the maximum adsorption capacity of phosphateis about 30 mg/g. In addition, Thomas adsorption rate constant kThwas found to decrease from 0.25 to 0.11–0.13 with an increase of the
inlet phosphate concentration from 10 mg/L to 30 mg/L.Fig. 12 showed the results obtained from a 13 hour intermittent
fixed bed operation process, which give full proof of an internaldiffusion-controlled process. As shown in Fig. 12, th e ef fluent
concentration immediately increased before reaching the interrup-tion concentration, where the ef fluent phosphate concentrationdropped sharply. This phenomenon suggests the intra-particlediffusion process as the rate-controlling step. The same conclusionhas also been reported in the removal of arsenic using polymeric/
inorganic hybrid sorbent by DeMarco et al. [37].
4. Conclusions
The ferric sludge, a by-product of drinking water treatment facilities
is shown to have a significant phosphate sorption capacity. The
Table 4
The operation parameters of column and parameters of Thomas model.
phosphate removal by ferric sludge depended on pH, initial phosphateconcentration, adsorbent dose and time. The amount of phosphateadsorbed on ferric sludge wasthe greatest at pH 5.5with adsorbent dose
of 3 g/L. The adsorption equilibrium and kinetics characteristicsare welldescribed by the M-Langmuir isotherm and pseudo-second ordermodel, respectively. Moreover, significant phosphate removal of 30 mg/g computed by Tomas model was achieved in the long-termcolumn experiments at inlet phosphate concentration of 30 mg/L.
Additionally, the intra-particle diffusion processis considered as theratecontrolling stepat the initialtime of the adsorption process.The bindingphosphate capacity of fixed bed depends on the packing mass of ferricsludge and inlet phosphate concentration, which is similar to the
capacity in the batch system. This study also suggests that phosphatemay be initially adsorbed as an inner-sphere complex, which isattributed to the porous structure of dewater ferric sludge. Inconsideration of the excellent phosphate binding ef ficiency, the ferric
sludge as substrate in engineered wetlands can be applied for the
treatment of phosphorus containing wastewater. Such developmentcomes with a cost-effective concept of “waste” reuse and the feature of ferric sludge's large availability at a time landfills spaces decreasing and
alternatives to ferric sludge disposal being sought.
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C t / C 0
0
0.2
0.4
0.6
0.8
1
1.2
0 500 1000 1500 2000 2500
time/min
h40
h20
13 hrs interruption
Fig. 12. Breakthrough curve for phosphate sorption onto ferric sludge in the column at
intermittent operation.
390 X. Song et al. / Desalination 280 (2011) 384– 390
