Chemical Engineering Journal 260 (2015) 616–622

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Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /ce j

Enhanced separation of nanoscale zero-valent iron (nZVI) usingpolyacrylamide: Performance, characterization and implication

http://dx.doi.org/10.1016/j.cej.2014.09.0421385-8947/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel./fax: +86 21 65985885.E-mail address: lishaolin@tongji.edu.cn (S. Li).

Wei Wang a, Shaolin Li a,⇑, Hong Lei a, Bingcai Pan b, Wei-xian Zhang a

a State Key Laboratory of Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Tongji University, Shanghai 200092, Chinab School of the Environment, Nanjing University, Nanjing 210023, China

h i g h l i g h t s

� nZVI settling was studied underdynamic and quiescent conditions.� Polyacrylamide (PAM) enhanced nZVI

settling and improved effluentquality.� PAM increased hydrodynamic size of

nZVI by 60 times and reduced itssurface charge.� The inhibition effect of PAM on nZVI

reactivity was negligible.� PAM makes nZVI more applicable to

the existing wastewater treatmentfacilities.

g r a p h i c a l a b s t r a c t

Polyacrylamide increases the separation efficiency of iron nanoparticles and improves the effluentquality, providing a facile way to adapt iron nanoparticles in the existing wastewater treatment facilities.

a r t i c l e i n f o

Article history:Received 5 August 2014Received in revised form 9 September 2014Accepted 12 September 2014Available online 19 September 2014

Keywords:Nanoscale zero-valent ironPolyacrylamideSeparationSettlingWastewater treatment

a b s t r a c t

Applications of nanoscale zero-valent iron (nZVI) in wastewater treatment demand fast and completeseparation of nZVI for its reuse and to ensure high-quality effluent. Gravity settling is a promising candi-date for nZVI separation due to its low capital and operational cost. Herein, we report polyacrylamide-enhanced separation of nZVI under quiescent and dynamic conditions. A considerable fraction(20–60 mg/L) of bare nZVI remained in supernatant as stable colloids, even with extended settling timeof 3 h and very low overflow rate (0.1 m3/m2 h). The addition of low dose PAM (610 mg/g-nZVI)increased the settling efficiency of nZVI by nearly 5 times and improved the effluent quality as well.The colloidal iron in the supernatant was reduced to less than 5 mg/L; the mean hydrodynamic size ofnZVI increased by nearly 60 times, from 7.6 lm to 474 lm, as measured by optical microscope and par-ticle size analyzer. Low dosage (610 mg-PAM/g-nZVI) of PAM reduced the surface charge (zeta potential)of nZVI while overdosing PAM (P50 mg-PAM/g-nZVI) may recharge and re-stabilize the iron nanoparti-cles. No inhibition effect of PAM on nZVI reactivity was observed, likely due to its low surface coverage.The study demonstrates a reliable and cost-effective solution for nZVI separation and also proves thatnZVI is fully compatible with conventional wastewater treatment processes.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Zero-valent iron is a mild reductant with a standard electrodepotential of �0.44 V [1]. Compared to larger iron particles such

as micro- or millimeter-sized iron, nanoscale zero-valent iron(nZVI) exhibits much higher reactivity toward a wide array oftoxic/recalcitrant compounds that are commonly found in waste-water, making it a promising material for wastewater treatment[2,3]. Owing to its diminished size, nZVI can be conveniently mixedwith wastewater in stirred tank reactors, offering a convenient wayfor its application in wastewater treatment [4,5]. However, this

W. Wang et al. / Chemical Engineering Journal 260 (2015) 616–622 617

new application demands fast and complete separation of nZVIfrom treated wastewater, in order to ensure effluent quality andincrease the material efficiency. This paper presents research onenhanced separation of nZVI via gravitational settling for wastewa-ter treatment.

In the past decade, studies on nZVI hydrodynamics have beenprimarily focusing on stabilizing iron nanoparticles via surfacecoating/modifications, for the purpose of better mobility in subsur-face transport [6–13]. The polymers, such as polyacrylic acids (PAA)and carboxymethyl-cellulose (CMC), are often used to promoteelectrostatic repulsion or steric effects among iron nanoparticlesand are designated to disperse nZVI in water [8–10]. Those poly-mers were often dosed in large amount (up to 8:1, polymer/nZVI)[8–13], raising concerns of their side effects such as nZVI reactivityloss and secondary pollution [7,14,15]. Meanwhile, reports on thenZVI separation via gravity settling are limited [4,5], and no poly-mers have been reported so far in terms of nZVI separation.

Gravity sedimentation for particle separation is an easy andinexpensive process in wastewater treatment [16]. The large den-sity of iron makes possible the nZVI separation via gravity settling.However, the uneven size distribution of iron nanoparticles bringscomplexity and uncertainty to its settling [17]. Although largeaggregates could be easily settled out of water, small-sized onesmay remain stable in water for the extended period of time. Fur-thermore, nZVI aging produces low density (hydro)oxides [18], ofwhich the settling properties are rather poor. While such informa-tion is crucial in determining the key design parameters of nZVI set-tling tank [16] (e.g., overflow rate and hydraulic retention time), noprevious study has focused a solution to enhance nZVI sedimenta-tion. Polyacrylamide (PAM, (C3H5NO)n) is often used in traditionalcoagulation process as coagulant-aid. Via adsorption, bridgingand/or charge neutralization, the high molecule weight polymerflocculates the hydrolysis products of coagulant, producing largeflocs and facilitating their gravitational separation [19]. However,the enhanced settling of nZVI using PAM has never been reported.

Objectives of this paper include: (1) to investigate the gravitysettling performance of nZVI before and after the addition ofPAM, under both dynamic and quiescent conditions; (2) to evalu-ate the corresponding changes in particle morphology, (hydrody-namic) size and zeta potential, which will help to understand thesettling performance of nZVI; and (3) to determine the possibleinhibition effects of PAM on nZVI reactivity. Results of this workcan enrich our knowledge on nZVI hydrodynamics, provide essen-tial information for process design and operation in wastewatertreatment and make nZVI more adaptable to the existing wastewa-ter treatment facilities.

2. Materials and methods

2.1. Chemicals

Zero-valent iron nanoparticles were synthesized using sodiumborohydride and ferric chloride following the procedure describedpreviously [20]. Nano Fe2O3 (30 nm) and nano Fe3O4 (20 nm) par-ticles were used as received (Aladdin, Shanghai, China). Anionicpolyacrylamide (PAM, (C3H5NO)n) was purchased from SinopharmChemical Reagent Co., China. The average molecule weight of PAMwas 10 million. The PAM-flocculated nZVI (PAM-nZVI) was pre-pared by mixing PAM solution with nZVI at the ratios between1:1000 and 1:10 (PAM:nZVI, w/w) as indicated in each test.

2.2. Settling test

Dynamic settling tests were performed to study the effects ofPAM on nZVI settling in continuous-flow clarifier. The tests were

conducted in nZVI process which consisted of an nZVI reactor, agravity settling clarifier and a recycling pump (Fig. 1A and B).The reactor and the clarifier were made of acrylic glass and wereboth 2.6 L in volume. The clarifier was a miniature vertical-flowsedimentation tank with area of 130 square centimeters (0.14square feet). Prior to the dynamic settling test, a number of poly-mers, such as anionic/cationic/nonionic PAM and polyethyleneoxide, were tested in 50 ml vials with nZVI to find the best polymerfor enhanced settling. The anionic PAM performed the best andwas selected in this study.

The influent contained 20 mg/L nickel ions and was prepared byNiCl2�6H2O (Sinopharm) and deionized water. Influent pH wasadjusted to 5.0. The influent was pre-dosed with 1 mg/L anionicPAM before being pumped into the reactor. A control test withoutPAM was conducted as comparison. The effluent was sampled atpredetermined intervals and analyzed for total and dissolvedmetal concentrations using ICP-OES (Inductively Coupled PlasmaSpectrometer, Aglient 720ES).

The quiescent settling column, 150 mm in diameter and3000 mm in height, was made of acrylic glass (Fig. 2A). The nZVIslurry (500 mg/L) was pre-mixed using a dispersion disk at600 rpm for 30 min prior to settling. For PAM-nZVI, 5 mg/L PAMwas pre-mixed with 500 mg/L iron nanoparticles. Five samplingports located evenly along the column. Five milliliter samples werewithdrawn at each sampling port each time. The samples wereacid-digested before ICP analyses for total iron concentration([Fe]T, mg/L). The measured [Fe]T was used to calculate the settlingefficiency (Eij) following the equation [21] below:

Eij ¼ 1� Cij

C0

� �� 100 ð1Þ

where Cij is the [Fe]T at depth i and time j, and C0 is the initial totaliron.

The removal iso-percent settling lines were then plotted andthe detail procedures can be found elsewhere [16,21]. The turbid-ities of supernatant were measured with a portable turbidimeter(2100P, Hach). The settling tests were duplicated three times atroom temperature (22 �C ± 1 �C).

2.3. Particle characterization

The particle size distribution (PSD) was acquired using a laserdiffraction particle size analyzer (MS3000, Malvern) equipped witha wet dispersion accessory (Hydro LV). An optical microscope(BA310, Motic) in conjunction with a digital camera was used tocharacterize the nanoparticles in aqueous condition. The nanopar-ticles were also characterized using a scanning electron micro-scope (XL30, Philips). Viscosity of nZVI suspension was measuredby a viscometer (NDJ-8S, Shanghai Sunny Hengping ScientificInstrument Co., Ltd).

Zeta (f) potentials of the nanoparticles were measured by laserscattering analyzer (Zeta-sizer Nano ZS, Malvern) and the testswere duplicated three times. The suspension prepared for the testscontained 1 g/L nanoparticles (i.e., nZVI, nFe2O3, nFe3O4) with var-ied PAM concentrations (0–100 mg/L) and 1 mM NaCl.

2.4. nZVI reactivity test

Effects of PAM on nZVI reactivity were probed by studying thesolution pH and oxidation reduction potential (ORP) of nZVI sus-pension and the reaction kinetics of nickel ions. Tests were per-formed at room temperature in a 1000-mL 7-neck flaskequipped with a mechanical rotating paddle. The solution waspurged with high-purity nitrogen gas throughout the test. pHand ORP electrodes were fitted into the flask to monitor solution

Fig. 1. Dynamic settling tests using nZVI and PAM-nZVI for Ni(II) removal: (A) the experimental setup of continuous-flow nZVI process, (B) photos of the reactor and clarifier;(C) total and dissolved Ni in the effluents; (D) suspended Fe in the effluents.

Fig. 2. Quiescent settling tests: (A) photos of the settling column; (B and C) iso-percent settling curves of nZVI and PAM-nZVI. Insert: nZVI settling in glass vials at differenttime intervals, with and w/o PAM.

618 W. Wang et al. / Chemical Engineering Journal 260 (2015) 616–622

pH and ORP. Initial pH was adjusted to 5.0. Nickel solution con-tained 100 mg/L Ni(II) and was pre-dosed with 20 mg/L PAMbefore reactions with nZVI (2 g/L). Control tests without PAM ornZVI were also conducted. Samples were taken at preset timeintervals and were filtrated with 0.45 lm PTFE syringe filtersprior to ICP analyses.

3. Results and discussion

3.1. nZVI settling under flow conditions

nZVI settling was investigated under dynamic (continuous flow)and quiescent (batch) conditions. Dynamic settling experiments

W. Wang et al. / Chemical Engineering Journal 260 (2015) 616–622 619

were conducted in a process showed in Fig. 1A, which consisted ofan nZVI reactor, a gravity-settling clarifier and a recycling pump.nZVI was mixed and reacted in the reactor, settled and separatedin the clarifier and returned to the reactor by the recycling pump.More details of the process can be found in our previous publica-tions [4,5].

Experiments were conducted separately using two influents;specifically, synthetic nickel solutions (Ni2+, 20 mg/L) were usedas influent but one of them was pre-dosed with 1 mg/L PAM. Efflu-ents under the two conditions, denoted as effluentB (for pure nickelsolution) and effluentP (for PAM-fed solution) were sampled at pre-set time intervals for nickel and iron analyses. Operational param-eters were kept the same, i.e., 30 g nZVI spiked into the nZVIreactors at the beginning of experiments, flow rate (Q) set at1.3 L/h corresponding to a hydraulic retention time of 2 h for thereactor. The overflow rate of the clarifier (0.1 m3/m2 h) was muchlower than those of conventional primary settling tanks (typicallyin the range of 1.2–2 m3/m2 h) in wastewater treatment facilities[16].

Photos in Fig. 1B provide a visual comparison of the settling per-formance under the two conditions. For the PAM-free influent, theeffluent carried large flocs that were clearly visible with naked eyesand the whole clarifier quickly turned black after just a few hoursof operation, due to the high level of suspended flocs and turbidityin the supernatant (Fig. 1B, left). For the PAM-fed influent, nZVIsettled completely in the clarifier and the supernatant showedlow level of turbidity and suspended solids (Fig. 1B, right).

The settling efficiency was quantified by analyses of the nickeland iron concentrations in the effluent, as shown in Fig. 1C andD. Higher concentration of nickel, 3 mg/L on average, was left inthe effluentB (Fig. 1C). Effluent quality rapidly deteriorated after50 h of operation, which might be caused by the corrosion prod-ucts (e.g., green rust) of iron with poor settling characteristics.Filtration of effluentB produced lower level of nickel (0.9 mg/L onaverage), suggesting that large fraction of the nickel content ineffluentB was carried by suspend solids and the unsettled sus-pended solids deteriorated the effluent quality. For the PAM-fedinfluent, the effluentP contained much lower concentration ofnickel (<1.5 mg/L), and the filtration did not bring much differenceto the nickel concentration (Fig. 1C). The removal capacities ofNi(II) were 77 and 87 mg-Ni/g-nZVI for bare-nZVI and PAM-nZVI,respectively. The removal capacity of PAM-nZVI appears to be a lit-tle higher, likely due to the enhanced separation efficiency as aresult of PAM flocculation. On the other hand, the removal capacityof PAM-nZVI is closed to those obtained from batch reactors usingbare nZVI and filtrated effluents (80–130 mg-Ni/g-nZVI) [22,23],indicating that the adverse effect of PAM on nZVI reactivity wasminor at experimental condition (4.3 mg-PAM/g-nZVI).

The iron content in suspended solid (Fig. 1D) of effluents pro-vided additional evidence on the separation efficiency of nZVIunder these two conditions: the effluentB contained much higherlevel of iron in suspended form, 37 mg/L on average, in comparisonwith that of 9.3 mg/L for effluentP. Based on the results, without theaids of PAM, 4.8 g nZVI (1/6 of the total dosage) flew out after 100-hoperation and the loss was reduced to 1.2 g using PAM-fed influent.The results demonstrated the poor settling ability of bare nZVI andthe enhanced gravity separation with the addition of PAM.

3.2. Quiescent settling

The column used for the quiescent settling was presented inFig. 2A and experimental results were shown in Fig. 2B and C.The height of the column was 2.5 m, close to the heights of settlingtanks in wastewater treatment. The initial concentration of nZVIwas 500 mg/L. The tests provide quantitative information that isapplicable to the design of nZVI settling tank.

Fig. 2B presents the iso-percent settling curves of bare nZVI. Thecurvilinear iso-percent lines suggested the coalescence and floccu-lation settling of iron nanoparticles. The total settling efficiency (R)was calculated using the following equation [16]:

R ¼Xn

h¼1

Dhn

H

� �En þ Enþ1

2

� �ð2Þ

where n is the number of equal percent settling curve, Dhn is thedistance between curves of equal percent settling (m), and H isthe height of total settling distance (m), 2.5 m in this case, and Eis the equal percent settling efficiency.

In the experiment with bare nZVI, 77% of the iron particles weresettled in the first 22 min, but settling of the remaining nZVI wasmuch slower. It took 32 min for the total settling efficiency to reach87%, but further increasing to 93% required nearly 2.5 h. Approxi-mately 30 mg/L iron (6% of the initial concentration) remained sus-pended after 3 h. Both quiescent and dynamic settling resultssuggested a significant fraction of nZVI remained considerably sta-ble in solution, even with extended detention time of 3 h or at lowoverflow rate of 0.1 m3/m2 h.

The above results suggested that bare nZVI could not be sepa-rated completely via gravity settling alone, which was in accor-dance with field data [4,5]. A clarifier equipped with inclinedtubes was used to separate nZVI from treated wastewater butunfortunately was unable to remove nZVI completely, at the over-flow rate of 0.67 m3/m2 h (the flow rate: 400 L/h, clarifier area:0.6 m2) [5]. An extra coagulation/sedimentation process was thusemployed to eliminate the suspended solids in effluent of nZVItreatment. This polishing process produced additional sludge andgenerated extra operational and capital costs. Since the configura-tion of nZVI process is in fact similar to those of polishing processand the nZVI corrosion is producing coagulant-like substances[24,25], the combination of nZVI process and flocculation was pro-posed to reduce the treatment cost.

Fig. 2C presents the iso-percent settling curves of PAM-floccu-lated nZVI (PAM-nZVI). 90% of iron was settled in 15 min and ittook just 30 min to achieve a total settling efficiency of 96%, com-pared to that of 150 min for the bare nZVI. It took much less set-tling time, approximately 20%, for PAM-nZVI to reach the highsettling efficiency. Less than 5 mg/L of iron was left suspendedafter 3 h settling and the supernatant turbidity was as low as 8.6NTU. The embedded photos in Fig. 2C provide a more direct visualcomparison of the enhanced settling with PAM. The settling exper-iments were conducted in 50-ml glass vials and photos were takenat the intervals denoted at the bottom of the vials. The two vialscontained the same level of nZVI, 500 mg/L. For PAM-nZVI, largerflocculates appeared in a few seconds immediately after the addi-tion of PAM (5 mg/L) and all the flocculates settled to the bottom injust 5 min. The supernatant turbidity was less than 3.0 NTU. Thebare nZVI settled much slowly and no clear supernatant or settlinginterface was observed. The vials remained dark after 5 min.56 mg/L of iron remained suspended in solution after 30 min ofsettling and the turbidity was as high as 150 NTU.

3.3. Characterization of iron nanoparticles

3.3.1. Optical micrographs and SEM imagesThe optical micrographs and SEM images of bare nZVI (A,B,C)

and PAM-nZVI (D,E,F) are presented in Fig. 3. While the SEMimages illustrate the morphology of a single iron nanoparticle,the optical microscope allows direct observation on nZVI aggre-gates under aqueous conditions, which are crucial to understandthe hydrodynamic properties of nZVI in wastewater reactors.

Fig. 3A and B show the optical micrographs of nZVI particles attwo different magnifications. In Fig. 3A, the black dot scattered

A

500 µµm

500 µm

D

50 µm5 µm

E

1 µm

C

1 µm

F

50 µm

BnZVI nZVI nZVI

PAM-nZVI PAM-nZVI PAM-nZVI

Fig. 3. Images of nZVI and PAM-nZVI obtained with optical microscope (A, B, D, E) and SEM (C, F).

Fig. 4. Particle size distributions of nZVI and PAM-nZVI.

Table 1Summary of hydrodynamic properties of bare nZVI and PAM-nZVI.

Particles Particle size distributions (lm) Viscosityc (mPa s)

d1a d10

a d50a d90

a D[4,3]b

nZVI 0.77 2.9 6.9 13 7.6 1.0 ± 0.1PAM-nZVI 27 103 355 984 474 1.1 ± 0.1

Notes:a d1, d10, d50, and d90 are the cumulative probability sizes at 1%, 10%, 50% and 90%

of volume fraction, respectively.b D[4,3] is the DeBroukere mean size denoted as the volume-weighted mean

diameter of all the particles measured.c Viscosity was determined in 1 g/L nZVI slurry with or without 10 mg/L PAM at

room temperature (22 ± 1 �C).

620 W. Wang et al. / Chemical Engineering Journal 260 (2015) 616–622

loosely in the scope view area. A closer look showed that the sizesof the black dots were mostly in the range of 1–15 microns(Fig. 3B). While single iron nanoparticles are in the range of afew tens to hundreds of nanometers (Fig. 3C), these black dotswere nZVI aggregates due to the self-agglomeration of ironnanoparticles.

The addition of PAM apparently produced larger flocculates ofapproximately 50–900 micrometers (Fig. 3D). A closer examinationindicated these giant flocculates were comprised of clusters of afew microns (nZVI aggregates) (Fig. 3E & insert). These images pro-vided direct evidences of PAM flocculation. In terms of the floccu-lation mechanisms, it has been reported that the anionic PAM mayinteract with nZVI surface via bidentate chelation and hydrogenbonding among its amine groups and iron oxide surface of nZVI[11,26].

3.3.2. Particle size distributionParticle size distributions (PSD) in Fig. 4 and Table 1 provided

quantitative analyses of the aggregates in Fig. 3. The median sizeof nZVI aggregates, denoted as d50 (cumulative probability sizesat 50%), was 6.9 lm and mean size (volume-weighted mean diam-eter) was 7.6 lm, indicating more than 50% of the nZVI aggregateswere below 8 lm. 90% of the bare nZVI aggregates were less than13 lm and 10% were less than 2.9 lm. Traditional settling tanks(or grit chambers) in wastewater treatment are mostly designed

to remove particles of greater than 200 lm [16], it was thusexpected that bare nZVI would not be able to settle completelyin such settling tanks. The PAM flocculation produced giant nZVIparticles of much greater size (Fig. 4 and Table 1). The median(d50) and mean size of PAM-nZVI, 355 lm and 474 lm, respec-tively, were nearly 60 times larger than those of bare nZVI, and99% of the flocs were larger than 27 lm. These results providequantitative information of the nZVI particle size increasing afterPAM addition.

Assuming these aggregates settled discretely in column/tanks,the settling velocities (vs) of these aggregates could be estimatedusing the Stokes equation [16]. For bare nZVI, 90% of the aggregateshad discrete settling velocities of less than 2.1 m/h. Nearly 10% ofthem, approximately 50 mg/L iron nanoparticles, had settlingvelocity of less than 0.1 m/h, which were sufficient to cause thedeterioration of effluent quality. The PAM flocculation producedgiant nZVI flocs of much greater discrete settling velocities: 99%of the flocs had discrete settling velocity of 8.8 m/h.

3.3.3. Zeta potentialFig. 5 presents the f potential of nZVI as a function of PAM con-

centration. nFe2O3 and nFe3O4 were also studied as comparison.Each of the suspension contained 1 g/L nanoparticles and 1 mMNaCl. pH values of nZVI, nFe2O3 and nFe3O4 suspension were 8.4,5.4 and 6.6, respectively. The addition of PAM didn’t alter the solu-tion pH significantly.

The bare iron nanoparticles were positively charged (+4.1 mV)under experimental condition, consistent with its IEP (iso-electricpoint) at around 8.3 in previous report [20]. f Potentials of nZVIdidn’t alter significantly at low PAM concentrations (610 mg/L)but turned more negative as PAM concentration increased. For

Fig. 5. Zeta potential changes of nZVI, nFe2O3, and nFe3O4 particles as a function ofPAM concentration.

Fig. 6. Effects of PAM on nZVI reactivity: (A) Ni(II) removal; (B) changes of solutionpH using nZVI and PAM-nZVI; (C) changes of solution ORP using nZVI and PAM-nZVI.

W. Wang et al. / Chemical Engineering Journal 260 (2015) 616–622 621

example, its f potential turned to �23 mV and �29 mV for 50 and100 mg/L PAM, respectively. As the magnitude of surface potentialdetermines the electrostatic repulsion between particles, theincrease of f potential implicated the possible restabilization ofnZVI at high PAM dosage. The zeta potential of nFe2O3 and nFe3O4

nanoparticles also showed the similar trend, �35 mV and �44 mVat 100 mg/L of PAM, respectively. As a conclusion, PAM overdose(P50 mg-PAM/g-nZVI) will result in restabilization of nZVI flocs,and a maximum dosage of 10 mg-PAM/g-nZVI was thus recom-mended and adequate for enhanced settling of nZVI.

3.4. Effects of PAM on nZVI reactivity

Reactivity of nZVI and PAM-nZVI was probed by studying thereactions with nickel ions and pH-ORP of nZVI slurry, as shownin Fig. 6. The initial concentration of nickel ions (C0) was 100 mg/L. 20 mg/L PAM and 2 g/L nZVI were used, corresponding to aPAM/nZVI ratio of 1:100. As shown in Fig. 6A, both nZVI andPAM-nZVI exhibited similar reactivity and quick sequestration ofnickel ions, with over 92% removal efficiency within 2 h. The testusing PAM showed no sign of Ni(II) removal. The pH-ORP measure-ments also showed similar changes: the pH values of slurries allincreased to 8.5 (Fig. 6B) and ORP values quickly decreased to�0.78 V (Fig. 6C) for both nZVI and PAM-nZVI.

The negligible effect of PAM on nZVI reactivity could beexplained by the structure of PAM-nZVI flocs in aqueous condition.As shown in Fig. 3, both nZVI and PAM-nZVI occurred primarily asaggregates of iron nanoparticles in aqueous solution. The SEMimages in Fig. 3C and F suggested no distinctive change in the mor-phology, size and shape of single iron nanoparticle before and afterPAM flocculation. A porous structure of nZVI aggregates was thussuggested under both conditions. Furthermore, the dosage (1%, or10 mg-PAM/g-nZVI) of PAM for enhancing nZVI settling was muchless than those (50–800%, polymer/nZVI) for particles stabilization[10–13]. The effect of PAM on nZVI reactivity was thus expected tobe negligible.

3.5. Implication for wastewater treatment

The nZVI process used in this research is similar to the tradi-tional returned activated sludge system, with the reactive nZVIanalogous to microbes in the biological process. While the extra-cellular polymeric substances (EPS) play an essential role in theseparation of microbes via aggregation and sedimentation

[27,28], their counterparts in the nZVI process are absent if onlybare nZVI is used. The addition of polymers is thus necessary toenhance its sedimentation. With negligible inhibition effect onthe nZVI reactivity and the convenience in dosing, PAM can func-tion as a counterpart for EPS.

The method described in this work also makes the iron nano-particles more adaptable to virtually all wastewater treatmentplants. The standard coagulation process, a mixing reactor in con-junction with a settling tank, is ubiquitously employed in primarytreatment prior to biological process in most wastewater plants[16]. The configuration of the coagulation-sedimentation processis similar to the nZVI process. The iron nanoparticles can be dosedand mixed in the same way as traditional coagulants. The corrosionof metallic iron produces charged poly-nuclear iron complexes(e.g., green rust) with enhanced adsorption properties which couldfunction as coagulants. The use of PAM renders nZVI fully compat-ible with conventional coagulation facilities, with the addition of apump for recirculation. No extra polishing process is required afterthe nZVI-enhanced coagulation, owing to the enhanced settlingand separation using PAM. As nZVI is a highly versatile agent fora wide range of recalcitrant compounds [2,3], this study providesan alternative approach to improve the effluent quality of manyindustrial wastewater treatments, with minimal increase in capitalcosts.

4. Conclusion

PAM-enhanced settling of zero-valent iron nanoparticles wasinvestigated under both quiescent and dynamic flow conditions.A considerable fraction of bare nZVI can form stable colloids, evenwith extended settling time. The residual flocs led to the deteriora-tion of effluent quality and may require extra polishing process,increasing both operational and capital costs. Addition of 1 mg/LPAM to the influent significantly enhanced the nZVI settling in clar-ifier and improved the effluent quality. PAM destabilized nZVI,enlarged the flocs, and increased nZVI settling velocity. The aver-

622 W. Wang et al. / Chemical Engineering Journal 260 (2015) 616–622

age diameter of nZVI aggregates increased from 7.6 lm to 474 lm.Low dose PAM also reduced the surface charge of iron particleswhile PAM overdose may recharge and re-stabilize the nanoparti-cles. PAM may interact with iron surface via hydrogen bonding byits amine groups and bidentate chelation. No inhibition effect onnZVI reactivity was observed, likely due to the low PAM coverageon the nZVI surface. Results suggested PAM can serve as an effec-tive nZVI-aid to improve its performance in wastewater treatment.

Acknowledgements

This work was financially supported by the National NaturalScience Foundation of China (Grants Nos. 21107082 & 21277102)and by the Fundamental Research Funds for the CentralUniversities.

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