Coagulation of Colloidal Particles with Ferrate(Vi)

Environmental Science Water Research & Technology

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Coagulation of colloidal particles with ferrateĲVI)†

Cite this: Environ. Sci.: Water Res. Dongyu Lv,ab Lei Zheng,b Huiqin Zhangbc and Yang Deng *b Technol.,2018,4,701

Coagulation and chemical oxidation have long been recognized as two major mechanisms of ferrateĲVI) 2− (i.e. FeO4 , an oxyanion containing FeĲVI)) in its environmental applications. Although ferrateĲVI) oxidation of various contaminants has been extensively studied, few efforts were made to appreciate the mechanisms and behaviors of ferrateĲVI)-driven coagulation in water. In this study, coagulation of colloidal kaolin particles with ferrateĲVI) in simulated natural water was investigated under the conditions related to drinking wa- −1 −1 ter treatment (initial turbidity 25.00 NTU, 0.0–9.0 mg L FeĲVI), pH 7.5, and 0.50–10.00 mg L DOC). FeĲIII) produced from FeĲVI) reduction initiated in situ coagulation. Lower minimum effective iron doses (MEIDs) were observed for FeĲVI) coagulation than for direct FeĲIII) coagulation at pH 6.5 and 7.5 (DOC = 2.00 mg L−1), at which the colloids were captured by iron precipitates principally via sweep coagulation. A citrate– ascorbate iron extraction method was used to reveal that FeĲVI) resultant flocs were composed of both amorphous and crystalline iron with an amorphous to crystalline Fe ratio of approximately 2.3 : 1.0 (pH 7.5, −1 −1 3.0 mg L FeĲVI), and 2.00 mg L DOC). FerrateĲVI) oxidation transformed natural organic matter (NOM) preferentially into more hydrophilic compounds, which have lower affinity with colloids and thus are less adsorbed on colloids to produce a less negatively charged surface. Therefore, ferrateĲVI) oxidation poten- tially promoted the aggregation of colloids through the alleviation of electrostatic repulsion. However, NOM at a high concentration (8.00–10.00 mg L−1 DOC in this study) could prevent the agglomeration of small iron oxide particles through the formation of a negatively charged NOM coating via adsorption, Received 28th January 2018, thereby preventing the growth of flocs. FerrateĲVI) coagulation, when combined with ferrateĲVI)oxidation, Accepted 9th March 2018 provides a more viable treatment option to address multiple water pollution in raw water, e.g. the presence of colloidal algal cells and dissolved algal toxins in water during a harmful algal bloom. The dual treatment DOI: 10.1039/c8ew00048d mechanisms enable a very effective treatment design with an economical physical footprint for supporting rsc.li/es-water a sustainable municipal drinking water supply.

Water impact

FerrateĲVI) coagulates colloids via sweep coagulation under drinking water treatment-related conditions. Coagulation with ferrateĲVI), distinct from traditional coagulation, gradually produces iron flocs containing amorphous and crystalline iron oxides in water. Meanwhile, ferrateĲVI) oxidation degrades NOM to favor particle aggregation. Effective ferrateĲVI) coagulation, combined with ferrateĲVI) oxidation, enables a dual-mechanism treatment to address multiple pollutants for supporting water supply sustainability.

Introduction FeĲVI) is reduced to more active intermediate high valence iron species (i.e. FeĲV) and FeĲIV)) and immediately to stable Ĳ 2− Ĳ Ferrate VI)(i.e. FeO4 ) is acknowledged as a promising treat- ferric ions (Fe III)) through electron transfer as a result of self- ment agent for drinking water treatment due to its ability to decay and/or reactions with reductants in water.2 Accompany- perform multiple functions with minimal production of dis- ing FeĲVI) reduction, FeĲIII)isin situ produced at an equimolar 1 infection by-products. After ferrateĲVI) is added to water, amount, serving as a potential coagulant for the removal of colloidal particles in water or as a precipitating agent for the a College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, elimination of undesirable soluble chemicals (e.g. phos- Gansu Province, 730000, China phate). At a typical water treatment pH condition (6.0–8.0), b Department of Earth and Environmental Studies, Montclair State University, ironĲIII) hydroxides are produced with a potential to further Montclair, New Jersey 07043, USA. E-mail: [email protected] adsorb certain water contaminants (e.g. heavy metal cations). c School of Civil Engineering, Architecture and Environment, Hubei University of Besides, these ferrateĲVI) resultant particles are able to cata- Technology, Wuhan, Hubei Province, 430068, China Ĳ 3–5 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ lyze ferrate VI) self-decomposition. Among multiple treat- c8ew00048d ment mechanisms (i.e. chemical oxidation/disinfection,

This journal is © The Royal Society of Chemistry 2018 Environ. Sci.: Water Res. Technol.,2018,4,701–710 | 701 Paper Environmental Science: Water Research & Technology coagulation, adsorption, and precipitation) sequentially oc- tificial raw water using a PDA. They measured the floccula- curring after ferrateĲVI) is dosed, chemical oxidation and coag- tion index (FI), which typically increases with the growth of ulation are recognized as the two principal ones for water flocs, and found that the FI of the FeĲIII) coagulation was treatment. Although many publications are available to report greater than that of the FeĲVI) at a specific iron dose of within the performance and mechanisms of ferrateĲVI) oxidation of 0.02–0.30 mM Fe, thus concluding that ferrateĲVI) underwent target contaminants in water, very few studies have been car- poorer coagulation. As a result, whether ferrateĲVI) may serve ried out to appreciate the coagulation behaviors of FeĲVI)in as a dual-function (i.e. chemical oxidation and coagulation) water,6 which is likely due to two major reasons. Firstly, agent in drinking water treatment is questioned.13 many previous studies were completed in a laboratory- Recently, research interests have been directed to the char- controlled phosphate or borate-buffered solution to facilitate acterization of ferrateĲVI)-induced particles in water. Goodwill 13 the investigation of FeĲVI) oxidation, because these buffer et al. compared FeĲVI) and FeĲIII) resultant particles pro- chemicals can both maintain pH and complex the produced duced in laboratory buffered water and reservoir water at pH FeĲIII) to prevent the formation of iron precipitates, thereby 6.2. The results showed that both particles had different size enabling an optical monitoring of FeĲVI) in water. However, distributions and morphologies. Moreover, the FeĲVI) resul- this experimental approach eliminates iron flocs and inhibits tant particles were composed of crystalline Fe2O3, which was the occurrence of coagulation and the ensuing flocculation. not detected in the FeĲIII) resultant particles, as well as more Secondly, it is technically difficult to separate the oxidation nanoparticles that generated a stable colloidal suspension. 14,15 and coagulation effects of ferrateĲVI) when ferrateĲVI)-induced Prucek et al. validated that the iron oxide particles from Ĳ γ – γ coagulation for the removal of dissolved natural organic mat- Fe VI) reduction exhibited a unique core ( -Fe2O3) shell ( - ter (NOM) is investigated.7 FeOOH) structure. Zheng and Deng16 also evaluated the Few efforts were made to investigate the mechanisms and settleability of FeĲVI)-induced particles in biologically treated performance of ferrateĲVI) coagulation in water. Conclusions municipal wastewater. They found that the majority of FeĲVI)- from the few available studies are inconsistent on what role induced particles was suspended in the secondary effluent, ferrateĲVI) plays in the removal of colloidal particles, particu- indicating the poor performance of coagulation and the ensu- 17 larly when compared with FeĲIII). Several groups reported that ing flocculation. Very recently, Cui et al. pointed out that ferrateĲVI) can behave better than FeĲIII) in terms of coagula- the ferrateĲVI) dose played a key role in the size distribution tion performance or can enhance particle removal in ensuing and coagulation of ferrate resultant particles. An increased 8 treatments. Jiang et al. reported that FeĲVI) was advantageous ferrateĲVI) dose augmented the fractions of soluble and colloi- − over ferric sulfate (2–12 mg L 1 as Fe) for the treatment of up- dal iron, while reducing the particulate portion, implying that land colored water, and could remove almost all turbidity at only a sufficiently high ferrateĲVI) dose could accelerate the 9,10 pH 7.5. Ma and Liu found that FeĲVI) pre-oxidation could aggregation of fine iron particles to facilitate downstream enhance a following alum coagulation for the removal of al- solid–liquid separation. Of note, the disparity among the gae and turbidity, but the coagulation driven by FeĲVI) alone aforementioned studies is likely due to different solution 11 was not investigated. Goodwill et al. reported that FeĲVI) chemistry conditions, operational conditions, and physical/ pre-oxidation could improve the effluent turbidity of a down- chemical characteristics of the target particulate matter. stream dual-media filter in continuous flow treatment experi- The objective of this study was to investigate the mecha- ments for the treatment of two U.S. Northeast surface waters. nisms and behaviors of ferrateĲVI)-driven coagulation for col- In contrast, other studies showed that FeĲVI) cannot pro- loidal particles in a typical drinking water treatment scenario. vide better coagulation than FeĲIII), a commonly used iron co- FeĲVI) coagulation and flocculation experiments were agulant. Tien and Graham12 applied a photometric disper- performed in simulated natural water with kaolin particles sion analyzer (PDA) instrument to compare the coagulation serving as model colloidal particles. Turbidity, zeta potentials behaviors of FeĲVI) and FeĲIII) in a kaolin suspension solution (ZPs), particle counts, and size distributions were measured without NOM. A PDA is an instrument for observing rapidly at varied pH (6.5 and 7.5) and different iron doses (0.0–9.0 −1 changing particle suspensions by an optical technique that mg L FeĲVI)). The results were compared with those with 7 analyses the light transmitted through a flowing suspension. direct FeĲIII) addition. The roles of NOM and FeĲVI) oxidation The results showed that sweep coagulation was observed for in particle stabilization were particularly investigated. − both iron coagulants at 2.8–11.2 mg L 1 Fe over pH 6–8, but the magnitude of floc formation with FeĲVI) was inferior com- Materials and methods pared to that with FeĲIII) and the floc growth rate with FeĲVI) Chemicals and reagents was slower due to the reduced FeĲVI) decomposition rate under a neutral or alkaline condition. Graham et al.7 also All the reagents used were at least analytical grade, except as Ĳ > ′ investigated Fe VI)-induced floc formation in humic acid solu- noted. Potassium ferrate (K2FeO4)(96%) and 2,2 -azino- tion in the absence of particulate matter. FerrateĲVI) achieved bisĲ3-ethylbenzothiazoline-6-sulfonate) (ABTS) were purchased comparable, or better, floc formation to ferric chloride, but from Sigma-Aldrich (St. Louis, MO, USA). All the other less dissolved organic carbon (DOC) removal. Yu et al.6 also chemicals were purchased from Fisher-Scientific (Fair Lawn, compared the coagulation behaviors of FeĲVI) with FeĲIII) in ar- NJ, USA). All the solutions were prepared using ultrapure

702 | Environ. Sci.: Water Res. Technol.,2018,4,701–710 This journal is © The Royal Society of Chemistry 2018 Environmental Science: Water Research & Technology Paper

2 water produced using a Milli-Q® Direct 8 water purification reaction kinetics pattern (R > 0.96) with respect to the FeĲVI) − − system. NOM-free simulated natural water was prepared with concentration with a rate constant of 0.138 L mg 1 min 1. 18 a chemical composition used in a previous study with mi- The rate of FeĲIII) production from FeĲVI) reduction relies upon Ĳ nor modifications: 1.5 mM NaHCO3, 1.0 mM CaCl2, 0.4 mM the Fe VI) degradation rate in water. The estimated times for Ĳ Na2SO4, 0.05 mM NaNO3, and 0.4 mM MgSO4. In order to 50% and 90% Fe VI) reduction were 2.4 and 65 min, synthesize NOM-containing simulated natural water, an ap- respectively. propriate aliquot of Suwannee River natural organic matter In the experiments to evaluate the effect of ferrateĲVI) pre- (SR-NOM) (IHSS, St. Paul, MN, USA) stock solution (100 mg oxidized NOM on the stability of kaolin particles, turbidity, −1 L DOC) was spiked. To prepare the simulated natural water DOC, UV254 absorbance, and ZPs of the kaolin particles (ini- with colloids, an appropriate weight of kaolin (The Clay Min- tial turbidity = 25.00 NTU) were measured in 5 different pre- erals Society, KGa-1b, Washington County, GA, USA), which treated water samples after 24 h of gravity sedimentation. served as the model colloids, was added to 2 L of the simu- The five water samples include: 1) NOM-free water without −1 lated natural water, followed by 1 min of rapid mixing (100 FeĲVI) pre-oxidation; 2) NOM-containing water (2.27 mg L rpm) on a magnetic stirrer. After one-night of sedimentation, DOC) without FeĲVI) pre-oxidation; and 3) three NOM − 1.5 L of the supernatant water layer containing the stable ka- containing water samples (2.27 mg L 1 DOC) that were pre- −1 olin suspensions was decanted as the colloid-containing sim- oxidized with 3.00, 6.00, and 9.00 mg L FeĲVI), respectively. ulated natural water. The simulated natural water was ad- In this study, the unit for the ferrateĲVI) dose is expressed as −1 justed to a desirable pH with 0.1 M NaOH or H2SO4 prior to mg L FeĲVI), which represents the mass concentration unit −1 use. Concentrated ferrateĲVI) (200 mg L as Fe) and ferric for elemental iron (Fe) in the ferrate anion. Iron particles in − (1000 mg L 1 as Fe) stock solutions were prepared by dissolv- the water samples were removed through 0.45 μm micro- ing certain amounts of K2FeO4 and ferric chloride in distilled filtration prior to use. water, respectively. These stock solutions were prepared immediately before use. The ferrateĲVI) concentration in the stock solution was confirmed by the ABTS method.19 Analyses FeĲVI) was spectrophotometrically measured by the ABTS method.19 The solution pH was measured using a pH meter Coagulation, flocculation, and sedimentation tests (Thermo Scientific Orion 5-Star Plus). The turbidity, particle Coagulation and flocculation tests were performed in 1 L bea- size, size distribution, and ZPs were measured using unfil- kers with 700 mL of simulated natural water on a four-paddle tered samples. The turbidity was quantified using a portable programmable jar tester (Phipps & Bird – 7790-950). The ex- turbidity meter (HACH, 2100Q). The ZPs were determined periments were initiated through the addition of an aliquot using a Nano Zetasizer (Malvern, ZEN 3690) without any sam- – of K2FeO4 from the stock solution. Within the first 30 s, the ple dilution. The size range for ZP measurement was 3.8 nm water was rapidly mixed (150 rpm) to completely disperse the 100 μm. The particle count and size distribution were mea- added ferrateĲVI). In the following 120 minutes, the water was sured using an automated dynamic imaging particle analyzer gently stirred (30 rpm) for the growth of flocs. The ferrateĲVI) (FlowCAM® VS-Series, Fluid Imaging Technologies, Inc.). The self-decomposition could release hydroxyl ions to cause a system was flushed with 2 mL deionized water at a flow rate − gradual pH increase. Therefore, if needed, pH was manually of 1 mL min 1, before the 1 mL sample was manually −1 maintained with a 0.1 or 0.01 M H2SO4 solution at a desirable injected at a flow rate of 0.5 mL min . When the particle- level during coagulation and flocculation. In order to deter- containing water passes through a flow cell, a camera in the mine the kinetics data of the FeĲVI) decomposition or particle FlowCAM® particle analyzer captures the particle images at count, 2 mL of the solution was withdrawn at designated 22 frames per second. These images were processed using sampling times. After the mixing was stopped, gravity-driven the VisualSpreadsheet® software (Fluid Imaging Technolo- sedimentation proceeded for 30 min. Thereafter, the superna- gies, Inc.). Dissolved organic matter was measured after the tant was collected for analysis. Control tests were performed samples were filtered through 0.45 μm syringe membrane fil- under identical conditions except that FeCl3, rather than ters (Thermo Scientific, cellulose acetate (CA), 30 mm diame-

K2FeO4, was added. Because ferric hydration led to the pro- ter). CA is a mechanically strong, hydrophilic, and low production of hydrogen ions in water, the solution pH was man- tein binding material for aqueous based samples. UV ually maintained at the designated level with 0.1 or 0.01 M absorbance and DOC were analyzed using a UV/vis spectro- NaOH solution. photometer (HACH DR 5000) and a total organic carbon ana- FeĲVI) decomposition is caused primarily by both self- lyzer (TOC-LCPH, Shimadzu Corp., Kyoto, Japan), respec- decomposition and reactions with reducing agents in water. tively. Particle morphology was determined with a Hitachi FeĲVI) reduction leads to the in situ production of FeĲIII) that H-7500 transmission electron microscope (TEM). To quantify may subsequently serve as a coagulant. Residual FeĲVI) and the amorphous iron in the produced iron oxides, a citrate– 20 accumulated FeĲIII) profiles during FeĲVI) decomposition (3.0 ascorbate iron extraction method was used with minor −1 mg L FeĲVI)) in simulated natural water are presented in modification. Briefly, 0.2 M Na-citrate and 0.05 M ascorbic Fig. SI-1.† FeĲVI) decomposition strictly followed a 2nd-order acid (pH 6.0) were added to the well-mixed samples

This journal is © The Royal Society of Chemistry 2018 Environ. Sci.: Water Res. Technol.,2018,4,701–710 | 703 Paper Environmental Science: Water Research & Technology

− immediately after the coagulation and flocculation tests (3.0 mg L 1). Generally, the ZPs dramatically increased with the −1 mg L FeĲVI)) with a reagent to solution weight ratio of 60 : 1. coagulant dose during FeĲVI)orFeĲIII) coagulation. At pH 7.5, − The suspensions were shaken on a horizontal shaker for 16 h as the iron dose increased from 0.0 to 9.0 mg L 1, the ZPs of and then centrifuged at 11 000 rcf for 30 min. The extractable the flocs in FeĲVI) and FeĲIII)-treated water samples increased dissolved iron in the suspensions was collected for analysis from −26.50 mV to −8.68 and −13.00 mV, respectively. At any of amorphous iron by inductively coupled plasma-mass specific iron dose, the FeĲVI) resultant flocs exhibited a spectroscopy (ICP-MS Thermo X-Series II, XO 472). The data slightly less negative charge compared to the flocs from allowed for the determination of the mass fraction of amor- direct FeĲIII) addition. −1 phous Fe in the total Fe (i.e. 3.0 mg L Fe) in the ferrateĲVI) At a low coagulant dose, iron coagulation was plausibly as- resultant oxide particles. All the experiments were run at least sociated with surface charge neutralization, which was as- in triplicates. The analytical results reported represent the cribed to two processes: 1) direct cationic FeĲIII) adsorption to mean of three replicate samples. Error bars in the figures the negatively charged kaolin surface; and 2) adsorption of represent one standard deviation. negative kaolin particles on iron hydrolysis products that are mononuclear and/or polynuclear species.21 As evidence, an −1 Results and discussion FeĲVI) dose as low as 1.5 mg L reduced the zeta potentials of the particles by 30% at pH 7.5 (Fig. 1). As the net surface Typical patterns of residual turbidity as a function of iron charge was decreasingly negative, the thickness of the diffuse − dose at pH 7.5 and 6.5 (2.00 mg L 1 DOC) are shown in Fig. layer surrounding the particles was accordingly reduced to SI-2(a) and (b),† respectively. The two pH levels fall within the minimize the energy required to move the particles into con- pH range used in typical coagulation practices.21 At either tact. However, as the coagulant dose increased, the role of pH, the residual settled turbidity of FeĲVI)orFeĲIII) coagula- sweep coagulation became increasingly dominant for the tion slightly decreased with the coagulant dose within a low FeĲVI)-driven coagulation of the colloidal kaolin suspension. dose range, but sharply dropped to <1.00 NTU as the iron Sweep coagulation is the entrapment or capture of particles dose reached a critical level, beyond which the settled turbid- by metal hydroxide precipitates that form when a coagulant 21 ity did not significantly increase or decrease. Here, the is added to water. It should be noted that effective FeĲVI) co- − threshold coagulant dose is defined as the minimum effec- agulation only occurred at an iron dose of ≥3.0 mg L 1 (i.e. tive iron dose (MEID). This finding is very similar to the ob- MEID), at which coagulation with FeĲIII) at either pH falls servation of Shin et al.22 who reported a minimum effective within the “sweep coagulation” region, as discussed else- alum dose in determining the alum dose for evaluating the where.21 Therefore, sweep coagulation appeared to play a roles of particles and NOM on coagulation. As seen in Fig. SI- more significant role in the effective FeĲVI)orFeĲIII) coagula- 2(a) and (b),† FeĲVI)-driven coagulation exhibited a very similar tion in this study. pattern of settled turbidity against the iron dose of the con- TEM analysis provided direct evidence to support the hy- trol group with direct FeĲIII) addition, except that lower pothesis. As shown in Fig. 2(a) and (b), kaolin particles −1 MEIDs were observed in the FeĲVI) coagulation (3.0 mg L Fe) remained suspended in simulated natural water due to −1 than in the FeĲIII) addition (4.0 mg L Fe). electrostatic repulsion (ZP = −26.40 mV at pH 7.5). Sizes of The initial ZP of kaolin particles in untreated simulated these stable particles ranged from several hundreds of nano- natural water (initial turbidity = 25.00 NTU) was −26.40 mV at meters to one or two micrometers. During coagulation with −1 pH 7.5, ensuring a stable colloidal suspension due to electro- 3.0 mg L FeĲVI), the kaolin particles were obviously static repulsion. Following the FeĲVI)orFeĲIII) coagulation, the entrapped by freshly produced iron precipitates as shown in ZPs of the flocs produced are shown in Fig. 1 (DOC = 2.00 Fig. 2(c). The sweep coagulation began with the nucleation of iron precipitates that likely occurred on the surface of kaolin and initiated a growth of iron precipitates entrapping the suspended kaolin particles in water. MEIDs for the FeĲVI) and FeĲIII) coagulation of the kaolin suspension with different initial turbidities at pH 7.5 were also measured (data not shown here). MEIDs for the FeĲVI) and FeĲIII) coagulation only slightly − increased from 3.00 to 4.00 mg L 1 and from 4.00 to 6.00 mg − L 1, respectively, when the initial turbidity was considerably increased from 2.5 to 100 NTU. A clear stoichiometry between the coagulant dose and particle concentration was not observed here (R2 < 0.75), again validating the prevailing role of sweep coagulation in this study. Of note, at any specific condition, a lower MEID was observed for the coagulation with FeĲVI) than with FeĲIII), though the two coagulations Fig. 1 Zeta potentials of particles produced from FeĲVI)orFeĲIII) coagulation (pH 7.5; DOC = 2.00 mg L−1; and initial turbidity = 25.00 exhibited similar patterns with coagulant dose (Fig. SI-1†) NTU). and shared the same principal coagulation mechanism in

704 | Environ. Sci.: Water Res. Technol.,2018,4,701–710 This journal is © The Royal Society of Chemistry 2018 Environmental Science: Water Research & Technology Paper

Fig. 2 TEM images of particles in simulated natural water: (a) and (b) kaolin particles suspended in water before FeĲVI) coagulation; and (c) kaolin −1 −1 particles incorporated into ferrateĲVI)-induced iron precipitates (3.0 mg L FeĲVI)) (pH 7.5; DOC = 2.00 mg L ; and initial turbidity = 25.00 NTU).

this study. It is of interest to explore the reasons resulting in with an amorphous iron to crystalline iron ratio of approxi- the disparity between the two coagulation behaviors. The cru- mately 2.3 : 1.0. The crystallization mechanisms in the FeĲVI) 13 cial difference between them was the different FeĲIII) sources, decomposition remain unclear. Goodwill et al. proposed − which could influence the coagulation, at least, in two as- that OH produced from the FeĲVI) oxidation of water and the pects, i.e. the formation of different iron precipitates and the protonation of ferrateĲVI) might accelerate the crystallization modification of the NOM impact on coagulation. of amorphous FeĲOH)3 through an aging process, resulting in Iron flocs produced from direct FeĲIII) coagulation are direct precipitation of crystalline iron oxides in water. Ĳ Ĳ 23,24 Ĳ amorphous ferric hydroxide (am-Fe OH)3 s)). In contrast, The Fe VI) induced flocs composed of both amorphous the chemical composition of the FeĲVI) resultant flocs is less and crystalline iron oxides exhibited distinct characteristics 13 understood. Goodwill et al. used X-ray photoelectron compared to the flocs from the FeĲIII) coagulation. Particle spectroscopy to characterize FeĲVI) resultant particles and counts (N) were measured for FeĲVI) and FeĲIII) coagulation −1 found that they contained hematite (α-Fe2O3), which was not over time (10–80 min) at pH 7.5 (3.0 mg L FeĲVI); and initial observed in FeĲIII) resultant particles. The results from X-ray turbidity = 25.00 NTU) (Fig. 3). The initial particle count of 14,15,25 4 powder diffraction (XRD) analyses in other studies kaolin was 2.84 × 10 particles per mL. Following the FeĲVI) showed that particles produced from FeĲVI) reduction in addition, the particle count was dramatically increased to 5 −1 deionized, AsĲV)-containing, and phosphate-containing water 2.10 × 10 particles per mL at 10 min at which 2.47 mg L were X-ray amorphous with diffraction maxima where the FeĲIII), accounting for 82% of the total added FeĲVI), had been most intensive diffraction line of maghemite (γ-Fe2O3) was produced from FeĲVI) reduction. Afterwards, the particle count expected. Prucek et al.14 further found core–shell nano- sharply decreased to 5.11 × 104 particles per mL at 25 min, γ γ × 4 architectures with a -Fe2O3 core and a lepidocrocite ( - and then gradually dropped to 1.97 10 particles per mL at 57 FeOOH) shell in FeĲVI) induced particles by Fe Mössbauer 60 min. A very similar trend was observed for FeĲIII) coagula- spectroscopy. tion. The particle count of FeĲIII) coagulation stabilized at Although the aforementioned studies validate the pres- 2.02–1.95 × 105 particles per mL at 10–25 min, followed by a ence of crystalline iron oxides in FeĲVI)-induced flocs, the quantitative information regarding the abundance of amorphous and crystalline iron oxides is unknown. In order to quantify the amorphous solid iron phase in FeĲVI)-induced floc samples, citrate–ascorbate iron extraction was under- taken in this study. The results showed that 73% of Fe in the –1 −1 iron flocs produced at 3.0 mg L FeĲVI), 2.00 mg L DOC, and pH 7.5 was amorphous, while only 27% was crystalline in the absence of kaolin particles. When kaolin particles were introduced to the simulated natural water (25.00 NTU), the amorphous Fe fraction slightly dropped to 69%, accompa- nied with a slight increase of the crystalline Fe fraction to 31% under identical experimental conditions. These observa- tions indicated that amorphous iron oxide was predominant Fig. 3 Experimental and modeled data on particle count (N) (particles over its crystalline counterparts during FeĲVI) coagulation, re- per mL) over time (pH = 7.5; iron dose = 3.0 mg L−1; DOC = 2.00 mg gardless of the presence or absence of colloidal particles, L−1; and initial turbidity = 25.00 NTU).

This journal is © The Royal Society of Chemistry 2018 Environ. Sci.: Water Res. Technol.,2018,4,701–710 | 705 Paper Environmental Science: Water Research & Technology rapid decrease to 6.04 × 104 particles per mL at 50 min. groups. The minimum detectable size was approximately as Thereafter, the particle count was not noticeably altered. Evo- low as 0.5 μm. As seen, the particle count of small particles lution of particle counts for FeĲVI) and FeĲIII) coagulation with continuously dropped from 10 to 60 min, suggesting consis- reaction time empirically followed eqn (1) and (2), tent aggregation of these small-sized particles. In particular, respectively. the number of small particles dramatically decreased from 1.01 × 105 to 1.32 × 104 particles per mL over 10–30 min, cor- 5 −0.077t 2 Fe(VI) coagulation : N = 5.04 × 10 e (R = 0.96) (1) responding to a reduction of 87%. The significant reduction at onset was ascribed to the presence of more iron particles produced from FeĲVI) reduction at the initial phase, which (2) provided more contact opportunities among small particles to develop larger particles. The particle count-time profile of the medium-sized particles relied upon two rates: 1) the Here, N is the particle count (particles per mL) and t is growth rate from small- into medium-sized particles; and 2) the time. The particle count after the addition of coagulant the formation rate from medium- to large-sized particles. The was much greater than the number of initial kaolin particles. consistently decreasing particle count of the medium particle Therefore, the evolution of particle counts primarily reflected group implied that the latter rate predominated over the for- the variation of iron hydroxide particles with time. At any mer one during coagulation. Of interest, the medium size specific time, a lower final particle count was observed for group, like the small particle group, had the greatest reduc- – FeĲVI) coagulation than FeĲIII) addition in this study, which is tion of particle count at 10 30 min. The rapid aggregation of 13 different from the findings of Goodwill et al. that FeĲVI) dos- medium-sized particles led to a buildup of large particles × 3 × 4 – ing generated more particles compared to FeĲIII) addition at from 1.33 10 to 1.02 10 particles per mL at 10 20 min. pH 6.2 in bicarbonate buffered deionized water or reservoir Afterwards, the continuous decrease in the number of large water. This disparity is most likely because a coagulant dose particles was due to the agglomeration among the large parti- equal to the MEID for effective FeĲVI) coagulation was used in cles. The number of particles of the medium and large-sized this study, which was less than the MEID for FeĲIII) addition. particle groups at 60 min in this study were close to those 13 ĲVI Consequently, FeĲIII) coagulation did not proceed under opti- reported by Goodwill et al. who counted Fe ) resultant par- mal conditions, leading to poor flocculation with a high particles in deionized water. ticle count. As a ubiquitous water matrix constituent, NOM played a Ĳ Goodwill et al.13 used a light blockage particle counter to more complicate role in the coagulation with Fe VI) compared ĲIII count particles from FeĲVI) addition in deionized water or res- with Fe ), because it did not only adsorb onto the particle ervoir water when flocculation was completed. They found surface, but also underwent chemical structural transforma- ĲVI that the majority of FeĲVI) resultant particles had a diameter tion due to Fe ) oxidation. As a result of the alteration in between 3 and 20 μm. Their measurements could only reach their molecular structure, NOM degradation products might ≥1 μm particles due to instrument limitation. In this study, influence the aggregation of iron oxide particles in a different particle counts in different size ranges were monitored manner compared to their parent compounds. In this study, during FeĲVI) coagulation of the kaolin suspension (pH = 7.5; three aspects associated with NOM were evaluated, including: −1 ĲVI 3.0 mg L FeĲVI); and initial turbidity = 25.00 NTU) (Fig. 4). 1) the varied impacts of NOM before and after Fe ) oxida- Three size ranges were arbitrarily categorized into small tion on the stability of kaolin particles; 2) the ZPs of iron pre- (<1.0 μm), medium (1.0–5.0 μm), and large (>5.0 μm) cipitates at different pH values in the absence of kaolin particles; and 3) the effect of NOM on the ferrateĲVI) coagulation of kaolin suspensions. In order to investigate the first aspect, a set of experiments were performed to monitor the sedimentation of kaolin particles in NOM-containing or free simulated natural water with or without FeĲVI) pre-oxidation (Fig. 5). In the experiments, kaolin particles, to achieve an initial turbidity of 25.00 NTU, were introduced into five dif- ferently pre-treated water samples. The five water samples in- cluded group A (control 1) that was NOM-free water without FeĲVI) pre-oxidation; group B (control 2) was NOM-containing −1 water (2.27 mg L DOC) without FeĲVI) pre-oxidation; and, groups C, D, and E (treatment) were NOM containing water − samples (2.27 mg L 1 DOC) pre-oxidized with 3.00, 6.00, and −1 9.00 mg L FeĲVI), respectively. It should be noted that iron precipitates produced from the pre-oxidation in groups C, D, Fig. 4 Particle counts in different size ranges during FeĲVI) coagulation −1 −1 μ of kaolin particles (pH = 7.5; 3.0 mg L FeĲVI); DOC = 2.00 mg L ; and and E were removed through 0.45 m membrane filtration initial turbidity = 25.00 NTU). prior to the addition of kaolin particles. As the sedimentation

706 | Environ. Sci.: Water Res. Technol.,2018,4,701–710 This journal is © The Royal Society of Chemistry 2018 Environmental Science: Water Research & Technology Paper

Fig. 5 Turbidity, DOC, SUVA, and zeta potentials of the kaolin suspension in NOM-containing or free simulated natural water with or without FeĲVI) pre-oxidation (pH = 7.5; initial turbidity = 25.00 NTU; DOC and SUVA were measured before the introduction of kaolin particles for sedimentation; zeta potentials were measured immediately after the introduction of kaolin particles at the onset of sedimentation; and turbidity was measured at 24 h sedimentation).

proceeded, turbidity gradually decreased in the five groups. FeĲIII) resultant flocs at different pH values were measured af- −1 After 24 h of sedimentation, the settled turbidity in group B ter the FeĲVI)orFeĲIII) coagulation experiments (3.0 mg L Fe) (5.40 NTU) was greater than that in group A (3.85 NTU), indi- were completed in kaolin-free simulated natural water at pH cating that NOM improved the stability of the kaolin parti- 7.5 (Fig. 6). The iron precipitates from the two different iron cles. The enhancement effect of NOM was ascribed to the ad- sources exhibited similar ZP profiles against pH in the ab- sorption of NOM on kaolin, which led to the formation of a sence of NOM. The pH values at the point of zero charge

NOM surface coating and thus a reduced ZP from −12.90 to (pHPZC) for the fresh FeĲVI) and FeĲVI) resultant precipitates −14.31 mV as shown in Fig. 5, favoring the stabilization of were approximately 6.58 and 6.36, respectively, indicating the kaolin suspension. Kaolin adsorption of aquatic NOM very close pHPZC without NOM. The values were similar to 26–29 has been well documented elsewhere. As the FeĲVI) dose the reported pHPZC of amorphous or crystalline iron oxides − − increased from 0.0 to 9.0 mg L 1 in the pre-oxidation step, in the literature.13,31,32 At 0.50 mg L 1 DOC, the surface the settled turbidity dropped from 5.40 to 4.35 NTU. The less charges of both iron precipitates decreased at a specific pH, particle stability was attributed to the increase of the ZP from leading to a pHPZC shift toward a lower pH. Of interest, the − − Ĳ 14.31 to 13.50 mV, which was caused by the structural al- extent of pHPZC shifting was different between the Fe VI) and ternation of NOM molecules due to FeĲVI) oxidation. As seen, FeĲIII) flocs. A greater pHPZC was observed for FeĲVI) precipi- the FeĲVI) pre-oxidation reduced DOC and SUVA from 2.27 to tates (approximately pH 3.48) compared to that for FeĲIII) pre- − − − 1.52 mg L 1 and from 4.02 to 1.67 L mg 1 m 1, respectively, cipitates (approximately pH 3.02). It is well known that NOM suggesting that NOM molecules became more hydrophilic can adsorb onto the iron oxide surface to form a surface coat- and less aromatic after FeĲVI) oxidation, which is in agree- ing. Although different adsorption mechanisms were pro- ment with the findings in our previous study.30 Because posed,33 the formation of a NOM coating on iron oxide was greater hydrophobic fractions of NOM have stronger affinity primarily ascribed to the ligand exchange between the car- towards kaolin surfaces,29 the more hydrophilic and lower boxyl/hydroxyl moieties of NOM and the iron oxide surface.34 molecular weight degradation products of NOM after FeĲVI) The negatively charged NOM coating led to a more negative oxidation adsorbed less to the kaolin surface. Therefore, ZP on the surface of iron oxides. However, as discussed ferrateĲVI) oxidation during coagulation is capable of above, ferrateĲVI) oxidation transformed NOM into more hy- destructing NOM to alleviate its inhibition effect in the for- drophilic and less aromatic compounds that unfavorably mation of iron flocs, thereby improving the FeĲVI) coagulation adsorbed to iron oxides. Consequently, fewer organic com- efficiency. pounds adsorbed onto the FeĲVI) resultant precipitates, which To determine the surface charges of FeĲVI) resultant precip- reduced surface charges less, thereby producing a greater itates in the absence of kaolin particles, ZPs of FeĲVI) and pHPZC than FeĲIII) oxide precipitates. Similar trends were

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Fig. 6 ZPs of FeĲVI)orFeĲIII) resultant precipitates in kaolin-free simulated natural water at different pH values: (a) FeĲVI); (b) FeĲIII) (flocs were −1 obtained after 3.0 mg L FeĲVI) decomposition at pH 7.5).

− observed in the presence of more NOMs. As DOC increased settled turbidity at 8.00–10.00 mg L 1 DOC suggested that − from 2.00 to 8.00 mg L 1, the ZP at any specific pH further NOM only at a high concentration played a significant role in decreased. Similarly, FeĲVI)-induced iron oxide precipitates the prevention of aggregation of iron oxide particles during had fewer negative surface charges compared to FeĲIII) flocs FeĲVI) coagulation. Although ferrateĲVI) oxidation was demon- under identical conditions. For example, at pH 7.5, the ZP strated to benefit the formation of iron floc, the fraction of dropped from −4.20 to −15.35 mV and from −8.89 to −18.20 for the FeĲVI) and FeĲIII) treated samples, respectively, with an − increased DOC from 0.50 to 8.00 mg L 1. Over the pH range of (3.0–9.0), all the surface charges were negative at 2.00–8.00 − mg L 1 DOC for either coagulant. The more negative surface charge at a higher DOC level more favorably stabilized the iron oxide particles. The effect of NOM on the FeĲVI) coagulation of kaolin particles is presented in Fig. 7. As NOM increased from 0.00 to 6.00 −1 −1 mg L DOC at pH 7.5, 3.00 mg L FeĲVI), and an initial turbidity of 25.00 NTU, the settled turbidity slightly increased from 0.45 to 2.45 NTU, indicating that NOM had a limited impact on the aggregation of iron precipitates within the DOC range. However, the settled turbidity sharply increased to Fig. 7 Settled turbidity and zeta potential during FeĲVI) coagulation of −1 24.10 NTU as NOM increased to 8.00 mg L DOC, and then al- kaolin particles at different NOM concentrations (pH = 7.5; 3.0 mg L−1 −1 most stabilized at 10.00 mg L DOC. Little reduction in the FeĲVI); and initial turbidity = 25.00 NTU).

708 | Environ. Sci.: Water Res. Technol.,2018,4,701–710 This journal is © The Royal Society of Chemistry 2018 Environmental Science: Water Research & Technology Paper

NOM oxidized by FeĲVI) was relatively insignificant at a high Deng's group at Montclair State University under the support DOC level. The adsorption of NOM and its degradation prod- of the Chinese Scholarship Council Fellowship Program and ucts on iron oxide, as discussed above, could produce a more Hubei Provincial Department of Education's Young Teacher negative particle coating to slow down the aggregation rate of Fellowship Program, respectively. Mr. Michael Cohrs is greatly iron oxide particles, thereby preventing the formation of iron appreciated for his help with the FlowCAM particle analysis. floc. The dramatic increase of settled turbidity occurring at a Dr. Y. Deng greatly appreciates the support, suggestion, and − high DOC level (8.00–10.00 mg L 1) might be also associated comments of Dr. Satish Myneni at Princeton University. with the steric arrangement of NOM molecules that inhibited the agglomeration of small particles.35 References

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