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Influence of the Mixing Energy Consumption Affecting Coagulation and Floc Aggregation Yamuna Srinivasan Vadasarukkai, and Graham A. Gagnon Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b06281 • Publication Date (Web): 16 Feb 2017 Downloaded from http://pubs.acs.org on February 16, 2017

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Environmental Science & Technology

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Influence of the Mixing Energy Consumption Affecting Coagulation and Floc Aggregation

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Yamuna S. Vadasarukkai and Graham A. Gagnon*

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Corresponding Author: Dr. G.A. Gagnon, Dalhousie University, Halifax, NS, B3J 1Z1

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e-mail [email protected], Tel: 902.494.3268, Fax 902.494.3108

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Abstract

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The operational significance of energy-intensive rapid mixing processes remains unaddressed in

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coagulation and flocculation of insoluble precipitates (flocs), which play an important role in the

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removal of impurities from drinking water supplies. In this study, the influence of rapid mixing

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and associated mixing energy on floc aggregation was examined for a surface water source

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characterized by a high fraction of aquatic humic matter. Infrared spectral analyses showed that

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the colloidal complexes resulting from ligand exchange between iron and dissolved natural

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organic matter (DOM) were not substantially influenced by the mixing energy input. This

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signified that DOM removal by coagulation can be achieved at lower mixing intensity, thereby

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reducing energy consumption. In contrast, macroscopic investigations showed the coagulation

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mixing energy affected floc size distributions during the slow mixing stage in flocculation and,

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to some extent, their settling characteristics. The results from analysis of floc properties clearly

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showed that more mixing energy was expended than necessary in coagulation, which is typically

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designed at a high mixing intensity range of 600-1000 s-1 in treatment plants. The key findings

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from this study have practical implications to water utilities to strategically meet water quality

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goals while reducing energy demands.

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Abstract Art

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1 ACS Paragon Plus Environment


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1. Introduction

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Urban water infrastructure systems require significant amounts of energy for proper operation of

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major processes, which may include water intake, conveyance, treatment, and distribution

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systems, to deliver a safe and high-quality drinking water to customers. Life cycle analysis

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(LCA) is often used to quantify energy usage at each stage1-3 as measured by energy

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benchmarks, such as kilowatt-hour of electricity consumed per unit volume of treated water.4

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Accordingly, the predominant use of electricity is directly associated with pumping (e.g., low lift

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and/or high lift pumps), which is connected to the pressure demands and operational

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characteristics of water distribution networks. Ultimately, pumping represents 60-85% of the

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total electricity consumption for surface water treatment plants.1, 5 Rapid mixing in coagulation is

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described as the second largest consumer of direct energy in a conventional water treatment

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plant.2, 5 A report from the Electric Power Research Institute (EPRI)5 estimated that rapid mixing

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processes had a total electricity consumption of approximately 308 kW-h/day in a representative

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surface water treatment plant with a intake of 45 million liters/day (MLD). In addition, the

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primary energy (e.g., fossil fuels, nuclear energy, renewable energy) used in the production of

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electricity for the operation of these systems is associated with greenhouse gas (GHG)

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emissions.6

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In water treatment practice, high energy is expended to ensure instantaneous dispersal of

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chemicals added during coagulation with the plant influent. A rapid-mixing tank with

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mechanical mixers is often a conventional means to accomplish mixing process in water

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treatment practice.7 Rapid-mixing tanks are typically designed for intense mixing with a root

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mean square velocity gradient (G-value) of 600-1000 s-1.8 Previous studies have identified

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several disadvantages in this tank design,7, 9 stating that the practice of intense mixing in

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coagulation is largely unnecessary. A major shortcoming is that a relatively large amount of

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energy is utilized to achieve a typical design mixing intensity since the average power supplied

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increases with the square of the G-value. For instance, Edzwald 7 calculated that the power

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requirement to achieve a typical G-value of 750 s-1 was 6.25 times greater when compared to 300

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s-1. This reflects a proportional increase in the energy cost and associated carbon emissions.

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Experimental investigations have supported the theory that the coagulation process has a great

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potential for energy savings with regards to rapid mixing conditions, without sacrificing water

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quality standards.10-12 For instance, Vadasarukkai, et al.11 have proposed a low-mixing intensity

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range of 110 s-1 < G < 450 s-1 for a combined effective removal of turbidity and dissolved natural

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organic matter (DOM). Alternatively, a more sustainable system for coagulation mixing, such as

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pipe and open channel static mixers, are now available to water plants.7 Nevertheless, there has

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been no study to date that examines how energy utilized in coagulation specifically influences

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the overall aggregation of metal hydroxide precipitates. Surface hydroxyl groups on these

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precipitates aid in effective sorption of turbidity, pathogens, inorganics, and DOM to their

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surfaces. Such precipitates can also grow into large sized aggregates through the formation of

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hydroxyl bridges between metal precipitates,13 collectively termed flocs. These flocs play a vital

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role in the fate and transport of contaminants incorporated in their structural matrix and their

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removal in subsequent solid-liquid separation processes.

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In efforts to reduce targeted impurities from drinking water supplies, it is important to

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understand the factors that may affect floc aggregation. The objective of this research was to

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understand how and to what degree the mixing energy consumed in coagulation influences the

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overall characteristics of flocs, including morphology, size distributions, and settling

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characteristics. Surface analyses of flocs formed immediately after the rapid mixing process (i.e.,

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microflocs) were characterized using Fourier transform infrared (FT-IR) spectroscopy. This was

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performed to obtain specific information on the mechanism of interaction(s) of sorbed

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constituents and the metal hydroxide precipitates. Additionally, morphology of microflocs

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formed during the rapid mixing stage was investigated using transmission electron microscopy

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(TEM) and scanning electron microscopy (SEM) combined with energy dispersive X-Ray

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spectrometry (EDS). A digital in-line holographic microscope (DIHM) was used to measure floc

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size distributions during slow mixing in the flocculation stage. The results from the present study

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provide opportunities for water utilities to strategically meet both water quality goals and energy

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reduction demands. In particular, it is estimated that an average of 10% reduction in annual

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energy costs in utilities across the U.S. represents a saving of more than $400 million annually to

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the water industry.14 It is anticipated that this study will contribute to the ongoing research and



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development in implementing technologies for an effective management of energy resources,

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costs, and carbon footprints.

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2. Materials and methods

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Water quality:

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Source water from Kelly Lake, drawn from the intake at the Louisbourg drinking water treatment

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plant (Sydney, NS, Canada), was used in this study. This raw water source is characterized by an

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average specific ultraviolet absorbance (SUVA) of 4 to 4.2 m-1 of absorbance per mg/L,

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indicating a high fraction of aquatic humic matter. Table 1 provides a description of the general

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characteristics of this source water, along with the results of elemental analysis measured using

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the inductively coupled plasma-mass spectrometry (ICP-MS) (Thermo Scientific XSeries II ICP-

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MS, Waltham, Massachusetts).

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Coagulation-flocculation experiments:

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Coagulation and flocculation experiments were tested at two pH conditions, a slightly acidic pH

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of 4.80 and a near neutral pH of 6.85, following a procedure similar to that of Vadasarukkai, et

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al..11 In that study, which used the same source water from Kelly Lake, an iron dose of 0.33 mM

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and 0.66 mM was determined as optimal coagulant concentration (OCC) of ferric sulphate at

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pHs 4.80 and 6.85, respectively. At the OCC, the authors showed total organic carbon (TOC)

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concentration and residual turbidity decreased following settling in the treated water. These

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optimal chemical conditions were used for enhanced coagulation in the present study to form

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iron-DOM floc aggregates.

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All experiments were carried out at a room temperature (21 ± 1oC) in a 1-L square jar test

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beaker, fitted with a flat paddle impeller (76 x 25 mm). A compact digital mixer (Cole-Parmer,

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Vernon Hills, Illinois), which had a mixing capacity ranging from 50 to 2500 rpm, was used to

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mix the water sample. The equivalent G-value for the flat paddle impeller configuration was

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determined from the laboratory velocity gradient calibration chart (G-curve) of Cornwell and

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Bishop.15 The solution was adjusted to the required pH (4.80 or 6.85) by adding a pre-determined

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volume of the pH adjusting chemicals (i.e., 1 N NaOH or 0.2 N HNO3 solutions). All reagents

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used were of analytical grade and stock solutions were prepared using deionized (DI) water.


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Energy calculations were based on a representative surface water treatment facility, with an

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average flow of 120 million liters/day. Equation 1 was used to calculate the daily consumption of

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energy during the rapid mixing operation in the coagulation process, expressed in kilowatt-

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hour/day. The average power supplied to stir the mechanical mixer was calculated from the

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previously determined G-value, assuming a motor efficiency of 85%.7 A water temperature of

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15°C, a retention time of 20 s in the rapid mixing tanks, and a power cost of $0.12 per kW-h was

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used as the plant design conditions.7

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 G 2  Qavg  t m    E  Pavg  t     t    

(1)

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where Pavg is the average power supplied in a mixing tank (J/s), t is the operating time period of

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the mixer (assumed as 24 hours), G is the root mean square velocity gradient (s-1), μ is the

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dynamic viscosity of water (kg/m.s), η is the motor efficiency, Qavg is the average daily plant

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flow rate (m3/s), and tm is the mean hydraulic retention time (s).

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Coagulation experiments were conducted at rapid mixing speeds ranging from 0 to 750 rpm,

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which gave an equivalent G-value range from 0 to 1450 s-1. An optimized dose of ferric sulphate

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and a calculated volume of pH adjusting chemicals were added at the start of the rapid mix

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process. Coagulation was conducted for 90 s at a specific G-value. Flocculation was achieved at

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a fixed G-value of 46 s-1 for 18 min. The energy consumed in the slow-mixing stage was not

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included in the total energy consumption of the plant, but accounts for 103 kW-h/day. The

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operating conditions used in this study were within the recommended standards followed in

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water treatment plants,8, 16 except for the G-value and 90 s of mixing time specified in the

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coagulation experiments. The use of more than one chemical (e.g., ferric sulphate, 1 N NaOH or

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0.2 N HNO3 solutions) in the present case required this additional retention time to simulate

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plant conditions.8

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Characterization of micro- and macro- structures of iron-DOM floc aggregates:

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Rapid mixing stage: Analyses of the functional groups in the resulting microflocs was performed

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using an attenuated total reflectance (ATR) Fourier transform infrared spectroscopy (FT-IR) in

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the mid-infrared region of 4000-650 cm-1 in the absorption mode (Cary 630 ATR FT-IR, Agilent 5 ACS Paragon Plus Environment


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Technologies, California). An approximate volume (~200 mL) of floc aggregates was collected

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following coagulation. The floc suspension was left undisturbed to settle for an hour in an acid-

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rinsed crucible while supernatant was carefully discarded using a 25 mL glass pipette.

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Flocs contain water molecules, which can absorb a significant portion of the infrared (IR) band

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due to the hydroxyl group (-OH). Therefore, water molecules can interfere with the spectral

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analysis of organic compounds (e.g., carboxylic, phenolic groups of DOM), as residual water are

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reported to absorb strong signals at 3700-3000 cm-1 and 1700-1600 cm-1.17 For this reason, many

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studies have concentrated and dried the floc samples .17-19 Fewer studies have preferred a diffuse

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reflectance infrared Fourier transform (DRIFT) spectroscopy for investigations of aquatic humic

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substances .20 Other adsorption studies have applied a thin layer of metal oxides on the crystal

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surface and then filled the ATR cell with organic acid solutions .21 In the present study, sample

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was oven dried at 35-40 °C for at least 3 days (or until the water was evaporated from the

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sample). The oven-dried floc aggregates were gently scraped and stored in a vial at 4 °C until

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analysis. A more specific description of the sample preparation method is illustrated in the

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Supporting Information (SI).

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An attenuated total reflectance flow cell with a zinc selenide (Zn-Se) crystal was used for this

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purpose. The Happ-Genzel method was used with 200 scans at a resolution of 2 cm-1.

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Backgrounds were taken with the crystal alone. Analysis of the spectral data was performed

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using optical spectroscopy software (Spekwin32, Germany).22

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Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) combined

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with energy dispersive X-Ray spectrometry (EDS) were used to take images of flocs generated

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from coagulation. Approximately 20 mL sample was collected from the rapid mixing stage and

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stored at 4oC until the analysis. As both electron microscopy techniques work in vacuum, a large

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quantity of water in floc sample must be removed as a sample preparation step. Initial

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investigation of floc images identified that flocs dried at 30-450C were irregular flat-plates,

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coarse/rough textured aggregates with sharp edges (data not shown), which may not effectively

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represent an amorphous floc precipitate commonly seen in water treatments. It was therefore

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concluded that the sample preparation at an elevated temperature was unsuitable for the SEM

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observation of floc structures. 6 ACS Paragon Plus Environment

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Alternatively, Cornelissen et al.23 found the resulting damage from drying of flocs at room

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temperature was not apparent at smaller floc sizes imaged using TEM. Therefore, a droplet of the

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TEM sample was carefully placed on a 200-mesh copper grid from Electron Microscopy

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Sciences (Hatfield, Pennsylvania). The grid was then allowed to dry at room temperature for 24

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hours. This allowed the water from the surface of floc structures to evaporate and prevented

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crystallization, which occurs at high-temperature (e.g., 30-104 °C) drying.17,19 A FEI Tecnai-12

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TEM (Hillsboro, Oregon) equipped with a Gatan 832 CCD camera and Gatan Microscopy suite

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software was used to take floc images. Duplicate samples were prepared for each analysis.

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The sample preparation was kept consistent for both the electron microscopy so to produce

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compatible results at different mixing conditions. That is, using a 10 mL pipette, a droplet of floc

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sample was carefully placed on a SEM sample mount, and the sample was dried at room

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temperature before image analysis. The samples were sputter coated with gold-palladium to

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prevent charge buildup on poorly conducting samples, such as flocs in the present case. A

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Polaron SC7620 Mini Sputter Coater (Quorum Technologies, Lewes) with a plasma current of

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18 mA for 120s was used for this purpose. The samples may ineffectually dissipate the resulting

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electron charge without such coating procedures, which can cause imaging artifacts.24 Both the

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elements were removed during the energy dispersive x-ray spectroscopy (EDS) analysis of

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elemental compositions. A Hitachi S-4700 FEG SEM (Hitachi, Tokyo) equipped with an energy

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dispersive x-ray spectroscope was used to provide analysis of elemental composition of samples.

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Each EDS spectrum was run for at least 30s, with an accelerating voltage of 20 kV and an

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emission current of 8.5 µA. A minimum of four EDS spectra per image was analyzed and the

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most representative images are presented. Each reported value was calculated by averaging the

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elemental composition from various representative SEM images (4-7 images), with at least four

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EDS spectra per image.

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Flocculation stage: A digital in-line holographic microscope (DIHM) from 4deep Inwater

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Imaging (Halifax, Nova Scotia, Canada) was used to measure the dynamic growth of floc sizes

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during the coagulation-flocculation experiments. The microscope was vertically mounted to one

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of the corners of the jar beaker, minimizing any interruption to the impeller rotations. Holograms 7 ACS Paragon Plus Environment


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of flocs were recorded continuously at 2 frames per second (fps), and they were stored as digital

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bitwise images. The formation of microflocs was not recorded at coagulation G-values > 450 s-1

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due to bubbles formed near the sampling port, which can cause significant errors in size

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measurements. Numerical reconstruction of holograms was performed to get images of floc

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aggregates, as described in Vadasarukkai, et al.25. Equivalent circular diameter (d) of floc size

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was calculated from the area of the identified flocs, which was expressed as a volume-based floc

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equivalent diameter.

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Settling properties:

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The treated water from the jar test beaker was poured into an Imhoff cone (Wheaton, Millville,

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New Jersey) immediately after the slow stirring operation in flocculation. It was allowed to settle

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quiescently for 24 hours. Samples were collected periodically from 10.3 cm below the water

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surface using a wide-mouth glass pipette. Residual turbidity measurements were recorded using

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a HACH 2100AN laboratory turbidimeter (Hach Company, Loveland, Colorado). The volume of

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settleable solids was recorded at the end of the experiment. This study did not account for a drop

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in water level height due to the withdrawal of approximately 40 mL of water samples during

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turbidity measurements, which can result in an estimated 5% discrepancy for the high settling

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velocity and at approximately a 30% discrepancy for the low settling velocity values.26 The

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linear range of the curve was identified, and it was used to determine the sedimentation rate27, as

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expressed in Equation 2.

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Slope( NTU,min) 

y 2  y1 x 2  x1

(2)

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where y1 and y2 are the values of turbidity in NTU of the linear slope. The slope of the regression

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line was calculated between the time period 0 (x1) and 30 (x2) minutes of the settling period.

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3. Results and discussion

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Floc morphology and size distributions at optimal coagulant concentrations (OCC):

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The iron to carbon ratio (i.e. > 0.8 mg Fe/mg TOC) used as OCC in the present study resulted in

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a rapid growth of ferric flocs in the rapid mixing stage itself kept at a G of 175 s-1, which was

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well within the optimal operating range (i.e., 110 s-1 < G < 450 s-1) of mixing energy input

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recommended for enhanced coagulation.11 This corresponds to an energy usage of 27 kW-h/day

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for a 120 million liters/day representative treatment facility. 8 ACS Paragon Plus Environment

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TEM micrographs in Figure 1 showed three sample population of flocs at the nanoscale,

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illustrating that a variety of floc morphologies were present at the optimal coagulant

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concentration. The darker regions of the TEM images in Figure 1b and 1c showed dense,

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spheroid-like or needle-like clusters that were closely packed to one another. The darker regions

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represented a floc size of 60 to 180 nm, as shown in the inserts. According to previous

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microscopic investigations,28-29 the association of coagulant species and humic networks was

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hypothesized to form similar electron-dense spots having a variety of shapes, including needle-

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like, globular or net-like structures. In the present case, this network of clusters was

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interconnected by what appear to be different sizes of granular structures that were lighter in

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appearance. The lighter regions were prominent near the edges of floc aggregate. Another type of

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floc morphology shown in Figure 1a was often seen in TEM micrographs at different scales of

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observation. The shape of the aggregate can be best described as an irregular, flaky or relatively

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tenuous precipitate made up of many particles of darker and lighter shades. Few of the darker

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particles were closely packed, while many of them were loosely joined to one another, as one can

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visually see from a large amount of void spaces in their structure compared to Figure 1b and c.

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Many larger floc aggregates observed under a different magnification (e.g., Figures S2 in SI)

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showed similar irregular geometric resemblance to the aggregate shown in Figure 1a. This visual

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observation of self-similarity in the shape of floc aggregates, regardless of the scale of

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observation, is consistent with the fractal nature of floc precipitates.23.30

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Differences in the optimal coagulant dose at pH 4.80 and 6.85 (i.e., an iron dose of 0.33 mM vs

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0.66 mM, respectively) resulted in a major difference in the overall floc size distributions

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between the two pHs, as illustrated in Figure 2. A significant difference in the variation of floc

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size distributions between the two experiments was confirmed using Levene’s test (α=0.05; N

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>30). The volume fraction of floc sizes ranged from 100 to 1000 μm at pH 4.80. The median floc

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diameter (D50) was 424 μm. At the higher iron dose (0.66 mM, pH 6.85), floc sizes were more

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variable and had a greater D50 value of 581 µm.

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Unlike the distinct differences in floc sizes, it was difficult to interpret the inherent differences in

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the floc morphology between the two experiments because of fractal nature of floc aggregates. 9 ACS Paragon Plus Environment


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Details on micrographs are included in the Supporting Information (SI) (i.e. Figures S2 in SI).

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Most of the TEM micrographs showed irregular, flake-like geometries at both pHs. At pH 6.85,

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many sections in the TEM grid had partially transparent patches of approximately 100 to 500 nm

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in size (inserts shown in Figure S2c and S2d in SI) that resembles a sheet of fabric attached to

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the precipitate. This TEM observation may represent amorphous hydroxide precipitates favoured

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at around neutral pH. 31

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Influence of mixing energy utilized in rapid mixing on coagulation, flocculation, and

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settling processes:

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This section discusses the influence of variation in the mixing energy consumed in coagulation

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on the overall characteristics of floc properties, including morphology, size distributions, and

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their settling characteristics. Energy calculations are representative of a 120 million liters/day

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surface water treatment facility.

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(a) Aggregation behaviour of primary particles (DOM) during coagulation

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Initial population has a strong influence on floc size formation.32 In the present study, a major

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constituent of primary particles was comprised of dissolved organic matter (DOM), which

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played an important role in the characteristics of flocs resulting from coagulation. Results from

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the FT-IR spectral analyses (Figure S3 in SI) showed similar surface complexation-ligand

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exchange reactions between DOM and iron hydroxides at both pHs, despite the differences seen

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in the floc size distributions. This phenomenon of adsorption of DOM on to metal oxide surfaces

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has been demonstrated previously in natural soil and aqueous systems.33-35 It resulted in similar

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IR peaks at 1559 and 1375 cm-1 at both the pHs in the present study (spectral results are

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presented in Figure S3 in SI). These absorption bands represented the stretching vibrations of the

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carboxylate groups (i.e., COO-) adsorbed onto the surface active hydroxyl functional groups of

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iron hydroxides (i.e., Fe-OH2 and Fe-OH stretches), which has been previously noted for the

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adsorption process on iron hydroxides.36

288 289

The adsorption of DOM on iron hydroxides were influenced by the pKa values of functional

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groups in the raw water, which had approximate values of 2.85 and 7.03 from acid-base titration

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curves (data not shown). At pH 4.80, disassociation of protons was expected, favouring the

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COO- functional groups to replace the dominant OH2 groups from surfaces of iron.37 Therefore,

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an addition of a 0.33 mM of iron at pH 4.80 resulted in a partial neutralization of the net positive 10 ACS Paragon Plus Environment

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charge on iron hydroxide surfaces and formed insoluble iron-DOM complexes. Alternatively,

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when the pH was near the pKa value of 7.03, more protons in the humic networks were

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disassociated, leading to an increase in the iron dose from 0.33 to 0.66 mM at pH 6.85.

297 298

A major difference between the two pHs occurred in the fingerprinting region below 1100 cm-1,

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which can be explained by the adsorption behaviour of sulphate. Peak, et al.38 had shown the

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spectrum of sulfate adsorbed on goethite had a sharply defined peak at 976 cm-1 for pH less than

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6, forming an inner-sphere sulfate complex with two distinctive splits formed at 1055 and 1133

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cm-1. These peaks were similar to the IR peaks formed at pH 4.80 in the current study at 975,

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1040, and 1111 cm-1. At pH 6.85, the sharp peak at 975 cm-1 was broadened and shifted to 969

304

cm-1. Aromatic ethers, carbohydrate, and polysaccharide groups also exhibit a broad band near

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1120-980 cm-1 with a characteristics peak near 1034 cm-1.39 It is possible that sulphate may have

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interacted with the co-adsorbed functional groups of DOM macromolecules and formed

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complexes, instead of being directly incorporated on to the surface of iron hydroxides. This

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observation was previously described for enhanced adsorption of trace metals onto iron oxide

309

surfaces through the formation of ternary complexes in the presence of high sulphate

310

concentrations.40

311 312

The relative peak intensity ratios in Table 2 provided a quantitative comparison of IR peaks

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formed at different amounts of energy used in coagulation. Details on the spectral comparison at

314

different mixing energy input are included in Figure S4 in SI. The ratios for the three prominent

315

peaks of adsorbed constituents (i.e., 1559, 1375 and 1039 cm-1) and the structural O-H vibration

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(i.e., 3177 cm-1) in the lattice structure of flocs were calculated as I1559/I3177, I1375/I3177, and

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I1039/I3177. It showed a fairly constant ratio for the adsorbed carboxylate groups (i.e., COO-) peaks

318

1559 and 1375 cm-1 at different mixing energy inputs. This suggests iron-DOM complexes

319

formed in the hypothesized size range of 60 to 180 nm were less vulnerable to the coagulation G-

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values. In other words, it implied DOM removal by coagulation can be achieved at reduced

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energy consumption. This disproves the theory that intense mixing is necessary to bring about

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particle-particle collision for effective interactions. However, a relatively large standard

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deviation for the adsorbed sulphate peak near 1039 cm-1 was evident at both pHs. This implies



324

that the coagulation G-values caused some changes to the shape of absorption spectra of sulphate

325

complexes, indicating changes in their composition.

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(b) Microstructure and their interactions

327

A scanning electron microscope with an energy dispersive X-ray spectroscopy (SEM-EDS)

328

analysis was used to compare the elemental composition of flocs formed at different mixing

329

energy inputs during coagulation. Although a consistent sample preparation technique was used

330

for both electron microscopy methods, floc images taken using SEM looked significantly

331

different from those observed by TEM, which is likely an artefact of drying process. This

332

suggests that drying at room temperature resulted in some perturbation of hydrated floc samples

333

for SEM observation. Cornelissen et al.23 found cryogenic freezing of the samples can improve

334

imaging of larger (1-100 µm) floc particles using SEM, without the loss of structural information

335

resulting from drying.

336 337

TEM images were more representative of flocs because of insignificant damage caused by the

338

drying process, which is likely due to the small size range of flocs analyzed using TEM.23 Floc

339

morphology can be evaluated using SEM despite the artefacts introduced by drying. SEM images

340

revealed qualitative differences in floc morphology as a function of mixing condition. For

341

instance, the SEM analysis showed the flocs resulting from a low mixing energy had flat plate-

342

like structures that were assembled in an elongated pattern. At higher mixing energies, smaller

343

flocs composed of globular aggregates with a compact structure were observed (Figure S5 in SI).

344

This phenomenon can be related to flocculation mixing studies.32,41 In those studies, a consistent

345

trend of reduction in floc sizes with increasing the mixing intensity was shown to form small and

346

compact flocs. The floc samples were investigated further using EDS analysis to understand the

347

elemental compositions that may have resulted in varied floc structures.

348 349

The EDS analysis are semi-quantitative as the elemental compositions are normalized to 100%

350

for the selected elements. A highly irregular surface of flocs resulting from the drying method

351

may cause increased variations at specific spots and between samples. A low concentration of

352

iron or DOM could be due to differences in surface area (e.g., small/globular, elongated/plate-

353

like structures), such uneven surfaces are suggested to increase variations in electron scattering

354

and absorption due to matrix effects.42 Therefore, the EDS results were not directly compared, 12 ACS Paragon Plus Environment

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355

but it was used to evaluate the percent weight trend (e.g., increasing/decreasing) of major

356

elements at different mixing conditions.

357 358

The results from the EDS analysis for coagulation experiments carried out at the mixing energy

359

input of 2 and 183 kW-h/day are illustrated in Figure 3. The elemental composition of flocs was

360

comparable between the two mixing conditions, with oxygen detected as a major element along

361

with carbon, iron, sulphur, and sodium co-existing in their structural matrix. Even though EDS

362

spectra detected appreciable percent weight of aluminium (Al) in the floc samples, it was not

363

included in the analysis because aluminium was also present in the SEM sample mount. Other

364

elements, such as calcium and magnesium, were detected at a low concentration. The relatively

365

higher percent weight of oxygen may have been a result of iron hydrolysis (e.g., Fe-OH and Fe-

366

OH2) and precipitation reactions (e.g., iron hydroxides), in addition to the oxygen present in

367

organic matter.

368 369

The EDS data, along with the FT-IR analyses, reinforces the interpretation that hydrolyzed iron

370

species interacted with DOM macromolecules, resulting in flocs with DOM-iron compositions

371

within their structure. However, the elemental analysis cannot confirm the nature of the

372

interactions, which may require investigations using X-ray photoelectron spectroscopy (XPS)

373

and/or solid-state nuclear magnetic resonance (NMR) results.43 There was a marginal difference

374

in the percentage composition of major elements present between the two mixing conditions at

375

pH 4.80. That is, carbon and oxygen were detected at slightly higher amounts in the floc samples

376

taken at 183 kW-h/day, while the percent weight of iron and sulphur contents was lesser when

377

compared to floc samples taken at 2 kW-h/day. Likewise, floc aggregates formed at pH 6.85

378

showed similarities between the elemental compositions at 2 and 183 kW-h/day (Figure 3

379

(bottom panel)). In this case, an increased fraction of iron (e.g., 8.3-18.2 %) was observed in

380

those flocs, which may have experienced more rapid precipitation reaction under higher pH

381

conditions.

382 383

The EDS analysis of elemental composition of flocs supported the hypothesis that the hydrolysis

384

of iron and their subsequent aggregation with DOM particles occurred irrespective of the mixing

385

condition. Previous microscopic investigations have found that iron and DOM interactions 13 ACS Paragon Plus Environment


386

resulted in aggregates of 60- 300 nm in size 28-29,44 Perikinetic or Brownian diffusion mechanism

387

initiated by thermal energy was likely to control particles-coagulant collisions during coagulation

388

that may result in successful attachment of particles ,45 while the importance of coagulation

389

mixing has been related to settling characteristics of flocs.29,46 A model proposed by Siéliéchi et

390

al.29 has indicated that low-intensity mixing favours small sediment volume due to intra-humic

391

colloid shrinkage and high-intense mixing is associated with a high collision rate and large

392

sediment volume.

393

(c) Floc size distribution

394

Mixing energy input in coagulation governed the floc growth during the slow mixing stage in

395

flocculation, as illustrated in Figure 4. Unless stated, the data represents the volume fractions of

396

floc size distributions measured using the in-line holographic microscopy in flocculation at 46 s-

397

1

398

floc size distribution at pH 4.80. On the contrary, a wide range of floc sizes were formed at pH

399

6.85 even without having a proper mixing condition in coagulation. The slow mixing energy

400

provided at a 103 kW-h/day in flocculation compensated for the no mixing operation in

401

coagulation and achieved necessary bridging between precipitates in that case. On a similar note,

402

a recent study had shown the overall water treatment performance of a full-scale drinking water

403

treatment plant remained unaffected during the accidental failures of mechanical mixer in the

404

rapid mixing tank.10

. It was apparent that no mixing energy during coagulation resulted in an obvious decline in the

405 406

For an average flow of 120 million liters/day, the energy consumed to operate at a specified

407

designed G-values of 600-1000 s-1 is in the range of 342 to 913 kW-h/day. As seen from Figure

408

4, operating at a mixing energy > 200 kW-h/day did not show any significant improvement in the

409

evolution of floc sizes in flocculation, in terms of median (D50) and 90th percentiles (D90).

410

Conversely, there was a slight drop in the median floc diameter at higher mixing energy. That is,

411

the decrease in the median floc size was from 375 to 275 μm at pH 4.8 and it dropped from 550

412

to 325 μm at pH 6.85. Likewise, Pivokonsky et al.41 found for a pilot-scale set up that a shallow

413

reduction in the average floc diameter was observed at G ≥ 200 s-1, with the value of the slope

414

being 0.34. This type of fragmentation of iron hydroxide flocs has been previously indicated for

415

tannery waste samples at > 170 s-1.47

416 14 ACS Paragon Plus Environment

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417

A general trend of decrease in floc sizes occurred in the case of higher energy usage,

418

representing aggregate breakage. This phenomenon has been demonstrated previously, with floc

419

aggregates becoming more compact and homogeneous in size with increasing G-values.32,41 Floc

420

breakage could be a result of variations in local G-value,48-49 which was not accounted for in the

421

present study. High mixing energy may contain maximum local G-values several orders of

422

magnitude higher than the average G-value. Bridgeman et al.48 found that a 500 μm particle was

423

subjected to 9.4 times the average G-value of 165 s-1 at every 5 seconds of rapid mixing. This

424

suggests that, in the present case, the frequency at which a floc was exposed to a maximum

425

(peak) local velocity gradient is expected to increase with increasing mixing energy. Further

426

analysis, involving floc size measurements are required to investigate whether shortening the

427

mixing time is necessary to promote floc aggregation and subsequently, improve particle

428

removal50 at different mixing conditions.

429 430

Floc breakage can pose an important practical problem by causing resuspension of impurities

431

and increased iron concentrations in settled water. An overall shift in the floc size distributions to

432

the left support this theory of an increase in the volume fractions of smaller flocs less than 100

433

μm in diameter. While an increase in small flocs population with increasing mixing regimes was

434

indicated, the size detection limit of DIHM was limited to provide a quantitative estimation of

435

small flocs less than 17 µm. The size range of the DIHM was suitable to measure flocs larger

436

than 50 μm, which are the major source contributing towards total surface area, volume, and

437

mass of flocs.51 Further analysis, using a Malvern Mastersizer 3000 52 and/or Zetasizer nano 44 ,

438

is recommended to examine the small floc size distribution that may have resulted at high energy

439

usage, as carryover of these flocs can pose significant challenges on downstream separation

440

processes (e.g., turbidity breakthrough in filters, reduced filter runtimes, increased organic

441

loading etc.). Recent work by Yu et al.13 suggests that broken flocs may not grow into large

442

aggregates due to a net reduction in the number of active surface hydroxyl groups. This may

443

greatly reduce the effectiveness of separation processes in the present case.

444

(d) Settling kinetics of flocs:

445

Comparison of slope values in Table 3 indicated the sedimentation rates were higher for the

446

optimal iron dose of 0.66 mM at pH 6.85 than for 0.33 mM at pH 4.8. This implies a faster rate



447

of floc formation and subsequent settling rates were achieved at the higher particle concentration

448

27

in the case of the near neutral pH.

449 450

The settled water turbidity curve in Figure 5 provides a comparative evaluation of the effect of

451

coagulation mixing energy on the settling characteristics of flocs. An overall decreasing trend of

452

residual turbidity with time indicated the settling process had a dampening effect on the

453

differences in floc characteristics due to coagulation mixing energy. In other words, there was no

454

significant difference in the settling kinetics after 60 min, particularly in the recommended

455

mixing energy range of 11 to 181 kW-h.11 The compact flocs (essentially smaller size particles)

456

resulting from a higher coagulation mixing energy were able to settle given enough time, except

457

for the data shown at 0 and 1879 kW-h/day. As expected, when the coagulation mixing energy

458

was at 0 kW-h/day (i.e., G = 0 s-1) and 1879 kW-h/day (i.e., G = 1450 s-1), the residual turbidity

459

was fairly high (> 2 NTU) even after a settling time of 90 min (at an overflow rate of 0.1

460

cm/min). This demonstrates that a minimum amount of energy is necessary to form settleable

461

flocs and for effective separation of those aggregates from the settled water in the subsequent

462

separation processes.

463

4. Practical implications for water treatment:

464

The key findings from the present study, which was done at bench-scale, are also critical to pilot-

465

scale and full-scale implementations. In most treatment plants, physicochemical conditions

466

related to the coagulant type and dose, temperature, and pH are routinely optimized at

467

laboratory-scale using jar tests and at pilot-scale. Mixing energy input associated with the tank

468

geometry, impeller types, and speed are not often considered for floc growth in existing testing

469

protocols. This shortcoming in the design of rapid mixing systems is too big to ignore in the

470

current scenario of rising demands for energy and water and consequent carbon emissions. As

471

shown in this study, more mixing energy is expended in current treatment plants than is

472

necessary if they operate at recommended design G-values (i.e., 600-1000 s-1). In the case of a

473

120 MLD water treatment system, reducing energy consumption from a 502 kW-h/day,

474

representing a G-value of 750 s-1, to less than 200 kW-h/day, representing a G-value of 450 s-1,

475

would result in an annual energy savings of nearly 170 MW-h/year without compromising

476

effluent water quality. That is, energy savings of over 90% can be achieved depending on plant 16 ACS Paragon Plus Environment

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Page 17 of 31


477

flow rate. It is pertinent that stakeholders and decision-makers consider the key results from this

478

study to capitalize on energy efficiency opportunities.

479 480

Flocs play a significant role in the transportation of dissolved natural organic matter (DOM) and

481

other impurities within engineered treatment systems and their removal from drinking water

482

supplies. In this study, floc characteristics for a surface water source were investigated at two

483

pHs from within a range (pH 4 to 6) associated with coagulation of high colour and TOC

484

waters.53 An important inference drawn was that the mixing energy did not have an influence on

485

the iron-DOM colloidal complexes formed in coagulation. While this phenomenon of

486

complexation of DOM on to the surface of metal oxide is well documented in early mechanistic

487

studies,33,34 it is expected the impact of the reported finding is applicable to other water sources

488

and coagulant types. Further investigations on the effect of different particle concentrations are

489

recommended.

490 491

The major difference in the overall floc size distributions resulting at the optimal coagulant

492

concentrations for the two pHs studied would have potential implications for a number of

493

treatment plants. Prior investigation of the spatial distribution of floc sizes at a direct filtration

494

plant suggests large, irregular shaped floc aggregates can reduce the effective filter run times.25

495

The data acquired in the present study on floc sizes can be calibrated to a treatment plant’s

496

specific case by operating at a definite pH, coagulant dose, and mixing energy. For plants

497

requiring dense and pin-point flocs, such as in a direct filtration facility, a small standard

498

deviation in floc sizes would be beneficial; this can be achieved at a slightly acidic pH condition.

499

While ferric salts based coagulation have been practiced widely in the pH 5s, plants that

500

coagulate at this pH and less may require pH adjustments of treated water prior to filtration.

501

Alternatively, in treatment plants with clarification processes (e.g. sedimentation) it may be more

502

desirable to operate at a higher pH since this resulted in a relatively high standard deviation in

503

floc size distributions which would aid in differential settling processes.

504



505 506

5. Associated content Author Information

507

Corresponding Author

508

*Phone: 902.494.3268; fax: 902.494.3108; e-mail: [email protected]

509

ORCID

510

Yamuna S. Vadasarukkai: 0000-0003-4689-6888

511

Notes

512

The authors declare no competing financial interest

513

6. Acknowledgements

514

The authors acknowledge the financial support of both NSERC and Halifax Water through the

515

NSERC/Halifax Water Industrial Research Chair program. The authors acknowledge the

516

Institute for Research in Materials (IRM) at Dalhousie University. The authors thank the

517

technical and in-kind supports of Patricia Scallion (IRM, Dalhousie University) during SEM

518

analysis and Dr. Ping Li (Biology Dept., Dalhousie University) in executing TEM analysis. We

519

thank Jon-Paul Sun (Ph.D. Student) from the department of Physics at Dalhousie University for

520

allowing us to use the FTIR instrument. The authors also acknowledge the technical support of

521

Heather Daurie at Dalhousie University’s Clean Water lab. We would like to thank the

522

Louisbourg water treatment plant authority, Nova Scotia, Canada for providing access to the

523

water treatment for sampling.

524

7. Supporting Information Available

525

Five figures (FT-IR spectral data, Figure S1; TEM micrographs, Figure S2; FT-IR spectral data,

526

Figures S3 and S4; SEM-EDS analysis, Figure S5) addressing additional experimental data are

527

included.

528

8.

529 530 531 532 533 534

1. Santana, M. V. E.; Zhang, Q.; Mihelcic, J. R. Influence of water quality on the embodied energy of drinking water treatment. Environ. Sci. Technol. 2014, 48 (5), 3084-3091.

References

2. Plappally, A. K.; Lienhard, J. H. Energy requirements for water production, treatment, end use, reclamation, and disposal. Renewable & Sustainable Energy Reviews 2012, 16 (7), 48184848. 18 ACS Paragon Plus Environment

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16. Ten State Standards of the Great Lakes-Upper Mississippi River Board of State and Provincial Public Health and Environmental Managers, Recommended Standards for Water Works; Health Research Inc., Health Education Services Division: Albany, New York, 2012.

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17. Sari, M.A.; Chellam, S. Mechanisms of boron removal from hydraulic fracturing wastewater by aluminum electrocoagulation. J. Colloid Interface Sci. 2015, 458, 103-111.

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18. El Samrani, A.G.; Lartiges, B.S.; Montarges-Pelletier, E.; Kazpard, V.; Barres, O.; Ghanbaja, J. Clarification of municipal sewage with ferric chloride: the nature of coagulant species. Water Res. 2004, 38 (3), 756-768.

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19. Goodwill, J.E.; Jiang, Y.; Reckhow, D.A.; Gikonyo, J.; Tobiason, J.E. Characterization of Particles from Ferrate Preoxidation. Environ. Sci. Technol. 2015, 49 (8), 4955-4962.

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20. Niemeyer, J.; Chen, Y.; Bollag, J.M. Characterization of humic acids, composts, and peat by diffuse reflectance fourier-transform infrared-spectroscopy. Soil Science Society of America Journal 1992, 56 (1), 135-140.

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21. Guan, X.; Shang, C.; Chen, G. ATR-FTIR investigation of the role of phenolic groups in the interaction of some NOM model compounds with aluminum hydroxide. Chemosphere 2006, 65 (11), 2074-2081.

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22. Menges, F. Spekwin32 - Optical Spectroscopy Software, Version v1.72.0; 2016; http://www.effemm2.de/spekwin/.

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23. Cornelissen, A.; Burnett, M.G.; McCall, R.D.; Goddard, D.T. The structure of hydrous flocs prepared by batch and continuous flow water treatment systems and obtained by optical, electron and atomic force microscopy. Water Sci.Technol. 1997, 36 (4), 41-48.

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24. Fischer, E.R.; Hansen, B.T.; Nair, V.; Hoyt, F.H.; Dorward, D.W. Scanning Electron Microscopy. In: Current Protocols in Microbiology; John Wiley & Sons, Inc., 2012. doi: 10.1002/9780471729259.mc02b02s25 25. Vadasarukkai, Y. S.; Gagnon, G. A. Characterization, fate and transport of floc aggregates in full-scale flocculation tanks. Environmental Science-Water Research & Technology 2016, 2 (1), 223-232. 26. Wong, K. B.; Piedrahita, R. H. Settling velocity characterization of aquacultural solids. Aquacult. Eng. 2000, 21 (4), 233-246. 27. Tassinari, B.; Conaghan, S.; Freeland, B.; Marison, I. W. Application of turbidity meters for the quantitative analysis of flocculation in a jar test apparatus. J. Environ. Eng. 2015, 141 (9), 04015015-1-04015015-8. 28. Myneni, S. C. B.; Brown, J. T.; Martinez, G. A.; Meyer-Ilse, W. Imaging of humic substance macromolecular structures in water and soils. Science 1999, 286 (5443), 1335-1337. 20 ACS Paragon Plus Environment

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29. Sieliechi, J. M.; Lartiges, B. S.; Kayem, G. J.; Hupont, S.; Frochot, C.; Thieme, J.; Ghanbaja, J.; de la Caillerie, J. B. D.; Barres, O.; Kamga, R.; Levitz, P.; Michot, L. J. Changes in humic acid conformation during coagulation with ferric chloride: Implications for drinking water treatment. Water Res. 2008, 42 (8-9), 2111-2123. 30. Gorczyca, B.; Ganczarczyk, J. Structure and porosity of alum coagulation flocs. Water Qual. Res. J. Can. 1999, 34 (4), 653-666. 31. Duan, J.M.; Gregory, J. Coagulation by hydrolysing metal salts. Advances in Colloid and Interface Science 2003, 100, 475-502. 32. Coufort, C.; Bouyer, D.; Line, A. Flocculation related to local hydrodynamics in a TaylorCouette reactor and in a jar. Chemical Engineering Science 2005, 60 (8-9), 2179-2192. 33. Gu, B. H.; Schmitt, J.; Chen, Z.; Liang, L. Y.; Mccarthy, J. F. Adsorption and desorption of different organic-matter fractions on iron-oxide. Geochim. Cosmochim. Acta 1995, 59 (2), 219229. 34. Guan, X.; Shang, C.; Chen, G. ATR-FTIR investigation of the role of phenolic groups in the interaction of some NOM model compounds with aluminum hydroxide. Chemosphere 2006, 65 (11), 2074-2081. 35. Vinkler, P.; Lakatos, B.; Meisel, J. Infrared spectroscopic investigations of humic Substances and their metal-complexes. Geoderma 1976, 15 (3), 231-242. 36. Rout, K.; Mohapatra, M.; Anand, S. 2-Line ferrihydrite: synthesis, characterization and its adsorption behaviour for removal of Pb(II), Cd(II), Cu(II) and Zn(II) from aqueous solutions. Dalton Transactions 2012, 41 (11), 3302-3312. 37. Liu, C., Huang, P. M. The influence of catechol humification on surface properties of metal oxides. In Humic substances structures, models and functions; Ghabbour, E. A.; Davies, G., Eds.; The Royal Society of Chemistry: Cambridge, UK, 2001; pp 253-270. 38. Peak, D.; Ford, R. G.; Sparks, D. L. An in situ ATR-FTIR investigation of sulfate bonding mechanisms on goethite. J. Colloid Interface Sci. 1999, 218 (1), 289-299. 39. Amir, S.; Jouraiphy, A.; Meddich, A.; El Gharous, M.; Winterton, P.; Hafidi, M. Structural study of humic acids during composting of activated sludge-green waste: Elemental analysis, FTIR and C-13 NMR. J. Hazard. Mater. 2010, 177 (1-3), 524-529. 40. Webster, J. G.; Swedlund, P. J.; Webster, K. S. Trace metal adsorption onto an acid mine drainage iron(III) oxy hydroxy sulfate. Environ. Sci. Technol. 1998, 32 (10), 1361-1368.



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41. Pivokonsky, M.; Bubakova, P.; Pivokonska, L.; Hnatukova, P. The effect of global velocity gradient on the character and filterability of aggregates formed during the coagulation/flocculation process. Environ. Technol. 2011, 32 (12), 1355-1366.

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52. Lee, S.Y.; Gagnon, G.A. Growth and structure of flocs following electrocoagulation. Separation and Purification Technology 2016, 163, 162-168.

42. McNaught, H.; Trenholm, R.; Claassen, D.; Scott, C.; Kodweis, G.; Vanderford, B.J.; Rexing, D.; Wert, E.C. No-lead brass service valve performance in Colorado River water. Journal American Water Works Association. 2015, 107 (8), E416-E424. 43. Lin, J.; Huang, C.; Dempsey, B. A.; Hu, J. Fate of hydrolyzed Al species in humic acid coagulation. Water Res. 2014, 56, 314-324. 44. Rahman, M. S.; Gagnon, G. A. Bench-scale evaluation of drinking water treatment parameters on iron particles and water quality. Water Res. 2014, 48, 137-147. 45. Benjamin, M.M.; Lawler, D.F. Water quality engineering: physical/chemical treatment processes; John Wiley & Sons: New Jersey. 2013. 46. Kan, C.C.; Huang, C.P.; Pan, J.R.S. Time requirement for rapid-mixing in coagulation. Colloids and Surfaces a-Physicochemical and Engineering Aspects 2002, 203(1-3), 1-9. 47. Rossini, M.; Garrido, J. G.; Galluzzo, M. Optimization of the coagulation-flocculation treatment: Influence of rapid mix parameters. Water Res. 1999, 33 (8), 1817-1826. 48. Bridgeman, J.; Jefferson, B.; Parsons, S. Assessing floc strength using CFD to improve organics removal. Chemical Engineering Research & Design 2008, 86 (8A), 941-950. 49. Hopkins, D.C.; Ducoste, J.J. Characterizing flocculation under heterogeneous turbulence. Journal of Colloid and Interface Science 2003, 264 (1), 184-194. 50. Garland C.; Weber-Shirk M.; Lion L.W. Revisiting Hydraulic Flocculator Design for Use in Water Treatment Systems with Fluidized Floc Beds. Environmental Engineering Science 2016, ahead of print. doi:10.1089/ees.2016.0174. 51. Li, D.H.; Ganczarczyk, J. Size distribution of activated-sludge flocs. Research Journal of the Water Pollution Control Federation 1991, 63 (5), 806-814.

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For Table of Contents Only

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Page 24 of 31

Table 1. Raw water quality for Kelly Lake

Water Quality Parameters pH Turbidity Total Organic Carbon Dissolved Organic Carbon UV absorbance at 254 nm SUVA Zeta Potential Elemental Analysis Magnesium (Mg) Iron (Fe) Sodium (Na) Aluminum (Al) Calcium (Ca) Potassium (K)

Values 6.0± 0.3 0.4± 0.1 NTU 3.8± 0.3 mg/L 3.6± 0.3 mg/L 0.15±0.02 cm-1 4.0± 0.2 L/mg.m -24± 0.4 mV Concentration (µg/L) 556 ± 2.7 330 ± 2.4 3900 ± 2.8 111 ± 1.0 1470 ± 11.3 196 ± 5.4

a

b

c

Figure 1. TEM images of microfloc formed at a G-value of 175 s-1 for the optimal coagulant concentration of 0.33 mM of Fe and pH = 4.80. Floc sizes shown in the insert were measured using ImageJ software v1.49. ACS Paragon Plus Environment

Page 25 of 31


Figure 2. Illustration of floc size distributions at a flocculation mixing intensity (G-value) of 46 s-1 for the optimal coagulant concentrations. The displayed histogram was estimated from three replicate experiments at pH 4.80 and 6.85, respectively.

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Page 26 of 31

Table 2. Peak intensity ratios calculated from the infrared spectra at various coagulation mixing intensities (G-value) and the corresponding energy consumed in the coagulation experiments at (a) 0.33 mM of Fe and pH = 4.80 (b) 0.66 mM of Fe and pH= 6.85.

G-values (s-1) Energy consumed at 120 MLD (kW-hr/day) Assignment of peak intensity ratios:

46

175

452

1450

2

27

183

1879

(a) 0.33 mM of Fe and pH = 4.80

Adsorbed caroboxylate(as ) Structural O  H

I1559 I 3177

1.77

1.50

1.57

1.51

Adsorbed caroboxylate(s ) Structural O  H

I1375 I 3177

1.61

1.32

1.46

1.33

Adsorbed SO4 Structural O  H

I1039 I 3177

2.46

1.72

1.99

1.71

Assignment of peak intensity ratios:

(b) 0.66 mM of Fe and pH = 6.85

Adsorbed caroboxylate(as ) Structural O  H

I1558 I 3177

1.29

1.44

1.34

1.32

Adsorbed caroboxylate(s ) Structural O  H

I1374 I 3177

1.11

1.23

1.13

1.11

Adsorbed SO4 Structural O  H

I1044 I 3177

1.41

1.37

1.38

1.07



Normalized weight (%)

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Elements Figure 3. Illustration of elemental composition of floc aggregates using SEM-EDS analysis for pH 4.80 (top panel) and 6.85 (bottom panel).


Equivalent volumetric diameter (μm)

Energy consumed (E), kW-h/day


Coagulation- mixing intensity (G-value) (s-1) Figure 4. Variation in floc size distributions with the change in the coagulation mixing intensities (G-value) and the corresponding energy consumed for the coagulation experiments conducted at 0.33 mM of Fe and pH = 4.80 (top panel) and 0.66 mM of Fe and pH= 6.85 (bottom panel). ACS Paragon Plus Environment

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Table 3. Influence of the coagulation mixing intensities (G-value) and the corresponding energy consumed on the sedimentation rate as calculated from the slope of turbidity-time regression line.

G-values (s-1) Energy consumed at 120 MLD (kW-hr/day)

0

24

112

452

1450

0

0.5

11

182

1879

Sedimentation rate (NTU/min):*

(a) 0.33 mM of Fe and pH = 4.80 -0.32

Sedimentation rate (NTU/min):*

-0.17

-0.10

-0.18

(b) 0.66 mM of Fe and pH = 6.85 -0.52

*

-0.17

-0.33

-0.32

-0.25

-0.61

The slope of the regression line was calculated between the time period 0 (x1) and 30 (x2) minutes of the settling period.


Residual Turbidity (NTU)


Coagulation- mixing intensity (G-value) (s-1) Figure 5. Influence of the coagulation mixing intensities (G-value) on settling kinetics for the coagulation experiments conducted at 0.33 mM of Fe and pH = 4.8 (top panel) and 0.66 mM of Fe and pH= 6.85 (bottom panel). ACS Paragon Plus Environment

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