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Environmental Processes

New Insights and Model for Understanding NOM Adsorption onto Mixed Adsorbents Siamak Modarresi, and Mark Benjamin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00849 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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Environmental Science & Technology

New Insights and Model for Understanding NOM Adsorption onto Mixed Adsorbents

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By

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Siamak Modarresi and Mark M. Benjamin*

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Dept. of Civil and Environmental Engineering, Box 352700, University of Washington, Seattle, WA 98195

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Abstract

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Adsorption-based processes are commonly used to remove natural organic matter (NOM)

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from drinking water sources and thereby mitigate its impacts on other water treatment

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processes and the quality of the finished water. These processes are complicated by the fact

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that NOM comprises multiple fractions that can exhibit disparate adsorption behaviors. Prior

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modeling of NOM adsorption has invariably focused on systems with a single adsorbent, but

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results presented here demonstrate surprising and counter-intuitive behavior in systems

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containing two or more adsorbents. Specifically, if the sequence of affinities of the different

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NOM fractions are the same for two adsorbents, then overall adsorption changes

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monotonically as one adsorbent is gradually replaced by the other. However, if the sequence

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differs for the two adsorbents, overall adsorption can increase even when the nominally

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stronger adsorbent is gradually replaced by the weaker one. This work demonstrates and

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explains such behavior for a particular mixture of adsorbents and introduces a mathematical

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model that illustrates how other mixtures might behave.

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Environmental Science & Technology

Abstract/TOC Art DOC adsorbed (mg /l)

2.5

PAC dose = Total dose − HAOPs dose

2.0

50 mg/l 1.5

20 mg/l

1.0

Model 1 0.5

Total adsorbent dose: 10 mg/l

0.0 0

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10

20

Model 2 30

40

50

HAOPs dose (mg/l)

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1.

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Introduction

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Natural organic matter (NOM) comprises a diverse group of organic compounds arising

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from plant degradation and microbial metabolism1,2. It can impart taste, odor and color to water;

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react with disinfectants to produce toxic and/or carcinogenic disinfection by-products (DBPs)3–5;

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increase the required doses of coagulant, adsorbent and disinfectant for effective water treatment;

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and foul microfiltration and ultrafiltration membranes6–10. For all these reasons, NOM is a major

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target of water treatment operations.

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The most commonly used process for NOM removal is coagulation with Al- or Fe-based

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salts, in which the primary removal mechanism is adsorption onto the hydrous metal oxide

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precipitates that the coagulants generate. Coagulation works well for removing high-molecular

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weight NOM with high specific UV absorbance at 254 nm (SUVA254), but it is less effective at

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removing low-molecular weight NOM with low SUVA25411. Adsorption by activated carbon has

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also been extensively studied for NOM treatment and has been found to depend strongly on the

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volume of pores in the secondary micropore and mesopore regions (0.8–5nm)12,13. In most

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studies, activated carbon adsorption and metal oxide coagulation have been found to target

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different NOM fractions, with coagulation preferentially collecting larger molecules (>30 kDa)

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and PAC more efficient at collecting smaller ones

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combined with PAC adsorption, DOC removal over a broad range of molecular weights can be

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attained. When this approach has been applied to enhance removal of DBP precursors or mitigate

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membrane fouling, the results have often been encouraging in laboratory studies, but those in

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pilot- or full-scale applications have been mixed17–24.

13–16

. As a result, when coagulation is

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Heated aluminum oxide particles (HAOPs) are a novel adsorbent first synthesized by Kim et

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al.25 At low doses, HAOPs remove more NOM than conventional coagulants (alum and ferric

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chloride) do, but at high doses, the removal efficiency reaches a plateau, indicating that a fraction

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of the NOM cannot adsorb onto HAOPs26. Much of the NOM that is not adsorbable on HAOPs

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can adsorb onto PAC26, so applying a mixture of HAOPs and PAC might be an effective

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treatment strategy.

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NOM is frequently characterized by fractionation based on the molecules’ hydrophobicity,

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acid/base characteristics, and/or molecular weight27. By comparing the concentrations of

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different fractions before and after treatment of the whole water, or by subjecting the isolated

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fractions to various treatment steps, researchers have attempted to infer which components of

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NOM are selectively targeted by the treatments, and which ones are responsible for adverse

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impacts such as DBP formation and membrane fouling5,14,28–30. While these studies have greatly

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enhanced our understanding of the chemical nature and reactivity of NOM, they have not led to

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many practical improvements in water treatment applications. This limitation arises both because

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the molecular properties of the NOM are distributed more or less continuously across a wide

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spectrum, rather than falling neatly into the well-defined ranges implied by the fractionation

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procedure, and because treatment processes are similarly imprecise in the types of molecules that

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they remove. As a result, designing and operating a practical treatment process that selectively

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targets a particular subset of the NOM molecules has proven extremely challenging.

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Rather than focusing on the chemical distinctions among different NOM fractions, some

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researchers have used adsorption directly to characterize the fractions31–34. In this approach, the

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NOM is represented as a mixture of several “fictive” components that compete for the available

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surface sites. The components are assigned adsorptive properties that, when considered in

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conjunction with a specific model for competitive adsorption, cause the overall NOM adsorption

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computed using the model to mimic the experimental findings, without ascribing any particular

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molecular properties to the different components. The research presented here evaluated the

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removal of NOM from water by mixtures of HAOPs and PAC and modeled the results in terms

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of fictive NOM components. The work yielded unexpected results and provided new insights

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into competitive adsorption in systems containing not only multiple adsorbates, but also multiple

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adsorbents.

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2.

Materials and methods

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Freshwater for the tests was collected from Lake Union (LU) in Seattle, WA. The water

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contained 2.4±0.3 mg/l DOC and had UV254 of 0.057±0.03 cm–1; these parameters were closely

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correlated with one another in the subsequent adsorption tests, and both were used as indicators

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of NOM concentration. The pH of the samples was adjusted to 7.0±0.05, and 0.5 mM each of

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NaCl and NaHCO3 was added to increase the ionic strength and buffer capacity. HAOPs were

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synthesized following the approach of Kim et al.25, and Norit SA SUPER PAC was purchased

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commercially (Cabot Co. Boston, MA). Some key properties of the adsorbents are provided in

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Table SI-1 of the Supplementary Information (SI).

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Batch adsorption was investigated by adding either one or both adsorbents to flasks

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containing the test water and mixing the suspensions on a rotary shaker for two hours. This

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mixing time was chosen both because it is at the upper end of exposure times for adsorption at

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water treatment plants and because, as shown in Figure SI-1, the rate of adsorption onto both

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adsorbents was very low at the end of that period. The solids were then removed by filtration

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through 0.45-µm filters and analyzed for DOC and UV254. UV254 was measured with a dual-

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beam Lambda-18 spectrophotometer (Perkin-Elmer Gmbh., Überlingen Germany) using a 1-cm

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quartz cell, and DOC was determined with a Sievers 900 TOC analyzer (GE Analytical

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Instruments, Boulder, CO).

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3.

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Batch adsorption of NOM

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Results and discussion

The removal of DOC by PAC and HAOPs in single-adsorbent systems is characterized in Figure 1. PAC removed more DOC than HAOPs did at all the adsorbent doses investigated.

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Because DOC removal by HAOPs approached a plateau at high adsorbent doses, adsorption

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in that system was modeled by treating the NOM as comprising two fictive components – one

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component (A) that is adsorbable on HAOPs and accounts for 45% of the DOC, and a second

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component (B) that cannot adsorb on HAOPs and accounts for the remaining 55%. With that

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assumption, the conceptual model corresponds to the model that leads to the classical Langmuir

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isotherm for a system containing a single adsorbate that binds to a single type of adsorbent site

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(Eqn.1):

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qH,A =

qmax,H K H,A cA

(1)

1 + K H,A cA

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where qH,A is the equilibrium adsorption density (mg DOC/g) of NOM (fictive) component A on

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HAOPs (‘H’), qmax,H is the maximum adsorption density on HAOPs, KH,A is the adsorptive

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equilibrium constant for binding of A to HAOPs, and cA is the dissolved concentration of A at

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equilibrium. The data could be fit very well by this isotherm equation, as shown in Figure 1.

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Adsorption of the NOM onto PAC was then modeled using the same two fictive

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components and, again, only one type of surface site. In this case, however, both the A and B

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components of the NOM were assumed to adsorb, albeit with different affinities for the surface;

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that is, the two fictive components competed with one another for the surface sites, so adsorption

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was modeled using the two-component competitive Langmuir isotherm (Eq.2a,b):

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qP,A =

qmax,P K P,A cA

(2a)

1 + K P,A cA + K P,BcB

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qP,B =

qmax,P K P,BcB

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(2b)

1 + K P,A cA + K P,BcB

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This modeling approach again yielded a very good fit to the experimental data. However, two

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sets of Langmuir parameters fit the data equally well – one with NOM component A adsorbing

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more strongly than component B (KP,A>KP,B), and one with the opposite order of affinities

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(KP,AKH,B (=0), if KP,A>KP,B, the NOM

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component that binds more strongly to HAOPs also binds more strongly to PAC (maximum

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overlap), and if KP,AKP,B) 0.876 0.065 0.221

HAOPs 1.140 0.000 0.185

PAC (KP,A