Experimental Evaluation of Turbidity Impact on the Fluence Rate

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Experimental evaluation of turbidity impact on the fluence rate distribution in a UV reactor using a micro-fluorescent silica detector Mengkai Li, Wentao Li, Dong Wen, Zhimin Qiang, and Ernest R. Blatchley Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02730 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 26, 2017

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Page 1 of 30

Environmental Science & Technology Submission to Environ. Sci. Technol.

by Li et al.

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Experimental evaluation of turbidity impact on the fluence rate

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distribution in a UV reactor using a micro-fluorescent silica

3

detector

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Mengkai Li †,‡, Wentao Li ‡, Dong Wen ‡, Zhimin Qiang ‡,*, Ernest R. Blatchley III †,§,*

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†

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United States

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‡

Lyles School of Civil Engineering, Purdue University, West Lafayette, Indiana 47907,

Key Laboratory of Drinking Water Science and Technology, Research Center for

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Eco-Environmental Sciences, University of Chinese Academy of Sciences, Chinese

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Academy of Sciences, 18 Shuang-qing Road, Beijing 100085, China

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§

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Lafayette, Indiana 47907, United States

Division of Environmental & Ecological Engineering, Purdue University, West

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*Corresponding authors.

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*

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Blatchley III)

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*Phone: +86 10 62849632; fax: +86 10 62923541; e-mail: [email protected] (Z.

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Qiang)

Phone: 1-765-494-0316; fax: 1-765-494-0395; e-mail: [email protected] (E.R.

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1

ACS Paragon Plus Environment


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ABSTRACT: Turbidity is a common parameter used to assess particle concentration

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in water using visible light.

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scattering, refraction, and reflection) in influencing the optical properties of aqueous

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suspensions complicates examinations of their effects on ultraviolet (UV)

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photoreactor performance.

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in a photoreactor containing various particle suspensions (SiO2, MgO, and TiO2) were

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measured using a micro-fluorescent silica detector (MFSD).

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particles, as well as transmittance and scattering properties of the suspensions were

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characterized at UV, visible, and infrared (IR) wavelengths.

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measurements indicated that the optical properties of all three particle types were

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similar at visible and IR wavelengths, but obvious differences were evident in the UV

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range.

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suspensions, the weighted average FR (WAFR) increased relative to deionized water.

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These increases were attributed to low particle photon absorption and strong

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scattering.

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TiO2 suspensions because of their high particle photon absorption and low scattering

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potential.

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transmittance at UV wavelengths can be used to quantify the effects of turbidity on

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UV FR distributions.

However, the fact that particles play multiple roles (e.g.,

To address this issue, UV fluence rate (FR) distributions

Reflectance of solid

The results of these

The FR results indicated that for turbidity associated with SiO2 and MgO

In contrast, the WAFR values decreased with increasing turbidity for

The findings also indicate that measurements of scattering and

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Key words: micro-fluorescent silica detector (MFSD); turbidity; scattering; reflection;

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fluence rate.

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by Li et al.

■ INTRODUCTION

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In recent years, ultraviolet (UV) technology has been used increasingly for

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disinfection, photolysis, and advanced oxidation processes in water and wastewater

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treatment.1, 2

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delivery, in particular the fluence distribution.

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applied to calculate fluence delivery, generally based on integrated application of

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sub-models to simulate the optical field (i.e., fluence rate (FR) distribution) and fluid

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mechanics, usually by application of computational fluid dynamics software.3, 4

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important component of accurate simulations of fluence delivery is an accurate

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description of the FR distribution.5–7

The performance of UV photoreactors is governed by fluence (dose) Numerical models have been widely

An

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Several mathematical models have been developed for simulation of FR

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distributions in UV reactors, including the multiple point source summation (MPSS),8

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line source integration (LSI),9 and extense source with volumetric emission (ESVE)

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models.10 Meanwhile, hardware and analytical methods to measure (local) FR have

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been developed to validate optical field models.11,12

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effective for quantification of the effects of dissolved compounds that may absorb UV

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radiation.13, 14 The fundamental optical behavior of these dissolved compounds (i.e.,

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absorbance) can be accurately measured using a spectrophotometer based on the

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Lambert-Beer Law.

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In general, FR models are

When UV radiation is imposed on particle suspensions, photon absorption and

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scattering (or reflection) by suspended particles will occur.

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behavior is difficult to simulate using existing models, and has not been examined in a

3


This complex optical



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systematic manner, largely because of the dispersed nature of the suspended particles.

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In fact, absolute scattering does not induce photon energy consumption; scattered

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photons remain available for disinfection or photolysis, while absorbed photons are no

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longer available for reactions.15, 16 Hence the Lambert-Beer law is insufficient for

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FR distribution simulation in the presence of particle suspensions, and measurements

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based on standard spectrophotometers do not allow independent characterization of

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the optical behavior of suspensions. Therefore, in the absence of numerical methods

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to account for the optical effects of suspended particles, experimental measurements

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of the FR field could be used to provide direct evidence of the effects of particles.

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Turbidity measurements are commonly used to characterize the particle

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concentration in water.17,18

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generally based on visible radiation, this commonly applied optical parameter has also

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been used to inform water quality assessments that are applied for performance

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analysis of UV photoreactors.

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measured using an electronic turbidity meter, usually expressed in nephelometric

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turbidity units (NTUs).

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approved by the United States Environmental Protection Agency (USEPA) and the

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International Organization for Standardization (ISO), including EPA Method 180.1,

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ISO7027, Great Lakes Instrument Method 2 (GLI 2), Hach Method 10133, etc.

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These methods are based on nephelometry, wherein a visible light beam and a

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photodetector are positioned to allow measurement of visible light scattered

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perpendicular to the incident beam.

Despite the fact that turbidity measurements are

At present, turbidity in water treatment is usually

Multiple turbidity measurement methods have been

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by Li et al.

To date, most studies of the effects of turbidity on UV FR fields have involved

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empirical investigations based on bench-scale reactors.

Passantino reported that clay

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turbidity up to 12 NTU does not have an obvious effect on MS2 inactivation.20

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Amoah found that UV inactivation of C. parvum and Giardia cysts was not impacted

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by 10 NTU of natural turbidity.21

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suspended particles have different inhibition impacts on the performance of MS2

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virus inactivation by UV.22

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sludge floc particles can protect MS2 coliphage and bacteriophage T4 from UV

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inactivation.23

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spherical actinometry and natural particles.24

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could yield increases in local FR, depending on the nature of the particles.

However, Malley observed that different

Templeton demonstrated that humic acid and activated

Mamane et al. examined the effects of turbidity on the FR field using Their work demonstrated that particles

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A micro fluorescent silica detector (MFSD) was developed for in-situ

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measurement of FR distributions in UV photoreactors in our previous research.12, 25

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The key features of this detector include 360-degree response, high stability, fast

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response, water-resistance, and small volume; these features have allowed it to be

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used for the examination of a wide range of optical characteristics of UV

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photoreactors, including the effect of inner-wall reflection,25 polychromatic lamp

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reactor behavior,14 and multi-lamp reactor behavior.26

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hypothesized that this detector would be suitable for the examination of the turbidity

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effect in UV photoreactors.

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impact of turbidity on the FR distribution in a UV reactor.

Therefore, it was

This work aimed to clarify, by use of the MFSD, the

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Environmental Science & Technology

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Submission to Environ. Sci. Technol.

by Li et al.

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A second hypothesis was that measurements of reflectance and scattering at the

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characteristic wavelength of the UV source would yield better indications of the

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effects of suspended particles on the FR field than traditional measurements of

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turbidity.

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namely SiO2, MgO, and TiO2) with a range of reflection coefficients were examined.

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The optical properties of the solid particles and suspension were characterized

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independently, and then UV FR distributions in various particle suspensions at various

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turbidities were measured.

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of suspended particles on the FR field of a UV photoreactor.

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promise for practical examination and monitoring of the particle effect on UV

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photoreactors used in water treatment.

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■ EXPERIMENTAL SECTION

With this in mind, three different substituted oxide powders (SOPs,

This study provides experimental evidence of the effects The results suggest

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Chemicals and analytical methods. Three SOPs, namely TiO2 (R902, DuPont

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Co., particle diameter (PD) ca. 400 nm, USA), SiO2 (Cabot M-5, PD ca. 300 nm,

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USA), and MgO (Sinopharm Chemical Reagent Co., PD ca. 1000 nm, China), were

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used to prepare particle suspensions with various turbidities.

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coefficients of the solid SOPs were measured at wavelengths of 254, 600, and 800 nm

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using a spectrophotometer (U-3900, Hitachi, Japan) equipped with an integrating

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sphere attachment; reported reflectance values were the sum of specular and diffuse

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measurements.

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individually for the reflection coefficient measurement.27

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reflection coefficients of the MgO powder than that of the initial standard plate of this

The reflection

Powders of all three SOPs were compressed into tablet mold

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Because of higher

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by Li et al.

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spectrophotometer (i.e., aluminum-evaporated plane) in the wavelength range of 240–

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280 nm, MgO powder was used as the standard plate in this study.

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reported reflection coefficients are relative to those of MgO.

Therefore,

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Aqueous suspensions of SOPs were prepared with deionized (DI) water.

A

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HACH 2100N turbidity meter was employed to measure the turbidity of aqueous

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particle suspensions.

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the range of 0−1000 NTU.

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halogen-filled tungsten filament lamp (emission spectrum spanned the wavelength

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range 400−1800 nm) to meet the reporting requirements of EPA Method 180.1. This

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method is based on the principles of nephelometry, and the photodetector, whose

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response spectral peak was reported to be between 400−600 nm, received light

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scattered perpendicular to the incident light path.

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meter did not allow measurements based on radiation in the UV portion of the

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electromagnetic spectrum, a fluorescence spectrophotometer (Cary Eclipse, Varian,

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Australia) with a 1 cm path-length cell was utilized to investigate the scattering

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behavior of each SOP suspension at specific wavelengths in the UVC (254 nm),

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visible (600 nm), and IR (800 nm) ranges.

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with the turbidity meter, the excitation and emission beams are perpendicular to each

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other.

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set to the same value.

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represent the scattering property of the test suspension at each of the chosen

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wavelengths.

Precision of 0.01 NTU was reported by the manufacturer for The turbidity meter was equipped with a stable

Additionally, because the turbidity

In the fluorescence spectrophotometer, as

For these measurements, the excitation and emission wavelengths were both Hence, the receiving “fluorescence” signal was used to

Note that in common spectrofluorometric applications, the emission

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wavelength is set to a longer value than the excitation wavelength, hence the excited

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fluorescent signal of the suspension cannot interfere the scattering property test.

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addition, absorbance at each of the three characteristic wavelengths (i.e., 254 nm, 600

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nm, and 800 nm) of each SOP suspension was measured with a spectrometer

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(UV2600, Shimadzu, Japan) in a 1 cm path-length cell.

In

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FR Distribution Measurement platform. Figure 1 represents a schematic

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diagram of the experimental platform for measuring the FR distributions in a

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single-lamp UV photoreactor.

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outside diameter), whose fluorescence was received by a BPW 34 B silicon PIN

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photodiode, and amplified and displayed by a multimeter, was used as the FR

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measurement detector in this study.

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diameter = 190 mm and inner length = 1000 mm) wrapped in a black cotton cloth was

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used; UV reflectance from the air/quartz/air interfaces of this reactor had previously

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been estimated at approximately 8%.24 A single low pressure mercury Hg lamp (GL

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Type, Xiashi Wanhua Co., China) with a quartz sleeve (radius = 11.5 mm) was housed

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in the center of the cylindrical quartz photoreactor.

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length = 297 mm; total output power 16 W; UVC efficiency at 254 nm = 26%.

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A cylindrical MFSD (1.0 mm length × 0.3 mm

A cylindrical quartz-wall UV reactor (inner

The lamp parameters were: arc

The MFSD was fixed onto a two-dimensional guideway with its vertical axis

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oriented parallel to the lamp axis (Figure 1).

This configuration allowed the detector

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to move either parallel (displacement precision of 1 mm) or perpendicular

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(displacement precision of 0.01 mm) to the lamp axis to measure the axial or radial

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FR distribution, respectively.

The calibration process of the MFSD at 254 nm has

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been described previously.12

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inserted into the gap between the lamp and the sleeve to monitor lamp output

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fluctuations.

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0.5ºC.

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A second MFSD, used as a reference detector, was

Water temperature was controlled by a recirculator with precision of

FR Distribution Measurement process.

The FR distributions in the reactor

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were measured for each of several particle suspensions.

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suspensions of SiO2 and TiO2, the SiO2 powder was first added to reach the turbidity

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requirement (e.g., 5.0 NTU for the SiO250%-TiO250% case with 10.0 NTU) and then

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the TiO2 powder was added to reach the final turbidity requirement (e.g., 10.0 NTU

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for the SiO250%-TiO250% case with 10.0 NTU).

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Note that for mixed

For each suspension, the test MFSD was placed parallel to the UV lamp at radial

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distances of 5.00 and 60.00 mm away from the quartz sleeve.

For each fixed radial

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distance, the detector was moved parallel to the lamp axis to measure the axial FR

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distribution in the UV reactor.

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lamp axis at the central cross-section of the lamp to measure the radial FR distribution.

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The detector was maintained at each test point for one minute to allow a data

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acquisition switch unit (34971A, Agilent Co., USA) to collect 30 readings from the

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test MFSD.

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The final result for each test location was divided by the corresponding reading of the

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reference detector to correct any potential error induced by the lamp output

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fluctuation.

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■ RESULTS AND DISCUSSION

Then the detector was moved perpendicular to the

For each location, the last ten readings were recorded and averaged.

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by Li et al.

Reflection coefficients of various solid particles.

Table 1 lists the reflection

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coefficients in the UVC region (i.e., 254 nm) and visible/IR region (i.e., 600 and 800

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nm) of the three solid SOPs (i.e., MgO, TiO2 and SiO2) that were compressed into

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plates.

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reflection coefficients for this material at all wavelengths were reported as 1.000.

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the visible/IR light region, all three SOPs displayed similar and high reflectance,

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while at 254 nm the reflection coefficient of TiO2 was substantially lower than other

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two SOPs.

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behavior in the visible/IR region; however, the low UV reflection coefficient of TiO2

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may be expected to yield a different UV FR distribution as compared to the other two

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SOP suspensions.

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study were pigment grade, which often contain other metal oxides (e.g., SiO2, Al2O2,

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ZrO2) as surface modifications.

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on the optical behavior of TiO2 particles.

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TiO2 particles may be different than those of other TiO2 particles.

211 212

Because MgO was used as the reference standard for reflection in this study, In

These results indicate that the all three SOPs may have similar optical

It should be noted that the TiO2 particles selected for use in this

These surface impurities can have profound effects As such, the optical characteristics of these

Table 1 Scattering properties of various suspensions.

Measurements of UVT values

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at 600 nm as a function of the turbidity for various SOP suspensions are shown in

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Figure 2a. Because of their similar reflection coefficients at 600 nm, all three SOPs

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displayed similar relationships between UVT600 and turbidity.

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suspensions did not display the same trends for UVT254 vs. NTU (Figure 2b) and

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scattering intensity (SI) at 254 nm, measured by the fluorescent spectrometer and

10


However, the particle

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by Li et al.

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defined by the strength of the signal scattered at an angle perpendicular to the incident

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beam (Figure 2c).

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suspension; therefore, as turbidity increases it is reasonable to expect a decrease in

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UVT254 and a corresponding increase in SI.

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suspensions, the SiO2 suspension demonstrated the fastest decrease of UVT254 and the

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fastest increase of SI, while the TiO2 suspension demonstrated opposite trends.

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These results suggest that these two parameters are complementary.

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measurements in the spectrometer and fluorescent spectrometer, incident UV photons

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will be scattered by an SOP suspension in the test cell, which would decrease the rate

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at which photons are received by the sensor oriented in the incident direction (i.e.,

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UVT254) and increase the rate at which photons are received by the sensor oriented

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perpendicular to the incident direction (i.e., SI).

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could be used to evaluate the scattering property of the suspension, and in this study

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the SiO2 and TiO2 suspensions displayed the strongest and weakest scattering ability,

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respectively.

A high particle concentration translates to high turbidity of a

However, among the three SOP

For the

Therefore, these two parameters

Figure 2

233 234

Note that the results of UVT254 and SI254 for the suspensions did not absolutely

235

agree with the relative reflection coefficient measurements reported at 254 nm in

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Table 1.

237

behavior of particles will depend on several factors, including particle size, particle

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shape, particle concentration, and chemical composition.

239

data in presented in Table 1 only express the effects of chemical composition.

We hypothesize that this is attributable to the fact that the scattering

11


The reflection coefficient By



240

contrast, the scattering property test (i.e., Figures 2b and 2c) is an integral

241

characterization for the suspension, providing a measure of optical (scattering)

242

behavior that accounts for all of the factors listed above.

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Radial FR distribution in various particle suspensions.

Figure 3 illustrates

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the measured radial FR distributions in the central cross-section of the UV reactor

245

filled with DI water, as well as the SiO2, TiO2, and MgO suspensions with various

246

turbidities. When the test MFSD was moved from 10 to 70 mm from the sleeve

247

surface, the FR decreased for all conditions.

248

3a), the local measurements of FR for all values of turbidity were higher than the

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corresponding measurements in DI water.

250

also increased.

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or redirect UV photons in various directions, thereby allowing photons to travel along

252

a longer optical path than they would in the absence of these particles. In theory, this

253

provides more opportunities for photons to interact with photochemical or

254

photophysical targets in the system, such as a suspended microorganisms, absorbing

255

dissolved chemicals, or an MFSD detector.

256

reflective particles could promote photoreactor efficiency.

257

reported previously with an aluminum clay suspension.23

258

For the SiO2 suspension (i.e., Figure

As turbidity increased, local FR values

These results imply that SiO2 particles in water will strongly reflect

Therefore, the presence of suspended, Similar results have been

Figure 3

259

Similar results were also observed with the MgO suspensions shown in Figure 3b.

260

The FR distribution in the MgO suspension increased with increasing turbidity and

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was consistently higher than with DI water.

However, for the TiO2 suspensions, a

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consistent increase of FR, relative to DI water, was evident only for the 2.0 NTU

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suspension.

264

locations more distant from the lamp.

265

the FR distribution dropped.

266

absorption at 254 nm (i.e., low refection coefficient) and low scattering ability of the

267

TiO2 particles, as compared to the other two SOPs.

268

reactor wall, photons had more opportunities to be absorbed by the TiO2 particles and

269

were less likely to be scattered than with the other two particle suspensions.

270

FR enhancement was more evident in the near-lamp region than for As turbidity increased from 2.0 to 20.0 NTU,

This was consistent with the relatively strong

As the MFSD approached the

Axial FR distribution in various particle suspensions.

The axial FR

271

distributions in the far-lamp region of the test UV reactor (60 mm from the sleeve

272

surface) for each SOP suspension are illustrated in Figure 4.

273

the MFSD was moved from the lamp center to the lamp end.

274

region, the FR values of all three SOPs were similar for conditions of turbidity of 2.0

275

NTU; at this relatively low value of turbidity, the effects of suspended particles on the

276

FR field were limited.

277

the SiO2 and MgO suspensions were higher than those of the TiO2 suspension at the

278

same turbidity.

279

scattering behavior of the particles (Figures 2b and 2c).

280

the near-lamp region (5 mm from the sleeve surface) for each particle suspension are

281

shown in Figure S1.

282

were similar in all turbidity cases.

283

The FR decreased as In the central lamp

For turbidities of 10.0 NTU and 20.0 NTU, the FR values for

These results were also consistent with the previously described The axial FR distributions in

Because of the short path-length, FR values of all three SOPs

Figure 4

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Mixed particle suspensions.


The particles in practical waters come principally

285

from soil (erosion).18, 19 The composition of natural particles is variable and depends

286

largely on local geology and land use practices.

287

comprise mixtures of silica and metal oxides, as well as organic materials.

288

for the practical applications of UV photoreactors, it is relevant to examine the

289

behavior of mixed particle suspensions involving mixtures of highly-reflective (e.g.,

290

SiO2) and minimally-reflective (TiO2) particles.

291

suspensions of TiO2 and SiO2 particles were prepared.

292

mixtures of SiO2 and TiO2 with turbidity of 10.0 NTU are shown in Figure 5.

293

Turbidity contribution ratios of SiO2 to TiO2 were 7:3, 5:5, and 3:7. Because of the

294

stronger scattering property of SiO2 as compared to that of TiO2, the FR distribution

295

of the suspension with SiO270%–TiO230% was higher than that of suspension with

296

SiO230%–TiO270%, and the FR distribution of the suspension with SiO250%–TiO250%

297

was intermediate to the other two suspensions. Similar results were observed in the

298

suspensions with turbidity of 20.0 NTU, as shown in Figure S2.

299 300

Natural particle suspensions likely Hence,

To address this issue, mixed Radial FR distributions for

Figure 5 Weighted average FR and minimum FR.

Tables 2 and S1 illustrate the

301

weighted average FR (WAFR) values and the FR values at 70 mm distance to the

302

sleeve surface in the central cross-section (regarded as the minimum FR) for DI water,

303

as well as for the SiO2, TiO2, and MgO suspensions with various turbidity values.

304

The WAFR values for the SiO2 suspensions with turbidity values of 2.0, 10.0, and

305

20.0 NTU were 1.05, 1.10, and 1.34 times higher than those of DI water, respectively.

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Similarly, the minimum FR values for the SiO2 suspensions with turbidity values of

307

2.0, 10.0 and 20.0 NTU were 1.02, 1.12, and 1.29 times higher than those in DI water,

308

respectively.

309

end of the dose distribution), which limits the performance of UV disinfection.28

The minimum FR will be related to the minimum dose (or low-dose

310

The results reported herein, which were developed through the use of the MFSD,

311

demonstrate that the presence of reflective suspended particles (i.e., particles that

312

scatter UV photons) can actually enhance the FR field and the overall performance of

313

a photoreactor.

314

This seems contrary to the conventional view of UV photoreactors in the sense that

315

turbidity is commonly regarded as an inhibiting factor for performance of these

316

systems.

317

the reactor.23

A previous study also demonstrated this enhancement effect.23

In fact, the reflection by the particles could extend the photon pathlength in

318

The low photon energy loss during the absorption/reflection process on SiO2

319

particles (high UV reflection coefficient) could reduce interactions of UV photons

320

with the reactor wall where they would be absorbed or inefficiently reflected, thereby

321

enhancing WAFR in the reactor.

322

can shield microbes from UV exposure, thereby counteracting the benefits described

323

above.

324

of UV disinfection systems can be quite complex.20–23, 29

325

Note that for UV disinfection, suspended particles

Therefore, the effects of suspended particles (turbidity) on the performance

Table 2

326

The WAFR values and minimum FR values for the MgO suspensions increased

327

with increasing turbidity, and all WAFR values and minimum FR values for the MgO

15




328

suspensions were higher than those for DI water.

For each turbidity condition (i.e.,

329

2.0, 10.0, or 20.0 NTU), because the MgO particles were less effective at scattering

330

UV254 radiation than SiO2 particles (Figure 2c), its WAFR value and minimum FR

331

values were lower than the corresponding values for the SiO2 suspension.

332

Calculated values of WAFR for the TiO2 suspensions dropped from 6.278 to

333

3.979 mW cm-2 as turbidity increased from 2.0–10.0 NTU. Meanwhile, the minimum

334

FR value decreased from 4.01 to 1.95 mW cm-2 in the turbidity range of 2.0–10.0

335

NTU.

336

that of DI water.

337

highest transmittance (Figure 2b) and the lowest scattering at 254 nm (Figure 2c).

338

Collectively, these attributes increase the likelihood of UV254 photons being incident

339

on and absorbed by the reactor wall.

340

TiO2 powder at 254 nm (i.e., 0.135) implies that a large fraction of the UV photons

341

were absorbed by the TiO2 particles in suspension.

342

MgO suspensions, an inhibition effect of TiO2 particles was observed.

343

At turbidity values of 10.0 and 20.0 NTU, the WAFR values were lower than At any given value of turbidity, the TiO2 suspensions had the

Moreover, the low reflection coefficient of

Therefore, unlike the SiO2 and

Table 2 also illustrates WAFR calculations for mixed suspensions with various

344

ratios of SiO2 to TiO2.

At a fixed value of turbidity, WAFR increased with SiO2

345

content. Moreover, as turbidity increased, the WAFR values of all three mixed

346

suspensions decreased, and the suspension with the highest TiO2 ratio demonstrated

347

the most rapid decrease.

348

SiO2 and TiO2 at fixed turbidity (5.0 NTU and 10.0 NTU) were greater than based on

349

TiO2 alone, and lower than those of SiO2 alone.

In addition, all WAFR values of the mixed suspensions of

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Discussion.

by Li et al.

This study illustrated the impact of turbidity, a conventional water

351

parameter used for characterizing the concentration of suspended particles in water,

352

on the FR field of a simple UV photoreactor.

353

optical properties of suspended particles can induce the FR distribution in a UV

354

reactor at an identical turbidity value.

355

measurements of turbidity involve visible radiation, whereas the optical properties of

356

particles in the UVC range are perhaps more relevant to the behavior of a UV

357

photoreactor.

358

differences in optical behavior in the visible and UVC ranges.

359

conventional turbidity measurements (based on visible light) are inappropriate for

360

characterization of the effects of suspended particles on the FR field.

361

measurements of reflectance and scattering at the characteristic wavelength of the UV

362

source are more representative of the effects of suspended particles on the FR field.

The results demonstrate that several

This is explained by the fact that conventional

Moreover, differences in particle composition are associated with Therefore,

As alternatives,

363

The measurements reported herein demonstrate that the optical field (FR

364

distribution or WAFR) was principally dependent on the photon scattering ability of

365

the suspension, which corresponded with measurements of photon absorption and

366

reflection by the solid powders in the suspension (reflection coefficient), as well as

367

other factors such as the particle size and shape.29

368

turbidity, particles that absorb poorly in a UVC wavelength range (e.g., SiO2 and

369

MgO) could enhance the FR distribution and performance of a photoreactor.

370

Conversely, for particles that absorb strongly in UVC range (e.g., TiO2 ), their

371

presence in suspension could diminish the FR distribution.

For suspensions with identical

17


Meanwhile, the



372

turbidity value could represent the particle concentration in the suspensions, implying

373

that the increasing turbidity could amplify the effect of the particles on the FR

374

distribution in the reactor, including positive effects (e.g., SiO2 and MgO), and

375

negative effects (e.g., TiO2).

376

The effects of suspended particles on the optical behavior of UV radiation were

377

also characterized by measurements of transmittance and scattering intensity

378

developed using a spectrometer and a fluorescence spectrometer, respectively.

379

two tests have the potential to be used as characterization methods for evaluating the

380

effects of turbidity on FR distributions.

381

■ ASSOCIATED CONTENT

382

Supporting Information.

383

distributions in the near-lamp region (5 mm from the sleeve surface) with various

384

particle suspensions with various turbidities (Figure S1), radial FR distributions with

385

mixed particle suspensions with various turbidities of 20.0 NTU (Figure S2), and

386

minimum FR values (Table S1)

387

■ AUTHOR INFORMATION

388

Corresponding Authors

389

*

390

Blatchley III)

391

*Phone: +86 10 62849632; fax: +86 10 62923541; e-mail: [email protected] (Z.

392

Qiang).

These

Two figures and one table are provided. Axial FR

Phone: 1-765-494-0316; fax: 1-765-494-0395; e-mail: [email protected] (E.R.

18


Page 19 of 30


by Li et al.

393

Notes

394

The authors declare no competing financial interest.

395

■ ACKNOWLEDGEMENTS

396

The authors gratefully acknowledge the financial support from the National

397

Natural Science Foundation of China (51408592, 21590814, 51525806), Edward M.

398

Curtis Visiting Professorship of Purdue University, and National Geographic Air and

399

Water Conservation Fund (GERC23-15).

400

■ REFERENCES

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wastewater by solar driven and UV lamp - TiO(2) photocatalysis: Effect on a multi

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drug resistant Escherichia coli strain. Water Res. 2014, 53, 145–52.

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halogenated organic byproducts during medium-pressure UV and chlorine coexposure

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of model compounds, NOM and bromide. Water Res. 2011, 45, (19), 6545–54.

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Page 23 of 30


Table 1. wavelengths.

by Li et al.

Relative reflection coefficients of various materials at different At each wavelength, the reflection coefficient was normalized

relative to the value recorded for MgO. Wavelength (nm)

MgO

TiO2

SiO2

254

1.000

0.135

0.849

600

1.000

0.929

0.947

800

1.000

0.916

0.954

23



Page 24 of 30


Table 2.

by Li et al.

Weighted average fluence rates for various particle suspensions. Weighted average fluence rate (mW cm-2)

Suspensions 0 NTU

2.0 NTU

10.0 NTU

20.0 NTU

SiO2

6.120

6.386

7.784

TiO2

6.278

4.870

3.979

MgO

5.973

6.244

6.454

5.0 NTU

10.0 NTU

20.0 NTU

SiO230%–TiO270%

5.911

4.890

3.996

SiO250%–TiO250%

N/A

5.111

4.831

SiO270%–TiO230%

5.985

5.314

5.213

Deionized water

5.824

24


Page 25 of 30


by Li et al.

Figure 1. Schematic diagram of the experimental platform for measuring the FR distributions in a UV photoreactor.

25



Page 26 of 30


by Li et al.

UVT600

1.1

(a)

1.0 0.9

SiO2

0.8

TiO2

0.7

MgO

UVT254

0.6 1.1

(b)

1.0 0.9 0.8 0.7

SI (A.U.)

0.6

(c)

600 400 200 0 0

10

20

30

40

50

Turbidity (NTU) Figure 2. UVT at 600 nm (a), UVT at 254 nm (b), and scattering intensity (SI) at 254 nm (c) as a function of turbidity.

All tests were repeated three times and all values of

relative standard derivation were lower than 10%.

26


Page 27 of 30


by Li et al.

DI water 2.0 NTU 10.0 NTU 20.0 NTU

20 16 12

(a)

Fluence rate (mW cm-2)

8 4 0 20

(b)

16 12 8 4 0 20

(c)

16 12 8 4 0 0

10

20

30

40

50

60

70

80

Distance from the lamp center (mm)

Figure 3. Radial FR distributions with various turbidities in (a) SiO2 suspension, (b) TiO2 suspension, and (c) MgO suspension.

27



Page 28 of 30


by Li et al.

20

(a)

SiO2

16

TiO2

12

MgO

8 -2

Fluence rate (mW cm )

4 0 20

(b)

16 12 8 4 0 20

(c)

16 12 8 4 0 0

3

6

9

12

15

Distance from the lamp center (mm)

Figure 4. Axial FR distributions in the far-lamp region (60 mm from the sleeve surface) in various particle suspensions with various turbidities of (a) 2.0 NTU, (b) 10.0 NTU, and (c) 20.0 NTU.

28


Page 29 of 30


by Li et al.

20

-2

Fluence rate (mW cm )

18 SiO2 70%-TiO2 30% SiO2 50%-TiO2 50% SiO2 30%-TiO2 70%

16 14 12 10 8 6 4 2 0

10

20

30

40

50

60

70

80

Distance from the sleeve surface (mm)

Figure 5. Radial FR distributions in mixed particle suspensions with turbidity of 10.0 NTU.

29



Table of Contents 211x198mm (72 x 72 DPI)


Page 30 of 30

Experimental Evaluation of Turbidity Impact on the Fluence Rate

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