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Remediation and Control Technologies

Online, Continuous, and Interference-Free Monitoring of Trace Heavy Metals in Water Using Plasma Spectroscopy Driven by Actively Modulated Pulsed Power Ching-Yu Wang, and Cheng-Che Hsu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b02970 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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Environmental Science & Technology

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Online, Continuous, and Interference-Free

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Monitoring of Trace Heavy Metals in Water Using

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Plasma Spectroscopy Driven by Actively Modulated

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Pulsed Power

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Ching-Yu Wang and Cheng-Che Hsu

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Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan

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E-mail: [email protected]

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KEYWORDS: Online monitoring, continuous monitoring, heavy metals detection,

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plasma in aqueous solution, plasma-liquid interaction, optical emission spectroscopy,

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and solution conductivity.

ACS Paragon Plus Environment

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ABSTRACT

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This work presents the development of an online continuous heavy metals monitoring

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system using optical emission spectroscopy of plasma in water. The plasmas were driven

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by actively modulated pulsed power (AMPP) in order to control the plasma and its

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emission behavior in solutions with a wide range of conductivity. The AMPP quantified in

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situ the solutions’ conductivity and modulated in real time the pulse width based on the

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conductivity. We demonstrated the online monitoring of the metallic elements. The results

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show that multiple metallic elements, namely Pb and Zn, can be independently and

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simultaneously detected with less than a 10% variation in the corresponding optical

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emission lines in solutions with a wide range of conductivity. An alert system was

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integrated to demonstrate the capability of an instant warning via e-mail once metallic


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elements were detected. Finally, we demonstrated that this system was robust even with

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the existence of several interferences and able to perform online continuous monitoring

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for days. We believe the system using plasma spectroscopy with AMPP for online

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monitoring of metals in water will have a significant impact on the fields of environmental

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monitoring and protection.

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Table of Contents 34


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1. INTRODUCTION

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Contamination of water by heavy metals has been a major public concern for

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decades because it poses adverse effects to human health1, 2. Heavy metals are

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generated from various sources, such as naturally occurring chemicals, industrial

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processes, agriculture activities, and human dwellings3-5. Upon being absorbed by human

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beings, heavy metals are difficult to decompose in the body and will lodge permanently

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in the kidneys and brain6. Chronic exposure to heavy metals can cause irreversible

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urinary and cardiovascular system diseases in adults, and retard cognitive ability and

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academic performance in children7, 8. Several lab-scale techniques for trace heavy metals

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detection have been conventionally employed, including inductively coupled plasma

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optical emission spectrometry (ICP-OES)9,

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spectrometry (ICP-MS)11,

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techniques require a bulky instrument and the sample pre-treatment and analysis process

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is time-consuming. In addition, a great number of analytical platforms have been

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developed and employed, for example, ion specific electrodes, electrochemical sensors15,

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inductively coupled plasma mass

and atomic absorption spectrometry (AAS)13,

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These


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colorimetric assay17, 18, and fluorescence spectroscopy19, 20. Although several of the

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abovementioned techniques offer advantages of high sensitivity, rapid analysis, and in

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situ monitoring, they only detect one or a few specific metallic elements at one time, and

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the test solution performance greatly depends on the compositions and properties of the

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solutions. In environmental field testing, test solutions with a wide range of pH or

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conductivity values or containing multiple inorganic and organic contaminants are

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commonly encountered. Such circumstances make it challenging to conduct quantitative

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analysis of metallic ions using the abovementioned techniques21-23. Therefore, a

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technique capable of simultaneously analyzing multiple metallic ions with zero cross-

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species interference is highly desired.

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Plasmas in and in contact with liquid have drawn great attention recently because

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of their wide range of applications24-28. One promising application is to detect and quantify

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heavy metallic ions in water using plasma spectroscopy29, 30. This technique detects the

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optical emission emanating from the plasma for identification and quantification of species

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for various types of samples. Using this technique for detection of heavy metallic elements


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in solution offers advantages, such as the capability of simultaneous detection of multiple

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elements, very little or no cross-element interference, process flexibility, and rapid

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detection. Although widely employed, the optical emission is greatly influenced by the test

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solution properties, especially the solution conductivity31, 32. The plasma behavior in such

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systems varies with the solution conductivity because it greatly influences the impedance

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of the whole system33. In order for these techniques to analyze solution with different

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conductivities, properly rearrange the plasma system and/or sample pre-treatment such

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as adding acid or salt29, 34, 35, preconcentrating, and pre-evaporation36-38 are required. The

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rearrangement or pre-treatment making it difficult for these techniques to analyze test

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solutions with varying conductivities. This also greatly limits the applicability of such a

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technique and makes it challenging to achieve rapid and online monitoring processes of

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solutions with different conductivity. Development of a technique allowing for rapid and

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online monitoring of the heavy metallic ions of various test solutions using plasma

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spectroscopy is therefore highly desired.


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In this work, we report the development of an online heavy metals monitoring

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system using plasma spectroscopy that allows for monitoring test solutions with a wide

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range of conductivity. The literature reports that the plasma behavior is greatly influenced

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by the power type, including direct current (DC)31, 39, alternative current (AC)40, 41, and

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pulsed power42, 43. We designed an actively modulated pulsed power (AMPP) source to

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drive the plasma and perform rapid and online monitoring. The AMPP actively modulated

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the pulse width so that the plasma behavior was well controlled in real time for plasma

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ignited in solutions with a wide range of conductivities. We demonstrated the continuous

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and online monitoring of the metallic elements Pb and Zn with a concentration from 0.5

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to 250 ppm in solutions with a conductivity from 300 to 1200 S/cm, even with the

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existence of other contaminants. We also showed that such a system is robust and allows

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for continuous monitoring for days. We believe this system will serve as a platform based

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on plasma spectroscopy allowing for monitoring of metallic element contamination in

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solutions in several fields, such as in-field monitoring or pollutant prevention stemming

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for environmental monitoring.


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2. MATERIAL AND METHODS

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Sampling and Plasma Generation Unit

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Figure 1 shows an illustration of an online heavy metals monitoring system based

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on plasma spectroscopy. This system consists of a sampling and plasma generation unit,

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an AMPP source, and an optical emission spectrum (OES) acquisition unit. The sampling

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and plasma generation unit consists of a glass cell 30 mL in volume with discharge and

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grounding electrodes inside. The discharge electrode is made of a platinum wire 0.5 mm

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in diameter and is covered by a glass tube 10 mm in outside diameter to restrict the area

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in contact with the test solution. The ground electrode is a bare platinum wire with the

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same diameter. With the application of the high voltage, bubbles with sizes of mm-to sub-

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mm in diameter were generated on the discharge electrode, followed by plasma ignition

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inside the bubble, as reported in the previous study42. We noted that in this work, the

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plasma was generated without external supply of gases such as Ar. A peristaltic pump is

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used to continuously inject the solution into the glass cell with a flow rate of 100 mL/min.

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Detailed information of the unit was listed in the supporting information (See Table S1).


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Figure 1. Schematic of the online heavy metals monitoring system. The inset shows a typical voltage waveform used in single detection.

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AMPP

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The AMPP source consisted of a high-voltage DC power source (GPR-100H15,

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GW Instek), a switching device, and a 5-ohm resistor connected in series with the power

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circuit. The switching device, composed of an insulated gate bipolar transistor (IGBT)


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driven by a microcontroller (Arduino, Arduino Uno), used the electrical signal to measure

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the conductivity of the test solution and actively modulated the duration and frequency of

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pulsed voltages applied to the discharge electrode. This integrated device allowed for

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providing various voltage pulse widths and intervals from s to min and offered great

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advantages to drive the plasmas and perform long-term online monitoring. The

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microcontroller provided pulsed signals to the IGBT with a user-defined width and

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frequency to modulate the pulse voltage applied to the electrode. Prior to the application

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of the pulsed signal to ignite the plasma, a short pulse was applied, and the voltage drop

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across the 5-ohm resistor was measured to quantify the conductivity of the test solution.

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An optimal width of the voltage pulse, based on the conductivity of the test solution, was

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then applied to the discharge electrode to ignite the plasma. The optimal voltage pulse

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for various solutions’ conductivity will be shown in a later section. The inset in Figure 1

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shows a typical voltage waveform used for online heavy metals monitoring in a single

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detection. A pulsed 500-s in width was first applied to quantify the conductivity of the

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test solution. The optimal pulse width to ignite the plasma was therefore selected and set

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automatically by the microcontroller. Depending upon the conductivity of the test solution,


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repetitive pulsed voltages with widths from 0.5 to 15 ms with a frequency of 20-Hz were

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applied. The time required between each conductivity-measurement and plasma ignition

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cycle was within 50 ms, which provided decent temporal resolution for rapid and real-time

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modulation of the pulse power. For continuous online monitoring, we applied a 500-s

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pulse for conductivity quantification and 400 pulses with optimal width for plasma ignition

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in a single detection. The time required for a single detection was 20 s, and the interval

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of each detection was 5 min. We noted that the duration of the interval can be flexible,

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which potentially could provide the ability for long-term continuous monitoring.

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OES Acquisition Unit

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The OES of the plasmas ignited on the surface of the discharge electrode was

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acquired and transmitted using two lenses (f=40 mm, LA4306, Thorlabs Inc.) and a fiber

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optics (QP400-2-UV-VIS, Ocean Optics Inc.) respectively, then analyzed with an optical

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emission spectrometer (USB 2000, Ocean Optics Inc.). The spectra were recorded using a

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laptop. The integration time for spectrum acquisition was 10 s unless otherwise noted. Several

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atomic emission lines were examined throughout this work, as listed in the supporting


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information (See Table S2). These emission lines were used as they were the most

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prominent emission lines for the corresponding elements. The intensities of each element

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were quantified after the background emission was subtracted. For quantitative analysis

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of each elements, the standard deviation (SD) of the average for each concentration was

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examined. The sensitivity was defined as the slope of the calibration line. The limits of

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detection (LOD) was calculated using the equation, 3 × σ/m, where σ is the SD of the

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blank solution and m is the slope of the calibration line.

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Plasma Diagnostics

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Plasma diagnostic tools included a differential probe (DP-8V, Pintek Electronics

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Co.) and a current probe (TCPA300, Tektronix) to monitor the voltage and current

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waveforms, respectively. A photomultiplier tube (PMT) (PD-439, Princeton Instruments) was

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used to measure the time-resolved total light intensity of the plasma within a pulse. This

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PMT was not attached with any light dispersion device. The PMT signal and the waveforms were

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recorded by an oscilloscope (GDS-2104E, GW Instek). We noted that PMT converted

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incident light into the electron current, the intensities are shown in negative values.


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Chemicals

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The following chemicals were purchased from commercial suppliers (Sigma-

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Aldrich, JT Baker, Honeywell, Acros) and used as received without further purification:

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lead (II) nitrate (99%), zinc nitrate (99%), sodium nitrate (99.5%), potassium nitrate (99%),

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cupric nitrate 2.5-hydrate (98%), lithium nitrite (99%), nickel (II) nitrate hexahydrate (98%),

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methylene blue (99%), orange II (99%), ethanol (95%), and nitric acid (69.5%). All test

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solutions were prepared using deionized water. Solutions containing Pb and/or Zn with

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concentrations from 0.5 to 250 mg/L were used as the heavy metal samples. Solutions

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without the addition of Pb and Zn were used as blank samples. Sodium nitrate (NaNO3)

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and potassium nitrate (KNO3) were added to the solutions in order to adjust the

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conductivity to its anticipated values, which ranged from 300 to 1700 S/cm.

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Alert System

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An alert scheme was developed and incorporated into this monitoring system. This

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alert scheme was developed and programmed using Matlab. It continuously and real-time

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loaded and recorded spectra acquired by the spectrometer. Emission intensities at target


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wavelengths were analyzed. When the emission intensity at target wavelengths exceeded

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certain threshold values, an alert message, such as an e-mail, could be sent out from the

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laptop. This alert system allowed for generating different type of alert messages and

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analyzing spectra using different algorithms.

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3. RESULTS AND DISCUSSION

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OES and Calibration Curve

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Figure 2a shows a representative OES of the plasmas ignited in the solution

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containing 250 mg/L Pb, 20 mg/L Na, and 40 mg/L K with a conductivity of 500 S/cm

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driven by 3000-s wide pulses. It was noted that a typical range of conductivity was

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150~500 S/cm for inland freshwater and 50~1500 S/cm for rivers in the United States

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according to the Environmental Protection Agency (EPA). The integration time of the

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spectra was 500 ms unless otherwise specified. Emission lines of Hα (656.3 nm), Hβ

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(486.1 nm), O (776.8 nm), OH (306.3 nm), Na (589.6 nm), and Pb (368.1 and 405.8 nm)

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were observed. With the application of a high-voltage pulse to the discharge electrode,

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gases were generated at the surface of the electrode through Joule heating and

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electrolytic reactions. When a sufficiently high pulse width and amplitude was applied, a

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gas layer containing water vapor, H2, and O2 was formed at the electrode surface, and

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the plasma was ignited in this gas layer. Multiple reactions, such as electron impact

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ionization, dissociation, and excitation, therefore occurred in the gas layer.


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Simultaneously, the species in the solution, namely water and salts, were heated and

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vaporized by the plasma. The vaporized species were also involved in the reactions in

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the plasma. Associated optical emission lines were then observed. We noted that the

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intensities of each emission line were a complicated function of many factors, such as the

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excitation energy, concentration, boiling point, properties of solutions, etc. We observed

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that OH was the most prominent emission line in the spectra for plasma ignited in solution

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with conductivity in such a range; this is similar to other reports in the literature31, 44.

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We then performed quantitative analysis of Pb using this system. Figure 2b shows

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a calibration curve with Pb concentrations from 0.5 to 250 mg/L. The inset shows the

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curve with Pb concentrations from 0.5 to 25 mg/L. The LOD of Pb was 0.35 ppm. The

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solution conductivity was 500 S/cm. Pulses with the width of 3000 s were applied. Each

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data point showed the average of five independent measurements (n = 5). The SD was

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indicated by the error bar. The R2 value of the fitted straight line was 0.998. We also

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demonstrated quantitative analysis for multiple metallic elements in solution with a

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conductivity in the range of 500-1700 S/cm with accumulation of 1200 pulses. Based on


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the calibration lines and the blank solutions (n=5), the LODs of Cu, K, Li, Na, Ni, Pb, Zn

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were calculated, as listed in the supporting information (See Table S3). These results

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indicate that the system is able to quantify the metallic elements in water with the

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conductivity within the range of typical industrial wastewater, such as battery

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manufacturing45,

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concentrations in wastewater for battery manufacturing, metal plating, and radiator

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industry are around 10, 10, 100 ppm, respectively. K, Cu, and Ni concentrations in metal

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plating industry are around 1, 100, and 1000 ppm, respectively. We noted that several

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strategies could be used to further decrease the LOD. For example, an increase in the

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amplitude of high-voltage pulse (shown in Figure S1), an increase in the integration time

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for OES acquisition for a better signal-to-noise level, or a modification of the optical

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system, such as using a lens with a larger diameter. Also, the addition of acid helped to

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lower the LOD, as reported in the literature49, 50.

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metal plating47,

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and radiator industry47. For example, typical Pb

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Figure 2. (a) Representative OES of the solution containing 250 mg/L Pb and (b) calibration curve with Pb concentrations from 0.5 to 250 mg/L with conductivity of 500 S/cm driven by 3000-s-wide pulses. The inset shows the calibration curve for Pb concentrations from 0.5 to 25 mg/L. Note that the spectrometer integral times for spectral acquisition in the spectrum in (a) and the calibration curve in (b) are 500 ms and 10 s, respectively. 226

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Selection of Pulse Width

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We then investigated how the width of high-voltage pulses influenced optical

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emission for solutions with conductivity ranging from 300 to 1200 S/cm. We noted that,


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for plasma in contact with solutions, plasma characteristics were strongly influenced by

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the solution’s conductivity. Figure 3a shows representative OES of the test solution for

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the Pb concentration of 250 mg/L and conductivity of 500 S/cm for high-voltage-pulse

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widths of 750, 3000, and 12000 s. It shows that the spectra were greatly influenced by

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the high-voltage pulse widths. The plasma was ignited intermittently when the pulse width

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was 750 s or less. Rather weak emission intensity was observed. With an increase of

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the pulse width to 3000 s, the characteristic peaks of each element were clearly shown

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in the respective spectra. A further increase in the pulse width to 12000 s led to the

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appearance of a spectral continuum from 300 to 800 nm. This continuum overlapped and

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interfered with the emission lines of the metallic elements of interest. The spectral

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continuum has been observed in previous studies33, 51. Its mechanism is complicated and

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has been discussed in several studies52,

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emission intensities at 405.8 nm for several high-voltage pulse widths. We first observed

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that the absolute intensities increased with the pulse widths. When the pulse width

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increased to 12000 and 24000 s, the intensities significantly fluctuated in time. Such an

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Figure 3b also displays the time-resolved


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observation suggests that there exists an optimal pulse width that gives high intensity and

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allows a temporally stable plasma intensity.

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We further identified the process window for the pulse width with various

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conductivity levels that resulted in different emission characteristics, namely, weak

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emission due to no or intermittent plasma formation, stable discharge, and unstable

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discharge due to a pulse width was too large. Figure 3c summarizes the process window.

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The pulse width led to stable discharge decreased with the increase in the conductivity.

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The dotted line shown in this figure indicates the optimal pulse width chosen for testing

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the solutions with varying conductivity in the following section. We note that based on the

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literature and our experimental observation, the selection of proper width relies mostly on

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the solution conductivities and the influence by the type of metallic element is rather minor.

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The process window shown in this figure is therefore universal for different type of metallic

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elements. The pulse-width tuning scheme for various solution conductivities is not only

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suitable for Pb but also for other elements. The Pb and continuum emission intensities

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for various pulse widths in solutions with 300, 500, and 1200 S/cm in conductivity are


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shown in Figure 3d to 3f, respectively. The continuum emission intensities were defined

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by calculating the average intensities from 395 to 415 nm, in the proximity to 405.8 nm.

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The Pb intensities were defined by the intensity at 405.8 nm deducting the continuum

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emission intensities. The error bar shown in each case is the SD of the intensities for 20

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spectral acquisitions. The results show that the minimum time required for plasma ignition

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decreased with increasing the conductivity of the solutions. Also, they show a similar trend

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for the characteristics of high continuum emission intensities and unstable discharge

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when the pulse width was too large. Such an observation indicates that the selection of

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proper/different voltage pulse widths is critical for obtaining reliable measurements when

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testing solutions with different conductivity.


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Figure 3. (a) Representative OES and (b) time-resolved emission intensities at 405.8 nm within 10 s in solution with 500 S/cm in conductivity driven by various pulse widths. (c) Qualitative analysis of emission behavior with the varying conductivity of the solutions and pulse widths applied. Pb and continuum emission driven pulse widths of 188 to 24000 s in solutions with conductivity of (d) 300, (e) 500, and (f) 1200 S/cm. 270

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Online Monitoring Driven by AMPP


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For testing solutions with different conductivity, using high-voltage pulses with

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proper pulse widths was extremely critical for obtaining reliable emission spectra. Thus,

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we developed a scheme of AMPP for analysis of solutions with different and unknown

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conductivity. As mentioned above, the central idea was to apply a short high-voltage pulse

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and measure the current through the electrode as the indication of the solution

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conductivity, based on which the proper pulse width could then be immediately

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determined and set. With such a scheme, the pulse width could be actively modulated.

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We used the AMPP to continuously monitor the Pb optical emission intensities of a test

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solution in a continuous flowing system with a solution conductivity ranging from 300 to

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1200 S/cm. For comparison, plasmas ignited with constant pulse widths of 500 and

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15000-s were also performed. Figure 4 shows the results of temporal-resolved Pb

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emission intensities at 405.8 nm for solutions with various levels of conductivity. The test

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solution contained a constant 50 mg/L of Pb. In these tests, the plasma was ignited for

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20 s with repetitive 20-Hz pulses in each detection, and the interval between each

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detection was 5 min. When a constant pulse width of 500 s was used, such a pulse width

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was not large enough to generate stable plasmas in solutions with low conductivity (below


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1000 S/cm). Stable Pb emissions are only observed when the solution conductivity was

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1200 S/cm. With a sufficiently large pulse width of 15000 s, the plasma was ignited in

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solutions with a wide range of conductivity. A rather large shot-to-shot variation of the Pb

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emission intensities with conductivity from 300 to 1200 S/cm was observed. Such a large

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variation led to poor performance for heavy metal monitoring. The voltage and current

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waveforms and the time-resolved emission intensities of the plasma driven with a

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constant voltage pulse of 5000 s ignited in the solutions with conductivity of 300, 500,

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and 1200 S/cm are shown in the supporting materials (See Figure S2). Such a finding

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clearly shows that it is critical to properly adjust pulse width when the conductivity of the

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test solution is not constant.

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With the AMPP, a short high-voltage pulse of 900 V and 500 s was applied and

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the microcontroller measured the voltage across a resistor of 5  in series with the power

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line within the pulse through the analog input port. The solution conductivity was

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estimated by measuring the voltage drop, an analog signal, across this 5- resistor. It

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shows linear relationship between the solution conductivity and this analog signal, as


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shown in the supporting information (Figure S3). This analog signal is therefore used to

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infer the solution conductivity. We note that for sample preparation, the solution

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conductivity was measured by the solution conductivity meter. Strictly speaking, the

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behaviors of ions in solution is more complicated especially driven by high voltage. Using

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this analog signal provides a simple and efficient way to quantify conductivity. Based on

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the quantified conductivity, the optimized pulse width was determined by using the results

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shown in Figure 3c. This optimized pulse was determined 50 ms prior to the application

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of the voltage pulse to ignite the plasma. When plasmas were ignited with actively

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modulated pulse width, stable Pb emission intensities were observed, as shown in the

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upper panel in Figure 4. A variation within 10% was observed for the emission intensities

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for solution conductivity ranging from 300 to 1200 S/cm. The AMPP based on rapid

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quantification of solution conductivity offered great advantages for real-time and

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continuous monitoring of the test solution with varying conductivity over a wide range.

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Figure 4. Online Pb monitoring in solution containing 50 mg/L Pb with 300, 500, 900, and 1200 S/cm in conductivity driven by AMPP (upper panel), 500-s constant pulse width (middle panel), and 15000-s constant pulse width (lower panel). Throughout these sets of tests, the plasma was ignited for 20 s with repetitive 20-Hz pulses in each detection, and the interval between each detection was 5 min. 318

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Online Monitoring for Multiple Metallic Elements Detection and Real-Time Alert System


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In this section, we will demonstrate that such a system was capable of

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simultaneous detection of multiple heavy metallic elements and immediately sending alert

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messages when the selected metallic elements were detected. Pb and Zn were chosen

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to demonstrate the online monitoring. For this demonstration, four solutions are selected:

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50 mg/L of Pb, 100 mg/L of Zn, blank, and a solution containing 50 mg/L of Pb and 100

325

mg/L of Zn. The conductivity of each solution varied from 500 to 1000 S/cm. We noted

326

that, when using the AMPP, solutions with a wide range of conductivity could be

327

effectively handled by modulating the pulse width. With the AMPP and continuous

328

acquisition of OES, monitoring of multiple elements could be performed by examination

329

of temporal-resolved emission intensities of designated wavelengths. This proved to be

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the key advantage of plasma spectroscopy. Figure 5 shows the temporal-resolved Pb

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and Zn emission intensities obtained from spectra at 405.8 and 481.1 nm, respectively,

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as well as the sample warning messages sent when corresponding metallic elements

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were detected. The interval between each detection was 8 min. For temporal-resolved

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intensities of 405.8 nm (Pb emission, red line in Figure 5), strong emission was observed

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when the test solution contained Pb, both with and without Zn. When either a Zn-


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containing and Pb-free solution or a blank solution was introduced, rather weak emission

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intensities in this wavelength were observed. The test for monitoring 481.1 nm (Zn

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emission, blue line in the same figure) also showed the system was able to detect the

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existence of Zn even with the existence of Pb in the test solution. This finding clearly

340

demonstrates that such a system allows for the detection of designated metallic elements

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with very low or no cross-element interference.

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The detection results and an alert system can be well integrated to send out

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warning messages upon the detection of designated metallic elements in real time. The

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inset of Figure 5 shows sample warning messages sent when Pb, Zn, or both were

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detected. This platform sent an e-mail as a warning message through the laptop. To

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extend this function to different metallic elements and send out corresponding messages

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could easily be done by monitoring different emission lines. These results clearly indicate

348

that the integration of the detection and alert system allows for online heavy metal

349

monitoring for rapid and multi-element detection with a real-time report.


29


Page 30 of 48

Figure 5. Simultaneous online Pb and Zn monitoring in solutions containing with Zn in 100 mg/L, blank, Pb in 50 mg/L, and with Pb and Zn in 50 and 100 mg/L, respectively. The insets show the warning e-mails sent from the alert system. 350

351

Test with Interferences of Dye, Acid, and Organic Compounds

352

Plasma spectroscopy is based on the detection of designated metallic optical

353

emission lines. Such a technique can be robust and nearly free of interference when the

354

analysis of test solutions contains various chemicals or it is done under a different

355

condition. We demonstrated the robustness of this technique by the intentional addition


30

Page 31 of 48


356

of interference during the process for continuous monitoring of heavy metallic elements.

357

The goal was to show the capability of this system in monitoring designated metallic

358

elements in chemically complicated test solutions. The test interferences included dye,

359

acid, and organic compounds. These test interferences represented contamination

360

sources frequently observed in various sources, such as wastewater drainage. Figure 6

361

shows the results for online Pb monitoring in solutions with the addition of the

362

interferences of 5-M methylene blue, 5-M orange II, pH-2 nitric acid, and 5-%-v/v

363

ethanol. The inset shows the visual appearance of the solutions with methylene blue and

364

orange II. A test solution containing Pb of 50 mg/L was used throughout this monitoring

365

process. We noted that, throughout this process, the conductivity of the solution was not

366

a constant and ranged from 300 to 1000 S/cm. The scheme with active pulsed width

367

modulation was used and the optimized pulse width was applied in each detection. The

368

results show that Pb can be properly detected even when interference is simultaneously

369

added. This indicates that this system offers the advantage of detecting Pb even when

370

the test solution contains interferences of dye, acid, or organic compounds. Here, it

371

should be noted that it has been reported in the literature that, in the acid-containing test


31


Page 32 of 48

372

solutions, the metallic emission intensities increases significantly49, 50. In this system, the

373

effect of the acid addition in the solution conductivity was controlled by properly setting

374

up the pulse width. A rather consistent intensity regardless of the type of interference

375

added was therefore observed. We also found that when ethanol was used as the

376

interference, characteristics peaks of ethanol, such as C2 (516 nm), were also shown in

377

the spectra.

378


32

Page 33 of 48


Figure 6. Online 50-mg/L Pb monitoring in solutions containing methylene blue with 5M, orange II with 5-M, nitric acid with a pH value of 2, and ethanol with 5 % v/v, respectively. The insets show the visual appearance of the solutions. 379

380

For continuous online monitoring, the stability of the system over a longer time

381

scale, such as several days, is a critical consideration. Several factors, such as erosion

382

and contamination on the discharge electrode surface, are usually concerned and have

383

been big issues in the plasma-liquid interactions system. These may greatly influence the

384

long-term operation of plasmas and, thus, the emission intensities. The use of pulsed

385

power with proper width and desired frequency could possibly result in significantly


33


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386

lengthening the electrode life and, consequently, the long-term stability of the system.

387

Therefore, we monitored time-resolved Na emission intensities to exam the long-term

388

stability of the system. Figure S4 shows a less than 10% variation for the emission

389

intensities of 100,000 pulses. This number of pulses allowed for a continuous monitoring

390

process for over one week if the test was performed every 10 min and each test acquired

391

the spectrum with 100 pulses. The use of proper pulse width and frequency not only

392

provided the system with long-term stability, but also offered it extra flexibility for the

393

operation. This feature is highly desirable for online monitoring.

394

Environmental Implications

395

An online continuous heavy metals monitoring system using plasma spectroscopy

396

driven by AMPP was developed. The AMPP quantified in situ the solution conductivity

397

and modulated the pulse width in real time for plasma ignition within 50 ms. By using

398

AMPP, stable and instant emissions of metal could be observed in solutions with

399

conductivity from 300 to 1200 S/cm, which is within the range of typical fresh water in

400

the environment. An alert system was integrated to provide a real-time warning once


34

Page 35 of 48


401

metals were detected. In addition, we demonstrated that this system is able to properly

402

detect metals in solutions containing several contaminants. We also showed that the

403

performance of this system will remain robust for days, which makes it suitable for long-

404

term and continuous monitoring. The key advantage of this present technique as opposed

405

to other plasma-based techniques is the capability analyzing test solution with various

406

conductivities without addition of extra gases and/or chemicals. The comparison between

407

this work and other plasma-based techniques, namely atmospheric pressure glow

408

discharge (APGD), solution-cathode glow discharge (SGGD), solution-anode glow

409

discharge (SAGD), dielectric barrier discharge (DBD), and microfluidic chip, are tabulated

410

in the supporting information (Table S4). Overall, this system using plasma spectroscopy

411

for detecting heavy metals offers a novel route for environmental monitoring.

412

413


35


Page 36 of 48

414

SUPPORTING INFORMATION

415

Table of geometry of the sampling and plasma generation unit; table of optical emission

416

lines used for measurement; table of detection performance for multiple metallic elements;

417

figures of spectra for plasma ignited in solutions driven by 800-1000 V pulses; figures of

418

time-resolved emission intensities and voltage and current waveforms within a single 5000-

419

s pulse in solutions with various conductivity; figure of the relationship between the analog

420

voltage and the conductivity of solutions; figure of time-resolved relative Na intensities across the

421

application of 100,000 pulses; table of comparison analytical sources using plasma OES.

422

423

ACKNOWLEDGEMENTS

424

This work is supported by the Ministry of Science and Technology, Taiwan (106-2221-E-

425

002-170-MY3).

426


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428

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