Hybrid Flow System for Automatic Dynamic Fractionation and

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Hybrid Flow System for Automatic Dynamic Fractionation and Speciation of Inorganic Arsenic in Environmental Solids Yanlin Zhang, Manuel Miró, and Spas D Kolev Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es505629a • Publication Date (Web): 03 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Environmental Science & Technology

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Hybrid Flow System for Automatic Dynamic Fractionation and Speciation of

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Inorganic Arsenic in Environmental Solids

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YANLIN ZHANG, † MANUEL MIRÓ*, ‡ SPAS D. KOLEV*, †

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†

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University of Melbourne, Victoria 3010, Australia.

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‡

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Palma de Mallorca, Spain

Centre for Aquatic Pollution Identification and Management (CAPIM), School of Chemistry, The

FI-TRACE group, Department of Chemistry, Faculty of Science, University of Balearic Islands,

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KEYWORDS: arsenic, dynamic fractionation, speciation, sequential injection analysis, gasdiffusion, hydride generation

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* *

Corresponding author phone: +61 3 83447931; fax: +61 3 9347 5180; e-mail: [email protected] Corresponding author phone: +34-971172746; fax: +34-971173426; e-mail: [email protected]

1 Environment ACS Paragon Plus


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Abstract

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An integrated flow analysis system and protocol are proposed for the first time for automatic

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dynamic flow-through fractionation of inorganic arsenic (arsenite and arsenate) in environmental

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solids in combination with its real-time speciation. Four extractants (i.e., (1) 0.05 M ammonium

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sulphate, (2) 0.05 M ammonium dihydrogen phosphate, (3) 0.2 M ammonium oxalate, and (4) a

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mixture of 0.2 M ammonium oxalate and 0.1 M ascorbic acid at 96 ºC) are applied sequentially to

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the sample to measure bio-accessible inorganic arsenic associated with (1) non-specifically sorbed

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phases, (2) specifically-sorbed phases, (3) amorphous plus poorly-crystalline hydrous oxides of iron

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and aluminium, and (4) well-crystallized hydrous oxides of Fe and Al, respectively. The kinetic

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extraction profiles of arsenite and total inorganic arsenic are obtained for each extractant by

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automatic collection of a given number of its aliquots (sub-fractions), exposed to the solid sample.

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Arsenite and total inorganic arsenic in each sub-fraction are converted to arsine sequentially by

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hydride generation at pH 4.50 and in 1.14 M hydrochloric acid, respectively. Arsine is absorbed

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into a potassium permanganate solution, the discoloration of which is related to the concentration of

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the corresponding arsenic species. The proposed method was successfully validated by analysing a

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soil reference material (NIST 2710a) and a sediment sample.

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30 31 32


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Introduction

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Arsenic pollution is of considerable environmental and health concern due to its ubiquitous nature

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and high toxicity (1). Natural arsenic sources play a major role in its environmental impact in some

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countries and areas (2, 3), while anthropogenic sources pose more environmental concerns in others

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(4). The toxicity of arsenic varies greatly depending upon its chemical form. In general, inorganic

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arsenic species are believed to be considerably more harmful than organoarsenicals (5). The

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dominant inorganic arsenic species are arsenite (As(III)) and arsenate (As(V)), with the former

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being considerably more toxic than the latter (6). As a consequence, determination of total inorganic

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arsenic (T-As = As(III) + As(V)) in environmental samples often provides insufficient information

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on its ecotoxicity, while speciation data give more information in environmental quality

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assessments. Another important factor determining the ecotoxicity levels of As in the environment

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is its bioaccessibility (i.e., ability of being leached) on the basis of the metalloid soil-phase

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associations, which determine its mobility and hence introduction into the food web (7). Therefore,

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novel analytical methodologies capable of assessing As mobility with concomitant information on

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speciation in environmental solid samples, thus allowing a comprehensive and holistic evaluation of

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the ecotoxicity of this metalloid, are highly desired.

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Numerous methods have been developed for the assessment of the toxicity of As in

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environmental solid samples based on sequential extraction procedures (SEP) (also termed

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fractionation) which involve the use of extraction reagents with increasing leaching ability for

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releasing As pools (or fractions) associated with different soil phases (8-11). Such methods are

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usually conducted under batch-wise end-over-end mixing conditions and are considered as routine

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tools for assessing risk exposure to metals and metalloids (9, 12). Although the batch SEP has been

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widely accepted and applied as a tool in risk exposure and assessment, the fractionation data

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provided could be viewed as questionable since naturally occurring processes are always dynamic

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while batch mode leaching protocols are based on a single equilibrium between the solid and liquid

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phases. Such equilibrium-based methodologies may result in errors and inaccurate conclusions



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regarding the actual mobility of As under environmentally changing conditions. Re-adsorption of

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the leached As by the solid sample itself may result in underestimation of bioaccessible As pools

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(13-15). Another common inherent shortcoming of these methodologies is that they are time and

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labour intensive and extended extraction times may result in the chemical transformation of As

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species during the extraction procedure (12).

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Based on these facts, flow-through dynamic extraction methods that mimic environmental

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conditions more realistically have been proposed for getting better knowledge of the bio-

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accessibility of As in solid samples under worst-case scenario conditions (16-19). Unfortunately,

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most of these methods only involve the determination of total As extracted in each reagent (16, 17,

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20-22). The use of continuous extraction followed by ion-exchange chromatography-inductively

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coupled plasma mass spectrometry (IC-ICP-MS) provided comprehensive information on the As

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species present in seafood and rice (23). However, the procedure involving two separate steps is

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time-consuming and requires the use of expensive instrumentation. Environmental samples can be

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either oxic or anoxic, depending on the actual environment from which they have been collected. In

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the former case, As(V) prevails over As(III) whereas As(III) may dominate in anoxic samples (24).

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Due to the higher toxicity of As(III) compared to As(V), knowledge of their relative concentrations

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is crucial for reliable risk assessment in potentially polluted areas. To the best of our knowledge

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flow-through SEP in combination with on-line speciation has not been reported as of yet.

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Two operational strategies have been described in the literature for dynamic SEP of As and

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involve the packing of a given amount of a solid substrate either in a stirred flow cell (22, 25, 26) or

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microcolumn (26, 27). Stirred flow cells have relatively large sample capacity and therefore require

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larger volumes of reagents for exhaustive As extraction while these volumes are substantially

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reduced in on-line dynamic leaching methods based on microcolumn extraction. In addition, the

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latter methods are generally faster, can provide better temporal resolution of fractionation data,

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offer more ruggedness upon slight changes in experimental conditions (solid to liquid ratio, flow

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rate and solid amount), and are conducted in entirely enclosed automatic systems (17, 28, 29).


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Therefore, the column approach has been adopted in this study.

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This paper reports on the development for the first time of a novel hybrid flow analyser based

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on the sequential injection analysis (SIA) concept for integrating on-line dynamic fractionation of

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inorganic As in environmental solids with subsequent on-line non-chromatographic speciation of

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the inorganic As species in the extracts. Based on the SEP proposed by Wenzel et al. (9), which is

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designed for anionic inorganic As species (30) as the most abundant As species in soil (31, 32), five

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fractions of As with different mobilities were considered in the present study. The first four

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fractions including arsenic associated with (1) non-specifically sorbed phases, (2) specifically-

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sorbed phases, (3) amorphous plus poorly-crystalline hydrous oxides of iron and aluminium, and (4)

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well-crystallized hydrous oxides of Fe and Al, respectively, were assessed by online sequential

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extraction using four leaching solutions. Each fraction consisted of sub-fractions with diminishing

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concentrations of bio-accessible As(III) and As(V). The residual As was determined as the fifth

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fraction after digestion of the solid sample with aqua regia. Unlike batch mode sequential extraction

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in which a prescribed volume of extractant is used for the leaching of As in each fraction, the online

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extraction of each fraction was performed by using a series of aliquots (sub-fractions) of the

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corresponding extractant which were sequentially pumped to the microcolumn containing the solid

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sample to obtain the dynamic extraction profile of the fraction. On-line As detection in the extracts

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is achieved by selective hydride generation (HG) of arsine from As(III) or T-As with subsequent

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arsine gas-diffusion separation and detection in the acceptor stream by a simple spectrophotometric

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method, reported previously by us (33), which involves the discoloration of the permanganate

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acceptor solution by arsine. The method is validated by analysing a soil standard reference material

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and a sediment sample collected in the State of Victoria, Australia.

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Experimental

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Instrumentation. An FIAlab 3200 SIA system (FIAlab Instruments, USA), shown schematically in

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Figure 1, which was run by FIALab for Windows 5.0 software (FIAlab Instruments, USA) was

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used for conducting on-line sequential extraction in accordance with the chemical composition and



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extraction temperatures of the various fractions proposed by Wenzel et al. (9) and subsequent on-

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line As speciation in the various extracts based on hydride generation and spectrophotometric arsine

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detection. The system was composed of two main syringe pumps, a ten-port selection valve (SV), 3

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auxiliary syringe pumps for on-line extract handling, and a gas-diffusion separation unit. Syringe

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pump P2 (Figure 1) had a capacity of 1 mL while all the other pumps had a capacity of 2.5 mL. A

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QE65 Pro Scientific-grade Spectrometer (Ocean Optics, USA) with an LS-1 tungsten halogen lamp

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as the radiation source and a SMA-Z flow cell with 1 cm path length and 20 µL capacity (FIAlab

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Instruments, USA) were used for the detection of KMnO4 discoloration at 525 nm. The light beam

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was brought from the lamp to the flow-through measuring cell and from there to the spectrometer

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through P600-20 optical fibres (FIAlab Instruments, USA) with bundle core diameter of 600 µm.

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Figure 1. Schematic diagram of the hybrid flow system for sequential extraction and speciation of

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inorganic As in solids samples (P1-P5, syringe pumps; D, spectrophotometric detector).


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To perform the hydride generation and arsine separation, a home-made gas-diffusion (GD)

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unit was made of Perspex composed of a donor and acceptor chambers separated by a filter paper

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disc (No 54, Whatman, Britain) sandwiched between two polypropylene microporous membranes

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(Cat. No. 320, Chemplex Industries, Inc. Eastchester, NY, USA). The polymer membranes are

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highly permeable but too flexible. The filter paper was used as a physical support for the

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polypropylene membranes. This minimized changes in the volumes of the donor and acceptor

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channels caused by pressure differences between them which ensured high repeatability of the

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results. Arsine, generated in the donor stream, diffused across the membranes into the acceptor

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solution located in the acceptor channel of the GD unit where it was oxidized by KMnO4. GD units

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with 2 different geometries of their donor and acceptor channels were compared. One of these units

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was equipped with hexagonal shaped channels with dimensions identical to those of a membrane

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separation unit used previously by us (34). In order to improve mass transfer between the donor and

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acceptor streams by enhancing mixing in the donor channel, the two channels of the other GD unit

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had meander shaped geometry, as shown schematically in Figure 1. The depths of the acceptor and

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the donor channels were 0.5 mm and 6 mm, respectively. The total length and width of the two

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meander channels were 100 mm and 1.8 mm, respectively, thus providing a volume capacity of

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90 µL for the acceptor channel and 1080 µL for the donor channel. The holding coil had an internal

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diameter (i.d.) of 1 mm and a volume capacity of 3 mL. The i.d. of the PTFE tubing used for

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connecting the components of the flow manifold was 0.5 mm. The outlet of the flow cell was

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connected to PTFE tubing of 3 m in length and 0.3 mm in i.d. to provide sufficient back-pressure to

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prevent the diffusion of H2 across the membrane, which would otherwise interfere with the

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analytical measurements. A 3 mL polypropylene syringe barrel (Terumo, The Philippines) was used

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as the pre-reduction reactor. Another 3 mL Terumo syringe barrel with a polyethylene frit with pore

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size of 20 µm at its bottom was packed with 0.5 g Dowex 50W-X8 cation-exchange resin (100 to

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200 mesh, Bio-Rad, Richmond, California, USA) and used for the collection of each sub-fraction.

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This syringe barrel is referred in Figure 1 as the extract vessel.



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A heating system incorporating a multifunction controller, a cylindrical heating element, a

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thermal sensor, and a temperature regulator (Watlow Electric Manufacturing Co., USA) was used

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for the heating of the fourth extractant and controlling of its temperature. The heating element had a

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length of 70 mm and a diameter of 12.5 mm. PTFE tubing with an i.d. of 1 mm and a volume

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capacity of 1.2 mL, used for the delivery of the extractant, was spirally wound around the heating

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element.

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A home-made Perspex cylindrical micro-column shown schematically in Figure 1, was used

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as a flow-through sample container. The column had an i.d. of 8.5 mm and a length of 10.5 mm,

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with a volume capacity of approximately 570 µL, or a solid sample capacity of approximately

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500 mg. However, 250 mg of homogeneous solid sample were used in the SEP to minimize the

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back-pressure and its effect on extraction. Inside the column, a polyethylene frit with pore size of

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20 µm was placed between the solid sample and the column outlet (Figure 1). With this sample size,

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there was a dead volume of about 50% of the column capacity and this dead volume was minimized

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to avoid dilution of the extract by inserting filter paper (Whatman No.40). In the extraction process,

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the column was positioned upright as shown in Figure 1 so that the extractants were propelled

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upwards since it was established that propelling solutions downwards caused higher back-pressure

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and blockage of the frits’ pores.

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A Perkin-Elmer Optima 4300 DV inductively coupled plasma optical emission spectrometer

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(ICP-OES), Perkin Elmer Inc., Shelton, USA）in combination with a home-made online hydride

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generation system, as shown in Figure S1 (Supplementary Information), was employed for the

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determination of total arsenic in each fraction after digestion with aqua regia.

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Readers are referred to the Supporting Information of this article for detailed information

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about the chemicals and materials used in the fractionation and speciation assays.

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Determination of As(III) and As(V). For the speciation and quantification of inorganic As in the

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extracts obtained on-line, separate determination of As(III) and T-As is needed. Similarly to

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Rupasinghe et al (34), the reduction of As(V) to As(III) was conducted in HCl medium (1 mol L-1) 8 Environment ACS Paragon Plus

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with KI (0.3%) as the reductant. To improve further the efficiency of the reduction process ascorbic

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acid was added to the reductant solution (35). In this study, 1250 µL of an extract or standard

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solution containing As(III) and/or As(V) were aspirated into the holding coil of the flow system by

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Pump P1 through Port 1 or Port 4 of the multiport selection valve (SV), followed by aspiration of

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500 µL of the reductant solution containing 4 M HCl, 1 % KI and 0.5 % ascorbic acid by Pump P1

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through Port 2 (Figure 1). The two solution zones were dispensed into the pre-reduction reactor

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connected to Port 9. Another 1250 µL of the same extract or standard solution were aspirated by

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Pump P1 and stored in the holding coil. At the same time, 1000 µL of the permanganate acceptor

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solution, 1250 µL of the NaBH4 solution, 625 µL of the citric acid/citrate buffer solution, and

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2500 µL of air were aspirated into Pumps P2, P3, P4, and P5, respectively. The air loaded in Pump

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P5 was dispensed into the pre-reduction reactor at a flow rate of 300 µL s-1 to promote mixing of

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the extract or standard solution with the reductant solution. 600 µL of the KMnO4 acceptor solution

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were propelled through the acceptor channel of the GD unit at a flow rate of 20 µL s-1 to displace

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the residual acceptor solution from the previous extract analysis, whereupon the flow was stopped.

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The extract or standard solution in the holding coil (1250 µL), 625 µL of the NaBH4 solution and

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the buffer solution, were then propelled towards the GD unit at flow rates of 4, 2 and 2 µL s-1,

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respectively. The extract or standard solution, flowing through Port 3 of SV, was merged with the

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buffer and NaBH4 solutions at confluence points a and b (Figure 1), respectively. Arsine was

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generated and diffused into the static acceptor solution located in the acceptor channel of the GD

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unit where it was oxidized by KMnO4 to arsenate. At the flow rates mentioned above, the extract or

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standard solution passed through the GD unit in 312.5 s. After a delay time of 10 s, the remaining

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400 µL of KMnO4 solution in Pump P2 were propelled towards the flow-through

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spectrophotometric measuring cell at 20 µL s-1 and a negative transient absorbance signal was

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recorded at 525 nm. The maximum decrease in permanganate absorbance relative to the baseline

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level was used as the analytical signal for the quantification of As(III).

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The acidic extract or standard solution stored in the pre-reduction reactor was then aspirated 9 Environment ACS Paragon Plus


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by Pump P1 and loaded into the holding coil. At the same time, 1000 µL of the acceptor solution

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was aspirated into Pump P2 and 600 µL of it was dispensed through the acceptor channel of the GD

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unit at a flow rate of 20 µL s-1 for cleaning of the acceptor channel. The extract or standard solution

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in the holding coil and the remaining 625 µL of NaBH4 solution in Pump P3 were propelled

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towards the GD unit at flow rates of 5.6 and 2 µL s-1, respectively. The two streams merged at

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confluence point b to generate arsine, which was detected in the same way as described above. The

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analytical signal in this case was related to the concentration of T-As. The concentration of As(V)

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was calculated as the difference between the T-As and As(III) concentrations. In this study, five

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standard solutions containing 0, 50, 100, 150 and 200 µg L-1 of As(III) and 0, 100, 200, 300 and

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400 µg L-1 of As(V), respectively, were used for the construction of the As(III) and T-As calibration

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curves. The use of different concentration ranges for As(III) and As(V) was based on the fact that

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As(V) was the dominant inorganic arsenic species in the solid samples studied.

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All As(III)/T-As standards were prepared in deionized water because preliminary experiments

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involving standards with matrices identical to the 4 extractants used in this study did not show any

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matrix effects on the determination of either As(III) or T-As.

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Integrated fractionation and As(III)/As(V) speciation analysis. An integrated operational

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procedure was developed in this study for the investigation of the extraction kinetics of As(III) and

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As(V) via online dynamic fractionation and subsequent As speciation in each single sub-fraction.

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This was performed by using 250 mg of solid sample packed in the micro-extraction column

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(Figure 1) connected to Port 10 of SV (Figure 1). The integrated fractionation procedure was

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conducted in triplicate for both the standard reference material and the sediment sample.

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To investigate the extraction profile of the first fraction, 2500 µL of the extractant was first

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loaded into the holding coil by Pump P1 from its reservoir through Port 5 of SV followed by

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dispensing the solution at a flow rate of 20 µL s-1 through the micro-extraction column (Figure 1).

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The extract was collected in the extract vessel, connected to Port 1. As(III) and T-As in the extract

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were determined as described above. By repeating the extraction and As(III)/T-As speciation


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analysis procedure under the same conditions and plotting the As(III) and As(V) concentrations

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against the corresponding sub-fractions, the kinetics profiles (extractograms) for As(III) and As(V)

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were obtained. The extractograms of the remaining three fractions were obtained in the same way.

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However, the temperature of the heating element in the extraction process of the fourth fraction was

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set at 98 ± 2 °C, resulting in extractant temperature at the entrance of the column of 96 ± 2 °C.

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The Dowex 50W×8 cation-exchange resin packed in the extract vessel was regenerated after

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the analysis of each individual sub-fraction by rinsing the bead particles with 2 mL of 2 M HCl

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solution aspirated via Port 4 of the SV, followed by washing with 2 mL of deionized water

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delivered by Pump P1 for three times. The used HCl solution and the waste water were disposed via

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Port 3.

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Method validation. In order to investigate the trueness of the flow-through SEP, a mass balance

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validation was undertaken by comparing the sum of extractable T-As in the four fractions plus the

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residual (immobile) As fraction with the value for T-As in the original sample. The residual fraction

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(Fraction 5) was determined by digesting the solid sample after completing the SEP. For this

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purpose the solid sample in the column was transferred to a 50 mL glass beaker with a watch glass

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on it and digested with 8 mL aqua regia on a hot plate (36). The sample was heated at about 230 °C

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until about 1 mL liquid was left in the beaker. The digest was diluted with deionized water to about

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10 mL and centrifuged at 3000 rpm for 20 min. The supernatant was transferred into a 25 mL

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volumetric flask and diluted with deionized water. T-As, which consisted of As(V) only due to the

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oxidative digestion procedure, was determined by the proposed flow method as described earlier.

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Appropriate dilutions were made when needed.

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The concentration of T-As in the original sample was determined by the direct digestion

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method outlined above. T-As was determined in 50 mg sample of the soil standard reference

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material while 250 mg of sediment samples were required due to their lower As concentration. In

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this determination, the sample solution was manually loaded in the extract vessel (Figure 1) so that

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potential interfering metal ions could be removed by the resin. The online SEP was also compared



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with its batch-wise counterpart developed by Wenzel et al. (9).

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The possibility of As(V) reduction to As(III) by the fourth extract containing ammonium

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oxalate and ascorbic acid at 96 °C was examined by a batch procedure in which sediment samples

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(0.5 g) with undetectable As(III) were extracted with 10 mL of the fourth extractant spiked with

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10 mg L-1 As(V) at 96 °C for 20 min. The suspension was centrifuged at 3000 rpm for 20 min and

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1 mL of the supernatant was diluted to 50 mL. The concentration of As(III) in this solution was

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determined by the proposed flow method described earlier.

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To further ascertain the reliability of the online fractionation and HG determination method,

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the reference material was sequentially batch extracted with the leaching solutions proposed by

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Wenzel et al. (9) and used in the online SEP and the extracts (F1 to F4) were digested with aqua

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regia to evaluate the eventual occurrence of organic forms of As. After offline converting As(V) to

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As(III) with KI and ascorbic acid solution, the total As in the digest was determined by HG-ICP-

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OES.

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Results and discussion

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Geometry of the gas-diffusion unit. It was established experimentally that the sensitivity of the

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analytical method was largely dependent on the flow channel configuration of the GD unit.

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Hexagon-shaped donor and acceptor channels were initially used similarly to a previous study (34)

272

involving As(III) determination in a flow system by HG and membrane separation. However, it was

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observed in the present study that the use of the flow channel configuration mentioned above

274

resulted in broad absorbance peaks leading to low sensitivity. By using such a flow channel

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configuration, shown in Figure 1, sharper absorbance peaks were recorded which resulted in

276

doubling the sensitivity. This could be attributed to improved arsine evolvement and membrane

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mass-transfer in the GD unit due to better mixing of the solutions in the donor and acceptor

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meander-shaped channels and the quick propelling of the discoloured KMnO4 solution to the flow-

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through measuring cell, which minimized peak broadening. An experiment involving the injection

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of a 1 mg L-1 As(III) standard showed that discolouring of the static KMnO4 solution in the acceptor 12 Environment ACS Paragon Plus

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channel due to the absorption of arsine was observed only in the first half of the channel length,

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thus indicating that arsine was quantitatively transferred across the membrane.

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Online speciation of As(III) and As(V). Ideally, HG of one of the two species (usually As(III))

284

should occur selectively under certain experimental conditions, while identical hydride generation

285

efficiency for the two species under different conditions can be achieved. The rates of arsine

286

generation from As(III) and As(V) with NaBH4 are strongly dependent upon the acidity of the

287

reaction medium and often differ significantly at lower acidity. As(III) is reduced much faster than

288

As(V) in low acidic medium and this has been employed for speciation of inorganic arsenic (37, 38).

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A common strategy in As speciation is to selectively reduce As(III) with NaBH4 under low acidic

290

conditions followed by simultaneous reduction of both As(III) and As(V) under highly acidic

291

conditions (37-40). However, even in highly acidic medium, quantitative conversion of As(V) to

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arsine is considerably slower than that for As(III), and heating of the reaction mixture and/or

293

prolonged reaction time are usually needed (37). The use of reductants, such as KI and ascorbic acid

294

(37, 41) with heating and cysteine (41-43) or thioglycolic acid (43) with or without heating has been

295

reported for pre-reduction of As(V) to As(III). It has been demonstrated that cysteine and

296

thioglycolic acid are more efficient reductants than KI (43). But it was observed in the present study

297

that these sulphur containing reductants were capable of generating volatile compounds with

298

reducing properties, possibly H2S, which could then be easily oxidized by KMnO4 thus producing

299

inflated results for As. Therefore, a mixture of HCl, KI and ascorbic acid was used for the reduction

300

of As(V) to As(III) which occurred in parallel with the determination of As(III) and the time

301

required for the determination of As(III) was sufficient for the quantitative reduction of As(V) to

302

As(III). Surplus amounts of KI (1%) and ascorbic acid (0.5%) in an acidic medium are usually used

303

for the reduction of As(V) (36) and the acid concentration is crucial for the reduction efficiency.

304

The effect of the HCl concentration in the reductant solution containing 1% KI and 0.5% ascorbic

305

acid on the reduction of As(V) in the pre-reduction step is shown in Figure S2 (Supplementary

306

Information). On the basis of these results 4 M HCl was used in preparing the reductant solution



307

which resulted in final concentration of HCl in the reaction mixture of 1.14 M.

308

Selection of parameters. The use of a NaBH4 solution with a concentration of around 0.5% (w/v)

309

has been shown to achieve high efficiency in the HG of As(III) (36). However, due to the poor

310

stability of NaBH4 and its strong pH dependence, the presence of NaOH at an appropriate

311

concentration in the NaBH4 solution is crucial. It was established that 0.05 M NaOH was capable of

312

suppressing the decomposition of NaBH4 in its reservoir. However, when EDTA was added to the

313

solution as a masking agent for transition metal ions as described in the Supplementary data, a

314

higher concentration of NaOH was required to neutralize EDTA to prevent it from rapidly

315

decomposing NaBH4. It was shown that NaOH concentration of 0.1 mol L-1 could effectively

316

suppress the decomposition of NaBH4 (0.5% (w/v)) in the presence of 1% (w/v) EDTA.

317

Experiments confirmed that under the conditions used for the determination of As(III), the

318

reduction of As(V) to arsine was negligible. The flow rate ratio between the extract/standard, buffer

319

and NaBH4 solutions in the determination of As(III) in all standards and extracts was maintained at

320

2:1:1. It was demonstrated that the flow rate of the standard/extract solution, varied in the range 2 -

321

20 µL s-1, had a significant effect on the analytical signal for As(III) (Figure S3, Supplementary

322

Information). As the flow rate was decreased, the signal increased considerably because of the

323

longer residence time of the extract zone in the GD unit which allowed more arsine to be generated

324

and transported into the acceptor solution. This improvement became insignificant for flow rates

325

below 4 µL s-1 as shown in Figure S3 and therefore the optimal values of the flow rates of the

326

extract/standard, citrate buffer and NaBH4 solutions were selected as 4, 2 and 2 µL s-1, respectively.

327

Under these conditions the time required for dispensing the 1250 µL of extract or standard solution

328

was 312.5 s. To maintain the same arsine generation conditions in the case of T-As determination,

329

where the original 1250 µL sample was mixed with 500 µL of reductant (final sample volume of

330

1750 µL), it was necessary to increase the sample flow rate from 4 to 5.6 µL s-1. Under these

331

conditions the 1750 µL sample was merged with the NaBH4 steam in 312.5 s, thus resulting in the

332

sensitivity for T-As equal to 96-100% of that for As(III) in the As(V) concentration range of 50 14 Environment ACS Paragon Plus

Page 14 of 30

Page 15 of 30


333

400 µg L-1. The insignificant difference between the analytical signals for T-As and As(III)

334

indicated that As(V) was practically quantitatively reduced to As(III) during the pre-reduction step.

335

Under such conditions linear calibration curves for both As(III) (mABS=0.2807[As] + 8, R2=0.9954,

336

[As(III)] range - 50-200 µg L-1) and T-As (mABS=0.2705[As] + 9, R2=0.9978, [T-As] range - 50-

337

400 µg L-1) were obtained, where mABS is the signal intensity in milli-absorbance units and [As] is

338

the concentration of As(III) or T-As in µg L-1. The detection limits based on three times of the

339

standard deviation for As(III) and T-As are 20 µg L-1 and 21 µg L-1, respectively. More detailed

340

operational parameters and merit data are given in Table S1 (Supporting Information).

341

Both transition metal ions and vapour/hydride forming elements might potentially give rise to

342

detrimental effects in the evolvement and detection of arsine. The results of detailed interference

343

studies are presented in the Supporting Information of this article.

344

Extraction profiles and quantification of As(III) and T-As in a soil standard reference

345

material and a sediment sample. The extractograms of the four fractions of the soil standard

346

reference material (NIST 2710a Montana Soil) shown in Figure 2 are the average of three replicate

347

runs. T-As in the first fraction, i.e. non-specifically sorbed fraction, was completely extracted with 7

348

sub-fractions，i. e. 17.5 mL of 0.05 M (NH4)2SO4, with an RSD of 4.5% for T-As, after which the

349

analytical signals for T-As decreased to baseline value. The second fraction, i.e. the specifically-

350

sorbed As, needed 42.5 mL for complete extraction, with an RSD of 5.3%. The third and the fourth

351

fractions, i.e. amorphous plus poorly-crystalline hydrous oxides of Fe and Al and the well-

352

crystallized hydrous oxides of Fe and Al, respectively, needed 25 mL and 17.5 mL of the

353

corresponding extractants for complete extraction of the inorganic As species studied, with RSD

354

values of 6.9 and 6.2%, respectively. This standard reference material contained relatively high T-

355

As concentration. For less As-contaminated samples, it can be expected that the online SEP will

356

consume lower volumes of extractants.



2

1

0 5

10

15

20

2

1

Extractant consumption (mL)

90 4

60

2

30

0

0 5

10

15

20

25

30

Extractant consumption (mL) 357

As(III) (mg/kg)

120

0

10

T-As (mg/kg)

As(III) (mg/kg)

150

6

0 0 5 10 15 20 25 30 35 40 45 50

180

8

20 10

25

F3

40 30


50

0

0 0

60

F2

100

F4

8

80

6

60

4

40

2

20

0

T-As (mg/kg)

1

As(III) (mg/kg)

2

3

T-As (mg/kg)

3

F1

T-As (mg/kg)

As(III) (mg/kg)

3

Page 16 of 30

0 0

5

10

15

20

25


Figure 2. Extractograms of As(III) ( ) and T-As () sub-fractions of the soil standard reference

358

material (NIST 2710a) where F1-F4 are Fractions 1-4; sample size is 250 mg, extract

359

flow rate is 20 µL s-1 and error bars are ± standard deviation.

360

The results in Figure 2 confirmed that the predominant form of As in all four fractions of the

361

soil standard reference material was As(V). Arsenate often accounts for the majority of overall As

362

in soil because of the usually high soil redox potential (44) and therefore the T-As concentration in

363

soil is in most instances a good estimate of the concentration of As(V). Even under the strongly

364

reducing conditions for the solubilisation of crystalline Mn and Fe oxyhydroxides, the concentration

365

of As(III) in the fourth fraction was found to be insignificant compared to that of As(V). This is

366

most likely due to the slow kinetics of the As(V)-As(III) transformation (44).

367

Table 1 presents the total concentrations of As(III) and T-As in each of the four fractions of

368

the soil standard reference material, calculated as the sum of the As(III) and T-As concentrations in

369

all corresponding sub-fractions.


Page 17 of 30


370

Table 1. Concentrations of As(III) and T-As (mg kg-1) in the 5 fractions of the soil standard

371

reference material NIST 2710a and sediment sample, determined by the proposed online SEP and

372

the batch-wise SEP. T-As (direct Certified Fraction

F1

Sediment

NIST 2710a

As(III)a 3.6±0.1

a, b

F3

F4

F5

ƩF

2.0±0.2

5.8±2.3

8.8±0.6

20.2±3.4

8.6±0.2

295±7

640±12

282±11

301±10 1527±39

As(III)b 4.2±0.3

3.2±0.6

7.4±1.5

15±2.0

29.8±4.2 344±14 1510±53

T-Asa

T-Asb

9.0±1.0

298±6

625±17

264±12

As(III)a

ND

ND

ND

0.3±0.1

digestion)

value

1584±36

1540±10

1584±36

1540±10

T-Asa 0.58±0.20 0.85±0.21 0.50±0.22 0.51± 0.13 0.21±0.08 2.42±0.33 2.65+0.32 As(III)b T-Asb

373

F2

ND

ND

ND

0.32±0.14

0.70±0.5 0.79±0.6 0.41±0.16 0.70±0.20 0.22±0.13 2.82±0.25 2.65+0.32

n=3, a online SEP , b batch-wise SEP

374

The extractograms of a sediment sample collected in the State of Victoria, Australia were also

375

obtained. In this case the four fractions were completely extracted with 12.5, 12.5, 12.5, and 15 mL

376

of the corresponding extractants, respectively. This can be attributed to the low concentrations of

377

the arsenic species in the sample (Table 1), indicating the sample-dependence of the extraction

378

procedure.

379

One of the main advantages of the proposed dynamic fractionation method is the

380

measurement of the concentrations of As(III) and T-As in each sub-fraction in real time. This

381

eliminates the need of determining the number of sub-fractions of each fraction in advance. This

382

number is determined dynamically on the basis of the T-As concentration in the sub-fractions.

383

Therefore, the duration and reagent consumption of the SEP are tailored to each particular solid

384

sample thus providing a considerable degree of flexibility and universality of the proposed approach.

385

Method validation. The sum of the concentrations of As(III) or T-As in all sub-fractions was 17 Environment ACS Paragon Plus


386

considered as a measure of overall leachable As(III) and T-As in the relevant fraction. For method

387

validation, the sum of overall leachable T-As and the As concentration in the residual fraction,

388

determined by the aqua regia digestion method outlined earlier, was compared with the certified

389

value of the soil standard reference material (NIST 2710a). For comparison purposes the

390

concentration of T-As in the soil standard reference material was also determined directly by

391

digesting 50 mg of fresh sample. No statistically significant differences at the 5% significance level

392

were found between the certified value of the reference material on one hand and the T-As

393

concentrations determined by the online SEP and the aqua regia digestion methods (Table 1). This

394

result demonstrates the reliability of the proposed online SEP and the negligible amounts of organic

395

As species in the leachates. The reference material was also analysed by the batch-wise SEP (9) and

396

the results are presented in Table 1. Despite the fact that discrepancies might be expected between

397

results obtained by batch-wise and flow-through fractionation protocols for metals (28, 30, 45-47), a

398

reasonable agreement between these approaches was found in the present study as evidenced by the

399

results for As(III) and T-As presented in Table 1. These results are in good agreement with those of

400

a previous study conducted by Fedotov et al. (30) where batch-wise and dynamic As fractionation

401

methods based on the Wenzel’s SEP (9) were compared. While the agreement for the first 3

402

fractions was satisfactory, discrepancies for the fourth fraction were observed which was probably

403

due to the fact that the dynamic method was operated at room temperature rather than at 96 oC as

404

specified in the batch-wise method (9).

405

There was no statistically significant differences at the 5% significance level between the T-

406

As concentration in the sediment sample, calculated as the sum of the T-As concentration in its 5

407

fractions (Fractions 1-4 obtained by the online SEP and Fraction 5 - by aqua regia digestion), and

408

the T-As concentration determined by the direct digestion method (Table 1). This result is another

409

confirmation of the trueness of the proposed SEP and shows that the amount of mobile organic As

410

species is negligible. Similarly to the soil standard reference material, there were no significant

411

differences between the proposed SEP and the batch-wise method (9), regarding the concentrations


Page 18 of 30

Page 19 of 30

412 413 414


of T-As in the individual fractions. Experiments ruled out the possibility of As(V) reduction to As(III) by the fourth extractant, thus further supporting the reliability of the proposed online SEP.

415

Figure S4 (Supplementary Information) illustrates the very good agreement between the

416

results for total arsenic in Fractions 1 to 4 of the standard reference material after digestion and the

417

corresponding T-As values determined by the integrated online fractionation method developed in

418

the present study. Statistical evaluation of these results using the t-test showed that there was no

419

statistically significant difference between the two methods at the 5% significance level for each of

420

the four fractions thus confirming the conclusion, based on the results in Table 1, that the leachable

421

arsenic in the standard reference material under the experimental conditions used in the present

422

study consists of predominantly inorganic arsenic species, i.e., arsenate and to a lesser extent

423

arsenite.

424

Compared to the batch-wise method (9), several advantages have been identified with the

425

proposed hybrid method. First, the fractionation, speciation and quantification processes have been

426

performed in an integrated and automatic flow system with minimum manual operations. As

427

opposed to previous flow-based configurations for As speciation (38, 40, 48) quantitative reduction

428

of As(V) to As(III) at room temperature is observed in our method. Second, the overall time for

429

completing the fractionation and speciation analysis (including calibration) in the proposed method

430

is approximately 10 h while the conventional batch-wise method requires 24.5 h for conducting the

431

sequential extraction steps only. It should be noted that at least another 5 h were needed for other

432

analytical operations, such as centrifugation, digestion and As determination by HG-AAS. Third,

433

relevant insight into the kinetics of the leaching process can be obtained, which is not provided by

434

the batch-wise method. Finally, due to the dynamic nature of the proposed fractionation method it is

435

possible to tailor the experimental extraction conditions (e.g., duration of extraction steps and

436

volume of extractants) as the analytical procedure is still underway by stopping the extraction step

437

as soon as T-As is entirely leached out by the extractant used and start the extraction of the next



438

fraction.

439

Acknowledgements

440

The authors would like to thank the Australian Research Council for financial support of this

441

research (Grant LP100100800).

442

Supporting Information Available

443

Additional supporting information as noted in the text is available free of charge via the Internet at

444

http://pubs.acs.org.

445 446 447


Page 20 of 30

Page 21 of 30


448

References

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Morabito, R.; Quevauviller, P. Toxic arsenic compounds in environmental samples:

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Jain, C. K.; Ali, I. Arsenic Occurrence, Toxicity and Speciation Techniques. Wat. Res. 2000, 34 (17), 4304-4312.

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Sharma, V. K.; Sohn, M. Aquatic arsenic: Toxicity, speciation, transformations, and remediation. Environ. Int. 2009, 35 (4), 743-759.

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copper, and lead in soils of a mining and agricultural zone in central Chile. Commun. Soil Sci.

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applicability of three-step BCR leaching schemes. J. Environ. Monit. 2005, 7 (1), 22-28.



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582 583


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Page 27 of 30


FIGURE CAPTIONS Figure 1. Schematic diagram of the hybrid flow system for sequential extraction and speciation of inorganic As in solids samples (P1-P5, syringe pumps; D, spectrophotometric detector). Figure 2. Extractograms of As(III) ( ) and T-As () sub-fractions of the soil standard reference material (NIST 2710a) where F1-F4 are Fractions 1-4; sample size is 250 mg, extract flow rate is 20 L s-1 and error bars are ± standard deviation.

1

ACS Paragon Plus Environment


TOC Art

2


Page 28 of 30

Page 29 of 30


Figure 1

3



Page 30 of 30

Figure 2

2

1

0 5

10

15

20

2

10


90 4

60

2

30

As(III) (mg/kg)

120

T-As (mg/kg)

As(III) (mg/kg)

150

6

0

0 0

5

10

15

20

25

0 0 5 10 15 20 25 30 35 40 45 50

180

8

20

1

25

F3

40 30


50

0

0 0

60

F2

100

F4

8

80

6

60

4

40

2

20

0

30

0 0

5

10

15

20

25



4


T-As (mg/kg)

1

As(III) (mg/kg)

2

3

T-As (mg/kg)

3

F1

T-As (mg/kg)

As(III) (mg/kg)

3

Hybrid Flow System for Automatic Dynamic Fractionation and

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