Tracing the Dynamic Changes of Element Profiles by Novel Soil

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Environmental Measurements Methods

Tracing the dynamic changes of element profiles by novel soil porewater samplers with ultralow disturbance to soil-water interface Zhaofeng Yuan, Williamson Gustave, Jonathan Bridge, Yi Liang, Raju Sekar, John Boyle, Chen-Yu Jin, Tong-Yao Pu, Yu-Xiang Ren, and Zheng Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b05390 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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Environmental Science & Technology

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Title: Tracing the dynamic changes of element profiles by novel soil

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porewater samplers with ultralow disturbance to soil-water interface

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Names of authors: Zhao-Feng Yuan 1, 2, Williamson Gustave

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Yi Liang 4,

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Ren2, Zheng Chen 2 *

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1

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Liverpool L69 7ZX, UK

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2

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University, 111 Ren'ai Road, Suzhou, Jiangsu 215123, P. R. China

1, 2,

Jonathan Bridge 3,

Raju Sekar 5, John Boyle 1, Chen-Yu Jin 2, Tong-Yao Pu2, Yu-Xiang

Department of Environmental Science, University of Liverpool, Brownlow Hill,

Department of Health and Environmental Sciences, Xi'an Jiaotong-Liverpool

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3

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St, 11 Sheffield S1 1WB, UK

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4

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Tianjin Motimo Membrane Technology Co.,Ltd, 11th Street, TEDA Tianjin 300160, P.

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R. China

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5

16

Road, Suzhou, Jiangsu 215123, P. R. China.

Department of Natural and Built Environment, Sheffield Hallam University, Howard

State Key Laboratory of Membrane Materials and Membrane Applications of

Department of Biological Sciences, Xi'an Jiaotong-Liverpool University, 111 Ren'ai

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* Corresponding author: Zheng Chen (E-mail: [email protected] or

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[email protected]; Tel: +86-512-81880471; fax: +86-512-88161899)

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ACS Paragon Plus Environment


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Abstract: In flooded soils, soil-water interface is the key zone controlling

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biogeochemical dynamics. Chemical species and concentrations vary greatly at micro-

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to cm-scales. Techniques able to track these changing element profiles both in space

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and over time with appropriate resolution are rare. Here, we report a patent-pending

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technique, the Integrated Porewater Injection (IPI) sampler, which is designed for soil

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porewater sampling with minimum disturbance to saturated soil environment. IPI

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sampler employs a single hollow fiber membrane tube to passively sample porewater

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surrounding the tube. When working, it can be integrated into the sample introduction

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system, thus the sample preparation procedure is dramatically simplified. In this study,

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IPI samplers were coupled to ICP-MS at data-only mode. The limits of detection of

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IPI-ICP-MS for Ni, As, Cd, Sb and Pb were 0.12, 0.67, 0.027, 0.029 and 0.074 μg·L-1,

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respectively. Furthermore, 25 IPI samplers were assembled into an IPI profiler using

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3D printing in a one-dimensional array. The IPI profiler is able to analyze element

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profiles at high spatial resolution (~2 mm) every ≥24 h. When deployed in

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arsenic-contaminated paddy soils, it depicted the distributions and dynamics of

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multiple elements at anoxic-oxic transition. The results show that the IPI sampler is a

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powerful and robust technique in monitoring dynamics of element profile in soil

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porewater at high spatial resolution. The method will greatly facilitate studies of

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elements behaviors in sediments of wetland, rivers, lakes and oceans.

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TOC Art

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Introduction

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Aquatic sediments play an important role as sinks for metals and metalloids 1. The

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chemical speciation and mobility of elements are mainly controlled by the redox

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condition, pH and organic matter content of the sediments

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conditions, a redox transition occurs along the soil-water interface (SWI), and the

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biogeochemical characters vary at the mm scale in sediment 5. The element profile

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along SWI varies when the external environment changes (e.g. pH, seasonal wet-dry

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cycle, exogenous pollutants introduction, microbial degradation, wetland plant roots

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activity)

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adsorption under oxidizing conditions, but mobilized by the desorption of As from

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mineral oxides under reducing conditions

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result in a distinct change of As concentration in a saturated soil profile. As a gate

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controlling the mass exchange between the sediment and waterbody, it has always

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been of great interest to study the spatial as well as the temporal distribution of

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elements along the SWI at high-resolution. However, a deep understanding of the

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elemental behavior in the environment has been hindered to date by the techniques

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available to sample this zone.

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6-9.

2-4.

Under natural

For example, arsenic (As) in soils is immobilized by mineral oxides

4, 10.

The redox changes along the SWI

Many techniques have been developed to study the behavior of elements along

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the SWI. Initially, slicing of frozen soil cores under anaerobic conditions was used

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11, 12.

5,

However, the soil slicing method is destructive, complex and easily 4


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contaminated 13. More recent work has focused on non-destructive sampling methods.

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The rhizon sampler is a typical non-destructive method that is at low cost and easy to

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operate

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measure the vertical distribution of elements 15. However, the sampling zone of rhizon

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samplers is poorly defined and generally greater than 1 cm, its application in

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high-resolution SWI studies is limited.

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14-18.

When the samplers are placed horizontally along a profile, they can

Compared to rhizon samplers, peeper, diffusive equilibration in thin films (DET) 4, 19-25.

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and diffusive gradient in thin-films (DGT) are much powerful

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DET/DGT

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diffusion/adsorption. When equilibrated, the liquid or gel is removed for element

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mapping analysis. The techniques have been used to map elements with high spatial

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resolution (μm to cm level). Although peeper and DET/DGT are powerful in mapping

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element profile, they can only be used once after deployment, which limits the

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application of the technique in tracking temporal element change 4.

probes

passively

sample

the

solutes

in

Peeper and

porewater

through

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In this study, a novel technique, called Integrated Porewater Injection (IPI)

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sampler, was developed to map spatial and temporal distribution of elements along

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the SWI. The IPI sampler uses a single hollow fiber membrane tube for passive

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sampling and active sample injection, thus the workload for sample preparation is

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minimized. We investigated the limit of detection (LOD), influences of carrier

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solution and humc acid (HA), and the characteristics of time-dependent diffusion 5



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across the sampling tube. We also reported the integration of an array of IPI samplers

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(IPI profiler) into a device that enables one-dimensional element profiling. The

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performance of IPI profiler was evaluated in field-collected paddy soils under a

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changing anoxic-oxic environment by pumping N2 or air.

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Materials and methods

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Reagents and Materials. The chemicals used in this study were of analytical or

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electronic grade and supplied by Aladdin Chemical Reagent Co., Ltd. (Shanghai,

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China), unless stated elsewise. Calibration standards, including nickel (Ni), arsenic

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(As), cadmium (Cd), antimony (Sb) and lead (Pb), were purchased from Guobiao

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(Beijing) Testing & Certification Co., Ltd (Beijing, China). Humic acid (HA) was

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provided by Alfa Aesar (catalog # 41747). All solutions were prepared with ultra-pure

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water (18.2 MΩ cm, Millipore Corp., Bedford, USA) deoxygenated by purging with

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pure N2 for more than 4 h. The sampling tube is made of a hollow fiber membrane

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tube (inner*outer diameter = 1.0*2.0 mm, polyvinylidene fluoride, Motimo

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Membrane Technology Co., Ltd, Tianjin, China). The membrane has a pore size of

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0.05 μm, and pore rate of 70-80 %. All the materials were ultrasonic washed with 2 %

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HNO3 and 1 % Decon solution for 15 min respectively, and rinsed five times with

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ultra-pure water. All the narrow tubes were oven-dried at 40 °C before use.

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The soil samples used in the study were collected from paddy fields in Gan Zhou

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(GZ, 25°30 'N, 114°36'E) and Qing Yuan (QY, 23°35'N, 113°20'E), China. The top 6


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20 cm of soil was sampled. Soil samples were wet sieved through a 1 mm diameter

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sieve. The soil characteristics are shown in Table S1.

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IPI sampler design and IPI profiler assembly. The structure of a typical IPI

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sampler is shown in Fig. 1A (Tidu Inc. Suzhou, China). The IPI sampler contains four

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components: 1. the hollow fiber membrane tube (20 mm in length), with a volume of

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16 μL; 2. the diffusion depressor (0.3*0.8*5 mm, inner*outer diameter*length, PTFE);

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3. pipe (silica tube, 0.5*1.5*180 mm, inner*outer diameter*length), with a volume of

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35 μL; 4. cap (carbon fiber, 0.8*20 mm, diameter*length). The hollow fiber

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membrane serves as a passive sampling tube during the sample collection (loading)

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stage. The diffusion depressor is designed to suppress the side diffusion of solutes

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from the hollow fiber membrane tube to the conduits through narrowing their

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connection. When connected to ICP-MS, the porewater sample in hollow fiber

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membrane tube is pumped out through the conduits. In order to avoid the negative

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influence of oxygen and dust in atmosphere, a carbon fiber cap is used to seal IPI

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sampler during sample loading stage. In order to extract soil porewater at different

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depth along SWI, IPI profiler was used. An IPI profiler is consisted of 25 IPI samplers,

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which are assembled side by side in a 3D printed holder (Fig. 1C). The IPI sampler

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and IPI profiler were stored in carrier solution before use.

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Fig. 1 Schematic diagram of IPI sampler. (A) the loading stage of IPI sampler, element ions diffuse through the hollow fiber membrane; (B) the sample injection stage of IPI sampler, the solution inside the hollow fiber membrane is pumped into ICP-MS; (C) photos of IPI profiler (from left to right, front, back and working in soil). Notes (A): 1. hollow fiber membrane; 2. the diffusion depressor; 3. pipe; 4). cap.

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The working procedure of IPI sampler. Once the IPI sampler is deployed into

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solution or saturated soil, the small molecules and ions diffuse into the hollow fiber

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membrane tube through the membrane (passive sampling stage, Fig. 1A). When

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equilibrium state is reached, the solution inside the tubes is introduced into ICP-MS

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(NexION 350X, PerkinElmer, Inc., Shelton, CT USA) (active sample introduction

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stage, Fig. 1B). When introducing the samples into ICP-MS, the caps were removed;

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the IPI sampler was integrated into the sampling system by replacing the sample loop

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in a six-way valve. The sample solution in an IPI sampler is pumped by ICP-MS

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peristaltic pump at the speed of 24 rpm. Twenty μg·L-1 rhodium in 4 % HNO3 was

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used as the internal standard. The ICP-MS conditions were as follows: STD mode;

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data only analysis; RF power 1600W; plasma gas flow rate 15 L·min-1; auxiliary gas

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flow 1.2 L·min-1; nebulized gas flow 0.94 L·min-1; the uptake flow rate is 0.5

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mL·min-1; nickel sampling and skimmer cones were used.

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Carrier solution test.

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μg·L-1 Ni, As, Cd, Sb and Pb before test. Six kinds of matrix condition were tested: i)

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ultra-pure water in acidic condition (pH 1); ii) ultra-pure water in near-neutral

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condition (pH 6); iii) 10 mM NaNO3 in pH 1; iv) 10 mM NaNO3 in pH 6; v) 10 mM

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NaCl in pH 1; vi) 10 mM NaCl in pH 6. The solution pH was adjusted by using

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NaOH or HNO3.

The IPI samplers were deployed in the test solution of 10

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The carrier effect of ultra-pure water and 10 mM NaCl carrier solutions in pH 6

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were also tested in flooded soils. The IPI samplers were deployed at a depth of 2 cm

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in two flooded paddy soils (GZ and QY) respectively. After 24 h incubation, the

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sample was analyzed by directly introducing to ICP-MS. 9



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Humic acid test. The humic acid (HA) was prepared according to the previous study

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26.

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mM NaCl solutions (20 and 100 μg·L-1) during a period of 0-48 h. Three levels of HA

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were tested: 1) 0 mg·L-1 HA ; 2) 5 mg·L-1 HA; 3) 20 mg·L-1 HA.

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The time-dependent response and calibration curve. The time-dependent response

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of IPI samplers were conducted in acidic and near-neutral conditions (pH 1 and 6; 10

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μg·L-1 Ni, As, Cd, Sb and Pb; 10 mM NaCl). The carrier solution was 10 mM NaCl.

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The solutions inside the samplers were measured after 0.5, 1, 3 and 6 h equilibrium

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time.

The influence of HA on Ni, As, Cd, Sb and Pb was investigated in pH 6 and 10

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When developing the calibration curve a series of standard solutions, containing

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1.0, 2.0, 5.0, 10, 20 μg·L-1 Ni, As, Cd, Sb and Pb at pH 1 and 10 mM NaCl, were

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measured. The carrier solution was 10 mM NaCl. The ICP-MS measurements were

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conducted after 24 h of equilibrium time.

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Comparison of IPI sampler and rhizon sampler. Rhizon sampler and IPI sampler

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with the same hollow fiber membrane tube were compared in flooded soils. The

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samplers were fixed alternately on a clean acrylic plate with an interval of 1 cm in

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triplicate, and deployed horizontally at a depth of 2 cm in GZ and QY soils. After 24

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h equilibration, the sample in IPI sampler was analyzed by ICP-MS. Then, 2 mL

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porewater was extracted by rhizon sampler and analyzed by ICP-MS.

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Non-destructive measurement of the temporal element profile change in SWI. An

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IPI profiler is made of 25 individual samplers with the assistance of a 3D printer. The

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IPI profiler is able to measure the elements distribution over a distance of 6 cm, with a

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resolution of ~2 mm. To detect the element profile, the IPI profilers were inserted into

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paddy soils which have been flooded for half a year, with 1 cm above SWI and 5 cm

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buried in paddy soils (Fig. 1C). The porewater element concentrations were measured

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every day under different environmental disturbance: Day 0, IPI profiler installation;

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Day 1, no disturbance; Day 2-3, continuously N2 bubbling in surface water; Day 4-6,

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continuously air bubbling in surface water; Day 7-8, continuously N2 bubbling in

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surface water. Nitrogen gas was provided by a high pressure tank, air by an air pump.

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Three pipettes were deployed evenly at 1 cm above the SWI in the container, and the

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bubbling in surface water was via these pipettes.

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Statistical Analysis. We used the statistical program R version 3.5.0 to analyze and

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plot the data. Standard deviation (SD) was used to show the variance. All raw data

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and the R scripts used to process and analyze the data can be found in Supporting

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Information. A one-way ANOVA (p 0.995. According to

The calibration curve was generated by deploying the

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IUPAC definition 39, 40, the LODs are 0.12, 0.67, 0.027, 0.029 and 0.074 μg·L-1 for Ni,

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As, Cd, Sb and Pb, respectively.

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Comparison of IPI sampler and rhizon sampler. The comparison of IPI sampler

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and rhizon sampler for measuring Ni, As, Cd, Sb and Pb was carried out in GZ and

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QY paddies (Table S3). Cadmium was below the LOD in both samples. The two

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methods showed no difference in measuring As and Sb. However, lower Ni and Pb

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were obtained by IPI sampler than rhizon sampler for both soils, which can be

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explained by the different sampling mechanisms for rhizon samplers and IPI samplers.

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Tension-free samplers, like IPI sampler, measure the metal species with the molecular

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size enabling them to pass through the diffusion pores on the sampler freely. In

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principle, free metals and colloidal metals with the size < 50 nm are able to be

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measured by the IPI sampler. However, larger size particles may be squeezed into

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rhizon samplers by the tension generated by syringes, which lead to an

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over-estimation of metal concentrations.

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some Ni, Cd and Pb form complexation with HA and cannot diffuse into IPI samplers,

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the complexation effect is insignificant for As and Sb(Fig. S3). Thus, in soil condition,

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IPI samplers have comparable performance with rhizon samplers for elements in ions

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or small moleculars, like As and Sb, but give smaller counts if huge colloidal metals

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exist.

33.

The experiment on HA addition showed

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Non-destructive spatial and temporal metal measurement by IPI profiler. With

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developed calibration curves, the IPI profiler is able to measure the dynamic changes

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of element profile in two As-contaminated paddy soils. The soils were exposed to

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different redox conditions by bubbling air or N2 gases. The vertical changes of As in

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GZ soil porewater are depicted in Fig. 3. At Day 1, the As concentrations were

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generally lower than 40 μg·L-1 with small variation. The As levels increased slightly

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with the depth. The deployment of IPI profiler at Day 0 was probably responsible for

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the low As concentration observed at Day 1 as it may introduce oxygen to soils.

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Similar As gradients along SWI were clearly observed in other days. The results

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represented a typical redox controlled element profile, which have been widely

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reported in other studies 2, 4, 9.

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The temporal changes of As profile in SWI were rarely reported. In this study, we

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are able to track the daily changes of As profile by using IPI profiler. In GZ soil, the

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As profile below 10 mm of SWI was not influenced by the surface water redox

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condition, but porewater As near SWI was sensitive to anoxic-oxic transition (Fig.

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4A&S4A). It increased under anoxic conditions (Day 2-3, Day 7-8) and decreased

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under oxic conditions (Day 4-6). The increase of As after pumping N2 (Day 2-3, Day

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7-8) around SWI could be attributed to the release of As during metal mineral

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reduction and dissolution when the redox conditions at SWI shifted from oxic to

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anoxic, and from day 4 to 6, the air pumped into surface water facilitated Fe oxides 20


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formation and As(III) oxidation at SWI, thus As was adsorbed by the oxides and

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decreased in porewater

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QY soil (Fig.S5A). The result reflects a quick matter movement in sandy GZ soil and

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a relative slow change in loamy QY soil

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profiler is a powerful and robust tool to trace the temporal change of elements.

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However, more tests are essentially needed before applying IPI profiler in other

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environments, like salty or acidic soils and sediments.

4, 41, 42.

However, no obvious As fluctuation was observed in

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This finding strongly supports the IPI

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Fig. 3 The vertical profile changes of soil porewater arsenic (As) along the 60 mm SWI in 8 days under different conditions. The error bars are standard deviations (SD, n=3)

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The dynamic changes of other elements, including Ni, Cd, Sb and Pb, were also

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tracked and presented in Fig. 4&Fig. S5. The concentrations of those elements were

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much lower than As. Nickle profile was constant with time in both soils (Fig. 4B&Fig. 21



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S5/6/7B). Spatially, porewater Ni concentrations were higher than Ni in surface water

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in QY soil. The concentrations of Cd in both soils were as low as

Tracing the Dynamic Changes of Element Profiles by Novel Soil

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