H Galet

D Saugnac

D Brugnara

D Ramos

P Beaumel

Blache

F Galtarossa
email: franco.galtarossa@lnl.infn.it

M Pierens

M Assié

V Delpech

F Galet

H Saugnac

D Beaumel

P Blache

M Chabot

F Chatelet

E Clément

F Flavigny

A Giret

A Gottardo

J Goupil

A Lemasson

A Matta

L Ménager

E Rindel

HeCTOr: the 3 He Cryogenic Target of Orsay for direct nuclear reactions with radioactive beams

Keywords: Direct nuclear reactions, Cryogenic target, Radioactive ion beams

Introduction

Direct nuclear reactions, where one or few nucleons are exchanged between projectile and target, have proved to be extremely powerful tools to study the structure of atomic nuclei [START_REF] Satchler | Direct nuclear reactions[END_REF] and to investigate a large variety of astrophysical scenarios [START_REF] Bardayan | [END_REF]. Their selectivity to the nature of the populated states and their sensitivity to the transferred angular momentum help to obtain a detailed spectroscopy of nuclei and access fundamental properties such as the quantum numbers and single-particle character of ground and excited states. These are the fundamental ingredients for the determination of nuclear shell properties and their evolution across the nuclide chart. The selectivity of the reaction mechanism results, in turn, in small cross sections, typically of few mbarn.

With the development of radioactive ion beam (RIB) facilities (see [3] and references therein) new possibilities are offered for the study of the structure of exotic nuclei via direct reactions in inverse kinematics, with the heavy unstable projectile impinging on the light target at energies ranging from few to several hundreds MeV/u [4]. RIB intensities are several orders of magnitude lower than stable beams, and thus require high detection efficiencies and the use of thick targets (typically few mg/cm 2 ).

For reactions involving hydrogen and deuterium, films of polypropylene like CH 2 and CD 2 are commonly used, thanks to their simple handling. The main drawback is the background generated by reactions on carbon, whose contribution needs in most cases to be estimated with a dedicated measurement at the expense of the effective beam time. Morover the presence of heavier elements increases, along with the target thickness, the energy and angular stragglings.

Part of these limitations can be overcome by developing pure targets where the gas is confined in a small volume and cooled down to cryogenic temperatures. In this way the density of scattering centers can be increased up to a factor 50. Many examples exist in the literature of hydrogen (H 2 ) and deuterium (D 2 ) cryogenic targets designed for low-and intermediate-energy nuclear physics experiments (see [5] and references therein).

Cryogenic targets of 4 He [6,7,8,9] and 3 He [10,11,12] are generally less widespread than H 2 targets and the existing ones are not specifically designed to be employed in experiments with low-energy (∼ 10 MeV/u) and low-intensity (∼ 10 4 -10 5 pps) beams for transfer reactions. In particular some are conceived for measurements, like for instance electron scattering or photo-absorption, where temperatures lower than 4 K and/or target thicknesses of few cm are needed to reach areal densities of tens or hundreds of mg/cm 2 [6,7,8,9,11]. Others involve measurements with polarized 3 He nuclei that can be performed also in storage rings, where densities of scattering centers of 10 16 -10 18 atoms/cm 2 are sufficient [10,12].

Valid alternatives conceived for low-energy nuclear reactions have been recently developed. Solid targets obtained by implantation [13,14,15,16] have the advantage to be very compact and easy to handle, whereas the employ of windowless and gas jet targets [17,18,19] allows to strongly reduce background and stragglings generated in the target windows. In both cases, though, target thicknesses of the order of 1 mg/cm 2 are hardly reached.

In this paper we report on a 3 He cryogenic target where densities of scattering centers of the order of 10 20 atoms/cm 2 , normally sufficient to cope with low RIB intensities, are obtained with a target thickness of few mm and an operation temperature higher than 4 K. These characteristics make it particularly suited to perform direct transfer reactions, such as ( 3 He,d), ( 3 He,p), ( 3 He,n) and ( 3 He,α), with radioactive beams in inverse kinematics at bombarding energies ranging from few to tens of MeV/u. These reactions allow to tackle several interesting physics cases, among which we mention shell evolution along neutron shell closures, neutron-proton pairing in unstable nuclei, or proton drip-line physics. The outreach of physics programs dedicated to such studies will strongly profit from the installation of the target in fragmentation or next-generation European post-accelerated ISOL facilities, SPES [20] and HIE-ISOLDE [21], or the low-energy branch of FAIR [22].

Constraints on the target design

Properties of the reactions of interest, such as beam intensity and energy, type of reaction and detected reaction products, strongly constrain the design of the target and lead to several specific requirements. In this Section we will report general considerations that should be taken into account in the design of a cryogenic target for our purposes, in Section 3 we will describe in detail the specific design of HeCTOr.

Dimensions. The target has to be coupled to existing detectors which are usually arranged in compact configurations around a traditional foil target of small dimensions. For this reason the dimensions of the target and the associated cryogenic equipment have to be as reduced as possible. At the same time, the target radius must be large enough to give the possibility to deal with both ISOL and fragmentation beams. Typical ISOL beams have dimensions on target σ ∼ 2-3 mm, while fragmentation beams can feature beam spots at least a factor of 2 larger. Thickness. To measure transfer cross sections usually of the order of few mb with beams of low intensity (< 10 5 pps), the choice of the target thickness must balance the need of sufficient luminosity and good energy resolution. These two requirements go in opposite directions. High luminosity calls for the use of thick targets, of the order of few mg/cm 2 , which can be obtained with a large target cell and operating at cryogenic temperatures. Good energy resolution, which mainly depends on energy losses, stragglings and re-interactions in the target, requires instead thin targets, usually of the order of 100 µg/cm 2 or less. The best choice usually depends on the specific physics case.

Window thickness and material. The target windows should be as thin as possible to reduce stragglings, energy losses and contributions coming from background reactions. At the same time they should be thick and elastic enough to withstand considerable deformations due to the high difference in pressure between gas inside the target cell and vacuum around it.

Transparency. The frame surrounding the gas target must have an opening around the target as large as possible to allow the emitted particles to reach the detectors, thus increasing the detection efficiency. Very large openings would require thicker windows and are difficult to obtain due to mechanical constraints. The target can then be designed to have a larger opening in the hemisphere (forward or backward) where the laboratory cross section of the reaction channel of interest is higher. The transparency to electromagnetic radiation (X and γ rays) emitted by the reaction products can be another important requirement. This once again calls for a reduction and an accurate choice of the materials, surrounding the gas target, that might "shadow" the γ-ray detector, absorbing part of the electromagnetic radiation emitted by the reacting nuclei.

Target versatility. A good versatility is an essential property to cover a large number of physics cases without deeply modifying the set-up between different experiments. For instance, the possibility to change the gas inside the target cell, to modify its pressure and temperature and to switch forward and backward sides gives considerable flexibility to the target and allows to perform a large variety of reactions.

HeCTOr design and operation

In this Section we will describe the characteristics and operation principles of HeCTOr, the 3 He cryogenic target developed at IJCLab and designed to perform direct reactions with radioactive beams in inverse kinematics from few to tens of MeV/u. All the requirements described in Section 2 have been carefully taken into account in its design. In Section 4 we will present the performance during the first in-beam experiment, where the target was coupled to the MUGAST [23] silicon array, the AGATA [24,25] γ-ray array and the VAMOS [26] large-acceptance magnetic spectrometer.

The target cell. The target cell, shown in Fig. 1, is composed of a main copper frame to which two conic copper flanges are screwed, one on each side of the frame. A thin foil, serving as target window, is then glued to each flange with epoxy resin glue. The diameter of the smaller base of the cone is 16 mm, the frame thickness is 15 mm and the nominal target thickness is 3 mm. The nominal target thickness is defined as the distance between the two windows when 3 He gas is at the nominal operation pressure of ∼ 1 bar inside the target, neglecting window deformations. An indium wire is placed between the conic flange and the frame to avoid leakages at cryogenic temperatures between the 3 He circuit and the vacuum around it. The different parts composing the target cell can be seen in the magnified view of Fig. 2.

A coil-shaped copper thermalization block is brazed at the bottom of the frame to ensure good thermal contact and operates as a heat exchanger to cool down the target by means of liquid 4 He flowing through it. The block contains a pipe with an inner and outer diameter of 4 mm and 6 mm, respectively.

The conic flange has a larger angular opening (130 • ) in the backward hemisphere to adapt for stripping reactions2 , like ( 3 He,d) or ( 3 He,p), where the light recoiling particles are mainly scattered at θ lab > 90 • . The target could also be filled with 4 He when necessary. For pick-up reactions, where the ejectiles are mainly scattered forward, like ( 3 He, 4 He) but also ( 4 He, 6 He) or ( 4 He, 8 Be), it can be rotated to adapt to the specific reaction.

For the target windows the considerations outlined in Section 2 led to the choice of 3.8-µm thick foils of Havar, an alloy of cobalt (42 %), chromium (20 %), nickel (2.7 %), tungsten (2.2 %) and other materials in smaller percentage. The corresponding areal density of each foil is 3.15 mg/cm 2 . The typical pressure of gas in the target is 1 bar absolute. The relatively high pressure and the elasticity of Havar induce a deformation of the windows, defined as the maximum distance along the beam direction between the nominal target thickness and the Havar window. Such deformation is measured using a micrometric probe placed in the center of the target window. A first measurement is made with the same (atmospheric) pressure in the gas target and around it. A second one is carried out setting a pressure of 2 bars inside the target and atmospheric pressure around it, in order to have a pressure difference of 1 bar, as in the case of nominal operation. The difference between the two values gives the window deformation, measured to be (0.70 ± 0.05) mm at room temperature. Thanks to the properties of Havar, a similar value is expected also at cryogenic temperatures (5 -6 K). When it is cooled down to such temperatures, the equivalent areal density of 3 He gas is ∼ 2 mg/cm 2 and the density of scattering centers is ∼ 3.5 • 10 20 atoms/cm 2 , about 2-3 orders of magnitude higher than what can be obtained with typical solid 3 He-implanted or gas jet targets. Since a small gradient of temperature is present along the cooling circuit in the gas cell region, the target temperature is determined as the average of the temperatures measured by two probes, visible in Fig. 1 (top), placed at the top and bottom of the target cell.

Cryostat description. The small volume of 3 He gas contained in the target cell is cooled down to cryogenic temperatures by circulating liquid 4 He (LHe) around the target region. The LHe is initially injected in the system from the top of the cryostat, defined as the assembly of HeCTOR, visible in Fig. 3, and the MUGAST vacuum chamber (refer also to Fig. 4, where the cryostat is enclosed by the dashed gray line). The LHe is then stored in the tank, or phase separator (T1), from which it can finally reach the target. To minimize the consumption and allow the LHe to maintain the cryogenic temperature, a thermal shield is provided by liquid nitrogen (LN 2 ) at atmospheric pressure and at a temperature of about 77 K, measured with a PT100 (TT02) placed at the bottom of the LN 2 tank (T2).

The target is insulated from the external environment through the outer wall of the cryostat, composed in the upper part of a cylindrical vessel made of AISI 304L stainless steel, with an outer diameter of 450 mm and a length of about 900 mm, and in the lower part of the MUGAST vacuum chamber. The LN 2 and LHe circuits, comprising the LN 2 and LHe tanks and the 3 He gas target, are hosted inside the outer wall. The target is supported by a glass-epoxy rod, a heat-insulating material, with a length of 812 mm, an inner diameter of 28 mm and an outer diameter of 30 mm, attached to the LHe tank T1. The total height of HeCTOr, from the top of the vessel to the bottom of the target cell, is approximately 1.73 m. The operation scheme of the cryostat is shown in Fig. 4. In this figure the outer wall, which can be seen as the first layer of insulation, is represented by the dotted gray line. A second layer, in orange, is maintained at LN 2 temperature and acts as a thermal shield to insulate the cold gas target from radiations from the vacuum vessel at room temperature. This shield is composed at the top of the LN 2 tank and its cover (orange dashed line) and in the central part of two 1.5-mm thick sheets of copper with an asymmetric hemicylindrical-like shape that connect the tank T2 to the target cell and enclose the glassepoxy rod (see Fig. 3 right). These sheets are mechanically and thermally anchored to the tank and cooled by thermal conduction. The bottom of the thermal shield covers the target cell, as shown in Fig. 1 (bottom), to protect the cold mass from the room temperature. A cut of the sheets is made at the level of the target windows to ensure transparency to the beam. The thermal shield is covered by a multi-layer insulation (MLI), or super-insulation), where each layer acts as a thermal shield, reducing the radiated heat transfer from the vacuum vessel to the shield. MLI consists of stacks of double-aluminized Mylar to reflect heat radiation, separated by low-conduction spacers to limit heat transfer by conduction. The temperature of each layer is left floating and then depends on the global thermal equilibrium of the system. MLI reduces the heat radiated to the cold mass by a factor ∼ 200 with respect to the case where it is not present. The upper part of the shield and the cold mass are covered by 30 and 10 layers of MLI, respectively.

The cooling circuit includes the tank T2 with an annular shape and a capacity of about 13 L, a filling and an exhaust line. The tank T2 is supplied with LN 2 flowing out of a Dewar connected via a flexible vacuum-insulated transfer line on top of the cryostat. The cold vapors are warmed at the outlet of the circuit before returning to the atmosphere, avoiding ice formation at the exhaust of the cryostat.

The cold mass, or Helium-4 circuit, shown in blue in Fig. 4 and indicating the 3 He gas target and its active cooling circuit, is located at the center of the system. LHe at atmospheric pressure is used to maintain the target temperature below 7 K. The circuit includes the cylindrical phase separator T1 with a useful capacity of about 11 L, filled by a LHe Dewar via a vacuuminsulated and flexible transfer line. The cold vapors return to the atmosphere through the outlet pipe where the presence of an electrical heater avoids the formation of ice. The 3 He cryogenic gas target is cooled down by means of LHe coming from T1 and passing through the coil-shaped thermalization block brazed at the bottom of the copper frame. Cold vapors from the boiling helium are returned to the top of the phase separator and flow out of the cryostat via the tank outgassing line.

The Helium-3 circuit includes, outside the cryostat, a setup based on a dry primary vacuum pump to inject or recover the gas in the target inside the cryostat and a tank with a volume of 90 L for the 3 He gas storage at sub-atmospheric pressure (28 mbar). To fill the target with 3 He gas, the pump is turned on and the valves SV01 and SV02 are opened. The needle valve NV01 moderates the flow to the target to limit sudden pressure changes. For the gas recovering procedure the pump is switched on and the valves SV03 and SV04 are opened. The back-pressure regulator BPR01 prevents the target from over- pressure, while BPR02 protects the pump. The 3 He circuit in the cryostat acts as a filling and recovery line to set the pressure in the target.

The isolation vacuum level is measured by means of a combined Pirani and Penning vacuum gauges. We have available a test bench with a dedicated small chamber where the target can be accommodated. In this configuration, the vessel with a volume of about 156 L is pumped by a turbomolecular pump with a pumping speed of 150 L/s in series with a primary rotary vane pump with a pumping speed of 8 m 3 /h, allowing to reach a vacuum of about 10 -5 mbar absolute at 300 K. When the system is cold, there is a gain of an order of magnitude on the vacuum level. The LN 2 Dewar is provided with a self-pressurization system and the transfer is controlled by an ON-OFF pneumatic valve (PV600). A high and low thresholds are set on the LN 2 level in the tank to automatically stop and start the transfer procedure. A PT100 thermometer (TT02) is installed at the bottom of the tank to check in particular the first filling during the cooldown process, since it represents the most delicate step. The LHe Dewar is equipped with a heater (EH01), a back-pressure regulator (BPR03) and a solenoid valve (EV01) to pressurize the Dewar. When a transfer is needed EV01 is closed, forcing the gas to flow through the back-pressure regulator BPR03 with a pressure inside the Dewar set to about 1.1 bar, and the heater is activated to produce vapors. This increases the pressure in the Dewar, allowing a transfer of LHe via the activation of the pneumatic valve PV01. A pressure transmitter (PTLHE) monitors the pressure of the Dewar. A LHe level sensor measures the quantity of LHe remaining in the Dewar while a second one monitors the level of the liquid in the tank inside the cryostat to start and stop the transfer. Cernox TM 1050 AA thermometers are located at the bottom of the LHe tank (TT01), on top and at the bottom of the target (TT04 and TT05), on the thermalization block (TT03) and on the outgassing of the cooling circuit of the target (TT06). The pressure of the 3 He storage tank and of the target are measured by piezoresistive pressure sensors (PIPT01 and PIPT02, respectively).

The acquisition and actuators controls are performed by means of a multiplexer Agilent 34970A equipped with two 34901A cards to measure resistances in four-wires mode, LHe level (4-20 mA) and pressure sensors (0-10 VDC). A 34903A relays card is used to activate the heater of the LHe Dewar and the ON-OFF valves. A LabVIEW program communicates with the multiplexer to manage LN 2 and LHe transfers and provides system supervision and data logging.

Cryostat operation. The first step in the procedure to cool down the target is to pump the vacuum vessel below 10 -5 mbar at room temperature. In the experimental configuration described in Sec. 4 the target is accommodated in the MUGAST reaction chamber, with a volume of 515 L. To make the vacuum we employed a primary pump Agilent IDP-10, two turbomolecular pumps Agilent Turbo-V750 TwisTorr (model 9696018) with a pumping speed of 700 L/s for N 2 and a cryogenic pump, composed of a compressor unity Coolpak 4000 Leybold and a cold head Cryo-plex 8 model 350 Oxford instruments. This pumping system allowed to reach a vacuum level of 10 -6 mbar.

The LN 2 circuit does not need a specific conditioning. By opening the PV600 valve, LN 2 starts to flow out of the Dewar towards the cryostat and the air remained in the circuit is flushed. For the LHe circuit, the high risk of blocking due to the possible solidification of air in the circuit prevents from carrying out the same simple procedure. The circuit is then conditioned by pumping it below 1 mbar and injecting 4 He gas slightly above the atmospheric pressure. This operation is then repeated at least three times. The 3 He circuit is pumped down to 10 -6 mbar via the purging port (HV01) with all solenoid valves opened and isolated by closing HV01 and then the solenoid valves.

The thermal shield is cooled down to 77 K by means of LN 2 . When the nominal working mode is reached the temperature is maintained stable keeping the LN 2 level between the high (LS01) and low (LS02) level until the end of the experiment. The 4 He circuit is cooled down by pressurizing the Dewar and opening the transfer valve PV01 until the tank T1 is filled. The cooling circuit of the target starts and, when the target reaches an equilibrium temperature of about 7 K, the target operation can start. The quantity of LHe needed to cool the target down to this temperature is about 200 L.

Once the cryostat is working in nominal conditions, the hand valve HV02 is opened and the target is filled with 3 He gas to atmospheric pressure and maintained during the experiment. The power consumption of the target at equilibrium is ∼ 1.7 W.

After the experiment, the LN 2 and LHe supply are stopped simultaneously and the cryostat begins to warm up until the room temperature is reached. The 3 He gas is recovered and stored in the tank at low pressure. The 3 He gas is not purified after each operation but only when a possible contamination with other gases might have occurred during the target operation. In such a case the tank can be connected to a specific purification tool, composed of a cold trap, to eliminate impurities from the gas.

Performance during the experiment

HeCTOr has been employed for the first time in an in-beam experiment in June 2019 at GANIL [START_REF]Grand Accélérateur National d'Ions Lourds[END_REF] in Caen, France. The aim of the experiment was the determination of the proton occupancies in the ground state of the N=28 46 Ar isotope, which can be achieved measuring the differential cross sections of the deuterons emitted in the proton stripping reaction 46 Ar( 3 He,d) 47 K.

In this Section we will describe the experimental set-up employed in GANIL and the performance of HeCTOr. This will allow us to extract and discuss important experimental quantities that must be evaluated when designing and analyzing an experiment with such a target, in particular stability over time (Section 4.1), energy losses (Section 4.2), energy resolution (Section 4.3), and transparency to γ rays (Section 4.4).

Experimental conditions and beam monitoring

The SPIRAL1 [START_REF] Villari | [END_REF] 46 Ar beam, produced by the fragmentation of 48 Ca, is re-accelerated at 10 MeV/u and impinges on the 3 He gas target with an intensity of approximately 4 • 10 4 pps. Along the beam line, about 2 m before the target, a beam tracking device, CATS [29], provides beam rate and profile monitoring and a signal for time-of-flight measurement. The detection set-up, shown in Fig. 5, is composed of the segmented silicon array MUGAST [23], coupled to the highresolution segmented HPGe array AGATA [24,25] and the large-acceptance magnetic spectrometer VAMOS [26]. This set-up allows the simultaneous detection in coincidence of both reaction partners and the γ rays emitted by the heavy recoil. The MUGAST array is composed of 12 Double-sided Strips Silicon Detectors (DSSD) surrounding the target: 5 trapezoidal and one annular detector in the backward hemisphere, two square detectors close to 90 • and 4 MUST2 [30] detectors at forward angles. The light ejectiles (deuterons in this case) are identified through E-ToF correlation and their scattering angle and energy are measured with an angular resolution better than 1 degree and an intrinsic energy resolution of ∼ 40 keV. The excitation energy of the heavy recoil ( 47 K) is then determined via twobody kinematics. AGATA is placed upstream with respect to MUGAST and detects the γ rays emitted by the reacting nuclei. The typical efficiency of AGATA at 18 cm from the target, with MUGAST installed, is 6-7 % for 1.3-MeV γ rays. VAMOS is placed at 0 • for the detection of the heavy residue and its identification in atomic number and mass. The coincidence with VAMOS allows to strongly suppress contributions from other reaction channels, in particular fusion-evaporation reactions on the target windows. Further details on the detection set-up can be found in Ref. [23].

The target system, inserted in the chamber from the top with a crane and secured to a mechanical support attached to the chamber, is placed 25 mm downstream with respect to the nominal target position, due to mechanical constraints, and cooled down to ∼ 6 K before the beginning of the beam time.

The 46 Ar beam ions passing through the target are detected in the focal plane detectors of VAMOS and separated depending on their magnetic rigidity Bρ, according to:

Bρ = 3.105 • βγ A q (1)
where A and q are the mass number and atomic charge of the ions, respectively, β = v/c (v is the velocity of the ions) and

γ = (1 -β 2 ) -1/2 .
When the target is empty the beam particles lose energy passing through the CATS detector and the two Havar windows. When the target is filled with 3 He the energy loss of beam and light ejectiles in the gas has to be considered as well. By monitoring the temperature and pressure of the gas in the target cell during the whole experiment, possible changes in the gas density can be identified and accounted for. Figure 6 shows the trend of the target temperature (in blue) and pressure (in red) as a function of time during 4 days of experiment. The t = 0 reference has been arbitrarily set at the midnight of the day when the experiment started and has been kept the same in Figure 7. We started to fill the target with 3 He at t ∼ 19 h, reaching the final pressure of 700 mbar at t ∼ 27.5 h. During the filling and the rest of the experiment the target temperature remained stable and close to 6 K. At this temperature and pressure the 3 He density is 4.5 mg/cm 3 . The maximum target deformation, measured to be 0.7 mm per side at 1 bar pressure, scales down to 0.5 mm for a pressure of 700 mbar. The corresponding areal density is ∼ 2 mg/cm 2 and the density of 3 He scattering centers is ∼ 3.5 • 10 20 atoms/cm 2 . The level of vacuum in the chamber, stable during the whole experiment, was 1.5 • 10 -6 mbar. The small increases in temperature and pressure occurring approximately each 6 hours, visible in Fig. 6, are due to the change of pressure occurring in the thermalization block when the LHe transfer process starts. Such change temporarily interrupts the LHe circulation in the pipe, causing a small increase of the temperature. Then, when LHe starts to flow again, the temperature eventually decreases.

The target thickness can be determined and monitored over time during the experiment by measuring the change in beam ion velocity after the target with VAMOS, according to Eq. 1, and computing the corresponding energy loss in the target. The top panel of Fig. 7 shows the experimental Bρ for a time interval of ∼ 40 h after the target filling procedure has been completed. Six main structures are visible, corresponding to the most intense charge states of beam ions detected in VAMOS (from 13 + to 18 + ). The intervals where no Bρ information is available represent periods of time where the beam was not delivered. The Bρ is clearly not constant over time but decreases smoothly, in turn indicating a progressive decrease of the velocity of beamlike ions reaching the VAMOS focal plane. This behavior might be explained assuming that the target thickness, instead of being constant, slightly increases over time. Since the temperature and pressure in the target are stable (see Fig. 6), the increase of target thickness could be associated to the thickening of layers of ice forming on the target windows through the deposit of frozen gas, probably coming from the MUGAST cooling system. The presence of other contaminants, like air coming from out of the reaction chamber or small quantities of gas coming from the gas detectors of the VAMOS focal plane, cannot be a priori excluded.

A rough estimation of the expected rate of ice growth can be obtained through simple considerations of kinetic theory of gases, in the approximation of ideal gas. Under the assumption that the velocity of the gas particles in the surroundings of the target follows the Maxwell's distribution, it can be shown that the number of collisions per unit time and unit area is given by:

N c = 1 4 nv = P √ 2πmk B T (2)
where n = P/(k B T ) is the volumic density of molecules, v = √ 8k B T/(πm) is the mean velocity of the Maxwell distribution, k B is the Boltzmann constant, T and P the gas temperature and pressure, respectively, and m the mass of the gas molecules. Due to the very low temperature of the surface, the molecules will stick to it after the collision. A monolayer of thickness d 0 is then formed in a time:

t m = 1 AN c = 4 d 2 0 nv (3)
where A = d 2 0 is the area of a molecule. The volume of a molecule in the ice can be approximated as d 3 0 = M/(N A ρ) where M is the molar mass, N A the Avogadro number and ρ ∼ 10 3 kg/m 3 the ice density. The resulting monolayer thickness is d 0 ∼ 3 Å. With the appropriate substitutions, Eq. 3 can be rewritten as:

t m = √ 2πk B T P ρ 2/3 N A M 1/6 ( 4
)
For the present experiment P = 1.5 • 10 -6 mbar, T = 300 K. Assuming in a first approximation that the gas is coming only from the cooling system of MUGAST, its composition is 50 % water (M H 2 O = 18.02 g/mol) and 50 % ethyl alcohol (M C 2 H 6 O = 46.07 g/mol), so an average value M ∼ 32 can be considered. A detailed study of the gas composition would require a dedicated measurement and is beyond the aim of this simple calculation. Substituting these values in Eq. 4, we obtain a monolayer formation time t m ∼ 1.6 s, corresponding to a rate of thickness growth per window of ∼ 22 µm/day. This value is not expected to change dramatically if some impurities are added to the gas. Different inputs for M and ρ, to account for varied gas compositions, result in values of thickness growth per day which do not deviate by more than 50 % from the present value.

The ice formation rate can be monitored experimentally by measuring the Bρ of beamlike ions in VAMOS and computing the correspondent energy loss in the target. To this aim, we performed simulations with the Zgoubi code [31], where the incoming ions are propagated through the optics of the spectrometer and the energy losses in the different materials along the ion path are taken into account using SRIM [32]. In the simulation we can vary the ice thickness, under the assumption that the ice forms at the same rate on the front and back window, until a good matching between simulation and data is found. The result is shown in the bottom panel of Fig. 7, where the simulated Bρ for the different charge states is superimposed in black on the experimental data. The ice thickness at the beginning of this plot (t ∼ 35 h) has been set to 37 µm to reproduce the Bρ value after all the other energy losses of the beam (in CATS, target windows and gas) have been considered as well. The experimental trend is correctly reproduced by setting in the simulation a rate of ice formation on the target windows of a total of 22 µm/day (11 µm per window), in fair agreement with the simple calculation discussed above. The total ice thickness at t ∼ 75 h is then approximately 74 µm. The simulation also shows that the Bρ structures for t > 65 h are the expected extension of those at t < 46 h, when the proper energy loss in the target is considered.

Impact of the ice formation

The energy spectrum of the deuterons emitted in the present reaction extends down to 2.5 MeV at the most backward laboratory angles, which is also the angular region where the transfer cross sections are higher. This feature can be common to other stripping reactions at similar bombarding energies, of course depending on the reaction Q value. These particles lose part of their energy passing through the gas composing the target, the Havar window and the ice layers on the window and may eventually reach the detector with not enough energy to overcome the detection threshold. For this reason the increase of ice thickness on the target windows over time turns out to be a highly undesired effect.

To give some numbers, 2.5-MeV deuterons lose about 1 MeV of energy in 40 µm of ice, the same amount of energy that they lose in about 9 µm of Havar or in 5.5 mm of 3 He at the temperature and pressure reached in the present experiment (energy losses computed with LISE++ [33]). These numbers also help to highlight the importance of the correct balance between target thickness and energy losses for this kind of reactions. Higher density of scattering centers (∼ 30 times more) could be obtained, for instance, operating the target with liquid 3 He at ∼ 4 K, condition that would allow to perform experiments similar to the one presented in this article with beam intensities of less than 10 4 pps. However, in such a case, the deuterons would not have enough kinetic energy to exit the target or to be detected. In this sense, the temperature, pressure and target thickness at which HeCTOr is operated represent an ideal compromise between the two opposite requirements.

Future developments of the system should foresee dedicated tests to better evaluate the rate at which the ice grows and how it depends on variables such as the vacuum level and the gas composition. A possible solution to overcome the problem, however, is to reheat the target each 2-3 days during the experiment. Such operation would allow to strongly reduce the ice thickness, though at the expense of the LHe consumption. For the experiment the target has been kept cold for about 11 days. In this time interval the consumption of LHe was 2400 L, or ∼ 200 L/day. This value could be significantly reduced by installing a system to recover the exhaust LHe.

Excitation energy resolution

An important observable in direct transfer reactions is represented by the excitation energy, E*, of the heavy reaction partner. The E* distribution is a measure of the strength of population of the levels in the heavy residue, 47 K in this case, in the transfer process. Such strength is proportional to the cross section for each specific transfer channel. With the present setup this information can be obtained from two-body kinematics, by measuring the energy and scattering angle of the light recoiling particles in MUGAST. The excitation energy resolution depends on many factors (target thickness, energy of the light ejectiles, angular and energy stragglings, intrinsic resolution of the detectors, beam dimensions, ...) and represents a relevant parameter for the design and analysis of an experiment.

The expected energy resolution for the present case has been determined through simulations performed with nptool [34], an open-source data analysis and Monte Carlo simulation framework developed for low-energy nuclear physics experiments and based on the Geant4 [35] simulation toolkit and the ROOT [START_REF] Brun | ROOT -An Object Oriented Data Analysis Framework[END_REF] data analysis framework. In nptool the target is created by specifying, besides the thickness, density and type of gas, the characteristics of the front and back windows (material, radius, thickness and possible deformations) and of the frame. A possible parametrization for the target and window deformation is represented by a hyperbolic cosine function. Given the cylindrical symmetry of the target cell with respect to the beam axis z, in the forward hemisphere (z > 0) this function has the form:

f (r) = z 0 + (d + 1) -cosh r R acosh(d + 1) (5)
where r represents the distance from the beam axis, R is the target radius, d the deformation at the center of the target and z 0 half of the target thickness (refer to Fig. 8, where R = 8 mm, d = 0.5 mm and z 0 = 1.5 mm). In the backward hemisphere the profile is described byf (r). We simulated the transfer reaction 46 Ar( 3 He,d) 47 K to the ground state of 47 K, in inverse kinematics at the bombarding energy in the laboratory frame E b = 460 MeV (10 MeV/u). In a first simulation we assumed a perfectly collimated point-like beam impinging on the target in (x,y) = (0,0). In this way we can determine the contributions to the excitation energy resolution coming only from the target and the detectors. The experimental set-up here considered is composed of HeCTOr, placed 25 mm downstream with respect to the nominal target position, and the MUGAST array. For the target configuration we set a thickness of 3 mm of 3 He gas. In the backward hemisphere, covered by MUGAST, we set a deformation of 0.5 mm, a 3.8µm thick Havar window and an ice layer of 35 µm. The gas pressure, temperature and density are 700 mbar, 6 K and 4.5 mg/cm 3 , respectively, corresponding to an areal density of ∼ 2 mg/cm 2 of 3 He. We assumed an isotropic angular distribution of emitted deuterons in the center-of-mass frame. The top panel of Fig. 9 shows the matrix where the energy of the deuterons right after the reaction, E lab , reconstructed by accounting for all the energy losses in the target and in the detector dead layers, is plotted versus their scattering angle in the laboratory frame, θ lab . The theoretical kinematic line is plotted in green. The inset shows the resulting excitation energy distribution of 47 K, centered in 0 as expected for the transfer to the ground state. The obtained excitation energy resolution is ∼ 1.3 MeV FWHM.

The relative contributions to this resolution can be computed performing different simulations considering, in each of them, only the effect of a single target "layer". We obtained for the target thickness ∆E t ∼ 0.8 MeV, for the target deformation ∆E d ∼ 0.4 MeV, for the Havar window ∆E w ∼ 0.6 MeV, for the ice layer ∆E l ∼ 0.6 MeV (all values are FWHM). The intrinsic resolution of the detectors is ∆E Si ∼ 40 keV. The total resolution is given by

∆E tot = ∆E t 2 + ∆E d 2 + ∆E w 2 + ∆E l 2 + ∆E Si 2 ∼ 1.
3 MeV, in agreement with the result of the simulation where all the effects are considered at the same time.

The experimental E* distribution can be obtained by gating on the 47 K ions detected in VAMOS and on the deuterons detected in MUGAST. Careful considerations need to be made when comparing it with the result of the simulation. The experimental E* distribution is expected to contain contributions from the different levels of 47 K populated in the transfer. The 1/2 + ground state of 47 K can be populated with an orbital angular momentum transfer L = 0 from 46 Ar, while the first excited states 3/2 + at 360 keV and (7/2 -) at 2020 keV can be populated via L = 2 and L = 3 transfer, respectively. Assuming the spectroscopic factors for the different L transfers to be comparable, at the most backward scattering angles in the laboratory frame, which correspond to the forward angles in the center-ofmass frame, the L=0 transfer to the ground state should have higher cross section than higher L transfer and the width of the E* distribution should reflect the experimental energy resolution. In the bottom panel of Fig. 9 we plot in black the E* distribution obtained with a gate only on the annular detector of MUGAST, which covers the most backward laboratory angles (θ lab > 160 • ).

To directly compare simulation and data one has to consider that the characteristics of the beam can significantly affect shape and width of the E* distribution. For this reason we performed a second simulation considering more realistic beam parameters, leaving the target characteristics unchanged from the first simulation. The beam profile on the xy plane is now repre-sented by a two-dimensional gaussian function with centroid in (x,y) = (0,0) and σ x =σ y = 3 mm. Typical spots of SPIRAL1 beams are of the order of ∼ 2 mm, but we have to consider the effect of the angular straggling of 0.9 mrad (computed with LISE++) occurring in the four 1.5-µm thick Mylar windows of the CATS detector placed 2 m before the target. Such angular straggling has been taken into account in the simulation. The number of events was reduced to adapt to the experimental statistics for a more direct comparison. The resulting E* spectrum is shown in red in the bottom panel of Fig. 9, superimposed to the experimental one. The two spectra are in reasonable agreement, though the experimental resolution is slightly larger (∼ 30 %) than the simulated one. This discrepancy can be explained considering that, in the experimental data, contributions from the low-lying 3/2 + state at 320 keV in 47 K, very close in energy to the ground state, may be present. Moreover the simulation does not consider other beam parameters which could not be precisely determined experimentally with a single beam profiler (CATS) detector, such as, for instance, possible impact positions on target dif-ferent from (0,0) or slight tilts in the beam direction. The small number of counts at negative excitation energy in the experimental distribution mainly comes from contributions from the deuteron break-up channel after the transfer. Due to the high cross section of the process and the limited resolution in particle identification in MUGAST, few protons, in coincidence with 47 K in VAMOS, can be tagged as deuterons and treated as such in the analysis. Their energy loss in the target is then overestimated, resulting in a negative excitation energy.

The complete experimental E* distribution and its physical implications will be further discussed in a forthcoming publication [START_REF] Brugnara | [END_REF].

Transparency to γ radiation

In experiments where absolute cross sections are extracted via γ-particle coincidence, the γ-ray efficiency of the detection set-up plays an important role in the determination of the relevant physical quantities. While MUGAST is nearly transparent to γ radiation [23], the thick frame and shield of the target, if intercepted by γ rays before they reach AGATA, might absorb part of them and lower the effective γ-ray efficiency. When the nucleus produced in the reaction de-excites instantaneously through the emission of γ radiation, the absorption of γ rays by HeCTOr is minimum, since they can pass through the entire large opening of the conic flange facing AGATA in the backward hemisphere. In the case that the decaying level has a long lifetime, of the order of the nanosecond or more, the nucleus might travel a few centimeters before de-exciting. Part of the γ rays will then be absorbed by the target frame before they reach AGATA and the efficiency of the γ-ray detector will be consequently reduced.

Even though transparency to γ rays was not one of the main constraints in the design of HeCTOr, it represents an important parameter to consider when designing and analyzing experiments involving long-lived nuclear excited states. In the present experiment, the 3/2 + state at 320 keV in 47 K, which decays to the ground state, has a lifetime τ ∼ 1.6 ns. Since the average β for 47 K ions is ∼ 0.14, in a time interval of the order of τ the nucleus travels approximately 6 cm and the γ-ray absorption due to the target can become significant. Such effect can be quantified by computing the loss of efficiency of AGATA when a calibration source is placed downstream with respect to the nominal target position, thus simulating the decay of a long-lived excited state. Therefore we placed a 152 Eu source at 85 mm from the target and acquired the corresponding γ-ray energy spectrum. A simulation was then performed with the AGATA simulation code [START_REF] Farnea | [END_REF], with the 152 Eu source placed in the same position, including HeCTOr and the reaction chamber. Figure 10 shows the comparison of the efficiency of the cores of the AGATA crystals as a function of the γ-ray energy between simulation (red squares) and data (black circles). The curves are fits obtained following the prescription of Ref. [39]. The absolute value and trend of the efficiency as a function of E γ are in excellent agreement with the simulation. Significantly wrong parameters for the target materials and dimensions would in fact result not only in a different offset but also very different shapes of the efficiency curve. As an example, the efficiency curve obtained in a simulation where the entire volume of HeCTOr was removed is shown in Fig. 10 with a dashed green line. The large difference, as compared to the other curves, clearly reflects the effect of γ-ray absorption by the target, which turns out to be, as expected, larger for γ rays of lower energies. This result confirms the importance of carefully accounting for the effect of γ-ray absorption by the target when analyzing the decay of isomeric excited states.

Conclusions

In this paper we reported on HeCTOr, a thick 3 He cryogenic target specifically designed to be employed for direct nuclear reactions in inverse kinematics. Among the strengths of the target are the high purity and density of scattering centers attainable at cryogenic temperatures, where thicknesses of the order of few mg/cm 2 (∼ 10 20 atoms/cm 2 ) can be obtained. These characteristics make it particularly suited for experiments with low-intensity radioactive beams in fragmentation and ISOL facilities. The target has been integrated in a compact experimental set-up consisting of three coincident detectors for a first in-beam experiment. It showed a correct operation of its components and an excellent stability of temperature and pressure over time. Dedicated Geant4 simulations have demonstrated to provide good control over different relevant experimental parameters, such as total target thickness, energy resolution and γ-ray absorption. Foreseen improvements of the set-up will focus on the reduction of both the thickness of ice layers forming on the target windows and the consumption of LHe needed to maintain the target at cryogenic temperatures.

Figure 1 :

 1 Figure 1: (Top) Photographs of the target cell without the thermal shield: a) backward side, from which the beam enters the target; b) forward side. (Bottom) CAD drawings of the target cell where the shield is added as a semitransparent layer: c) backward side; d) forward side. The rod connecting the target cell to the LHe tank is also visible.

Figure 2 :

 2 Figure 2: Magnified view of the target cell. From left to right: conic flange, Havar window, indium wire and target frame. In the forward side the sequence is repeated in inverted order.

Figure 3 :

 3 Figure 3: (Left) Photograph of HeCTOr. (Right) CAD drawing of HeCTOr. The cylindrical vessel is semi-transparent to allow the visualization of the LN 2 tank. The two asymmetric hemicylindrical sheets enclosing the glass-epoxy rod connect the bellow to the target cell.

Figure 4 :

 4 Figure 4: Scheme illustrating the piping and instrumentation diagram of HeCTOr (see text for details).

Figure 5 :

 5 Figure 5: A drawing of the experimental set-up at GANIL.

Figure 6 :

 6 Figure 6: Plot of the temperature in K (blue) and pressure in mbar (red) of the target during 4 days of experiment. The small spikes occurring approximately each 6 hours, and visible mainly in the temperature curve, are in correspondence of the filling of the LHe tank.

Figure 7 :

 7 Figure 7: (Top) Experimental Bρ as a function of time for ∼ 40 h after the filling of the target with 3 He. (Bottom) Same as the top panel but with the simulated Bρ distribution superimposed in black.

Figure 8 :

 8 Figure 8: (Left) A simplified drawing of the target cell where the different parameters of Eq. 5 are indicated. The red curves represent the target deformation. (Right) An expanded view of the target generated in the Geant4 simulations.

Figure 9 :

 9 Figure 9: (Top) Two-dimensional plot energy vs scattering angle of the deuterons obtained in the simulation of the reaction 46 Ar( 3 He,d) 47 K at E b = 10 MeV/u for the transfer to the ground state of 47 K. The assumed angular distribution of the emitted deuterons is isotropic in the center-of-mass frame. The green line represents the theoretical kinematic line. The inset shows the resulting simulated excitation energy distribution of 47 K and the value of resolution FWHM. (Bottom) Comparison between simulated (red) and experimental (black) E* distributions of 47 K. The number of simulated events and the binning have been adapted for a clearer comparison with the experimental spectrum.

Figure 10 :

 10 Figure10: Comparison of the AGATA core efficiency as a function of the γray energy between simulation (red squares) and data (black circles) for γ rays emitted by a 152 Eu calibration source placed at 85 mm downstream from the target. The green triangles and dashed line represent a second simulation where HeCTOr is not present. The curves are fits obtained following the prescription of Ref.[39].

The expressions stripping and pick-up may be confused when the reaction occurs in inverse kinematics. Here and later in the text we always refer these expressions to the light particle, so in a stripping (pick-up) reaction the light target particle loses (gains) nucleons.

Acknowledgments

The authors acknowledge the GANIL staff for the invaluable support in the phases of installation of the target and during the experiment. We also wish to thank Emmanuel Dartois for his help with the kinetic theory of gases.

This work was supported by the European Union's Horizon 2020 research and innovation program under grant agreement No 654002. A. Gottardo acknowledges financial contribution from the MIUR PRIN 2017 call for funding, project 2017P8KMFT.


Junmin Deng

Tao Chen

Yara Arbid

Mathieu Pasturel

Sungjun Bae

Khalil Hanna
email: khalil.hanna@ensc-rennes.fr

Aging and reactivity assessment of nanoscale zerovalent iron in groundwater systems

Keywords: NZVI, aging effect, groundwater, reactivity, secondary minerals

In this study, changes in the reactivity of nanoscale zerovalent iron (NZVI) in five different groundwater (GW) systems under anoxic and oxic conditions were examined over a wide range of aging time (0-60 d). p-nitrophenol (p-NP) was used as a redoxsensitive probe, whereas nalidixic acid (NA), a typical antibiotic found in the natural environment, was used as a sorbing compound. Investigation of the p-NP reduction in pure water systems showed that NZVI lost 41% and 98% of its reductive activity under anoxic and oxic conditions after 60 d, while enhancement of its reactivity was observed after short-term aging in GW (1-5 d), followed by a further decline. This behavior has been ascribed to the formation of secondary Fe(II)-bearing phases, including magnetite and green rust, resulting from NZVI aging in GW. Adsorption experiments revealed that GW-anoxic-aged NZVI samples exhibited a good affinity toward NA, and a greater NA adsorption (~27 µmol g -1 ) than that of pristine NZVI (~2 µmol g -1 ) at alkaline pH values. Surface complexation modeling showed that the enhanced adsorption of NA onto secondary minerals can be attributed to the Fe(II)-NA surface complexation. This considerable change in the reductive ability and the adsorption capacity of NZVI arising from groundwater corrosion calls for greater attention to be paid in assessment studies, where NZVI is injected for long-term remediation in groundwater.

Introduction

Nanoscale zerovalent iron (NZVI) is one of the most extensively studied nanomaterials of environmental clean-up technologies [START_REF] Li | Heavy metal removal using nanoscale zero-valent iron (NZVI): Theory and application[END_REF][START_REF] Phenrat | Electromagnetic induction of zerovalent iron (ZVI) powder and nanoscale zero valent iron (NZVI) particles enhances dechlorination of trichloroethylene in contaminated groundwater and soil: Proof of concept[END_REF][START_REF] Wei | Influence of nanoscale zero-valent iron on geochemical properties of groundwater and vinyl chloride degradation: A field case study[END_REF][START_REF] Xia | Genesis of pure Se(0) nano-and microstructures in wastewater with nanoscale zero-valent iron (NZVI)[END_REF]. Because NZVI is an environmentally safe material with high surface area and great reactivity, it has been widely applied in treatments of contaminants, including persistent organic compounds [START_REF] Joo | Oxidative degradation of the carbothioate herbicide, molinate, using nanoscale zero-valent iron[END_REF]), toxic inorganic contaminants [START_REF] Ryu | Reduction of highly concentrated nitrate using nanoscale zero-valent iron: Effects of aggregation and catalyst on reactivity[END_REF], and even radioactive nuclides [START_REF] Tsarev | The short-term reduction of uranium by nanoscale zero-valent iron (NZVI): Role of oxide shell, reduction mechanism and the formation of U(v)-carbonate phases[END_REF].

Despite NZVI having been frequently applied in in-situ groundwater remediation [START_REF] Libralato | Environmental effects of NZVI for land and groundwater remediation[END_REF][START_REF] Mueller | Application of nanoscale zero valent iron (NZVI) for groundwater remediation in Europe[END_REF], the remedial performance of NZVI is sensitive to many geochemical factors in groundwater such as organic matter [START_REF] Giasuddin | Adsorption of humic acid onto nanoscale zerovalent iron and its effect on arsenic removal[END_REF][START_REF] Zhou | Increasing the electron selectivity of nanoscale zero-valent iron in environmental remediation: A review[END_REF], cations [START_REF] Chen | Effects of pH value and calcium hardness on the removal of 1,1,1-trichloroethane by immobilized nanoscale zero-valent iron on silica based supports[END_REF], and anions [START_REF] Su | Effects of various ions on the dechlorination kinetics of hexachlorobenzene by nanoscale zero-valent iron[END_REF]. For instance, inhibition of As(V) and As(III) removal by NZVI was reported in the presence of humic acid [START_REF] Giasuddin | Adsorption of humic acid onto nanoscale zerovalent iron and its effect on arsenic removal[END_REF], while the presence of Cl -/SO4 2- /Cu 2+ enhanced the reduction of hexachlorobenzene [START_REF] Su | Effects of various ions on the dechlorination kinetics of hexachlorobenzene by nanoscale zero-valent iron[END_REF]. Groundwater constituents may affect not only the NZVI reactivity but also its fate or mineralogical transformation over long-term aging in the natural environment. Indeed, NZVI can be completely or partially oxidized into various Fe(II)/Fe(III) (hydr)oxides depending on the groundwater conditions [START_REF] Bae | Advances in surface passivation of nanoscale zerovalent iron (NZVI): A critical review[END_REF]). In the presence of HCO3 -, the formation of magnetite/carbonate green rust/iron carbonate hydroxide as aging products of NZVI have been observed [START_REF] Xie | Influence of anionic cosolutes and pH on nanoscale zerovalent iron longevity: Time scales and mechanisms of reactivity loss toward 1,1,1,2-tetrachloroethane and Cr(VI)[END_REF], whereas the presence of SO4 2-enabled the formation of magnetite/schwertmannite [START_REF] Reinsch | Chemical transformations during aging of zerovalent iron nanoparticles in the presence of common groundwater dissolved constituents[END_REF]. Further investigations of NZVI aging in synthetic groundwater revealed that magnetite was a major aging product with traces of goethite/amakinite under anoxic conditions [START_REF] Schöftner | Electron efficiency of NZVI does not change with variation of environmental parameters[END_REF], with lepidocrocite being the only iron phase found after 60 d aging under oxic conditions [START_REF] Dong | Fate of As(V)-treated nano zero-valent iron: Determination of arsenic desorption potential under varying environmental conditions by phosphate extraction[END_REF]). However, a comprehensive understanding of short-and long-term aging effects on NZVI behavior in real groundwater is still limited. Despite very few investigations [START_REF] Adeleye | Persistence of commercial nanoscaled zero-valent iron (nZVI) and by-products[END_REF]) on the long-term environmental fate of commercial NZVI in groundwater, little is known about the reactivity of the generated secondary iron phases which can further influence interactions with co-existing contaminants.

Herein, a comprehensive kinetic assessment of reductive and adsorptive capacities of NZVI and aged NZVI in various real groundwater was performed under oxic and anoxic conditions to determine the NZVI reactivity versus corrosion over a wide range of aging time (0-60 d). Specifically, the main aim was to assess the interaction mechanisms of "aged" NZVI particles with p-nitrophenol (p-NP), a redox-sensitive probe, and nalidixic acid (NA), a sorbing antibiotic contaminant in natural systems.

Five different groundwater samples from different places in France were used in this study. The reduction of p-NP by fresh NZVI in pure water (PW) and the five groundwater samples was first evaluated, and the mineral transformation of NZVI particles after the reaction was analyzed by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM). Structural and compositional analyses were performed for aging products at the desired aging time under anoxic and oxic conditions, and additional reduction experiments were performed to investigate the changes in the reductive activity of NZVI aging products. NA adsorption on the NZVI aging products was also evaluated as a function of pH, and a surface complexation model was developed to describe the impact of surface-bound Fe(II) of aged NZVI samples on NA adsorption.

Materials and methods

Chemicals and materials

Complete information on chemicals used in this study is presented in the Supporting Information (SI). NZVI (50-100 nm) was synthesized by following our previous method [START_REF] Parkhurst | User's guide to PHREEQC (Version 2): A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations[END_REF]. It exhibited a typical chain-like core-shell structure as per high-resolution transmission electron microscopy (HR-TEM) analysis (Fig. S1).

Maghemite (γ-Fe2O3, >99.0%) was purchased from Alfa Aesar (Ward Hill, MA, USA). Magnetite (Fe3O4) [START_REF] Cheng | Influence of magnetite stoichiometry on the binding of emerging organic contaminants[END_REF], lepidocrocites (γ-FeOOH) [START_REF] Paterson | Iron oxides in the laboratory: Preparation and characterization[END_REF],

carbonate green rust ([Fe 2+ 4Fe 3+ 2(HO)12] 2+ •[CO3 2-•mH2O] 2-) (

Groundwater characterization

Physicochemical characteristics including pH, electrical conductivity (EC), total dissolved solids (TDS), total organic carbon (TOC), total hardness (TH), total phosphate (TP), common anions, and cations (Table S1) of the groundwater samples were measured following the guidelines of "Standard Methods For The Examination of Water and Wastewater American Public Health Association" [START_REF] Eaton | Standard methods for the examination of water and wastewater[END_REF].

Particularly, anions in groundwater were determined using ion chromatography (IC; Dionex Corp., Sunnyvale, CA, USA), while cations were measured by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7700x; Agilent Technologies, Santa Clara, CA, USA). The pH and EC were determined using a pH meter (HANNA, HI991003; Hanna Instruments, Smithfield, RI, USA) and a conductivity meter LF 340 (WTW, Weilheim, Germany), respectively. Total hardness was determined by the EDTA titration method, and TDS was measured gravimetrically [START_REF] Patnaik | Handbook of environmental analysis: Chemical pollutants in air, water, soil, and solid wastes[END_REF]. Silicate and phosphate analyses were performed according to the molybdenum blue method [START_REF] Grasshoff | Sorption of two aromatic acids onto iron oxides: Experimental study and modeling[END_REF], and the ammonium molybdate spectrometric methods [START_REF] Standard | Water quality: Determination of phosphorusammonium molybdate spectrometric method[END_REF], respectively. The concentration of carbonate species was measured by the acid titration method [START_REF] Eaton | Standard methods for the examination of water and wastewater[END_REF].

Characterizations of NZVI and aged NZVI particles

The aged NZVI was prepared by using NZVI (10 g L -1 ) suspensions in 15-mL vials containing DDIW or deoxygenated GW1. Subsequently, the NZVI suspension was placed under ambient air condition (oxic aging) or in an anaerobic chamber (anoxic aging) and allowed to age for 1, 5, 15, 30, and 60 d. Given that the particles settle in the vial with increasing aging time, thus potentially reducing the contact time with water, the suspensions were vigorously mixed by hand shaking once a day [START_REF] Dong | Physicochemical transformation of Fe/Ni bimetallic nanoparticles during aging in simulated groundwater and the consequent effect on contaminant removal[END_REF].

The pH variation during aging was monitored using a portable pH meter. Aged NZVI particles were separated by centrifugation at the desired aging time and dried in a vacuum freeze drier (-52 °C, 24 h) for further characterization and experiments.

XRD (Bruker D8 Advance, Cu K1 radiation,  = 1.5406 Å) was used to identify the aging products of NZVI. The samples were transferred to sample holders and treated with 1: 1 (v/ v) glycerol solution to avoid surface oxidation during the XRD analysis [START_REF] Bae | Influence of riboflavin on nanoscale zero-valent iron reactivity during the degradation of carbon tetrachloride[END_REF]. The morphological information of aged NZVI particles was obtained by HR-TEM (JEM-2100; JEOL, Tokyo, Japan) with selected area electron diffraction (SAED). The dried NZVI aging products were transferred to ethanol solution and sonicated for 30 min. Finally, a drop of the diluted suspension was placed on the copper TEM grid and analyzed at an acceleration voltage of 200 kV.

Reduction and adsorption experiments

Experiments to measure the reactivity of fresh NZVI in different waters were carried out in 200-mL flasks in an anaerobic chamber to avoid the effect of dissolved oxygen during the reaction. An exact amount (1 mL) of 20 mM p-NP stock solution was transferred to the flask containing 200 mL of filtered groundwater or DDIW, followed by stirring at 500 rpm to prepare an initial p-NP concentration of 0.1 mM.

Then, 10 mg of NZVI was introduced into the solution to initiate the reduction of p-NP by NZVI. At each sampling time, 1 mL of the sample was taken from the suspension and filtered through a 0.2-µm filter (Whatman) for high-performance liquid chromatography (HPLC) analysis.

Similar procedures were conducted to investigate the reductive activity of aged NZVI particles. Briefly, aged NZVI suspension (10 g L -1 ) was firstly bath-sonicated for 1 h. The aged NZVI suspension (1 mL) was added to 200 mL of p-NP solution (0.1 mM). Then, samples were taken at the desired time intervals and analyzed by HPLC.

In contrast to the reduction experiments, adsorption batch experiments of NZVI aging products were initiated by transferring 10 mg of aged NZVI particles into 200 mL of NaCl solution (10 mM). The pH was then adjusted to the required value using 0.1 M and/or 1 M NaOH/HCl solutions. After 2 h of adsorption, an aliquot was taken and filtered (0.2-μm, Whatman) to measure NA and dissolved Fe(II). To investigate the role of each mineral phase on the reactivity of aged NZVI, additional reduction and adsorption assessments of iron-containing minerals (i.e., magnetite, maghemite, carbonate green rust, lepidocrocite, and goethite, 50 mg L -1 ) were carried out following the same procedures described above. All experiments in this study were performed in triplicate, and the data reported here were the average of the three replicated experiments, and error bars represent the relative standard deviation.

Analytical Methods

The aqueous concentration of p-NP, p-AP, and NA were determined using an HPLC (Waters 600 controller; Waters Corporation, Milford, MA, USA) equipped with a photodiode array detector (Waters 996; Waters Corporation) and a reversed-phase C18 column (250 mm × 4.6 mm; i.d., 5 μm). The UV detector was set to 273, 317, and 258 nm for p-NP, p-AP, and NA analyses, respectively [START_REF] Bae | Effect of NaBH4 on properties of nanoscale zero-valent iron and its catalytic activity for reduction of p-nitrophenol[END_REF], Xu et al. 2017c).

The mobile phase (mixture of acetonitrile/water (50/50, v/v) containing 0.1% formic acid) was prepared and used at the flow rate of 1 mL min -1 in the isocratic mode for the measurement of p-NP, p-AP, or NA. The dissolved Fe(II) concentration was measured using the 1,10-phenanthroline method at a wavelength of 510 nm with a UV-vis spectrophotometer (U-3310, Hitachi; Hitachi Ltd., Tokyo, Japan) [START_REF] Fadrus | Suppression of iron(III) interference in the determination of iron(II) in water by the 1,10-phenanthroline method[END_REF].

Surface complexation modeling

The NA adsorption by NZVI aging products at different pH values was described using surface complexation modeling. The geochemical speciation code PHREEQC and the "Minteq" database were used (D. L. Parkhurst and C. A. J. Appelo 1999). The pKa of NA is 6.19, and the logarithm of the formation constant of NA-Fe + (aq) is 3.99 [START_REF] Vincent | Prototropic and metal complexation equilibria of nalidixic acid in the physiological pH region[END_REF]. The surface complexation model for iron oxides was used, as previously reported, and the protonation of Fe surface sites was formulated following a 2-pKa approach (Eq. 1 and 2) [START_REF] Cheng | Influence of magnetite stoichiometry on the binding of emerging organic contaminants[END_REF][START_REF] Grasshoff | Sorption of two aromatic acids onto iron oxides: Experimental study and modeling[END_REF].

≡ 𝐹𝑒𝑂𝐻 2 + ⇋ ≡ 𝐹𝑒𝑂𝐻 + 𝐻 + ; 𝑝𝐾𝑎 1 (1) ≡ 𝐹𝑒𝑂𝐻 ⇋ ≡ 𝐹𝑒𝑂 -+ 𝐻 + ; 𝑝𝐾𝑎 2 (2)
The charge-potential relationship was described according to the constant capacitance model (CCM), and the modeling parameters are presented in Table S2. We adapted the parameters of the three plane model (TPM) to use the CCM in PHREEQC (version 2) [START_REF] Cheng | Influence of magnetite stoichiometry on the binding of emerging organic contaminants[END_REF]. Correlation coefficient (r 2 ) ranges from 0.95 to 0.99.

Results and discussion

Reduction of p-NP by NZVI suspensions in various groundwater systems

The removal kinetics of p-NP in various water matrices showed that p-NP was completely removed by NZVI within a 2 h-reaction in all experiments under anoxic conditions (Fig. S2). The aqueous concentrations of p-AP after the reaction revealed a complete mass balance of the conversion reaction of p-NP into p-AP (Table S3),

indicating the negligible adsorption of p-NP or p-AP on NZVI surfaces in this study.

The pseudo-first-order kinetic rate constants (kobs, min -1 ) were determined by considering the early stage of kinetics data over 30 min of reaction time (Fig. 1a). The highest rate constant was observed in GW2 (0.07 min -1 ), compared to PW and other groundwater systems.

In contrast to the black color of NZVI in the PW system, a change of suspension color to bluish-green was observed in the groundwater samples. The XRD analysis revealed various corrosion products of NZVI depending on the water matrix (Fig. 1b).

In PW, diffraction peaks were observed at 18.4°, 30.2°, 35.5°, 43.2°, 53.6°, 57.2°, and 62.8°; these peaks can be assigned to maghemite (γ-Fe2O3)/magnetite (Fe3O4).

Carbonate green rust was detected in all groundwater systems, together with calcite in GW3 and GW5 and aragonite in GW4 (Fig. 1b) (Dong et al. 2020, Kontoyannis and[START_REF] Kontoyannis | Calcium carbonate phase analysis using XRD and FT-Raman spectroscopy[END_REF].

The pH and concentration of the main dissolved species including anions and TOC in the five groundwater matrices were determined after the reaction (Table S4). The suspension pH in groundwater systems increased to approximately 9, owing to the anaerobic corrosion of NZVI (Fe(0) + 2H2O → Fe 2+ + 2OH -+ H2) (Deng et al. 2020).

As the distribution of carbonate species is strongly dependent on the pH value (Fig. S3), an increase in CO3 2-species is expected, which can result in the formation of carbonatebearing minerals (e.g., CaCO3 and carbonate green rust). The precipitation of CaCO3 occurred mainly in GW3, GW4, and GW5 systems because of the relatively higher initial concentration of calcium (2.44-2.61 mM) and bicarbonate (4.2-4.3 mM) (Table S1). Indeed, a drop in the aqueous concentration of dissolved carbonates was observed, especially in GW3, GW4, and GW5, which was comparable to the Cl -and SO4 2- concentrations after the reaction (Table S4). Up to 88% of the initial NO3 -concentration was eliminated in groundwater samples after the reaction, likely because of the reductive removal of NO3 -by NZVI [START_REF] Hwang | Mechanism study of nitrate reduction by nano zero valent iron[END_REF][START_REF] Su | Simultaneous removal of cadmium and nitrate in aqueous media by nanoscale zerovalent iron (nZVI) and Au doped nZVI particles[END_REF]. For silicate concentration, up to 90% loss was detected after the reaction, probably due to its strong binding affinity toward iron minerals [START_REF] Kanematsu | Silicate binding and precipitation on iron oxyhydroxides[END_REF]). However, no significant change in Total Organic Carbon (TOC) was observed before and after reaction with NZVI (Table S4).

In contrast, the NZVI corrosion products, particularly Fe(II)-containing minerals, could further contribute to the reduction of p-NP. Indeed, previous studies have reported the greater reductive ability of green rust than that of magnetite [START_REF] Digiacomo | Immobilization of Cr(VI) by sulphate green rust and sulphidized nanoscale zerovalent iron in sand media: batch and column studies[END_REF][START_REF] Usman | Magnetite and green rust: Synthesis, properties, and environmental applications of mixed-valent iron minerals[END_REF]). An independent set of experiments using synthetic Fe-minerals confirmed that carbonate green rust led to 43% of p-NP removal in 5 h, whereas only 8% of p-NP was removed by magnetite, and almost no removal of p-NP by Fe(III)oxyhydroxides was observed (Fig. S4). Therefore, the carbonate green rust generated in NZVI-groundwater suspensions could additionally reduce p-NP. Other groundwater constituents, such as nitrate and silicate, may decrease the p-NP reduction efficiency owing to the inhibitory effects caused by the competitive reduction reaction of nitrate with p-NP [START_REF] Alowitz | Kinetics of nitrate, nitrite, and Cr(VI) reduction by iron metal[END_REF]Scherer 2002, Westerhoff and[START_REF] Westerhoff | Nitrate removal in zero-valent iron packed columns[END_REF], and the blocking of the reactive sites by silicate [START_REF] Meng | Combined effects of anions on arsenic removal by iron hydroxides[END_REF]. This may explain why the highest rate constant for p-NP reduction was observed for GW2, which contained a lower nitrate concentration. Nevertheless, to avoid the formation of CaCO3 phases, which could potentially interfere with the NZVI aging process, GW1 was selected and used for the kinetic assessment (reduction and adsorption) of aged samples. Kinetics of removal of p-NP by different aged NZVI particles were monitored to check the impact of aging time on the overall reduction ability of NZVI suspension (Fig. S5). For PW systems, kobs values continuously decreased as the aging time increased under both anoxic and oxic conditions (Fig. 2). Almost 50% of p-NP was reduced into p-AP in the 60 d-aging sample under anoxic conditions, while no reduction was observed for the 60 d-aged samples under oxic conditions (Fig. S5). A bell-shaped curve was observed for kobs variation with aging time for GW1-aged NZVI; i.e., kobs values increased over the early stage of aging (0-5 d) and then decreased with further aging.

Effects of NZVI aging products on the reduction of p-NP

In particular, the reductive ability of NZVI in groundwater reached a maximum after 5 d aging, which was much higher (~1.8 times) than that of the fresh materials. In general, the samples of anoxic-aged NZVI showed better p-NP reduction than that of the oxicaged NZVI. Similar trend of NZVI reactivity against aging time in simulated groundwater under anoxic conditions has been previously reported [START_REF] Xiao | A comparative study on the physicochemical properties, reactivity and long-term performance of sulfidized nanoscale zerovalent iron synthesized with different kinds of sulfur precursors and procedures in simulated groundwater[END_REF].

This behavior could be explained by the preservation of fresh NZVI and/or the occurrence of more reactive Fe phases under anoxic conditions.

In order to assess the phase transformation of NZVI in PW and GW1 systems, XRD analyses of secondary iron minerals were conducted under oxic and anoxic conditions (Fig. 3). The abundance of the α-Fe phase decreased gradually for all the samples. For PW-anoxic-aged NZVI, the XRD peaks of Fe(0) remained until 5 d, and magnetite/maghemite was detected as the main aging product (Fig. 3a), while the disappearance of the Fe(0) peak coupled with the appearance of carbonate green rust and magnetite/maghemite was observed in the 5 d-aged sample of PW-oxic-aged NZVI (Fig. 3c). In contrast, peaks of carbonate green rust and magnetite/maghemite were detected in GW1-anoxic-aged NZVI, with the disappearance of Fe(0) observed at longer aging times (Fig. 3b). In GW1-oxic-aged NZVI, an increase in peaks of carbonate green rust and magnetite/maghemite was observed at only 1 d of aging (Fig. 3d). Then, a significant increase in Fe(III)-oxyhydroxides (lepidocrocite and/or goethite) with a decrease in peaks of carbonate green rust at 60 d was observed, suggesting the conversion of carbonate green rust into lepidocrocite and/or goethite under oxic conditions (Legrand et al. 2004, Schwertmann and[START_REF] Schwertmann | The formation of green rust and its transformation to lepidocrocite[END_REF]. More details about the changes in solution chemistry and pH values of NZVI suspensions along the aging process are given in the SI (Fig. S6). Collectively, these results showed that NZVI gradually lost its reactivity under both anoxic and oxic conditions in the PW system, probably due to the consumption of Fe( 0)

and accumulation of an oxyhydroxide layer coating the NZVI surface. In groundwater systems, though an initial enhancement of p-NP removal was observed over a short aging time (1-5 d), a decline was observed at later aging times. The greater reduction ability of carbonate green rust compared to that of magnetite, maghemite, lepidocrocite, or goethite (see Fig. S4) may explain the higher kobs values observed in groundwater systems over the initial aging period. Previous work has also shown that the breakdown of the iron oxide shell coating of the ZVI may occur during the early stage of aging, thereby allowing better access to the core Fe(0) [START_REF] Xie | Influence of anionic cosolutes and pH on nanoscale zerovalent iron longevity: Time scales and mechanisms of reactivity loss toward 1,1,1,2-tetrachloroethane and Cr(VI)[END_REF]. Furthermore, as previously reported, the divalent cationic metal contained in groundwater may bind to surface coatings and induce surface oxide layer dissolution or NZVI depassivation [START_REF] Liu | Depassivation of aged Fe0 by divalent cations: correlation between contaminant degradation and surface complexation constants[END_REF]. NA adsorption to differently aged NZVI particles was investigated over a wide pH range (5-12) (Fig. 4). The NA adsorption of NZVI aging products generally increased with increasing aging time, especially under acidic/neutral conditions. However, a different scenario of NA adsorption was observed for groundwater-aged NZVI samples.

Effects of NZVI aging products on the adsorption of nalidixic acid

NA adsorption by GW1-anoxic-aged NZVI decreased sharply as the pH increased, but the adsorption envelope exhibited a bell-shaped curve with increasing aging time. In contrast, GW1-oxic-aged NZVI exhibited an inversed S-shaped adsorption curve.

Considering the different binding affinity of NA to different iron minerals [START_REF] Cheng | Influence of magnetite stoichiometry on the binding of emerging organic contaminants[END_REF], Xu et al. 2017b), this adsorption behavior by aged NZVI could be attributed to the change in mineral composition of NZVI aging products. The adsorption capacity of individual Fe oxyhydroxides, including magnetite, maghemite, carbonate green rust, lepidocrocite, and goethite, were evaluated over the same pH range (5-12) (Fig. S7). A negligible amount of NA was adsorbed by fresh NZVI, whereas Fe-(hydro)oxides exhibited significant NA adsorption capacity, especially in acidic to neutral pH. Under alkaline conditions, green rust exhibited better NA adsorption performance (when the adsorbed amount was normalized to mass and not surface area) than magnetite.

However, a quantitative comparison of adsorption capacities must be made with caution since these synthetic minerals have different properties, including crystallinity, specific surface area, and surface charge.

According to the shape of the NA adsorption envelope (Fig. S7), more adsorption at circumneutral pH was observed with Fe(II)-Fe(III) mixed valent minerals (e.g., magnetite and carbonate green rust), while Fe(III) bearing minerals (e.g., lepidocrocite, goethite, and maghemite) exhibited an S-shaped curve or downward trend. Generally, organic ligands binding to metal-oxides show maximum adsorption at a pH near the pKa value (6.19 for NA). Indeed, previous reports showed typical NA adsorption by Fe-oxides with maximum NA adsorption at pH around 6 [START_REF] Robberson | Adsorption of the quinolone antibiotic nalidixic acid onto anion-exchange and neutral polymers[END_REF], Xu et al. 2017b). However, the pH-adsorption curve of NA shifted to larger pH values in the case of magnetite and green rust, probably because of the presence of released Fe(II) in solution. As previously observed for magnetite, the addition of dissolved Fe(II) in suspension increased the NA adsorption at neutral to alkaline pH values [START_REF] Cheng | Influence of magnetite stoichiometry on the binding of emerging organic contaminants[END_REF]). Here, Fe(II) release during NA adsorption was monitored for the 60 d-aged sample and presented in Fig. S8. Ferrous ion release was detected in all cases at pH 5 and 7 and became negligible at pH above 9. Interestingly, the pH dependence of NA adsorption is fully consistent with that of dissolved Fe(II) concentration. For instance, a high dissolved Fe(II) amount was observed for anoxic-aged NZVI samples, which exhibited greater adsorption performance even at high pH values. PW-oxic-aged NZVI samples showed more NA adsorption than GW-oxic-aged NZVI under alkaline conditions. This correlation suggests that the amount of dissolved Fe(II) or the mineral bound-Fe(II) plays a key role in NA adsorption behavior onto NZVI secondary phases.

Surface complexation modeling for nalidixic acid binding onto the 60 d-aged sample

First, determining the mineral composition of the aged samples is a prerequisite to describing the NA binding to secondary iron phases. In this study, the most aged sample (60 d) was selected to determine the proportion of identified secondary minerals (see HR-TEM images confirmed that the initial chain-like aggregated structures and large-sized spherical aggregates (approximately 200 nm) of NZVI (Fig. S1) were transformed into magnetite/maghemite after 60 d of aging in a PW-anoxic system (Fig. 5a). In GW1-anoxic-aged NZVI samples, the particle size of magnetite/maghemite decreased significantly (approximately 5 nm), and magnetite/maghemite seemed to be attached to the surface of the hexagonal shape of carbonate green rust (Fig. 5b). Under oxic conditions, carbonate green rust was observed in both PW-and GW1-aged products (Figs. 5c,d). Particularly, the co-existence of magnetite with carbonate green rust was detected in PW-oxic-aged NZVI samples, while needle-shaped particles of lepidocrocite were observed in GW1-oxic-aged NZVI. All the SAED patterns of specific Fe mineral phases in Figure 5 are also provided in Figure 6, which shows the typical patterns of each mineral [START_REF] Araújo-Neto | Monodisperse sodium oleate coated magnetite high susceptibility nanoparticles for hyperthermia applications[END_REF][START_REF] Génin | Fougerite and FeII-III hydroxycarbonate green rust; ordering, deprotonation and/or cation substitution; structure of hydrotalcite-like compounds and mythic ferrosic hydroxide Fe(OH)(2+x)[END_REF][START_REF] Krystofiak | Multiple morphologies of goldmagnetite heterostructure nanoparticles are effectively functionalized with protein for cell targeting[END_REF][START_REF] Liao | Effects of Al substitution on local structure and morphology of lepidocrocite and its phosphate adsorption kinetics[END_REF].

Figure 6. Electron diffraction patterns of selected areas from Fig. 5. Note: M, magnetite/maghemite; GR, green rust; L, lepidocrocite.

To describe the mechanism underlying NA adsorption onto the 60 d-aged sample, we applied a surface complexation model. We assumed that NA binds to two surface hydroxy groups by involving its carboxylate and its keto-group as follows : (Cheng et al. 2018) 2 ≡FeOH + 2 H + + NA -⇌ (≡Fe)2NA + + 2 H2O log NA K (3)

Only the predominant phases were taken into consideration, and the mixed valent iron minerals (i.e., magnetite and green rust) were supposed to exhibit similar behavior with respect to NA adsorption. Consequently, the magnetite parameters were used in the adsorption model in PW-anoxic-aged NZVI, GW1-anoxic-aged NZVI, and PWoxic-aged NZVI samples. In the case of GW1-oxic-aged NZVI, the dominant phases were maghemite and lepidocrocite. NA adsorption to maghemite or lepidocrocite was first calculated, and all modeling parameters are shown in Table S2. The complexation of NA-Fe(II) was accounted for by using dissolved Fe(II) concentration as input parameters in this model. The model satisfactorily predicted the pH dependence of NA adsorption to aged NZVI but overestimated the NA adsorption at pH 5 in the anoxic-aged NZVI system (Fig. 7). This may be ascribed to the higher amount of dissolved Fe(II) under acidic conditions (see Fig. S8), which can affect NA-Fe (II) complexation in solution and thus ternary surface complexation (i.e., surface-metal-ligand complex) [START_REF] Cheng | Influence of magnetite stoichiometry on the binding of emerging organic contaminants[END_REF].

We also simplified the model by considering only the predominant phase of mixedvalence oxides (magnetite) in anoxic-aged NZVI samples despite the co-existence of green rust. Nevertheless, the good accuracy of the experimental and modeling data at neutral and alkaline pH confirms the key role of surface-bound Fe(II) in the adsorption of NA.

In summary, aged NZVI samples showed a good capacity for NA adsorption, which increased with aging time. The high NA adsorption onto NZVI products at neutral to alkaline pH can be ascribed to the surface-bound Fe(II) in the secondary phases.

Conclusions

We investigated the aging and reactivity of NZVI in various groundwater systems.

First, the nature of secondary iron phases resulting from NZVI aging in groundwater is strongly dependent on the groundwater composition. The final products of NZVI after reaction were mainly magnetite in pure water system, while green rust was predominant in groundwater systems. With the aging process (up to 60 d), NZVI continuously lost its reductive activity in the PW system. However, in the groundwater this reductive ability was first enhanced in the short-term (1-5 d) and then declined at longer aging times. Adsorption tests revealed higher NA binding to NZVI aging products (~ 30 µmol/g at pH 7), while NA uptake by fresh NZVI was negligible. Notably, the adsorption capacity of NA generally increased with aging time, with unexpected greater adsorption at neutral to alkaline pH values. Additional experiments and surface complexation modeling suggested that this strong adsorption at alkaline conditions can be attributed to the surface-bound Fe(II). These results shed light on an overlooked aspect of interactions of environmental compounds with the secondary Fe minerals resulting from NZVI corrosion in groundwater. Our findings will also help to predict the fate of NZVI in groundwater, and the impact of the secondary Fe minerals on the mobility and transformation of contaminants in soil and groundwater.

Figure 1 .

 1 Figure 1. (a) Pseudo-first-order kinetic rate constants (kobs, min -1 ) of p-NP reduction by

Figure 2 .

 2 Figure 2. Variation of pseudo-first-order kinetic rate constants (kobs, min -1 ) of p-NP

Figure 3 .

 3 Figure 3. XRD pattern of fresh and NZVI aging products: (a) PW-anoxic-aged NZVI,

Figure 4 .

 4 Figure 4. Nalidixic acid adsorption versus pH by fresh and different aged NZVI samples:

Figure 5 .

 5 Figure 5. Transmission electron microscopy images of NZVI aging products at 60 days

Figure 7 .

 7 Figure 7. Experimental (symbols) and modelled (dashed lines) variation of nalidixic

Table S5 )

 S5 . Almost 100% of the magnetite/maghemite phase was present in PW-anoxic-

	aged NZVI, while GW1-anoxic-aged NZVI contained approximately 55% of
	magnetite/maghemite, 10% of goethite, and 35% of carbonate green rust. For PW-oxic-
	aged NZVI, magnetite/maghemite was the main aging product (~86%) with 14% of
	carbonate green rust, while GW1-oxic-aged NZVI contained various Fe mineral phases
	(magnetite/maghemite, ~55%; goethite, ~8%; carbonate green rust, ~7%; and
	lepidocrocite, ~30%).

Acknowledgments

This work was supported by the Institut Universitaire de France (IUF) and the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2022R1A2C2005791). We gratefully acknowledge Isabelle Soutrel (LC/UV) and Vincent Dorcet (THEMIS platform, ScanMAT, UAR 2025 University of Rennes 1-CNRS; CPER-FEDER 2007-2014) for their assistance.