Marco Faroni
email: m.faroni003@unibs.it

Manuel Beschi
email: manuel.beschi@itia.cnr.it

Antonio Visioli
email: antonio.visioli@unibs.it

Predictive Inverse Kinematics for Redundant Manipulators: Evaluation in Re-Planning Scenarios

Keywords: robot manipulators, redundant robots, model predictive control, inverse kinematics, joint limits, optimal redundancy handling, re-planning

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INTRODUCTION

Redundant manipulators have been deeply investigated by the robotics community due to the well-known advantages they present. As a matter of fact, the redundant degrees of freedom can be exploited to satisfy secondary goals, such as dexterity improvement [START_REF] Lee | Redundancy resolution for dual-arm robots inspired by human asymmetric bimanual action: Formulation and experiments[END_REF], collision avoidance [START_REF] Maciejewski | Obstacle avoidance for kinematically redundant manipulators in dynamically varying environments[END_REF], manipulability maximization [START_REF] Faroni | A global approach to manipulability optimisation for a dual-arm manipulator[END_REF], joint range availability [START_REF] Liegeois | Automatic supervisory control of the configuration and behavior of multibody mechanisms[END_REF], imitation of human-like motions [START_REF] Lamperti | A redundancy resolution method for an anthropomorphic dual-arm manipulator based on a musculoskeletal criterion[END_REF]. By contrast, the presence of redundant degrees of freedom leads to more complex planning and control algorithms. In particular, the Inverse Kinematics (IK) allows infinite solutions and the choice of a suitable one is usually not trivial. Different approaches have been proposed to cope with this issue. As a first classification, methods in the literature can be subdivided into global and local ones [START_REF] Nakamura | Advanced Robotics: Redundancy and Optimization[END_REF]. The former ones typically consider the whole desired task in a single optimization problem to find a globally-optimal solution with respect to a given metrics. The latter ones solve the IK point-by-point, by taking into account just information about the current configuration of the system (i.e., the current joint configuration and the desired motion at the current cycle). Obviously, global methods give better results in terms of global execution of the task. However, they present relevant drawbacks, mainly related to their high computational complexity. First of all, they lead to complex nonlinear optimization problems that are usually solved by means of approximated methods. Moreover, their computational burden does not make them suitable for online implementation, thus, they cannot cope with re-planning strategies. For these reasons, local methods are often preferred. They usually address the IK at differential level [START_REF] Flacco | Control of redundant robots under hard joint constraints: Satu-ration in the null space[END_REF]. In the typical approach, the Jacobian of the manipulator is inverted by means of a generalized pseudo-inverse, in order to calculate joint velocities or accelerations. Then, joint configurations are computed by numerical forward integration [START_REF] Siciliano | Kinematic control of redundant robot manipulators: A tutorial[END_REF]. A Closed Loop Inverse Kinematics (CLIK) strategy is then used to recover from possible position errors given by numerical drift, out-of-path initial configuration, or re-planning [START_REF] Chiacchio | Closed-loop inverse kinematics schemes for constrained redundant manipulators with task space augmentation and task priority strategy[END_REF]. Secondary tasks are usually handled by projecting them into the null space of the Jacobian, so that they do not affect the execution of the primary task [START_REF] Siciliano | A general framework for managing multiple tasks in highly redundant robotic systems[END_REF][START_REF] Ott | Prioritized multi-task compliance control of redundant manipulators[END_REF]. The main disadvantage of this approach lies in its inability of handling the robot kinematic and/or dynamic constraints. As a consequence, if the computed solution exceeds the physical limits of the robot, saturations may occur, leading to the deformation of the desired task. Recent methods address this issue by converting the IK problem into a constrained Quadratic Program (QP) [START_REF] Flacco | Control of redundant robots under hard joint constraints: Satu-ration in the null space[END_REF][START_REF] Kanoun | Kinematic control of redundant manipulators: generalizing the task priority framework to inequality tasks[END_REF][START_REF] Rocchi | Opensot: a whole-body control library for the compliant humanoid robot Coman[END_REF][START_REF] Zanchettin | Motion planning for robotic manipulators using robust constrained control[END_REF]. Constraints are included in the QP in the form of linear matrix inequalities, while hierarchical tasks can be handled in a priority framework [START_REF] Kanoun | Kinematic control of redundant manipulators: generalizing the task priority framework to inequality tasks[END_REF][START_REF] Flacco | Control of redundant robots under hard joint constraints: Satu-ration in the null space[END_REF][START_REF] Escande | Hierarchical quadratic programming: Fast online humanoidrobot motion generation[END_REF] or in terms of a multi-objective cost function with different weights (according to the task priority) [START_REF] Liu | Generalized hierarchical control[END_REF]. The use of quadratic programming ensures the solution to be within the robot constraints and to minimize the deviation with respect to the desired tasks. However, as this approach is purely local, the solution only takes into account the current information of the system. The use of information about the future evolution of tasks, constraints, and kinematic variables could be exploited to improve the performance of the IK in global terms. For this reason, few recent works applies model predictive techniques to address this kind of problems [START_REF] Schuetz | Predictive online inverse kinematics for redundant manipulators[END_REF][START_REF] Tassa | Controllimited differential dynamic programming[END_REF][START_REF] Zube | Cartesian nonlinear model predictive control of redundant manipulators considering obstacles[END_REF][START_REF] Faroni | A predictive approach to redundancy resolution for robot manipulators[END_REF]. In particular, [START_REF] Faroni | A predictive approach to redundancy resolution for robot manipulators[END_REF] propose a fast predictive method to handle online redundancy resolution under kinematic constraints. This method models the robot joints as a series of multiple integrators. Then, the joints are coupled in a single linear state-space representation. The task is considered at velocity level at each point of the predictive horizon. At each cycle, the prediction of the joint configuration (calculated at the previous iteration) is used to compute the predicted Jacobians needed in the task expression. In this way, the task is linearized and the problem is converted into a QP. A continuous-time parametrization of inputs and outputs is applied to reduce the required computational time of the algorithm and this allows its online implementation. As previously mentioned and as demonstrated in [START_REF] Faroni | A predictive approach to redundancy resolution for robot manipulators[END_REF], the use of such strategy improves the purely local resolution of the IK by taking into account the future evolution of the system. However, the behavior of such technique in case of online task re-planning has not been investigated yet. This is of significant interest given that task re-planning is often exploited in recent developments in the field of collaborative robotics and robotic applications in unstructured environments. Re-planning strategies can be useful in several contexts. For example, the original task can be modified according to changes in the environment (e.g., in case of moving obstacles) or when an unexpected event occurs in the proximity of the robot [START_REF] Dyllong | Planning and real-time modifications of a trajectory using spline techniques[END_REF]. However, the abrupt change of the planned task could be a potential source of instability behaviors, as the solution of predictive methods is based on the predicted task and the prediction of joint states (which can result to be incorrect in case of modification of the task). For these reasons, in this paper, we investigate the behavior and the performance of the predictive redundancy resolution proposed in [START_REF] Faroni | A predictive approach to redundancy resolution for robot manipulators[END_REF] in case of different re-planning scenarios. For this purpose, the algorithm is suitably modified to cope with issues related to re-planning. In particular, the desired motion at the re-planning occurrence could result to be infeasible. This is the case, for example, of re-planned paths that present position and/or velocity discontinuities with respect to the current state of the robot. For this reason, the primary task is included in the online optimization problem as part of the quadratic cost function. This allows the task deformation when the desired motion is not realizable. Moreover, a CLIK scheme is implemented to recover from possible position errors. The evaluation of the method is organized as follows. Firstly, the predictive method is compared against the local one in the case without re-planning to show the advantages of the technique. Secondly, the behavior in case of re-planning is investigated. Three re-planning scenarios are defined to simulate different situations. In the first case, the re-planned task has continuous velocity with respect to the current robot state. In the second one, the re-planned path has just continuous position, and in the third one the new desired path is discontinuous with respect to the current robot configuration, simulating an abrupt change in the planned motion. Moreover, different implementations are analyzed. Given the predictive nature of the algorithm, the re-planning can occur instantaneously or only after the end of the predictive horizon. In other words, in the second case, the re-planned task is visible to the controller as soon as it enters the predictive horizon. The paper demonstrates the robustness of the method in all these different scenarios. Furthermore, a comparison against local methods is presented to highlight the improvements given by the predictive strategy in terms of tracking error and secondary task satisfaction.

BACKGROUND

The redundancy resolution method proposed in [START_REF] Faroni | A predictive approach to redundancy resolution for robot manipulators[END_REF]) is briefly recalled. Considering an n-degreeof-freedom manipulator, the joint-space kinematic model is derived by modeling each joint as an rth-order integrator in a single linear state-space representation. In this work, r = 2 will be used, so the outputs are given by the joint positions and velocities, while the control inputs are represented by the joint accelerations. The predictive equations are derived by considering the model in the continuous-time domain, as detailed in (Faroni et al., 2017a). The basic principle of the strategy consists in choosing a reduced number of predictive and control time instants along the respective horizons, in order to reduce the number of variables involved in the model and to speed up the execution of the resulting algorithm. In this work, the prediction and control time instants are chosen to be coincident and are denoted by the vector τ ∈ R p , where p ∈ N is the number of time instants. Therefore, the predictive equations at time t 0 can be written in matrix form as:

q (τ ) = L 0 ξ 0 + F 0 u (τ ) (1) q(τ) = L 1 ξ 0 + F 1 u (τ )
(2) where q (τ ) , q (τ ) ∈ R np are respectively the joint position and velocity vectors predicted at time t 0 + τ , u (τ ) ∈ R np is the input vector along the control horizon, ξ 0 ∈ R 2np is the state vector at time t 0 , where ξ := [ q 1 , q1 , . . . , q n , qn ] T , L 0 , L 1 ∈ R np×2n , and F 0 , F 1 ∈ R np×np are predictive matrices calculated as in [START_REF] Faroni | A predictive approach to redundancy resolution for robot manipulators[END_REF]. Now, consider an m-dimensional primary task, with m < n, which typically represents the desired pose of the robot in the Cartesian space. The satisfaction of the task along the predictive horizon is obtained by imposing that f (q(t)) = x(t) for all t ∈ {t 0 + τ }, where x is the desired Cartesian pose over time, and f : R n → R m , q → f (q) is the forward kinematics of the manipulator. Such task is considered at velocity level as:

J(q(t)) q(t) = ẋ(t) ∀t ∈ {t 0 + τ }.

(3) where J = df /dq ∈ R m×n is the Jacobian matrix of the robot, and ẋ = dx/dt is the desired task velocity. This can be rewritten in matrix form as J(q (τ ) ) q(τ) = Ẋ(τ) (4) where:

J q (τ ) := blkdiag J q(t 0 + τ 1 ) , . . . , J q(t 0 + τ p ) ( 5)

Ẋ(τ) := ẋ(t 0 + τ 1 ) T , . . . , ẋ(t 0 + τ p ) T T . (6)
The operator blkdiag(•) generates a block-diagonal matrix from the given matrices. Condition (4) is nonlinear in the control action u (τ ) for the Jacobians nonlinearly depend upon the joint configuration q. This condition is linearized by calculating the predicted joint configuration q(τ) at each cycle by means of (1) and, then, using it to compute the predicted Jacobians at times t 0 + τ . The analysis conducted in this paper deals with replanning strategies. For this reason, infeasible desired motions and position errors may occur. In order to cope with this situations, changes in the original algorithm are introduced. Firstly, the linearized task (4) is rewritten in terms of a quadratic cost function that minimizes the deviation with respect to the desired task:

u (τ ) = argmin J(q (τ ) )F 1 u (τ ) + J(q (τ ) )L 1 ξ 0 -Ẋ(τ) 2 (7)
Secondly, a CLIK scheme is implemented to recover from position errors that could arise due to infeasible motions or to the re-planning itself (if the desired path changes discontinuously when re-planning occurs). The CLIK strategy consists in modifying the velocity reference by adding a term proportional to the position error e p at the current cycle. The position error is calculated as e p = x(t 0 ) -f (q(t 0 )). Details about the implementation of CLIK strategies with different Cartesian representations are well explained in [START_REF] Caccavale | The role of Euler parameters in robot control[END_REF]. Thus, (7) becomes:

u (τ ) = argmin J(q (τ ) )F 1 u (τ ) + J(q (τ ) )L 1 ξ 0 -( Ẋ(τ) +K p e p ) 2 (8
) where K p ∈ R m×m is a positive-definite matrix. Kinematic constraints are now straightforwardly included in the formulation thank to their linearity in the control action u (τ ) . As a matter of fact, reminding the predictive equations ( 1) and (2), position, velocity, and acceleration limits can be imposed along the predictive horizon as:

q min ≤ L 0 ξ 0 + F 0 u (τ ) ≤ q max qmin ≤ L 1 ξ 0 + F 1 u (τ ) ≤ qmax qmin ≤ u (τ ) ≤ qmax (9)
where q min , q max , qmin , qmax , qmin , qmax ∈ R np are the minimum and maximum joint position, velocity, and acceleration limits respectively. Finally, secondary tasks should be written in terms of a quadratic cost function, in order to include it in the optimization problem. For example, if the secondary objective task is the maximization of the joint range availability [START_REF] Liegeois | Automatic supervisory control of the configuration and behavior of multibody mechanisms[END_REF][START_REF] Atawnih | Kinematic control of redundant robots with guaranteed joint limit avoidance[END_REF], the online optimization problem can be expressed in quadratic form as min

u (τ ) (F 0 u (τ ) + L 0 ξ 0 -q) T W JA (F 0 u (τ ) + L 0 ξ 0 -q) (10)
where q ∈ R np is given by the middle value of the joint range, and W JA ∈ R np×np is a suitable positive-definite weighting matrix. Alternatively, if the secondary task is the minimization of the joint velocity norm, the quadratic problem results to be min

u (τ ) (F 1 u (τ ) + L 1 ξ 0 ) T W VEL (F 1 u (τ ) + L 1 ξ 0 ) (11)
where W VEL ∈ R np×np is a suitable positive-definite weighting matrix. Equations ( 8), (9), and (10) (or ( 11)) can now be clustered in a constrained QP that has to be updated and solved at each cycle. The higher priority of the primary task (8) with respect to the secondary one (10) (or ( 11)) can be handled in a hierarchical-QP framework (see [START_REF] Kanoun | Kinematic control of redundant manipulators: generalizing the task priority framework to inequality tasks[END_REF][START_REF] Escande | Hierarchical quadratic programming: Fast online humanoidrobot motion generation[END_REF] for details) or by using a weighted approach (as well exposed in [START_REF] Liu | Generalized hierarchical control[END_REF]).

RE-PLANNING SCENARIOS

Consider the task function x : R → R m , γ → x(γ), where γ is a curvilinear abscissa that parametrizes the task curve and is typically utilized to define the timing law of the trajectory. For the sake of simplicity and without loss of generality, assume γ corresponds to the normalized longitudinal length along the curve. Time dependency is given by γ : R → [0, 1], t → γ(t), assuming γ ∈ C 1 (R).

In case online re-planning occurs at time t = t re-pl , the resulting task function can be expressed as:

x(γ) = x orig (γ(t)) if t < t re-pl x re-pl (γ(t)) if t ≥ t re-pl (12)
where x orig is the original desired task and x re-pl is the new task after the occurrence of re-planning. Functions x orig and x re-pl are reasonably assumed to be at least of class C 1 ([0, 1]).

The smoothness of the resulting task x affects the system response. For this reason, three re-planning scenarios are devised, depending on the smoothness of x(γ):

(1) x ∈ C 1 ([0, 1]). In this case, discontinuity may occur at acceleration level, thus, the task can potentially be performed without generating tracking errors. This scenario is sketched in Figure 1a.

(2) x ∈ C 0 ([0, 1]). The task can present discontinuous velocity with respect to the current state of the robot.

As the manipulator presents bounded accelerations, the velocity discontinuity is not feasible, therefore a position error will surely occur. The magnitude of such error can depend on the state of the robot when the re-planning occurs. This scenario is sketched in Figure 1b. (3) x is discontinuous at γ = γ(t re-pl ). This means that, right after the re-planning occurs, this scenario intrinsically introduces a large position error, as the robot results to be far away from the new desired task position. In this phase, the robot motion will be mainly driven by the CLIK controller, until the position error is recovered. Note that, in a real application, such scenario is not likely to happen, as the high-level planner should be in charge of generating a continuous path (linking the discontinuous path if needed). However, this evaluation is important for testing the robustness of the algorithm in case a sudden and abrupt change of the task occurs. This scenario is sketched in Figure 1c.

Moreover, re-planning is handled by the predictive method by following two counterposed approaches:

• an instantaneous approach, in the sense that the predicted task seen by the predictive controller changes abruptly when the re-planning occurs (similarly to local methods). • a "predictive" approach, that is, the predictive controller is able to foresee the re-planned task as soon as it enters the predictive horizon.

SIMULATION RESULTS

The simulation platform consists in the kinematic model of the 7-degree-of-freedom manipulator KUKA LWR IV. A 0.2m-long tool is mounted on the robot, aligned with the rotation axis of the last joint. Configuration and velocity ranges are taken from the datasheet of the robot and are given by: q max = [ 2.96, 2.09, 2.96, 2.09, 2.96, 2.09, 2.96 ] T rad, qmax = [ 1. 74, 1.92, 1.74, 2.26, 2.26, 3.14, 3.14 ] T rad/s, q min = -q max , qmin = -qmax . Acceleration bounds are set equal to ±6 rad/s 2 for all joints. The tuning parameters of the predictive method are chosen as p = 3, {τ } = {1, 40, 150} ms, and K p = 20I. First of all, a comparison of the local and the predictive methods is proposed. For this purpose, consider the primary task x = [ 0.35, 0.55 + 0.25 cos(4πγ), 0.4 + 0.25 sin(4πγ) ] T m, where γ ∈ [0, 1] represents the normalized longitudinal length along the path and is given by the following timing law

γ(t) = 6 T 5 tot t 5 - 15 T 4 tot t 4 + 10 T 3 tot t 3 (13)
where T tot = 12 s is the execution time of the task. Two secondary tasks are tested. In the first case, the robot should maximize the joint range availability (i.e., (10), with W JA = I). In the second case, it should minimize the 0.65 norm of the velocity vector (i.e., (11), with W VEL = I).

The following parameters are evaluated: the maximum and mean position error along the task (e max and e mean respectively), and the secondary task satisfaction. In case of joint availability maximization, the task satisfaction is represented by the index

i JA = 1 T tot Ttot 0 q -q(t) dt, ( 14
)
where q is the vector given by the middle values of the joint ranges. In case of velocity minimization, the secondary task is evaluated by means of the index

i VEL = 1 T tot Ttot 0 q(t) dt. (15)
The smaller i JA and i VEL , the better the secondary task is satisfied. Results are shown in Tables 1 and2, which show that the predictive approach ensures better satisfaction of the secondary tasks, compared to the local approach. The error values for the joint-availability case are comparable.

In the case of velocity minimization, however, the local approach gives a larger maximum error, which is avoided by the predictive method.

A second analysis is conducted to evaluate the behavior and the performance of the predictive method in case of re-planning. Referring to (12), the original desired primary task is given by the circle x orig = [0.35, 0.35 + 0.125 cos(4πγ), 0.4 + 0.125 sin(4πγ)] T m, and γ is calculated by using the timing law (13). The secondary task consists in the maximization of the joint range availability. The re-planning occurs at time t re-pl = 6 s. From that moment on, depending on the re-planning scenario, the new desired task is:

• Note that the definition of the three scenarios is compliant with that given in Section 3. Moreover, as mentioned in Section 3, the re-planning is handled both in instantaneous and predictive manner.

Results are shown in Tables 3,4, and 5. The evaluated parameters are the same as those in Table 1. As expected, note that the first scenario always shows a small error. This is clear also from Figure 2a, where the Cartesian tasks

emax [m] emean [m] i JA [rad] Local 1.25 • 10 -1 2.2 • 10 -3 2.55 Predictive (instant.) 1.25 • 10 -1 2.3 • 10 -3 1.78 Predictive (predict.) 1.25 • 10 -1 2.3 • 10 -3 1.78
obtained by the predictive and the local methods almost overlap the desired path. The second scenario presents a significant maximum deformation of the task (due to the infeasible velocity discontinuity), and the third one has a maximum error equal to the distance of the original and the re-planned task when the re-planning occurs. In almost all cases, the maximum and mean error values are comparable for the two methods, showing that the occurrence of re-planning does not generate unpredictable behaviors of the predictive method. Moreover, in case the re-planning is handled in the predictive way, it is also possible to reduce the position error that arises in the second scenario. This behavior is clarified in Figure 2b, which shows the Cartesian task performed by the robot with instantaneous and predictive re-planning. The zoomed window shows that the predictive approach converges to the new path with a smaller deformation of the task. This does not happen in the third scenario, for the position discontinuity is mainly recovered by the CLIK scheme, which has no predictive nature. In this case, the two methods converges to the new task in a similar way, as shown in Figure 2c.

Finally, for all scenarios, the predictive redundancy handling ensures remarkable improvements of the secondary task satisfaction, similarly to the case without re-planning. The case of velocity norm minimization leads to analogous findings, however, numerical results are omitted for the sake of brevity.

CONCLUSIONS

This paper has presented an evaluation of the predictive redundancy resolution in case of different re-planning scenarios. First of all, the original method was slightly modified to cope with re-planning issues. In particular, the primary task was handled in the cost function of the QP. Secondary tasks can be handled in a priority framework. Then, an outer CLIK controller was implemented to recover from position errors. Simulations on a 7-degree-of-freedom KUKA LWR IV were performed to assess the performance of the method. The predictive method was compared to the local one without re-planning. The predictive method shows a significant improvement of the secondary task satisfaction, as the predictive nature of the algorithm allows a better exploitation of the redundancy.

In case of re-planning, the predictive method still ensures better satisfaction of the secondary task, compared to the local method. As regards the tracking error, the predictive and the local methods give similar results. This is an important result to assess the robustness of the predictive method, as a sudden change of the predicted task does not give rise to unstable or undesired behaviors. The only difference in terms of tracking error arises when the re-planned task presents a velocity discontinuity. In this case, by handling the re-planning in a predictive way, the method is able to reduce the maximum deformation of the desired task. Summarizing, the predictive method shows significant advantages over its local counterpart in all re-planning cases, confirming the benefits assessed without re-planning.

( a )

 a First scenario: acceleration discontinuity. (b) Second scenario: velocity discontinuity. (c) Third scenario: position discontinuity.

Fig. 1 .

 1 Fig. 1. Re-planning scenarios based on discontinuity of the kinematic states. 4. SIMULATION RESULTS

  Scenario 1: x re-pl = [ 0.35, 0.6 -0.125 cos(4πγ), 0.4 + 0.125 sin(4πγ) ] T m. • Scenario 2: x re-pl = [ 0.35, 0.6 -0.125 cos(4πγ), 0.4 -0.125 sin(4πγ) ] T m (note that the motion direction becomes counterclockwise). • Scenario 3: x re-pl = [ 0.35, 0.725-0.125 cos(4πγ), 0.4+ 0.125 sin(4πγ) ] T m.

  scenario: comparison of task executions.

Fig. 2 .

 2 Fig. 2. Task executions. Dotted black: original path. Dotted red: re-planned path. Solid blue: task executed by the predictive method. Solid magenta: task executed by the local method (blue and magenta lines are sometimes indistinguishable due to overlapping).

Table 1 .

 1 Comparison of local and predictive redundancy resolution without re-planning in case of maximization of the joint range availability.

		emax [m]	emax [m]	i JA [rad]
	Local	2.95 • 10 -4 1.53 • 10 -4	2.90
	Predictive 2.95 • 10 -4 1.49 • 10 -4	1.69
	Table 2. Comparison of local and predictive redun-
	dancy resolution without re-planning in case of min-
		imization of the velocity norm.
		emax [m]	emax [m] i VEL [rad/s]
	Local	1.50 • 10 -3 2.21 • 10 -4	1.04
	Predictive 3.15 • 10 -4 2.15 • 10 -4

Table 3 .

 3 Scenario 1: Comparison of local and predictive methods.

		emax [m]	emean [m]	i JA [rad]
	Local	2.47 • 10 -4 1.30 • 10 -4	1.91
	Predictive (instant.) 2.47 • 10 -4 1.31 • 10 -4	1.82
	Predictive (predict.) 2.47 • 10 -4 1.31 • 10 -4	1.82
		Table 4. Scenario 2: Comparison of
		local and predictive methods.
		emax [m]	emean [m]	i JA [rad]
	Local	1.60 • 10 -2 3.06 • 10 -4	2.28
	Predictive (instant.) 1.57 • 10 -2 2.98 • 10 -4	1.81
	Predictive (predict.) 1.24 • 10 -2 2.45 • 10 -4	1.81

Table 5 .

 5 Scenario 3: Comparison of local and predictive methods.

ACKNOWLEDGEMENTS

The research leading to these results has received funding from the European Union's Horizon2020 research and innovation program under grant agreement No. 780488 (PickPlace).


E Demangeat

M Pédrot

A Dia

M Bouhnik-Le-Coz

F Grasset

K Hanna

M Kamagate

F Cabello-Hurtado

Colloidal and chemical stabilities of iron oxide nanoparticles in aqueous solutions: the interplay of structural, chemical and environmental drivers

Keywords: semiconductor, magnetic resonance 50 imaging, pigment, biomedicine, drug delivery, heterogeneous 51

Environmental significance statement

Amongst engineered nanoparticles, iron oxide nanoparticles (IONPs) appear particularly relevant toward the environment. Indeed, considering their potential uses in environmental fields (water and soil depollution, agrochemistry, etc.), it becomes critical to determine their behavior and fate in the environment. Their stability and reactivity are dependent upon the environmental parameters, and both intrinsic and surficial properties of IONPs. This work focused on the aggregation behavior of IONPs (especially magnetite, maghemite, hematite), their chemical transformation (oxidation of magnetite) and the impact of a surface coating that mimic natural interactions possibly occurring in the environment. Understanding these interactions and transformations will thus bring key insights regarding the mobility, bioavailability and chemical reactivity of IONPs in prevailing environmental conditions.

Their origin is also anthropogenic as they are increasingly used in 7 various fields such as for industrial, medical and environmental 8 purposes [3][4][5][6] . Iron oxides nanoparticles (NP) respective stability, 9 specific surface area, porosity, dissolution rate as well as 10 transformation kinetics are controlled by their mineral structure 11 and determine their fate and behaviour in the environment 7 .

12 Amongst them, iron oxide nanoparticles, which also occur as 13 ubiquitous phases in both soils and waters, play a critical role in the 14 chemical dynamics resulting from environmental condition changes 15 since nano minerals are highly reactive 8, 9 . In the natural conditions, 16 only a small fraction of engineered iron oxide NP persists in its 17 original form 10 . Biotransformation, oxidation/reduction, dissolution, 18 precipitation, sorption and photochemical transformation may 19 occur amongst other biogeochemical driven processes 11,12 . In turn, 20 this transformation can affect the aggregation, mobility, stabilize iron oxide NP against aggregation [34][35][36] shapes of the nanoparticles (Fig. 1). Magnetite and maghemite 325 oxides both displayed rounded shapes and similar sizes with 326 average diameters of 7 ± 2 nm and 6 ± 2 nm (Table 1), suggesting 327 that the oxidation of magnetite to maghemite did not impact the 328 particle size. Accordingly, BET surface area for maghemite (131 m²g -329 1 ) is relatively close to that of magnetite (115 m²g -1 ) (Table 1 showed successful coating of HA and PC on magnetite surfaces (Fig.

Introduction 1 Understanding the prevailing processes controlling chemical 2 exchanges

 12 at soil/water interfaces is an environmental priority with 3 regards to their impacts on both ecosystems and soil services, as 4 well as water resources. Iron (Fe) oxides are ubiquitous minerals 5 constituent of soils, sediments, aquifers and geological materials 1, 2 . 6

26 Engineered 18 .

 2618 and fate of iron oxide NP in the 25 environment as well as their impacts on the living environment13 . iron oxide NP will become new tools for the treatment 27 of contaminated waters and soils 14, 15 . As compared to conventional 28 macroscale materials, nanomaterials exhibit a high surface-to-29 volume ratio and high reactivity. Thus, their colloidal size may give 30 them high mobility in porous media and their properties allow them 31 to be particularly suitable for the treatment of contaminated soils 32 or aquifers. Iron oxide NP are involved in (i) the dechlorination of 33 organic solvents (CCl 4 , C 2 Cl 4 ), (ii) the detoxification of pesticides 34 (such as Lindane, DDT), (iii) the transformation of fertilizers () the immobilization of metals and metalloids (As, Pb, Cr, If remediation projects using iron oxide NP are currently 37 focused on local-scale pollution, a scale change is initiated to act 38 against diffuse pollution. Moreover, their use in nano-39 agrochemistry could be developed in the years ahead and would 40 represent an intentional diffuse source of iron oxide NP within the 41 environment 19 . A widespread use of iron oxide NP for in situ 42 remediation or in agriculture also raises the question of their 43 environmental compatibility. 44 Amongst engineered iron oxide NP of environmental concern, 45 magnetite (Fe 3 O 4 ), maghemite (ɤ-Fe 2 O 3 ) and hematite (α-Fe 2 O 3 ) are has only one type of cation, Fe 3+ in its structure 28, 29 . 64 In aqueous solution, iron oxides (IO) are amphoteric solids that 65 acquire a surface charge in the protonation and deprotonation 66 reactions of Fe-OH surface sites. The ensuing electrostatic, steric or 67 combined stabilization layers that develop at the surface of the 68 particles are significant in driving iron oxide NP colloidal stability 30 . 69 The interfacial equilibrium at the surface of iron oxide NP is thus 70 affected by the adsorption and desorption of the various 71 constituents of the soil and soil solution in addition to the solution 72 pH 31, 32 . The surface charge of iron oxide NP is also dependent upon 73 their own structural organization such as isomorphic substitutions, 74 which give iron oxide NP permanent charges. 75 As part of the Natural Organic Matter (NOM) occurring within 76 soils and waters, macromolecular polyfunctional humic acid (HA), 77 which displays a particular affinity for metal ions and their various 78 acidic functional groups (carboxylic, phenolic, carbonyl), favours the 79 formation of surface complexes on the Fe-OH sites of iron oxides 80 via chemisorption .33 . As a consequence, interactions between iron 81 oxide NP and HA should be further investigated. Several studies 82 have shown that, in aqueous solution, the HA coating on iron oxide 83 NP likely imparts a negative charge to the nanoparticle surfaces, 84 increasing their surface potential and the ensuing propensity to 85

165 2 -Fig. 1 :

 21 Fig. 1: High Resolution TEM (HRTEM) images of a) Fe3O4, b) ɣ-Fe2O3, c) α-Fe2O3, d) HA-Fe3O4 and e) PC-Fe3O4

342 2 )Table 1 :

 21 . In the spectra of HA-Fe 3 O 4 , the presence of an acid carboxylic 343 group was proven by the vibrational bands at 1410 cm -1 and 1614 344 cm -1 58-60 . In the spectra of PC-Fe 3 O 4 , the absorption bands at total Fe(II)/Fe(III) ratio versus time for magnetite 351 suspension exposed to air is characterized by a two-steps' behavior. 352 Over the four first days (0-96 h), a sharp decrease in total Physicochemical properties of a) Fe3O4, b) ɣ-Fe2O3 and c) α-Fe2O3 obtained from TEM, BET analyses and potentiometric titration.

Fig 2 :Fe 3 O 4 ,(Table 2 :

 2342 Fig 2: ATR-FTIR spectra of HA-magnetite and PC-magnetite in the range 780-1800 cm -1 obtained from 0.5 g L -1 NP suspensions at pH= 6.

Fig 3 :Fig 4 : 7

 347 Fig 3: Evolution of the total Fe(II)/Fe(III) ratio (blue circles) and bound Fe(II)/Fe(III) ratio (red diamond) as a function of time. The reaction was performed with 2.5 g L -1 magnetite NP at pH = 7.5 in oxic conditions.

8 |

 8 643 soil solution have to be considered in order to accurately assess the 644 fate of NP in the environment. NP intrinsic properties, such as their 645 surface chemistry, are also of prime importance since they are 646 involved in both electrostatic and steric interactions, surficial aerobic environments, NP likely 649 undergo many surficial modifications and become embedded in 650 matrixes or functionalized with specific molecules 84 . As a result, 651 these attached molecules can both passivate and/or confer their 652 own properties to mineral surfaces; e.g. polyanionic HA coating on 653 magnetite, resulting in a lower pHzpc and enhanced colloidal 654 stability. In addition, HA coating -as well as PC covering -might 655 passivate iron oxide NP surfaces to yield a steric barrier that 656 prevents Fe leakage from mineral surfaces and the ageing of the 657 magnetic core by oxidation. HA, which are ubiquitous in most 658 aquatic systems, display a specific affinity for iron oxide NP as well 659 as trace elements and therefore HA-iron oxide NP complexed 660 structures may also enhance ETM adsorption to NP. However, 661 although natural coatings demonstrated improved iron oxide NP 662 stability and reactivity, their effects depend upon the amount and 663 chemical composition of the substances 4 . 664 The rapid oxidation of magnetite into maghemite is important since 665 iron plays an active redox catalytic role in many energy transfer and 666 electron transfer processes 85-87 . The iron oxide redox state also 667 plays a key role in specific anaerobic environments where Fe(II) and 668 Fe(III) are used as energetic catalysts for bacteria 88, 89 . Finally, the 669 transformation of magnetite into maghemite is also relevant 670 considering its adsorption capacity towards environmental 671 compounds 90 . Several questions are raised from these 672 environmental considerations, especially, what are the impacts of 673 these modifications regarding iron oxide NP reactivity? J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Conclusions 675 The pH appears to be a key driver in controlling iron oxide NP 676 colloidal behaviour as resulting from protonation and 677 deprotonation reactions of surface hydroxyl-groups. Surface 678 modifications also implied considerable effects on iron oxide NP 679 colloidal stability as they promoted electrostatic interactions. In 680 contrast to PC, HA was shown to be particularly efficient in 681 enhancing magnetite colloidal stability. Magnetite seems to be 682 highly unstable in aerobic conditions. The oxidation of magnetite 683 into maghemite modified its surface chemistry and surface area, 684 and these modifications resulted in higher aggregation at most pH 685 values. Compared to maghemite, hematite appeared to be less 686 sensitive to pH and displayed a lower surface area and surface 687 potential. Both hematite and maghemite displayed high 688 aggregation size distributions at different pH values. 689 Considering the widespread use of iron oxide NP and their 690 subsequent release in the environment, their fate and behaviour in 691 soils and natural waters raise many environmental questions not 692 only in terms of their possible impacts on living organisms but also 693 for their mobility and fate in ecosystems. Therefore, more attention 694 should be paid for the understanding of colloidal and chemical 695 stabilities of iron oxide NP as it is likely involved in many major 696 interactions within the environment.

  697

1

 1

141 Experimental 142 Iron oxide nanoparticles and coated magnetite NP 143 Synthesis of bare iron oxide NP. Magnetite

  Fe 2 O 3 ) was synthesized by the forced hydrolysis of acid

		Please do not adjust margins
	156	Maghemite (ɣ-Fe 2 O 3 ) was synthesized following the method of	105	In the environment, soils and soil solutions are prior interfaces
	157	Anna et al. 52 , through the oxidation of the previously synthesized	106	for iron oxide NP interactions 46 occurring at mineral surfaces and in
	158	magnetite by adding 5 wt% NaOCl aqueous solution (16 mL) and	107	water. In addition to particle interactions, NP are prone to various
	159	sonicating for 2 hours. The solid product was magnetically	108	chemical and physical modifications which foster the complexity of
		separated and washed with water.				109	the reactions and likely affect both NP stability and reactivity.
	161					110	Electrostatic, steric or combined electrosteric forces can result from
	162	Fe(III) salt solutions at 98°C from 0.02 M Fe(NO 3 ) 3 following the	111	these interactions and promote repulsive forces whereas Van der
		protocol described in Schwertmann & Cornell (2003)	1 .			112	Waals and magnetic dipolar interactions foster attraction between
						113	particles that likely aggregate. The intrinsic physicochemical
						114	properties of iron oxide NP (size, shape, surface area, chemistry,
						115	superparamagnetism, etc.), their ensuing high reactivity, together
						116	with the surrounding solution composition (dissolved species, pH,
						117	ionic strength, nanoparticle concentration), are the main drivers of
						118	iron oxide NP chemical and colloidal stability in aqueous media 47, 48 .
						119	Considering the geochemical, biological and technological
						120	significance of iron oxide NP	49 , understanding the mechanisms of
						121	their stability may turn out to be relevant to unravel the fate and
						122	behaviour of iron oxide NP in the environment as well as their
						123	impacts on living organisms. Therefore, the aim of this work is to
						124	investigate the dual colloidal and chemical stability of magnetite NP
						125	in order to (i) understand the effects of their intrinsic properties
						126	(size, morphology, surface chemistry) and those of the
						127	environmental drivers (especially the effect of pH) on their
						128	aggregation state, (ii) assess the oxidation kinetics of magnetite, (iii)
						129	investigate the link between colloidal and chemical stability of NP,
						130	as well as (iv) identify the resulting environmental impacts.
						131	Hematite has also been studied to better understand the impact of
						132	intrinsic structural properties on iron oxide NP colloidal stability as
						133	the structure of hematite differs from both magnetite and
						134	maghemite. Indeed hematite NP size and morphology as well as
						135	internal crystal structure, generally confer a higher stability and
						136	crystallinity to hematite. Last, the method we chose to study the
						137	iron oxide NP aggregation state allowed us to study their size
						138	distribution over a wide range of sizes (that spread from one tenth
						139	of a nm to µm-scale aggregates) which allowed us to determine NP
						140	behavior in a context comparable to those observed in natural
							waters.
	92				38
	93	demonstrated that surface-bound lipids inhibit the iron core from	144	(Fe 3 O 4 ) was
	94 95	oxidizing, even in the presence of bacteria, by forming a hydrophobic pocket 38, 39 . In addition, several authors have shown	145 146	prepared by the co-precipitation of iron salts according to the modified literature procedure 50, 51 . In brief, FeCl 2 .4H 2 O (1.988 g)
	96	that a phospholipid coating likely decreases nanoparticle size	147	and FeCl 3 .6H 2 O (5.406 g) were dissolved in 5 mL HCl (2 M) and 20
	97	dispersion and favours narrow-sized nanoparticles	40,	41 .	148	mL H 2 O, respectively, and then mixed with magnetic stirring to
	98	Furthermore, the instability of magnetite in air causes NP to	149	obtain an iron solution with a molar ratio of Fe(II)/Fe(III) = 0.5.
	99	undergo a partial oxidation to maghemite (ɣ-Fe 2 O 3 ), ending up with	150	Magnetite NP were then precipitated by the dropwise addition of
	100	a core-shell structure in which the thickness of the oxidized layer is	151	the iron salt solution into a 0.7 M NaOH-NaNO 3 base solution (250
	101	a function of the particle size 42, 43 . This phase transformation	152	mL). The black precipitate was left to settle in anaerobic conditions
	102	directly affects the surficial properties and internal characteristics	153	for a few hours to get rid of the supernatant. Three washings were
	103 104	of NP, which are deeply involved in driving iron oxide chemical stability and reactivity 44, 45 .	154 155	then conducted in anaerobic conditions with deoxygenated deionized water and 5 10 -3 M NaCl solution.
		Please do not adjust margins

. Nevertheless, Hadju 86 et al. 37 demonstrated that coating magnetite with HA could either 87 foster or hinder Fe 3 O 4 -NP colloidal stability depending on the 88 amount of HA present at the mineral surfaces. Other natural 89 constituents such as amphiphilic phospholipids (integral major 90 component of cell membranes) likely affect iron oxide NP stability. 91 Using phospholipids as a coating on pyrite surfaces, Hao et al. 160 Hematite (α-163 Coatings with humic acid and phosphatidylcholines. Fe 3 O 4 -NP 164 were coated with HA (Elliott Soil Humic Acid Standard IV) and PC (1,

  ).

	330
	331	Hematite displayed different morphologies with rhombohedral
	332	shapes measuring 28 nm-wide and 39 nm-long on average, with a
	333	surface area of 49 m²g -1 (Table 1). No significant changes were
	334	reported for neither HA-nor PC-coated magnetite from TEM
	335	images, suggesting that the coating process did not modify the
	336	morphology of magnetite. Although the surface area was not
	337	determined for coated materials, no significant modification is
	338	expected, according to previous findings	57 .
	339
		ATR-FTIR spectroscopy

340

The IR spectrum recorded for of Fe 3 O 4 , HA-Fe 3 O 4 -and PC-Fe 3 O 4 341

Table 2

 2

	583		). This enhanced
	584	stabilization may stem from a HA steric barrier that helped maintain
	585	a space between the particles to prevent the aggregation of NP	70, 71 .
	586	Similar to the HA coating, the PC surface modification did not affect
	587	the size and morphology of magnetite. Magnetite coated with PC
	588	displayed a different colloidal behaviour as compared to bare
	589	magnetite. As PC imparted both negative and positive surface
	590	groups on the surfaces of the NP (glycerophosphate group,
	591	trimethylethanolammonium group), and because of the complex
	592	and heterogeneous surface structures that are generated by
	593	adsorbed PC	72 , PC-coated nanoparticles likely displayed variable
	594	colloidal behaviours. As compared to magnetite, PC-NP showed
	595	similar size proportions and repartition although the magnetite-PC
	596	coarse fraction displayed a few large aggregates (hence its wider
	597	size range, see Table 2) regardless of the pH. PC-NP were thus
	598	probably able to form highly contrasted structures (according to
	599	their size) suggesting that PC-coated magnetite did not entirely
		depend on electrostatic interactions but also on steric forces 72 .

600

Effects

of iron oxide intrinsic physicochemical properties. The

			621	displaying coarse aggregates with homogeneous sizes suggests that
			622	other mechanisms are involved at basic pH. Further investigations
			623	are thus needed to fully assess iron oxide NP colloidal stability	74-76 .
			624	Environmental perspectives. Aggregation and stabilization have
			625	significant effects on the mobility, reactivity and environmental fate
			626	of NP. Once released in surface waters, unstable particles tend to
			627	agglomerate, sediment or flocculate or they preferentially adsorb
			628	onto bigger particles or surfaces 77 . As a result, aggregated NP that
			629	form larger colloids with a possible smaller surface area are less
			630	easy to transport and do not get through the soil as easily as smaller
			631	particles 78 . As aggregated NP become less mobile, they may
			632	become more ineffective in acting as nutrient carriers	79 . It is also
			633	more difficult for these colloids to be taken up by plants and living
			634	organisms as they are less soluble than their smaller counterparts 80 .
			635	The dissolution of NP induces the mobilization of ions which are
			636	likely complexed with molecules that foster their absorption by
			637	plants and animals. In addition, depending on their concentration
			638	and speciation as well as on the physiology of the organism that
			639	absorbs it, metal atoms or ions can be either positive nutrients or
			640	toxic elements. On the other hand, aggregation may directly affect
			641	NP toxicity towards living organisms since smaller NP have been
				shown to penetrate the cell walls of certain species more easily	81, 82 .
	601
	602	oxidation of magnetite (Fe 3 O 4 ) to maghemite (ɣ-Fe 2 O 3 ) did not
	603	change morphology and particle size. Maghemite still displayed a
	604	higher surface area (131 m² g -1 ) than magnetite (115 m² g -1 ) and
	605	oxidized NP very likely yielded intrinsic compositional differences.
	606	These modifications probably resulted in a higher surface potential
	607	thereby explaining the variable size distributions observed as the
	608	pH changed. Maghemite NP aggregation was favoured at acidic pH
	609	(equal or below 5), which would be in good agreement with their
	610	high surface potential. Indeed, the tendency to form large
	611	aggregates at these pH values likely resulted from a high energetic
	612	barrier that had to be overcome at the mineral surfaces	73 . Indeed,
	613	increasing NP surface to volume ratio leads to a higher surface
	614	energy which induces aggregation between particles. As hematite
	615	displayed a much lower surface area, the higher colloidal stability
	616	observed over a wide pH range may result from a lower energetic
		barrier at the surface of the particles in accordance with NP having
	620

617 a relatively lower surface potential. Hematite stability would thus 618 be related, in part, to its smaller surface area and surface potential. 619 However, the typical aggregation behaviour of hematite NP 642 Considering the pH effect, the physicochemical properties of the

Acknowledgements


My Ph.D. dissertation explores the plant-object and more specifically the "roots" of plants, from tubers to rhizomes and bulbs. As this is a curious object to study, I will start by showing you some images to introduce my talk here. My interest in studying the roots of plants is more obvious when one sees those images: the symbolic meaning of the underground part of the plant is extraordinarily rich and complex. But how do we tell a natural history of the root part of the plant? How could it be of any interest to unravel the scientific knowledge of early modern Europe? How could underground objects shed light on the historical discipline? (Slide 2) European ways of thinking are commonly based on a hierarchy of beings which one generally calls "Chain of Beings" or "Scala naturae" (for "Scale of beings"). These terms define how an individual of the early modern times envisioned the natural world and the beings which lived in this world. Roots and bulbs were the very last level, the lowest one in the food chain. (Slide 3) As you can see on Jan Brueghel and Henrick van Balen's painting, the root vegetables are placed on the ground, next to animals (rabbits which are meant to eat them). They were called "crawling livings". They were opposed to lemons or apples and pears which are painted higher on this allegoric depiction of seasons. Fruits are nobler edible resources than roots. However, this "noble status" was contradicted by the authors of herbaria and medical treatises. As a matter of fact, "roots" were one of the most valued parts of the plants, commonly used as medicinal herbs. At first, the root part of the plant was described as a secret remedy. It was often surrounded by magical rituals, such as the mandragora (or mandrake), which I could not forget to mention in this presentation. (Slide 4) Mandrake, bryony, mallow and other famous roots were jealously kept secret as occult knowledge, shared only between practitioners of such "wizardry" : it was shared among women mostly, because they were the one in charge of the health issues in their home, and it was also shared between travelling and itinerant merchants, and even some apothecaries. Roots were also the domain of sorcerers and women capable of witchcraft. They were synonyms of the underground earth, the act of collecting roots was embedded with the act of digging the ground, which was more of a wild animal gesture. Roots were used both as medicinal talismans and therapeutic edible remedies. It was not surprising, then, that the mandrake would shriek when extracted out of the ground, by night, during a specific moon. (Slide 5) It also led to particularly ridiculous fashions and rituals, such as depicted in the satirical play by Niccolò Machiavelli (The Mandrake, 1524), or in Othello, the tragedy written in 1603 by William Shakespeare. Roots were inevitably viewed as universal remedies, a panacea used for every human ache and illness, but more frequently used as an aphrodisiac, a poison or as a psychotropic drug. The mandrake was called Anthropomorphos by Greek authors. John Gerard, in The Herball, or Generall Historie of Plante (1636), reminds us of the biblical origins of the famous root, especially in Rachel and Jacob biblical parabola about sterility. John Gerard said:

There have been many ridiculous tales brought up of this plant, whether of olde wives or some runnagate surgeons or phisickmongers, I know not, (a title bad inough for them: ) but sure some one or moe that sought to make themselves famous in skillfull above others were the first brochers of that errour I spake of. […] They fable further and affirm, that he who woulde take up a plant thereof must tie a dogge thereunto to pull it up, which will give a great shrike at the digging up; otherwise if a man should do it, he should certainly die in short space after […]1 (Slide 6) Moreover, I came across some root images by zooming on big digitized canvasses and luckily, I could find some interesting scenes of practices around the roots of plants in bigger sceneries and landscapes. This is, as The Routledge Handbook of Material Culture in Early Modern Europe says, a "stimulating view of materiality". It questions us "about the whole material world as a context for lived experience […], material interactions at all social levels" 2 . This visual method perfectly complements my study of herbaria and other treatises that I compare and intertwine with iconographic data in which we can observe, surrounding the vegetal objects, work environments and practices, as well as gestures of consumers, farmers or merchants, whose knowledge and technics were various. These research crossovers between written and iconographic data also make it possible to find connections between diverse social environments. Images also allow us to question cultures where writing and reading are not common practices. Once we look at these iconographic representations, we can view the interactions between things and history, the material object studied here being the root. The way we see the plant object in all its dimensions, material, symbolic, cultural, teaches us more about the tastes of people in modern times, but also the constraints of their environment: their climatic constraints, their social constraints as well, etc. The plant objects in the paintings or in the illustrated plates of the herbaria, depicted a slightly bigger picture, portraits of individuals or groups of individuals in history, which could be described as "larger narratives"3 .

Root vegetables are often used to represent the scarcity and harshness of winters, as well as the difficulties of farmers in the fields. They could also picture poverty in its most figurative and literal depiction. The image of poverty is often featured in the paintings of 16 th and 17 th centuries. Those paintings were not just the portraits of people, women, and men, who bore the scars of this poverty, as objects are indeed a proof of this poverty and of the necessity of root vegetables in times of shortage. (Slide 7) Root vegetables also added theatrical dramaturgy to the scenes. For instance, hermits and pilgrims ate sparingly and rarely, as we can see on Jan Wildens canvas. Root vegetables were painted here to show their frugality and accentuate the rigor of their living conditions.

The opposite metaphor could be expressed in paintings too. (Slide 8) In fact, in some paintings, sins were underlined by plant objects. Thus, appetite, lust, gluttony, excess, are traits that were put forward by painters to better criticize them or to make fun of a society that evolved amid such negative human characteristics. Pieter Aertsen, Frans Snyders and Jan Wildens' canvasses are particularly good examples. Market scenes show two worlds apart: one being the world of wealthy people and excess of food, characterized by the generosity and fertility of earth, represented by big piles of vegetables and the lady with a hat, certainly from the bourgeoisie, buying vegetables. And on the other hand, the second world was made of social hardship, poverty, and food scarcity, symbolized by the market sellers, in dark clothes and recluse positions. Root vegetables, parsnips, carrots, onions, etc., were significant during the 17 th and 18 th centuries: they were easy to grow, and they were considered as humble (but not noble) vegetables. Christian ideals at the time highlighted moral values such as moderation, sobriety, and temperance. These critical views were also described in scientific literature such as herbaria or medical treatises. It was flagrant in "husbandry" and gardening treatises. Excess and gluttony were sins, as were drunkenness and lust. Excess was even seen as an illness and an imbalance of galenic humors in the body of a sick man or woman. (Slide 9) Thus, human beings had to cultivate their garden, as said in A New Orchard, and Garden (1653) by William Lawson : "What else are trees, in comparison with the earth, but as haires to the body of a man?". In his horticultural treatise, William Lawson saw plants and trees as anthropomorphic reflections of the human body. If a man were healthy because of his humoral balance, then, one should treat the trees as a balance between tops (Lawson meant the branches and fruits) and roots:

I utterly dislike the opinion of those great gardners, that following their books, would have the maine roots cut away: for tops cannot grow without roots. And because none can get all the roots, and removal is an hinderance, you may not leave on all tops, when you set them : For there is a proportion betwixt the top and root of a tree, even in the number (at least) in the grouth. If the roots are many, they will bring you many tops […]4 (Slide 10) As for social behaviors and symbolic meanings of plants, women were sometimes portrayed alongside men in suggestive positions, as Pieter Aertsen liked to paint them. At first glance, one only sees sellers or market gardeners, but at a second glance, one sees sexual attributes, external genitalia, through the image of vegetables and fruits. The roots thus represent male phallic attributes, melons and round fruits represent female attributes. Some vegetables and fruits were unrealistically painted out of the imagination of the artists. The symbolic value of such vegetal objects shows how people were thinking natural things from the Renaissance to the 18 th century. The anthropomorphic resemblance with human organs was benefiting the new medical market of remedies and medicinal recipes. Such simples (or medicinal herbs) would see their status and value increase, while being described and displayed in medical treatises as scientific data. By way of illustration, the parsnip or Pastinaca, "panais" in french, was a root vegetable of little interest and value. However, the seeds and roots became one of the most affordable and easily grown medicinal drugs on the market, after the rereading of medical treatises from Antiquity by Renaissance humanists which gave fertility powers to those parts of the plants, as Philip Miller, who was an English botanist from a Scottish descent, explained (Slide 11):

The Root and Seed of the second Sort [of parsnip] is sometimes used in Medicine; but it is seldom cultivated in Gardens, the Markets being supply'd from the Fields: yet the Druggists commonly sell the Seeds of the Garden Kind for it, which they may purchase at an easy Price, when it is too old to grow. 5 (Slide 12) In Adam Lonicer's Kreüterbuch, first published in 1557, Renaissance humanists already demonstrated and indicated how to grow soils more fertile by preparing the roots of plants and trees. This essential part of the plant would provide all the energy and vitality to a plant, because, as the physicians and naturalists of the 16 th century believed, plants were feeding from the soil. In one of Adam Lonicer's illustrations, we can see a gardener holding a root in his right hand and a tool in his left hand. Other instruments and tools are lying on the ground: a shovel or a spade, a pickaxe, a smaller ax and what looks like a small hoe, serving as a root divider, all made for digging or cutting plants. Until the 18 th century, gardening tools had shown little variations since the Renaissance (and even since the Middle Ages). Early modern utensils were nevertheless used with new technics, for horticultural and agricultural functions. (Slide 13) The continuously increasing number of herbs gardens, fields and plantations, and the growing demand for food in the 18 th century, prefigured the massive use of tools and mechanical machines in the new agricultural system generated by physiocratic principles, later advocated by the Scottish philosopher and economist Adam Smith. Noël Chomel shows a revamped rake to remove roots from the ground in his Oeconomical Dictionary 6 in this new fashion for efficacy. It was used for cleaning up the soils before planting vegetables. Root vegetables, as we have seen in Jan Brueghel and Hendrick van Balen's Garland of Fruits, were also valued as livestock food. Beets and turnips were given to pigs, horses, cows, ewes, and goats, as described by Noël Chomel in the aforesaid dictionary. Such technical and scientific knowledge about plants were exported to the new territories colonized by European settlers. Due to their particularly strong ability to grow in hostile, cold, damp, or hot climates, turnips, parsnips, onions, and carrots were brought in colonial settlements. However, it did not prevent the naturalists from discovering new types of roots in the field. (Slide 14) Cassava was one of those botanical discoveries in the "New World". Cassava is toxic and poisonous when not prepared correctly. Botanists observed how the Indigenous people from Canada, the Caribbean islands or in South America were using it. The nutritional quality of cassava became manifest when the settlers saw they could make flour for cooking recipes. Cassava went from being an obscure discovery to a scientific finding which mainly served as food for slaves, servants, and workers, but also for the rich colonists, who used it for the tapioca starch. The root was first encountered by the Spanish in Brazil but described by the French through what André Thevet had heard of after his long trip from 1555 to 1557. He gave a brief description in his book Les Singularitez de la France antarctique (1557), naming cassava « racines de Manihot ». Jean de Léry in his L'Histoire d'un voyage fait en la terre du Brésil autrement dit Amérique (1578) mentioned the cassava root as an edible resource. A century after, Maria Sibylla Merian, one of the first female botanists to travel, went to Surinam and might have been the first one to have given a realistic vision of the cassava in her watercolor drawing, compared to André Thevet's fantasist vision. If the leaves are looking roughly the same, the root is different from one drawing to the other. The engravers employed by Thevet adopted a European style and gave what I call "a carrot or parsnip look" to the cassava tuber.

(Slide 15) What is more, the cassava root allows us to study the relationships between slaves, Indigenous people, and European settlers. Jean-Baptiste Labat in his Nouveau voyage aux isles de l'Amerique (1724), wrote about his experience as a botanist and administrator in the West Indies from 1696 to 1704 (you can compare this description to the image I am showing you here) :

J'Ay di ten quelques endroits cy-devant que la Cassave & la farine de Manioc servent de pain à la plûpart des habitans blancs, noirs & rouges des Isles, c'est-à-dire aux Européens, aux Negres & aux Sauvages. […]

Quand ces racines sont arrachées, les Negres destinez à cet ouvrage, en gratent ou ratissent l'écorce avec un méchant couteau comme on fait aux navets, & les jettent dans un canot plein d'eau où on les lave bien, après quoi on les grage, c'est-à-dire qu'on les réduit en une espece de farine fort humide qui ressemble à de la grosse scieure de bois, ce qui se fait en passant fortement la racine sur une rappe de cuivre, comme on passe le sucre. […] Le Negre qui grage met un bout de la planche où la grage est attachée dans un canot ou auge de bois, & appuye l'autre bout contre son estomach, il y a à côté de lui un panier où sont les racines bien gratées & bien lavées, il en prend une à chaque main, & la passe & repasse sur la grage en l'y appuyant fortement, jusqu'à ce qu'il l'ait réduite en farine. 7 The task of grating the cassava root must have been tedious to do. The materiality of the cassava unveils information about the European domination on slaves and the hybridization, cultural blending and mixing of knowledge between the enslaved African people (featured on this image by their skilled labor force), the Indigenous people (which are symbolized here by the transmission of the cassava flour technic 7 LABAT, Jean-Baptiste, Nouveau voyage aux isles de l'Amerique, 1724, p. 1127-128. Translation: « I have already told before that Cassava & Manioc flour serve as bread for most of the White, Black & Red inhabitants of the Islands, that is to say: to Europeans, Negroes & Savages. […] When these roots are pulled up, the Negroes intended for this work, scrape or rake the bark with a sharp knife as we do with turnips, and throw them into a canoe full of water where they are washed well, afterwards. They are grated, that is to say, they are reduced to a very moist kind of flour which resembles coarse sawdust, which is done by passing the root strongly over a copper stroke, as we do with sugar. […] The Negro who grates, puts one end of the board where the grater is attached in a canoe or wooden trough, & leans the other end against his stomach. There is, next to him, a basket where the roots are scraped & well washed, he takes one in each hand, and passes it again and again on the grater, pressing it strongly, until it has been reduced into flour. » through objects like the basket or the canoe/trough) and the European settlers (whose presence is shown here in the European forms of houses and shelters).

To conclude, roots make me travel through times and places. Printed books, paintings and engravings show different aspects of the same vegetal object. Early modern texts reveal more of a process originating from scientific methods and rational thoughts. Whereas paintings impart a more detailed idea to the technics and gestures which were used at that time. Materiality emerges from both written and visual dimensions of the root part of the plant. They serve as real indicators and illustrations of scientific notions and technics and they reinforce the image of social and cultural conditions around the plant object to help us write a natural history of the roots along with a history of human mentalities.

Bibliography:

Images:

1) GRIECO,

  Tassanee ALLEAU Doctorante contractuelle au Centre d'études supérieures de la Renaissance (Université de Tours, CESR, UMR 7323) Communication in Material and Visual Culture Seminar Series 28 October: Scientific Knowledge | Session Chair: Peter Davidson Title: "'At the root' of practices and knowledge: natural histories of the underground plant-object in a visual iconographic database" 20 min paper https://www.eca.ed.ac.uk/material-and-visual-culture-seminar-series (Slide 1) First, I would like to thank Anna Myers and Georgia Vullinghs for taking care of the organization of this seminar at the University of Edinburgh. I feel really honored to talk here about my recent Ph.D. research.

  Allen J., « Les plantes, les régimes végétariens et la mélancolie à la fin du Moyen Age et au début de la Renaissance italienne » In : Le Monde végétal (XIIe-XVIIe siècles) : Savoirs et usages sociaux [en ligne]. Saint-Denis : Presses universitaires de Vincennes, 1993. 2) Jan Brueghel et Hendrick van Balen, Krans van vruchten en bloemen rond een allegorie op de landbouw, Garland of Fruit surrounding a Depiction of Cybele Receiving Gifts from Personifications of the Four Seasons, oil on panel, c. 1615/18, collection Mauritshuis. 3) "The Mandrake" from John Gerard, The Herball Or Generall Historie of Plante, Volume 1, Adam Islip Joice Norton and Richard Whitakers, 1636. 4) Pierre Boaistuau, Histoires prodigieuses, [1559]. Londres, Wellcome Library, ms. 136, f. 138v. 5) Jan Wildens, Gezicht op Antwerpen, oil on canvas, 1636, collection Rijksmuseum, Amsterdam 6) Jan Wildens, Pilgrimage place in a rocky landscape, oil on canvas, c. 1635-53. 7) Frans Snyders and Jan Wildens, Greengrocery Stall (Market scene outside a town), Flanders, oil on canvas, c. 1618/21, Hermitage Museum. 8) LAWSON, William, A New Orchard, and Garden (…), Whereunto is newly added, the art of propagating plants, etc. W. Wilson, 1653, p. 14-15. 9) Pieter Aertsen, Market scene, oil on panel, 1569, Hallwyl Museum's collection. 10) MILLER, Philip, The Gardener's Dictionary: Containing the Methods of Cultivating ... the ... Garden, ... Conservatory and Vineyard. Abridg'd from the Folio Edition, Etc., Volume 2, 1735. Bibliothèque nationale de France, Gallica. 11) LONICER, Adam, Kreüterbuch, 1557, p.29.

GERARD, John, The Herball, or Generall Historie of Plante, volume 1, 1636, p.281.

RICHARDSON, Catherine, HAMLING, Tara, GAIMSTER, David (dir.), The Routledge Handbook of Material Culture in Early Modern Europe, Routledge, 2017, p.1.

Read RIELLO, 2009, RIELLO and GERRITSEN, 2014, as quoted in The Routledge Handbook of Material Culture in Early Modern Europe, p.80.

LAWSON, William, A New Orchard, and Garden (…), Whereunto is newly added, the art of propagating plants, etc. W. Wilson, 1653, p.14-15.

MILLER, Philip, The Gardener's Dictionary: Containing the Methods of Cultivating ... the ... Garden, ... Conservatory and Vineyard. Abridg'd from the Folio Edition, Etc., Volume 2, 1735.

CHOMEL, Noël, Dictionnaire oeconomique, A Lyon, Chez Pierre Thened, 1709 (première édition), p. 869-870.


Mengistu Teramage

Yuichi Onda

Hiroaki Kato

Xinchao Sun

Impact of forest thinning on the dynamics of litterfall derived 137 Cs deposits in coniferous forest floor after Fukushima accident

Keywords: Ageing, Degradation, Drying and wetting, Organic matter, Radiocesium

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N°dossier : PUB20200145 Référence : TERAMAGE MENGISTU, ONDA Yuichi, KATO Hiroaki, SUN Xinchao Impact of forest thinning on the dynamics of litterfall derived 137 Cs deposits in coniferous forest floor after Fukushima accident Chemosphere, 239, (Elsevier 2019) 124777 | 10.1016/j.chemosphere.2019.124777 (ACL) Auteurs dans l'ordre de la publication : 1, TERAMAGE, MENGISTU , CORRESPONDING AUTHOR Institut de Radioprotection et de Sûreté Nucléaire, PSE-ENV/SRTE/LR2T, France 2, ONDA, Yuichi Universite Tsukuba, Center for Research in Isotopes and Environmental Dynamics, Tsukuba, Japon 3, KATO, Hiroaki University of Tsukuba, Japon 4, SUN, Xinchao Tianjin University, Tianjin, Chine Type : Publication écrite Titre : Impact of forest thinning on the dynamics of litterfall derived 137 Cs deposits in coniferous forest floor after Fukushima accident Résumé d'auteur : The effects of a 50% forest thinning intensity on Fukushima-derived 137Cs deposition by litterfall and its discharge by runoff in hillslope coniferous forest were monitored using four litterfall traps and a hillslope erosion plot. The observation was underway during the pre-and post-thinning periods. Results demonstrated that during the pre-thinning period a total 150 ± 13 g m g -2 of litterfall deposited about 924 ± 69 Bq m g -2 of 137Cs. This accounts for 11% of the local 137Cs fallout recorded for the study site in the aftermath of the accident. After thinning, both litterfall and 137Cs increased by more than six-and twofold, respectively. This is possibly owing to the slow individual tree recovery rate assisted by the change on the running space provided by canopy openings, which can accelerate even the normal gust wind to gain damaging power on the unshielded mechanically injured parts of the contaminated residual trees. In both cases, litterfall generally transferred about 37% (3 ± 0.2 kBq m g-2 ) of the local 137Cs fallout onto the forest floor over the observation period. The eroded litter-associated 137Cs increased by about a factor of two after thinning, which only accounted for less than 1% of 137Cs deposited by litterfall. This implies that the forest floor retains 137Cs and remains contaminated regardless of the size of the eroded litter material. But this could become a potential secondary contamination source for the downstream resources such as water bodies and villages, especially at the time of flooding, which in turn calls a serious attention in designing decontamination schemes.

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Introduction

Several studies have indicated that soil water content affects the availability of radionuclides (e.g., [START_REF] Degryse | Enhanced sorption and fixation of radiocesium in soil amended with K-bentonites, submitted to wetting-drying cycles[END_REF][START_REF] Sohlenius | Inferences about radionuclide mobility in soils based on the solid/liquid partition coefficients and soil properties[END_REF][START_REF] Tanaka | Influence of drying period on the migration behavior of radionuclides in aerated soil layer[END_REF]. [START_REF] Rosen | Effect of clay content and wetting-anddrying on radiocesium behavior in a peat and a peaty podozol[END_REF] showed that succession of dry and wet cycle compared to wet conditions increased the bioavailability of 134 Cs that generated a greater uptake by rye grass from organic soil. [START_REF] Popov | Effect of periodic wetting and drying on selective sorption of 137 Cs by mixtures of soil and organomineral sorbent[END_REF] also demonstrated that the addition of 5% organomineral sorbent (OMS) on sandy soddypodzolic soil increased the effect of the drying and wetting cycles on the selective sorption of radiocesium ( 137 Cs). Therefore, depending on the binding strength, 137 Cs in soil can be available and mobile by rain, soil water as well as exudates secreted by plants. In a laboratory incubation experiment using constant soil moisture content, [START_REF] Teramage | The impacts of 137 Cs input forms on its extractability in Fukushima forest soils[END_REF] recently demonstrated that the extractability of 137 Cs in forest soils depends on its input forms (i.e., aqueous 137 Cs vs. solid organic sources of 137 Cs). They found that both the water-and acetate-extractable 137 Cs fractions were greater for aqueous amended soils than for solid organic amended soils. Results show that the extracted 137 Cs fraction with water was less than 1% for the aqueous amended soils and below detection limit for the solid organic amended soils. Likewise, with an acetate reagent, the extractable 137 Cs fraction ranged from 46 to 56% for aqueous amended soils and from 2 to 15% for the solid organic amended soils. Indeed, in field environments, the soil moisture content fluctuates depending on the seasons, climatic conditions and unforeseen weather situations such as tropical cyclones, which can modify the solution/solid elemental distribution. In Japan, including in the contaminated Fukushima region, the fluctuation of soil moisture triggered by seasonal changes, floods and typhoons followed by dry weather events, is a commonly occurring phenomenon. Therefore, studies on the impact of drying and wetting cycles in the laboratory can provide information on the behavior of 137 Cs in soils of varying moisture contents. The objective of this study was to complete the previous study (i.e., [START_REF] Teramage | The impacts of 137 Cs input forms on its extractability in Fukushima forest soils[END_REF] by including and investigating the impact of drying and wetting cycles on the 137 Cs availability in soils.

Material and methods

The full description of the incubation experiment, including the incubation set up, experimental units and conditions, sampling schemes and sample analysis has been described elsewhere [START_REF] Teramage | The impacts of 137 Cs input forms on its extractability in Fukushima forest soils[END_REF]. In short, the contaminated litter (OL) and fragmented (OF) layers were carefully separated by hand while the underlying soils at a depth of 8 -20 cm representing uncontaminated mineral soils, were collected under both broadleaf (BL) and cedar (CED) forests in the contaminated Fukushima area in Japan. The type of both soils used was andosol with a siltyloam texture. Although micaceous minerals were not detected, the clay content was about 13±2% in BL and 6±2% in CED soils composed of quartz, anorthite, magnetite and kaolinite [START_REF] Coppin | Radiocaesium partitioning in Japanese cedar forests following the "early" phase of Fukushima fallout redistribution[END_REF]. The carbon content of the soil in the CED forest (8.2±0.3%) was more than twice that in the BL (3.8±0.2%). The pH of both soils was acidic (CED: 5.3±0.04, BL: 4.7±0.01). The carbon content in the added organic materials ranged from 35±4% (CED-OF) to 44±4% (CED-OL).

The respective mineral soils were contaminated by mixing either 137 Cs-contaminated water, BL-W and CED-W (initial contamination rate of ~ 6.0 ± 0.2 kBq g -1 for both BL and CED, carrier 133 Cs addition represent less than 1‰ of 133 Cs content in soil) or 137 Cs contaminated OL/OF materials. Based on the availability of the OL and OF materials and to keep the carbon content as close as to its content in the top soil layers at the field [START_REF] Coppin | Radiocaesium partitioning in Japanese cedar forests following the "early" phase of Fukushima fallout redistribution[END_REF], a ratio of about 7:1 (dry mass) mineral soils to 137 Cs contaminated OL or OF for BL and 2:1 (OL) and 1.3:1 (OF) for CED were thoroughly mixed (initial contamination rates of 2.4±0.2 Bq g -1 (BL-OL), 13.0±0.7 Bq g -1 (BL-OF), 12 ± 0.04 Bq g -1 (CED-OL) and 35 ± 0.03 Bq g -1 (CED-OF)). The six respective contaminated mixtures (BL-W, -OL, -OF and CED-W, -OL, -OF) were transferred into incubation bottles (130 to 200 g dry weight) with two replications and the experiment set in motion.

The moisture regime of the incubated samples over the entire observation period is illustrated in Figure 1. As indicated in Figure 1, after completing the constant moisture incubation experiment (~40% for BL and ~ 60 -80% for CED, defined by the field moisture contents at the time of sampling), the remaining samples were exposed to four complete drying-and-wetting cycles (i.e. approximately 40-day cycles) starting from 286 days after incubation. The moisture content fluctuated from very wet (BL: 134% and CED: 230%) to nearly dry (both conditions: ~2%).

Samples from the incubated bottles were taken to monitor different parameters after three complete dry and wetting cycles from all experimental units (i.e. after 377 days of incubation) and after the fourth complete cycle (i.e. after 440 days of incubation) only for the liquid contamination treatment for BL conditions (due to insufficient remaining samples in the BL-OL and BL-OF treatments).

At each sampling time, samples of about 3 g (from aqueous contamination) and 6 g dry mass (from solid contamination) for successive extraction and about 10 g dry mass (from each contamination type) were taken to monitor the 137 Cs activity, moisture and carbon contents.

The successive extraction technique was carried out in tubes containing desorption reagents to soil ratio of 10ml g -1 for BL-W and CED-W and 20ml g -1 for BL-OL, BL-OF, CED-OL and CED-OF treatments. The tubes were then shaken for 24h (water) or 4h (acetate reagent) and centrifuged to separate the supernatants. Thus, the same process was repeated four times, refilling the residue with a similar amount of each reagent, and fully dispersing the soil at the beginning of each stage.

The extracted 137 Cs concentrations at each four successive extractions were used to estimate the coefficient of distribution, 𝑘 𝑑 ′ (as the slope of the straight line) of the extractable fraction and the fixed 137 Cs fraction in solid materials (as the y-intercept) from the graph plotting the final concentration of 137 Cs in the solids against the final 137 Cs concentration in the solution obtained at each extraction stage. The detailed sampling and the analysis of the various parameters were the same as described by [START_REF] Teramage | The impacts of 137 Cs input forms on its extractability in Fukushima forest soils[END_REF]. ANOVA at the 0.05 significance level was used for statistical analysis and comparison whenever required.

Results and discussion

Dynamics of organic carbon degradation

The evolution of organic carbon content (Corg) in soils contaminated with water or organic amendments (with OL or OF) are provided in Figure 2. A single first order kinetic law can explain the Corg degradation over time, suggesting the same degradation rate throughout our experiments.

Compared to the soils contaminated from liquid input, for which the Corg degradation was very weak, the Corg degradation rate was higher in the soils contaminated by the organic materials particularly for the OL conditions. This is obviously due to the increase of the degradable plant material (DPM), which constitutes a labile carbon pool by the added organic materials [START_REF] Jenkinson | Calculating net primary production and annual input of organic matter to soil from the amount and radiocarbon content of soil organic matter[END_REF]. For each treatment, the magnitude of Corg degradation kinetics remained unaffected by the drying and wetting cycles at least during the observation period. Indeed, several studies have indicated that an optimum moisture, which is usually somewhere near field capacity (i.e. macropore space air-filled and micropore spaces water-filled), is required for the decomposer communities in the soil to promote during Corg degradation (e.g., [START_REF] Chen | Effects of temperature and moisture on carbon respired from decomposing woody roots[END_REF][START_REF] Davidson | Effects of soil content on soil respiration in forest and cattle pasture of eastern Amazonia[END_REF][START_REF] Gabriel | Examining moisture and temperature sensitivity of soil organic matter decomposition in a temperate coniferous forest soil[END_REF][START_REF] Ilstedt | Optimum soil water for soil respiration before and after amendment with glucose in humid tropical acrisols and a boreal mor layer[END_REF][START_REF] Moorhead | Climate and litter quality controls on decomposition: An analysis of modeling approaches[END_REF]. [START_REF] Linn | Effect of water-filled pore space on carbon dioxide and nitrous oxide production in tilled and nontilled soils[END_REF] have also indicated that the microbial respiration decreased both in very wet and dry soils due to limited O2 diffusion in soil pore spaces and reduction soluble organic carbon substrates in water films, respectively. However, Cortufo et al. (2010) have reported that the degradation rate of Arbutus unedo leaf litter was higher for wet treatments (Mean residence time-MRT: 2.99 years)

and lower for dry treatments (MRT: 4.26 years) compared to the control treatment (MRT: 3.23 years) in a laboratory incubated experiment. [START_REF] Borken | Reappraisal of drying and wetting effects on C and N mineralization and fluxes in soil[END_REF] also explained that the additional mineralization during the wetting of soil could be associated with several processes such as exposure of previously protected organic matter by wetting pulse, while frequent oscillating soil moisture diminishes the wetting pulse and hence the degradation. However, this was not the case in our observation, where the trends of Corg degradation were not affected by drying and wetting cycles. This is possibly either due to the presence of diverse microbe communities which become active at both extreme moisture conditions that compensated and maintained the Corg degradation, or our observation time was not sufficient for the effect to manifest. Another explanation could be that the choice of the initial soil moisture content close to the field capacity resulted in optimum

Corg degradation, and the oscillated degradation rate due to the drying and wetting cycle averaged out to keep this optimum value.

Impact of drying and wetting on the availability of 137 Cs and k'd

The evolution of 137 Cs in terms of water-extractable fraction (w-available) for the liquid contamination only, the acetate-extractable fraction (Ac-available) for both contaminations and the associated distribution coefficient (k'd) during the entire observation period, including the drying and wetting treatment is given in Table 1. Except for the results related to drying and wetting treatment, the rest of the observations have been discussed in detail elsewhere [START_REF] Teramage | The impacts of 137 Cs input forms on its extractability in Fukushima forest soils[END_REF]. The information is provided here for further comparison and discussion of the results related to the impact of drying and wetting cycle treatments.

Soils contaminated by liquid 137 Cs input

For this contamination treatment, the k'd values for both water-and acetate-extractable 137 Cs fractions in the BL and CED soils remained nearly the same with time except for a relatively high value at the initial stage for the water-extractable fractions (Table 1). This was possibly due to pseudo or temporary retention of 137 Cs on to the soil particles at zero contact time. The k'd values were also unaffected by the drying and wetting cycle treatments, indicating the dynamics of 137 Cs distribution between the solution and the solid phase is similar regardless of the time and treatments. The water-extractable 137 Cs fraction for both BL and CED was very small with a general decreasing trend, including values obtained after the drying and wetting cycle treatments (Table 1). This fraction decreased from 1.00±0.03 % to 0.29±0.03 % for CED and from 0.72±0.03 % to 0.29±0.01 % for BL at the beginning and at the end of the observation period, respectively.

By taking the whole observation data as illustrated in Figure 3, the rate constant of the waterextractable 137 Cs fraction was calculated as 4.4 x 10 -3 d -1 for CED and 1.7 x 10 -3 d -1 for BL.

As expected, the acetate-extractable 137 Cs fraction was higher than the water-extractable fraction and decreased with time. The trend was similar for the two different periods, constant moisture or drying and wetting cycles, with a decrease from 55% to 30% for BL and 56% to 37% for CED.

This implies that drying and wetting cycles weakly affects the 137 Cs availability and its progressive ageing in time. In good agreement, [START_REF] Rosen | Effect of clay content and wetting-anddrying on radiocesium behavior in a peat and a peaty podozol[END_REF] reported that the acetate-extractable 137 Cs fraction across eight drying and wetting cycle treatments applied to contaminated peat and peaty podzol soils in the laboratory experiment was nearly the same across the cycles, confirming no clear effect of the drying and wetting cycles. Though not a direct desorption study, [START_REF] Tanaka | Influence of drying period on the migration behavior of radionuclides in aerated soil layer[END_REF] shown that drying had no influence on 137 Cs and 60 Co concentrations in the top layers of sandy soil in a laboratory experiment. Taking the data, including the drying and wetting treatments, the calculated rate constant values for acetate-extractable fraction were relatively lower (BL: 8.2 x 10 -4 ; CED: 9.4 x 10 -4 d -1 ) than that of their water-extractable counterparts (Figure 3).

Statistically, both the water-and acetate-extractable 137 Cs fractions were not significantly different between BL and CED for their respective treatment types (Single factor ANOVA, P = 0.918936 for water-extractable BL vs. CED; P = 0.852618 for acetate-extractable of BL vs. CED), which indicates that water-derived 137 Cs contamination pattern is not influenced by the small differences existing between these soils, mainly related to the OM content. Furthermore, the general decreasing pattern observed for the water-and acetate-extractable fraction in both forest soils indicates that the stabilization of water-and acetate-extractable fractions are not responsive to the three and four drying and wetting cycle treatments. Saki et al. (2015) obtained a measurable leached 137 Cs amount from contaminated litter soaked alone in water than that soaked in water with minerals. However, in our observation it is clear that the water-extractable 137 Cs fraction for this contamination treatment was below a measurable amount, even after drying and wetting cycles. Both low 137 Cs in the organic contaminated material or desorption and resorption of 137 Cs by mineral particles in the way beyond the water desorption capacity is highly probable, which would result in no water-extractable 137 Cs [START_REF] Teramage | The impacts of 137 Cs input forms on its extractability in Fukushima forest soils[END_REF], and hence it is not discussed here.

Soils contaminated by solid 137 Cs inputs

The k'd values of the acetate-extractable 137 Cs fraction remained nearly the same across the observation time, except for low values observed at time zero (Table 1). The lower k'd values at the initial observations may indicate the tendency of more acetate-extractable 137 Cs fraction present in the solution phase from less tightly bound fractions on the surface of the organic materials. According to the observation, there were no definite effects of drying and wetting cycle treatments on the k'd values.

Compared to the acetate-extractable 137 Cs fraction from the soils contaminated with liquid 137 Cs input, the acetate-extractable 137 Cs fraction from the soils contaminated with solid (OL and OF) 137 Cs inputs was generally very low and practically unchanged with time, implying the nature of contamination affects the 137 Cs availability and the absence of further 137 Cs ageing. Further, the stability of the acetate-extractable 137 Cs fraction from the soils contaminated with solid is in line with the absence of impact of drying and wetting treatment on the carbon degradation as the same observed in the constant moisture treatments. The acetate-extractable 137 Cs fraction was larger in BL-OL (11 -17%) and BL-OF (4 -7%) than in their CED counterparts (2 -5%). Statistically, the acetate-extractable 137 Cs fraction in BL-OL treatment was significantly different from the BL-OF (P = 0.00017), CED-OL (P=0.0000039) and CED-OF (P=0.0000036) treatments, highlighting that the acetate-extractable 137 Cs fraction from the contaminated litter tends to depend on the litter types and its degradation status. In connection to this, several studies from laboratory and field conditions have revealed that the leaching and migration of 137 Cs in deciduous forest floors are greater than from coniferous floors (e.g., [START_REF] Hashida | Disparate radiocesium leaching from two woody species by acceleration of litter decomposition using microbial incubation[END_REF][START_REF] Koarashi | Forest type effects on the retention of radiocesium in organic layers of forest ecosystems affected by the Fukushima nuclear accident[END_REF][START_REF] Kurihara | Radiocesium migration in the litter layer of different forest types in Fukushima, Japan[END_REF][START_REF] Nakanishi | 137 Cs vertical migration in a deciduous forest soil following the Fukushima Dai-ichi Nuclear Power Plant accident[END_REF]. For example, [START_REF] Koarashi | Forest type effects on the retention of radiocesium in organic layers of forest ecosystems affected by the Fukushima nuclear accident[END_REF] reported that approximately 80% of the deposited 137 Cs in deciduous broad-leaved forest migrated to minerals located below the organic layer, whereas more than half of the deposited 137 Cs remained in the organic layer of the coniferous forest. [START_REF] Kurihara | Radiocesium migration in the litter layer of different forest types in Fukushima, Japan[END_REF] also observed faster 137 Cs output flux from decomposing broadleaf forest litter than from the cedar forest litter. These collectively confirm that the litter type does affect the 137 Cs dynamics in soil. [START_REF] Rigol | Sequential extractions for the study of radiocesium and radiostrontium dynamics in mineral and organic soils from western Europe and Chernobyl area[END_REF] reported a 2-3-fold decrease of exchangeable 137 Cs fraction after drying and wetting cycles for 9 months in podsols and peaty-podsols. Moreover, [START_REF] Park | Enhanced irreversible fixation of cesium by wetting and drying cycles in soil[END_REF] have reported that repeated drying and wetting cycles changed expandable clay minerals (e.g. vermiculite) to non-expandable structure (e.g. illite like minerals) which drastically reduced the available Cs fractions. However, in our observation (where the clay minerals dominated by kaolinite), the impact of drying and wetting cycles had statistically no effect on the acetateextractable 137 Cs fraction, which can be associated with the nature of the clay minerals. In fact, an increase of approximately 28% of the acetate-extractable fraction in the BL-OL condition was observed after three complete cycles (17% at t-377) as compared to the immediate value before the treatment (12.2% at t-205) (Table 1). However, as the shape of the curve didn't change from the trend of the regular moisture treatment, it was hard to confirm that the observed result was due to the effect of the drying and wetting treatment, and not to the incubation time.

Conclusion

If taking the entire observation period into account, including the incubation at constant moisture and the incubation during drying and wetting cycles, 137 Cs ageing was observed in soils contaminated with liquid 137 Cs input whereas this was not the case for soils contaminated with solid 137 Cs inputs. Nevertheless, it is possible to conclude that the 137 Cs availability in soil likely depends on the nature of contamination, the litter type and a certain extent on its degradation status.

Given the limited data, observation period and the need for further investigation, the influence of drying and wetting cycles on organic matter degradation and on 137 Cs availability was not really clear.

Figure 1 .Figure 2 .Figure 3 .

 123 Figure 1. The moisture regime of the soil during incubation period for liquid contamination (W), added contaminated litter (OL) and added contaminated fragmented litter material (OF).

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	Domaine : Secteur 6 -Recherche en Radioprotection
	Axe-programme : AP 003/10 -Risques Environnementaux
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	Intitulé de la revue ou ouvrage : Chemosphere
	Abrégé ISSN : Chemosphere
	Editeur : Elsevier
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	N°ISSN : 0045-6535
	Date de publication : 05/09/2019
	Volume : 239
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	Pagination : 124777
	Identifiant DOI : 10.1016/j.chemosphere.2019.124777

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Table 1 .

 1 The evolution of the extractable fraction of 137 Cs and its k'd with respect to the incubation time. The shaded cells represent the values obtained after the third (at t-377) and fourth (at t-440) complete drying and wetting cycles. (n = 2)

			Extracted by water	Extracted by acetate
			137 Cs liquid contamination	137 Cs liquid contamination	137 Cs solid contamination
			W		W		OL		OF
			k'd		k'd		k'd		k'd
	Forest	Time	(L kg -1 )	Extractable	(L kg -1 )	Extractable	(L kg -1 )	Extractable	(L kg -1 )	Extractable
	type	(d)		%		%		%		%

Acknowledgments

We are grateful for the comments and review insights provided by Dr. Martin Garin Arnaud and Dr. Fevrier Laureline. We extend our thanks to Mr. Orjollet Daniel for his support during the sample activity measurements. This work was supported by the ANR (French National Research