L G A N D O I S A

Modelling trace metal extractability and solubility in French forest soils by using soil properties

Soil/solution partitioning of trace metals (TM: Cd, Co, Cr, Cu, Ni, Sb, Pb and Zn) has been investigated in six French forest sites that have been subjected to TM atmospheric inputs. Soil profiles have been sampled and analysed for major soil properties, and CaCl 2 -extractable and total metal content. Metal concentrations (expressed on a molar basis) in soil (total), in CaCl 2 extracts and soil solution collected monthly from fresh soil by centrifugation, were in the order:

respectively. Metal extractability and solubility were predicted by using soil properties. Soil pH was the most significant property in predicting metal partitioning, but TM behaviour differed between acid and non-acid soils. TM extractability was predicted significantly by soil pH for pH < 6, and by soil pH and Fe content for all soil conditions. Total metal concentration in soil solution was predicted well by soil pH and organic carbon content for Cd, Co, Cr, Ni and Zn, by Fe content for Cu, Cr, Ni, Pb and Sb and total soil metal content for Cu, Cr, Ni, Pb and Sb, with a better prediction for acidic conditions (pH < 6). At more alkaline pH conditions, solute concentrations of Cu, Cr, Sb and Pb were larger than predicted by the pH relationship, as a consequence of association with Fe colloids and complexing with dissolved organic carbon. Metal speciation in soil solutions determined by WHAM-VI indicated that free metal ion (FMI) concentration was significantly related to soil pH for all pH conditions. The FMI concentrations of Cu and Zn were well predicted by pH alone, Pb by pH and Fe content and Cd, Co and Ni by soil pH and organic carbon content. Differences between soluble total metal and FMI concentrations were particularly large for pH < 6. This should be taken into account for risk and critical load assessment in the case of terrestrial ecosystems.

Modélisation de l'extractabilité et de la solubilité des métaux traces dans les sols forestiers franc ¸ais en utilisant les propriétés du sol.

Résumé

La partition entre phases solide et solution des Eléments Traces Métalliques (ETM : Cd, Co, Cr, Cu, Ni, Sb, Pb and Zn) a été étudiée dans six sols forestiers franc ¸ais impactés par des apports atmosphériques. Les profils de sol ont été échantillonnés et les principaux paramètres des sols, ainsi que les concentrations en ETM totales et extractibles au CaCl 2 ont été déterminés. Les concentrations des éléments (en moles) totales, extractibles et dissoutes dans les solutions de sol extraites par centrifugation se classent respectivement ainsi :

L'extractabilité et la solubilité des ETM dans les sols ont été prédites à partir des paramètres du sol. Le pH du sol est le paramètre le plus significatif pour la prédiction de la partition des ETM. Cependant, il a été mis en évidence une dynamique différente pour des pH inférieurs et supérieurs à 6. L'extractabilité des ETM peut être correctement prédite par le pH du sol en conditions acides et par le pH du sol et la teneur en Fer pour toute la gamme de pH considérée. Les concentrations totales dans la solution de sol sont prédites de manière significative par le pH du sol, couplé à la teneur en carbone organique (pour Cd, Co, Cr, Ni et Zn), en fer (pour Cu, Cr, Ni, Pb et Sb), et à la teneur totale en métal (Cu, Cr, Ni, Pb, Sb). La prédiction est améliorée pour les valeurs de pH les plus basses (pH < 6). En effet, pour les valeurs de pH les plus élevées, la complexation des ETM par le carbone organique dissous et l'association avec des colloïdes inorganiques (hydroxydes de fer) entraîne une augmentation des concentrations totales en solution. La spéciation des ETM en solution a été déterminée grâce au logiciel WHAM-VI. Les concentrations en ion libre en solution sont significativement corrélées au pH du sol. Ainsi, les concentrations en ion libres de la plupart des métaux peuvent être efficacement prédites à partir du pH du sol du pH et de la teneur en Fer pour Pb, et du pH et de la teneur en CO dans le cas de Cd, Co et Ni. Les différences de concentrations observées entre le métal total et l'ion libre sont plus importantes pour les pH élevés. Ceci devrait être pris en compte dans le cadre de l'évaluation des risques et des charges critiques pour les écosystèmes terrestres.

Introduction

Natural processes as well as anthropogenic activities lead to trace metal (TM) (cadmium, Cd; cobalt, Co; chromium, Cr; copper, Cu; nickel, Ni; antimony, Sb; lead, Pb; and zinc, Zn) dispersion in the atmosphere [START_REF] Nriagu | Quantitative assessment of worldwide contamination of air, water and soils by trace metals[END_REF][START_REF] Nriagu | A global assessment of natural sources of atmospheric trace metals[END_REF][START_REF] Pacyna | Current and future emissions of selected heavy metals to the atmosphere from anthropogenic sources in Europe[END_REF]. TMs can reach remote areas [START_REF] Steinnes | Metal contamination of surface soil from long-range atmospheric transport. Existing and missing knowledge[END_REF]. Surface soils in the northern hemisphere undergo, to a large extent, atmospheric contamination by TMs [START_REF] Steinnes | Metal contamination of surface soil from long-range atmospheric transport. Existing and missing knowledge[END_REF] and French forest soils were shown to be contaminated by atmospheric inputs of some TMs by [START_REF] Hernandez | Heavy metal distribution in some French forest soils: evidence for atmospheric contamination[END_REF]. Forest ecosystems are particularly sensitive to atmospheric contamination because the canopy interacts with atmospheric metallic pollutants and may concentrate them [START_REF] Lovett | Dry deposition and canopy exchange in a mixed oak forest as determined by analysis of throughfall[END_REF]. Therefore, metal behaviour and potential impact of TM in forest soils has to be evaluated.

TM availability and potential toxicity to sensitive organisms are governed by their concentration in soil solution, because soil solution is a vector of available metal to plant, microorganism and soil invertebrates. Soil extraction by dilute salts allows the evaluation of the exchangeable and easily mobilized fraction of TM [START_REF] Aten | On heavy metals in soil; rationalization of extractions by dilute salt solutions, comparison of the extracted concentrations with uptake by ryegrass and lettuce, and the possible influence of pyrophosphate on plant uptake[END_REF] in soil samples. Total metal concentration in soil solution is used to assess metal leaching from soils to surface water. TM speciation plays an important role in metal availability and toxicity, since free metal ion content (FMI) is often recognized as the most toxic form [START_REF] Florence | The speciation of trace metals in waters[END_REF]. Critical loads assessment is based on the determination of a critical limit, which is the maximum tolerable FMI concentration in soil solution for soil organisms [START_REF] Lofts | Deriving soil critical limits for Cu, Zn, Cd, and Pb: a method based on free ion concentrations[END_REF][START_REF] De Vries | Impact of soils properties on critical concentrations of cadmium, lead, copper, zinc and mercury in soil and soil solution in view of ecotoxicological effects[END_REF]. Protection of terrestrial ecosystems from atmospheric pollution at the European scale is based on the calculation of critical loads (defined as 'a quantitative estimate of an exposure to one or more pollutants below which important harmful effects on specified sensitive elements of the environment do not occur according to present knowledge': Nilsson & Grennfelt, 1988). In order to protect forest ecosystems, knowledge on TM availability and concentration in soil solution is thus needed for a large variety of ecosystems.

Total metal content and some other soil properties are frequently measured and available in data bases for numerous terrestrial ecosystems at national scales. However, data on TM concentration in soil solution are scarce on a large scale. Soil solution compositions are thus predicted with transfer functions [START_REF] De Vries | Impact of soils properties on critical concentrations of cadmium, lead, copper, zinc and mercury in soil and soil solution in view of ecotoxicological effects[END_REF], linking soil solution and soil properties to TM concentration in soil solutions. Numerous studies discuss TM (mostly Pb, Cd, Cu, Zn and more rarely Ni) partitioning in soils between solid and dissolved phases [START_REF] Jopony | The solid-solution equilibria of lead and cadmium in polluted soils[END_REF][START_REF] Mcbride | Solubility control of Cu, Zn, Cd, and Pb in contaminated soils[END_REF][START_REF] Römkens | Cd, Cu and Zn solubility in arable and forest soils: consequences of land use changes for metal mobility and risk assessment[END_REF][START_REF] Sauvé | Solid-solution partitioning of metals in contaminated soils: dependence on pH, total metal burden, and organic matter[END_REF][START_REF] Krisnamurti | Solid-solution speciation and phytoavailability of copper and zinc in soils[END_REF][START_REF] Tipping | The solid-solution partitioning of heavy metals (Cu, Zn, Cd, Pb) in upland soil of England and Wales[END_REF], but mostly for contaminated soils, whereas only a few [START_REF] Sauvé | Solid-solution partitioning of Cd, Cu, Ni, Pb and Zn in the organic horizons of a forest soil[END_REF][START_REF] Watmough | Metal partitioning and uptake in central Ontario forest[END_REF] examine weakly contaminated forest soils. Consequently, forest soils are under-represented in data sets [START_REF] Pampura | Validation of transfer functions predicting Cd and Pb free metal ion activity in soil solution as a function of soil characteristics and reactive metal content[END_REF]. These studies show that soil (or soil solution) pH is always an important property for predicting TM partitioning. Other soil properties, such as organic carbon content or soil metal content were also found to be important. The difficulty of taking into account the influence of soil solution composition, particularly dissolved organic carbon (DOC), on TM partitioning has been highlighted [START_REF] Tipping | The solid-solution partitioning of heavy metals (Cu, Zn, Cd, Pb) in upland soil of England and Wales[END_REF][START_REF] Sauvé | Solid-solution partitioning of Cd, Cu, Ni, Pb and Zn in the organic horizons of a forest soil[END_REF].

Most of the existing studies are based on estimates of TMs in soil solution (water or dilute salt extraction). An accurate assessment of the properties that significantly predict TM extractability and solubility in forest ecosystems is needed. For that purpose, we investigated several French forest soils, which were very diverse in terms of vegetation cover and soil types. This study is aimed at (i) determining concentrations for eight TMs (Cd, Co, Cu, Cr, Ni, Sb, Pb and Zn) in different soil pools (total, extractable and soil solution) for French forest soils, covering a wide range of soil properties, (ii) evaluating TM extractability and availability by investigating the partition between soil phases and solution speciation, (iii) defining transfer functions between solid phase and soil solution based on appropriate soil properties, to predict soil solution composition for critical load modelling, and (iv) evaluating the processes that control TM solubility and speciation.

Materials and methods

Study sites

Six sites were selected for the study which belonged to the French network RENECOFOR (Réseau National de suivi à long terme des Ecosystèmes Forestiers, i.e. National Network for the longterm Monitoring of Forest Ecosystem; [START_REF] Ulrich | Le réseau RENECOFOR: objectifs et réalisation[END_REF], managed by the ONF (Office National de Forêts: the National Forest Board). These sites are integrated into the ICP (International Co-operative Programme) forest European network (Level II sites) and are located all over France (Figure 1).

The selected sites are representative of various chemical and physical properties of French forest soils. Forests of three main tree species were considered (spruce, Picea abies L.; beech, Fagus sylvatica L. Karst; and silver fir, Albies Alba Mill.) and four main types of soils (podzol, andosol, cambisol and luvisol (IUSS, 2006)), covering a wide range of pH values (3.9-8.5), organic carbon (9-126 g kg -1 ) and iron (0.4-8%) contents. The main characteristics of the study sites are summarized in Table 1.

Sampling procedure and soil pre-treatment

All soil layers were sampled in two pits from each forest site. Twelve profiles, with a total of 58 samples, were thus collected down to the deepest soil layer. Soil pits (described by [START_REF] Brêthes | RENECOFOR -Caractéristiques Pédologiques des 102 Peuplements du Réseau[END_REF] were cleaned with clean plastic tools before sampling. In the laboratory, soil samples were dried at

Atlantic Ocean

Mediterranean Sea

France

Figure 1 Location of the study sites.

room temperature, sieved with a 2-mm nylon mesh and kept in polypropylene bags before analysis of total metal content, extractable pool and main soil properties. In addition, fresh soils were sampled monthly over 1 year (August 2007to July 2008) in order to take into account spatial and temporal variability of soil solution chemical composition. For that purpose, soil samples (n = 310) were collected after removal of the humus layer, using an auger in four locations around the pits previously sampled. Potential contamination of soil solution by using an auger was checked and was found to be negligible. Soil samples were collected according to soil layers down to 60 cm. Finally, a composite sample was made by pooling the four sub-samples corresponding to each soil layer. The samples were kept in plastic bags, brought back to the laboratory and treated within 48 hours.

Sample treatment

Dilute salt extraction. TM availability in soils was investigated by using 0.01 m CaCl 2 extraction [START_REF] Houba | State of the art and future developments in soil analysis for bioavailability assessment[END_REF][START_REF] Menzies | Evaluation of extractants for estimation of the phytoavailable trace metals in soils[END_REF] according to the protocol described by [START_REF] Houba | Soil analysis procedures using 0.01 M calcium chloride as extraction reagent[END_REF]. Three grams of dried soil (<2 mm fraction) were shaken for 2 hours with 30 ml of 0.01 m CaCl 2 in polypropylene tubes (previously rinsed with 0.1 m HCl and extra pure water). Each tube was centrifuged for 20 minutes at 3000 g. The supernatant was removed. An aliquot was used for pH measurement. The supernatant was then filtered (SFAC filters, 0.2 μm) and acidified (2% ultrapure HNO 3 ) for analytical work. This procedure was applied to all samples (n = 58).

Soil solution extraction.

Fresh soil samples collected each month from each soil layer were centrifuged by using Teflon double bottomed centrifugation devices (adapted from [START_REF] Giesler | Soil solution chemistry -effects of bulking soil samples[END_REF] on a Beckman Coulter Aventi J26 XPI centrifuge (Beckman Coulter France S.A.S., Paris, France), equipped with four 1-litre swinging buckets. Centrifuge speed was 3400 g, in order to extract soil solution at a pF around 4 [START_REF] Ranger | Forest soil dynamics during stand development assessed by lysimeter and centrifuge solutions[END_REF]. Each month, 38 fresh samples were centrifuged. Depending on the sample moisture content, soil solution extraction was not always possible (for approximately 20% of the samples): a total of 310 soil solution samples were extracted.

Analytical procedure Soil analysis.

Soil solution speciation

Soil solution speciation was estimated with WHAM-VI software [START_REF] Tipping | Humic ion-binding model VI: an improved description of the interactions of protons and metal ions with humic substances[END_REF]. According to previous studies [START_REF] Tipping | The solid-solution partitioning of heavy metals (Cu, Zn, Cd, Pb) in upland soil of England and Wales[END_REF][START_REF] Pampura | Validation of transfer functions predicting Cd and Pb free metal ion activity in soil solution as a function of soil characteristics and reactive metal content[END_REF][START_REF] Buekers | Modelling the effects of ageing on Cd, Zn, Ni and Cu solubility in soils using an assemblage model[END_REF], dissolved organic matter was assumed to contain 50% carbon comprising 65% of active fulvic acids (FA). Equilibrium with atmospheric CO 2 was assumed. Input data included soil solution pH, FA content, and major cation and anion (Ca 2+ , K + , Na + , Mg 2+ , Cl -, NO - 3 , SO 4 2 -) and trace element (Cd, Co, Cu, Ni, Pb and Zn) concentrations. Total concentrations of Fe were measured but Fe 3+ activity was assessed by considering equilibrium with Fe hydroxides. A value of K so = 2.7 was selected [START_REF] Tipping | The solid-solution partitioning of heavy metals (Cu, Zn, Cd, Pb) in upland soil of England and Wales[END_REF]. Fe 3+ values predicted by WHAM-VI using total Fe inputs were always larger than theoretical Fe 3+ in equilibrium with hydroxides, indicating that soil solution was over-saturated with regard to Fe hydroxides. The predicted Fe 3+ activities, derived from hydroxides equilibrium, were used as input data. For Al, the expected values of Al 3+ in equilibrium with Al hydroxides were calculated with a value for K so of 8.5 [START_REF] Tipping | The solid-solution partitioning of heavy metals (Cu, Zn, Cd, Pb) in upland soil of England and Wales[END_REF]. The measured dissolved Al concentrations were usually below the calculated values in equilibrium with Al hydroxides, indicating that Al solubility in soil solution was not controlled by Al hydroxides. Consequently, total Al measured concentrations were used as input data. Sometimes, predicted Al 3+ values with WHAM-VI exceeded Al 3+ calculated values (for alkaline pH solutions). In that case, calculated Al 3+ activities based on hydroxides equilibrium were used as input data. Because of the small amount of extracted water, the data set was not complete and only 224 samples could be used for speciation analysis.

Statistical analysis

All statistical analyses were performed with STATISTICA software (Statsoft, Tulsa, OK, USA). Because of the sampling procedure, temporal trends for TM concentrations could not reasonably be investigated. Sampling was considered as independent repetition. For each layer considered, a mean value (calculated from n = 4 to n = 12, depending on the success of soil solution extraction) was calculated from samples taken at different periods. The typical relative standard deviation for a single layer was 15-70%.

Results

Total metal content in soils

Trace metal concentrations were very variable ( 1). Lead, Sb and Cd had larger concentrations in organic layers, which might indicate the influence of atmospheric inputs in some soils [START_REF] Hernandez | Heavy metal distribution in some French forest soils: evidence for atmospheric contamination[END_REF]. The enrichment factor (EF), calculated by using cesium [START_REF] Roussiez | Background levels of heavy metals in surficial sediments of the Gulf of Lions (NW Mediterranean): an approach based on 133 Cs normalization and lead isotope measurements[END_REF] and deeper soil layers as references, exceeded 2.0 in upper soil layers of sites EPC 08 (Cd, Co, Pb, Sb and Zn), EPC 63 (Cd, Pb and Sb), EPC 87 (Sb) and SP 57 (Pb). EPC 08, located in the northern part of France, is the site most affected by anthropogenic inputs and with larger total metal content and EF, as already reported by [START_REF] Hernandez | Heavy metal distribution in some French forest soils: evidence for atmospheric contamination[END_REF].

Correlations between soil properties and total metal contents in soils were performed to determine the main solid phases containing TMs (Table 2). Some TMs are more likely to be associated with soil fine fractions (clays and silts) and Cr and Ni were positively correlated with clay and silt contents and negatively with sand. TM affinity for fine fractions is related to their larger sorption surfaces [START_REF] Hernandez | Heavy metal distribution in some French forest soils: evidence for atmospheric contamination[END_REF]. Cobalt, Cr, Zn and especially Ni total concentrations were positively correlated with soil pH, and negatively with Sb. These patterns are probably linked to processes such as weathering or leaching of soil minerals [START_REF] Aubert | Distribution and origin of major and trace elements (particularly REE, U and Th) into labile and residual phases in an acid soil profile (Vosges Mountains, France)[END_REF][START_REF] Watmough | Metal partitioning and uptake in central Ontario forest[END_REF]. Lead, Sb and Cd were positively correlated with OC content. Lead and Cd can be absorbed by soil OC [START_REF] Basta | Path analysis of heavy metal adsorption by soil[END_REF][START_REF] Mcbride | Solubility control of Cu, Zn, Cd, and Pb in contaminated soils[END_REF] and they were enriched in surface layers, which had the largest OC content. Moreover, Pb and Sb had a specific behaviour because they were strongly correlated with each other but not with Fe, in contrast to Cr, Cu, Ni, Zn and Co, which were strongly correlated with each other and with Fe (and to a lesser extent with Al and Mn). This is consistent with the wellknown association of this group of TMs with secondary minerals or hydroxides, according to soil type [START_REF] Hernandez | Heavy metal distribution in some French forest soils: evidence for atmospheric contamination[END_REF].

Extractable trace metals in soils

Extractable TM concentrations. The concentrations of extract- able TM in studied soils are given in Table 3. On the basis of extracted amounts (nmol kg -1 ), extractable TM can be classified as follows: Zn > Cu > Pb = Ni > Co > Cd > Cr. Extractable Sb concentrations were below detection limits and thus not considered in this part of the study. Extractable amounts With: clay, silt and sand in %, OC = organic carbon (g kg -1 ), total metal content in μg g -1 .

varied over several orders of magnitude, as indicated by the range (minimum-maximum values). The mean percentage of extractable metal depends also on the element (Table 3a) and ranged from 0.015 to 25.5% for Cr and Cd, respectively. When expressed on a percentage basis for extractable TMs in soils, TM can be ordered as follows:

Cd > Cu > Zn > Co > Ni > Pb > Cr.
Modelling metal extractability using soil properties. The partition coefficient between the extractable (M CaCl2 ) and total trace metal (M tot ) pool in soils is a useful tool to evaluate TM behaviour, particularly for case studies with small TM contents, where sorption sites are unlikely to be saturated. Extractability of TMs can be defined as follows [START_REF] Rieuwerts | The influence of soil characteristics on the extractability of Cd, Pb, and Zn in upland and moorland soils[END_REF]:

Extractability = Log(M CaCl2 /M tot ), (1)
where M CaCl2 and M tot are expressed in μmol g soil -1 . The influence of soil properties on TM extractability was investigated through correlation analysis, using log (TM extractability) as a Trace metals in soil solution: prediction and speciation TM concentrations in soil solution. Trace metal concentra- tions in soil solutions vary over several orders of magnitude (Table 5). Important variations were observed within soil profiles and between the sites. Zinc had the largest concentration in soil solution, with the smallest deviation. As a whole, TM concentrations can be ordered as follows:

Zn > Ni > Cu = Pb > Co > Cr > Cd > Sb.
Correlations were made between TM concentrations in soil solutions and soil solution properties (Table 6). Zinc, Cd and Mn concentrations were significantly correlated with each other and with Al concentration. These elements are negatively correlated with pH and positively with Ca concentration. Another group of elements composed of Pb, Sb, Cu and Cr were significantly and positively correlated with Fe and DOC concentrations in soil solution.

Relationships between TM in soil solution and soil properties

Influence of pH. Inverse relationships between TM concentration in soil solution and soil pH were seen for all elements (Figure 3). For pH values <6, a significant negative linear relationship was observed. For pH values above 6 (open symbols, Figure 3), there was no relationship and concentrations were greater than expected from the relationship established for values <6. The difference between measured and expected concentrations depends on the element considered. This difference is in the order:

Pb > Cr > Sb > Cu > Cd > Co > Ni > Zn.
Ni dependence on pH for acid conditions is very weak.

Multiple regression between TM concentration in soil solution and soil properties.

The influence of soil properties on TM con- centrations in soil solution was assessed by correlation and multiregression analysis. As for extractable TMs, we used logarithms of the values for concentrations. Correlation analysis indicated that soil pH, OC content, soil Fe content and, for some elements, total metal content were significantly correlated with TMs in soil solution. Multiple regressions were performed (Table 7) between log (TM concentration in soil solution) and soil pH, OC content, Fe content and total metal content. Regression analysis was made with all samples and also with a subset of samples with pH < 6. Equation ( 2) is derived from the competitive sorption model [START_REF] Mcbride | Solubility control of Cu, Zn, Cd, and Pb in contaminated soils[END_REF][START_REF] Sauvé | Solid-solution partitioning of metals in contaminated soils: dependence on pH, total metal burden, and organic matter[END_REF], which states that metallic cations compete with protons for sorption on soil exchange sites (here OC and Fe). The relationship for TMs in all soil solutions was:

log(TM) sol. = a + b pH + c log OC + d log Fe + e log(M) tot , (2)
where (TM) sol is the concentration of the element in soil solution (mol l -1 ), OC is soil organic carbon content (in g kg -1 ), Fe is soil iron content (%) and (M) tot is soil total metal content (in mol kg -1 ). For Cu, Cr, Sb and Pb, DOC was a significant variable and was then included in the regression and the equation became: (3) Non-significant variables (P > 0.05) were excluded from the regressions. Prediction of TM concentration in soil solution was improved when soils of pH < 6 only were considered (Table 7). Organic C content was negatively correlated with Cd, Zn and Co concentrations in soil solutions and positively correlated with Cr and Ni concentrations. Soil Fe content was negatively correlated with Cu, Cr, Pb Ni and Sb. Total metal content was significant in predicting Cu, Cr, Ni, Pb and Sb concentrations in soil solution. As a whole, multiple regressions using only soil properties were able to predict total metal concentrations in soil solutions. Including DOC concentrations (positive correlation) slightly improved Cu, Cr, Sb and Pb total concentration prediction over the whole pH range compared with using soil properties only.

TM speciation in soil solution

The concentration of free metal ions (FMI) in soil solution has been suggested to be a useful measure of toxicity to organisms [START_REF] Florence | The speciation of trace metals in waters[END_REF][START_REF] Lofts | Deriving soil critical limits for Cu, Zn, Cd, and Pb: a method based on free ion concentrations[END_REF]. WHAM-VI software was used to predict FMI concentration in soil solution for six of the studied elements: Cd, Cu, Co, Ni, Pb and Zn. As done previously for total metal concentration, correlations were performed on logtransformed data of all sets of samples in order to determine which soil properties predicted effectively FMI concentration in soil solution. Multiple regressions indicated (Table 8) that pH was significant (negative correlation) for each TM. Organic C content also made a significant contribution to the prediction of Cd, Co and Ni FMI concentrations.

Contrary to what was observed for total concentration in soil solution, the relationships between free TM concentration and pH were significant for the whole range of soil pH, except for Co and Ni (Figure 4). For more alkaline pH values, these two elements had larger FMI concentrations than predicted by the linear relationship with pH. This might indicate that their complexation with DOC at alkaline pH is underestimated by WHAM-VI, as already noted for Ni by [START_REF] Van Laer | Speciation of nickel in surface waters measured with the Donnan membrane technique[END_REF].

Prediction of FMI concentration in soil solution can thus be efficiently achieved by using simple soil properties, such as pH and OC content, depending on the metal. The percentage of variance explained ranged from 57% (Co) to 87% (Cu). As previously observed for total metal concentration in soil solutions, pH, OC (Cd, Co, Ni) and soil Fe content (Pb) alone can efficiently predict FMI concentration in soil solutions. For these weakly contaminated forest soils, total soil metal content is thus a less important characteristic.

Discussion

Extractable vs soluble pool of metal in soils

Both extractable and soluble TM pools have been investigated in weakly contaminated French forest soils. The order of concentration of TMs in CaCl 2 -extracted solution and soil solution was almost similar. Zinc was the most concentrated element in all pools. Cd was the most extractable element, in line with its labile characteristics [START_REF] Maiz | Evaluation of heavy metal availability in contaminated soils by a short sequential extraction procedure[END_REF][START_REF] Hernandez | Heavy metal distribution in some French forest soils: evidence for atmospheric contamination[END_REF] and probable enhanced extraction because of complex formation with chloride [START_REF] Lebourg | Trace metal speciation in three unbuffered salt solutions used to assess their bioavailability in soil[END_REF][START_REF] Degryse | Soil solution concentration of Cd and Zn can be predicted with a CaCl 2 soil extract[END_REF]. Metals in both extractable fraction and soil solution were significantly negatively correlated with soil pH (except extractable Cr and Cu), for pH values < 6. Extracted TMs correspond to extractable and potentially available pools [START_REF] Houba | State of the art and future developments in soil analysis for bioavailability assessment[END_REF][START_REF] Menzies | Evaluation of extractants for estimation of the phytoavailable trace metals in soils[END_REF]. However, CaCl 2 extracts and soil solution concentrations were very different. Chromium concentrations were larger by a factor of two to three in the soil solution compared with the extract. For all the other elements, concentrations in CaCl 2 extracts were greater (by 2-fold for Ni, 5-fold for Zn, Cd and Pb, and 20fold for Co and Cu). Exchange with Ca, as well as complexation with Cl -(especially for Cd) may have induced a greater release of TM in the CaCl 2 extracts. No significant relationship was observed between the two sets of concentrations. The exception was for Cr (r = 0.67, P < 0.001) and Pb (r = 0.61, P < 0.001), but with a very large distribution of the data. Moreover, for more alkaline soil pH, the results showed that metal extractability did not increase, contrary to concentration trends of some elements (especially Cu, Cr, Pb and Sb) in soil solutions.

It has been previously suggested that differences in soil solution and CaCl 2 extracts were partly due to differences in DOC concentrations [START_REF] Degryse | Partitioning of metals (Cd, Co, Cu, Ni, Pb, Zn) in soils: concepts, methodologies, prediction and application -a review[END_REF]. In our study, DOC concentrations in CaCl 2 extracts were, on average, five times smaller than in soil solutions. Several processes might explain the lower DOC concentration in the extract, including an increase of concentration from re-wetting dry soils, DOC coagulation because of large Ca concentrations and dilution as the soil/solution ratio was larger than in soil solution [START_REF] Borken | A climate change scenario for carbon dioxide and dissolved organic carbon fluxes from a temperate forest soil: drought and rewetting effects[END_REF][START_REF] Amery | The copper-mobilizing-potential of dissolved organic matter in soils varies 10-fold depending on soil incubation and extraction procedures[END_REF][START_REF] Fest | Partitioning of organic matter and heavy metals in a sandy soil: effects of extracting solution solid to liquid ratio and pH[END_REF]. Therefore, DOC measured in CaCl 2 extracts does not mimic soil solution DOC under field conditions, whereas DOC partly controls metal speciation in solutions (Pedrot et al., 2008). Thus, CaCl 2 extraction should not be considered as a proxy for soil solution composition in the case of weakly contaminated forest soils.

On the other hand, CaCl 2 extracts provide useful information about the TM pool that can be easily leached (and hence accessible to other pathways), rather than TM bioavailability, although good correlations with biological response have been observed [START_REF] Owojori | Role of clay content in partitioning, uptake and toxicity of zinc in the earthworm Eisenia fetida[END_REF]. However, because extractability increases when pH becomes more acidic, the link between TM extractability and soil pH must be taken into account when considering the soil acidification process.

Influence of pH, organic matter and Fe oxides on TM solubility

Our study showed that, in weakly contaminated soils, TM partitioning between soil and soil solution was strongly related to soil pH. A negative significant correlation was observed between soil pH, TM extractability and total metal concentration in soil For each element, the first line for each metal corresponds to the regression for all samples. The second line corresponds to the regression analysis for samples with pH < 6. All variables are significant (P < 0.05). For Cu, Cr, Sb and Pb, a second group of regression analyses is shown, which includes DOC for the whole pH range. (TM) sol is concentration of the element in soil solution (mol l -1 ); OC is soil organic carbon content (g kg -1 ); Fe is soil iron content (%); M tot is soil total metal content (mol kg -1 ) and DOC is dissolved organic carbon in soil solution (mg l -1 ).

solution for pHs below 6 and free metal ion concentrations for all elements studied. This suggests a competition between protons and TMs for sorption or complexation sites on soil components [START_REF] Sauvé | Solid-solution partitioning of metals in contaminated soils: dependence on pH, total metal burden, and organic matter[END_REF]. For extractable and total trace metal in soil solution, this process occurred under acidic conditions. For more alkaline pH conditions, the weak effect of pH on total metal concentrations was the result of increased complexation. In the case of FMI concentrations, a negative correlation between concentration and soil pH was observed for all pH values.

In the context of this study, two main soil components could be identified for TM sorption on solid phases: soil organic matter and iron oxides. According to [START_REF] Mcbride | Solubility control of Cu, Zn, Cd, and Pb in contaminated soils[END_REF], [START_REF] Sauvé | Solid-solution partitioning of metals in contaminated soils: dependence on pH, total metal burden, and organic matter[END_REF] or [START_REF] Tipping | The solid-solution partitioning of heavy metals (Cu, Zn, Cd, Pb) in upland soil of England and Wales[END_REF], soil organic matter exhibits a strong affinity for TMs. In our study, this was confirmed for Cd, Co and Zn as a negative correlation was observed between the concentration of these elements in soil solution and soil organic matter content. However, the lack of a significant correlation between some elements (Cu, Pb, Sb) and OC, as previously reported [START_REF] Mcbride | Solubility control of Cu, Zn, Cd, and Pb in contaminated soils[END_REF][START_REF] Sauvé | Solid-solution partitioning of metals in contaminated soils: dependence on pH, total metal burden, and organic matter[END_REF], does not mean that sorption to this phase did not occur. In fact, interaction with DOC can lead to inconsistent relationship to OC. For Ni and Cr, a positive relationship was observed. Larger OC content in the solid phase can induce greater DOC release and consequent increase of TMs in soil solution by the formation of DOC-TM complexes [START_REF] Sauvé | Solid-solution partitioning of Cd, Cu, Ni, Pb and Zn in the organic horizons of a forest soil[END_REF][START_REF] Rieuwerts | The influence of soil characteristics on the extractability of Cd, Pb, and Zn in upland and moorland soils[END_REF]. The negative correlation observed between soil iron content and extractable (Cr, Cd and Co) and dissolved (Cu, Cr, Ni, Sb and Pb) TMs, indicated that TMs were strongly associated with iron oxides in forest soils [START_REF] Hernandez | Heavy metal distribution in some French forest soils: evidence for atmospheric contamination[END_REF]Probst et al., 2003a).

Specific interaction between TM, DOC and Fe

For soil pH above 6, measured concentrations of TMs in soil solution did not depend on soil pH. In these conditions, greater TM concentrations were measured compared with values obtained at pH close to 6. This increase depended on the element concerned; thus while Cd, Co, Ni and Zn concentrations only increased a little, Cu, Cr, Sb and Pb concentrations increased with increasing pH by several orders of magnitude. This could be partly related to the influence of soil pH on soil organic matter partitioning. From pH 5 to 8, large DOC concentrations were observed (Figure 5a), as previously shown by [START_REF] Yin | The importance of organic matter distribution and extract soil : solution ratio on the desorption of heavy metals from soils[END_REF] and [START_REF] Lofts | Modelling the solid-solution partitioning of organic matter in European forest soils[END_REF]. Moreover, more alkaline pH enhanced complexing between TMs and dissolved organic matter (Figure 5b, see Pb as an example) because in these conditions proton competition is weaker and cation hydrolysis facilitated. The influence of DOC on TM speciation was thus predominant.

In the soil solutions, significant (P < 0.001) correlations between TMs (Cr, Cu, Sb and Pb), DOC and Fe concentration were found (Table 6). Including DOC concentration in multiple regressions improved Cr, Cu, Sb and Pb total concentration in soil solution prediction. In soil solution, TMs are associated with iron oxides, DOC and/or organo-mineral colloids [START_REF] Pokrovsky | Fe-Al-organic colloids control of trace elements in peat soil solution: results on ultrafiltration and dialysis[END_REF][START_REF] Grybos | Is trace metal release in wetland soils controlled by organic matter mobility or Feoxyhydroxides reduction[END_REF][START_REF] Lofts | The chemical speciation on Fe (III) in freshwaters[END_REF], because dissolved organic matter stabilizes ferric-TM colloids [START_REF] Pedrot | Insights into colloid-mediated trace element release at the soil/water interface[END_REF]. The order of the increase of concentration measured for pH > 6, compared with concentrations that would be expected from the linear relationship with pH, was Pb > Cr > Sb > Cu > Cd >

Co > Ni > Zn. This order was in accordance with the order of strength of complexes formed by TMs with soil humic material [START_REF] Pandey | Stability constants of metal/humic acid complexes and its role in environmental detoxification[END_REF]. Zinc, Cd (and Co and Ni) concentrations did not increase to a large extent for pH > 6. These elements have a smaller affinity for DOC in soil solution [START_REF] Römkens | Cd, Cu and Zn solubility in arable and forest soils: consequences of land use changes for metal mobility and risk assessment[END_REF][START_REF] Weng | Complexation with dissolved organic matter and solubility control of heavy metals in a sandy soil[END_REF][START_REF] Citeau | Colloidal facilitated transfer of metals in soils under different land use[END_REF]. Chromium, Cu, Sb and Pb concentrations increased at pH > 6. The large affinity of Fe, Cu, Cr and Pb for organic colloids has been described previously [START_REF] Sauvé | Copper solubility and speciation of in situ contaminated soils: effects of copper level, pH and organic matter[END_REF][START_REF] Kalbitz | Mobilization of heavy metals and arsenic in polluted wetland and its dependance on dissolved organic matter[END_REF] et al., 2003;[START_REF] Pedrot | Insights into colloid-mediated trace element release at the soil/water interface[END_REF]. Because various types of colloids (organic and organo-metallic, association of DOC and Fe (III) hydroxides) can be found in soil solution, different affinities between the present elements and these colloids can thus be assumed. In our study, no distinction could be made between TM complexing with DOC or Fe hydroxides, although a large affinity of Cr for iron oxides was evident. Chromium extractability was correlated only with Fe content, and in soil solution the correlation between Fe and Cr was the strongest (r 2 = 0.88, n = 37), which confirmed the association between the two elements in soil solution. Antimony was also less significantly correlated with DOC than with Fe in soil solution. This indicated a greater affinity of Sb for Fe hydroxides than for DOC in soil solutions. Antimony affinity for DOC in soil solution depends on its oxidation state (III or V) [START_REF] Filella | Antimony in the environment : a review focused on natural waters: II. Relevant solution chemistry[END_REF]. Affinity of Sb for Fe (III) hydroxides has been already observed in other soils [START_REF] Mitsunobu | Comparison of antimony behavior with that of arsenic under various soil redox conditions[END_REF].

Ability of soil properties to predict TM dynamics in soils

These results for weakly contaminated forest soils lead to simple metal-specific models, based on a limited number of soil properties that could accurately predict both TM partitioning (extractability) and solubility. The partitioning of the elements studied showed an obvious similarity: the same dependence on soil pH (<6) for extractability and solubility was observed. TM partitioning in soils depends on element affinity for solid (OC, iron oxides) and dissolved (DOC, iron hydroxides) phases. TM (Cd, Ni, Zn and Pb) extractability could be predicted by soil pH (below pH 6 for Pb) and OC and Fe content in the case of Cd, Co, Ni and Zn. Cr extractability could be predicted by soil Fe content alone.

In soils of pH < 6, pH combined with OC content (for Cd, Co, Cr, Ni and Zn) or Fe content (for Cu, Cr, Pb and Sb) predicted accurately total metal concentration in soil solution. For less acidic pH values, soil properties alone were less effective in predicting total metal concentration in soil solution because processes occurring in soil solution mainly controlled TM solubility and speciation. In the case of Cu, Cr, Sb and Pb, including DOC concentration in the regressions improved prediction of the total concentration for the whole pH range.

TM partitioning between FMI and soil depended on soil pH at all pH values. pH alone and combinations of pH and OC for Cd, Co and Ni, and pH and Fe for Pb accurately predicted FMI in the soil solutions. The efficiency of the prediction (variance) was similar to that previously obtained for FMI concentration prediction in soil solutions [START_REF] Tipping | The solid-solution partitioning of heavy metals (Cu, Zn, Cd, Pb) in upland soil of England and Wales[END_REF][START_REF] Pampura | Validation of transfer functions predicting Cd and Pb free metal ion activity in soil solution as a function of soil characteristics and reactive metal content[END_REF][START_REF] De Vries | Impact of soils properties on critical concentrations of cadmium, lead, copper, zinc and mercury in soil and soil solution in view of ecotoxicological effects[END_REF], when considering 'reactive' metal content in soil, and soil solution pH. Therefore, for these French forest soils, simple transfer functions could be used for FMI prediction in soil solution. No information was needed about soil solution pH and total metal content in soils.

In summary, in this study, total metal content was a significant property with which to predict total metal concentration in soil solution only for Cu, Cr, Sb and Pb, and was never significant for FMI prediction. According to [START_REF] Watmough | Metal partitioning and uptake in central Ontario forest[END_REF], total metal content in soils with small inputs does not vary to a large extent, and has thus a weak influence on metal concentration in solution.

Risk assessment and critical limits

An original aspect of our study was to consider separately the behaviour of TM in soil solution for pH > 6: forest soils of this pH range are rarely investigated [START_REF] Römkens | Cd, Cu and Zn solubility in arable and forest soils: consequences of land use changes for metal mobility and risk assessment[END_REF][START_REF] Sauvé | Solid-solution partitioning of Cd, Cu, Ni, Pb and Zn in the organic horizons of a forest soil[END_REF]. Even if taken into account, these pH conditions are not generally considered separately, even if for some elements a different behaviour is observed [START_REF] Watmough | Metal partitioning and uptake in central Ontario forest[END_REF]. Our results showed that relatively large total concentrations in soil solutions of Cr, Cu, Pb and Sb were measured at pH > 6. Moreover, soil solutions were filtered (<0.22 μm) and consequently, only small colloids were included in the analysis (1-0.22 μm). The measured concentration would be greater if colloidal concentrations of up to 1 μm were considered. Total concentrations in soil solution should therefore be considered for TM mobility in soil profile and leaching to surface and ground waters [START_REF] Citeau | Colloidal facilitated transfer of metals in soils under different land use[END_REF][START_REF] Pedrot | Insights into colloid-mediated trace element release at the soil/water interface[END_REF].

Critical limits are based on FMI concentration [START_REF] Lofts | Deriving soil critical limits for Cu, Zn, Cd, and Pb: a method based on free ion concentrations[END_REF] as TMs are mostly available in FMI form [START_REF] Florence | The speciation of trace metals in waters[END_REF] in aquatic ecosystems. Availability of certain small organometallic complexes has been shown by [START_REF] Brandt | Evidence for bioavailable copper-dissolved organic matter complexes and transiently increased copper bioavailability in manure-amended soils as determined by bioluminescent bacterial biosensors[END_REF]. Moreover, small colloids carrying TM can be dissociated or ingested by living organisms [START_REF] Van Gestel | Physico-chemical and biological parameters determine metal bioavailability in soils[END_REF] and this might lead to a significant toxic effect, particularly in alkaline soils. In such soil conditions, FMI certainly under-estimates TM toxicity in soil solutions, and TM critical limits for soil organisms cannot be estimated only on this basis (Bur et al., in press). The observed increase of total TM concentration at pH values > 6 through complex formation with organo-mineral colloids can not be neglected when considering TM impact on ecosystems. These data will be integrated into critical load data bases to improve critical load calculations, which have been already tentatively made at a national scale for lead (Probst et al., 2003b).

Conclusions

In this study, TM (Cd, Co, Cr, Cu, Ni, Pb, Sb and Zn) partitioning between total, CaCl 2 extractable and soil solution in relation to soil properties was investigated in some French forest soils under the influence of long-range atmospheric pollution. The main findings are as follows. TM abundance in soil was Zn > Cr > Pb > Ni > Cu > Co > Sb Cd. Metal concentrations were within the range of background values in mineral layers, but in northern, eastern and central sites, surface organic layers have been impacted by atmospheric inputs (EF > 2 for Zn, Cd, Sb and Pb). TM extractability and solubility strongly depended on the element. In soil solution, metal concentration was in the order Zn > Ni > Cu > Pb > Co > Cr > Cd > Sb, whereas regarding extractability, metal could be classified as follows: Cd > Cu > Zn > Co > Ni > Pb > Cr. Differences in metal concentrations were noted between extractable and soluble fractions, especially for pH > 6. Under these conditions, CaCl 2 extraction could not be considered strictly as a proxy for soil solution composition in the case of forest soils with small TM inputs.

TM partitioning in forest soils between total, extractable and soluble phases mainly depended on soil pH. Nevertheless, TM behaviour in soils was different in acidic (pH < 6) and less acidic conditions. In these weakly contaminated soils, competition between protons and metallic cations for adsorption sites mainly controlled TM behaviour at pH < 6. In soil solution, FMI concentration for Cd, Cu, Pb and Zn was significantly correlated to soil pH over the whole pH range. Organic carbon and Fe content were the main metal adsorption components in forest soils. At pH > 6, complexing with DOC and Fe (III) hydroxides increased Cu, Cr, Sb and Pb total concentrations.

Simple models based on soil pH and including some soil sorbing phases (OC, Fe), predicted effectively TM partitioning. TM extractability was predicted by pH only for pHs < 6 and by pH and soil Fe for the whole pH range. Fe content alone explained Cr extractability. Total metal concentration in solutions was modelled by soil pH, OC content (Cd, Co, Cr, Ni and Zn) and Fe content (Cu, Cr, Ni, Pb and Sb). Total metal concentration in soil is an important property to predict total metal concentration in solution only for Cu, Cr, Ni, Pb and Sb. More precise predictions were obtained at pH < 6, and for less acidic pH, soil solution composition had a stronger influence on TM solubility and speciation. Including DOC concentration improved total metal concentration prediction in soil solution for the whole pH range. In contrast, FMI concentration in soil solution was efficiently simulated for all pH values using only a few simple soil properties (pH and OC for Cd, Co and Ni, and Fe for Pb).

Neutral to alkaline conditions should be considered separately in order to accurately predict total metal concentration in solutions from forest soils. Small colloids (organic carbon or hydroxides) were efficient at complexing metals, and they increased concentrations in soil solution as shown for Cu, Cr, Sb and Pb. The increase in concentration order was Pb > Cr > Sb > Cu > Cd > Co > Ni > Zn. Because risk assessments are presently based only on FMI concentrations, TM impact on soil organisms may be under-estimated for these pH conditions. Critical load assessment must thus take this into account in the future.

Figure 2

 2 Figure 2 Relationships between metal extractability (log (M CaCl2 /M tot )) and soil pH for all the studied soils. (a) Cd; (b) Co; (c) Cu; (d) Ni; (e) Pb; (f) Zn. The lines represent the relationship between TM extractability and soil pH for pH < 6, and open symbols are for pH > 6.

  log(TM) sol = a + b log OC + c pH + d log Fe + e log(M) tot + log(DOC).

Figure 3

 3 Figure 3 Relationships between total metal concentrations in soil solution (log(TM) sol ) and soil pH: (a) Cd; (b) Co; (c) Cr; (d) Cu; (e) Ni; (f) Pb; (g) Sb; (h) Zn. Each data-point is a mean value of four to twelve measurements. The lines represent the relationship between log(TM) sol and soil pH for soil pH < 6 and open symbols are for pH > 6.

  *P < 0.0001, ***P < 0.001, **P < 0.01 -P > 0.05. n = number of samples; r 2 = Pearson correlation coefficient; P = level of significance.

Figure 4

 4 Figure 4 Relationship between free metal ion (FMI) concentration (log (M 2+ ) Sol ) in soil solution and soil pH for Cd (a), Co (b), Cu (c), Ni (d), Pb (e) and Zn (f). The lines represent the relationship between log (M 2+ ) Sol and soil pH for the entire pH range.

Figure 5

 5 Figure 5 (a) Relationship between dissolved organic carbon (fulvic acids: FA) in soil solution and soil pH. (b) Relationship between the percentages of lead associated with dissolved organic matter and soil pH (WHAM-VI output data).

Table 1

 1 Main soil characteristics of the study sites. Mean and range (minimum and maximum) values are shown

	Total metal content / μmol kg -1

Table 1

 1 ) in the studied soils. They can be considered as either not, or slightly, contaminated because values fall within background values (Kabata-Pendias & Pendias, 1992), except for Pb and Co. The order of metal abundance (μmol kg -1 ) in mineral layers was Cr > Zn > Ni > Cu > Pb > Co > Sb > Cd. Pb, Zn and Cr concentrations were very variable, depending on soil type (Table

Table 2

 2 Correlation matrix (Pearson correlation coefficients) between total metal concentrations in soils and soil properties (n = 58; bold numbers are for P < 0.001, underlined numbers for P < 0.01 and those in italics for P > 0.01)

		Clay Silt Sand OC	pH water Cr		Cu	Ni	Zn	Co	Pb	Cd	Sb	Al	Fe	Mn
	Clay	1	0.08 -0.80 -0.04	0.40	0.58	0.32	0.54	0.21	0.04	0.10	0.28	0.11	0.00 -0.01 -0.18
	Silt		1	-0.40	0.20 -0.01	0.27	0.13	0.03	0.23	0.13	0.18	0.34	0.26	0.27	0.22	0.20
	Sand			1	-0.11 -0.21 -0.58	-0.35 -0.27	-0.11	0.04 -0.29	-0.39	-0.39	-0.12 -0.02	0.16
	OC				1	-0.36 -0.31	-0.03 -0.27	0.20	0.01	0.85	0.59	0.68	-0.09 -0.01	0.16
	pH water					1	0.52 -0.03	0.76	0.46	0.43 -0.42	0.27	-0.44	0.15	0.34	0.13
	Cr						1	0.71	0.81	0.50	0.58 -0.20	0.27	0.01	0.59	0.67	0.26
	Cu							1	0.49	0.35	0.41	0.14	0.24	0.36	0.56	0.58	0.27
	Ni								1	0.73	0.75 -0.30	0.34	-0.26	0.44	0.69	0.43
	Zn									1	0.84 -0.05	0.60	-0.12	0.65	0.81	0.77
	Co										1	-0.21	0.37	-0.21	0.59	0.92	0.85
	Pb											1	0 . 4 8	0.89	-0.20 -0.18 -0.12
	Cd												1	0 . 4 2	0.19	0.41	0.39
	Sb													1	-0.12 -0.10 -0.14
	Al														1	0.77	0.48
	Fe															1	0.79
	Mn															1

Table 3

 3

		(a) Extractable concentrations (nmol kg -1 ) and percentage of
	metal extracted by CaCl 2 (% total metal content). Mean and range
	(minimum and maximum) values are shown
	(a) n	Extractable amount / nmol kg -1 Extractable proportion / %
	Cd 36 2.17 (2.3 × 10 -4 -14.73)	25.5 (0.92-84.5)
	Co 38 3.20 (2.5 × 10 -4 -22.44)	1.32 (0.04-8,79)
	Cu 46 59.47 (4.1 × 10 -4 -192.66)	8.3 (0.03-58.7)
	Cr 58 0.53 (6.97 × 10 -5 -2.27)	0.015 (0-0.13)
	Ni 48 10.38 (3.010 -3 -61.68)	1.2 (0.01-6.18)
	Pb 53 10.12 (4.2 × 10 -4 3 -124.57)	0.5 (0.006-4.56)
	Zn 54 288.29 (2.7 × 10 -3 -1489.2)	6.3 (0.02-40.7)

Table 3

 3

	(b)	Organic carbon	pH	Fe
	Cd	-0.028	-0.876	-0.526
	Co	0.042	-0.770	-0.647
	Cu	0.234	0.241	-0.078
	Cr	0.113	-0.683	-0.824
	Ni	0.049	-0.740	-0.539
	Pb	0.149	-0.718	-0.480
	Zn	0.120	-0.927	-0.414
	variable because extractability values ranged by several orders
	of magnitude for a given element. The log-transformed data had
	a normal distribution (Shapiro-Wilk Test) and thus the Pearson
	correlation coefficient was used. Organic carbon was never
	significantly correlated with TM extractability (Table 3b). Soil
	Fe content was significantly negatively correlated with Cr (P <
	0.001) and Co (P < 0.01) extractabilities. All TM extractabilities,
	except Cu, were significantly negatively correlated with soil pH.
	However, the influence of pH on TM extractability was stronger in
	acid conditions (pH < 6, see Figure 2), especially for Pb. Above
	pH 6 (open symbols in Figure 2), extractability did not depend
	on pH. As seen in Table 4, significant linear regressions between

(b)

Pearson coefficient between trace metal extractability and soil properties (bold numbers are for P < 0.001, underlined numbers for P < 0.01 and those in italics for P > 0.01) TM extractability and soil pH were found for pH < 6 for Cd, Co, Ni, Pb and Zn. In order to predict TM extractability for the whole pH range, other soil properties were included in a multiregression. Only soil Fe concentration was a significant factor for Cd, Co, Cr and Zn: soil Fe content predicted 80% of variance in Cr extractability.

Table 4

 4 Coefficients of multiple regressions between log (TM extractability) and soil properties. The first group corresponds to correlation with pH only, for pH < 6. The second group corresponds to multi-regression for the whole pH range Pearson correlation coefficient; n = number of samples; P = level of significance: ****P < 0.0001, ***P < 0.001.

	pH			Constant	pH	Log Fe	r 2	n	P
	<6	C d	L o g( C d Extract. )	1 . 8 3± 0.36	-0.54 ± 0.08	-	0.60	36	* * * *
		Co	Log (Co Extract. )	2 . 0 4± 0.71	-0.91 ± 0.15	-	0.53	34	* * * *
		Ni	Log (Ni Extract. )	0 .11 ± 0.57	-0.46 ± 0.12	-	0.30	37	* * *
		Pb	Log (Pb Extract. )	1 . 6 7± 0.36	-0.91 ± 0.14	-	0.38	40	* * * *
		Zn	Log (Zn Extract. )	2 . 3 8± 0.57	-0.80 ± 0.12	-	0.52	43	* * * *
	All	Cd	Log (Cd Extract. )	1 . 2 0± 0.22	-0.38 ± 0.05	-0.39 ± 0.15	0.73	42	* * * *
		Co	Log (Co Extract. )	0 . 0 9± 0.37	-0.42 ± 0.08	-0.79 ± 0.19	0.66	36	* * * *
		Cr	Log (Cr Extract. )	-3.76 ± 0.06	-	-1.31 ± 0.13	0.78	56	* * * *
		Ni	Log (Ni Extract. )	-0.32 ± 0.28	-0.34 ± 0.06	-0.30 ± 0.17	0.56	43	* * * *
		Zn	Log (Zn Extract. )	1 . 5 0± 0.27	-0.59 ± 0.06	-0.52 ± 0.17	0.79	49	* * * *
	r 2 =

Table 5

 5 Concentrations of DOC (mg l -1 ) and TMs (nmol l -1 ) in soil solutions. Mean, minimum, maximum and standard deviation are shown:

	n = 37
		Mean	Standard deviation	Minimum	Maximum
	DOC	25	18	3	78
	Cd	7.91	7.62	0.67	33.6
	Co	28.2	2 2 .6	2 .57	87.1
	Cu	37.1	2 5 .8	2 .46	90.6
	Cr	21.2	1 8 .2	0 .20	67.1
	Ni	76.9	6 3 .2	9 .13	330
	Pb	37.3	5 6	0 .18	168
	Sb	1.84	1.83	0.01	6.12
	Zn	943	624	137	2340

Table 6

 6 Correlation matrix between total soluble TM and major component in soil solutions (n = 37; bold numbers are for P < 0.001, underlined numbers for P < 0.01 and those in italics for P > 0.01)

		pH	DOC	Fe	Al	Ca	Mn	Cd	Cr	Co	Cu	Ni	Sb	Pb	Zn
	pH	1	0.05	-0.02	-0.69	0.79	-0.56	-0.46	-0.12	-0.27	-0.19	-0.12	-0.38	-0.37	-0.68
	DOC		1	0.78	0.30	0.14	-0.07	0.12	0.70	0.07	0.62	0.17	0.53	0.66	-0.01
	Fe			1	0.14	0.06	-0.01	0.32	0.88	0.10	0.79	0.37	0.63	0.84	0.00
	Al				1	-0.75	0.57	0.58	0.40	0.55	0.42	0.17	0.62	0.53	0.75
	Ca					1	-0.39	-0.47	-0.19	-0.37	-0.11	0.01	-0.41	-0.31	-0.70
	Mn						1	0.68	0.22	0.74	0.38	0.56	0.29	0.37	0.57
	Cd							1	0.54	0.72	0.68	0.68	0.53	0.62	0.76
	Cr								1	0.42	0.83	0.51	0.67	0.88	0.20
	Co									1	0.44	0.62	0.31	0.37	0.58
	Cu										1	0.60	0.71	0.87	0.39
	Ni											1	0.09	0.47	0.33
	Sb												1	0.80	0.48
	Pb													1	0 . 4 3
	Zn														1

Table 8

 8 Multiple regressions between log FMI and soil properties: (M 2+ ) sol is FMI concentration in soil solution (mol l -1 ), OC is soil organic carbon content (g kg -1 ) and Fe is soil iron content (%)

			Constant	Log OC	pH	Log Fe	n	r 2	P
	Cd	log (Cd 2+ ) sol	-5.07 ± 0.36	-0.36 ± 0.12	-0.58 ± 0.06	-	37	0.74	****
	Co	log (Co 2+ ) sol	-5.85 ± 0.27	-0.40 ± 0.09	-0.27 ± 0.05	-	37	0.58	****
	Cu	log (Cu 2+ ) sol	-1.70 ± 0.47	-	-1.40 ± 0.09	-	37	0.87	****
	Ni	log (Ni 2+ ) sol	-5.72 ± 0.28	-0.34 ± 0.09	-0.23 ± 0.05	-	37	0.49	****
	Pb	log (Pb 2+ ) sol	-1.68 ± 0.48	-	-1.36 ± 0.10	-0.76 ± 0.30	37	0.85	****
	Zn	log (Zn 2+ ) sol	-3.47 ± 0.21	-	-0.57 ± 0.04	-	37	0.84	****
	****P < 0.0001, ***P < 0.001 -P < 0.05.
	n = number of samples; r 2 = Pearson correlation coefficient; P = level of significance.

Acknowledgements

This project was supported by ADEME (French Agency for Environment) and received logistic support from the ONF (French Forest board). Laure Gandois received a fellowship from ADEME. The authors warmly thank the ONF staff (Vincent Borthelle, Jean-Pierre Chassagne, Joël Clamart, Gilles Fournel, Jean Luc Fiol, René Gregoire, Alain Jacquemard, Franc ¸ois Mouchot, Florence Pertile, Pierre Trithardt and Jérôme Vany) for invaluable help during field work and Erwin Ulrich for facilitation, Manuel Nicolas (ONF-INRA Nancy) for help in soil sampling, Gael Durbe and Alain Alric (EcoLab, Université de Toulouse, UPS-CNRS-INP) for help in field work and technical assistance in the laboratory, Daniel Boutaud and Jean Claude Harrichourry for centrifugation device design and production, Frederic Candaudap (LMTG, Université de Toulouse, UPS-CNRS-IRD) for ICPMS analysis assistance and Frederic Julien (EcoLab, Université de Toulouse, UPS-CNRS-INP) for major anion analysis. Two anonymous reviewers are thanked for their helpful comments.


A F F Saad
email: afsaad@zu.edu.eg

Michel Fromm

Mona H Ibraheim

A A A El-Namrouty

Aya M Nwara

S A A Kandil

Mahmoud S Dawood

Loss of chemical bonds induced by high doses of γ-radiation in a PADC polymer film: the influence of dose and dose rate on radiation chemical yields

Keywords: PADC, γ-ray, FT-IR, Molecular bonds, G-value, Dose, Dose rate

published or not. The documents may come

Loss of chemical bonds induced by high doses of γ-radiation in a PADC polymer film: The influence of dose and dose rate on radiation chemical yields

Introduction

Since the end of the seventies of the last century, poly allyl diglycol carbonate (PADC) polymer material, CR-39, is used for radiation detection [START_REF] Cartwright | A nuclear-track-recording polymer of unique sensitivity and resolution[END_REF][START_REF] Cassou | Properties and applications of CR-39 polymeric nuclear track detector[END_REF]. PADC is commonly used in various branches of science and technology such as nuclear and particle physics, cosmic rays, neutron dosimetry and radiation biology [START_REF] Nikezic | Formation and growth of tracks in nuclear track materials[END_REF][START_REF] J O U R N A L P R E -P R O O F Price | Science and technology with nuclear tracks in solids[END_REF]. Exposure to ionizing radiation such as gamma and X-rays, alpha particles and heavy ions can play a prominent role in modifying physical, chemical, molecular, structural, optical, electrical and thermal properties of the polymeric materials [START_REF] Marletta | Chemical reactions and physical property modifications induced by keV ion beams in polymers[END_REF][START_REF] Steckenreiter | Chemical modifications of PET induced by swift heavy ions[END_REF][START_REF] Saha | Chemical modification of polypropylene induced by high energy carbon ions[END_REF][START_REF] Saad | Radiation-induced modifications on spectroscopic and thermal properties of CR-39 and SR-90 nuclear track detectors[END_REF][START_REF] Khan | Measurement of neutron fluence with CR-39 using a UV spectrophotometer[END_REF][START_REF] Singh | A comparison of modifications induced by Li 3+ and O 6+ ion beam to Makrofol-KG and CR-39 polymeric track detectors[END_REF][START_REF] J O U R N A L P R E -P R O O F Sharma | Effect of gamma irradiation on the optical properties of CR-39 polymer[END_REF][START_REF] Kumar | Study of optical, structural and chemical properties of neutron irradiated PADC film[END_REF][START_REF] Raghuvanshi | Effect of gamma irradiation on the optical properties of UHMWPE (Ultra-highmolecular-weight-polyethylene) polymer[END_REF]Saad et al., 2014a,b). In general, chain scission of PADC polymer is seen as the major process of the degradation of the main chain of that polymer. Such scission process caused by ionizing radiation (UV radiation, X-rays, gamma rays, swift ions), is known to release carbon mono oxide and carbon dioxide [START_REF] Yamauchi | Formation of CO 2 gas and OH groups in CR-39 plastics due to gamma-ray and ions irradiation[END_REF][START_REF] Saad | Radiation-induced modifications on spectroscopic and thermal properties of CR-39 and SR-90 nuclear track detectors[END_REF] and references therein.

In order to describe molecular and chemical damages formed in PADC and other particle track detector (PTD) polymers by ionizing radiations; an international collaboration that aimed at studying radiation damage using spectroscopic measurements has been set up between Japan and France (T. Yamauchi, T. Kusumoto, R. Barillon, and their co-workers) [START_REF] Barillon | Chemical bond scission induced by 1 H + , 16 O 8+ , and γrays in a cellulose nitrate detector (O-082)[END_REF]Yamauchi et al., 2005a,;Yamauchi et al., 2008a,b;[START_REF] Mori | Radiation chemical yield for loss of carbonate ester bonds in PADC films exposed to gamma ray[END_REF][START_REF] Yamauchi | Thresholds of Etchable Track Formation and Chemical Damage Parameters in Poly (ethylene terephthalate), Bisphenol A polycarbonate, and Poly (allyl diglycol carbonate) Films at the Stopping Powers Ranging from 10 to 12,000 keV/µm[END_REF][START_REF] Mori | Radiation chemical yields for loss of ether and carbonate ester bonds in PADC films exposed to proton and heavy ion beams[END_REF][START_REF] Mori | Vacuum effects on the radiation chemical yields in PADC films exposed to gamma rays and heavy ions[END_REF][START_REF] Barillon | Chemical cross sections induced by ions in solid organic detectors: Experimentation and simulation[END_REF][START_REF] Kusumoto | Yields on the formation of OH groups and the loss of CH groups along nuclear tracks in PADC films[END_REF][START_REF] Kusumoto | Radiation chemical yields for the losses of typical functional groups in PADC films for high energy protons registered as unetchable tracks[END_REF][START_REF] Yamauchi | Distinct step-like changes in G values for the losses of typical functional groups in poly (ethylene terephthalate) along boron ion tracks around the detection threshold[END_REF]. This team was able to highlight which are the chemical units of the PADC structure that are involved in the destructive process at the origin of ionizing radiation damage trails as mentioned in a recent review paper [START_REF] Fromm | Role of intermediate species in the formation of ion tracks in PADC: a review[END_REF]. That vast study is based on analyses of spectroscopic J o u r n a l P r e -p r o o f experimental data of the loss/appearance of specific chemical functions further to PADC/PTD polymer exposure to various ionizing radiations such as alpha particles, gamma rays and heavy ions. The main achievements of these studies are connected with a quantitative determination of the sensibility of PADC chemical functional groups to ionizing radiations expressed in terms of cross-sections and G-values (radiation chemical yields) for the loss of given chemical groups. As previously mentioned, it was also shown by other groups and further confirmed, that carbon dioxide formation goes together with radiation damage [START_REF] Rickards | Gas ejection from CR-39 under ion bombardment[END_REF][START_REF] Chong | UV-VIS and FTIR spectral studies of CR-39 plastics irradiated with X-rays[END_REF][START_REF] Saad | FT-IR spectroscopy of carbon dioxide in CR-39 and SR-90 track detectors irradiated with ions and gamma-rays at different energies and fluences[END_REF][START_REF] Yamauchi | Raman and near-IR study on proton irradiated CR-39 detector and the effect of air-leak on damage formation[END_REF][START_REF] Malek | Generation of CO2 in γ-ray-irradiated CR-39 plastic[END_REF]Yamauchi et al., 2005b;[START_REF] Saad | Radiation-induced modifications on spectroscopic and thermal properties of CR-39 and SR-90 nuclear track detectors[END_REF]. In addition, it has also been established that the amount of CO 2 in the detector is closely related not only to the latent track formation mechanism but also to the track detection sensitivity [START_REF] Saad | FT-IR spectroscopy of carbon dioxide in CR-39 and SR-90 track detectors irradiated with ions and gamma-rays at different energies and fluences[END_REF]. Carbon dioxide absorbed in the polymer material plays indeed a positive role in the detector's sensitization [START_REF] Fujii | Sensitization of polymeric track detectors with carbon dioxide[END_REF][START_REF] J O U R N A L P R E -P R O O F Csige | Post-irradiation sensitization of CR-39 track detector in carbon dioxide atmosphere[END_REF][START_REF] Fujii | Effect of vacuum, oxygen and carbon dioxide on the track registration in SR-90 and CR-39[END_REF][START_REF] Baradacs | CO2 treatment and vacuum effects in proton beam micromachining of PADC[END_REF]d'Errico et al., 2017). [START_REF] O'sullivan | Correlative studies of track-etch behaviour and chemical development of lithographic polymer resists[END_REF] measured G value of γray (an average of 1.25 MeV from 60 Co) scissions in PADC per 100 eV as ≈ 10, from the correlation between the minimum value of the primary ionization rate and the G values for various types of polymers based on the observed bulk etching rates. [START_REF] Mori | Radiation chemical yield for loss of carbonate ester bonds in PADC films exposed to gamma ray[END_REF] studied the effect of γray irradiation on the radiation chemical yield, for loss of carbonate ester bonds in PADC films by means of FT-IR spectrometry. They found that the G value is 20 (scissions/100 eV) and is somewhat higher than that from heavy ion irradiations (147 MeV Fe ions) (Yamauchi et al., 2008a;[START_REF] Mori | Radiation chemical yields for loss of ether and carbonate ester bonds in PADC films exposed to proton and heavy ion beams[END_REF]. These authors also evaluated the chemical radiation yield for other kinds of heavy ions and protons to different functional groups in PADC films, for example, ether and ethylene. They also found that the highest G values were observed in PADC films irradiated by protons, namely about 20 scissions/100 eV for both bonds, namely J o u r n a l P r e -p r o o f ether and carbonate ester. The values decreased down to about 5 scissions/100 eV for films irradiated with heavier ions. Studying gamma irradiation of PADC at higher doses can be an effective way to understand the latent track formation, because the local dose in a heavy ion track core exceeds several MGys in heavy ions. Recently it was also shown that a critical dose of ~50-60 kGy exists above which carbonate ester density in PADC start decreasing [START_REF] Kusumoto | Evidence for a critical dose above which damage to carbonate ester bonds in PADC appear after gamma ray and ultra-soft X-ray exposures[END_REF]. For ether bonds, such threshold was not observed.

In the current paper, we propose to study the changes of the molecular structure induced by γ-radiation at different high doses over the range 0.5 to 3 MGy. For this purpose, FT-IR spectrometer as a standard technique of spectroscopic analysis of polymeric materials is used.

Experimental procedure

Fig. 1 shows the chemical structure of a poly allyl diglycol carbonate (PADC) high polymer which has a length of chain between two adjacent carbonate groups in symmetrical positions. PADC sheets of TASTRAK used in this investigation were manufactured by Track Analysis Systems Ltd. (TASL), Bristol, UK. Polymeric detector samples of a uniform thickness of 750 µm were cut to a size of 2×3 cm 2 . The detector samples were irradiated in atmosphere at room temperature using a commercial gamma irradiator of type MC-20 (Russia) with 60 Co energy source which provides a dose rate of 1.2 kGy/h. The PADC samples were irradiated with different doses ranging from 0.5 to 3 MGy of gamma rays. The samples were irradiated for various times in order to get variable doses. These irradiations were performed at the Gamma Cell, Nuclear Research Centre, Inshas , Atomic Energy Authority .Egypt. The FT-IR spectrometer was operated in attenuated total reflection (ATR) mode. The working range of the spectrometer is 4600-400 cm -1 . A resolution of 4 cm -1 , scan numbers of J o u r n a l P r e -p r o o f 16 and apodization of cosine were used. The Fourier transform infrared (FT-IR) spectrometer model of Vertex 70, made in Germany is used. FT-IR spectroscopy was performed in the wave number range 4000-400 cm -1 at room temperature on the pristine and irradiated CR-39 samples.

Results

FT-IR spectral changes due to γ-ray irradiation

FT-IR spectra in ATR mode for pristine and irradiated samples of PADC films are shown in Fig. 2. It is easy to find a significant difference in FT-IR spectra between irradiated and unirradiated samples of PADC because changes in the molecular structure due to our irradiation conditions were high, thus it can be easily detected in ATR mode. Absorption bands were identified. The broad absorption peak around 2920 cm -1 is due to C-H stretching.

Two strong absorption peaks at 1734 and 1240 cm -1 are assigned to C=O and C-O-C bonds, respectively, which compose carbonate ester bonds [START_REF] Gagnadre | IR spectroscopy studies of polycarbonate irradiated by H+ and Li+ ions[END_REF][START_REF] Darraud | Optical modifications of polymers by ion beam irradiation[END_REF]Lounis-Mokrani wt al., 2003;[START_REF] Saad | FT-IR spectroscopy of carbon dioxide in CR-39 and SR-90 track detectors irradiated with ions and gamma-rays at different energies and fluences[END_REF]. Three weak absorption peaks at 1132, 1076 and 1022 cm -1 are assigned to ether bonds. In addition, an absorption peak at 959 cm -1 is assigned to wagging vibration of the double bond C=C. Two other absorption peaks at 878 and 785 cm -1 are assigned to C-H rocking out-of-plane deformation [START_REF] Gagnadre | IR spectroscopy studies of polycarbonate irradiated by H+ and Li+ ions[END_REF]Lounis-Mokrani wt al., 2003). Indeed, the OH groups at around 3500 cm -1 are clearly suppressed in FT-IR spectra in ATR mode for the pristine and exposed samples of PADC as shown in Fig. 2. This is due to that the yield of OH group formation is below the detection limit in that mode compared to the case of another transmission mode, as presented in our previous studies [START_REF] Saad | FT-IR spectroscopy of carbon dioxide in CR-39 and SR-90 track detectors irradiated with ions and gamma-rays at different energies and fluences[END_REF][START_REF] Saad | Radiation-induced modifications on spectroscopic and thermal properties of CR-39 and SR-90 nuclear track detectors[END_REF].

J o u r n a l P r e -p r o o f

A new broad band (as a shoulder) at wave number 1627 cm -1 , which could be identified as C=C bond, is found in the FT-IR spectra of gamma exposed samples of PADC films as also shown in Fig. 2. It can be seen that these shoulder absorption bands are strongly affected by gamma irradiation in their shapes and/or intensities. The band area of C=C at 1627 cm -1 is plotted as a function of irradiation dose, as shown in Fig. 3. The net area of the C=C absorption band, which actually represents the net absorbance, ∆A, is given by Eq.

∆A = A irr -A o (1)
where A irr and A o are the area under the absorption peak of the irradiated and unirradiated PADC samples, respectively. The band area of C=C is quickly enhanced with increasing dose up to 1 MGy, then it somewhat decreases with higher dose at 1.5 MGy. Beyond 1.5 MGy the net absorbance sharply increases up to a maximum at 2.5 MGy, and then sharply decreased at 3 MGy as can be seen in Fig. 3. In fact, the response of PADC film to the gamma dose of 2.5

MGy reveals a differing behavior. This inconsistent response to that dose was also observed by other standard analysis techniques as reported in our recent study [START_REF] Saad | Modifications in the optical and thermal properties of a CR-39 polymeric detector induced by high doses of γradiation[END_REF]. In general, the PADC film might have undergone a very considerable degree of scission of its chain groups, generating a new broad band of C=C at 1627 cm -1 , as a result of the γ-ray exposure particularly for that dose. This will be the subject of a separate study. The band assignments for most important chemical functional groups in a PADC polymer were given in the literature and are enlisted in Table 1.

The relative changes in the bonds IR absorption intensities have been evaluated from the relative increase or decrease in the intensity of the peak associated to the functional groups present in the PADC high polymer. IR absorption bands as obtained from the pristine and the irradiated samples of PADC polymer for different doses ranging from 0.5 to 3 MGy.

It is apparent that the absorption bands observed at 1734,1400,1240,1132,1076,1022,959,878 and 785 cm -1 decrease in height due the γ-ray exposure. It is clear that the absorbance J o u r n a l P r e -p r o o f decreases after irradiation for all the peaks except for the triplet bands at 2960, 2925 and 2853 cm -1 which show an opposite trend. However, there is a little shift in the peak positions of some absorption bands after irradiation with high doses of γ-rays. In the next paragraphs, we will pay attention to the two strong bands of the carbonate ester bond and the three weak absorption peaks of the ether bond.

Loss of chemical bonds in a PADC film

Loss of carbonate ester bonds

The normalized absorbance, A/A 0 , was attained for each examined absorbed dose, where A is the absorbance of the band considered after exposure and A 0 is that of the pristine one. The normalized absorbance is equal to the survival fraction, N/N 0 , which is the ratio of the number density of the considering chemical bond, N, to that of pristine, N 0 . That is, A/A 0 = N/N 0 . Fig. 4 shows the changes in the normalized absorbance, A/A 0 , of the two peaks for carbonate ester bonds as a function of γ-ray dose over the range 1.5 to 3 MGy (just to focusing on the higher doses). Each peak reduced in height monotonically in the examined regions of dose. This figure shows the results for both C=O and C-O-C bonds, presenting a clear linear decline of the normalized absorbance against the absorbed dose for both bonds up to 3 MGy. Trends for the two peaks that compose the carbonate ester bond are almost similar.

We used the following simple formula [START_REF] Mori | Radiation chemical yield for loss of carbonate ester bonds in PADC films exposed to gamma ray[END_REF]:

N/N 0 =1 -k i . D ( 2
)
where D is the absorbed dose in a unit of Gy and k i is an experimental constant for the bond i in Gy -1 . The constant k i corresponds to the slope of the fitting lines in Fig. 4. Normalized absorbances are converted to G values for the loss of C=O and C-O-C bonds per 100 eV, using the following simple relation [START_REF] Mori | Radiation chemical yield for loss of carbonate ester bonds in PADC films exposed to gamma ray[END_REF]:

J o u r n a l P r e -p r o o f

G = 9.64 ×10 6 N 0 k i (3)
where N 0 is in mol/kg. N 0 is 7.30 mol/kg for the carbonate ester bond in PADC. The calculations give G values of 4.7 and 6.3 for C=O and C-O-C bonds, respectively. Finally, the average result, G value for the loss of carbonate ester bonds in PADC is determined to be about 5.5 as tabulated and listed in Table 2.

In the current study, the G value for gamma ray is considerably lower than that obtained in a previous study [START_REF] Mori | Radiation chemical yield for loss of carbonate ester bonds in PADC films exposed to gamma ray[END_REF]. With PADC as a target and 60 Co as a gamma source in both studies; the dose rate used by Mori et al. was 4.5 kGy/h (1.25 Gy/s) and we used 1.2 kGy/h (0.33 Gy/s), in atmospheric conditions within dose range 0.5 to 3 MGy. An important decrease of the G-value from 20 [START_REF] Mori | Radiation chemical yield for loss of carbonate ester bonds in PADC films exposed to gamma ray[END_REF] to about 6 (this work) in conjunction with a decrease of the dose rate (by a factor 3.75) is observed. The decrease of dose rate lowers thus radiation chemical yields for destruction of C=O and C-O-C bonds. The so-called dose rate effects (DRE) in polymer properties under irradiation in an oxygen environment are known to be very significant and connected with chain reactions of oxidizing radiation destruction initiated by radiolysis species of radical type [START_REF] Briskman | Dose Rate Effects in Polymer Materials Irradiated in Vacuum[END_REF]. DRE yields either from physical or chemical aspects. At high dose rates diffusion limited oxidation (DLO) effects become evident cause for dose rate effect and at low dose rates more complex chemical reactions determine the magnitude of dose rate effect [START_REF] Sipilä | Methods used in evaluating severity of dose rate effect on low dose rates[END_REF]. Generally, high dose rates of ionizing radiation (typically > 100 kGy/h) have less impact on the properties of polymers compared to lower dose rates (this work 1-5 kGy/h). It was for example shown that at equal total doses the carbonyls and hydroperoxide yields in LDPE (Low density polyethylene) are greatly enhanced when decreasing the dose rate from 0.69 to 0.04 kGy/h [START_REF] Buttafava | Dose rate effects on the radiation induced oxidation of polyethylene[END_REF]. Herein, dose rates (namely 4.5 kGy/h and 1.2 kGy/h) belong to the low or intermediate dose rates. DRE and thus oxidizing radiation destruction generally follows three steps in polymers, namely: a free radical reacts with J o u r n a l P r e -p r o o f molecular oxygen to generate peroxy radicals which themselves abstract hydrogen, then the formed peroxide decomposes to create carbonyl functions. It is clear from Fig. 2 that no carbonyl function is created, therefore conventional DRE in an oxygen environment may not be invoked here as a major pathway. Moreover, a recent study of the radiation degradation of Poly (ethylene-co-vinyl acetate) (EVA), containing a carbonyl moiety, demonstrated that the DRE is negligible in the range 0.4 -1.5 kGy/h [START_REF] Fabiani | Inverse effect in simultaneous thermal and radiation aging of EVA insulation[END_REF]. Additionally, it may be quite unlikely to invoke the commercial origin of the polymers used as the cause of observed variations of G-values. Baryotrack TM used by Mori et al. and Tastrak TM (this work) are both PADCs, thus with identical structures but with small differences in their initiator compositions (< 5%).

Coming back to the dose rates and especially to low or intermediate dose rates, it has been shown that in the case of aliphatic compounds; when irradiation is carried out at low dose rates in air, the effects are strongly dependent on the dose rate [START_REF] Wilski | Radiation stability of polymers[END_REF]. It is indeed generally assumed that the rate of radicals generation is proportional to the dose rate [START_REF] Sarcinelli | Inverse response of polypropylene to gamma radiation under vacuum[END_REF], hence with a dose rate ratio of 3.75 between Mori et al. experiments and the present study it is quite probable that the lower G-value we obtained (~8 instead of 20

for Mori et al.) is a clear manifestation of dose rate dependence.

Loss of ether bonds

Fig. 5 shows the changes in the normalized absorbance, A/A 0 , for ether bonds as a function of the γ-ray dose. The first two peaks of ether group (peaks number: 6 and 7) at 1132 and 1076 cm -1 have more or less the same trend on the γ-ray dose up to 3 MGy, showing almost a linear decrease that can be expressed by Eq. (2). Beyond 2 MGy, the absorbance of the first peak at 1132 cm -1 completely disappear up to 3 MGy, while the absorbance of the second peak at 1076 cm -1 (peak number: 7) is still observed till the end of the dose range as clearly seen in Fig 2 . The third peak of that group at 1022 cm -1 (peak number: 8) behaves J o u r n a l P r e -p r o o f differently when compared to the two other peaks. It is difficult to assess the absorbance for that peak. So, we assess the constant k i for the ether bonds from peaks with wave numbers 1132 to 1076 cm -1 (peaks number: 6 and 7). Eq. ( 3) gives G-values for the loss of ether bonds to be 7.3 and 6.2 at absorption bands of 1132 and 1076 cm -1 , respectively. The average G value for the loss of the ether bonds in PADC is determined to be about 6.9 (scission/100 eV) as seen in Table 2. Based on the loss measurements for both carbonate ester and ether bonds, it is found that the rate of reduction in the normalized absorbance of ether bonds is barely higher than that of carbonate ester bonds. On the other hand, the peak No. 6 (one of ether groups at 1132 cm -1 ) has completely disappeared after 2 MGy. This means that it has suffered complete radiation destruction, so its G-value cannot be determined at the higher doses used.

As noticed above, the molecular bonds of ether functions in the PADC polymer have suffered a serious degradation due to γ-irradiation, at different high doses over the range 0.5 to 3 MGy. Overall, the radiation molecular yield, G value, has a dependence on the gamma dose in PADC. At high doses over the range 0.5 to 3 MGy, the yield for the loss of ether bonds is barely higher than that of carbonate ester bonds, confirming that the ether part being generally considered more radio-sensitive than the carbonate ester one [START_REF] Mori | Radiation chemical yields for loss of ether and carbonate ester bonds in PADC films exposed to proton and heavy ion beams[END_REF][START_REF] Mori | Vacuum effects on the radiation chemical yields in PADC films exposed to gamma rays and heavy ions[END_REF]. Here again, the G-value we obtain is smaller than the one measured by [START_REF] Mori | Vacuum effects on the radiation chemical yields in PADC films exposed to gamma rays and heavy ions[END_REF]. G-values obtained herein both for ether and carbonate ester function loss are relatively close together (average values of respectively 6.9 and 5.5) as those obtained by Mori et al. are quasi the triple (typically 20 for both chemical functions) [START_REF] Mori | Radiation chemical yields for loss of ether and carbonate ester bonds in PADC films exposed to proton and heavy ion beams[END_REF][START_REF] Mori | Vacuum effects on the radiation chemical yields in PADC films exposed to gamma rays and heavy ions[END_REF][START_REF] Kusumoto | Radiation chemical yields for the losses of typical functional groups in PADC films for high energy protons registered as unetchable tracks[END_REF]. So, we may argue that such difference militates in favor of a dose rate effect. We can observe that when the dose rate of 60 Co gamma rays decreases by a factor 3.75, exposing PADC sheets in air leads to a reduction of radiation chemical yields of a factor 3 -3.6 for ether and carbonate ester function loss.

J o u r n a l P r e -p r o o f

Discussion and conclusion

Molecular modification due to γ-ray damage in the PADC film has been examined by means of FT-IR spectrometry, which was exposed to γ-radiation at incident energies 1.25

MeV with different doses ranging from 0.5 to 3 MGy. The radiation molecular yield, i.e., G values, have been determined to be 5.5 (scission per 100 eV) for the loss of the carbonate ester bonds in PADC film exposed to gamma ray. Similar analyses have been performed for the ether bonds at the same doses. The G-value for the loss of these bonds was about 6.9. In this study, dose and dose rate dependence was observed, for both ether and carbonate ester function loss. It is quite interesting to note that the dose rate effect observed corresponds to a reduction of the dose of 3.75 in comparison with the study of others [START_REF] Mori | Radiation chemical yield for loss of carbonate ester bonds in PADC films exposed to gamma ray[END_REF] 7.5 6.2 6.9 ≈ 7

J o u r n a l P r e -p r o o f  G values for the loss of carbonate ester and ether bonds are 5.5 and 6.9, respect.  Radiation chemical yields are influenced by dose and dose rate of gamma rays.

J o u r n a l P r e -p r o o f

  and that this reduction yields a decrease of experimentally determined G-values by typically 3 -3.6 folds.The results presented herein demonstrate for the first time the existence of a dose rate effect in PADC exposed to Co-60 γ-radiation. If this last remark does not constitute in itself a major advance (dose rate effects in polymers are well-known); it indicates whatever that dose rate may be an interesting tool to be used in further studies. In spite of Co-60 gamma ray dose is clearly affects the G-values, variations of dose rates could also be an interesting way to adjust G-values in order to simulate heavy-ion irradiation effects using gamma rays. Actually, the radiation chemical yield for the loss of carbonate ester bonds obtained in the present study is close to those measured by[START_REF] Yamauchi | a. Loss of carbonate ester bonds along Fe ion tracks in thin CR-39 films[END_REF] for 147 MeV Fe ions. The used gamma rays (Co-60) are characterized by an electronic LET of 0.4 keV/µm as the Fe ions we have J o u r n a l P r e -p r o o f

Fig

  Fig. 1. J o u r n a l P r e -p r o o f

Table 1

 1 Assignments of FTIR bands of functional groups in PADC polymer film.

	Peak	Band	Functional Groups	Reference
	Number	position
		(cm‾¹)
	1	2926	Symmetric CH 2 stretching vibration	(Gagnadre et al., 1993; Saad et al.,
			of aliphatic hydrocarbons	2001; Yamauchi et al., 2001)
	2	1734	Ester C=O stretching vibration of the	(Gagnadre et al., 1993; Saad et al.,
			alkyl carbonate -C-CO-C	2001; Yamauchi et al., 2001)
	2*	1627	Hydrocarbon C=C stretching of the	Present work
			vinyl group
	3 4 5 6 7 8 9	1452 1400 1240 1132 1076 1022 959	-CH=CH 2 CH 2 deformation vibration and OCH 2 wagging vibration of -CO-O-CH 2 In-plane -CH 2 -bending vibration of-CO-O-CH 2 Asymmetric stretching vibration of C-O-C of aliphatic ester Symmetric stretching vibration of the C-O-C group of the saturated aliphatic ether Asymmetric stretching vibration of the C-O-C group of the saturated aliphatic ether Symmetric stretching vibration of the C-O-C group of the saturated aliphatic ether Wagging vibration of the double bond C=C J o u r n a l P r e -p r o o f (Lounis-Mokrani et al., 2003) (Lounis-Mokrani et al., 2003) (Yamauchi et al., 2001) (Gagnadre et al., 1993; Darraud et al., 1994; Lounis-Mokrani et al., 2003) (Darraud et al., 1994; Lounis-Mokrani et al., 2003) (Darraud et al., 1994; Lounis-Mokrani et al., 2003) (Lounis-Mokrani et al., 2003)
	10	878	C-H rocking out-of-plane	(Lounis-Mokrani et al.,
			deformation	2003)
	11	785	Ether group or C-H rocking out-of-	(Saad et al., 2001)
			plane deformation

Table 2

 2 Radiation chemical yields for loss of carbonate ester and ether bonds in a PADC polymer film.

	Bonds

5200 keV/µm. The G-value we measured for the loss of ether moiety is close to the one measured for 46 MeV Ar ions (LET = 3880 keV/µm) (Yamauchi et al., 2008).

Conflict of Interest Forms

No conflict

J o u r n a l P r e -p r o o f

]]][-Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: