Environ. Sci. Technol. 2010, 44, 9463–9469
Implications of the Use of As-Rich Groundwater for Agricultural Purposes and the Effects of Soil Amendments on As Solubility C . D E L A F U E N T E , * ,† R . C L E M E N T E , † ´ LEZ,‡ AND J. A. ALBURQUERQUE,† D. VE M. P. BERNAL† Centro de Edafologı´a y Biologı´a Aplicada del Segura, CSIC. PO Box 164, 30100 Murcia, Spain, and Instituto de Agroquı´mica y Tecnologı´a de Alimentos, CSIC. PO Box. 73, 46100 Burjassot, Valencia, Spain
Received June 14, 2010. Revised manuscript received September 24, 2010. Accepted November 9, 2010.
An agricultural site in Segovia province (Spain) contains high levels of arsenic (As) of geological origin in its groundwater, which is used intensively for irrigation. Crops, irrigation waters, and soils were analyzed to evaluate the occurrence of As in this area and its potential impact on the food chain. High As mobility was found in the agricultural soils, related to the application of As in the irrigation waters (14.8-280 µg As L-1) and the general alkaline and sandy character of these soils, which imposes a low capacity for As sorption and therefore enhances plant uptake. The use of amendments can also affect the solubility of As in these soils. Evidence for this was evaluated based on a study of the effect of organic (compost) and inorganic (iron oxides-rich rolling mill scale and phosphate fertilizer) amendments. Arsenic solubility in soil and plant uptake were high, but not significantly affected by organic matter or phosphate addition, while As immobilization was associated with addition of iron oxides with the rolling mill scale, although this did not result in a decrease of As uptake by the tested plants.
1. Introduction Arsenic (As) is a ubiquitous, toxic metalloid that is distributed extensively worldwide. Trace concentrations of this element are commonly found in the environment, but high levels have been found widely in soils and groundwater as a consequence of natural and/or human processes. Arsenic is present mainly as the inorganic species arsenite (As(III)) and arsenate (As(V)) in natural environments, but organic As species can be found also in soils and waters as a result of the methylation of inorganic As forms by microorganisms and where organic As compounds have been used as pesticides or defoliants (1). Several epidemiological studies have demonstrated that drinking water and foods are the main sources of As for humans (2, 3), leading to the appearance of pathologies associated with chronic exposure to As (e.g., several types of cancer). However, the total As concentration in foods and waters cannot provide reliable information about possible * Corresponding author phone: +34 968 396200; fax: +34 968 396213; e-mail:
[email protected]. † Centro de Edafologı´a y Biologı´a Aplicada del Segura. ‡ Instituto de Agroquı´mica y Tecnologı´a de Alimentos. 10.1021/es102012s
2010 American Chemical Society
Published on Web 11/23/2010
human health risks, since inorganic As is the most toxic fraction of this metalloid, and it is used by the World Health Organization (WHO) to establish the guideline limit (15 µg inorganic As kg-1 body per week) for As intake by humans (4). The percentage of inorganic As in the edible tissues of vegetables depends on the species, growth stage, organ, etc. Mun ˜ oz et al. (5) found a wide range of inorganic As (from 28 to 100% of the total As concentrations) in various raw vegetables (garlic, potato, carrot, and beetroot). Thus, speciation analysis of the As content of foods is needed in a wide range of edible products, to provide a reliable assessment of the health risk. Arsenic accumulation in edible plant tissue is frequently low (2.0 µg g-1 dry weight) have been reported for rice grown in As-polluted agricultural soils from southeast Asia (3). However, As concentrations in crops depend on different parameters, such as the solubility of this metalloid in the soil and the plant ability to take up As and translocate it to the target organs. Arsenic solubility in soils is governed largely by adsorption-desorption reactions, but other processes such as precipitation can occur (1). Therefore, remediation strategies for As-polluted soils based on the reduction of As solubility can reduce or prevent the transfer of this metalloid to the food chain and waters, although their effectiveness is closely related to different properties of soils such as pH, texture, or concentrations of competing ions (7). Different materials such as iron, aluminum and manganese oxides and hydroxides, and clay minerals are efficient As adsorbents. In this context, the use of different, low-cost organic and inorganic industrial byproduct as soil amendments is an option that is being considered increasingly for the reclamation of soils and waters contaminated with As (8); also, this can be a valuable way to recycle these wastes. High As contents of a geological origin have been reported since the year 2000 in the groundwater of the northwestern part of Segovia province (Spain). Currently, a delimited area of approximately 1700 km2 is affected, where groundwater As concentrations are usually above the allowable limit established for drinking water of 10 µg L-1 (9). In a field study in a nearby, polluted area, Calvo et al. (10) showed high levels of As in groundwater: 88% of the water samples analyzed had more than 10 µg As L-1 and around 10% of the sampling points showed concentrations above 100 µg L-1. So, the irrigation of agricultural soils with these waters could increase both the total and available fractions of As in soils, and thus As uptake by plants. The aims of the present study were to evaluate the accumulation of As in this agricultural land and the relationships between As concentrations in soils, waters, and plants, and to assess the effects of different amendments on As solubility in soil and plant uptake, with regard to limiting As transfer into the food chain.
2. Materials and Methods 2.1. Field Sampling. The study area (130 km2 approximately, Supporting Information (SI) Figure S1) lies in the southeastern part of the Tertiary Duero Basin of the North Iberian Meseta. Water, soil, and plant samples were collected from agricultural areas at the Chan ˜ e, Remondo, Vallelado, Villaverde de Iscar, and Sanchonun ˜ o sites in Segovia province (Spain), in which high levels of As were reported in groundwater by Garcı´aSa´nchez et al. (11). Waters, soils, and plants were collected in July 2008 in order to determine the general presence of As in the area VOL. 44, NO. 24, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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and to evaluate its relationship with the different soil properties. In order to obtain further information about As occurrence, a second sampling (November 2008) was carried out at Chan ˜ e and Vallelado, as they had shown higher soil As concentrations than the rest of the locations in the first sampling. Finally, the Vallelado site was sampled again, in April 2009, because it showed the highest levels of As in the irrigation waters and soils and, in consequence, the highest potential risk of As transfer to the human food chain. Therefore, a soil collected in the latter sampling was selected in order to evaluate the effects of different amendments on As bioavailability, in a pot experiment. Eleven samples of irrigation water, including shallow (n ) 7) and deep groundwater (n ) 4), were analyzed for total As concentration. The water samples were kept in polyethylene bottles (100 mL) and acidified in situ (1% HCl v/v) to avoid metal precipitation. Two nonacidified subsamples were refrigerated for subsequent elemental analysis (total organic carbon and total nitrogen). The soils were selected on the basis of two criteria: the present land use (irrigated or nonirrigated) and the origin of the irrigation water (shallow or deep groundwater: collected at 100 m depth, respectively). Thus, a total of 30 top-soil samples (0-20 cm depth) were collected from different agricultural plots: 11 under nonirrigated systems and 19 under irrigated systems (7 irrigated with deep groundwater and 12 with shallow groundwater). In addition, a total of 5 soils from pine forests within the same area (7.9) and higher electrical conductivity values were found in the waters from shallow-dug wells (an average of 2.5 dS m-1) than in waters from deep-drilled wells (an average of 0.73 dS m-1). The concentrations of As and the pH and electrical conductivity values in the collected waters are within the range reported by other authors in groundwater collected from the same area (10, 11). 3.2. Soil Characteristics and As Concentration and Solubility. All soils collected showed a sandy character, more than 90% of their total weight generally belonging to the coarse sand fraction (>65 µm). Low contents of CaCO3 were found in the sampled soils (SI Table S3), with the exception of two agricultural soils from the Villaverde site, which showed elevated values (>15% CaCO3). Significantly higher concentrations of Fe, Zn, and Cu were found in the agricultural soils than in the pine forest soils. By contrast, organic matter concentrations were higher in the pine forest soils (SI Table S3). The present irrigation system of the agricultural soils did not affect the general properties of the soils, as no significant differences were found in the soil pH, EC (Table 1), or Zn, Fe, Cu, or OM concentrations (SI Table S3) when non- and irrigated agricultural lands were compared. All the agricultural soils sampled were alkaline (an average of 8.1), while the soils collected from the pine forests were slightly acidic (an average of 6.1). Total As concentrations in the agricultural soils were usually within the range 0-20 µg g-1, this being the background concentration established by Garcı´a-Sa´nchez et al. (20) for natural soils of a nearby province (Salamanca, Spain). The values found in this study are also clearly lower than the guideline limit proposed for agricultural soils (50 µg As g-1, ref 21). The agricultural soils showed higher total As concentrations than the pine forest soils (3.9 and 0.5 µg As g-1, respectively), but no significant differences existed between the total As concentrations in irrigated (4.1 µg As g-1) and nonirrigated (3.7 µg As g-1) agricultural soils. Positive, significant correlations were found between total As concentrations in soils and the concentrations of Fe (r ) 0.853, P < 0.001), Zn (r ) 0.777, P < 0.001), and Cu (r ) 0.743, P < 0.001). The fraction of As adsorbed in the soil (NaHCO3extractable) was evaluated in the three soils with the highest concentrations of total As (Table 1): one from Chan ˜ e (CH1: 4.8 µg As g-1) and two from Vallelado (VA1: 12.8 µg As g-1 and VA2: 9.9 µg As g-1). Despite their different total As concentrations, the NaHCO3-extractable fractions were similar: 0.067, 0.074, and 0.086 µg g-1 in soils CH1, VA1, and VA2, respectively, accounting for 1.4, 0.6, and 0.9% of the total As. In the second sampling (November 2008), the soils collected from Chan ˜ e and Vallelado showed total As concentrations similar to those found in the first sampling (July 2008) in these locations (Table 1). The agricultural soils irrigated with deep or shallow groundwater did not show significant differences in their concentrations of total As. A high concentration of As in the agricultural soil at Vallelado was also found in the third sampling (April 2009), the total concentration being 16.9 µg As g-1. An elevated proportion of the total As concentration in this soil occurred in the adsorbed fraction (NaHCO3-extractable), reaching 1.6 µg As g-1 of soil (equivalent to 9.4% of the total As concentration). The available phosphorus in this soil was also very high (56 µg g-1). A high proportion of water-soluble As was also found in this soil (1.0 µg As g-1), accounting for 5.8% of the total As concentration. This concentration exceeds the guideline limit (0.04 µg water-extractable As g-1) proposed by Bohn et al. (22) for agricultural purposes. The concentration of water-soluble As in this soil is similar to the highest VOL. 44, NO. 24, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Total As Concentrations (µg g-1), pH, and Electrical Conductivity (EC: dS m-1) in the Soils Sampled in July 2008 and November 2008a first sampling (July 2008)
second sampling (November 2008)
present land use
Id
As
pH
EC
nonirrigated soil
SCH1 CH1 CH2 RE1 RE2 VA1 VL1
1.9 4.8 1.9 1.1 2.2 12.8 1.4
8.1 8.1 8.3 8.2 8.3 7.8 8.4
0.09 0.21 0.09 0.05 0.07 0.19 0.07
irrigated soil
SCH2 CH3 CH4 RE3 RE4 VA2 VA3 VL2
1.3 4.6 0.5 1.4 2.2 9.9 9.7 3.2
6.8 8.1 8.0 8.0 8.1 7.6 8.4 8.3
pine forest soil
SCH3 CH5 RE5 VL3
0.5 0.5 0.4 0.3
6.0 6.4 5.7 6.2
Id
As
pH
EC
CH6 VA4 VA5 VA6
5.0 12.1 12.3 15.0
8.2 8.0 8.1 8.1
0.19 0.16 0.16 0.22
0.18 0.20 0.05 0.08 0.16 2.61 0.16 0.10
CH7b CH8b CH9b CH10c VA7b VA8b VA9b VA10c VA11c VA12c
3.0 0.3 4.9 4.5 15.6 1.8 16.5 15.1 12.4 15.5
8.3 8.2 8.5 8.4 8.7 8.8 8.0 7.7 7.9 8.1
0.11 0.09 0.24 0.10 0.32 0.16 0.23 1.43 0.53 0.27
0.02 0.03 0.04 0.05
VA13
0.3
6.7
0.06
a Sites: SCH, Sanchonun˜o; CH, Chan˜e; RE, Remondo; VA, Vallelado; VL, Villaverde de Iscar. b Soils irrigated with deep and shallow groundwater, respectively. c Soils irrigated with deep and shallow groundwater, respectively.
FIGURE 1. The concentrations of total and inorganic As (µg g-1, DW) and the soil-plant transfer coefficients for As (TC ) [Asplant]/[Assoil]) in the edible crops. * Mean value of crops collected from different agricultural plots (barley n ) 5, carrot n ) 5, potato n ) 2). value (0.96 µg As g-1) reported by Moyano et al. (23) in a field survey of agricultural soils (n ) 26) from a nearby area. 3.3. Arsenic Concentrations in Plants. The total As concentration in the edible part of the crops (barley, wheat, carrot, potato, sugar beet, cabbage, green pepper, red cabbage, curly endive, and leek) ranged from 0.05 to 0.93 µg g-1 (DW; Figure 1), while As concentrations expressed on a fresh weight basis varied between 0.004 and 0.76 µg g-1. These values are within the range for vegetables and cereals set by the recent report of the European food safety authority (24). No significant correlation existed between plant total As concentration and soil total As concentration. In general, the total As concentrations found in this work were slightly lower than those reported by Moyano et al. (23) for the edible parts of wheat, carrot, sugar beet, and potato (1.3, 0.4, 2.8, and 0.5 µg g-1 DW, respectively) collected from a nearby, polluted agricultural land. 9466
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The majority of the soil-plant transfer coefficients (TC) ranged between 0.01 and 0.10 (74% of the sampled crops; Figure 1), typical values for crops (25). However, some of the sampled crops (26% of the total) showed higher TC values (0.1-0.5), which are within the range (0.2-1.6) found by Cao and Ma (26) for carrot and lettuce plants grown in As-polluted soils. Inorganic As concentrations varied between 0.04 and 0.55 µg g-1 (DW, Figure 1). When inorganic As concentrations were expressed on a fresh weight basis, cereals showed higher values (0.06-0.40 µg g-1) than vegetables (0.004-0.070 µg g-1). An important percentage of the total As concentration in the plants was found in the inorganic form (49 to 100% of the total As) (Figure 1). The proportions of inorganic As found in this study are in agreement with the results of Dı´az et al. (27), who also found high percentages of inorganic As (67-100% of the total As) in different crops from an arsenic endemic area of Chile. 3.4. Effects of the Amendments on As Solubility in the Soil and Plant Uptake. The amendments used in the pot experiment did not provoke any phytotoxic effect, and the plants showed similar yields in the controls and with the different treatments (data not shown). In general, the rolling mill scale decreased the solubility of As in soil, compared to the control, throughout the incubation experiment. The main effects of this amendment occurred at day 14 of incubation, when the soluble As was decreased, on average, 12.9, 7.6, and 6.2% in the water-soluble, (NH4)2SO4-, and NaHCO3extractable As fractions, respectively, in the rolling mill scale treatment compared to the control soil (Table 2). Compost did not produce significant effects on As solubility in the soil, while the inorganic fertilizer addition led to ambiguous results: a significant decrease in the water-soluble fraction, but also a slight increase in the NaHCO3-extractable forms of As at day 14 (Table 2). After the plant harvest, the rolling mill scale treatment also gave the lowest As concentrations in the water-soluble and (NH4)2SO4-extractable fractions. Neither compost nor fertilizer produced a significant effect on As solubility in soil
TABLE 2. Dynamics of the Soil pH and As Concentrations (µg g-1) in Different Extractable Fractions of the Soil through the Incubation Phase (days 2 and 14) and after the Harvest Time (Day 56), And Concentrations of Total As (µg g-1, DW) and the Bioconcentration Factor (BCF = [As]plant/[AsH2O]soil) in plantsa treatment parameter
fertilizer
compost
ANOVA
day
control
rolling mill scale
pH
2 14 56
7.7 7.8 8.0
7.7 7.7 8.0
7.7 7.8 7.9
7.7 7.8 8.0
NS NS NS
water-soluble As
2 14 56
1.03ab 1.01a 0.85ab
0.98b (- 4.9) 0.93b- 7.9) 0.87a (+ 2.4)
1.11a (+ 7.8) 1.00a (- 1.0) 0.86a (+ 1.2)
0.95b (- 7.8) 0.88b (- 12.9) 0.80b (- 5.9)
** *** **
(NH4)2SO4-extractable As
2 14 56
1.58 1.31a 1.57ab
1.56 1.32a (+ 0.8) 1.52b (- 3.2)
1.53 1.30a (- 0.8) 1.62a (+ 3.2)
1.43 1.21b (- 7.6) 1.41c- 10.2)
NS ** ***
NaHCO3-extractable As
2 14 56
1.55a 1.45bc 1.84
1.58a (+ 1.9) 1.62a (+ 11.7) 1.79
1.35b (- 12.9) 1.51ab (+ 4.1) 1.91
1.28b (- 17.4) 1.36c (- 6.2) 1.84
** ** NS
3.1 3.7
3.9 4.5
4.1 4.8
3.3 4.1
NS NS
Soils
Plants As concentration BCF
a Values in parentheses denote the percentage change in As concentration, calculated as (([Astreatment] - [Ascontrol]/ [Ascontrol])100). NS, ***, and **: not significant and significant at the probability levels P < 0.001 and P < 0.01, respectively. Mean values denoted by the same letter in a row are not statistically different according to Tukey’s test at P < 0.05.
after 56 days (Table 2). Total As concentrations in red cabbage were similar in all the treatments, and clearly higher than those found in the same species in the field survey (0.9 µg g-1). Similar bioconcentration factor values were found for all the treatments (Table 2).
4. Discussion 4.1. Arsenic in Collected Field Samples (Waters, Soils, and Plants). The origin of the high levels of As in the groundwater of this area has been considered geological, mainly related to As mobilization from the Fe oxy-hydroxides in the deep aquifer minerals (11). However, in this study, no significant differences were found between deep and shallow groundwater at the different sampling times. Vega et al. (28) proposed that the intense exploitation of both the deep and shallow aquifers, and the recirculation of excess irrigation water, could cause a considerable mixing of groundwater from different aquifers. Thus, the flow of As between aquifers may be facilitated by the high levels of soluble As in these soils, which can leach rapidly through soil and therefore may reach shallow groundwater. The water levels of the wells (shallow groundwater) in this area vary largely due to the balance between drainage for irrigation and recharge by rain (11). Therefore, the occurrence of a very dry period in April 2009 could have provoked an intensive evaporation and therefore a significant concentration of As in the shallow groundwater, explaining the fact that the highest As concentration was obtained in this sampling. In contrast, Calvo et al. (10) did not report statistically significant variations in the As concentrations in the irrigation waters in a field survey carried out in the affected area, which included different rainfall periods (June 2002 to July 2003). The total As concentrations and physicochemical properties of the soil were similar in nonirrigated and irrigated soils, probably due to the crop rotation, a common practice in this area that frequently alternates cereals (nonirrigated) and vegetables (irrigated) in successive seasons in the same plot. However, the higher concentrations of As in the agricultural soils than in the pine forest soils suggest that irrigation with
As-rich groundwater has led to the enrichment of As in the agricultural areas. This finding is in agreement with Moyano et al. (23), who also found higher concentrations of total As in agricultural soils irrigated with As-rich waters than in soils irrigated with nonpolluted groundwater. Although the concentrations of total As in the agricultural soils were not high, an elevated solubility of this metalloid was found. Arsenic solubility in soils generally depends more on soil texture and mineral composition (e.g., iron (hydro)oxides and clay content) than on soil total As content (1). Adsorption of As on soil mineral surfaces may not be favored in the soils studied here, due to their sandy character and their low clay and Fe contents. These results support the findings of Warren et al. (25), who observed increasing As bioavailability in the soil with increasing sand and decreasing clay content. The positive correlation between the total Fe and As concentrations suggests that iron (hydro)oxides can enhance the adsorption of arsenic in these soils, and thus decrease As solubility. However, As retention by iron (hydro)oxides and clays is limited at high pH, which could favor As solubility in these soils. Pierce and Moore (29) reported that As(V) (the main fraction of As in well-aerated soils) is preferentially sorbed to Fe (hydro)oxides between pH 4 and 7, with an optimal adsorption pH of about 4. As the soil pH increases, hydroxyl ions replace As anions on the soil sorption sites, and As is released into solution (30). The alkaline character of these soils has been associated with the weathering of some parent materials such as marls and limestone (23). The traditional use of organic amendments of an alkaline character (e.g., farmyard manure) in these agricultural soils can also have contributed to increased soil pH and these materials usually provide high amounts of phosphorus (31), both limiting As retention in the soil (32). However, the results obtained can be interpreted partly in terms of the source of the contamination. The high solubility of As in these soils is a consequence of both the addition of very-soluble As forms in the irrigation waters and the low As adsorption capacity ´ lvarez-Benedı´ et al. (32). Therefore, of these soils, reported by A an intensive use of very As-rich irrigation waters during dry VOL. 44, NO. 24, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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periods, such as April 2009, could have enhanced the mostsoluble fraction of As in the soils. Several crops of this area exceeded the transfer coefficient values usually reported in the literature for vegetables and cereals (25), indicating a potential risk of As transfer to the human food chain. The presence of high levels of easily available As forms in the soils could favor the accumulation of this metalloid in the sampled crops. This is in good agreement with Calvo et al. (33), who showed a positive correlation between As concentrations in curly endives (Cichorium endivia L.) and As concentrations in soil, in a pot experiment using As-rich irrigation waters. Therefore, an intensive monitoring network for the groundwater should be developed to avoid the use of very As-rich irrigation waters, a potential risk for As entry into the human food chain. Arsenic in plants was present mainly as inorganic forms, which are more toxic than organic species, but the consumption of edible crops from this area poses no serious risk to human health if it forms part of a normal diet (200 g fresh mass per day), according to the limit of inorganic As intake proposed by the WHO (15 µg of inorganic As kg-1 body weight per week, ref 4). 4.2. Effects of the Amendments on As Solubility in the Soil and Plant Uptake. Rolling mill scale is worth considering as an amendment to reduce As solubility in these soils. However, the immobilization effects on As found in this experiment were lower than those reported by other authors using iron oxides as amendments in As-polluted soils (25, 34). In this context, several properties of this soil, such as the high concentration of available phosphorus, the low clay content and the alkaline pH, could limit As adsorption on the surfaces of iron oxides from the rolling mill scale (7). Therefore, more-detailed studies of the kinetics of the adsorption processes in rolling mill scale-amended soils are needed to optimize its use as a low-cost adsorbent of As in this type of soil. Although it is well-known that phosphorus can increase As solubility in soils through direct competition with As for sorption sites (32) and for uptake on transport proteins of cell membranes (26), the addition of P in the inorganic fertilizer did not alter significantly the solubility of As in the soil. This could be related to the high ratio P/As in the NaHCO3-extractable fraction (56 µg g-1 of P and 1.6 µg g-1 of As) found in this soil. This is in good agreement with Lei et al. (35), who also showed an inverse relationship between the P/As ratio (1:2, 1:1, and 2:1) and As adsorption in soil. Controversial results have been found about the influence of composts on As mobility (26, 36). Beside the type of organic matter and the presence of competing ions, the pH also might play an important role in the effects of composts on As solubility in soils (37). In this experiment, compost did not change soil pH throughout the duration of the pot experiment, which could partially explain the negligible effects produced by this amendment on the solubility of As. Thus, in terms of future management, the use of this type of compost does not appear to be a suitable option for As immobilization in these soils. Very similar concentrations of As were found in plants when the different treatments were compared, this being associated with the small differences shown among treatments regarding the solubility of As in soil. It is interesting to note that, although the P/As ratio was higher in the compost- and inorganic fertilizer-amended soils than in the control, the plants did not take up more As in P-amended soil. Lower As uptake than that found in this experiment has been reported, even in soils that are heavily polluted with As (38). This finding highlights the low As sorption ability of this soil and, therefore, a potential risk of As transfer into the food chain if very rich-As waters are used to irrigate these agricultural soils. 9468
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Acknowledgments This work was funded by Caja Segovia and by the Ministerio de Ciencia e Innovacio´n through project CTM2007-66401C02-01/TECNO. We thank the Universities of Murcia, Auto´noma of Madrid and IATA-CSIC for the support in the As analysis, Dr. David J. Walker for the English revision, and Dr. Juan Cegarra and CAESA for providing the compost and the rolling mill scale used in this work, respectively.
Supporting Information Available Additional tables and figure. This material is available free of charge via the Internet at http://pubs.acs.org.
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