Arsenic Mobility in As-Containing Soils from Geogenic Origin: Fractionation and Leachability

Research Article Arsenic Mobility in As-Containing Soils from Geogenic Origin: Fractionation and Leachability Diego Martiñá-Prieto , Javier Cancelo-González , and María Teresa Barral Departamento de Edafolox´ ıa e Qu´ ımica Agr´ ıcola,FacultadedeFarmacia,CampusVida,Universidadede Santiago de Compostela, 15782 Santiago de Compostela, Spain Correspondence should be addressed to Javier Cancelo-Gonz´ alez; [email protected] Received 6 September 2017; Accepted 24 December 2017; Published 12 February 2018 Academic Editor: Nabeel K. Niazi Copyright © 2018 Diego Marti˜ n´ a-Prieto et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The lithogenic arsenic in soils of the Anll´ ons River basin (Spain) was quantified, its chemical fractions were determined, and its leachability characteristics were compared under various experimental conditions. Fifty soil samples of C horizons, sampling an area of 50 km2along the river course, were submitted to an exploratory analysis. Pseudo-total arsenic ranged between 2 and 489 mg kg−1. Arsenic leachability using the standard methods DIN 38414-S4 and TCLP was less than 0.25% of pseudo-total As. Then the effect of pH (3, 6, and 9), solid : liquid (S : L) ratio (1 : 10 and 1 : 50), phosphate (10 mM), and contact time (24 and 240 h) on arsenic mobilization was studied in nine soils with the highest As concentration. Arsenic mobilization increased at alkaline pH and lower S : L ratio; the most decisive factor was the addition of phosphate, increasing arsenic leachability up to 1,000 times, and this increased even 2.3 times when the contact time was extended from 24 hours to 240 hours. The results suggest that the mobilization of arsenic may be underestimated in short-term water leaching tests and that the environmental conditions favouring arsenic mobilization should be taken into account for a sound evaluation of the transfer risk of arsenic towards aquatic ecosystems. 1. Introduction Arsenic (As) is a metalloid widely distributed in natural environments [1]. In the earth’s crust, As is the 20th most abundant trace element [2] whose average concentration has been fixed at 1.7 mg kg−1 [3]. In soils, the baseline As concentration is generally in the order of 5–10 mg kg−1 [1], althoughtheconcentrationsinsoilsdevelopedondeposits of sulphides can reach hundreds of mg kg−1.Inparticular, arsenic is frequently a significant component in gold deposits, and gold mining has frequently been associated with arsenic contamination. The processing of gold bearing sulphide minerals which contain arsenopyrite and other complex arsenic sulphide minerals results in arsenic containing emissions and effluents which must be given careful consideration in relation to clean air and clean water standards. In fact, there are numerous cases at the global level where gold mining operations have caused a release and redistribution of arsenic into the environment [4–13]. The occurrence of geogenic As represents a major concern for groundwater quality in many countries [14]. Arsenic mobilization from rocks and soils, as well as its subsequent incorporation into aquatic systems, is of environmental concernbecausethisisahighlytoxicelementthatposesaserious threat to human and ecosystem health, especially as a result of the contamination of drinking water and food [1, 15, 16]. Several processes have been identified as being responsible for the mobilization of As from the solid phase to the aqueous phase: ion displacement, desorption (or limited sorption) at pH values above 8.5, reduction of arsenate to arsenite, and mineral dissolution, particularly reductive dissolution of Fe and Mn (hydr)oxides [17]. Pseudo-total As concentrations are not a good indicator of mobilization and potential environmental impact because As fractions differ in their solubility, particularly under different environmental conditions. Fractionation studies and leaching tests may help to assess arsenic mobilization and potential risk of transfer to aqueous systems. Sequential Hindawi Journal of Chemistry Volume 2018, Article ID 7328203, 14 pages https://doi.org/10.1155/2018/7328203 2Journal of Chemistry extraction procedures (SEP), such as those of Lombi et al. [18] or Wenzel et al. [19], make it possible to distinguish trace element fractions with different solubility, which are empirically related to mobilization classes [20]. In turn, leaching tests are used to predict both the potential mobilization of metals and metalloids to groundwater and their bioavailability to plants under natural conditions [21]. Standard tests have been developed for the evaluation of leachability that are mainly based on a washing in water under a specific soil : extractant ratio and contact time. Nevertheless, based on the geochemical characteristics of arsenic, we hypothesize that experimental conditions may drastically affect its solubility and consequently the result of this evaluation. To our knowledge, limited work has been carried out on lithogenic arsenic leaching experiments in soils [22–27] and references on As leachability are mostly focused on polluted soils of anthropic origin, mine soils and wastes, or other types of residues. In this study, pseudo-total As contents, chemical fractions, and As mobilization under various experimental conditions were determined in soils from the Anll´ ons basin, where high geogenic As concentrations were detected, with the final aim of evaluating the potential risk of As transfer to aqueous systems. The results of this study would be of interest to achieve a more comprehensive knowledge of arsenic pollution in the area under study, which is partly considered aSiteofCommunityImportance,asdefinedintheEuropean Commission Habitats Directive (92/43/EEC). The study would be also of general interest to evaluate arsenic mobilization in different environmental scenarios and to design stabilization/rehabilitation strategies in polluted areas. Also, refining of leaching tests based on the results of this study may be applied for the assessment of the environmental impact of mining operations which imply soil remobilization and accumulation in mine spoils, with risk of lixiviation of arsenic to water. 2. Materials and Methods 2.1. Study Area. The Anll´ onsRiverislocatedintheregionof Galicia (NW Spain); it has a length of about 63 km and flows into the Atlantic Ocean via the Laxe-Ponteceso R´ ıa (Figure 1). The river drains a rural catchment of 516 km2mainly dedicated to agriculture, forestry, and cattle raising activities. Two main human settlements are located in the catchment: Carballo, with a population of over 31,000, and Ponteceso, with a population of about 6,000. The river runs over schists in the upper area, turning into a smooth profile in the middle area of the river, characterized by basic rocks (gabbros and amphibolites). Finally, the lower stretch of the river runs over two-mica granite, followed by biotitic gneiss at the mouth, where the largest area of alluvial materials is also found [28] (Figure 1). The most abundant soil type in the catchment is CambicUmbrisol[29].Intheareasofscarceorganicmatter, ochric A-horizons are recognized, resulting in Dystric and Ferralic Cambisols. Leptosols and Regosols are also found in steep areas of the catchment, and Fluvisols along the fluvial banks. In the catchment, high arsenic concentrations have been detected in various environmental compartments, due to the presence of arsenopyrite (AsFeS) mineralizations associated with gold ores in hydrothermal quartz veins [30]. Arsenic concentrations in the rocks of the area are usually around 1%, but in mineralized zones with semimassive arsenopyrite they can reach up to 10%. High As concentrations have been found in the bed sediments of the Anll´ ons River, downstream of the mineralized area, with a maximum of 264 mg kg−1,which have been attributed to natural geogenic arsenic enrichment, exacerbated by gold mining activities carried out since Roman times [31–33]. At some points along the river course, these high As concentrations have been identified as being responsible for ecotoxicity [34, 35]. Arsenic concentrations exceeding ten to sixty times the legal limit for As in drinking water, set at 10 𝜇gL −1 by WHO [36], have been detected in groundwater near the mineralized area. Also, increased arsenic levels have also been detected in the river water, downstream of the mining area and at the river mouth [37]. However, in spite of these evidences, the arsenic content, chemical fractions, and leachability in the soils of the Anll´ ons basinhavenotbeenstudiedsofar,althoughtheymay contribute significantly to As in groundwater and in the riverine ecosystem, posing a potential risk for environment and human health. With the purpose of achieving the defined objectives of this study, the experimental procedure outlined in Figure 2 has been followed. 2.2. Exploratory Analysis. Fortheidentificationoflithogenic As enrichment, a set of fifty soil samples was selected from the “Geochemical Atlas of Galicia” collection [38], made up of approximately 30,000 samples, one per square kilometre, covering the entire territory of this region. Samples were takenfromCsoilhorizonstodetectlithogenicconcentrations ofseveralelementswithoutanydisturbancefromexogenous contamination. The C soil horizons were sampled in the central 10 cm, whose lower limit is generally between 60 and 80 cm depth, with the exception of the soils developed on basic rocks whose lower limit is approximately between 80 and 120 cm depth. Samples were taken using probes or directly from the soil profile exposed in roadside talus after careful elimination of the outer 25 cm. The samples selected in the present work covered an area of 50 km2at both sides of theAnll´ ons River course, with a more comprehensive study in the As-Au mineralized area. Soil samples from the collection, which had been previously milled and passed through a 200 𝜇m sieve and preserved in dry and dark conditions in air tight containers, were milled again in an agate mortar and pestle and passed through a 50 𝜇msievetocarryout the corresponding analyses. In these conditions of preservation, it is not expected that there have been changes in the concentration of the element or in its speciation. 2.2.1. Pseudo-Total Arsenic Concentrations. Pseudo-total As was determined by X-ray Fluorescence Spectrometry (custom built, equipped with a Philips high-voltage generator and a Mo anode of 2.2 kW as the X-ray source). Certified reference Journal of Chemistry 3 0 3,5 7 N 4768000 4774000 4780000 4786000 4792000 4768000 4774000 4780000 4786000 4792000 500000 510000 520000 530000 540000 500000 510000 520000 530000 540000 Schists Granodiorites Ultrabasic rocks Amphibolites Alluvial Marshes Gneiss Granite River Sample points Specific sample points 14 (km) Figure 1: Location and geology of the Anll´ ons River catchment and location of the sampling points for the exploratory analysis and detailed specific analysis. material BCR-277b (European Commission, Joint Research Centre, Geel, Belgium) was employed to check the accuracy of pseudo-total As measurements. The As concentration (mg kg−1) obtained for this certified reference material was 45.4 ±4.1 (certified value 47.3 ±1.6). 2.2.2. Mapping of Pseudo-Total Arsenic Contents. Spatial data analysis was carried out using ArcGIS 10.3 software (ArcGIS Desktop: Release 10, Environmental Systems Research Institute, Redlands, CA). Inverse distance weighting (IDW) was used in this research to interpolate pseudo-total As contents. 4Journal of Chemistry Evaluation of soil As fractionation and leachability under various environmental conditions Exploratory analysis (50 soil samples of C- horizon from Anllóns River catchment) Total As contents Detailed analyses (9 soil samples with the highest As contents) As fractionation As leachability TCLP Effects of changes in the environmental conditions Effect of pH Effect of P and time As solubility in water Effect of S :Lratio Figure 2: Scheme of the experimental procedure. All interpolation methods were developed based on the theory that points closer to each other have more correlations and similarities than those farther away [39]. IDW is a linear combination of the observed values, inversely weighted by the distances of the observation locations from the interpolation point. This interpolation method is based on the assumption that the influence of a known data point is inversely related tothedistancefromtheunknownlocationthatisbeing estimated [40]. 2.2.3. Arsenic Solubility in Water. The German standard method DIN 38414-S4 [41] for the examination of water, Journal of Chemistry 5 waste water, sediments, and sludge by the determination of leachability by water was followed, using Milli-Q water at a 1 : 10 soil : water ratio and 24 h end-over-end agitation. Subsequently, extracts were filtered through 0.45 𝜇m syringe filters (WhatmanPuradisc25 polyethersulfone filter devices, General Electric Company, Freiburg, Germany) and frozen (−80∘C) until analysis of pseudo-total As concentrations using Inductively Coupled Plasma Spectrometry (ICP-MS). A Varian 820-MS ICP-MS, equipped with collision reaction interface (CRI) technology to reduce polyatomic interferences, was employed for this purpose (Varian Inc., Palo Alto, CA). Iron concentrations were also determined in the extracts. The detection limits for As and Fe were 3.38 ng L−1 and 85.06 ng L−1, respectively. The certified reference material EnviroMat Drinking Water EP-H-1 (catog. number: 140-025- 032, SCP Science) was used for quality control. 2.3. Detailed Analyses of Arsenic Speciation and Leachability. Nine samples with the highest As contents were selected for a more detailed study of As mobilization by determining As solid fractions using a fractionation sequential procedure and solubility under different experimental conditions. Soil samples were numbered according to their geographical location from left to right and top to bottom. 2.3.1. As Fractionation. The sequential extraction procedure (SEP) described by Lombi et al. [18] was applied using 1 g of soil and 25 mL of each extractant. This procedure considers the following “operationally defined” chemical pools: F1: exchangeable (0.05 M (NH4)2SO4, 1 h shaking), F2: specifically sorbed (0.05 M NH4H2PO4, 1 h shaking), F3: associated with organic matter and Al (0.05 M NH4FpH7.0, 1 h shaking), F4: bound to amorphous Fe oxides (0.2 M NH4- oxalate buffer pH 3.25, 4 h shaking in the dark), F5: bound to crystalline Fe oxides (0.2 M NH4-oxalate buffer + 0.1 M ascorbic acid pH 3.25, 30 min in a water bath at 96∘C), and F6: residual phase (calculated as the difference between pseudototal As content and the sum of As extracted from steps F1 to F5). The extracts resulting from each phase were centrifuged at 5,000 rpm for 15 min at room temperature, filtered using 0.45 𝜇m Whatman Puradisc 25 polyethersulfone syringe filters, and frozen (−80∘C) until analysis of pseudo-total As concentrations using ICP-MS. All reagents were prepared with deionized Milli-Q water. All experiments were run in triplicate and blanks were run simultaneously. Pseudo-total As content of these specific samples was determined by microwave-assisted acid digestion at 150∘C. To this end, 0.2 g soil was introduced into Teflonmicrowave digestion vessels with 10 mL acid solutions formed by 9 mL HNO3(conc)and 1mL HF (conc)[33]. After digestion, extracts were made up to 100 mL with 2.5% H3BO3solution added to “neutralize” theexcessHFandcomplexfluoride(formingtetrafluoroboric acid) in solution, as described by Wilson et al. [42]. This dilution with H3BO3solution also makes it possible to maintain the HNO3concentrations below 10%, as required for ICP-MS measurements. Extracts were filtered using 0.45 𝜇m Whatman Puradisc 25 polyethersulfone syringe filters and frozen and pseudo-total As concentrations were measured using ICP-MS. 2.3.2. Arsenic Leachability (1) TCLP. Arsenic mobilization was estimated by applying the Toxicity Characteristic Leaching Procedure (TCLP) standard method, which simulates worst-case leaching conditions of landfill wastes and represents a method to evaluate the potential toxicity of a waste material. The EPA Method 1311 [43] was applied, consisting of 24 h extraction in Milli-Q water adjusted with acetic acid at pH 4.5, using a 1 : 20 soil : water ratio. After the extraction step, the suspensions were centrifuged at 2,000 rpm for 15 min and the extracts were filtered using 0.45 𝜇m Whatman Puradisc 25 polyethersulfone syringe filters and frozen until analysis of As concentration using ICP-MS. (2) Effect of Changes in the Environmental Conditions. The effect on As mobilization of solid : liquid ratio, pH, phosphorous, and time was also evaluated. The effect of solid : liquid ratio on As mobilization was tested in a 24 h extraction for 1:10and1:50ratiosinMilli-Qwater.TheeffectofpHwas studied at pH values 3, 6, and 9, using 1 : 10 solid : liquid ratios and 24 h end-over-end shaking. The pH was adjusted with either 1 M HNO3or 1 M NaOH; a saline background of 0.01 M NaNO3in Milli-Q water was used to counteract the differences in ionic strength due to the addition of acid or base and to more closely simulate the conditions of the soil solution. The effect of P (added as NaH2PO4)onAsmobilization was evaluated by performing an extraction of 1 g soil in 10 mL of 0.01 M P prepared in 0.01 M NaNO3; the suspensions were end-over-end-shaken for 24 and 240 h to simultaneously evaluate the effect of phosphate and extraction time. After the extraction steps, all extracts were filtered using Whatman Puradisc 25 polyethersulfone syringe filters and frozen (−80∘C) until analysis of pseudo-total As concentrations using ICP-MS. All experiments were run in triplicate and with the corresponding blanks, at room temperature (20 ±2∘C). 2.4. Statistical Analyses. Arsenic mobilization for the aforementioned environmental conditions was evaluated using either Student’s 𝑡-test analysis or one-way analysis of variance (ANOVA). Single Pearson correlations were calculated to analyse the possible relationships between pseudo-total As content and As mobilized under the different environmental conditions. Principal component analysis (PCA), a multivariate statistical technique widely used as a tool for reducing the number of dimensions, was applied for a comprehensive study of As mobilization in soil samples. Namely, pseudototal As and Fe, As fractionation, and As mobilization under different environmental conditions, as well as Fe mobilization in phases F4 and F5 of As fractionation, were introduced as inputs in the PCA analysis. The SPSS 20.0 statistical package was used for all the statistical analyses 6Journal of Chemistry 502000 508000 514000 520000 526000 502000 508000 514000 520000 526000 4775000 4775000 4780000 4780000 4785000 4785000 4790000 4790000 0361,5 As (km) 2–60 61–120 130–180 190–240 Anllóns catchment 250–300 310–360 370–420 Sample points River 430–490 N Figure 3: Mapping of pseudo-total As. (IBM SPSS Statistics for Windows, Version 20.0. Armonk, NY). 3. Results and Discussion 3.1. Exploratory Analysis. The spatial distribution of As in the soil samples from the C horizons is represented in Figure 3. Pseudo-total As concentrations varied between 2 and 489 mg kg−1,withameanvalueof85mgkg −1.Theconcentrations were within the range of those detected by Devesa-Rey et al. [31] in polluted sediments of the Anll´ ons River, which reached a maximum value of 264 mg kg−1. The statistical distribution of As concentrations can be visually inspected using a box plot (Figure 4(a)). The 25th, 50th, and 75th percentiles (14, 25, and 94 mg kg−1, resp.) were represented as lines in vertical boxes, with error bars representing the 10th and 90th percentiles (5 and 344 mg kg−1, resp.).FivesoilsampleslocatedintheAu-Asmineralizedarea [28] exceeded the 90th percentile and may be considered outliers. The value inferred from the 25th percentile (14 mg kg−1) canbeconsideredalocalbackground[44],whichisslightly lower than the limit allowed for agricultural soils in Galicia, set at 25–30 mg kg−1 [45], whose upper limit was surpassed by 23 samples. The general reference level for As, fixed at 50 mg kg−1 for soils in the Spanish region of Galicia [46], was exceeded in 21 soil samples (42%). Concentrations surpassing 50 mg kg−1 are also considered dangerous in several countries and above this threshold soil remediation procedures are recommended [47]. Soluble As concentrations obtained using the German DIN 38414-S4 [41] method ranged between 0.13 and 9.85 𝜇gL −1, which are much lower than the acceptable leaching limit values for inert waste landfills determined under similar conditions [48]. Expressed on a dry weight basis, the As extracted ranged between 1.24 and 98.45 ng kg−1, with a mean value of 8.96 ng kg−1.The25th,50th,and75th percentiles corresponded to 1.94, 3.88, and 8.10 ng kg−1, respectively (Figure 4(b)), while the 10th and 90th percentiles corresponded to 1.65 and 20.05 ng kg−1,respectively.Out of five outliers identified for soluble As, only one was also Journal of Chemistry 7 0 100 200 300 400 500 600 Total As content (mg kg−1) (a) 0,00 0,02 0,04 0,06 0,08 0,10 0,12 Water-soluble As (mg kg−1) (b) 0 2 4 6 8 10 12 0 100 200 300 400 500 Total As content (mg kg−1) Water-soluble As (ug ，−1) (c) Figure 4: (a) Box plot for pseudo-total As content. (b) Box plot for water-soluble As. (c) Mobilization of water-soluble As in function of pseudo-total As contents in soil samples. The median and 25th and 75th percentiles were represented as lines in vertical boxes, with error bars representing the 10th and 90th percentiles. an outlier for pseudo-total As. The percentage of pseudototal As mobilized in these experimental conditions was very low and ranged between 0.002 and 0.03%. No correlation between pseudo-total As and water-soluble As was observed (Figure 4(c)). 3.2. Detailed Analyses of As Speciation and Leachability. Nine samples with the highest As contents (108–489 mg kg−1)were submitted to a more detailed study including As fractionation and As leachability in various extractants. In these soil samples,thepHvariedbetween5.6and7.1(Table1),Mnconcentration ranged between 0.2 and 3.0%, and pseudo-total Fe content varied between 1.3 and 6.7%. No correlation was found between pseudo-total As and Fe either in the selected samples or in the exploratory analysis (data not shown). Semiquantitative mineralogical analysis of soil samples using X-ray diffraction did not identify As minerals. Soil organic matter was not determined because this component is scarce in soil C horizons. 3.2.1. As Fractionation. The results of As fractionation following the procedure of Lombi et al. [18] are shown in Table 1. The following decreasing order of abundance of the fractions was observed: F5>F4∼F6>F2∼F3> F1. The predominant fraction was F5, which corresponds to As bound to crystalline Fe oxides, with percentages varying between 37.4 and 60.7% with respect to pseudo-total As, followed by F4 (As bound to amorphous Fe oxides), with percentages varying between 15.8 and 39.8%, and F6 (As in the residual phase), with values between 2.1 and 30.7%. Arsenic associated with aluminium and organic matter (F3) accounted for 2.2–6.4% of pseudo-total As and was similar to specifically sorbed As (F2) (between 2.4 and 5.7% of pseudototal As). The exchangeable As (F1) was the least abundant fraction, accounting for only 0.01–0.05% of pseudo-total As. In summary, the results of the fractionation procedure revealed that As was mainly associated with amorphous and crystalline Fe oxides, which together represent 63 to 86% of pseudo-total As and justify the low water-soluble As concentrations. Only 2–6% of pseudo-total As corresponded to F1 and F2 fractions that are considered the most mobile [49]andresponsibleforAstoxicityinsoils[50]. The As distribution in the soils of the Anll´ ons Basin was slightly different from that observed in the bed sediments of 8Journal of Chemistry Table 1: Soil pH, pseudo-total Fe, Mn, and As content, and As fractionation in soil samples following the SEP procedure described by Lombi et al. (1999). Sample pH FeXRF (%) MnXRF (mg kg−1) AsT,XRF (mg kg−1) F1 (mg kg−1) F2 (mg kg−1) F3 (mg kg−1) F4 (mg kg−1) F5 (mg kg−1) F6 (mg kg−1) 1 6.4 2.2 313 489 0.13 (0.04%) 12.19 (3.70%) 20.87 (6.33%) 86.28 (26.18%) 123.37 (37.43%) 86.71 (26.31%) 2 6.6 3.5 401 387 0.16 (0.05%) 13.74 (3.95%) 10.44 (3.00%) 75.50 (21.72%) 184.79 (53.17%) 62.91 (18.10%) 3 7.1 4.6 533 194 0.08 (0.04%) 5.86 (3.49%) 5.70 (3.39%) 28.70 (17.08%) 102.01 (60.71%) 25.69 (15.29%) 4 6.8 4.5 531 213 0.07 (0.05%) 6.21 (3.94%) 5.42 (3.44%) 29.33 (18.60%) 92.83 (58.87%) 23.82 (15.11%) 5 6.3 3.0 372 359 0.09 (0.03%) 8.82 (2.61%) 9.33 (2.76%) 56.89 (16.82%) 168.76 (49.89%) 94.35 (27.89%) 6 6.2 3.2 324 413 0.08 (0.02%) 9.32 (2.44%) 8.43 (2.21%) 71.52 (18.76%) 201.13 (52.75%) 90.83 (23.82%) 7 6.0 2.9 329 425 0.08 (0.02%) 9.53 (2.42%) 9.61 (2.44%) 77.11 (19.60%) 225.82 (57.39%) 71.31 (18.12%) 85.61.3200169 0.06 (0.05%) 7.21 (5.72%) 8.12 (6.43%) 50.21 (39.78%) 57.98 (45.94%) 2.68 (2.13%) 9 6.9 6.7 2981 108 0.01 (0.01%) 3.18 (3.46%) 2.66 (2.89%) 14.56 (15.83%) 43.35 (47.15%) 28.21 (30.68%) Figures in parentheses in the fractionation analysis indicate percentage of pseudo-total As solubilised in each step. thisriver,wheretheresidualphasepredominates(upto75% of pseudo-total As in some samples), followed by As bound to Fe oxides, representing up to 55% of pseudo-total l As [31, 33]. However the results of the fractionation are in agreement with those found in vineyard soils by N´ ovoa-Mu˜ noz et al. [51], who have observed a predominance of the As fraction associated with crystalline Al and Fe oxides; they also agree with those of Moreno-Jim´ enez et al. [49] who also have found a predominance of As retained by Al and Fe hydrous oxides in soils adjacent to an old mine site and with those of Li et al. [25] analysing the speciation of geogenic arsenic in soils. 3.2.2. Arsenic Leachability. The results of the extraction in TCLP showed low concentrations of soluble As (0.27 and 1.63 𝜇gL −1), with the exception of sample 1, for which the As concentration in the extracts reached 41.86 𝜇gL −1 (Table 2). These concentrations were much lower than the maximum allowable concentration for TCLP extracts, fixed at 5 mg L−1 byUSEPA[52],andalsofarfromEC 20 determined by Rubinosetal.[53]forAs Vand AsIII using the Microtoxacute toxicity bioassay (which is usually conducted with TCLP extracts), set at 4.4 and 10.2 mg L−1,respectively. Expressed in dry weight basis, TCLP-soluble As ranged from0.01 to 0.03 mg kg−1, which represents 0.006 to 0.012% of pseudo-total As; sample 1 was an exception, because soluble arsenic reached 0.84 mg kg−1 and represented 0.25% of pseudo-total As. The low As solubility in the studied soils can be explained by the predominant association of As with Fe oxides that are poorly soluble in the acetic solutions (Table 3) (the highest concentration of solubilized Fe was 5.00 mg kg−1 representing 0.008% of pseudo-total Fe). Arsenic concentrations in TCLP extracts were in the order of those obtained in the DIN 38414-S4 [41] extracts and significant positive Pearson correlations (𝑝 < 0.01) were found between the two standard methods (Table 4). Nevertheless, none of the methods showed significant positive correlations with pseudo-total arsenic. For the evaluation of the influence of environmental conditions on As mobilization, the effect of solid : liquid ratio was first investigated in a 24 h extraction for 1 : 10 and 1 : 50 ratios in Milli-Q water. Arsenic concentrations in the extracts were significantly higher (𝑝 < 0.05) for the 1 : 50 S : L ratio than for the 1 : 10 S : L ratio (Figure 5(a)). Notwithstanding, even for the highest concentration, they were much lower than the leaching limit values established for waste acceptable at landfills, set at 0.05 mg L−1 for inert waste [48]. Expressed on a dry weight basis, As mobilization increased approximately 7 to 29 times with the decrease in S : L ratio (Table 2). Thus, whereas for a 1 : 10 ratio it varied between <0.01 and 0.10 mg kg−1 (<0.01% to 0.03% of pseudo-total As), for a 1 : 50 ratio it ranged between 0.05 and 1.10 mg kg−1 (0.05 to 0.33% of pseudo-total As). Increasing As solubility with increasing liquid : solid ratio was also observed for sediments of the Anll´ ons River and related to higher risk of As mobilization during resuspension events [33]. For the study of the effect of pH, As solubility in 0.01 M NaNO3at pH values 3, 6, and 9 was tested (Table 2). Arsenic leachability increased significantly (𝑝 < 0.05)with increasing pH and was 6 to 78 times higher at pH 9 than at pH 3. The maximum percentage of As released, observed at pH 9, reached 0.71% of pseudo-total As, whereas the lowest percentage,observedatpH3,onlyreached0.001%.The environmental relevance of the pH effect on As solubility is revealed in Figure 5(b), where it can be observed that for six soil samples As concentration in the extracts at pH 9 exceeded Journal of Chemistry 9 Table 2: As leachability in TCLP and the effect of pH, solid : liquid ratio, phosphorous, and time on As mobilization. Sample AsT1 (mg kg−1) TCLP EffectofS:Lratio EffectofpH EffectofPandtime AsTCLP (mg kg−1) As1:102 (mg kg−1) As1:50 (mg kg−1) AsPH=3 (mg kg−1) AsPH=6 (mg kg−1) AsPH=9 (mg kg−1) As𝑡=24h (mg kg−1) As𝑡=240h (mg kg−1) 1 329.61 0.84 (0.25%) 0.10a (0.03%) 1.10b (0.33%) 0.12a (0.04%) 0.36b(0.11%) 0.73c (0.22%) 10.50a (3.19%) 17.05b (5.17%) 2 347.53 0.03 (0.007%) 0.02a (0.005%) 0.52b (0.15%) 0.01a (0.003%) 0.10b(0.03%) 0.67c (0.19%) 10.97a (3.16%) 16.23b (4.67%) 3 168.04 0.02 (0.01%) 0.04a (0.02%) 0.26b (0.16%) 0.01a (0.006%) 0.12b(0.07%) 0.38c (0.23%) 3.63a (2.16%) 8.48b (5.05%) 4 157.68 0.03 (0.02%) 0.04a (0.03%) 0.51b (0.32%) 0.02a (0.01%) 0.11b(0.07%) 0.38c (0.24%) 4.54a (2.88%) 7.53b (4.77%) 5 338.24 0.03 (0.009%) 0.01a (0.004%) 0.20b (0.06%) 0.01a (0.003%) 0.11b(0.03%) 0.62c (0.18%) 8.13a (2.40%) 14.34b (4.24%) 6 381.30 0.03 (0.009%) 0.01a (0.002%) 0.12b (0.03%) 0.01a (0.003%) 0.09b(0.02%) 0.96c (0.25%) 8.62a (2.26%) 13.38b (3.51%) 7 393.47 0.02 (0.005%) 0.01a (0.003%) 0.10b (0.02%) 0.01a (0.003%) 0.08b(0.02%) 0.49c (0.13%) 8.21a (2.09%) 14.38b (3.65%) 8 126.21 0.02 (0.02%) 0.01a (0.006%) 0.07b (0.05%) 0.01a (0.01%) 0.18b(0.14%) 0.89c (0.71%) 3.75a (2.97%) 3.34a (2.64%) 9 91.96 0.01 (0.006%) 3.7 10−3a (0.004%) 0.05b (0.05%) 810−4a (0.001%) 0.01b(0.01%) 0.05c (0.06%) 1.99a (2.16%) 2.05a (2.23%) 1Pseudo-total As content determined by microwave digestion used for the calculation of the percentages of As extracted; 2As extracted using DIN 38414-S4 [38]; a,b,csuperscript letters indicate significant differences (𝑝 < 0.05) between the tested experimental conditions for each parameter; figures in parentheses indicate the percentage of As mobilized with respect to the pseudo-total As concentrations. Table 3: Fe leachability in TCLP and the effect of pH, solid : liquid ratio, phosphorous, and time on Fe mobilization. Sample FeT(%) TCLP Effect of S : L ratio Effect of pH Effect of P and time FeTCLP (mg kg−1) Fe1:10 (mg kg−1) Fe1:50 (mg kg−1) FePH=3 (mg kg−1) FePH=6 (mg kg−1) FePH=9 (mg kg−1) Fe𝑡=24h (mg kg−1) Fe𝑡=240h (mg kg−1) 12.90 1.73 (0.01%) 0.02 (<0.01%) 0.22 (<0.01%) 1.74 (0.01%) 0.07 (<0.01%) 0.56 (<0.01%) 0.10 (<0.01%) 0.20 (<0.01%) 2 4.97 0.40 (<0.01%) 0.01 (<0.01%) 1.67 (<0.01%) 3.96 (0.01%) <d.l.2<d.l.2<0.01 (<0.01%) 0.02 (<0.01%) 3 6.19 0.77 (<0.01%) 0.85 (<0.01%) 0.43 (<0.01%) 8.34 (0.01%) 0.04 (<0.01%) 0.26 (<0.01%) 0.03 (<0.01%) 0.07 (<0.01%) 4 6.50 5.00 (0.01%) 1.01 (<0.01%) 0.54 (<0.01%) 5.67 (0.01%) <d.l.21.62 (<0.01%) 0.03 (<0.01%) 0.10 (<0.01%) 5 4.47 0.59 (<0.01%) 0.06 (<0.01%) 0.34 (<0.01%) 1.19 (<0.01%) <d.l.20.66 (<0.01%) 0.10 (<0.01%) 0.10 (<0.01%) 6 4.38 1.01 (<0.01%) <0.01 (<0.01%) 0.12 (<0.01%) 2.79 (0.01%) <d.l.21.59 (<0.01%) 0.15 (<0.01%) 0.08 (<0.01%) 7 4.26 0.74 (<0.01%) 0.01 (<0.01%) 0.09 (<0.01%) 1.62 (<0.01%) <d.l.20.16 (<0.01%) 0.08 (<0.01%) 0.10 (<0.01%) 81.20 0.50 (<0.01%) 0.05 (<0.01%) 0.17 (<0.01%) 2.22 (0.02%) <d.l.21.63 (0.02%) 0.43 (<0.01%) 0.43 (<0.01%) 9 6.59 <d.l.10.03 (<0.01%) 0.66 (<0.01%) <d.l.2<d.l.2<d.l.20.03 (<0.01%) 0.05 (<0.01%) 1d.l.:detectionlimitofFeinTCLPmethodequalto1.7 ⋅ 10−3 mg kg−1;2d.l.: detection limit of Fe in the study of pH effect equal to 8.5 ⋅ 10−4 mg kg−1;figures in parentheses indicate the percentage of Fe mobilized with respect to the pseudo-total Fe concentrations. or was close to the leaching limit value for waste acceptable at landfills for inert waste, set at 0.05 mg L−1 [45]. These results agree with other studies reporting the higher solubility of As at alkaline pH in soils and sediments [32, 33, 54, 55]. Alkaline pH values favour As mobilization due to the displacement of As by hydroxyl ions competing for sorption sites and the more negative surface charges of active soil components at this pH, which hinder the subsequent adsorption. In cases where higher As mobilization has been observed at acidic pH in comparison with circumneutral pH values, it has been