Arsenic in Soils: Natural Concentration and Adsorption by Oxisols Developed From Different Lithologies

Arsenic (As) is a toxic and a carcinogenic element naturally occurring in the environment. Therefore, studies on As natural concentration in soils and its adsorption process are important tools for the evaluation of potential risks of soil contamination in order to adoption of control actions or monitoring of potential As-contamination sources. The objective of this study was to evaluate the natural levels of As and determine the maximum adsorption capacity of As (MACAs) of six Oxisols (Latossolos) of Minas Gerais State, Brazil, developed from different lithologies. The soil sample’s total As content was determined using the USEPA 3051A method. The adsorption experiments were performed using different As concentrations, and the MACAs was measured by the Langmuir isotherm. On average, the natural As content in Oxisols was 13.13 mg kg, which is above the reference value of soil quality (RVQ) for As, in Brazil (8 mg kg). The levels of As in Oxisols originated from metamorphic/igneous rocks were significantly higher than those of Oxisols from sedimentary rocks. Globally, the evaluated soils showed a mean MACAs equal to 2,548 mg kg. Soil horizon Bw showed a higher MACAs than that of A horizon. In general, the levels of clay, iron oxides, iron forms (especially poorly-crystallized) and organic carbon had a positive influence on MACAs. Although the RVQ for As is well below the MACAs in all soils, the soil adsorbed As naturally present, rendering it unavailable in the soil aqueous phase. Therefore, there was no risk of contamination for human health.


Introduction
Arsenic (As) is a toxic and carcinogenic element.After the analytical detection of elements in solution has been improved in the last decades, relationships were established between very low As concentrations and (i) some cancer forms (skin, lung, liver, bladder, kidney, and colon), (ii) other common diseases (cardiovascular diseases, diabetes, and anemia), (iii) neurological changes, and (iv) reproductive and immunological dysfunctions in humans (Aviñó et al., 2008).This metalloid is widely distributed in the environment and occurs in the Earth's crust at concentrations between 0.5 and 2.5 mg kg -1 .However, contents above 13 mg kg -1 can be found in clayey sediments (Kabata-Pendias & Mukherjee, 2007).According to the World Health Organization (WHO, 2001), the levels of As naturally occurring in soils vary from 1 to 40 mg kg -1 , with an average of 5 mg kg -1 .Arsenic mobility depends largely on geochemical environmental characteristics, mainly pH, redox potential, and adsorption and desorption equilibrium, which are controlled by soil mineral and organic matrices.Oxide surfaces, especially iron (Fe), aluminum (Al), and manganese (Mn) oxides, have a high affinity for As (Cui & Weng, 2013), influencing the mobility of this element in the environment.Brazilian climatic conditions increase the weathering degree of Oxisols (Latossolos), wherein the clay fraction is predominantly composed of kaolinites, and Fe and Al oxides (Fontes & Weed, 1991).jas.ccsenet.
Given the defined gu reference (RVI).The (Biondi et Gerais, the 2009 The soils profiles were characterized using samples collected at depths of 0-20 cm (A horizon) and 50-70 cm (B horizon), according to soil description and collection criteria (Santos et al., 2005).Two samples of approximately 2 kg were collected from the central part of each soil surface (A) and subsurface (Bw) horizons, characteristic of each profile.The soil samples were dried, crushed, and sieved (2-mm mesh) to obtain the air-dried fine earth (ADFE).Soil was classified according to the Brazilian System of Soil Classification (EMBRAPA, 2013) and the U.S. Soil Taxonomy (Soil Survey Staff, 2014) (Table 2).

Natural Levels of Arsenic in the Soils
In the laboratory, soil sub-samples were finely ground in an agate mortar in triplicate and then digested using the USEPA 3051A method (USEPA, 2007) in a microwave oven Milestone Ethos Pró (Milestone, Sorisole, Italy) for the determination of As natural contents.The determination was performed using standard samples (SS-1) (EnviroMAT SCP Science®, Baie D'Urfe, Quebec, Canada) with certified values as controls.
Analytical extracts were used to measure As natural contents by inductively coupled plasma-optical emission spectrometry (ICP-OES Optima 3.300 DV, Perkin-Elmer, Norwalk, CT, USA) with a limit of detection of 7 µg L -1 .Calibration curve solutions (0 to 100 ppm) were prepared from the dilution of the Merck-Titrisol As standard (1000 ppm).HNO 3 (Merck, Rio de Janeiro, Brazil) was used for the preparation of the solutions and preservation and dilution of the samples.Arsenic levels obtained in the analyses were compared using the Student's t-test for non-homogeneous variances at 1% probability level.

Arsenic Adsorption
Arsenic adsorption was assessed in triplicate for the A horizon and B horizon soil samples, using 0.001 mol L -1 NaNO 3 solutions (Merck, Rio de Janeiro, Brazil) containing Na 2 HAsO 4 •7H 2 O (Sigma-Aldrich, São Paulo, Brazil) at the following concentrations: 0.00, 0.08, 0.16, 0.24, 0.40, 0.56, 0.72, 0.88, 1.12, 1.36, and 1.60 mmol L -1 of As (V), with pH adjusted to 5.5.A mass of 0.5 g of each soil was added to 50 mL of As(V) solution, in polypropylene tubes.These soil suspensions were mixed by vertical stirring at 45 rpm for 24 hours.The suspensions were centrifuged, and the supernatants were filtered, for determination of As contents by ICP-OES.
The maximum adsorption capacity of As(V) (MACAs) was evaluated using the Langmuir isotherm, according to Equation 1.
where, x/m is the amount of element adsorbed per unit of weight of absorbent (mg g -1 ), b is the maximum adsorption capacity of element by the soil (mg g -1 ), C is the element concentration in the equilibrium solution, or final solution, after the adsorption experiments (mg L -1 ), and a is a constant related to the binding energy between element and the soil (L mg -1 ) (Fontes, 2012).

Soil Physical Characterization (Grain Size Distribution)
The evaluated soils presented a well-differentiated textural class, ranging from sandy-loam to clayey (Table 4).Red-Yellow Oxisol (LVA1, LVA2, and LVA3) presented high levels of clay, with a texture ranging from clayey to very clayey.On the other hand, Red Oxisol (LV4, LV5, and LV6) presented a wide range of clay contents, from sandy-loam to very clayey, which may be due to the heterogeneity of their source materials.-----------------dag kg -1 -------------- The maximum adsorption capacity of As (MACAs) varied between 3580.0 and 1769.0 mg kg -1 .The highest value was found in a soil developed from mafic/ultramafic rocks (LV4-Bw), while the lowest one was observed in a soil developed from sandstones (LV6-A).The LVA presented a mean MACAs of 2541.7 mg kg -1 , which is slightly lower than that of the LV soil class (2556.5 mg kg -1 ).The overall mean of MACAs of the six soils studied here was 2549.1 mg kg -1 .

Discussion
Physical, chemical and/or mineralogical differences between soil classes may be due to the heterogeneity of their parent material and it are characteristics that promote differences in natural levels of As present in the soils and in the As adsorption by soils.As mentioned, the MACAs of LV is slightly higher than that presented by LVA (Table 6).However, natural levels of As are lower in LV class when compared to those of LVA (Table 5).The LV soil class (the one with the lowest overall As content) presented both the highest MACAs in LV4 and the lowest MACAs in LV6.These results indicate that the natural levels of As in this soil class had no direct influence on As adsorption.
The Bw horizon of the studied soils presented As adsorption higher than that of surface horizons.It might be due to the lower total organic carbon (TOC) contents of Bw in relation to the A horizons, which reduces competition for adsorption sites, besides a higher content of poorly crystallized Fe, as evidenced by the high values of Fe OX content and Fe OX /Fe D ratio (Table 3).
Iron and aluminum oxy-hydroxides, especially those poorly crystallized, with high surface area, have favorable characteristics for As adsorption.Chemical reactivity of inner-coordinate surface functional groups makes these surfaces strong sorbents of As (McBride, 1994).Since the adsorption process, involves the strong affinity of the Fe and Al oxy-hydroxides, for As, it is strongly attracted to the adsorption sites on solid surfaces and is effectively removed from solution (Johnston & Heijnen, 2001).
The LV4 presented the highest MACAs, which was due to its very clayey texture (Table 4) together with a predominance of Fe oxides, as it is developed from mafic/ultramafic materials (Table 2).This Fe was predominantly in a poorly crystallized form (Table 3), which confers a high specific surface to this mineral and hence to the soil, thus increasing adsorption sites.LV6, which was developed from sandstones (Table 2) and has a sandy-loam texture, stands out for the lowest MACAs of the study (1769.0 and 1780.0 mg kg -1 , respectively for A and Bw horizons).In this soil, allied to the low amount of clay was a predominance of Fe in well-crystallized form, that is, a high Fe D value (Table 3), which seems to corroborate its lower As adsorption capacity.
Soil texture influence on As adsorption was evidenced in the evaluation of MACAs in diverse Japanese soils (Sultana & Kobayashi, 2016).The authors found the highest As adsorption in a clay-loam soil, followed by a loam soil, and last, by adsorbing less, a sandy-loam soil.
Similar results were found in the evaluation of MACAs using as adsorbent the clay fraction of different soils located in Antarctica (Poggere et al., 2017).These authors obtained a mean MACAs of 3554.0 mg kg -1 and they concluded that a soil mineralogical composition is more important than its clay content for MACAs evaluations since poorly-crystallized Fe and Al oxides are mainly responsible for As adsorption.It is worth mentioning that Fe oxides and hydroxides, especially those with low crystallinity, have favorable characteristics as anion adsorbents for having numerous OH -ions on the surface.These ions have unsatisfied valences responsible for exchange of binders and electrostatic attraction interactions.Studies using Fe oxides, such as goethite, for remediation of As-contaminated water sources concluded that As retention on goethite surface occurs by diverse complexation mechanisms: (i) predominantly monodentate complexation, for low-load adsorbent surfaces; (ii) bidentate bonding, for high-load surfaces, and (iii) mono and bidentate bonding, for medium-load adsorbent surfaces (Fendorf et al., 1997).
The evaluation of As adsorption in three different soil classes in Taiwan showed that MACAs increased as the amount of Fe, Al, and Mn oxides increased in the soils (Chien et al., 2015).Studies in the literature show similar results and conclude that one of the leading causes of As(V) adsorption to adsorbent materials is a high concentration of oxides, such as Fe oxides (Zhang et al., 2013).
The values of MACAs lower than a mean of this study (2549.1 mg kg -1 ) were found in one study that evaluated 17 Brazilian Oxisols, whose mean MACAs was 2013.0 mg kg -1 (Campos et al., 2007).The authors concluded that the largest MACAs were found in soils rich in clay and Fe and Al oxides.
Such statements corroborate the fact that one of the aspects culminating in a higher or lower As adsorption in soils refers to the content of oxides therein.The evaluated Oxisols presented a high adsorption capacity for As, which is common in this soil class.In a study of three soil classes in China, the authors inferred that soils classified as Oxisol have the highest MACAs if compared to a Mollisol (Chernossolo) or to those derived from volcanic rocks (Feng et al., 2013).
Assessing the relationship between physical and chemical properties, another study on 16 Chinese soils (Jiang et al., 2005) concluded that 93.8% of As(V) adsorption variability in soils can be described by their contents of TOC, clay, and Fe D .In soils with higher binding energy, 87.7% of As(V) adsorption was attributed to TOC, clay content, Fe D contents and to As natural concentrations.However, regardless of binding energy, these authors stated that the most influencing characteristic for As(V) adsorption to soil is related to Fe D .
Conversely, our findings showed that As adsorption capacity in Oxisols was mainly related to factors such as soil texture, TOC, and Fe forms (well or poorly crystallized) since the clay-fraction mineralogy of the studied soils remained relatively unchanged throughout the soil profile.For example, LV4 (soil with higher MACAs) and LV6 (soil with the lowest MACAs) were composed of the same minerals, e.g., kaolinite, gibbsite, hematite, and illite, differing only by the presence of maghemite in LV6 (Table 3).

Conclusion
Oxisols developed from sedimentary parent materials presented a natural arsenic (As) content inferior to those in soils from granite/gneiss and mafic/ultramafic rocks.
In most of the studied Oxisols, As levels were above the thresholds of Reference Value of Quality and Reference Value of Prevention for As in soil.However, their high maximum adsorption capacity of As (MACAs) provides the retention of this contaminant in the soil solid phase.
The MACAs was efficiently described by the Langmuir isotherm for all the studied soils.B horizons showed higher As adsorption due to the smaller content of total organic carbon in this layer, as well as the predominance of Fe oxides in poorly-crystallized forms.
Among the Oxisols evaluated, As adsorption capacity had no relationship with the natural content of this element in the soils deemed "uncontaminated".Red Oxisols class presented the lowest natural level of As, but it also comprised both soils with high and low MACAs.
Among the studied soils, Dystrofic Red Oxisol (Acrudox), which has a very clayey texture and was developed from mafic/ultramafic rocks, stood out for the largest MACAs.In contrast, the lowest MACAs were presented by Dystrofic Red Oxisol (Hapludox), which has a sandy-loam texture and was formed from sandstones.

Table 3 .
Chemical and mineralogical characterization of clay fraction in the studied soils

Table 4 .
Physical characterization of the studied soils

Table 6 .
Parameters related to the Langmuir equation for As adsorption in the studied Oxisols (Latossolos) 3.2 Natural Levels of ArsenicNatural levels of As in the soil classes evaluated are shown in the Table5.The concentrations of As of LV (Red Oxisol), whose LV5 and LV6 originate from sedimentary material (Table2), were lower than those of LVA (Red-Yellow Oxisol), which was formed by metamorphic/igneous rocks.When horizons were compared, the levels of As in Bw of LV were superior to those of the surface (or A) horizon.Yet, in LVA, no statistical