PHYSICOCHEMICAL EVALUATION OF SOILS AND SEDIMENTS NEAR MINING OPERATIONS IN ASIENTOS, AGUASCALIENTES

The Fourth International Symposium on Environmental Biotechnology and Engineering, Mexico City, Mexico, 09/09/2014

PHYSICOCHEMICAL EVALUATION OF SOILS AND SEDIMENTS NEAR MINING OPERATIONS IN ASIENTOS, AGUASCALIENTES

K. Mitchell, M. Ramos-Gómez, A. Guerrero-Barrera, L. Yamamoto-Flores; F. Thalasso-

Siret, F. Avelar-González

UNIVERSIDAD AUTÓNOMA DE AGUASCALIENTES, Aguascalientes, México; CINVESTAV- IPN, México DF, México; ABSTRACT

Soils and sediments near mining activities are frequently direct receptors of mining waste and as such present a potential environmental risk. The study of this potential risk should include not only total metal quantification, but also the study of physicochemical characteristics that play an important role in their solubility, mobility and toxicity in affected sites. Thus, the aim of this study was to evaluate total metal concentrations and the physicochemical characteristics that may affect metal mobility in nearby soils and sediments of the Piedras Negras River in Asientos, Aguascalientes; an area recently affected by a substantial increase in mining activity. Soil and sediment sampling were carried out once during the dry season and once during the rainy season. Soil moisture and organic matter were initially determined. After pretreatment which consisted in drying and sieving through a 2mm mesh, cation exchange capacity and total metal content were determined. pH, redox potential, soluble salts were determined in a 1:10 dilution soil:water. Sediments from all sites were classified as very polluted according to guidelines established by the United States Environmental Protection Agency, and above the maximum permitted limits for commercial, residential and farm use according to the NOM-147-SEMARNAT/SSA1-2004 in at least 2 metals, in select cases, more than 200 times guideline values. 70% of soils from the 10 sample sites were classified in the same way. Though neutral conditions were predominant, 20% of samples from both substrates were moderately acidic in nature (<6.5). 80% of soil and sediment samples presented highly reductive conditions (-100 to -300mv). Both substrates had medium to low (<25 cmol(+)/kg) cation exchange capacities. Salinity measured as conductivity in µS/cm in both substrates were found to be normal, though values obtained in sediment samples were twice as high as those obtained in soil samples. Differences between soil and sediment samples were found in values obtained for soluble salts. Nitrate concentrations were 1.5 times higher in sediments. Sulfides and sulfates had 7 and 15 times higher concentrations respectively in sediments.

It was concluded that the sample area was affected by nearby mining activities. High concentration of metals and soluble salts in sediment samples indicate that the river is a direct receptor of mine drainage; whilst soils appear to be affected by wind carried mine tailings. Current physicochemical properties may favor mobility, and thus represent an environmental toxicity risk.

Key words: metals, mining, pollution, mobility, solubility, physicochemical


Introduction

Soils and sediments naturally contain trace metals due to the geology of the parent material from which they were formed (McLean and Bledsoe, 1992). But at higher concentrations, all metals, including essential metals, may cause toxic effects (Liu et al., 2008). Elevated concentrations may be due in part to atmospheric deposition, erosion or leaching of metals in soil, sediment re-suspension or evaporation from water sources (Nriagu, 1989). However, very high concentrations in soils and sediments are more frequently the result of nearby industrial, agricultural, mining and smelting activities (Bradl, 2002; Tchounwou et al., 2012.). Accumulation of metals can degrade soil quality and negatively impact the health of human, animals, and the ecosystem (Nagajyoti et al., 2010).

In order to accurately evaluate the potential environmental risk presented by metals in soils and sediments, it is necessary to study not only total metal quantification, but also the physicochemical characteristics (Howari and Banat, 2001) that play and important role in their solubility, mobility and toxicity.

pH is a physicochemical parameter that directly and indirectly affects the retention of metals in soils and sediments (Gabler, 1997) given that metal solubility decreases with increasing pH. At lower pH, higher concentrations of metals can be found in the soil solution (Peng, 2009; Sherene, 2010). However, pH is dependent on other physicochemical characteristics. Studies have shown that moisture conditions affect the pH; at saturation there is a tendency to neutrality (Van den Berg and Loch, 2000; Zheng and Zhang, 2011). The capacity for organic matter to retain metals by chelation and complexation is also affected by the pH (Kashem and Singh, 2001; Sherene, 2010). Metal retention is also affected by the soil´s or sediment´s cation exchange capacity. As a measure of the total capacity of the soil to hold exchangeable cations, the cation exchange capacity is used to indicate the negative charge present on a unit mass of soil (Peverill et al., 1999). The higher the cation exchange capacity, more easily the metal cations are attracted to these negative sites in solid particles (Atwell et al., 1999).

Another important physicochemical characteristic is salinity measured in terms of conductivity. This parameter is related to the presence of soluble salts in the soil and sediments. It has been shown that an increase in salinity increases mobility of metals due to the complex formation capacity of anions derived from metal salts and competition of salts derived from metal cations with positively charged species for the adsorption sites on the solid phase (Acosta et al., 2011; Paalman et al., 1994).

Thus, the aim of this study was to evaluate total metal concentrations and the physicochemical characteristics that may affect metal mobility in nearby soils and sediments of the Piedras Negras River in Asientos, Aguascalientes; an area recently affected by a substantial increase in mining activity. Materials and Methods Field Sampling

Samples of sediments and soils surrounding the riverbed of Piedras Negras river located in the mining town Asientos, Aguascalientes were taken from 10 sites once


during the rainy season and once during the dry season, using the simple cross-sectional sampling method as established by the USEPA (2002).

Soil and sediment samples were classified in three groups: original sample, particulate sample and liquid extract sample using standardized techniques according to APHA-AWWA-WPCF (1998). Original sample The following parameters were determined in unmodified samples.

• Humidity: determined using the thermo-gravimetric method with a drying time of 24 hours at 105oC (Alvarez- Benedi and Muñoz-Carpena, 2004).

• Concentration initial of metals: quantified by atomic absorption spectrophotometry after extraction and acid digestion using an acid mixture of H2O: HCl: HNO3 (USEPA, 2007).

Particulate sample

The samples were dried at a temperature of 55oC for 10 days, then ground and sifted to obtain a particle size of less than 0.5mm (Robledo-Santoyo and Maldonado-Torres, 1997). The following parameters were quantified with the obtained particulate sample.

• Organic matter content: determined using the Walkley-Black metric titration method, which included the oxidation of organic matter with potassium dichromate under acid conditions (Robledo-Santoyo and Maldonado-Torres, 1997).

• Cation exchange capacity: determined by the extraction method with 1N ammonium acetate / pH 7.0 (Robledo-Santoyo and Maldonado-Torres, 1997).

Liquid extract

To obtain samples of liquid extract, 5g of the particulate sample were placed in 50ml of distilled water to obtain a solution of 1:10 (m/v). Samples were then placed in a rotating agitator for 24 hours and then left to stand for another 24 hours (Robledo-Santoyo and Maldonado-Torres, 1997). The following parameters were determined once the liquid extract was obtained.

• pH: determined using the potentiometric method (Robledo-Santoyo and Maldonado-Torres, 1997).

• Conductivity: determined using the potentiometric method (Carter and Gregorich, 2008).

• Redox Potential: determined using the electrode method (FAO, 2006). • Soluble salts: determined using standardized HACH spectrophotometric methods

o Sulfides: methylene blue method #8131 o Sulfates: sulfaver 4 method #8051 o Cyanide: pyridine-pyrazalone method #8027

Statistical analysis

Obtained data were statistically analyzed using MiniTab Express 1.1 and Pearson´s StatChrunch. Results and Discussion


Total Cu concentration is shown Figure 1a for samples collected during rainy

season and dry season of soils and sediments. 90% of the collected sediment samples were classified as likely to have severe toxic effects in accordance with the scale used by Lindsay (1979) and Reichman (2002), while only 20% of collected soil samples presented such characteristics. All sediment samples and 75% of soil samples were classified as very polluted according to reference guidelines established by the USEPA (1977). Figure 1b shows the total concentration of Zn in both sampling groups of soils and sediments.

FIGURE 1. Total metal concentration (mg/kg) for Cu, Zn, Pb and Cd in soil and sediment samples collected in two groups 1(rainy season) and 2 (dry season) from the riverbed and surrounding areas of the Piedras Negras River, Asientos, Aguascalientes. Black line indicates limit above which severe toxic effects is likely.


All sediment samples were classified very polluted and likely to have severe toxic effects. 80% of soil samples were considered to be very polluted and 70% were considered likely to cause severe toxic effects. Higher concentrations of both metals were observed in sediments samples as compared to soil samples. This is indicative of the riverbed being a possible direct receptor of waste material from the nearby mining activity.

The presence of these metals in such high concentrations is of importance due to their implications in environmental toxicity. According to Hambige and Krebs (2007), zinc is an essential mineral of exceptional biologic importance. However, in excessive amounts, it is damaging to many life forms (Eisler, 1993; Fosmire, 1990). Copper is also an important essential trace element for all living organisms needed for the activity of a variety of enzymes involved in critical areas of metabolism (Leone et al., 1999), however, in high concentrations it causes toxicity, due to the generation of reactive oxygen species damaging proteins, lipids and DNA (Brewer, 2010).

Values for Pb concentrations can be observed in Figure 1c All sediment samples were classified as very polluted and very likely to have severe toxic effects. 75% of soil samples were classified as very polluted while 60% presented conditions likely to have severe toxic effects. All soil and sediment samples contained concentrations of Pb above the maximum permitted limits for commercial, residential and farm use according to the NOM-147-SEMARNAT/SSA1-2004. Figure 1d shows values for Cd concentrations in samples where 85% of sediment samples were classified as very polluted and 80% were likely to cause severe toxic effects. 45% of soil samples were categorized as very toxic and 40% likely to cause severe toxic effects.

According to the US Environmental Protection Agency (USEPA) and the International Agency for Research on Cancer (IARC), as cited by Tchounwou et al. (2012), Pb and Cd are classified as either “known” or “probable” human carcinogens based on epidemiological and experimental studies. They have no established biological functions and are considered as nonessential metals and systemic toxicants that are known to induce multiple organ damage, even at lower levels of exposure.

The presence of these four metals in such high concentrations in sediment samples from this area is evidence of the riverbed being a direct receptor of mine drainage. Soil samples presented lower but significant concentrations when compared to sediment samples, and may be the result of wind dispersion of mine tailings.

It has been established that in order to study the potential risk of toxic metal pollution in soils and sediments, it is necessary to evaluate the physicochemical characteristics that play a role in the solubility and mobility of these metals (Howari and Banat, 2001). According to the classification used by Robledo and Maldonado (1997) 95% of the sediments sampled in both dry and rainy seasons had a neutral pH, 5% are classified as moderately acid (Table 1a). Although no significant differences were found, the median during the dry season was lower than during the rainy season. From Table 1b, it can be observed that 75% of the sampled soils were classified as neutral, while the remaining 25% were classified as moderately acid. No significant differences between the values obtained from the different seasonal periods were observed, however, as observed in the case of sediment samples, lower pH values were more prevalent during the dry season.


pH is an important factor in the mobilization of metals in soils and sediments (Fijałkowski et al., 2012) due to the hydrolysis reactions of cations caused by an increase in the concentration of protons due to the competition between them and ligands (OH-, SO4

2- etc.,) resulting in a decrease of adsorption sites (McLean and Bledsoe, 1992). This results in an increase in solubility and therefore the mobility of the metals in inverse proportion to the pH; such that an alkaline pH is not conducive to high metal mobility (Ramos-Gómez et al., 2012). Predominantly neutral conditions of the sampled sites in this study may indicate a temporary reduction in mobility. pH however is highly dependent on soil moisture content. In this study, higher percentages were observed in sediment samples, both sample groups being affected by the prevalent hydrological conditions. As moisture content increases towards saturation, the pH tends to neutrality. TABLE 1. Values for physicochemical characteristics of sediment samples collected from the riverbed and surrounding areas of the Piedras Negras River, Asientos, Aguascalientes a.

Sediments

Sample,Zone Rainy, Dry Rainy, Dry Rainy, Dry Rainy, Dry Rainy, Dry1 7.10 7.13 -183.00 -37.50 79.01 91.30 4.40 4.82 19.06 21.122 7.00 6.00 -185.00 -227.40 75.56 59.60 2.60 5.63 14.22 19.123 7.30 7.45 -168.60 7.40 56.87 45.60 4.48 4.53 16.46 9.794 7.30 6.65 -192.70 -279.90 69.55 51.30 5.89 4.96 17.11 19.715 7.00 7.31 -176.40 -75.80 68.06 78.20 7.53 5.47 25.96 20.536 7.30 6.95 -181.80 -197.30 41.92 43.50 8.48 5.98 6.31 5.667 7.10 7.18 -189.70 -36.90 41.37 49.60 6.54 6.24 9.44 7.918 7.50 6.96 -162.80 -230.10 25.48 30.80 3.89 5.12 12.98 17.119 7.10 7.05 -149.50 -271.90 39.79 42.10 4.31 4.96 9.32 23.0110 7.00 6.96 -160.30 -374.20 32.11 38.40 9.65 6.87 9.50 16.17

pH Redox,Potential,(mV) Soil,Moisture,(%) Organic,Matter,(%)Cation,Exchange,

Capacity,(cmol(+)/kg)

b.

Soils

Sample*Zone Rainy* Dry Rainy* Dry Rainy* Dry Rainy* Dry Rainy* Dry1 7.00 7.39 -296.50 -83.30 19.40 18.20 5.38 4.98 16.58 24.782 6.00 7.26 -193.00 -54.60 17.20 17.10 4.68 3.91 11.80 26.203 7.30 7.23 -304.90 -113.30 14.90 18.90 4.44 2.45 24.54 25.784 7.20 7.40 -306.50 -50.70 19.60 20.10 4.17 3.97 16.11 16.585 7.20 7.02 -289.00 -87.60 17.30 17.70 8.20 6.12 23.01 22.246 7.20 6.12 -349.40 -295.10 12.40 13.50 1.88 3.25 16.11 19.777 7.80 6.33 -362.00 -226.80 17.40 17.90 1.34 1.91 9.44 13.228 7.20 7.32 -306.30 -208.80 14.50 15.90 5.78 2.28 19.47 18.599 7.10 6.46 -283.20 -381.60 17.30 18.00 5.18 2.89 16.52 16.2810 7.40 6.36 -491.10 -229.90 18.00 18.00 3.50 2.97 16.11 15.52

pH Redox*Potential*(mV) Soil*Moisture*(%) Organic*Matter*(%)Cation*Exchange*

Capacity*(cmol(+)/kg)

The redox potential of the substrate is another factor that must be evaluated as it also plays a major role in the mobility of metals. Studies have shown that there can be an increase the mobility due to reduction reactions even under neutral conditions (McBride, 1994; Bollinger et al., 2003). Table 1a and b shows that 80% of soils and sediments presented highly reductive conditions according to the classification used by the USEPA (1994), the remaining 20% being classified as moderately reduced. During the dry season, obtained values showed greater variation. This can be attributed to moisture gradients across the sampled areas as a result of the prevailing hydrological conditions of the season (Yu et al., 2006). This is reflected in less reductive conditions in certain sample sites. This prevalent reductive condition in the area studied can greatly


affect metal transport possibly contributing to an accelerated migration (McLean and Bledsoe, 1992).

Metal mobility or lack thereof is also affected by the organic matter content of the studied substrate. According to the classification used by Fernández Linares et al. (2006), 90% of the sampled sediments had high organic matter content above 4% as shown in Table 1a, however, only 45% of soil samples fall within the same category. As with other physicochemical characteristics, organic matter has a direct impact on the behavior of metals, affecting solubility with complexation and chelation reactions (Kashem and Singh, 2001). Studies have shown that during decay, humic and fulvic acids whose functional groups are capable of associating with metal cations (Kyziol, 2002) are produced. However, under acidic conditions such groups are ionized resulting in the remobilization of these cations (Carrillo-Gonzalez et al., 2006), further demonstrating the close relationship between the pH and other physicochemical characteristics (Sherene, 2010), including organic matter content.

As a measure of the soil´s capacity to hold exchangeable cations, cation exchange capacity is used to indicate the net negative charge present on a mass unit of soil (Peverill et al., 1999). The higher the cation exchange capacity, more easily metal cations are attracted to the negative sites on soil and sediment particles (Atwell et al., 1999). According the classification used by Robledo and Maldonado (1997), 85% of soils and 60% of sediment samples were classified as having average cation exchange capacity. 15% and 40% respectively presented low cation exchange capacity, conditions that may contribute to a less immobilization (Atwell et al., 1999).

As per the classification of the USDA (2002), all soils (Figure 2a) are in conditions of normal salinity (<2000 µS/cm). In contrast, 5% of the sampled sediments present slightly saline conditions. However, that salinity is increasing with respect to time. This has important implications given that an increase in salinity results in an increase in mobility of metals in the soil due to the complex formation capacity of anions derived from metal salts and competition of salts derived from metal cations with positively charged species for the adsorption sites (Acosta et al., 2011; Paalman et al., 1994).

The distribution of sulfates in mg/L is shown in Figure 2c in both soils and sediments. Sediment samples contained concentrations up to 7 times more than in soils. These values indicate the presence of contamination by soluble salts, in this case sulfates, a common product of oxidation-neutralization of metal ores or waste reagents from flotation processes used in the mining industry (Gutiérrez-Ruiz et al., 2007). Sulfide concentrations were also higher in sediment samples (Figure 2d), up to 15 times higher in some cases. The presence of higher than typical concentrations of sulfides in soils and sediments may indicate constant reception of mine wastes, at a higher rate than that of the oxidation process sulfides normally undergo. The oxidation process is also important given that it produce sulfates resulting in a decrease in pH of the substrate, an important factor in the mobility and availability of metals (Clemente et al., 2003). Additionally in substrates where specific adsorption sites become saturated with cations and anions already present, exchange reactions predominate, resulting in competition for these sites between metals and soil ions (McLean and Bledsoe, 1992). In the presence of these inorganic ligands, the adsorption of certain metals on soil and soil constituents can be inhibited (Boulding, 1996), as such contribute to an increase in the concentration for the free metal in the liquid phase of the soil.


FIGURE 2. Salinity measured as conductivity in µS/cm and concentration of soluble salts (sulfates, sulfides and cyanide) in mg/L in soil and sediment samples collected in two groups 1(rainy season) and 2 (dry season) from the riverbed and surrounding areas of the Piedras Negras River, Asientos, Aguascalientes.

Certain mining activities requires the use of great quantities of alkaline solutions containing high concentrations of sodium cyanide, free cyanide and metal-cyanide complexes (Korte and Coulston, 1998). As such, cyanide concentrations were also measured in both substrates. Sediment samples showed higher values (Figure 2b). Cyanide biodegrades under aerobic conditions with the initial formation of ammonia, which is converted to nitrite and nitrate in presence of nitrifying bacteria (Wiemeyer Eisler, 2004). Using the soil values to represent typical geological characteristics, the high cyanide concentrations in sediment samples is indicative of direct and constant pollution from mining activities. Conclusion

Soils and sediments from the Piedras Negras River, Asientos, Aguascalientes were classified as very polluted with Zn, Cu, Pb, Cd according to guidelines established by the USEPA, at some sites, more than 200 times guideline values, and above the maximum permitted limits for commercial, residential and farm use according to the NOM-147-


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PHYSICOCHEMICAL EVALUATION OF SOILS AND SEDIMENTS NEAR MINING OPERATIONS IN ASIENTOS, AGUASCALIENTES

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