Lead sorption and leaching from an Inceptisol sample amended with sugarcane vinasse

Sorção e lixiviação de chumbo em amostra de um Cambissolo tratado com vinhaça de cana-de-açúcar

Abstracts

Vinasse is produced at high amounts by the ethanol and sugarcane spirits industries. Although the effects of vinasse on soil attributes are well elucidated, little is known about its effects on sorption and leaching of lead (Pb) from soils. This work evaluated the Pb sorption and leaching from disturbed topsoil samples (0-20 cm) of an Inceptisol, under the influence of sugarcane vinasse. Pb sorption was evaluated by batch sorption method, using air-dried samples (< 2 mm) treated with pure (100%), and diluted (20%, and 50%) vinasse. The amended soil samples were further reacted with 0.05; 0.10; 0.25; 0.50, and 0.75 mmol L-1 of Pb. To measure the Pb leached, soil columns were packed in PVC tubes, 12-cm long with 4-cm internal diameter filled with 200 g of treated soil. The vinasse doses applied over the columns corresponded to 0 (control), 150 and 300 m³ ha-1. Seven days later, 5 mL of a Pb solution (100 mmol L-1) was applied on the columns; they were successively leached and Pb was measured in the leachates. The vinasse increased the maximum sorption capacity and binding energy of Pb with the soil when at equilibrium conditions. However, at non-equilibrium conditions, the vinasse enhanced the Pb leaching. From these results, it is advised that special attention must be given to Pb contaminated areas where vinasse is applied.

sugarcane cultivation; organometallic complexes; organic waste; Pb mobility

A vinhaça é produzida em grandes quantidades pela indústria de etanol e de aguardente (cachaça). Embora os efeitos da vinhaça nos atributos do solo sejam bem conhecidos, pouco se sabe sobre eles no comportamento sortivo de chumbo (Pb). Avaliaram-se a sorção e a lixiviação de Pb em amostras da camada superficial (0-20 cm) de um Cambissolo Háplico Tb Distrófico típico (CXbd) sob influência da vinhaça. Para o ensaio de sorção, as amostras foram peneiradas em malha de 2 mm, tratadas com vinhaça diluída 20% e 50% e ao natural (100%), empregando água destilada nas amostras controle. Em seguida, soluções com 0,05; 0,10; 0,25; 0,50 e 0,75 mmol L-1 de Pb foram adicionadas às amostras de solo. Para o ensaio de lixiviação, foram preparadas colunas de solo em tubos de PVC de 12 cm de altura e 4 cm de diâmetro interno, contendo 200 g de solo. As doses de vinhaça aplicadas sobre as colunas foram correspondentes a 150 e 300 m³ ha-1. As colunas foram mantidas incubadas com vinhaça por sete dias. Após esse período, foram aplicados sobre cada coluna 5 mL de uma solução de concentração 100 mmol L-1 de Pb e, posteriormente, realizadas sucessivas lixiviações, coletando-se o percolado para determinação do teor de Pb. A vinhaça aumentou a capacidade máxima de adsorção de Pb e sua energia de ligação com o solo em condições de equilíbrio (ensaio de sorção), mas em condições de não equilíbrio (ensaio de lixiviação em colunas), a vinhaça aumentou a lixiviação de Pb. Com base nesses resultados, atenção especial deve ser dada a áreas que estejam recebendo vinhaça ao longo dos anos e que estejam ocasionalmente sujeitas à contaminação por Pb.

cana-de-açúcar; complexos organo-metálicos; resíduo orgânico; mobilidade de Pb

SOILS AND PLANT NUTRITION

Lead sorption and leaching from an Inceptisol sample amended with sugarcane vinasse

Sorção e lixiviação de chumbo em amostra de um Cambissolo tratado com vinhaça de cana-de-açúcar

Bruno Teixeira Ribeiro^I,*; José Maria de Lima^II; Luiz Roberto Guimarães Guilherme^II; Luiz Gustavo Fernandes Julião^III

^IIF SULDEMINAS - Campus Machado/CGPP, Rod. Machado - Paraguaçú, km 3 - Campus Machado - 37750-000 - Machado, MG - Brasil

^IIUFLA - Depto. de Ciência do Solo, C.P. 3037 - 37200-000 - Lavras, MG - Brasil

^IIIUNICAMP/Faculdade de Engenharia Agrícola, C.P. 6011 - 13083-875 - Campinas, SP - Brasil

ABSTRACT

Vinasse is produced at high amounts by the ethanol and sugarcane spirits industries. Although the effects of vinasse on soil attributes are well elucidated, little is known about its effects on sorption and leaching of lead (Pb) from soils. This work evaluated the Pb sorption and leaching from disturbed topsoil samples (0-20 cm) of an Inceptisol, under the influence of sugarcane vinasse. Pb sorption was evaluated by batch sorption method, using air-dried samples (< 2 mm) treated with pure (100%), and diluted (20%, and 50%) vinasse. The amended soil samples were further reacted with 0.05; 0.10; 0.25; 0.50, and 0.75 mmol L^-1 of Pb. To measure the Pb leached, soil columns were packed in PVC tubes, 12-cm long with 4-cm internal diameter filled with 200 g of treated soil. The vinasse doses applied over the columns corresponded to 0 (control), 150 and 300 m³ ha^-1. Seven days later, 5 mL of a Pb solution (100 mmol L^-1) was applied on the columns; they were successively leached and Pb was measured in the leachates. The vinasse increased the maximum sorption capacity and binding energy of Pb with the soil when at equilibrium conditions. However, at non-equilibrium conditions, the vinasse enhanced the Pb leaching. From these results, it is advised that special attention must be given to Pb contaminated areas where vinasse is applied.

Key words: sugarcane cultivation, organometallic complexes, organic waste, Pb mobility

RESUMO

A vinhaça é produzida em grandes quantidades pela indústria de etanol e de aguardente (cachaça). Embora os efeitos da vinhaça nos atributos do solo sejam bem conhecidos, pouco se sabe sobre eles no comportamento sortivo de chumbo (Pb). Avaliaram-se a sorção e a lixiviação de Pb em amostras da camada superficial (0-20 cm) de um Cambissolo Háplico Tb Distrófico típico (CXbd) sob influência da vinhaça. Para o ensaio de sorção, as amostras foram peneiradas em malha de 2 mm, tratadas com vinhaça diluída 20% e 50% e ao natural (100%), empregando água destilada nas amostras controle. Em seguida, soluções com 0,05; 0,10; 0,25; 0,50 e 0,75 mmol L^-1 de Pb foram adicionadas às amostras de solo. Para o ensaio de lixiviação, foram preparadas colunas de solo em tubos de PVC de 12 cm de altura e 4 cm de diâmetro interno, contendo 200 g de solo. As doses de vinhaça aplicadas sobre as colunas foram correspondentes a 150 e 300 m³ ha^-1. As colunas foram mantidas incubadas com vinhaça por sete dias. Após esse período, foram aplicados sobre cada coluna 5 mL de uma solução de concentração 100 mmol L^-1 de Pb e, posteriormente, realizadas sucessivas lixiviações, coletando-se o percolado para determinação do teor de Pb. A vinhaça aumentou a capacidade máxima de adsorção de Pb e sua energia de ligação com o solo em condições de equilíbrio (ensaio de sorção), mas em condições de não equilíbrio (ensaio de lixiviação em colunas), a vinhaça aumentou a lixiviação de Pb. Com base nesses resultados, atenção especial deve ser dada a áreas que estejam recebendo vinhaça ao longo dos anos e que estejam ocasionalmente sujeitas à contaminação por Pb.

Palavras-chave: cana-de-açúcar, complexos organo-metálicos, resíduo orgânico, mobilidade de Pb

Introduction

Vinasse is produced at the rate of 12-13 L per liter of ethanol or sugar cane spirits (Freire and Cortez, 2000; Martinelli and Filloso, 2008). About 27 billion liters of ethanol (IBGE, 2009) and 350 billion liters of vinasse are produced yearly in Brazil. It consists mainly of water, organic matter (organic acids and sugars) and cations such as K, Ca, and Mg (Freire and Cortez, 2000; Doelsch et al., 2009).

In the past, vinasse was dumped into watercourses, causing great environmental impacts (Günkel et al., 2007); nowadays, it is being applied to soil, mainly in sugarcane crops, as liquid industrial waste fertilizer (Freire and Cortez, 2000). Some of the effects of the vinasse on physical, chemical and biological attributes of soil were studied by Camargo et al. (1983), Leal et al. (1983), Neves et al. (1983), Mattiazzo and Glória (1987), Sengik et al. (1988), Franco et al. (2008) and Doelsch et al. (2009). However, little is known about its effect on the sorption of heavy metals, such as Lead (Pb), especially in Brazil.

Pb can be present in soil due to weathering processes, mining and smelting activities (Kabata-Pendias and Pendias, 2000; Sharma and Dubey, 2005), and also by the agricultural use of phosphate fertilizers, sewage sludge or biosolids (Silveira et al., 2003; Borges and Coutinho, 2004; Oliveira et al., 2005; Campos et al., 2005; Silva et al., 2006; Camilotti et al., 2007; Torri and Lavado, 2008; Galdos et al., 2009).

The soil organic matter is the most important attribute that account for Pb sorption in soils (Sipos et al., 2005; Vega et al., 2007). The organic matter-Pb complexes can be soluble and leach in the soil, contaminating groundwater (Jalali and Khanboluki, 2007) whereas the mineral solid phase may account for its immobilization (Madrid and Díaz-Barrientos, 1998).

We here hypothesize that the presence of vinasse in soil, especially due to their soluble organic compounds, will increase leaching of Pb. The main objective was to evaluate the vinasse effect both on Pb sorption and Pb leaching in a topsoil sample from an Inceptisol.

Material and Methods

Sample preparation and incubation with vinasse for Pb batch sorption experiment

Topsoil samples (0-0.2 m) of a Typic Dystrustepts (Cambissolo Háplico distrófico, according to Brazilian System of Soil Classification), located at Lavras, Minas Gerais State, Brazil, were used for this study. Selected physical and mineralogical attributes, determined according to Embrapa (1997), are in Table 1.

The samples were air-dried for 72 hours and sieved to < 2 mm. Then, 50 g were placed in stainless-steel micro columns (6.0 cm high and 3.5 cm internal diameter), whose base was perforated. Then, they were saturated by capillarity action for 48 h with distilled water (control - T0), diluted vinasse at 20% (T1), 50% (T2) with distilled water and natural vinasse (T3). After saturation, the columns were incubated for seven days, and air dried. Then, soil samples were removed from the columns, air-dried for 72 h and sieved in a 2-mm mesh for chemical characterization, according to Embrapa (1997) (Table 2).

The vinasse (from sugar cane spirits production) presented acidic character (pH = 3.5), potassium (2.5 g L^-1), solid residue (9.6 g L^-1) and total organic carbon (9.0 g L^-1). Potassium content was determined directly from the natural vinasse via a flame photometer. Total organic carbon was determined after the digestion of 0.5 mL of natural vinasse with potassium dichromate and titration with ammoniacal ferrous sulfate. Pb content was determined in an Atomic Absorption Spectrophotometer, with an air/acetylene flame (detection limit around 0.040 mg L^-1), and not detected in the vinasse.

For the Pb sorption, 0.25 g of vinasse pre-treated soil was placed in 30-mL bottles in suspension with 25 mL of Pb solution, at the concentrations of 0; 0.05; 0.10; 0.25; 0.50 and 0.75 mmol L^-1. They were prepared from Pb(NO₃)₂ in a 5 mmol L^-1 Ca(NO₃)₂ solution, the pH adjusted to 5.5 (± 0.2) with a saturated solution of Ca(OH)₂ (11.12 mmol L^-1). Samples were shaken at room temperature (22ºC ± 2) and normal pressure conditions, for 72 h, alternating 12 h shaking and 12 h repose. After that, the samples were centrifuged for 10 min at 500 x g. The supernatant solution was collected and immediately analyzed for Pb. Each bottle was weighed to determine the volume of entrained solution. Sorbed Pb was calculated by the difference between the initial and final Pb concentrations and was expressed in millimols per kilogram:

where Pb_initial and P_final are the initial and final solution phase Pb concentrations (mmol L^-1), and V is the solution volume (L).

To measure Pb desorption, 25 mL of a solution of a 5 mmol L^-1 Ca(NO₃)₂ solution (pH 5.5 ± 0.2) were added to the remaining soil residue. Bottles were shaken under the same conditions as previously described, centrifuged, and the supernatant was analyzed for the Pb concentration. The desorbed Pb was:

where Pb_d is the Pb concentration in the desorption supernatant, V_d is the volume of Ca(NO₃)₂ solution added during desorption step, and V_entr is the volume of solution entrained into the bottles after the sorption step. The residual Pb was calculated by the difference between the sorbed and desorbed Pb.

Sorption isotherms were obtained in order to describe the Pb sorption from the samples and data was adjusted according to Langmuir's model:

where K is the Langmuir's constant (related to sorption binding energy) and Pb_max is the maximum sorption capacity of Pb (mmol kg^-1). K and Pb_max were obtained from the linear form of Langmuir's model. These data were submitted to F Test and the averages were compared by Scott-Knott Test (p < 0.05).

Pb leaching experiment

Soil columns were packed in PVC tubes with 12-cm height and 4-cm internal diameter, with 200 g of aggregates (1 - 2 mm) with an average density of 1.00 ± 0.04 g cm^-3 and total porosity of 0.59 ± 0.02 cm³ cm^-3. The bottom of the columns was perforated at the center (7-mm diameter) and had an acrylic screen (1 mm mesh) to prevent the presence of particles in the leachate. The vinasse's doses applied to the soil columns corresponded to 0 (control), 150 and 300 m³ ha^-1. The highest dose was enough to reach the field capacity of the soil in the columns (0.30 cm³ cm^-3 or 1/2 pore volume). Distilled water was added to the 150 m³ ha^-1 dose treatment so that the field capacity could also be reached. The control samples' field capacity was reached by only adding distilled water. The soil columns were incubated during seven days at room temperature (22ºC ± 2) and normal pressure conditions. The field capacity was kept constant by replacing the evaporated water with distilled water on a daily-basis. The evaporation from soil columns ranged from 1.5 to 2 mm day^-1.

After the incubation period, 5 mL of a Pb(NO₃)₂ solution (100 mmol L^-1 Pb) were applied on the top surface of the columns. The amount of Pb applied was equivalent to 2.5 mmol kg^-1 of soil. In the next day, 20 mL (0.16 pore volume) of distilled water (simulating a 10 mm irrigation or rainfall) were applied. Leachate was collected to determine the Pb as described in the batch sorption experiment. This procedure was repeated five times, every other day, totaling six leachings (one pore volume) and a total irrigation or rainfall of 60 mm.

Results and Discussion

Batch sorption experiment

In all initial Pb solutions, there was an increase in the sorbed and residual Pb, in the samples treated with vinasse, mainly in the initial Pb solutions of 0.25, 0.50 and 0.75 mmol L^-1(Figure 1).

The presence of colloidal organic compounds of vinasse accounted for increased Pb sorption on the solid phase. Fructose, glucose, sucrose, glycerol, galactose, acetate, oxalate, citrate, among others, are found in sugarcane vinasse (Doelsch et al., 2009). A small increase in organic matter content was observed in the samples (Table 2). The higher Pb sorption in the samples that were treated with vinasse can also be associated with the formation of organometallic complexes on the soil mineral surfaces (Schwab et al., 2005). These interactions can be influenced by the affinity between the organic compounds and the components from the inorganic fraction of the soil, especially the oxides. This would enable the formation of "metal-organic oxide-ligand" ternary complexes, increasing the Pb retention capacity in soils (McBride, 1994). In this context, it was found a greater sorption capacity of the organic compounds from vinasse on the Fe oxides synthetics (goethite and hematite), which was approximately fivefold the sorption on the surface of kaolinite (Benke et al. (1999).

In comparison to other metals, Pb tends to be more sorbed in the soils due to its lower electronegativity as related to oxygen,which favors covalent bonds, and its high capacity to hydrolyze (McBride, 1994; Pierangeli et al., 2001c; Pierangeli et al., 2004). Additionally, Pb sorption in the soil is related to the solid phase composition; the organic matter is considered the most important soil component that is responsible for the retention capacity of this metal in the soil (Sipos et al., 2005). In soils with variable charge, Fe oxides also have great influence on the sorptive behavior of Pb (Pierangeli et al., 2001b), especially at high pH contitions (Pierangeli et al., 2001a).

Madrid and Díaz-Barrientos (1998), Hooda and Alloway (1998), Schwab et al. (2005) and Wong et al. (2007) have demonstrated the role of organic compounds present in residues from different origins on the sorptive behavior of metals in soils. The metal sorption in organic compounds is related to the sharing of electrons between metal surface and the organic functional groups. A stable bonding between the metal and the organic compound can take to either the occurrence of organometallic complexes in the solid phase or in the soil solution (Canellas et al., 1999). Besides, the oxidation of oxygen functional groups that form the bonds can determine the metal release in the soil solution. The influence of vinasse was also observed on copper adsorption in calcareous soils (Rodriguez-Rubio et al., 2003).

The addition of vinasse to the soil can contribute to the increase the cation exchange capacity (Leal et al., 1983; Sengik et al., 1988; Silva et al., 2007), which can also increase the Pb sorption. However, the present study did not show significant vinasse effect on the cation exchange capacity and the point of zero salt effect (PZSE) that could lead to an increase in the negative charge (Table 2). In fact, a slight decrease in the water-pH and an increase in KCl-pH were observed in the samples that were treated with vinasse, indicating a reduction of the negative charges (Tan, 1993). Initially, because of its low pH, the addition of vinasse led to a decrease of pH in the soil solution. Then, the soil pH increased, because of the reductive environment (which consumes H⁺ ions) that was imposed by the presence of organic compounds in vinasse, which presents high biochemical demand of oxygen (Leal et al., 1983; Mattiazzo and Glória, 1987; Doelsch et al., 2009). This, indirectly, can lead to an increase in the Pb sorption due to a higher CEC of the colloids with variable charges.

Pb sorption by the soil samples was well described by the sorption isotherms adjusted to Langmuir's model (Figure 2A). Isotherms had an L-type form, which were first characterized by a high affinity between the metal and the soil and reached the highest degree where the saturation of the sorption sites occurred (Pierangeli et al., 2007; Wong et al., 2007). Samples treated with 50%diluted vinasse and natural vinasse had an increase in the Pb_max (Figure 2B), as compared to the control-T0 and 20% vinasse-T1. An increase in the sorption binding energy was also observed when the samples were treated with natural vinasse.

Pb leaching experiment

The amount of Pb added to the columns (2.5 mmol kg^-1 of soil) was bellow the maximum sorption capacity (Pb_max) (Figure 2A). Nevertheless, after the experiment, a small fraction of non-adsorbed Pb (< 0.5% of total applied) was influenced by the presence of vinasse in the columns (Figure 3). Less Pb was released from control samples, as compared to those with vinasse (150 and 300 m³ ha^-1) (Figure 4). The highest dose of vinasse account for higher amount of accumulated Pb in the leachate. This is related to the binding between organic compounds of vinasse and Pb in the soil solution, which enhanced mobility through the soil columns, under the successive leaching. Similar results were found for soil columns under the influence of EDTA in an Oxisol (Gabos et al., 2009), and citrate and oxalate in a Spodosol (Freitas et al., 2009), which shows the interaction of chelate with Pb. Similarly, leaching of Zn and Cu from homogeneous soil columns (the same size used in this work) was enhanced when soils were treated with two wastewaters from an agricultural industry (olive-mill wastewater and concentrated sugarbeet vinasse) (Madrid and Díaz-Barrientos, 1998).

Comparison of the results between batch and leaching experiments

The increase in the Pb sorption in the samples treated with vinasse (Figure 1 and 2), if interpreted based on the results of the batch sorption experiment, may be due to the formation of organometallic compounds which were adsorbed on the soil mineral particles (Schwab et al., 2005). When analyzing the Pb in the leachate of soil columns (Figures 3 and 4), it was observed that a small fraction of the metal that was present in the soil solution moved because of the presence of vinasse, due to the formation of the soluble organometallic complexes in the soil solution. Dispersion of particles occurring in the batch sorption procedures, increased the surface for reaction; this is caused by the constant agitation and desorbed species can be resorbed by the matrix. Thus, the results of the batch sorption experiment may overestimate the real sorption capacity in natural conditions. In soil columns, since they are open systems, the unbalanced conditions are more closely related to natural conditions, where the desorbed species are removed and the reverse reactions are much less probable to occur.

Based on the results of the Pb leaching experiment, which is more closely related to the field conditions, special attention must be given to the agricultural areas that receive large quantities of vinasse and are also exposed to contamination with Pb; for example: through the use of phosphate fertilizers (Campos et al., 2005) with sewage sludge or biosolids of urban and industrial origins (Silveira et al., 2003; Borges and Coutinho, 2004; Oliveira et al., 2005; Silva et al., 2006; Torri and Lavado, 2008; Galdos et al., 2009). This can happen where sewage sludge is used as a source of N and vinasse is used as a source of K as may occurs in sugarcane crops (Franco et al., 2008). In this case, the sewage sludge may contain a significant amount of Pb and accounts for soil contamination (Camilotti et al., 2007).

According to the quantity and frequency of the vinasse amended to the soil, it may reach the groundwater (Lyra et al., 2003), carrying along sorbed Pb, thus affecting the ground water quality. The Pb may be passively co-transported by organic matter which also enhances its mobility in soils by the formation of highly mobile organic colloids (Lang et al., 2005).

Conclusions

At equilibrium conditions, the applied vinasse increased the Pb's maximum sorption capacity and binding energy in the soil. However, at non-equilibrium conditions, the vinasse enhanced Pb leaching. The results of the batch sorption experiment may overestimate the real sorption capacity in natural conditions.

Acknowledgements

To Fapemig and CNPq for the financial support to this research.

Received May 08, 2009

Accepted March 31, 2010

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Publication Dates

• Publication in this collection
  02 Aug 2010
• Date of issue
  Aug 2010

History

• Accepted
  31 Mar 2010
• Received
  08 May 2009