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Chromium Movement in Columns of Two Highly Weathered Soils a
M. A. K. de Alcântara & O. A. de Camargo
a
a
Soils and Agroenvironmental Resources Center, Institute of Agronomy (IAC-CSRA), Campinas, São Paulo, Brazil Version of record first published: 20 Aug 2006
To cite this article: M. A. K. de Alcântara & O. A. de Camargo (2004): Chromium Movement in Columns of Two Highly Weathered Soils, Communications in Soil Science and Plant Analysis, 35:5-6, 599-613 To link to this article: http://dx.doi.org/10.1081/CSS-120030346
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COMMUNICATIONS IN SOIL SCIENCE AND PLANT ANALYSIS Vol. 35, Nos. 5 & 6, pp. 599–613, 2004
Chromium Movement in Columns of Two Highly Weathered Soils M. A. K. de Alcaˆntara and O. A. de Camargo* Soils and Agroenvironmental Resources Center, Institute of Agronomy (IAC-CSRA), Campinas, Sa˜o Paulo, Brazil
ABSTRACT The movement of contaminants through soils is an important environmental task. Chromium (Cr), a heavy metal that is potential hazardous to the environment (including human health), has been sometimes added to agricultural soils of some regions through the disposal of tannery wastes. There is a lack of knowledge about the movement of this metal in highly weathered soils frequently found in the tropics. The purpose of this work was to study the movement of Cr in soil columns considering the influence of the pH and the horizons of the soils. It was also applied a mathematical model to quantify some parameters of this movement, although in an exploratory way. Miscible displacement experiments were carried out in A and B soil horizon samples, that received two liming levels,
*Correspondence: O. A. de Camargo, Soils and Agroenvironmental Resources Center, Institute of Agronomy (IAC-CSRA), P.O. Box 28, Campinas 13020-902, Sa˜o Paulo, Brazil; E-mail:
[email protected]. 599 DOI: 10.1081/CSS-120030346 Copyright & 2004 by Marcel Dekker, Inc.
0010-3624 (Print); 1532-2416 (Online) www.dekker.com
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collected in Sa˜o Paulo State, Brazil: a Typic Eutrorthox (TE), and a Typic Haplorthox (TH). Liming showed little effect on the elution curve forms for both soils. The Cr movement in the A horizon for the TE soil was greater than the observed in the B horizon, possible due the formation of soluble compounds between the metal and fulvic acids. It was also applied a simplified version of the MRTM (Multireaction and Transport Model), which showed good fit to experimental data. Key Words: Oxisols; Heavy metal; Chromium; Movement.
INTRODUCTION In some regions of the tropics, as in Brazil, it is a common practice the disposal of tannery industrial wastes to agricultural soils, in order to improve some physical and chemical attributes. The tannery wastes may contain chromium in the trivalent form. This metal may be present with variable concentrations, depending on the tanning process during leather production. The presence of this metal implies in a potential risk for the groundwater contamination, an important task concerning environmental pollution studies.[1] If the adsorption capacity of a soil is overcome, the metal becomes potentially free to move through soil profile. The adsorption capacity may vary according to the attributes of the different soil horizons. The pH is an important soil attribute that influences both the adsorption and the solubility of Cr in the soil. Increasing the soil pH, increases the adsorption of Cr to the soil, and diminishes its solubility.[2,3] In fact, the influence of the pH on Cr behavior is much more complex, once it may modify the presence of the trivalent and hexavalent forms. The Cr(III) and Cr(VI) have distinct behaviors in the environment.[2] In view of this, it is expected that liming, a common practice used to increase soil pH, could reduce the mobility of Cr in soils. In metal movement studies, it is common the usage of mathematical models. The complexity of the existent models is variable. As much complex is the model, more difficult its application is in practice. Frequently, to solve an environmental problem it is demanded rapid solutions. The usage of simple models, that minimize the necessity of specific information for each soil and also need simple calculus, may be advantageous in this sense, even furnishing only an exploratory prevision of the contaminant front displacement through the soil, considering a long period.[4]
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The simulation of the movement of solutes has been done in a simplified way, considering saturated flow. Although in the field, the real flow condition is usually not saturated, this simplification has been used because do not imply errors to short term estimation.[5] Theoretically, the maintenance of a saturated flow would result in a more conservative approach to solute movement. The objective of this work was to study chromium movement in columns of two highly weathered soils from Brazil, considering the influence of the superficial liming of the soils, an the influence of the soil horizons. Also, a simplified formulation of MRTM (Multireaction and Transport Model) was applied, in order to quantify, although in an exploratory way, the Cr movement.
Model Theory Several models have been applied to predict the movement and the sorption of solutes in soils. The MRTM is one of them. It was used previously by Selim et al.[6] to predict the movement of Cr(VI) and Hg in soil columns.[6–8] Ma and Selim[9] also used this model in order to predict the movement of some agro toxics like atrazine. The MRTM considers that the movement of an element may be described by the Eq. (1), the general equation for the solute transport in soils[10]:
@S @C @2 C @C þ ¼ D 2 q Q @t @t @z @z
ð1Þ
where S is the amount of solute associated to the soil solid phase (mg kg1); C is the solute concentration in soil solution (mg dm3 or mg L1); Q is the solute removal (or supply) not included in S (mg cm3 h1); D is the dispersion-diffusion coefficient (cm2 h1); q is the Darcy flux (cm h1); is the specific mass of the soil, also called bulk density (g cm3); is the soil moisture content (cm3 cm3); z is the spatial variable (cm), and t is the time (h). In the Eq. (1), the @S=@t term describes the reversible process between the solution and the solid phases. Some simplifications for the model can be made.[11] The MRTM considers that the reversible retention is kinetic with multiple reactions (multi-sites), where the solid phase S is composed by two phases, as shown in Eq. (2): S ¼ se þ s1
ð2Þ
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k1 se
KF
k2 C
kirr
sirr
Figure 1. Schematic representation of the considered reactions between Cr in the soil solution and soil solid phases—simplified from Selim and Amacher.[8]
In the original version of the model, there are other phases, as s2 and s3 that react consecutively with the s1 phase. But these additional phases will not be focused in this work. The Q term in the Eq. (1), referred to the removal (or supply) term not considered in S, represents the irreversible kinetics reactions between the element in the solution and the element in the solid phase. Figure 1 is a schematic representation of the reactions between the different solid phases and the element in the soil solution. In the simplified version of MRTM, it is assumed that the solute is present in soil solution C and may be adsorbed in three phases, that represent the retained metal by the soil (se, s1, and sirr), where the liquid and the solid phases are expressed in mg L1 and mg kg1, respectively. The se phase is considered as the amount of heavy metal permanently adsorbed in equilibrium with the solute in the soil solution C. The mechanism that governs the equilibrium reaction may be explained by Eq. (3): S ¼ KF Cen
ð3Þ
where Ce is the concentration of the solute in solution in equilibrium with the solid phase (mg dm3 or mg L1); KF is the Freundlich adsorption coefficient (kg1 n cm3n kg1) and n is an estimate of the adjust parameter (dimensionless). It is considered that the metal present in soil solution C reacts kinetically and reversibly with s1, very slowly and reversibly with s2 and irreversibly with sirr. The kinetic reaction between C and s1 may be represented by: @s1 ¼ k1 C n k2 s1 @t
ð4Þ
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where kl and k2 are respectively the direct and reverse coefficients (h1) and n is the reaction order (dimensionless)—if n 6¼ 1, the reaction is not linear. When studying the movement of Cr(VI) and Hg in soil, it is possible to make some remarks about the theoretical assumptions of the MRTM.[6] The model considers that the water flux is constant, in a soil with homogenous moisture. In other words, q and do not vary along the time and depth. The model also assumes that a certain solute with a known concentration C0 was applied on the soil surface during a time tp, and in sequence it was applied a displacing solution without the metal being studied. To achieve these experimental conditions in practice, it is necessary to carry out experiments using the miscible displacement theory. To apply the model, or, to simulate the movement of solutes in soil, it is necessary to solve the second order differential equations presented here. Different analytic or numeric solutions are proposed for these equations. The MRTM consists basically in a computational program wrote in Fortran language that solves these equations, considering the boundary and initial conditions presented here. An extensive review about the MRTM can be found in Selim et al.[6] and Alcaˆntara.[12]
MATERIALS AND METHODS The study was conducted using samples of the A and B horizons of two highly weathered tropical soils: a Typic Eutrorthox (TE) and a Typic Haplorthox (TH). The physical and chemical attributes of these soils are shown in Table 1. The soil samples were passed through a 2-mm sieve and stored at approximately 80% field capacity humidity until being used in the leaching experiments. In order to study the effect of the soil pH on the movement of Cr, the A horizon of the two soil samples were incubated with calcium carbonate, in doses corresponding to 3.5 and 3.1 t ha1, respectively, to raise the percent base saturation to 70%. For the study of the soil organic matter effect, it was utilized the A and B soil horizons samples, with no liming. This procedure was adopted because the inorganic chemical and mineralogical compositions of these two horizons for each soil are quite similar, differing just in the quantity and quality of the soil organic matter (Table 1). It was carried out leaching experiments in soil columns, using the miscible displacement theory, detailed by Kirkham and Powers.[13] The soils initially stored wet, were dried at 100–110 C, and passed through a
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Chemical and physical properties of the soils. Soils
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Attributes pH in 1 M KCl (initial) pH in KCl (after liming) Organic C (g kg1) Humic acid (g kg1) Fulvic acid (g kg1) Humine (g kg1) Fe2O3 in H2SO4 (g kg1) Al2O3 in H2SO4 (g kg1) Mn easily reducible (mg kg1) Clay (g kg1)
TE TE TH TH A Horizon B Horizon A Horizon B Horizon 4.34 5.50 15.5 3.0 3.2 9.3 209 186 146 560
5.26 — 3.8 0.5 1.4 1.9 222 214 45 580
3.58 5.70 8.3 2.9 1.9 3.6 52 71 1.8 120
3.93 — 2.9 0.3 1.2 1.4 52 86 2.5 150
2-mm sieve. Then, bulk soil from each horizon was packed into a 4.5-cm diameter glass tube. The length of the soils inside the column was 10 cm. The packing for each column was made in 2.5 cm height steps using a long rod funnel in order to minimize segregation of aggregates with different sizes. In each step, the soil was compacted in order to achieve a predetermined density (Table 2). In the base and in the top of the soil were used geotextile blanket, to avoid whirling and surface sealing. The leaching system was carried out according to the following steps: (1) slow soil saturation (beginning at the soil base), using as displacing solution 5 mmol L1 Ca(NO3)2; (2) application of the displacing solution at the top of the soil column, until achieving a constant flux q (Table 2); (3) application of 200 mL of the pulse containing 500 mg L1 Cr as CrCl3 salt (it was considered t ¼ 0, the beginning of the chromium pulse application, that coincided with the first chromium sample collected in the effluent); and (4) application of the displacing solution, until the whole chromium elution. During all the steps, the saturated flux was maintained constant by applying a constant 1 cm head of water at the top boundary. The total volume collected in the effluent is equal to the sum of the partial volumes Vi of each sample collected during the steps 3 and 4. The total volume V considered during the elution also corresponds to the sum of the applied pulse volume with the displacing solution volume applied after the pulse. The characteristics of the columns are presented in Table 2.
A A B A A B
TE TE TE TH TH TH
no with no no with no
Liming 310.13 310.13 310.13 357.84 357.84 357.84
Soil mass (g) 1.3 1.3 1.3 1.5 1.5 1.5
Bulk density (kg dm3) 0.59 0.59 0.47 0.44 0.44 0.45
Total porositya (cm3 cm3) 500 500 500 500 500 500
Applied chromium (mg L1)
Characteristics of the studied columns.
2.49 6.58 1.95 27.44 5.15 15.00
Flow density (cm h1)
200 200 200 200 200 200
Pulse volume (cm3)
12.8 10.7 15.3 16.9 12.5 14.4
Pore volumeb
b
Saturated flow: the volumetric wetness () is equal to the total porosity of the soil. Applied pore volume ( p), obtained by the ratio: p ¼ V/V0 , where V is the pulse volume þ displacing solution volume, and V0 is the volume occupied by the pores in the column.
a
Horizon
Soil
Column
Table 2.
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The effluent chromium concentration C of the samples collected during the steps 3 and 4 were determined by Induced Coupled Plasma (ICP-AES), in order to obtain the breakthrough curves (BTCs). The BTCs were obtained by plotting the relative concentrations C/C0 vs. the pore volumes p ¼ V/V0, were V0 is the volume occupied by the soil pores in the column. It was applied a chloride pulse as 30–40 mMol L1 CaCl2, followed by application of the displacing solution, in order to obtain the dispersion-diffusion coefficient. This coefficient was obtained calculating the slope of the chloride BTC when C/C0 ¼ 0.5.[13] In this work, Cr was applied to the columns as CrCl3, to furnish Cr in the trivalent form. In Cr movement studies, it is a common practice the usage of a source containing Cr in the hexavalent form. However, the trivalent form is the predominant one (if not the only one) present in tannery wastes.[14] As the columns were conducted under saturated flux, it was believed that the redox conditions would not allow changes of the Cr(III) forms to Cr(VI), although any analysis in this sense were made. Otherwise, eventual Cr(VI) forms that could be present, should be reduced to Cr(III). To avoid problems with the different oxidation species, the authors quantified only the total Cr. The MRTM allows different formulations, according to the considered phases. Due to the exploratory character of this study, it was used only an arbitrary formulation of the model, containing the element in soil solution C, the solid phases se, s1 and the irreversible phase sirr. The considered parameters were consequently KF, k1, k2, and kirr. It should be remarked that the used formulation was an arbitrary one, and not necessarily represents the best fit to experimental data. Also, the validation of the model was beyond the scope of this work. The KF was obtained in two ways. The first one was experimentally (KF exper), by adjusting de Freundlich coefficient according to Eq. (3). The second manner was a manual optimization of the KF (KF adjust) in order to fit the simulated and the experimental BTCs. The sirr was manually optimized to fit the C/C0 max obtained (as the C/C0 max obtained experimentally decreased, the sirr assumed lower values). However, it was just a mathematical adjustment, since no effort was made to verify if the Cr was really irreversibly bound to the soil. The fitting of the model to the experimental data was verified calculating de correlation coefficients (r) between experimental and predicted data. As an example of model utilization, two columns of the B horizon were selected, and the simulation of Cr movement was made considering an arbitrary time after the pulse application.
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RESULTS AND DISCUSSION
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MRTM Simulation Curves The correlation coefficients (r) between experimental and simulated data, as well as the fitting parameters are shown in Table 3. All the r-values varied from 0.846 to 0.976 and were significant at 1% probability, indicating a good fit between experimental and simulated data. Selim et al.[6] in leaching experiments for Cr(VI), applied a metal initial concentration C0 five times smaller than the one utilized here, a smaller flow rate, varying from 0.17 to 1.13 cm h1, and the correlation coefficients obtained varied from 0.965 to 0.986. Figure 2 shows simulated and experimental curves. It was observed good fit between experimental and simulated data for all the columns. However, it should be noticed that the fitting for almost all the curves was reached using the KF adjust, instead of the KF exper. For some columns of TH, it was possible to observe a good visual fitting, even at smallest r-values. This may be due to the differences in the top width of the experimental and simulated curves. The top width of the curve depends on the pulse application time, tP, a parameter measured directly during the leaching in the columns. Then, this parameter cannot be modified during the model simulation. In this work, applying a 1 cm layer of the liquid to the surface of the soil do not ensure a flux perfectly
Table 3. Parameters values for simulating the elution curves with MRTM and correlation coefficients.
Soil Horizon Liming TE TE TE TH TH TH
A A B A A B
no yes no no yes no
r 0.928 0.976 0.962 0.881 0.846 0.976
KF exper KF adjust KF exper/ k1, k2 kirr (cm3 kg1) KF adjust (h1) 63.9 104.2 78.0 23.7 35.7 21.5
15.0 10.0 25.0 15.0 10.0 5.0
4.26 10.42 3.12 1.58 3.57 4.3
0.1 0.1 0.1 0.1 0.1 0.1
0.35 1.10 0.48 0.00 0.00 0.00
KF exper: Freundlich adsorption coefficient; experimentally obtained from chromium adsorption isotherms. KF adjust.: adsorption coefficient obtained to fit the experimental data. k1 and k2: considered constant for all the columns. : 1% significance by t-test.
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608 TYPIC EUTRORTHOX
TYPIC HAPLORTHOX
0.80
0.80
0.60
0.60
C/C0
C/C0
A HORIZON - NO LIME 1.00
0.40 0.20 0.00 0.00
0.40 0.20
3.00
6.00
9.00
0.00 0.00
12.00 15.00 18.00
3.00
6.00
p = V/V0
0.80
0.60
0.60
C/C0
C/C0
1.00
0.80
0.40 0.20
0.40 0.20
3.00
6.00
9.00
0.00 0.00
12.00 15.00 18.00
3.00
6.00
p = V/V0
9.00
12.00 15.00 18.00
p = V/V0
B HORIZON - NO LIME
B HORIZON - NO LIME 1.00
0.80
0.80
0.60
0.60
C/C0
1.00
0.40 0.20 0.00 0.00
12.00 15.00 18.00
A HORIZON - LIMED
1.00
0.00 0.00
9.00
p = V/V0
A HORIZON - LIMED
C/C0
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A HORIZON - NO LIME 1.00
0.40 0.20
3.00
6.00
9.00
p = V/V0
12.00 15.00 18.00
0.00 0.00
3.00
6.00
9.00
12.00 15.00 18.00
p = V/V0
Figure 2. Experimentally obtained breakthrough curves (dots) and MRTM simulated elution curves (solid line).
constant during the whole leaching in some columns. This fact could explain in part the results obtained. The KF used in the simulation (KF adjust) for almost all columns were generally much smaller than the adsorption coefficients obtained in the batch experiments, adjusting the adsorption isotherms (KF exper). These optimizations were necessary to fit the beginning of the recuperation data for Cr in the model to those experimentally obtained. It works because, when the value of KF adjust becomes small, the whole curve is displaced to the left side and when its value becomes high, the reverse occurs. Modifications in some parameters values obtained in adsorption studies
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to adjust the simulated curves to experimental data was also made by Amacher and Selim.[15] Possibly, the experimental conditions involved in acquiring this parameter may explain the results. The KF exper were obtained under static conditions, by shaking the contaminant salt with the soil, without considering the forward moving of the solute front through the soil. Under dynamic conditions, the contact between the adsorbent (soil surface) and the adsorbate (Cr) would be much less than the obtained under static conditions, when the contaminant liquid is vigorously shacked with the soil during a relatively greater time period. Then, as the KF expresses the relationship between the element in solution and the adsorbed, the smallest contact under dynamic conditions in column could explain the smallest values obtained for this parameter compared to that obtained in adsorption studies. Liming increased the ratio KF exper/KF adjust for the two soils (in TE soil this ratio was modified from 4.26 to 10.42 and, in TH soil, from 1.58 to 3.57). This enlarged ratio caused by liming could be misinterpreted as an incapacity of the model to predict the increase in adsorption of the element as a consequence of the soil pH rising. However, this hypothesis may be discarded if one considers the liming effect on the kirr parameter. The kirr. corresponds to the irreversible reaction rate between the element in the solution C and the element irreversibly adsorbed to the solid phase sirr. This parameter determines the height of the curve or, in other words, the maximum value obtained for the relative concentration in the effluent, C/C0 max. Considering a same curve, increasing the kirr, value, causes a decrease in the C/C0 max obtained in the effluent. For TH soil, all the columns presented a C/C0 max ¼ 1.0 and a kirr equal to zero. For TE soil, liming modified the kirr value from 0.35 up to 1.10 h1. Then, according to the theoretically considerations made for the model, liming would cause an increase on the irreversible adsorption of the element, and not an increase of the adsorption of the element to the exchangeable forms, as predicted by the KF parameter. For TH soil, since the C/C0 max was not affected by liming, it was not possible to evidence this effect. However, once more it is necessary to remember that no effort was made to verify in practice if irreversible reactions really occurred in the soil. It was made only a mathematical adjust of the kirr to fit the data. But, if irreversible reactions occurred or not, these results agree with the expected higher retention values for the element with the soil pH increase. In this study, the values for k1 and k2 parameters were arbitrarily maintained constant and equal to 0.1 h1. The adoption of a constant value facilitated too much obtaining the other parameters. Theoretically, kl and k2 should be adjusted individually for each column, once they express the rates for direct and reverse reactions between C and s1. Beside
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this, the individual adjustment would result in a higher value for the k1 than for the k2. However, this simplification, even with a dose of arbitrariness, did not cause a great alteration in the forms of the curve. In several simulations realized, it was observed a small (sometimes not perceptible) modification on the forms of many curves varying the k1 and k2 parameters from 0.1 to 50.0 h1. The small influence of the k1 and k2 on the elution curves shapes, suggests that the kinetic reactions are not important in the model description of Cr movement.
Movement Simulation Example: A Case Study Two columns for simulating the Cr movement were selected: the B horizons of TE and TH soils. The selected simulation times were 7.07 and 1.71 h after starting pulse application, that correspond to 1 h after the pulse application was over. These selected time, although arbitrary, have a practical meaning: simulates the Cr movement through the soil profile 1 h after a chromium front was applied. The obtained simulations are shown in Fig. 3. Using the MRTM, it was simulated the Cr quantities in soil solution C and the total adsorbed S, considering 0.25 cm depth increments. A great portion of the applied Cr in the TE soil remained in the soil 1 h
Solution (C)
Adsorbed (S)
concentration , mg L-1 0
250
500
concentration , mg kg-1 0
750
0
0
-2
-2
-4
-4
deep , cm
deep , cm
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610
-6 -8 -10
TE TH
250
500
750
-6 -8
TE TH
-10
Figure 3. Simulated concentrations for chromium in solution (C) and adsorbed chromium (S) 1 h after the pulse application to the B horizons of the two soil columns.
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after the pulse application. A part of it is adsorbed, decreasing the risk of further movement of the metal. However, in this soil, nearby 5 cm deep, it was also found maximum concentration of Cr in solution C (approximately 280 mg L1). In the TH soil, the reduced amounts, either in solution or adsorbed, indicated that the Cr was almost totally leached out the soil profile. Furthermore, for this soil, the MRTM suggests that there is no Cr adsorbed in the irreversible soil phase (kirr ¼ 0) that would be difficult to leach. In other words, these results show that almost all the applied Cr leached over the 10 cm depth. The Cr in solution is, at least in theory, available to be transported to a greater depth if more displacing solution is applied to the soil surface. Then monitoring the soil solution Cr concentrations assumes an important role from a practical point of view, because it indicates the readily available quantity for displacement, that may occur either by applying a displacing solution (in laboratory) or by rainfall or irrigation water (in field conditions). Until nowadays, metal leaching studies using the miscible displacement theory are scarce in Brazilian tropical conditions. The simulations presented here resulted from an exploratory application of the MRTM. The use of one model formulation, made it possible to evaluate only a small portion of its capability concerning the Cr movement in highly weathered soils. However, the results obtained indicate that it is a promising tool for predicting heavy metals movement and, should receive special attention by the researchers and by environmental protection agencies in tropical countries like Brazil.
CONCLUSIONS The MRTM application resulted in a good fit of the experimental data obtained in highly weathered soil columns. The KF coefficients obtained in static adsorption studies were not adequate to the mathematical fitting of the experimental data. Liming showed little effect on the elution curves forms for the studied soils. However, for the TE soil, the kirr value obtained in the column with liming was greater than that for the no limed column, indicating an increase in the irreversible reaction rates. The Cr movement in the A horizon for the TE soil was greater than the observed in the B horizon for this soil, possible due the formation of soluble compounds between the metal and the fulvic acids.
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ACKNOWLEDGMENTS
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Financial support from the National Research Council (CNPq) and FAPESP are gratefully acknowledged.
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Chromium Movement
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