INTRODUCTION

The deposition of industrial waste, mining activities, incidental accumulations, atmospheric deposition, agricultural chemicals, etc., are some sources for the pollution of soils with heavy metals (1). The mobile forms of those heavy metals constitute a risk as they may leach into

groundwater that could be later used for human or animal consumption (2). Soil pH, other factors such as the presence of competing ligands, the ionic strength of the soil solution and the simultaneous presence of competing metals are known to significantly affect sorption processes and leaching potential through a soil profile (3). The absorption of heavy metals differs in the different soil horizons, due to texture composition in different soil horizonts.

In order to avoid the accumulation of metal and their movement within soil profile, different remediation techniques were developed. Among them those based on adding materials capable of immobilizing mobile forms of metals (4), (5), like compost and biosolid are adequate (6) are adequate. Another technology is the phytoremediation, based on the absorption of heavy metal by different plant species, which latter are removed (7). Perez-de-Mora et al. (8) found that the use of amendments (like biosolid compost) and plant cover was important for in situ remediation of heavy metal contaminated soils.

Our objective was to evaluate the effects of biosolid compost and phytoremediation separate or simultaneously applied on the leaching of cadmium, copper, lead and zinc, through the different horizons of a superficially polluted soil.

MATERIALS AND METHODS

The horizons A, Bt and BC of a Typical Argiudoll located in the province of Buenos Aires (34[degrees]8' S, 59[degrees]4'W), Argentine were collected. The soil main characteristics are included in Table 1. Mineralogy values obtained by Castiglioni et al. (9) in nearby Tipic Argiudolls are present (X-ray decomposition Program, DECOMPXR) (10). The horizon A was enriched with cadmium copper, lead and zinc applied as nitrate. The polluted soils were moistened to field capacity and allowed to dry in cycles of approximately 15 days within a 3-month period in order to reach equilibrium with soil colloids (11).

Table 1: Main characteristics of the studied horizons. S = Smectite, I
= Illite, I/V = Interstratified illite-vermiculite. The relative
proportion expressed in area percentage in diffraction diagrams were
after Castiglione et al.(9)

                                       Mineralogy
Soil       OC        Clay  Silt  Sand
Horizons    %   pH     %     %     %   S   I+I/V      Texture

A         2.02  5.8  31.3    57  11.7  44     53  Loamy clay-silty
Bt        0.83  6.3  62.9  28.3   8.8  64     33  Clayey
BC        0.21  6.7  42.3  46.8  10.9  81     18  Clayey-silty

The soils were put in PVC tubes (0.15 m diameter) of three heights. Columns of 0.20 contained 0.12 m A horizon (A), columns of 0.35 contained 0.12 m A+0.15 m Bt horizon (B) and columns of 0.48 m contained 0.12 m A+0.15 m horizon Bt+0.13 m horizon BC (C).

The treatments were: 1) polluted soil (control), 2) polluted soil+plant (Plant), 3) polluted soil+50 Mg biosolid compost [ha.sup.-1] soil (Compost) +50 Mg biosolid compost [ha.sup.-1] +plant (Compost-Plant). The plant used was Festuca rubra. The experiment was designed as a random block test with three repetitions per treatment. The compost was prepared with sawdust as the structuring material and biosolids (1:1, v:v) obtained in the sewage treatment plant located in San Fernando, province of Buenos Aires. Its most relevant characteristics are shown in Table 2a.

Table 2a: Main characteristics of the biosolidcompost. CEC, capacity
exchangeable cationic, TC, total carbon, SC, soluble carbon, TN, total
nitrogen, HA, humic acid, FA, fulvic acid, IL, ignition losses, DM, dry
matter

pH         CEC        TC       SC       TN    HA   FA  IL  DM

(1:5)  [Cmol.sub.c]           g                             %
       [kg.sup.-1]        [kg.sup.-1]

6.9        16        4.2      0.03     0.44  0.5  0.3  41  65

Leachates were collected after adding water to columns: A: 1000 mL, B: 1200 mL, C: 2000 mL. Leachates were obtained out after harvesting vegetal material, every 7-8 weeks, totaling 4 leachings. No leachates were produced between sampling. Cadmium, Cu, Pb and Zn were determined using plasma emission spectrometer technique (ICP) (12).

Data are presented as a concentration of metals and as a total mass of leached metals. It was calculated by multiplying concentration and the volume of leachate divided by the volume of water that entered each column. All data were statistically analyzed through the analysis of variance (ANOVA) and the difference among means was checked with the least significant differences (LSD, p<0.05).

RESULTS AND DISCUSSION

The content of heavy metals in the compost (Table 2b) was below the limits established by the Argentinean regulations (13). Therefore, there would be no significant heavy metal addition in treatments using compost.

Table 2b: Heavy metals in biosolid compost, mg [kg.sup.-1]

Ni   Ba   Cr   Cu   Zn    Pb   Hg   As  Ag  Se  Cd

109  443  230  727  1183  383  4.3  17  21  <7  <4

The impact the treatments had on the leachates obtained from the soil columns is showed in Table 3. There was a general variability of concentrations within each treatment, which caused the lack of significant differences among them. However, in the treatment compost, Cd concentration in some leachings of columns whit A and C horizons were significantly higher (p<0.05) if compared to the Control. Besides, in Control, there was a significant increase of Cd concentrations (p<0.05) in successive leachings. Antoniadis and Alloway (14) observed that the application of an organic waste in polluted soils increased Cd concentrations in leachates. This is possibly due to the formation of complexes with soluble fractions of organic matter. Even though the concentration of Soluble Organic Carbon (SOC) of the compost used was low (15), Table 2, it could cause of the increase in Cd solubility, thus favoring its movement within the columns.

Table 3: Average concentrations of Cd, Cu, Pb and Zn among the four
leachings collected in the different columns, mg [L.sup.-1]. 1[degrees]
letter: Significant differences within leachings, 2[degrees] letters:
Significant difference within treatment. Values followed by the same
letter do not differ significantly (p<0.05)

Columns  Leachings  Control         Compost          Plant

Cd
A                1  0.01327  a-ab   0.00613  b-b     0.00833  ab-b
                 2  0.00220  b-b    0.00453  b-a     0.00133  b-b
                 3  0.00787  a-ab   0.01027  a-a     0.00900  ab-ab
                 4  0.01027  a-ab   0.00840  a-ab    0.00627  ab-b
B                1  0.00024  b-nd   0.00156  b-nd    0.00045  b-nd
                 2  0.00093  b-nd   0.00264  b-nd    0.00040  b-nd
                 3  0.00773  a-ab   0.00747  a-a     0.00693  ab-ab
                 4  0.00648  a-nd   0.00707  a-nd    0.00627  ab-nd
C                1  0.00000  c-b    0.00100  c-a     0.00053  b-ab
                 2  0.00112  bc-ab  0.00033  c-b     0.00040  b-b
                 3  0.00208  ab-c   0.01587  a-a     0.00720  ab-b
                 4  0.00320  a-nd   0.00680  b-nd    0.00000  b-nd

Cu

A                1  0.01760  nd-b   0.02590  a-b     0.02367  a-b
                 2  0.00807  nd-nd  0.00600  b-nd    0.01360  a-nd
                 3  0.00880  nd-nd  0.00533  b-nd    0.01573  ab-nd
                 4  0.00880  nd-nd  0.00560  b-nd    0.00227  b-nd
B                1  0.00760  a-nd   0.01364  nd-nd   0.01505  nd-nd
                 2  0.00573  a-nd   0.00360  nd-nd   0.00648  nd-nd
                 3  0.00747  b-nd   0.00240  nd-nd   0.00547  nd-nd
                 4  0.00320  ab-nd  0.00540  nd-nd   0.00120  nd-nd
C                1  0.00273  nd-nd  0.00273  nd-nd   0.00167  nd-nd
                 2  0.00533  nd-nd  0.00336  nd-nd   0.00610  nd-nd
                 3  0.00266  nd-nd  0.00600  nd-nd   0.00460  nd-nd
                 4  0.00040  nd-nd  0.00493  nd-nd   0.00000  nd-nd

Pb

A                1  0.00593  b-nd   0.00875  bc-nd   0.01013  b-nd
                 2  0.00000  b-b    0.00000  c-b     0.00633  b-a
                 3  0.01280  a-nd   0.02267  a-nd    0.02987  a-nd
                 4  0.01680  a-nd   0.01360  ab-nd   0.01427  ab-nd
B                1  0.00087  b-nd   0.00456  abc-nd  0.00875  b-nd
                 2  0.00000  b-nd   0.00000  c-nd    0.00000  b-nd
                 3  0.00980  a-nd   0.01320  a-nd    0.01773  a-nd
                 4  0.01520  a-a    0.01253  ab-ab   0.00215  b-c
C                1  0.00000  b-nd   0.00000  c-nd    0.00000  b-nd
                 2  0.00000  b-nd   0.00000  c-nd    0.00040  b-nd
                 3  0.00000  a-nd   0.01700  a-nd    0.00920  a-nd
                 4  0.01220  a-nd   0.01093  bc-nd   0.00000  b-nd

Zn

A                1  0.73547  a-a    0.66155  a-a     0.44807  a-b
                 2  0.53000  b-ab   0.63967  ab-a    0.43920  a-b
                 3  0.11133  c-nd   0.09453  b-nd    0.13600  b-nd
                 4  0.09920  c-ab   0.11400  b-a     0.06160  b-b
B                1  0.65130  a-a    0.45444  a-ab    0.29398  a-b
                 2  0.45800  b-a    0.41648  ab-a    0.34095  a-b
                 3  0.09453  be-a   0.07900  ab-a    0.04693  b-b
                 4  0.03560  c-nd   0.05733  b-nd    0.06200  b-nd
C                1  0.18080  b-nd   0.34100  a-nd    0.28567  a-nd
                 2  0.43900  a-nd   0.37776  a-nd    0.37153  a-nd
                 3  0.08297  bc-b   0.08440  b-a     0.04340  b-b
                 4  0.02900  c-nd   0.08440  b-nd    0.04493  b-nd

Columns       Compost-plant

Cd

A              0.02047  a-a
               0.00173  c-b
               0.00540  b-b
               0.00667  b-b
B              0.00120  b-nd
               0.00127  b-nd
               0.00580  a-b
               0.00360  b-nd
C              0.00040  b-ab
               0.00007  b-b
               0.00453  a-bc
               0.00180  ab-nd

Cu

A              0.04760  a-a
               0.00650  b-nd
               0.00720  b-nd
               0.00533  b-nd
B              0.00500  nd-nd
               0.00573  nd-nd
               0.00520  nd-nd
               0.00347  nd-nd
C              0.00133  nd-nd
               0.00193  nd-nd
               0.00533  nd-nd
               0.00173  nd-nd

Pb

A              0.00873  nd-nd
               0.00000  nd-b
               0.00920  nd-nd
               0.01387  nd-nd
B              0.00060  b-nd
               0.00000  b-nd
               0.00533  a-nd
               0.00640  a-bc
C              0.00000  b-nd
               0.00000  a-nd
               0.00307  b-nd
               0.00000  b-nd

Zn

A              0.82760  a-a
               0.40700  b-b
               0.08347  c-nd
               0.05333  c-b
B              0.48200  a-ab
               0.36580  b-b
               0.04787  c-b
               0.04800  c-nd
C              0.26167  b-nd
               0.36050  a-nd
               0.05840  c-b
               0.04064  c-nd

Copper concentration increased significantly (p<0.05) in the first leachate of A column, in the Compost-Plant treatment. Cu concentrations diminished considerably (p<0.05) in the successive leachings of all treatments with Compost and Compost-Plant. Gove et al. (16) observed in soil columns of 0.40 m that losses due to leachings were generally greater during the first leachings, thus assuming that there was a partial decrease of Cu in the labile pool.

Lead concentrations were not affected in general terms by the treatment. This could be accredited to the low solubility of Pb in the soil (17). Zinc concentrations in the leachates diminished significantly (p<0.05) when compared Plant treatment to the Control in certain columns and leachings.

The mass of leached metals of each treatment is showed in Fig. 1. In general terms, the same leaching pattern was observed in the four analyzed metals and in all treatments. There was a greater mass of heavy metals in the leachates in A column, if compared to B and C columns. No significant variation was found into B and C columns. Horizon Bt, with 62.9% of clay, presents evidently a barrier for the passing of metals, and caused a true abrupt reduction of the total mass of leached metals. Due to their high specific surface, clay minerals play an important role in the immobilization of heavy metals through the superficial complexion effect (18). Bt horizon with higher pH and clay content characterized by smectite (Table 1), respect to A horizon. B horizon, had the greatest metal sorption capacity due the presence of this clay mineral provides the soil with a large cation exchange capacity. Appel and Ma (19) observed that the presence of smectite as the dominant clay ensures high metal sorption capacity been a factor regulating the sorption of heavy metals by soils (Table 4). The BC horizon contains even more proportion of smectite than Bt horizon (Table 1), however its clay content is lower. In general, the B and C columns present the same leached metal content, reason why for this soil in study, the critical clay to limit the leaching are over 40% (Table 1). Considering the quality of clays this level critical is over 53.4 g smectite [g.sup.-1] soils (value calculated with data Table 1).

Table 4: Total heavy metals in the polluted superficial horizon and
maximum limits accepted by regulations (mg [kg.sup.-1])

         Total contents in the      Maximum limit as per
           polluted horizon     the different regulations *

Cadmium           7.22                          3
Copper            57.9                         50
Lead             149.2                        100
Zinc             166.4                        150

*: Lower limits found in USEPA, CCE and other countries' guidelines (22)

[FIGURE 1 OMITTED]

In B columns, the mass of leached Cd was greater in Compost or Plant treatments if compared to the Compost-Plant treatment. In a soil column experiment, another authors (20), (21) also observed that the presence of fescue or organic amendments increased Cd mobility. Copper showed a pattern similar to Cd, but no significant differences were observed among the treatments in any of the horizons. Pb evidenced a greater leached mass (d.s., p<0.05) in horizons A and B in Plant treatment. Zinc showed significant differences (p<0.05) only between Plant and Control treatments

Cadmium, Cu, Pb and Zn contents of the polluted soil horizon exceeded the limits established by international standards (Table 3). However, the concentration of those metals in the leachates from the different horizons evidenced that they scarcely exceed the limits established for the different uses of water (22).

CONCLUSIONS

This leaching experiment through the horizons of a typical soil of the Pampas region, showed that horizon Bt presents a barrier to metal leaching. Both concentration of clay and type of clay appears to immobilize heavy metals in those soils. The clay content over 40% and/or 53.4 g smectite [g.sup.-1] soil reduce the heavy metal leaching. The application of organic amendment or occurrence of plant eventually used in remediation techniques did not influence on the leaching of metals.

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Marta Susana Zubillaga, Emiliano Bressan and Raul Silvio Lavado

Catedra de Fertilidad y Fertilizantes, Facultad de Agronomia, Universidad de Buenos Aire, Av San Martin 4453-1417, Ciudad de Buenos Aires, Argentina

Corresponding Author: Marta Susana Zubillaga, Catedra de Fertilidad y Fertilizantes, Facultad de Agronomia, Universidad de Buenos Aire, Av San Martin 4453-1417, Ciudad de Buenos Aires, Argentina Tel.: +54-11-45248022