Heavy metal mobility in polluted soils: effect of different treatments

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.