Changes on chemical fractions of heavy metals in Chilean soils amended with sewage sludge affected by a thermal impact.
In central-southern Chile as many as 5100 forest fires occurred from October 2004 through May 2005. In addition, prescribed agricultural burns are frequent events to fix macronutrients, to remove vegetatation, and to control some soil fungi. Heat impact modifies the physical, chemical, and biological characteristics and properties of soils, as well as the organic and mineral reserves (Antilen 2002). The ecological effect of fire on soils and plants is widely recognised, and has been an important objective in different studies (Garcia-Corona et al. 2004; Lafleur et al. 2005; Molinari et al. 2005).
Soils derived from volcanic materials in central-southern Chile constitute around 70% of the country's arable land. These acid soils are rich in iron oxides and organic matter contents, and have pH-dependent variable surface charge and cation exchange capacity (CEC) and high phosphate accumulation. Their surface charge distribution changes with pH and ionic strength of the soil solution. Metal-humus complexes determine properties such as consistency, bulk density, and retention of water (Besoain 1999).
Before the installation of wastewater treatment plants, the collected wastewater in Chile was directly discharged and sewage sludge did not exist. With the commencement of wastewater treatment, sewage sludge is accumulating awaiting final disposal, and land application is the primary option under consideration.
Municipal sewage sludge contains organic matter, essential plant nutrients, and dissolved minerals, and has buffering capacity (Eriksson 1998; Zhang et al. 2002a, 2002b; Escudey et al. 2004a, 2004b; Pasquini and Alexander 2004). When land-applied, they may replenish the depleting nutrient reservoirs in these soils under cultivation, allowing the recovery of soil organic matter lost either during a forest fire or in degradation processes due to adverse environmental conditions and unsuitable agricultural practices (Margherita et al. 2006), but may also involve the input of variable quantities of heavy metals. Due to this potential pollution, Chile, like other countries, has set regulations with maximum values for metal concentrations in agricultural soils where sewage sludge may be applied (CONAMA 2000). The periodic application of sewage sludge increases the heavy metal concentration of soils. Thus, in soils with a mineralogy dominated by crystalline compounds and with lower organic matter content than volcanic soils, Sukkariyah et al. (2005) found that a negligible movement of trace metals through the soil profile occurred after 17 years of sludge application, and Chang et al. (1984) found that >90% of metals such as Cd, Cr, Cu, Ni, Pb, and Zn added in that way remained in the surface layer (0-0.15 m) after 6 years. Unlike others contaminants, most metals do not undergo microbial or chemical degradation in the soil; therefore, metal concentrations will remain without significant changes for a long period of time (Guo et al. 2006a).
The evaluation of the total metal content in soils or sewage sludge is useful for a global index of contamination, but it does not provide information about pollutant chemical fractions. On the other hand, it has been widely recognised through biochemical and toxicological studies that the environmental impact of heavy metal pollution can be related to soluble and exchangeable fractions that decide bioavailability, mobility, and toxicity in soils (Rauret 1998; Lock and Janssen 2001; Guo et al. 2006b)
Sequential extraction (Tessier et al. 1979; Sposito et al. 1982; Wenzel et al. 2001) is a useful technique for determining chemical fractions of metals, which are important in evaluating the risk of leaching involving contamination of downstream waterbodies, as well as the accumulation and bioavailability of heavy metals in agricultural and polluted soils. Studies carried out on the surface horizons of non-volcanic Chilean soils irrigated with untreated sewage effluent for up to 50 years, analysed with sequential extraction of heavy metals, showed for all heavy metals that a very low percentage of the total extracted was in exchangeable and adsorbed fractions (Schalscha et al. 1980).
Forest fires and prescribed burns are events that take place quite frequently; the lost organic matter can be recovered through amendment with sewage sludge. In that case, it is important to study the potential effect of a heat on heavy metal chemical fractions considering the application of sewage sludge before and after the fire. Thus, the objective of this study was to determine the chemical fractions of Cu, Zn, Pb, and Ni in volcanic soils amended with sewage sludge, and to determine the effects before and after the application of a thermal impact to simulate a forest fire.
Materials and methods
Samples of the surface horizons (0-0.15 m) of 2 volcanic soils located in southern Chile were used. One Ultisol (Collipulli; fine, mixed, thermic typic Rhodoxeralf) and one Andisol (Ralun; medial, mesic, Acrudoxic Hapludands) were collected, sieved at 2 mm, and stored at field moisture content.
The sewage sludge (SS) from a domestic wastewater treatment plant near the city of Santiago was used. The amount of sludge was calculated for each soil considering the OM content lost after heating at 400[degrees]C, and added to all sewage sludge-treated samples. To recover the soil organic matter lost after a 400[degrees]C heating, the equivalent of 11.7 and 33.1 t/ha of SS was added to the respective soils. To simulate the heat impact, all samples were placed in 2-3-cm layers into a furnace for 40 min to ensure a homogeneous heating. (Antilen 2002). The soil temperature during heating was measured by a type K, Ni-Cr thermocouple; 400[degrees]C heating temperature was chosen because it is the temperature reached at 0-5 mm depth after 40 min of surface heat impact under laboratory controlled conditions (Antilen et al. 2006). Five different conditions for each soil sample were considered: (i) control soil (C); (ii) control soil with addition of sewage sludge (C + SS); (iii) control soil burnt at 400[degrees]C (C + 400); (iv) control soil amended with sewage sludge and then burnt at 400[degrees]C (C + SS + 400); and (v) soil burnt at 400[degrees]C, then amended with sewage sludge and incubated for 2 months (C + 400 + SS + 2m).
Control and sewage sludge samples were characterised for organic carbon content using the Walkley-Black method (Allison 1965) adapted to Chilean soils (Sadzawka 1991), pH, and electrical conductivity (1:2.5 w/v soil : water). The CEC was estimated as the total amount of K + Ca exchanged in soil or sewage sludge pH. The isoelectric point (IEP) was carried out by micro-electrophoresis in a Zeta-Meter ZM-77 (Galindo et al. 1992).
The Walkley-Biack method when applied to volcanic soil following the procedure described by Sadzawka (1991) gives soil PC content in agreement with the combustion method; thus, when applied on 60 samples of Andisols the relationship O[C.sub.W-B] = 1.065 * O[C.sub.c] + 0.0029 ([R.sup.2] = 0.853) was obtained (data not shown). The Walkley-Black method was used to determine the destruction of OM due to heating and to estimate the sewage sludge addition to each sample.
Total content and chemical fractions of heavy metals
The chemical fractions of heavy metals were obtained by a sequential extraction procedure proposed by Sposito et al. (1982) carried out in duplicate. Heavy metal fractionation involves exchangeable, sorbed, organic, carbonate, and residual fractions. Samples (2 g) were sequentially treated with 25 mL of the following reagents: 0.5 mol/L of KN[O.sub.3] maintained in contact for 16 h; deionised water for 2 h (3 times, the extracts are combined); 0.5 mol/L of NaOH solution for 16h; 0.05 mol/L of [N.sub.a2]EDTA solution for 6 h; and 4 mol/L of HN[O.sub.3] at 80[degrees]C for 16h. After each extraction the suspension was centrifuged and the supernatant was filtered through a 0.45-[micro]m membrane filter. The content of Ni, Cu, Zn, and Pb in each extract was determined by ICP-OES spectroscopy using a Perkin Elmer Optima 2000 DV instrument.
The content of Ni, Cu, Zn, and Pb in the soil samples determined by ICP-OES on the superuatants of extracts obtained after digestion of 2 g of sample with 12.5 mL of HN[O.sub.3] (4 mol/L) at 80[degrees]C overnight was designed as total content (Sposito et al. 1982).
Results and discussion
The organic carbon content of Collipulli soil was lower than Ralun soil. In general, Andisols are richer in organic matter than Ultisols, and its accumulation is probably associated with mineralogy dominated by low crystalline compounds (Escudey et al. 2001). Sewage sludge has a pH 6.9, which is 1.7-2.5 times higher in alkalinity than the soils. The exchangeable cation content of the sewage sludge is about 18-47 times that of the soils, Ca being most important. Based on EC, the soluble mineral content of sewage sludge is orders of magnitude higher than that of the soils.
As the sewage sludge pH is close to neutrality, the amended soils showed a pH higher than control soils. As a consequence, the negative surface charge of amended soils will be increased accordingly with the isoelectric point values of samples (Table 1).
In variable surface charge soils, when pH increases the negative surface change increases and, consequently, the CEC of soils; thus, the adsorption of heavy metals may be also increased, a consequence of the amendment process. In accordance with the well-known buffer capacity of volcanic soils, the pH of amended samples declines to the control soil pH after an incubation period of 2 months (Antilen 2002). Therefore, in amended Chilean volcanic soils, after a period the CEC of amended soil will decrease and the excess of previously adsorbed heavy metals may be released to the soil solution.
Total content and chemical fractions of heavy metals
The chemical fractions and total content of Cu, Ni, Pb, and Zn in sewage sludge and control soils are presented in Table 2. The total metal content in the sewage sludge follows the sequence Zn > Cu > Pb > Ni. From fractionation data, it is observed that Zn and Cu are mainly associated with highly insoluble fractions, such as carbonates and residual fraction (26.4, 47.4 for Zn; 33.4, 11.3% for Cu, respectively). In general terms, these results obtained for Chilean volcanic soils are in agreement with those obtained for a polluted Chinese Phaeozem (Guo et al. 2006b). In control soils the total content of heavy metals follows the sequence Zn > Cu > Ni > Pb for Collipulli soil, and Cu > Zn > Ni > Pb for Rahin soil. In Collipulli control soil the high total content of Zn and Cu (71.5 and 73.8 mg/kg, respectively) is mainly present as highly insoluble and consequently low available residual fraction, associated with sulfides and oxides (Table 2). Cadmium is also a heavy metal important as soil pollutant, but it was not considered in the present study because of its concentration in soils (Collipulli 1.0 mg/kg, Ralun 0.4 mg/kg) and sewage sludge (6.9 mg/kg), and its low mobility in depth; thus, Cd has not been detected in leachates from studies with sewage sludge treated soil columns carried out on both soils, with a detection limit of 5 [micro]g/kg (Escudey et al. 2006).
As expected, in sewage sludge amended samples, the total content of heavy metals increased in proportion to sewage sludge application, around 2 and 13 times for Zn, 3 and 14 times for Cu, and 3 and 48 times for Pb for Collipulli and Ralun, respectively, and almost 2 times for Ni in both soils, compared with control soils content (Tables 2 and 3).
Following Sposito et al. (1982), the exchangeable, sorbed, organic, carbonate, and sulfide fractions of trace metals are believed to correlate well with the amount extracted by KN[O.sub.3], [H.sub.2]O, NaOH, EDTA, and HN[O.sub.3]. Even so, in this study the fractionation of the 4 trace metals will be reported according to the extracting reagent employed, instead of the expected soil solid-phase fraction.
[FIGURE 1 OMITTED]
The total heavy metal content is often lower than the amount determined by the sequential extraction procedure; this apparently unusual result has been previously observed (Sposito et al. 1982; Chang et al. 1984). The effectiveness of single extraction, even with an intensive reagent such as HN[O.sub.3], is lower than a multiple-step extraction procedure where the same reagent is also involved. The effectiveness is even lower after the thermal treatment, where the relative content of hardly soluble fractions, previously associated to alkaline-soluble organic fractions, increases.
In samples dried at 105[degrees]C, the loss of weight due to heating at 400[degrees]C was 3.6% for Collipulli, 11.8% for Ralun, and 34.0% for sewage sludge. In soils and sewage sludge as well as in sewage-sludge amended samples (C + SSs + 400), the weight lost was higher than the organic matter loss. In addition to organic matter oxidation, heating results in dehydroxilation of allophane and other inorganic compounds (Escudey and Moya 1989), and probably involves heavy metal vaporisation, as has been reported during sewage sludge incineration (Abanades et al. 2005; Menard et al. 2005).
In control and amended soil samples, the KN[O.sub.3] (sorbed) and [H.sub.2]O (exchangeable) extractable heavy metal fractions, considered the most available, represent <10% of the total content (Figs 1 and 2), with the exception of ZnKN[O.sub.3] (32%) and Zn-[H.sub.2]O (28%) in Ralun control soil. The sewage sludge application rate, application of temperature, or incubation time does not affect significantly the heavy metal content in the KN[O.sub.3] and [H.sub.2]O extracts. In Figs 1 and 2, the chemical fractions of heavy metals, expressed as relative content (wt %), as a function of the treatment are presented. The heavy metal mass balance for C and C + 400 samples presents a standard deviation of 12%. For sewage sludge amended samples, with or without thermal treatment (C + SS, C + SS + 400, and C + 400 + SS), the mass balance presents a standard deviation of 13%. Temperature may result in heavy metal vaporisation, particularly if metal chlorides are formed. However, Escudey et al. (2006) reported that after heating a similar sewage sludge at 500[degrees]C, heavy metal content showed differences within experimental error. Thus, heavy metal vaporisation after heating treatment is not a major process.
Amended soils (C+SS) showed changes in the distribution determined by chemical fractionation when compared with control samples (C). For all heavy metals, fractions associated to NaOH and EDTA extractants become more important, modifying the relative contribution of the residual form (HN[O.sub.3]) in the mass balance (Figs 1 and 2). The most important changes were observed for Cu and Zn, in proportion to the sewage sludge added to each soil.
[FIGURE 2 OMITTED]
In heated soil samples (C + 400), the content of the organic fraction decreased in relation to the control samples (C); this fraction was redistributed into the HN[O.sub.3] and EDTA fractions after organic matter oxidation. The Cu and Zn distributions were the most affected; Pb was detected only in Collipulli and mainly distributed in the EDTA fraction.
After the thermal treatment of sewage sludge treated samples (C + 400 + SS + 2m), the Cu-NaOH fraction in both soils, Ni- and Zn-NaOH in Ralun, Pb-HN[O.sub.3] in Collipulli, and Zn-EDTA in both soils significantly increased as result of organic matter oxidation. In relation to incubation time, only slight changes from NaOH to HN[O.sub.3] fractions were observed in all samples.
Due to thermal treatment of sewage sludge amended soils (C + SS + 400), the NaOH fraction turned into EDTA and HN[O.sub.3] fractions, resulting in more insoluble compounds such as carbonates and oxides associated with the residual fraction (Table 4). Similar results had been previously observed by Tack and Verloo (1993) and Obrador et al. (2001), for sewage sludge after a thermal treatment. As a consequence of a thermal impact on sewage sludge treated soils, an increase in the residual and more insoluble fractions is observed, and a reduction in metal available and leachable compounds must be expected, because these are associated to more soluble chemical fractions.
The pH of soil increases with sewage sludge amendment, and decreases later due to the buffering capacity of variable surface change volcanic soils. These changes will affect the adsorption and release of heavy metals to soil solution.
Heavy metals content increases several-fold after sewage sludge amendment; residual fractions of Cu and Zn associated with sulfides and oxides become the most important.
Thermal treatment reduces the NaOH fractions of heavy metals, because of the oxidation of soil organic matter. In amended and 2-month incubated samples, a slight redistribution of organic complexes from sludge to carbonate and residual fraction associated with insoluble species was observed.
Temperature application to sewage sludge amended soils results in the formation of more insoluble heavy metal fractions, reducing their mobility and groundwater contamination potential.
This study was supported by DIPUC No. 2003 16/E2 and FONDECYT (grant 1030778).
Manuscript received 16 January 2006, accepted 14 June 2006
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Monica Antilen (A,D), Nadia Araya (A), Margarita Briceno (B), and Mauricio Escudey (C)
(A) Facultad de Quimica, Pontificia Universidad Catolica de Chile, Vicuna Mackenna 4860, 6904411 Santiago, Chile.
(B) Departamento de Quimica, Universidad Arturo Prat, Av. Arturo Prat 2120, Casilla 12, Iquique, Chile.
(C) Facultad de Quimica y Biologia, Universidad de Santiago de Chile, Av. Lib. B.O'Higgins 3363, Santiago, Chile.
(D) Corresponding author. Email: email@example.com
Table 1. Chemical and physical properties of soils and sewage sludge Characteristics Collipulli Ralun Soil Order/source Ultisol Andisol Soil Class Fine, mixed, Medial, mesic, thermic typic Acrudoxic Rhodoxeralf Hapludands Latitude 36[degrees]]58'S 41[degrees]32'S Longitude 72[degrees]09'W 73[degrees]05'W Altitude 120-400 600-1400 Rainfall (m/year) 1.2-1.5 4.0-5.0 Annual mean temp. 15.8 10.5 ([degrees]C) CEC (cmol( + )/kg) 8.7 [+ or -] 0.5 7.1 [+ or -] 0.2 Bulk density (g/mL) 1.24 0.90 EC (mS/cm) 0.1 [+ or -] 0.0 0.3 [+ or -] 0.0 OC(wt%) l.8 [+ or -] 0.0 6.9 [+ or -] 0.0 Total P (mg/kg) 576 [+ or -] 15 901 [+ or -] 36 Chemical comp. (mg/kg) Cu 71.5 14.8 Pb 10.3 0.0 Zn 73.8 10.6 Ni 25.3 6.7 C-N analysis (%) C 2.33 9.83 N 0.20 0.74 C/N 11.15 13.23 Exchang. cations (cmol( + )/kg) Na 0.1 0.1 K 0.1 0.1 Ca 5.2 2.4 Mg 2.5 0.5 Isoelectric point 2.7 [+ or -] 0.1 2.8 [+ or -] 0.2 Mineralog. comp. Kaolinite Allophane >50% Total porosity (%) 50 61 Characteristics Sewage sludge Soil Order/source Water treatment plant Soil Class Solid waste Latitude 33[degrees]10'S Longitude 70[degrees]45'W Altitude 500-600 Rainfall (m/year) 0.3-0.4 Annual mean temp. 14.5 ([degrees]C) CEC (cmol( + )/kg) 105. 5 [+ or -] 8.2 Bulk density (g/mL) n.d. EC (mS/cm) 1.9 [+ or -] 0.1 OC(wt%) 18.0 [+ or -] 0.5 Total P (mg/kg) 10 200 [+ or -] 35 Chemical comp. (mg/kg) Cu 401.0 Pb 110.0 Zn 1779.6 Ni 44.8 C-N analysis (%) C n.d. N n.d. C/N n.d. Exchang. cations (cmol( + )/kg) Na 3.8 K 1.1 Ca 78.8 Mg 4.1 Isoelectric point 6.4 [+ or -] 0.2 Mineralog. comp. n.d. >50% Total porosity (%) n.d. n.d., Not determined. Table 2. Total and chemical fractionation (mg/kg) of heavy metals in soils and sewage sludge Extractant Cu Pb Zn Ni Collipulli KN[O.sub.3] 0.5 0.9 3.0 0.4 [H.sub.2]O 0.7 2.0 1.5 0.0 NaOH 17.1 0.0 1.6 0.0 EDTA 2.8 2.9 10.2 1.8 HN[O.sub.3] 46.6 11.4 93.3 18.3 Sum 67.7 17.2 109.6 20.5 Total content 71.5 10.3 73.8 25.3 Ralun KN[O.sub.3] 0.1 0.0 3.6 0.6 [H.sub.2]O 0.1 0.0 3.2 0.4 NaOH 5.2 0.0 1.6 1.4 EDTA 1.9 0.0 3.0 0.5 HN[O.sub.3] 2.9 0.0 0.0 6.0 Sum 10.2 0.0 11.4 9.0 Total content 14.8 0.0 10.6 6.7 Sewage sludge KN[O.sub.3] 12.6 21.0 2.0 [H.sub.2]O 12.8 49.0 33.0 11.7 NaOH 157.0 11.0 476.0 11.9 EDTA 37.3 1.0 956.1 7.6 HN[O.sub.3] 110.0 60.0 533.0 19.1 Sum 329.0 97.0 2019.0 52.0 Total content 401.0 110.0 1779.6 44.8 Table 3. Total and chemical fractionation (mg/kg) of heavy metals in sewage sludge amended soils Extractant Cu Pb Zn Ni Collipulli KN[O.sub.3] 1.0 0.1 2.6 0.5 [H.sub.2]O 2.2 0.0 5.6 0.7 NaOH 78.5 0.1 3.6 2.6 EDTA 40.3 3.8 71.4 1.9 HN[O.sub.3] 58.7 19.6 52.7 28.7 Sum 180.8 23.6 135.9 34.3 Total content 197.8 29.6 150.6 27.8 Ralun KN[O.sub.3] 2.1 0.0 4.6 0.7 [H.sub.2]O 3.0 2.3 5.0 0.9 NaOH 92.8 5.1 50.4 3.9 EDTA 43.8 8.1 72.6 2.1 HN[O.sub.3] 44.0 24.0 16.3 6.0 Sum 185.6 39.5 148.9 13.6 Total content 199.5 47.8 165.5 11.1 Table 4. Total and chemical fractionation (mg/kg) of heavy metals in amended soils heated at 400[degrees]C Extractant Cu Pb Zn Ni Collipulli KN[O.sub.3] 0.4 0.1 2.8 0.4 [H.sub.2]O 2.7 0.0 5.9 3.9 NaOH 3.0 0.1 0.0 0.8 EDTA 50.0 4.9 65.0 2.8 HN[O.sub.3] 107.4 22.5 81.7 29.0 Sum 163.5 27.6 155.4 36.9 Total content 206.3 34.7 164.9 30.3 Ralun KN[O.sub.3] 0.3 0.0 0.8 0.2 [H.sub.2]O 2.3 1.2 3.8 0.3 NaOH 1.1 0.5 2.3 0.5 EDTA 83.7 8.3 75.8 3.1 HN[O.sub.3] 42.4 30.7 61.4 5.9 Sum 163.5 40.7 144.1 10.0 Total content 156.5 46.3 125.6 8.4