**5. Heavy metals in crops grown on soils amended with sewage sludge**

#### **5.1. Concentration in plants**

available concentrations by DTPA and Mehlich 3, may occur by applying high sludge rates. Liming reduces the availability of Zn and hence the potential for contamination of the soil, but only DTPA is able of detecting this reduction. Mehlich 1 extractant, which is widely used in routine soil analysis (soil testing) in Brazil, seems to be inefficient to assess whether a soil is contaminated by heavy metals. Likewise, Mehlich 3 and DTPA are not always efficient. The alternative extractant 0.1 mol L–1 HCl has also shown poor performance. As the factors associated to inefficiency of the extractants are variables (e.g., type of heavy metal, soil class, type of crop, cropping sequence and origin of sewage sludge) and they may act in combination, the available concentration can not yet be considered a consistent and relatively safe indicator for assessing the risk of heavy metals contamination in soils amended with sewage sludge. Nevertheless, extractable concentrations by DTPA and Mehlich 1 may be useful to monitor the leaching of heavy metals in soils amended with sewage sludge. Relatively low sludge rates (~ 10 Mg ha–1 year–1) applied for a long time (5 years) stimulate the leaching of Cu, Ni and Pb, as indicated by increase in extractable concentrations by Mehlich 1 in depth. High sludge rates (~ 20 Mg ha–1 year–1) with high concentration of Zn in its composition (> 1,300 mg kg–1) have the potential to leach Zn from the superficial layers of the soil in a short time (2 years), as suggested by the increase in extractable concentrations by DTPA in depth. Thus, the extractable concentration, used in the monitoring of leaching, can help in assessing the risk of groundwater contamination by these heavy metals added to the soil by sewage sludge application to land. The results of fractionation of heavy metals, albeit very limited, have shown opposite trends for Zn and Ni. Zn added to the soil by the sludge is preferably bound to Fe and Al oxides. In this form, its solubility is limited and thus the risk of being excessively absorbed by plants or leached is reduced. On the other hand, added Zn which binds to SOM is most often associated with fulvic and humic acids. As these humic substances are poorly stable, its transformation tends to release Zn retained in soil organic matrix, thereby increasing the risk of contamination. In contrast, added Ni which binds to SOM is mainly in humin fraction. Since this fraction is very stable, it restricts the solubility of Ni, minimizing the risk of contamination of crops and

Speciation of heavy metals in soils amended with sewage sludge is still incipient. We found only one work on this topic, which is a MS thesis. The results of this thesis, however, have shown significant trends. Applications of sewage sludge rates as low as 20 Mg ha–1 year–1 for seven years declined forms of Cu and Zn (Cu2+ and Zn2+) that are preferentially absorbed by plants, but increased forms of these metals (Cu-DOC and DOC -Zn) that are easily leached. This means that the sewage sludge, when applied in low rates over a long period, decreases the risk of phytotoxicity and contamination of the food chain, but increases the risk of

Simultaneous use of different measures to evaluate heavy metals in soils can increase security in the risk assessment of environmental contamination due to the application of sewage sludge to land. From this perspective, total concentration, available concentration, chemical fractions and chemical species of metals should be assessed together to better characterize the dynamics of possible contamination. These different measures represent a gradient of solubility with immediate and potential impact on plant uptake and leaching of heavy metals in soils. Thus,

groundwater.

groundwater contamination.

894 Environmental Risk Assessment of Soil Contamination

Land application of sewage sludge can successively increase heavy metals availability in soil, uptake by plants and accumulation in plant tissues. Thus, concentration in plant tissue can be used as an indicator of heavy metals transfer from soils amended with sludge to plants and of the entry of hazardous elements in the food chain. It can also be used to evaluate the phyto‐ toxicity and contamination of harvested products by heavy metals added to the soil by sewage sludge application, thus presenting important role in determining crop performance and quality of food originated in field. The concentrations of heavy metals in plants vary depending on several factors, including rate of sewage sludge, type of heavy metal, type of plant and analyzed plant part. These factors were studied in field experiments conducted in Brazil, particularly in São Paulo state. Their main results are summarized below.

Galdos et al. [77] evaluated the concentrations of Cu, Ni and Zn in the middle third of the leaf below the ear of maize grown for two years on a clayey Oxisol amended with sewage sludge to supply up to twice the N requirement of the crop. The maximum rates were 21.6 Mg ha–1 in the first year and 20.5 Mg ha–1 in the second year. Cu concentration in the leaf was changed only in the first year, increasing with application of the maximum rate of sludge. The waste had no effect on the concentration of Ni, which was measured only in the second year. For Zn, the concentration in the leaf increased progressively with sewage sludge rates in the two years. The concentrations of Cu and Zn were within the range of concentrations suitable for the crop [99], indicating no phytotoxicity, which is consistent with the absence of adverse effects of the waste on productivity of grains.

Oliveira et al. [69] applied sewage sludge rates up to 10 Mg ha–1 year–1 to two Oxisols (Typic Haplorthox and Typic Eutrorthox) for five years and measured the concentrations of Cu, Mn, Ni, Pb and Zn in shoot and grains of maize in the 5th year. There was no effect of the sludge on concentration of Cu in any case. The Mn concentration varied only in the shoot, decreasing in Typic Haplorthox and not having clear effect on Typic Eutrorthox. The sludge did not change the concentration of Ni in shoot and grains, except for concentration in grains in the Typic Eutrorthox which was below the LOD of the analytical method (AAS). The Pb concen‐ tration was not changed in the shoot and it was below the LOD in grains. The concentration of Zn increased in shoot and grains, with the exception of the concentration in the grains in Typic Eutrorthox, which has not changed. Increased concentrations of Zn were below the tolerance limit in foods as corn (50 mg kg–1) established by Brazilian Health Surveillance Agency [100].

Silva et al. [88] applied sewage sludge from Franca and Barueri municipalities to a clayey Oxisol in three successive annual crops of maize and measured the concentrations of Cu, Mn, Ni, Pb and Zn in middle third of leaf opposite and below the ear and in its grains. Sludge rates were defined to supply until eight times the amount of N required by the crop, reaching maximum values of 30 and 64 Mg ha–1 for sludges from Franca and Barueri, respectively. The concentration of Cu in leaf increased only in the first crop with Barueri sludge application and in third crop with Franca sludge application. Despite the increase, the values are below the toxic level (> 50 mg kg–1) for maize leaf presented in Barbosa Filho et al. [99]. The concentration of Mn in the leaf increased with increasing sludge rates, except for the first crop when was used Franca sewage sludge. Suitable concentrations of Mn in maize leaf can be as high as 214 mg kg–1 [99]. No concentration exceeded this value suggesting that there was no toxicity of Mn. The concentration of Ni increased only in second crop by Franca sludge application. On the other hand, the Zn concentration increased with increasing rates of both sludges and in all crops. The only situation in which the concentration of Zn in leaf was slightly above the appropriate range of concentrations (15-100 mg kg–1) presented in Barbosa Filho et al. [99] was in third crop with application of higher Barueri sludge rate. However, this excess Zn in leaf must not have been toxic to the plant, since no symptoms of toxicity were reported. In grains, there was much less cases of elevated concentrations of heavy metals when compared with the leaf, suggesting some limitation in the redistribution of these metals to the harvested plant part. Consistent increases were observed for Mn in second crop with sludge application, Ni also in second crop, but with Barueri sludge application, and Zn in the second and third crops with application of the sludges. In the first crop, metals concentrations were below the LOD of the analytical method used (ICP-OES). The concentrations of Cu, Ni, Pb and Zn in grains were below the maximum limits for cereals in general (30, 5.0, 8.0 and 50 mg kg–1, respectively) by Brazilian Food Industry Association [101] and Mn concentrations were below the critical range presented in Kabata-Pendias and Pendias [102] for grains of plants grown in contami‐ nated soils with Mn.

Oliveira et al. [71] evaluated the concentrations of Cd, Cr, Pb and Zn in stem, leaves, straw, ear husk, cobs and grains of maize grown on a clayey Oxisol after nine years of annual applications of sewage sludge rates (Table 10). The Cd concentrations were below the LOD of the analytical method (AAS) (0.06 mg kg–1 in grains and 0.2 mg kg–1 in other plant tissues). Cr concentration increased in stem and leaves in response to sludge application and was below the LOD in ear husk, cob and grains. Pb concentration increased in stem, leaves and ear husk, did not change in cob and was below the LOD in grains. Except for grains, all plant parts had increases in Zn concentration. These results show that stem and leaves were the most sensible plant tissues for expressing the effect of sewage sludge rates on Cr, Pb and Zn concentrations in maize.

[99], indicating no phytotoxicity, which is consistent with the absence of adverse effects of the

Oliveira et al. [69] applied sewage sludge rates up to 10 Mg ha–1 year–1 to two Oxisols (Typic Haplorthox and Typic Eutrorthox) for five years and measured the concentrations of Cu, Mn, Ni, Pb and Zn in shoot and grains of maize in the 5th year. There was no effect of the sludge on concentration of Cu in any case. The Mn concentration varied only in the shoot, decreasing in Typic Haplorthox and not having clear effect on Typic Eutrorthox. The sludge did not change the concentration of Ni in shoot and grains, except for concentration in grains in the Typic Eutrorthox which was below the LOD of the analytical method (AAS). The Pb concen‐ tration was not changed in the shoot and it was below the LOD in grains. The concentration of Zn increased in shoot and grains, with the exception of the concentration in the grains in Typic Eutrorthox, which has not changed. Increased concentrations of Zn were below the tolerance limit in foods as corn (50 mg kg–1) established by Brazilian Health Surveillance

Silva et al. [88] applied sewage sludge from Franca and Barueri municipalities to a clayey Oxisol in three successive annual crops of maize and measured the concentrations of Cu, Mn, Ni, Pb and Zn in middle third of leaf opposite and below the ear and in its grains. Sludge rates were defined to supply until eight times the amount of N required by the crop, reaching maximum values of 30 and 64 Mg ha–1 for sludges from Franca and Barueri, respectively. The concentration of Cu in leaf increased only in the first crop with Barueri sludge application and in third crop with Franca sludge application. Despite the increase, the values are below the toxic level (> 50 mg kg–1) for maize leaf presented in Barbosa Filho et al. [99]. The concentration of Mn in the leaf increased with increasing sludge rates, except for the first crop when was used Franca sewage sludge. Suitable concentrations of Mn in maize leaf can be as high as 214 mg kg–1 [99]. No concentration exceeded this value suggesting that there was no toxicity of Mn. The concentration of Ni increased only in second crop by Franca sludge application. On the other hand, the Zn concentration increased with increasing rates of both sludges and in all crops. The only situation in which the concentration of Zn in leaf was slightly above the appropriate range of concentrations (15-100 mg kg–1) presented in Barbosa Filho et al. [99] was in third crop with application of higher Barueri sludge rate. However, this excess Zn in leaf must not have been toxic to the plant, since no symptoms of toxicity were reported. In grains, there was much less cases of elevated concentrations of heavy metals when compared with the leaf, suggesting some limitation in the redistribution of these metals to the harvested plant part. Consistent increases were observed for Mn in second crop with sludge application, Ni also in second crop, but with Barueri sludge application, and Zn in the second and third crops with application of the sludges. In the first crop, metals concentrations were below the LOD of the analytical method used (ICP-OES). The concentrations of Cu, Ni, Pb and Zn in grains were below the maximum limits for cereals in general (30, 5.0, 8.0 and 50 mg kg–1, respectively) by Brazilian Food Industry Association [101] and Mn concentrations were below the critical range presented in Kabata-Pendias and Pendias [102] for grains of plants grown in contami‐

waste on productivity of grains.

896 Environmental Risk Assessment of Soil Contamination

Agency [100].

nated soils with Mn.

In other long term experiment, Cd, Cr and Pb concentrations were evaluated in leaf apposite and below the ear, whole plant and grains of maize after 11 annual applications of sewage sludge rates up to 20 Mg ha–1 to an Oxisol [73]. There was no effect of sludge on Cd and Pb concentrations in leaf. For Cr in this plant part, its concentrations were below the LOD (< 0.19 mg kg–1) of the analytical method (AAS). Similarly, concentrations of these heavy metals were below the LOD in grains (0.03, 0.15 and 0.19 mg kg–1 for Cd, Cr and Zn, respectively). When considered the whole plant, Cd, Cr and Pb concentrations were not significantly changed by sewage sludge applications.

There are also results for sugarcane. Camilotti et al. evaluated the concentrations of Cd, Cr, Ni and Pb in stem and leaves of sugarcane cultivated in a clayey Oxisol after three [103] and four [75] annual applications of sewage sludge rates, which reached a maximum of 15 Mg ha–1, with the goal of supplying 100 % and 200 % of N required by the crop. There was no effect of three applications of sewage sludge in concentrations of Cr in stalk and leaves, Ni in leaves and Pb in stalk and leaves, as well as Cd concentrations in all plant parts analyzed. Cr in leaves and Ni in stalk were below the LOD of the analytical method used (AAS). Similarly, the fourth sludge application did not affect the concentrations of Pb in leaves, but all other cases the concentrations of evaluated heavy metals were below the LOD.

In an experiment conducted on Ultisol, Nogueira et al. [78] applied sewage sludge rates up to 10.8 Mg ha–1 at sugarcane planting for supplying N required by the crop and evaluated As, Cd, Cr, Cu, Ni, Pb, Se and Zn concentrations in leaf (with top visible dewlap), stalk and juice at the harvest time of plant cane (first year) and ratoon cane (second year). In these two evaluations, As concentration increased in leaf and was not affected in stalk and juice by sludge application. Cd concentration increased in leaf, stalk and juice in both crops. On the other hand, Pb concentration decreased in stalk and juice of plant cane and was not affected in other situations. Cu concentration decreased in stalk of plant cane and leaf of ratoon cane and was not affected in other cases. Ni concentration also was not generally affected, but it increased in stalk of plant cane and decreased in stalk of ratoon cane. For Pb concentration, there was no effect of sludge rates. Se concentration was affected only in leaf of plant cane, increasing with sludge application. Zn concentration increased in all situations. There were no cases of concentration below the LOD probably because the authors used ICP-MS.


†Means within a column in each plant tissue followed by the same letter are not significantly different according to Tukey test (*p* < 0.05).

‡2.5 Mg ha–1 in 1st, 2nd, and 3rd years and 20 Mg ha–1 in 4th, 5th, 6th, 7th, 8th and 9th years.

Source: Nogueira et al. [71].

**Table 10.** Cr, Pb and Zn concentrations in stem, leaves, ear husk, cob and grains of maize after nine annual applications of sewage sludge to a clayey Oxisol from São Paulo State, Brazil.

#### **5.2. Risks of contamination**

**Sewage sludge rate Heavy metal in plant tissue Annual Cumulative Cr Pb Zn**

898 Environmental Risk Assessment of Soil Contamination

Mg ha–1 \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ mg kg–1 \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

 0 0.13 c† 0.68 b 15.76 c 45 0.33 b 0.61 b 19.11 c 90 0.45 a 0.83 a 26.62 b 2.5 + 20‡ 127.5 0.19 c 0.47 c 44.35 a

 0 0.74 b 1.10 c 29.35 b 45 1.20 a 1.43 b 28.54 b 90 1.48 a 1.72 a 39.34 ab 2.5 + 20‡ 127.5 0.67 b 1.64 ab 50.77 a

 0 < 0.3 4.28 ab 13.03 b 45 < 0.3 3.95 b 12.91 b 90 < 0.3 4.86 a 17.71 ab 2.5 + 20‡ 127.5 < 0.3 4.70 ab 20.52 a

 0 < 0.3 0.88 a 13.49 b 45 < 0.3 0.98 a 15.93 b 90 < 0.3 1.04 a 27.47 a 2.5 + 20‡ 127.5 < 0.3 0.98 a 32.35 a

0 0 < 0.09 < 0.4 32.36 a 5 45 < 0.09 < 0.4 29.32 a 10 90 < 0.09 < 0.4 35.77 a 2.5 + 20‡ 127.5 < 0.09 < 0.4 36.70 a †Means within a column in each plant tissue followed by the same letter are not significantly different according to

‡2.5 Mg ha–1 in 1st, 2nd, and 3rd years and 20 Mg ha–1 in 4th, 5th, 6th, 7th, 8th and 9th years.

applications of sewage sludge to a clayey Oxisol from São Paulo State, Brazil.

**Table 10.** Cr, Pb and Zn concentrations in stem, leaves, ear husk, cob and grains of maize after nine annual

Tukey test (*p* < 0.05).

Source: Nogueira et al. [71].

Stem

Leaves

Ear husk

Cob

Grains

The results reviewed above have shown contrasting effects of sewage sludge on the concen‐ trations of heavy metals in maize and sugarcane. Sludge effects on Cu, Pb and Ni concentra‐ tions ranged greatly. On the other hand, its effects were more consistent for Zn. Sludge application generally increased Zn concentrations in plants, but not in phytotoxic levels. Increases occurred even in edible parts of crops, such as grains of corn, but concentrations did not exceed the limit established by Brazilian legislation. For sugarcane, in addition to Zn accumulation, Cd concentrations may also increase with sludge application. However, even increasing, Cd concentrations remained very low.

These findings support the view that the sewage sludge applied to land has low potential to contaminate maize and sugarcane with heavy metals. However, this does not mean that risks of contamination of other crops are also low. Vegetable crops, for example, can easily be contaminated by heavy metals since they have contact with sewage sludge applied to soil or bed. Because of this high risk of contamination, Brazilian law prohibits the agricultural use of sewage sludge for production of vegetable crops and also other crops [67].
