Abstract

Key Words

Copper, zinc, vegetable soils, metal speciation

Introduction

The concentrations of heavy metals in soil are associated with biological and geochemical cycles and are influenced by anthropogenic activities such as agricultural practices, industrial activities, and waste disposal. Heavy metals in soils may be inherited from the parent materials or added through use of organic and chemical fertilizers and pesticides. The knowledge of both the total concentration and chemical speciation is necessary to characterize the behavior of heavy metals in soil. In fact, it is well known that metals are present in soil in different chemical forms, which influence their reactivity and hence their mobility and bioavailibility. Different sequential extraction techniques such as the five-step procedure of Tessier et al. (1979) are commonly applied to evaluate both the actual and potential mobility of metals in the environment. This extraction scheme allows the division of the total metal content into five fractions: exchangeable, carbonate bound iron/manganese oxide bound and residual fraction. The scheme was developed for sediments but many studies have used these procedures for soils (Abollino et al , 2002; Lu et al 2003, Lu et al , 2004) . However, this scheme may not be suitable for soils which do not contain carbonate. Rauret (1998) also elaborated that the extractants used for the fraction of metals bound to carbonates (ie acetic acid and sodium acetate ) and the iron and manganese oxides (ie. hydroxylamine in acid solution ) were not completely suitable. Both carbonates and oxides may not be completely attacked. Shuman (1979) proposed a scheme to study microelements in acid soils that do not contain carbonates or sulphides. This scheme included exchangeable , organic matter, Fe oxide , sand, silt and clay. Another speciation scheme was developed by the EC Standards , Measurement and Testing Programme., formerly BCR (Bureau Community of Reference). This scheme proposed only four fractions: ie: exchangeable; (acetic acid),. reducible (hydroxylamine hydrochloride), oxidisable species (hydrogen peroxide and nitric acid ) and residual (aqua regia), (Rauret, 1998). The BCR procedure had been tested for sediments (Thomas et al. 1994) and soils (Davidson et al. , 1998).

In P. Malaysia, studies on heavy metal concentrations in the common agriculture soils have only started in the last decade or so and speciation studies are still lacking. An earlier study on the assessment of contamination of agricultural soils and crops in Peninsular Malaysia (Fauziah et al. 2001) showed that some of our soils already contain elevated values of Cd and Zn and that cabbages may show high values of Cu. The Malaysian soil investigative levels for heavy metals was also established and proposed to be the 95^th percentile.

Ultisols and Inceptisols (two of the common soil Orders in Malaysia ) were chosen for this study. These soils are all located in vegetable farms. The main objectives of this study were to determine the concentration ranges of total heavy metals (Cu, Pb, Zn and Ni) and the chemical speciation in four soil phases (i.e exchangeable, amorphous Fe oxides, organic matter and residual).

Methods

The soil samples (0-20 cm) were collected using a stainless steel auger. Eighty six soil samples developed over granite (Ultisols) and 25 samples developed over riverine alluvium (Inceptisols) were collected from some major vegetable growing areas in Cameron Highlands in the state of Pahang and Johore state. Twenty two soils from uncultivated areas developed over granite were also sampled for background values. The soils were air dried and analysed for chemical properties such as pH, organic C and cation exchange capacity (CEC). Soil pH was measured in a 1:2.5 (w/v) ratio of soil to water. Organic carbon was measured using Walkley-Black method and cation exchange capacity was determined as described by Van Ranst et al ((1999). The total heavy metal concentrations (Cu, Pb, Zn and Ni) were determined by the aqua regia method ( Van Ranst , 1999). Some selected soil samples (26 of cultivated Ultisols and 10 of cultivated Inceptisols) were analysed for chemical partitioning. The sequential extraction procedures used are modified from the schemes developed by Tessier et al. (1979) and Shuman (1979). The extraction was modified for our soils because these soils do not contain carbonates and appreciable sulphides and manganese. The first two steps (for exchangeable and organic bound fractions) are those from Tessier’s scheme while the third step ( for amorphous iron oxides ) followed that of Shuman (1979) . Step 4 is the determination of heavy metals in the residual fraction using aqua regia instead of nitric, hydrofluoric and perchloric acid. All the analyses were performed in duplicate. Extractions were carried out on 1.0 g of soil and involved the following steps:

F1: Eight ml of 1 M MgCl2 were added to the sample and suspension was shaken for 1 h and then centrifuged (20 min, 4000 rpm).
F2: Six ml of 0.02 M HNO3 and 10 ml of 30% H2O2 were added to the residue obtained from the first extraction, and the suspension was shaken for 5h at the temperature of 85±2°C. After cooling, 10 ml of 3.2 M CH3COONH4 were added and shaken for 30 min and centrifuged
F3: The samples were extracted with 20 ml of solution 0.2 M ammonium oxalate and 0.2 M oxalic acid, shaken in the dark for 4 h and centrifuged.
F4: The heavy metals contents in the residual fractions were determined by aqua regia method.

Cu, Ni, Zn and Pb were determined by atomic absorption spectroscopy (AAS).

Results

Chemical properties and total heavy metal concentrations

Table 1 shows the mean of pH, cation exchange capacity, organic carbon for the cultivated Ultisols and Inceptisols and the uncultivated soils Included in this table are also values for Ultisols and the 95^th percentile for 241 agricultural soils from an earlier study by Fauziah et al. (2001) in order to compare the results of the present study.

Table 1 : Mean values for total heavy metal concentrations and chemical properties of Ultisols and Inceptisols

	pH	O.C. (%)	CEC (mmolc kg-1)	Cu	Pb (mg	Zn kg-1)	Ni
Cultivated Ultisols n= 86	6.06	1.76	179	21.4	19.4	59.9	15.3
Cultivated Inceptisols n=25	6.18	2.62	195	43.8	13.2	80.1	7.9
Uncultivated Ultisols n=22	5.62	0.9	62.9	7.4	17.7	18.4	5.7
*Ultisols (n=58)	5.49	1.77	103	13.6	31.3	53.0	20.4
*95th percentile of 241 samples of agricultural soils				47.3	65.8	92.0	41.3

Table 1 shows that soil pH, organic carbon and CEC in the cultivated soils are higher than the uncultivated soils. This shows that these 3 chemical properties have increased due to the soil amendments such as addition of chicken dung and liming.

Zinc content is the highest in both soil Orders, followed by Cu, Pb and Ni. Comparing the heavy metals between the cultivated and uncultivated Ultisols using T test, Cu and Zinc have shown significant increase (p< 0.001) in the cultivated soils. The potential source of both these metals may be the chicken dung which is added in large quantities to the soils for the production of vegetables.

Total Cu and Zn in the the Inceptisols show much higher values than those in the Ultisols. This may be due to the length of time that these soils have been used. The farms in the Inceptisols area have been operating for more that 20 years while those in the Ultisols have only been recently developed.

Values of heavy metal concentrations in the cultivated Ultisols in the study area show close to values of Ultisols in other parts of the country. In general, all the heavy metal values are lower than the 95^th percentiles established in an earlier study by Fauziah et al (2001), thus indicating that these soils are still not considered contaminated. However, Cu and Zn in the Inceptisols show that they are close to being in the contaminated zone.

Correlation studies (Table 2) of total heavy metal concentrations and soil chemical properties show that only total Zn and Pb is positively correlated to pH, Zn with organic carbon , while Cu is negatively correlated with CEC.

Table 2 : Correlation coefficients between total heavy metal contents and chemical properties of cultivated Ultisols.

	Cu	Pb	Zn	Ni
CEC	-0.35**	ns	ns	ns
pH	ns	0.41**	0.27**	ns
Organic Carbon	ns	ns	0.30**	ns

ns not significant
** significant at the 0.01 probability level

Speciation of Heavy Metals

The Cu, Pb, Zn, and Ni fractions expressed as percentages of the sum of individual chemical fractions are presented in Table 3 and Table 4. In the cultivated Ultisols, Cu and Zn show the highest concentration in the Fe oxide fraction, while Pb and Ni are highest in the residual fraction. The percentage of Cu fractions follows the order : Fe oxides > residual > organic > exchangeable. For Pb, the percentage of Pb fractions follows the order: residual > amorphous Fe oxides > exchangeable > organic . Percentage of Zn fractions follows the order: Fe oxides > residual > exchangeable > organic. The percentage of Ni fractions follows the order: residual > amorphous Fe oxides > exchangeable > organic .

The amounts of non-residual fractions (F1, F2 and F3) represents the amounts of active heavy metals while those of the residual fractions may be considered to be the stable form and thus not available to plants for a reasonable period. In this study, the non-residual fractions of Cu, Pb, Zn and Ni in the Ultisols average 60.8%, 51.1%, 67.7% and 25.4% which suggests that the mobility and bioavailability of the four metals are in the order: Zn > Cu > Pb > Ni.

Table 3 : Mean values of Cu, Pb, Zn, and Ni fractions expressed as percentage of sum of fraction (%) for the Cultivated Ultisols

	F1 Exchangeable	F2 Organic	F3 Amorphous Fe Oxides	F4 Residual
Cu	1.3	2.5	57	39.2
Pb	7.9	6.3	36.9	48.9
Zn	2.4	1.3	64.0	32.3
Ni	9.0	1.3	15.2	74.6

The trend is not similar in the longer cultivated Inceptisols (Table 4). The highest percentages of all the heavy metals were found to be in the residual fractions. This trend was also found by Adamo et al. (2003) for volcanic soils in southern Italy which was irrigated for a long time with contaminated river water. More than 50% of Ni, Pb and Zn in these soils were held in the residual fraction . This trend may be explained by residence time effect which may reduce metal mobility and bioavailability due to complexation, adsorption and precipation of metal ions in the soil particle surface. Lu et al. (2004) investigated the time effect on the fractionation of heavy metals in soils and found that soluble metals added were transformed from easily extractable fractions to more stable fractions. The non-residual fractions of Cu, Pb, Zn and Ni in the cultivated Inceptisols average 32.1%, 11.2%, 7.7 % and 9.3 % This suggest that the mobility and bioavailability of the 4 metals are in the order Cu> Pb > Ni > Zn.

These results showed that the order of mobility of metals differ in these two types of cultivated soils which are both amended with chicken dung and also the amount in the residual fractions.

Yu et al. (2004) who studied the copper fractionation and extractability in two contaminated soils found that most of the anthropogenic Cu was associated with the mobile fractions in his Inceptisol after a 6 week incubation whereas in the Ultisol, the mobile became dominant only at the higher rate of Cu amendments. The types of chicken dung and other soil amendments may also be responsible for this difference. Stacy et al. (2001) who studied the effect of aging biosolids on the availability of cadmium and zinc in soil found that the release of both these metals depended on the biosolid composition. While two sludges showed no change in plant available metal pools (L values) , the third sludge showed an increase.

Table 4 : Mean values of Cu, Pb, Zn, and Ni fractions expressed as percentage of sum of fraction (%) for the Cultivated Inceptisols

	F1 Exchangeable	F2 Organic	F3 Amorphous Fe Oxides	F4 Residual
Cu	4.9	8.8	18.4	67.9
Pb	5.7	2.2	3.3	88.8
Zn	0.8	2.3	4.6	92.3
Ni	1.6	1.9	5.8	90.7

Conclusion

Total heavy metal concentrations in cultivated Ultisols and Inceptisols fall within the typical range for unpolluted soils. Heavy metals speciation using a modified Tessier’s sequential extraction procedure showed that all the metals in the Inceptisols are dominantly in the residual phase. The general trend in the Ultisols for Pb and Ni is residual > oxalate > exchangeable > organic. For Zn and Cu, the oxalate extractable phase is highest followed by the residual phase. Zinc and Pb contents in the Ultisols are also positively correlated to pH of the soil.

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