African Journal of Biotechnology Vol. 10(62), pp. 13505-13513, 12 October, 2011 Available online at http://www.academicjournals.org/AJB DOI: 10.5897/AJB11.225 ISSN 1684–5315 © 2011 Academic Journals
Full Length Research Paper
Speciation of heavy metals in paddy soils from selected areas in Kedah and Penang, Malaysia Habibah Jamil1, Lee Pei Theng1, Khairiah Jusoh1, Ahmad Mahir Razali1, Fouzi B. Ali2 and Ismail B. S.1* 1
School of Environmental and Natural Resource Sciences, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia. 2 Muda Agricultural Development Authority, 05990 Alor Setar, Kedah, Malaysia. Accepted 1 September, 2011
This study was carried out with the intention of evaluating heavy metal contamination in cultivated paddy areas. The speciation of heavy metals in paddy soils was determined in order to gain insight regarding their origin and distribution in soils. Five sampling sites were randomly selected from Kedah (Yan, Kota Setar, Kubang Pasu district) and Penang (Bumbung Lima district), where the soils constituted marine alluvial deposits. A site in Langkawi, where organic paddy farming is practised was used as the control. The sequential extraction method was adopted in order to obtain the four heavy metal fractions namely the easily leachable and ion exchange (ELFE), acid reducible (AR), oxidizable organic (OO) and resistant (RR) fractions. This study shows that the soil samples were clayey (82 to 96% of grain size Mn>Cr>Cd>Pb>Zn>Cu. Heavy metals such as Pb, Cu, Cr and Zn predominantly occurred in the insoluble form (RR fraction), with the oxides of Fe and Mn incorporated into the clay minerals. Although, the fertilizers and pesticides studied contained low amounts of heavy metals, the elevated amount of amount of Mn and Cd in the soils (ELFE fraction) could possibly be attributed to the longterm and repeated application of these materials to the cultivated paddy areas. Key words: Heavy metals, paddy soil, pesticide, fertilizer. INTRODUCTION Soils consist of heterogeneous media comprising decomposed rock fragments, clay minerals, oxides of Fe, Al and Mn, organic materials, organo-metallic complexes and soil solutions (Alloway, 1995). In cultivated paddy areas, the soils cannot be considered natural, as they have been highly modified by management practices including levelling, cultivation, puddling, submerging and fertilizing (Prakongkep et al., 2008). Apart from supplying micronutrients for the plant growth, some of the fertilizers and pesticides might introduce toxic heavy metals into the soils. According to Adriano (1986), phosphatic fertilizers contain varying amounts of Zn, Cd and other trace elements that originate in the phosphate rock. As for the
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manufactured fertilizers, the insufficiently purified product could contain several impurities including heavy metals (Gimeno-Garcia et al., 1996). Various types of fertilizers and pesticides that contribute to the heavy metals content in soils have been identified by McLaughlin et al. (2000). It was reported that the application of phosphate fertilizers in New Zealand and Australia contributed to the elevated levels of Cd in agricultural soils. Giuffré de López Camelo et al. (1997) reported that rock phosphate and NPK fertilizers contained heavy metal values several times higher than their natural levels in soils. More also, the application of rock phosphate to cultivated paddy areas in India caused an increase in the Cd content of the rice grain (Ramachandran et al., 1998), and in Malaysia, small amounts of Cd in the bioavailable form were found to be present in paddy soils at a few sites in Kedah (Khairiah et al., 2009a). However, the contribution from pesticides and fertilizers to the Cd contamination
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in these sites has yet to be determined. Due to their cumulative behaviour and toxicity, heavy metals are potentially hazardous not only to crop productivity, but also to human health through food consumption (Das et al., 1997; Melamed et al., 2003; Kyuma, 2004). Heavy metals are also harmful to the environment because of their direct toxicity to biota and indirect threat to human health from groundwater contamination and accumulation in food crops (Martínez and Motto, 2000). The total metal concentration in soils cannot provide insight regarding their mobility and bioavailability (Powell et al., 2005). Thus, the sequential extraction technique has been adopted by many researchers for diverse types of soils including the upland soils (Khairiah et al., 2006) and paddy soils (Wong et al., 2002; Manceau et al., 2005; Khairiah et al., 2009a). Such procedures were proven to provide information on heavy metal speciation, including their origin, mode of occurrence, bioavailability, mobilization and transport in soils (Pérez-Cid et al., 1999). At the northwest region of Peninsular Malaysia, paddy is cultivated in vast low lying plains consisting of the marine, fluvial and peat deposits. For decades, various types of fertilizers and pesticides have been applied to these areas to improve rice productivity and control pests. However, little effort has been done regarding the investigation of the effect of fertilizer and pesticide consumption to heavy metal contamination in paddy soils. In the current study, the sequential extraction method was adopted to determine heavy metal speciation in paddy soils. The objective of the study was to determine the status of heavy metal contamination in the paddy soils and factors that could influence their bioavailability to the paddy plants. The study will provide valuable insight regarding heavy metal contamination in paddy soils.
weakly bound to the clay surface area, secondary minerals and organic materials. Ten gram soil samples were put into Kartell bottles followed by the addition of 50 ml 1.0 M NH4CH3OO (pH 7). The samples were then shaken for 1½ h, and centrifuged at 3000 rpm for 30 min before being filtering through 0.45 µm millipore filter paper and made up to 50 ml with distilled water. Samples were washed with 50 ml distilled water, followed by further shaking and centrifugation as described earlier. Consequently, a total of 50 ml NH2 OH.HCl (pH 2) was added in order to extract the metals from the acid reducible (RA) fraction using the procedure earlier described. Metals extracted from this fraction mainly came from those which were strongly bound to secondary minerals. Metals in the organic oxidation (OO) fraction were extracted by adding 15 ml of H2O2 to the sample placed in a water bath for 1 to 1½ h, followed by the addition of 50 ml NH4CH3OO (pH 3.5), representing the metal bound to the organic matter. Samples were then digested using HNO3:HClO4 at 25:10 ratio in a sand bath at 100°C as in the RR extraction method. The digestion process was repeated until the samples turned whitish. The determination of heavy metal concentration was carried out using AAS (atomic absorption spectrophotometry) with the Perkin Elmer model (1100B). Organic carbon analysis and soil pH were also carried out by using the Walkey-Black (1934) and Duddridge and Wainright (1981) methods, respectively, while the
