Comparative Accumulation of Heavy Metals in Selected Vegetables, Their Availability and Correlation in Lithogenic and Nonlithogenic Fractions of Soils from Some Agricultural Areas in Malaysia

Heavy metal content was determined in selected vegetables cultivated in some highland and lowland areas in Peninsular Malaysia. Leafy vegetables were represented by convolvulus (Ipomoea aquatica) and green mustard or sawi (Brassica rapa var. parachinensis), tubers and bulbs by sweet potato (Ipomoea batatas) and onion (Allium cepa), and fruity vegetables by chilly (Capsicum annuum), brinjal (Solanum melongena) and long bean (Vigna sinensis), respectively. Heavy metals from lithogenic and nonlithogenic soil fractions were studied at Cameron Highlands situated in the Pahang state and at lowland areas in Klang, Bangi, Gombak and Sepang districts in the Selangor state. The aim of the study was to investigate the availability of heavy metals and their potential uptake by vegetables in selected agricultural areas. The metals analysed were ferrum (Fe), zinc (Zn) cadmium (Cd), manganese (Mn), plumbum (Pb), copper (Cu) and chromium (Cr). Three soil samples were collected from each area and sampling was done at 1-30 cm depth. Extraction of heavy metals was carried out using sequential extraction and four fractions were produced comprising the easily leachable and ion exchange fraction, the acid reducible fraction, the oxidation organic fraction and the resistant fraction respectively. Heavy metal content in plant and soil samples were determined by atomic absorption spectrophotometry following standard methods (AOAC). Most metals were found at concentrations normally observed in vegetables grown in uncontaminated agricultural areas, with zinc (Zn) and manganese (Mn) content being highest, followed by copper (Cu), plumbum (Pb) and cadmium (Cd). However, the levels of potentially toxic metals such as Pb, Cd and Cr in the vegetables studied were found to be below the stipulated levels. Analysis of soil samples showed that the highest concentrations of heavy metals were obtained from the resistant fraction as compared to the other soil fractions. Concentration of Fe and Pb was found to be high in Sepang, whereas that of Cu was highest in Gombak and Cd levels were generally high in Sepang and Gombak. In contrast, the concentration of metals in the easily leachable and ion exchange fractions were low. Since differential uptake and accumulation of metals in the various plant parts are influenced by the availability of metals from the latter two fractions of the soils, the results indicate that availability of heavy metals to the cultivated plants (and thus, its consequent health risk to consumers) is also low. Based on the results obtained, the availability of heavy metals can be arranged as follows: Zn > Mn > Cd > Cu > Pb > Fe. The agricultural soils were found to contain high levels of Fe, Mn and Zn, whilst Cd and Cr were found in very low levels, well below the critical soil levels listed for arable land.