Assessment of Heavy Metal Concentrations in Different Soil Types and Vegetables Grown in Muar, Johor

Large areas of land in Muar district, Johor, are especially used for vegetable cultivation. Four major cultivation areas were selected for this study namely, Solok, Pagoh, Sawah Ring and Sengkang which had different types of soils. In these areas, 53 sampling points with a total of 159 soil samples comprising of 66 samples of organic clay and muck (OCM), 42 samples of Holyrood series, 27 samples of Durian series and 24 samples of Jerangau series were collected from the vegetable cultivation areas. The soils were sampled at 0-20 cm, 20-40 cm and 40-60 cm depths. From each of the sampling areas, one sample of vegetable was collected thus, making a total of 53 vegetable samples. Forty-three samples of background topsoils (0-20 cm) from each location that were considered as unaffected or minimally affected by man were sampled. The soil samples were analyzed for pH, organic carbon (OC), cation exchange capacity (CEC), texture and mineralogy. The aqua-regia method was used to determine the total heavy metal concentrations and three extractants i.e. ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) and 0.1 M hydrochloric acid (HCl) were used to extract the available heavy metal concentrations. The chemical partitioning of heavy metals in cultivated mineral and background topsoils were determined using Tessier’s sequential extraction. Dry ashing method was used to determine the contents of heavy metals in vegetable samples. Flame Atomic Absorption Spectrophotometry (AAS) was used to analyze all heavy metals (Cd, Cr, Cu, Ni, Pb and Zn) that were extracted from soil and vegetable samples. Statistically, the pH for all of the soil series showed no significant differences at p ≤ 0.05 with depths. Organic clay and muck and Durian series had a significantly higher OC contents in topsoils than subsoils. There were no significant differences for OC contents in Holyrood and Jerangau series with depths. The differences of CEC between soil depths were not statistically significant (p ≤ 0.05) for Holyrood, Durian and Jerangau series but were statistically significant for OCM, between 0-20 cm and 40-60 cm depths. The total heavy metal concentrations in cultivated topsoils showed a significant increase as compared to the background topsoils. The ranges of total Cd, Cr, Cu, Ni, Pb and Zn concentrations in the cultivated soils were 0.06-3.72 mg kg-1, 0.14-77.8 mg kg-1, 0.74-128.0 mg kg-1, 0.2-258.0 mg kg-1, 0.02-39.6 mg kg-1 and Zn 6.2-208.2 mg kg-1, respectively. Generally, total concentrations of Cd, Cr, Cu and Zn were significantly higher in the topsoils of OCM and Holyrood series than Durian and Jerangau series. Most of total heavy metal concentrations in OCM and Holyrood series showed significant differences with depths. Chromium, Pb and Zn in Durian series showed significant differences with depth whereas in Jerangau series, only Cr and Pb showed significant differences with depth. In the cultivated mineral topsoils, Cd, Cu, Ni, Pb and Zn were dominated in the residual fraction and were least present in Fe-Mn oxide for Cd, Cr and Cu whereas Ni, Pb and Zn were least present in the organic fraction. The percentages of Ni, Pb and Zn in the residual fraction of cultivated mineral topsoils were much higher than the background mineral topsoils. The total Zn had a good correlation with pH (r = 0.74**, n = 22), total Pb and Cr with CEC (r = 0.95**, n = 8; r = 0.51*, n = 22). No correlations were found between total heavy metal concentrations in soil and heavy metal uptake by vegetables. Cadmium and Pb in vegetables showed positive correlation with CEC (r = 0.77**, n = 14; r = 0.54**, n = 22) whereas Pb and Zn in vegetables showed positive correlation with OC (r = 0.63*, n =14; r = 0.60*, n = 14). Zinc in vegetables also showed positive correlation with pH (r = 0.73*, n = 9). Available Cu in soil extracted by DTPA was positively correlated with Cu (r = 0.44*, n = 22) in vegetable and available Cd extracted by 0.1 M HCl with Cd (r = 0.84*, n = 8) in vegetables. Most of the heavy metals in the cultivated and background topsoils were below the 95th percentile and Dutch standard, except for Cd. Among the soil types, topsoils of OCM and Holyrood series clearly showed that Cd had exceeded both limits whereas Zn was only above 95th percentile value. The heavy metal contents in the leafy vegetables were significantly higher than fruit vegetables, except for Cr on fresh weight basis. The heavy metal contents in vegetables were still below the maximum permitted concentration (MPC) that was allowed by Malaysian Food Act (1983) and Food Regulation (1985).

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