Availability and phytoremediation of zinc and copper in two Malaysian soils treated with sewage sludge

The disposal of municipal sewage sludge face serious challenges. Currently, over 3.2×106 metric tons of domestic sewage sludge in Malaysia are in need of safe disposal. Phytoremediation provides an efficient soil remediation solution since it uses plants to remove contaminants. A glasshouse experiment were conducted to assess the potential of four plant species (Jatropha curcas, Hibiscus cannabinus, Acacia mangium and Syzigium cumini) to the phytoremediation of excess Zn and Cu in Oxisol and in Ultisol amended by different rates of sewage sludge: 0, 5 and 10% v/v. The results showed that the addition of 10% e sludge enhanced the nutritional status of the Oxisol and Ultisol, shown by the increase in soil pH from 5.36±0.01 to 5.84±0.02 in the Oxisol, and from 4.77±0.02 to 5.37±0.01 in the Ultisol, the CEC increased from 8.00±0.57 to 9.39±0.01 cmolc kg-1 in Oxisol and from 10.33±0.01 to 11.22±0.06 cmolc kg-1, available P increased from 10.70±0.05 mg kg-1 to 24.00±0.02 mg kg-1, in Oxisol and from10.90 ±0.05 mg kg-1 to 26.80±0.01 mg kg-1 in Ultisol. Among the four plants examined, it was found that J. curcas and H. cannabinus were capable of accumulating more Zn and Cu than those of the A. mangium and S. cumini, which was shown by their high translocation factor (TF>1). H. cannabinus had the highest TF value of Zn (2.43±0.02), while J. curcas had the highest TF value of Cu (1.52±0.02). Fractionation of metals showed that the Zn and Cu existed in the residual form, while the sludge application into the soils tended to shift the forms of Zn and Cu away from residual fraction to water soluble and exchangeable fractions that might be more available for plant uptake. A leaching study was conducted to determine movement of Zn and Cu in the tested soils. The results showed that the application of 10% sewage sludge seemed to increase the concentration of Zn and Cu in the leachates of the soils, the maximum concentration of Zn in the leachates from the Ultisol was 82.35±0.45 mg L-1 which was higher than that of the Oxisol 62.91 ±0.25 mg L-1. For Cu, it was 8.69±0.15 mg L-1 in the leachates of the Ultisol which was lower than that of the Oxisol of 11.67±0.05 mg L-1. The downward movement of Zn and Cu in the soil columns after the leaching process was different among the metals, whereby Zn had a lower concentration (1.12±0.03 mg kg-1) compared that to Cu (5.6±0.07 mg kg-1) in the both soils, especially for the 0-5 cm layer. The results adsorption study showed that the sewage sludge application had significant effect on the processes of adsorption-desorption of Zn and Cu. This is shown by the systematic change of the distribution coefficients (Kd). Comparison between Kd values for both soils indicated the following selectivity of metals: Cu (Kd= 3.42) > Zn (Kd=2.82). It is clear that Zn adsorption was lower than that of Cu. The adsorption isotherms of Zn and Cu of both soils were well fitted to linear Freundlich and Langmuir equations (R2 = 0.96 - 0.99). This study suggests that sewage sludge possessed the ability to improve the fertility of highly weathered soils. However, the presence of Zn and Cu is a negative side effect of using sewage sludge. The excess Zn and Cu in treated soils can be successfully removed by phytoremediators, such as J. curcas and H. cannabinus