Heavy Metal Distribution In Cropland Soils Amended With Sewage Sludge And Heavy Metal Uptake By Maize

As the acceleration of urbanization proceeding, the rate of urban sewage treatment is increasing year by year. At the same time, sludge production of urban sewage treatment plant is increasing rapidly. To convert the huge amount of sludge into a harmless and recyclable resource is one of the focus topics in world-wide environment research. Therefore, applying sewage sludge to farmland is increasingly being given more attention. The risk of heavy metals to environment caused by using sludge has to be deeply explored and researched.The physical-chemical properties and fertility factors in sludge and soil samples were analyzed. The modified three-step sequential extraction procedure proposed by the Community Bureau of Reference (BCR) was used to predict Pb, Cd, Cu and Zn fractional distribution in maize soil amended with sludge at doses of 10 t hm-2, 30 t hm-2 and 60 t hm-2 for two years. A high-pressure airtight digestion technique and ICP-MS were used to determine the distribution, translocation and accumulating of Pb, Cd, Cu and Zn in the parts of maize was grown on soils amended with sludge at different doses. The conclusions were shown as follow.With increasing dose of sludge application, the contents of organic matters, total nitrogen, available nitrogen, available phosphonium and available kalium in soils have increased. By the evaluation of sludge-amended soil fertility, soil fertility level had been improved remarkably. Therefore sewage sludge can be used as a kind of good organic fertility. The main existing forms of Pb,Cd,Cu,Zn in sludge were exchangeable and weak acid solublefraction, reducible fraction and oxidizable fraction, which could be directly or indirectly used by organism.The contents of Pb,Cd,Cu,Zn in sludge-amended soils have increased with increasing dose of sludge application. According to the Soils Standard of Environmental Quality of China (GB15618-1995), The contents of Pb,Cu,Zn were much lower than the limits. The Cd contents in reference soil and sludge-amended soils were relatively high. The fractional distribution of Pb, Cu were the similar, It follows residual fraction > oxidizable fraction > reducible fraction > exchangeable and weak acid soluble fraction. Cu in the soil samples mainly associate with residual fraction, but Pb mainly associate with oxidizable fraction, reducible fraction, exchangeable and weak acid soluble fraction. The fractional distributions of Cd, Zn were the similar. With increasing dose of sludge application, the contents of oxidizable fraction, reducible fraction, exchangeable and weak acid soluble fraction have increased while the content of residual fraction has reduced. The order of exchangeable and weak acid soluble fraction yields for these four heavy metals or the sequence of their activity in weak acidic soils was followed with Cd>Zn>Pb>Cu. The classification of the soil samples in this study according to our modified RAC based on the content percentage of exchangeable and weak acid soluble fraction. The safe period of sludge continuous application should be considered at least 6 years (sludge at dose of 6 times)The order of average content of Pb, Cd, Cu and Zn was Zn>Cu>Pb>Cd. It was showed significantly positive relationships between the contents in maize and the metal contents in soils with the different doses. With increasing dose of sludge application the contents of Pb, Cd, Cu and Zn in different parts of maize rise. According to the Safe Controlling Standards of Food (GB2762-2005), the maximum authorized contents of Pb and Cd in maize seeds is 0.2 mg.kg-1. The contents of Cd in maize seeds from reference soil to 12 times dose did not exceed the standard limit. But all the contents of Pb including reference did exceed the standard level. The TF and EC values for Pb, Cd, Cu and Zn in maize were lower than 1.0. The orders of TF and EC in maize with the same dose of sludge were same. The order of maize TF values was followed by Pb>Cd>Cu>Zn; The order of maize EC values was followed by Cd>Cu>Zn>Pb. The relative sequence of uptake by maize parts predicted here for Pb, Cd was: stems and leaves > roots > seeds; in contrast, for Cu was: roots > seeds > stems and leaves, for Zn was irregularity. The root has a higher capacity of absorbing and accumulating Cu, Zn; whereas the seeds, stems and leaves have a higher capacity of absorbing and accumulating Pb, Cd.