| Large-scale pig farms have produced a lot of piggery waste,such as pig manure and sewage.Pattern of farmland utilizationis one of the main modes of pig farm waste disposal,which requires the aquaculture wastewater must reach the emission standard,and then it could be used as water and fertilizer when the cultivated fields are in need.This strategy makes piggery wastewater an alternative source of water for irrigation to alleviate the shortage of agricultural water resources.The soil irrigated with piggery wastes in a long history might be contaminated with heavy matals,because the piggery wastes were not treated properly in tha past.At the present time,swine wastewater can reach the discharge standard after being treated,but The risk of heavy metal pollution has always existed,that has limited agricultural utilization stratege development.Adsorption is the main physicochemical behavior of heavy metals when they go into the soil along with the piggery wastewater.Most of heavy metals will accumulate in the soil by adsorption.However,the land use of wastewater makes the soil environment complex.Accumulation and transformation of heavy metals in soil may be affected by adsorption and leaching.Heavy metals that are brought into the soil with swine wastewater will accumulate or migrate in the soil,which are closely related to the properties of swine wastewater and soil.Soil heavy metal pollution of fecal irrigated area was systematically studied based on a breeding pig farm in Chongqing.Besides,three types of soil were selected as research objects,yellow brown soil and paddy soil were collected from irrigated area of the pig farm,while the purple soil which was the most widely cultivated soil in the southwestern region was collected from the suburb of Chongqing.The adsorption and migration characteristics of heavy metals in the soils,and factors affecting the process were explored in the laboratory by batch adsorption experiment.In addition,soil column experiment was conducted to reveal the mechanism of migration and transformation of heavy metals in the soil when irrigated by swine wastewater that was up to the emission standard.Consequently,the environmental behavior of heavy metals in different soils was fully understood by the combination of dynamic and static methods,which laid a theoretical foundation for the sustainable development of agricultural ecology with piggery wastewater.The main conclusions were as follows:(1)The topsoil(0~20cm)samples were collected from different land use types in irrigated area surrounding the pig farm,pH and content of Cu,Zn,Cd,Pb were measured,and the fractions of heavy metals were tested by BCR sequential extraction method.Single factor pollution index,Nemerow pollution index,geo-accumulation index was used to assess heavy metal pollution.In addition,bioavailability and potential ecological risk of heavy metal were evaluated.The results showed that the soil was weakly alkaline,and the average contents of Cu,Zn,Cd and Pb were 30.80 mg/kg,91.33 mg/kg,0.55mg/kg and 29.34 mg/kg respectively,which were all over the background value of the region,but within Environmental Quality Standard for Soil.The variation coefficient ranked as follows,Cd(1.06)>Cu(0.31)>Zn(0.23)>Pb(0.14).Cu,Zn and Pb were mainly in residual fraction,while Cd was mainly in reducible fraction.There was moderate Cd pollution in somewhere around the pig farm,and without other heavy metal pollution in the soil.The bioavailability showed Cd>Zn>Pb>Cu.The comprehensive potential ecological risk of heavy metals in the study area was in a slight level,and the contribution rate of Cd was the highest.(2)In order to understand the adsorption process and mechanism of heavy metals,and provide a theoretical basis for the prevention and control of soil pollution contrapuntally,adsorption kinetics and thermodynamics of Cu,Zn,Cd and Pb in yellow brown soil,paddy soil and purple soil,as well as the effect of soil organic matter and solution pH on the adsorption were researched by batch experiment.Results showed that the adsorption of soil to the heavy metals could be balanced in 12h,the equilibrium adsorption of soil to Cu and Pb was much greater than that of Zn and Cd,the pseudo-second-order kinetic model with R~2 more than 0.99 fitted the adsorption kinetics well.The adsorption isotherms could be described by Langmuir and Freundlich equation,the adsorption and buffering capacity of Cu and Pb were higher than of Zn and Cd in the tested soils,the isothermal adsorption process was spontaneous with physical and chemical effects.Adsorption process of Cu and Cd in the three kinds of soil,as well as Zn in yellow brown soil and paddy soil,and Pb in purple soil were endothermic.While the adsorption process of Zn in purple soil,Pb in yellow brown soil and paddy soil were exothermic.Adsorption quantity of soil removal of organic matter was decreased,but it increased with the initial pH value of solution.(3)Accumulation and transformation characteristics of heavy metals in soil were studied by soil column experiments when piggery wastewater up to the standard was used as leaching water.The results showed that soil pH after leaching was somewhere in between the swine wastewater and pH value of soil background.Leaching of swine wastewater increased soil organic matter content.Cu,Zn,Cd and Pb could not be accumulated largely when leaching of piggery wastewater that was up to the standard.However,if high concentration of heavy metals added,Cu mainly accumulated in the soil depth of 0~35cm,Zn,Cd and Pb are mainly accumulated in 0~45cm.The cumulative proportion of heavy metals in three kinds of soil was Cd<Zn<Cu<Pb.Although the theoretical adsorption capacity of soil to heavy metals were large,the migration ability of heavy metals was enhanced under leaching.In the leached soil with swine wasterwater,Cu,Zn,Cd and Pb was mainly existed in the residue,but the residue fraction of Pb would be transformed into oxidizable and reducible with the increase of the amount of wastewater.In the leached soil with artificial synthesis wastewater,Cu and Zn in the three kinds of soil mainly existed in residual;Cd in yellow brown soil mainly existed in oxidizable,and in paddy soil and purple soil,Cd mainly existed in residual;Pb existed mainly in residual at low irrigation level,and at a higher irrigation level,the proportion would decrease in yellow brown soil and purple soil,but increase in paddy soil.The risk of heavy metal pollution in shallow groundwater with short-term irrigation of swine wastewater was very low. |
PDF Full Text Request