The Release Of Copper And Sulfur In Flooded Polluted Paddy Soils Treated By Different Sulfur Fertilizers

With the rapid development of industry and agriculture, the improper discharge of industrial exhausts, waste water irrigation and improper use of pesticides and fertilizers, large areas of paddy soils in our country were contaminated by heavy metals. As a biogenic element, sulfur has a strong influence on the bio-geochemical process of heavy metals. Recently, sulfur and heavy metals were found to co-exist in the colloid particles, which greatly enhanced the mobility of heavy metals in soils. With enhanced application of different kinds of sulfur fertilizers, it is vital to investigate the mechanism of the co-transformation and co-transportation between the heavy metals and sulfur in paddy soils to control the transfer of heavy metals in soil-rice system, which could provide safety of ecosystem and agricultural products, and ensure human health. In this study, we collected the paddy soils polluted by heavy metals near a copper smelter in Fuyang city of Zhejiang Province. Under simulated soil column experiment in the lab, the paddy soils were treated by different sulfur fertilizer and leached after different flooding periods. By combined using the synchrotron-based microscopic X-ray fluorescence (μ-XRF) spectroscopy and other microstructure analysis methods, we studied the basic physicochemical properties of soils and the variability of the availabilities of heavy metals and sulfur in the paddy soils, and further reveal the micro-distribution of heavy metals and sulfur in the soils and the collected colloids. The main conclusions were as follows:(1) The use of different kinds of sulfur fertilizers and different flooding periods markedly changed the basic physicochemical properties (pH/EC/Eh) of paddy soils polluted by heavy metals, the availability and the spatial distribution of copper(Cu) and sulfur(S) in paddy soils. Ammonium sulfate (AS) could significantly reduce the pH value of soil solutions, restrain the release of the soil available Cu, and increase the EC value and oxidizability of paddy soils mainly in the early flooding periods. However, sulfur coated urea (SCU) could dramatically increase the pH value, EC value and reducibility of soil solutions, and restrain the release of the soil available Cu mainly in the late flooding periods. Both AS and SCU could inhibit the release of the soil available S in the early flooding periods (7d), but promote the release of the soil available S in late flooding periods(60d). Results from u-XRF analysis demonstrated that Cu/S/Fe distributed heterogeneously in the interested areas. Elemental fluorescence counts were further studied by regression analysis and the results demonstrated that AS could promote the co-transformation of Cu and S in paddy soils, but SCU had a more complicated effects on the co-transformation of Cu and S because of its sustained-release character. Both adding sulfur fertilizers and extending flooding time could visibly increased the correlation of Cu and Fe, which maybe revealed that sulfur fertilizers and long flooding time promoted the absorption of Cu to Fe oxides or the formation of Cu and Fe complexes, especially for SCU.(2) Cu and S were observed to be co-exist and co-transport in the nano-particles under flooding conditions, and the kind of sulfur fertilizers affected the co-transportation in the nano-particles. Results from Dynamic Light Scattering (DLS) analysis demonstrated that the size of colloids from leaching solutions ranged from176.1nm to452nm, which revealed that most of colloidal particulates were nano-particles. Further SEM-EDX and ICP-AES analysis revealed the detected Cu, S, Fe, and Al present in the colloids. These results indicated Cu and S could be co-exist and co-transport in the nano-particulates. A notable proportion of Cu and S in the leaching solution were found to be colloidal Cu and S, while most of Fe and Al in the leaching solution were found to be colloidal Fe and Al. Different sulfur fertilizations had different effects on colloidal Cu and S. Adding sulfur fertilizations promoted the release of colloidal S, which was more clearly illustrated by the applicationi of AS. SCU could effectively restrain the release of colloidal Cu, while AS increased the release of colloidal Cu. Results from μ-XRF analysis demonstrated that both sulfur fertilizations and increasing flooding time could promote co-transportation of colloidal Cu/S and colloidal Cu/Fe. SCU had a relative greater comtribution to the co-transportation of colloidal Cu/S, and AS had a more positive impact on the co-transportation of colloidal Cu/Fe(3) Considering that the rice growth period is long, we suggest selecting SCU as a sulfur fertilizer for controlling heavy metal pollution in paddy soils. During a long flooding period, SCU can more effectively restrain the release of soil extractable Cu and colloidal Cu than AS, so SCU can better control the mobilization of heavy metals and reduce the environmental risk of heavy metals in the paddy soil.