| Colloids can enhance the movement of strongly sorbing contaminants, a phenomenon called colloid-facilitated contaminant transport. In the presence of mobile colloids, contaminants may move faster and farther than in the absence of colloids. We hypothesized that the introduced water dispersible colloids from different manure not only can act as carrier particles to enhance the transport of heavy metals, but also can interact with the original soil colloids, hence enhance or inhibit the colloid facilitated transfer of metals in soils. In present study, the characteristic of water-dispersible colloids derived from swine manure was investigated, as well as heavy metal distribution in colloids and molecular speciation of colloid-metal bound. The aim of our study was to clarify the water-dispersible colloid facilitated transportation mechanism of heavy metals. The main results were as follows:There was no difference of the organic functional group among water-dispersible colloids with different sizes from swine manure.The mineral quartz was found in the colloids. Heavy metals in swine manure were largely associated with particles with> 10μm in size. Water extractible Cu, Zn, Fe, and Mn were found mostly in water dispersible colloids, whereas water extractible Ni, Cr, Cd, and As were observed as dissolved. Colloidal Cu largely accumulated in 0.02-0.2μm colloids. Colloidal Zn, Fe and Mn were largely associated with 1-10μm colloids. The higher metal/organic carbon (OC) ratio of colloids than that of dissolved fraction might result in the distribution of water extractible heavy metals. The highest accumulation of Cu was found in the 0.02-0.2μm fraction of colloids, in which organic carbon was the major component. The Cu in the 1-10μm colloid fraction existed as both inorganic compounds and organic associations, whereas it mainly existed as organic complexes in colloids<1μm (<0.53μm, specifically). Cu ligand was estimated by fitting the XANES spectra with linear combinations of the references. There was no ion Cu was found in colloids. Cu was found in colloids as complexation or chelation. Copper in organic complex was probably bound with hydroxyl group in cellulose and lignin, or bound with microorganism..The 1-10μm colloids had a strong potential to maintain a low Cu concentration in solution and a high buffering capacity with respect to external loading of Cu. Meanwhile, the high Ceo of the 0.02-0.2μm colloids demonstrated that this size range of colloids might serve as a sustained source of Cu input to uncontaminated zones of soil or groundwater aquifers. Large colloids (1-10μm) of swine manure were partially filtered out as they passed through the sand particles, and fine colloids (<0.2μm) facilitated the transport of Cu. The formation of organic complexes was hypothesized to enhance the mobility of Cu.The result of colloid facilitated transport of metals through un-contaminant soil showed that the transport of Cu, Zn, Cd and Cr could not be affected by the organic colloids in acid condition. However, in neutral pH condition, the water-dispersible colloids can facilitate the transport of heavy metals. The concentration of metals in effluent was positively correlated to the content of colloids. Fifty percent of Cu was found transport as colloidal combination, while Zn, Cd, and Cr were mainly transport as dissolved. In Ca-dominant alkalinity soil, Cu probably transported as organic colloidal-complex, carbonate or hydroxide precipitate. And Zn and Cr seemed to react with Fe-and Mn-oxides or mineral, and then co-transport through soil. The colloid facilitated transport of heavy metals might be affect by the dynamic adsorption of colloids on metals, chemical disturbance of soil solution, physiochemical charactistics of soil matrix and the nature of heavy metal. Water-dispersible colloids reduce the adsorption capacity of soil colloid on metals. With high affinity for metals and facilitated transport, organic colloids might provide significant risks for groundwater contamination in areas receiving heavy applications of agricultural solid wastes amendment.Column experiments were conducted to assess the importance of organic colloids from different sources for metal transport in historically contaminated soils. Dissolved and colloidal fractions of metals were separated by ultracentrifugation, and colloids were studied by synchrotron scanning transmission X-ray microscopy (STXM) and micro-X-ray absorption near edge structure (μ-XANES). The results showed that a small proportion of Cu was found to be associated with colloids, which was significantly higher in the presence of leaf soil (LS) and poultry manure (PM) colloids than either EM colloids (colloids released from EM manure) or the control. Cadmium and Mn were also effectively dissolved, while a great proportion of Pb and Zn were found to be associated with colloids. Organic colloids from LS increased the elution of colloidal Pb significantly. Quite a few Cu-Zn associations were clearly visible in the colloidal phase of leachates, which indicated a certain similarity of these two metals in nature. The Zn-bearing colloids were either organic or mineral, depending on the nature of the introduced organic colloids. Zn phyllosilicate species were dominant in the treatment of control and LS colloids, while organic colloid-associated Zn was prevalent in the treatments with EM and PM colloids. Because of the increase in quantities of phyllosilicate-associated Zn in the presence of LS colloids and the decrease in the presence of PM and EM colloids compared to the control, it was suggested that the characteristics of the organic colloids, such as aromaticity, hydrophobicity, and polysaccharide-type organic matter, most likely exerted significant effects on the colloidal transport of the metals. The introduced organic colloids from different manures not only acted as carrier particles to enhance the transport of heavy metals (such as Zn in EM and PM colloids) but also interacted with the original soil colloids to enhance (LS colloids) or inhibit (EM and PM colloids) the clay-facilitated transfer of metals in soil. |
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