| Due to the rapid development of the current industry and the interference of human activities, heavy metal pollution of farmland soil poses a serious threat to food safety and human health. Among the remediation technologies for soil heavy metal pollution control, chemical passivation remediation is prospective because of its high remediaiton rate, short time, simple, and low cost. Therefore, it has been received a closed attention by many researchers. It is very important to choose the effective passivators for the successful remediation.In this study, firstly, four passivators, namely steel slag, phosphate rock, palygorskite and charcoal, were selected by field survey and pot experiment. The single extraction(diethylenetriamine five acetic acid (DTPA) extraction)and chemical fractionations (sequential extraction) for metal bioavailabilty analysis were used to evaluate their effectiveness of immobilizing heavy metals with different concentrations (0,5% and 20%) of passivators addition. Secondly, the effects of our passivators on biomass and metal uptake in Apium graveolens, Daucus carota, and Lycopersicon esculentum Millertomato were examined by pot experiment. Finally, the phase composition, specific surface area and pore characteristics of four immobilizing agents were also analyzed by X-ray diffraction (XRD) and specific surface area-pore size distribution analyzer. The passivation mechanisms between heavy metals and four passivators were preliminarily studied by isothermal adsorption experiment. The results are as follows:(1)The soil concentrations of heavy metals, including lead (Pb), zinc (Zn), copper (Cu),cadmium(Cd) and arsenic(As), all exceeded the Environmental Quality Standard for Soils (GB 15618-1995)(Grade â…¡). The bioavailable fraction of five metals decreased significantly and the soil pH value increased after addition of palygorskite,steel slag or phosphate rock at a ratio of 20% compared with the control.(2)Resuts from chemical fractionations showed that exchangeable Pb decreased, while residual Pb increased significantly by addition of 20% palygorskite. Exchangeable Cd was decreased by applying 20% palygorskite, steel slag or phosphate rock. Exchangeable Zn was also decreased by additions of 20% steel slag or 20% phosphate rock. Carbonate-bound Zn (a fraction easily taken up by plants) was decreased by 20%phosphate rock or 20% steel slag additions. Residual Zn significantly increased by application of palygorskite. Similar result was also observed in residual Cu when steel slag (5% and 20%) or 20% phosphate rock were added. The Ca-As fractions, aninert fraction for plant uptake,increased significantly with applying 20% phosphate rock.(3)Compared with the control, the plant heights of Daucus carota,Lycopersicon esculentum Millertomato significantly increased by phosphate rock, phosphate rock, and charcoal addition, and the plant height of Daucus carota and also significantly increased by palygorskite. The biomass of Daucus carota significant increased by palygorskite or steel slag. The biomass of Apium graveolens increased significantly after the four passivators addion. The content of As in Daucus carota significantly decreased by steel salg or phosphate rock, and the content of As in edible parts of lycopersicon esculentum millertomato significantly decreased by palygorskite, steel salg, phosphate rock treatments. The content of As in the leaf of the Apium graveolens significantly decreased by four passivators. The content of Cd in root, stems of the Apium graveolens and underground part of Daucus carota significantly decreased by four passivators. The content of Pb in the stems of Apium graveolens significantly decreased by four passivators. The content of Pb in the underground part of Daucus carota and edible parts of Lycopersicon esculentum Millertomato significantly decreased by palygorskite or phosphate rock treaments, and the steel salg also significantly decreased the content of Pb in the underground part of Daucus carota.(4) The results from the adsorption isotherm experiment showed that steel slag,palygorskite, phosphate rock have a good absorbing ability for Cu2+, Cd2+, Pb2+, As5+ and Zn2+. The adsorption isotherm fited well in both Langmuir and Freundlich models. The adsorption capacity of the four passivators increased with the increasing of the initial concentration for the five heavy metal ions. Under the same initial concentration conditions, four passivators have different adsorption capacity for five heavy metals, In the same equilibrium concentration, the adsorption capacity of four passivators was followed by palygorskite>steel slag>phosphate rock>charcoal for Cu2+, steel slag>phosphate rock, palygorskite>charcoal for Cd2+, phosphate rock>palygorskite>steel slag>charcoal for Pb2+, and steel slag>palygorskite> phosphate rock>charcoal for Zn2+. As for As5+, at the low concentration, the adsorption capacity of four passivators was steel slag>phosphate rock>palygorskite >charcoal, but at the high concentration, no significant difference was observed in adsorption capacity among charcoal, phosphate rock and palygorskite.(5) The results from desorption experiment showed that steel slag had a low desorption capacity for Cd2+, As5+ and Zn2+), and the desorption capacity of phosphate rock, charcoal and palygorskite increased with the increasing adsorption capacity for the five heavy matals. In the largest adsorption capacity, the desorption capacity of four passvitors was followed by charcoal>phosphate rock>palygorskite>steel slag for Zn2+, palygorskite>charcoal>phosphate rock>steel slag for As5+, charcoal>palygorskite >steel slag>phosphate rock for Pb2+, charcoal>palygorskite>phosphate rock>steel slag for Cd2+ nd charcoal>steel slag>phosphate rock>palygorskite for Cu2+, respectively. Among four kinds of passvitors, charcoal has the biggest desorption capacity for five heavy metals.(6)Among four kinds of passvitors, the main immobilizing mechanism of charcoal and palygorskite was physical adsorption and ion exchange adsorption due to their large specific surface areas and pore volumes, while chemical precipitation was dominant immobilizing mechanism for steel slag and phosphate rock because of their high pH values. |
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