| With the rapid development of the national economy, heavy metal pollution problems have become increasing apparently, have caused serious harm for the ecological environment, food safety, human health and sustainable development of agriculture, and have obtained widespread concern in the community. At present, the total concentrations of heavy metals usually are used as the evaluation criteria of heavy metal pollution in soils, however, the evaluation results are often incompatible with the actual situation. Many of the research results show that the bioavailability of heavy metals and its environmental effect can not be predicted by the total concentrations of heavy metals in soil, to a greater extent determined by the speciation distribution of heavy metals. The different speciation of heavy metals can produce the different environmental and biological effects. As we all know, free state of heavy metals have a direct relationship with its toxic effects, however, the methods which can directly measure free state of heavy metals in soil are relatively low. In this paper, cadmium as a representative of toxic heavy metals, a new diffusive gradients in thin films (imprinted functionalized silica gel) device were developed to measure free state of Cd(II) in soil by the diffusion layer and binding agent. Performance of imprinted functionalized silica gel device was studied. The effects of pH and ionic strength on the validation of imprinted functionalized silica gel device were also studied. Comparison of imprinted functionalized silica gel device and ion-selective electrodes for the measurement of free Cd(II) was assessed. The concentrations of free Cd(II) in different samples by imprinted functionalized silica gel device were measured. The objective of the paper will provide a simple and reliable tool for measurement of free Cd2+in soils, and a potential technical support for the assessing the environmental and health risks of heavy metals. Conclusions are as follows:1. A simple procedure was developed to synthesize Cd(II)-imprinted thiocyanato-functionalized silica gel (SCN/IIP) and Cd(Ⅱ)-imprinted amino-functionalized silica gel (N3/IIP) sorbents by combining a surface molecular imprinting technique with a sol-gel process. The maximum static adsorption capacities of SCN/IIP and N3/IIP were72.8T77.2mg·g-1, respectively. Two sorbents offered a fast kinetics for the adsorption of Cd(Ⅱ) for20min, had a substantial binding capacity in the range of pH4-8, could be used repeatedly, and were found to adsorb selectively Cd2+in the presence of Co(Ⅱ), Ni(Ⅱ), Zn(Ⅱ), Pb(Ⅱ) and Cu(Ⅱ) interferences in the same medium. The equilibrium data of SCN/IIP fitted perfectly with Redlieh-Peterson isotherm model. The equilibrium data of N3/IIP fitted perfectly with Langmuir isotherm model. Kinetic studies of SCN/IIP and N3/IIP indicated that the adsorption for Cd(II) by two sorbents followed a pseudo-second-order model. Various thermodynamic parameters such as△G0,△H0and△S0were evaluated. Negative values of△G0of SCN/IIP and N3/IIP indicated that the adsorption process was spontaneous. The positive standard enthalpy change of two sorbents suggested that the adsorption of Cd(II) by sorbents was endothermic. The positive values of△S0exhibited that the randomness increased with the adsorption of Cd(Ⅱ) on two sorbents.2. The diffusion coefficients of Cd(II) ions through PES membranes with different pore diameters (0.22and0.45μm) were8.5×10-6和2.6×10-5cm2s-1, respectively. The PES membrane with0.22μm of pore diameter was suitable as the diffusion layer.3. Two imprinted functionalized silica gel devices using SCN/IIP and N3/IIP as the binding agents and PES membrane with0.22μm of pore diameter as the diffusive layer, namely, PES-SCN/IIP-imprinted functionalized silica gel and PES-Nj/IIP-imprinted functionalized silica gel, had been systemically investigated. The ratios of Ci/Cbulk for free Cd(II) in NaNO3solution of0.01mol·L-1by PES-SCN/-imprinted functionalized silica gel and PES-N3/-imprinted functionalized silica gel were0.9869±0.0492and0.6246±0.0678, respectively. PES-SCN/-imprinted functionalized silica gel and PES-N3/-imprinted functionalized silica gel were deployed in synthetic solution with the different molar ratios of Cd(II)ions/EDTA of4:4,4:3,4:2and4:1. The ratios of Ci/Cbuik for free Cd(II) by PES-SCN/-imprinted functionalized silica gel were0,14.95±1.21,28.91±1.73and42.84±1.92%, respectively. There was a noted difference between theoretical and experimental values. The ratio of Ci/Cbuik for free Cd(II) by PES-N3/-imprinted functionalized silica gel were0,24.63±1.42,49.25±1.96and75.49±2.48%, respectively and were very close to the theoretical percentages of free metals in the solutions. The PES-N3/-imprinted functionalized silica gel for can accurately determine free Cd(II) in synthetic solution, while the measured value by PES-SCN/-imprinted functionalized silica gel was low. The PES-N3/-imprinted functionalized silica gel had a substantial binding capacity at pH5-8and concentrations of competitive Na+up to0.1mol·L-1. The measurements of PES-N3/-imprinted functionalized silica gel and Cd(II) ion selective electrode for free Cd(II) in river water were equivalent.4. The PES-N3/-imprinted functionalized silica gel for can accurately determine free Cd(II) in soil. The measurements of free Cd(II) by PES-N3/-imprinted functionalized silica gel was feasible. Compared with chemical extraction method, PES-N3/-imprinted functionalized silica gel device can be used as a new tool for measurement of free Cd(II).The extraction of Imprinted functionalized silica gel for free Cd(II) is superior to other methods of morphological analysis. The results showed that the Imprinted functionalized silica gel can become a kind of new method of extracting ree Cd(II) in soil.In summary, imprinted functionalized silica gel can used as a new tool for the quantitative measurement and sampling of free Cd(II) in soil and can provide strong technical support for assessing the environmental and health risks of heavy metals. |
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