| Diffusive Gradients in thin-films（DGT）is a technique that uses the principle of free diffusion（based on Fick’s First law）to study the gradient diffusion of the target in the diffusion layer and its buffering dynamic process,determining techniques for obtaining available state content,spatial distribution and kinetic information in environmental media.Copper（Cu）,lead（Pb）and cadmium（Cd）are common heavy metal pollutants and malignant pollution events have occurred in China.Based on DGT research and development domestic and overseas,literature metrology analysis was carried out on published relevant studies.This study innovative developed two new DGT binding phases,Ch-90Na resin type with high adsorption efficiency and natural polysaccharide chitosan gel type with biodegradability,respectively.The isothermal saturation adsorption curve,adsorption kinetics curve and adsorption process curve were measured.Then the diffusion coefficient of DGT based on the combination of the two new materials was measured,and then applied in natural water.The concentrations of Cu,Pb and Cd in small lakes were measured.The main conclusions are as follows:1.The adsorption capacity of CH-90Na resin gel phase is 42.66～159.48μg.The adsorption capacity of chitosan gel phase is 37.17～80.60 μg.The elation efficiency of Ch-90Na resin gel was 63.7%～95.4%for 5%HNO3,and 57.5%～96.4%for 5%HCl.The elation efficiency of chitosan gel was 54.9%～73.1%for 5%HNO3,and 50.3%～82.5%for 5%HCl.Ch-90Na resin gel had a more stable elation effect of 5%HNO3,and chitosan gel had a better elation effect of 5%HCl.2.Both CH-90Na resin gel and chitosan gel have good adsorption effect on Cu,Pb and Cd,and the adsorption capacity of CH-90Na resin gel is higher than that of chitosan gel.The adsorption of Cu and Pb by CH90Na resin gel is more consistent with Langmuir model and quasisecond-order kinetic fitting,and the adsorption of Cd is more consistent with Temkin model and quasi-first-order kinetic fitting.The Langmuir model was more suitable for chitosan gel adsorption,the quasi-firstorder kinetic model was more suitable for Cu adsorption and higher concentration of Pb and Cd adsorption,and the quasi-second-order kinetic model was more suitable for Lower concentration of Pb and Cd adsorption.The liquid film diffusion model is more suitable to describe the gel adsorption process of CH-90Na resin.The intro-particle diffusion model is more suitable to describe the adsorption of Pb,Cd and Cu at low concentration on chitosan gel,and the liquid film diffusion model is more suitable to describe the adsorption of Pb and Cd at low concentration on chitosan gel.The FTIR spectra of chitosan gel before and after the adsorption of heavy metals showed that there were both competition and coupling adsorption.3.A method for detecting Cu,Pb and Cd in DGT with CH-90Na resin gel as adsorption phase was established.The diffusion coefficients of DGT with CH-90Na resin gel as adsorption phase ranged from 1.25×10-6 to 2.32×10-6 cm2s-1.The detection limits of CH-90Na gel DGT were Cu 1.0 ng/L,Pb 5.0 ng/L and Cd 27.4 ng/L for 7 days,and Cu 0.2 ng/L,Pb 1.2 ng/L and Cd 6.4 ng/L for 30 days.A method for detecting Cu,Pb and Cd in chitosan gel as adsorption phase DGT was established.The diffusion coefficients of chitosan gel as adsorption phase DGT ranged from 6.14×10-7 to 1.25 ×10-6 cm2s-1.The detection limits of chitosan gels were 47.2 ng/L,22.1 ng/L and 33.6 ng/L for 7 days,and 34.6 ng/L,5.2 ng/L and 7.8 ng/L for 30 days according to the triples of blank standard deviation.Comparison of two DGT detection limits lower than the National standard "Determination of 65 elements in water Quality by Inductively coupled Plasma mass spectrometry"4.From May to July 2021,Qingnian Lake of Beiyangyuan Campus of Tianjin University was taken as the object to set up three monitoring sites,with three parallel DGTS for each site.The average values of Cu,Pb and Cd were 0.276 μg/L,0.550 μg/L and 0.324 μg/L.The average values of Cu,Pb and Cd were 1.621 μg/L,1.039 μg/L and 4.490 μg/L,respectively.The concentration is far lower than "Surface water environmental quality standard GB 3838-2002" national water quality standard.The DGT of chitosan gel was higher than that of CH-90Na resin gel.5.The experimental results were analyzed and summarized,especially the field experimental data of the two DGTS were compared.The average values of Cu,Pb and Cd were 0.276 μg/L,0.550 μg/L and 0.324 μg/L respectively.The average values of Cu,Pb and Cd were 1.621 μg/L,1.039 μg/L and 0.490 μg/L,respectively.From the perspective of parallelism,CH-90Na resin gel DGT has the best monitoring effect on Cd,followed by Pb and Cu.In terms of parallelism,chitosan gel DGT had the best monitoring effect on Pb,followed by Cu and Cd.However,compared with the DGT monitoring data of the two kinds of chitosan gel,the DGT of chitosan gel was higher than that of CH-90Na resin gel.Cu,Pb and Cd were 486.42%,89.07%and 50.97%on average per month.In conclusion,innovation,and application of two new monitoring Cu,Pb,and Cd DGTS were carried out in this study,and comparative analysis was made from material characterization,adsorption fitting and field natural water monitoring results,and good results and scientific conclusions were obtained.In the end,the shortcomings and prospects are proposed.DGT,as an increasingly and widely used in-situ sampling technique,can be used in combination with high-resolution imaging technology in the future studies. |
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