Development Of Sulfhydryl Modified Silica Cryogel As Binding Phase Of DGT For The Enrichment Of Dissolved Hg(Ⅱ) In Seawater

Mercury is a highly toxic heavy metal element,and Hg²⁺and its complex（Hg（Ⅱ）for short）are harmful to the environment and humans due to their strong mobility,non-degradability,and bioaccumulation.Numerous industrial activities and convenient shipping are discovered in the estuary,where is also a suitable place for marine aquaculture.Therefore,the concentration detection of Hg（Ⅱ）in the estuary is of great significance to the land-sea coordination for the sustainable development of the marine economy.Diffusive gradients in thin films（DGT）technology can be adopted to long-term in-situ enrichment and sampling of trace Hg（Ⅱ）,it can accurately measure the Hg（Ⅱ）concentration with the combination of instrumental analysis.However,the study focus on in-situ enrichment of dissolved Hg（Ⅱ）in seawater is rare due to the adsorption performance of the binding phase tend to decline at alkaline pH,and the abundant chlorine and other ions can have interfered with the enrichment.In this study,sulfhydryl-modified（SiO₂-SH）cryogel particles with a wide range of pH,tolerance to chloride ions,high selectivity were prepared by sol-gel method and freeze-drying method,which were used as the binding phase to form SiO₂-SH-DGT to establish the in-situ enrichment and accurate determination method of Hg（Ⅱ）in seawater,and it was applied to the fresh water and typical mariculture areas in Qinzhou Bay.The main conclusion of the study is summarized as follows:（1）SiO₂-SH cold gel was prepared by using water glass as the precursor and mercaptopropyltrimethoxysilane as the modifier,and the optimal preparation was found by the response surface method optimization:modifier/SiO₂ molar ratio0.37,modification time 9.5 h,modification temperature 54℃.SiO₂-SH cryogel was amorphous silica,and the particles were porous spherical with a diameter of10-30 nm.It retained the main structure of SiO₂,but blocked part of the internal pores causing the specific surface area to decrease.（2）SiO₂-SH cryogel particles maintained high selectivity and adsorption properties for Hg（Ⅱ）in a wide pH range（1-9）,which were not affected by high concentrations of sodium chloride.The adsorption equilibrium could be reached within 1 min,the maximum adsorption capacity was 709.2 mg/g,and the adsorption capacity could still be achieved well after five adsorption/desorption cycles.The high adsorption capacity of the modified material mainly could be attributed to the chemical adsorption of Hg（Ⅱ）by the sulfhydryl group,which partly came from the physical adsorption by electrostatic attraction and high surface area.The adsorption was an exothermic reaction.（3）SiO₂-SH-DGT was composed with agarose gel as the diffusion phase and SiO₂-SH cryogel embedded in polyacrylamide gel as the binding phase.The aqua regia solution could effectively elute the mercury in the binding phase（95.7%）.The adsorption capacity of SiO₂-SH-DGT was high and parallel,and the diffusion coefficient of Hg（Ⅱ）was 1.32×10^-5 cm²·s^-1,which was 1.45-3.08 times that of the previous studies.The performance of SiO₂-SH-DGT was stable in the presence of pH 3-9,ionic strength 10-800 m M and Na⁺,Ca²⁺,and Cl^-.However,the presence of Mg²⁺in the solution could interfere with the in-situ enrichment of Hg（Ⅱ）,and the diffusion coefficient D_S of SiO₂-SH-DGT in seawater was 45%lower than that in pure water.Moreover,a positive correlation was still maintained between the adsorption quality and the enrichment time,the concentration of dissolved Hg（Ⅱ）could also accurately quantified with the D_S in seawater.（4）SiO₂-SH-DGT was applied to enrich the Hg（Ⅱ）in the spiked pure water and spiked simulated seawater,the measured C_DGT/C_SOL was in the range of 0.90-1.01,proving that the method was of high accuracy.Based on the method,the concentration of dissolved Hg（Ⅱ）in Cui Lake of Guangxi University was 7.08±0.02 ng/L,which was close to the concentration of 6.69 ng/L when the lake water was directly determined.The device was deployed in three typical mariculture areas in Qinzhou Bay.With correcting the thickness and coefficient of diffusion phase,the dissolved Hg（Ⅱ）concentration along the inner bay to the offshore mariculture area was 2.39-8.49 ng/L.