|Other Abstract||The ocean has superior condition of environment and resources, which is closely related to the survival and development of mankind. Due to the multiple impacts of the land, sea and human activities, the offshore area becomes the most polluted ocean area. As an essential element, copper plays an important role in the process of carbon and nitrogen cycle and iron transport in ocean system. Meanwhile, it will also participate in the synthesis of various enzymes maintaining the organisms’ living activities. Thus, in order to further explore the changes and cycle of copper ions (Cu2+) in ocean systems, especially in the offshore seawater, it is necessary to develop sensitive and accurate methods to detect and analyze Cu2+.
Lots of techniques have been developed to determine the copper ions in water samples so far. However, most of them need pretreatment steps such as purification, removing turbidity and color, enrichment and so on. Common methods include adsorption, extraction, precipitation and co-precipitation, etc. However, these methods are limited to use because of many disadvantages such as vast need of chemical reagents, complexity and poor reproducibility, time-consuming and so on. Electrochemical method has many advantages such as portability, indigenous miniaturization, high selectivity, great sensitivity, and time-saving when it is used as a preconcentration process. The electro-deposition process also can enhance the analytical signal and eliminate interference from sample matrices. Just because of those, electrochemical enrichment has already been employed to provide concentrations required for analysis by flameless atomic absorption spectrometry, anodic stripping voltammetry, and inductively coupled plasmas mass spectroscopy.
In view of the present problems, such as serious background interference of the medium, the low enrichment efficiency, large-scale instrument and complex operation of pretreatment process in the determination of copper ions in seawater, a series of pint-sized electrochemical preconcentration/media transfer equipment was set up and used as the pretreatment process for the quantitative determination of copper ions in seawater by ultraviolet-visible spectrophotometry and stripping voltammetry. The research work of this paper mainly involves the following three aspects:
1. Electrochemical medium transition procedure used to detect copper ions in turbid water. In this part, electrochemistry and UV-visible spectrophotometry method was combined to detect copper ions in turbid or colored water. The electrochemical deposition process served as medium shift pretreatment step while the spectrophotometric analysis was in charge of the quantitative analysis. This combination would give the chance to measure Cu2+ in water samples under conditions without interferences of turbidity, color or high salinity due to the advantages of electrodeposition. Under the optimized conditions, a linear relationship between the absorbance and the concentration of Cu2+ was obtained in the range of 16 μmol/L to 160 μmol/L with a detection limit of 2.5 μmol/L. This method had excellent reproducibility and selectivity and was applied to the determination of Cu2+ in actual turbid water with satisfactory results.
2. Electrochemical preconcentration method used to detect copper ions in standard seawater. In this part, a large volume electrochemical cell was designed to improve the enrichment efficiency and applied to detect Cu2+ in artificial seawater and standard seawater. The cell contained an inner large size working electrode, namely a glass carbon piece electrode (GCP) with effective diameter of 9 mm. To further increase the effective working surface area of the electrode, 40 nm gold nanoparticles (AuNPs) were electrochemically deposited onto the electrode surface (AuNPs/GCP). According to the Cu2+ concentration before and after the preconcentration, the enrichment efficiency of GCP and AuNPs/GCP was improved to 9.4% and 18.6% respectively. Under the optimized conditions, the liner range was segmented (0.01 ~ 0.1 μmol/L and 0.1 ~ 1 μmol/L) with a low detection limit of 6.5 nmol/L (S/N=3). It has been successfully applied to determine copper ions in standard seawater with different salinity and the result showed great accuracy.
3. Electrochemical enrichment/transition technique used to detect copper ions in real seawater. In this part, a multifunctional electrochemical cell was designed on the basis of the large size cell mentioned above and applied to detect Cu2+ in coastal seawater samples. The cell consisted of three main parts: a large enrichment cell, a mini stripping cell and an interior three-electrode system with a large-sized glassy carbon piece electrode (GCP) (9 mm in diameter). The enrichment step was acted in the large enrichment cell and the stripping step was in the mini stripping cell. The working electrode was AuNPs modified glassy carbon piece electrode (AuNPs/GCP) and xylenol orange (XO) was chosen as the color developing agent for spectrophotometry. The conditions such as acidity, developer dosage and developing time were optimized. After combining the preconcentration and transfer method under optimal conditions, the concentration of Cu2+ was 30 times higher than that in the original solution. Moreover, this system has been successfully applied to the detection of copper ions in actual seawater, the results are in good agreement with ICP-MS, and it had been successfully applied to the detection of Cu2+ in sea water for 8 consecutive days.|