Development Of Hg-Au Microelectrodes For Measuring DO, Mn²⁺ And Fe²⁺ In Pore Water Of Marine Sediments

Hg-Au microelectrodes were developed from an Au microelectrode by using electrochemical method.This research studied the preparation method of Au microelectrodes,the conditions of electroplating and strong polarization procedures, the calibration method of Hg-Au,as well as electrochemical parameters for practical measurements.By adopting three-electrode cell and square wave voltammetry,Hg-Au microelectrode can measure dissolved oxygen,Mn²⁺,Fe²⁺ and sulfide during a single potential scan from -0.10 V to -1.75 V versus saturated calomel electrode. Experimental results indicated that the microelectrode has the sensitivity and detection limit of 0.16 nA/(μmol/L),6μmol/L for dissolved oxygen;0.35 nA/ (μmol/L),3μmol/L for Mn²⁺;0.22 nA /(μmol/L),5μmol/L for Fe²⁺ and 34 nA/ (μmol/L),0.03μmol/L for sulfide.It was found that the microelectrode exhibits good reproducibility with an error of within 3%after measuring 15 sediment samples consecutively.Hg-Au microelectrodes were applied to measure concentration profiles of the redox chemical species such as dissolved oxygen,Mn²⁺ and Fe²⁺ within sediments from South China Sea.Microbial aerobic and anaerobic respiration depths and characteristics within sediments were elucidated.Under oxic condition, microorganisms would preferentially use oxygen as electron acceptor during organic matter oxidation.When the oxygen consumption rate is higher than oxygen diffusive transportation rate within the sediments,microorganisms tend to utilize manganese dioxide and iron oxides as electron accepters during anaerobic respiration.Exponential regressions were conducted based on the concentration profiles of oxygen within sediments.It was calculated that sediments from three stations(2Y90, Y10 and Y20) of South China Sea had oxygen fluxes of 0.28mmolO₂·m^-2·d^-1,0.31 mmolO₂·m^-2·d^-1,and 0.36 mmolO₂·m^-2·d^-1 respectively across the sediment-seawater interface with an average of 0.32 mmolO₂·m^-2·d^-1.Through derivative calculation mathematically,the oxygen consumption rates at the three stations were 9.67 mmolO₂·m^-3·d^-1,11.90 mmolO₂·m^-3·d^-1,and 21.44 mmolO₂·m^-3·d^-1 respectively with an average of 14.34 mmolO₂·m^-3·d^-1.These data were then translated into organic carbon degradation rates within surface sediments by applying Redfield ratio. Combined with measurements of organic carbon contents,the residence time of organic carbon within the surface sediments were calculated to be 24.5,17.9,and 11.0 years respectively with an average of 17.8 years.