Dongxing Yuan

Electrochemical removal of chromium from aqueous solutions using electrodes of stainless steel nets coated with single wall carbon nanotubes

An electrochemical technique was adopted to investigate the removal of Cr(VI) species and total chromium (TCr) from aqueous solution at a laboratory scale. The electrodes of stainless steel nets (SSNE) coated with single wall carbon nanotubes (SWCNTs@SSNE) were used as both anode and cathode. Three parameters, including solution pH, voltage and electrolyte concentration, were studied to explore the optimal condition of chromium removal. The optimal parameters were found to be pH 4, voltage 2.5 V and electrolyte concentration 10 mg/L. Under these conditions, the Cr(VI) and TCr removal had a high correlation with the amount of SWCNTs coated on the electrodes, with coefficients of the regression equations 0.953 and 0.928, respectively. The mechanism of Cr(VI) removal was also investigated. X-ray photoelectron spectroscopy (XPS) study and scanning electron microscope (SEM) picture showed that the process of chromium removal involved the reduction of Cr(VI) to Cr(III) on the cathode, and then the adsorption of Cr(III) by SWCNTs on the cathode. The study results indicated that the proposed method provided an interesting means to remove chromium species from aqueous solution, especially Cr(VI) in acidic condition.

In-field determination of nanomolar nitrite in seawater using a sequential injection technique combined with solid phase enrichment and colorimetric detection.

A novel sequential injection method for the determination of nitrite at nanomolar level in seawater samples has been developed. The pink azo compound was formed based on the Griess reaction and quantitatively adsorbed onto a Sep-Pak C18 cartridge. The enriched azo compound was rinsed with water and ethanol (28%, v/v) in turn, and then eluted with an eluent containing 26.6% (v/v) ethanol and 0.108 mol L(-1) H(2)SO(4). Finally the azo compound was measured using a spectrophotometer at 543 nm. Under the optimized conditions, the linear calibration ranges were 0.71-42.9 nmol L(-1) for a 150-mL sample and 35.7-429 nmol L(-1) for a 15-mL sample. The relative standard deviation of 8 measurements was 1.44% for 14.3 nmol L(-1) nitrite. For the 150 mL sample, the detection limit was estimated to be 0.1 nmol L(-1). The throughput of the method was about 4 samples per hour. The proposed method has been successfully applied to the in-field determination of nanomolar concentrations of nitrite in seawater.

A sensitive flow-batch system for on board determination of ultra-trace ammonium in seawater: Method development and shipboard application

Combining fluorescence detection with flow analysis and solid phase extraction (SPE), a highly sensitive and automatic flow system for measurement of ultra-trace ammonium in open ocean water was established. Determination was based on fluorescence detection of a typical product of o-phthaldialdehyde and ammonium. In this study, the fluorescence reaction product could be efficiently extracted onto an SPE cartridge (HLB, hydrophilic-lipophilic balance). The extracted fluorescence compounds were rapidly eluted with ethanol and directed into a flow cell for fluorescence detection. Compared with the common used fluorescence method, the proposed one offered the benefits of improved sensitivity, reduced reagent consumption, negligible salinity effect and lower cost. Experimental parameters were optimized using a univariate experimental design. Calibration curves, ranging from 1.67 to 300nM, were obtained with different reaction times. The recoveries were between 89.5 and 96.5%, and the detection limits in land-based and shipboard laboratories were 0.7 and 1.2nM, respectively. The relative standard deviation was 3.5% (n=5) for an aged seawater sample spiked with 20nM ammonium. Compared with the analytical results obtained using the indophenol blue method coupled to a long-path liquid waveguide capillary cell, the proposed method showed good agreement. The method had been applied on board during a South China Sea cruise in August 2012. A vertical profile of ammonium in the South East Asia Time-Series (SEATS, 18° N, 116° E) station was produced. The distribution of ammonium in the surface seawater of the Qiongdong upwelling in South China Sea is also presented.

An automatic gas-phase molecular absorption spectrometric system using a UV-LED photodiode based detector for determination of nitrite and total nitrate

An automatic gas-phase molecular absorption spectrometric (GPMAS) system was developed and applied to determine nitrite and total nitrate in water samples. The GPMAS system was coupled with a UV-light emitting diode photodiode (UV-LED-PD) based photometric detector, including a 255 nm UV-LED as the light source, a polyvinyl chloride (PVC) tube of 14 cm as the gas flow cell, and an integrated photodiode amplifier to measure the transmitted light intensity. The UV-LED-PD detector was compact, robust, simple and of low heat production, comparing with detectors used in other GPMAS works. For nitrite measurement, citric acid was used to acidify the sample, and ethanol to catalyze the quantitative formation of NO(2). The produced NO(2) was purged with air flow into the UV-LED-PD detector, and the gaseous absorbance value was measured. The total nitrate could be determined after being reduced to nitrite with a cadmium column. Limits of detection for nitrite and nitrate were 7 μmol/L and 12 μmol/L, respectively; and linear ranges of 0.021-5 mmol/L for nitrite and 0.036-4 mmol/L for nitrate were obtained. Related standard deviations were 1.81% and 1.08% for nitrite and nitrate, respectively, both at 2 mmol/L. The proposed method has been applied to determine nitrite and total nitrate in some environmental water samples.

Simultaneous determination of nanomolar nitrite and nitrate in seawater using reverse flow injection analysis coupled with a long path length liquid waveguide capillary cell

A reverse flow injection analysis (rFIA) method coupled with 1m liquid waveguide capillary cell and spectrophotometric detection for simultaneous determination of nanomolar nitrite and nitrate in seawater was developed. The design of two analytical channels sharing the same detection system in the proposed method allowed the analysis of both nitrite and nitrate with single sample injection. Different strategies of reagent injection were investigated to obtain a higher sensitivity and a better peak shape. A dual-wavelength detection mode was chosen to eliminate the light source shifting and sample matrix interference. Experimental parameters were optimized based on a univariate experimental design and the matrix effect from seawater was preliminarily investigated. The proposed method had high sensitivity with detection limit of 0.6 nmol L(-1) for both nitrite and nitrate. The linearity was 2-500 nmol L(-1) for both analytes, and the upper limit could be extended by choosing a lower sensitivity detection wavelength. The analytical results of 26 surface seawater samples obtained with the proposed method showed good agreement with those using a reference method operated using an automated segmented flow analyzer. The proposed method could greatly minimize the trouble introduced by bubbles in the segmented flow analyzer. It also had the advantages of high precision and high sample throughput (nitrite and nitrate detected in triplicate; 5 h(-1)). Compared to normal flow injection analysis, the rFIA method is superior due to its lower reagent consumption, less dispersion of sample, as well as higher sensitivity.

On-Line Solid Phase Extraction and Spectrophotometric Detection with Flow Technique for the Determination of Nanomolar Level Ammonium in Seawater Samples

The determination of low-level ammonium in seawaters suffers from low sensitivity and high contamination; therefore, it is desirable to develop highly sensitive methods for automatic measurements. A highly sensitive and automated flow technique system for nanomolar level ammonium measurement is described. Reagents for Berthelot reaction were automatically added into seawater samples. After a 10 min reaction at 40°C, the formed indophenol blue compound was on-line extracted onto an Hydrophilic-Lipophilic Balance (HLB) cartridge. The enriched compound was rinsed with water and ethanol solution (30%, v/v) and, in turn, eluted with an eluent containing 30% (v/v) ethanol and 5.0 mM of NaOH, and determined with a spectrophotometer at 640 nm. Parameter, including extraction conditions, reagent concentrations, pH, temperature, and reaction time, were optimized. Under the optimized conditions, the detection limit was 3.5 nM and the linear range was 0–428 nM. The relative standard deviations were 5.7% (n = 8) for 44.6 nM standard solution and less than 6.0% (n = 3–5) for samples within concentrations of about 52.4–288.7 nM; the recovery was in the range 93.6 to 108.5%. The sample throughput was 3 h−1. The proposed method provides a simple, cheap, and automatic way to determine ammonium in seawater samples without complicated sample treatment.

Reverse flow injection analysis method for catalytic spectrophotometric determination of iron in estuarine and coastal waters: A comparison with normal flow injection analysis

A method for determining iron in seawater had been developed by coupling reverse flow injection analysis (rFIA) and catalytic spectrophotometric detection with N,N-dimethyl-p-phenylenediamine dihydrochloride (DPD). With a seawater sample or a standard solution as the carrier, the mixture of DPD and buffer was injected into the carrier stream quantitatively and discretely. After mixing with H(2)O(2), the DPD was oxidized to form two pink semiquinone derivatives that were monitored at 514 nm wavelength with a reference at 700 nm. The method detection limit was 0.40 nmol L(-1), lower than half of that of normal flow injection analysis (nFIA) method. The sample throughput was 10h(-1) with triplicate determination, compared with 4h(-1) for nFIA-DPD method. The analysis results of the certified seawaters CASS-4 (12.33 ± 0.18 nmol L(-1)) and NASS-5 (3.47 ± 0.23 nmol L(-1)) well agreed with the certified values (12.77 ± 1.04 and 3.71 ± 0.63 nmol L(-1), respectively). The typical precision of the method for a 2.97 nmol L(-1) iron sample was 4.49% (n=8). Interferences from copper and salinity were investigated. An instrument was assembled based on the proposed method and applied successfully to analyze total dissolvable iron (TDFe) in surface seawater samples collected from the Pearl River Estuary, the results of which revealed non-conservative behavior of TDFe during the estuarine mixing. Results for these samples with both rFIA-DPD and nFIA-DPD methods showed good agreement with each other. The proposed method was superior to the currently used nFIA-DPD method, particularly when it is adapted for field and in situ deployment, due to its lower reagent consumption, higher sample throughput and keeping the manifold tubing clean.

Distribution characteristics of total mercury and methylmercury in the topsoil and dust of Xiamen, China

The levels and distribution of mercury (Hg) species, including total mercury (THg) and methylmercury (MeHg) in the topsoil and dust collected from twenty sampling stations located in different land function areas of Xiamen, China, were investigated. The THg concentrations in topsoil ranged from 0.071 to 1.2 mg/kg, and in dust ranged from of 0.034 to 1.4 mg/kg. For stations where the THg of dust was less than 0.31 mg/kg, THg concentrations in the topsoil were significantly correlated to those in the corresponding dust (r = 0.597, n = 16, P = 0.014). The MeHg concentrations in topsoil were varied between 0.14 and 5.7 microg/kg. The ratios of MeHg/THg in the topsoil ranged from 0.069% to 0.74%. The range of MeHg concentration in the dust were 0.092-2.3 microg/kg. The ratios of MeHg/THg in the dust were at the same level as those in the topsoil. The MeHg concentrations in both topsoil and dust were linked to corresponding THg concentrations and soil organic matter. Neither THg nor MeHg concentration in the topsoil and dust was obviously linked to the land function.

Mercury Species in Seawater and Sediment of Xiamen Western Sea Area Adjacent to a Coal-fired Power Plant

The purpose of this study was to investigate the concentrations and spatial distributions of mercury (Hg) species in seawater (including dissolved, particle, and total Hg) and sediment (including total and methyl Hg) of the Xiamen western sea area adjacent to a coal-fired power plant. The influence of the wastewater discharged from the seawater desulphurization (De-SO2) system of the power plant is discussed. Concentration of the three species of mercury in seawater from 18 sampling sites varied: dissolved ranged from 0.70 to 4.65 ng/L (mean 1.47 ng/L; median 1.12 ng/L); particulate ranged from not detected to 90.52 ng/L (mean 10.47 ng/L; median 1.26 ng/L); and total was 1.51 to 92.88 ng/L (mean 11.94 ng/L; median 2.84 ng/L). High concentrations of total Hg and particulate Hg, more than 70 ng/L, were observed in the area adjacent to the outfalls of the power plant. The Hg from the waste seawater of the power plant might be re-emitted to the atmosphere because it exists in seawater mainly in particulate attached form. The sediment total Hg concentrations ranged from 0.055 to 0.201 microg/g, with a mean of 0.126 microg/g and a median of 0.125 microg/g. Low methyl Hg concentrations in the sediment were observed in the study area, ranging from 0.017 to 0.256 ng/g as Hg, with a mean of 0.087 ng/g and a median of 0.081 ng/g. The ratios of methyl Hg to total Hg in the sediment were low, with a mean and median of 0.069%. Both total and methyl Hg were significantly linked to the sediment organic carbon (SOC).

Rapid Analysis of Heavy Metals in Coastal Seawater Using Preconcentration with Precipitation/Co-precipitation on Membrane and Detection with X-Ray Fluorescence

State Key Laboratory of Marine Environmental Science at Xiamen University through MEL [MELRI1001]