Investigating organic sulfur in estuarine and offshore environments: A combined field and cultivation approach.

Estuarine-offshore sediments accumulate substantial particulate organic matter, containing organic sulfur as a key component. However, the distribution and sources of organic sulfur in such environments remain poorly understood. This study investigated organic sulfur in the Yangtze River Estuary and adjacent East China Sea. Dissolved organic sulfur varied from 0.65 to 1.99 µmol/L (molar S:C 0.006-0.018), while particulate organic sulfur ranged from 0.42 to 2.69 µmol/L (molar S:C 0.007-0.082). Sedimentary organic sulfur exhibited a similar molar S:C ratio (0.014-0.071) to particulate organic sulfur in bottom water, implying that particulate matter deposition is a potential source. Furthermore, sediments exposed to frequent hypoxia harbored significantly higher organic sulfur and S:C values compared to non-hypoxic areas. Laboratory incubation experiments revealed the underlying mechanism: sustained activity of sulfate-reducing bacteria in hypoxic sediments led to a substantial increase in sedimentary organic sulfur (from 15 to 53 µmol/g) within 600 days. This microbially driven sulfurization rendered over 90 % of the organic sulfur resistant to acid hydrolysis. Therefore, this study demonstrates that, alongside particle deposition, microbial sulfurization significantly contributes to organic sulfur enrichment and likely promotes organic matter preservation in estuarine-offshore sediments, particularly under hypoxic conditions. This finding advances our understanding of organic sulfur sources in these vital ecosystems.

[Advances and Prospect of Sampling Techniques and Analytical Methods for Trace Elements in the Ocean: Progress of Trace Element Platform Construction in Xiamen University].

Trace elements, which are important chemical components in the ocean, generally refer to those chemical elements with concentrations below 10 µmol·kg-1in seawater. Some trace elements, such as Fe and Zn, serve as essential micronutrients for marine organisms, which regulate marine primary productivity and are closely related to the biogeochemical cycle of carbon and nitrogen and therefore affect the global environment and climate change. In contrast, some elements, such as Pb, are anthropogenic pollutants largely released by human activities. In addition, some trace elements and their isotopes can be used as tracers for oceanographic processes and proxies for paleoceanography. However, the high saline matrix and extremely low trace element concentrations in seawater, as well as the contamination from research vessels, sampling equipment, and the surrounding environment during the process of sample collection, pretreatment, and analysis, have restricted researchers from obtaining reliable trace element data in the ocean for a long period of time. Nevertheless, high quality samples and accurate data are prerequisites for investigating the biogeochemical and environmental behavior of marine trace elements. This paper reviews the development of sampling techniques and analytical methods for trace elements in seawater, introduces the research history and platform construction activities in Xiamen University in this field, summarizes the advantages and disadvantages of various sampling and analytical techniques and methods, and presents the perspectives on future developments in the research on trace elements in the ocean.

The Hg behaviors in mangrove ecosystems revealed by Hg stable isotopes: a case study of Maowei mangrove.

As one of the most productive marine ecosystems in the tropics and subtropics, mangroves are an important part of the global mercury (Hg) cycling. The environmental processes and effects of Hg in mangroves are complex and affect human Hg exposure, and it is crucial to understand Hg behaviors in the mangrove ecosystem. However, clarifying Hg behaviors in the mangrove ecosystem remains difficult because of an insufficient understanding of the dominant pathways. In this study, measurements of mercury (Hg) concentration and isotope ratios in sediment and plant tissues from a mangrove wetland were used to investigate Hg isotope fractionation in mangrove plants and sediments. Spatial patterns in Hg concentration and isotope signatures indicate that Hg re-emission in the sediment was suppressed by mangrove plants. The ratio of Δ199Hg/Δ201Hg was 0.93 for all sediments, indicating that Hg mass-independent fractionation in the mangrove ecosystem was primarily affected by photoreduction, while the ratios of Δ199Hg/Δ201Hg and Δ199Hg/δ202Hg for plant tissues suggested that natural organic matter reduction of Hg(II) was occurred in the plants. The distinct positive Δ199Hg values found in mangrove plants were supposed to be the results of the unique physiological characteristics of mangroves. The exterior Hg sources from atmosphere and seawater emphasize the role of mangrove ecosystems in the global Hg biogeochemistry. Our study highlights the distinct Hg isotope signatures in the mangrove from that in forests and indicates unique Hg behaviors in the mangrove ecosystem.

Distribution of mercury isotope signatures in Yundang Lagoon, Xiamen, China, after long-term interventions.

Isotope signatures of mercury (Hg) were determined for Hg fractions in seawater, sediments, porewaters, core sediments and fish from the Yundang Lagoon, Xiamen, China. Sequential extraction was used to extract Hg fractions in sediments and the purge-trap method was used to preconcentrate Hg in seawater. A large variation in mass dependent fractionation (δ202Hg: -2.50‰ to -0.36‰) was observed in the lagoon. Seawater and fish samples showed positive mass-independent fractionation (Δ199Hg: -0.06‰-0.45‰), while most of sediment and porewater samples displayed insignificant mass-independent fractionation (Δ199Hg: -0.10‰-0.07‰). Ancillary parameters (total organic carbon, sulfide, pH, Eh, water content and grain size) were also measured in the sediments to investigate correlations with Hg isotopes. Three sources (domestic sewage, sediments and atmospheric deposition) were identified as the main sources of Hg in the lagoon seawater. Photochemical reaction was the main process causing isotope fractionation in seawater. Through Hg partitioning and deposition, light isotopes were enriched from dissolved Hg to particulate Hg, then to sediments, and then to porewaters. Finally, Hg isotope signatures were used to identify the Hg sources and fractionation processes in core sediments from different depths. Our results demonstrate that Hg isotopes are powerful tools for tracing Hg sources and arriving at a better understanding of Hg biogeochemical cycling in the lagoon after long-term interventions.

Study of mercury transport and transformation in mangrove forests using stable mercury isotopes.

Mangrove forests are important wetland ecosystems that are a sink for mercury from tides, rivers and precipitation, and can also be sources of mercury production and export. Natural abundance mercury stable isotope ratios have been proven to be a useful tool to investigate mercury behavior in various ecosystems. In this study, mercury isotopic data were collected from seawater, sediments, air, and plant tissues in two mangrove forests in Guangxi and Fujian provinces, China, to study the transport and transformation of mercury in mangrove sediments. The mangroves were primarily subject to mercury inputs from external sources, such as anthropogenic activities, atmospheric deposition, and the surrounding seawater. An isotope mixing model based on mass independent fractionation (MIF) estimated that the mangrove wetland ecosystems accounted for <40% of the mercury in the surrounding seawater. The mercury in plant root tissues was derived mainly from sediments and enriched with light mercury isotopes. The exogenous mercury inputs from the fallen leaves were diluted by seawater, leading to a positive Δ199Hg offset between the fallen leaves and sediments. Unlike river and lake ecosystems, mangrove ecosystems are affected by tidal action, and the δ202Hg and Δ199Hg values of sediments were more negative than that of the surrounding seawater. The isotopic signature differences between these environmental samples were partially due to isotope fractionation driven by various physical and chemical processes (e.g., sorption, photoreduction, deposition, and absorption). These results contribute to a better understanding of the biogeochemical cycling of mercury in mangrove wetland ecosystems.

Application of an Isotope Binary Mixing Model for Determination of Precise Mercury Isotopic Composition in Samples with Low Mercury Concentration.

An isotope binary mixing model was applied for high precision measurement of mercury isotope ratios in samples with low mercury concentrations by multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). Standard addition was used to evaluate the precision and accuracy of the isotope composition calculations resulting from the isotope binary mixing model. A high, steady 202Hg signal of approximately 2.13 V was achieved, with the mercury concentration reaching 3 ng/mL. The isotopic composition of three standards (NIST SRM 1646a; NIST SRM 1575a; BCR 482) and natural samples were precisely determined. The standards and natural samples were diluted to low mercury concentrations (low to 0.90 ng/mL) and mixed with standard solutions (NIST SRM 3133) with high mercury concentrations (50 ng/mL); the isotopic compositions of low mercury concentration samples were calculated using an isotope binary mixing model after the isotopic compositions of the mixing solutions were measured. The results showed that the uncertainty of the calculated mercury isotopic compositions was in an acceptable range and the calculation isotope data were in good agreement with direct measurements. Our method allows the precise determination of mercury isotope composition in mercury solutions of concentrations (0.90 ng/mL) below the detection limit of the current system (3.00 ng/mL).

Spectrophotometric flow injection determination of dissolved titanium in seawater exploiting in-line nitrilotriacetic acid resin preconcentration and a long path length liquid waveguide capillary cell.

A sensitive spectrophotometric method for the determination of dissolved titanium (Ti) in seawater is developed. It involves in-line preconcentration and a long path length liquid waveguide capillary cell (LWCC). Nitrilotriacetic acid (NTA) resin is used to preconcentrate Ti from ∼25 mL seawater sample at pH 1.7, and elution is accomplished with 0.8 mol L-1 hydrochloride acid. The eluted Ti solution is buffered to pH 6.0 with 1.0 mol L-1 ammonium acetate and mixed with 1.5 mmol L-1 Tiron solution. The mixture is then injected into LWCC and measured by spectrophotometry at 420 nm. Before the preconcentration step, the sample is treated with 7 mmol L-1 ascorbic acid to reduce Fe(III) to Fe(II), in order to eliminate the Fe interference. The method is not interfered by Fe(III) and Cu(II) present in seawater samples at concentrations 50-fold higher in relation to Ti, and by Cd(II), Pb(II), Cr(VI), Mn(II), Al(III), Zn(II), and Ni(II) at concentrations 100-fold higher in relation to Ti. It is time efficient (7.5 minutes per sample), sensitive (0.10 nmol L-1 detection limit), precise (1.40% measurement RSD at 1.00 nmol L-1 Ti) and is characterized by a linear range of 0.50-5.00 nmol L-1 Ti. The method was applied to analysis of natural water samples collected from the Jiulongjiang Estuary, Fujian, China.

Redox speciation analysis of dissolved iron in estuarine and coastal waters with on-line solid phase extraction and graphite furnace atomic absorption spectrometry detection.

An automatic on-line solid phase extraction (SPE) system employing the flow injection (FI) technique directly coupled to a graphite furnace atomic absorption spectrometer (GFAAS) was established for speciation and determination of dissolved iron in estuarine and coastal waters. Fe(II) was mixed with ferrozine solution in a sample stream to form the Fe(II)-ferrozine complex which was extracted onto a C18 SPE cartridge, eluted with eluent and detected with GFAAS. In a parallel flow channel, Fe(III) was reduced to Fe(II) with ascorbic acid and then detected in the same way as Fe(II). The home-made interface between FI-SPE and GFAAS efficiently realized the sample introduction to the furnace in a semi-automated way. Parameters of the FI-SPE system and graphite furnace program were optimized based on a univariate experimental design and an orthogonal array design. The salinity effect on the method sensitivity was investigated. The proposed method provided a detection limit of 1.38 nmol L(-1) for Fe(II) and 1.87 nmol L(-1) for Fe(II+III). With variation of the sample loading volume, a broadened determination range of 2.5-200 nmol L(-1) iron could be obtained. The proposed method was successfully applied to analyze iron species in samples collected from the Jiulongjiang Estuary, Fujian, China. With the 2-cartridge FI-SPE system developed, on-line simultaneous determination of Fe species with GFAAS was achieved for the first time.

Effect of environmental factors on the complexation of iron and humic acid.

A method of size exclusion chromatography coupled with ultraviolet spectrophotometry and off-line graphite furnace atomic absorption spectrometry was developed to assess the complexation properties of iron (Fe) and humic acid (HA) in a water environment. The factors affecting the complexation of Fe and HA, such as ionic strength, pH, temperature and UV radiation, were investigated. The Fe-HA complex residence time was also studied. Experimental results showed that pH could influence the deprotonation of HA and hydrolysis of Fe, and thus affected the complexation of Fe and HA. The complexation was greatly disrupted by the presence of NaCl. Temperature had some influence on the complexation. The yield of Fe-HA complexes showed a small decrease at high levels of UV radiation, but the effect of UV radiation on Fe-HA complex formation at natural levels could be neglected. It took about 10 hr for the complexation to reach equilibrium, and the Fe-HA complex residence time was about 20 hr. Complexation of Fe and HA reached a maximum level under the conditions of pH 6, very low ionic strength, in the dark and at a water temperature of about 25°C, for 10 hr. It was suggested that the Fe-HA complex could form mainly in freshwater bodies and reach high levels in the warm season with mild sunlight radiation. With changing environmental parameters, such as at lower temperature in winter or higher pH and ionic strength in an estuary, the concentration of the Fe-HA complex would decrease.

Vertical distribution of total mercury and methylmercury in sediment of the Fugong mangrove area at Jiulong River Estuary, Fujian, China.

The concentrations and vertical distributions of total mercury (Hg) and methylmercury (methyl Hg) in the sediment of the Fugong mangrove area, located at the Jiulong River Estuary, Fujian, China, were investigated. The concentrations of total mercury were between 0.12-0.17 and 0.11-40.14 microg/g, while concentrations of methylmercury were between 0.15-1.8 and 0.081-0.58 ng/g (as mercury), in the dry and rainy seasons, respectively. The total mercury concentration was not correlated with the sampling depth. As the depth increased, methylmercury concentrations first increased to their maximum level at a depth of 10-25 cm, and then decreased; this was similar to the vertical distribution characteristics of methylmercury/total mercury ratios. The mangrove ecosystem was considered as a source of methylmercury for adjacent areas, due to the higher average methylmercury concentration in the mangrove sediment than other sediments nearby. Statistically significant logarithmic correlations, conic correlations, and negative correlations were observed for methylmercury and sulfide concentration, sediment organic matter, and sediment pH, respectively.

[Bioaccumulation of mercury in Crassostrea sp. exposed to waste seawater discharged from a coal-fired power plant equipped with a seawater flue-gas desulfurization system].

A field experiment was conducted to study mercury (Hg) bioaccumulation in Crassostrea sp. exposed to waste seawater discharged from a coal-fired power plant equipped with a flue gas desulfurization system. Oysters were cultured in the discharge outlet of the power plant (studying site) and a control site, respectively. The total Hg (THg) concentrations (all counted as dry weight) of seawater in the studying and control sites were determined as (120.6 +/- 55.5) ng x L(-1) (n = 5) and (2.7 +/- 1.0) ng x L(-1) (n = 5), respectively, while methyl Hg (MeHg) concentrations were (0.30 +/- 0.44) ng x L(-1) (n = 5) and (0.28 +/- 0.31) ng x L(-1) (n = 5), respectively. The THg in oyster at the studying site increased dramatically from (138.3 +/- 14.3) ng x g(-1) (n = 6) to (3 012 +/- 289) ng x g(-1) (n = 6) within 7 d, and remained at high levels of 2 935-4 490 ng x g(-1) for the next 34 d. In contrast, the THg in oyster at the control site showed no significant change, and kept at low levels of 60.7-137.5 ng x g(-1). After 41 d exposure, the MeHg in oyster at the studying site had no significant change, ranging from 55.4 ng x g(-1) to 73.1 ng x g(-1), and the content at the control site showed a slight decrease, ranging from 15.6 to 55.6 ng x g(-1). The study showed that THg in the waste seawater discharged at the coal-fired power plant could be quickly bioaccumulated by oyster to a great extent, the potential risk can thus not be ignored. MeHg concentration in the waste seawater was quite low, and no obvious bioaccumulation was found in oyster. Under the study conditions, no self-synthesis of MeHg or transformation of inorganic Hg into MeHg was found.