Characterization of dissolved organic matter in anoxic rock extracts and in situ pore water of the Opalinus Clay

Dissolved organic matter (DOM) from the Opalinus Clay, a potential host rock for the disposal of radioactive waste, was isolated under strictly anoxic conditions from ground rock material and compared with DOM of in situ pore water samples. For the extractions, deionized water, synthetic pore water (SPW, water containing all major ions at pore water concentrations but no organic matter) and 0.1 M NaOH were used. The influence of the solid-to-liquid ratio, extraction time, acid-pretreatment and O₂ exposure of the rock material on the isolated DOM were investigated. Liquid chromatography coupled with a total organic C detector (LC-OCD) and reverse-phase ion chromatography were used to characterize the DOM size distributions and to determine the low molecular weight organic acid (LMWOA) contents in the pore water samples and the rock extracts.The results revealed that only a small portion of the total organic C of the rock material (<0.38%) was extractable, even after removal of carbonates by acid-pretreatment. The concentrations of dissolved organic C (DOC) were found to range from 3.9 ± 0.4 to 8.0 ± 0.8 mg/L in the anoxic extracts. The pore waters exhibited similar DOC concentrations ranging from 1.2 to 15.8 ± 0.5 mg/L. The analysis by LC-OCD showed that the DOM extracted under anoxic conditions and the pore water DOM mainly consisted of hydrophilic compounds of less than 500 Da. The DOM extracted with SPW was most similar in size to the pore water DOM. Grinding the rock under oxic conditions increased the DOC yields and shifted the size distribution toward higher molecular weight compounds compared to the strictly anoxic treatment. Acetate, lactate and formate were identified in all extracts and in the pore water. In total, LMWOA accounted for 36% of the total DOC in both pore water and SPW extracts. The results imply that controlled anoxic conditions and the use of SPW as an extractant are required to isolate DOM from Opalinus Clay rocks which most resembles the in situ pore water DOM with respect to its size distribution and the LMWOA contents.     

Nature and reactions of dissolved organic matter in the interstitial waters of marine sediments

Two organic rich sediments, an oxic muddy sand and a silty mud containing sulphate reducing and methane producing metabolic zones, were sampled from Loch Duich, a fjord type estuary in the N.W. coast of Scotland. Dissolved organic carbon (DOC), as measured by dry combustion and UV absorption, remained constant (8.3–15.8 mg C/l) with depth in the oxic pore waters at a concentration at least twice that of the overlying seawater. DOC in the anoxic pore waters increased linearly with depth from 13.6 at the surface to 55.9–70.5 mg C/l at 80cm. Most of the DOC was present in the high molecular weight (HMW) fraction as separated by ultrafiltration; the low molecular weight (LMW) fraction remained constant (10.0 mg C/l) in both oxic and anoxic pore waters. Spectroscopic data showed the ‘humic’ fraction of the HMW dissolved organic matter was mainly fulvic acid, a small proportion (approx 1%) of humic acid, and a third fraction, possibly melanoidins, which increased relative to fulvic acid with depth. These data confirm the pathway of humification (NissenBaum et al, 1971; nissenbaum and Kaplan, 1972) where HMW organic matter accumulates in pore waters as condensation products of LMW organic substances.     

Coal weathering and the geochemical carbon cycle

The weathering rate of sedimentary organic matter in the continental surficial environment is poorly constrained despite its importance to the geochemical carbon cycle. During this weathering, complete oxidation to carbon dioxide is normally assumed, but there is little proof that this actually occurs. Knowledge of the rate and mechanisms of sedimentary organic matter weathering is important because it is one of the major controls on atmospheric oxygen level through geologic time.We have determined the aqueous oxidation rates of pyrite-free bituminous coal at 24° and 50°C by using a dual-cell flow-through method. Coal was used as an example of sedimentary organic matter because of the difficulty in obtaining pyrite-free kerogen for laboratory study. The aqueous oxidation rate obtained in the present study for air-saturated water (270 μM O₂) was found to be on the order of 2 × 10⁻¹² mol O₂/m²/s at 25°C, which is fast compared to other geologic processes such as tectonic uplift and exposure through erosion. The reaction order with respect to oxygen level is 0.5 on a several thousand hour time scale for both 24° and 50°C experiments. Activation energies, determined under 24° and 50°C conditions, were ≈40 kJ/mol O₂ indicating that the oxidation reaction is surface reaction controlled.The oxygen consumption rate obtained in this study is two to three orders of magnitude smaller than that for pyrite oxidation in water, but still rapid on a geologic time scale. Aqueous coal oxidation results in the formation of dissolved CO₂, dissolved organic carbon (DOC), and solid oxidation products, which are all quantitatively significant reaction products.     

Geochemical fractionation of manganese in the Riogrande II reservoir, Antioquia, Colombia

The Riogrande II reservoir in Colombia has a total storage capacity of 240 million m³ and lies 2,270 m above sea level. The reservoir is used for power generation, water supply and environmental improvement. Dissolved manganese (Mn) is removed from reservoir water dedicated to domestic use by purification processes. Removal of Mn, however, poses a major challenge to purification processes and warrants the study of ways to naturally reduce dissolved Mn levels in the reservoir. The source of Mn within the reservoir is not well understood, however, presumably arises from sediment mobilization initiated by variation in pH, redox potential (ORP or Eh), dissolved oxygen (O₂) and ionic strength conditions. This study investigated conditions within the reservoir to further understand Mn transfer from the sediment into the water column. O₂, pH, oxidation–reduction potential (ORP or Eh), organic matter content and electric conductivity were measured in water samples and sediment from the reservoir. Sequential extraction (SE) procedures were used to test the specific effects exerted by each of these conditions on Mn mobilization from the sediments. The European Community Bureau of Reference (BCR) sequential extraction procedure was used to quantify metals in sediment (referred to as the BCR extraction below). Statistical analysis of geochemical data from water samples (both water column and sediment pore water) and sediments demonstrated the conditions under which Mn can be released from sediments into the water column. The results indicated a primarily oxic water column and anoxic reducing conditions in the sediment (ORP or Eh ≤ ?80 mV). The pH of water in contact with bottom sediments varied from 7.6 to 6.8. The pH of sedimentary pore water varied from 6.8 to 4.7. The sediments contained significant amounts of organic matter (20 %). Chemical extractions showed that the exchangeable fraction contained over 50 % of the total Mn within sediments. Microscopic analysis using scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS) indicated that Mn does not occur within well-crystallized mineral phases in the Riogrande II sediments. A large proportion of Mn exists instead as material adsorbed onto the surfaces of recently deposited sediment particles. Bacterial oxidation of organic matter may cause the observed anoxic conditions at the bottom of the reservoir. Mineralization of organic matter therefore contributes to reducing conditions within the sediments. Mobilization of Mn from the sediment into the water column may result from reductive dissolution of this fraction. Manganese release by this mechanism diminishes the water quality of the Riogrande II reservoir and warrants further study.     

Factors affecting dissolved organic matter dynamics in mixed-redox to anoxic coastal sediments

Mixed-redox (suboxic, or oscillating between oxidizing and reducing conditions) to anoxic marine sediments from the Raritan -New York Bay complex and the Inner New York Bight of the eastern U.S. were studied to investigate the factors controlling the accumulation of pore-water dissolved organic carbon (DOC). DOC increased with depth at each of four study sites, but accumulation was generally limited in the mixed-redox zone relative to the anoxic zone. Humic-like fluorescence intensity also differed between mixed-redox and anoxic zones of the sediment, such that anoxic pore waters were relatively enriched in fluorescent, humic-like compounds.A pore-water DOC model was tested for its capacity to explain these geochemical patterns. Model results for a heavily irrigated and a non-bioturbated site both suggest that, excluding the uppermost few cm of the sediment column, pore-water DOC is predominantly comprised of poorly-reactive material with estimated degradation rate constants on the order of 10⁻³ yr⁻¹. Model results are also consistent with the suggestion that DOC accumulation is suppressed in the mixed-redox compared to the anoxic zones of the sediment due to rapid oxidation of high-molecular-weight DOC, and limited production and enhanced oxidation of the less reactive polymeric low-molecular-weight component of the DOC pool. An assessment of sorptive behavior of DOC in the surface sediments of the study area suggests that sorption can influence pore-water DOC distributions under conditions of high sedimentation and low bioirrigation.     

Dissolved organic carbon and volatile fatty acids in marine sediment pore waters

The results of a study of the contribution of microbial metabolic products to total dissolved organic carbon (DOC) levels in coastal sediments are presented. The data indicate that acidic volatile compounds make up a substantial fraction of pore water DOC's in both oxic and anoxic pore waters of coastal marine sediments. Formic, acetic and butyric acids are the principal volatile species identified at levels exceeding 10 μM. Acid concentrations are up to five times higher in anoxic pore waters than in oxic waters. Volatile organic acids show promise as indicators of diagenetic processes in marine sediments and of the ecological succession of microorganisms, in particular.     

Manganese(II) oxidation driven by lateral oxygen intrusions in the western Black Sea

Mn(II) oxidation in the suboxic zone of the water column was studied at four stations in the western Black Sea. We measured Mn(II) oxidation rates using ⁵⁴Mn tracer and tested the hypothesis of alternative oxidants for Mn(II) other than dissolved oxygen. In anoxic incubation experiments with water from different depths of the chemocline, Mn(II) was not oxidized by nitrite, nitrate, or iodate. In the presence of light, Mn(II) also was not oxidized under anoxic conditions as well. Anaerobic Mn(II) oxidizing microorganisms could not be enriched. In oxic incubation experiments, the addition of alternative oxidants did not significantly increase the Mn(II) oxidation rate. The lack of an anaerobic Mn(II) oxidation in our experiments does not unambiguously prove the absence of anaerobic Mn(II) oxidation in the Black Sea but suggests that dissolved oxygen is the only oxidant for biologically catalyzed Mn(II) oxidation. Lateral intrusions of modified Bosphorus water were shown to be the main mechanism providing dissolved oxygen in the suboxic and the upper anoxic zones and explaining observed Mn(II) oxidation rates. Maximum in situ Mn(II) oxidation rates in the suboxic zone were 1.1 nM Mn(II) per h in the central Black Sea, 25 nM Mn(II) per h on the Romanian continental slope and 60 nM Mn(II) per h on the Anatolian continental slope. These rates correlate with the amount of particulate Mn and the number of Mn-oxide particles and are in agreement with rates measured 13 yr before. Our study highlights the importance of lateral intrusions of oxygen for the ventilation of the suboxic zone and the anoxic interior and for the regulation of different oxidation-reduction processes in the chemocline, including Mn(II) oxidation, which may be significant for other anoxic basins as well.     

Dissolved carbon in pore waters from the carbonate barrier reef of Tahiti (French Polynesia) and its basalt basement

This paper deals with dissolved inorganic carbon (DIC) and organic carbon (DOC) in pore waters from a 150 m deep hole drilled through the carbonate barrier reef of Tahiti and its underlying basalt basement. Alkalinity-pH measurements were used to calculate the DIC species concentration, and DOC was analysed according to the high temperature catalytic oxidation technique. Salinity was used as a conservative tracer to help identify water origin and mixing within the hole. Water mixing, calcium carbonate dissolution and mineralization of organic carbon combined to form three distinct groups of pore water. In the deeper basalt layers, pore water with alkalinity of 1.4 meq kg^–1 pH of 7.6 and p(CO₂) of 1.2 mAtm was undersaturated with respect to both aragonite and calcite. In the intermediate carbonate layer, pore water with alkalinity of more than 2.0 meq kg^–1, pH of 7.70 and p(CO₂) of 1.4 mAtm was supersaturated with respect to both aragonite and calcite. The transition zone between those two groups extended between 80 and 100 m depth. The shift from aragonite undersaturation to supersaturation was mainly attributed to the mixing of undersaturated pore waters from the basalt basement with supersaturated pore waters from the overlaying limestone. In the top of the reef, inputs from a brackish water lens further increased p(CO₂) up to 5.6 times the atmospheric P(CO₂).     

Dissolved carbon in pore waters from the carbonate barrier reef of Tahiti (French Polynesia) and its basalt basement

This paper deals with dissolved inorganic carbon (DIC) and organic carbon (DOC) in pore waters from a 150 m deep hole drilled through the carbonate barrier reef of Tahiti and its underlying basalt basement. Alkalinity-pH measurements were used to calculate the DIC species concentration, and DOC was analysed according to the high temperature catalytic oxidation technique. Salinity was used as a conservative tracer to help identify water origin and mixing within the hole. Water mixing, calcium carbonate dissolution and mineralization of organic carbon combined to form three distinct groups of pore water. In the deeper basalt layers, pore water with alkalinity of 1.4 meq kg^?1 pH of 7.6 and p(CO₂) of 1.2 mAtm was undersaturated with respect to both aragonite and calcite. In the intermediate carbonate layer, pore water with alkalinity of more than 2.0 meq kg^?1, pH of 7.70 and p(CO₂) of 1.4 mAtm was supersaturated with respect to both aragonite and calcite. The transition zone between those two groups extended between 80 and 100 m depth. The shift from aragonite undersaturation to supersaturation was mainly attributed to the mixing of undersaturated pore waters from the basalt basement with supersaturated pore waters from the overlaying limestone. In the top of the reef, inputs from a brackish water lens further increased p(CO₂) up to 5.6 times the atmospheric P(CO₂).     

Variation in DOC and trace metal concentration along the heavily urbanized basin in Kathmandu Valley,Nepal

Water samples were analyzed for DOC and trace metals from Bagmati River within Kathmandu valley, Nepal, to understand the variation trends of DOC and trace metals and their relationship along the drainage network. The variability in organic matter and wastewater input within the Bagmati drainage basin appeared to control DOC and most of the trace metal concentration. The large input of organic matter and wastewater creates anoxic condition by consuming dissolved oxygen and releasing higher concentrations of DOC, trace elements such as nickel, arsenic, barium, cadmium, and copper with downstream distance. Concentrations of DOC and trace metals like barium and zinc showed strong relationships with human population density and suggest that human activities have strong control on these parameters along the drainage network. The DOC and most of the trace metal concentration increased with downstream distance and appeared to be directly associated with human activities. The variation trends of most of the trace metals appeared to be the same; however, concentration varied widely. Inputs of organic matter and wastewater due to human activities appeared directly to be associated for the variation of DOC and trace metals along the Bagmati drainage network within Kathmandu valley.     

Insights into estuarine benthic dissolved organic carbon (DOC) dynamics using δC-DOC values, phospholipid fatty acids and dissolved organic nutrient fluxes

Benthic dissolved organic carbon (DOC) flux rates and changes in DOC isotope ratios, along with nutrient fluxes, phospholipid fatty acids concentration and carbon isotope ratios were measured in productive estuarine sediments over a diel cycle to determine the mechanisms driving benthic-pelagic coupling of DOC. There was uptake of DOC during the dark and efflux during the light at all sites. DOC uptake rates were related to benthic respiration (dark O₂ uptake) and effluxes were coupled to the trophic status (ratio of production to respiration) of the sediments. Highest uptake and efflux rates were observed at two high nutrient concentration sites. The DOC:DON ratio of water column dissolved organic matter (DOM) decreased during the dark and increased during the light indicating preferential uptake and release of carbon rich dissolved organic matter. The calculated carbon isotope ratio of the DOC taken up by the benthos was significantly more depleted than the bulk water column DOC pool, suggesting preferential uptake of selected components of the water column DOC pool. Generally the isotope ratio of the DOC released during the light was more enriched than that taken up during the dark, which suggests that the benthos has the potential to significantly alter the estuarine DOC pool. Uptake and efflux were coupled to respiration and algal grazing/mineralization, therefore increased nutrient loading may shift the composition of the estuarine DOC pool through changes in the magnitude of benthic DOC fluxes. A combination of biological (diel shifts in DOC production and consumption) and abiotic processes (flocculation) appear to be driving the observed benthic DOC dynamics at the study sites. This study was the first to measure carbon isotopic changes in the water column DOC pool due to benthic processes, and shows that the benthos can alter the estuarine DOC pool through diel differences in DOC uptake and efflux.     

Bulk organic matter characteristics in the Pichavaram mangrove – estuarine complex,south-eastern India

The Pichavaram mangrove ecosystem is located between the Vellar and Coleroon Estuaries in south-eastern India. To document the spatial-depth-based variabilities in organic matter (OM) input and cycling, five sediment cores were collected. A comparative study was carried out of grain-size composition, pore water salinity, dissolved organic C (DOC), loss-on-ignition (LOI), elemental ratios (C/N and H/C), pigments (Chl a, Chl b, and total carotenoids), and humification indices. Sand is the major fraction in these cores ranging from 60% to 99% followed by silt and clay; cores from the estuarine margin have high sand content. In mangrove forests, pore-water DOC concentrations are high (32 ± 14 mg L⁻¹), whereas salinity levels are low (50 ± 5.5‰). Likewise, LOI, organic C and N, and pigment concentrations are high in mangroves. OM is mainly derived from upstream terrestrial matter and/or mangrove litter, and marine OM. The humification indices do not vary significantly with depth because of rapid OM turnover. The bulk parameters indicate that the Vellar and Coleroon Estuaries are more affected by anthropogenic processes than mangrove forests. Finally, greater variability and sometimes lack of specific trends in bulk parameters implies that the 2004 tsunami caused extensive mixing in sediments.     

Dissolved organic carbon in interstitial waters from sediments of Middle Valley and Escanaba Trough,Northeast Pacific,ODP Legs 139 and 169

Oxidation measurements of nonvolatile dissolved organic carbon (DOC) have been determined by high temperature combustion for pore waters from sediments of Middle Valley and Escanaba Trough, Northeastern Pacific, sampled by the Ocean Drilling Program (ODP) Legs 139 and 169, as well as for overlying and near bottom seawater. The DOC values in the interstitial waters are generally greater than those in the overlying water column, ranging from 0.1 to 158 mg of C per liter (mg C/L). Some of the profiles of DOC in the pore waters are similar to total organic C and total inorganic C profiles. DOC maxima at shallow depths of Site 858 are probably enriched due to higher temperature alteration of organic matter. At Sites 856 and 1035, the DOC increases at greather depths due to the hydrothermal activity. The DOC values correlate with MnO and MgO profiles in Hole 856B. The changes below 30 mbsf in the DOC depth profiles of Sites 858 and 1036 parallel those of the volatile hydrocarbon gases. The reference Site 1037 in Escanaba Trough has DOC values that increase with depth according to the increase in thermal stress.     

The potential formation of acid mine drainage in pyrite-bearing hard-coal tailings under water-saturated conditions: an experimental approach

 Annually, an amount of approximately 13 million cubic meters of hard-coal tailings must be disposed of in the German Ruhr Valley. Besides the waste of land in a densily populated region, the disposal of the pyrite-bearing material under atmospheric conditions may lead to the formation of acid mine drainage (AMD). Therefore, alternative disposal opportunities are of increasing importance, one of which being the use of tailings under water-saturated conditions, such as in backfilling of abandoned gravel pits or in the construction of waterways. In this case, the oxidation of pyrite, and hence the formation of AMD, is controlled by the amount of oxygen dissolved in the pore water of tailings deposited under water. In case the advective percolation of water is suppressed by sufficient compaction of the tailings, oxygen transport can be reduced to diffusive processes, which are limited by the diffusive flux of dissolved oxygen in equilibrium with the atmospheric pO₂. Calculations of the duration of pyrite oxidation based on laboratory experiments have shown that the reduction of oxygen is mainly controlled by the content of organic substance rather than the pyrite content, a fact that is supported by results from oxidation experiments with nitrate. A "worst case" study has lead to the result that the complete oxidation of a 1.5-m layer of hard-coal tailings deposited under water-saturated conditions would take as much as several hundred thousand years. Received: 6 May 1996 · Accepted: 2 August 1996     

Trace metal behaviour in riverine sediments: Role of organic matter and sulfides

Three sediment cores were collected in the Scheldt, Lys and Spiere canals, which drain a highly populated and industrialized area in Western Europe. The speciation and the distribution of trace metals in pore waters and sediment particles were assessed through a combination of computational and experimental techniques. The concentrations of dissolved major and trace elements (anions, cations, sulfides, dissolved organic C, Cd, Co, Fe, Mn, Ni, Pb and Zn) were used to calculate the thermodynamic equilibrium speciation in pore waters and to evaluate the saturation of minerals (Visual Minteq software). A sequential extraction procedure was applied on anoxic sediment particles in order to assess the main host phases of trace elements. Manganese was the most labile metal in pore waters and was mainly associated with carbonates in particles. In contrast, a weak affinity of Cd, Co, Ni, Pb and Zn with carbonates was established because: (1) a systematic under-saturation was noticed in pore waters and (2) less than 10% of these elements were extracted in the exchangeable and carbonate sedimentary fraction. In the studied anoxic sediments, the mobility and the lability of trace metals, apart from Mn, seemed to be controlled through the competition between sulfidic and organic ligands. In particular, the necessity of taking into account organic matter in the modelling of thermodynamic equilibrium was demonstrated for Cd, Ni, Zn and Pb, the latter element exhibiting the strongest affinity with humic substances. Consequently, dissolved organic matter could favour the stabilization of trace metals in the liquid phase. Conversely, sulfide minerals played a key role in the scavenging of trace metals in sediment particles. Finally, similar trace metal lability rankings were obtained for the liquid and solid phases.     

A climate-related oxidizing event in deep-sea sediment from the Bering Sea

Many cores from the deep basins of the Bering Sea have a thin oxidized zone within otherwise reduced sediment. This oxidized zone began to form about 6000 yr ago and represents an interval of about 3200 yr. Mineralogically, the oxidized and reduced sediments are similar, but chemically they differ. Concentrations of Fe and C are lower, and concentrations of Mn, Ba, Co, Mo, and Ni are higher in the oxidized than in the reduced sediment. Mn is enriched about 10-fold in the oxidized zone relative to its concentration in the reduced sediment, Mo about threefold, and Ba, Co, and Ni about twofold. These data suggest that the oxidized zone developed diagenetically as the result of the balance between the flux of organic matter and the available dissolved oxygen in bottom and interstitial waters.We propose that the Bering Sea was substantially ice covered when global glacial conditions prevailed. during the transition to global interglacial conditions, seasonal meltwater from thawing sea ice formed a lens of fresh water that decreased organic productivity. During the winter seasons, however, sea ice reformed and caused downwelling of dense, oxygen-rich waters to recharge bottom waters. The combination of lower organic productivity and more oxygen-rich bottom water allowed oxidized sediment to accumulate. Once full interglacial conditions were established, the volume of sea ice produced was insufficient to affect either productivity or the supply of dissolved oxygen and so bottom conditions again became reducing.Similar events probably occurred during the onset of global glacial conditions, and similar oxidized layers probably formed at these times. Such oxidized zones are highly unstable, however, in a reducing environment and, once buried beyond the influence of bacterial and infaunal activities, are depleted of their available oxygen and converted to reduced sediment.     

The Dynamics of Benthic Respiration at a Mid-Shelf Station Off Oregon

Mid-shelf sediments off the Oregon coast are characterized as fine sands that trap and remineralize phytodetritus leading to the consumption of significant quantities of dissolved oxygen. Sediment oxygen consumption (SOC) can be delayed from seasonal organic matter inputs because of a transient buildup of reduced constituents during periods of quiescent physical processes. Between 2009 and 2013, benthic oxygen exchange rates were measured using the noninvasive eddy covariance (EC) method five separate times at a single 80-m station. Ancillary measurements included in situ microprofiles of oxygen at the sediment–water interface, and concentration profiles of pore water nutrients and trace metals, and solid-phase organic C and sulfide minerals from cores. Sediment cores were also incubated to derive anaerobic respiration rates. The EC measurements were made during spring, summer, and fall conditions, and they produced average benthic oxygen flux estimates that varied between ?2 and ?15 mmol m^?2 d^?1. The EC oxygen fluxes were most highly correlated with bottom-sensed, significant wave heights (H _s). The relationship with H _s was used with an annual record of deepwater swell heights to predict an integrated oxygen consumption rate for the mid-shelf of 1.5 mol m^?2 for the upwelling season (May–September) and 6.8 mol m^?2 y^?1. The annual prediction requires that SOC rates are enhanced in the winter because of sand filtering and pore water advection under large waves, and it counters budgets that assume a dominance of organic matter export from the shelf. Refined budgets will require winter flux measurements and observations from cross-shelf transects over multiple years.     

江汉平原高砷地下水中DOM三维荧光特征及其指示意义

水体中溶解性有机质(dissolved organic matter, DOM)是含水层中砷释放的主控因素之一.江汉平原河湖众多、沟渠广布,地表水体与浅层地下水的交互作用使得DOM的组分特征及其强度有显著差异.为查明江汉平原地下水中溶解性有机质在砷迁移转化过程中的作用,对江汉平原地表水和浅层地下水进行三维荧光光谱分析,使用平行因子分析法提取水体中有机质的分子组成、功能特点和荧光特征,并分析各组分相对含量与地下水中砷与铁的关联.江汉平原水体中DOM包括3种主要组分,组分C1、C2为类腐殖质,C2是生物降解过程中产生的小分子,C3为类蛋白物质.地下水DOM以类腐殖质组分C1、C2为主,地表水以类蛋白类物质C3和小分子腐殖质C2为主.高砷地下水中DOM以陆源为主,主要通过两种途径促进As的迁移转化:(1) DOM的腐殖质组分充当微生物群落的电子运输工具,促进微生物作用下的有机质氧化和铁氧化物的还原,并伴随As的释放及大量溶解性有机碳(dissolved organic carbon, DOC)和HCO₃-的产生;(2) As以铁等金属阳离子为桥接物与腐殖质结合,通过形成As-Fe-DOM络合物,导致地下水中砷的迁移.      

Seasonal Dynamics of Dissolved Trace Metals in the Scheldt Estuary: Relationship with Redox Conditions and Phytoplankton Activity

Seasonal dynamics of dissolved trace metals (Cd, Co, Cu, Ni and Zn) and its relationship with redox conditions and phytoplankton activity has been studied in the Scheldt estuary, during nine surveys carried out between May 1995 and June 1996. Seasonal profiles of dissolved trace metals and general estuarine water quality variables are compared, to identify the geochemical and biological processes responsible for the observed trace metal distributions. In keeping with previous studies, the behavior of dissolved Cd, Cu, and Zn can be explained by the presence of anoxic headwaters and the restoration of dissolved oxygen within the estuary. In the river water, the concentration of dissolved Cu and Zn is generally low, except during winter when dissolved oxygen is present in the water column, although highly undersaturated. Mobilization of particle-bound Cd, Cu, and Zn occurs as dissolved oxygen increases with increasing salinity, possibly because of oxidation of metal sulfides in the suspended matter. The geochemistry of dissolved Co is also related to the redox conditions but in an opposite way. Dissolved Co is mobilized in the anoxic upper estuary, along with the reduction in Mn (hydro) oxides, and subsequently coprecipitated with Mn (hydro) oxides when dissolved oxygen is restored. Conservative behavior is observed for dissolved Ni within the estuary. In the middle estuary, Cd and Zn are readsorbed during phytoplankton blooms, as suggested by the low concentrations of these metals during the most productive periods in spring and early summer. The removal may be caused by direct biological uptake and/or increased adsorption to suspended matter because of the pH increase associated with algae blooms. In the lower estuary, chemical gradients are much weaker and dilution with seawater is the dominant process.     

洱海沉积物间隙水中溶解有机质的地球化学特性

对洱海沉积物间隙水中溶解有机质(DOM)含量、紫外可见吸收、荧光以及分子量等的垂直分布特征进行了研究。结果表明,DOC含量在沉积物水界面明显富集,随后急剧下降,6cm处达到最小值,随后呈上升趋势。DOC与吸光度值、荧光发射光谱强度之间具有一定的线性相关关系。DOM的E3/E4值范围在1～6之间,绝大多数在1～35之间。表征DOM中腐殖质来源的指标荧光指数值处于148～159之间,说明DOM以陆源输入为主。此外,洱海沉积物间隙水DOM分子量分布呈多峰分布模式,重均分子量(M_w)值在1462～1953Da之间,数均分子量(M_n)值在547～900Da之间,多分散性系数ρ值在202～305之间。随着沉积深度增大,M_w和M_n有略微的增大趋势,但变化不大。沉积物的氧化还原条件、微生物活动以及铁、锰氧化物等在沉积剖面的差异是控制间隙水中DOM各种地球化学特性的主要因素。