A fast and sensitive method for the continuous in situ determination of dissolved methane and its d13C-isotope ratio in surface waters

A fast and sensitive method for the continuous determination of methane (CH4) and its stable carbon isotopic values (d13C-CH4) in surface waters was developed by applying a vacuum to a gas/liquid exchange membrane and measuring the extracted gases by a portable cavity ring-down spectroscopy analyser (M-CRDS). The M-CRDS was calibrated and characterized for CH4 concentration and d13C-CH4 with synthetic water standards. The detection limit of the M-CRDS for the simultaneous determination of CH4 and d13CCH4 is 3.6 nmol L21 CH4. A measurement precision of CH4 concentrations and d13C-CH4 in the range of 1.1%, respectively, 1.7& (1r) and accuracy (1.3%, respectively, 0.8& [1r]) was achieved for single measurements and averaging times of 10 min. The response time s of 5765 s allow determination of d13C-CH4 values more than twice as fast than other methods. The demonstrated M-CRDS method was applied and tested for Lake Stechlin (Germany) and compared with the headspace-gas chromatography and fast membrane CH4 concentration methods. Maximum CH4 concentrations (577 nmol L21) and lightest d13C-CH4 (235.2&) were found around the thermocline in depth profile measurements. The M-CRDS-method was in good agreement with other methods. Temporal variations in CH4 concentration and d13C-CH4 obtained in 24 h measurements indicate either local methane production/oxidation or physical variations in the thermocline. Therefore, these results illustrate the need of fast and sensitive analyses to achieve a better understanding of different mechanisms and pathways of CH4 formation in aquatic environments

Arsenic in surface sediments of a harbor sludge dumping site in the Helgoland Mud Area, North Sea

In order to analyse differences in concentration, speciation and total mobility of arsenic two different locations were studied near the Helgoland Mud Area, North Sea. The first location is characterised by natural sedimentation, the second by deposited sediments dredged from the port of Hamburg. Porewater as well as sediment profiles were analysed with respect to arsenic compounds (As (III) and total As) and major redox species as total and reactive manganese and iron. The sediment samples were handled under inert atmosphere before and during extraction by water, phosphate, hydrochloric acid and aqua regia. Total element contents in porewater and leachable extracts of sediment fractions were analysed. The results show a strong redox coupling of arsenic with manganese and iron. Oxidized arsenic seems to adsorb to manganese- and iron-oxyhydroxides in surface sediments. In contrast to the solid samples, the pore water data shows a release of As (III) into porewater when manganese- and ironoxyhydroxides are reduced in the upper part of the cores. Also a remobilisation of As (V) occurs. Downward diffusing arsenic can be fixed by carbonate below the zone of manganese and iron reduction. In the anoxic parts of the sediments As (III) and As (V) are released and could be fixed at authigenic iron sulphide or arsenic sulphides formation. A sulfidic precipitation of arsenic in iron-dominated systems is limited by the occurrence of HS-. Total solid-phase contents in leachable extracts of sediment fractions of the natural area show significant higher arsenic concentrations than the core of the anthropogenic dumping area. This is due to the higher fines content of the Helgoland mud area. Higher total porewater contents of iron and arsenic in the core of the anthropogenic dumping area thus due to higher turnover rates of organic matter by iron reduction. Higher concentrations of arsenic may be due to a higher availability of iron in the dumped sediments

Phosphate Induced Arsenic Mobilization as a Potentially Effective In-Situ Remediation Technique—Preliminary Column Tests

Arsenic (As) contamination of groundwater is commonly remediated by pump and treat. However, this technique is difficult to apply or maintain efficiently because the mobility of arsenic varies depending on the geochemical aquifer conditions. Arsenic interacting with the sediment can cause strong retardation, which is counteracted by ions competing for sedimentary sorption sites like silica, bicarbonate and phosphate. Phosphate competes most effectively with arsenic for sorption sites due to its chemical similarity. To accelerate an ongoing but ineffective pump and treat remediation, we examined the competitive effect of increasing phosphate doses on contaminated aquifer material of different depths and thus under distinct geochemical conditions. In the columns with phosphate addition, significant amounts of arsenic were released rapidly under oxic and anoxic conditions. In all tests, the grade of leaching was higher under anoxic conditions than under oxic conditions. As(III) was the dominant species, in particular during the first release peaks and the anoxic tests. Higher amounts of phosphate did not trigger the arsenic release further and led to a shift of arsenic species. We suggest that the competitive surface complexation is the major process of arsenic release especially when higher amounts of phosphate are used. Commonly arsenic release is described at iron reducing conditions. In contrast, we observed that a change in prevailing redox potential towards manganese reducing conditions in the oxic tests and iron reducing conditions in the anoxic column took place later and thus independently of arsenic release. The reduction of As(V) to As(III) under both redox conditions is presumed to be an effect of microbial detoxification. A loss of sulphate in all columns with phosphate indicates an increased microbial activity, which might play a significant role in the process of arsenic release. Preliminary tests with sediment material from a contaminated site showed that phosphate additions did not change the pH value significantly. Therefore, a release of other metals is not likely. Our results indicate that in-situ application of phosphate amendments to arsenic-contaminated sites could accelerate and enhance arsenic mobility to improve the efficiency of pump and treat remediation without negative side effects. The novelty of this approach is the use of only small amounts of phosphate in order to stimulate microbial activity in addition to surface complexation. Therefore, this method might become an innovative and cost-effective remediation for arsenic contaminated sites