Determination of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in freshwaters by sequential injection spectrophotometry with on-line UV photo-oxidation

Recommendations and good practices for dissolved organic carbon (DOC) analyses at low concentrations

Numerous protocols for dissolved organic carbon (DOC) measurements on natural water are used in the literature. An ISO protocol for the determination of DOC exists since 2018, but it is certified for DOC values ≥ 1 mg L-1, while many publications report DOC values much lower. In addition, this ISO protocol does not include indications on vials cleaning, filtering material, and type of caps and septa to be used. The purpose of this study was to evaluate protocols for measurements of low DOC concentrations (≤ 1 mg L-1). The effect of the sample container, type of septum, filtration material, nature of acid used for storage, and matrix effects on DOC concentration were evaluated.•The use of glass vials decontaminated at 450 °C or 500 °C for at least 1 h, 0.45 µm hydrophilic polytetrafluoroethylene (PTFE) membranes previously rinsed with 20 mL ultra-pure water and HCl acidification gives the lowest DOC contamination,•Sulfides (ΣH2S), sodium (Na+) or calcium (Ca2+) do not induce high matrix effect for the analysis (≤ 10%),•At low DOC concentrations (≤ 1 mg L-1), the use of pierced PTFE septa with acidified samples induce slight DOC contamination after storage at 4 °C, and dramatic contamination after storage at -18 °C.

A high-throughput atomic emission analyzer for simultaneous field detection of dissolved inorganic and organic carbon in seawater and lake water

Dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) are two important indicators of global carbon cycle. However, there are no portable analyzers available to simultaneously accomplish high-throughput field detection of them in the same sample. Herein, a simple analyzer comprising a dual-mode reactor to accomplish both chemical vapor generation and headspace sampling, and a miniature point discharge optical emission spectrometer (μPD-OES) was developed for simultaneous and high-throughput detection of DIC and DOC in seawater and lake water. Phosphoric acid and persulfate were successively injected into sample solutions to convert DIC and DOC to CO₂ under the conditions of magnetic stirring and UV irradiation, respectively. Subsequently, the generated CO₂ was swept into the μPD-OES for quantitation of DIC and DOC via monitoring carbon atomic emission at 193.0 nm. Under optimal conditions, limits of detection for DIC and DOC (as C) were both 0.01 mg L^-1 with relative standard deviations (n = 20) better than 5% and sample throughput of 80 samples per hour. Compared to conventional analyzers, the proposed instrument provides the advantages of high throughput, compactness, low energy consumption and eliminates expensive instruments. The accuracy of the system was validated by simultaneous determination of DIC and DOC in various water samples in laboratory and field environments.

Stimulation of high-concentration dissolved nitrogen and reactive phosphorus in Lake Taihu sediments on the initiation and maintenance of cyanobacterial blooms

The spatio-temporal variation of dissolved inorganic nitrogen (DIN: NH₄⁺-N, NO₂^--N, and NO₃^--N) and dissolved reactive phosphorus (DRP) in Meiliang Bay of Lake Taihu sediments and their potential effects on the cyanobacterial blooms were studied. Monthly sampling was performed using high-resolution dialysis sampling devices (HR-Peeper) and two important results were observed in April (the initiation period of cyanobacterial bloom) and June-August (the maintenance period of cyanobacterial blooms). In April, high concentrations of dissolved NO₂^--N and NO₃^--N, probably caused by the groundwater influx, were observed in deep anoxic sediments (below 110 mm). NO₂^--N and NO₃^--N are good electron acceptors for the mineralization of organic P under anaerobic conditions and should lead to an increase in DRP concentrations in sediments, DRP released from sediments thus further stimulating the cyanobacterial growth and the outbreak of severe cyanobacterial blooms in May due to the extremely low concentrations of DRP in the water body. From June to August, high concentrations of NO₂^--N, NO₃^--N, and DRP were observed in the surface sediment, which was caused by the release of NH₄⁺-N from the mineralization of cyanobacterial debris. This should play an important role in maintaining cyanobacterial growth, especially in stimulating the occurrence of cyanobacterial blooms during September, when N and P were co-limited. This study revealed high-concentration DIN and DRP in Lake Taihu sediments potentially stimulated the initiation and maintenance of cyanobacterial blooms.

Combined use of high-resolution dialysis, diffusive gradient in thin films (DGT) technique, and conventional methods to assess trace metals in reservoir sediments

Recently, reservoirs in southern China are witnessing incidents involving black water, which are harmful to the aquatic ecosystem. This study unravels the cause of the black water events by studying the occurrence and the ecological risks of contaminants (Pb, Cu, Cd, Zn, Ni, TFe, Mn, S, P, and DOC) in sediments of Tianbao reservoir. Due to the significantly high concentration of TFe, Mn, and P in the sediments, the study further used the thin film diffusion gradient (DGT) technology and high-resolution dialysis method to investigate the movement of Fe²⁺, Mn²⁺, S^2-, and reactive P within the sediments. The ecological risk assessment (threshold effect level and probable effect level) showed that the sediments had a low concentration of Pb, Cu, Cd, Zn, and Ni. High organic matter from the Eucalyptus plantation surrounding the reservoir, as well as the intense thermal stratification of the reservoir, caused the hypolimnion to be hypoxic (DO < 2 mg/L). The diffusion fluxes at the water-sediment boundary (WSB) demonstrated a significant movement of Fe²⁺, Mn²⁺, and PO₄^3- from the sediments into the overlying water, while the movement of S^2- was in both directions due to hypoxia. A high correlation Fe-DOC (r = 0.9), Fe-S (r = 0.8), and Mn-S (r = 0.7) and the redox interaction of Fe²⁺, Mn²⁺, S^2-, P, and DOC at the hypoxic WSB caused the production of black substances in the hypolimnion contributing to the so-called black water reservoir.

A comprehensive understanding of enhanced Pb mobilization in sediments caused by algal blooms

A good understanding of lead (Pb) mobilization in eutrophic lakes is a key to the accurate assessment of Pb pollution. In this work, dissolved and labile Pb was determined by both high resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) in sediment-water profiles of the hyper-eutrophic Meiliang Bay of Lake Taihu on a monthly basis during one year. The drinking water standards for dissolved Pb of the World Health Organization (10μg/L) and those of China were exceeded in the overlying water (20.79-118.5μg/L). Out of which, a total of five months even exceeded the fisheries water quality limitation (50μg/L) in China. The algal blooms created an anaerobic environment in the surface sediments in July. The reductive conditions led to the dissolution of Fe/Mn and this caused the release of Pb, followed by organic matter complexation. This was supported by the coincident changes of dissolved Pb with dissolved organic matter (DOM) in sediments under anaerobic incubation. Algae residue decomposition in October caused another distinct release of Pb, but this process should be considerably suppressed by increased sulfide precipitation and pyrite adsorption of Pb ion. These results indicated that Pb mobilization in sediments can be significantly enhanced by algal blooms in eutrophic lakes, indicating that further attention should be paid to Pb pollution in waters with harmful algal blooms.

Peak Chromium Pollution in Summer and Winter Caused by High Mobility of Chromium in Sediment of a Eutrophic Lake: In Situ Evidence from High Spatiotemporal Sampling

To study the mechanisms of chromium (Cr) mobilization in sediments of eutrophic lakes, monthly sampling was performed in the Meiliang Bay of Lake Taihu, China, combined with laboratory experiments. High-resolution dialysis and diffusive gradients in thin film (DGT) sampling techniques were used. Results indicated that in July 2016 and January 2017, the concentrations of soluble Cr and DGT-labile Cr(VI) in the overlying water exceeded both drinking and fishery water quality standards, resulting from the high mobility of Cr in sediments. In July (summer), the high concentration of soluble Cr (134.04 ± 7.20 μg/L) detected in the anaerobic sediments was primarily caused by the complexation of Cr(III) with dissolved organic matter (DOM). This mechanism was supported by an observed simultaneous increase of soluble Cr and DOM under simulated anaerobic conditions. In January (winter), the high concentrations of soluble Cr (97.55 ± 9.65 μg/L) and DGT-labile Cr(VI) (25.83 ± 1.25 μg/L) in aerobic sediments were primarily caused by reoxidation of Cr(III) by Mn(III/IV) oxides as evidenced by the lowest concentrations of soluble and DGT-labile Mn(II). This study sheds light on the full-year variation and mechanisms of Cr mobilization in eutrophic lake sediments and suggests the urgent need for remediation of Cr pollution especially for winter.

A simplified coulometric method for multi-sample measurements of total dissolved inorganic carbon concentration in marine waters

A new system with high precision based on a coulometric method requiring greatly reduced operator intervention has been developed for the determination of [DIC] in marine waters.

Selective determination of cyanides by gas diffusion-stopped flow-sequential injection analysis and an on-line standard addition approach

A highly selective sequential injection (SI) method for the automated determination of weak-acid-dissociable cyanides is reported. The analytical procedure is based on the on-line reaction of the analyte with ninhydrin in carbonate medium to form a coloured product (lambda(max)=510 nm). Cyanides are removed from sample matrix by acidification through a gas-diffusion step incorporated in the SI manifold. The effect of instrumental and chemical variables was studied. By adopting an on-line standard addition protocol, the sensitivity of the proposed method was enhanced drastically, without affecting the determination range. The assay was validated in terms of linearity (up to 200 microg L(-1)), limit of detection (c(L)=2.5 microg L(-1)), limit of quantitation (c(Q)=7.5 microg L(-1)), precision (s(r)<2.5% at 100 microg L(-1)) and selectivity. High tolerance against critical species such as sulfides and thiocyanates was achieved. The applicability of the method was demonstrated by analyzing tap and mineral water samples at levels below the limits established by international E.U. and U.S. organizations. The percent recoveries were satisfactory in all cases, ranging between 94.2 and 103.6%.