Acid volatile sulfide and simultaneously extracted metals in the sediment cores of the Pearl River Estuary, South China

The acid-volatile sulfide (AVS), simultaneously extracted metals (SEM), total metals, and chemical partitioning in the sediment cores of the Pearl River Estuary (PRE) were studied. The concentrations of total metals, AVS, and SEM in the sediment cores were generally low in the river outlet area, increased along the seaward direction, and decreased again at the seaward boundary of the estuary. The amounts of AVS were generally greater in deeper sediments than in surface sediments. SEM/AVS was > 1 in the surface sediments and in the river outlet cores. The ratio was < 1 in the sediments down the profiles, suggesting that AVS might play a major role in binding heavy metals in the deep sediments of the PRE. The SEM may contain different chemical forms of trace metals in the sediments, depending on the metal reaction with 1 M cold HCl in the AVS procedure compared with the results of the sequential chemical extraction. The SEM/AVS ratio prediction may overestimate trace metal availability even in the sediments with high AVS concentrations. (c) 2004 Elsevier Inc. All rights reserved.

Approccio SEM-AVS (Simultaneously Extracted Metals-Acid Volatile Sulfides) per la valutazione del rischio ambientale dei metalli nei sedimenti della Pialassa Baiona

La Pialassa Baiona è una laguna sottoposta a diversi vincoli normativi, visto il suo inquadramento tra le aree SIC e ZPS, e soggetta a diverse sorgenti di disturbo antropico, più intense negli anni ‘50-‘70. Questa tesi si propone lo scopo di valutare il rischio associato ai metalli bivalenti presenti nei sedimenti della Pialassa Baiona attraverso tre approcci: il primo riguarda la determinazione della frazione biodisponibile dei metalli presenti nei sedimenti attraverso la determinazione dei solfuri acidi volatili (AVS) e dei metalli simultaneamente estraibili (SEM), nonché la valutazione della potenziale tossicità dei sedimenti attraverso la valutazione del rapporto molare SEM/AVS, il secondo approccio considera invece il contenuto pseudo totale dei metalli bivalenti (Cd, Cu, Ni, Pb e Zn) e il loro confronto sia con i valori tipici di fondo naturale del Mar Adriatico che con i valori guida di riferimento internazionale (Threshold Effect Level, TEL e Probable Effect Level, PEL) al fine di valutare lo stato di qualità dei sedimenti della zona d’indagine. Il terzo approccio considera l’influenza del gradiente naturale terra-mare tipico delle zone di transizione e del gradiente antropico legato alla vicinanza dell’area industriale alla Pialassa Baiona, sulla distribuzione spaziale dei metalli oggetto di questo studio. I risultati ottenuti evidenziano che l’area più prossima alla zona industriale e al contempo più lontana dall’effetto del ricambio delle acque e di dilavamento ad opera del mare, è risultata quella con livelli significativamente più elevati per la maggior parte dei metalli analizzati. Questo permette di ipotizzare un’influenza diretta delle sorgenti di inquinanti, ma anche un effetto dispersivo della circolazione. Gli AVS hanno invece evidenziato un gradiente terra-mare; ciò comporta che nelle zone più prossime all’influenza del mare si sono riscontrate concentrazioni minori di AVS. La valutazione della potenziale tossicità dei metalli in termini di rapporto SEM/AVS non ha evidenziato la presenza di siti a rischio per il biota acquatico, se non per un unico sito prossimo all’area industriale.

Sulphur and iron species in Dvurechenskii mud volcano sediments

The deep Black Sea is known to be depleted in electron-acceptors for sulphide oxidation. This study on depth distributions of sulphur species (S(II), S(0),S(n)**2-,S2O3**2-,SO3**2-,SO4**2-) in the Dvurechenskii mud volcano, a cold seep situated in the permanently anoxic eastern Black Sea basin (Sorokin Trough, 2060 m water depth), showed remarkable concentrations of sulphide oxidation products. Sulphite concentrations of up to 11 µmol L**1-, thiosulphate concentrations of up to 22 µmol L**1-, zero-valent sulphur concentrations of up to 150 µmol L**1- and up to five polysulphide species were measured in the upper 20 cm of the sediment. Electron-acceptors found to be available in the Dvurechenskii mud volcano (DMV) for the oxidation of hydrogen sulphide to sulphide oxidation intermediates are iron-minerals, and probably also reactive manganese phases. Up to 60 µmol g**1- of reactive iron-minerals and up to 170 µmol L**1- dissolved iron was present in the central summit with the highest fluid upflow and fresh mud outflow. Thus, the source for the oxidative power in the DMV are reactive iron phases extruded with the mud from an ancient source in the deeply buried sediments, leading to the formation of various sulphur intermediates in comparably high concentrations. Another possible source of sulphide oxidation intermediates in DMV sediments could be the formation of zero-valent sulphur by sulphate dependent anaerobic microbial oxidation of methane followed by disproportionation of zero-valent sulphur. Sulphide oxidation intermediates, which are produced by these processes, do not reach thermodynamic equilibrium with rhombic sulphur, especially close to the active center of the DMV due to a short equilibration time. Thus, mud volcano sediments, such as in the DMV, can provide oxidizing niches even in a highly reduced environment like the abyssal part of the Black Sea.

Biogeochemical processes in anoxic sediment from Lake Plußsee, Germany

In anoxic environments, volatile methylated sulfides like methanethiol (MT) and dimethyl sulfide (DMS) link the pools of inorganic and organic carbon with the sulfur cycle. However, direct formation of methylated sulfides from reduction of dissolved inorganic carbon has previously not been demonstrated. When studying the effect of temperature on hydrogenotrophic microbial activity, we observed formation of DMS in anoxic sediment of Lake Plußsee at 55 °C. Subsequent experiments strongly suggested that the formation of DMS involves fixation of bicarbonate via a reductive pathway in analogy to methanogenesis and engages methylation of MT. DMS formation was enhanced by addition of bicarbonate and further increased when both bicarbonate and H2 were supplemented. Inhibition of DMS formation by 2-bromoethanesulfonate points to the involvement of methanogens. Compared to the accumulation of DMS, MT showed the opposite trend but there was no apparent 1:1 stoichiometric ratio between both compounds. Both DMS and MT had negative d13C values of -62 per mil and -55 per mil, respectively. Labeling with NaH**13CO3 showed more rapid incorporation of bicarbonate into DMS than into MT. The stable carbon isotopic evidence implies that bicarbonate was fixed via a reductive pathway of methanogenesis, and the generated methyl coenzyme M became the methyl donor for MT methylation. Neither DMS nor MT accumulation were stimulated by addition of the methyl-group donors methanol and syringic acid or by the methyl-group acceptor hydrogen sulphide. The source of MT was further investigated in a H2**35S labeling experiment, which demonstrated a microbially-mediated process of hydrogen sulfide methylation to MT that accounted for only <10% of the accumulation rates of DMS. Therefore, the major source of the 13C-depleted MT was neither bicarbonate nor methoxylated aromatic compounds. Other possibilities for isotopically depleted MT, such as other organic precursors like methionine, are discussed. This DMS-forming pathway may be relevant for anoxic environments such as hydrothermally influenced sediments and fluids and sulfate-methane transition zones in marine sediments.