Acid Volatile Sulfide Research Articles

Rape straw biochar enhanced Cd immobilization in flooded paddy soil by promoting Fe and sulfur transformation

Cd is normally associated with sulfide and Fe oxides in flooded paddy soil. The mechanisms of biochar enhanced Cd immobilization by promoting Fe transformation and sulfide formation are unclear. Rape straw biochar (RSB) pyrolyzed at 450 °C (LB) and 800 °C (HB) was added to Cd-contaminated paddy soil at 1% (LB1, HB1) and 2% (LB2, HB2) doses. The results showed that Fe/Mn oxide–Cd (Fe/Mn–Cd) and free Fe oxide (Fed) concentrations decreased in the first 12 days and then rose, while Fe2+ in pore water (W–Fe2+) tended to rise first and then fall. The electron transfer rate of soil in the HB2 treatment was 4.9-fold higher than that in the treatment without biochar (CK). Fe oxide reduction was enhanced by RSB, with a maximum increase in W–Fe2+ by 62.1% in HB2 on Day 12. The negative correlation between W–Fe2+ and Fed showed that Fe2+ promoted the reformatted of seconded Fe minerals after Day 12, and the Fed in the HB2 treatments increased by 31.5% in this period. RSB addition also promoted the reformation of poorly crystallized Fe oxide (Feo) by increasing soil pH, which increased by 17.2% and 15.1% on average in the LB2 and HB2 treatments, respectively, compared to CK. Compared to Day 7, the increased rate of Fe/Mn–Cd on Day 30 in RSB was approximately twice that of CK. Compared to the molybdate group, the maximum decrease in CaCl2–Cd was 29.1% in LB2 on Day 12. LB2 increased SO42− and acid-volatile sulfide concentrations by 6.9- and 4.1-fold, respectively, compared to CK. These results suggested that RSB, particularly HB, promoted more Cd adsorption in Fe minerals by increasing Fe hydroxylation and recrystallization processes. LB increased the contribution of sulfide to Cd immobility.

Reactive vanadium and iron fluxes in different modern sedimentary environments

This study evaluated the removal of the metals (Me) vanadium and iron associated with the operationally defined HCl-extractable (MeHCl), pyrite (Mepyr), and reactive (Mereac = MeHCl + Mepyr) fractions in the sediments of five contrasting sedimentary environments: (1) the oxygen minimum zone (OMZ; San Lázaro and San Blas basins); (2) the anoxic/anoxic-sulfidic sediments of the California Continental Borderland basins and Todos Santos Bay Canyon; (3) the oxic sediments of the Patton Escarpment, Baja California continental shelf, and deep sediments of the Gulf of Mexico (GoM); (4) the Guerrero Negro Hypersaline System (GNHS) in Baja California Sur, Mexico; and (5) the turbiditic deposits of the GoM. The average VHCl concentration ranged from 8 ± 15 nmol g−1 in turbidites to (7.9 ± 4.3) × 102 nmol g−1 in the OMZ, respectively. The Vpyr concentrations were within the narrow range of 6.6 ± 4.2 nmol g−1 (oxic sediments) to 8.6 ± 5.4 nmol g−1 (turbidites), which indicates that the pyrite fraction is not an important reservoir of reactive V. The relative consistency of the Vpyr concentrations (7.2 ± 5.2 nmol g−1; n = 1098), regardless of the sedimentary environment and redox state of the system, allowed us to calculate a global burial value for oceanic sediments of 4.2 ± 3.0 Gg y–1. The VHCl enrichment observed in OMZ and anoxic/anoxic-sulfidic sediments may be due to its incorporation into acid volatile sulfide. The lowest average values of the degree of V pyritization corresponded to the OMZ (0.98 ± 0.68%) while the highest average values corresponded to turbidites (69 ± 32%), with the latter being due to their extremely low VHCl values. The mass accumulation rate (MAR) calculations for reactive Fe and V show that the sediments of all OMZ regions could annually incorporate (12.1 ± 8.6) × 104 Gg and (6.3 ± 3.8) × 102 Gg of reactive Fe and V, respectively. Although the GNHS exhibited the greatest MAR value of reactive Fe and V [(32 ± 28) × 102 g m−2 ky−1 and 35 ± 35 g m−2 ky−1, respectively], this environment currently does not contribute an important percentage of the MAR at the global level due to its small geographical area. However, in the geological past when its geographical area was much greater, the MAR values of reactive Fe and V may have been more important. Although deep sediments (>1000 m water depth) cover a substantial area, they contribute a relatively small, yet significant proportion of the total vanadium (V) deposited in ocean sediments, ranging from 9.7% to 24.8%.

Sediment Spatial Distribution and Quality Assessment of Metals in Chinook Salmon and Resident Killer Whale Marine Habitat in British Columbia, Canada.

At-risk resident killer whale (Orcinus orca) populations of the northeastern Pacific, Canada, and their main prey, Chinook Salmon (Oncorhynchus tshawytscha), are exposed to a variety of contaminants including chemical elements from both natural and anthropogenic sources, which may be constraining their recovery. Concentrations of 36 chemical elements in subtidal surface sediments (1-435m depth) collected from 98 sites along the British Columbia coast were used to characterize coast-wide patterns, and a subset of metals (mercury (Hg), cadmium (Cd), arsenic (As), nickel (Ni), copper (Cu), and lead (Pb)) were selected to assess Chinook Salmon and resident killer whale marine habitat quality. Principal component analysis (PCA) showed a dominance of Hg, antimony (Sb), Pb, Cu, and zinc (Zn) for Prince Rupert Harbour, Victoria Harbour, and Burrard Inlet, suggesting local sources. Based on the PCA, geochemical properties such as total organic carbon (TOC), acid volatile sulfide (AVS), and pH explained the spatial distribution of all elements in sediment (p < 0.001). Mercury, Cd, As, Ni, Cu, and Pb hotspots were identified along the coast of Vancouver Island, the central and north coast, in the Strait of Georgia, and Haida Gwaii. Bischof Island of Haida Gwaii and Ardmillan Bay on the central coast were most contaminated and enriched by Cd, determined by geoaccumulation index (Igeo) and enrichment factor (EF), respectively. Marine habitat quality was assessed by comparing metal concentrations to Canadian Sediment Quality Guidelines (SQGs). Chinook Salmon populations may be indirectly affected by metal toxicity (As > Cd and Cu > Ni > Hg > Pb) to lower trophic level prey species. Toxicity related impacts to benthic organisms as a result of exposure to elevated Cd and As concentrations in Northern Resident Killer Whale critical habitat and to Hg, Cd, As, Ni, Cu, and Pb concentrations in Southern Resident Killer Whale critical habitat may indirectly pose a threat to resident killer whale populations, highlighting a need for management actions to reduce risks associated with these metals.

Evaluation of bioaccumulation and toxicity of Tubifex tubifex exposed to contaminated river sediment by potentially toxic elements — A case study of the Middle Black Sea, Turkey

To determine the ecotoxicological assessment of the Black Sea in the seasonal and spatial dimensions, the concentrations of potentially toxic elements (PTEs) in sediment were measured at five sampling sites selected as Kızılırmak (T1), Engiz River (T2), Mert River (T3), Abdal River (T4), and Yeşilırmak (T5). In addition, a 28-day toxicity test was conducted with T. tubifex to evaluate the accumulation and toxicity of the PTEs contamination. The results of PTEs concentration in sediment indicated that the sampling sites are ranked according to the degree of elemental pollution from highly polluted to less polluted as Yeşilırmak > Kızılırmak > Abdal River > Engiz River > Mert River. Both natural and anthropogenic activities including industrial and agricultural effluents, municipal wastewater, and traffic-related pollution were found as the main sources of PTEs. Additionally, the acid volatile sulfide (AVS) – simultaneously extracted metal (SEM) concentrations were measured to determine the potential availability and toxicity of the presence of PTEs. ∑SEM–AVS < 0 were found in 60 % of the samples taken from five different sampling points while ∑SEM– AVS > 0 were observed in 40 % of the samples. Accordingly, the toxicity of PTEs is expected to be limited at 60 % of the samples. The results indicated no mortality in the tested sediment samples. The highest reproduction rate (16.67 ± 1.53) and lowest PTEs content were measured in the Mert River. The results of bioaccumulation factors (BAF) revealed that the most accumulated PTEs were Cu > Zn > Ni > Pb.

Geochemical controls on the distribution of total mercury and methylmercury in sediments and porewater from the Yangtze River Estuary to the East China Sea

A comprehensive investigation was conducted to explore the distributions of total mercury (THg) and methylmercury (MeHg) in sediments and porewater along a typical transect from the Yangtze River Estuary (YRE) to the East China Sea (ECS) open shelf. THg concentrations in the surface sediments exhibited large variations across sites, higher in the estuary mixing region, especially within the turbidity maximum zone (TMZ). The spatial and vertical distributions (0–20 cm) of THg in sediments were highly controlled by sediment grain size and total organic carbon (TOC), due to the strong binding between Hg and fine-grained sediments that enriched with organic matter. In contrast, MeHg concentrations in surface sediments were higher in the estuary mixing region and the ECS open shelf than in the river channel, with remarkably higher MeHg/THg ratios in sediments and porewater at the open shelf sites, identified as the regional hotspots of the net in situ MeHg production. Considering the large gradients of physiochemical properties of sediments, porewater and the overlying water, the results of this study suggested that the higher net Hg methylation potential in the open shelf region was largely attributed to the lower acid volatile sulfide, lower TOC and higher salinity, which facilitated the partitioning of inorganic Hg into porewater that highly bioavailable for Hg-methylation bacteria. Moreover, the estimated diffusive fluxes of MeHg at the sediment-water interface were positive at all the tested sites, and pronouncedly higher within the TMZ (driven by the higher THg loading and higher porosity) that requires special attention.

Min‐Trap® Samplers to Passively Monitor In‐Situ Iron Sulfide Mineral Formation for Chlorinated Solvent Treatment

AbstractThe objective of this work was to field‐demonstrate the Min‐Trap® sampler technology, a new in‐situ passive monitoring tool that offers distinct advantages for collecting mineralogical data. The Min‐Trap consists of a solid porous medium (e.g., silica sand) contained within a water‐permeable mesh and support housing that is deployed inside a monitoring well. The porous medium serves as a solid substrate upon which target minerals passively form. Analysis of the sample medium using chemical, microscopic, spectroscopic, or other techniques can provide direct evidence of geochemical conditions and the formation of target minerals in‐situ. The abiotic degradation of chlorinated volatile organic compounds via the reducing power stored in reactive minerals (e.g., iron sulfides) is a recognized important process, and Min‐Traps are well suited to document the formation of these minerals during remediation. Min‐Traps were tested in multiple wells at two field sites undergoing active remediation where reactive iron sulfide minerals were expected to be actively forming. Min‐Trap samples were analyzed by a variety of analytical techniques, including total iron, acid volatile sulfide (AVS), chromium extractable sulfide (CrES; also known as chromium reducible sulfur; CRS), weak acid soluble (WAS) iron, strong acid soluble (SAS) iron, and scanning electron microscopy–energy dispersive X‐ray spectroscopy (SEM‐EDS). The results provided direct verification of iron sulfide minerals in all cases (n = 9) where they were expected or interpreted to be possibly present based on groundwater data. Min‐Traps offer the potential for lifecycle cost savings because they do not require the collection of soil core samples, they can aid in better remedy implementation and performance, and they can potentially support earlier transition from active to passive remedy phases.

Remediate black-odorous sediment by slow-release calcium nitrate: Migration, transformation and microbial succession

Calcium nitrate has been widely used in-situ remediation of black-odorous sediment, which microorganisms mainly utilized nitrate to redox substances through denitrification. However, this technique has the problem that the local nitrate concentration was too high to inhibit the activity of functional microorganisms, which affected the oxidation effect and caused the abnormal release of inorganic nitrogen from the sediment. The slow release of calcium nitrate by tube dosing can effectively solve this disadvantage. However, the specific layout of the dosing method needs further research. Therefore, a method of tube-dosing distribution points was proposed by explored the migration and transformation laws of N-S-Fe in the vertical and horizontal directions of the sediment and the changes in the microbial community structure after the fixed-point dosing of calcium nitrate in this study, which further optimized the calcium nitrate dosing process and provided a new dosing method and idea in practical application. Within one month, nitrate migrated and transformed vertically and horizontally, accompanied by the accumulation of nitrite, oxidation of acid volatile sulfide (AVS) and ferrous iron. Among them, within the nitrate migration distance of 0–6 cm (vertical) and 0–8 cm (horizontal), the removal rate of AVS reached 100% and ferrous iron was effectively oxidized. In addition, in the process of vertical and horizontal migration, bacteria closely related to sulfide and ferrous iron-driven denitrification, such as Thiobacillus, Sulfurimonas and Thioalkalispira, were found to be dominant. These results demonstrate that after exploring the vertical and horizontal migration and transformation laws of nitrate in different contaminated sediments remediated, the reasonable layout of dosing methods can be carried out in practical application.

Sustained-release nitrate combined with microbial fuel cell: A novel strategy for PAHs and odor removal from sediment

Nitrate addition is a biostimulation technique that can improve both the oxidation of acid volatile sulfide (AVS) through autotrophic denitrification and the biodegradation of polycyclic aromatic hydrocarbons (PAHs) via heterotrophic denitrification. However, during the remediation, parts of the dissolved nitrate in the sediment migrates from the sediment to the overlying water, leading to the loss of effective electron acceptor. To overcome this limitation, a combined approached was proposed, which involved nitrocellulose addition and a microbial fuel cell (MFC). Results indicated the nitrate could be slowly released and maintained at a higher concentration over long term. In the combined system, the removal efficiencies of PAHs and AVS were 71.56% and 89.76%, respectively. Furthermore, the voltage attained for the MFC-nitrocellulose treatment was maintained at 146.1 mV on Day 70, which was 5.37 times higher than that of the MFC-calcium nitrate treatment. Sediments with nitrocellulose resulted in lower levels of nitrate and ammonium in the overlying water. Metagenomic results revealed that the combined technology improved the expression of nitrogen-cycling genes. The introduction of MFC inhibited sulfide regeneration during incubation by suppressing the enzyme activity like EC4.4.1.2. The enhanced biostimulation provided potential for in-situ bioremediation utilizing MFC coupled with slow-released nitrate (i.e., nitrocellulose) treatment.

Effect of microbial network complexity and stability on nitrogen and sulfur pollutant removal during sediment remediation in rivers affected by combined sewer overflows

Discovering the complexity and improving the stability of microbial networks in urban rivers affected by combined sewer overflows (CSOs) is essential for restoring the ecological functions of urban rivers, especially to improve their ability to resist CSO impacts. In this study, the effects of sediment remediation on the complexity and stability of microbial networks was investigated. The results revealed that the restored microbial community structure using different approaches in the river sediments differed significantly, and random matrix theory showed that sediment remediation significantly affected microbial networks and topological properties; the average path distance, average clustering coefficient, connectedness, and other network topological properties positively correlated with remediation time and weakened the small-world characteristics of the original microbial networks. Compared with other sediment remediation methods, regulating low dissolved oxygen (DO) shifts the microbial network module hubs from Actinobacteria and Bacteroidetes to Chloroflexi and Proteobacteria. This decreases the positive association of networks by 17%–18%, which intensifies the competitiveness among microorganisms, further weakening the influence and transmission of external pressure across the entire microbial network. Compared with that of the original sediment, the vulnerability of the restored network was reduced by more than 36%, while the compositional stability was improved by more than 12%, with reduced fluctuation in natural connectivity. This microbial network succession substantially increased the number of key enzyme-producing genes involved in nitrogen and sulfur metabolism, enhancing nitrification, denitrification, and assimilatory sulfate reduction, thereby increasing the removal rates of ammonia, nitrate, and acid volatile sulfide by 43.42%, 250.68% and 2.66%, respectively. This study comprehensively analyzed the succession patterns of microbial networks in urban rivers affected by CSOs before and after sediment remediation, which may provide a reference for reducing the impact of CSO pollution on urban rivers in the subsequent stages.

Assessment of the Spatial Variations of Mercury and Methylmercury in the Sediment of a Lake and Its Inflow River Estuaries

The contents of total mercury (THg) and methylmercury (MMHg) were measured in Nansi Lake and its inflow river estuaries to investigate their spatial distribution characteristics and the geochemical relationships between THg, MMHg, and methylation rate (%MMHg) with mineral phases of the sediment. The contents of THg, MMHg, and %MMHg in the lake sediment were lower than those in the inflow river estuary sediment. Inputting of inflow rivers was primary source to THg and MMHg in Nansi Lake. The THg contents in the sediment decreased from the upper to the lower lake, while the MMHg contents indicated the opposite trends of spatial variations. The THg was significantly positively correlated with MMHg and %MMHg in estuarine sediment but has a significant negative correlation with MMHg and %MMHg in lake sediment. Total organic carbon (TOC), acid volatile sulfide (AVS), and clay have great influence on the distribution of THg in sediment. Methylmercury in the estuary sediment was positively correlated with THg, AVS, TOC, and clay, while these relationships were not revealed in the lake sediment. Cluster and principal component analysis results showed that THg, clay, and TOC were derived from the same sources in the sediment of Nansi Lake. It implied that for improving the water environmental quality of lakes, the environmental management of rivers entering the lake should be strengthened.

Vertically stratified methane, nitrogen and sulphur cycling and coupling mechanisms in mangrove sediment microbiomes

BackgroundMangrove ecosystems are considered as hot spots of biogeochemical cycling, yet the diversity, function and coupling mechanism of microbially driven biogeochemical cycling along the sediment depth of mangrove wetlands remain elusive. Here we investigated the vertical profile of methane (CH4), nitrogen (N) and sulphur (S) cycling genes/pathways and their potential coupling mechanisms using metagenome sequencing approaches.ResultsOur results showed that the metabolic pathways involved in CH4, N and S cycling were mainly shaped by pH and acid volatile sulphide (AVS) along a sediment depth, and AVS was a critical electron donor impacting mangrove sediment S oxidation and denitrification. Gene families involved in S oxidation and denitrification significantly (P < 0.05) decreased along the sediment depth and could be coupled by S-driven denitrifiers, such as Burkholderiaceae and Sulfurifustis in the surface sediment (0–15 cm). Interestingly, all S-driven denitrifier metagenome-assembled genomes (MAGs) appeared to be incomplete denitrifiers with nitrate/nitrite/nitric oxide reductases (Nar/Nir/Nor) but without nitrous oxide reductase (Nos), suggesting such sulphide-utilizing groups might be an important contributor to N2O production in the surface mangrove sediment. Gene families involved in methanogenesis and S reduction significantly (P < 0.05) increased along the sediment depth. Based on both network and MAG analyses, sulphate-reducing bacteria (SRB) might develop syntrophic relationships with anaerobic CH4 oxidizers (ANMEs) by direct electron transfer or zero-valent sulphur, which would pull forward the co-existence of methanogens and SRB in the middle and deep layer sediments.ConclusionsIn addition to offering a perspective on the vertical distribution of microbially driven CH4, N and S cycling genes/pathways, this study emphasizes the important role of S-driven denitrifiers on N2O emissions and various possible coupling mechanisms of ANMEs and SRB along the mangrove sediment depth. The exploration of potential coupling mechanisms provides novel insights into future synthetic microbial community construction and analysis. This study also has important implications for predicting ecosystem functions within the context of environmental and global change.8kPXxAybXxhPRn3YMgQLJxVideo

Reactive sulfide dynamic models for predicting metal hazardous in sediments of two northern Egyptian Lakes

To track pollution status and bioavailability of Cu, Zn, Cd, Pb, and Ni, the current study's acid-volatile sulfide (AVS) and simultaneously extracted metals (SEM) models were compared to previous studies in Edku and Mariut lakes prior to 8 years. Sediment samples were collected in winter and summer 2019 from the two lakes. Metal pollution is higher in the winter than in the summer, according to SEM/AVS models. Metal toxicity is reported to have increased slightly in both lakes. According to risk quotients (RQSEM-PEL and RQSEM-TEL) the two lakes are moderately polluted. The results of the Fe/AVS ratio were in the range of 6.77 to 226.87 and 2.88 to 36.38 μmol/g for Edku and Mariut lakes, respectively. This indicated that [SEM]/[AVS] ratios overestimate the availability of metals. A positive correlation was reported between total organic matter and ∑SEM (r = 0.74 and 0.39 at p < 0.05) in Mariut and Edku lakes, respectively. Multiple-year SEM-AVS studies are recommended.

Sediment Properties and Benthic Fauna Associated with Stock Enhancement and Farming of Marine Bivalve Populations in Xiangshan Bay, China

This study was designed to examine the impacts of ark shell Scapharca subcrenata enhancement and suspended oyster Crassostrea sikamea farming on sediment properties and fauna communities in the benthic zone of Xiangshan Bay, China. Ark shell enhancement took place over 319 days, after which sediment samples were collected from each of the three treatment sites: the enhancement area, an oyster farm, and a control area. Sediments from the oyster farming area had significantly higher moisture and acid volatile sulfide content, and the highest Cu, Zn, Pb, and Cd concentrations in all three treatment areas. The ark shell enhancement area has the lowest total organic carbon. No significant difference was found between the mean grain size in each of the three areas. Benthic fauna communities in the ark shell enhancement area were similar to those in the control area. Among the 16 faunal taxa identified, Ruditapes philippinarum was the most abundant species, followed by Glycera onomichiensis and Musculus senhousia. Suspension feeders were not found in the sediment beneath the oyster rafts. Cluster analysis showed a distinct separation between the benthic fauna communities in the oyster farming area and the other two treatment areas. Ark shell enhancement was thus shown to have minimal impact on the benthic environment, while the raft-farmed oysters had significant effects on the sediment structure and the benthic fauna after more than ten years of farming. The differences between the impacts of ark shell enhancement and oyster farming could be attributed to the slowing of water flow caused by oyster rafts.

Nitrogen and phosphorusas major factors shaping microbial community structures in surface sediments in Xiangxi River, Three Gorges Reservoir

香溪河是三峡库区最大的入库支流,目前关于香溪河沉积物微生物群落结构特征及其主要影响因素尚不明确。本研究监测香溪河上、中、下游上覆水及沉积物的主要理化指标,通过16S rDNA高通量测序研究香溪河表层沉积物中微生物群落结构特征,使用统计学软件分析细菌与理化指标间的关系。结果表明,上覆水中氨氮(NH₃-N)、叶绿素a(Chl.a)浓度自下游向上游逐渐升高,其中NH₃-N浓度从下游的2.04 mg/L升高到上游的3.14 mg/L。不同点位上覆水中磷酸盐(PO^3-₄)浓度在0.14~0.18 mg/L之间波动;同一点位间隙水中NH₃-N、PO^3-₄浓度明显高于上覆水,10~15 cm沉积物间隙水中NH₃-N与PO^3-₄浓度分别是0~5 cm沉积物间隙水的3.4倍和1.7倍。沉积物中Fe(Ⅱ)、Fe(Ⅲ)、酸可挥发性硫化物和铬还原性硫化物含量均表现为上游点位高于中游、下游,底层沉积物含量高于中层和表层。香溪河中游沉积物的微生物多样性和丰富度高于下游和上游,其中变形菌门(Proteobacteria)是表层沉积物中的优势菌门,鞘脂单胞菌属(Sphingomonas)为主要菌属。不同代谢功能的微生物在空间分布上存在差异,比如具有降解多环芳烃能力的外新鞘脂菌属(Novosphingobium)和土生单胞菌属(Terrimonas)在下游点位的相对丰度最高,分别达到5.8%和5.4%,硝化螺旋菌属(Nitrospira)和黄杆菌属(Flavobacterium)在中游的相对丰度最高,而铁氧化菌水杆菌属(Aquabacterium)主要分布在上游点位。冗余分析(RDA)表明总有机碳、硫酸根、Fe(Ⅱ)、氨氮和总磷是影响香溪河沉积物中微生物群落结构的主要环境因子。本研究有助于进一步了解周边环境和人为活动对香溪河水质稳定及沉积物中微生物的影响,为三峡库区水环境改善和管理提供参考。;Xiangxi River is the largest inflow tributary in the Three Gorges Reservoir area. Structural characteristics and influence factors of the microbial communities in sediments of Xiangxi River are not clear so far. This study investigated the main physical and chemical parameters of the overlying water (i.e., surface, middle and bottom layers) and sediments of the Xiangxi River. 16S rDNA high-throughput sequencing was used to reveal the characteristics of microbial community structures in the sediments. In addition, the relationship between bacteria and physicochemical parameters were analyzed statistically. Chlorophyll.a (Chl.a), concentrations of ammonia nitrogen (NH₃-N) in the overlying water increased gradually from the downstream (2.04 mg/L) to the upstream (3.14 mg/L). The concentrations of phosphate (PO^3-₄) in overlying water fluctuated between 0.14 and 0.18 mg/L at different sampling sites. In addition, the concentrations of NH₃-N and PO^3-₄ in the pore-water at the same site were significantly higher than those in the overlying water. Pore-water NH₃-N and PO^3-₄ concentrations at 10-15 cm depth were 3.4 and 1.7 times higher than those at 0-5 cm depth in sediments, respectively. Higher contents of Fe (Ⅱ), Fe (Ⅲ), acid volatile sulfide and chromium reducing sulfide were observed in the upstream sampling sites and bottom layers of the sediments. The microbial diversity and richness in the sediments of midstream were higher compared to the sediments of downstream and upstream. Proteobacteria was the dominant phylum in the surface sediments, and Sphingomonas was the main genus. The spatial distributions of microorganisms for different metabolic activities varied. Typical bacteria with the ability to degrade polycyclic aromatic hydrocarbons, such as Novosphingobium and Terrimonas, were mainly observed in the sediments of downstream sites with percentage reaching 5.8% and 5.4%, respectively. The percentages of Nitrospira and Flavobacterium were highest in the midstream. The iron-oxidizing bacteria (i.e., Aquabacterium) were rich in the upstream sites. Redundancy analysis (RDA) showed that total organic carbon (TOC), sulfate (SO^2-₄), Fe (Ⅱ), NH₃-N and total phosphorus (TP) were the main environmental factors shaping the microbial community structures in sediments of Xiangxi River. This study will help to further understand the impacts of surrounding environment and human activities on the stability of water quality and the microorganisms in the sediments of Xiangxi River, and provide a reference for the improvement and management of the water environment in the Three Gorges Reservoir area.

Inorganic sulfur cycles in sediments of the Pearl River Estuary: Processes, mechanisms, and isotopic indicators

The burial of inorganic sulfur is one of the important components in the global sulfur cycle. In this study, the cycling of inorganic sulfide species and the underlying control mechanisms were examined in sediments found in distinct sub-regions of the Pearl River Estuary (PRE). Specifically, the content and isotopic composition of SO42−, acid-volatile sulfides (AVS), and pyrite were determined, along with sedimentary sulfate reduction rates (SRR). The results show that the formation of iron sulfides (such as AVS and pyrite) was mainly controlled by the content of available reactive iron in the sediments of the PRE, and therefore formed mainly in the shallow sediments, with little variation in the deeper parts of the sediments. The δ34S values of AVS and SO42− increased synchronously with depth in the middle and lower part of the sediment, indicating a closed diagenetic environment. In contrast, the δ34S value of pyrite only increased slowly with depth in the case of a small portion of AVS converted to pyrite, which was controlled by the burial time. There are obvious differences in the formation and burial of pyrite in sediments of different sub-regions of the PRE. The surface layer of stations QA and HQ in the upper estuary and brackish coast was an open environment due to sedimentary reworking, and the contents of AVS and pyrite in the sediment was extremely low due to re-oxidation, while the oxidation of 32S-rich H2S also resulted in lower δ34S values of sulfate in the sediment pore water than in the overlying seawater. Station GS in the estuarine mouth had a much higher percentage of AVS converted to pyrite in the sediments than that at stations QA and HQ due to the relatively long burial time. In addition, near the sulfate methane transition zone (SMT), only the estuarine mouth (station GS) produced iron sulfides (AVS) synchronously with the anaerobic oxidation of methane (AOM), and then converted to pyrite gradually; in contrast, the effect of AOM on the formation of iron sulfides in the sediments of the upper estuary and the brackish coast was almost negligible. The results show that the contribution of AOM processes to the final burial of inorganic sulfur is generally small in PRE.

Assessment of Bacillus subtilis applied in rice-crayfish coculture system on physicochemical properties, microbial sulfur cycling, Cd accumulation and bioavailability

Bacillus subtilis is frequently used commercially in rice-crayfish coculture system (RCCS) to modify the physicochemical characteristics of soil and water. However, given the intensification of cadmium (Cd) contamination in RCCS and potential implications for food safety, the impacts of this modulation on the Cd bioavailability and accumulation need to be more thoroughly assessed. Here, B. subtilis was inoculated into RCCS containing different levels of soil Cd through an outdoor mesocosm experiment. Results showed that the abundance of B. subtilis in the soil was highest on the 1st day after inoculation and appeared the second peak on the 2nd week. 40–128-fold increase in B. subtilis abundance in crayfish intestines was found in the B. subtilis added groups compared to the non-B. subtilis added groups. Additionally, the Cd accumulation in crayfish and rice tissues rose significantly when the soil Cd content raised. However, with the improvement of soil pH, oxidation-reduction potential (ORP), nitrification potential (NP), acid volatile sulfide (AVS) and sulfur reducing bacteria abundance, and the decrease of organic matter (OM) at 2 W and 8 W, the addition of B. subtilis into RCCS with various Cd contents significantly reduced the Cd accumulation and the soil Cd bioavailability. The low bioavailability of Cd was characterized by the reduction of exchangeable Cd (EXC-Cd) and the grows of Fe–Mn oxides bound Cd (FeMnOx-Cd) and residual Cd (RES-Cd). Specifically, the soil EXC-Cd was negatively correlated with soil pH (−0.647), OPR (−0.663) and NP (−0.811). In short, the obtained results demonstrated that B. subtilis was able to stabilize the physicochemical properties of soil and water in RCCS, while mineralizing Cd and attenuating Cd accumulation in the crayfish and rice, suggesting that B. subtilis can be used as a cleaning probiotic applied in rice and aquatic animals coculture system.

Effects of silver nanoparticles on denitrification and anammox in sediments of hypertrophic and mesotrophic lakes

The abundant production and wide usage of silver nanoparticles (Ag NPs) inevitably lead to their release into aquatic ecosystems. However, it is still unclear about how Ag NPs influence denitrification and anammox (DA) in freshwater sediments. To address this, the sediments of hypertrophic and mesotrophic lakes were exposed to 0.5 and 50 mg/L Ag NPs under anaerobic conditions for 7 days to explore the effects of Ag NPs on environmental variables, including redox potential (Eh), pH, organic matter (OM) and acid volatile sulfide (AVS), and the resulting influence on DA. Experimental results indicated that NO3−-N and NH4+-N levels were increased by the low (p > 0.05) and high doses of Ag NPs (p < 0.05) in comparison with the non-Ag control, revealing an inhibitive impact on DA. The level of total nitrogen (TN) was notably increased by the low and high doses of Ag NPs (p < 0.05), perhaps due to inhibited enzyme activity and corresponding encoding gene abundance, which were related to generating gaseous nitrogen such as N2O and N2. In addition, environmental factor Eh was significantly raised by Ag NPs (p < 0.05), further inhibiting DA. Moreover, the quantitative analysis unveiled that denitrifying and anammox bacteria in hypertrophic lakes evinced a stronger resistance to Ag NPs toxicity than those in mesotrophic lakes. Overall, our study revealed that short-term exposure to Ag NPs could inhibit DA in sediments. These findings provide an understanding enabling evaluation and prediction of the environmental risks of Ag NPs in freshwater lakes.

Historical Global Review of Acid-Volatile Sulfide Sediment Monitoring Data

Acid-volatile sulfides (AVS) are strongly associated with the bioavailability of some divalent metals such as cadmium, copper, lead, nickel and zinc. However, the global spatial variability of AVS for aquatic systems is unknown. The specific goals of this study were to: (1) summarize all available AVS monitoring data from all types of freshwater and saltwater waterbodies (streams/creeks, rivers, lakes/ponds/reservoirs and estuarine/marine areas) and (2) compare AVS concentrations from these various types of waterbodies considering both soil type classification and biomes. AVS measurements were reported from 21 different countries. A total of 17 different soil types were reported for all waterbody types and both podzols and luvisols were found in all waterbody types. Nine different biomes were sampled for all waterbody types. The temperate broadleaf and mixed forest biome was sampled for AVS in all waterbody types. Mean AVS concentrations ranged from 0.01 to 503 µmoles/g for 140 different waterbody types and the 90th centile for all these waterbodies was 49.4 µmoles/g. A ranking of waterbody type means from low to high AVS measurements showed the lowest mean value was reported for streams/creeks (5.12 µmoles/g; range from 0.1 to 39.8 µmoles/g) followed by lakes/ponds/reservoirs (11.3 µmoles/g; range from 0.79 to 127 µmoles/g); estuarine/marine areas (27.2 µmoles/g; range from 0.06 to 503 µmoles/g) and rivers (27.7 µmoles/g; range from 1.13 to 197 µmoles/g). The data provided in this study are compelling as it showed that the high variability of AVS measurements within each waterbody type as well as the variability of AVS within specific locations were often multiple orders of magnitude differences for concentration ranges. Therefore, a comprehensive spatial and temporal scale sampling of AVS in concert with divalent metals analysis is critical to avoid possible errors when evaluating the potential ecological risk of divalent metals in sediment.

Developing ecological risk assessment of metals released from sediment based on sediment quality guidelines linking with the properties: A case study for Kaohsiung Harbor

This study aimed to introduce sediment properties (total organic carbon (TOC), acid-volatile sulfides (AVS), particle size distribution) into sediment quality guideline-based risk quotients to assess the potential toxicity of metals (Ni, Cu, Zn, Cr, Cd, and Pb) released from sediments. Sediment was collected at three times points in 20 sampling sites in Kaohsiung Harbor. The Microtox® toxicity test was used to assess the sediment toxicity and the relationship between sediment toxicity and risk quotient estimated based on the metal concentration was constructed. To improve the toxicity prediction and modify the risk quotient according to the sediment properties, stepwise multiple linear regression (MLR) models that have been tested over wide ranges of TOC, AVS, and particle size distribution to determine the key sediment properties. Common multimetal indices, including the pollution load index, modified degree of contamination index, Nemerow pollution index, potential ecological risk index, and total toxic risk index, were compared with sediment toxicity to evaluate the degrees of correlation. By modifying the relationship between metal toxicity and the risk quotient by including TOC and AVS, the prediction showed that sediments in Kaohsiung Harbor were generally of slight acute toxicity to acute toxicity to organisms, with sampling sites near an industrial zone showing a higher probability of high acute toxicity. In particular, the acute risk of adverse effects on aquatic organisms from sediments in the Salt River estuary was significantly higher than that at other sites, which was consistent with the results of assessment based on the multimetal indices. This study suggests that the MLR-based approach may facilitate the adoption of updated site-specific metals standards that more accurately account for the parameters affecting metal bioavailability than metal concentration standard alone.

Comparison of acid volatile sulphide, metal speciation, and diffusive gradients in thin-film measurement for metal toxicity assessment of sediments in Lake Chaohu, China.

The toxicity of heavy metals in sediments is inseparable from their forms in the environment. Traditional sediment toxicity assessment systems, such as total metals, dissolved metals in pore water, metals extracted by the Community Bureau of Reference procedure, and acid volatile sulphide (AVS)-simultaneously extracted metal (SEM), have their own limitations. This study revealed the horizontal and vertical distribution characteristics of AVS and SEM in Lake Chaohu and three typical groups of two-dimensional profiles of diffusive gradients in thin-film (DGT)-labile S(-II) were obtained at representative sampling sites. There was a positive correlation between DGT-labile S(-II) and AVS due to sulphate-reducing bacteria and a negative correlation due to the high sulphate reduction rate induced by high total organic carbon. Moreover, there was no correlation between DGT-labile S(-II) and AVS when bioturbation was dominant in the sediments. To realise the application of DGT measurement in toxicity assessment of heavy metals in sediment through the sandwich relationship of DGT-labile metals vs. metals speciation vs. sediment toxicity assessment, the key relationship of DGT-labile metals vs. metals speciation was explored. DGT-labile Ni showed potential to reveal this relationship.