Nutrients in overlying water affect the environmental behavior of heavy metals in coastal sediments

Thermochemical conversion of microbial mats: A case study on Cr(VI) removal from freshwater

In response to increasing anthropogenic pollution and metal accumulation in aquatic environments, bioremediation has gained great importance. Microbial consortia are emerging as a promising solution due to their diverse pollutant metabolizing pathways. This study evaluates the role of biotic and abiotic components in microbial mats for Cr(VI) removal from freshwater. For this purpose, microbial mats were modified by high-temperature treatments using an autoclave and a muffle furnace, and modified and unmodified mats, both chemically characterized by SEM-EDS, ATR-FTIR, XRD, and XPS, were used in Cr(VI) removal assays. The concentrations of total Cr and Cr(VI) were measured, which were initially 15 mg/L and after 17 days reached values between 0.22 and 4.99 mg/L depending on the treatment. The removal efficiency was nearly 99% in unmodified mats, while in autoclaved ones it reached 95% and in calcined mats 67%. These findings demonstrate that all components contribute to Cr removal. Both trivalent and hexavalent Cr penetrated the porous structure reaching deep layers where they were adsorbed by electrostatic attraction or complex formation with the mat components, while the insoluble compounds formed, such as Cr2O3 and CrPO4, precipitated on the mat. Microbial mats with all their components, including their living microbial communities, provide abundant sites for surface removal phenomena and offer greater potential for the reduction of Cr(VI) to Cr(III), thereby achieving higher Cr removal efficiency.

Emerging environmental risks to the largest coastal lagoon (Pinqing Lagoon) on the Chinese mainland: Assessment through a water-sediment-ecological perspective

Coastal lagoons are vital yet vulnerable marine ecosystems. This study analyzes a five-year dataset to evaluate changes in water quality and their impacts on biota in Pinqing Lagoon (PQL). Seasonal surveys conducted from 2019 to 2023 across 14 sites revealed significant variability in water and sediment quality parameters. During the wet summer season, water quality deteriorated significantly as nutrient levels peaked. In contrast, the dry winter season posed a high ecological risk from heavy metals (HMs), specifically Cu, Hg, and Cd, in sediments. Sediment core analysis indicates upward trends in HMs since the 1950s. Salinity and pH are the primary drivers shaping the benthic communities in transitional zones, while nutrients and HMs primarily affect organisms in the inner bay. Nutrient enrichment has exacerbated HM accumulation and ecological degradation, with tolerance to HMs emerging as a critical factor for benthic survival. This comprehensive assessment of PQL highlights the seasonal patterns of pollution in urban lagoons, providing valuable insights for future conservation efforts.

Water exchange frequency affects the fractions of sulfur and heavy metals in mariculture sediments

Water exchange is a key step in mariculture activities. However, the impact of water exchange frequency (WEF) on the environmental behavior of sulfur and heavy metals is not well understood. In this study, the migration and transformation of sulfur and heavy metals in mariculture sediments under different WEFs were investigated. The results indicated that high WEF would be unfavorable to overlying water quality, while increased acid volatile sulfur (AVS) in the sediments (11.13 μmol/g). High WEF accelerated the release of heavy metals from bottom sediments and their enrichment in the surface layer, leading to an increase in heavy metal content in the surface sediments, with Cd showing particularly significant changes (CV, ∼20%). Moreover, the WEF also had an obvious effect on the Cd fraction (CV>10%). The increase in acid-soluble Pb inhibited the conversion of AVS to chromium (II)-reducible sulfur (CRS) in the sediments, enhancing the sediment aging process. An appropriate frequency of water exchange (once every 5 days) could increase the abundance and diversity of bacteria and help to shape specific microorganisms. Changes in heavy metals in the surface sediments caused Firmicutes to become the most affected bacterial species by the WEF. The functional flora involved in the sulfur cycle were lesser affected by the WEF (CV, ∼5%), whereas those involved in the nitrogen cycle were more affected (CV >17%). The findings provide guidance for scientific mariculture.

The effects of phosphorus on the fate and transformation of sediment-associated cadmium from river sediments

The coexistence of phosphorus (P) and cadmium (Cd) in river sediments poses a significant challenge for remediating these contaminants in aquatic environments, given the ongoing debates regarding their interactions. This study aimed to elucidate the impact of water-soluble phosphorus (PO43-) on the fate and transformation of sediment-associated Cd under varying conditions. The findings revealed that the impact of PO43- on the release of sediment-associated Cd depends on the presence or absence of cations such as Ca2+ and Mg2+, with Ca2+ exerting a more pronounced effect than Mg2+. In the absence of Ca2+ and Mg2+, PO43- effectively inhibits the release of sediment-associated Cd by facilitating the formation of more stable precipitates, including (Cd)3(PO4)2. Conversely, when Ca2+ and Mg2+ are present, they compete with Cd for PO43- binding sites, reducing the formation of (Cd)3(PO4)2 and indirectly enhancing the release of sediment-associated Cd. Furthermore, redox conditions play a significant role in Cd release from sediments depending on its fractions. However, this influence may be less prominent compared to that caused by PO43-. Notably, increasing sediment ageing time diminishes the enhancing effect exerted by PO43-, likely attributed to the conversion of Cd into a more stable residual fraction. This study offers further insights into the impact of PO43- on the fate and transformation of sediment-associated Cd within river sediments, necessitating simultaneous consideration of multiple variables such as cations, redox conditions, and sediment ageing.

Interaction of heavy metals and polycyclic aromatic hydrocarbons in soil-crop systems: The effects and mechanisms

In natural environments, the removal and degradation of two major pollutants, heavy metals (HMs) and polycyclic aromatic hydrocarbons (PAHs), are explored through targeted experimental investigations. However, these endeavors reveal that outcomes in situ may significantly diverge from the idealized effects observed in laboratory settings due to the complex interaction between HMs and PAHs, underscoring a pressing need for thorough research into their mutual impacts. This review examines the origins and migratory pathways of compound pollution stemming from HMs and PAHs. Concurrently, it provides an overview of the farmland ecosystem's response to combined HMs-PAHs pollution. This encompassed the assessments of changes in the soil's physical and chemical properties, the intricacies of the migration and transformation processes of the combined pollution within plants, and the consequential impact on the physiological functions of soil microorganisms. The varying concentration ratios of HMs and PAHs can modulate the permeability of plant root cell membranes, thereby influencing the translocation of these substances within the plant via symplastic and apoplastic pathways. Recent research has uncovered the mechanisms underlying cation-π interactions between HMs and PAHs. This review aims to offer a comprehensive overview of the current state of HMs-PAHs co-pollution, offering both qualitative and quantitative insights into their interaction patterns within the farmland ecosystem. The ultimate goal is to establish a robust theoretical foundation to support the in-situ remediation of these pollutants in agricultural practices and to provide a theoretical basis for soil health management in agricultural production.

Effect of organic matter on the environmental behavior of sulfur and heavy metals in mariculture sediments during the aging process

Organic matter (OM) significantly impacts the environmental behavior of sulfur and heavy metals. In this study, the effects of OM on the migration and transformation of sulfur and heavy metals in mariculture sediments were investigated. The results indicated that baiting had a strong impact on the accumulation of acid volatile sulfur (AVS) (P < 0.05) and increased the environmental risk of sulfide in sediments. The addition of bait promoted the generation of chromium (II)-reducible sulfur (CRS); however, the resistance of AVS to CRS conversion increased with increasing bait addition. The addition of bait considerably influenced Cd accumulation. The acid-soluble fractions of Cr and Cu and the oxidizable fraction of Cd were primarily affected by the bait addition (coefficient of variation>15 %). An increase in the reducible fraction promoted the conversion of AVS to CRS, which reduced the degree of sediment aging. Higher OM levels reduced the diversity and abundance of the bacterial communities. The sulfate respiration functional microbiota was particularly affected by OM.

Different seasonal dynamics, ecological drivers, and assembly mechanisms of algae in southern and northern drinking water reservoirs

The role of environmental factors on the community structure of algae has been intensively studied, but there are few analyses on the assembly mechanism of the algal community structure. Here, changes in the community structure of algae in different seasons, the effects of environmental variables on the algal community structure, and the assembly mechanism of the algal community structure in northern and southern reservoirs were investigated in this study. The study revealed that Bacillariophyta, Cyanophyta, and Chlorophyta were the predominant algal species in the reservoirs, with Bacillariophyta and Cyanophyta exhibiting seasonal outbreaks. Compared to the northern reservoirs, the algal diversity in the southern reservoirs was greater. The diversity and algal community structure could be significantly impacted by variations in water temperature and nitrogen level. According to the ecological model, the interaction among algal communities in reservoirs was primarily cooperation. The key taxa in the northern reservoirs was Aphanizomenon sp., while the outbreak in the southern reservoirs was Coelosphaerium sp. The community formation pattern of reservoirs was stochastic, with a higher degree of explanation observed in the southern reservoirs compared to the northern reservoirs. This study preliminarily explored the assembly mechanism of the algal community, providing a theoretical basis for the control of eutrophication in drinking water reservoirs.

Management of dredged marine sediments in Southern France: main keys to large-scale beneficial re-use

Fifty million cubic meters of marine sediments are dredged each year in France in order to maintain harbor activities and sustain the economy of littoral territories. Because of anthropogenic activities in and around harbors, sediments can contain significant amounts of chemical and organic pollutants whose behavior during dredging must be addressed in order to avoid releasing risks for humans and the environment. French regulations come to govern the management of dredged sediments, considering them "safe" and possible to be dumped at sea or "contaminated" and needed to be treated on land as waste. In recent years, new constraints have been pushed toward the management of land. This management is, however, challenging as few channels are proposed to reuse marine sediments, and elimination appears to be economically and environmentally unsustainable. This study provides an overview of the technical and regulatory aspects related to dredged marine sediment management in France and aims to identify and discuss the limits of their valorization. Dredged sediments are mainly composed of particles with heterogeneous grain size, some being known for many applications such as building materials and growing media. However, several reasons have been put forward to explain why these particles are not reused when extracted from dredged sediments. Several technical, socio-economic, and regulatory obstacles explain the low demand for dredged sediments. This demand can be stimulated by government incentives and a good regulatory framework. National regulations could help streamline their reuse by removing their "waste" status and creating a regulated market for dredged sediment.