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Mercury (Hg) and methylmercury (MeHg) are easily released from sediments to overlying water and cause secondary contamination. In general, Hg concentrations are low in natural aquatic environments, but Hg toxicity is high. Therefore, it is important to assess the mobility and release risks of Hg and MeHg from surface sediment using in situ high-resolution sampling techniques. The profile distribution of Hg and MeHg was obtained for samples from Weishan sub-lake (WL) and Dushan sub-lake (DL) of Nansi Lake, China, by high-resolution dialysis (HR-Peeper probes) and the diffusive gradients in thin films (DGT) technique at mm-resolution. Furthermore, Hg mobility and release risks in sediments were evaluated by combining BCR (European Community Reference Bureau) extraction and DGT-measured data. The soluble concentrations of Hg in surface sediments in WL and DL were 21.70 and 19.38 ng L−1 and the DGT-labile concentration of Hg were 8.21 and 10.30 ng L−1, respectively. The soluble and labile Hg and MeHg concentrations were higher in the surface sediments (from −40 to 0 mm) than in deep sediments. The distribution of the labile-Hg was controlled by the ferrimanganic (hydr)oxide and total nitrogen rather than organic carbon content. The non-residual components accounted for a greater proportion of the interface, which further confirmed Hg was more active on the surface layer of the sediment. The resupply ability indicated that the release of Hg from sediment was insufficient to maintain the initial concentration in the porewater before consumption. The MeHg fluxes in WL (6.18 ng m−2 day−1) were twice those in DL (2.89 ng m−2 day−1), and the risk assessment code revealed a higher risk in the surface layer (25.21–61.88%) than in the deep layer (0–27.75%). Dissolved Hg and MeHg accumulated on the surface of the sediments and were more active than in the deeper sediments. The DGT-labile state can be used for a better understanding of the bioavailability and mobility of Hg. The diffusion direction of Hg and MeHg was from sediment to the overlying water. The release risks of Hg and MeHg from surface sediments (especially in WL) were found to be worthy of concern.

The occurrence of harmful algae blooms has been increasing in large lakes worldwide. The mechanism of heavy metals mobilization in sediments during algae blooms is not well understood. As a major pollutant in the sediments of Taihu Lake, cobalt (Co) has been selected to study heavy metal mobilization during algae blooms. Rhizon and HR-Peeper sampling techniques have been used for in situ investigation and indoor simulation experiments to collect information on dissolved Co, manganese (Mn), and UV absorbance at 254 nm (UV254) in sediments. Excitation–emission matrix (EEM) was combined with parallel factor (PARAFAC) to determine the change of dissolved organic matter (DOM) components during algae blooms. The chemical morphology of Co in pore water was analyzed by visual MINTEQ model. The Stern–Volmer model was used to characterize the stability of different DOM components and Co(II). Algae blooms significantly increased the dissolved Co concentration in sediments. The release of Co was closely related to DOM in the algae bloom sediments, which was reflected by the similar distribution and significant positive correlation between the dissolved Co and DOM in pore water, during both in situ and laboratory simulation algae blooms experiments. On the other hand, the saturation of oxygen in the sediment–water interface (SWI) rapidly decreased from 100 to 0% during algae blooms, resulting in high mobilization of Co and reduction of Mn oxides in sediments. This was supported by the simultaneous increase of dissolved Co and Mn and significant positive correlation between dissolved Co and Mn in the simulation aerobic–anaerobic sediments. The transformation of most Co(II) into DOM-Co(II) complexes, as calculated by the Visual MINTEQ model, further demonstrated that the mobilization of Co was mainly controlled by the DOM content in the sediments during algae blooms. Further studies revealed that tyrosine-like substance released by algae played a major role in their complexation with Co, possibly due to their relatively high content and high stability after binding Co. The mobilization of Co in sediments during algae blooms was mainly controlled by DOM through complexation, reflected by the observation that dissolved Co concentration and UV254 increased simultaneously and had significant positive correlation during in situ monitoring and indoor simulations experiments. More than 80% of dissolved Co in the pore water during algae blooms was DOM-Co complexes, supporting this conclusion. It was further observed that tyrosine-like substances played an important role in Co complexation.

Internal phosphorus (P) loading can increase the P level in the water column and further sustains cyanobacterial blooms. This study focused on the role of benthic fauna bioturbation in affecting the sediment P release and the P level of water column in a eutrophic lake, Lake Taihu. The macrofauna density decreased from 4766.56 ± 10541.80 ind/m

The heavy metal contamination of the aquatic ecosystem is still prevalent even after reduction of the external anthropogenic inputs of the metals. The release of labile heavy metals from the sediments into the water is a potential risk, responsible for the contamination of the aquatic system. Herein, samples of sedimentary column cores were collected in Nansi Lake, and the distribution profiles of the labile and soluble metals (Cd, Cu, Ni, Pb, and Zn) were obtained by the diffusive gradient in thin films (DGT) and the high-resolution dialysis (HR-peeper) technique. Furthermore, the mobility, bioavailability and release risk of the heavy metals were assessed using the results of geochemical sequential extraction, DGT as well as the DGT-induced fluxes in sediments (DIFS) model. The results showed that the profile characteristics of the DGT-labile and soluble heavy metals showed irregular distribution in the sediment cores and Cd, Pb, Zn had an obvious positive correlation with Fe/Mn (p

A new diffusive gradients in thin films technique (HR-MPTS DGT) with mercapto-functionalized attapulgite in a binding gel was developed for simultaneous two-dimensional (2-D) chemical imaging of As

For soils and sediments, which are complex and heterogeneous, root-triggered processes, bioturbation of benthic communities, and local decomposition of reactive organic materials can greatly increase their biogeochemical heterogeneity. In practice, common monitoring methods can only be used to determine the centimeter- to millimeter-scale heterogeneity from vertical or horizontal one-dimensional profiles. However, significant heterogeneity exists in both soils and sediments on the millimeter to sub-millimeter scales. Therefore, in-situ and high-resolution sampling and imaging methods are essential to capture the spatial and temporal heterogeneity of soils and sediments. This article reviews micro-scale sampling methods in soil and sediments, including diffusive equilibration in thin films (DET), diffusive gradient in thin films (DGT), planar optodes (PO), nanoparticle- and microparticle-based luminescence imaging, and soil zymography. Besides, we introduce their imaging principles and approaches in detail, and discuss some problems and solutions during imaging. Micro-scale sampling and imaging methods have given us many insights into the biogeochemical processes in soils and sediments, but the full potential of these methods has not been exhausted yet. Finally, the potential directions for the development of micro-scale sampling methods in the study of complex biogeochemical processes in heterogeneous soils and sediments are prospected.

The complex seawater matrix has significantly influenced the determination of estuarine dissolved Hg(II), hindering its monitoring and risk assessment in maricultural areas. In this work, SiO

The spatio-temporal variation of dissolved inorganic nitrogen (DIN: NH

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.

Geochemical properties of rare earth elements (REEs) have the potential to represent the provenance and depositional history of surface sediment in aquatic environments. In this study, both surface and core sediment samples were collected from Dongting Lake to investigate the distribution characteristics and source of REEs by combining the methodologies of geostatistics, positive matrix factorization (PMF) model, discriminant function (DF), and provenance index (PI) based on REEs geochemical parameters. The results indicated that the total REEs content in sediment samples ranged from 129.12 to 284.02 μg g−1, with the average REEs content calculated to be 197.95 μg g−1. Light REEs (LREEs) comprised >90% of the total REEs, indicating that there was an enrichment of LREEs in the sediment samples. The REEs of the surface sediment showed strong spatial variation, with relatively high values located in Eastern Dongting Lake and relatively lower levels in Western Dongting Lake. Moreover, the vertical distributions of ∑REEs, ∑LREEs and ∑HREEs at most sampling sites behaved similarly with rapid increase until about 6–8 cm, followed by a downward trend with some irregularities. The strong association between most REEs confirmed that they often have a positive correlation and co-existence in sediment. The PMF model revealed that most of the REEs in the surface sediment were derived from natural sources with some anthropogenic inputs also serving as contributing sources. The DF and PI results indicated that the REEs distribution pattern in the surface sediment of Dongting Lake was similar to that of the Yangtze River, suggesting that Yangtze River had been more of an influence on sediment loads than the upstream tributaries. This study highlights the broader applicability of the REEs tracing method in sediment transport processes and can provide new knowledge regarding source apportionment analysis of sediment-related contaminants in aquatic environments.

Dredging is used worldwide to remove polluted sediments from water bodies. However, the dredging efficacy remains hard to identify. Here, we studied the efficacy of dredging engineering as a means to remove Cu, Cd, and Pb from polluted lake sediments, after six years of completion. Dissolved metals and DGT-labile metals were quantified in the non-dredged and post-dredged sediments by high-resolution dialysis (HR-Peeper) and diffusive gradients (DGT) in thin films techniques. April and July measurements showed that dredging was effectively remediate the polluted sediments. The dissolved Pb, Cd, and Cu contents decreased up to 30%, 44%, and 26%, and the DGT-labile contents decreased up to 51%, 27%, and 33% compared with the contents in the non-dredged zone. Dredging was thus proven efficient in decreasing the labile metal fractions, increasing the capacity of available solids to bind metals, and slowing the leaching of metals from available solids in the post-dredged sediments. In October and January, the dredging efficacy was counteracted by the decomposition of algae, which increased the dissolved and DGT-labile metal concentrations in the post-dredged zone.

Studies of dredging effectiveness, especially the ones that last for several years, are scarce. In this study, we evaluated effectiveness of dredging performed for six years on controlling cobalt (Co), zinc (Zn), and nickel (Ni) contamination of sediments. High-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) methods were applied to analyze the non-dredged and post-dredging sediments. The soluble and DGT-labile Co and Ni content declined by 22% and 44% (soluble) and by 16% and 26% (labile) in April, July and October in the post-dredging region. In contrast, their concentrations increased by 105% and 9% (soluble) and 322% and 27% (labile) in January. These changes in the dredging effects were caused by the corresponding changes in the reducible and residual fractions of Co and the residual fraction of Ni in sediments in the dredged site, respectively. Soluble and DGT-labile Zn decreased on average by 23% and 29% in July and October and increased on average by 151% and 52% in April and January in the post-dredging region. The different Zn mobility in the post-dredging region was controlled by the reducible fraction of Zn. The results revealed positive influence of dredging engineering in summer, autumn and/or spring and negative one in winter. Therefore, an accurate assessment of dredging effectiveness should take its seasonal variation into consideration.

Antimony (Sb) mobilization in sediments and its impact on water quality remained to be studied. In this study, high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) technique were used to measure soluble Sb and labile Sb in sediment-overlying water profiles for a full year in a eutrophic region of Lake Taihu. Results showed that the highest mean concentrations of soluble Sb in overlying water (11.27 and 6.99 μg/L) appeared in December 2016 and January 2017, due to oxidation of Sb(III) to Sb(V) by Mn and Fe oxides, all of which exceeded the surface or drinking water limits set by China, United States and European Union. From April to November 2016, the concentrations of soluble Sb remained low with small monthly fluctuations and mean values ranging from 1.79 to 2.93 μg/L. This was attributed to the predominance of insoluble Sb(III) in sediments under anoxic conditions. The concentration of soluble Sb was slightly higher in summer than in autumn, due to the complexation of Sb(III) with DOM, as shown under anaerobic incubation. The mobility of inorganic Sb in sediments was mostly determined by the transition between Sb(III) and Sb(V), with Sb pollution in bottom water during winter being of concern.

Recycling waste materials as adsorbents for stabilising contaminated marine sediment is a low-cost and sustainable remediation method. The objective of this research was to evaluate the efficacy of organic (lignite and green waste compost) and inorganic wastes (acid mine drainage sludge (AMDS) and coal fly ash (CFA)) on metal stabilisation and assess the leachability of minerals from the adsorbents. The kinetic results illustrated that Zn (0.21 mg L−1) and Cu (0.16 mg L−1) quickly released from the sediment within 30 min. The continuous column leaching tests showed that the addition of carbonaceous lignite effectively reduced leaching of Zn (4.5–7.0 mg L−1) and Pb (0.05–0.06 mg L−1) after 100 pore volumes, although the stabilisation capacity was lower than that of activated carbon. In comparison, two inorganic industrial by-products (AMDS and CFA) outcompeted the stabilisation performance of organic materials, which reduced the long-term leaching concentrations of Cu and Zn to below 1 mg L−1. The AMDS even provided comparable efficiency to the commercial zero-valent iron due to its abundant sorption sites and alkaline earth metals for contaminant adsorption and precipitation. However, the addition of waste adsorbents resulted in elevated leaching of Mn, Fe, and Al from the lignite- and AMDS-amended sediment, which may pose toxic risks to benthic organisms. the proposed waste adsorbents present a low-cost and low-carbon treatment for in-situ contaminated sediment remediation.

Iron (Fe) and manganese (Mn) reactions have been regarded as the primary factors responsible for the mobilization of phosphorus (P) in lake sediments, although their individual roles are hard to distinguish. In this study, in situ mobilization of P, Fe and Mn in sediments was assessed by high resolution spatio-temporal sampling of their labile forms using diffusive gradient in thin films (DGT) and suction device (Rhizon) techniques. It was found that the monthly concentration distributions showed greater agreement and better correlation coefficients between labile P and labile Fe, than those between labile P and labile Mn, implying that Fe plays a key role in controlling P release in sediments. Furthermore, better correlations were observed between hourly changes in concentrations of soluble reactive phosphorus (SRP) and soluble Fe(II), than those between SRP and soluble Mn. Changes were observed under simulated anaerobic incubation conditions, suggesting that P release was caused by the reductive dissolution of Fe oxides. This was supported by the lack of influences on P release from reductive dissolution of Mn oxides in the sediment-water interface and top sediment layers under the anaerobic incubations. In simulated algal bloom experiments, positive correlations and consistent changes were observed between SRP and soluble Fe(II) concentrations, but not between SRP and soluble Mn concentrations. This further demonstrated the Fe-dependent and Mn-independent release of P in sediments. Therefore, Fe redox reactions have a high impact on P mobilization in sediments, while Mn redox reactions appear to have negligible influences.

In this study, red seaweed (Gracilaria lemaneiformis) food waste with high carbohydrate content was valorized into levulinic acid (LA) and algae hydrochar through microwave-assisted low-temperature hydrothermal treatment in dilute acid solution. Various parameters including treatment temperature (160–200 °C), reaction time (1–40 min), acid concentration (0–0.6 M), and biomass-to-liquid ratio (1%–10%, w/v) were examined. The energy efficiency and carbon recovery of the proposed process were investigated. Under the experimental conditions of 5% (w/v) biomass loading, 0.2 M H2SO4, 180 °C, and 20 min, the highest levulinic acid yield of 16.3 wt% was produced. The resulting hydrochar showed approximately 45–55% energy yield and higher heating values of 19–25 MJ kg−1. The energy efficiency of the present study (1.31 × 10−6 g LA/J) was comparable to those of the conventional hydrothermal treatment of lignocellulosic biomass, while the reaction time (20 min) was much shorter with a high carbon recovery (73.3%).

Phosphorus (P) availability is closely related to the distributions of pH, O2 and phosphatase activities in the rhizosphere of plants growing in soils and sediments. In this study, the P uptake processes and mechanisms of Vallisneria natans (V. natans) during two vegetation periods (i.e., week three and six) were revealed using three noninvasive 2D imaging techniques: planar optode (PO), diffusive gradients in thin films (DGT) and zymography. The results showed that increased phosphatase activity, O2 concentration and root-induced acidification were observed together in the rhizosphere of root segments and tips. In week three, when V. natans was young, the flux of DGT-labile P accumulated more in the rhizosphere in comparison with the bulk sediment. This was because increased phosphatase activity (of up to 35%) and root-induced acidification (with pH decreasing by up to 0.25) enhanced P acquisition of V. natans by the third week. However, the flux of DGT-labile P turned to depletion during weeks three to six of V. natans growth, after Fe plaque formed at the matured stage. The constant hydrolysis of phosphatase and acidification could not compensate for the P demand of the roots by the sixth week. At this stage, Fe plaque become the P pool, due to P fixation with solid Fe(III) hydroxides. Subsequently, V. natans roots acquired P from Fe plaque via organic acid complexation of Fe(III).

Routine waterway dredging activities generate huge volumes of dredged sediment. The remediation of dredged contaminated sediment is a worldwide challenge. Novel and sustainable ex-situ remediation technologies for contaminated sediment have been developed and adopted in recent years. In this review paper, the state-of-art ex-situ treatment technologies and resource utilisation methods for contaminated sediment were critically reviewed. By applying different techniques, sediment could been successfully transformed into sustainable construction materials, such as ceramsite, supplementary cementitious materials, fill materials, paving blocks, partition blocks, ready-mixed concrete, and foamed concrete. We highlighted that proper remediation technologies should be cleverly selected and designed according to the physical and chemical characteristics of sediment, without neglecting important aspects, such as cost, safety, environmental impacts, readiness level of the technology and social acceptability. The combination of different assessment methods (e.g., environmental impact assessment, cost-benefit analysis, multi-criteria decision analysis and life cycle assessment) should be employed to comprehensively evaluate the feasibility of different sustainable remediation technologies. We call on the scientific community in a multidisciplinary fashion to evaluate the sustainability of various remediation technologies for contaminated sediment.

Assessing the environmental quality of coastal systems is important not only for the management and protection of such areas, but also for improving the quality of water resources. Since sediment itself can often be a source of certain toxic elements, in addition to information on the distribution of metals in the water column and in the sediment itself, it is useful to determine the bioavailable forms of individual elements, particularly toxic ones. In this study, water and sediment geochemical data were supplemented with oxyanion mobility in sediments estimated by diffusion gradients in thin film (DGTs). The data obtained indicate that the chemical composition of the water in the Raša River estuary primarily reflects the high input of suspended sediment from the catchment, the mixing of freshwater and seawater, and to a lesser extent the effects of anthropogenic activities. Although sediment composition is primarily determined by geological and hydrodynamic conditions in the catchment, it also indicates moderate enrichment in Co, Cr, Mo and Ni. In contrast, the distribution of oxyanions in sediment pore water indicates the influence of sediment as a source of some elements in the bottom water, e.g., sediment contributes to 40% of the arsenic bottom water budget. The obtained depth profiles of the oxyanion distribution in the sediment pore water indicate an early onset of suboxic to anoxic conditions in Raša Bay, which is prone to rapid sedimentation. All this demonstrates the need to consider the bioavailable forms of elements when assessing environmental quality, as the lack of such information can lead to an incomplete assessment, especially in dynamic coastal systems such as estuaries.

Root-triggered microscale variations in O

Eutrophication and metal pollution are global environmental problems. The risk of metal pollution is high in the eutrophic lakes because of high mobility of metal in sediments. However, the mechanism of cadmium (Cd) mobility in sediments is still unclear. Here we study the mobilization of Cd in sediments from the eutrophic Lake Taihu via monthly field monitoring of mobile Cd using diffusive gradient in thin films (DGT) and high resolution dialysis (HR-Peeper) techniques. We found a high mobility of Cd in sediments in February and March, resulting from reductive dissolution of Mn oxide mediation by high microbial activities, as shown by the similarities in distribution patterns of DGT-labile Cd and Mn. A two orders of magnitude increase in dissolved Cd concentrations (about 28 μg L−1) was observed in May and June, with dissolved Cd concentrations in overlying water about 110 times higher than the criteria continuous concentration set by Environmental Protection Agency. Hourly changes were found to coincide and correlate between dissolved Cd and dissolved organic matter (DOM) under simulated anaerobic conditions, strongly suggesting that the sudden outbreak of Cd pollution observed in the field resulted from the complexation of DOM with Cd in sediments. This was further supported by the NICA-Donnan model that more than 71% of dissolved Cd in the pore water in May and June was present as Cd-DOM complexes. Three components of DOM including humic-, tryptophan-, and tyrosine-like components in the sediments in June was identified using the fluorescence excitation emission matrix-parallel factor analysis. We found that Cd was stable complexed with tyrosine-like component. The Fourier transform infrared and two-dimensional correlation spectroscopy further revealed that Cd was bound to phenolic O H, alkene C C, alcoholic C O, aromatic C H, and alkene CH groups. Our study effectively promotes the understanding of Cd mobilization in sediments and highlights the risk of sudden Cd pollution events in the eutrophic lakes.

In this work, a novel electrochemical Hg2+ biosensor with high sensitivity and excellent reusability was presented. The sensor was based on tunable vertical single-walled carbon nanotubes (v-SWCNTs) and a target recycling strategy. A facile and scalable approach involving the conformational regulation of self-assembled monolayers was established for the fabrication of v-SWCNTs with tailored orientation and homogeneity. The obtained v-SWCNTs exhibited superior properties including a large specific area, high electrical conductivity, and excellent substrate binding strength, opening up a wide horizon for advanced electrochemical applications. Meanwhile, an efficient Hg2+ recycling strategy was designed using exonuclease III. In this strategy, a trace amount of Hg2+ triggered consecutive nicking reactions, and numerous report probes were released to bind with v-SWCNTs through π–π interactions. Based on the innovative design, an ultralow detection limit of 3 fM (S/N = 3), a wide linear range from 10 fM to 1 μM, high selectivity, and good reliability were achieved for a Hg2+ assay in water and serum samples using the prepared biosensor. Besides, due to the reversibility of π–π interactions, the stable v-SWCNT interface was regenerated for 50 consecutive measurements without obvious signal loss, making it a promising candidate for routine and efficient Hg2+ monitoring.

A new diffusive gradients in thin films technique (ZrO-AT DGT) with zirconium oxide, A-62 MP and T-42H resins containing in a single binding gel was developed for simultaneous measurement of nitrate (NO3-N), ammonium (NH4-N) and phosphate (PO4-P). The DGT uptake was found to be independent of pH variation from 3.2–8.7. Ionic strengths below 5, 10 and 750 mmol·L−1 NaCl did not affect DGT uptake of NH4-N, NO3-N and PO4-P, respectively. This new DGT was deployed in natural freshwater environments, with in situ measurements of the three nutrients found to be accurate. It ensured that rinsing the exposed surface of the DGT device at 3-day intervals can prevent biofouling. Additionally, a hybrid sensor comprising the novel DGT binding layer overlying an O2 planar optrode was tested in sediments to evaluate the dynamics of O2 and the three nutrients. Results showed that PO4-P and NO3-N fluxes decreased while fluxes of NH4-N increased under aerobic conditions. Nearly simultaneous variation in O2 and NO3-N was observed at the sediment-water interface (SWI) and transformation of NO3-N and PO4-P was found to be sensitively influenced by O2 dynamics.

Mobilization of trace metals in the rhizosphere of macrophytes is controlled by root-driven chemical changes, especially the steep gradients of O2 and pH from the rhizosphere to bulk sediments. Here, the O2 and pH dynamics, and the distribution of trace metal, in the rhizosphere of Vallisneria spiralis were obtained using planar optodes and diffusive gradients in thin films, respectively. Radial O2 loss (ROL) and acidification occurred on all visible roots of V. spiralis and exhibited highly spatiotemporal dynamics depending on the root growth and various environmental conditions. Trace metals showed different mobilization mechanisms in the rhizosphere. ROL and produced Fe(III) (oxyhydr)oxides decreased the mobility of Fe, As, Co, V and W in the rhizosphere. However, Mn, Ni and Cu exhibited greater mobility in the rhizosphere than bulk sediments as a result of the oxidation of metal sulfide and proton-induced dissolution of minerals. In particular, Co and Ni presented increased activity at the interface between rhizosphere and bulk sediment, which was attributed to the redox dissolution processes of Fe and Mn as a result of ROL and rhizosphere acidification. These results provide new insights into the roles of macrophyte root-induced O2 and pH changes in controlling trace metal mobility in sediments.

Phosphorus influence on arsenic bioavailability in soils and its toxicity to plants is widely recognized. This work compares competitive influence of P on As bioavailability in dry and flooded soils. Pot experiments were carried out in dry and flooded soils, respectively. Bioavailable As in soils was measured using diffusive gradients in thin films (DGT), soil solution concentration, and three single chemical extraction methods. P concentration at 50 mg/kg promoted wheat growth in dry soil. At concentrations above 50 mg/kg, P competition inhibited wheat growth and enhanced As toxicity. In flooded soil, the rice height and biomass decreased with the increase of P addition. P concentrations above 800 mg/kg were lethal to the rice. The content of As absorbed by wheat and rice roots as well as shoots increased with the increase of P concentration. The bioavailability of As in wheat- and rice-grown soils, determined by all methods, also increased with the increase of P concentration. The correlation analysis between the bioavailable As measured by the all three methods and the content of As in plants showed a significant positive correlation. The Pearson correlation coefficient for the DGT method was higher comparing to all other methods. DGT-induced fluxes in soils (DIFS) modeling further showed sharp decreases of Tc (the characteristic time to reach equilibrium between available solid As pool and soil solution As from DGT perturbation) and increases of desorption and adsorption rate constants (k1 and k−1) of As in P-amended soils, reflecting that the kinetic release of As from available solid As pools became much easy from P competition. P competition in both dry and flooded soils could significantly increase bioavailability of As and further increase its toxicity. Competition effect was more pronounced in flooded soil. DGT is a more accurate method for As bioavailability evaluation in both dry and flooded soils.

Stabilization/solidification (S/S) is a low-cost and time-efficient method for soil remediation, however, delayed hydration reactions and high carbon footprint are major limitations for the treatment of high-Pb contaminated soil. This study develops a novel and low-carbon approach that combines ground granulated blast furnace slag (GGBS) and ordinary Portland cement (PC) with phosphate-/sulphate-rich byproducts to produce low-alkalinity, high-compatibility, high-strength binary cement (BC) for S/S process. Results show that contaminated soil with a large fraction of exchangeable and soluble Pb (e.g., shooting range sites) severely disturbed the formation of hydration products in conventional S/S treatment, whereas BC system could mitigate the Pb interference via precipitation and sorption evenly distributed on the BC hydrates as shown by elemental mapping. Thus, BC presented superior environmental performance in terms of toxicity characteristic leaching procedure (TCLP) and semi-dynamic leaching tests, which were substantiated by quantitative X-ray diffraction and thermogravimetric analyses. The addition of potassium dihydrogen phosphate reduced TCLP leachability of Pb by 86.9% and Pb diffusion coefficients by 69.4% due to the formation of insoluble Pb3(PO4)2. Similarly, incinerated sewage sludge ash enhanced Pb stabilization, whereas waste phosphogypsum increased early strength via precipitation of PbSO4, although the effectiveness of physical encapsulation was compromised due to reduction in hydration products. Therefore, the proposed binary binders with selected additives present a new and low-carbon S/S treatment for high-Pb shooting range soil remediation.

Management of dredged sediment has been a worldwide challenge. Conventional off-site marine disposal of contaminated sediment incurs substantial cost implications and secondary pollution risks. This study developed value-added and time-effective approaches that transform contaminated sediment into construction materials to be used as fill materials, partition blocks, and paving blocks. The technical viability and economic feasibility of different binder/mixture designs and production methods were assessed. Thermogravimetric and X-ray diffraction analyses showed that contaminated sediment hindered hydrates formation and adversely influenced the properties of sediment-based products. Samples with 5 wt% binder could only fulfil strength requirements of fill materials. Tailored mixture designs with dry-mix/press method improved the precipitation reactions and densified the porous structure, which enhanced the performance and conformed to the requirements of partition blocks and even paving blocks. The leaching concentrations of potentially toxic elements and organic contaminants from the dredged sediment were substantially reduced by stabilization/solidification, which satisfied the leachability acceptance criteria for its on-site reuse. Moreover, cost and benefit assessment illustrated that the production of all three studied sediment-based products was potentially profitable, of which the scenario recycling sediment into paving blocks showed the highest profit. This novel solution can effectively mitigate burden of dredged sediment disposal and produce green construction materials in a sustainable way.

Navigational dredging is an excavation of marine/freshwater sediment to maintain channels of sufficient depth for shipping safety. Due to historical inputs of anthropogenic contaminants, sediments are often contaminated by metals/metalloids, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and other contaminants. Its disposal can present significant environmental and financial burdens. This study developed a novel and green remediation method for contaminated sediment using stabilization/solidification with calcium-rich/low-calcium industrial by-products and CO2 utilization. The hydration products were evaluated by quantitative X-ray diffraction analysis and thermogravimetric analysis. The incorporation of calcium carbide residue (CCR) facilitated hydration reaction and provided relatively high 7-d strength. In contrast, the addition of Class-F pulverized fly ash (PFA) and ground granulated blast furnace slag (GGBS) was beneficial to the 28-d strength development due to supplementary pozzolanic and hydration reactions. The employment of 1-d CO2 curing was found to promote strength development (98%) and carbon sequestration (4.3 wt%), while additional 7-d air curing facilitated cement rehydration and further carbonation in the sediment blocks. The leachability tests indicated that all studied binders, especially CCR binder, effectively immobilized contaminants in the sediments. The calcium-rich CCR and GGBS were regarded as promising candidates for augmenting the efficacy of CO2 curing, whereas GGBS samples could be applicable as eco-paving blocks in view of their superior 28-d strength. This study presents a new and sustainable way to transform contaminated sediment into value-added materials.

Iron redox cycle plays a primary role in controlling the mobility of P in sediments. It is crucial to better understand how lanthanum (La) modified bentonite (LMB, Phoslock®), an increasingly employed capping agent, immobilizes P from sediments by altering Fe redox-coupled P cycling. Batch adsorption experiments found that LMB effectively adsorbed Fe(II) with a capacity of 8.51 mg g−1. Fe(II)-preloaded LMB effectively retained P during a 518-hour equilibration, while up to 16.7% of adsorbed P was release-sensitive in LMB without Fe(II) preloading. A 60-day incubation experiment was performed using sediment cores, with an LMB amendment dosage of up to 200 LMB/Pmob (w/w, Pmob denotes the amount of mobile P in the surface 40 mm sediment layer). The concentrations of pore water soluble reactive P (SRP) and labile P were measured by high resolution dialysis (HR-Peeper) and by diffusive gradient in thin films (DGT), respectively, at a vertical millimeter scale. They stratified into static layers with extremely low concentration distribution in the top 16–22 mm sediments (mean SRP ≤ 0.28 mg L−1 and mean DGT-labile P ≤ 0.051 mg L−1) and active layers with decreased upward diffusion potential (≤5.85 for SRP and ≤12.7 for DGT-labile P) below the static layer, when the applied dosage reached 60 LMB/Pmob. The LMB amendment reduced the pore water Fe and DGT-labile Fe in sediments, while considerable amounts of Fe and Fe-bound P existed in the LMB binding layer (25% of the total P in 200 LMB/Pmob treatment). These findings show that the adsorption of Fe by LMB plays a significant role in the stabilization of LMB-bound P, possibly by adsorbing release-sensitive P initially bound to the rhabdophane surface. LMB adsorbed Fe and P were not released until the redox potential decreased to extremely reductive conditions (−150 mV to −300 mV), possibly due to the re-adsorption of Fe and P by LMB. This study reveals synergistic effects of Fe adsorption and provides new insight into the immobilization mechanisms of P by LMB application.

Herein, novel and versatile electrochemical aptasensors were constructed on a self-supported nanoporous gold (np-Au) microelectrode, integrating with an exonuclease III (Exo III) induced signal amplification strategy. Self-supported np-Au microelectrode with 3D bicontinuous nanoporous structures possesses tremendously large specific area, clean surface, high stability and biocompatibility, bringing about significant advantages in both molecular recognition and signal response. As paradigms, two analytes of bisphenol A (BPA) and ochratoxin A (OTA) were selected to demonstrate the superiority and versatility of designed aptasensors. Trace amounts of mDNA (associated with BPA or OTA concentration) hybridized with cDNA strands assembled on np-Au microelectrode, activating the cleavage reaction with Exo III. Thus, cDNA was digested and mDNA was released to undergo a new hybridization and cleavage cycle. Finally, residual cDNA strands were recognized by methylene blue labelled rDNA/AuNPs with the assistance of hDNA to generate the electrochemical signals, which were used to quantitatively monitor targets. Under the optimized conditions, prepared aptasensors exhibited wide linear ranges (25pg/mL to 2ng/mL for BPA, 10pg/mL to 5ng/mL for OTA) with ultralow detection limits (10pg/mL for BPA, 5pg/mL for OTA), excellent selectivity and stability, and reliable detection in real samples. This work opens a new horizon for constructing promising electrochemical aptasensors for environmental monitoring, medical diagnostics and food safety.

The effectiveness of sediment dredging for the control of internal phosphorus (P) loading, was investigated seasonally in the eutrophic Lake Taihu. The high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were used to measure the concentrations of soluble Fe(II) and soluble reactive P (SRP) as well as DGT-labile Fe/P in the non-dredging and post-dredging sediments. The P resupply kinetics from sediment solids were interpreted using DGT Induced Fluxes in Sediments (DIFS) modeling. The results showed no obvious improvement in water and sediment quality after dredging for 6years, due to their geographical proximity (a line distance of approximately 9km). However, dredging significantly decreased the concentrations of soluble Fe(II)/SRP and DGT-labile Fe/P in sediments, with effects varying at different depths below the sediment-water interface; More pronounced effects appeared in January and April. The diffusive flux of pore water SRP from sediments decreased from 0.746, 4.08 and 0.353mg/m2/d to 0.174, 1.58 and 0.048mg/m2/d in April, July and January, respectively. DIFS modeling indicated that the P retention capability of sediment solids was improved in April in post-dredging site. Positive correlations between pore water soluble Fe(II) and SRP as well as between DGT-labile Fe and P, reflect the key role of Fe redox cycling in regulating dredging effectiveness. This effect is especially important in winter and spring, while in summer and autumn, the decomposition of algae promoted the release of P from sediments and suppressed dredging effectiveness. Overall, the high-resolution HR-Peeper and DGT measurements indicated a successful control of internal P loading by dredging, and the post-dredging effectiveness was suppressed by algal bloom.

In this study, a Tulsion® CH-95 binding gel-based diffusive gradient in the thin film (TCH DGT) method was developed for the simultaneous measurement of CH3Hg+ and Hg2+. The binding kinetics of CH3Hg+ and Hg2+ in a mixed solution indicated rapid uptake to the TCH gel. The Hg2+ exhibited slower absorption rates compared to the CH3Hg+, which did not affect the DGT uptake overall. Stable elution efficiencies were obtained for CH3Hg+ and Hg2+ using a mild reagent containing 0.1 M HCL and 2% thiourea. The diffusion coefficients of CH3Hg+ and Hg2+ in agarose gel were calculated on the basis of the time-series deployment method and were 9.94 (×10−6 cm2 s−1, 25 °C) and 7.65 (×10−6 cm2 s−1, 25 °C), respectively. The DGT uptakes were independent of pH for CH3Hg+ (pH 4.10–8.10) and Hg2+ (pH 4.10–10.03), with no effect of the ionic strengths between 0.1–1000 mM NaCl. The capacity of TCH DGT for CH3Hg+ and Hg2+ according to linear and theoretical responses were calculated to be 33.97 μg cm−2 and 30.34 μg cm−2, respectively. The performance of this technique was further tested in sediment. Two distinct peaks of CH3Hg+ and Hg2+ appeared at similar sediment depths, reflecting that the localized remobilization of CH3Hg+ and Hg2+ may occur through similar mechanisms. This confirms that the TCH DGT is a useful in situ monitoring tool.

The release of phosphorus (P) from sediments may have significant influences on the concentration of water P and the trophic status of lakes. This study investigated the effects of temperature on the mobilization of P in sediments and its release to the overlying water in an incubation experiment and in the field of a shallow Lake Taihu (China). The ZrO-Chelex diffusive gradients in thin films (ZrO-Chelex DGT) and dialysis sampler (Peeper) were used to simultaneously measure labile P/Fe and soluble P/Fe in sediments at vertical resolutions of 2 mm and 4 mm, respectively. The results showed that the mean DGT-labile P and soluble reactive P (SRP) concentrations at 25 °C increased by 279% and 125% in comparison with those at 7 °C in the microcosm experiment, respectively, while in the field the proportions became 460% and 189% respectively. The microbial activity in sediments also increased with the rising temperature. Moreover, positive correlations (p

Microbial activity can enhance the sequestration of phosphorus (P) in sediments, but little is known about the mechanisms behind it. In this study, sediment cores were sampled from the most eutrophic Meiliang Bay of Lake Taihu, and three treatments were set up in a laboratory incubation experiment, involving (a) the non-treated sediment cores, (b) inoculation, and (c) sterilization. The dissolved and labile iron (Fe) and P were obtained by high-resolution dialysis and the diffusive gradients in thin films (DGT) technique, respectively. AgI-based DGT was used for measuring the 2D distribution of labile sulfide. The bacterial community was investigated using a scanning electron microscope and 16S rRNA high throughput sequencing technique. The results showed that sterilization reduced the capacity of sediment to immobilize P, and that the critical sediment depth layer for microbial P sequestration was 0-10 mm. In addition, sterilization or inoculation significantly changes the structure of bacterial communities. Fe or S oxidation under micro-aerobic or anaerobic conditions played an important role in bacterial retention of P in the sediments. Nitrate-reducing coupling Fe(II)-oxidizing bacteria (Acidovorax) in the inoculated sediment and electrogenic sulfur-oxidizing bacteria (Candidatus Electronema) in the non-treated sediment were identified as the key bacterial genera responsible for the retention of P in sediments. This implies that bacterial communities could quickly establish the ability for negative feedback regulation by inoculation once the function and structure of indigenous sediment bacteria are seriously impaired, although this needs further validation in the field.

Alpine ecosystem has a potential to intercept the transport of atmospheric metals, while the regulation mechanisms with variations in altitude and slope direction remain unclear. In this study, the soil and moss samples on the northern and southern slopes of Shennongjia Mountain were collected with altitude to quantitatively identify the sources of lead (Pb) and to decipher the regulation mechanisms of altitude and slope on the Pb distribution. The results showed that the concentrations of Pb decreased evidently with soil depth, and in the O (organic soils) and A (surface mineral soils) horizons they increased with altitude. The Pb isotopes and moss biomonitoring revealed that Pb was mainly from atmospheric deposition, and the sources included fossil fuel combustion, ore mining and smelting. Based on a binary mixing model of Pb isotopes, the percentage of atmospheric Pb in the O and A horizons and mosses averaged 58.8%, 43.7% and 71.0%, respectively. Atmospheric wet deposition strikingly controlled the distribution of soil Pb along the altitude. Canopy filtering and leaching also impacted the accumulation of Pb in the forest floor. The significant difference in the atmospheric Pb accumulation in the soils between the two slopes was not observed as expected, since atmospheric dry deposition from northwestern China contributed to the Pb accumulation on the northern slope according to the Pb isotopic ratios and air mass trajectories. The results of this study indicate that altitude determines the distribution pattern of atmospheric Pb, while slope direction screens the source region of Pb in alpine ecosystems.

Root-triggered microscale variations in O2 distribution in the rhizosphere of young Phragmites australis are important for nutrient removal in sediments. In this study, the micro-scale O2 dynamics and the small-scale changes of soluble reactive phosphorus (SRP) and ammonium (NH4+) in the rhizosphere of P. australis were investigated using planar optodes and high-resolution dialysis (HR-Peeper), respectively. Results suggested that root O2 leakage has a highly variable distribution depending on the stage of root growth, the site of O2 leakage gradually shift from the entire emerging main roots to the main root tip and subsequently shifted the emerging lateral roots. The O2 concentration increased in the rhizosphere with increasing light intensity and O2 levels in the overlying water. Continuous O2 release from the lateral roots causes the formation of iron plaque on the surface of lateral roots, which reduce the mobility of P by adsorption of iron plaque in the rhizosphere. The oscillation of oxic-anoxic root zones improves nitrogen removal through the processes of anammox, heterotrophic denitrification and nitrification. This work from the micro-scale demonstrates that the O2 concentration is the spatio-temporal variations in the rhizosphere, and it presents an important role for nutrient removal in sediments.

The combined use of capping (lanthanum modified bentonite; LMB) and oxidizing (calcium nitrate; CN) agents was investigated to immobilize arsenic (As) in sediments. The vertical changes in labile As and dissolved As were measured using diffusive gradients in thin films (DGT) and Rhizon devices. The results showed that the combined application of LMB and CN had the optimal effect on the immobilization of both DGT-labile As and dissolved As, compared to single treatments using LMB or CN. After 60 days of incubation, the maximum reduction efficiencies of DGT-labile As at sediment depths were 76.4%, 70.8%, and 44.9% of those treated with LMB + CN, CN, and LMB, respectively. After 32 days of incubation, the average concentrations of dissolved total As throughout the depths decreased from 7.71 μg/L after the control treatment without any amendments to 5.25, 4.03, and 3.15 μg/L after the addition of LMB, CN, and LMB + CN, respectively. The larger part of exchangeable As at sediment depths was converted into the reducible As mainly bound Fe/Mn oxide-hydroxides after combining LMB and CN. Due to the As(III) existing mainly in the form of electrically neutral H3AsO3 in sediments, it is hard to adsorb As(III) for the LMB and iron/manganese oxide-hydroxides formed by the oxidation effect of calcium nitrate. Thus, the single or combined LMB and CN use had much weaker effect on the immobilization of As(III) compared with As(V). The results of current study indicated that the combined use of LMB and CN could be a promising method to control the potential release of As from the sediment to the overlying water. However, this method needs further improvement to achieve a better immobilization effect on As(III) in sediments.

Cobalt (Co) cycling is often dominated by its role as a micronutrient in marine, while little is known on its cycling in a shallow eutrophic lake. Monthly sampling was performed in eutrophic Meiliang Bay of Lake Taihu, combining two laboratory control experiments and in situ Co limitation bioassay experiments. The high-resolution dialysis and the diffusive gradients in thin films technique were used to detect dissolved and labile Co, respectively. The positive correlations between dissolved/labile Co and Mn in the sediments for 6 or 7 months demonstrated that the mobility of Co in the sediments was primarily controlled by Mn redox cycling in the field. However, it is unexpected that the dissolved and labile Co only showed a small change over one year irrespective of the significant fluctuation in dissolved/labile Mn, with the concentrations being as low as 1.08 ± 0.22 μg/L and 0.246 ± 0.091 μg/L for dissolved and labile Co in the surface 20 mm sediment, respectively. Cyanobacterial bloom simulation and aerobic-anaerobic-cyanobacterial addition experiments indicated that the level of Co in the sediment-overlying water system was strongly regulated by cyanobacterial uptake, followed by the degradation of Co-enriched cyanobacterial biomass, which offset the influence of Mn redox cycling on Co mobility in the sediment. The significant enhancement of Microcystis spp. biomass by Co addition further indicated that Co was the potential limiting nutrient for cyanobacterial blooms. This work provides new ideas for better management strategies of eutrophication in shallow lakes.

Geo-engineering is recognized as a promising technique to reduce the release of phosphorus (P) from sediments to water. To investigate the efficiency of lanthanum modified bentonite (LMB), in this study, a microcosm experiment with LMB capping of sediment cores, was performed up to 110 days. High-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) were employed to measure soluble reactive P (SRP) and labile P in overlying water-sediment profiles, respectively, at spatial resolutions from millimeter to submillimeter scale. Results showed that the concentrations of SRP and DGT-labile P decreased with treatment time. A top static layer, with low concentration and small variation (

A millimeter-scale investigation is key to the understanding of the competitive effects of phosphate(P) on arsenic(As) mobility in sediments by taking the great biogeochemical heterogeneity of the sediments into consideration. In this study, a microcosm experiment was performed in this aspect using high-resolution dialysis and diffusive gradients in thin films (DGT) to simultaneously measure dissolved and labile P, As, and iron (Fe) in sediments, respectively. With the increase of P content in water from 0.02 mg L −1 to 0.20 and 2.4 mg L −1 , consistent release of As from sediments was observed. The concentrations of DGT-labile As increased significantly especially in the upper sediment layer (up to 12 times of the 0.02 mg P L −1 treatment) due to the competition of phosphate, which corresponded well to the increase in DGT-labile P. There was limited increase in dissolved P and As contents due to the buffering provided by sediment solids, while the concentrations of both dissolved and DGT-labile Fe in sediments decreased. A stoichiometric calculation showed that 47% and 8% of the added P were removed through Fe(II) precipitation for the 0.20 and 2.4 mg P L −1 treatments, respectively, which greatly suppressed the release of As induced by P competition for the 0.20 mg P L −1 treatment. The DGT-induced fluxes in sediments (DIFS) modeling showed an increase in solid resupply to pore water As from elevation of water P through the increases of the desorption rate constant from 5.4 to 31( × 10 −7 ) s −1 and the sorption rate constant from 1.8 to 22( × 10 −4 ) s −1 .