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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. [ABSTRACT FROM AUTHOR]

Abstract 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. Graphical abstract Image 1 Highlights • Dredging effects change during the different seasons as reflected by soluble and labile concentrations. • Dredging was most effective in April, July and October and had a negative effect in January. • Efficacy of Co was affected by the reducible and residual fractions in sediments. • Efficacies of Zn and Ni were respectively affected by reducible and residual fractions. [ABSTRACT FROM AUTHOR]

Abstract 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. Graphical abstract Image 1 Highlights • Monthly distribution of labile P concentration correlated with labile Fe concentration in sediments. • Hourly change of SRP correlated with soluble Fe(II) concentrations under anoxic conditions. • Phosphorus was not released during the reductive dissolution of Mn oxides. • Consistent changes in P and Fe but not in Mn occurred in algal bloom sediments. • Fe but not Mn redox reactions should control mobilization of P in sediments. Evidence that Fe redox reactions play a key role in controlling P mobilization in sediments, while Mn redox reactions do not have a significant influence. [ABSTRACT FROM AUTHOR]

Abstract This study assessed the effectiveness of dredging in controlling arsenic (As), selenium (Se), and antimony (Sb) contamination in sediments, by examining contaminant concentrations in sediments six years after dredging was completed. High-resolution diffusive gradients in thin films (DGT) and dialysis (HR-Peeper) techniques were used to monitor the concentrations of DGT-labile metalloids and soluble metalloids in sediments, respectively. Results revealed that dredging effectively remediated metalloid contamination in sediments only in April, July and/or January. Compared to non-dredged sediments, the concentrations of soluble and DGT-labile As, Se, and Sb in dredged sediments decreased on average by 42%, 52%, and 43% (soluble), and 54%, 50%, and 53% (DGT), respectively. The effectiveness of the dredging was primarily due to the transformation of metalloids from labile to inert fractions, which increased the ability of the sediments to retain the metalloids, and the slowed rate of resupplied metalloids from available solid pools. In contrast, negligible/negative effects of dredging were seen in October, and the concentrations of soluble and DGT-labile metalloids even increased in some profiles of dredged sediments. This was mainly caused by a release of the metalloids from algal degradation, which may offset the dredging effectiveness. Graphical abstract Image 1 Highlights • Dredging effects varied by season, reflected by metalloid concentrations. • Dredging was most effective in April, July and/or January. • Efficacy due to higher fraction of inert metalloids with slower kinetic response. • Dredging had negligible and negative effects in October. • Algal decomposition and metalloid release suppressed dredging efficacy. [ABSTRACT FROM AUTHOR]

Algal growth causes a drastic change in aquatic conditions over a diel cycle, which may induce sensitive feedback systems in sediments, causing P release. In this study, a microcosm experiment was performed using a suction sampler (Rhizon) to observe changes in soluble reactive phosphorus (SRP) and soluble Fe(II) concentrations in the top 20 mm sediment layer on a 3-h time interval, at different phases of harmful algal bloom (HAB) development. The results showed that the algal blooms prevailed up to 15 days after incubation, after which the process of bloom collapse proceeded until the 70th day. The concentrations of pore-water soluble Fe(II) and SRP increased throughout the incubation period. Compared to day 1, maximum increases of 214% in soluble Fe(II) and 387% in SRP were observed at night during the bloom and collapse periods, respectively. The diffusive fluxes of Fe and P at the sediment-water interface (SWI) generally corresponded to their changes in concentrations. Hourly fluctuation in soluble Fe(II) and SRP concentrations were observed with two distinct concentration peaks occurred at 21:00 p.m. and 06:00 a.m. (or 03:00 a.m.), respectively. These findings suggest that Fe-P coupling mechanisms are responsible for the release of P from sediments. During the collapse period, soluble Fe(II) concentrations were suppressed by the increase of labile S(-II) at night. Meanwhile, SRP concentrations were decoupled from Fe cycling with small fluctuations (<11% RSD) on an hourly timescale, and the decomposition of algae was a dominant source contributing to the release of P from sediments. These results significantly improved the understanding of processes and mechanisms behind the stimulated release of P from sediments during HABs. [ABSTRACT FROM AUTHOR]

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 6 years, due to their geographical proximity (a line distance of approximately 9 km). 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 0.353 mg/m 2 /d to 0.174, 1.58 and 0.048 mg/m 2 /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. [ABSTRACT FROM AUTHOR]

The release of arsenic (As) from sediments poses a risk to human health especially at high pH levels. Despite this, the distribution and kinetic response of mobile As remains unclear under varying pH conditions. In this study, a microcosm incubation experiment was performed, using sediment cores in combination with dialysis (Peeper) and thin film diffusive gradients (DGT), to investigate the distributions of mobile As (soluble As in pore water and DGT-labile As) at high vertical resolutions (2-4 mm). Results show that the concentrations of soluble As present in the water column increased 1.5-fold with an increase in pH from 5.4 to 11.2. Both soluble As in pore water and DGT-labile As exhibited stable low-level distributions in the uppermost layer beneath the sediment-water interface, followed by increasing concentration distribution with decreasing layers to middle depths. The mean concentrations of mobile As species increased with increased water pH in both sediment profiles and with upward diffusion gradients, showing a 0.2-fold increase of soluble As in the top 20-mm layer and a 0.6-fold increase in deeper 20-52-mm layers, while DGT-labile As showed a 1.0- and 1.1-fold increase in these two layers, respectively. Modeling of DGT-induced flux in sediments (DIFS) showed that the desorption rate constant increased more rapidly than the absorption rate constant, resulting in the increased availability of solid As pools, therefore resupplying the soluble As in pore water from sediments. [ABSTRACT FROM AUTHOR]

Antimony (Sb) is more mobile in lacustrine sediments with seasonal warming. However, the mechanisms of Sb mobility in sediments are still unclear, especially considering the interactions among Sb, iron (Fe), manganese (Mn), and dissolved organic matter (DOM). In this study, high-resolution dialysis (HR-Peeper) and multi-spectral techniques simultaneously investigated changes in Sb, Fe, Mn, and DOM in two different ecological types (algal and grass) sediments with increasing temperature. We found that the dissolved Sb rapidly increased with the increase in temperature. The oxidation of Sb(III) to Sb(V) by Fe/Mn oxides in oxygen (O 2) rich overlying water and surface sediment layers was one of the reasons for Sb concentration enhancement in pore water. Further, using excitation-emission matrix and parallel factor analysis (EEM-PARAFAC), synchronous fluorescence (SF) spectroscopy, fourier transform infrared (FTIR) spectroscopy, and two-dimensional correlation spectroscopy (2D-COS) revealed that complexation with DOM was the other reasons for Sb concentration increasing in sediments. This was demonstrated by the similar distribution pattern and significant correlation between Sb and tryptophan-like components. Titration experiments further revealed that Sb was more stably bound to tryptophan-like components in the aromatic C–H (660 cm−1), alcoholic C–O (1115 cm−1), alkene C C (1615 cm−1), and carboxylic acid OH (3390 cm−1) groups. The tryptophan-like components from the algae region had a higher binding force than that from the macrophytes region. Our study effectively promotes an understanding of Sb mobilization in lacustrine sediments. [Display omitted] • Dissolved Sb rapidly increased with the increase in temperature. • The migration of Sb in sediments was mainly controlled by the Fe and Mn oxides. • Complexation with DOM was the other reason for Sb increasing in sediments. • Sb(V) was more stably bound to tryptophan-like components. [ABSTRACT FROM AUTHOR]

The effects of bivalve ( Corbicula fluminea ) bioturbation on the lability of phosphorus (P) in sediments were investigated. The high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were employed to obtain soluble and labile P/Fe profiles at a vertical resolution of 2 and 1 mm, respectively. The bivalve bioturbation increased the concentrations of soluble reactive P (SRP) in pore water and DGT-labile P up to 116% and 833% of the control within the sediment depths from the sediment water interface (SWI) to − 64 mm and − 44 mm, respectively. The sediments with bioturbation had a smaller distribution coefficient than the control (1964 vs. 3010 cm 3 g − 1 ), reflecting a weaker ability in retaining P. Meanwhile, the sediments with bioturbation had a greater ratio of the concentration of DGT-labile P to that of SRP (0.20 vs. 0.03), demonstrating a stronger ability to resupply pore water SRP by the sediment solids when they are affected by the bioturbation. The DGT-induced fluxes in sediments (DIFS) modeling further showed a much shorter response time (277.9 vs. 18,670 s) and a much higher rate (0.192 vs. 0.002 day − 1 ) of the solids in release of P with the bioturbation. Correspondingly, the flux of P to the overlying water from the bioturbation treatment increased up to 157% of the control. The bivalve bioturbation significantly increased the concentrations of soluble Fe(II) and DGT-labile Fe up to 84% and 334% of the control from the SWI to − 46 mm, respectively. The SRP and DGT-labile P were highly correlated with respective soluble and DGT-labile Fe. It was concluded that the release of P from the sediments with bioturbation to the pore water and the overlying water was promoted by the reductive dissolution of easily reducible Fe(oxyhydr)oxides due to the depletion of oxygen in the top sediments from bivalve respiration. [ABSTRACT FROM AUTHOR]

The effects of chironomid larvae bioturbation on the lability of phosphorus (P) in sediments were investigated through sediment incubation for 140 days. High-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were applied to obtain soluble and labile P/Fe profiles at a millimeter resolution, respectively. The larvae bioturbation decreased concentrations of soluble/labile P and Fe by up to over half of the control at the sediment depths of influence up to 70 and 90 mm respectively. These effects continued over 116 days and disappeared on the 140th days due to eclosion of chironomid larvae. Labile P was highly correlated with labile Fe, while a weak correlation was observed between soluble P and soluble Fe. It was concluded that Fe(II) oxidation and its enhanced adsorption were the major mechanisms responsible for the decreases of soluble and labile P. [ABSTRACT FROM AUTHOR]

Root-triggered microscale variations in O 2 distribution in the rhizosphere of young Phragmites australis are important for nutrient removal in sediments. In this study, the micro-scale O 2 dynamics and the small-scale changes of soluble reactive phosphorus (SRP) and ammonium (NH 4 +) in the rhizosphere of P. australis were investigated using planar optodes and high-resolution dialysis (HR-Peeper), respectively. Results suggested that root O 2 leakage has a highly variable distribution depending on the stage of root growth, the site of O 2 leakage gradually shift from the entire emerging main roots to the main root tip and subsequently shifted the emerging lateral roots. The O 2 concentration increased in the rhizosphere with increasing light intensity and O 2 levels in the overlying water. Continuous O 2 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 O 2 concentration is the spatio-temporal variations in the rhizosphere, and it presents an important role for nutrient removal in sediments. [Display omitted] • Micro-scale O 2 dynamics in the rhizosphere of P. australis was studied by planar optode. • Radial O 2 loss was highly spatio-temporal dynamic as the roots grow. • Radial O 2 loss was associated with irradiance and O 2 level in the overlying water. • The nutrient reduction in the rhizosphere attribute to the radial O 2 loss. O 2 release from the lateral roots contributed to the oxidizing environment in the rhizosphere of P. australis , and supported the removal of excess nutrients in sediments. [ABSTRACT FROM AUTHOR]

Lake eutrophication and algal blooms may result in the mortality of macrozoobenthos. However, it is still not clear how macrozoobenthos decomposition affect phosphorus (P) mobility in sediments. High-resolution dialysis (HR-Peeper) and the diffusive gradients in thin films (DGT) technique were used in this study to assess the dissolved organic matter (DOM), dissolved/DGT-labile iron (Fe), P, and sulfur (S(–II)) profiles at a millimeter resolution. The decomposition of Bellamya aeruginosa significantly increased the internal loading of sediments P. The Fe(III) and sulfate were reduced under anaerobic conditions and promoted P desorption from sediments. This was supported by the significant increase in DGT-labile S(–II) and dissolved/DGT-labile P, Fe(II) and the significant positive correlation between Fe and P on day 8. The simultaneous increase in DOM and soluble reactive phosphorus (SRP) and the significant positive relationship between these factors were observed during the decomposition of B. aeruginosa. This suggested that complexation of DOM with metals may promotes the release of P from sediments. [Display omitted] • Decomposition of benthic organisms significantly increased sediment P levels. • Reduction of Fe(III) and sulfate promoted P release from sediments. • DOM complexes with metals reduced sediment P immobilization. During macrozoobenthos decomposition, the reduction and dissolution of Fe(III) oxides and the complexation of DOM with metal ions caused the massive release of P from sediments and increased the risk of lake eutrophication. [ABSTRACT FROM AUTHOR]

This study investigated seasonal variations in spatial distribution, mobilization kinetic and toxicity risk of arsenic (As) in sediments of three representative ecological lakes in Lake Taihu. Results suggested that the bioavailability and mobility of As in sediments depended on the lake ecological types and seasonal changes. At the algal-type zones and macrophyte-type zones, elevated As concentrations were observed in April and July, while these occurred at the transition areas in July and October. The diffusion flux of soluble As ranged from 0.03 to 3.03 ng/cm2/d, indicating sediments acted as a source of As. Reductive dissolution of As-bearing iron/manganese-oxides was the key driver of sediment As remobilization. However, labile S(-II) caused by the degradations of algae and macrophytes buffered sediment As release at the algal-type and macrophyte-type zones. Furthermore, the resupply ratio was less than 1 at three ecological lakes, indicating the resupply As capacity of sediment solid phase was partially sustained case. The risk quotient values were higher than 1 at the algal-type zones and transition areas in July, thereby, the adverse effects of As should not be ignored. This suggested that it is urgently need to be specifically monitored and managed for As contamination in sediments across multi-ecological lakes. [Display omitted] • As distribution in sediments are dependent on the lake types and seasonal changes. • Seasonal warming and low DO concentrations lead to increases in As mobility. • Increases in labile S(-II) in sediments can buffer the mobility and release of As. • High As ecotoxicological risk occurs at algal-type lake zones and transition areas. [ABSTRACT FROM AUTHOR]

Sediments have a heterogeneous distribution of labile redox-sensitive elements due to a drastic downward transition from oxic to anoxic condition as a result of organic matter degradation. Characterization of the heterogeneous nature of sediments is vital for understanding of small-scale biogeochemical processes. However, there are limited reports on the related specialized methodology. In this study, the monthly distributions of labile phosphorus (P), a redox-sensitive limiting nutrient, were measured in the eutrophic Lake Taihu by Zr-oxide diffusive gradients in thin films (Zr-oxide DGT) on a two-dimensional (2D) submillimeter level. Geographical information system (GIS) techniques were used to visualize the labile P distribution at such a micro-scale, showing that the DGT-labile P was low in winter and high in summer. Spatial analysis methods, including semivariogram and Moran’s I, were used to quantify the spatial variation of DGT-labile P. The distribution of DGT-labile P had clear submillimeter-scale spatial patterns with significant spatial autocorrelation during the whole year and displayed seasonal changes. High values of labile P with strong spatial variation were observed in summer, while low values of labile P with relatively uniform spatial patterns were detected in winter, demonstrating the strong influences of temperature on the mobility and spatial distribution of P in sediment profiles. [ABSTRACT FROM AUTHOR]

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 < 0.001) existed between DGT-labile P and labile Fe and between SRP and soluble Fe under different temperature, implying that the mobilization of P and its release to the overlying water was dominantly controlled by Fe redox cycling. This study provided strong evidence that the rising temperature enhanced the microbial reduction of P-hosted Fe (oxyhydr)oxide, while an on-site accumulation and degradation of algal material in the sediment surface may lead to a greater effect on P mobilization. [ABSTRACT FROM AUTHOR]

There is a great heterogeneity in the distribution of mobile phosphorus (P) in natural sediments, while the assessment of P immobilization by amendment agents has mostly neglected this feature. In this study, the effects of aluminum sulfate (ALS) on P immobilization were investigated via a 60-day microcosm experiment using sampled sediment cores. The high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were first employed to establish the profiles of soluble reactive P (SRP) and DGT-labile P in aluminum-amended sediments at 2 and 1 mm resolutions, respectively. Both concentrations of two mobile P forms decreased with increasing ALS dosages. The static layers with extremely low P concentrations (≤0.060 mg L −1 for SRP and ≤0.024 mg L −1 for DGT-labile P) were observed in the upper 6–16 mm sediment layers at 6–15 ALS/P mobile treatments (mobile P is the sum of NH 4 Cl-P, BD-P, and NaOH-nrP; mol mol −1 ). The static layer was followed by an active layer in which the upward release potential ( R AL ) decreased from 33.4 to 21.9 for SRP and from 92.2 to 51.0 for DGT-labile P, respectively. As the formation of the static layer is a key to sustaining P immobilization in sediments, the minimal dosage of 9 ALS/P mobile is required for ALS capping. Modeling with DGT Induced Fluxes in Sediments (DIFS) showed a greater increase in adsorption rate constant ( k 1 , maximum 7.2-fold) compared to adsorption rate constant ( k 1 , maximum 2.2-fold), demonstrating that the release of P from sediment solids became increasingly difficult after ALS amendment. [ABSTRACT FROM AUTHOR]

Arsenic (As)-contaminated water restoration is extremely challenging because As remobilization from sediments can result in episodic or long-term release of As to the overlying water. In this study, by combining high-resolution imaging techniques with microbial community profiling, we examined the feasibility of utilizing the rhizoremediation of submerged macrophytes (Potamogeton crispus) to decrease As bioavailability and regulate its biotransformation in sediments. Results showed that P. crispus considerably decreased the rhizospheric labile As flux to lower than 4 pg cm−2 s−1 from larger than 7 pg cm−2 s−1, suggesting its effectiveness in promoting As retention in sediments. Iron plaques induced by radial oxygen loss from roots decreased the mobility of As by sequestering it. Additionally, Mn-oxides may act as an oxidizer for the oxidation of As(III) to As(V) in the rhizosphere, which can further increase the As adsorption owing to the strong binding affinity between As(V) and Fe-oxides. Furthermore, microbially mediated As oxidation and methylation were intensified in the microoxic rhizosphere, which decreased the mobility and toxicity of As by changing its speciation. Our study demonstrated that root-driven abiotic and biotic transformation contribute to As retention in sediments, which lays a foundation for applying macrophytes to the remediation of As-contaminated sediments. [Display omitted] • Abiotic and biotic oxidation of Fe(II) promoted the formation of Fe plaques. • Fe plaque sorption decreased the bioavailability of As in the rhizosphere. • Oxidation of As(III) was promoted by Mn-oxides and aio A gene-related bacteria. • Activity of microbial communities involved in As methylation was elevated. [ABSTRACT FROM AUTHOR]

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 . [ABSTRACT FROM AUTHOR]

Submerged plants and lanthanum-modified bentonite (LMB) have important applications for the remediation of contaminated sediments; however, their combined effect on arsenic (As) removal has not been comprehensively evaluated. In this study, the physicochemical properties and changes in soluble As in sediments treated with LMB, Vallisneria spiralis (V. spiralis), and LMB + V. spiralis were observed at three time points (days 15, 35, and 66), and the changes in microbial and As species in sediments on day 66 were analyzed. LMB + V. spiralis treatment was the most effective for As removal. On day 66, the average concentrations of soluble As at a depth of 0–100 mm decreased by 12.71%, 48.81%, and 59.73% following treatment with LMB, V. spiralis , and LMB + V. spiralis , respectively. Further analysis showed that LMB is more effective at removing As(V) than V. spiralis , while V. spiralis is more effective at removing As(III), and the combination of LMB + V. spiralis is more effective for removing both As(III) and As(V) than individual LMB and V. spiralis treatments. LMB + V. spiralis enhanced the transformation of mobile As to Fe 2 O 3 /oxyhydroxide–bound As in sediments and the activity of As-oxidizing microorganisms. LMB promoted the growth of V. spiralis and enhanced the removal of As. This study indicates that this combination is an effective method for removing mobile As from sediments, and could effectively inhibit the release of As from sediments to overlying water. • V. spiralis had better immobilization effect on As(Ⅲ) compared with LMB. • Combined LMB and V. spiralis could reduce the flux of As. • V. spiralis enhanced arsenic uptake via LMB for growth promotion. • Combined LMB and V. spiralis had great removal effect on As(Ⅲ) and As(Ⅴ). [ABSTRACT FROM AUTHOR]

• Macrofauna density has dropped by 93% in Lake Taihu from 2007 to 2020. • Ventilation causes highly dynamic redox oscillation in the burrows of G. taihuensis. • G. taihuensis increases O 2 penetration depth by more than 20 mm via bio-irrigation. • G. taihuensis bioturbation significantly increases the P retention in sediments. • Sharp decline in macrofauna density partly leads to high water column P level. 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/m2 in 2007 to 345 ± 447.63 ind/m2 in 2020 due to the frequent bottom-water hypoxia in Lake Taihu. The reduced macrofauna density majorly resulted from Grandidierella taihuensis, Limnodrilus hoffmeisteri , and Tanypus chinensis larvae, and their total density decreased by approximately 97% in 2020 compared to 2007. G. taihuensis , one of the major benthic faunas, was further used as a representative to investigate the effects of bioturbation on sediment P release using high-resolution sampling and imaging techniques. The results show that G. taihuensis can increase the O 2 penetration depth by more than 20 mm through bio-irrigation, and causes the redox conditions in burrows and surrounding sediments to change dramatically within a few minutes due to the intermittent ventilation. Subsequent oxidation of the soluble Fe(II) led to the formation of Fe-oxide bound P in the surface sediments, thereby increasing the P retention in the sediments. When the G. taihuensis density was 1563 ind/m2 at the sampling site, approximately 0.12 g m−2 yr−1 P can be retained in sediments. As previous studies have shown that L. hoffmeisteri and T. chinensis played a similar role in increasing the P retention in sediments through their bioturbation activities, the sharp decline in benthic fauna density and burrowing activities in Lake Taihu should be an important reason for maintaining the high P level in the water column by decreasing the P retention in sediments. [Display omitted] [ABSTRACT FROM AUTHOR]

Phosphorus (P) availability is closely related to the distributions of pH, O 2 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, O 2 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). [Display omitted] • Combination of pH/O2 PO, zymography and DGT imaging techniques applied. • Increased phosphatase activity and acidification appeared when V. natans was young. • DGT-labile P accumulated in the rhizosphere when V. natans was young. • DGT-labile P depletion co-occurred with increased ACP when roots matured. • Roots could acquire P via organic acid complexation of Fe(Ⅲ) from Fe plaque. [ABSTRACT FROM AUTHOR]

A two-dimensional (2D) imaging technique, planar optode (PO) technique, is developed to investigate biogeochemical processes at a high resolution in heterogeneous sediments and soils, especially at the sediment-water interface. Compositional changes with depth in sediments are usually considered to be relatively steady. However, the activities of benthic organisms can generate significant heterogeneity and complex 3D transport-reaction patterns over millimeter to meter scales in the surficial sediments. Spatial and temporal quantification of the variables such as O 2 in the bioturbated sediments promoted the development of the planar optode. Different planar optode techniques including composite planar optode for multi-analytes and the combination of planar optode with diffusive gradients in thin films (DGT) have been developed and used to study the 2D solutes distributions and/or dynamics of O 2 , pH, partial pressure of CO 2 (p CO 2), temperature, exoenzymes, and metal/metalloid ions in sediments and soils. New findings observed in laboratory-based microcosm experiments and field-based in situ studies using planar optode techniques, have significantly improved our understanding of heterogeneous biogeochemical reactions and processes. In this article we provide a critical review on the: 1) research progress using planar optode techniques; 2) principles, configurations, and devices used for planar optode systems; 3) characteristics and interferences associated with planar optode measurements; and 4) applications of planar optode in the environment. We have suggested the barriers, advantages, and research needs for the use of planar optodes. Unlabelled Image • The research progresses of 2D imaging PO technique were reviewed. • The principles, configurations and devices used for PO system were introduced. • The characteristics and interferences seen with PO measurements were evaluated. • PO applications in combination with several research cases were discussed. [ABSTRACT FROM AUTHOR]