Successful control of phosphorus release from sediments using oxygen nano-bubble-modified minerals

Multi-imaging platform for rhizosphere studies: Phosphorus and oxygen fluxes

Rhizosphere is a soil volume of high spatio-temporal heterogeneity and intensive plant-soil-microbial interactions, for which visualization and process quantification is of highest scientific and applied relevance, but still very challenging. A novel methodology for quick assessment of two-dimensional distribution of available phosphorus (P) in rhizosphere was suggested, tested, and development up to the application platform. Available P was firstly trapped by an in-situ diffusive gradients in thin-films (DGT) sampler with precipitated zirconia as the binding gel, and subsequently, the loaded gel was analyzed with an optimized colorimetric imaging densitometry (CID). The imaging platform was established linking: i) DGT, ii) planar optode, and iii) soil zymography techniques to simultaneously determine available P, oxygen, and acid phosphatase in rhizosphere at sub-millimeter spatial scales. The DGT identified available P level in rice rhizosphere were spatially overlapping to the localized redox hotspots and phosphatase activity. The spatial relationship between available P and acid phosphatase activity was dependent on root development. The root radial oxygen loss (ROL) remained active during the experimental observations (2-3 days), while a flux of available P of 10 pg cm-2 s-1 was visualized within 2-3 mm of roots, confirming the correlative response of rice roots to oxygen secretion and P uptake. Summarizing, the established imaging platform is suitable to capture spatial heterogeneity and temporal dynamics of root activities, nutrient bioavailability, ROL and enzyme activities in rhizosphere.

Simultaneous immobilization of heavy metals and nutrient elements in contaminated sediment using a novel composite agent product

In this research, an innovative type of sediment resource treatment agent (SRA) was synthesized successfully, which could immobilize ammonia nitrogen (NH₃-N), total phosphorus (TP), potassium (K), and simultaneously stabilize cadmium (Cd), lead (Pb), chromium (Cr), copper (Cu), nickel (Ni), and zinc (Zn) in dredged sediment. The effects of SRA dosage on stabilizing the nutrient elements and heavy metals were investigated. The results demonstrated that the increase of SRA dosage significantly enhanced the stabilization of nutrients and heavy metals. The 14-day rainwater infiltration and rainwater scouring experiments were carried out. With the simulation test of rainwater infiltration, the stabilization ratios of Cr, Cu, Ni, Pb, Zn, Cd, NH₃-N, TP, and K with 2% SRA addition reached 80.8%, 76.8%, 80.3%, 77.5%, 78.0%, 72.7%, 64.3%, 73.9%, and 73.9%, respectively. Under the action of rainwater scouring, the stabilization ratios of Cr, Cu, Ni, Pb, Zn, Cd, NH3-N, TP, and K with 6.4% SRA addition reached 84.6%, 84.0%, 77.6%, 87.3%, 80.0%, 61.5%, 76.2%, 77.8%, and 91.7%, respectively. Therefore, the results demonstrate that SRA is an excellent composite material in stabilizing heavy metals while reserving the nutrients in dredged sediment, thus showing great potential in the application for dredged sediment resource treatment.

Oxygen micro-nanobubbles for mitigating eutrophication induced sediment pollution in freshwater bodies

Sediment hypoxia is a growing problem and has negative ecological impacts on the aquatic ecosystem. Hypoxia can disturb the biodiversity and biogeochemical cycles of both phosphorus (P) and nitrogen (N) in water columns and sediments. Anthropogenic eutrophication and internal nutrient release from lakebed sediment accelerate hypoxia to form a dead zone. Thus, sediment hypoxia mitigation is necessary for ecological restoration and sustainable development. Conventional aeration practices to control sediment hypoxia, are not effective due to high cost, sediment disturbance and less sustainability. Owing to high solubility and stability, micro-nanobubbles (MNBs) offer several advantages over conventional water and wastewater treatment practices. Clay loaded oxygen micro-nanobubbles (OMNBs) can be delivered into deep water sediment by gravity and settling. Nanobubble technology provides a promising route for cost-effective oxygen delivery in large natural water systems. OMNBs also have the immense potential to manipulate biochemical pathways and microbial processes for remediating sediment pollution in natural waters. This review article aims to analyze recent trends employing OMNBs loaded materials to mitigate sediment hypoxia and subsequent pollution. The first part of the review highlights various minerals/materials used for the delivery of OMNBs into benthic sediments of freshwater bodies. Release of OMNBs at hypoxic sediment water interphase (SWI) can provide significant dissolved oxygen (DO) to remediate hypoxia induced sediment pollution Second part of the manuscript unveils the impacts of OMNBs on sediment pollutants (e.g., methylmercury, arsenic, and greenhouse gases) remediation and microbial processes for improved biogeochemical cycles. The review article will facilitate environmental engineers and ecologists to control sediment pollution along with ecological restoration.

Effect of aeration on water quality and sediment humus in rural black-odorous water

Black-odorous water, which exists widely in rural areas of China, affects the living environments of residents and causes a loss of ecosystem functions, and improper treatment may even cause secondary pollution. Moreover, the transformation relationships among the components of humus in the sediments of black-odorous water are largely unknown. Therefore, it is necessary to select appropriate methods for treating black-odorous water in rural areas and to understand the characteristics of humus in sediment during the treatment process. In this study, the use of aeration in treating black-odorous water and interstitial water in rural areas was studied and the effects of different treatment methods on the contents and spectral characteristics of humus components in sediments were explored. It was found that the quality of the treated water improved from severe black-odorous to nonblack-odorous, the colour of the sediment changed from black to greyish-brown to turquoise, and the odour changed from strong to weak. The removal rates of ammonia nitrogen, total nitrogen, total phosphorus, and chemical oxygen demand in the sediment aeration group reached more than 50% in the upper water, and more than 70% in the interstitial water. After treatment, the content of fulvic acid, the main black substance in sediment, decreased by 0.36-1.58 g/kg, and the molecular structure of the humus was simplified.

Organic phosphorus regeneration enhanced since eutrophication occurred in the sub-deep reservoir

Lake eutrophication remains a serious environmental problem of global significance, and phosphorus (P) plays a key role in lake eutrophication. Internal P loading, as a result of P release from sediments, is gathering more and more recognition as an important source governing the P availability in these ecosystems. Anoxic condition can promote the release of P associated with Fe oxides, which has already been a consensus. However, it is still unknown whether the anoxic conditions induced by eutrophication act to intensify or weaken the regeneration of organic P (P_org) in sediments. We selected the Hongfeng Reservoir, a typical sub-deep lake, to study the regeneration behaviours of C and P in the sediments buried before and after eutrophication. The results showed that P_org did not significantly increase with the rapid increase in organic C (C_org) since eutrophication occurred. Furthermore, the organic C/P ratio was much higher in sediments buried after eutrophication than in those buried before, which indicated that P_org regeneration had been significantly enhanced since eutrophication occurred. Based on C/P ratios, our estimation suggested that the P_org regeneration and P release from sediment to water approximately enhanced 45.2% ± 8.7% and 34.5% ± 9.8%, respectively. Elevated primary productivity (algae) and the corresponding hypoxic/anoxic condition, both caused by eutrophication, promoted P biogeochemical cycle in the sub-deep reservoir. This study further verifies the significant contribution of regenerated P_org to the internal P load, and highlights the importance of controlling P release from sediments in order to restore clear water ecosystems in sub-deep lakes or reservoirs.

Oxygen Nanobubbles for Lake Restoration—Where Are We at? A Review of a New-Generation Approach to Managing Lake Eutrophication

Nutrient enrichment of lakes from anthropogenic activities is a significant and increasing issue globally, impairing the health, biodiversity and service provisioning from lakes, with impacts on cultural, recreational, economic and aesthetic values. Internal nutrient loads from lakebed sediment releases are a primary cause of lake eutrophication and have necessitated geoengineering methods to mitigate releases and speed up recovery from eutrophication. Our objective in this review was to evaluate the use of oxygen nanobubbles as a geoengineering technology to remediate low oxygen conditions at the lake sediment/water interface, as a precursor to alleviating eutrophication linked to high internal nutrient loads. Oxygen nanobubbles (NBs) are bubbles < 1000 nm formed at the interface of solid surfaces and aqueous solutions. These bubbles have higher density than water, persist for longer and facilitate greater oxygen solubility than larger bubbles. Methods have been developed to enable NB formation at the surface of carrier materials, which are then used in conjunction with modified local soils (MLSs), to ‘floc, lock and oxygenate’ to strip nutrients from the water column, locking them in lakebed sediments and oxygenating the sediments to prevent re-release of nutrients. Most studies of NBs for lake restoration have thus far only demonstrated their potential for this purpose, using short-term, small-scale core incubations conducted mainly in laboratory settings. Work is required to (1) address scalability, including procurement and cost, (2) extend laboratory incubation studies to large outdoor enclosures and pond/lake trials, (3) examine longevity of the effects in the natural environment, including potential for MLSs to smother benthos and/or have toxic effects, and (4) extend to a range of lake environments and MLS types. Legal, cultural and social acceptance of the technology is another prerequisite of applications in the natural environment and requires individualised analysis. Until these issues are addressed in a systematic way that addresses scalability and recommends suitable carrier materials and MLSs, NBs may continue to remain largely untried as a geoengineering method to address lake eutrophication.

Internal phosphorus cycling in macrophyte-dominated eutrophic lakes and its implications

Macrophyte-dominated eutrophication (MDE) generally exhibits different characteristics from phytoplankton-dominated eutrophication (PDE). However, the significance of P cycling on eco-environmental management of MDE lakes is still not fully recognized. In this study, P-cycling mechanism was studied in a typical MDE lake (Wuliangsu Lake, China) based on a Before-After-Control-Impact design, taking advantage of the contrasting states between its 13 sub-lakes (with/without macrophytes and with/without external water flow). Our study demonstrated that P cycling predominantly occurs through "macrophyte ↔ sediment" in the MDE lakes, rather than "water ↔ sediment" as in PDE lakes; the biodynamics of the macrophytes acts as a primary driving force for this self-enforced P cycling. Our findings challenge the present lake eutrophication management strategies, which primarily limited to the water nutrient content, and demonstrate that successful MDE lake restoration should focus on stressors caused by the sustainable "macrophyte-sediment" P cycling. Macrophyte harvesting immediately before withering is recommended as the most sustainable technique in environment management for periodically frozen shallow MDE lakes. By this technique, sediment P can be gradually pumped up by the overgrown macrophytes each year until the advent of an alternative stable state (low sediment P, small biomass, and higher diversity), thereby forming a positive feedback loop "macrophyte harvesting → less sediment P → less macrophyte → higher diversity." Also, the catastrophic shift from MDE to PDE is no longer a concern. Furthermore, the macrophyte debris will not pose a problem as long as the macrophytes are removed during the harvest.

Simulation of Denitrification Process of Calcium Nitrate Combined with Low Oxygen Aeration Based on Double Logarithm Mode

In situ remediation of sediment pollution is an important measure for the treatment of urban black-odorous water. In this study, the process of calcium nitrate dosing and low oxygen aeration was used to repair the sediment of black-odorous water body in a glass container. The variation trend and removal efficiency of ammonia nitrogen and nitrate nitrogen in sediment and overlying water were investigated during the process. By establishing the double logarithm model of calcium nitrate sediment repair process, the change law of ammonia nitrogen and nitrate nitrogen content in sediment under different calcium nitrate dosing conditions was studied, and the denitrification process of different calcium nitrate dosing and low oxygen aeration was simulated. The results showed that by establishing the double logarithm model of calcium nitrate sediment remediation process, when the dosage of calcium nitrate was 6%, the inhibition rate of calcium nitrate on nitrate nitrogen release was the largest. The stable inhibitory concentration of nitrate nitrogen was 11.65 mg/g, and the stable inhibited concentration of ammonia nitrogen was 382.95 mg/kg. The stable inhibitory concentration of nitrate nitrogen and ammonia nitrogen in overlying water was 8.34 mg/L and 16.47 mg/L. Moreover, excessive calcium nitrate (8%) may increase the risk of microbial ecological environment in sediment and weaken the inhibitory effect. The optimum parameters were the calcium nitrate dosage of 6%, the reaction time of 21 days, and the aeration rate of 30 mL/min. Under these conditions, the removal effect of ammonia nitrogen in sediment and overlying water was further improved, and the concentration of nitrate nitrogen was effectively controlled. The stable inhibitory content of nitrate nitrogen in sediment was 5.55 mg/g, and the stable inhibitory content of ammonia nitrogen was 982.79 mg/kg. The stable inhibitory concentration of nitrate nitrogen and ammonia nitrogen in overlying water was 6.55 mg/L and 118.20 mg/L. Based on a simulation, this study provides important technical support for the formulation of a refined endogenous pollution control scheme by controlling the process of calcium nitrate remediation and low oxygen aeration.

Dynamics of sediment phosphorus affected by mobile aeration: Pilot-scale simulation study in a hypereutrophic pond

Controlling the release of phosphorus (P) in sediments is important to prevent eutrophication and harmful algal blooms in water bodies. Here we explored the effect of mobile aerators on the control of P release from sediments in a eutrophic pond. The dissolved oxygen in the water body recovered significantly after six months of aeration, becoming 4.2-5.8 times higher than in the control. The pH and Eh values at the sediment-water interface considerably increased, while the concentration of soluble reactive phosphorus (SRP) in pore water significantly decreased, resulting in the alteration of SRP fluxes from 1.69 mg/m² d to -53.49 mg/m² d. Moreover, the inert P in sediments increased by 5.2% of the total P at the end of the study compared with the initial state, and the calcium-bound phosphorus (HCl-P) increased by 96.6%. However, although aeration reduced the concentration of SRP in the water column, the total P concentration was 2.45 times higher than that of the control, and the content of redox-sensitive P (BD-P) in the sediment also increased by 200%. Overall, although mobile aeration can maintain the microenvironment of the sediment interface and increase the inert P content in the sediment to reduce the P flux, it cannot reduce the risk of release of mobile P.

Reducing arsenic toxicity using the interfacial oxygen nanobubble technology for sediment remediation

The arsenic (As)-bearing eutrophic waters may suffer from the dual conditions of harmful algal blooms and release of As, driven by algal-induced hypoxia/anoxia. Here, we investigate the use of interfacial oxygen (O₂) nanobubble technology to combat the hypoxia and control As exposure in simulated mesocosm experiments. It was observed that remediation of algal-induced hypoxia at the sediment-water interfaces (SWI) by application of O₂ nanobubbles reduced the level of dissolved As from 23.2 μg L^-1 to <10 μg L^-1 and stimulated the conversion of As(III) to the less toxic As(V) (65-75%) and methylated As (10-15%) species. More than half of the oxidation and all the methylation of As(III) resulted from the manipulation by O₂ nanobubbles of microbes responsible for As(III) oxidation and methylation. Hydroxyl radicals were generated during the oxidation of reductive substances at the SWI in darkness, and should be dominant contributors to As(III) abiotic oxidation. X-ray absorption near-edge structure (XANES) spectroscopic analysis demonstrated that surface sediments changed from being sources to acting as sinks of As, due to the formation of Fe-(hydr)oxide. Overall, this study suggests that interfacial O₂ nanobubble technology could be a potential method for remediation of sediment As pollution through the manipulation of O₂-related microbial and geochemical reactions.

New insights into restoring microbial communities by side-stream supersaturated oxygenation to improve the resilience of rivers affected by combined sewer overflows

Combined sewer overflows (CSOs) are a dominant contributor to urban river pollution. Therefore, reducing the environmental impacts of CSOs and improving the self-purification capacity of water bodies are essential. In this study, the side-stream supersaturation (SSS) oxygenation was applied to restore microbial function of rivers which are affected by CSOs to improve the self-purification capacity. The results showed that apart from the dissolved organic matter inputs from CSO event, the sediment had become an important contributor to pollution in the studied river. After the long-term (46 d) implementation of SSS oxygenation, dissolved oxygen and the oxidation-reduction potential of the river water increased by 98% and 238%, respectively, compared to emergency control measures implemented following individual CSO events. The NH₃-N concentrations and the chemical oxygen demand also decreased by 20% and 45%, respectively. In addition, the occurrence of microbial functions related to information storage and processing, and cellular process and signaling, increased by 1.87% and 0.82% in response to SSS oxygenation, respectively, and the Shannon index of the sediment microbial community increased by more than 15%. The frequencies of genes related to nitrification and sulfur oxidation also increased by 20-450% and >50%, respectively. This research provides new insights into the ecological restoration of rivers affected by CSOs.

Alleviation of ammonia inhibition via nano-bubble water supplementation during anaerobic digestion of ammonia-rich swine manure: Buffering capacity promotion and methane production enhancement

Anaerobic digestion (AD) of ammonia-rich swine manure (SM) with nano-bubble water (NBW) supplementation was studied in this work with the expectation of ammonia inhibition alleviation, buffering capacity promotion, and methane production enhancement. Results indicated that cumulative methane yield was elevated by 12.3-38.7% in NBW groups. Besides, the reduced methane production rate and elongated lag phase under ammonia inhibition were increased and shortened by NBW supplementation, respectively. The rapid increase of total alkalinity (TA) and partial alkalinity (PA) could be observed with NBW supplementation, as well as the rapid decline of VFA/TA, thus improved buffering capacity and alleviated ammonia inhibition. Moreover, higher level of extracellular hydrolases and coenzyme F₄₂₀ could be detected in NBW groups. In conclusion, NBW with higher mobility and zeta potential (absolute value) could be a promising strategy for the alleviation of ammonia suppression during the AD of SM.

Phosphorus Release from Sediments in a Raw Water Reservoir with Reduced Allochthonous Input

Following successful abatement of external nutrient sources, one must shift the focus to the role of phosphorus (P) release from sediment. This enables us to better assess the causes for sustained eutrophication in freshwater ecosystem and how to deal with this challenge. In this study, five sediment cores from the shallow YuQiao Reservoir in northern China were investigated. The reservoir serves as the main raw water source for tap water services of Tianjin megacity, with a population of 15.6 million. Sediment characteristics and P fractions were determined in order to assess the role of the sediments as the P source to the water body. The total P content (TP) in sediments was similar to what was found in catchment soils, although the P sorption capacity of sediments was 7–10 times greater than for the catchment soils. Isotherm adsorption experiments documented that when P concentration in overlying water drops below 0.032–0.070 mg L−1, depending on the site, the sediment contributes with a positive flux of P to the overlying water. Adsorbed P at different depths in the sediments is found to be released with a similarly rapid release rate during the first 20 h, though chronic release was observed mainly from the top 30 cm of the sediment core. Dredging the top 30 cm layer of the sediments will decrease the level of soluble reactive phosphate in the water being sustained by the sediment flux of P.

Cable bacteria extend the impacts of elevated dissolved oxygen into anoxic sediments

Profound biogeochemical responses of anoxic sediments to the fluctuation of dissolved oxygen (DO) concentration in overlaying water are often observed, despite oxygen having a limited permeability in sediments. This contradiction is indicative of previously unrecognized mechanism that bridges the oxic and anoxic sediment layers. Using sediments from an urban river suffering from long-term polycyclic aromatic hydrocarbons (PAHs) contamination, we analyzed the physicochemical and microbial responses to artificially elevated DO (eDO) in the overlying water over 9 weeks of incubation. Significant changes in key environmental parameters and microbial diversity were detected over the 0-6 cm sediment depth, along with accelerated degradation of PAHs, despite that eDO only increased the porewater DO in the millimeter subfacial layer. The dynamics of physicochemical and microbial properties coincided well with significantly increased presence of centimeter-long sulfide-oxidizing cable bacteria filaments under eDO, and were predominantly driven by cable bacteria metabolic activities. Phylogenetic ecological network analyses further revealed that eDO reinforced cable bacteria associated interspecific interactions with functional microorganisms such as sulfate reducers, PAHs degraders, and electroactive microbes, suggesting enhanced microbial syntrophy taking advantage of cable bacteria metabolism for the regeneration of SO₄^2- and long-distance electron transfer. Together, our results suggest cable bacteria may mediate the impacts of eDO in anaerobic sediments by altering sediment physiochemical properties and by reinforcing community interactions. Our findings highlight the ecological importance of cable bacteria in sediments.

An application of Bayesian Belief Networks to assess management scenarios for aquaculture in a complex tropical lake system in Indonesia

A Bayesian Belief Network, validated using past observational data, is applied to conceptualize the ecological response of Lake Maninjau, a tropical lake ecosystem in Indonesia, to tilapia cage farms operating on the lake and to quantify its impacts to assist decision making. The model captures ecosystem services trade-offs between cage farming and native fish loss. It is used to appraise options for lake management related to the minimization of the impacts of the cage farms. The constructed model overcomes difficulties with limited data availability to illustrate the complex physical and biogeochemical interactions contributing to triggering mass fish kills due to upwelling and the loss in the production of native fish related to the operation of cage farming. The model highlights existing information gaps in the research related to the management of the farms in the study area, which is applicable to other tropical lakes in general. Model results suggest that internal phosphorous loading (IPL) should be recognized as one of the primary targets of the deep eutrophic tropical lake restoration efforts. Theoretical and practical contributions of the model and model expansions are discussed. Short- and longer-term actions to contribute to a more sustainable management are recommended and include epilimnion aeration and sediment capping.

The effect of secondary capping on the control of phosphorus release from sediment by activated thin-layer capping with Al-PIA

It is well-known that the activated thin-layer capping covering by secondary capping of contaminated sediment poses a threat to the inactivation of activated material. In this study, the static simulation experiment was conducted to study the effect of secondary capping thickness by sediment on the control of TP release from the sediment by aluminum-based P-inactivation agent (Al-PIA), and to propose the phosphorus adsorption pathway of Al-PIA. The results showed that Al-PIA could effectively reduce the release of phosphorus pollutants from the sediment at the capping intensity of 2 kg/m². When the secondary capping thickness of sediment were 0, 2, 4, 7, 10, and 15 mm, the average removal rates of TP were 87.57%, 76.39%, 61.22%, 51.32%, 41.93%, and 32.11%, respectively, indicating that the removal efficiency of phosphorus decreased with the increase of the secondary capping thickness of the sediment. The adsorbed phosphorus by Al-PIA was mainly non-apatite inorganic phosphorus (NAIP) in inorganic phosphorus. With the increase of the secondary capping thickness of sediment, the NAIP proportion of phosphorus adsorbed by Al-PIA increased. Meanwhile, the removal rate of phosphorus in the activated capping system showed a first increase and then decrease trend, and the removal rates of total phosphorus (TP), inorganic phosphorus (IP), and organic phosphorus (OP) were obvious except for that of organic phosphorus (OP).

Minerals loaded with oxygen nanobubbles mitigate arsenic translocation from paddy soils to rice

Inhibiting reductive transformation of arsenic (As) in flooded paddy soils is fundamentally important for mitigating As transfer into the food chain. In this study, oxygen-nanobubble-loaded-zeolites (ZON) and -vermiculites (VON) were tested as a novel approach for supplying oxygen to paddy soils to inhibit As influx into rice. The dynamic physio- and bio-chemical variations in the rhizosphere and bulk soil were profiled in a rhizobox experiment. Upon adding ZON and VON, the redox potential and dissolved oxygen consistently increased throughout the cultivation period. The improved redox environment inhibited As(III) release into porewater and increased As(V) adsorbed on crystalline Fe (hydr)oxides, following the reduction of arsC and arrA gene abundances and enhancement of the aioA gene. Moreover, adding ZON and VON promoted root iron plaque formation, which increased As retention on iron plaque. Both ZON and VON treatments mitigated As translocation from soil to rice, meanwhile increasing root and shoot biomass. ZON was superior to VON in repressing As transfer and promoting rice growth due to its higher oxygen loading capacity. This study provides a novel and environment-friendly material to both mitigate the As translocation from paddy soil to rice and improve rice growth.

Interception of phosphorus release from sediments using Mg/Fe-based layered double hydroxide (MF-LDH) and MF-LDH coated magnetite as geo-engineering tools

A magnesium/iron-based layered double hydroxide (MF-LDH) and a composite of MF-LDH and magnetite (MF-LDH@Fe₃O₄) were synthesized, characterized and used as solid-phase phosphorus (P)-sorbents (SPPSs) to control the release of sedimentary P. The behavior and mechanism of phosphate adsorption onto MF-LDH and MF-LDH@Fe₃O₄ were studied. The effect of MF-LDH capping and amendment on the migration of P in sediments were comparatively investigated, and the impact of fabric-wrapped and unwrapped MF-LDH@Fe₃O₄ capping on P mobilization in sediments were also comparatively investigated. Results showed that both MF-LDH and MF-LDH@Fe₃O₄ had good phosphate adsorption performance, and the adsorption mechanisms included cation exchange, electrostatic attraction, ligand exchange and inner-sphere complex formation. Sediment capping and amendment using MF-LDH both could dramatically reduce the risk of the release of soluble reactive P (SRP) and diffusive gradient in thin-films-labile P (P-DGT) from sediments into overlying waters (OLY-Ws), and the MF-LDH capping had a better suppressing efficiency of sediment-P release into OLY-W than the MF-LDH amendment. Sediment capping with the fabric-wrapped and unwrapped MF-LDH@Fe₃O₄ both greatly decreased the risk of SRP and P-DGT released from sediment into OLY-W, and the efficiency of the prevention of SRP released from sediment into OLY-W by the fabric-wrapped MF-LDH@Fe₃O₄ capping layer (about 81-90%) was slightly lower than that by the unwrapped MF-LDH@Fe₃O₄ capping layer (about 94-99%). The reduction of P-DGT in the top sediment and the direct interception of the soluble P from pore water (POR-W) to OLY-W by the MF-LDH@Fe₃O₄ capping layer were the keys to the management of P released from sediment by the MF-LDH@Fe₃O₄ capping. From the standpoint of the efficiency of sedimentary P suppression, the convenience of application and the sustainability of sediment remediation, sediment capping with the fabric-wrapped MF-LDH@Fe₃O₄ is a promising approach to manage the release of sedimentary P into OLY-W.

Control of internal phosphorus release from sediments using magnetic lanthanum/iron-modified bentonite as active capping material

The non-magnetic capping materials are difficult to be recycled from the water bodies after their application, leading to the increase in the cost of the sediment remediation. To address this issue, a capping material, i.e., magnetic lanthanum/iron-modified bentonite (M-LaFeBT) was prepared by loading lanthanum onto a magnetic iron-modified bentonite (M-FeBT) and used to control the internal phosphorus (P) loading in this study. To determine the capping efficiency and mechanism of M-LaFeBT, the impact of M-LaFeBT and M-FeBT capping on the mobilization of P in sediments was investigated, and the stabilization of P bound by the M-LaFeBT and M-FeBT capping layers was evaluated. Results showed that M-LaFeBT possessed good magnetic property with a saturated magnetization of 14.9 emu/g, and exhibited good phosphate adsorption ability with a maximum monolayer sorption capacity (Q_MAX) of 14.3 mg P/g at pH 7. Moreover, M-LaFeBT capping tremendously reduced the concentration of soluble reactive P (SR-P) in the overlying water (OL-water), and the reduction efficiencies were 94.7%-97.4%. Furthermore, M-LaFeBT capping significantly decreased the concentration of SR-P in the pore water and DGT (diffusive gradient in thin films)-labile P in the profile of OL-water and sediment. Additionally, most of P bound by the M-LaFeBT capping layer (approximately 77%) was stable under natural pH and reducing conditions. The phosphate adsorption ability for M-LaFeBT was much higher than that for M-FeBT, and the Q_MAX value for the former was 4.86 times higher than that for the latter. M-LaFeBT capping gave rise to a higher reduction of DGT-labile concentration in the profile of OL-water and sediment than M-FeBT capping. The P adsorbed by the M-LaFeBT capping layer was more stable than that by the M-FeBT capping layer. Results of this study demonstrate that M-LaFeBT is promising for utilization as an active capping material to intercept sedimentary P release into OL-water.

Dredging mitigates cyanobacterial bloom in eutrophic Lake Nanhu: Shifts in associations between the bacterioplankton community and sediment biogeochemistry

Cyanobacterial blooms are a worldwide environmental problem, which is partly attributed to their access to excessive nitrogen (N) and phosphorus (P). Preventing the blooms by reducing N and P from internal inputs is viewed as a challenge. To evaluate the effects of dredging on cyanobacterial abundances and bacterioplankton communities, water and sediment samples were collected from eutrophic Lake Nanhu (Wuhan, China) before dredging (2017) and after dredging (2018). After dredging, significant decreases were observed for sediment nutrients (e.g., C, N, and P sources); C-, N-, P-, and S-cycling-related enzyme activity; N- and P-cycling-related gene abundance; microbial abundance; and dramatic changes were observed in the composition of the sediment microbial community. The release rates of nutrient including nitrogen, phosphorus, and organic matter decreased after dredging, and sediment biogeochemistry was closely correlated to nutrient release rates. Additionally, our observations and analyses indicated that the abundance and diversity of the bacterioplankton community decreased significantly, the composition and interaction of the bacterioplankton community dramatically changed, and the bacterioplankton community function (e.g., N, P-cycling-related enzymes and proteins) down regulated after dredging. Water and sediment physicochemical factors explained 72.28% variation in bacterioplankton community composition, and these physicochemical factors were significantly correlated with diversity, composition, and function of bacterioplankton community. Our findings emphasized that cyanobacterial blooms in freshwater ecosystems were closely correlated with noncyanobacterial bacterioplankton that were largely conserved at the phylum level, with Proteobacteria, Actinobacteria, and Bacteroidetes as the main taxa. To our knowledge, this is the first report clarifying the mechanism of cyanobacterial blooms mitigation by dredging, via changing the association between the bacterioplankton community and sediment biogeochemistry. Our findings are of significance and indicate that dredging is effective for mitigating cyanobacterial blooms.

Novel, recyclable active capping systems using fabric-wrapped zirconium-modified magnetite/bentonite composite for sedimentary phosphorus release control

A zirconium-modified magnetite/bentonite composite (M-ZrFeBT) was synthesized, characterized and combined with water-permeable fabric to construct novel, recyclable active capping systems for sedimentary phosphorus (P) release control. Three fabric-wrapped M-ZrFeBT capping devices with different shapes were designed, i.e., CAP-1, CAP-2 and CAP-3, and they are disc-shaped, cuboid-shaped and spindle-shaped capping devices, respectively. The behavior and mechanism for phosphate adsorption onto M-ZrFeBT was studied. The impact of CAP-1, CAP-2 and CAP-3 capping on the mobilization of P in sediments was investigated. The results showed that M-ZrFeBT possessed good phosphate adsorption ability, with a largest monolayer adsorption capacity of 8.02 mg P/g. The replacement of Fe/Zr bound hydroxyl groups with phosphate through ligand-exchange reactions to generate the inner-sphere Fe-O-P and Zr-O-P bonding played a key part in the uptake of phosphate from water by M-ZrFeBT. Sediment capping with fabric-wrapped M-ZrFeBT not only brought about a significant decline in the concentrations of soluble reactive P (SRP) and DGT (diffusive gradient in thin films)-labile P (LP_DGT) in the overlying water, but also gave rise to the diminished SRP and LP_DGT concentrations in the upper sediment. Most (96.5%-98.2%) of P bound by the M-ZrFeBT in the capping layers was in the form of NaOH extractable inorganic P, HCl-extractable P and residual P, which were considered to be hard to be released back into the water column under common pH and oxygen-deficient conditions. The reduction of pore water SRP and LP_DGT in the upper sediment layer induced by the adsorption of SRP on the M-ZrFeBT-based capping layer played a key part in the interception of SRP liberation from the sediment solid into the overlying water. Results indicate that fabric-wrapped M-ZrFeBT capping is promising for controlling the internal P loading from sediments in shallow freshwater bodies.

Oxygenation and synchronous control of nitrogen and phosphorus release at the sediment-water interface using oxygen nano-bubble modified material

Synchronously controlling the nitrogen (N) and phosphorus (P) release from sediments is an important basis for eutrophication management in lakes, but it is still a technical challenge at present. Loading nano-bubbles on the surface of natural minerals to increase dissolved oxygen(DO) level at the sediment-water interface (SWI)provides a possible solution to this problem. In this study, oxygen nano-bubble modified mineral (ONBMM) technology was developed, and its efficiency of oxygenation at the SWI and effect on the removal of internal nutrient input were evaluated under simulated conditions. The results showed that ONBMM effectively improved DO levels near the SWI; the highest concentration reached 6.55 mgL^-1. Meanwhile, adding ONBMM remarkably reduced the concentrations of total P(TP), total N(TN) and ammonia N(NH₃-N) in the overlying water. Compared with the control group, the fluxes of TP, NH₃-N, and TN loading from sediments in simulation cores treated with ONBMM reduced by 96.4%, 51.1%, and 24.9%, respectively. The high-resolution data obtained by DGT showed that ONBMM effectively inhibited the reduction and release of FeP through increasing the oxygen level at the SWI. The results of 16S rRNA high-throughput sequencing showed that adding ONBMM strengthened the role of nitrobacteria, denitrifying bacteria, and ammonia oxidation bacteria at the SWI. The ONBMM technology provides a new tool to achieve oxygenation at the SWI and in situ control of internal pollution in eutrophic lakes.

Combined use of calcium nitrate, zeolite, and anion exchange resin for controlling phosphorus and nitrogen release from sediment and for overcoming disadvantage of calcium nitrate addition technology

Ca(NO₃)₂ addition has proved to have a high potential to immobilize internal phosphorus (P) in sediments; however, it cannot effectively stop the release of ammonium-nitrogen (NH₄⁺-N) from sediments into overlying waters (OL-waters). Additionally, the addition of Ca(NO₃)₂ alone has high risk of nitrate-nitrogen (NO₃^--N) releasing into OL-waters. To overcome the shortcoming of the Ca(NO₃)₂ addition method, we reported an integrated method, i.e., a combined method based on Ca(NO₃)₂ injection, zeolite capping, and anion exchange resin (AERN)-contained floating system suspending (Ca(NO₃)₂/zeolite/AERN). The effectiveness and mechanism of the Ca(NO₃)₂/zeolite/AERN method for simultaneously controlling the release of soluble reactive P (SRP) and NH₄⁺-N were investigated, and the NO₃^--N releasing risk of this method was evaluated. It was found that the joint use of Ca(NO₃)₂ injection, zeolite capping, and AERN-contained floating system suspending not only could effectively suppress the release of SRP and NH₄⁺-N from sediments into OL-waters simultaneously, but also had much less risk of NO₃^--N releasing into OL-waters as compared to the single Ca(NO₃)₂ injection method and the combined Ca(NO₃)₂/zeolite method. The inhibition of the reductive dissolution of the P-bound Fe(III) oxides/hydroxides by the presence of nitrate and the adsorption of ammonium on the zeolite played very important roles in the interception of SRP and NH₄⁺-N releasing into OL-waters by the Ca(NO₃)₂/zeolite/AERN method. After the sediment remediation using the Ca(NO₃)₂/zeolite/AERN approach, the increase in the content of residual P in the sediment layer of 0-50 mm, the decrease of mobile P in the sediment layer of 0-10 mm, and the increased NH₄⁺-N adsorption capacity for the sediment layer of 0-10 mm would be conductive to the interception of SRP and NH₄⁺-N liberation in the long run. Results of this research suggest a promising application potential of the Ca(NO₃)₂/zeolite/AERN method in the simultaneous control of the release of SRP and NH₄⁺-N from sediments.

Innovative Feasibility Study for the Reclamation of the Cascajo Wetlands in Peru Utilizing Sustainable Technologies

Wetlands are an important feature for our society that provides versatile benefits, such as habitat for diverse wildlife, shoreline erosion protection, flood control, and mitigation of climate change through capture and storage of carbon. The aim of this work was to assess the application of nanotechnologies for the restoration of the water quality in the Cascajo Wetlands, Peru, where the water quality was deteriorated. Ceramic-based bio-filters (CBBFs) were used to reduce and buffer the contamination rates of pollutants, whereas micro-nano bubbles (MNBs) were applied to increase the dissolved oxygen and release free radicals in water. Additionally, bio-fence was implemented to prevent water intrusion from the ocean. Remote sensing data through the Normalized Difference Vegetation Index (NDVI) and the Enhanced Vegetation Index (EVI) was used to monitor the water surface condition. With treatment of CBBFs and MNBs for 13 months, we observed reduction in the chemical oxygen demand (COD), biological oxygen demand (BOD), total nitrogen (TN), and total phosphate (TP) in the water body, showing removal percentages of 98.5%, 97.5%, 98.1%, 98.5%, and 94.6%, respectively, in comparison with values before starting the implementation. The trends of NDVI and EVI over seasons are not completely aligned with the results taken from the wetlands treated with MNBs, CBBFs and bio-fence. While TN was highly correlated with the empirical value of TN based on remote sensing, no correlation was observed between COD and empirical COD. The use of eco-friendly techniques has performed efficiently to remove the pollutant.

Oxidation of flooded paddy soil through irrigation with water containing bulk oxygen nanobubbles

Nanobubbles (NBs) with a diameter of <1 μm have unique properties, including a long lifetime in water. Our previous study showed that irrigation with water containing bulk oxygen NBs (oxygen NB water) reduced seasonal methane (CH₄) emissions from flooded rice cultivation by 21%. We therefore hypothesize that the CH₄ emission reduction attributed to soil oxidation. To test this hypothesis, we carried out three sequential soil-column experiments under flooded and rice-unplanted conditions. They shared the same experimental settings, except for the lengths of soil storage before experiment (2-3 years) and of aerobic soil preincubation (7-13 days). The columns were manually irrigated with aerated tap water (control) or oxygen NB water. A tubing pump was operated to constantly drain leaching water from the column bottom. During the 57-day experimental periods, we regularly measured dissolved greenhouse gases in the drainage and leached heavy metals. The greater amount of leached manganese than of leached iron indicates that the reductive soil conditions were relatively weak in the three experiments. Oxygen NB water reduced the total dissolved CH₄ emissions by 20-28% compared to control water. However, a significant difference (p < 0.05) was found only when the CH₄ emission was greatest in the experiment with the shortest length of soil storage due to a high availability of soil labile carbon. Soil oxygen profiling using a microelectrode revealed that oxygen depletion at shallow depths (4-15 mm from the soil surface) was ameliorated by oxygen NB water on day 35 of experiment when the CH₄ emission was smaller due to a limited soil labile carbon. The results confirm that irrigation with oxygen NB water reduces CH₄ production in a flooded paddy soil through the oxidation of shallow soil.

Effects of different dosing modes of calcium nitrate on P locking in sediment and nutrient concentrations in waters

Sediment is an endogenous pollution source, which often leads water systems to eutrophication due to the release of nutrients, especially phosphorus (P). Calcium nitrate (CN) was dosed to the water systems under different modes to control P release from the sediments in this study. A 63-day static laboratory test was conducted to explore the effects of intermittent dosing and one-time dosing modes of CN on P locking in the sediment and the concentrations of nitrogen (N) and P in waters. Results showed that 89% total phosphorus (TP) in the overlying water and 91% TP in the interstitial water of sediment were reduced in the intermittent dosing reactor, which were 4% and 13% higher than those in the one-time dosing reactor, respectively. Thus, the concentration of TP in the overlying water of the dosing reactors was both below 0.1 mg/L during the whole experiment. Meanwhile, the mean values of oxidation-reduction potential (ORP) in the sediment increased to - 110.7 ± 42.02 mV when CN was added intermittently, which were significantly higher than those of the one-time dosing reactor (- 158.3 ± 44.61 mV) and control reactor (- 320.7 ± 0.05 mV). Compared with one-time dosing mode, the intermittent dosing not only reduced the maximum concentrations of NO₂^--N from 9.21 to 1.79 mg/L and NO₃^--N from 92.42 to 27.58 mg/L but also shorten their retention time in the overlying water, which might depress the toxic threats to aquatic animals in water environments. Therefore, the intermittent dosing of CN could not only improve the P locking effect but also minimize the risks to aquatic animals in water environments under the premise of reasonable dosage selected. In a word, this research provided an effective operation mode for locking P with CN in the heavily polluted water bodies, which is also advantageous to avoid toxic threats to aquatic animals in water environment.

Assessment of iron-modified calcite/zeolite mixture as a capping material to control sedimentary phosphorus and nitrogen liberation

Calcite/zeolite mixture (CZ) can be used to construct a capping layer for the simultaneous management of phosphorus (P) and nitrogen (N) liberation from sediments into the overlying water (OVER-water). However, its control efficiency of sedimentary P release still needs to be improved. To address this issue, an iron-modified CZ (Fe-CZ) was synthesized, characterized, and employed as a capping material to simultaneously prevent P and N release from sediments into OVER-water. Batch and microcosm incubation experiments were performed to study the efficiency and mechanism for the control of P and N release from sediments by capping Fe-CZ. Results showed that sediment capping with Fe-CZ resulted in the significant reduction of soluble reactive P (SRP) and ammonium-N (NH₃-N) in OVER-water, with reduction rates of 77.8-99.7% and 54.0-96.7%, respectively. Furthermore, the Fe-CZ capping layer decreased the SRP concentration in the pore water (PORE-water) at depth of 0-30 mm and reduced the concentration of PORE-water NH₃-N at depth of 0-50 mm. Moreover, the Fe-CZ capping layer gave rise to the great decrement of the concentration of the labile P measured by DGT (diffusive gradient in thin films) technology (P-DGT) in the profile of OVER-water and sediment. Additionally, the Fe-CZ capping resulted in the reduction of redox-sensitive P (P-BD) in the 0-50 mm sediment and caused the transformation of P-BD to calcium-bound P (P-HCl) and residual P (P-RES) in the 0-10 mm sediment as well as to P-RES in the 10-20 mm sediment. Results of this work indicate that the Fe-CZ capping has a high potential for the simultaneous management of P and N release from sediments, and the decrease of the contents of sediment P-DGT, sediment P-BD, PORE-water SRP and PORE-water NH₃-N as well as the conversion of mobile P to more stable P in the top sediment should have a significant role in the simultaneous interception of sedimentary P and N liberation into OVER-water by the Fe-CZ capping.