Anoxia remediation and internal loading modulation in eutrophic lakes using geoengineering method based on oxygen nanobubbles

Journal Pre-proof Anoxia remediation and internal loading modulation in eutrophic lakes using geoengineering method based on oxygen nanobubbles

Honggang Zhang, Jun Chen, Mingli Han, Wei An, Jianwei Yu PII:

S0048-9697(20)30276-X

DOI:

https://doi.org/10.1016/j.scitotenv.2020.136766

Reference:

STOTEN 136766

To appear in:

Science of the Total Environment

Received date:

29 November 2019

Revised date:

31 December 2019

Accepted date:

16 January 2020

Please cite this article as: H. Zhang, J. Chen, M. Han, et al., Anoxia remediation and internal loading modulation in eutrophic lakes using geoengineering method based on oxygen nanobubbles, Science of the Total Environment (2020), https://doi.org/10.1016/ j.scitotenv.2020.136766

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© 2020 Published by Elsevier.

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Anoxia remediation and internal loading modulation in eutrophic lakes using geoengineering method based on oxygen nanobubbles Honggang Zhang1,2*, Jun Chen1, Mingli Han1, Wei An1, Jianwei Yu1 1 State Key Laboratory of Environmental Aquatic Chemistry, Research Center for

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Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

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2 Yangtze River Delta Branch, Research Center for Eco-Environmental Sciences, Chinese

Corresponding author: [email protected].

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*

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Academy of Sciences, Yiwu 322000, China

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Abstract

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Benthic anoxia and internal P release, widely occurring in eutrophic lakes, are major

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factors threatening the health of aquatic ecosystems. In this paper, we experimentally evaluated the efficacy of a new type of “flock-lock” geoengineering method based on oxygen nanobubble technology to remediate sediment anoxia and reduce the internal P release in waters with and without algal blooms. Oxygen-carrying materials (OCM) modified from natural zeolites were used as capping agents and an oxygen-locking layer consists of OCM and the oxidized sediment was formed between anoxic sediment and overlying water. The synergy of diffusion and retention of oxygen in this layer contributes to both the increase of DO and reversal of anoxic conditions. By capping with OCM, the DO in overlying water improved instantly from around 1.5 1

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mg/L to 3.5-4 mg/L and 5-6 mg/L in the systems with algal blooms and without algal blooms, respectively, and maintained throughout the incubation period. The oxygen penetration depth in the sediment can be significantly enhanced from around 0 cm to 3 cm and form an oxygen-locking layer at the end of the experiment by capping with OCM. The labile P was effectively retained via the re-oxidation of ferrous iron in this

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layer compared with the obvious release of labile P and Fe in control. More

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importantly, the oxygen depletion and labile P increase at the sediment-water interface

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caused by the decomposition of the deposited algal biomass can be substantially

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eliminated after capping with OCM. The study shed insights on the sustainable

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modulation of sediment anoxia and internal loading in eutrophic waters.

geoengineering

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1. Introduction

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Keywords: Oxygen nanobubbles, internal phosphorus, algal blooms, sediment anoxia,

Harmful algal blooms as a key consequence of eutrophication, have become a global environmental problem and pose serious threats to the environment and public health (Paerl and Huisman 2008). The formation of harmful algal blooms on the surface of water restricts light penetration and depletes oxygen, thereby deteriorating water quality and adversely affecting the ecosystem (Carey et al. 2012). In addition, they will precipitate onto the surface of sediment after dies, in where the settled algal cells can release massive nutrients (i.e., algae-sourced nutrients) as they undergo 2

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decomposition (Li et al. 2010, Sun et al. 2014). The algae-sourced nutrients, especially for phosphorus (P), may return back to water column and contribute to sustaining the eutrophication (Chen et al. 2018). More importantly, the originally anoxic environment in bottom water would be exacerbated by decomposition of the settled algal biomass (Pearce et al. 2013). Sediment anoxia is commonly considered

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as an incentive factor that results in internal phosphorus release from the sediment

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into water column. The internal loads of P from sediment can continuously contribute

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to water eutrophication and subsequently induce algal blooms even after a reduction

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of external inputs (Wan et al. 2019). Therefore, there is a pressing need to establish

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reliable measures to reduce both the concentrations of P in the water column and

biomass.

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internal P loading from the anoxic sediment as well as from the decomposed algal

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In-situ geo-engineering technologies have been well accepted and increasingly used to remove P from freshwaters by using some adsorbing materials (Huser et al. 2016b, Spears et al. 2014). To date, many P adsorbents such as aluminum-, calciumand iron-rich substrates and lanthanum-modified clay (i.e., Phoslock®) have been tested in both laboratory and field studies for controlling internal P loading (Huser et al. 2016a). These P adsorbents are usually expected to reduce both water column soluble reactive P (SRP) and P release from sediments. However, massive P may be stored in algal cells within the established algal bloom waters and the SRP in the water column may be far below the threshold values for eutrophication. Most of P 3

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adsorbents have little effect on both precipitating particulate P and removing algal blooms from water column (Lürling and van Oosterhout 2013). To overcome this problem, some scientists have developed a synergistic method with combined use of flocculation and capping treatment, which is considered as “flock-lock” (Lürling and van Oosterhout 2013, Pan et al. 2012). The first step of this treatment is to precipitate

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the algal blooms together with total P onto the surface sediment and hereafter the

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internal P loading is blocked or retained in the sediment by in-situ capping with

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modified materials (Ding et al. 2018, Yin et al. 2013). However, the P immobilization

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capability of capping materials could be hampered by both anoxic conditions and

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released P from the sedimented algal biomass (Lürling and Faassen 2012, Wang et al. 2016a). Also, the redox conditions, pH, and organic matter may also affect the

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stability of many P inactivating agents (Lurling et al. 2014, Meis et al. 2013, Wang

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and Jiang 2016). Unfortunately, these capping materials possess little effects on sediment anoxia remediation, which results in P release from sediment due to reductive dissolution of Fe(III) oxyhydroxides and the release of Fe-bound P from sediments (Wang et al. 2016a). Oxygen controls the P release from lake sediments is a long-lasting paradigm in limnology, although the sedimentary P exchange ought to be considered as a complex process (Hupfer and Lewandowski 2008). Many previous studies focus on the oxic or anoxic condition as the primary factor controlling the redox status of iron (Ollikainen et al. 2016, Pearce et al. 2013, Smolders et al. 2017). For this reason, efforts designed 4

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to replenish benthic dissolved oxygen and remediate the anoxic environment is proposed for the restoration of eutrophic lakes and reservoirs, primarily acting by inhibiting internal P loading from the sediments (Zhang et al. 2018a). A conventional approach to increase oxygen levels in bottom water is mechanical aeration, which can directly oxygenate the bottom water (Ma et al. 2015). However, this method is

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constrained to apply in large-scale or deep waters due to the high energy consumption

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and relatively low efficiency (Bormans et al. 2016). Moreover, the aeration can

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substantially result in re-suspension of the anoxic sediment and trigger the depletion

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of oxygen. Some chemical oxidants such as hydrogen peroxide (H2O2) (Matthijs et al.

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2012) and calcium peroxide (CaO2) (Wang et al. 2019) are capable of killing the cyanobacterial algae and improving the oxygen level by releasing peroxide as reacting

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with water. However, the application of chemical oxidants has remained contentious

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due to the increase of pH and other side-effects on the aquatic organisms. It is thus important for developing more cost-effective and eco-environmentally benign methods to tackle the internal loading. Nanobubble technology (NB), which is considered as a technological revolution due to its huge potential in various application, has been introduced into the environmental engineering (Lyu et al. 2019) in addition to medicine (Bhandari et al. 2017, Khan et al. 2019, Khan et al. 2018). Benefited from the advanced nanobubble technology, capping with oxygen modified materials can sustainably enhance the oxygen level and reverse the redox potential at the sediment-water interfaces (SWI) 5

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(Zhang et al. 2018a), which makes it possible to explore the dynamic of variations in internal P loading during the remediation of anoxic conditions across the SWI. Accordingly, a comprehensive analysis of the P speciation in lake sediment profiles amend with oxygen-carrying materials (OCM) was investigated in a sediment-water simulated experiment. In this study, the control effectiveness and stability of oxygen

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nanobubble technology for both sediment anoxia remediation and internal P loading

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control under the effect of algal bloom sedimentation was investigated. The oxygen

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nanobubble modified zeolites and natural zeolites were selected as capping agents. A

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series of water chemistry variations, including the pH, redox conditions, oxygen

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levels, P concentrations, as well as P forms and Fe concentrations were determined in water and sediments. The results of this study will provide theoretical support for the

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in situ geo-engineering method.

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2. Materials and methods

2.1. Sample collection and preparation

The surface lake sediment, at a depth of 0-10 cm, was sampled by a column sampler at Tanxi Bay (120°,07′,47E, 31°25′55N) in Lake Taihu, China in July 2017. The sediments were filtered through a 1.8-mm sieve and homogenized. Lake water was sampled at the same location at a depth of approximately 0.5 m. The lake water was filtered through 5 μm-mesh net. Fresh algal blooms were also sampled from the surface of lake water at the same location of Tanxi Bay. Lake Taihu is one of the five 6

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largest freshwater lakes in China, and Tanxi Bay is a typical area confronted with algal blooms annually. The dominant species is Microcystis. The sampled algae were concentrated by a 64 μm-mesh phytoplankton net. The concentrated fresh algae had a moisture content of 92.7% and contained approximately 7.77 mg-dry algae/g. The sediment, lake water, and algae were stored at 4 °C and used for study within 2 d.

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Soils and flocculants used for settling the algal biomass onto the sediment were

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prepared according to our previous study (Zhang et al. 2018b). The natural zeolite

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particles were obtained from Yongjia Natural Minerals Ltd., Hebei province of China.

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The oxygen-carrying materials (OCM) were prepared by oxygen nanobubble

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modification based on the principle of pressure swing-adsorption which was described in previous literature (Zhang et al. 2018a). Briefly, the zeolite particles were firstly

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calcined at 400-450 °C to remove both physically adsorbed and crystal water in

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micropores and then placed into a pressure-resistant and airtight container in which a vacuum with pressure to−0.1MPa was last for 2 h to remove gas from the micropores of zeolite particles. Thereafter, pure O2 (99.99%) was replenished into the container and held at a pressure of 0.15 MPa for 4 h to load the O2. The oxygen nanobubble loading process was repeated three times to achieve the supersaturation of O2 in the particle micropores. Based on our previous studies, it is expected to not only generate oxygen nanobubbles at zeolite particle-water interfaces after adding OCM into the water but also release oxygen nanobubbles into the surrounding water (Zhang et al. 2018a). 7

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2.2 Experimental microcosms

Forty-eight columns (inner diameter: 10cm and height: 100cm) were prepared and divided into three groups, i.e., control, the columns capping with natural zeolites and OCM, respectively. Each group had 16 columns and was divided into two sub-groups: One was with the addition of algal bloom and the other was without the addition of

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algal bloom. Accordingly, six treatments were conducted (i.e., control without algal

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blooms (C), control with algal blooms (ZC), capping with natural zeolites in the

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columns without algal blooms (CFC), capping with natural zeolites in the columns

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with algal blooms (ZFC), capping with OCM in the columns without algal blooms

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(CFOC), and capping with OCM in the columns with algal blooms (ZFOC)). Each

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treatment had eight replications (Fig. S1). Wet sediments were added into the columns and resulted in approximately 20 cm thick sediments. Lake water (6 L) was slowly

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added into the columns to avoid sediment resuspension. After standing 3 days, 20 g of fresh algal biomass was added. After the addition of fresh algae, a small part of the algae sunk or suspended in the water column immediately, while most of algae floated in the surface water with a thickness of approximately 1-2 cm. The thickness of algae adopted herein according to that in natural environment of the sample site. All columns were incubated at 22-25 °C and 12/12 hours of light/dark cycles and stabled for 1 day. In lakes, algal blooms are commonly a mixture of alive and dead algae cells, as well as microorganisms and other particles. Algal bloom sedimentation could happen 8

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during the entire algae bloom period and algal biomass would eventually be incorporated into sediments after sedimentation. In this experiment, the “flock-lock” method which combined a low-dose flocculent (“flock”) with the OCM (“lock”) was applied. First, the algal bloom sedimentation was facilitated by flocculation using 3 mg/L chitosan modified soils, which was described in previous literature (Li and Pan

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2015). After algae sedimentation, 100 g natural zeolite particles were added into the

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columns of CFC and ZFC, respectively whereas 100 g OCM were used for capping

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materials in CFOC and ZFOC columns. The capping treatment resulted in a 1.5-2.0

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cm thick capping layer on top of the sediments.

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2.3 Sampling and analysis

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During each sampling event, two columns under the same treatment (treated as

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duplicates) were randomly visited for sediment and water sampling. The sediment samples (the top 5 cm) were collected from 0-5cm layer by using a Plexiglass tube with 1 cm internal diameter and immediately stored at 4 °C, and a portion of them was freeze-dried, ground, and sieved to 100-mersh for further analysis. The overlying water samples (100 mL) were collected from 5 cm above the sediment using a syringe with a siphon. The water samples were immediately stored at 4 °C before chemical analyses. Total phosphorus (TP) and total dissolved phosphorus (TDP) (i.e., filtered through 0.45 μm Millipore filter paper) were measured by Mo-Sb-Vc colorimetric method, and soluble reactive phosphorus (SRP) was determined using the

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molybdenum blue method (APHA 1998). The particulate phosphorus (PP) was obtained from the difference between TP and TDP. Temperature, pH, DO, and ORP were directly monitored in the columns using a YSI handheld multi-parameter instrument (Professional Plus, YSI Incorporated, USA). To determine the sediment cores' micro-profiles, oxygen concentration and redox

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Unisense,

Denmark),

respectively.

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measurements

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and

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potential profiles were measured using the oxygen and Eh microelectrodes (OX-50

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microelectrodes were carried out directly in the columns at 2.5mm depth increments

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equipped with an internal reference and a guard cathode (Chen et al. 2016). The DO

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concentration in the bottom water (5 cm above the sediment) was adjusted to the same

water column.

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as that in situ conditions (i.e., DO <2 mg/L) by bubbling an air-N2 mixture into the

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The forms of P were determined on wet samples by extracting P scheme (Christophoridis and Fytianos 2006). Specifically, 200 mg sediment was first extracted using 1 M NH4Cl (NH4Cl-P, loosely adsorbed P), followed by 0.11 M Na2S2O4/0.11 M NaHCO3 (BD-P, P forms sensitive to redox potentials, such as P bound on the surface of Fe(III) and Mn oxides and hydroxides), the next step includes treatment with 1 M NaOH (NaOH-P, exchangeable with hydroxide ions), which extracts the P fraction adsorbed on the surface of aluminum hydroxides and the interior of ferric oxides of the sediment particles. The last step is treatment with 0.5 M HCl (HCl-P, Ca-bound fraction), which represents the amount of P found in Ca and 10

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Mg minerals. The residual-P, consisting mainly of refractory P of the Si crystal lattice and organic P fraction. To explore the correlations between variations of labile Fe and P along with the sediment profiles, the labile P and labile Fe were sampled using ZrO-Chelex DGT according to the methods described by (Ding et al. 2016). DGT probes were inserted

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into the sediment core for 24 h at the end of the experiment. After removal and

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cleaning of probes, the ZrO-Chelex binding gel was horizontally sliced into 2 mm

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sections using a cutter made of stacked ceramic blades. Each slice was then eluted

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with 400 mL of 1.0M HNO3 and then 400 mL of 1.0M NaOH solutions, with extracts

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collected and used for the analysis of labile Fe and labile P concentrations.

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2.4 Data treatment and statistical analysis

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Concentrations of DGT-labile P and DGT-labile Fe (CDGT) in the sediment-water profile were calculated according to Equation (1)(Davison and Zhang 2012): 𝐶𝐷𝐺𝑇 =

𝑀∆g⁄ 𝐷𝐴𝑡

(1)

Where, M (μg) is the mass accumulated over the deployment time; A (cm2) is the exposure area of the gel; t (s) is the deployment time; ∆g (cm) is the thickness of diffusion layer and D (cm2/s) is the diffusion coefficient in the agarose gel. D values for P and Fe have been reported by (Wang et al. 2016b). All statistical analyses were performed using SPSS v20.0 software. Correlations between pairs of variables were analyzed using the Pearson correlation coefficient.

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The differences in DO, ORP, P forms, TP and SRP concentrations between different treatments were analyzed by pairwise comparisons using one-way analysis of variance (ANOVA) with post-hoc Turkey’s test, at a p < 0.05 level of significance.

3. Results and discussion

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3.1 Variation of the water chemistry

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3.1.1 pH, DO and ORP

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The pH of the overlying water ranged from 7.75 to 8.25 during the 30 days incubation (Fig. S2). There were no significant differences between the treated groups and

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control groups regardless of the addition of algal bloom, suggesting that flocculation

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and capping treatment did not change the pH in the overlying water (p>0.05). At the

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beginning of the experiment, the dissolved oxygen (DO) in the bottom water of all the systems was adjusted to lower than 2 mg/L by bubbling an air-N2 mixture according to the oxygen level at SWI in the in-situ sampling site. As shown in Fig.1a, the DO concentrations in all the control groups (i.e., C and ZC) were around 0.5 mg/L and remained anoxic state throughout the experiment. One exception is that the concentration in the system with the addition of algae (ZC) increased to 2.2 mg/L on the 3rd day and then dropping to around 0.5 mg/L again. Contrary to control systems, DO was significantly increased on the 1st day after the capping treatment regardless of algae addition or not. The DO levels could be enhanced instantly resulted from the

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diffusion of oxygen nanobubbles from OCM. It was found that the dissolution of oxygen nanobubble is much faster than that of an air nanobubble due to higher solubility of oxygen in water (Yasui et al. 2019). It should be noted that, however, the oxygen levels in the systems capping with natural zeolite (i.e., CFC and ZFC) pulled back to the initial status from the elevated concentrations (i.e, 3-4 mg/L) after day 5

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and remaining anoxic conditions until the end of experiment. Interestingly, the

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oxygen levels were increased from 0.5 mg/L to nearly 6 mg/L in the systems capping

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with the OCM (i.e., COFC and ZFOC) and maintained a higher level throughout the

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test period of 30 days despite a slight decrease on the 3rd day. Especially for the

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systems without algae (CFOC), in which DO concentration could stay above 5.5 mg/L at the end of the experiment. The DO in the column with algae addition (ZFOC) could

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also maintain at about 4 mg/L at the end of experiment, which is significantly lower

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than that of group without algae (p<0.05), presumably due to the oxygen consumption during the degradation of algal biomass under the capping layer. It can be seen from Fig.1 that both algal and algae-free systems can maintain aerobic conditions throughout the experimental period by capping with OCM, which can be further confirmed by the reversion of oxidation-reduction potential (ORP) in CFOC and ZFOC (Fig.1b). The initial ORP of the overlying water in all systems were around -350 mV (Fig. b). The ORP values of the C and ZC increased slightly in the early stage of the experiment and remained at around -200 mV from 1st day to 30th day, indicating a reductive condition in the bottom water. Conversely, for all the 13

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treated groups, ORP values were drastically increased and even reversed from -200 mV to 200 mV on the first day after capping treatment. However, obvious different trends were found between the groups capping with natural zeolites and OCM (p<0.05). The ORP, in both CFC and ZFC, showed a rapidly declining trend after increased to about 180 mV at the 1st day, and even dropped to -250 mV on the 10th

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day, which was significantly lower than the value of the control systems (p<0.05). For

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CFOC and ZFOC, the ORP was reversed to nearly 250 mV after capping and

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stabilized at 150 mV until day 30. The sustainable reversals of ORP values in both

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algae and algae-free systems indicate that the anoxic conditions in the bottom water

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can be repaired by capping with the oxygen-carrying material. However, unlike DO improvement, no obvious differences in ORP values between COFC and ZFOC

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(p>0.05), although the slightly lower values were found in ZFOC, which suggests the

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presence of algal bloom under the capping layer might not affect the redox potential in the overlying water.

3.1.2 Phosphorus

As expected, the TP concentration in ZC was significantly higher than that in the algae-free system (C) due to the algae-sourced phosphorus (P<0.05) (Fig.2 a). In the control systems, TDP and SRP were much higher in ZC than those in C (p<0.05), indicating that some of the algae-sourced P is released into the overlying water in addition to the phosphorus carried by the algal cells.

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For both algal and algae-free systems, TP concentrations after treated by flocculation-capping treatment were significantly lower than those of the controls (p<0.05). However, the concentration of dissolved phosphorus (TDP and SRP) after flocculation-capping treatment (CFC and CFOC) in the algae-free system was slightly lower than those of the control (C) (p>0.05). On the contrary, the TDP and SRP in the

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overlying water of the ZFC and ZFOC were significantly lower than those of the ZC

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after flocculation-capping treatment (p<0.05). This result indicates that the

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flocculation-capping treatment has little effect on the dissolved P present in the water

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column whereas it is very effective for the water body experiencing algal blooms. It is

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no wonders that during a bloom we can have concentrations of tens of thousands to millions of cells/mL, and P stored in algal cells in the water column can go up to

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hundreds of mg/L, while water column SRP will be far below the level of detection

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(Lürling and van Oosterhout 2013). In this situation, an established bloom may be a sufficient P reservoir to maintain high risk for ongoing cyanobacterial blooms (Noyma et al. 2016). Thus, the results showed in Figure 2 confirmed that the “flock-lock” strategy provides a promising way to remove both the algal blooms and dissolved P in the water column. It should be noted that TP concentration in ZFC obviously increased on the 5th day compared with that in ZFOC due to the contribution of increasing PP concentration (Fid. 2d). The algal cells under the cover layer experienced relatively faster degradation due to the anoxic condition in ZFC and release part of the cell debris 15

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adsorbing phosphorus. On the contrary, the anoxic condition in ZFOC was reversed to provide additional oxygen for algal cell respiration sustaining alive algal cells in a certain period. These results also agree with our previous findings that deposited algal cells easily decayed due to the inhibition of photosynthesis and respiration rate after flocculation-capping treatment (Zhang et al. 2018b). Moreover, the concentrations of

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TDP and SRP in the systems with capping with the OCM (CFOC and ZFOC) were

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lower than those in CFC and ZFC, respectively, which further implies that the

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remediation of the anoxic condition can reduce the P release from sediment. By using

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flocculation-capping treatment, it can not only precipitate the algal blooms together

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with total P onto the surface sediment but also can block or retain these P in the

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sediment by in-situ capping with modified materials.

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3.2 Remediation of anoxic condition across SWI

Bottom water oxygenation is an increasingly common lake management strategy for mitigating hypoxia/anoxia and associated deleterious effects on water quality (Bierlein et al. 2017). The traditional way of relying on the pumping oxygen, air gas or oxygen-rich water into the bottom where experiencing hypoxia usually results in resuspension of the sediment, causing additional consumption of oxygen by both organic matters and reduced chemical substances from sediment. Besides, it will return to the anaerobic state after turning the pump off (Bierlein et al. 2017, Ollikainen et al. 2016). In the present study, capping with the natural zeolites (CFC

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and ZFC) could provisionally increase the oxygen levels in the early stage of experiment, while the DO concentration dropped to the initial level again after several days after capping (Fig. 1). This result revealed that capping with materials without oxygen-carrying capacity would not sustainably remediate the anoxic conditions at SWI. However, capping with oxygen-carrying zeolites instead of natural zeolites was

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demonstrated to remediate the anoxic condition across SWI through gravity settling

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with minimum energy consumption. The remediating effects could remain throughout

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the whole experiment period (Fig. 1).

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The release of oxygen from the OCM can not only improve the bottom water

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oxygen levels but also provide oxygen to penetrate beyond the sediment-water interface into the sub-layer sediment (Fig. 3). This is coincidence with the results of

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ORP distribution along with the sediment profiles (Fig.4).

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The oxygen penetration depths by capping with OCM were deeper than those of control sediments, and the penetration depths were approximately 1.7 cm at the beginning stage. Surprisingly, the penetration depth in ZFOC increased to 4 cm on day 30th whereas oxygen level in ZFC obvious decreased across the sediment profiles (Fig.3). The sediments typically possess a thin oxic layer which can only extend tens of microns to several millimeters into the sediment bed even under oxygen-rich bottom waters (Wang et al. 2014). The penetration depth of oxygen in the sediment induced by the capping layer is vitally important for controlling the internal P loading. This resulted in an oxygen-locking layer consisting of capping layer and oxidized 17

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sediment between the anoxic sediment and overlying water, which can be proved by the elevated ORP and oxygen content across the SWI (Fig.3 and 4). The resulting oxygen- locking layer not only released oxygen into the water but also retained oxygen which could significantly mitigate and even contribute a persistent reversal of

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3.3 Variation of phosphorus forms in the sediment

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the anoxic condition at the SWI (Zhang et al. 2018a)

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Phosphorus exists in lake sediments in both organic and inorganic forms. Inorganic phosphorus is often associated with amorphous and crystalline forms of Fe, Al, Ca

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and other elements. Logically, investigation of bioavailable P in the environment

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(especially sediment) treated by flock-lock treatment is essential to understanding the

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applicability of the geo-engineering method because macrophyte and phytoplankton

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can directly utilize P from sediment for growth. Moreover, P cycling and bioavailability in estuarine and coastal areas depend on P speciation (Yang et al. 2016). In terms of potential bioavailability, the extracted P fractions in lake sediments may be characterized as the loosely absorbed P (NH4Cl-P), the reductant soluble P (BD-P), the metal oxide bound P (NaOH-P) and the calcium bound P (HCl-P) (Zhou et al. 2001). The initial P fractions in the different surface sediments (0-5 cm) are shown in Figure 5. NH4Cl-P, NaOH-P, and BD-P in sediments were considered as a potential P resource for the overlying water and also prefer to utilized by macrophytes and algae (Wang et al. 2017).

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Among all forms of P, NH4Cl-P was the lowest, only average of 10 mg/kg, mainly because it is easily released due to changes of physical and chemical factors including pH, temperature, water dynamics, and redox conditions. Previous study found that NH4Cl-P can form by adsorption of phosphate, released during organic matter degradation, onto the surface of minerals in sediments (Yang et al. 2016). This may

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help explain why the NH4Cl-P concentrations were higher in sediment with algae

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addition (ZFC in Fig. 5). Both BD-P and NaOH-P are also considered as bioavailable

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P, which refers to the phosphorus that is physiochemical bound by hydroxides of Fe,

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Al, and Mn (Rydin 2000). In our study, these two forms of P accounting for more than

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51% to TP in the sediments (Fig. 5). This indicates that those P fractions in sediments can be released in certain conditions, implying the unstable character of this pool. The

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result means that they likely to dissolve and release from the sediment under

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anaerobic or alkaline conditions. HCL-P can be converted to soluble phosphate under low pH or high acidic conditions, although HCl-P was generally found to be stably bound in the sediment (Gao et al. 2005). This can partly explain the results here that HCL-P concentrations in the sediments decreased obviously in both ZFOC and CFC (Fig. 5). The TP in the sediment of the control systems (C and ZC) was lower than that of before the treatment, indicating the part of phosphorus released from the sediment. On the contrary, the retention capacity of P in the sediment after capping treatment obviously increased compared with that of control groups (Fig. 5). However, the 19

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results also showed significant differences between the sediments capping with natural zeolites and OCM. P fractionation in the treated groups indicates that the enhanced phosphate retention by capping with OCM is mainly due to the increased fractions of BD-P, NaOH-P and Residual-P (Fig.5). Those results suggest that the transformation of P between different fractions occurred after treatment. The release

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of BD-P and NaOH-P was more obvious under anoxic conditions than under oxic

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ones, indicating that the low redox potential induces mobilization of this P fraction.

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To the best of our knowledge, other commonly P-inactivation agents including Al-,

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Fe-, and Ca-salts, have little effect on transforming mobile P to Residual-P in lake

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sediment (Wang et al. 2017). From our results, we can expect that the synergistic effects of persistent reversal of bottom hypoxia and the capping layer are expected to

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prevent the bioavailable P released from the sediment, which can provide a window

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opportunity for the growth of submerged macrophytes. Once the submerged vegetation successfully re-establishment, they can continue to uptake these bioavailable P from sediment, and then effectively mitigating eutrophication (Liu et al. 2019).

3.4 Assessment of oxygenation effects

Lake sediments play a crucial environmental role in nutrient cycling because they can convert between the source and sink of nutrients as the ambient condition changes. This is especially true of the SWI, where has frequent substance turnover and nutrient

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exchange due to its susceptibility to various physical, chemical, and biological interactions (Han et al. 2015). Oxygen level is one of the crucial factors that affect the P release from sediments. Unfortunately, hypoxia/anoxia is a global threat to aquatic ecosystems, especially in eutrophic deep waters where often occurs “dead zones” at the bottom regions (Feist et al. 2016). Previous studies found that the availability of P

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adsorbed to iron drastically increases as oxygen concentrations decline, owing to the

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reduction of iron. (Boynton et al. 2018). Up to now, our understanding of the

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interactions between P and Fe under different oxygen levels at the SWI has been

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limited due to the lack of in-situ remediation of anoxia and high-resolution monitoring

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technologies (Ding et al. 2016). Most previous studies were based on aeration methods, which can easily disturb the settled sediment-water interfaces, as well as the

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ex-situ sampling technology, often changes the original properties of samples thereby

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causing considerable analytical errors. Those partly resulted in the uncertainty that oxygenation trail to control internal P may not always effective for water quality management of lakes (Bormans et al. 2016). In this study, the sediment anoxia was remediated by capping with OCM by which the disturbances to the SWI can be minimized through gravity settling. The one-dimensional vertical distribution of labile Fe and P across the sediment-water profile were simultaneously measured by ZrO-Chelex DGT on the 30th day (Fig. 6). The distribution of labile P and Fe across the SWI profiles exhibited a similar shape in the same treatment group. This is consistent with the fact that the redox-driven release 21

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of P is generally verified by simultaneous accumulations of soluble Fe (II) at the SWI (Chen et al. 2018). Previous studies found that the anoxic condition at SWI induced by the formation of algal blooms causes the reductive dissolution of Fe (III) oxyhydroxides and the release of Fe-bound P (Ding et al. 2016, Hupfer and Lewandowski 2008). In the present study, compared with the system without algae

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addition, peak values of labile P and Fe were found from 1cm to -3cm, implying the

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release of the algae-sourced P and Fe during decomposition of the deposited algal

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cells. This indicates that the algal sedimentation could not only directly influence the

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internal P loading via the release of the algae-sourced nutrients but also substantially

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affect the redox potential, which can strongly change the P and Fe (Han et al. 2015). In the later stages of the experiment, a large part of the adding algal biomass

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deposited onto the surface of sediment due to cell death, which also naturally occurs

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during the algal bloom period in lakes. The algal cells buried under the capping layer would undergo a fast dying period due to a lack of light and oxygen supply (Zhang et al. 2018b). This resulted in the release of algae-sourced P showed in Figure 2 a and d, which may cause variable control of lake eutrophication by using other inactivating agents (Wang et al. 2016a). To minimize both the algal biomass return the water column and algae-sourced P release, slow down the speed of the algal decomposition and prevent the anoxic conditions is desirable. Capping with OCM after algal sedimentation can provide a promising way to solve this bottleneck problem. The decomposition of the deposited algal biomass can consume the oxygen and 22

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change the redox potential at SWI, which was reflected by the results that ORP profiles in ZFC significantly lower than that of CFC. However, the ORP profiles in ZFOC and CFOC were obviously higher than those in ZFC and CFC, respectively (Fig. 4). This indicates that oxygen released from the oxygen-locking layer could compensate for the consumption caused by the degradation of the algal biomass. The

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peak values of labile P and Fe were eliminated after capping with OCM, which was

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mainly attributed to the reversal of the anoxic conditions. However, it is also should

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be noted that the sediment under the oxygen-locking layer is still in anaerobic state

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(Fig. 3), which can explain the gradual rise of labile P and Fe below -3 cm (Fig. 6).

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The reduction of ferric hydroxide to the soluble form of ferrous hydroxide leads to the dissolution of the adsorbed fraction of P, which is widely considered as a major reason

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of internal P release under anoxic conditions (Chen et al. 2019). Those processes may

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have enhanced the release of labile P and Fe, and then may have induced the mobility of labile P and Fe from the bottom to the surface sediments, as shown as Figure 6. In addition to quickly improve the DO levels in the water column, the oxygen-locking layer formed between the anoxic sediment and overlying water enhanced the P retention capacity of sediment. The re-oxidation of upward diffusion ferrous iron to Fe (oxyhydr) oxides in oxic layer, which provides efficient adsorption sites to retain P. The higher thickness of the surface oxic layer decrease the flux of P released from sediment (McManus et al. 1997).

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3.5 Implications for lake restoration

Today most of the studies are focused on reducing nutrient inputs to the lakes in order to reduce eutrophication, but by helping lake itself to deal with the internal phosphorus we can create a turbo effect in the battle against eutrophication by geoengineering efforts to bring oxygen into the bottom of lakes. Unfortunately, the

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lake geo-engineering method has remained contentious because of the variable results

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reported in the literature (Spears et al. 2014). The present study demonstrated that by

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using the geo-engineering method developed based on oxygen nanobubbles, it is

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possible to deliver oxygen into benthic anoxic regions through gravity settling, which

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can achieve both replenishing oxygen consumption in the “dead zone” with minimum

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energy consumption and minimizing the disturbance to the water stratification and surface sediment. Oxygenation, especially at sediment-water interfaces, will create the

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necessary conditions for the establishment of new ecosystems and opportunity for re-establishing submerged vegetation that enables lake itself to deal with eutrophication. However, the in-situ experiment of this “flock-lock” technology within the whole year round in lakes need to be further investigated.

5. Conclusion

The study demonstrated the efficacy of a new type of “flock-lock” geoengineering method base on the oxygen nanobubble technology for simultaneous remediation of the sediment anoxia and control of the internal P loading. The synergy of diffusion 24

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and retention of the oxygen in the OCM contributed to quickly improve the DO concentrations in the water column and to oxygen penetration downwards to the deeper layer of sediment. More importantly, a 3 cm oxygen-locking layer consists of the oxygen-rich capping layer and the oxidized sediment layer formed between the anoxic sediment and overlying water after 30 days incubation. This layer plays

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multiple roles in the reversal of anoxic condition, reoxygenation of Fe2+ and adoption

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of P released from the deeper anoxic sediments. We also found that the oxygen

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released from the oxygen-locking layer can compensate for the oxygen depletion even

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under intensive algae decomposition period at the SWI. This is crucial for the

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restoration of lakes experiencing an established algal bloom in where reconstruction

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Acknowledgments

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of submerged macrophytes can hardly be achieved.

The research was supported by the National Natural Science Foundation of China (41877473, 41401551), Beijing Natural Science Foundation (8162040) and the Funds for Major Science and Technology Program for Water Pollution Control and Treatment (2018ZX07701001).

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References

APHA,1998. Standard methods for the examination of water and wastewater. American Public Health Association, Washington DC. 20th Edition. Bhandari, P.N., Cui, Y., Elzey, B.D., Goergen, C.J., Long, C.M. and Irudayaraj, J.,2017. Oxygen nanobubbles revert hypoxia by methylation programming. Sci

of

Rep-Uk.

ro

Bierlein, K.A., Rezvani, M., Socolofsky, S.A., Bryant, L.D., Wuest, A. and Little,

-p

J.C.,2017. Increased sediment oxygen flux in lakes and reservoirs: The impact of

re

hypolimnetic oxygenation. Water Resources Research 53 (6),4876-4890.

lP

Bormans, M., Marsalek, B. and Jancula, D.,2016. Controlling internal phosphorus

na

loading in lakes by physical methods to reduce cyanobacterial blooms: a review. Aquat. Ecol. 50 (3),407-422.

Jo ur

Boynton, W.R., Ceballos, M.A.C., Bailey, E.M., Hodgkins, C.L.S., Humphrey, J.L. and Testa, J.M.,2018. Oxygen and Nutrient Exchanges at the Sediment-Water Interface: a Global Synthesis and Critique of Estuarine and Coastal Data. Estuaries and Coasts 41 (2),301-333. Carey, C.C., Ibelings, B.W., Hoffmann, E.P., Hamilton, D.P. and Brookes, J.D.,2012. Eco-physiological adaptations that favour freshwater cyanobacteria in a changing climate. Water Res. 46 (5),1394-1407. Chen, M., Ding, S., Chen, X., Sun, Q., Fan, X., Lin, J., Ren, M., Yang, L. and Zhang, C.,2018. Mechanisms driving phosphorus release during algal blooms based on 26

Journal Pre-proof

hourly changes in iron and phosphorus concentrations in sediments. Water Res. 133 153-164. Chen, M., Ding, S., Wu, Y., Fan, X., Jin, Z., Tsang, D.C.W., Wang, Y. and Zhang, C.,2019. Phosphorus mobilization in lake sediments: Experimental evidence of strong control by iron and negligible influences of manganese redox reactions.

of

Environ. Pollut. 246 472-481.

ro

Chen, M.S., Ding, S.M., Liu, L., Xu, D., Gong, M.D., Tang, H. and Zhang, C.S.,2016.

-p

Kinetics of phosphorus release from sediments and its relationship with iron

re

speciation influenced by the mussel (Corbicula fluminea) bioturbation. Sci. Total

lP

Environ. 542 833-840.

Christophoridis, C. and Fytianos, K.,2006. Conditions affecting the release of

na

phosphorus from surface lake sediments. J. Environ. Qual. 35 (4),1181-1192.

Jo ur

Davison, W. and Zhang, H.,2012. Progress in understanding the use of diffusive gradients in thin films (DGT) – back to basics. Environ. Chem. 9 (1),1-13. Ding, S., Chen, M., Cui, J., Wang, D., Lin, J., Zhang, C. and Tsang, D.C.W.,2018. Reactivation of phosphorus in sediments after calcium-rich mineral capping: Implication for revising the laboratory testing scheme for immobilization efficiency. Chem. Eng. J. 331 720-728. Ding, S.M., Wang, Y., Wang, D., Li, Y.Y., Gong, M.D. and Zhang, C.S.,2016. In situ, high-resolution evidence for iron-coupled mobilization of phosphorus in sediments. Sci Rep-Uk 6 27

Journal Pre-proof

Feist, T.J., Pauer, J.J., Melendez, W., Lehrter, J.C., DePetro, P.A., Rygwelski, K.R., Ko, D.S. and Kreis, R.G.,2016. Modeling the Relative Importance of Nutrient and Carbon Loads, Boundary Fluxes, and Sediment Fluxes on Gulf of Mexico Hypoxia. Environ. Sci. Technol. 50 (16),8713-8721. Gao, L., Zhou, J.M.M., Yang, H. and Chen, J.,2005. Phosphorus fractions in sediment

of

profiles and their potential contributions to eutrophication in Dianchi Lake.

ro

Environ. Geol. 48 (7),835-844.

-p

Han, C., Ding, S.M., Yao, L., Shen, Q.S., Zhu, C.G., Wang, Y. and Xu, D.,2015.

re

Dynamics of phosphorus-iron-sulfur at the sediment-water interface influenced by

lP

algae blooms decomposition. J. Hazard. Mater. 300 329-337. Hupfer, M. and Lewandowski, J.,2008. Oxygen Controls the Phosphorus Release

na

from Lake Sediments - a Long-Lasting Paradigm in Limnology. International

Jo ur

Review of Hydrobiology 93 (4-5),415-432. Huser, B.J., Egemose, S., Harper, H., Hupfer, M., Jensen, H., Pilgrim, K.M., Reitzel, K., Rydin, E. and Futter, M.,2016a. Longevity and effectiveness of aluminum addition to reduce sediment phosphorus release and restore lake water quality. Water Res. 97 122-132. Huser, B.J., Futter, M., Lee, J.T. and Perniel, M.,2016b. In-lake measures for phosphorus control: The most feasible and cost-effective solution for long-term management of water quality in urban lakes. Water Res. 97 142-152. Khan, M.S., Hwang, J., Lee, K., Choi, Y., Seo, Y., Jeon, H., Hong, J.W. and Choi, 28

Journal Pre-proof

J.,2019. Anti-Tumor Drug-Loaded Oxygen Nanobubbles for the Degradation of HIF-1α and the Upregulation of Reactive Oxygen Species in Tumor Cells. Cancers (Basel) 11 (10),1464. Khan, M.S., Hwang, J., Seo, Y., Shin, K., Lee, K., Park, C., Choi, Y., Hong, J.W. and Choi, J.,2018. Engineering oxygen nanobubbles for the effective reversal of

of

hypoxia. Artif Cell Nanomed B 46 S318-S327.

ro

Li, H. and Pan, G.,2015. Simultaneous Removal of Harmful Algal Blooms and

re

Sci. Technol. 49 (10),6249-6256.

-p

Microcystins Using Microorganism- and Chitosan-Modified Local Soil. Environ.

lP

Li, K.Y., Liu, Z.W. and Gu, B.H.,2010. The fate of cyanobacterial blooms in vegetated and unvegetated sediments of a shallow eutrophic lake: A stable isotope

na

tracer study. Water Res. 44 (5),1591-1597.

Jo ur

Liu, M., Ran, Y., Peng, X., Zhu, Z., Liang, J., Ai, H., Li, H. and He, Q.,2019. Sustainable modulation of anaerobic malodorous black water: The interactive effect of oxygen-loaded porous material and submerged macrophyte. Water Res. 160 70-80. Lürling, M. and Faassen, E.J.,2012. Controlling toxic cyanobacteria: Effects of dredging and phosphorus-binding clay on cyanobacteria and microcystins. Water Res. 46 (5),1447-1459. Lürling, M. and van Oosterhout, F.,2013. Controlling eutrophication by combined bloom precipitation and sediment phosphorus inactivation. Water Res. 47 29

Journal Pre-proof

(17),6527-6537. Lurling, M., Waajen, G. and van Oosterhout, F.,2014. Humic substances interfere with phosphate removal by lanthanum modified clay in controlling eutrophication. Water Res. 54 78-88. Lyu, T., Wu, S., Mortimer, R.J.G. and Pan, G.,2019. Nanobubble Technology in

of

Environmental Engineering: Revolutionization Potential and Challenges. Environ.

ro

Sci. Technol.

-p

Ma, W.X., Huang, T.L. and Li, X.,2015. Study of the application of the water-lifting

re

aerators to improve the water quality of a stratified, eutrophicated reservoir.

lP

Ecol.Eng. 83 281-290.

Matthijs, H.C., Visser, P.M., Reeze, B., Meeuse, J., Slot, P.C., Wijn, G., Talens, R. and

na

Huisman, J.,2012. Selective suppression of harmful cyanobacteria in an entire

Jo ur

lake with hydrogen peroxide. Water Res. 46 (5),1460-1472. McManus, J., Berelson, W.M., Coale, K.H., Johnson, K.S. and Kilgore, T.E.,1997. Phosphorus regeneration in continental margin sediments. Geochimica et Cosmochimica Acta 61 (14),2891-2907. Meis, S., Spears, B.M., Maberly, S.C. and Perkins, R.G.,2013. Assessing the mode of action of Phoslock (R) in the control of phosphorus release from the bed sediments in a shallow lake (Loch Flemington, UK). Water Res. 47 (13),4460-4473. Noyma, N.P., de Magalhães, L., Furtado, L.L., Mucci, M., van Oosterhout, F., Huszar, 30

Journal Pre-proof

V.L.M., Marinho, M.M. and Lürling, M.,2016. Controlling cyanobacterial blooms through effective flocculation and sedimentation with combined use of flocculants and phosphorus adsorbing natural soil and modified clay. Water Res. 97 26-38. Ollikainen, M., Zandersen, M., Bendtsen, J., Lehtoranta, J., Saarijarvi, E. and Pitkanen, H.,2016. Any payoff to ecological engineering? Cost-benefit analysis of

of

pumping oxygen-rich water to control benthic release of phosphorus in the Baltic

ro

Sea. Water Resour Econ 16 28-38.

-p

Paerl, H.W. and Huisman, J.,2008. Climate - Blooms like it hot. Science 320

re

(5872),57-58.

lP

Pan, G., Dai, L.C., Li, L., He, L.C., Li, H., Bi, L. and Gulati, R.D.,2012. Reducing the Recruitment of Sedimented Algae and Nutrient Release into the Overlying Water

na

Using Modified Soil/Sand Flocculation-Capping in Eutrophic Lakes. Environ. Sci.

Jo ur

Technol. 46 (9),5077-5084.

Pearce, A.R., Rizzo, D.M., Watzin, M.C. and Druschel, G.K.,2013. Unraveling Associations between Cyanobacteria Blooms and In-Lake Environmental Conditions in Missisquoi Bay, Lake Champlain, USA, Using a Modified Self-Organizing Map. Environ. Sci. Technol. 47 (24),14267-14274. Rydin, E.,2000. Potentially mobile phosphorus in Lake Erken sediment. Water Res. 34 (7),2037-2042. Smolders, E., Baetens, E., Verbeeck, M., Nawara, S., Diels, J., Verdievel, M., Peeters, B., De Cooman, W. and Bakens, S.,2017. Internal Loading and Redox Cycling of 31

Journal Pre-proof

Sediment Iron Explain Reactive Phosphorus Concentrations in Lowland Rivers. Environ. Sci. Technol. 51 (5),2584-2592. Spears, B.M., Maberly, S.C., Pan, G., Mackay, E., Bruere, A., Corker, N., Douglas, G., Egemose, S., Hamilton, D., Hatton-Ellis, T., Huser, B., Li, W., Meis, S., Moss, B., Lurling, M., Phillips, G., Yasseri, S. and Reitzel, K.,2014. Geo-Engineering in

of

Lakes: A Crisis of Confidence? Environ. Sci. Technol. 48 (17),9977-9979.

ro

Sun, Q., Chen, J., Zhang, H., Ding, S.M., Li, Z., Williams, P.N., Cheng, H., Han, C.,

-p

Wu, L.H. and Zhang, C.S.,2014. Improved Diffusive Gradients in Thin Films

re

(DGT) Measurement of Total Dissolved Inorganic Arsenic in Waters and Soils

lP

Using a Hydrous Zirconium Oxide Binding Layer. Anal. Chem. 86 (6),3060-3067. Wan, L., Chen, X., Deng, Q., Yang, L., Li, X., Zhang, J., Song, C., Zhou, Y. and Cao,

na

X.,2019. Phosphorus strategy in bloom-forming cyanobacteria (Dolichospermum

Jo ur

and Microcystis) and its role in their succession. Harmful Algae 84 46-55. Wang, C., Bai, L., Jiang, H.-L. and Xu, H.,2016a. Algal bloom sedimentation induces variable control of lake eutrophication by phosphorus inactivating agents. Sci. Total Environ. 557-558 479-488. Wang, C., He, R., Wu, Y., Lürling, M., Cai, H., Jiang, H.-L. and Liu, X.,2017. Bioavailable phosphorus (P) reduction is less than mobile P immobilization in lake sediment for eutrophication control by inactivating agents. Water Res. 109 196-206. Wang, C., Shan, B.Q., Zhang, H. and Rong, N.,2014. Analyzing sediment dissolved 32

Journal Pre-proof

oxygen based on microprofile modeling. Environ. Sci. Pollut. Res. 21 (17),10320-10328. Wang, C.H. and Jiang, H.L.,2016. Chemicals used for in situ immobilization to reduce the internal phosphorus loading from lake sediments for eutrophication control. Crit. Rev. Environ. Sci. Technol. 46 (10),947-997.

of

Wang, Y., Ding, S.M., Gong, M.D., Xu, S.W., Xu, W.M. and Zhang, C.S.,2016b.

ro

Diffusion characteristics of agarose hydrogel used in diffusive gradients in thin

-p

films for measurements of cations and anions. Anal. Chim. Acta 945 47-56.

re

Wang, Y., Wang, W.H., Yan, F.L., Ding, Z., Feng, L.L. and Zhao, J.C.,2019. Effects

lP

and mechanisms of calcium peroxide on purification of severely eutrophic water. Sci. Total Environ. 650 2796-2806.

na

Yang, B., Liu, S.-M., Wu, Y. and Zhang, J.,2016. Phosphorus speciation and

Jo ur

availability in sediments off the eastern coast of Hainan Island, South China Sea. Cont. Shelf Res. 118 111-127. Yasui, K., Tuziuti, T. and Kanematsu, W.,2019. High temperature and pressure inside a dissolving oxygen nanobubble. Ultrason. Sonochem. 55 308-312. Yin, H.B., Kong, M. and Fan, C.X.,2013. Batch investigations on P immobilization from wastewaters and sediment using natural calcium rich sepiolite as a reactive material. Water Res. 47 (13),4247-4258. Zhang, H., Lyu, T., Bi, L., Tempero, G., Hamilton, D.P. and Pan, G.,2018a. Combating hypoxia/anoxia at sediment-water interfaces: A preliminary study of 33

Journal Pre-proof

oxygen nanobubble modified clay materials. Sci. Total Environ. 637–638 550-560. Zhang, H., Shang, Y., lyu, T., Chen, J. and Pan, G.,2018b. Switching Harmful Algal Blooms to Submerged Macrophytes in Shallow Waters Using Geo-engineering Methods: Evidence from a 15N Tracing Study. Environ. Sci. Technol. 52

of

(20),11778-11785.

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Zhou, Q.X., Gibson, C.E. and Zhu, Y.M.,2001. Evaluation of phosphorus

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na

lP

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Chemosphere 42 (2),221-225.

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bioavailability in sediments of three contrasting lakes in China and the UK.

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Fig. 1 Variation of DO and ORP in the overlying water during the incubation. C:

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control without algal blooms, ZC: control with algal blooms, CFC: capping with

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natural zeolites in the columns without algal blooms, ZFC: capping with natural

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zeolites in the columns with algal blooms, CFOC: capping with OCM in the columns

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without algal blooms, ZFOC: capping with OCM in the columns with algal blooms.

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The significant differences in DO and ORP among different groups are indicated by lowercase letters. For each parameter, the groups without the same letter are

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significant differences (p<0.05).

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Fig. 2 Variation of total phosphorus (TP), total dissolved phosphorus (TDP), soluble

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reactive phosphorus (SRP) and particle phosphorus (PP) in the overlying water during

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the incubation. C: control without algal blooms, ZC: control with algal blooms, CFC: capping with natural zeolites in the columns without algal blooms, ZFC: capping with

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natural zeolites in the columns with algal blooms, CFOC: capping with OCM in the columns without algal blooms, ZFOC: capping with OCM in the columns with algal blooms. The significant differences in TP, TDP, SRP and PP among different groups are indicated by lowercase letters. For each parameter, the groups without the same letter are significant differences (p<0.05).

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Fig. 3 Variations of oxygen with depth in the sediment-water profiles of ZC, ZFC and ZFOC. ZC: control with algal blooms, ZFC: capping with natural zeolites in the

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blooms.

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columns with algal blooms, ZFOC: capping with OCM in the columns with algal

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Fig. 4 Variations of ORP with depth in the sediment-water profiles of different treatments. C: control without algal blooms, ZC: control with algal blooms, CFC:

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capping with natural zeolites in the columns without algal blooms, ZFC: capping with natural zeolites in the columns with algal blooms, CFOC: capping with OCM in the columns without algal blooms, ZFOC: capping with OCM in the columns with algal blooms.

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Fig. 5 Concentrations of the various forms of P in the surface sediments at the end of the experiment (30 days). C: control without algal blooms, ZC: control with algal

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blooms, CFC: capping with natural zeolites in the columns without algal blooms, ZFC:

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capping with natural zeolites in the columns with algal blooms, CFOC: capping with OCM in the columns without algal blooms, ZFOC: capping with OCM in the columns with algal blooms.

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Fig. 6 The vertical distribution of DGT-labile P and DGT-labile Fe. The horizontal dashed line at 0 cm indicates the SWI. C: control without algal blooms, ZC: control with algal blooms, CFOC: capping with OCM in the columns without algal blooms, ZFOC: capping with OCM in the columns with algal blooms.

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Journal Pre-proof Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Graphical abstract

Highlights OCM can sustainably improve oxygen levels in both water and sediment.

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OCM significantly enhances oxygen penetration depth downward into

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P retention in the oxygen-locking surface layer via re-oxidation of Fe2+ to

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

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sediment.

Oxygen-locking layer is crucial for anoxia remediation and internal P modulation.

Oxygen-locking layer replenishes oxygen depletion caused by algal

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

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

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Fe3+.

decomposition.

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6