Mechanisms of the photochemical release of phosphate from resuspended sediments under solar irradiation

doi:10.1016/j.scitotenv.2017.04.039

Science of The Total Environment

Volume 595, 1 October 2017, Pages 779-786

https://doi.org/10.1016/j.scitotenv.2017.04.039 Get rights and content

Highlights

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Dissolved P release increased when resuspended sediments were exposed to simulated solar irradiation.
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The fraction and composition of organic phosphorus has been changed after solar irradiation.
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OH plays an important role in PO₄^3 − photo-released.
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Photochemical processes offer the potential to PO₄^3 − released from sediment in the aquatic environment.

Abstract

In previous studies, resuspended sediments that were exposed to simulated solar irradiation could release dissolved phosphate (PO₄^3 −). However, the mechanisms of phosphate release remain unclear. In this research, a battery of experiments was performed to reveal the mechanisms of the photochemical release of phosphate from resuspended sediments of a shallow eutrophic lake under solar irradiation. The results show that the PO₄^3 − released in resuspended sediments was significantly higher than that in the dark control or in water alone after treatment with solar irradiation for 6 h. The results of sequential chemical extractions showed that the concentrations of labile organic, moderately labile organic and residual organic phosphorus decreased in the resuspended sediment after 6 h of solar irradiation; of these, moderately labile organic phosphorus was the greatest contributor to the release of dissolved phosphate in resuspended sediment. Orthophosphate, phosphate monoesters, phosphate diesters and pyrophosphate were detected with ³¹P NMR. It is worth mentioning that the diester-P and pyro-P species disappeared after 6 h of irradiation. In addition, enzyme activity and radical trapping experiments were applied to identify the roles of biomineralization and photochemical degradation during phosphate release from resuspended sediments under solar irradiation. The amount of PO₄^3 − released in fresh sediment was greater than that in the autoclaved sediment, which should be attributed to the higher alkaline phosphatase activity in the fresh sediment. However, the PO₄^3 − released from the photochemical degradation of organic phosphorus is the primary phosphate source during sediment resuspension under 6 h of solar irradiation. The phosphate photorelease was inhibited when methanol was added to the suspension and decreased significantly when the concentration of methanol was increased from 0.5 M to 2.0 M. All of these results suggest that photochemical processes may lead to PO₄^3 − release from sediment in aquatic environments.

Graphical abstract

Introduction

Phosphorus, an essential element for organisms in aquatic environments, has been recognized as the most significant nutrient that influences the trophic status of a lake (Conley et al., 2009, Meinikmann et al., 2015). Internal phosphorus in the sediment is a main component of phosphorus in the aquatic environment and a significant source of phosphorus (Qin et al., 2015, Zhu et al., 2015). Phosphorus stored in sediment can be released to the overlying water through physical, chemical and biological processes, such as the diffusion of surface sediment, mineralization or sediment resuspension (Gardolinski et al., 2004, Jensen et al., 2006, Li et al., 2011, Kim et al., 2015). Sediment resuspension is a frequent event in shallow lakes, resulting in the direct exchange of phosphorus with the water column due to it is immediate contact with the overlying water; this mechanism could represent an important pathway of phosphorus cycling (Morin and Morse, 1999).

A recent study demonstrated that dissolved nutrients could be released when resuspended marine and estuarine sediments were irradiated by sunlight (Kieber et al., 2006, Mayer et al., 2009, Helms et al., 2014). In a suspension constituted with autoclaved sediment (< 30 μm) and filtered sea water, more dissolved PO₄^3 − was released under solar irradiation than in dark conditions. Because biological effects had been eliminated, the increase in PO₄^3 − in the suspension was attributed to the photochemical decomposition of organic phosphorus from sediments (Southwell et al., 2010). These findings suggested that photocatalysis had a significant impact on organic phosphorus production into the surface waters, leading to the increase of the concentration of available phosphorus (Southwell et al., 2011). These earlier studies have found that photochemical processes play an important role in nutrient recycling in aquatic environments, but there is scant research of environmental controls and mechanisms of the phosphate released from resuspended sediments under solar irradiation.

The photochemical transformation of organic matter in the environment includes direct and indirect photolysis. Sometimes, organic phosphorus itself can absorb solar energy and direct photolysis (Nowack, 2003, Lesueur et al., 2005); this phenomenon is correlated with the structure and morphology of organic phosphorus. More commonly, as shown in previous work, organic phosphorus can decompose to release PO₄^3 − by reactions with reactive oxygen species (ROS) (Senthilnathan and Philip, 2011, Li et al., 2015), such as hydroxyl radicals ( radical dot OH) (Keen et al., 2014, Xie et al., 2015, He et al., 2015). Natural constituents of water, such as nitrate (NO₃⁻) and iron ions (Fe^3 +), can take part in the photodecomposition of organic matter in natural water (Kim and Zoh, 2013). For example, NO₃⁻ and Fe^3 + can produce OH in water when excited by solar light (Passananti et al., 2013, Shah et al., 2015, Boucheloukh et al., 2012); these radicals can mediate the photochemical decomposition of organic matter in the natural environment. As far as we know, the relation between the produced radical dot OH and the PO₄^3 − released from organic phosphorus photodegradation during sediment resuspension under solar irradiation has not been reported.

Here, we undertook a detailed study of PO₄^3 − release from resuspended sediments of a shallow eutrophic lake, under simulated solar irradiation. The initial concentration of the resuspended sediment, the particle size of the sediment and the radiation spectrum were considered when the effects of environmental parameters on the photorelease of PO₄^3 − were explored. Sequential chemical extractions and ³¹P NMR were used to investigate variations in the phosphorus speciation during sediment resuspension under solar irradiation; these techniques are useful in understanding the major forms of phosphorus and the pathways of phosphorus photorelease. In addition, enzyme activity and radical trapping were applied to identify the biomineralization and photochemical degradation of the phosphate released from resuspended sediments under solar irradiation. The results will be useful to understanding the phosphorus cycle in aquatic environment.

Section snippets

Sample collection and processing

The water and sediment used in the experiment were collected from Lake Nanhu in Wuhan, China, on May 16, 2015, and the specific sampling location is shown in Fig. S1. Lake Nanhu is a shallow, hypertrophic lake with an average depth of 1.5–3 m that receives urban rainwater and is polluted by municipal wastewater. The surface sediment was collected from the lake using a Peterson dredge, put into clean plastic bags, and transported back to the lab; the overlying water was collected at the same site

Release of PO₄^3 − during sediment resuspension

To understand the effect of the photodegradation of the dissolved organic phosphorus on the amount of PO₄^3 − released from the resuspended sediment under solar irradiation, the PO₄^3 − released in the lake water sample and suspension was filtered with a 0.2 μm polysulfone membrane. As shown in Fig. 1a and b, the initial concentration of PO₄^3 − in the filtered water and filtered autoclaved sediment resuspension was 0.18 and 0.30 mg/L, respectively. The initial concentration of the PO₄^3 − in the

Conclusions

This study revealed the mechanism of PO₄^3 − release from resuspended sediments under solar irradiation. The amount of PO₄^3 − that was released dramatically increased when the sediment resuspension occurred under solar irradiation, and the initial concentration of the resuspended sediment, the particle size of the resuspended sediment and radiation spectrum affected this process significantly. ³¹P NMR and the sequential extraction of organic phosphorus clearly demonstrated that the organic

Acknowledgments

This research was supported by the Fundamental Research Funds for the Central Universities (2662016PY061), the National Natural Science Foundation of China (41230748, 41401547), the Fok Ying Tong Education Foundation, China (151078) and the China Postdoctoral Science Foundation (2013M540619, 2015T80855).

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Citation Excerpt :
This could be an effective pretreatment method for industrial wastewater treatment. Our previous studies have shown that DOP in the resuspended sediment from shallow lakes could release dissolved Pi under simulated sunlight irradiation and further determined the content and species of organic phosphorus using sequential extraction and 31P-NMR methods (Guo et al., 2020; Li et al., 2019, 2017). However, due to the 31P-NMR technology is limited by the signal detection limit and low sensitivity to low molecular weight organic compounds, it is necessary to further understand DOP with an adequate molecular resolution in order to evaluate its role in eutrophic shallow lakes.
Dissolved organic phosphorus (DOP) is a key factor in the water eutrophication process because of its high potential bioavailability and inorganic phosphate (Pi) compensation ability through bio- and photo-mineralization. However, the research on the characterization and transformation of DOP is insufficient owing to their complex composition. This study investigates the release of dissolved Pi from DOP photo-mineralization in Lake Dong based on Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis. The results showed that the photo-release of dissolved Pi is spatially heterogeneous in Lake Dong and is consistent with the distribution of DOP concentration. The FT-ICR MS results showed that the simulated irradiation decreased the relative abundance (RA) of the DOP molecular formulae with higher molecular weight (MW) and higher double bond equivalence values (DBE), while the RA of DOP molecular formulae with lower MW and lower DBE value increased or remained. Besides, the total RA of lipid-like formulae increased from 49.09% to 55.90%, while the oxy-aromatic-like formulae decreased from 50.91% to 44.10%, suggesting that simulated irradiation would influence the potential bioavailability of DOP. As the main photolysis medium during DOP photo-mineralization, the hydroxyl radicals (∙OH) are mainly derived from dissolved organic matter (DOM) compared to the nitrate (NO₃⁻) and iron ion (Fe³⁺) in Lake Dong. These results are important in understanding the ability and mechanism of DOP photo-mineralization and provide suggestions for cycling phosphorus in eutrophic shallow lakes.

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Mechanisms of the photochemical release of phosphate from resuspended sediments under solar irradiation

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Sample collection and processing

Release of PO43 − during sediment resuspension

Conclusions

Acknowledgments

Talanta

Water Res.

Appl. Catal. B Environ.

Geomorphology

Water Res.

Org. Geochem.

Chemosphere

Sci. Total Environ.

Chemosphere

Chem. Eng. J.

Chemosphere

Mar. Chem.

J. Hydrol.

Mar. Chem.

Water Res.

Water Res.

Sci. Total Environ.

J. Hazard. Mater.

Chem. Eng. J.

J. Photochem. Photobiol. A Chem.

Mar. Chem.

Water Sci. Eng.

Particuology

Release of PO₄^3 − during sediment resuspension