Mechanisms of the photochemical release of phosphate from resuspended sediments under solar irradiation
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 PO43 − was released under solar irradiation than in dark conditions. Because biological effects had been eliminated, the increase in PO43 − 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 PO43 − by reactions with reactive oxygen species (ROS) (Senthilnathan and Philip, 2011, Li et al., 2015), such as hydroxyl radicals (OH) (Keen et al., 2014, Xie et al., 2015, He et al., 2015). Natural constituents of water, such as nitrate (NO3−) and iron ions (Fe3 +), can take part in the photodecomposition of organic matter in natural water (Kim and Zoh, 2013). For example, NO3− and Fe3 + 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 OH and the PO43 − released from organic phosphorus photodegradation during sediment resuspension under solar irradiation has not been reported.
Here, we undertook a detailed study of PO43 − 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 PO43 − 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 PO43 − during sediment resuspension
To understand the effect of the photodegradation of the dissolved organic phosphorus on the amount of PO43 − released from the resuspended sediment under solar irradiation, the PO43 − 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 PO43 − in the filtered water and filtered autoclaved sediment resuspension was 0.18 and 0.30 mg/L, respectively. The initial concentration of the PO43 − in the
Conclusions
This study revealed the mechanism of PO43 − release from resuspended sediments under solar irradiation. The amount of PO43 − 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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2022, Water ResearchCitation 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.