Phosphorus is an important biogenic element but excess dissolved phosphate in water bodies contributes to eutrophication (Davies et al. 2014). Investigations of the source of phosphorus and cycling process in freshwater bodies are difficult and limited by methodology. Some studies have indirectly investigated the sources, migration, and transformation of phosphorus in water bodies using sediment fingerprinting (Lübeck et al. 2020), microbial community fingerprinting (Heathwaite and Johnes 2015), and spatio-temporal variation in phosphorus speciation (Heathwaite and Johnes 2015). However, challenges remain with quantitative analysis of phosphorus sources and the phosphorus cycle.
Stable isotope analysis is an effective tool for tracing the sources and cycles of elements in nature. Carbon and nitrogen are widely used in the stable isotope analysis of sources and cycles (Huang et al. 2019). Because there is only one stable isotope of phosphorus, it is impossible to use the stable isotope of phosphorus to trace its sources and cycle (Liu et al. 2021). However, most phosphorus in nature exists in the form of orthophosphate (PO43−), and oxygen has three stable isotopes (16O, 17O, and 18O) (Jaisi and Blake 2014). Therefore, the phosphate oxygen isotope (δ18OP) can be used to trace phosphorus sources and the phosphorus cycle. Research in this area is currently in a stage of rapid development. This technology involves enriching phosphate in environmental samples, separation, and purification to obtain a bright yellow Ag3PO4 solid. The phosphorus stable isotope ratio is then measured by high temperature pyrolysis-stable isotope ratio mass spectrometry (Gross et al. 2013). The main issues hindering wide application of this technology are the cumbersome pretreatment method, large sample requirements, and poor method adaptability. Phosphate-selective adsorption materials could be used to achieve in situ phosphate enrichment and highly efficient laboratory elution to optimize the pretreatment method (Tcaci et al. 2019). This will avoid the need for collection of a large number of samples and simplify the subsequent purification steps (Liu et al. 2020). When optimizing the phosphate oxygen isotope pretreatment method, phosphate-selective adsorption materials could be used to prepare an efficient in situ enrichment device to achieve rapid enrichment and efficient laboratory elution of phosphate.
Hydrated zirconia is a phosphate-selective adsorption material that has promising application prospects (Liu et al. 2018). The metal-based inner layer of this material can provide selective adsorption of phosphate (Li et al. 2020). To date, many studies have investigated preparation methods and the adsorption performance of hydrated zirconia (Feng et al. 2021; Wang and Wei 2021; Li et al. 2019; Bui et al. 2021). However, because hydrated zirconia is a powder, it is not suitable for release and recovery in rivers. To solve these issues, hydrated zirconium oxide loaded on the surface of a porous granular material could be used as a selective adsorption material for phosphate. Various studies have investigated the preparation and adsorption performance of zirconium support materials, such as a hydrated zirconium oxide support resin (Sowmya and Meenakshi 2014), natural zeolite (Vera-Puerto et al. 2020; Mosa et al. 2020), activated carbon (Almanassra. et al. 2021; Velu et al. 2020), ceramsite (Deng et al. 2019; Qiu et al. 2012), oyster shells (Lee et al. 2005; Tran et al. 2020), activated carbon fibers (Sakamoto et al. 2021), and bentonite (Xi et al. 2021; Angkawijaya et al. 2020). The maximum adsorption capacity range for these materials is 4.43–26.12 mg·g−1. Most of these materials have been tested in highly concentrated phosphate solutions with the aim of obtaining high-performance phosphate-selective adsorbents for remediation of eutrophic water bodies. Little attention has been paid to phosphate elution from the support materials. In addition, the maximum adsorption capacity and time to reach adsorption equilibrium are not important when evaluating the enrichment effect for use in freshwater with a low phosphate concentration.
To address these issues, we used hydrated zirconia as a phosphate-selective adsorption material, and comprehensively investigated the macroporous adsorption, mechanical strength, economics, and availability. We evaluated zeolite, D001 macroporous resin, activated carbon, and ceramsite as potential support materials. The optimum support material was selected after laboratory experiments, and an in situ phosphate-enrichment blanket was prepared using this material. Response surface methodology was used to determine the optimum in situ enrichment time, material dose, and elution time for rapid in situ enrichment and highly efficient elution of phosphate in the pretreatment method. This method could help promote the application of phosphate oxygen isotope analysis in freshwater bodies.