Rapid industrial development, urban construction, and changes in various human activities have resulted in a significant increase in the levels of nitrogen (N) and phosphorus (P) entering coastal waters, rivers, and lakes. Agriculture and urban activities are the main sources of P and N in aquatic ecosystems. This influx of N and P has subsequently led to diverse problems such as toxic algal blooms, loss of oxygen, fish kills, loss of biodiversity, loss of aquatic plant beds and coral reefs, and other problems (Carpenter et al.1998). The excessive addition of N and P to water bodies has a detrimental effect on aquatic ecosystems, leading to a decline in their health and functioning. This, in turn, negatively impacts the usability of water for various purposes such as drinking, industrial processes, agriculture, recreation, and more. Therefore, it is imperative to prioritize the removal of N and P from these aquatic ecosystems to mitigate these detrimental effects.
Among many pollution control technologies, adsorption is very suitable for treating low concentration polluted water bodies due to its advantages of selective removal, reusability, simple operation, etc. It is crucial to develop low-cost adsorbent and apply it to practical projects. At present, it is one of the research hotspots to choose environment-friendly industrial and agricultural wastes as raw materials to prepare adsorption materials.
Fly ash is a kind of particulate waste discharged from the combustion of coal-fired power plants. It is reported that the amount of fly ash generated by coal combustion in China has exceeded 100 million tons, and the emissions have increased year by year (Lin et al.2022). The stockpiling of a large amount of fly ash not only occupies a large amount of land resources, but also causes dust and pollutes the atmosphere and groundwater. At present, fly ash is mainly used as raw material or auxiliary material of cement in the construction industry, such as clinker partially replacing Portland cement and magnesium sulfate cement(Chen et al.2021). However, the utilization rate of this method is limited. It is reported that the global utilization rate of fly ash is 16% – 25% (Morales-Ospino et al.2020). Therefore, the development of new methods for forming high value-added products is the focus of attention of all countries, and the synthesis of zeolite is one of the utilization methods.
Due to the similar composition of fly ash and zeolite, many researchers have synthesized zeolite for the purpose of ammonia removal. Among the various synthesis methods, the hydrothermal method is the most common and stable synthesis method. However, a large amount of waste alkali liquor will be produced during the synthesis process, causing secondary pollution. Zeolite is classified as a hydrated aluminosilicate compound. This framework comprises tetrahedrally coordinated aluminum, silicon, and oxygen atoms. The surface of zeolite carries a net negative charge and therefore has a limited ability to phosphorus adsorption.
As for the utilization of waste alkali liquor, Xie et al (2014) used the waste lye and lanthanum chloride to synthesize lanthanum hydroxide. They found that the generated lanthanum hydroxide is amorphous, has a higher specific surface area and higher phosphate adsorption performance than the commercially available lanthanum hydroxide.
As for studies on phosphorus removal by zeolite, the absorption capacity of zeolite phosphate is usually enhanced by introducing metals, such as calcium, iron, magnesium or rare earth elements. While lanthanum is a rare earth element, research shows that lanthanum has high affinity for phosphate and can form solid and insoluble LaPO4 (Liu et al.2020). Lanthanum can be introduced into zeolite through alkali solution immersion and calcinations (Liu et al.2017). Lanthanum-modified zeolite has received more attention in removing phosphorus from water (Min et al.2019; Copetti et al. 2016). It is reported that the adsorption capacity of lanthanum-modified zeolite is about 2 times higher than that of natural and synthetic zeolite.
The above research has done some exploration work in the resource utilization of waste lye and the modificarion method of zeolite with lanthanum under alkaline conditions, but there are still the following problems: (1) Only the waste lye and lanthanum chloride in the synthetic zeolite from fly ash were used to synthesize lanthanum hydroxide, and it was found that the generated lanthanum hydroxide had high P adsorption performance. There are few reports on the removal of N and P by loading waste lye and lanthanum chloride into synthetic zeolite; (2) lack of systematic and comprehensive research on the mechanism of N and P removal based on loading waste lye and lanthanum chloride into synthetic zeolite, including P removal mechanism under the condition of the coexistence of lanthanum and other metal ions in waste lye (Fe, Ca) and the efficiency of N removal after lanthanum-modified synthetic zeolite; (3) Most current adsorbent materials are based on performance evaluations in a laboratory setting, and less consideration is given to actual use in the environment.
In this work, we developed a synthesis zeolite based on fly ash for the removal of N and P by the modification method of waste lye and lanthanum chloride. In experiment condition, the preparation of an efficient adsorbent by combining waste lye, zeolite synthesized from fly ash, and Lanthanum chloride was analysis. Then, the adsorption performances and mechanisms of the developed adsorbent were evaluated. Finally, for the consideration of practical application potential, the adsorbents were made into granular pellets to evaluate the effectiveness of their application in Subsurface Flow Constructed Wetland (SFCW) for N and P removal.