Enhanced removal of phosphate and fluoride from phosphogypsum leachate by cerium/iron-based zeolite: Adsorption behaviors and competition mechanisms.

Phosphogypsum is a by-product produced during the production of phosphate fertilizer, calcium hydrogen phosphate, and industrial phosphoric acid [[1], [2], [3]]. Currently, the phosphorus chemical industry has experienced unprecedented growth, producing a large amount of phosphogypsum [4]. At present, the stockpile of phosphogypsum in China has exceeded 800 million tons, with an annual discharge of over 70 million tons. Although the comprehensive utilization rate of phosphogypsum is more than 50 %, there is still a large amount of phosphogypsum stored in the phosphogypsum reservoirs, which is a big challenge for the ecological and environmental protection of the Yangtze River in China. The composition of phosphogypsum is complex. In addition to calcium sulfate, there are other species with high content including phosphorus and fluoride [5]. In the stacking process of phosphogypsum, the soluble phosphorus and fluoride will migrate from the slag to the water body, causing long-term pollution to the surrounding environment. Nevertheless, phosphogypsum is also an important and valuable secondary resource because it contains large amounts of phosphorus and fluoride, which are critical elements that are extensively consumed in agriculture and chemical industries [6,7]. Therefore, developing an effective method to remove and recover phosphate and fluoride from phosphogypsum leachate is significantly important for the sustainable development of industry.

At present, many technologies have been exploited for the treatment of phosphogypsum leachate including crystallization [8], biological treatment [9], adsorption [10], precipitation [11], and membrane separation [12]. Among them, adsorption has become an effective technical means for removing pollutants due to its low initial cost, simple operation, good repeatability, and high selectivity [13]. The exploitation of novel adsorbents with favorable adsorption has become a hotspot in treating phosphogypsum leachate. Zeolite is an aluminosilicate mineral, having unique structural properties, high surface area, ion-exchange and adsorption features [14]. Utilizing zeolites for wastewater treatment has attracted extensive attention for achieving sustainable environmental management [15,16]. However, due to the rapid development of the chemical industry, the discharged wastewater has become more complex and contains multiple pollutants, which might restrict the adsorption performance of the adsorbents [17,18]. Therefore, modifying zeolites with substances possessing high binding ability to targeted pollutants is a promising way to improve their adsorption capacities and selectivity.

Among various materials, the metal oxides possess excellent affinity for many anions. Particularly, the rare-earth metal oxides possess a relatively strong adsorption capacity for various anions such as fluoride and phosphate, and can overcome many shortcomings that other adsorbents exhibit. Their main mechanism of action is ion exchange and adsorption, which are caused by protonation reactions related to surface hydroxyl groups. Previous studies have verified the special selectivity of the abundant and cost-effective cerium (Ce) for the capture of phosphorus and fluoride [[19], [20], [21], [22]]. Due to this reason, many cerium-based elemental materials have been prepared and used in wastewater treatment. For example, Wang et al. prepared Ce³⁺-enriched biochar material (Ce-BC) by using a simple impregnation-precipitation-pyrolysis technology, which exhibited rapid adsorption for phosphate, reaching completion within 10??min [23]. Wujcicki et al. developed a cerium (IV)-modified chitosan-based hydrogel (Ce-CTS), achieving a removal efficiency of over 98 % for phosphate [24]. Wang and Luo fabricated cellulose beads with trapped CeO₂ nanoparticles (CeO₂@CBs) and applied them to treat low-concentration fluoride-containing wastewater. When the initial concentration was 10.0 mg/L, 99.15 % of F^– could be removed with a dosage of 1.0 g [25].

However, compared with single metal oxides, oxides composed of two metals can inherit the advantages of individual metals, enabling them to complement each other’s strengths and weaknesses [[26], [27], [28], [29]], which can significantly improve the adsorption performance. Zhu et al. developed a new La/Al bimetallic organic framework to enhance the utilization of La sites [30]. Lin et al. proposed a novel La-Zr bimetallic-modified magnetic adsorbent (La-Zr@Fe₃O₄), which had a high affinity for phosphate and could quickly separate and recover phosphorus from fish farm sludge [31]. Zhao et al. synthesized a La-anchored Zr-based organic framework material to effectively remove fluoride, which was highly effective within the pH range of 3-9, and exhibited an adsorption capacity of 57.23 mg/g [32]. Overall, although cerium oxides have great potential for removing phosphorus and fluoride, the introduction of a second metallic element (such as cost-effective Fe) into the original adsorbent is a feasible strategy for improving its properties in practical aqueous environments.

In the present study, a series of Ce/Fe oxides-loaded zeolite (Ce/Fe-ZL) adsorbents with different cerium/iron ratios were synthesized and used for removing phosphate and fluoride to enhance the treatment efficiency. In particular, the work aims at: (i) studying the influence of Fe doping on the adsorption performance of Ce/Fe-ZL, and exploring how it is affected by various environmental factors such as pH and coexisting anions; (ii) clarifying the competition between the adsorption of phosphate and fluoride, and developing potential adsorption mechanisms for them; (iii) evaluating the actual performance of Ce/Fe-ZL in phosphogypsum leachate through a dynamic column experiment. Overall, the work contributes to the exploitation of novel and efficient materials for phosphate and fluoride removal, and provides guidance on the harmless treatment of phosphogypsum leachate.