# Removal of Phosphate from Aqueous Solution by
## 1 Introduction  
As an essential nutrient element, phosphorus supports the growth and metabolism of living organisms, and its content and supply capacity directly regulate the production level of the plant [1]. Recently, large quantities of phosphorus have been mined from mineral deposits and used as fertilizer in agricultural activities to satisfy the demand for food [2]. Commonly, a high amount of fertilizer input ensures crop production yield in the soil [3]. However, the low utilization efficiency of fertilizer results in a considerable quantity of nutrients remaining in the soil [4], especially phosphorus, for which the utilization rate is only 10-15% [5]. On the one hand, phosphorus enters the water body with rainfall andwater flow scouring, leading to water pollution and even eutrophication [6, 7]. Several studies have revealed that high concentrations of phosphorus stimulate the uncontrolled growth of algae and excessive consumption of dissolved oxygen, resulting in the extinction of fish and shrimp and the deterioration of water quality, thus affecting human health [8, 9]. On the other hand, it was evinced that the large-scale exploitation of phosphorus resources will eventually lead to a shortage [10]. Therefore, removing phosphorus from water is highly needed to prevent and control water pollution [11]. There are several technologies that can be utilized for removing phosphate from water, including physiochemical technologies and biological methods [12, 13, 14, 15]. These methods are costly, operate under sensitive conditions, and do not recover phosphorus resources [16, 17]. Presently, the adsorption method has become a common and effective water treatment method for removing phosphate because it is easy to use, cheap, and convenient [8]. In addition, after adsorbing phosphorus, the adsorbent can be used again as a phosphorus resource [18, 19]. However, the selection of the adsorbent is crucial for phosphorus removal by adsorption [20]. Therefore, it is vital to make a cheap and easy-to-operate adsorbent in order to use the adsorption methods universally. Natural minerals such as zeolite, montmorillonite, and diatomite have been widely used as adsorbents to improve the adsorption of phosphate [21, 22, 23]. Among them, zeolite is considered as a wastewater treatment material with a wide application prospect due to its abundant resources, low price, and stable structure [24]. However, due to the negative charge resulting from the homogeneous substitution of cations in the crystal lattice, zeolite has excellent adsorption for removing cation pollutants from water. In contrast, it has low affinities for anions such as phosphorus [25]. Numerous studies reported that phosphate adsorption by zeolite was enhanced by its thermal pyrolysis and acid modification [26]. This means that after thermal pyrolysis and acid modification, the zeolite significantly enhanced the adsorption of phosphorus in water [27, 28]. Biochar, a multifunctional product made from agricultural waste or household waste, has gained popularity worldwide [29, 30]. In addition to the improvement of agricultural soil, enhancement of agricultural production, and carbon sequestration [31], biochar has been widely utilized in the remediation of soil polluted by heavy metals [32] and water pollution control [33]. However, the implications of biochar application on water pollution, such as phosphorus removal, are regulated by the variation of biochar physicochemical properties, which result from the diverse feedstock and pyrolysis regimes [34, 35]. The main obstacles to the widespread use of biochar to remove phosphorus from water are that biochar has a low density, large volume, and inconvenient transportation. In addition, biochar lacks attachments and cannot be easily separated from the liquid, which also hinders its application [36].