Phosphate and fluoride are important contaminants in water bodies. The discharging of phosphate and fluoride mainly come from industrial activities. For instance, the phosphate and fluoride concentration of wastewater generating from fertilizer production could achieve 4540 and 9720 mg·L^?1, respectively (Bagastyo et al., 2017). And wastewater mixed with fluoride and phosphate is also produced in semiconductor manufacturing (Park et al., 2008). Surveys revealed that high level of phosphate and fluoride in groundwater occurred in many areas around the word such as China, Mexico and India (Kong et al., 2019). The excessive phosphate in water causes eutrophication and enormous death of aquatic lives, while long-term intake of fluoride often results in dental and skeletal fluorosis (Koilraj and Sasaki, 2017; Zhao et al., 2018). The International Agency for Research on Cancer (IARC) has classified phosphate and fluoride as carcinogenic to humans (International Agency for Research on Cancer, 2021). Moreover, the coexistence of phosphate and fluoride exposes more severe damage to human security (Kong et al., 2019). Therefore, it has become a global environmental requirement to remove the phosphate and fluoride from water systems.

Several technologies such as ion exchange (Dong et al., 2020; Grzegorzek et al., 2020), adsorption (Murambasvina and Mahamadi, 2020; Oginni et al., 2020), chemical precipitation (Huang et al., 2017) and coagulation (Bakshi et al., 2020; Dubey et al., 2018) have been used for phosphate and fluoride removal. However, most of these techniques have disadvantages and limitations in practical application for the high energy consumption and complex operation process. Adsorption is considered as a promising technique for the removal of pollutants from aqueous solution with multiple virtues of wide material availability, easy operation and low operational cost (Saadat et al., 2018). In recent years, diverse adsorbents including metal oxides/hydroxides, biochar, industrial and agricultural by?products have been developed for phosphate and fluoride removal from wastewater (Goswami and Kumar, 2018; Kong et al., 2020; Kumari et al., 2020; Liu et al., 2018; Ye et al., 2020). Solid wastes, such as steel slag (Claveau-Mallet et al., 2013; Sellner et al., 2019), fly ash (Hermassi et al., 2017; Ye et al., 2019) and basic oxygen furnace slag (Islam and Patel, 2011; Xue et al., 2009), have attracted extensive attention among these sorbents on account of their easy?gained and low?cost. These solid wastes are usually composed of oxides, such as SiO₂, MgO, CaO, Fe₂O₃ and Al₂O₃, that would promote the removal of phosphate and fluoride through electrostatic attraction, precipitation, and ligand exchange (Lee et al., 2020; Li et al., 2020b; Xu et al., 2015; Yin et al., 2017). Additionally, the large specific surface areas and abundant porous structure of these solid wastes can greatly enhance their adsorption capacity (Qiu and Duan, 2019).

Straw ash (SA) is a kind of solid waste after the thermo?chemical transformation of straw, and has the characteristics of low density, high porosity and large surface area. SA has been proved to be a promising adsorbent for the removal of heavy metals (Arshadi et al., 2014; Zhang et al., 2019). It could immobilize heavy metals through chemical precipitation, ion exchange and surface complexation. Besides, the alkaline of SA equips it with excellent acid neutralization capacity. However, there have been no relevant reports on the use of SA as phosphate and fluoride adsorption material. Previous studies have showed that alkaline residue is an effective adsorbent for the removal of anionic dye and phosphate (Yan et al., 2014). And the main components of SA (i.e., CaO, Fe₂O₃, SiO₂ and Al₂O₃) enable it to adsorb or precipitate phosphate and fluoride. Therefore, SA can be considered as a potential adsorbent for the removal of phosphate and fluoride from aqueous solution.

To explore the feasibility of the SA to be used as adsorbent for phosphate and fluoride removal, the effects and mechanisms of the SA on removing phosphate and fluoride from artificially acid solutions were evaluated in this work for the first time. The aims are to exhibit the potential application of the SA for the treatment of industrial wastewater contaminated with phosphate and fluoride, and provide theoretical support for its practical application.