Adsorption recovery of phosphate from waste streams by Ca/Mg-biochar synthesis from marble waste, calcium-rich sepiolite and bagasse

In this study, marble waste and calcium-rich sepiolite were employed as Ca and Mg sources, and the agricultural waste was used as carbon source to synthesize two modified-biochar composites (Mar-BC800 and Sep-BC800) for removing phosphate from waste streams. The results show that Mar-BC800 and Sep-BC800 exhibited excellent performance for phosphate adsorption from aqueous solution in a wide pH range of 3–11, and the maximum adsorption capacity of phosphate by Mar-BC800 and Sep-BC800 was 263.17 and 128.21 mg/g, respectively. The adsorption process was best described by the pseudo second-order model and Langmuir model, and the dominating sorption mechanism for phosphate is that Ca2+ or Mg2+ with phosphate to form Ca5(PO4)3OH, Mg3(PO4)2 and Mg3(PO4)2·4H2O precipitate. Furthermore, Mar-BC800 and Sep-BC800 are simple to produce, the methods used to produce modified biochar are green, economic and environmentally-friendly, and the phosphate adsorbed modified biochar can applied as fertilizer to farmland soil, which have the functions of increasing soil fertility and improving soil physical and chemical properties.