Preparation and application of novel rice husk biochar–calcite composites for phosphate removal from aqueous medium

Excessive phosphorus (P) in water bodies causes eutrophication, which threatens the health of the environment. In the present study, a novel rice husk biochar–calcite composite (BRH-C) was prepared. The pyrolysis conditions and phosphate removal processes were optimized using central composite design. The optimized pyrolysis conditions for the preparation of BRH-C were as follows: a pyrolysis temperature of 700 °C, pyrolysis time of 2.3 h, and rice husk–calcite (RH-C) ratio of 4.2:1 (w/w). The maximum removal and adsorption capacities of optimized BRH-C were 87.3% and 1.76 mg/g, respectively, at low phosphate concentrations. The pyrolysis temperature had positive linear and quadratic effects on the phosphate removal, whereas the pyrolysis time had a negative quadratic effect. The RH-C ratio had a positive quadratic effect. The maximum phosphate removal (54.2%) and adsorption capacity (10.72 mg/g) were achieved at phosphate concentration of 95 mg/L, 0.24 g of BRH-C, pH 5.4, and contact time of 11.75 h. Phosphate removal by BRH-C was maximum at lower concentrations (10–25 mg/L), whereas phosphate removal by calcite was maximum at higher concentrations (75–125 mg/L). Calcite altered the yield and textural properties of BRH-C and imparted characteristics calcite functional groups and minerals to BRH-C. Freundlich adsorption isotherm and second-order kinetic model suggested heterogeneous and multilayer chemisorption of phosphate onto both BRH-C and calcite. Presence of NaCl, NaNO3, KCl, and NaHCO3 in solution reduced the amount of phosphate removal by BRH-C. Promising results for phosphate removal were achieved only for up to two cycles of BRH-C regeneration. BRH-C is a cost-effective and ecofriendly adsorbent prepared from rice husk and calcite, with no need for sophisticated instruments or chemicals. It was developed as an optimized adsorbent for the efficient removal of phosphate at low concentrations. [Display omitted] •An optimized rice husk biochar–calcite composite (BRH-C) for phosphate removal was prepared.•Optimized pyrolysis conditions were 700 °C, 2.3 h, and rice husk–calcite ratio of 4.2:1.•The calcite affected the textural and functional properties of BRH-C.•BRH and calcite synergistically removed phosphate better than calcite at low concentrations.•Phosphate removal occurred via pore filling, electrostatic interaction, and precipitation.