Phosphorus adsorption by calcium chloride-modified buckwheat hulls biochar and the potential application as a fertilizer

This study aimed to investigate the adsorption and desorption properties of phosphorus on CaCl2-modified buckwheat hulls biochar (labelled BBC) and the potential application of BBC saturated with phosphorus as a fertilizer. The findings indicated that phosphorus adsorption was most effective when the buckwheat hulls and CaCl2 mass ratio was 1:1, the mixture was pyrolyzed at 700 °C (labelled BBC1:1-700), and the BBC1:1-700 dosage was 2.0 g L−1. Thermodynamic analysis revealed that phosphorus adsorption onto BBC1:1-700 was a spontaneous process, with a ΔG0 of −1.12 kJ mol−1. The adsorption isotherm and kinetic models indicated that phosphorus adsorption was a monolayer chemisorption process. XRD, FT-IR and XPS characterization revealed that a Ca–P chemical reaction occurred between BBC1:1-700 and phosphorus to form CaHPO4. Pot experiments showed that, compared with the soils treated with calcium-containing phosphorus fertilizer (P2O5,12%), soils treated with BBC1:1-700 saturated with phosphorus (labelled BBC1:1-700-P) exhibited lower soil bulk weight (from 1.23 to 1.09 g cm−3), increased organic matter content (from 3.62 to 15.83 g kg−1) and phosphorus content (from 0.68 to 1.38 g kg−1), and promoted the growth of buckwheat plants, with root lengths ranging from 1.47 to 4.90 cm and stem lengths ranging from 19.23 to 25.33 cm. These findings indicated that BBC1:1-700 can be used to simultaneously solve the high phosphorus in water bodies and insufficient phosphorus-fertilizer in soils.