Adsorption and desorption characteristics of nitrogen, phosphorus and potassium by biochars from different raw materials

Objectives The adsorption and desorption of nitrogen, phosphorus and potassium by biochars derived from corn stalk, rice husk and rice stalk were studied. The solid storage and slow release capacity of nitrogen, phosphorus and potassium from the biochars will provide a reference for adjusting the properties of organic fertilizers using biochar as additives.

Methods The Langmuir and Freundlich equations were used for fitting the isothermal adsorption characteristics of nitrogen, phosphorus and potassium under pH 6.75 ± 0.25 at 25℃. Five extractions were made for the desorption of the absorbed N, P and K. The first four extractions were all by water. The fifth extraction agent for N was 1 mol/L KCl, for P was 0.5 mol/L NaHCO₃, and for K was 1 mol/L CH₃COONH₄.

Results The isothermal adsorption of N, P and K could be fitted by both Langmuir and Freundlich equations. Langmuir equation fitted better in the adsorption of nitrogen and phosphorus, while the Freundlich equation did better in that of K. Among the three tested biochars, the one derived from rice stalk performed the best in adsorption of N, P and K, with the maximum adsorption amounts of (2.44 ± 0.15) mg/g, (2.91 ± 0.12) mg/g and (4.97 ± 0.22) mg/g, respectively. 2) Biochar derived from different raw materials were different in both the single and total desorption rates of N, P and K. The maximum adsorption of biochar to cations (NH₄⁺ and K⁺) was affected by the amount of cation exchange and the number of acidic functional groups and pH_pzc, and the maximum adsorption of P was mainly related to the amount of cation exchange and the number of basic functional groups.

Conclusions The adsorption of N, P and K by the three biochars occurs in both monolayer and multi-layers, and the adsorption rates are varied among them. The physical adsorption by biochar contributes more than chemical ones on N and K, and chemical adsorption contributes higher on P. The solid storage and sustained release ability of NPK are different among the three biochars, rice stalk biochar is the best of them.