Purpose Biochar amendments can alter phosphorus (P) availability in soils, though the influencing mechanisms are not yet fully understood. This work investigated the adsorption and desorption of P on ferrihydrite (F, a Feoxide widely distributed in surface environments) in order to evaluate the interactions between P and Fe-oxide in the absence or presence of biochar (F or ferrihydrite-biochar (F-B) interaction) in soils. Materials and methods Biochar was produced by pyrolysis of rice straw at 600°C in steel ring furnaces. Two-line ferrihydrite was synthesized by dropwise addition of 1 mol L −1 KOH into Fe(NO 3 ) 3 solution until the pH reached 7-8 while stirring vigorously. An F-B complex was prepared under similar conditions, except that a mixture of 10 g biochar and the Fe(NO 3 ) 3 solution was used as the starting material instead of Fe(NO 3 ) 3 alone. A batch equilibration method was used to determine sorption or desorption of P. The mechanisms of P adsorption on F and F-B complex materials were discussed. Results and discussion Adsorption of P on F decreased as the pH was increased from 3.0 to 10, but the adsorption capacity of F decreased by about 30-40% in the presence of biochar. The P chemisorption rates on F also decreased in the presence of biochar. The Freundlich model showed that the active adsorption sites on the surface of the F-B complex were energetically heterogeneous. The desorbability of adsorbed P on F was enhanced by combination with biochar. The mechanisms of P adsorption on F and F-B complex materials are different.Conclusions The results showed that the amount and rate of P adsorption on the surface of ferrihydrite decreased with the presence of biochar, and the desorbability of adsorbed P on ferrihydrite can be enhanced when combined with biochar. Thus, the presence of biochar can decrease P adsorption on the Fe-oxides and enhance P availability in soils.

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Two kinds of porous magnetic ferrite nanowires containing manganese (MnFe 2 O 4 and Mn doped Fe 3 O 4 ) have been successfully synthesized by thermal decomposition of organometallic compounds, using nitrilotriacetic acid (NA) as a chelating agent to coordinate with various ratios of Fe(II) and Mn(II) ions.The resultant MnFe 2 O 4 and Mn doped Fe 3 O 4 nanostructures are superparamagnetic, and have magnetization saturation values of about 45.9 and 48.7 emu g À1 for MnFe 2 O 4 and Mn doped Fe 3 O 4 , respectively. The Brunauer-Emmett-Teller specific surface areas of the MnFe 2 O 4 and Mn doped Fe 3 O 4 are 37.8 and 45.4 m 2 g À1 , respectively. The as-prepared porous MnFe 2 O 4 and Mn doped Fe 3 O 4 nanowires exhibit excellent ability to remove heavy metal ions and organic pollutant in waste water. In addition, these porous magnetic ferrites may be useful in other fields such as biomedicine and Li-ion batteries.