Silicate minerals control the potential uses of phosphorus-laden mineral-engineered biochar as phosphorus fertilizers

Silicate minerals constitute the main components in silicon(Si)-rich biomass,affecting the phosphorus(P)adsorption and release competencies of mineral-engineered biochar;however,the mechanisms underlying their differences remain largely unresolved.To examine these interactions,we investigated the mineralogical compositions and quantified the P-adsorption capacities of Al-,Fe-,Mn-,Zn-,and Mg-engineered biochars from Si-rich rice husk material.The potential uses of P-laden mineral-engineered biochar for P fertilizers were assessed using citric acid extraction.The results from X-ray diffraction,scanning electron microscopy,and Fourier transform infrared spectrometry revealed that mixed metal(oxyhydr)oxides and metal-silicate compounds precipitated in the biochar structure and acted as the main P adsorbents.Micro-crystalline silicates derived from the biomass-induced metal-silicate precipitates in all engineered biochars,which effectively retained the aqueous P with varying excellent capacities(25.6-46.5 mg/g)but relatively slow kinetics(48 h).The suitability of the Zn-,Mg-,Mn-,and Fe-biochars as P-recycled fertilizers was confirmed by the high amounts of citric acid extractable P(19-69%of the total P).Varying amounts of Zn,Mg,and Mn(34-47%of the total host metals)were also released from the engineered biochar through ligand-promoted dissolution.Our data shed light on the novel potential utilization of Mn-,Mg-and Zn-biochars from Si-rich biomass for P retrieval and their use for P,Mg,and micronutrient(Mn and Zn)fertilizers.Regarding the P removal capacity,the mineral-engineered biochar needed a longer adsorption period than conventional metal-engineered biochar.