Development of novel biochar adsorbent functionalized with layered double hydroxides for phosphate removal and its application potential as fertilizer

リン除去のための層状複酸化物を担持させたバイオ炭系新規吸着材の開発とその肥料としての応用の可能性

Development of novel biochar adsorbent functionalized with layered double hydroxides for phosphate removal and its application potential as fertilizer

Degree Grantors Yamaguchi University

The reduction of excessive discharge of phosphate into water bodies is a dominant theme to combat the critical eutrophication issue and requires the development of high-performance materials for effective phosphate treatment. In this study, rice straw was used as a raw material for the synthesis of biochar functionalized with layered double hydroxides (BC-LDHs) as efficacious phosphate adsorbents, and their successful synthesis was corroborated via characterization analysis. Experimental investigations, including pH, coexisting anion, reaction time, and initial phosphate concentration effects were systematically performed with selected BC-LDHs 6 and pure LDHs. An optimum pH of 3.0 was observed in both samples. Kinetic and isotherm studies indicated that phosphate adsorption on these samples was controlled by the pseudo-second-order model and the Freundlich model. Comparative kinetic tests also demonstrated that BC-LDHs 6 and pure LDHs reached the equilibrium within 24 h and 3 h, respectively. Nonetheless, the maximum adsorption capacity of the composite was 192 mg/g, which was higher than that of pure LDHs (166 mg/g). The coexistence of various anions negligibly affected the removal efficiency of the composite; however, fluoride was the most competitive anion for adsorption on pure LDHs. The adsorption mechanisms of the composite involved electrostatic interaction, inner-sphere complexation, pore diffusion, precipitation, and reconstruction. Furthermore, phosphate adsorbed on both materials could be easily recovered by 0.1 M NaOH solution owing to the displacement reaction between phosphate and hydroxyl ions. Additional evidence from reusability experiments exhibited that the composite could maintain its good adsorption performance even after three adsorption-desorption cycles. The transformation of BC-LDHs 6 after its usage in phosphate treatment (P-BC-LDHs 6) into a fertilizer was further explored by using seed germination and early growth assays of lettuce through a comparison with phosphate-loaded LDHs (P-LDHs). Lettuce seeds germinated in all P-BC-LDH 6 treatments showed undesirable growth characteristics compared with the controls, while total germination failure was observed under high concentrations of P-LDHs. In the latter experiments, the optimal application rates for plant growth were 2.5% for P-BC-LDHs 6 and 1.0% for P-LDHs. The considerably greater biomass development and length of lettuce were visible in samples delivered from P-BC-LDHs 6 compared to those from P-LDHs. The results obtained suggest that BC-LDHs 6 is a promising adsorbent for phosphate treatment and post-adsorption BC-LDHs 6 has the application potential to serve as a fertilizer for horticultural crop production.

Creators Jittrera Buates