Mineral Iron-Polyphenol Interplay on Heterogeneous Biochar Surfaces: Implications for Circular Nutrient Recovery in Sustainable Agriculture

Abstract

Two environmentally friendly iron-functionalized pine-wood-derived biochar adsorbents were developed and compared for phosphate recovery and reuse: (i) ferric chloride-modified biochar (Fe-BC) and (ii) green tea–mediated iron-modified biochar (GT-Fe-BC). Batch experiments demonstrated effective phosphate recovery by Fe-BC (>80%) and GT-Fe-BC (>70%) within ∼1 h. Adsorption kinetics for both materials were best described by the pseudo-second-order (PSO) model, indicating a chemisorption-dominated process. Elovich and intraparticle diffusion analyses further confirmed that phosphate uptake involved chemisorption accompanied by both film diffusion and intraparticle diffusion. Freundlich isotherm fitting indicated heterogeneous surface adsorption for both materials, with GT-Fe-BC exhibiting a broader distribution of active sites. The maximum P adsorption capacities were 9.12 mg/g for Fe-BC and 14.31 mg/g for GT-Fe-BC. Characterization analyses (SEM–EDS and FTIR) confirmed successful iron incorporation and phosphate complexation with iron-containing phases. XPS analysis further revealed phosphate adsorption through Fe–O–P inner-sphere complexation and outer-sphere interactions. In comparison to Fe-BC, GT-Fe-BC worked effectively (>98% phosphate removal) over a wide pH range of 2–10, making it a robust adsorbent. The desorption studies showed sustained phosphate and iron release from both systems. Compared to Fe-BC, GT-Fe-BC showed higher release of both species, possibly due to more reactive green tea–mediated iron phases that continuously release Fe²⁺/Fe³⁺ bound to phosphate. The study suggests that both adsorbents have strong potential for phosphate recovery and subsequent reuse as slow-release fertilizers, with their application tailored to specific nutrient recovery and delivery requirements.