Fe₃O₄Nanoparticles Dispersed on Douglas Fir Biochar for Phosphate Sorption

Surface area, particle aggregation, pressure drop in columns, nanotoxicity, and commercialization difficulties limit the use of nanoparticle adsorbents. Magnetic primary nano-Fe3O4 particles (∼16.7 nm diameter) were dispersed on high-surface-area (695 m2/g) Douglas fir biochar (MBC). A cheap, commercial fast pyrolysis biochar, a syngas byproduct, was modified by chemical coprecipitation of Fe3O4 from Fe3+/Fe2+ aqueous NaOH, served as a matrix, aiding magnetite nanoparticle dispersion and reducing the extent of particle aggregation. This MBC removed ∼90.0 mg/g of phosphate from water, approximately 20 times the capacity reported for neat (∼39 nm) magnetite particles (∼5.1 mg/g). MBC was robust in fixed-bed column sorption with 82.5 mg/g (at pH 3) capacity, showing no significant equilibrium or kinetic limitations in flow versus batch sorption. The biochar support serves as an added adsorption phase for heavy metals and organic contaminants, adsorbing poorly on magnetite. MBC enables magnetic separation of exhausted adsorbent from a batch process, an alternative to filtration. The neat and phosphate-laden hybrid sorbents were was characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray, point of zero charge, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), elemental analysis, vibrating sample magnetometry, and Brunauer–Emmett–Teller surface-area and pore-volume measurements. The chemisorption mechanism versus pH, evaluated by XPS and existing literature, characterized the dominant phosphate complexes adsorbed on magnetite. The pH effect on phosphate sorption and the P 2p XPS binding energy shifts at pH from 1 to 13 are reported. A solution pH of 1–3 facilitates the formation of bidentate monoprotonated phosphate complexes [(Fe–O)2-PO2H]− at Fe–OH surface functions. H2PO42– predominates in solution at pH ∼4–6.5, which favors the formation of [Fe–O–PO3H]− at these pH values. At strongly basic pH (10–13) values, PO43– predominates and forms deprotonated chemisorbed monodentate [Fe–O–PO3]2– and bidentate [(Fe–O)2PO2]2–. Multilayer phosphate sorption and precipitation of iron phosphates were considered.