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. H2PO4
2– 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, PO4
3– 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.
