Modification of pyrogenic carbons for phosphate sorption through binding of a cationic polymer

This study reports on the development of modified pyrogenic carbonaceous materials (PCMs) for recovering orthophosphate (PO₄-P). The PCMs include softwood and hardwood biochars and a commercial granular activated carbon (GAC) that were modified by irreversible adsorption of the quaternary ammonium polymer, poly(diallyldimethylammonium) chloride (pDADMAC), which reverses electrokinetic charge and increases PO₄-P sorption. MgO-doped biochars were prepared by a literature method for comparison. Imaging and spectroscopic analyses characterize pDADMAC coverage, MgO doping, and binding of PO₄-P. At environmentally relevant concentrations, PO₄-P sorption by the pDADMAC-treated biochars was ~100 times greater than that of the corresponding unmodified biochars, and was comparable to that of the corresponding MgO-doped biochars on a coating content basis. The pDADMAC-coated carbons bind PO₄-P by ion exchange, while the MgO-doped biochars bind PO₄-P principally by forming an amorphous Mg phosphate species. Susceptibility to competition from other relevant anions (Cl^-, NO₃^-, HCO₃^-/CO₃^2-, SO₄^2-) and poultry and dairy manure extracts was moderate and comparable for the two types of modified softwood biochars. Sorption to the pDADMAC-treated biochars appears to be more reversible than to the MgO-doped biochars using stepwise water extraction. Greater reversibility may be advantageous for trapping and recycling phosphate.