Adsorption of orthophosphate anions in aqueous solution by cationized milled solid wood residues was characterized as a function of sorbate-to-sorbent ratio (≈0.001-2.58 mmol of P/g substrate), pH (3-9), ionic strength, I (no I control; 0.001 and 0.01 M NaCl), reaction time (4 min to 24 h), and in the presence of other competing anions (0.08-50 mM SO 4 2-; 0.08-250 mM NO 3 -). Sorption isotherms revealed the presence of two kinds of adsorption sites corresponding to high and low binding affinities for orthophosphate anions. Consequently, a two-site Langmuir equation was needed to adequately describe the data over a range of solution conditions. In addition to higher sorption capacity, cationized bark possessed a higher binding energy for orthophosphate anions compared to cationized wood. The sorption capacity and binding energy for bark were 0.47 mmol of P g -1 and 295.7 L mmol -1 , respectively, and for wood, the corresponding values were 0.27 mmol g -1 and 61.4 L mmol -1 . Both the sorption capacity and binding energy decreased with increasing I, due to competition from Clions for the available anion-exchange sites. The surface charge characteristics of cationized bark (pH zpc ) 7.9) acted in concert with orthophosphate speciation to create a pH-dependent sorption behavior. Orthophosphate uptake was quite rapid and attained equilibrium levels after 3 h. Both SO 4 2and NO 3influenced percent removal but required high relative competing anion to H 2 PO 4molar ratios, i.e., 2.5-3 for SO 4 2and 25 for NO 3 -, to cause appreciable reduction. These results support our hypothesis that adsorption of orthophosphate anions on cationized bark involves ion exchange and other specific Lewis acidbase interactions.

More efficient adsorption media are needed for removing dissolved phosphorus in surface water runoff. We studied the use of cationized pine bark as a sorbent for dissolved phosphorus in water. Cationized pine bark was prepared by treating extracted milled pine bark with polyallylamine hydrochloride (PAA HCl) and epichlorohydrin (ECH) in aqueous medium. Attachment of PAA HCl to the bark was monitored by percentage of weight gain and nitrogen content. Changes in bark surface chemistry were characterized by zeta potential and inverse gas chromatography measurements. Energy dispersive x-ray analysis and scanning electron microscopy were used to determine changes in chemical composition and morphology of the cationized bark. The cationized bark showed a decrease in the dispersive component of the surface energy and a barely measurable change in the surface acid-base properties. In the pH range 2.5 to 7.9, the cationized bark showed a positive zeta potential, which changed to negative at pH 7.9 or higher. By comparison, the zeta potential of control extracted bark remained negative throughout the pH range of 2.5 to 9. We propose that the reaction of bark with PAA HCl results in a surface network of fixed cationic sites of quaternary ammonium that have mobile chloride ions as counter anions. Preliminary results from batch adsorption experiments indicate that cationized milled bark has an estimated maximum adsorption capacity of approximately 12.65 mg phosphate/g, which compares favorably with that of other well-known phosphorus sorbents.