Solution chemistry effects on orthophosphate adsorption by cationized solid wood residues

Abstract

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 SO4 2-; 0.08-250 mM NO3-). 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 Cl- ions for the available anion-exchange sites. The surface charge characteristics of cationized bark (pHzpc = 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 SO42- and NO3- influenced percent removal but required high relative competing anion to H2PO4- molar ratios, i.e., 2.5-3 for SO42- and 25 for NO3-, to cause appreciable reduction. These results support our hypothesis that adsorption of orthophosphate anions on cationized bark involves ion exchange and other specific Lewis acid-base interactions.

Citation

Karthikeyan, K.G.; Tshabalala, Mandla A.; Wang, D.; Kalbasi, M. 2004. Solution chemistry effects on orthophosphate adsorption by cationized solid wood residues. Environmental science & technology. Vol. 38, no. 3 (2004): Pages 904-911