Orthophosphate sorption onto lanthanum-treated lignocellulosic sorbents

Inorganic/organic hybrid adsorbents for removing orthophosphate from water were prepared by lanthanum (La) treatment of bark fiber, a lignocellulosic material obtained from juniper (Juniperus monosperma). The La was anchored to the juniper bark (JB) fiber by ion exchange with Ca in the bark and was responsible for removing orthophosphate. Two La concentrations (0.01 and 0.1 M) were used that resulted in loadings of 0.198 (La/JB01) and 0.302 (La/JB02) mmol of La g(-1) of fiber, respectively. At circumneutral pH conditions, La exhibited a strong affinity for bark, but significant La desorption occurred under acidic conditions (pH < 4.5). For La/JB02, 86% of the initial La loading was released at pH 2.5, and only 0.1% at pH 7.3. Initial La loading in the bark fiber significantly influenced its orthophosphate sorption capacity, which was determined for La/JB01 and La/JB02, respectively, to be 0.188 and 0.233 mmol of P g(-1) (sorption envelopes) and 0.211 and 0.351 mmol of P g(-1) (sorption isotherms when surface site saturation occurred). The P-to-La molar ratio on the bark surface was extremely high (0.12-1.77for La/JB01 and 0.13-2.58 for La/JB02), implying that orthophosphate removal occurred by other mechanisms in addition to surface complexation. From the orthophosphate surface loading levels and the shape of the sorption isotherms, it appears that adsorption occurred at low sorbate-to-sorbent (P-to-La-treated JB) ratios and transitioned to surface precipitation at higher ratios. When surface precipitation occurred, orthophosphate uptake continued to increase with increasing initial P levels. Results from X-ray diffraction and infrared spectrometric analyses are also suggestive of the occurrence of surface precipitation during the interaction of orthophosphate with La-treated lignocellulosic materials.