Influence of humic substances on phosphate adsorption by aluminium and iron oxides

In order to elucidate the influence of humic substances on phosphate adsorption, the amounts of phosphate adsorbed by synthetic aluminium oxide (Al(OH)3), ferrihydrite (Fe5O3(OH)9) and goethite (α-FeOOH) were determined at different concentrations, adsorption times, origins of humic acid (HA) and fulvic acid (FA) and order of addition of adsorbates. HA and FA were isolated from a Danish Histosol, two Podzols and an Umbrisol and used after purification for mineral matter (ash contents <3%). The experiments were performed with up to 14 mmol L−1 of HA-C or FA-C corresponding to 0.8–1.7 mmol L−1 of carboxylic groups (active surface complexation groups). These concentrations are comparable to concentrations of naturally occurring dissolved organic compounds (DOC) in mineral soil layers and to the phosphate concentrations tested (0–1.6 mM). Phosphate adsorption data were well-fitted by the Langmuir adsorption equation and the phosphate adsorption maximum (b) and binding constant (K) were calculated for various combinations of adsorbent, adsorbate, concentration and adsorption time. For adsorption onto goethite, b and K were independent of adsorption time (3–28 days), whereas b increased 10–40% and K 2–3 times during 3 to 28 days with aluminium oxide and ferrihydrite as adsorbents. The well-crystallised goethite adsorbed 2.7 μmol m−2 of phosphate, while monolayer coverage of the poorly ordered oxides corresponded to 5 μmol m−2 (ferrihydrite) and 4 μmol m−2 (aluminium oxide). Together with a 2–4 times higher K for the goethite–phosphate system, these figures indicate formation of a binuclear surface complex between phosphate and goethite while phosphate probably forms a mononuclear complex on the ferrihydrite and aluminium oxides surfaces. The presence of HA or FA together with phosphate in the adsorption solution had limited effect on adsorbed phosphate. Although small decreases of b and K were found after 3 days of equilibration with aluminium oxide and ferrihydrite as adsorbents, prolonged equilibration eliminated the differences between the amounts of phosphate adsorbed in the presence and absence of HA or FA. Moreover, compared to simultaneous addition of the two adsorbates, pre-adsorption of HA to goethite 3 days before phosphate addition did not change the b and K parameters. Therefore, humic substances can be concluded to have limited influence on phosphate adsorption by aluminium and iron oxides.