Adsorption of zinc on colloidal (hydr)oxides of Si, Al and Fe in the presence of a fulvic acid

The adsorption of Zn (total concentration 10−6 M) to colloidal quartz, hydrargillite and goethite (50, 300 and 70 mg/l, respectively) was studied by a batch technique at a constant ionic strength (0.01 M) but with variation of pH (3–10) and fulvic acid (FA) concentration (0, 2 or 20 mg/l). The adsorption had similar pH-dependence in all systems in the absence of FA giving a pH50 (pH of 50% adsorption) of 7.6 under these conditions. The presence of the FA reduced the overall adsorption to quartz (pH50 of 7.9 at 2 mg FA/I and 9.3 at 20 mg/1) and shifted the adsorption curves downwards (pH50 of 6.8) in the hydrargillite and goethite systems at 2 mg FA/l. At 20 mg FA/l, the adsorption in the two latter systems was increased at pH <6.5 and reduced at pH >6.5. The results reflect the affinity of the surfaces for FA as well as the formation of Zn–FA complexes (in solution and on solid surfaces).