MOLYBDENUM ADSORPTION BY ALUMINUM AND IRON-OXIDES AND HUMIC-ACID

It is hypothesized that soil adsorption properties can be estimated from contents and adsorption properties of individual soil components (additivity principle), if the data are consistent. Therefore, adsorption of molybdenum (VI) by synthetic aluminum oxide, ferrihydrite, goethite, extracted humic acid and a sandy soil sample was determined as a function of Mo concentration (0-1.2 mmol/L) and pH (3.5-8). For each pH and adsorbent, the Langmuir adsorption maximum (adsorption capacity) was calculated. For the oxides, plots of these capacities against pH resulted in adsorption envelopes exhibiting maxima at pH 4-5, i.e., near the pK(a1) and pK(a2) for molybdenum acid. At this pH, adsorption corresponded to 1.8 mumole/m2 (aluminum oxide), 2.4 mumole/m2 (ferrihydrite), and 3.6 mumole/m2 (goethite), suggesting formation of binuclear surface complexes involving monomeric Mo. The corresponding plot for humic acid differed in shape by rapidly decreasing adsorption from its maximum at pH 3.5, indicating a different mechanism, probably complex formation between carboxyl and phenol groups on humic acid and Mo in octahedral coordination. Good agreement between measured Mo adsorption by the soil and the amount calculated from the adsorption capacities of the pure oxides and humic acid and the contents of oxalate- and dithionite-extractable Al and Fe and total carbon in the soil for the entire pH range emphasizes the importance of the Al and Fe oxides and organic matter for soil Mo adsorption but also indicates that adsorption properties of various constituents in soil are additive.