The Adsorption of Humic and Fulvic Acids by Clays

Summary The adsorption from suspension of the humic‐ and fulvic‐acid components of a soil organic‐matter extract by bentonite was extremely rapid, and was unaffected by temperature. For the humic fraction the extent of adsorption was: Ca and Mg systems > acid systems > K and Na systems; for the fulvic fraction: acid > Ca > Mg > K and Na systems. Only in the K systems was there an approach to the saturation of adsorption, although cation‐exchange‐capacity reductions ceased at a low level of adsorption. In the absence of exchangeable calcium there was a sharp fall in the extent of adsorption with rise in pH, particularly above pH 3.5. Only a small proportion of the organic carbon in both the fulvic and humic fractions was usually adsorbed by the bentonite. The remainder of both fractions was found to be relatively unadsorbable, while the re‐extracted adsorbed proportion remained highly adsorbable. Adsorption by kaolin in K systems was as extensive as that by bentonite, but in the acid and Ca systems it was much less. For adsorption of humic carbon the order of effectiveness was Ca > acid > K, and for fulvic carbon, acid > Ca > K, as in the bentonite systems. Lepidocrocite, goethite, β‐Fe 2 O 3 .H 2 O, and gibbsite also removed much fulvic and humic carbon from suspension, particularly at lower pH values. Mechanisms for the adsorption of organic colloids by clays are considered in the light of these results.