Copper(II) binding by free and kaolinite-sorbed humic substances

Humic preparations isolated from different sources—soils (a soddy-podzolic soil and a typical chernozem), high-moor peat, and brown coal—have been used. To analyze the binding of copper ions by humic substances (HSs), the preparations were obtained in two forms: solutions and humic-clay complexes (HSs irreversibly sorbed on kaolinite). With this approach, the binding of copper(II) ions by HSs has been studied in different systems: (1) Cu(II)-HSs irreversibly sorbed on kaolinite, (2) Cu(II)-dissolved HSs, and (3) Cu(II)-dissolved HSs-HSs irreversibly sorbed on kaolinite. In the systems containing both dissolved HSs and humic-clay complexes, HSs of similar structure isolated from the same source were used. The quantitative estimation of the copper binding was based on the constant of sorption (K) for HSs in humic-kaolinite complexes and the stability constant (β) of complexes for free (dissolved) substances. Both parameters were expressed in similar units: L/kg. The values of logK = 3.31—3.33 are independent of the quantity and quality of the HSs in the sorption complexes but reliably exceed the K value for pure kaolinite (2.92). The value of β is not affected by the presence of insoluble HSs together with their soluble forms, but it depends on the source of HSs. The value of logβ varies in the range from 5.62 to 6.93, which significantly exceeds K and indicates a significantly higher affinity of dissolved HSs for copper ions than that of irreversibly sorbed HSs. The revealed regularities have shown that the content of HSs in the soil solution can significantly affect the mobility of a heavy metal bound to the soil organic matter.