Effect of tea polyphenols on copper adsorption and manganese release in two variable-charge soils

Batch experiments were conducted to investigate the effects of tea polyphenols (TPs) on Cu(II) adsorption and Mn2+ released in two variable-charge soils (an Oxisol and an Ultisol). The results confirmed that TPs enhanced Cu(II) adsorption onto and the release of Mn2+ from the two soils. The adsorption of TPs decreased the positive charge of soils or made negatively charged soil more negative. TPs can enhance Cu(II) adsorption by the two soils through an electrostatic mechanism, but its effects mainly occur through a non-electrostatic mechanism; namely, formation of soil-tea polyphenols-Cu2+ ternary complexes on soil surfaces. Tea polyphenols increased not only the maximum adsorption capacity of the two soils for Cu(II), but also the binding strength of Cu(II) on the soils. The presence of TPs increased the release of Mn2+ from the two soils through reductive dissolution of soil Mn oxides by TPs, and this was more evident in the Oxisol due to greater amount of easily reducible manganese in the soil. The amount of Mn2+ released from the soils increased with decreasing pH, and this was more evident in the presence of TPs. Cu2+ and Mn2+ competed with each other for adsorption sites on the soils, which increased the release of Mn2+ from the soils and decreased the adsorption of Cu(II). Overall, TPs enhanced Cu(II) adsorption by variable charge soils and could therefore increase the immobilization of Cu(II) in the soils. However, TPs increased the amount of Mn2+ released from variable-charge soils and thus the activity of manganese in the soils.