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Adsorption and desorption mechanisms of Cu2+ on amended subsurface riverbank soils

To explore the adsorption and desorption mechanisms of Cu2+ on different amended subsurface (20–40 cm) riverbank soils, two kinds of modified pomace [i.e., saponified grapefruit pomace (SGP) and saponified orange pomace (SOP)] were added to the subsurface soils of Cangxi, Nanbu, Jialing, and Hechuan from the Jialing River; Santai and Chuanshan from the Fu River; and Quxian and Guangan from the Qu River in Sichuan and Chongqing, China, respectively, at a mass ratio of 5%. Then, the batch method was used to study the isothermal adsorption and desorption characteristics of Cu2+ on all the amended soil samples. Furthermore, the morphological changes of Cu2+ on the tested soil samples were analyzed. The following results were obtained: (1) The adsorption isotherms of Cu2+ on each tested soil sample were “L” type and accorded with the Langmuir adsorption model. The Cu2+ adsorption amount of soil samples from the Fu and Jialing rivers was larger than that from the Qu River, and the maximum adsorption capacity was 269.46 mmol/kg (Jialing soil amended by SOP). (2) The stability of Cu2+ adsorption on soil samples amended by SGP was stronger than that by SOP, and the maximum desorption rate was less than 22%. The Cu2+ adsorption stability of soil samples from the Fu and Jialing rivers was stronger than that from the Qu River. (3) The main existing form of Cu adsorbed by SGP-amended soil was carbonate-bound form, followed by iron–manganese oxidation form. Cu in iron–manganese oxidation form was the highest on SOP-amended soils. (4) Soil pH, cation exchange capacity (CEC), and specific surface area (SBET) were the key factors that determine the Cu2+ adsorption amount (SBET) and morphological changes (pH and CEC).