Adsorption and Desorption Characteristics of Cd, Cu and Pb in Different Soil Aggregates Through Soil Profile under Single and Ternary Systems

As the fundamental unit of soil, aggregates exhibit significant variations in their ability to adsorb or desorb trace elements, depending on their size. Batch experiments were conducted to investigate the characteristics of adsorption and desorption of Cd, Cu and Pb on different sizes of soil aggregates from each genetic horizon of calcareous soil profiles. The results showed that both Langmuir and Freundlich models were successfully fitted to describe the isothermal adsorption process of single Cd, Cu, and Pb on different soil aggregates. Additionally, soil aggregates at the bottom layer showed the highest maximum adsorption capacity and required the lowest energy for Cd, Cu and Pb adsorption compared to aggregates from upper soil horizon. The physical-chemical properties of various soil aggregates were found to govern the adsorption and desorption process of heavy metals rather than aggregate sizes, wherein the content of Fe/Al oxides and organic matter were the most crucial influencing factors accounted for this process. Cd displayed higher mobility than Cu and Pb among the different soil aggregates, and the maximum adsorption capacity of the metal ions studied followed the order: Pb > Cu > Cd, while their desorption rate followed the order: Cd > Cu > Pb. Additionally, the < 0.053 mm size of soil microaggregates presented the lowest desorption rate for Cd, Cu, and Pb compared to other sizes of soil aggregates in each soil horizon. Furthermore, Orthogonal experiment results demonstrated the competition for metal adsorption occurred on soil aggregates in ternary heavy metal ions system, however, only the desorption of Pb was significantly affected by the coexistence of Cd and Cu.