[Adsorption Characteristics and Mechanism of Cd and Pb in Tiered Soil Profiles from a Zinc Smelting Site].

Vertically tiered soil profiles, comprising miscellaneous fill (S1), plain fill (S2), silty clay (S3), and completely weathered slate (S4), were collected from a zinc smelter site in Zhuzhou City, Hunan Province, and their Cd and Pb adsorption characteristics were examined. Static batch experiments were conducted with different initial Cd and Pb solution concentrations, at temperatures of 288-308 K and pH values of 2-6. The results showed that a pseudo first-order model could be fitted to the kinetics of Cd/Pb adsorption in these soils. The soil profiles had a large retention capacity for Cd and Pb. The Cd and Pb adsorption isotherms for these soils conformed to the Freundlich isotherm, with maximum adsorption at 298 K of 2097-4504 mg ·kg-1 for Cd and 4376-10564 mg ·kg-1 for Pb, based on the Langmuir isotherm. The adsorption capacity of Cd and Pb increased with an increase in initial pH and temperature. The Cd and Pb adsorption process were a spontaneous physical and chemical process, and the soil profiles were ranked by their Cd and Pb adsorption capacities in the following order:completely weathered slate (S4)>miscellaneous fill (S1)>silty clay (S3)>plain fill (S2). The variation in adsorption capacities resulted from the differences in physical and chemical properties of the soil, mainly Fe/Al content and cation exchange capacity. Fourier transform infrared and SEM-EDS analysis showed that the main adsorption mechanism is the exchange of Cd and Pb with Fe/Al, while -OH/C＝O sites in soils were the predominant adsorption sites for Cd and Pb. In the study area, exogenous Cd and Pb discharged by smelting activity accumulated predominantly in surface soil, and their concentration gradually decreased with depth. These results provide a scientific basis for the prevention and control of heavy metal pollution in the soil and groundwater of a smelting site.

[Mechanism of Synergistic Adsorption of Arsenic and Cadmium by Aluminium-substituted Ferrihydrites].

The synergistic process and mechanism of aluminum (Al)-substituted ferrihydrites on arsenic[As(Ⅴ)] and cadmium[Cd(Ⅱ)] were studied under laboratory conditions. The results showed that synergistic adsorption and coprecipitation of As and Cd by Al-substituted ferrihydrites was clearly affected by both the pH of solution and the order in which heavy metals were added. The solution in which As co-existed with Cd for 72 hours, at a pH of 6.0 to 6.5, the As and Cd adsorption capacity of Al-substituted ferrihydrites containing 20% Al (AF20) reached 60.9 mg·g-1 and 17.1 mg·g-1, respectively. The removal rates of As and Cd were 96.0% and 73.0%, respectively. Arsenic and Cd were synergistically adsorbed into the internal pores of AF20 particles, and the synergistic adsorption effect of AF20 on As and Cd was clear. Adding Cd to the solution containing As, for 72 hours, and with a pH of 6.1 to 6.5, the As and Cd adsorption capacity of AF20 was 58.1 mg·g-1 and 12.4 mg·g-1, respectively. The removal rates of As and Cd were 96.0% and 48.3%, respectively. Adsorption of As limited the fixation of Cd by AF20. When adding As to the solution containing Cd, for 72 hours, with a pH of 9.5 to 9.8, fixed amounts of As and Cd on AF20 were 20.9 mg·g-1 and 24.4 mg·g-1, respectively. The removal rates of As and Cd were 38.8% and 98.9%, respectively. The coprecipitation of As and Cd by AF20 was clear. The resulting insoluble As and Cd compounds distributed the Cd distribution in a sparse strip and impeded the further adsorption of As. The results show that Al-substituted ferrihydrites can synergistically adsorb and coprecipitate As and Cd in contaminated environmental media.