Sorption of As, Cd, and Pb by soil amendments: in situ immobilization mechanisms and implementation in contaminated agricultural soils

The contamination of agricultural soils by toxic heavy metals, such as As, Cd, and Pb, is of great concern for crop safety as well as environmental and public health. Various adsorbents for the in situ immobilization of these metals have been widely studied, but researches on the potential and superiority of metal adsorption in agricultural soil amendments are still lacking. This study was conducted to investigate the nature of their sorption processes on soil amendments including slaked lime (SL), phosphogypsum (PG), bone meal (BM), and biochar (BC) using a series of laboratory batch tests. The Langmuir adsorption isotherm was used to predict sorption parameters. The experimental data fitted reasonably well on the Langmuir model with high correlation coefficients ( R 2 = 0.64–0.99) suggesting that monolayer sorption/complexation/precipitation was the dominant mechanism. Among the amendments, SL achieved the highest maximum adsorption capacity ( q max ) for As and Cd at 714.3 and 2000 mg g −1 , respectively, while PG had the highest q max for Pb at 196.08 mg g −1 . The results indicate that there is no direct correlation between sorption stability and maximum adsorption capacity. Among the sorbents, BC had the highest sorption stability for As (0.007 L mg −1 ), Cd (0.121 L mg −1 ), and Pb (2.273 L mg −1 ), respectively, albeit the q max values for these three metals were not high. This indicates that the As, Cd, and Pb sorbed on biochar tended to be more stable than those retained on other amendments. While a large sorption capacity is important, our results provide important insights into the metal sorption stability/energy of adsorbents that will aid in the development of long-term management efficiency strategies to rehabilitate metal-contaminated arable soils.