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 (R2 = 0.64-0.99) suggesting that monolayer sorption/complexation/precipitation was the dominant mechanism. Among the amendments, SL achieved the highest maximum adsorption capacity (qmax) for As and Cd at 714.3 and 2000 mg g-1, respectively, while PG had the highest qmax 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 qmax 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.

Coexistence of Cr and Ni in anthropogenic soils and their chemistry: implication to proper management and remediation.

In anthropogenic soils, there have been relatively limited studies focusing on Cr and Ni contaminants because they exhibit less toxic effects to overall ecosystem and human health than other metal contaminants. In recent years, however, soil contamination with Cr and Ni has become a serious concern in several parts of the world because of the continuously increasing concentrations of these metals due to accelerated industrialization and urbanization. To investigate the status of soil contamination with Cr and Ni by anthropogenic activities, relevant global data sets in different land-use types reported by several studies were reviewed. This review presents the significant work done on Cr and Ni concentrations in roadside, central business district (CBD), and industrial soils in 46 global cities and evaluated their correlation by global data in the past few years. The highest concentrations of Cr and Ni were observed in industrial soils. Furthermore, a significant relationship was found between Cr and Ni concentrations in the soils, which might be because both metals are released from the same sources or anthropogenic activity processes. We also discuss the state of knowledge about the chemistry and distribution of Cr and Ni in the soil environment to understand how their processes such as redox reaction, precipitation-dissolution, and sorption-desorption affect the remediation of Cr- and Ni-contaminated soils using in situ immobilization technology. Application of organic and inorganic immobilizing agents (e.g., lime, compost, and sulfur) for the clean-up of Cr- and Ni-contaminated soils has received increasing interest from several researchers worldwide. Several immobilizing agents have been suggested and experimentally tested with varying degrees of achievement in Cr- and Ni-contaminated soils. Overall, the use of sulfur-containing amendments and pH-increasing materials could be considered the best options for the remediation of co-contamination of Cr and Ni in soil.