Investigating colloid-associated transport of cadmium and lead in a clayey soil under preferential flow conditions

Colloids have a high adsorption capacity and can be mobile under preferential flow, and so may facilitate heavy metal migration. Heavy metal migration with soil colloids in a clayey soil under preferential flow conditions was investigated through experiments. Adsorption tests were carried out to determine the adsorption of Cd2+ and Pb2+ to the clay and colloids. The preferential flow characteristics in the soil column were investigated by dye tracing tests. The mobility of soil colloids in the soil column was studied by breakthrough tests. Leaching tests of cadmium and lead with and without colloids were carried out conditions. The adsorption tests showed that soil colloids adsorbed more cadmium and lead than the silty clay. The dye tracing tests showed that moderate preferential flow in the soil column can be obtained by choosing clod-size distribution and dry density. The co-leaching test showed that the outflow of cadmium and lead was 1.49 and 33.88 times greater with colloids than without, respectively. The heavy metals adsorbed onto clay and the pore concentrations were both lower with colloids than without, indicating more heavy metals migrated downward with colloids. The migration of cadmium and lead was greatly enhanced by colloids under preferential flow conditions.

Characteristics of Heavy Metal Migration In Farmland

Compared with water and air pollution, soil pollution is highly concealed, has poor self-purification ability, and has high risks associated with accumulation. Characteristics of heavy metal migration directly affect the quality of the environment, and comparative studies utilizing column leaching and natural leaching experiments have rarely been performed. In this study, we used farmland soil samples from Xiba Town in the Wutongqiao district to determine the differences in leaching characteristics between column leaching and natural leaching tests. The results indicate the following. (1)The release of heavy metals in soil is divided into two stages: the first stage is a rapid release period, while the leaching solution has an extremely low heavy metal concentrations during the second stage. The cumulative amount released during the second stage exhibits regular fluctuations, while the heavy metal release rate is consistent with the heavy metal adsorption properties of the soil. (2) The release and accumulation of heavy metals in the soil are influenced by many factors that may interact with each other, which leads to low correlations between the cumulative heavy metals released in the column leaching and natural leaching tests. Simulating natural heavy metal migration trends using the column leaching test is effective to some extent, but there are significant differences between the accumulation sites and accumulated amounts. This study provides a theoretical basis for improving the remediation of soil contaminated by heavy metals.

Effect of insect feces (Hermetia illucens) on rice growth and heavy metal migration from polluted soil to rice plant

Insect feces are a new kind of biological organic fertilizer. Little is known about the influences of insect feces on rice growth and heavy metal migration from soil to rice plant. In this study, the effects of different amounts (CK (0%), T1 (2%), T2 (4%), T3 (6%), and T4 (8%) ) of black soldier fly larvae (BSFL) feces on the rice growth and the migration/accumulation of heavy metals (Cd and Pb) were investigated by pot experiments within two years. The application of insect feces remarkably increased the contents of soil pH, organic matter, ammonium nitrogen, available phosphorus and potassium. Meanwhile, the insect feces application reduced the weak acid soluble contents of soil Cd and Pb by 8.3–56.8%, but increased those in the oxidizable (by 22.4–165.7%) and residual (by 1.8–225.6%) states. Except for the T4 treatment in the first year, all fertilization treatments increased the rice yield (up to 43.7% and 195.5% higher than those of CK within two years). Moreover, the insect feces application reduced the contents of Cd (8.3%-66.7%) and Pb (6.4%-61.8%) in different parts of rice. Under the same treatment, the metal contents in each part of rice in the second year were lower than those in the first year. The insect feces application decreased the absorption coefficients (24.4%-57.5%) and secondary transport coefficients (3.6%-44.1%) of Cd and Pb by rice plant.