Alginate-encapsulated biochar as an effective soil ameliorant for reducing Pb phytoavailability to lettuce (Lactuca sativa L.).

The alginate-biochar formulation for metal removal from aquatic environments has been widely tried but its use for lowering phytoavailability of metals in the soil-crop continuum is limited. Biochar has been increasingly used as a soil amendment due to its potential for soil carbon sequestration and sorption capacity. Handling of powdery biochar as a soil top-dressing material is, however, cumbersome and vulnerable to loss by water and wind. In this experiment, biochar powder, which was pyrolyzed from oak trees, was encapsulated into beads with alginate, which is a naturally occurring polysaccharide found in brown algae. Both batch and pot experiments were conducted to examine the effects of the alginate-encapsulated biochar beads (BB), as compared to its original biochar powdery form (BP), on the Pb adsorption capacity and phytoavailability of soil Pb to lettuce (Lactuca sativa L.). The BB treatment improved reactivity about six times due to a higher surface area (287 m2 g-1) and five times due to a higher cation exchange capacity (50 cmolc kg-1) as compared to the BP treatment. The maximum sorption capacity of Pb was increased to 152 from 81 mg g-1 because of surface chemosorption. Adsorption of Pb onto BB followed multiple first-order kinetics and comprised fast and slow steps. More than 60% of the Pb was adsorbed in the fast step, i.e., within 3 h. Also, the BB treatment, up to the 5% level (w/w), increased soil pH from 5.4 to 6.5 and lowered the phytoavailable fraction of Pb in soil from 5.7 to 0.3 mg kg-1. The Pb concentrations in lettuce cultivated at 5% for the BP and BB treatments were similar but 63 and 66% lower, respectively, than those of the control soil. The results showed that the encapsulation of biochar with alginate enhanced adsorption by the biochar.

A DOC coagulant, gypsum treatment can simultaneously reduce As, Cd and Pb uptake by medicinal plants grown in contaminated soil.

The efficiency of gypsum, as a dissolved organic carbon (DOC) coagulator, for the simultaneous immobilization of two heavy metals (Cd and Pb) and one metalloid (As) in agricultural soils near an abandoned mining site was examined. The agricultural soil was defined as long-term contaminated as As (1540mgkg-1), Cd (55mgkg-1) and Pb (1283mgkg-1) concentrations exceeded the Korean guideline values for As (25mgkg-1), Cd (4mgkg-1), and Pb (200mgkg-1). Gypsum was incorporated into the contaminated soil at 3% (w/w). In comparison two commonly using immobilizing agents (lime and compost), together with a mixture (lime+gypsum) were also included in the pot trial for the cultivation of two medical plants (A. gigas and A. macrocephala) and to evaluate the effectiveness of gypsum on As, Cd and Pb immobilization. The results showed that even though pH change-induced immobilizing agents such as lime were more effective than gypsum at immobilizing Cd and Pb, addition of gypsum also effectively reduced heavy metal phytoavailability as indicated by decreases in the concentration of Cd and Pb in medicinal plants. Furthermore, gypsum and gypsum+ lime were also most effective in reducing As concentrations in both plants studied. This was mainly attributed to significant decreases in soil DOC (48-64%) when gypsum and gypsum+lime were applied to the soil. Consequently, it was concluded that enhanced DOC coagulation with gypsum, could be considered as a promising technique for the immobilization of both metals (Cd and Pb) and metalloids (As) in agricultural soils.

An integrated approach to safer plant production on metal contaminated soils using species selection and chemical immobilization.

In order to examine the species specific accumulation of heavy metals in medicinal crops, seven different common medicinal plants were cultivated on a Cd (55mgkg(-1)) and Pb (1283mgkg(-1)) contaminated soil. Subsequently, the effect of various immobilizing agents, applied in isolation and in combination, on Cd and Pb uptake by two medicinal plant species was examined. Cadmium and Pb root concentrations in medicinal plants grown in the control soil varied between 0.5 and 2.6mgkg(-1) for Cd and 3.2 and 36.4mgkg(-1) for Pb. The highest accumulation occurred in Osterici Radix (Ostericum koreanum) and Ginger (Zingiber officinale) and the lowest in Yam (Dioscorea batatas). Application of immobilizing agents significantly reduced both Cd and Pb concentrations in all medicinal plants examined, where the most effective single immobilizing agent was lime fertilizer (LF). Application of combination treatments involving sorption agents such as compost together with lime further decreased Cd and Pb concentrations from 1.3 and 25.3mgkg(-1) to 0.2 and 4.3mgkg(-1), respectively, which was well below the corresponding WHO guidelines. Thus appropriate immobilizing agents in combination with species selection can be practically used for safer medicinal plant production.