Immobilization and phytotoxicity reduction of heavy metals in serpentine soil using biochar

Purpose: Serpentine soils derived from ultramafic rocks release elevated concentrations of toxic heavy metals into the environment. Hence, crop plants cultivated in or adjacent to serpentine soil may experience reduced growth due to phytotoxicity as well as accumulate toxic heavy metals in edible tissues. We investigated the potential of biochar (BC), a waste byproduct of bioenergy industry in Sri Lanka, as a soil amendment to immobilize Ni, Cr, and Mn in serpentine soil and minimize their phytotoxicity.

Materials and methods: The BC used in this study was a waste byproduct obtained from a Dendro bioenergy industry in Sri Lanka. This BC was produced by pyrolyzing Gliricidia sepium biomass at 900 °C in a closed reactor. A pot experiment was conducted using tomato plants (Lycopersicon esculentum L.) by adding 1, 2.5, and 5 % (w/w) BC applications to evaluate the bioavailability and uptake of metals in serpentine soil. Sequential extractions were utilized to evaluate the effects of BC on bioavailable concentrations of Ni, Cr, and Mn as well as different metal fractionations in BC-amended and BC-unamended soil. Postharvest soil in each pot was subjected to a microbial analysis to evaluate the total bacterial and fungal count in BC-amended and BC-unamended serpentine soil.

Results and discussion: Tomato plants grown in 5 % BC-amended soil showed approximately 40-fold higher biomass than that of BC-unamended soil, whereas highly favorable microbial growth was observed in the 2.5 % BC-amended soil. Bioaccumulation of Cr, Ni, and Mn decreased by 93–97 % in tomato plants grown in 5 % BC-amended soil compared to the BC-unamended soil. Sequentially extracted metals in the exchangeable fraction revealed that the bioavailabile concentrations of Cr, Ni, and Mn decreased by 99, 61, and 42 %, respectively, in the 5 % BC-amended soil.

Conclusions: Results suggested that the addition of BC to serpentine soil as a soil amendment immobilizes Cr, Ni, and Mn in serpentine soil and reduces metal-induced toxicities in tomato plants.