Evaluating the capability of tannic acid-modified biochar in sequestering metal ions and in enhancing soil health

Pollution of soils is one of the currently significant environmental concerns. Heavy metals that contaminate the soil can have harmful effects on crops, enter the food chain, and pose a threat to human health. Various approaches have been employed in the past to remediate heavy metal-contaminated soils for agricultural use, including the use of biochar and its modified forms through physical or chemical methods. In the present study, the suitability of tannic acid as a biochar modifier was evaluated for the first time with the aim of enhancing its ability to adsorb Cd and Pb, thereby reducing their transfer from soil to plants. A mesocosm trial was set-up using three treatments: control, unmodified biochar, and tannic acid-modified biochar and their impact was evaluated on soil characteristics and plant physiology. The FT-IR spectra demonstrate that tannic acid-modified biochar considerably improved its capability to absorb Cd but worsened those for Pb and this occurred due to an increase in oxygen-containing functional groups and a reduction in total pore volume, respectively. Tannic acid-modified biochar application increased the carbon and nitrogen concentrations in the soil, as well as the microbial biomass and activities. Lettuce plants grown on soil amended with modified biochar accumulated a low Cd amount in roots but a high Pb amount in leaves that reduced the photosynthetic pigments and the photosynthetic rates and increased the antioxidant response. It is found that tannic acid treatment of biochar is effective for the remediation of Cd-contaminated soils but not for those polluted by Pb.