Effects of physical aging processes on the bioavailability of heavy metals in contaminated site soil amended with chicken manure and wheat straw biochars

The physicochemical properties of biochars undergo slow changes in soils due to the natural aging processes, which influences their interaction with heavy metals. The effects of aging on immobilization of co-existing heavy metals in contaminated soils amended with fecal and plant biochars possessing contrasting properties remain unclear. This study investigated the effects of wet-dry and freeze-thaw aging on the bioavailability (extractable by 0.01 M CaCl2) and chemical fractionation of Cd and Pb in a contaminated site soil amended with 2.5% (w/w) chicken manure (CM) biochar and wheat straw (WS) biochar. Compared to that in the unamended soil, the contents of bioavailable Cd and Pb in CM biochar-amended soil decreased by 18.0% and 30.8%, respectively, after 60 wet-dry cycles, and by 16.9% and 52.5%, respectively, after 60 freeze-thaw cycles. CM biochar, which contained significant levels of phosphates and carbonates, effectively reduced the bioavailability of Cd and Pb and transformed them from the labile chemical fractions to the more stable ones in the soil during the accelerated aging processes, mainly through precipitation and complexation. In contrast, WS biochar failed to immobilize Cd in the co-contaminated soil in both aging regimes, and was only effective at immobilizing Pb under freeze-thaw aging. The changes in the immobilization of co-existing Cd and Pb in the contaminated soil resulted from aginginduced increase in oxygenated functional groups on biochar surface, destruction of the biochar's porous structure, and release of dissolved organic carbon from the aged biochar and soil. These findings could help guide the selection of suitable biochars for simultaneous immobilization of multiple heavy metals in co-contaminated soil under changing environmental conditions (e.g., rainfall, and freezing and thawing of soils).

Automated summary

The aging processes of biochars in soils can slowly change their physicochemical properties, influencing their interaction with heavy metals. The effects of aging on immobilization of co-existing heavy metals in contaminated soils amended with contrasting biochars remain unclear. This study investigated the effects of wet-dry and freeze-thaw aging on the bioavailability and chemical fractionation of Cd and Pb in contaminated soil amended with chicken manure (CM) biochar and wheat straw (WS) biochar. The results showed that CM biochar effectively reduced the bioavailability of Cd and Pb, while WS biochar only immobilized Pb under freeze-thaw aging. The changes in heavy metal immobilization resulted from aging-induced alterations in biochar surface functional groups, porous structure, and dissolved organic carbon release. These findings are important for selecting suitable biochars for immobilizing multiple heavy metals in co-contaminated soil under changing environmental conditions.