Identifying key processes driving Cd long-term adsorption and immobilization by biochar in soils and evaluating combined aging effects under simulated local climates

Abstract

To investigate the effects of major environmental processes on biochar aging in field soils, four individual aging processes (dry-wet cycling, freeze-thaw cycling, leaching, and acidification) and a combination treatment including these four individual aging processes were conducted. The effects of five aging treatments on Cd transport between soils and biochar and Cd immobilization by pristine and KMnO4-modified biochars in soils were quantified. The results showed that the percentages of the total net effects of the aging treatments (the percentage sum of the decrease in Cd released into the environment and the increase in relatively stable Cd fractions in the soil) were in the following order: combined aging (−81.79% to −60.66%) < dry-wet cycling (−32.27% to −20.99%) < acidification (−21.93% to −14.46%) < freeze-thaw cycling (−22.69% to −6.91%) < leaching (8.51%–13.76%) for the CK, pristine biochar, and modified biochar treatments. The combined aging treatment had the most negative net effect on Cd immobilization because the effects of the four individual aging treatments superimposed to a certain degree. Modified biochar was more effective for Cd immobilization than pristine biochar under all five aging treatments. These results indicated that dry-wet cycling is a key environmental process among four individual aging processes in this study area, and that leaching is the only process that produces positive outcomes. Therefore, field soil moisture conditions can be adjusted to facilitate Cd immobilization by biochar. Moreover, combined environmental effects rather than individual aging processes should be considered under local climatic conditions when assessing biochar aging in field soils.