Insights into effects of ageing processes on Cd-adsorbed biochar stability and subsequent sorption performance

The conversion of agricultural biomass wastes into biochar has enormous potential to improve soil quality. Particularly, biochar particles introduced into the natural environment readily bind environmental pollutants. The interaction of biochar and adsorbed pollutants will, however, be impacted by long term ageing. The mechanism of biochar adsorption performance that could be affected by such early-adsorbed pollutant is not understood. Herein, we study the effects of different ageing processes on Cd-adsorbed biochar stability by K₂CrO₇-H₂SO₄ oxidation method for carbon loss evaluation, and sorption capacity towards diethyl phthalate (DEP). We adopted artificially accelerated ageing methods to simulate various processes: HNO₃/H₂SO₄, H₂O₂ oxidative, leaching, high temperature, freeze-thaw cycles, and dry-wet cycles. The results showed that all the Cd-adsorbed aged biochars had more C-C/C-H functional groups and exhibited higher carbon stability than pristine aged biochars. Specially, the carbon loss (20.2-25.2%) of Cd-adsorbed biochar showed almost two times lower than pristine biochar (34.5-35.4%) after high temperature and leaching ageing due to enhancing Cd-π bonding, which could resist strong K₂Cr₂O₇/H₂SO₄ oxidation. Such interaction synergistically promoted the sequestration of adsorbed Cd on biochars. The subsequent sorption performance for DEP on Cd-adsorbed biochars was slightly higher than pristine biochar after H₂O₂ oxidative and leaching ageing. Whereas, the dominate mechanism of DEP removal on Cd-adsorbed biochars changed to hydrophobic partitioning after high temperature, freeze-thaw cycles and dry-wet cycles ageing, leading to a decreased DEP sorption capacity. Hence, the biochar application must be comprehensively considered the susceptibilities due to reciprocal effects of early adsorbed pollutant and natural ageing environment, which helps us both assess and take measures to minimize potential risks in the environment. The present study suggests the continuous irrigation and tropical or temperature climate regions would be suitable for long-term biochar application in soil remediation and carbon sequestration.