Yang Kai, Wang Xilong, Cheng Hefa, Tao Shu
College of Water Sciences, Beijing Normal University, Beijing, 100875, China.
MOE Key Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing, 100871, China.
Environ Pollut. 2023 May 1;324:121414. doi: 10.1016/j.envpol.2023.121414. Epub 2023 Mar 7.
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 CaCl) 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 aging-induced 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).
由于自然老化过程，生物炭的物理化学性质在土壤中会发生缓慢变化，这会影响它们与重金属的相互作用。老化对添加了具有不同性质的粪便和植物生物炭的污染土壤中共存重金属固定化的影响尚不清楚。本研究调查了干湿交替老化和冻融老化对添加2.5%（w/w）鸡粪（CM）生物炭和小麦秸秆（WS）生物炭的污染场地土壤中镉（Cd）和铅（Pb）的生物有效性（可被0.01 M氯化钙提取）和化学形态的影响。与未添加生物炭的土壤相比，经过60次干湿循环后，添加CM生物炭的土壤中生物可利用态Cd和Pb的含量分别降低了18.0%和30.8%，经过60次冻融循环后，分别降低了16.9%和52.5%。CM生物炭含有大量的磷酸盐和碳酸盐，在加速老化过程中，主要通过沉淀和络合作用，有效地降低了Cd和Pb的生物有效性，并将它们从不稳定的化学形态转化为土壤中更稳定的形态。相比之下，WS生物炭在两种老化条件下均未能固定污染土壤中的Cd，仅在冻融老化条件下对固定Pb有效。污染土壤中共存Cd和Pb固定化的变化是由于老化导致生物炭表面含氧官能团增加、生物炭多孔结构破坏以及老化生物炭和土壤中溶解有机碳的释放。这些发现有助于指导在变化的环境条件（如降雨、土壤冻融）下选择合适的生物炭用于同时固定共污染土壤中的多种重金属。
