非均性多孔介质中生物炭胶体的迁移行为研究

生物炭颗粒在多孔介质中的迁移行为不仅决定了其在环境中的归趋，也极大地影响了被吸附污染物的环境行为.以往的研究主要集中在生物炭胶体在均性多孔介质中的迁移行为，但实际环境介质通常是非均性的，目前对生物炭胶体在非均性多孔介质中的迁移行为知之甚少.本研究采用两种不同粒径的石英砂构建了上下两层非均性填充柱（上层细颗粒，下层粗颗粒），研究了生物炭胶体在非均性多孔介质中的迁移和截留行为，考察了溶液离子强度和pH对生物炭胶体迁移能力的影响.研究结果表明在非均性多孔介质中，生物炭胶体具有很高的迁移能力，在离子强度为1~50 mmol·L^-1，pH为4.0~11.0条件下，生物炭胶体的迁移率达40.2%~88.0%.非均性介质中生物炭胶体的截留曲线表现为非单调型曲线，截留量峰值往往出现在细-粗石英砂的交界面处（细石英砂侧），这与非均性介质中显著的电荷异质性、介质尺寸异质性和迁移过程中传质通量异质性有关.生物炭胶体在上层细石英砂中的截留量显著大于其在下层粗石英砂中的截留量，表明上层细石英砂是影响生物炭胶体迁移行为的关键层.随着溶液离子强度增加，生物炭胶体自团聚作用增强，其与石英砂介质间界面作用能垒降低，因而生物炭胶体的迁移能力减弱.由于生物炭胶体与细石英砂间的物理张力作用趋于显著，增加离子强度提高了生物炭胶体在上层细石英砂中的截留比率.中性和碱性条件下生物炭胶体的迁移能力较强，而在酸性条件下，生物炭胶体表面电负性显著降低，团聚体粒径增大，生物炭胶体的迁移能力较弱.降低溶液pH增加了生物炭胶体在上层细石英砂中的截留比率.本研究的结果将有利于人们更好地了解生物炭胶体在复杂多孔介质中的迁移行为，为全面评估生物炭的潜在环境风险提供理论支持.

The migration behaviors of biochar colloids in heterogeneous porous media

The migration behavior of biochar particles in porous media not only determines the fate of biochar in the environment, but also greatly affects the environmental behavior of the adsorbed pollutants. Previous studies have mainly focused on the migration behavior of biochar colloids in homogeneous porous media, but little is known for heterogeneous porous media. In this study, the heterogeneous columns with two layers (fine particles in the top layer and coarse particles in the bottom layer) were constructed by using two different particle sizes of quartz sand. The effects of solution ionic strength and pH on the migration behavior of biochar colloids were investigated. Results showed that biochar colloids had a high migration ability in the heterogeneous porous media. Under the conditions of ionic strength of 1~50 mmol·L^-1 and pH 4.0~11.0, the outflow percentage of biochar colloids were 40.2%~88.0%. The retention profiles of biochar colloids were non-monotonic in heterogeneous media, and the peak value of retention usually appears at the interface of fine-coarse quartz sand (in fine quartz sand), which was mainly due to the significant charge heterogeneity, size heterogeneity of media and the mass transfer flux heterogeneity during migration. The retention of biochar colloids in the upper fine quartz sand was significantly larger than that in the bottom coarse quartz sand, indicating that the upper fine sand was the key layer affecting the migration behavior of biochar colloids. The migration ability of biochar colloids decreased with increasing of ionic strength due to the enhanced homo-aggregation of biochar colloids and the decrease of interaction energy barriers between biochar and fine quartz sand. The increase of ionic strength promoted the retention of biochar colloids in the upper fine quartz sand because of the enhanced straining. The migration ability of biochar colloids was weaker in acidic condition than those in neutral and alkaline condition, since the surface negativity of biochar colloids was weaker and the size of aggregate was bigger in acidic solutions. The retention ratio of biochar colloids in the upper fine quartz sand increased with the decrease of solution pH. Results of this study will help to better understand the migration behavior of biochar colloids in complex porous media and provide theoretical support for the comprehensive assessment of the potential environmental risks of biochar.