Evaluating the role of biochar in the transport behavior of chlorinated phenols in alluvial soil systems

• Biochar amendment increases CPs retention, extending breakthrough times by 12–40 times. • Retention of CPs correlates strongly with their polarizability and other molecular properties. • In the presence of biochar, CPs retention shifts from hydrophobic to polar interactions, reducing mobility. This study investigates the influence of soil properties and biochar amendments on the transport behavior of chlorinated phenols (CPs) in alluvial soil systems. The primary focus was to evaluate the adsorption mechanisms governing CP mobility and retention, with particular attention to how the physicochemical properties of the native and biochar-amended soils interact with the molecular characteristics of CPs, such as hydrophobicity, molecular weight, and ionization potential. Therefore, column experiments with an alluvial soil and biochar amended soil were conducted to evaluate the transport of four types of CPs. The advection-dispersion equation (ADE) was used to calculate the retardation factor ( R d ). The primary retention mechanisms identified in alluvial soils were hydrophobic, polar, and electron-donor-acceptor interactions, rather than charge-based attractions, though exceptions, such as for pentachlorophenol, were noted. The chemical properties of CPs, including acid dissociation constant (p K a ), molecular weight, and hydrophobicity, played a crucial role in their transport behavior. The addition of biochar significantly enhanced CP retention, increasing breakthrough times by a factor of 12 to 40 and yielding R d values between 42 and 155. A strong correlation was found between retention and molecular polarizability. Furthermore, addition of biochar into the soil modified the type of interaction from hydrophobic to polar impact on CPs retention during the transport. These findings underscore the potential for mitigating organic pollutant leaching by biochar amendment, highlighting that effectiveness depends on both the physical-chemical properties of the soil/amendment and the physicochemical characteristics of the pollutants. The research emphasizes biochar’s role in environmental remediation, particularly in controlling the migration of organic pollutants and protecting ground- and surface water quality. Additionally, the study evaluates biochar’s structural and functional properties, underscoring its potential to immobilize hydrophobic and slightly polar contaminants.