# A Critical Review of the Modelling Tools for the Reactive Transport of Organic Contaminants
## 2 Processes
### Sorption  
Adsorption is a mass transfer process where substances accumulate at the interface of two phases, such as liquid-liquid or liquid-solid (aquifer matrix). Adsorbents play a vital role in immobilizing certain pollutants from groundwater [186]. The sorption of organic chemicals in soils is a highly intricate process influenced by both matrix properties and contaminant characteristics, which dictate interactions between the sorbed contaminant and the solid surface. Binding to the solid is contingent upon factors like reactive groups, specific surface area, electrostatic charge, hydration sphere, ionic radius, steric effects, hydrophobicity, and more [187]. Various forces attract solute molecules to the solid surface, with electrostatic binding predominating in physical or chemical sorption scenarios [188]. Despite numerous studies on pollutant adsorption across various materials, including within remediation frameworks, there remains a need for deeper understanding of these complex mechanisms, often overlooked in favour of remediation efficacy [189]. There are several interactions that are responsible for the removal of pollutants from water by sorption, such as physical absorption, surface adsorption, ion exchange, complexation, chelation, acid-base interactions, proton displacement, precipitation, pore filling, van der Waals forces, electrostatic attraction, hydrogen bonding, hydrophobic interactions (\(\pi\)-\(\pi\) interactions, Yoshida interactions), inclusion complexes, diffusion into the network of the material, and covalent bonds [190; 191]. Physisorption is inherently non-specific and involves interactive forces that are relatively weak. This process has been studied with regard to the immobilisation of organic pollutants on activated carbon [192]. Adsorption of organic compounds on porous materials, i.e., pore filling, is a recognised mechanism [193; 194; 195] and the maximum sorption by this process is explicitly related to the volume of micropores of the sorbent and is expressed by the Dubinin-Polanyi equations [196]. Van der Waals forces, also known as London dispersion forces, refer to the interactions between two neutral atoms, molecules or particles separated by a distance greater than their dimensions. Despite the weakness of the bond, these forces play a crucial role in understanding the mechanism of adsorption and the transformation processes of hydrocarbon molecules [197; 198]. The mechanism of electrostatic attraction involves the interactions between the adsorbed substance (e.g., an organic pollutant) and the sorbent based on their electrical charges. Sorption by electrostatic interaction depends on the pH of the solution and the point of zero charge (PZC) of the sorbent [199]. Modelling studies have shown that \(\pi\)-\(\pi\) interactions and H-bonding interactions play an important role in the sorption of PAHs (polycyclic aromatic hydrocarbons) [200; 201; 202; 203]. \(\pi\)-\(\pi\) interactions are weak non-covalent interactions that influence the chemical structure by providing significant stabilisation and strengthening of the bonds. In general, hydrogen bonds can form A-H-B type complexes, where A is any atom that is more electronegative than H because it removes an electron from H, making the hydrogen electron-deficient, and atom B is another electronegative atom, such as oxygen, nitrogen, halogens, etc. [204]. Sorption is a recognised effective technique for the removal of organic pollutants using activated carbon [205], resins [206], nanomaterials [207; 208; 209], zeolites [210; 211], goethite [212], and stable organic substances [213]. The sorption and retardation of pollutants can be estimated by laboratory experiments in which contaminated water is mixed with aquifer materials containing different amounts of adsorbents. The sorption capacity of these materials is analysed by equilibrating known quantities of a solid with solutions of the compound in question.