Adsorption of organic micropollutants onto biochar: a review of relevant kinetics, mechanisms and equilibrium
Abstract: As an alternative to activated carbon, biochar has been considered for removal of organic micropollutants from water and wastewater via adsorption. This review elaborates on the fundamental basis of adsorption kinetics, mechanisms, and equilibrium with respect to biochar-based adsorption of micropollutants. The objectives include: 1) linking biochar surface properties with adsorption abilities, 2) categorizing the kinetics of adsorption of aqueous-phase organic compounds onto biochar, 3) categorizing the molecular-scale interactions between organic micropollutants and biochar, and 4) reviewing existing quantitative methods for characterizing adsorption equilibrium of organic micropollutants from water onto an adsorbent surface. To fulfill these goals, the relationships among biochar surface properties, adsorption kinetics, mechanisms, and equilibrium were clarified as current literature often lacks such discussion or may include conflicting descriptions. Due to its heterogeneous nature, research on biochar's adsorption potential for micropollutants is ambiguous. By adapting adsorption theories to biochar application specifically, this review helps to inform future research in terms of addressing knowledge gaps in characterizing and improving biochar adsorption.
Summary (1 min read)
Q = 𝐾𝐾𝑑𝑑𝐶𝐶𝑆𝑆𝑛𝑛 Eq. 11
- The more drastically the binding affinity changes (indicating the existence of remarkably strong affinity sites) across the adsorbent surface, the smaller the n value.
- The distribution coefficient K used in derivation of isotherms is related to the thermodynamic equilibrium constant (Kc=exp (-ΔG0/RT)), and the surface site distribution function Φ𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 is also related to K. 5.2 Thermodynamics of adsorption Thermodynamics, like isotherms, play an essential role in characterizing adsorption equilibrium and mechanisms.
- For adsorption of organic dyes on biochar, Figure 4A shows isosteric heat changing with surface coverage.
- A negative ΔGO value indicates that the driving forces promote adsorption rather than hinder adsorption.
2) Categorize the kinetics of adsorption of aqueous-phase organic compounds onto biochar.
- The kinetics of aqueous-phase adsorption can be categorized in terms of the rate-limiting step.
- Diffusion-controlled kinetics are usually observed for weak physical adsorption, whereas reactioncontrolled kinetics most often characterize chemical adsorption or strong physical adsorption such as coulombic attraction, which stems from charged functional groups on both the biochar’s surface and on micropollutant molecules.
3) Categorize the molecular-scale interactions between organic micropollutants and biochar.
- The intermolecular forces discussed in this review include those associated with organics and biochar, including hydrophobic interaction, dipole interaction, π-interaction, H-bonding, coulombic attraction, and covalent bonding.
- Binding mechanisms are closely related to the molecular structures of the organic micropollutants and surface properties of biochar.
- Qualitative categorization of mechanisms is essential for calculating the contribution of each mechanism to total free energy change of adsorption in thermodynamic models 27.
- The engineered solid surface properties can be improved to facilitate dominant binding accordingly.
4) Review existing quantitative methods for characterizing adsorption equilibrium of organic micropollutants from water.
- Adsorption equilibrium can be characterized using isotherm models.
- Kinetics and mechanisms of adsorption can be further elucidated.
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