Abstract
Humic acids (HA) are promising green materials for water and wastewater treatment. They show a strong ability to sorb cationic and hydrophobic organic pollutants. Cationic compounds interact mainly by electrostatic interaction with the deprotonated carboxylic groups of HA. Other functional groups of HA such as quinones, may form covalent bonds with aromatic ammines or similar organic compounds. Computational and experimental works show that the interaction of HA with hydrophobic organics is mainly due to π–π interactions, hydrophobic effect and hydrogen bonding. Several works report that sorbing efficiency is related to the hydrophobicity of the sorbate. Papers about the interaction between organic pollutants and humic acids dissolved in solution, in the solid state and adsorbed onto solid particles, like aluminosilicates and magnetic materials, are reviewed and discussed. A short discussion of the thermodynamics and kinetics of the sorption process, with indication of the main mistakes reported in literature, is also given.
Highlights
Hazardous waste poses an elevated risk to human health and to the environment if it is not managed and disposed of safely
An ∆ist H which does not vary with the surface coverage suggests that the equilibrium of the adsorption system investigated should be well modelled by a Langmuir isotherm
Humic acids (HA) are the soluble fraction of humic substances at an alkaline pH and are produced by the biotic and abiotic degradation of dead organic matter
Summary
Hazardous waste poses an elevated risk to human health and to the environment if it is not managed and disposed of safely. More specific interactions between HA functional groups and organic pollutants may occur It was shown, for example, that soil and sediment humic acid quinones covalently bind aromatic ammines [54,55] and form stable charge–transfer complexes with electron donor herbicides [56], resulting in the formation of non-extractable residues (NER) of xenobiotics. In addition to covalent bonding, other processes such as a slow sorption and diffusion of the pollutants through the micropores of HA may contribute to NER formation [57] Given their solubility in water at neutral and alkaline pH, HA are the main carriers affecting the mobility of pollutants in the environment [58,59]. We collect the results of the relevant literature about the sorption of organic pollutants onto humic acids in three different configurations: HA in solution, HA in solid state and HA immobilised onto solid materials as supports
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