Sorption of Organic Pollutants by Humic Acids: A Review

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.

Macromolecular Structure of a Commercial Humic Acid Sample

“…The outside HA concentration was monitored by measuring the absorbance at 450 and 280 nm through a spectrophotometer (Perkin Elmer, Lambda 40, Waltham, MA, USA). The peak centred on 280 nm is the result of absorbance by the aromatic ring portion of HA structure, while HAs have a specific absorption band near 450 nm as widely reported in the literature [3,5,24]. The pH of the tested suspension was 8.6.…”

“…As reported in the literature [3][4][5] HA can bind cations by electrostatic interactions, as well as non-ionic organic compounds by means of hydrogen bonds, dipole-dipole interactions, and hydrophobic interactions. Because of these properties, HA have been proposed as an adsorbent in wastewater purification plants [5][6][7]. On the other hand, HA cannot be removed from water by using conventional oxidation processes; however, several approaches are effective for treating recalcitrant compounds, such as adsorption [8,9] and advanced oxidation processes [10,11].…”

“…HA consist of aromatic rings and alkyl chains bearing a diversity of functional groups, notably carboxyl, phenol, hydroxyl, and quinone groups [3]. As reported in the literature [3][4][5] HA can bind cations by electrostatic interactions, as well as non-ionic organic compounds by means of hydrogen bonds, dipole-dipole interactions, and hydrophobic interactions. Because of these properties, HA have been proposed as an adsorbent in wastewater purification plants [5][6][7].…”

Cr(VI) Sorption from Aqueous Solution: A Review

“…As easily inferred from any experimental isotherm curve (with the exception of linear isotherms), the q e /C e ratio is not constant (i.e., it varies with sorbent coverage [85]), thus Equation ( 25) does not represent an equilibrium constant and cannot be used in the van't Hoff equation (Equation ( 24)). As already observed, a correct determination of K from the q e /C e ratio could be obtained by extrapolating its value at zero sorbent coverage [86][87][88].…”

Magnetite-Functionalized Horse Dung Humic Acid (HDHA) for the Uptake of Toxic Lead(II) from Artificial Wastewater

“…Isotherm and Kinetics Sorption Experiment. The isotherm adsorption experiment was performed by contacting 20 mg sorbent with 25 mL of artificial wastewater which has various Pb(II) concentrations (20,40,50,60,80,100,150,200, and 400 mg/L) for 180 min. The kinetics adsorption experiment was conducted by contacting 20 mg of sorbent with 25 mL artificial wastewater containing 25 mg/L Pb(II) for 5 min.…”