Adsorption of humic and fulvic acids onto a range of adsorbents in aqueous systems, and their effect on the adsorption of other species: A review

Humic acid (HA) and fulvic acid (FA) are the prime components of natural organic matter (NOM) and can cause several problems in potable water such as odor, taste, and color. They can also produce hazardous disinfectant by-products (DBPs), especially on water chlorination, and can reduce the amount of dissolved oxygen in the aquatic environment. Therefore, removal of NOM from water systems is highly desirable. This review discusses the results found for adsorption of HA and FA on a wide range of adsorbents such as carbonaceous materials, nanosized metal (oxy-hydro) oxides, low-cost agricultural materials, clays, zeolites, biopolymers, composites and miscellaneous adsorbents. Studies investigating the effect of adsorbed humic material on the adsorption of other aqueous species are also presented. The adsorbents used have been assessed in terms of their maximum HA or FA adsorption capacity with special emphasis given to environmental conditions such as contact time, solution pH, initial HA or FA concentration, temperature, and adsorbent dosage. Adsorption mechanisms of HA and FA are discussed taking into account the results of kinetic, isotherm, surface complexation modeling, and thermodynamic studies, together with available spectroscopic evidence. Natural and agricultural waste materials and bio-sorbents were found to exhibit excellent HA and FA adsorption performance. This review aims to provide an overview of research on the removal HA and FA from aqueous systems by various adsorbents. It also shows the strengths and weaknesses of current research and suggests ideas for future research that will improve our ability to remove HA and FA from natural and waste-water systems.