Adsorption of phosphates and organic acids on aluminum hydroxide in aquatic environment : mechanisms and interactions

Phosphorus, which is the nutrient accelerating eutrophication in fresh water, is discharged to surface waters either from agricultural soils or from the effluent of wastewater treatment facilitates. However, the interactions and mechanisms of various phosphate species with minerals in soils or metal coagulants in the phosphorus removal processes have not been understood completely due to the neglect of the importance of other phosphate species rather than orthophosphate and the competitions among different phosphates. The important roles of condensed phosphates and organic phosphates in water quality management have not been recognized until recently due to the advancement of analytical methods for phosphorus. Natural organic matter (NOM) is of significant environmental concern as NOM controls mineral growth and dissolution, trace-metal cycling and mobility of both inorganic and organic pollutants in the environment. Therefore, the fate of phosphates in aquatic environment cannot be described properly without considering the presence of NOM. Furthermore, the competition of phosphates and NOM exerts another important impact on removal of disinfection byproducts (DBPs) precursors in water treatment because both levels of NOM and phosphorus in surface waters are increased due to the eutrophication. Therefore, a comprehensive study focusing on the mechanisms, kinetics and interactions of phosphates and NOM in the adsorption process with aluminum hydroxide, a major adsorbent in both soils and water treatment systems was conducted as my PhD research work with the following objectives to: 1) investigate the adsorption mechanisms of condensed phosphates and organic phosphates on aluminum hydroxide; 2) study the competitive adsorption between orthophosphate and other phosphates; 3) identify the adsorption mechanisms of NOM on aluminum hydroxide; and 4) evaluate the competitive adsorption between orthophosphate and NOM.The adsorption mechanisms of condensed phosphates and organic phosphates on aluminum hydroxide were investigated with ATR-FTIR spectroscopy and theoretical calculation with the Gaussian 03 Program. A series of small organic acids bearing carboxylic and/or phenolic groups were employed to evaluate the adsorption mechanisms of NOM on aluminum hydroxide with ATR-FTIR spectroscopic and kinetic studies. Batch adsorption experiments were conducted to evaluate the competitive adsorption between orthophosphate and other phosphates or NOM.The major findings of my research include: 1) electron delocalization and polarization has been confirmed to be effective in describing the peak shift as well as the complexation of condensed/organic phosphates with aluminum hydroxide; 2) the structures of the complexes formed upon the adsorption of pyrophosphate, tripolyphosphate and myo-inositol hexaphosphate on aluminum hydroxide have been clarified with ATR-FTIR spectroscopic investigation; 3) it has been revealed that the addition sequence has a pronounced influence on the competitive adsorption of phosphates on aluminum hydroxide over the pH range examined in this study; the amount of total phosphorus adsorbed on aluminum hydroxide has been found to be dependent of the fraction as well as the intrinsic binding affinity of each phosphate species for surface sites of the adsorbent; 4) the aromatic carboxylates have been found most likely to be adsorbed to the first surface layer of the hydroxyl groups and water molecules without forming coordinative bonds with the aluminum hydroxide surface through strong hydrogen bonds formed in this process; 5) the involvement of the carboxylic moiety in NOM adsorption has been identified to be important at low pH while the phenolic groups are of primary importance for the structures and stability of the complexes formed at higher pH, and the carboxylic groups also play an important role in the complexation when the two hydroxyls are not properly situated; and 6) the efficiency of orthophosphate in reducing humic acid (HA) adsorption increases with increasing orthophosphate concentration whereas orthophosphate adsorption is only moderately suppressed by HA; HPLC-SEC and SUVA analyses have discovered that the HA molecules with high molecular weight are adsorbed preferentially but easily displaced by the specifically adsorbed orthophosphate.Keywords: condensed phosphate, organic phosphate, orthophosphate, ATR-FTIR, competitive adsorption, natural organic matter, complexation, eutrophication.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2005