Behaviors And Mechanisms Studies Of Natural Organic Matter Adsorption On Amine-adsorbent In Source Water

Natural organic matter (NOM), commonly present in naturanl waters and soils, are formed from the decomposition of plant and animal biomass.The presence of NOM in natural waters can cause various environmental and health problems. So, it is important to minimize the presence of NOM in water.Amine-adsorbents exhibit high adsorption capacity for NOM in aqueous solution. In the study, polyaniline (PAN) and amine-functionalized magnetic nanoadsorbent (Fe₃O₄@ SiO₂-NH₂) were prepared to remove humic acid (HA) and tannic acid (TA) from aqueous environment. Factors affecting the adsorption amount of NOM on amine-adsorbents were also investigated, including equilibrium concentration, adsorption time, ionic strength and solution pH values. Based on the results of adsorption tests and adsorbent characterization, adsorption mechanism was advised. In the study, some important conclusions are as follow:Adsorption of HA and TA on PAN(1) PAN was prepared by the conventional chemical oxidation, the surface area was 29.97 m2/g, the diameter was 80¹20 nm, IEP of PAN was 5.8, and synthesized PAN was in its emeraldine salt state.(2) PAN showed high adsorption capcity for TA and HA, the maximum adsorption amount were 117.6 mg/g and 115.0 mg/g at 25℃, respectively. Adsorption equilibrium was achieved with in 300⁴00min. Adsorption isotherms at different temperature obeyed Langmuir model, and adsorption kinetics followed pseudo-second-order equation. Adsorption process of TA and HA on PAN was endothermic. The free energy was negative, indicated that the adsorption process is spontaneous.(3) TA and HA adsorption on PAN was highly dependent on solution pH and the maximum adsorption amount of TA was found to be at pH 6.0, and the HA adsorption on PAN decreased monotonously with increasing pH. The presence of cations (Ca²⁺, K⁺, Na⁺) can improve the adsorption and the enhanced adsorption was in the order of Ca²⁺>K⁺≈Na⁺, which indicated that the electrostatic interaction may play an important role in HA and TA adsorption on PAN. (4) TA and HA adsorbed on PAN can be effectively desorbed in NaOHsolution.Adsorption amount of TA and HA had no apparent decrease after third desorption-regenetation cycles, which indicated the regenerated PAN can be used repeatedly.Adsorption of TA on PAN Fe₃O₄@ SiO₂-NH₂(1) Fe₃O₄@ SiO₂-NH₂ with a core-shell structure, the average diameter of 17.9 nm, amina group of 9.24%, was synthesized. The adsorbent was superparamagnetic, and can be sepatated magnetically.(2) The adsorbent showed high adsorption capacity for aqueous TA with the maximum adsorption amount 158.9 mg/g at 25℃. TA adsorption on Fe₃O₄@SiO₂-NH₂ could be described well by Langmuie model and pseudo-second-order equation can fit the experimental date very well.(3) TA adsorption decreased with pH increasing, and maximum adsorption amount was found to be at pH 6.0. TA adsorption amount on adsorbent increased with the increasing ionic concentration, and Ca²⁺ ion can evidently enhance the adsorption.(4) The loaded TA adsorbent can be desorbed in 1M HCl solution, and after third desorption-regeration cycles, it still exhibited strong adsorption affinity for TA.(5) The XPS analysis of before and after TA adsorption contirmed that TA molecules were adsorbed on the adsorbent by surface complexation with protonated amino group of the adsorbent.The results in this study indicated that PAN and Fe₃O₄@ SiO₂-NH₂ can eliminate NOM in solution effectively, with the advantages of large adsorption amount,fast adsorption speed and good regeneration. The main mechanisms were electrostatic interaction and surface complexation.