Adsorption Behaviorsand Mechanisms Of Aqueous Humicacid On Amino-functionalized Magnetic Nano-adsorbent

Humic acid (HA), ubiquitously present in soils and natural waters, areformed from a series of chemical and biological reactions of plant and animalbiomass. The presence of HA in natural waters may cause a harmful effect on theenvironment and human health. Therefore, it is important to remove the HA fromwaters efficiently.Amino-functionalized magnetic nanoparticle adsorbents exhibit highadsorption capacity for HA in aqueous solution. In the study, magnetic PANIadsorbents (Fe3O4@SiO2-PANI) and magnetic PEI adsorbents (Fe3O4@SiO2-PEI)were prepared to remove HA from aqueous environment. Factors affecting theadsorption amount of HA on magnetic nanoparticle adsorbents were alsoinvestigated, including the reaction temperature, the different initiativeconcentration of HA, reaction time，solution pH and ionic strength. Based on theadsorbents characterization of adsorbents and the results of adsorption tests,some important conclusions are as follow:Adsorption of HAon Fe3O4@SiO2-PANI:(1) Fe3O4@SiO2-PANI with the core of Fe3O4and the shell of SiO2wassynthesized, PANI has already successfully masked on the magnetic silica andthe average particle sizes of Fe3O4, Fe3O4@SiO2, and Fe3O4@SiO2–PANI are10.3,15.3and21.6nm, respectively. The surface coating did not induce theobvious change of the crystal structure of Fe3O4. The adsorbent wassuperparamagnetic and can be separated from the adsorbate easily.(2) The Fe3O4@SiO2-PANI adsorbent showed high adsorption capacity forHA, and PANI on the surface of Fe3O4@SiO2contribute to the enhanced HAadsorption. In the study, the maximum adsorption amount was36.52mg/g at25℃. Adsorption equilibrium was achieved with700~800min. Adsorptionisotherms at different temperature obeyed Langmuir model, and HA are adsorbed in the form of homogenous coverage on the adsorbents’ surface. The behavior ofadsorption is favorable, and adsorption kinetics followed pseudo-second-orderequation. The adsorption rate constant decreased with the increasing initialconcentration of HAincreasing.(3) The HAadsorption on Fe3O4@SiO2–PANI decreased with increasing pH.The order of cations effects on adsorption is Ca2+﹥K+≈Na+, and the exist ofCa2+can evidently enhance the adsorption.(4) Desorption and regeneration of HA on Fe3O4@SiO2-PANI wereconducted in5cycles, and the regenerated adsorbent still possessed the highadsorption capacity, hence can be used repeatedly for the removal of HA inaqueous solution.(5) The XPS analysis of before and after HA adsorption confirmed that thecarboxylic and phenolic groups of HA molecules could reacted with protonatedimine and amino groups of PANI on the adsorbent.Adsorption of HAon Fe3O4@SiO2-PEI:(1) Based on the analysis of TEM, XRD and FTIR, the PEI has effectivelycovered on the surface of Fe3O4@SiO2. The diameter was20nm, and the crystalstructure of Fe3O4didn’t change. Fe3O4@SiO2-PEI was superparamagnetic.(2) The maximum adsorption amount of Fe3O4@SiO2-PEI synthesized bythe molecular weight of PEI in600and1800were132.59mg/g and50.42mg/g,respectively. HA adsorption on Fe3O4@SiO2–PANI could be well described byFreundlich model and adsorption kinetics followed pseudo-second-order model.(3) HA adsorption on Fe3O4@SiO2–PEI decreased with increasing pH. Thepresence of cations (Ca2+, K+, Na+) can improve HA adsorption and the enhancedHAadsorption was in the order of Ca2+﹥K+≈Na+.