Aggregation Kinetics Of Nano MnO₂ And Its Application In Enhanced Conventional Treatment Process For Trace Thallium Removal

In recent years, heavy metal pollotion accidents are constantly happening. These polution accidents have certain effect on the ecological safety and national economy, as well as drinking water supply. In the national “Twelfth Five Year” Plan, specific requirements about the comprehensive prevention and control of heavy metals contamination had been proposed. The strict stipulatio for heavy metal were listed in “Standards for Drinking Water Quality” of China. Due to its extremely high toxicity, the allowed concentration of Tl in drinking water is the lowest. In water treatment processes, reduction of potassium permanganate(KMn O4) or oxidation of dissolved manganese can produce nanosized manganese dioxide(n Mn O2). There are lots of surface hydroxyl groups on manganese oxides(i.e., more negatively charged functional groups), and good adsorption properties for cationic heavy metal therein. Moreover, nano materials, characterized by small size, huge specific surface area, have more activity sites, and thus high reaction activity. Therefore, n Mn O2 may have excellent adsorption properties. In water treatment processes, nano materials will also undergo aggregation, which may impact the removal efficiency of heavy metals for relevant waters. The present work firstly investigated the property of adsorption and oxidation Tl by n Mn O2; then the aggregation kinetics of n Mn O2 and reduction-induced aggregation and/or dissolution of n Mn O2 were also studied by time-resolved dynamic light scattering(TR-DLS); and the efficiency and mechnisms of trace Tl removal from drinking water through enhanced conventional treatment process by n Mn O2 was investegated at last.The adsorption properties of n Mn O2 for Tl(I) was investigated by constant concentration of Tl(I). Adsorption of Tl by n Mn O2 reached equilibrium in 15 min and large max. adsorption amount(~672 mg/g) was obtained. Competitive cations impact Tl(I) adsorption on n Mn O2 to a certain extent. Ca(II) and Mg(II) hindered Tl(I) adsorption, while 100 m M Na(I) only has ignorable effect. The complexation anions also reduced the adsorption of Tl(I) on n Mn O2 under high initial Tl(I) concentration, while no effect was observed at low initial Tl(I) concentration. Data also showed that p H plays a important role in the interaction between Tl(I) and n Mn O2. n Mn O2 Tl(I) adsorption capacity increased with the rising of p H. At acidic condition(i.e., p H4.0), n Mn O2 can oxidize Tl(I), generate Mn(II) and Tl(III), and thus cause the n Mn O2 aggregation and dissolution of n Mn O2. While at natural and basic condition(i.e., p H7.0 and 9.0), the adsorption amount of Tl(I) on n Mn O2 was larger than that at acidic condition. But there is no redox reaction between Tl(I) and n Mn O2, and no n Mn O2 aggregation and disolved Mn was observed. Aquatic humic acid(HA) reduced Tl adsorption amount by n Mn O2 in all p H range, and hindered the aggregation of n Mn O2 under acidic conditions n Mn O2.The aggregation kinetics of n Mn O2 in aquatic solution exhibit interaction of classical DLVO type interactions. Fitting calculation of classical DLVO theory to experimental data indicated that Hamerker constant of n Mn O2 was 7.84×10-20 J. The critical coagulation concentration(CCC) of monovalent and divalent electrolyte were for 28, 0.8, and 0.45 m M for Na NO3, Mg(NO3)2, and Ca(NO3)2, respectively. The interactions between colloids were more complicated when macromolecular organic matter(e.g., humic substances(HS), polysaccharides, proteins, and surfactants) was present. The steric repulsive forces, originated from organic layers adsorbed on Mn O2 colloidal surfaces, may be mainly responsible for their stabilizing effects. Humic substances and biomacromolecules tested slowed Mn O2 colloidal aggregation rates greatly. Protein(i.e., bovine serum albumin, BSA) was most effective, followed by aquatic humic acid(i.e., Suwannee River Humic Acid, SRHA), soil humic acid(i.e., SHA), polysaccharide(i.e., alginate), and aquatic fulvic acid(Suwannee River Fulvic Acid, SRFA). The EPM data was interpreted by the Ohshima’s Soft Particle Theory. The thickness of adsorbed layer can be obtained, and the results were consistent with aggregation experiments, indicating that staric interactions may play a primary role of in stabilizing n Mn O2. Little change was observed for n Mn O2 stabilization in the presence of surfactants(i.e., polyvinylpyrrolidone and dodecylbenzenesulfonate).X ray photoelectron spectroscopy(XPS) analysis confirmed that the excellent aggregation of n Mn O2 in the presence of Mn(II) additive may mainly attributed to their adsorption onto n Mn O2 surface, reducing steric repulsive effectively, while no evidence was found for redox reactions. The role of functional groups in the reduction-induced aggregation and/or dissolution of n Mn O2 was investigaed by TR-DLS. The introduction of reductive organic monomers resulted in a reduction-induced aggregation of Mn O2 colloids. As the redox reactions proceeded, hydrodynamic diameter of Mn O2 colloids firstly decreased slightly despite the decreased electrostatic repulsion between them, then an accelerated rate aggreg ation of Mn O2 colloids was observed accompanying further decrease of electrostatic repulsion, and finally the Mn O2 aggregates settled down. Organic monomer structure significantly impact the n Mn O2 aggregation and/or dissolution. Aggregation data showed that the higher reactivity organics have, the greater aggregation was observed. On the contrary, in the presence of organics with stronger chelating ability, n Mn O2 dissolved more. The rate of n Mn O2 aggregation and/or disolution increased with the increasing of monomer concentration. Additionally, the reaction reacitivy between organics and n Mn O2 decrease with the increasing of p H, thus lower aggregation and/or dissolution rate was obtained correspondingly. In the presence of citric acid, a autocatalyzed dissolution of n Mn O2 was measured, and the decreasing of p H and increasing of citric acid concentration accelerated its autocatalytic dissolution.Aluminum sulfate coagulation or quartz sand filtration can not remove trace Tl effectively. However, higher Tl removal efficeincy were obtained when these two conventional treatment process were enhanced by n Mn O2. Tl removal efficeincy was incereased with the increasing of n Mn O2 dosage. Due to the concentration of Tl was extremely low, the valence of Tl did not significantly impact Tl removal efficiency by the enhanced coagulation at wide p H range(i.e., 4.0-9.0). The influence of Ca(II) and HA was different. The dosage of n Mn O2 impact the initial efficiency of sand filtration. Similar Tl removal efficiency was not obtained until enhanced filtration run a period of time at different n Mn O2 dosage. Tl(III) removal efficiency was lower than Tl(I), due to Tl(III) has better performance of complexation with natural organic matter. The increase of Ca(II) and HA concentration inhibited the enhanced filtration to remove Tl. In the presence of SRHA, large amount of dissolved Mn was determined in the filtrated water.