| Humic acid (HA) is a high-molecular which widely exists in the nature and the precursor which can produce harmful substances in the chlorine disinfection of drinking water, such as trihalomethane and three halogen acid. The harmful substances may lead to carcinogenesis, teratogenesis and mutagenesis. In the conventional treatment process of drinking water, HA is difficult to remove. This paper used multiwalled carbon nanotubes (MWCNTs) to remove HA in micro-polluted source water after magnetizing and modifying by calcium chloride. In the thesis, the influence law of preparation conditions of magnetic MWCNTs coated calcium on HA removal performance was studied, the characterization of structural features of magnetic MWCNTs coated calcium was analyzed, the influence law of water quality factors on the HA removal performance was systematically investigated, and the HA removal kinetics, thermodynamics and mechanism was discussed.In the process of preparation, The influence of preparation conditions of magnetic MWCNTs coated calcium on the HA removal performance was studied, such as mass ratio of MWCNTs to Fe3O4, mass ratio of MWCNTs to CaCl2, the molar ratio of hydroxyl ions to carbonate ions (OH·/CO32-), reaction time, reaction temperature, stirring speed and aging time. The results showed that mass ratio of MWCNTs to Fe3O4, mass ratio of MWCNTs to CaCl2, reaction temperature and stirring speed were the factors that influence largely, except the molar ratio of hydroxyl ions to carbonate ions, reaction time and aging time. The greater the mass ratio of MWCNTs to Fe3O4, the better the HA removal rate. The mass ratio of MWCNTs to CaCl2 reduced from 4-1 to 1:1, HA removal rate was gradually increasing from 76.36% to 87.59%, and continued to decrease the mass ratio, HA removal rate decreased. As the reaction temperature rose to 60 ℃, HA removal rate increased to 91.32%, then the HA removal rate was falling when the temperature was to continue to raise, and the curve tend to be stable after 75℃. And with the stirring speed raised to 300r/min, the removal rate increased to 92.21%. After continuing to increase the stirring speed, the removal rate had a tendency to decline. The orthogonal test on preparation of magnetic MWCNTs coated calcium showed that the order of factors affecting the algae removal performance was: m(MWCNTs)/m(Fe3O4)> stirring speed(S)> reaction temperature(T)> m(MWCNTs)/ m(CaCl2). The optimum preparation condition of magnetic MWCNTs coated calcium sample was:m(MWCNTs)/m(Fe3O4)=2.5:1, m(MWCNTs)/m(CaCl2)=2:1, T=60℃ S=400r/min.The thesis studied the effect of magnetic MWCNTs coated calcium dosage, HA initial concentration, adsorption time, stirring speed, pH and temperature on HA removal by magnetic MWCNTs coated calcium. The results showed that the removal efficiency of HA increased along with the increase of the magnetic MWCNTs coated calcium dosage and the removal rate increased slowly after the dosage increases to 0.5 mg/L. With the increase of the HA initial concentration, adsorption quantity increased, but the removal rate was reduced. HA removal rate increased quickly at the beginning of contact, then turned to be slow. After 5 hours the reaction reached adsorption equilibrium and the equilibrium adsorption quantity was 39.41 mg·g-1. The removal efficiency of HA reached 91.84% as the stirring speed increased to 225 r/min and decreased after more than 225 r/min. The removal rate increased with the increase of pH, and reached the higest of 92.24% at 5 of pH, then it reduced with the increase of pH. The removal rate of HA by magnetic MWCNTs coated calcium decreased along with the increase of temperature. It showed that the low temperature was beneficial to the adsorption of HA and the adsorption process was exothermic reaction..The magnetic MWCNTs coated calcium was characterized by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS), X-ray diffraction (XRD), infrared spectrum (FTIR), specific surface area (BET) and vibrating sample magnetometer (VSM). The results showed that CaCO3 and magnetic iron oxide which mainly involves Fe3O4 and y-Fe2O3 were adhered on the MWCNTs successfully. The kinetic of HA adsorption by magnetic MWCNTs coated calcium was studied using the well-known used kinetic models:pseudo-first order kinetic model, pseudo-second-order model, liquid film diffusion model and Particle diffusion model. It showed that the reaction kinetic was according with pseudo-second-order model. The adsorption mechanism may be controlled by two step:migration of HA from the bulk of the solution to the external surface of magnetic MWCNTs coated calcium, and the adsorption of HA at an active site on the surface of the magnetic MWCNTs coated calcium. Three kinds of isothermal adsorption model were used to fit the adsorption process. The adsorption data was better fitted to Temkin isotherm than the Langmuir and Freundlich isotherm which showed that the magnetic MWCNTs coated calcium adsorbed HA via chemical adsorption mainly. The thermodynamic parameter ΔG0<0, Δ0<0,ΔS<0, demonstrate the spontaneous and exothermic entropy reduction process. Adsorption mechanism mainly included the adsorption sites that MWCNTs provided, π-π bonds between MWCNTs and HA and coordination bonds between Ca and HA. |
PDF Full Text Request