A Study Of Electrosorption Desalination And Selectivity Of Ions By Activated Carbon Fiber

Electrosorption desalination technology is a promising method for ion separation,has the advantages of low-power、high efficiency、no pollution. Adsorption capacity andadsorption rate are the main indicators of the performance of electrosorption. Thefactors which affect electrosorption include: electrode material、the equipmentstructure、operating conditions、water quality. So, carrying out the studies on the basic theory andpractical applications of electrosorption, not only has important scientific value, but alsohas profound practical significance.In this paper, activated carbon fibers were taken as electrode, the electrochemicalproperties、the kinetic and isotherm of electrosorption were analyzed, the selectivity ofalkali metal、alkaline earth metal ions in multi-component solutions were studied. Themain results are as follows：(1) This paper has built on capacitance model of electric double layer, since thepore size distribution is considered, the main factors which affect the capacitance ofelectric double layer was studied by cyclic voltammetry. The results show that: thecapacitance of electric double layer was composed of microporous and mesoporouscapacitance; Scan rate、the concentration、the material structure are the main factors ofaffecting capacitance; The capacitance value with a high scan rate is little different fromthe capacitance value with a low scan rate for high concentration solution; For lowconcentration, the capacitance value of the high scan rate is much smaller than thecapacitance value of the low scan rate because of the slow response of charge anddischarge of microporous, which has mass transfer resistance, resulting in a lowerutilization of the specific surface area and less contribution to the capacitance of electric double layer.(2) By analyzing the electrical impedance spectrum of solution with differentconcentration, an equivalent circuit was obtained, and the equivalent circuit componentvalues was found, the interface state of solution was further analyzed. The resultsshowed that: electrosorption process in this experiment can be fitted byR(C(RW)).Under the same operating conditions, the migration resistance Rs in highconcentration solution is small, the actual voltage applied to the external electrodematerial is relatively large, the capacitance value is larger.(3) The effects of voltage、concentration and temperature on the electrosorptioncapacitance and rate were analyzed by static electrosorption experiments under variousconditions. Adsorption kinetics under various conditions can be described by Lagergrenmodel; The activated carbon fiber adsorption rate constant of NaCl under1.2Vincreased by13%compared to the open-circuit. The increase of the rate of adsorption isdue to the increase of electrostatic attraction. Adsorption isotherm under each conditioncan be described by Langmuir and Freundlich isotherm model, and the former is abetter fitting result. The maximum adsorption capacitance of NaCl by activated carbonfibers under1.2V voltage increased by2.14times than the open-circuit, the main reasonattributed to the increase of the surface charge density and adsorption point, there isnothing to do with redox reaction. Electrosorption thermodynamics parameter displaythat electrosorption process of this experiment is endothermic, non-spontaneous andentropy change is negative.(4) On the basis of electrochemical properties、 electrosorption kinetics、thermodynamics research of activated carbon fiber, the electrosorption selectivity ofdifferent inos by activated carbon fiber was studied in single-component andmulti-component alkali metal、alkaline earth metal ions solutions, the changes of ionconcentration after electrosorption in multi-component solutions was determined, theelectrosorption capacitance of electrode was calculated. The results showed that: theselective electrosorption performance of ions by activated carbon fiber can bedetermined by ion hydration radius、ion valence and the initial concentration of ions.For the ions with same concentration and same valence, the ion with small hydratedradius is more likely to be adsorbed; For the ions with same concentration and different valences, divalent ions are preferentially adsorbed over monovalent ion; Ions with highconcentration are preferentially adsorbed, the adsorption capacitance of ions with lowconcentration is smallest. Ca2+can substitute other adsorbed ions during dynamicelectrosorption process.