| Gypsum whisker, which is also called calcium sulfate whisker(CSW), is a kind of low cost, highly cost-efficient, and widely applicated new inorganic materials and is getting more and more applications in coating, rubber, plastic, papermaking, and other fields. Some people had attempted to use gypsum whisker to purify phosphorus, oil or dyes-containing wastewater, showing a certain effect. However, research on the application of gypsum whisker in the field of environmental engineering is still lacking, because there are some problems required to solve systematically, such as affinity with contaminants, poor stability, and so on. Research work on surface modification of calcium sulfate whisker and enhancing its ability to remove common contaminants will undoubtedly have practical significance for potential expansion of gypsum whiskers in the environmental field.The two conventional preparation methods of calcium sulphate whisker are thermal method and atmospheric pressure acidification method, but shotcomings such as more complicated process, high production costs, and other issues are still exist. In this paper, reactive crystallization of calcium sulfate whisker is adopted, and the operating conditions on the CSW growth habit and morphology impact are discussed, such as the concentration of the polymer additive P123, SI degree of supersaturation solution, temperature, solution p H, reaction time and so on. The results showed that: under experimental conditions, the degree of supersaturation, temperature, and reaction time are the key factors for whisker morphology. The moderate amount o f P123 helps improve whisker aspect ratio, and is not influenced by pH obviously. At 30 ~ 60℃, 0.25 ~ 0.57 supersaturation, the solution p H = 7, and under the conditions of the reaction time of 4~12h, large aspect ratio calcium sulfate whisker can be prepared with uniform morphology. The reaction crystallization method has advantages of simple process, low production cost, high efficiency.In order to improve the affinity of gypsum whiskers with heavy metal, we adopted C hitosan-adpic acid to modified calcium sulfate whisker preparated by crystallization method. The influence of modification reagent dosage, modification temperature, p H, stirring speed, and other operating parameters of the modified product on the Hg2+ purifying efficiency in simulated wastewater were investigated, and the results showed that: modified temperature, modified pH, stirring speed have effects on the mercury adsorption of Modified whiskers, with temperature, stirring speed increasing, the modified whiskers’ s adsorption efficiency of mercury showing upward trend; when modified solution p H value is about 7.5, the mercury adsorption efficiency is about 95%; the effect of the amount of modifier, namely the ratio of the mass percentage of chitosan and adipic acid, is not obvious.To study the laws and mechanisms of Hg purification by gypsum whiskers modified with chitosan- adipic acid, batch adsorption experiments were conducted and the effects of operating conditions such as adsorption time, pH value, and adsorption temperature et.al on mercury removal were studied. Mercury adsorption kinetics and adsorption isotherms were also investigated. at last, the mechanism about removal of mercury were analyzed by means of scanning electron microscopy(SEM), infrared spectroscopy, and thermal gravimetric analysis. The results showed that, pH is a critical factor affecting mercury adsorption efficiency of gypsum whisker, with mercury adsorption efficiency firstly increasing with improved p H value and then becoming flat. When pH = 7.5, the mercury removal efficiency of whiskers is greater than 95%, and the mercury purification process is completed in about 2 ~ 3h.When the initial concentration of mercury is in the range of 1000 ~ 4000ug/L, whiskers’ s adsorption meets Langmuir adsorption isotherm model and pseudo-second order adsorption kinetics model. The mechanism for the reaction of the gypsum whisker with mercury is that the chitosan’s amino and hydroxy coated on the surface of whiskers react with mercury forming stable inner complex salt(-N-Hg-O-). This process is mainly a chemisorption process and physical adsorption of mercury on the gypsum whiskers may also exist. |
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