【作者】 柳听义；

【导师】 赵林；

【作者基本信息】 天津大学 ， 环境科学， 2011， 博士

【摘要】 由于含铬废水(主要为Cr(VI))的不合理排放,造成地表水环境的严重污染,甚至会进一步危害动物、植物和人类的健康。近年来,将纳米零价铁(Nanoscale zero-valent iron,NZVI)用于Cr(VI)污染水体的治理是一种新的污染控制技术,然而,在实际应用中NZVI仍然存在易团聚、易被氧化和难于工程实施等问题。针对这些问题,本论文通过柠檬酸改性和壳聚糖(CS)球包埋,成功制备出了有很好的分散性能和稳定性能的NZVI材料,然后通过碳纤维和表氯醇改性提高CS-NZVI球的机械强度。主要研究内容如下:通过柠檬酸表面改性的NZVI粒子平均粒径为65.2 nm,有很好的稳定性。随着Cr(VI)初始浓度、温度、pH值和腐殖酸(Humic acid, HA)初始浓度升高,NZVI对Cr(VI)去除效率降低;NZVI投加量的增加,去除率也随之增加。NZVI去除Cr(VI)符合一级反应动力学方程。制备壳聚糖-纳米铁(CS-NZVI)球的比较理想条件为:加热温度55℃,加热时间4h,出样口与NaOH液面之间的高度h为10cm,醋酸的浓度0.5%,CS的浓度5.0 g/L,NaOH的浓度选为0.5mol/L。CS-NZVI球为规则均一的黑色球体,球的粒径大约为3.1mm。CS-NZVI球内部具有大小不均一孔状结构,孔径大小分布在9.5-108.8μm之间,孔径尺寸的平均值为42.6μm。CS-NZVI球中的NZVI颗粒有很好的空气稳定性和分散性。CS-NZVI球对Cr(VI)的去除率受NZVI投加量、pH、Cr(VI)初始浓度和温度影响。在Cr(VI)初始浓度为20 mg/L、pH=3.9、NZVI投加量为5.0 g/L和20℃条件下,Cr(VI)去除率达到99.8%。CS-NZVI球对Cr(VI)的吸附是一个吸热熵增的过程,去除机理包括CS对Cr(VI)的吸附富集和NZVI还原去除Cr(VI)。CS-NZVI球对Cr (VI)的去除率比NZVI颗粒的略微低一些。CS-NZVI球去除Cr(VI)符合一级反应动力学方程。Freundlich等温吸附方程能够更好地对其进行描述,这表明CS-NZVI球对Cr(VI)的吸附是多层吸附。通过碳纤维和表氯醇对新制备的CS-NZVI球进行改性,提高了其机械强度。随着Cr(VI)初始浓度和pH值升高,ECH-CS-NZVI球和碳纤维CS-NZVI球对Cr(VI)去除效率降低;随着溶液温度和NZVI投加量的增加,去除率也随之增加。在相同条件下,ECH-CS-NZVI球对Cr(VI)的去除率与CS-NZVI没有明显的差别,而碳纤维CS-NZVI球对Cr(VI)的去除率与CS-NZVI的相比则有明显的降低,所以采用ECH提高CS-NZVI球的机械强度比较理想。更多还原

【Abstract】 Due to the unreasonable discharge of wastewater, surface water was contaminated by hexavalent chromium (Cr (VI)). Cr (VI) is a potential carcinogen and often causes both short term and long term adverse effects to humans, animals, and plants. As a result, removal of Cr (VI) from surface water has caused more and more attention from researchers. In recent years, nanoscale zero-valent iron (NZVI) has been widely introduced into water treatment processes to remove numerous heavy metals. However, the agglomeration, oxidation by non-target compounds and higher mobility in the aqueous solution are the major challenges for NZVI use in environmental remediation. To overcome these problems, entrapped Fe0 nanoparticles have been successfully prepared. NZVI coated by citric acid and chitosan (CS) beads shows good dispersibility and stability. Carbon fiber (CF) and epichlorohydrin (ECH), were used to enhance the mechanical strength of CS-NZVI beads. The main objectives of the study are to:NZVI modified by citric acid are nearly spherical in shape and uniform in size with a mean diameter of 65.2 nm. NZVI particles are better protected by citric acid and oxidation is hindered. The reduction capacity for Cr (VI) increases with increasing temperature and NZVI dosage but decreases with the increase in initial concentration of Cr (VI) and humic acid (HA) and pH values. HA have a better ability to chelate iron ions or oxide surface and will hinder the formation of Fe(III)-Cr(III) precipitate. The chelation reaction will reduce reactive sites of NZVI available to Cr (VI), so HA adversely affects Cr (VI) removal from the wastewater.The optimum conditions of preparation of CS-NZVI beads are: an initial acetic acid solution concentration of 0.5 % (v/v), an initial NaOH concentration of 0.5 M, an initial CS concentration of 5.0 g/L, a height between the exit of the sample and the NaOH surface of 10 cm, 55℃and 4 h. CS-NZVI beads are black, nearly spherical in shape and uniform in size with a mean diameter of 3.1 mm. CS-NZVI beads are macroporous and the pore sizes in the CS-NZVI beads are heterogeneous. The pore size ranges from 9.5 to 108.8μm with an average aperture size of around 42.6μm. Entrapment of NZVI in CS beads prevents the particles from aggregation and oxidation. The Cr (VI) removal rates depended on the dosage of NZVI, the pH value, the initial concentration of Cr (VI) and the reaction temperature. With an initial Cr (VI) concentration of 20 mg/L, a NZVI dosage of 5.0 g/L, a solution pH of 3.9 and 20℃, the removal rate was more than 99.8% after 1 h.Cr (VI) removal by CS-NZVI beads is an endothermic process. The removal mechanism may include both physical adsorption of Cr (VI) on the surface or inside of CS-NZVI beads and subsequent reduction of Cr (VI) to Cr (III). The results indicate that there is no significant difference between the reaction rates of bare NZVI and entrapped NZVI. Cr (VI) reduction kinetics follows a pseudo-first-order rate expression. Freundlich isotherm agrees better than Langmuir isotherm with experimental data, which indicates that the adsorption of Cr (VI) by CS-NZVI beads is heterogeneous adsorption.Through either physical or chemical modifications by carbon fiber (CF) and epichlorohydrin (ECH), the mechanical strength of CS-NZVI beads were enhanced. The reduction capacity for Cr (VI) increases with increasing temperature and NZVI dosage but decreases with the increase in initial concentration of Cr (VI) and pH values. At the same condition, the results indicate that there is no significant difference between the reaction rates of CS-NZVI beads and ECH-CS-NZVI. However, the reaction rates of CF-CS-NZVI beads are obvious lower than these of CS-NZVI. ECH may be a better choice to enhance the mechanical strength of CS-NZVI beads.更多还原