【作者】 王向宇；

【导师】 刘惠玲；

【作者基本信息】 哈尔滨工业大学 ， 环境科学与工程， 2009， 博士

【摘要】 氯代有机物作为重要的化工原料、中间体和有机溶剂,被广泛应用于诸多工业,如化工、医药、制革、电子、农药等行业,它们通过多种途径进入环境,严重污染了大气、土壤、特别是地下水和地表水,以直接或间接的方式对人类的健康造成危害。以零价铁促进氯代有机物还原脱氯成为一个非常活跃的研究领域。相比其它的脱氯方法,零价铁脱氯材料廉价易得,无毒无害,对氯代有机物的脱氯反应在常温常压下即可进行,反应所需工艺设备简单,因而具有更多的优势。在零价铁表面加入催化剂钯可以极大地促进还原脱氯的速率。纳米级零价铁或钯/铁双金属颗粒具有高比表面积和表面反应活性,比普通金属铁脱氯速率快。本文采用液相还原法制备纳米铁颗粒,分别使用氯钯酸钾的水溶液和醋酸钯的乙醇溶液为钯化液对纳米铁颗粒进行钯化,经抽真空加热干燥处理制备出干燥状态下的纳米钯/铁双金属颗粒,并通过比表面积测定(BET–N2)、扫描电镜(SEM)、透射电镜(TEM)、X射线衍射光谱(XRD)、X射线荧光光谱(XRF)和能谱分析(EDS)等分析检测手段,表征了纳米钯/铁双金属颗粒的比表面积、表面形貌、颗粒形状和粒径、晶体结构、钯化率和钯负载均匀度。结果表明采用醋酸钯的乙醇溶液为钯化液制备的纳米钯/铁双金属颗粒不易团聚,粒径较均匀,颗粒纯度高,比表面积约为51.4 m2/g,颗粒直径为20?50 nm,纳米铁主要以体心立方的α-Fe0形式存在。采用纳米钯/铁双金属颗粒对氯代甲烷(包括四氯化碳、氯仿和二氯甲烷)进行催化还原脱氯,比较不同单质铁还原体系(包括纳米钯/铁双金属颗粒、纳米铁颗粒和普通微米级还原铁粉)对氯代甲烷的脱氯效果。考察了纳米钯/铁双金属颗粒钯化率、颗粒投加量、反应体系初始pH值和目标污染物初始浓度对氯代甲烷脱氯率的影响。结果表明纳米钯/铁双金属颗粒对氯代甲烷的脱氯率高于纳米铁颗粒和普通铁粉。氯代甲烷的脱氯率随颗粒投加量的增加而提高,纳米钯/铁双金属颗粒钯化率为0.20 wt%、溶液初始pH值为7的反应条件下,氯代甲烷的脱氯效果最佳。考察了纳米钯/铁双金属颗粒钯化率、颗粒投加量、反应体系初始pH值等因素对氯乙酸(包括三氯乙酸、二氯乙酸和一氯乙酸)脱氯率的影响。当纳米钯/铁双金属颗粒钯化率小于0.10 wt%时,氯乙酸的脱氯率随着钯化率的增高而增加,钯化率大于0.10 wt%时氯乙酸脱氯率随着钯化率的增高而减小。增加投加量可以促进脱氯反应的进行,纳米钯/铁双金属颗粒对氯乙酸的催化还原脱氯有较大的pH适用范围。纳米钯/铁双金属颗粒对氯乙酸的脱氯率随着氯乙酸分子中氯取代数的减少而降低。采用聚乙烯醇、戊二醛和聚乙二醇等为主要亲水化试剂,对聚偏氟乙烯(PVDF)微孔滤膜进行亲水化改性,以聚丙烯酸(PAA)为螯合剂,以亲水化改性PVDF膜为载体对纳米钯/铁双金属颗粒进行固定化,制备出负载纳米钯/铁双金属颗粒PAA/PVDF复合膜,采用等离子发射光谱、SEM和傅立叶红外光谱(FT-IR)对负载纳米钯/铁双金属颗粒PAA/PVDF复合膜的颗粒负载量、膜表面和截断面形貌及表面基团组成进行了分析,以三氯乙酸(TCAA)为目标污染物考察复合膜的催化还原脱氯效果。结果表明,经亲水化和PAA交联处理的负载纳米钯/铁双金属颗粒的复合膜上颗粒分散效果最好,载入的颗粒量最大,对三氯乙酸的催化还原脱氯效果优于未亲水化及未PAA交联的复合膜。负载纳米钯/铁双金属颗粒的复合膜对三氯乙酸的脱氯反应速率高于非固定化的纳米钯/铁双金属颗粒和负载纳米铁颗粒的复合膜。负载纳米钯/铁双金属颗粒的复合膜对三氯乙酸的催化还原脱氯效果受钯化率、负载颗粒投加量和目标污染物初始浓度等因素的影响。钯化率为0.534 wt%、颗粒投加量为0.391 g/L时复合膜对三氯乙酸的去除率最高。三氯乙酸初始浓度亦影响其去除率,随着初始浓度的升高,三氯乙酸的去除率降低。负载纳米钯/铁双金属颗粒的PVDF复合膜重复使用4次对三氯乙酸去除率基本不变,复合膜具有一定的稳定性。复合膜上PAA对Fe2+的螯合作用可避免Fe2+对水体的污染,也可防止铁颗粒表面氢氧化物钝化层的形成,即有利于避免负载纳米钯/铁双金属颗粒的失活。水的存在对脱氯反应起到重要作用,它促使零价铁发生腐蚀反应产生氢,贵金属钯在纳米钯/铁双金属颗粒对目标污染物进行脱氯反应中起到了催化的作用。将负载纳米钯/铁双金属颗粒的复合膜保存于无溶解氧的超纯无水乙醇中可防止膜变质和颗粒的失活,使得复合膜保持较高的反应活性。更多还原

【Abstract】 Chlorinated organic compounds (COCs) have been extensively used as synthetic intermediates and solvents in industrial applications, such as manufacture of chemicals, pharmaceuticals, leatherwares, electro-products, and pesticides. Due to leaks, spills, and releases from industrial sources, they inevitably contaminate the environment. COCs are ubiquitous and hazardous pollutants found in atmosphere, soil, surface water and groundwater. They are highly toxic and poorly biodegradable, and present a serious threat to the environment and human health. Reductive dechlorination of COCs by zerovalent iron (ZVI) has gained acceptance as a promising treatment technology. Compared with other treatment methods, ZVI technology has obvious advantages due the lowcost and nontoxicity of iron. Dechlorination of COCs can be conducted at ambient temperature and atmospheric pressure by relatively simple technique and equipment. The dechlorination reaction rate can be significantly increased by using iron nanoparticles post-coated with palladium. The enhanced reactivity of Pd/Fe bimetallic nanoparticles is attributed to the increased surface area and the deposition of the second metal Pd as a catalyst onto the surface of iron.Pd/Fe bimetallic nanoparticles were prepared by the liquid phase method, a simple two-step synthetic method where Pd was deposited onto the Fe surface. An aqueous solution of potassium hexachloropalladate and an ethanol solution of palladium acetate were used as palladization solution, respectively. Pd/Fe bimetallic nanoparticles were dried in an oven under vacuum, and extensively characterized in terms of BET-N2, SEM, TEM, XRD, XRF and EDS to identify the specific surface area, morphology, particle shape, size, crystal structure, Pd content, and surface elemental composition. The results suggest that Pd/Fe bimetallic nanoparticles palladized by using an ethanol solution of palladium acetate had smaller aggregation tendency and were more evenly sized than Pd/Fe bimetallic nanoparticles palladized by using an aqueous solution of potassium hexachloropalladate with a diameter in the range of 20?50 nm and a surface area of 51.4 m2/g. The crystal structure of iron was a regular bccα-Fe crystalline state.Dechlorination efficiencies of chlorinated methanes (including tetrachlorinated methane (CT), trichlorinated methane (CF), and dichlorinated methane (DCM)) by Pd/Fe bimetallic nanoparticles were higher than that by iron nanoparticles or microscale iron powders. The effects of some essential parameters, such as Pd loading, Pd/Fe nanoparticle addition, initial pH value of reaction system, initial concentration of target pollutants, on the dechlorination efficiency of chlorinated methanes were systematically studied. Dechlorination efficiency increased with increasing nanoparticle addition. For chlorinated methane solution with initial concentration of 100 mg/L, the optimal dechlorination efficiency was obtained at Pd/Fe nanoparticle addition of 10 g/L, Pd loading of 0.20 wt%, and initial pH value of 7.Effects of Pd loading, nanoparticle addition, initial pH value of reaction system on dechlorination efficiency of chloroacetic acids (including trichloroacedic acid (TCAA), dichloroacetic acid (DCAA), and monochloroacetic acid (MCAA)) were investigated. Dechlorination efficiency of chloroacetic acids increased with the increase of Pd loading at Pd loading < 0.10 wt%, while it was found to decrease at Pd loading > 0.10 wt%. Increasing Pd/Fe nanoparticle addition resulted in the increase of dechlorination efficiency. Dechlorination of chloroacetic acid by Pd/Fe bimetallic nanoparticles can be conducted in a wide range of pH value. Dechlorination efficiency of chloroacetic acid with low-chlorinated chloroacetic acid was greater than that of high-chlorinated chloroacetic acid.PVDF microfiltration (MF) membrane was hydrophilized by coating with a mixture of polyvinyl alcohol (PVA), glutaraldehyde, and polyethylene glycol (PEG). Polyacrylic acid (PAA) was used as chelating reagent to functionalize hydrophilized PVDF MF membrane. Pd/Fe bimetallic nanoparticles were immobilized in modified PAA/PVDF MF membrane matrix. Pd/Fe composite membrane system showed high reactivity in catalytic dechlorination of TCAA. The amount and dispersal of Pd/Fe nanoparticles immobilized in PAA/PVDF MF membrane were largely influenced by hydrophilization and PAA crosslinking treatment of the pristine membrane. The metal content, morphology, and surface chemical structure of composite membrane were measured by using ICP-OES, SEM, and FT-IR, respectively. No obvious aggregation of Pd/Fe bimetallic nanoparticles were found in hydrophilized and PAA crosslinked PVDF MF membrane. Compared with unmodified Pd/Fe composite membrane, the hydrophilized and crosslinked PAA functionalized Pd/Fe composite membrane showed higher reactivity for catalytic dechlorination of TCAA.Removal efficiency of TCAA by Pd/Fe PVDF composite membrane was greater than that by nonimmobilized Pd/Fe bimetallic nanoparticles. Some important parameters, such as Pd loading and Pd/Fe nanoparticles addition contained in the composite membrane, and initial concentration of target pollutant, were found to have effects on the removal efficiency of TCAA. The highest removal efficiency of TCAA with initial concentration of 5 mg/L was obtained at Pd loading of 0.534 wt% and Pd/Fe nanoparticles addition of 0.391 g/L. Removal efficiency decreased with the increase of initial concentration of TCAA. The stability and longevity of Pd/Fe membrane system were preliminarily studied by a multi-spiking batch experiment where complete removal of TCAA was achieved for four times although the aging of the composite membrane was found. With ferrous chelating by PAA, the secondary pollution caused by the dissolved metal could be avoided. Water plays a key role in dechlorination reaction process. No removal of TCAA was detected in an ethanol solution of TCAA, indicating that no dechlorination reaction took place in the absence of water, which is the donor of hydrogen from the Fe corrosion reaction, and the hydrodechlorination reaction requires the presence of some water to facilitate the corrosion. Being stored in pure ethanol solution without water and dissolved oxygen would benefit the maintenance of catalytic dechlorination activity of Pd/Fe composite membrane.更多还原