【作者】 张珍；

【导师】 徐新华；

【作者基本信息】 浙江大学 ， 环境工程， 2013， 博士

【摘要】 用纳米级零价铁(nZVI)或双金属体系对氯代脂肪烃、氯酚、PCBs和含氯杀虫剂等含氯有机污染物进行还原脱氯,使含氯有机物转化为低氯或无氯有机物,以达到低毒、可生物降解的目的,为含氯有机污染物的处理提供了一种全新的降解途径。然而该技术在实际应用中,却存在催化剂表面易钝化、纳米催化剂易团聚等诸多制约因素。为了较为系统的考察纳米级双金属体系对含氯有机污染物的催化还原脱氯降解效果,本文重点研究了纳米级Ni/Fe和Pd/Fe对水中氯苯和PCBs等含氯有机物的催化还原脱氯效果,考察了温度、pH、催化剂用量、初始浓度等因素对纳米级双金属去除水中氯苯和PCBs的影响,初步探讨了纳米级双金属体系去除水中氯苯和PCBs等含氯有机物的机理。结果表明：无论是1,3-二氯苯,还是1,2,4-三氯苯,均能在5h左右被纳米级Pd/Fe有效还原脱氯。纳米级Pd/Fe在5h内对1,3-二氯苯和1,2,4-三氯苯的去除效率分别为96.2%和86.7%,之所以1,2,4-三氯苯的去除效率比1,3-二氯苯的低,这一方面是因为相同结构时,苯环上的氯原子个数越多越难发生催化还原脱氯反应；另一方面,主要是因为1,2,4-三氯苯在邻位上存在氯原子,而邻位上的氯原子则比间位上的氯原子更难脱氯。也就是说,在含氯有机污染物的脱氯过程中,氯原子个数较多的含氯有机物比相同结构氯原子个数较少的含氯有机物难进行催化还原脱氯反应。腐植酸(HA)和S042-对纳米级Pd/Fe催化还原1,3-二氯苯和1,2,4-三氯苯脱氯均具有抑制作用,但其作用机理各不相同。尽管HA中的活性基团会促进纳米级Pd/Fe对有机氯污染物的还原脱氯,但是HA在纳米级Pd/Fe表面易被吸附,催还还原受到抑制,而且这种作用占了绝对的主导作用,从而导致HA的存在会抑制纳米级Pd/Fe催化还原脱氯反应的进行。而S042-一方面会攻击零价铁的表面,在Fe的表面形成一层钝化膜,减小Fe的腐蚀速度,进而在一定程度上抑制反应的进行；另一方面S042-中的S会引起双金属催化剂中毒,从而降低其反应性。Cl-、Cu2+以及Ni2+对纳米级Pd/Fe催化还原1,3-二氯苯和1,2,4-三氯苯脱氯均具有促进作用,但其作用机理也有所不同。Cl-主要是进入到铁表面的氧化钝化层中,并分散氧化层增加其渗透性；同时,金属离子水解使Cl-生成HC1,HCI又阻止Fe表面氧化物和氢氧化物钝化层的形成,在一定程度上保持了纳米级Pd/Fe较高的反应性,从而在一定程度上促进纳米级Pd/Fe催化还原1,3-二氯苯和1,2,4-三氯苯脱氯反应的进行。而Cu2+和Ni2+则是与Fe反应,生成良好的加氢催化剂Cu和Ni,促进纳米级Pd/Fe催化还原1,3-二氯苯和1,2,4-三氯苯脱氯反应的进行。此外,较高的纳米级Pd/Fe投加量和较高的钯化率均有利于纳米级Pd/Fe催化还原1,3---氯苯和1,2,4-三氯苯脱氯反应的进行。对纳米级Pd/Fe去除水中PCB14的研究同样发现,HA会占据纳米级Pd/Fe表面的活性反应位,从而对纳米级Pd/Fe催化还原脱氯产生抑制作用。随着HA浓度由0,25增加到50mg/L,反应24h纳米级Pd/Fe对PCB14的去除效率分由99.3%,98.8%下降到98.3%。此外,Cl-、HCO3-、CH3COO"的存在对纳米级Pd/Fe对PCB14催化还原脱氯具有一定的促进作用,其作用机理主要是缓解了催化剂表面氢氧化物钝化层的形成,加速了铁的腐蚀,从而促进了脱氯反应的进行。纳米级Ni/Fe和纳米级Pd/Fe均能使Aroclor1242在较短的时间内脱氯,反应5h时纳米级Ni/Fe对Aroclor1242的去除效率就达到近80%,之后在10h和24h分别达到95.6%和95.8%。高氯代PCBs同系物逐步脱氯为低氯代PCBs同系物,最后脱氯成为联苯,反应过程中并没有发现C-C化学键的断裂。PCBs的脱氯产物为低氯代PCBs同系物和联苯。此外,研究还发现,高氯代PCBs同系物较低氯代PCBs同系物更难脱氯。高的镍化率和纳米级Ni/Fe投加量均有利于催化还原脱氯反应的进行。Ni/Mg和Mg粉的脱氯性能明显优于纳米级Ni/Fe和纳米级Fe,主要是因为Mg的还原性明显优于Fe。在自然环境中,不同地方PCBs污染的同系物种类以及污染浓度不尽相同,而大多数的研究只关注于某一种特定PCBs同系物的治理,而自然环境中不同PCBs污染往往同时存在,因此研究纳米级Ni/Fe和纳米级Pd/Fe催化还原Aroclor1242具有很好的应用前景。含氯有机物在纳米级双金属作用下脱氯过程大致可以分为三个阶段：(1)吸附作用,水中Fe的腐蚀以及H的产生；(2)第2种金属(以Pd为例)表面氢的分离,形成金属氢化物或者可供含氯有机污染物脱氯反应的氢；(3)含氯有机污染物吸附在纳米级Pd/Fe表面形成Pd-RCln,然后一个C-Cl键断裂,C1原子被金属氰化物或者高活性氢取代,形成RHCln-1。此外,含氯有机物在双金属作用下的脱氯是逐步完成的,即含氯有机物苯环上的氯原子被氢逐步代替,最终氯苯脱氯成为苯,而PCBs则脱氯成为低氯代的PCBs同系物及联苯。无论是纳米级Ni/Fe还是纳米级Pd/Fe,其催化还原2,4-DCP脱氯均是先将DCP转化为CP,然后进一步脱氯产生P,P和Cl-则是2,4-DCP脱氯的终产物。HA会吸附在纳米级Ni/Fe和Pd/Fe表面,占据其表面的活性反应位,抑制纳米级Ni/Fe以及纳米级Pd/Fe催化还原2,4-DCP脱氯反应的进行。在纳米级Ni/Fe催化还原2,4-DCP脱氯时,20mg/L HA的存在,会使反应2h时2,4-DCP的去除率由不加HA时的99.7%降低到84.1%,P的产率也相应的由85.7%降低62.7%,反应过程中o-CP的最高浓度分别由10min时的0.025mM变为30min时的0.039mM。此外,NO3-对纳米级Pd/Fe催化还原2,4-DCP以及CP脱氯具有明显的抑制作用。随着NO3-的初始浓度由0,10,30增大到50mg/L,反应60min时2,4-DCP的去除率相应由42.9,40.7,38.6降低到37.5%；与此同时,P的产率也由16.5,15.2,14.3降低到13.5%。此外,较高的镍化率(钯化率)、较高的纳米级Ni/Fe (Pd/Fe)投加量均有利于纳米级Ni/Fe(Pd/Fe)双金属颗粒催化还原2,4-DCP脱氯反应的进行。更多还原

【Abstract】 Zero valent iron and its compounds have been used widely in the remediation of chlorinated organics compounds (COCs), such as chlorinated aliphatic hydrocarbons, chlorophenol, organochlorine pesticides, polychlorinated biphenyls (PCBs), and so on. The reasons were that on the one hand they had low cost and were easy to obtain, and on the other hand, they could convert COCs to lower chlorinated, biodegradable, lower toxicity or non-toxicity organics compounds. This gave a new approach in the remediation of COCs. But their real engineering application has been severely circumscribed since their still existed some indeterminate factors. The catalytic dechlorination of2,4-dichlorophenol, PCBs, chlorobenzenes by Ni/Fe and Pd/Fe bimetallic nanoparticles were investigated in order to understand their applicability for in-situ remediation of groundwater. Moreover, the mechanism and the factors, such as temperature, pH, Ni/Fe dosage and initial concentration, which influenced the catalytic dechlorination reactions were all investigated. The experimental results indicated:No matter1,3-dichlorobenene, or1,2,4-trichlorobenzene could be effectively removed by Pd/Fe nanoparticles in water. The removal efficiencies of1,3-dichlorobenene and1,2,4-trichlorobenzene by Pd/Fe nanoparticles were sepatately96.2%and86.7%in5h.1,3-dichlorobenene had higher removal efficiencies than that of1,2,4-trichlorobenzene in the same time at the same experimental conditions, on the one hand for chlorine atoms of1,3-dichlorobenene on benzene ring is smaller, less chlorine atoms on benzene ring are dechlorinated faster than more chlorine atoms on the benzene ring, and the more chlorine atoms on the benzene ring have, the difficult to dechlorinated for the COCs is.On the other hand, Chlorine atoms on para-or meta-positions are dechlorinated faster than those on the ortho-position. The ortho-position Chlorine atoms on1,2,4-trichlorobenzene made it difficulty to dechlorinate than1,3-dichlorobenene.Humic Acid (HA) and SO42-all had inhibition role on the dechlorination of1,3-dichlorobenene and1,2,4-trichlorobenzene, but their mechanism is difference. HA on the one hand adsorbed on the surface of Pd/Fe nanoparticles, and competed for reaction sites with COCs, the accumulation of adsorbed HA on Pd/Fe nanoparticles surface would reduce the dechlorination reaciton rate, but on the other hand, the function in HA may act as electron shuttle promoting electron transfer, this would has positive effective enhancing the dechlorination reaciton rate by Pd/Fe nanoparticles, but HA’s inhibition role was the dominant role as a whole, leading HA had inhibition role on the dechlorination reaction. On one hand, sulfate could attack Fe surface although it is amild corrosion stimulator which corrodes Fe vigorously, this may be attributed to the thick film formed in presence of sulfates which reduce the corrosion rate, further decreasing the dechlorination reaction rate. On the other hand, sulfate could made the bimetallic catalyst poisoning, which also could decrease the dechlorination reaction rate.The dechlorination rate of1,3-dichlorobenene and1,2,4-trichlorobenzene underwent significant enhancement in the presence Cl", Cu2+and Ni2+. Chloride is known to attack the protective Fe(OH)2film locally, causing pitting corrosion. Even small amounts of Cl" are known to break down the film, the corrosion rate increasing rapidly with Cl-concentration. The high solubility of FeCl2further destabilizes the film of insoluble Fe(OH)2allowing Fe to corrode and enter into solution as the chloride, and making the surface active and causing further dissolution of Fe. The consequence was the PCB dechlorination rate increased significantly compared to the control. Ni and Cu are well-known excellent catalysts for the hydrogenolysis. Co-existence of Ni(Cu) and Fe in the particles has been proved to be very effective to accelerate the dechlorination. When Cu2+and Ni2+existed in the aqueous, Cu2+and Ni2+could react with zero valent iron and forming Ni and Cu, enhancing the dechlorination reaction. Besides, high Ni(Cu) content and Ni/Fe(Pd/Fe) nanoparticles dosage favored the catalytic dechlorination of1,3-dichlorobenene and1,2,4-trichlorobenzene.HA adsorbed on the surface of Pd/Fe nanoparticles, and competed for reaction sites with PCB14, the accumulation of adsorbed HA on Pd/Fe nanoparticles surface would reduce the dechlorination reaciton rate of PCB14. With HA concentration increasing from0,25to50mg/L, the removal percentages of PCB14by Pd/Fe nanoparticles decreased from99.3%,98.8%to98.3%in24h, respectively. Besides, Cl-、HCO3-、CH3COO-all enhanced the dechlorination rate of PCB14by Pd/Fe nanoparticles. This may be attributed to the extremely high solubility of (CH3COO)2Fe, FeCl2, Fe(HCO3)2, as compared to Fe(OH)2, leading to rapid dissolution of Fe. Further, they also increased conductivity of the solution which meant enhancing Fe corrosion rates at the Pd/Fe galvanic cells.Pd/Fe and Ni/Fe nanoparticles could effectively remove Aroclor1242in water. Approximately80.0%Aroclor1242underwent dechlorination within5h by Ni/Fe nanoparticles, then it arrived at95.6%in10h and95.8%in24h, accordingly. The higher chlorinated congeners were gradually dechlorinated to the lower chlorinated congeners. And during the reduction of PCBs, C-C bond cleavage reaction did not appear, the proposed dechlorination products with Ni/Fe nanoparticles here was the low chlorinated PCBs and biphenyl. High Ni/Fe nanoparticle dosage and high Ni content in Ni/Fe nanoparticles favored the catalytic dechlorination reaction. Moreover, a comparison of different types of catalysts on the dechlorination of Aroclor1242indicated that Ni/Mg and Mg powders showed a greater reactivity than Ni/Fe and Fe nanoparticles, respectively.The dechlorination mechanism of COCs by Ni/Fe and Pd/Fe nanoparticles is proposed as described as follows:(1) Adsorption, hydrogen produced and the corrosion of iron in aqueous.(2) The dissociation of H2on the Pd, leading to the formation of metal hydride or hydrogen radicals for the dechlorination.(3) The COCs adsorbed on the Pd surface and formation of the complex Pd-CIR, then breaking down of C-Cl, and the production of hydrogen by the active metal hydride or replacement of the chlorine atoms to form phenol by hydrogen radicals.No matter Ni/Fe, nor Pd/Fe bimetallic nanoparticles, could effectively remove2,4-DCP in water. And2,4-DCP was first adsorbed by the nanoparticles, then reduced to o-CP and p-CP, and later converted to phenol, phenol was the sole final organic product. No other chlorinated intermediates or final organic products were detected. HA adsorbed on the surface of nanoscale Ni/Fe and Pd/Fe particles, and competed for reaction sites with COCs, the accumulation of adsorbed HA on the nanoscale Ni/Fe and Pd/Fe particles surface would reduce the dechlorination reaciton rate. The concentration of2,4-DCP decreased rapidly and the removal percentage of2,4-DCP and the production rates of P reached99.7%and85.7in120min for Ni/Fe nanoparticles in the absence of HA. In contrast, they reached only about84.1%and62.7%in the presence of20mg/L HA during the same reaction periods, respectively. At the same time, the maximum concentration of o-CP changed from0.025mM in10min in the absence of HA to0.039mM in30min in the presence of20mg/L HA during the reaction periods. Moreover, the existence of NO3-also had a significant inhibition effect on dechlorination efficiency. With initial NO3-concentration increased from0,10,30to50mg/L, the removal percentage of2,4-DCP reached42.9,40.7,38.6and37.5%in60min. Meanwhile, the production rates of P increased from42.9,40.7,38.6to37.5%, respectively. Besides, high Ni(Pd) content and high Ni/Fe(Pd/Fe) nanoparticles dosage favored the catalytic dechlorination of2,4-DCP.更多还原