| 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. |
PDF Full Text Request