Controllable Precipitation Of Layered Double Hydroxide During Purification Of Electroplating Waste Water And Application Of Sludge-derived Catalyst In Decomposing Environmental Gaseous Pollutants

With the fast development of city and industry, China faces various environmentalproblems to be solved, including heavy metal-organic polluted water and earth, haze-characterized atmospheric pollution. Industry process results in electroplating wastewater, which usually contains transition metals (such as Fe, Cr, Zn, Ni) and organiccomplex ions. If it was discharged without treatment, both nature and human beingwould be damaged potentially. At present, plating water is generally treated byprecipitation cost-effectively. However, it can result in great amount of sludge, whichis much more difficult to deal with. As a result, it is urgent to find a new way to treatplating water and sludge. On the other hand, control of NOxand SF6is a worldwidetopic, which is fairly related to "haze" and "climate warming". From the viewpoints ofrecycling sludge into high added-value product and utilizing waste to treat pollution,this paper was carried out. Various plating waters were used to produce catalystprecursor with controllable and adjustable structure by in-situ precipitation andcomponent matching, resulting in catalyst characterized by carbon-doped transitionmetal. This catalyst was used to decompose typical environmental gaseous pollutions,including NOxand SF6. The following conclusions have been obtained:(1) Effective purification of heavy metals could be achieved by in-situ LDHprecipitation and component matching (EMW) of electroplating waste waterElectroplating waste wasters with different components were usually mixed randomly.Thus, complicated components in the waste water were difficult to be co-removed bychemical precipitation. Moreover, in the CN-containing plating water, transition metalcyanide complexes co-existed with each other. Three different methods were used totreat plating water, including in-situ components matching (EWM), adsorption duringsynthesis (ADS) and adsorption after synthesis (AAS). Without any additional metalsor other adsorbents, several plating waters were treated by EMW. These water mainlycontained Zn(189mg/L), Ni(392mg/L), Cr(131mg/L), Fe(1927mg/L) and CN(42.3mg/L). As a result,100%Zn,98%Ni,100%Cr and100%Fe were removed. Yet,63%of free and complex CN were removed by intercalation into the formed LDHinterlayer. The obtained precipitation was a kind of sludge containing obviousNiZnCr-Fe(CN)4-6-LDH structure.(2) EWM can regularize the structures of layer and interlayer as required inLDH In order to regularize the structures of layer and interlayer as required in LDH through EWM of heavy metal waste water, the corresponding ratio changes of variousmetal hydroxyl and oxide bonds in FTIR were compared, by controlling the molarratios in MgxAl-LDH (x=2~4) layer. When Mg:Al increased from2:1to4:1, thecontent ratio of OH-Mg2Al:OH-Mg3decreased from95.2:4.8to58.9:41.1.Furthermore, the corresponding ratio changes of interlayer ferricyanide andferrocyanide in FTIR were compared, by controlling the molar ratios in ZnxCr-LDH(x=2~4) layer. When Zn:Cr was2:1and4:1, charge density of LDH cation layer was4.00and2.40e/nm2, respectively. In order to balance the charge density betweenLDH layer and interlayer with NO-2-3and CO3, about85%Fe(CN)3-6(3.37e/nm2) and66%Fe(CN)4-6(4.42e/nm2) transformed to each other. Therefore, LDH precipitationcould be obtained with certain transition metal species, layer structures and aniondistributions in the interlayer by EWM of heavy metal waste water.(3) LDH-type sludge could be used to produce nano magnetic ferrite and sludge-derived catalyst There were various LDH structures in transition metal sludgederived from in-situ precipitation and component matching of plating water. Thus,this LDH-type sludge was aimed to produce sludge-derived catalyst. As a result,NiZnCr-Fe(CN)4-6-LDH transformed into (NiZn)Fe2O4structure at a temperatureabove600oC. Furthermore, EWM-formed NiZnCr-Fe(CN)4-6-LDH could betransformed into nano magnetic ferrite oxide (NiZn)Fe2O4, with the crystallite size of10~25nm and the saturation magnetization up to40emu/g. In addition, there was2~7%carbon, little amount of Si, Ti, Zr et.al. in the activated sludge. Therefore, bycontrolling decomposition temperature, LDH-type plating sludge could betransformed into sludge-derived catalyst, featured by carbon-doped transition metaloxides.(4) Effective reduction of NO by plating sludge-derived catalyst Various transitionmetals and organic matters co-exist in sludge, which was precipitated from platingwater. In order to reduce NOxby transition metal sludge, the behavior of NiFe sludge-derived catalyst in decomposing nitric oxide (NO) was tested. The determining factorsof this process were compared, including sludge microstructure, carbon content,reaction temperature and space velocity. The sludge catalyst as well as NO wasspecially characterized before and after reaction. As a result,100%of NO reductionwas obtained at as low as300oC, evolving N2and CO2. Furthermore, this catalystcould decompose NO steadily at a wide temperature window from300to750oC,with a stable removal of3mmol/g. A microstructure analysis showed that NiFe were well-dispersed and doped by carbon in the catalyst. These results showed that thereaction between NO and C could be catalyzed by NiFe alloy in sludge-derivedreagent, resulting in the reduction of NO. The whole process is a typical SCR.(5) Effective decomposition of greenhouse gas SF6via sludge-derived catalystSulfur hexafluoride (SF6) is a typical greenhouse gas, and its catalytic decompositionwas seldom reported. From the viewpoint of plating sludge recycling, the feasibilityof CrCuFe sludge-derived reagent in decomposing SF6was tested. As a result, thereagent remarkably removed SF6at a capacity of1.10mmol/g at600oC. Furthermore,the evolved gases were SO2and SiF4, with no toxic SOF4, SO2F2and SF4beingdetected. These generated gases could be readily captured by NaOH solution. Theseresults showed that the reaction between SF6and SiO2could be catalyzed by CrCuFeoxides in sludge-derived reagent, resulting in the reduction of SF6. In order to exhibitthe catalytic role of mixed transition metal oxides, kirschsteinite-dominant stainlesssteel slag (SSS) was utilized to decompose SF6. At600oC, SSS could decompose0.14mmol/g SF6with a mechanism similar with that of sludge-derived catalyst. TheCaFeMgMn component in SSS catalyzed the decomposition of SF6.