LDH-Based Nanomaterials For Adsorption Of Cr(?)and Dyes And Their Preliminary Applications In Tanning

Clean and green leather production was crucial for sustainable development.The key of leather making is tanning,which gives leather better thermal stability and corrosion resistance.So far,the best route is chrome tanning,but the utilization ratio of chrome tanning agent is only 60-70%.The Cr(III)ions in tannery wastewater are partially oxidized to Cr(VI)ions under certain conditions,which can easily cause biological mutation and serious pollution of the ecosystem.Acid black 10b used in leather dyeing is one of important anionic azo dyes,which is difficult to degrade in the ecosystem.Therefore,it is of great significance to develop an efficient and ecofriendly treatment method for wastewater containing chromium and azide dyes.In the present thesis,on the basis of layered double hydroxide(LDH),a hierarchical structured composite oxide adsorbent was obtained by a citric acid-decorated aqueous phase coprecipitation,and used in adsorption of Cr(VI)oxyanions and acid black 10b,exhibiting excellent adsorption properties.A series of single M2+3M3+-LDH materials were prepared by coprecipitation method and their applications in the process of less chrome tanning were preliminarily explored.The main points of the thesis are as follows:(1)A hierarchical structured hybrid precursor rGO@NiMgAl-LDH(rGO:reduced graphene oxide),synthetized by a facile citric acid-assisted aqueous phase coprecipitation method,shows the NiMgAl-LDH nanosheets(?90.1×8.2 nm)vertically staggered grown on both sides of rGO monolayer.Then,upon the calcination of the obtained precursor at 500? in N2 flow,a hierarchical layered composite oxide adsorbent rGO@NiMgAl-LDO was obtained,which maintains the nanosheet array structure with slightly decreased dimensions of LDO nanosheets(?85×7.5 nm),while the specific surface area(SBET)is significantly increased to 275.3 m2 g-1 along with abundant mesoporous structure.The adsorption properties of rGO@NiMgAl-LDO for Cr(VI)oxyanions and acid black 10b(AB10b)were studied in detail compared with pure NiMgAl-LDO and corresponding precursors.Kinetic studies reveal that both the Cr(?)and acid black 10b adsorption onto rGO@NiMgAl-LDO and related samples were well fitted by the pseudo-second-order model.The isotherms were well simulated by Langmuir model.Especially,rGO@NiMgAl-LDO composite has the theoretical maximum adsorption capacity to Cr(VI)and AB10b of 106.3 mg/g and 270.9 mg/g,respectively.The adsorption capacity of Cr(VI)is higher than those of the most recently reported LDH and LDO-based adsorbents,and the adsorption of AB10b is much higher than those of conventional inorganic adsorbents.The excellent adsorption capacity for Cr(VI)can be attributed to the larger SBET and abundant mesoporous structure,electrostatic interactions between Cr(VI)oxygen anions and the positively charged NiMgAl-LDO in nanosheet array structure,"memory effect" of rGO@NiMgAl-LDO composite and the adsorption-coupling reduction for Cr(VI)by the surface M-OH groups on the layers of rGO@NiMgAl-LDO.The excellent adsorption capacity for AB10b can be mainly ascribed to the larger SBET and abundant mesopores,as well as the electrostatic interactions between AB10b anions and the positively charged NiMgAl-LDO.Meanwhile,the three cycle removal ratio of rGO@NiMgAl-LDO for Cr(VI)and AB10b were 79.5%and 81.5%,respectively.A possible adsorption mechanism was tentatively presented for Cr(VI)and AB10b,respectively.(2)A series of CO32--intercalated M2+3M3+-LDH(M2+:Zn2+or Mg2+,M3+:Cr3+,Al3+or Fe3+)materials were prepared by coprecipitation method,and applied in the process of less chrome tanning of sheep skin.Especially,the ZnCr-LDH presented the best improving tanning effect.At initial pH value of 3.0-4.0 in tanning solution,3%of ZnCr-LDH with 3%of chromium powder(in acid-soaked leather mass)as tanning agent reached the shrinkage temperature of 95.3?,which is comparable to 97.2? in the traditional chrome tanning with 6%of chromium powder.The strengthened effect of the ZnCr-LDH on less chrome tanning can be attributed to the appropriate size of LDH nanosheets via in-situ acid etching and rich layer hydroxyl groups,which can easily penetrate into the collagen voids and bind electrostatically/covalently with collagen thus enhancing the humidity and heat resistance of the collagen.The improved mechanism of LDH nanosheets in less chrome tanning was tentatively proposed.