Fabrication Of Metakaolin Geopolymer-Based Adsorbents And Their Applied Performance For Heavy Metal Retention

Environmental contamination becomes a deterrent to human and economic development with the rapid growth of industrial society. The harm of heavy metal pollution is serious, including cancergenic, teratogenic, mutagenic effect, from basic engineering works, mining, metallurgy, fertilizers, electroplate, leather industry, etc. The heavy metal pollution becomes the bottleneck of sustainable human development, many researchers focus on the treatment and removal of heavy metals.Various techniques such as chemical precipitation, ion exchange, membrane processes and adsorption are used to treat with heavy metal waster water. Recently, adsorption has become the central research focus due to its protocol simplicity, effectiveness and low cost. However, many adsorbents are unstable and easily decomposed to secondary pollution, and they are difficult to operate and separate as powders, which can’t be packed in columns for continuous treatment of industrial waster water. In this work, geopolymers which are stable and environmentally friendly materials with 3-D network structure, chosed be as adsorbents for treatment heavy metal. A novel porous metakaolin-based geopolymeric spheres (PGS) had been firstly synthesized by a suspension and solidification method. Geopolymer/Ca-alginate hybrid gel beads (GAB) were obtained by a blending method. The morphology and structure of PGS and GAB were characterized by SEM, XRD, FTTR,27A1,29Si NMR and pore-size distribution studies. The adsorption characteristics and mechanism of Cu(Ⅱ) and Pb(Ⅱ) by PGS and GAB had been discussed. This study focused on solving the key technical problems for heavy metal removal methods, which will provide the theory and scientific basis for geopolymer-based adsorbent widely applied. The principal results are as follows:1) The n(SiO2)/n(Na2O) ratios in sodium silicate solution, n(Na2O)/n(Al2O3) and n(H2O)/n(Na20) were decided by single factor experiment. Influence of three factors on the structure of geopolymers had been analyzed, the n(SiO2)/n(Na2O) ratios in sodium silicate solution was key factor affecting microstructure and adsorption properties of geopolymers. The optimum formula was n(SiO2)/n(Na2O)=1.6 in sodium silicate solution, n(Na2O)/n(Al2O3)=1, n(H2O)/n(Na20)=22. The adsorption of Cu(Ⅱ) by geopolyers was followed the pseudo-second-order kinetic model and Langmuir equations. The maximum adsorption capacity of Cu(Ⅱ) by geopolyers calculated from Langmuir isotherm model was 43.48 mg/g, when volume of Cu(Ⅱ) solution was 100mL at pH=5, C0= 50 mg/L. The results showed that the adsorption of Cu(II) by geopolymer was physicchemical adsorption process and followed a favorable adsorption.2) The optimum formula and technology for making porous geopolymeric spheres (PGS) by a suspension and solidification method have been established by orthogonal experiments. The optimum formula was n(SiO2)/n(Na2O)=1.6 in sodium silicate solution, n(Na2O)/n(Al2O3)=1, n(H2O)/n(Na2O)=16, H2O2%=0.5%, K12%=1.5%. The BET specific surface area of PGS was 53.95 m2/g with an average pore diameter of 5.38 nm. PGS was characterized by SEM, XRD, FTIR and 27A1,29Si NMR, the results showed that the hardening mechanism of PGS followed with the typical geopolymerization:source of silica and alumina that is readily dissolved in the alkaline solution can be used as a source of geopolymer precursor species and undergoes geopolymerization.3) The adsorption characteristics and mechanisms of PGS has been researched. The adsorption process could be well fitted by pseudo-second-order kinetic model and Langmuir isotherm model. The maximum adsorption capacity of Cu(II) and Pb(II) by PGS calculated from Langmuir isotherm model was 52.63 mg/g and 131.98 mg/g, respectively. PGS showed higher adsorption capacity than the commercial spherical 4A molecular sieve and some other reported spherical materials. The continuous removal of Cu(II) from an effluent was conducted in fixed-bed column. The column breakthrough curves were analyzed. The high bed depth, slow flow rates and low initial concentration could prolong the breakthrough time. The breakthrough time for Cu(II) and Pb(II) were 35 h and 69 h, respectively, when bed depth was 2.2 cm, flow rates was 1 mL/min and initial concentration was 50 mg/L4) Geopolymer/Ca-alginate hybrid gel beads (GAB) was prepared by a blending method. The GAB easily formed beads and was stable in water by a blending method. The optimum formula and technology for making GAB had been established by orthogonal experiments. The optimum formula was n(SiO2)/n(Na2O)=1.6 in sodium silicate solution, n(Na2O)/n(Al2O3)=1, n(H20)/n(Na20)=16, m(Geo)/m(SA)=1:0.16. The SEM photos showed that GAB was a interpenetrating and core-shell structure, the thicknesses of the GAB surface layers was 40-50 μm, the inner structure of GAB was cellular. The BET specific surface area of GAB was 16.19 m2/g with an average pore diameter of 11.51 nm.5) The batch adsorption test and column test of GAB had been discussed. The adsorption kinetics followed the pseudo-second-order model. The adsorption rate of Pb(II) was much faster than Cu(II). The maximum adsorption capacity of Cu(II) and Pb(II) by GAB calculated from Langmuir isotherm model was 62.53 mg/g and 149.70 mg/g, respectively. The value of the sorption energy from D-R model indicated adsorption of Cu(II) and Pb(II) by GAB was chemical process and physical process, respectively. GAB showed higher adsorption capacity than some other reported Ca-alginate hybrid materials. The fixed-bed packed by GAB, the breakthrough curves had been analyzed at different flow rate, bed depth and initial Cu(Ⅱ) concentration. The breakthrough time was 120 h, when bed depth was 3 cm, the flow rate was 1 mL/min and initial concentration was 50 mg/L.