Study On Adsoption Characteristic And Mechanism Of Heavy Metals In Wastewater By Crayfish Carapace And Its Biomimetic Compound

The building of an energy-efficient and environment-friendly society is the thought after plundering and destroying the nature in excess. And it is necessary to maintain social order and concord between the nature and man. Renewable natural resources, especially wastes were used to developed biosorption materials, which was in order to treatment environmental pollution. That became an international hot topic to study and develop one after the other. It was well know that adsorption of heavy metals by chitin and chitosan in crayfish carapace. However, it was inevitable that preparations of these products cause some new environmental problems and high cost. As a natural mineralized material, crayfish carapace was controlled by organic matrix in a living organism during the mineralization process. Biominerals prepared had highly ordered structure. Therefore, study of the adsorption characteristic and mechanism of heavy metals by low-cost wastes is of great significance and to promote its application.In this study, we utilize a high-frequency oscillatory type ball mill to make the particle size of carapace to break through the traditional range. Then it is used to remove heavy metal ions. The adsorption capacities of crayfish carapace micropowder for heavy metal ions (Cu, Cd, Zn and Pb) were studied at adsorbent dosages, initial metal concentrations, particle sizes, solution pH and temperature. Several models were used to study kinetic, thermodynamics and isotherm. Adsorption characteristic of crayfish carapace was compared with partial decalcification crayfish carapace, chitin and chitosan extracted from crayfish carapace. Scanning electron microscopy, X-ray diffraction and Fourier transform were adopted to study constituent of crayfish carapace and adsorption mechanism. At last, organic chitosan and amino acids as the mineralized template, the calcium carbonates were prepared and studied the crystal form. The main results indicated as follows:1. Adsorption capacity increased with increasing pH values. The good results were obtained at the pH value 3.5. Small particle size favors for adsorption, but when mean diameter decreased to 82.36μm, the adsorption capacity did not change significantly any more. The kinetic studies indicated that adsorption processes of copper, cadmium, zinc and lead finally reached the adsorption equilibrium within 120 min,30 min,90 min and 120 min, respectively. The data correlated with the pseudo second-order model and correlative parameters (R2) were about 0.95. The study of adsorption thermodynamics showed that adsorption of copper was exothermal. Zinc and cadmium were endothermic. However, there was nonlinear between adsorption of lead and temperature. Crayfish carapace micropowder recorded maximum uptakes of 200.00,217.39,80.00 and 322.58 mg/g, respectively. Besides, adsorption capacity of partial decalcification crayfish carapace decreased with increasing decalcification ratios.2. The extraction ratio of chitin, Chitosanl and Chitosan2 (degrees of deacetylation of 70.9% and 88.6%) prepared from crayfish carapace were 18.40%,16.25% and 15.6%, respectively. However, adsorption capacity of crayfish carapace for metal ions was about 2 to 3 times higher than chitin and chitosan extracted from it. The analyses of Scanning Electron Microscopy showed that the surface of the partial decalcified sample washed by acid solution was much smoother than the untreated. With the increasing of decalcification ratios, the crystallinity were 32.76%、40.88% and 45.54%, respectively. Carbonates of metals were formed on the surface of the crayfish carapace exposed to metal ions solutions. The analyses of X-ray diffraction and Fourier transform infrared spectroscopy showed that calcite in crayfish carapace played the most significant role in adsorption reactions, and chitosan was also an effective constitute in it. The removal mechanism of metal ions by ball-milled micropowder of crayfish carapace resulted primarily from the dissolution of calcium carbonate followed by precipitation of carbonate of metals on the surface of the crayfish carapace. Chitosan and protein also could participate in this reaction.3. The analyses of SEM, XRD and FT-IR showed that calcite formed in the pure water. The intensity of 20 diffraction peak at 29.40°was the greatest. Calcium carbonate formed in mineralized template of chitosan with different molecular weight, contained calcite and vaterite. The mount of vaterite increased with decreasing molecular weight of chitosan from 100 kDa to 300 kDa. The size of the carbonate calcium was all smaller than 10μm. More and more calcite produced with the extension of mineralized time. The maximum size of carbonate calcium, about 30μm, was obtained on the day 15. The increase of pH values favors for the formation of calcite. Carbonate calcium size were about 5μm at the pH value 6. When the pH value was alkaline, the size exceeded 10μm. Chitosan and amino acids as the mineralized template together, the mount of vaterite increased with the concentration of amino acids. And the concentrations of amino acids of 0.6% lead to a big change of the crystal transition. In addition, the adsorption capacity of mineralized calcium carbonate for heavy metals recorded a large mount, and there was no significant difference among different calcium carbonate. Vaterite contributes more significantly to adsorption than calcite due to the special surface structure.4. Desorption of crayfish carapace micropowder for copper, cadmium, zinc and lead ions were incredibly effective and superior to chitn, chitosan and mineralized calcium carbonate. Desorption reaction brought into equilibrium within 2 h. It also had high removal efficiency (more than 80%) for heavy metal ions in wastes from the paper industry and electroplating effluent, which were superior to chitosan and calcium carbonate.