Preparation Of Functional Composites Based On The Gelatin And Their Application In The Removal Of Heavy Metals

Heavy metal pollution is one of the most serious environmental problems due to its high toxicity, covertness, persistence and irreversibility, which poses unprecedented threats to human health. In recent years, numerous health-related pollution incidents such as lead or cadmium poisoning have raised public concern in the world. During the period of 12 th and 13 th Five-Year Planing for Comprehensive Prevention and Control of Heavy Metal Pollution, positive progress has been made in environmental protection work. Heavy metals have been found widely in various environmental media(including soil, water, air and food). These species may enter the human body through direct ingestion of soil and water, dermal contact of contaminated soil and water, and consumption of vegetables grown in contaminated fields. Thus, how to effectively remove undesirable metal ions is a very important but still a challenging task for environmental science.Nowadays, different kinds of technologies have been proposed for efficient removal of heavy metal from water system, including but not limited to adsorption, chemical precipitation, ion exchange, membrane filtration and electrochemical technologies. Among these strategies, adsorption offers flexibility in its design and operation, high efficiency, simple running program and low maintenance cost resulted from regeneration processes. Thus, various sorbents are all considered in the literature to be promising choices for the effective removal of heavy metals. Unfortunately, these nanomaterials show low removal efficiency and are proven to have certain environmental toxicity, which will lead to the secondary pollution. Gelatin is a traditional water-soluble biopolymer with the distinctive advantages of nontoxicity, biodegradability and low cost. It can act as a sorbent to form complexes with metal ions. In this study, two kinds of gelatin functional composite materials were prepared to the removal of heavy metal ions during sewage irrigation and in drinking water. This study provides the theoretical basis for the preparation of sorbents with high adsorption capacity. Furthermore, it can be used for the pollution treatment and resources recycling of wastewater containing heavy metals, which reveals great scientific value and exciting perspective of practical application in the future.Part I: Preparation of polyethylenimine-grafted gelatin sponge and its application in the removal of heavy metalsObjective: Wastewater irrigation is known to contribute significantly to heavy metal pollution in soil and then accumulate in vegetable crops. In this study, a polyethylenimine(PEI)-grafted gelatin sponge was prepared for the highly efficient removal of heavy metals during wastewater irrigation.Methods: Gelatin sponge was modified by 3, 3?-dithiodipropionic acid through the reaction of the carboxylic acid groups with the amine groups. Subsequently, the PEI-grafted gelatin sponge was obtained by the reaction of the carboxylic acid groups(3, 3?-dithiodipropionic acid and gelatin) with the amine groups of PEI. The number of amino groups increased significantly based on a cascade effect. The adsorption abilities of the gelatin sponge and PEI-grafted gelatin sponge were studied in detail and then the PEI-grafted gelatin sponge was spread onto the surface of soil planted with garlic for removal of heavy metal ions during wastewater irrigation.Results: The SEM micrographs, FT-IR spectra and Zeta potentials of the gelatin sponge with and without PEI modification were evaluated to study the Preparation and characterizations of resulting composites. Based on the strong water adsorption ability, wastewater remained in the PEI-grafted gelatin sponge for a sufficient period of time for the heavy metals to interact with the sorbents. The adsorption capacities of lead and cadmium on the PEI-grafted gelatin sponge were 65 mg g-1 and 66 mg g-1, which were much higher than those on the gelatin sponge(19 mg g-1 and 20 mg g-1). Subsequently, the PEI-grafted gelatin sponge was spread on the surface of soil planted with garlic and then sprayed with synthetic wastewater. The concentrations of cadmium and lead in the garlic leaves were 1.59 mg kg-1 and 5.69 mg kg-1, respectively, which were much lower than those(15.78 mg kg-1 and 27.98 mg kg-1) in the garlic leaves without the gelatin sponge, and the removal efficiencies were 89.9% and 79.7%.Conclusions: The PEI-grafting gelatin sponge had the strong water adsorption and then wastewater could remain in the PEI-grafted gelatin sponge for a sufficient period of time. PEI-grafting gelatin sponge had high adsorption capacity for heavy metals and could effectively remove heavy metals during wastewater irrigation, which could improve the soil quality and reduce the accumulation of heavy metals in plants. It is significant for not only environmental quality control but also decreasing the health risks of heavy metal contamination.Part II: Preparation of Chitosan-PEI grafted magnetic gelatin and its application in the removal of heavy metalsObjective: The development of environmentally friendly sorbents with a high adsorption capacity is the essential problem in the removal of heavy metals from drinking water. In this study, chitosan/PEI-grafted magnetic gelatin was prepared using TGase as a cross linker to remove heavy metals from drinking water.Methods: Magnetic gelatin was prepared in an easy and effective way using TGase as a cross linker, and it exhibited a high adsorption capacity, good dispersion and high magnetization. To further improve the adsorption capacity and the mechanical stability, chitosan grafted polyethylenimine(PEI) cationic copolymers were synthesized and then grafted onto the surface of magnetic gelatin. Subsequently, it was used as a sorbent to remove heavy metals from drinking water at a low initial concentration and the removal properties were evaluated through the adsorption isotherm and adsorption kinetics. Furthermore, the effects of the temperature, p H value and coexisting anions/cations in drinking water on the adsorption capacities of the magnetic composite were studied.Results: In this study, magnetic gelatin was prepared using transglutaminase as a cross-linker, which could only catalyze an acyl-transfer reaction between the lysine and glutamine residues of the gelatin and not affect other amino groups. It effectively resolved the main problem of traditional sorbents that amino groups were not only the cross-linked groups for maintaining the mechanical stability but also the recognition sites to improve the adsorption capacity. After further modification with the chitosan/polyethylenimine cationic copolymers, the number of amino groups was greatly increased, and then, the magnetic composites exhibited a high adsorption capacity, excellent water compatibility and low cost. The adsorption capacities of lead and cadmium on the magnetic composite were 341 mg g-1 and 321 mg g-1, respectively, and the pseudo-second-order kinetic and Langmuir isotherm models fit the adsorption data well. Magnetic composites could be recycled for up to five cycles without a significant decrease in their adsorption capacity.Conclusions: In this study, a new type of enzyme crosslinking system was proposed and effectively resolved the problem that amino groups were not only the crosslink groups to maintain the mechanical stability but also the recognition sites. A green, environmental friendly sorbent with high adsorption capacity for low concentration of heavy metal ions in drinking water was successfully prepared. It is expected that chitosan/PEI-grafted magnetic gelatin has broad applications for the adsorption and preconcentration of heavy metal ions from aqueous systems.