Synthesis Of Industrial Wastes-based Geopolymer And Its Adsorption Capacity For Pb²⁺ And Cu²⁺

Environmental problems are one of the inevitable products of the development of industrialization.Since the 1980s,environmental water pollution incidents caused by heavy metal have occurred frequently all over the world,bringing serious health risks to human beings.Due to the non-biodegradable characteristic,high toxicity level and ability to bio-accumulate of heavy metal,once they flow into the human body through the food chain,they can easily cause damage to the kidneys,liver,brain tissue and other organs.Therefore,there are a few approaches have been applied to remove heavy metals from wastewater,such as chemical precipitation,ion exchange,electrochemistry,membrane filtration and adsorption.Among them,the adsorption method is favored due to its advantages of simple preparation,convenient operation and low energy consumption.Geopolymers are inorganic polymer materials prepared via alkali activation of aluminosilicate precursors.In this study,coal gangue and red mud were used as raw materials to prepare noval geopolymer adsorbents through adding different types of alkaline activators.Furthermore,the correlation between the geopolymer gel content and the Pb²⁺and Cu²⁺adsorption properties were studied utilizing the hydrochloric acid dissolution method and spectroscopic techniques.XRD,FTIR,BET and SEM/EDS were used to investigate the crystal composition,functional groups,pore structure and distribution,and surface morphology of the materials.The single factor condition experiment method was used to explored the adsorption performance of geopolymers for Pb²⁺and Cu²⁺under different p H,reaction time and initial concentration and determined the type of adsorption using adsorption kinetics and adsorption isotherms.The main conclusions are as follows:（1）Without the addition of aluminum source and silica source,the gel content in the geopolymer will show a trend of increasing first and then decreasing by changing the amount of Na OH and red mud doping during the preparation process.Combined with the characterization and the adsorption test data,the geopolymer gel content variation exhibited a linear correlation with its Pb²⁺and Cu²⁺adsorption capacities.After comprehensive analysis of XRF and SEM,the external silica and aluminum sources promoted Al₂O₃ and Si O₂ dissolution in the raw material,which increased and decreased the specific surface area,respectively;however,both sources improved the Pb²⁺and Cu²⁺adsorption capacities of the geopolymers to varying degrees through changing the content and properties of the geopolymer gel.Furthermore,the leaching concentration of heavy metals in geopolymers is lower than the allowable limit（GB5085.3-2007）.Compared with coal gangue and red mud,geopolymers show excellent leaching resistance and high solidification ability to heavy metal.（2）Through analysis of BET,the doping with red mud can increase the specific surface area of the geopolymers.Furthermore,the addition of a silica source can increase the specific surface area of the geopolymers,while aluminum source will cause the specific surface area of the geopolymers to decrease.Na Al-MGP can still maintain a large adsorption capacity.This phenomenon occurs because the Na⁺from the alkaline activator provided the[Al O₄]^-charge balance formed by the active aluminum component of the raw materials.In previous studies,the heavy metal adsorption mechanism of geopolymers was mainly ion exchange,in which the Na⁺played a significant role,and the increased[Al O₄]^-amounts introduced more cations,which increased heavy metal ion exchange and the adsorption capacity.（3）The results of the adsorption test showed that the optimal p H for the geopolymer to adsorb Pb²⁺and Cu²⁺were 6 and 5,respectively,and the optimal reaction time was 250 min.The adsorption of Pb²⁺and Cu²⁺in aqueous solution by the geopolymers primarily occurred through chemical adsorption,which followed the pseudo-second-order kinetics model,and the adsorption processes were well fitted to the Langmuir isotherm model,indicating the adsorption of Pb²⁺and Cu²⁺by geopolymers belongs to single-layer adsorption,and the interaction force between adsorbed ions is negligible.The q_max of the four geopolymers for Pb²⁺and Cu²⁺were32.4 and 18.80 mg g^-1（CG–GP）<53.8 and 35.50 mg g^-1（MGP）<106.8 and 66.13 mg g^-1（Na Al–MGP）<137.7 and 90 mg g^-1（Na Si–MGP）and the adsorption process was favorable.（4）Competitive adsorption test results showed that there is a competitive adsorption phenomenon between Pb²⁺and Cu²⁺during the adsorption process.The presence of one kind of heavy metal will hinder the adsorption of another heavy metal by geopolymers to a certain extent.Additionally,after five cycles of recycling,the Pb²⁺and Cu²⁺adsorption capacities of Na Si–MGP still maintain at a high level,and the adsorption capacity for Pb²⁺and Cu²⁺can still reach 81.9%and 52.1%of the original.