Complete transformation of aluminum waste into zeolite and its use in the removal of pollutants from aqueous solution

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Zero Waste concept aims to minimize the negative effects of the generation and management of waste on human health and the environment, as well as to reduce the use of natural resources. The novelty of this Thesis comes from the development of a simple procedure to completely transform a hazardous aluminum waste into added-value materials of great industrial interest, as zeolites, which are used later as adsorbents for the removal of heavy metals and ammonium from aqueous effluents. There are not previous works related to the use of the hazardous aluminum waste as a raw material for the synthesis of zeolites. In this work the following stages were developed: i) Characterization of different aluminum wastes to get a representative sample. ii) Lab-scale hydrothermal synthesis and characterization of zeolites from aluminum waste to define the optimal experimental conditions. iii) Scaling up of the process (bench-scale synthesis) to assess its industrial application. iv) Development of a more sustainable synthesis process by alkaline effluents recycling in order to achieve a zero waste and zero effluents process. v) Using the zeolites as adsorbents for the removal of heavy metals and ammonium to decontaminate water. Aluminum waste, due to its high aluminum content (Al2O3 ~ 66 wt.%), was characterized and used as the only aluminum raw material for the lab- and bench-scale synthesis of zeolites in the Na2O-Al2O3-SiO2-H2O system. The synthesized zeolites exhibited high cation exchange capacities (CEC) and suitable adsorption properties, resulting in potential adsorbents for the removal of heavy metals and ammonium from aqueous solutions. The most relevant hazardous features of aluminum waste derive from its mineralogical and chemical composition and very fine granulometry (2.5-93 μm). It exhibits spontaneous and exothermic reactivity because its high contents of metallic aluminum (12.8 wt.%) and aluminum nitride (13.1 wt.%), in contact with water, can generate toxic gases such as ammonia (71 Nm3 per ton of waste) and/or inflammable and explosive gases like hydrogen (162 Nm3 per ton of waste). Besides, it contains heavy metals, which can be released by leaching under incorrect management and disposal. The development of a simple and one-step hydrothermal synthesis process enabled the complete transformation of the waste in zeolite. No other solid residues were generated in the zeolitization process. The lab-scale synthesis process resulted into three types of zeolites: NaP1, Sodalite (SOD), and Analcime (ANA), and led to reaction yields of 2.5 kg of zeolite per kg of waste. The preparation of the zeolites has mainly been designed by selecting temperature and alkalizing agent (NaOH) concentration. The optimal lab-scale synthesis conditions were: · The NaP1 zeolite was obtained at 120 ºC for 6 h, using 1 M NaOH and liquid/solid ratios of 15-25 mL/g, resulting in a Si/Al ratio of 1.85. · The SOD zeolite was prepared at 120 ºC for 6 h, using the highest alkali concentration (5 M NaOH) and a liquid/solid ratio of 25 mL/g, leading to a Si/Al ratio of 1.02. · The ANA zeolite was synthesized at the highest temperature (200 ºC) for 6 h, using 1 M NaOH and a liquid/solid ratio of 25 mL/g, resulting in a Si/Al ratio of 1.73. The different zeolites (NaP1, SOD, and ANA) were also synthesized via lab-scale synthesis with mother liquor recycling, involving a decrease of the raw materials consumption. The characterization of the obtained NaP1, SOD, and ANA zeolites revealed that the resulting zeolites showed characteristics and properties similar to those of zeolites prepared from both pure chemical reagents and other waste sources (fly ash, rice husk, kaolin, etc.). From an industrial point of view, NaP1 is the zeolite with most interest in water treatment applications due to its high CEC (2.73 meq NH4 +/g). The scaling up of the synthesis of NaP1 with recycling of alkaline effluents (mother liquor and rinse water) was performed for the optimal conditions to assess the feasibility and reproducibility of the process. It also led to the complete conversion of the waste into highly crystalline zeolite, achieving high reaction yields (2.5 ton of zeolite per ton of waste). The bench-scale zeolitization process involved not only a reduction of the NaOH and water consumptions, but also a significant cost reduction. The resulting NaP1 zeolites obtained in the scaling up with alkaline effluents recycling showed high CEC (2.27-2.37 meq NH4 +/g) as well as structural, morphological, textural, and physical-chemical characteristics similar to those synthesized from fresh NaOH solutions. The removal of Pb2+, Cd2+, and Hg2+ from aqueous solutions was studied, evaluating the effects of adsorption parameters on the single- and multi-cation adsorption process. The kinetic of single-cation adsorption process was found to be very rapid, achieving high removal efficiencies: 98.9, 93.3, and 99.3 % for Pb2+, Cd2+, and Hg2+, respectively, in the first 15 min for zeolite doses of 0.5-5 g/L. The experimental maximum removal capacities of the zeolite were: 183.0, 4.37, and 0.23 mg/g for Pb2+, Cd2+, and Hg2+, respectively. The metal cations could be removed by the zeolite through a homogeneous and physical adsorption process. The zeolite showed the greatest affinity for Pb2+, due to its smallest size compared with Cd2+ and Hg2+. The multi-cation removal efficiency of Pb2+ remained practically unchanged in presence of Hg2+ and Cd2+, reaching high removal efficiencies (almost 100 %) both at very low contact times (1 min) and at longer times (30 min) for zeolite doses of 2-10 g/L. The elimination of NH4 + from aqueous solutions was also performed using the zeolite obtained from the waste as adsorbent. The uptake NH4 + showed a fast kinetic, leading to removal percentages of 88 % in the first 15 min for a zeolite dose of 5 g/L. The experimental maximum removal capacity of the zeolite was 37.9 mg/g for NH4 +, similar to that found for other sorbent materials. Finally, it can be concluded that the developed zeolitization process would enable to reduce the amount of aluminum waste, which is generally disposed of in secure deposits (involving high treatment cost), preserving natural resources and hence helping the sustainability of environment. Accordingly, it implies a synergic effect on the environmental protection: firstly, the transformation of the hazardous aluminum waste into a zeolite can contribute to its end-of-waste condition, and secondly, the zeolite obtained from aluminum waste can be considered as a promising adsorbent used for the treatment of aqueous effluents contaminated by heavy metals (endocrine disruptors) and other inorganic compounds (ammonium).
Mención Internacional en el título de doctor
Heavy metals, Hazardous aluminum waste, Zeolites, Adsorption, Hydrothermal synthesis, Water polution, Recycling
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