Publication: Complete transformation of aluminum waste into zeolite and its use in the removal of pollutants from aqueous solution
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Publication date
2018
Defense date
2018-07-27
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Tutors
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Abstract
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).
Description
Mención Internacional en el título de doctor
Keywords
Heavy metals, Hazardous aluminum waste, Zeolites, Adsorption, Hydrothermal synthesis, Water polution, Recycling