Synthesis of porous biomass fly ash-based geopolymer spheres for efficient removal of methylene blue from wastewaters
Graphical abstract
Introduction
The removal of dyes from industrial wastewaters is of the utmost importance, not only because most of them are toxic and show carcinogenic properties, but also because treated industrial wastewaters may provide a vital source of clean water to mitigate the most pressing concern of our society – water scarcity. In the next 30 years, around 40% of the world's population will live in areas with extreme water scarcity (OECD, 2012). Effective and low cost technologies for wastewater treatment are being eagerly pursued. Adsorption is considered the most effective, simple and universal technique for water decontamination (Gupta et al., 2012), activated carbon being the benchmark adsorbent material (Mestre et al., 2014). Despite the unique adsorption capacity of this material, its high production cost (Rafatullah et al., 2010) has led to extensive research for lower cost alternatives. One exciting approach could be the use of low cost and eco-friendly inorganic polymers (also known as geopolymers). Inorganic polymers are synthesised at near-ambient temperatures (Novais et al., 2016b) by chemical activation of aluminosilicate sources, such as metakaolin (Zhu et al., 2018), fly ash (Novais et al., 2016c) and waste glass (Novais et al., 2016a). They have a negatively charged aluminosilicate network, balanced by cations such as sodium or potassium, which may in turn be exchanged with cations is solution. This feature suggests the feasibility of using inorganic polymers as dye or heavy metal adsorbent materials. Despite this, the study of the use of geopolymers as adsorbents is fairly recent (Minelli et al., 2018; Novais et al., 2016d). Moreover, the vast majority of the investigations are focussed on the use of powdered adsorbents (Falah et al., 2016; Liu et al., 2016), which cannot be easily recovered, and so cannot be used in field applications or directly in packed beds. Powdered adsorbents may require the use of support materials (e.g. porous ceramics, polymer foams) (Zhang et al., 2016) to allow their industrial application or a separation step (e.g. pressure filtration) after wastewater treatment process, both being detrimental to the wastewater treatment cost, besides increasing the process complexity.
Recently, the use of porous geopolymer monoliths for methylene blue (MB) extraction from wastewaters was reported by the authors (Novais et al., 2018a). The MB removal capacity of the monolithic bodies reached 15.4 mg/g, while the adsorbent could be reused up to 5 times. These promising results demonstrated the feasibility of using porous bodies, not powders, to extract MB from polluted wastewaters. Nevertheless, the adsorbents's removal efficiently dropped significantly, to around 65%, when the MB initial concentration reached 50 ppm (Novais et al., 2018a). Therefore, additional investigations addressing the use of bulk porous geopolymers should shed light on the most influential parameters affecting the dye adsorption by the geopolymers. One possibility to enhance the geopolymers' MB adsorption capacity, in comparison with the use of cylindrical discs (thickness = 3 mm; d = 22 mm) (Novais et al., 2018a), could be the use of porous geopolymer spheres (GS) (2–3 mm). The authors have recently reported the synthesis of waste-containing GS by a suspension-solidification approach (Novais et al., 2017), and their subsequent use as pH regulators in anaerobic digesters (Novais et al., 2018c), while other investigations have studied the use of GS as a heavy metal adsorbent (Ge et al., 2017) or as a photocatalytic material (Li et al., 2016). Nevertheless, the use of GS as MB adsorbents has never been reported, up to now. This is the first ever investigation regarding the use of geopolymer spheres and formulations made from biomass fly ash (FA) waste as MB adsorbent materials from synthetic wastewaters. The FA waste used was obtained from a local Portuguese paper industry, thus enhancing the circular economy aspect of this work. These FA-based GS are expected to show much higher surface area than the previously reported geopolymer monoliths (Novais et al., 2018a) and, therefore, higher MB removal capacity. The influence of contact time, MB initial concentration and adsorbent amount on the removal efficiency of this pollutant by the GS was studied. Desorption and regeneration tests were carried out to evaluate the MB extraction efficiency and the feasibility for multiple reuse of the adsorbent. The high removal efficiency shown by these innovative adsorbents, and their multiple recycling, demonstrates the adsorbents' interesting potential for wastewater treatment systems. The present study is a significant step forward in comparison with previous investigations (Novais et al., 2018a), clearly demonstrating the performance advantage of the proposed solution (the use of geopolymer spheres instead of cylindrical discs) for wastewater treatment.
Section snippets
Materials
Metakaolin (Argical™ M1200S; Univar) and biomass FA waste were used as sources of reactive silica and alumina. The FA was sieved, and then only the portion below 63 μm was used in the geopolymer synthesis. The FA chemical composition, presented in Table 1, shows that the waste is a silica-rich material (34 wt.%). However, its low alumina content results in a SiO2/Al2O3 ratio ∼2.5. For this reason, metakaolin was also used in the compositions (50 wt.% regarding the aluminosilicate sources) to
Geopolymer spheres characterisation
Fig. 1a shows a representative optical micrograph of the FA-based GS, while Fig. 1b presents their size distribution measured by image analysis. The geopolymers have an elongated spheroidal shape, with average diameter and length being 2.6 ± 0.2 mm and 2.9 ± 0.3 mm, respectively. These results demonstrate that the production method described here is effective and reproducible, leading to a narrow size distribution. The mm size spheres can be directly used in packed beds and easily retrieved
Conclusions
In this investigation, and for the first time, FA-based geopolymer spheres were evaluated as a methylene blue adsorbent material. The MB uptake by the porous geopolymer spheres is affected by the dye initial concentration, adsorbent amount and contact time. Results showed a faster (20% smaller equilibrium time) and higher MB uptake (twofold increase) in comparison with the other bulk-type geopolymer reported to date, demonstrating the performance advantage of using spheres (d = 2.6 mm) instead
Acknowledgements
This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. R.C. Pullar thanks the FCT for funding under grant IF/00681/2015.
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