Research article
Highly efficient lead extraction from aqueous solutions using inorganic polymer foams derived from biomass fly ash and metakaolin

https://doi.org/10.1016/j.jenvman.2020.111049Get rights and content

Highlights

  • Waste-derived inorganic polymer foams with tailored porosity were synthesised.

  • The cm-size foams were used as adsorbent to extract lead from aqueous solutions.

  • The foams exhibited impressive lead removal capacity (105.9 mg/g, pH 5, 23 °C).

  • The lead uptake is the highest reported to date for bulk-type inorganic polymers.

  • The foams' lead uptake is much higher and faster than the previous best report.

Abstract

This work reports a simple and safe, but powerful, route to depollute lead-containing aqueous solutions. Inorganic polymer foams (cm-size) were used as bulk-type adsorbents. The influence of the specimens' porosity and activator molarity on the foams' physical properties and on their lead extraction ability was studied. Then, the best performing samples were deeply evaluated as lead adsorbents by studying the impact of pH, lead concentration, contact time, ionic strength and solution volume. Lead sorption kinetics is strongly affected by the pollutant concentration, pH and the solution ionic strength. Under the most favourable conditions the foams showed an impressive removal capacity (105.9 mg/g at pH 5, 23 °C, C0 = 800 ppm, deionised water), surpassing all other reported values on the use of bulk-type inorganic polymers. The foams’ lead uptake is 2.3 times higher than the previous best performing bulk-type specimens (mm-size spheres), and sorption is 12.5–15 times faster. The foams can be easily regenerated using mild acidic conditions, and then reused as adsorbent, suggesting that the main adsorption mechanism is ion exchange.

Introduction

The availability of fresh water has long been recognised as a global systemic risk (Mekonnen and Hoekstra, 2016), which is expected to reach critical values in the near future. Industrial wastewaters could be a non-conventional water source, provided that toxic elements frequently found in effluents coming from various industries are removed, ideally using low-cost, environmentally benign and very effective methods. Amidst the numerous heavy metals found in industrial effluents, lead is one of the most toxic (Siyal et al., 2018), known to have severe effect on human health, and therefore we have use it as model heavy metal.

The elimination of metal ions from wastewaters using different techniques (Alharbi et al., 2020; Gopinath et al., 2020) and materials (Hu et al., 2020; Pang et al., 2019) has been deeply investigated. Among the various existing techniques, adsorption stands out as the most widely used procedure due to its simplicity and efficiency. Activated carbon is the benchmark adsorbent, owing to its very high specific surface area (SSA) endowing excellent extraction ability. Nevertheless, the production of these materials has very high associated costs hindering their wider use (Fu and Wang, 2011). Alternative low-cost materials are in great demand. Inorganic polymers (IPs) are promising candidates because of the possibility of being synthesised with relatively low energy input and using industrial wastes or by-products as precursors. Besides the projected economic and environmental advantages over activated carbons, these materials are intrinsically nano- (Kriven et al., 2003) and micro-porous (Bai and Colombo, 2018) and have a negatively charged framework balanced by cations such as Na+ or K+, that can be substituted by other cations via ion exchange to enable their use as adsorbent (Novais et al., 2019b). Despite being a fairly recent research topic, the existing literature demonstrates the huge potential of this technology for wastewater treatment (Novais et al., 2020; Siyal et al., 2018). Nevertheless, in most of the studies μm-sized IP powders are used to depollute wastewaters. This strategy ensures a high heavy metal uptake but requires a post-separation step to recover the adsorbent, increasing the complexity and cost of wastewater treatment. Recently, a safer and simpler strategy was proposed: the use of IP foams (mm or cm-size) (Ge et al., 2015; Novais et al., 2016b) that can be used without the need for support materials. Despite being a very promising route, the available literature is still limited. Up to now, there are only four studies where bulk-type IP adsorbents were used to extract lead from synthetic waters. One of these works addresses the use of cylindrical discs (22 mm diameter and 3 mm thickness), the maximum observed uptake being 6.3 mg/g (24 h sorption) (Novais et al., 2016b). Faster and higher lead uptake (4 h, 16.5 mg/g) was reported when using IP granules (4 < d < 11.2 mm) (Bumanis et al., 2019), while the use of IP supported zeolites resulted in a slightly lower uptake (15.8 mg/g) than in (Bumanis et al., 2019) when using similar sorption time (6 h), but much higher values were seen when extending the sorption to 120 h (37.9 mg/g) (Khalid et al., 2018). The highest lead uptake ever reported for bulk-type (mm or cm size; not powders) adsorbents was achieved when using IP spheres (2 < d < 4 mm). The spheres showed an impressive lead uptake of 45.6 mg/g, but again using very long sorption times (60 h) (Tang et al., 2015). These studies suggest that the use of bulk-type IPs as lead adsorbent is feasible. However, additional studies are required to fully demonstrate the viability of this innovative strategy in comparison with the conventional approach (the use of powdered adsorbents), and to provide insights into influencing factors on lead adsorption by cm-size IP adsorbents that have not been considered up to now (e.g. activator molarity; initial lead concentration; ionic strength).

In wastewater treatment systems, pH plays a critical role on the heavy metals extraction (Alwan, 2012, 2008) and should be strictly controlled and adjusted to preserve high efficiency levels. Lead adsorption by IPs is known to be strongly affected by the solutions pH. At low pH values (<2) the adsorption is much lower than that seen when the pH is between 4 and 5, and this feature has been associated with the competition between the H+ and the Pb2+ ions for the adsorption sites (Al-Zboon et al., 2011; Cheng et al., 2012).

In the present work, cm-size waste-based IP foams were evaluated as a lead adsorbent material. The foams were intentionally prepared using different activator concentrations and foaming agent contents, to evaluate the influence of the NaOH molarity and specimen porosity on the lead removal ability of the foams. The adsorption of Pb2+ was studied as a function of sorption time, pH, pollutants initial concentration, adsorbent dosage and the solution’ ionic strength. This study provides additional insights on the use of cm-size IP foams as lead adsorbent material aiming to decrease the existing knowledge gap regarding these bulk-type environmentally friendly and low-cost materials.

Section snippets

Materials

Two solid precursors were used to synthesise the IPs: biomass fly ash, deriving from the burning of residual biomass coming from forestry operations, and metakaolin (Argical™ M1200S, Univar®). The precursors were chemically activated using a mixture of sodium silicate (SiO2/Na2O = 3.1, 62.1 wt% H2O, Quimialmel, Portugal) and sodium hydroxide (ACS reagent, 98%; AkzoNobel) solutions (8 or 10 M, depending on the composition).

To promote in-situ foaming of the slurry, a commercial aluminium powder

Inorganic polymer foams characterisation

Fig. S1 presents the bulk density and total porosity of the various specimens before and after the washing step. The extraction of alkalis from the samples led to a major drop in their bulk density, between 9 and 20% depending on the composition. The lowest bulk density was observed for the foam 10 M_0.10Al (0.34 g/cm3). The decrease in the sample's density was found to be dependent of the mixture composition: samples prepared using higher NaOH molarity (10 M) and foaming agent (0.10 wt%)

Conclusions

Herein, highly porous IP foams were synthesised using a simple, environmentally friendly, and low-cost protocol using an industrial waste as the main solid precursor. The foams were then used as bulk-type adsorbents to extract lead from aqueous solutions, differing from the conventional route employing powdered IPs whose recovery after use is complex. Results showed that low pHs and the presence of competing ions (e.g. Na+) are detrimental to Pb2+ adsorption. At pH 5, the foams exhibited very

CRediT authorship contribution statement

Rui M. Novais: Conceptualization, Methodology, Validation, Writing - original draft, Writing - review & editing, Supervision. João Carvalheiras: Investigation, Validation, Writing - review & editing. Maria P. Seabra: Writing - review & editing. Robert C. Pullar: Writing - review & editing. João A. Labrincha: Writing - review & editing, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Novais (CEECIND/00335/2017), Carvalheiras (SFRH/BD/144562/2019) and Pullar (IF/00681/2015) wish to thank FCT for supporting their work. This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 & UIDP/50011/2020, financed by national funds through the Foundation for Science and Technology/MCTES.

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