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

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

Synthesis of red mud derived M-type barium hexaferrites with tuneable coercivity

Hexagonal ferrites can be employed in a multitude of applications, the most common hexaferrites are the M ferrites such as BaFe12O19 (barium hexaferrite, BaM). It is known that if Fe3+ is substituted with a combination of Ti4+/Co2+ the coercivity of BaM can be reduced to produce soft M ferrites with easily switchable magnetisation. They can be utilised as powders, films or bulk ceramics, and can be manufactured from a wide variety of synthesis methods. The production of hexaferrites usually requires commercial raw materials, but if an industrial waste can be utilised, this will help to ease waste disposal and storage costs, valorise a waste material and encourage circular economy. In this study, bauxite residue (red mud) from the production of alumina was used to synthesise M-type hexaferrites, using a simple ceramic process. BaCO3, or BaCO3+Co3O4, were added to the red mud, blended and heated at 1000 °C to produce the M-type hexaferrites. Without cobalt addition up to 81.1 wt% M ferrite was produced, and with Co addition up to 74.3 wt% M ferrite was formed. Without cobalt, the M ferrite phase closely resembled BaFe9Al3O19, and was a hard ferrite with a magnetisation of 12–19 A m2/kg for the whole powder (up to 23.6 A m2/kg for the M ferrite phase) and a coercivity of ~290 kA/m. When cobalt was added, secondary titanate phases vanished, and Ti4+/Co2+ partially substituted very soft M ferrite was formed with a low coercivity of ~16 kA/m but a higher magnetisation of 24.5 A m2/kg for the whole powder (up to 34.9 A m2/kg for the M ferrite phase). Therefore, not only can good quality magnetic materials be easily produced from this common waste material, but its magnetic properties can be tuned by varying the 2 + ions added during the process

Unravelling the Affinity of Alkali-Activated Fly Ash Cubic Foams towards Heavy Metals Sorption

In this work, alkali-activated fly ash-derived foams were produced at room temperature by direct foaming using aluminum powder. The 1 cm3 foams (cubes) were then evaluated as adsorbents to extract heavy metals from aqueous solutions. The foams’ selectivity towards lead, cadmium, zinc, and copper ions was evaluated in single, binary, and multicomponent ionic solutions. In the single ion assays, the foams showed much higher affinity towards lead, compared to the other heavy metals; at 10 ppm, the removal efficiency reached 91.9% for lead, 83.2% for cadmium, 74.6% for copper, and 64.6% for zinc. The greater selectivity for lead was also seen in the binary tests. The results showed that the presence of zinc is detrimental to cadmium and copper sorption, while for lead it mainly affects the sorption rate, but not the ultimate removal efficiency. In the multicomponent assays, the removal efficiency for all the heavy metals was lower than the values seen in the single ion tests. However, the superior affinity for lead was preserved. This study decreases the existing knowledge gap regarding the potential of alkali-activated materials to act as heavy metals adsorbents under different scenarios

Effective mechanical reinforcement of inorganic polymers using glass fibre waste

Glass fibre waste (GFW) coming from wind turbine blade production was used for the first time as a reinforcement agent in inorganic polymer (geopolymer) production. The influence of glass fibre content and length on the inorganic polymer microstructure, apparent density, and compressive and tensile strength was evaluated. Results demonstrate that the use of short, randomly distributed fibres significantly enhance the geopolymers' compressive strength (by up to ∼162%) and tensile strength (by up to ∼77%), while overcoming the brittle nature of the geopolymers. It was also found that the glass fibre content, rather than the fibre length, is the dominant factor affecting the geopolymers' mechanical properties. These results demonstrate the possibility of using an unexplored waste stream (glass fibre waste) as a reinforcement agent in inorganic polymers, which may reduce the amount of wastes in landfill and lower inorganic polymer production costs, while simultaneously contributing towards the circular economy

Innovative application for bauxite residue: Red mud-based inorganic polymer spheres as pH regulators

In this study, and for the first time, red mud (RM)-based geopolymer spheres were synthesised, with varying porosity and RM content, and then their use as pH regulators was evaluated. The aluminosilicate sources of these inorganic polymers were 100% waste-based, consisting of a mixture of RM and fly ash wastes. Geopolymer spheres containing up to 60 wt.% RM were successfully produced, while higher RM contents distorted the specimens’ spherical shape. Results showed that alkalis leaching from the spheres over time can be controlled by their porosity, while the RM content induces only minor changes to leaching. The RM-based spheres leached up to 0.0237 mol/dm3g of OH− ions from their structure, this being among the highest values ever reported for geopolymers. This allowed a continuous and prolonged pH buffer capacity with narrow pH decay over the 28 days (2.4 pH units), suggesting the use of the RM-based spheres as pH buffering materials in wastewater treatment and anaerobic digestion systems

Unexplored alternative use of calcareous sludge from the paper-pulp industry in green geopolymer construction materials

Calcareous sludge is an alkaline waste produced by the paper pulp industry that is commonly disposed of in land-fill. However, recent studies and the European regulations discourage such practice. This work investigates an alternative and innovative way to recycle and reuse this waste, as filler, in the production of green geopolymeric mortars intended for applications in construction. This is the first time that this calcareous sludge is used to produce novel waste-based materials, in both construction and geopolymer technology. The novel alkali-activated mortar also uses biomass fly ash \u2013 another slightly investigated waste stream \u2013 to substitute the metakaolin (70 wt% substitution) and the manufacture process is performed at ambient conditions. All of that reduces the overall process footprint. The implemented mix design is aimed at maximising the waste incorporation and improving the material properties, such as workability and mechanical performance. The main results demonstrate that 10 wt% of calcareous sludge can be efficaciously used as filler in the geopolymeric mortars, generating up to 30% improvement in the mechanical resistance. This alternative use of such wastes will contribute to increase the sustainability of the novel construction materials also granting environmental advantages and a financial surplus for the industry

In-depth investigation of the long-term strength and leaching behaviour of inorganic polymer mortars containing green liquor dregs

Green liquor dregs are the most challenging waste stream coming from the pulp and paper industry. Despite tremendous efforts, there are not currently any viable recycling alternatives for this massively produced waste (2 Mt/year), which inevitably ends up in landfills. Urgent actions must be undertaken to tackle this. In this work, a substantial amount of dregs was incorporated into eco-friendly, waste-based inorganic polymer (geopolymer) mortars as fine filler. Then, and for the first time, the long-term strength performance (up to 270 days) and heavy metals leaching behaviour of the dregs-containing mortars was evaluated. The effect of the mixture composition and dregs incorporation content on the fresh- and hardened-state properties of the mortars was also studied. Dregs were found to increase the initial and final setting time of the slurries, thus extending the open time before their in-situ application. The use of dregs as fine filler effectively enhances the compressive strength of the mortars, and decreases their water absorption levels. These eco-friendly building materials showed excellent long-term performance, as their strength continuously increases up to the 270 th day (after mixture), and no signs of efflorescence formation were detected. Moreover, the heavy metals leaching levels of the mortars were well below the contamination limits in soil, which demonstrates the feasibility of this recycling methodology

Sustainable and efficient cork - inorganic polymer composites: An innovative and eco-friendly approach to produce ultra-lightweight and low thermal conductivity materials

In this investigation cork was used as a low density aggregate in the production of ultra-lightweight and low thermal conductivity inorganic polymer (geopolymer) composites. This novel and highly sustainable material, synthesised at room temperature (23 °C), may decrease the energy losses inside buildings, thus contributing to the United Nations development goals regarding energy and climate change. The ultra-low density (260 kg/m3) and low thermal conductivity (72 mW/m K) shown by the cork-composites are the second lowest ever reported for inorganic polymer composites, only being surpassed by that of polystyrene-inorganic polymer composites. However, cork is a fully renewable and sustainable resource, while polystyrene is manufactured from non-renewable fossil fuels, and for that reason our strategy has an additional sustainability advantage. Moreover, the cork-inorganic polymer composites do not release any toxic fume when under fire conditions, which is a major advantage over polymeric-based foams

Incorporation of glass fibre fabrics waste into geopolymer matrices: An eco-friendly solution for off-cuts coming from wind turbine blade production

In this investigation, an innovative and sustainable solution for glass fibre off-cuts is proposed. For the first time, glass fibre fabrics (non-woven mat) produced during wind turbine blade manufacture were used to produce reinforced inorganic polymer composites. The influence of fabric amount on the composites’ microstructure, water absorption, strength and apparent density was investigated. Results show that the fabrics exhibited an impressive flexural strength gain (up to 144%) at the 28th day, when compared to the non-reinforced geopolymer, while simultaneously increasing the composites’ ductility. This feature may extend the application range of such geopolymers. Moreover, the fabric prevents the complete collapse of the composites upon failure, while the matrix alone fails catastrophically under load