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

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, C 0 ¼ 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.

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

“…The foams' total porosity reaches 77.8% (see Table 1) and their bulk density is 0.53 g/cm 3 . These values slightly differ from those recently reported by the authors (84.0% total porosity and 0.38 g/cm 3 ) [20], despite the fact that the same recipe has been used in their synthesis. The explanation for the higher apparent density observed in the present study is attributed to the modification of the foams' synthesis protocol, particularly the geometry of the mould.…”

“…The alkali-activated foams were produced using a composition developed by the authors [18,19] containing a 70/30 fly ash and metakaolin blend activated with a 75/25 sodium silicate/sodium hydroxide solution (in weight). The molarity of the sodium hydroxide solution (8 M) and the foaming agent amount (0.1 wt.% aluminium plus 0.05 wt.% surfactant) were selected following our previous findings [20] and considering both technical and economic reasons. After mixing, the paste was transferred to rubber moulds (1 cm 3 ), their top surface covered with a plastic film, and the specimens were cured at room temperature (23 • C) for 1 day.…”

“…In the single sorption tests, the heavy metal element (Pb 2+ , Cd 2+ , Zn 2+ , Cu 2+ ), concentration (10-800 ppm), and the contact time (1-6 h) between the adsorbent and the ion solution was varied. It should be noted that the maximum contact time (6 h) was defined considering a previous study by the authors showing that within this period the alkali-activated foams can efficiently extract lead from aqueous solutions [20]. The heavy metals initial concentrations varied between 100-800 ppm: 4.83 × 10 −5 -3.86 × 10 −3 mol/L for Pb 2+ , 8.90 × 10 −5 -7.12 × 10 −3 mol/L for Cd 2+ , 1.53 × 10 −4 -1.22 × 10 −2 mol/L for Zn 2+ , 1.57 × 10 −4 -1.26 × 10 −2 mol/L for Cu 2+ ).…”

“…Adsorption was performed at room temperature (23 • C) on 1 cm 3 foams (as cubes), using 100 mL of solution, and at a fixed pH of 5. This pH value was selected considering previous investigations showing that the lead [20,23], cadmium [24], zinc [25], and cop-per [26,27] sorption is favoured at this value, but also to avoid metal hydroxide precipitation that might take place at higher pHs. After this assessment, binary and multicomponent sorption tests were performed at the optimised conditions (C 0 = 10 ppm, contact time: 6 h) to evaluate the selectivity of the foams in bi-component and multi-component sorption systems.…”

“…The apparent density was determined by measuring the weight and volume of seventy specimens. The total porosity was then calculated as in [21], considering a true density of 2.37 g/cm 3 [20]. The water uptake by the foams was measured upon 24 h immersion in distilled water.…”