Interactions of lead(II) ions with crushed concrete fines Nichola Coleman, Chiara L. Coleman, Aimee A. Coleman, Michael Rivett 1 School of Science, Faculty of Engineering & Science, University of Greenwich, Chatham Maritime, ME4 4TB, UK Construction and demolition activities generate approximately two thirds of the world’s refuse, with concrete- based waste accounting for the greatest proportion [1]. Crushing and sieving, to recover and recycle the primary aggregate, gives rise to a large volume of cement-rich fine material (<5 mm) for which further utilisation is required. In this respect, a number of studies has demonstrated that these crushed concrete fines (CCF) can be used to remove dissolved metal cations (e.g., Cu 2+ , Zn 2+ , Pb 2+ , Cr 3+ , Ni 2+ , Cd 2+ and Co 2+ ) and oxyanions (e.g., AsO 3 3− , CrO 4 2− , H n PO 4 (3−n)− and MoO 4 2- ) from contaminated water [1-3]. In most cases, irrespective of metal speciation, the mechanism of interaction is the alkali-mediated precipitation of solubility-limiting phases on the surface of the CCF. However, the immobilisation and retention of Pb 2+ ions takes place via substitution for Ca 2+ ions within the hydrated cement phases [2]. The present study models the removal kinetics of Pb 2+ by CCF using the pseudo-first- and pseudo-second-order rate equations [3], and also investigates the equilibrium removal of Pb 2+ ions using the Langmuir, Freundlich, Elovich, Halsey, Harkin-Jura, Temkin and Dubinin-Radushkevich (D-R) isotherm models [4]. The uptake of Pb 2+ by CCF was determined by static batch sorption using 2.5 g of 1-2 mm CCF in contact with 100 cm 3 solutions of Pb(NO) 3 at various concentrations between 10 and 1000 ppm for up to 120 h at 20 °C [2]. Solution concentrations of Pb 2+ were determined by inductively coupled plasma spectroscopy and the recovered Pb 2+ -laden CCF particles were analysed by scanning electron microscopy with energy dispersive X-ray analysis. Removal of Pb 2+ by CCF followed a pseudo-second-order reaction (k 2 = 6.7 × 10 −5 g mg −1 min −1 , R 2 = 0.997). Equilibrium was established at 48 h with a maximum sorption capacity ( q max ) of 37.2 mg g -1 and the experimental isotherm was most closely described by the Freundlich (R 2 = 0.997), Halsey (R 2 = 0.997) and D-R (R 2 = 0.963) models. The D-R model predicted q max to within 4% of the experimental value and estimated the mean free energy of adsorption ( E ads ) to be 9.4 kJ mol -1 . The ubiquitous and plentiful supply of CCF arising from demolition may represent a cost-effective option for the managementof metal-contaminated wastewater in small treatment works. Further research is now warranted to consider the sorption characteristics of CCF exposed to ‘real’ multicomponent wastewater streams. References 1. V.K. Elmes, N.J. Coleman, Interactions of Cd 2+ , Co 2+ and MoO 4 2− ions with crushed concrete fines. J. Compos. Sci. 2021, 5, 42. 2. N.J. Coleman, W.E. Lee, I.J. Slipper, Interactions of aqueous Cu, Zn and Pb ions with crushed concrete fines. J. Hazard. Mater. 2005, 121, 203-213. 3. A.P. Hurt, A.A. Coleman, N.J. Coleman, Interactions of Cr 3+ , Ni 2+ , and Sr 2+ with crushed concrete fines. Crystals 2022, 12, 717. 4. J. Liu, X. Wang, Novel silica-based hybrid adsorbents: lead(II) adsorption isotherms. Sci. World J. 2013, 897159.
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