Interaction of carboxymethylchitosan and heavy metals in cement media

Abstract

The performance of an etherified chitosan, carboxymethyl chitosan (CMCH), when added to cement mortars doped with heavy metals, was assessed. In the presence of heavy metals (Cr, Pb, Zn) strong modifications of the fresh-state properties were evaluated. The addition of the polymer was seen to be useful in minimising some of these modifications, as those related to the setting time. A competitive mechanism for adsorption between the oxoanionic form of the metals and the carboxylate groups of the chitosan derivative was established. Studies on the metal chelating ability of the polymer and leaching from the hardened specimens showed scarce complexation under alkaline conditions, pointing to physical entrapment based on metal adsorption. However, significant chelation of metals was proved at near-neutral pH, suggesting the potential usefulness of the polymer as a agent for removing heavy metals from polluted waters and subsequently immobilizing them in cement mortars. Leaching tests carried out on polymer-metal complex-bearing samples showed a significant reduction in the amount of released Pb and Zn.

Solidification/stabilization of toxic metals in calcium aluminate cement matrices

“…In fresh and hardened state, mortars properties were calculated according to the standardized procedures reported elsewhere [5,23]: consistency (measuring the slump through the mini-spread flow test), water retention capacity, open time and compressive strength.…”

“…Afterwards, pore size distributions (PSD) by mercury intrusion porosimetry (MIP) and mineralogical composition (X-ray diffraction, XRD) were determined [5]. Based on the diffraction peaks intensities, the relative amount of the different crystalline phases was established in a semi-quantitative approach [24] that took into account the difficulties related to the proper quantitative estimation of the CAC systems.…”

“…The solid compounds were separated by centrifuging and the supernatant was filtered through a 0.45 m filter. Solid phase was assessed by XRD and, when deemed necessary, by Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy, while residual metal concentration of this clear solution was determined by atomic absorption spectrometry (AAS) (Perkin-Elmer AAnalyst-800) [23]. Steadystate data were fitted to the Langmuir and Freundlich models [25].…”

“…Eluates were collected at eight times (0.25, 1, 2.25, 4, 9, 16, 36 and 64 days) and the residual metal concentrations were determined by AAS. Cumulative leachings were calculated as described elsewhere [5] and the leaching mechanism was determined from the slope of the linear regression analysis as detailed in the norm [26].…”

“…Cement is the most adaptable binder currently available for this immobilization. A growing number of papers have dealt with this subject, mainly using ordinary Portland cement (OPC), some of its phases or OPC modified by calcium aluminate, calcium sulphate, and/or admixtures [1][2][3][4][5][6][7], occasionally forming new phases with the metallic cations. In addition, G. Qian and co-workers have extensively studied the S/S processes of toxic metals in fly ashcalcium sulfoaluminate cement, Friedel's salt or fly ashes [8][9][10].…”