Effect of Remineralization on Heavy-Metal Leaching from Cement-Stabilized/Solidified Waste

Abstract

Crushed samples of stabilized/solidified (s/s) waste were leached at constant leachate pH in the pH range 4-7 with nitric acid solutions to evaluate the influence of remineralization on metal release. The s/s waste consisted of synthetic heavy-metal sludge containing 0.1 mol L(-1) copper nitrate, 0.1 mol L(-1) zinc nitrate, and 0.1 mol L(-1) lead nitrate mixed with ordinary Portland cement. Unleached and leached particles were characterized by scanning electron microscopy and energy-dispersive X-ray spectrometry. Two consecutive leaching fronts advancing from the surface of the particles toward the center were identified: the first front was associated with the dissolution of portlandite and partial reaction of the calcium silicate hydrate gel, while the second front was associated with the dissolution of calcium-aluminum hydroxy sulfates such as ettringite and monosulfate. At pH 4 and 5, a remineralization zone rich in heavy metals formed immediately behind the second leaching front. The shell extending from the remineralization zone to the surface of the particles was depleted in calcium, sulfate, and heavy metals. As a result of remineralization, heavy-metal releases to the leachate were reduced by factors ranging between 3.2 and 6.2 at pH 4 and between 74 and 193 at pH 5. At pH 6 and 7, remineralization of Pb and Zn occurred further behind the second leaching front and closer to the surface of the particles. The amount of heavy-metal release depended on both the leachate pH and the remineralization factor.

Degradation of cement at the reservoir/cement interface from exposure to carbonated brine

“…Islam et al (2004a) also found a diffusion-dependent rate of reaction in cement paste being attacked by acid in their batch experiment.…”

Solidification/stabilization and leaching behavior of PbCl2 in fly-ash hydrated silicate matrix and fly-ash geopolymer matrix

“…Mohammad found the consumption of ANC in cement matrix forms two consecutive leaching fronts, the leaching of Zn and Pb only occurs between the second front and the specimen surface, and the leaching behavior of metals is modeled by the shift of the fronts (Mohammad et al 2004a). Mohammad also found the shift of the fronts is influenced by pH (Mohammad et al 2004b). These conclusions have supported our inference about the three-stage release.…”

Compositional changes in cement-stabilized waste during leach tests—comparison of SEM/EDX data with predictions from geochemical speciation modeling

“…Each leaching test lasted for approximately 150 h. Leachate samples were collected at regular time intervals during the tests and analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The concentrations of major ions and trace metals in the leachate as a function of leaching time are presented in Islam et al [16].…”

“…The condition of equilibrium would require that the composition of pore water be uniform in the crushed material and equal to that of the extract. However, backscattered SEM images revealed that the leaching front marking the dissolution of Ca-Al hydroxy sulfates only advanced between 30 and 220 µm from the surface toward the interior of s/s waste particles leached for 150 h at constant pH in the range pH 4-7 [16,17]. Since a large change in pore water composition occurs across this front, uniformity of pore water composition (i.e., equilibrium) was not achieved even for this large extraction time.…”

“…Although leaching also causes measurable changes in the composition of the solid phases in the s/s waste matrix, little research has been done to assess whether these changes can be predicted by existing leach models. Islam et al [16,17] found that SEM/EDX observations of leached s/s waste conflicted with the assumptions made by certain leach models. These observations revealed that two consecutive leaching fronts are formed when cement particles are exposed to acidic or neutral leaching solutions.…”