Interaction of gypsum with lead in aqueous solutions

doi:10.1016/j.apgeochem.2010.04.007

Applied Geochemistry

Volume 25, Issue 7, July 2010, Pages 1008-1016

https://doi.org/10.1016/j.apgeochem.2010.04.007 Get rights and content

Abstract

Sorption processes on mineral surfaces are a critical factor in controlling the distribution and accumulation of potentially harmful metals in the environment. This work investigates the effectiveness of gypsum (CaSO₄⋅2H₂O) to sequester Pb. The interaction of gypsum fragments with Pb-bearing solutions (10, 100 and 1000 mg/L) was monitored by performing macroscopic batch-type experiments conducted at room temperature. The aqueous phase composition was periodically determined by Atomic Absorption Spectrometry (AAS), Ion Chromatography (IC) and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP–OES). Regardless of the [Pb_aq]_initial, a [Pb_aq]_final < 4 mg/L was always reached. The uptake process was fast (t < 1 h) for [Pb_aq]_initial ⩾ 100 mg/L and significantly slower (t > 1 week) for [Pb_aq]_initial = 10 mg/L. Speciation calculations revealed that after a long time of interaction (1 month), all the solutions reached equilibrium with respect to both gypsum and anglesite. For [Pb_aq]_initial ⩾ 100 mg/L, sorption takes place mainly via the rapid dissolution of gypsum and the simultaneous formation of anglesite both on the gypsum surface and in the bulk solution. In the case of [Pb_aq]_initial = 10 mg/L, no anglesite precipitation was observed, but surface spectroscopy (proton Rutherford Backscattering Spectroscopy, p-RBS) confirmed the formation of Pb-bearing surface layers on the (0 1 0) gypsum surface in this case also. This study shows that the surface of gypsum can play an important role in the attenuation of Pb in contaminated waters.

Introduction

Although several international organizations classify Pb at the top of the priority list of the most hazardous substances (immediately after As according to the US Environmental Protection Agency), it is still extensively used in many industrial applications such as in batteries, pigments, ammunition, devices to shield X-rays, roofing and photographic material. Although its use as an additive to gasoline has been banned in most industrialized countries, it is still in use in several developing countries. The same applies for the use of Pb in applications such as water pipes (Tukker et al., 2006). As a result, the worldwide anthropogenic inputs of Pb to freshwaters and the atmosphere are in the range of several thousand of tonnes/a that gradually accumulate in the geo- and biosphere (Nriagu and Pacyna, 1988). Even so, the most catastrophic effects of Pb emission arise when its release to the environment occurs inevitably in areas close to tailings and waste water from mines (Simón et al., 1999).

It is well known that the degree of Pb mobility depends on several geochemical factors, pH and the specific mineralogy of the surrounding areas being among the most important ones (Lin et al., 1995). Lead sorption processes taking place at the mineral – aqueous solution interface can lead to the removal of Pb from polluted waters with an efficiency close to 100% under certain conditions (Traina and Laperche, 1999, Godelitsas et al., 2003a, Martin et al., 2008). Several minerals, such as silicates, oxides/oxyhydroxides (Cortina et al., 2003), carbonates (Godelitsas et al., 2003a) and the apatite mineral group (Valsami-Jones et al., 1998, Lower et al., 1998a, Lower et al., 1998b) have been proposed as effective candidates for the immobilization of Pb. More recently, synthetic materials derived from phosphate technology, such as Apatite II™ (Conca and Wright, 2001), a by-product of the fishing industry, have been proposed for Pb remediation (Conca and Wright, 2006).

Following this research line, an experimental study of the interaction of Pb-bearing aqueous solutions with gypsum (CaSO₄⋅2H₂O) fragments is presented here. The choice of this material is based on the fact that its chemical behavior in the presence of Pb is expected to be similar to that of Ca carbonates and Ca phosphates, namely rapid dissolution of the substrate followed by the formation of a sparingly soluble Pb-bearing solid phase, presumably PbSO₄ (anglesite) in this case. Among sedimentary rock-forming minerals, gypsum emerges as potentially useful to take up a broad spectrum of dissolved metals, particularly Pb, because of (1) the high reactivity of its surfaces, which makes it much more soluble (K_sp = 10^−4.58; Parkhurst and Appelo, 1999) than CaCO₃ and apatite; (2) the relative non-toxicity of both dissolved Ca and SO₄ (in comparison to PO₄); and (3) its abundance in nature. Few studies have examined the interaction of ${SO}_{4}^{2 -}$ ions with dissolved Pb and most of them have focused on acid mine drainage and weathering of sulfidic mineral wastes (Bigham and Nordstrom, 2000, Blowes et al., 2005, Lottermoser, 2007). In such environments, gypsum is one of the most abundant and widespread of the post-mining secondary minerals (Blowes et al., 2005). Gypsum re-dissolution will release its component ions back into solution (Lottermoser, 2007) and as a consequence, new reactions will occur. Sulfate ions will quickly react with Pb to form highly insoluble anglesite (K_sp = 10^−7.79; Parkhurst and Appelo, 1999) according to the following equations: ${CaSO}_{4} \cdot 2 H_{2} O_{(s)} + n H_{2} O_{(l)} \to {Ca}_{(aq)}^{2 +} + {SO}_{4 (aq)}^{2 -} + n H_{2} O_{(l)}$ ${SO}_{4 (aq)}^{2 -} + {Pb}_{(aq)}^{2 +} \to {PbSO}_{4 (s)}$

According to this the presence of gypsum may contribute to removing significant quantities of Pb, providing a natural attenuation mechanism in mine-drainage water (Lin and Herbert, 1997, Lin, 1997). The reaction of ${SO}_{4}^{2 -}$ with Pb to give anglesite has also been described in soils where the flux of Pb is high and constant, such as in the soil of shooting ranges, where the Pb-input comes from shotgun pellets (Lin et al., 1995).

No previous detailed experimental study has been carried out to date on this system. The main objective of this work is, therefore, to gain a better understanding of the kinetics of the dissolution/precipitation reactions occurring as a result of the interaction between gypsum and Pb-bearing waters. In this way, a study is presented on the effectiveness of gypsum as a Pb-sequestering agent using macroscopic batch-type experiments, having monitored the changes in Pb-bearing aqueous solutions with wet-chemical analyses and characterized the solids by X-ray diffraction, high-resolution microscopy and surface spectroscopy.

Section snippets

Macroscopic experiments in batch reactors

The gypsum specimens (selenite variety) used in all experiments were crystals from Teruel, Spain. X-ray fluorescence spectroscopy (XRF) and X-ray diffraction (XRD) analysis confirmed that the starting material corresponded to highly pure gypsum containing less than 0.2 wt.% impurities. The investigation was based on a macroscopic closed batch-type treatment procedure. Experiments were carried out by placing 2 g of crushed gypsum crystals (1.0–1.4 mm fragments obtained using an agate mortar) and 100

Macroscopic experiments in batch reactors

Fig. 1a shows the evolution of Pb concentrations as a function of time for the sorption experiments performed with 2 g of 1.0–1.4 mm gypsum fragments in contact with an aqueous solution containing [Pb_aq]_initial of 1000 mg/L. The Pb concentration fell dramatically during the first minutes of the experiments. In order to observe more accurately the evolution of the Pb concentration, a broken axes graphic is inserted in Fig. 1a. After 5 min of reaction, more than 60% of the Pb was removed from

Discussion

The experimental results indicate that the removal of Pb from an aqueous solution in contact with gypsum fragments occurs through at least two different sorption mechanisms, adsorption and precipitation, taking place at the gypsum–aqueous solution interface. The predominance of a specific mechanism depends on the [Pb_aq]_initial. Thus, the experimental data in Fig. 1, Fig. 3 show that, when the initial concentration of Pb in the aqueous solutions is high or moderate ([Pb_aq]_initial = 1000 and 100

Conclusions

Under the conditions of these experiments, Pb reacts easily with gypsum in aqueous environments. Both microscopic and macroscopic observations indicate that Pb is incorporated onto gypsum by surface precipitation of anglesite crystals through a simultaneous dissolution–precipitation process when the initial concentration of Pb in the solution ranges from 100 to 1000 mg/L. As a consequence, the concentration of Pb in the aqueous solution declines to reach values controlled by the relatively low

Acknowledgments

This work has been financially supported by the Spanish government under Contract CGL2007-65523-C02-01). Financial support has also been provided by the Research Group 910148 (UCM-Comunidad de Madrid, Spain). The Complutense University of Madrid has also provided financial support for Jose Manuel Astilleros through a travel grant. We sincerely thank the Microscopy Centre and the X-ray Diffraction Central Service of the Complutense University of Madrid for technical assistance. We also thank two

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Interaction of gypsum with lead in aqueous solutions

Abstract

Introduction

Section snippets

Macroscopic experiments in batch reactors

Macroscopic experiments in batch reactors

Discussion

Conclusions

Acknowledgments

Surf. Sci.

Appl. Geochem.

J. Crystal Growth

Environ. Pollut.

Geochim. Cosmochim. Acta

Appl. Geochem.

Geochim. Cosmochim. Acta

Sci. Total Environ.

Resour. Conservat. Recycl.

Chem. Geol.

Iron and aluminum hydroxysulfates from acid sulfate waters

Passive in situ remediation of metal-polluted water with caustic magnesia: evidence from column experiments

Environ. Sci. Technol.