A multi-site mechanism model for studying Pb and Cu retention from aqueous solutions by Fe-Mg-rich clays

Abstract

The retention mechanisms of metal ions during interaction of clay with metal-rich aqueous solutions is usually investigated by sorption isotherms. Although classical isotherm models may provide sufficient information about the characteristics of the solid–liquid system, they do not distinguish among the various retention mechanisms. This study presents a methodological approach of combining batch experiment data and geochemical modelling for the characterization of the interaction of Mg-Fe-rich clay materials with monometallic solutions of Pb and Cu. For this purpose, a palygorskite clay (PCM), an Fe-smectite clay (SCM) and a natural palygorskite-Fe-smectite mixed clay (MCM) were assessed for their effectiveness as metal ion sorbents. The sorption capacity of the materials follows the order MCM > SCM > PCM and ranges between 27.6–52.1 mg g–1 for Pb and 7.7–17.6 mg g–1 for Cu. Based on the experimental results that allowed the speciation calculations, fitting of sorption isotherms and the investigation of relationships between protons, Mg and the metals studied we suggest that a combination of sorption mechanisms occurs during the interaction of clay materials with metal solutions. These involve surface complexation, ion exchange and precipitation of solid compounds onto the solid surface. A three-term isotherm model was employed to quantify the role of each of the above mechanisms in the overall retention process. The superior performance of mixed clay among the materials tested is attributed to the synergetic effect of exchange in the interlayer and specific sorption on the clay edges.

Micro-column testing of the dynamic adsorption of Pb by Fe-Mg clay–quartz beds

“…The respective experimental and model-obtained adsorption capacities under both static and dynamic conditions are compared in Table 6. a Data acquired by Kypritidou & Argyraki (2018) The static adsorption capacities were somewhat higher than the dynamic ones, which can be attributed to the better contact between the clay particles and the solution. In static experiments, the solid particles were fully dispersed within the solution, allowing the full contact of Pb ions with the solid surface.…”

“…Static experiments were conducted using a solid:liquid ratio of 10 g L -1 undergoing 4 h of agitation, a pH 3.5 solution and initial Pb concentrations up to 800 mg L -1 . The Sips isotherm equation was used to fit the batch experimental data and to estimate the maximum adsorption capacities of the clay materials (Kypritidou & Argyraki, 2018). The respective experimental and model-obtained adsorption capacities under both static and dynamic conditions are compared in Table 6.…”

“…The structure and mineralogy of the clay phases in these deposits have been studied extensively by Gionis et al (2006Gionis et al ( , 2007, Chryssikos et al (2009) and Kaufhold et al (2019), revealing the complex nature of these materials. A rigorous characterization of the respective clay samples (PCM, SCM and MCM) regarding their mineralogy, structure, chemistry and physicochemical properties was reported by Kypritidou & Argyraki (2018) and is summarized in Table 1. More details regarding the structural, mineralogical and physicochemical properties of the samples are given in the Supplementary Materials.…”

“…This inlet solution pH favours both the solubility of Pb and its diffusion within the clay mass for all of the beds. A greater adsorption capacity was observed for the MCM-Qz bed because this mixed clay material combines both the large surface area of the palygorskite phase and the great CEC of the Fe-smectite phase (Table 1; Kypritidou & Argyraki, 2018). The adsorption capacity of the MCM-Qz bed (0.48 mg g -1 ) exceeded the CEC of the MCM clay (0.29 mg g -1 ), indicating the contributions of both ion-exchange and surface-complexation reactions in Pb retention.…”

“…Therefore, the presence of Fe-smectite plays a key role in increasing the adsorption capacity of clays with increasing the inlet solution pH. By increasing the inlet solution pH, the solubility of Pb decreases, and the interactions with the clay surface become stronger.Furthermore, the retained Pb ions may diffuse within the interlayer of Fe-smectite where they exchange the interlayer cations (mainly Mg)(Kypritidou & Argyraki, 2018) https://doi.org/10.1180/clm.2022.45 Published online by Cambridge University Press…”