A calcined clay fixed bed adsorption studies for the removal of heavy metals from aqueous solutions
Graphical abstract
Introduction
Water pollution by toxic heavy metals through the discharge of industrial waste is a worldwide environmental problem (Bo et al., 2020). The presence of heavy metals in streams, lakes, and groundwater reservoirs exerted several deleterious health problems (Bhagat et al., 2020; de Freitas et al., 2018). Heavy metals contamination originated by the discharge of various liquid effluents such as metal plating, mining, tanneries, as well as agricultural sources where fertilizers, amendments and irrigation (Bo et al., 2020; Huang et al., 2020a).
Once a toxic metal ion reaches human tissues via food chain, it causes irreversible damages for kidneys, nervous system, liver, and brain (Mapenzi et al., 2020). Therefore, it is necessary to treat metal contaminated effluents before their final discharge to the receiving water bodies (Khalfa et al., 2020). Many treatment systems like ion exchange, coagulation, chemical precipitation, membrane filtration, solvent extraction, and adsorption were proposed for heavy metal removal (Tran et al., 2017). Most of those methods are known for their high costs, toxic waste production, low efficiency and high energy consumption (Rasaki et al., 2019; Zhou et al., 2019). Among all those routine methods, adsorption is the most appropriate process due to its simplicity, higher removal capacity, and low operating costs (Athman et al., 2020). Nevertheless, further investigations are needed to deal with locally available adsorbents for potential environmental applications, including the elimination of metals from industrial wastewaters. To this end, clay minerals are among the most effective adsorbents for heavy metals due to their high specific surface area, intercalation, ion exchange abilities, low cost and ubiquitous presence in most soils (Sipos et al., 2018). In most cases, those natural adsorbents are tested via the application of batch experiments at the laboratory level, but not for industrial scale treatment (Sdiri et al., 2016; Uddin, 2017).
Thus, the evaluation of clay efficiency in the removal of metals under continuous feeding is expected to deepen knowledge about the sorptive mechanisms. This study has been performed to evaluate the effectiveness of the calcined clay in the removal of several heavy metals from aqueous systems (in single and binary systems) through a dynamic adsorption. A designed fixed bed was useful for a better description of breakthrough curves. Metals of concern included Lead (Pb), Chromium (Cr) and Cadmium (Cd) because of their abundance and toxicity even at very low concentrations.
The effects of the bed height, the flow rate, and the initial feed concentration on metal sorption in single and mixed systems, were investigated. Various theoretical models (i.e., Thomas, Yoon-Nelson, Adams-Bohart and Advection diffusion models) were also fitted to the experimental data.
Experimental data of Pb(II), Cr(VI) and Cd(II) removal by clay was fitted to various mathematical models, particularly the most appropriate predictions of (i) Thomas, (ii) Yoon and Nelson, (iii) Bohart and Adams and (iv) Advection diffusion models. Fixed bed adsorption models usually exhibit some difficulties to find satisfactory compromise between experimental and predicted data. As a result, simplifications are usually introduced for a better prediction of the breakthrough curves. Those models are fundamentally important to set up an efficient scheme of a well-designed fixed bed. They were used for an accurate determination of the maximum solid-phase concentration that can be estimated by an approximation of those models at 50% of bed saturation. The determination of the axial dispersion coefficients under different experimental conditions may help the sizing of industrial scale adsorbers. Mass transfer from the solution may be limited by axial dispersion, which represents the deviation from the ideal flow of the solution through the clay layer.
This model is among the most commonly used equations to predict the parameters of a breakthrough curve. Calculation of the adsorption capacity can be the first step for the design of a fixed bed removal apparatus (Bulgariu and Bulgariu, 2013).
Determination of the model parameters (i.e., rate constant KTh and maximum adsorption capacity qmax) can be performed through the linearized Thomas equation (Gomaa et al., 2018; Thomas, 1948). described as following:where C is the concentration of metal in the effluent (mg/L); C0 is the initial concentration (mg/L);
F is the flow rate (mL/min); KTh is the Thomas adsorption rate constant (L mg−1 min−1); m is the calcined clay mass (g); qmax is the maximum adsorption capacity (mg/g).
Bohart- Adams model assumed that adsorption efficiency depended on both the adsorbent and the adsorbate (Bohart and Adams, 1920), as expressed by the following equation:where C is the concentration of metal in the effluent (mg/L); C0 is the initial concentration (mg/L); KBA is the adsorption kinetic constant (L mg−1 min−1); N0 is the maximum volumetric adsorption capacity (mg/L); u is the power speed.
Yoon and Nelson model is based on the decrease of the adsorption probability (Recepoğlu et al., 2018).
In single system, the linearized form of Yoon and Nelson model can expressed as follows (Hoang et al., 2019; Yoon and Nelson, 1984):where KYN is the constant of Yoon- Nelson (min−1); t50 is the time required for 50% adsorbate breakthrough (min).
Experimental determination of both KYN and t50 are required to determine theoretical adsorption parameters.
Advection-dispersal-reaction (ADR) model addresses the axial dispersion of a solution along the column based on the assumptions of constant linear velocity, temperature and initial concentration of the solution. Other assumptions included same axial dispersion; absence of chemical reactions and a significant mass transfer by convection without radial dispersion.
The model equation can be written as following (Lapidus and Amundson, 1952):where DLis the axial dispersal coefficient (m2/s); H is the fixed bed height (m); v is the solution flow (m/s); Vi is the total volume of the solution to the cross-section surface of the clay layer (m3); Vmin is the minimum volume of the solution needed to saturate the clay layer on the cross-section surface (cm3/cm2).
DL and Vmin were determined using a non-linear regression; SSE and RMSE were calculated to evaluate the discrepancies between the experimental and calculated values (i.e., the ADR model values).
The estimate of the axial dispersion coefficient is crucial to determine the Peclet number (Pe) and the transfer coefficient of the studied material.
Peclet number was determined by the equation:
Axial dispersion occurs through the bed column (voids) and not inside the adsorbent pores. Its influence is related to the axial dispersion coefficient.
Section snippets
Materials and methods
A natural clay sample, collected from Elhamma area, Gabes district, southern Tunisia, was used for the sorbent preparation. A portion of the collected clay sample was washed with distilled water, dried to 60 °C and crushed by hands using a stainless-steel mortar and pestle. A less than 1 mm powdered sample was stored in a polypropylene bottle for subsequent uses.
Results and discussion
An attempt to develop a custom designed process for the retention of several toxic metals (i.e., Pb(II), Cr(VI) and Cd(II)) by calcined clay in dynamic conditions has been made. The effectiveness of a natural clay in removing the studied metals was evaluated accordingly.
Adsorption of metal ions in a column-wise packed clay (calcined at 550 °C for 24 h) focused on several parameters, including the effects of the adsorbent bed height, feeding solution flow and the initial concentration of
Conclusions
This study has been performed to evaluate applicability of a calcined clay, from southern Tunisia, in a fixed bed adsorption. The effects of various parameters were studied in detail for an efficient treatment of metal loaded effluents. Dynamic removal of the studied metals indicated that the breakthrough time decreased with the flow rate and initial concentration, but increased with the bed height. Adsorbed amounts depended on the bed height and initial concentration, and the flow rate.
CRediT authorship contribution statement
Leila Khalfa: Conceptualization, Methodology, Writing - original draft, Writing - review & editing. Ali Sdiri: Writing - original draft, Writing - review & editing, Formal analysis, Data curation, Validation, Supervision, Visualization. Mohamed Bagane: Conceptualization, Validation, Funding acquisition, Supervision. Maria Luisa Cervera: Conceptualization, Methodology, Funding acquisition, Supervision, Validation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The manuscript was written through contributions of all authors. We would like to gratefully acknowledge the help of the national engineering school, University of Gabes, Tunisia, and the fnancial support of the Ministerio de Economia y Competitividad-FEDER, Project CTQ2012- 38635 and CTQ_2014-52841 and Generalitat Valenciana PROMETEO- II-2014-077, Spain.
Special thanks go to the esteemed reviewers for their precious time, comments and suggestions to improve the manuscript.
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Leila Khalfa and Ali Sdiri contributed equally to this work as first authors.