Exothermic adsorption of chromate by goethite
Graphical abstract
Introduction
Chromium (Cr) is one of the most common groundwater contaminants released from industrial plants. This is because Cr is used widely to produce stainless steel, to passivate metal surfaces, and to tan leather (Shi et al., 2020). Chromium ions in different oxidation states have different mobilities. Cr(VI) is the most soluble form of Cr, but is mutagen, teratogen, and carcinogen, and poses therefore strong risks to public health and to ecosystems (Zhitkovich, 2011). Inverse correlations between Mn(II) and Cr(VI) suggest that reductive dissolution of Mn oxides is the major pathway of Cr(VI) mobilization in the aquifers (Oze et al., 2007; Manning et al., 2015; Izbicki et al., 2015; Vengosh et al., 2016; Guo et al., 2020). Cr(V) is even more mobile and toxic than Cr(VI) but has a relatively short half-live and disproportionate to Cr(VI) and Cr(III). The reduced species Cr(III) undergoes hydrolysis to form very insoluble oxides, such as Fe(III) and Al(III), and is usually immobile in aquatic environments. The high mobility of Cr(VI) in soil solution and groundwater can only be prevented by reducing all of the Cr(VI) into immobile Cr(III) species, e.g., through adsorption by Fe oxides containing Fe(II) or reductive co-precipitation with other phases containing Fe(II) (Gröhlich et al., 2017; Bompoti et al., 2019; Paternoster et al., 2020). Methanogenic Archaebacteria have been found to contribute to anaerobic chromate reduction by microorganisms in hot springs in Yellowstone National Park (Chrencik and Marsh, 2012).
Surface complexation models can be used to interpret experimental data to describe adsorption processes and estimate ion partitioning at the solid–solution interface (Davis and Leckie, 1980). Numerous studies of oxyanion (e.g., arsenate, phosphate, and silicate) sorption by iron oxides have recently been published. Despite the increasing number of modelling studies of chromate surface complexation, the effect of temperature on adsorption has not yet been investigated, and no quantitative molar enthalpy ΔrHT and entropy ΔrST data for the chromate adsorption reactions are available. This information is important for numerous water purification applications. Real thermodynamic data would be useful because adsorption onto iron oxides is a cost-effective way of removing Cr from (often hot) industrial wastewater, of studying Cr-containing canister material used in underground nuclear waste deposits in which long-term production of residual heat by the fuel cells occurs, and of studying Cr behaviour in geothermal water. It was recently suggested in a flow microcalorimetry study an exothermic inner-sphere surface complexation (Kabengi et al., 2017), but thermodynamic data for the complex were not reported. Adsorption of an anion onto an oxide surface is usually qualitatively considered to be exothermic (i.e., to have a negative enthalpy value), meaning the adsorption bond should become weaker as the temperature increases (Kulik, 2006). Unfortunately, very few experimental results of oxyanion adsorption by oxides at elevated temperatures have been published, and none are available for chromate. The aim of this study was to derive thermodynamic constants describing the temperature-dependent formation of chromate surface complexes, as available for other oxyanions (Kersten and Vlasova, 2009a, 2009b, 2013).
Section snippets
Batch adsorption experiments
Deionized water through which Ar had been bubbled was used to prepare each solution and suspension to avoid cross-contamination with carbonate, which could interfere strongly with chromate adsorption (Villalobos et al., 2001). Synthetic goethite was prepared from ferric nitrate solution under highly alkaline conditions and then aged at 90 °C for 7 days. The goethite had a relatively low Brunauer–Emmett–Teller (BET) surface area of 25 m2 g−1, which was stable at elevated temperatures. Each batch
Adsorption modelling at room temperature
The data for chromate adsorption onto goethite at different ionic strengths and Cr concentrations for room temperature are plotted against pH in Fig. 1. The percentage of chromate adsorbed was highest at pH 3 (the lowest pH used) and decreased as the pH increased to almost 0% at circumneutral pH values. This pattern is common for many oxyanions. The adsorption edge became steeper as the total Cr concentration decreased. Similar trends have been found for other metal oxyanions, such as molybdate
Conclusions
Chromate adsorption by goethite was found to occur through both inner- and outer-sphere surface complexation reactions. The batch equilibrium experimental results indicate that maximum chromate adsorption occurred at acidic pH values and that the amounts of chromate adsorbed decreased as the pH increased towards the zero-point-of-surface-charge (pHPZC 9.1) of the adsorbent surface. The ionic strength and temperature both affected outer-sphere surface complex formation. Adsorption of Cr(VI)
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
This study was supported by the German Science Foundation through grant no. KE508-27. Regina Walter helped with the ICP-MS measurements.
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