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Sorption and desorption of Cr(VI) ions from water by biochars in different environmental conditions.

Tytłak A, Oleszczuk P, Dobrowolski R - Environ Sci Pollut Res Int (2014)

Bottom Line: The Langmuir model has better fitting of adsorption isotherms than the Freundlich model.The sorption process can be described by the pseudo second-order equation.The results indicated that the sorption mechanism of Cr(VI) on biochar involves anionic and cationic adsorption combined with Cr(VI) species reduction.

View Article: PubMed Central - PubMed

Affiliation: Department of Environmental Chemistry, Faculty of Chemistry, Maria Curie-Sklodowska University, Maria Curie-Skłodowska Square 3, 20-031, Lublin, Poland.

ABSTRACT
In the present research, the potential of two biochars produced by the thermal decomposition of wheat straw (BCS) and wicker (BCW) for Cr(VI) ions removing from wastewater was investigated. The pH and the presence of chlorides and nitrates were also investigated. The Freundlich and Langmuir models were applied for the characterization of adsorption isotherms. The Langmuir model has better fitting of adsorption isotherms than the Freundlich model. The sorption process can be described by the pseudo second-order equation. The optimal adsorption capacities were obtained at pH 2 and were 24.6 and 23.6 mg/g for BCS and BCW, respectively. X-ray photoelectron spectroscopy (XPS) studies confirmed that Cr(III) ions were the most abundant chromium species on the biochars' surface. The results indicated that the sorption mechanism of Cr(VI) on biochar involves anionic and cationic adsorption combined with Cr(VI) species reduction.

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The pH influence on Cr(VI) ions’ adsorption onto BCS and BCW; m = 0.2 g, V = 50 mL, CCr(VI) = 5.2 mg/L, t = 24 h, T = 25 °C
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Fig5: The pH influence on Cr(VI) ions’ adsorption onto BCS and BCW; m = 0.2 g, V = 50 mL, CCr(VI) = 5.2 mg/L, t = 24 h, T = 25 °C

Mentions: It has been proved that pH is one of the most important parameters having an influence on the adsorption capacity of adsorbent for heavy metal ions removal from the aqueous solutions (Kołodyńska et al. 2012; Aliabadi et al. 2012; Deveci and Kar 2013). In Fig. 5, the adsorption abilities of chromium in an equilibrium pH are shown depending on the biochar tested. The variations in chromium(VI) sorption ability in the examined pH range may be partly related to pH dependency of the chromium species present in the aqueous solution and onto the BCs’ surface. At pH 2, Cr(III) ions exist in aqueous solution as [Cr(H2O)5]3+ which has associated water molecules, whereas Cr(VI) is unstable. These molecules can be exchanged with the hydroxyl ions which depend on the pH values. Changes in acidic groups present on the surface of the biochar cause the variations of pH. The maximum adsorption of Cr(VI) ions onto BCs was achieved at pH 2. For both of BCs in the pH range from 1 to 7, the same trend was observed, and the differences between biochars were only noticeable above pH 7. It is caused by the uncompleted reduction of Cr(VI) and mixed Cr(III) and Cr(VI) adsorption processes. At pH lower than 2, in acidic conditions, the reduction of Cr(VI) to Cr(III), followed by the adsorption by nonspecific sorption or surface chelation, is the main mechanism of Cr(VI) adsorption. At pH 2.0–3.2, Arslan and Pehlivan (2007) noticed that carboxylic acid centers could be appreciably deprotonated and exist in the form of COO− which can bind to Cr(III) ions. Furthermore, Gardea-Torresdey et al. (2000) reported that Cr(III) can be bound to the carboxyl groups containing oxygen. In higher pH values, acidic surface groups enhanced the adsorption of Cr(III) onto the biochar. These kinds of groups were identified by FTIR (Fig. 1) and X-ray photoelectron spectroscopy (XPS) (Fig. 8) (which will be discussed later) techniques onto the BCs’ surface.Fig. 5


Sorption and desorption of Cr(VI) ions from water by biochars in different environmental conditions.

Tytłak A, Oleszczuk P, Dobrowolski R - Environ Sci Pollut Res Int (2014)

The pH influence on Cr(VI) ions’ adsorption onto BCS and BCW; m = 0.2 g, V = 50 mL, CCr(VI) = 5.2 mg/L, t = 24 h, T = 25 °C
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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Fig5: The pH influence on Cr(VI) ions’ adsorption onto BCS and BCW; m = 0.2 g, V = 50 mL, CCr(VI) = 5.2 mg/L, t = 24 h, T = 25 °C
Mentions: It has been proved that pH is one of the most important parameters having an influence on the adsorption capacity of adsorbent for heavy metal ions removal from the aqueous solutions (Kołodyńska et al. 2012; Aliabadi et al. 2012; Deveci and Kar 2013). In Fig. 5, the adsorption abilities of chromium in an equilibrium pH are shown depending on the biochar tested. The variations in chromium(VI) sorption ability in the examined pH range may be partly related to pH dependency of the chromium species present in the aqueous solution and onto the BCs’ surface. At pH 2, Cr(III) ions exist in aqueous solution as [Cr(H2O)5]3+ which has associated water molecules, whereas Cr(VI) is unstable. These molecules can be exchanged with the hydroxyl ions which depend on the pH values. Changes in acidic groups present on the surface of the biochar cause the variations of pH. The maximum adsorption of Cr(VI) ions onto BCs was achieved at pH 2. For both of BCs in the pH range from 1 to 7, the same trend was observed, and the differences between biochars were only noticeable above pH 7. It is caused by the uncompleted reduction of Cr(VI) and mixed Cr(III) and Cr(VI) adsorption processes. At pH lower than 2, in acidic conditions, the reduction of Cr(VI) to Cr(III), followed by the adsorption by nonspecific sorption or surface chelation, is the main mechanism of Cr(VI) adsorption. At pH 2.0–3.2, Arslan and Pehlivan (2007) noticed that carboxylic acid centers could be appreciably deprotonated and exist in the form of COO− which can bind to Cr(III) ions. Furthermore, Gardea-Torresdey et al. (2000) reported that Cr(III) can be bound to the carboxyl groups containing oxygen. In higher pH values, acidic surface groups enhanced the adsorption of Cr(III) onto the biochar. These kinds of groups were identified by FTIR (Fig. 1) and X-ray photoelectron spectroscopy (XPS) (Fig. 8) (which will be discussed later) techniques onto the BCs’ surface.Fig. 5

Bottom Line: The Langmuir model has better fitting of adsorption isotherms than the Freundlich model.The sorption process can be described by the pseudo second-order equation.The results indicated that the sorption mechanism of Cr(VI) on biochar involves anionic and cationic adsorption combined with Cr(VI) species reduction.

View Article: PubMed Central - PubMed

Affiliation: Department of Environmental Chemistry, Faculty of Chemistry, Maria Curie-Sklodowska University, Maria Curie-Skłodowska Square 3, 20-031, Lublin, Poland.

ABSTRACT
In the present research, the potential of two biochars produced by the thermal decomposition of wheat straw (BCS) and wicker (BCW) for Cr(VI) ions removing from wastewater was investigated. The pH and the presence of chlorides and nitrates were also investigated. The Freundlich and Langmuir models were applied for the characterization of adsorption isotherms. The Langmuir model has better fitting of adsorption isotherms than the Freundlich model. The sorption process can be described by the pseudo second-order equation. The optimal adsorption capacities were obtained at pH 2 and were 24.6 and 23.6 mg/g for BCS and BCW, respectively. X-ray photoelectron spectroscopy (XPS) studies confirmed that Cr(III) ions were the most abundant chromium species on the biochars' surface. The results indicated that the sorption mechanism of Cr(VI) on biochar involves anionic and cationic adsorption combined with Cr(VI) species reduction.

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