Cyclic voltammetric and FTIR studies of powdered carbon electrodes in the electrosorption of 4-chlorophenols from aqueous electrolytes

et al. 2014

Reac Kinet Mech Cat

Activated carbons with large and medium-sized S BET , carbon blacks with different surface areas as well as graphite and heat-treated carbon of extremely low porosity were used in this study. Cyclic voltammograms were recorded for these materials in neutral electrolyte (0.1 M Na 2 SO 4 ) solutions containing various chlorophenols (CPs; 4-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol) as depolarizers. The changes in the FTIR spectra of the electrode materials caused by the adsorption and/or coupling of CPs (and/or their degradation products) were recorded. HPLC was used for analyzing the concentration of CPs and their degradation products. The dependence of the electrochemical behavior of CPs with various chlorine contents on the character of carbon materials is discussed. The possible mechanism of the initial stages of CP degradation by electro-oxidation/ reduction depends on the kind of electrode material. Since different carbons vary in their electrochemical behavior and in their voltammetric response to the presence of different CPs, the application of the method for selecting electrode materials better suited to removing organochlorine impurities from ground-water seems feasible. The efficiency of the process depends closely on the kind of carbon and on the tendency for its surface to become covered by a polymeric (conducting or nonconducting) film of electro-degradation products.

Section: Resultsmentioning

confidence: 99%

Electro-oxidation of chlorophenols on powdered carbon electrodes of different porosity

Biniak

Pakuła

et al. 2014

Reac Kinet Mech Cat

“…The extent of electrodegradation (a four-fold drop in organics concentration) observed for the carbon black with the larger surface area indicates the dominant role of surface (porosity) structure in these processes. As in the case of chlorophenol electrooxidation, the shape of the CV curves can be explained the hydrolytic removal of an acetic acid molecule and then by the electooxidation of the chlorophenol unit, which leads to the observed quinone/hydroquinone couple [14]. In the presence of a depolarizer, the course of the CV curves (Fig.…”

Section: Resultsmentioning

confidence: 99%

Behavior of graphitized carbon blacks in the electrodegradation and sorption of chlorophenoxyacetic acids

Biniak

Pakuła

et al. 2015

Reac Kinet Mech Cat

This paper compares the properties of two graphitized carbon blacks in the sorption and first stage of electrodegradation (dechlorination) of chlorophenoxyacetic acids and discusses the influence of surface area on the electrochemical behavior of these carbon materials. Two carbon blacks with ten-fold different surface areas (100 and 10 m 2 /g) were used as sorbents and electrode materials as received. Two chloroorganics with various numbers of Cl atoms in the molecule, 4-chlorophenoxyacetic acid and 2,4-dichlorophenoxyacetic acid, were adsorbed on the carbon samples from neutral aqueous electrolyte solutions containing low concentrations of adsorbate. Cyclovoltammetric studies of the graphitized carbon blacks were performed using powdered electrode techniques, and the drop in organics concentration during cyclization was measured using HPLC. For carbon black with a smaller surface area, the presence of faradaic reactions was not masked by capacitive currents, and well-shaped faradaic couple peaks characteristic of a quinine/hydroquinone-like adsorbed system were observed. This indicates that these materials are active in chloroorganics electrodegradation (by dechlorination). The extent of electrodegradation (a four-fold drop in organics concentration) observed for carbon black with a larger surface area indicates the dominant role of surface (porosity) development in these processes.

“…Granulowany węgiel aktywny R3-ex (Norit, Holandia) został odpopielony za pomocą stężonego kwasu fluorowodorowego i chlorowodorowego, a następ-nie utleniony stężonym HNO 3 (3 h, 80°C) lub wygrzewany przez 3 godziny w NH 3 w temperaturze 900°C [17]. Najważniejsze właściwości fizykochemiczne charakteryzujące powierzchnię węgli aktywnych przedstawiono w tabeli 1.…”

Section: Materiały I Metodyka Badańunclassified

“…Dodatkowo, zasadowy charakter powierzchni węgla aktywnego sprzyja reakcjom grup funkcyjnych adsorbentu z grupami -COOH w cząsteczkach herbicydu. Podobne zjawisko zwiększenia zdolności adsorpcyjnych węgli aktywnych modyfikowanych amoniakiem zaobserwowano w przypadku fenolu [21], 4-chlorofenolu [17] czy też 2,4-dichlorofenolu [22].…”

Section: Wyniki I Dyskusjaunclassified

The Influence of Surface Chemistry of the Activated Carbons on the Adsorption of 2,4-dichlorophenoxyacetic Acid

Kuśmierek

2016

Eng. Prot. Environ.

The aim of this study was to investigate the effect of surface chemistry of the activated carbons on the adsorption of 2,4-dichlorophenoxyacetic acid (2,4-D). The 2,4-D was adsorbed on non-modified Norit R3-ex activated carbon (AC-NM) as well as on activated carbons modified by oxidation with concentrated nitric acid (AC-HNO3) and by heat treatment in ammonia at 900°C (AC-NH3). Adsorption isotherms of the 2,4-D on the activated carbons were analyzed using the Freundlich, Langmuir and Langmuir-Freundlich models. The Langmuir equation was slightly better fitted to the experimental data with the correlation coefficients better than 0.99. The values of the Langmuir maximum adsorption capacity (qm) were 2.945, 2.740 and 3.297 mmol/g for the AC-NM, AC-HNO3 and AC-NH3 activated carbons, respectively. The adsorption capacity of the activated carbons increased in the order: AC-HNO3 < AC-NM < AC-NH3. The best adsorbent was activated carbon with basic properties, while the worst adsorption properties were observed for the activated carbon with acidic properties. The acid treatment of activated carbon produced a large number of oxygen-containing functional groups on the carbon surface as it increases its acidic property and, in consequence, reduces the adsorption of the 2,4-D. The treatment of activated carbon with ammonia at high temperature leads to the formation of nitrogen-containing groups. The basic properties of the carbon surface enhance the interaction between activated carbon and acid molecules (dipole-dipole, H-bonding, covalent bonding) causing the increased adsorption of the 2,4-D from water. The effect of pH on the adsorption of 2,4-D onto activated carbons was also studied. The adsorption of the 2,4-D was almost constant at the pH range of 2.0-2.6 and decreased with the further increasing in the pH. The solution pH determines the adsorbent charge and the protonation or dissociation of the adsorbate. The pKa of 2,4-D is 2.8, and at a pH greater than the pKa value, the herbicide existed predominantly in anionic forms. As the pH increased, the degree of dissociation of 2,4-D increased, thereby making it more negatively charged. The values of the point of zero charge (pHPZC) were 6.10, 3.35 and 7.85 for the AC-NM, AC-HNO3 and AC-NH3 activated carbons, respectively. At pH of less than the pHPZC, the surface of the carbon had a net positive charge; at a pH greater than pHPZC, the surface had a net negative charge. The large reduction in the 2,4-dichlorophenoxyacetic acid adsorption at highly basic conditions can be attributed to the electrostatic repulsion between the negatively charged activated carbons and the dissociated 2,4-D molecules. The experimental results demonstrate that the surface chemistry of the activated carbons affects significantly the adsorption of the 2,4-D and should be taken into account when choosing an adsorbent for the removal of the herbicide from water.