**2.1.2 Surface characterization**

The textural characterization of the carbon samples was based on nitrogen adsorption isotherms at 77K. These experiments were carried out with Surface Area & Porosimetry Analyzer, Micromeritics ASAP 2010 apparatus. Prior to the adsorption testing, the samples were outgassing at 240 0C for 24 h under a pressure of 10-3 Pa. The apparent surface areas were determined from the adsorption isotherms using the BET equation; the Dubinin-Raduskhevich and B.J.H. methods were applied respectively to determine the micro- and mesopores volume. The oxidation treatment resulted in reduction of the apparent surface area with mesopores formation (Table 1).

The carbon's point zero charge (pH*PZC* values) were obtained by acid–base titration (Sontheimer, 1988). pH*PZC* decreases when the carbon surface is treated with nitric acid (Table 1). The parent carbons and their oxidized forms were characterized by elemental and proximate analyses using an Automatic CHNS-O Elemental Analyzer and a Flash EATM 1112 (Table 2). The oxygen content significantly increases when the carbon surface is treated with nitric acid.

The carbon surface was also characterized by temperature-programmed desorption with a Micromeritics TPD/TPR 2900 equipment. A quartz microreactor was connected to a mass spectrometer set up (Fisons MD800) for continuous analysis of gases evolved in a MID (multiple ion detection) mode. Surface oxygen groups on carbon materials decomposed

Comparison of the Thermodynamic Parameters Estimation for

pH to prevent introduction of any new electrolyte into the systems.

deviation of the adsorption parameters was under 1.5 %.

nm according to the standard procedure.

The heat of adsorption of the first monolayer is much stronger than the heat of adsorption of the second and all following layers. Typical for Chemisorption case

**2.3 Supporting theory** 

1], ( Christmann, 2010).

**2.2 Adsorption process analysis** 

**2.2.1 Batch experiments** 

the Adsorption Process of the Metals from Liquid Phase on Activated Carbons 99

Batch laboratory techniques were utilized to study the equilibrium of Cr (III) adsorption on Norit and Merck activated carbons. The adsorption isotherms were obtained at four different temperatures: 22, 30, 40 and 50 0C. All adsorption isotherms were determined at initial pH of the resulting Cr (III) solution i.e. 3.2, without adding any buffer to control the

The batch tests were conducted by loading a desirable amount of sorbent to the 250 ml Erlenmeyer flasks containing the Cr(III) solution of fixed (at 200 ppm, which is 10 times lower than the initial concentration present in the tannery wastewater) concentration. Each of the 10 samples used for one experiment consisted of a known carbon dosage from a range 1.2 – 20 g/l in 25 ml of Cr(III) 200 ppm solution, which were shaking on a gyratory shaker at 180 rev/min for 1-7 days (depending on the temperature of the experiment). Each experiment was performed for both initial and post-treated with peroxide, 1 М and acid forms of Norit and Merck carbons, thus generated a total of 1022=40 samples for each experimental temperature. Furthermore, in some cases, for the batch tests the conditions were changed for fixed carbon loading at 4.8 g/l, whereas Cr(III) concentration were varied from 50 to 2000 ppm. Experiments were duplicated for quality control. The standard

At the end of the experiments, the adsorbent was removed by filtration through membrane filters with a pore size of 0.45 m. The chromium equilibrium concentration was measured spectrophotometrically, using UV-Visible GBC 918 spectrometer, at fixed wavelength =420

In a typical adsorption process, species/materials in gaseous or liquid form (the adsorptive) become attached to a solid or liquid surface (the adsorbent) and form the adsorbate [Scheme

Monolayer adsorption Multilayer adsorption

Scheme 1. Presentation of the typical adsorption process (after Christmann, 2010)

The heat of adsorption of the first layer is comparable to the heat of condensation of the subsequent layers. Often observed during Physisorption

upon heating by releasing CO and CO2 at different temperatures (Table 3). The assignment of the TPD peaks to the specifics surface groups was based on the data published in the literature (Figueiredo, 1999). Thus, a CO2 peak results from decomposition of the carboxylic acid groups at low temperatures (below 400 0C), or lactones at high temperatures (650 0C); carboxylic anhydrous decompose as CO and CO2 at the same temperature (around 650 0C). Ether (700 0C), phenol (600-700 0C) and carbonyls/quinones (700-980 0C) decompose as CO. The treatment by nitric acid resulted in an increase in carboxylic acids and anhydrous carboxylic, lactones and phenol groups.


Table 1. Textural and surface characteristics of the studied activated carbons.


Table 2. Proximate and elemental analyses of the studied activated carbons


Table 3. Surface oxygen functionality of the studied activated carbons

All chemicals used were of an analytical grade. Salt Cr2(SO4)2OH2 , which is used in the tanning industry, was used as a sources of trivalent chromium. Metal standard was prepared by dissolution of Cr (III) salt in pure water, which was first deionized and then doubly distilled. The initial pH of the resulting Cr (III) solution was 3.2. The chromium solution was always freshly prepared and used within a day in order to avoid its aging.
