**2.2 Adsorption process analysis**

### **2.2.1 Batch experiments**

98 Thermodynamics – Interaction Studies – Solids, Liquids and Gases

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

> Vmicro, (cm3/g)

Moisture Volatile Ash C H N O

CO2 CO CO/CO2

Smeso, (m2/g) Smicro,

Elemental analysis (wt %)

(m2/g) pHPZC

Vtotal, (cm3/g)

Merck\_ initial 755 0.33 0.31 41 714 7.02

Merck\_1 M HNO3 1017 0.59 0.55 40 977 3.41 Norit\_initial 770 0.40 0.32 41 729 6.92

Norit\_1 M HNO3 945 0.43 0.41 72 873 4.41

Norit\_initial 3.9 6.7 2.8 95.2 0.40 0.48 3.90 Norit\_1M HNO3 1.8 7.9 2.0 87.9 0.60 2.60 8.90 Merck\_initial 2.0 9.1 3.2 92.8 0.25 0.40 6.50 Merck\_1M HNO3 1.7 12.8 2.0 86.3 0.30 0.54 12.80

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

Proximate analysis (wt %)

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

Table 3. Surface oxygen functionality of the studied activated carbons

Carbons Oxygen evolved, (g/100g)

Norit\_initial 0.49 1.18 2.41 Norit\_1M HNO3 3.18 5.94 1.86 Merck\_initial 0.44 1.15 2.61 Merck\_1M HNO3 3.05 18.7 6.22

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

carboxylic, lactones and phenol groups.

(m2/g)

Carbons SBET,

**Carbons** 

its aging.

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

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 deviation of the adsorption parameters was under 1.5 %.

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 nm according to the standard procedure.
