Influence of Ionic Strength and Temperature on the Adsorption of Reactive Black 5 Dye by Activated Carbon: Kinetics, Mechanisms and Thermodynamics

Abstract

The aim of this work was to investigate the influence of ionic strength and temperature on the adsorption of Reactive Black 5 dye on commercial powdered activated carbon. Adsorption experiments were performed at 45 °C with the addition of NaCl (c₀ = 0.01, 0.05, 0.10 and 1.00 M) and Na₂SO₄ (c₀ = 0.01 M). The results were compared with those obtained for both salts (c₀ = 0.01 M) at three additional temperatures: 25, 35 and 55 °C. For all adsorption experiments, kinetic and thermodynamic studies were performed. This research showed that the addition of NaCl, even in the concentration of only c₀ = 0.01 M, significantly enhanced dye adsorption and that higher NaCl concentration resulted in higher adsorption capacity. In addition, slightly higher adsorption was observed when Na₂SO₄ was added to the dye solution at the same concentration as NaCl, as well as at a higher temperature, regardless of the salt added to the dye solution. It was also shown that adsorption is kinetically controlled, assuming a pseudo-second-order model, and that intraparticle diffusion is not the only process that influences the adsorption rate. Finally, calculated thermodynamic parameter values for both salts (c₀ = 0.01 M) indicate that adsorption was a spontaneous endothermic process.

Keywords: Reactive Black 5; activated carbon; ionic strength; isothermal adsorption; kinetics; thermodynamics.

Figure 1

Chemical structure of Reactive Black 5 dye.

Figure 2

FTIR spectra of activated carbon before (—) and after treatment with 0.01 M NaCl (—) and 0.01 M Na₂SO₄ (—).

Figure 3

FTIR spectra of activated carbon before (—) and after RB5 dye adsorption (—), as well as of RB5 dye (—).

Figure 4

Electrokinetic ζ-potential of the activated carbon in sodium chloride (—) and sodium sulphate (—) aqueous solutions as a function of pH value.

Figure 5

SEM images of activated carbon surface enlarged: (a) 20,000× and (b) 30,000×.

Figure 6

Effect of NaCl concentration on the RB5 dye concentration in the liquid phase (c_t) after appropriate time of adsorption (t) at 45 °C (◯ c₀ = 0.01 M; ◇ c₀ = 0.05 M; Δ c₀ = 0.10 M; ☐ c₀ = 1.00 M).

Figure 7

Effect of ionic strength (c₀ = 0.01 M) on the efficiency of decolouration (E_d) after appropriate time of adsorption (t) at 45 °C (◯ without salt added; ◇ NaCl; ☐ Na₂SO₄).

Figure 8

Effect of temperature on the efficiency of decolouration (E_d) after appropriate time of adsorption (t) for NaCl and Na₂SO₄ concentration of c₀ = 0.01 M (◯ NaCl, 25 °C; ◇ Na₂SO₄, 25 °C; ☐ NaCl, 55 °C; Δ Na₂SO₄, 55 °C).

Figure 9

pH profiles for RB5 dye adsorption at 45 °C at the beginning of adsorption, after 30 min, 2 h and 16 h for all NaCl concentrations (■ c₀ = 0.01 M; ■ c₀ = 0.05 M; ■ c₀ = 0.10 M; ■ c₀ = 1.00 M) and Na₂SO₄ concentration of c₀ = 0.01 M (■).

Figure 10

Root time plot for the adsorption of RB5 dye on activated carbon at 45 °C for all NaCl concentrations (◯ c₀ = 0.01 M; ◇ c₀ = 0.05 M; Δ c₀ = 0.10 M; ☐ c₀ = 1.00 M) and Na₂SO₄ concentration of c₀ = 0.01 M (✴).

Figure 11

J. H. van’t Hoff plot for adsorption of RB5 dye on activated carbon with the salt addition in concentration of c₀ = 0.01 M for NaCl (◇) and Na₂SO₄ (◯).