Bio Adsorption: An Eco-friendly Alternative for Industrial Effluents Treatment 43

3.3 Dosage Curves

Once the adsorbent has been selected and treated only by washing with water, drying and sieving

(Figure 3.1), the working conditions should be selected, for which the so-called dosage curves are

carried out to examine the adsorbent performance for the removal of each pollutant. This means

performing two kinds of experiments: (i) varying the adsorbent mass in contact with a specified

volume of a given concentration solution, and (ii) varying the concentration of a given solution

volume with a fixed adsorbent mass. To ensure that equilibrium is reached, both types of experiments

should be performed with the proper agitation, at a fixed pH and temperature, for a determined

amount of time.

On the basis of these experiments, working conditions can be selected to allow the necessary

kinetic and equilibrium studies to be carried out.

As an example of this procedure, the results found using peanut shells as adsorbent, and nickel

as contaminant are presented.

Figure 3.1. Peanut shells before and after washing, drying and sieving.

These curves were developed at pH = 6, corresponding to the pH of the effluents to be treated,

with a volume of 50 mL of solution and continuous stirring for 24 hr, verifying that equilibrium was

reached after that time and that the contaminant was detectable in the remaining solution. The range

of concentrations that can be measured in the atomic absorption spectroscopy equipment for nickel

was in 0.1 to 8 mg L^–1 so the concentrations used for the dosing test were between 10 and 50 mg L^–1.

The adsorbent mass range was 0.30 to 1.0 g established based of the initial 24 hr experiment.

When the weight of the adsorbent increases, the percentage of nickel removal increases, but

the adsorption capacity decreases as it can be observed in Figure 3.2a. Secondly, when the weight

of the adsorbent is fixed and the nickel concentration increases, the removal percentage decreases,

but the adsorption capacity increases as can be observed in Figure 3.2b. The maximum adsorption

capacity was 30 mg Ni²⁺ L^–1.

a

b

Figure 3.2. Peanut shell dosage curves. a. As a function of adsorbent mass. b. As a function of initial nickel solution

concentration.

The compromise relation selected was 0.3 g of adsorbent and 20 mg Ni²⁺ L^–1 as the initial

concentration. For this ratio the percentage of removal obtained was close to 80% and the adsorption

capacity close to the maximum. This compromise relation also allows measurable results to be

obtained within the concentration range suitable for the measuring instrument.