3.3. Adsorption Experiments

Potassium dichromate (K₂Cr₂O₇) was used as the source of Cr(VI). A stock solution was prepared in deionised water by dissolving 2.8300 g of potassium dichromate in 1000 cm³. The solution had a concentration of 1000 mg·dm⁻³ and a pH of about 4.5. The solutions used in the experiments were prepared from the stock solution by diluting it with deionised water. Most of the adsorption studies were performed using a Cr(VI) solution with a concentration of 100 mg·dm⁻³. This concentration can be found in waters located in the vicinity of electroplating plants.

The adsorption studies were carried out by a batch method at room temperature. Concentrations of Cr(VI) were determined by measuring the absorbance at the characteristic wavelength (545 nm) using a UV-visible spectrophotometer Evolution 201 (Thermo Fisher Scientific, Madison, WI, USA), accordingly to the diphenylcarbazide method.

In all adsorption studies, schwertmannite suspensions were used directly after synthesis rather than dried material. The volume of the suspension samples was recalculated to dry schwertmannite content. Each experiment was repeated three times under the same conditions. The dry matter content in 1 cm³ of the suspension was 8.00 mg and 31.0 mg for SCH_A and SCH_B, respectively. It can be observed that the dry matter content for SCH_B is approximately four times higher than for SCH_A, which significantly affects the adsorption results.

Studies of Cr(VI) adsorption were carried out for both schwertmannite A (SCH_A) and schwertmannite B (SCH_B). Removal of Cr(VI) was tested in batch studies as a function of adsorbent dosage, pH, contact time, and initial Cr(VI) concentration. All adsorption studies were carried out using the batch method at room temperature for 24 h. The effect of pH was investigated by adjusting the pH of Cr(VI) solutions using 0.1 M HCl and 0.1 M NaOH. In this set of experiments, 1 cm³ of adsorbent (SCH_A or SCH_B) per 10 cm³ of 100 mg·dm⁻³ Cr(VI) standard solution were taken. The effect of the adsorbent dose was studied by adding the varying volume of adsorbent (SCH_A or SCH_B) from 0.05 to 1 cm³ to Cr(VI) solution at an initial concentration of 100 mg·dm⁻³ The kinetic experiments were conducted in batch mode by shaking 1 cm³ of adsorbent (SCH_A or SCH_B) with Cr(VI) solution at a constant pH (~4.5). Cr(VI) concentration was monitored for 24 h. The adsorption isotherms were performed in two measurement series—in the range of low initial Cr(VI) concentration (1–100 mg·dm⁻³) and in the range of high initial Cr(VI) concentration (10–1000 mg·dm⁻³). The adsorbent SCH_A or SCH_B (1 cm³) was added to 10 cm³ Cr(VI) solution (1–1000 mg·dm⁻³). The pH of the solutions was 4.5 and the equilibrium time for this experiment was 24 h.

The amount of Cr(VI) adsorbed (removal in percentages) is calculated as follows (Equation (10)) [28]:

where c₀ and c are the initial and the final concentrations of Cr(VI) in the aqueous solution (mg·dm⁻³).

The equilibrium adsorption capacity was calculated from the standard equation (Equation (11)) [28]:

where c₀ and c (mg·dm⁻³) are the initial and equilibrium Cr(VI) concentrations, respectively; V (cm³) is the volume of the solution and m (g) is the amount of adsorbent.