Comparative Studies of Adsorption of Chromium(VI) Ions onto Different Industrial Wastes

Chromium is found in different drinking water sources in many countries. Higher levels of chromium in drinking water are carcinogenic to consumers. Chromium removal can be achieved by employing various technologies, but not all are cost-effective to meet drinking water standards. In this work, Cr(VI) removal using three different industrial solid wastes, namely, fly ash, red mud, and dolochar, were explored and compared. Scanned electron microscopy was used to obtain the images of adsorbents both before and after adsorption. Adsorption experiments were executed using batch and column modes. The effect of pH (2–10), adsorbent dosage (5–35 g/L), contact time (5–180 min), agitation speed (90–210 rpm), and adsorbate concentration (5–150 mg/L) on the adsorption for all three adsorbents was studied in the batch experiments. At a pH value of 2 and an adsorbate dose of 25 g/L, chromium removal was found to be maximum for all three adsorbents. After a contact period of 120 min, it was found that the adsorption of chromium reached equilibrium for all three adsorbents. Dolochar was found to be the most effective one among the three adsorbents. Of the various adsorption isotherm models, the Langmuir isotherm appeared to be the best fit (R² > 0.99). From the fixed-bed column study at a bed depth of 10 cm, the breakthrough times were computed as 147, 193, and 219 min for fly ash, red mud, and dolochar, respectively. Similarly, the exhaust times were observed as 651, 780, and 631 min for fly ash, red mud, and dolochar, respectively. These results indicate that these industrial wastes can be utilized as low-cost alternatives to commercial adsorbents for the removal of Cr(VI).