Evaluation of Banana Peels, Pumice and Charcoal Potential to Adsorb Chromium Ions from Tannery Wastewater

Abstract

Heavy metal contamination has been a serious problem throughout the world because of the hazardous effects on the health of humans. Of major concern is chromium heavy metal which is a major pollutant from the tannery industry. Chromium often occurs in the trivalent, Cr (III), and hexavalent, Cr (VI), states under natural environmental conditions. The hexavalent state of chromium is highly toxic and carcinogenic. Although technology advancements in heavy metal removal in wastewater have been observed, conventional methods of chromium removal and recovery are expensive to implement particularly in developing countries. Research has hence focused on the use of low-cost adsorbent materials to reduce and remediating chromium metals from tannery wastewater. This research study investigated the use and effectiveness of pumice, charcoal and banana peels as adsorbents to remove chromium from stock chromium solutions in batch experiments and from tannery wastewater in a pilot study. The three adsorbents were prepared by rinsing and drying in an oven before use. Batch experiments were conducted to evaluate the effect of contact time, pH and initial concentration on chromium removal by the three adsorbents. To evaluate the practical applicability of these materials, field studies were conducted in a local tannery in Thika, Kenya. Aspects of time and adsorbent bed depth were varied to evaluate their effect on chromium removal from the tannery wastewater. The three adsorbent materials, showed high chromium removal for both batch and field studies with a range of 69.3- 98.9%. Chromium removal was observed to be effective at low pH of between 3 and 6, with reduction in the rate of chromium adsorption being observed with time. Adsorption equilibrium time for pumice and charcoal were observed at 120 min while that of banana peels was observed at 60 min. The adsorption of chromium increased with increase in concentration of the metal in solution. Adsorption results were modeled using kinetic and isotherm models. Experimental data indicated that chromium adsorption by all the three adsorbents follow the second-pseudo-order kinetics model (R2> 0.987). The three adsorbents were also observed to have fitted well with both the Langmuir and Freundlich isotherm models, with separation factor (RL<1) and n values (n>1) showing favourability for the adsorption of chromium. Adsorbent depth was observed to have no significant influence on the removal of chromium from tannery wastewater. Pumice and charcoal were not significantly different in the removal of chromium, and had high desorption capabilities (>90.5%). This demonstrates their practical applicability in real life industrial waste treatment. However, banana peels applicability in the field is not viable due to the biodegradation of the peels with time, as well as the low desorption levels (29.6%). This study also recommends the evaluation of effective modification methods that would improve the adsorptive capacities of the three adsorbents.