Removal of chromium in wastewater with natural clays in southern Malawi

Sammanfattning: To live a healthy life, people all around the world need access to safe water. A lot of industries,together with the fast growing population in Blantyre, a city in southern Malawi, pose a threat to theaccess of safe water for the citizens. Several of the industries in Blantyre release contaminated waterto the nearby streams. One serious pollutant emitted from a match factory is chromium (Cr),especially in the occurrence of Cr(VI) which is carcinogenic for humans. Earlier studies have shownthat the concentration of chromium in the match factory’s wastewater was higher than WHO:sguidelines. It has also been published that natural clay minerals can be used for adsorption of Cr(III).This study investigates the removal of Cr(VI) through the adsorption of Cr(VI) to clay minerals or byreduction of Cr(VI) to Cr(III) which is precipitated from solution.The laboratory work performed in this study includes both experiments for adsorption of Cr(VI) andreduction of Cr(VI) to Cr(III). The reducing agents investigated were two clays and Fe(II) sulfate. Foradsorption of negatively charged Cr(VI) compounds, the two clays where used at lower pH and themineral bauxite was also tried as adsorbent agent. Wastewater from the match factory was dilutedand mixed with the removal agents and the concentration of total chromium was measured beforeand after the mixing process. For the agent with highest potential for Cr(VI) removal, the optimalconditions due to pH, dosage of agents and contact time were inspected.None of the investigated reducing agents served its purpose which means that no Cr(VI) was reducedto Cr(III) in this study. The adsorption of Cr(VI) with clay 1 at lower pH was also not successful, butthe adsorption worked for clay 2 at lower pH and for bauxite. In the experiments bauxite adsorbed ahigher amount of Cr(VI) than clay 2. The adsorption with bauxite turned out to be independent interms of pH. The optimal conditions for bauxite in the experiments were with a dosage of 3.5 gbauxite in 50 mL wastewater and a contact time of 40 minutes. The results showed an adsorption of93 percent of total chromium with bauxite at optimal conditions.