Removal of Cd2+ and Cr3+ ions from Aqueous Solution by Modified Polypropylene Plastic Waste: Equilibrium Study

Abstract

Heavy metals pollution of water poses a major environmental challenge due to its antagonistic effects. They are not biodegraded and tend to accumulate in human body to toxic levels linked to some health effects. Therefore, there is need for their removal in water. Technologies that exist such as membrane filtration are limited by their high operation cost. However, adsorption is a cheap, efficient, and easy technique of removal of pollutants from wastewater. There are limited information available on value addition of plastic waste for application as adsorbents. Global production of polypropylene amount to about 56 million metric tons with a large percent of these going to waste. Application of polypropylene plastic waste was explored in this study for remediation of Cd2+ and Cr3+ ions from model solution. This was achieved by first chemically modifying the plastic by use of hydrogen peroxide and hydrochloric acid then applying it for adsorption of Cd2+ and Cr3+ ions. The chemically modified plastic waste was characterized and applied for equilibrium adsorption batch experiments. The images obtained from SEM analysis confirmed availability of pores in the adsorbent which are critical for Cd2+ and Cr3+ ions adsorption. The FTIR results confirmed the attachment of hydroxyl groups on the adsorbent which are vital for removal of heavy metals. Effect of adsorbent dosage, contact time, solution pH and initial metal ion concentration were investigated and the concentration of the resulting filtrate after adsorption determined using ICP-AES. The results from optimization experiments showed that Cd2+ and Cr3+ ions adsorption was optimal at initial metal ion concentration values between 15mg/L and 20mg/L and contact time of 45 mins and 60 mins respectively. Optimum adsorbent mass of 0.1g and optimal pH values between 4.5 and 5.5. Equilibrium experimental results showed adsorption capacities of 7.395mg/g (Cd2+) and 6.225mg/g (Cr3+) for Langmuir isotherm model with R2˃0.99 indicating chemisorption process.