Biosorption by Green Alga Chlorella vulgaris for Decontamination of Pb(II) from Wasters at Near Benign Concentration

Abstract

Lead ion, i.e., Pb(II), is one of the most toxic heavy metal ions (HMI) that constitutes a significant threat to aquatic ecosystems and human health. It is non-biodegradable and, therefore, could build up in human bodies to a harmful level via biomagnification and bioaccumulation, even when it exists at a very low level in the environment. Its presence in the environment is mainly due to human activities such as mining, burning fossil fuels, and industrial operations, which contaminate natural water sources through pathways such as precipitation of dust in the contaminated atmosphere, runoffs of contaminated soils, and discharge of contaminated wastewater. Corrosion of pipelines may also cause leakage of heavy metals into tap water. The levels of HMI, including Pb(II), in contaminated natural waterbodies are mostly at near-benign concentrations (10-100 ppb), which is harmful but very difficult to decontaminate with conventional methods. This study investigated the potential of utilizing growing green alga Chlorella vulgaris in removing Pb(II) from water sources contaminated with near-benign concentrations (10-100 ppb). The growth and behavior of C. vulgaris cells under Pb(II) exposure were examined. It was first established that the biosorption of Pb(II) by growing C. vulgaris in culture containing near-benign level Pb(II) was very similar to that of non-growing cells with short contact time. Then, biosorption tests were carried out with non-growing cells to analyze the effects of various factors such as pH, contact time, biomass concentration, and Pb(II) concentration. Results indicate that C. vulgaris exhibits promising biosorption abilities, with efficient removal of Pb(II) attributed mainly to surface adsorption mechanisms. PH played an important role in biosorption, with increasing pH enhancing biosorption in the tested range of pH 2.0 - pH 7.0. The rapid adsorption kinetics with the plateau achieved within 5 minutes contact time and the desorption of over 85.7% of lead from cells using EDTA buffer support the hypothesis that the biosorption of Pb(II) at near-benign levels is surface adsorption, which has been well reported for the biosorption of Pb(II) and other HMI by microalgae at high-level biosorption tests. Results of the study show that using C. vulgaris could effectively decontaminate Pb(II) contaminated water to meet drinking water standards within a well-defined operation window.