Biosorption of lead heavy metal from aqueous solutions using spirulina microalgae

Today, the removal of heavy metals from industrial effluents is significantly increasing, which can play an important role in protecting the environment and human health. In recent years, researchers have resorted to biological methods, especially biological and microbial methods to remove toxic heavy metals. Biosorption of heavy metals is a relatively new technology for the treatment of industrial effluents, and its purpose is to remove toxic metals and clean the environment, as well as recycle precious metals. This process is a rapid reaction between the metal and the cell surface (extracellular bonding) in which the metal adheres to the surface of molecules like a surface protein. Conventional biological processes in the biological purification of metals are biosorption, bioaccumulation and bio-sedimentation. In the present study, sprulina platensis ability to absorb heavy metal lead from aqueous solutions was investigated by optimizing the absorption conditions for live spirulina microalgae. The aim of the present study is to provide a solution for biological removal of lead heavy metal using biosorption method using spirulina microalgae.

Three parameters of algae concentration (0.5, 1, 1.5 and 2 g/l), lead concentration (20, 40 and 60 mg/l) and contact time (30, 60, 90, 180 and 360 minutes) was considered.as experimental parameters. In order to perform the tests, 12 culture samples were placed. By examining the growth rate and after the cultures reached the desired concentration, three concentrations of lead were added to the samples and after certain periods of time, the culture was sampled and the adsorption rate was measured. The adsorption isotherm was then plotted to determine the theoretical capacity of the contaminant absorption.

The results showed that the optimum growth occurred in light conditions of 3000 lux and pH 10 and the maximum optical density was 1.421. The results showed that higher concentrations of algae had a higher rate of biosorption and the maximum absorption efficiency was obtained at a concentration of 2 g/l spirulina. Examination of contact time effect shows that by increasing contact time up to 180 minutes, the amount of biosorption increases and then decreases, which is probably due to the toxicity of the remaining heavy metal on spirulina microalgae cells. The maximum removal of heavy metal was 81.33% and occurred at a concentration of 2 g/l spirulina and a dose of 20 mg/l lead in a contact time of 180 minutes. The results showed that the Langmuir isotherm could fit the experimental data better than other models.

The process of biological adsorption by spirulina consists of two stages of rapid absorption (first 20 minutes) and slow absorption thereafter. The absorption efficiency is upward for about 60 minutes and then gradually decreases. Also, the high absorption rate in the first 30 minutes (more than 50% of the total biological absorption in all samples) indicates the existence of a rapid absorption phase in the first minutes. The reason for the rapid absorption in the first 30 minutes is due to the rapid physical adsorption at the active sites of the algal cell, known as the negative sites, and the adsorption occurs by electrostatic force.