Document Type: Review Paper
Department of Chemical Engineering. Faculty of Engineering, University of Abuja, P.M.B. 117, Airport Road, main campus, FCT, Abuja, Nigeria.
Effluents containing heavy metals are hazardous to human health and the environment even at low concentrations. It is costly and unsustainable to use conventional methods to remove heavy metals from dilute effluents. Microalgal biomass owing to its high metal biosorption capacity, is a promising alternative biosorbent for treating dilute heavy metal solutions. However, the application of freely suspended algal biomass for metal removal has a number of drawbacks such as small particle size, low chemical resistance, low mechanical strength, and difficulty in separation of biomass and effluent. The present article reviews the techniques used to address these drawbacks. It also discusses the key factors affecting biosorption efficiency including initial concentration of metal ions, contact time, solution pH, solution temperature, biosorbent concentration, agitation rate, and competing ions. Biomass cross-linking with appropriate agents such as polysolfane, formaldehyde, or chlorohydrin could improve mechanical strength, chemical resistance, and separation of the biomass from the effluent. However, cross-linked biomass usually shows low sorption capacity and slow rate of metal uptake. These disadvantages could be minimized by using physical and/or chemical pretreatments prior to biomass cross-linking. Alkaline detergent, sodium hydrogen carbonate without autoclaving, sodium hydroxide or sodium carbonate plus autoclaving, or supercritical carbon dioxide at mild conditions are among the most effective pretreatments. Apart from liberating more latent metal binding sites on the biomass, supercritical CO2 could also improve the porosity of the biomass thereby improving sorption rate of the cross-linked biomass. High sorption capacity and rapid metal uptake will allow substantial reduction in size of biosorption columns, which will consequently improve the economic and sustainability features of algal-based metal biosorption processes.
- Biosorbents for heavy metals removal were reviewed.
- Microalgae show promising biosorption capacity.
- Microalgae need to be cross-linked for effective biosorption.
- Biomass pretreament prior to cross-linking could improve sorption capacity and rate.