Biofiltration of toxic elementary by Azolla biomass

The aquatic fern Azolla binds heavy metals in a wide range of concentrations with high effectiveness, and it can therefore be used for the decontamination of polluted solutions. We have tested the application of its biomass as a practical biofilter for industrial waste treatment. In this report, we describe the properties of Azolla biomass which has been dried, rewetted and packed in columns, for use as an ion-exchange matrix. We used scanning electron microscopy in conjunction with an elemental analysis of the Azolla biomass to investigate its structure and to localize exchangeable cations. Azolla elemental content and model solutions containing Sr²⁺ eluted through the biofilter were analyzed. The Azolla biofilter bound Sr²⁺ ions efficiently by ion exchange with K⁺, Na⁺, Ca²⁺ and Mg²⁺. K⁺ and Na⁺ ions were eluted during the initial phase of Sr²⁺ binding, due to their lower affinity to the cation-binding groups in the Azolla cell wall. Ca²⁺ and Mg²⁺, bound with a higher affinity to the Azolla cation-binding groups, were exchanged during the secondary phase of Sr²⁺ binding. Pre-loading of the metal-binding groups with K⁺ increased the removal of heavy metals from solution by the Azolla biofilter. Our observations explain the mechanism of heavy-metal adsorption to the Azolla matrix.