Macrophyte biomass productivity for heavy metal adsorption

Abstract

The search for low cost adsorbents that have metal-binding capacities has intensified in the last decades. Some natural aquatic macrophytes have been studied as adsorbents to remove heavy metals. Macrophytes ease to propagate converts them into plague for many ecosystems while they are also considered by some activities as a residue; therefore its resignification implies positive environmental effects. Whereas these macrophytes can be obtained from water bodies where they develop naturally, controlled production in greenhouses may be more appropriate for its use as filling in fixed-bed reactors that must operate continuously throughout the year. This work focused on obtaining the macrophytes growth parameters in order to calculate the most proper greenhouse containers dimensions and their required cultivation periods to be employed in a determined fixed-bed reactor with a certain effluent flow to filter. These parameters include: the biomass yield, the area required to obtain certain dry weight of a given biosorbent, and the relative growth and propagation rates. Selected macrophytes species were Azolla pinnata, Salvinia molesta, Limnobium leavigatum, Lemna minor and Pistia stratiotes. The characterization of each biomass and the study of their performance as biosorbents were conducted for the removal of Cu (II), Pb (II) and Cr (VI) ions from aqueous solutions, always procuring the WHO guidelines for drinking water. Azolla pinnata resulted in the species with the highest percentage of dry weight (6.56%), and the lowest values of relative growth and propagation rates. This species was as well the most efficient in removal of Cu and Pb (96.7% and 99.4%, respectively), while Pistia stratiotes was better adsorbent for Cr with a removal of 58.8%. The possibility of metal recovery and macrophyte biomasses reuse was also proven. Given their natural abundance, elemental growth conditions and propagation rates, macrophytes represent a low cost alternative to the most efficient commercial adsorbents.

Keywords: Adsorption; Biomass productivity; Greenhouse cultivation; Heavy metal.