Metal adsorption on mosses: toward a universal adsorption model

Abstract

This study quantifies the adsorption of heavy metals on 4 typical moss species used for environmental monitoring in the moss bag technique. The adsorption of Cu2+, Cd2+, Ni2+, Pb2+ and Zn2+ onto Hypnum sp., Sphagnum sp., Pseudoscleropodium purum and Brachytecium rutabulum has been investigated using a batch reactor in a wide range of pH (1.3–11.0) and metal concentrations in solution (1.6 μM–3.8 mM). A Linear Programming Model (LPM) was applied for the experimental data to derive equilibrium constants and the number of surface binding sites. The surface acid–base titration performed for 4 mosses at a pH range of 3–10 in 0.1 M NaNO3 demonstrated that Sphagnum sp. is the most efficient adsorbent as it has the maximal number of proton-binding sites on the surface (0.65 mmol g−1). The pKa computed for all the moss species suggested the presence of 5 major functional groups: phosphodiester, carboxyl, phosphoryl, amine and polyphenols. The results of pH-edge experiments demonstrated that B. rutabulum exhibits the highest percentage of metal adsorption and has the highest number of available sites for most of the metals studied. However, according to the results of the constant pH “Langmuirian” isotherm, Sphagnum sp. can be considered as the strongest adsorbent, although the relative difference from other mosses is within 20%. The LPM was found to satisfactorily fit the experimental data in the full range of the studied solution parameters. The results of this study demonstrate a rather similar pattern of five metal adsorptions on mosses, both as a function of pH and as a metal concentration, which is further corroborated by similar values of adsorption constants. Therefore, despite the species and geographic differences between the mosses, a universal adsorption edge and constant pH adsorption isotherm can be recommended for 4 studied mosses. The quantitative comparison of metal adsorption with other common natural organic and inorganic materials demonstrates that mosses are among the most efficient natural adsorbents of heavy metals.