Characterization of waste roots from the as hyperaccumulator Pteris vittata as low-cost adsorbent for methylene blue removal

The perennial fern Pteris vittata is an Arsenic-hyperaccumulator plant able to grow in hydroponic cultures and hence used for phytoremediation of contaminated water. In order to abate the costs linked to the disposal of As contaminated biomass, in this work Pteris vittata waste roots were tested as a new low-cost bio-adsorbent for the removal of Methylene Blue (MB). The latter was selected as a representative cationic dye since its wide usage in industrial applications. The pH of zero charge was evaluated to be 6.2, hence all the adsorption tests were performed at a neutral pH. Isotherms at 20 °C and 40 °C showed a typical Langmuir trend with a maximum adsorption capacity (qmax) of 112 mg/g and 154 mg/g respectively. Kinetic tests were also carried out for different solid-liquid ratios and fitted by a mathematical model. The effective diffusivity of MB in the solid was estimated using the maximum likelihood method and the values of 5.99·10−8± 9.6·10−9 cm2/s and 9.56·10−8± 4.5·10−9 cm2/s were obtained at 20 °C and 40 °C respectively. After calculating the Biot number it was found out that both the intra-particle resistance and the external mass transfer resistance are important for the description of the rate of the dye uptake. This paper sets the basis for further investigation of such material in continuous systems in order to verify its feasibility of application in industrial apparatus.