Fabrication of carboxymethyl-functionalized porous ramie microspheres as effective adsorbents for the removal of cadmium ions

Ramie fiber, one of the strongest and oldest natural fibers, has gained much interest in adsorption of heavy metal ions in water, however, it is severely limited due to the low response rates and low adsorption capability to heavy metal ions. A novel type of carboxymethyl-functionalized porous ramie microspheres (CPRM) were prepared with a combined emulsification-evaporation method and a carboxymethylation reaction, in order to effectively remove Cd(II) in water. XPS and FTIR characterizations confirmed that the carboxyl groups were introduced onto the CPRM. The diameters of the CPRM mainly lied within the range of 8–12 μm, and the BET surface area of the CPRM was 47.6 m2/g. SEM images confirmed a porous structure was present on the CPRM surface. The CPRM obtained a high adsorption capacity of Cd(II) (173.1 mg/g) at 303 K, which was higher than most reported conventional adsorbents. The adsorption isotherms and the kinetics were in agreement with the Langmuir isotherm model and the pseudo-second-order kinetic model. The process was both endothermic and spontaneous. The CPRM could also remove Cd(II), Pb(II), Ni(II), and Zn(II) from wastewater with various efficiencies. The Cd(II)-loading CPRM could be successfully regenerated by the HCl and NaOH treatment. The CPRM show good potential for purifying wastewater.