Functionalization of Shirasu-balloons Surface for Removal of Cadmium Ions from Contaminated Soil

Microspheres of aluminosilicate glass from volcanic sediment, whose surfaces were modified with functional groups, Shirasu-balloons (SB), were investigated as an adsorbent to remove cadmium ions from contaminated soil. The introduction of thiol groups (–SH) on the surface of SB enabled it to adsorb cadmium ions. Meanwhile, the introduction of an alkyl group (–ODS) made the surface of SB hydrophobic, and consequently increased the mechanical strength of SB. Those adsorbents could float up on the water surface after the adsorption process. The prepared adsorbent was characterized by FTIR spectrophotometry, elemental analysis and optical microscopy for making photographic images. The adsorption of cadmium ions on the adsorbent was investigated by comparing the results by a plain SB with those by thiol-functionalized SB. The adsorption maximum for cadmium ions was 3.75 mg/g for SH-SB and 2.62 mg/g for SH-ODS-SB, respectively. The adsorption of cadmium ions on functionalized SH-SB and SH-ODS-SB fitted well to the Langmuir isotherm equation. After a shaking time of 24 h, the recovering ratios of the absorbents from soil were 73 wt% for SH-SB and 98 wt% for SH-ODS-SB absorbents. In addition, the kinetic and adsorption-desorption properties of the adsorbent were also considered.