Removal of Zn(II) and Hg(II) from aqueous solution on a carbonaceous sorbent chemically prepared from rice husk

A carbonaceous sorbent was prepared from rice husk via sulfuric acid treatment. Sorption of Zn(II) and Hg(II) from aqueous solution was studied varying time, pH, metal concentration, temperature and sorbent status (wet and dry). Zn(II) sorption was found fast reaching equilibrium within similar to 2 h while Hg(II) sorption was slow reaching equilibrium within similar to 120 h with better performance for the wet sorbent than for the dry. Kinetics data for both metals were found to follow pseudo-second order model. Sorption rate of both metals was enhanced with temperature rise. Activation energy, E(a), for Zn(II) sorption, was similar to 13.0 kJ/mol indicating a diffusion-controlled process ion exchange process, however, for Hg(II) sorption, E(a) was similar to 54 kJ/mol indicating a chemically controlled process. Sorption of both metals was low at low pH and increased with pH increase. Sorption was much higher for Hg(II) that) for Zn(II) with higher uptake for both metals by rising the temperature. Hg(II) was reduced to Hg(I) on the sorbent surface. This was confirmed from the identification of Hg(2)Cl(2) deposits on the sorbent surface by scanning electron microscopy and X-ray diffraction. However, no redox processes were observed in Zn(II) sorption. Sorption mechanism is discussed. (C) 2009 Elsevier B.V. All rights reserved.