Removal of Zn and Ni by adsorption in a fixed bed of wheat straw

Zn⁺² and Ni⁺² in a solution were removed by biosorption in a fixed bed of wheat straw Triticum aestivum. The removal rate and the mass transfer coefficient for Zn⁺² and Ni⁺² were found to be proportional to the liquid superficial velocity to the power of 0.31 for the range of the particle Reynolds number from 18 to 445 (equivalent liquid rates of 0.00070–0.0175 m³ m⁻² s⁻¹). This agrees well with reported literature for mass transfer in a packed bed of solid particles under a laminar flow regime. Effect of the solution pH, temperature and the particle size (0.5, 1.0, 1.5 and 2.0 in.) on biosorption of Zn⁺² and Ni⁺² was also investigated. Biosorption of both Zn⁺² and Ni⁺² increased significantly with the solution pH from 4.0 to 7.0. On the other hand, Zn⁺² and Ni⁺² removal appeared to be insensitive to liquid temperature from 25 to 30 °C. Nevertheless, a 25% increase in the percentage removal of metal ions was observed with further increase of liquid temperature from 30 to 35 °C. However, the biosorbent particle size did not seem to have a systematic effect on the biosorption of Zn⁺² and Ni⁺². In addition, biosorption of Zn⁺² was not affected considerably by co-adsorption of the bimetal solution while biosorption of Ni⁺² decreased about 14%.