Batch and continuous fixed-bed column biosorption of Cd2+ and Cu2+

Biosorption of Cd2+ and Cu2+ by wheat straw (Triticum sativum) using a batch system and a continuous up-flow fixed-bed column was studied. For the batch system, the effect of pH over a range from 3.0 to 7.0 and the temperature from 20 to 40 °C on the metal removal was investigated. Various initial metal concentrations from 20 to 150 mg L−1 were used. Adsorption of metal ions was observed to increase with pH and temperature. By increasing the temperature from 20 to 40 °C at an initial concentration 100 mg L−1, the metal uptake increased from 12.2 to 15.7 mg g−1 and 9.1 to 12.2 mg g−1 for Cd2+ and Cu2+, respectively. Similarly, by increasing the solution pH from 3.0 to 7.0, adsorption capacity of Cd2+ and Cu2+ increased from 2.7 to 14.4 mg g−1 and 3.4 to 12.4 mg g−1, respectively. Among the three widely used isotherms, namely the Langmuir, Freundlich and Temkin models, the experimental data better fitted the Langmuir isotherm.For the continuous up-flow mode in a fixed bed of wheat straw, experiments were performed over a range of flow rates from 1.10 × 10−3 to 3.67 × 10−3 m3 s−1 (0.3–1.0 LPM) and varied bed heights from 0.5 to 2.0 m. The results obtained agree to the bed-depth service-time (BDST) model well. In addition, for estimations of the parameters necessary for the design of a large-scale fixed-bed adsorber, the experimental data were also fitted to the Thomas, Bohart–Adams and Yan models. The Thomas model appeared to describe the experimental results better. A mass transfer model for adsorption of metal ions in a fixed bed of wheat straw was also developed and found to agree with the experimental data fairly well.