Modeling of Pb(II) adsorption by a fixed-bed column

ABSTRACT

Removal of Pb(II) from an aqueous environment using biosorbents is a cost-effective and environmentally benign method. The biosorption process, however, is little understood for biosorbents prepared from plant materials. In this study, the biosorption process was investigated by evaluating four adsorption models. A fixed-bed column was prepared using a biosorbent prepared from the aquatic plant Hydrilla verticillata. The effect of bed height and flow rate on the biosorption process was investigated. The objective of the study was to determine the ability of H. verticillata to biosorb Pb(II) from an aqueous environment and to understand the process, through modeling, to provide a basis to develop a practical biosorbent column. Experimental breakthrough curves for biosorption of 50 mg L⁻¹ aqueous Pb(II) using a fixed-bed column with 1.00 cm inner diameter were fitted to the Thomas, Adams-Bohart, Belter, and bed depth service time (BDST) models to investigate the behavior of each model according to the adsorption system and thus understand the adsorption mechanism. Model parameters were evaluated using linear and nonlinear regression methods. The biosorbent removed 65% (82.39 mg g⁻¹ of biosorbent) of Pb(II) from an aqueous solution of Pb(NO₃)₂ at a flow rate of 5.0 ml min⁻¹ in a 10 cm column. Na₂CO₃ was used to recover the adsorbed Pb(II) ions as PbCO₃ from the biosorbent. The Pb(II) was completely desorbed at a bed height of 10.0 cm and a flow rate of 5.0 ml min⁻¹. Fourier transform infrared (FT-IR) analysis of the native biosorbent and Pb(II)-loaded biosorbent indicated that the hydroxyl groups and carboxylic acid groups were involved in the metal bonding process. The FT-IR spectrum of Pb(II)-desorbed biosorbent showed an intermediate peak shift, indicating that Pb(II) ions were replaced by Na⁺ ions through an ion-exchange process. Of the four models tested, the Thomas and BDST models showed good agreement with experimental data. The calculated bed sorption capacity N₀ and rate constant k_a were 31.7 g L⁻¹ and 13.6 × 10⁻⁴ L mg⁻¹ min⁻¹ for the C_t/C₀ value of 0.02. The BDST model can be used to estimate the column parameters to design a large-scale column.

KEYWORDS:

• Bed depth service time model
• biosorption
• desorption
• fixed-bed column modeling
• Hydrilla verticillata
• Pb(II)

Funding

The authors express their gratitude to the National Research Council of Sri Lanka for their financial support under grant no. NRC 13-087.