A comprehensive investigation of external mass transfer and intraparticle diffusion for batch and continuous adsorption of heavy metals using pore volume and surface diffusion model

Abstract

Separation of heavy metals from water bodies is a need of hour, to protect human kind, marine life, and the environment. In this context, adsorptive separation of heavy metal over cheap and environmental friendly biosorbent remains promising. Numerous studies have been carried out to characterize the adsorption of heavy metal ions through isotherms, kinetics, and simple mathematical models. However, such studies lacked in detailed mass transfer models evaluating heavy metal adsorption in batch and continuous modes. This work reports the application of pore volume and surface diffusion (PVSD) model investigating mass transfer processes of Cu (II), Cd (II), and Pb (II) adsorptions over alkali-treated Caryotaurens seeds, chemically modified Albizia lebbeck pods, and Schleichera oleosa bark, respectively, in batch and continuous modes. Heavy metal-biosorbent systems are represented by Freundlich, Temkin, and Langmuir isotherms, respectively. PVSD model equations (algebraic, ordinary, and partial differential equations) for batch and continuous modes are solved in gPROMS (General Process Modelling System). For validation, concentration decay and breakthrough curves concerning batch and continuous modes, respectively, are predicted and matched excellently with in-house experimental data and justified with R2 and RMSE values. In batch mode, external mass transfer coefficient suggested no constraint for heavy metal external transport, whereas, surface and pore volume diffusion coefficients were found to control the adsorption process. For continuous mode, breakthrough curves are predicted concerning metal solution flowrate, initial metal concentration, and bed height, and parameters viz. breakthrough time, exhaustion time, adsorption column capacity, and mass transfer zone are calculated. Axial dispersion and mass transfer coefficients signify negligible resistance for metal ions' bulk transport within the column. Surface and pore volume diffusions affected continuous adsorption process, with surface diffusion being dominant. The current PVSD model may represent other metal-biosorbent systems to estimate transport parameters suitable for continuous adsorber operation.